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General introduction

The current concern in the disease diagnosis focuses on the early detection of diseases by a simple and non-invasive way. At the moment, the diagnosis of diseases is based on the analysis of the external symptoms appeared on the patient. This means that the patient is already in advanced stages of the disease which complicates the disease containment in the most cases.

Precisely, in some cases, the infection by some diseases can increase the concentration of some species in the blood. Liver or kidney diseases are associated with the increase of ammonia concentration in the blood. In a normal case, the excess of ammonia in the body is transformed into urea by the liver as presented in Figure 1a. The capacity of liver to remove ammonia, formed from the degraded amino acids, may be reduced because of cirrhosis or massive liver necrosis.

The blood bypasses the liver through the portal-systemic anastomoses in case of cirrhotic problem with portal hypertension which causes further inability to remove ammonia by the body.

In the case of portal hypertension-related esophageal varices rupture and in presence of a massive bleed, the swallowed blood develops even another source of ammonia. Blood is rich in proteins which degrades in the intestines and transforms into amino acids and further into ammonia.

Excess of ammonia in the blood, resulting from any of these problems, has a potentially toxic effect on the brain. The concentration of ammonia increases in the brain as shown in Figure 1b. It is considered to play an important nosogenesis role in hepatic encephalopathy disease [1].

The detection of ammonia excess in the blood faces many problems such as the invasiveness and the difficulties in measurements. Such analysis to detect ammonia in the blood is complicated and cannot be available for everybody. Because ammonia has high vapor pressure, it is emitted outside the blood throughout the exhaled breath. Thus, the concentration of ammonia gas in the exhaled breath increases in the case of liver problems. The detection of ammonia in the breath is offering a simple and noninvasive method for the diagnosis of liver diseases. For the diagnosis of liver disease by breath analysis, the first requested thing is to have a selective detection of ammonia because human breath contains a huge amount of volatile organic compounds (VOCs) in addition to nitrogen, oxygen, water vapor and carbon dioxide and so on. Such detection technique should be simple, small in size and low cost in order to be available for everybody and to be integrated into a smart portable device for example. Chemical sensors are offering a simple, low cost and small size sensing device. Recently, chemical sensors field has witnessed a great development. It is enough to count the number of publications in the specialized scientific journals "Sensors and Actuators" as well as in the specialized congresses "Eurosensors" (the 31th edition will take place in Paris, September 2017)

and "International Meeting on Chemical Sensors". Figure 2 illustrates the evolution of the number of articles related to gas sensors listed on ScienceDirect since the 1974s. In parallel to the development of research, the industrial market for chemical sensors has also shown very strong growth (+9.6%/year) since the end of 2000s with a volume estimated by Global Industry Analysts Inc. about $13 billion in 2011 and it is expected to reach $31.2 billion by 2020 [3]. The necessity of chemical sensors in many fields such as environmental and food safety, air quality automobile, and recently in exhaled breath analysis leads to notable progress. Among the most developed sensors to date, gas sensors based on semiconductor materials are not only well adapted to microelectronic techniques but also incorporate a wide variety of materials such as metal oxides, semiconductor polymers and other composites. Chemical sensors based on metal oxides are the most used because of their good sensitivity to gases. They were developed and marketed for the first time by Seiyama and Taguchi in the 1960s [4]. They used ZnO and then SnO 2 as sensitive materials for the detection of liquefied petroleum gases. Consequently, much research has been carried out to improve the performance of these chemical sensors. Among these oxides, tin dioxide is the most used and studied sensor because of its good sensitivity to gases and its chemical stability during operation in a polluted atmosphere. Despite these advantages, SnO 2 sensors faced major shortcomings such as a lack of selectivity and high power consumption due to the high operating temperature (300-500 °C). In order to meet the breath analysis application, one of the major challenges is to have a selective gas sensor. In addition to selectivity, an ideal gas sensor should be sensitive to low concentration of gases with fast response and recovery times. Metal oxides surface functionalization by organic molecules could be incorporated in clinical and laboratory diagnostic tool in order to fulfil these requirements [5].

number of articles

In the laboratory of SPIN center in Saint Etienne, especially in the group of Process of Solid Transformation and Instrumentation (PTSI), SnO 2 sensors have been well studied during the past 30 years for many applications, mainly for the industrial ones. The enhancements of the SnO 2 sensors, depending on the application, were achieved in different ways such as doping by other metal or metal oxide. For the breath analysis application, special improvement of SnO 2 sensor should be applied to fulfil all the requirements. In addition, such sensor is supposed to be integrated in the smart portable devices, so it should operate at room temperature.

The group of Valérie Stambouli at LMGP laboratory of Grenoble works on SnO 2 surface functionalization for the development of biosensors. They have experience on the functionalization of metal oxide surfaces especially SnO 2 .

So, the idea is to try to incorporate functionalized sensors in the application of breath analysis.

This work focuses on the organic functionalization of existing SnO 2 sensor from Saint Etienne laboratory. The functionalization is first started by the attachment of APTES because it can act as a substrate for grafting of different molecules. Functionalization of APTES can be achieved in vapor and liquid phases. Vapor phase silanization was carried out at LMGP according to previous reported synthesis procedures. Liquid silanization was developed at PTSI by studying the effect of different synthesis parameters. SnO 2 functionalized by APTES was used as substrate to attach different functional groups. The different functionalized sensors were tested under ammonia and other gases in a dedicated test bench at Saint Etienne.

Therefore, this is an approach to obtain organic-inorganic materials which are expected to have special interactions with gases at low operating temperature.

This thesis is divided into four chapters as following:

Chapter 1 presents a bibliographic study on chemical sensors, in particular SnO 2 based gas sensors. The advantages and the blocking points of SnO 2 sensors are highlighted as well as the possible ways tested in the literature to improve their performance. Afterwards, a short review about breath analysis for disease diagnosis is presented. Finally, a brief review about organic surface functionalization is proposed.

Chapter 2 presents the experimental procedures for the preparation of SnO 2 sensors to be functionalized. The synthesis procedures of functionalization as well as the different characterization techniques used in this work are exposed.

Chapter 3 is essentially dedicated to the physico-chemical characterizations of pure and functionalized SnO 2 sensors by multiple techniques. In addition, the influence of synthesis parameters of APTES modified SnO 2 is investigated.

Chapter 4 is devoted to the electrical characterization of the different functionalized sensor. Tests are carried out under ammonia and other interfering gases such as ethanol, carbon monoxide and acetone. A sensing mechanism is proposed in this chapter.

In the end, the conclusion and some outlook of the present work are proposed.

Bibliographic study Chapter 1.

General review about chemical gas sensor

A chemical sensor is a device that transforms chemical information, such as concentration or composition of a sample to be analyzed into a useful signal (electrical or optical). Typically, chemical sensors consist of two main parts, a receptor and a transducer arranged as illustrated in Figure 1-1. The receptor is composed from a sensitive material allowing the recognition of the target compound with which it interacts. Transducer system transforms the interaction between the target compound and the sensitive element into a measurable quantity. This interaction is recognized in most cases by a variation of the physical characteristics of the sensitive material (conductance, temperature, permittivity, mass, etc.). A chemical gas sensor is therefore capable of providing information representative of the presence or concentration of a chemical compound in a gaseous mixture. Separator can be introduced in a sensor to eliminate the particles in the gas sample, e.g. a membrane. A chemical sensor is not an autonomous system but is one of the essential components of an analyzer. Other parts such as transporting the analyzed sample to the sensor, conditioning the sample, processing the signal from the sensor, etc. may complement the chemical sensor according to the specifications of the application.

Different families of chemical gas sensors

The chemical sensors are classified according to the principle of transducer. Several transduction principles are now being commercially exploited for the detection of gases. At the present time, measurements of gas concentrations (gas analyzers) are generally based on physical principles such as Mass Spectrometry, Ultraviolet or Infra-red Absorption Spectrophotometry or Chromatography. These systems are very efficient but also very voluminous and very expensive, which reduces their use in the analysis of samples of gases taken from the real environment. The so-called "real-time" measurement has therefore lead to numerous sensor developments based on different principles. Table 1-1 gives an overview of various families of gas sensors. These include sensors based on infrared absorption that are most commonly used in safety systems and for high-precision gas analyzers. Each type of the sensors presented in this simplified table has advantages and disadvantages in terms of certain characteristics such as selectivity, stability, energy consumption and cost. Despite their lack of selectivity and long-term stability, semiconductor gas sensors has been marketed and widely spread since 1968, because they offer many advantages such as price, portability, sensitivity, response time, etc. They are widely used in automotive, indoor/outdoor controlling of gases and healthy applications. For the advantages stated above, semiconductor gas sensors are the object of particular attention both in research and industrial level. However, some blocking points remain problematic such as selectivity which prevents the desired performance with these systems from being achieved. From several decades, research laboratories are still trying to develop new sensors always more efficient, while remaining small in size and inexpensive.

Main characteristics of gas sensors

In the ideal case, a gas sensor must provide information about the nature and concentration of a chemical compound. As with any instrument, several criteria can generally be taken into account when defining the performance of a sensor. In the field of chemical sensors, these criteria are often the sensitivity, selectivity and stability. In addition, reversibility, response and recovery time, limit of detection and reproducibility should be taken in consideration.

Sensitivity

The first quality that is sought for a sensor is its sensitivity to gases. The sensitivity is a parameter which expresses the variation of the sensor response as a function of the variation of the concentration of a gas. A gas sensor is said to be sensitive if a small change in concentration causes a large variation in the output signal. In other word, the sensitivity is the slope of the calibration curve. The curve of sensor response versus the concentrations of the target gas is usually named the calibration curve. This curve is generally highly non-linear, so the sensitivity is not constant. Therefore, it is defined, for a given concentration of gas, by the relative or fractional variation of conductance (or resistance or other variable). The general formula of sensitivity is thus given as equation (1).

S 𝑖 = 𝑑𝑅 𝑑[𝐶] 𝑖 (1)
With S i : the sensitivity to gas i, dR: variation in sensor response (sensor output, resistance or conductance, etc.), d[C] i : the change in concentration of gas i, and

𝑑𝑅 𝑑[𝐶] 𝑖

: the variation of sensor response over the concentration variation of gas i.

The sensitivity is measured in units of output quantity per units of input quantity (Ω/ppm, Hz/ppm, etc.).

In order to compare different sensor sensitivities, relative and differential responses should be used. According to the literature, the sensor response can be written in four forms as shown in equations 2, 3, 4 and 5 depending on the value of response with respect to the initial state of the sensor (base line). When the response is in decreasing form with respect to the base line, equations 2 or 4 can be used, otherwise equations 3 or 5 is applied.

If we consider "G" (conductance) as sensor response, relative sensor response calculations are given in equations 2 and 3. Normalized sensor response calculations are shown in equations 4 and 5.

𝐺 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 = 𝐺 0 -𝐺 𝐺 0

(2)

𝐺 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 = 𝐺 -𝐺 0 𝐺 (3)
With G relative : relative sensor response.

G 𝑛𝑜𝑟𝑚𝑎𝑙𝑖𝑧𝑒𝑑 = 𝐺 𝐺 0 (4)
G 𝑛𝑜𝑟𝑚𝑎𝑙𝑖𝑧𝑒𝑑 = 𝐺 0 𝐺 (5)
With G normalized : normalized sensor response, G 0 : the conductance value under base gas atmosphere (generally air) or a fixed concentration value of the given target gas, G: Corresponds to the value of the conductance under a concentration of the target gas.

Selectivity

Selectivity is defined as the ability of a sensor to respond to a certain gas in the presence of interfering gases. It is a parameter to be taken into account regarding the applications in real atmospheres, because the sensor is often used to detect a gas in an atmosphere containing several gases.

Stability

The notion of stability is associated with problems of temporal drifts. These drifts are detected by an evolution of the responses (amplitude, shape) for a given gas or by the evolution of the baseline with the time under the same conditions. These drifts can have several origins, such as problem of reversibility or the instability of its surface [2]. There are two types of drifts: shortterm drift (e.g. observed after switching on) and long term (due to the instability of sensor).

Reversibility

It relates to the ability of the sensor to return to its initial state in absence of the target compound to be detected. In the case of a non-return to the initial conditions, we speak about poisoning of the sensor.

Response and recovery times

The response time is often defined as the time taken by the sensor to reach 90 % of the steadystate sensor response when exposed to the gas. The recovery time is the time taken to return to 10% above the initial value in air after cutting off the gas injection. The sensor is said to be with high performance, when it responds with less time needed. The response-recovery time depends not only on the sensor itself but also on the measurement conditions such as the dimensions of measuring chamber, gas flow, and readout electronics. Thus, the conditions where the test is carried out should be mentioned with the value of the sensor response and recovery times.

Limit of detection (LOD)

LOD corresponds to a signal equal 3 times the standard deviation of the conductance baseline noise. Values above the LOD indicate the presence of target gas, while values below LOD indicate that no analyte is detectable.

Reproducibility

The reproducibility of a gas sensor reflects its ability to produce the same response for the same gaseous atmosphere. The system is reproducible if it responds to a gas in the same way regardless of the number of measurements and the time between measurements. Reproducibility includes response-recovery time and mainly the sensitivity.

Moreover, there is the notion of technological reproducibility from sensor to sensor. It is possible to manufacture two sensors having the same physical and geometrical characteristics. The deposition of sensitive layers is particularly important because it determine the reproducibility of the sensors in most cases.

As a conclusion, each of these performances is of course more or less crucial depending on the applications: an application such as the air quality in the passenger compartment of a car will prefer the response time and the reproducibility rather than the low detection threshold. Another application such as human breath analysis will be more seeking for the lower detection limits of gases. In these two cases, the smallness of the sensor is not necessarily a priority, contrary to an instrumented textile application. This shows that it is almost impossible to design a generic sensor without considering dedicated application.

Metal oxide gas sensors

Historical background

Conductometric semiconductors metal oxide also known as chemo-resistive gas sensors, are based on metal oxide sensing layer. Actually they constitute one of the most investigated groups of gas sensors due to their variety of sensitive material and preparation methods. They have attracted the attention in gas sensors applications under atmospheric conditions due to their simplicity of use, large number of detectable gases in many application fields, flexibility in production and low cost [3][4][5][6]. In addition, chemical sensors based on metal oxides have other advantages such as portability, very large change in film conductance upon exposure to a reference gas, and fast response time etc. In the 1950s, Brattain et al. [7] and Heiland [8] have found that the electrical properties of a porous layer of semiconductor oxide deposited on a ceramic substrate are substantially affected in the presence of a low concentration of oxidizing or reducing gas. [10,11]. These oxides are operated at elevated temperature between 300 °C and 650 °C, which does not represent favorable operating conditions of the sensors in terms of stability, repeatability and energy consumption [12]. Among these oxides, tin dioxide is the most used and studied because of its good sensitivity to gases and its chemical stability during operation in a polluting atmosphere.

Tin dioxide (SnO 2 ) gas sensors

Tin dioxide is the inorganic compound with the formula SnO 2 . Tin (IV) dioxide also called stannic oxide is the most extensively studied gas sensing material and it is the dominant choice for solid state gas sensors. SnO SnO 2 materials is also used as electrode material in solar cells, light emitting diodes, flat panel displays, and other optoelectronic devices [14]. SnO 2 can be in different forms that have several useful properties, including that of a transparent conductive film which is used to define LCD alphanumeric and graphic patterns. To achieve robustness and durability in its role as a gas sensor, SnO 2 is usually prepared in the form of a ceramic [15] which is sintered onto a substrate, commonly the alumina. The first gas sensor based on SnO 2 for the detection of flammable/explosive gases was developed by TAGUSHI in 1962 [16]. In 1968, the research on metal oxide gas sensors led to the commercialization of the first sensor based on semiconductor oxide under the name Figaro TGS (Taguchi Gas Sensor) for the detection of domestic gas leakage [17]. This Japanese company then proposed different versions of SnO 2 -based sensors for the detection of natural gas (1980), hydrogen sulfide (1981), CO (1983). In total, the number of SnO 2 sensors currently used in Japan can be estimated at more than 80 million. Today, there are many companies offering this type of sensors, such as Figaro, FIS, MICS, etc. [START_REF] Bicerano | Prediction of polymer Properties[END_REF].

Electrical properties of SnO 2 thick film: effect of gas adsorption

In general, for a semiconductor, the conduction type originates from the creation of structural defects arising from stoichiometric deviations or doped impurities. In a real case, surface of a metal oxide can be imagined as a crystal which is cut, where the bonds between atoms on the surface are broken and defects in topology (gaps) appear. This intrinsic state is due to the abrupt discontinuity of the crystal lattice. Extrinsic states are due to the presence of foreign species on the surface of the solid and the interaction with the surrounding gaseous phase.

Usually SnO 2 is operated in ambient air which contains about 20.95% of oxygen. In the initial electronic situation (vacuum), oxygen interacts with surface vacancies of SnO 2 which induces upward band bending (i.e. surface energy barrier for electrons that trying to travel from the bulk to the surface) [6] as shown in Figure 1-3. In this case, the surface is charged negatively and the concentration of the electrons in the SnO 2 conduction band decreases leading to the reduction of the conductivity compared with flat band. Oxygen acts as electron acceptor on SnO 2 surface. In the vicinity of the surface of solid (bulk), there is therefore a zone of depletion poor in major carriers, comprising only positively ionized defects. The conductivity in the vicinity of the surface is consequently low. This interaction is favored at 300-450 °C because maximum amount of O -appears on the surface at this temperature range [4]. Conversely, if the gas is donor that is created for example by the adsorption of hydrogen, the surface is positively charged and there is accumulation of the free carriers in its vicinity. In both cases, the electron transfer between the volume and the surface will stop when the Fermi levels of the surface and the solid are equal. However, this transfer will result in a curvature of the conduction and valence levels in order to ensure the continuity of the latter between the surface and the solid.

The reaction mechanisms of gas detection in air are governed by the oxygen concentration and the type of species adsorbed on the surface of the sensitive material. The type and the concentrations of the adsorbed oxygen on the surface depend mainly on the operation temperature.

Adsorption of oxygen at the surface of SnO 2

In the case of metal oxides material such as SnO 2 , the presence of oxygen is necessary for the detection of reducing gases. In this case, the reducing gases react preferentially with the chemisorbed oxygen and rarely with the material directly [2,19]. The adsorbed oxygen is therefore the precursor of the detection by oxidation reaction of the reducing gases. In addition, oxygen reacts with the SnO 2 and has an effect on the concentration of charge carriers of the material, which consequently modifies its conductivity as shown before. Depending on the operating temperature, oxygen can exist as O 2 , O 2 -, O -, and O 2-adsorbed on the surface (with different reactivity) of the sensitive layer [4,[START_REF] Barsan | Conduction Model of Metal Oxide Gas Sensors[END_REF]. The evolution of these species on the surface of SnO 2 with the temperature can be summarized in Figure 1234. The reported surface oxygen species (Figure 1-4) were mainly observed with spectroscopic techniques (TPD, EPR and IR) on the surface of SnO 2 [4,[START_REF] Barsan | Conduction Model of Metal Oxide Gas Sensors[END_REF]. At the temperatures of interest in this study, which is between 25 and 200 °C, the oxygen species exist in neutral (O 2 ) and ionic molecular (O 2 --s) forms adsorbed on the adsorption sites (s) of SnO 2 grains. Equations ( 6) and (7) show how the physisorption of oxygen gas on SnO 2 takes place.

At elevated temperature (>400 °C), the adsorbed oxygen ion transform to double ionic as shown in equation (9). Here, chemosorbed oxygen takes electrons from the conduction band of SnO 2 .

O --s + e -↔ O 2--s (9)
Each of these reactions (equations ( 7), ( 8) and ( 9)) leads to decrease in the conductance of the whole metal oxide film by extracting electrons from its conduction band. Furthermore, these various types of oxygen at different temperatures affect the response of sensor to oxidizing and reducing gases. The description of the reaction mechanisms for detecting a gaseous compound by a sensitive surface of SnO 2 requires attention to its surface morphology. Generally, the semiconductor films used in gas detection consist of contiguous crystallites (Figure 12345). During the adsorption of negatively charged species at the surface (O -for example), an electronic depletion zone is formed at the surface of the grains and also at the grain boundaries. This results

in the formation of a potential barrier that modulates the electron flow from one grain to another.

This surface resistance will thus dominate the overall resistance of the material and leads to a decrease in the conductivity of the whole film. The length of the depletion layer between two neighbor grains is known as Schottky barrier. When Schottky barrier between the grains increases, the transfer of electrons become more difficult, so conductance decreases. As a result, any phenomenon tending to vary the oxygen content adsorbed will cause a change in the electrical conductance of the sensitive material. For example, a reduction in the amount of adsorbed oxygen is induced either by decreasing the oxygen content in the surrounding atmosphere or by consuming this oxygen by heterogeneous reaction at the surface. This is the basic principle that generates the electrical responses of the SnO 2 -based gas sensors. For these devices, the conductance of the sensitive layer is expressed by equation (10). Moreover, the conductivity is the product of two important factors: the number of carrier electrons or holes and the carrier mobility (eVs), which is defined as the ease with which a carrier moves through a material. Hence, the conductance is controlled by the temperature, which modulates the electrons number in the conduction band of SnO 2 .

𝐺 = 𝐺 0 . exp (- 𝑒𝑉 𝑠 𝑘 𝐵 𝑇 ) (10)

Principle of detection of reducing and oxidizing gases

When SnO 2 is exposed to gases other than oxygen present in the air, a chemical reaction may occur with the pre-adsorbed oxygen species. The reactions are essentially redox reactions. For example, reducing gas such as carbon monoxide (CO), interacts with the adsorbed oxygen ion and is transformed to carbon dioxide (CO 2 ) as shown in equation (11). CO gas consumes the adsorbed surface oxygen, giving back electrons to the conduction band. This will reduce the depletion layer which leads to increases in conductance. The reaction of CO on the surface of SnO 2 is well studied before because it is the most simple gas (i.e. typical reaction gives CO 2 ) [START_REF] Boyle | The effects of CO, water vapor and surface temperature on the conductivity of a SnO 2 gas sensor[END_REF][START_REF] Emiroglu | In situ diffuse reflectance infrared spectroscopy study of CO adsorption on SnO 2[END_REF]. This reaction takes place preferentially at sensor operating temperature between 250 °C and 400 °C.

CO + O --s → CO 2 + e -+ s (11) In contrast, the reaction of oxidizing gas with the surface results in more oxygen ions in the surface. This reaction will lead to decrease in conductance of the sensor. Therefore, SnO 2 exhibits two types of responses: increasing or decreasing in conductance in different magnitude, depending on the concentration, the type of gas and the operating temperature.

Adsorption of water (H 2 O)

Water vapor is an essential element to be investigated at ambient temperature. Indeed, water vapor in the breath of the human is very elevated (100% of relative humidity at 37 °C). Water vapor acts as an interfering gas for the detection of gases related to the detection of diseases by its interaction with the surface of SnO 2 . As the functionalization will be carried out at low temperature (<100 °C), the adsorbed water on the surface of SnO 2 will later participate in the attachment of organic molecules on the surface. Therefore, the effect of water is of great interest for our study.

The results from the literature of TPD and IR studies are summarized in Figure 1-6 [START_REF] Barsan | Conduction Model of Metal Oxide Gas Sensors[END_REF]. At temperatures of our interest (<100 °C), water is adsorbed by physisorption or hydrogen bond on the surface of the SnO 2 in molecular form. But, at temperature higher than 200 °C, water is adsorbed in the form of hydroxyl groups (OH) and molecular water is no longer present. IR and TPD have shown the presence of OH groups on SnO 2 . However, the interaction mechanism of adsorbed water with SnO 2 once the temperature increases is still under discussion. Many studies assume homolytic dissociation of adsorbed water molecule on the surface of SnO 2 into two OH groups. One is an "isolated" hydroxyl bond to Sn and the other is a "rooted" OH group generated from lattice oxygen [START_REF] Henzler | Oberflächenphysik des Festkörpers[END_REF][START_REF] Hernandez-Ramirez | Water vapor detection with individual tin oxide nanowires[END_REF]. Others authors say that the hydroxyl groups are generated from an acid/ base reaction of HOH sharing its electronic pair with Sn (Lewis acid site), and leaving the weakly bonded proton (H + ). This proton can react with the lattice oxygen which acts as Lewis base or with adsorbed oxygen [START_REF] Barteau | Site requirements of reactions on oxide surfaces[END_REF].

It is experimentally proven that SnO 2 based sensors exhibit reversible increase in surface conductance in presence of water vapor at temperature above 200 °C. These reversible changes in conductance are related to the presence and disappearance of OH groups on the surface [START_REF] Gillot | Study of the oxidation kinetics of finelydivided magnetites. II -Influence of chromium substitution[END_REF].

Likely, at room temperature, the conductivity of SnO 2 increases on exposure to water vapor [START_REF] Kuang | High-Sensitivity Humidity Sensor Based on a Single SnO2 Nanowire[END_REF][START_REF] Li | Water pre-adsorption effect on room temperature SnO 2 nanobelt ethanol sensitivity in oxygen-deficient conditions[END_REF]. There are three types of mechanism to explain the increase of surface conductivity. Two different types have been proposed by Heiland and Kohl [START_REF] Heiland | Physical and Chemical Aspects of Oxidic Semiconductor Gas Sensors A2 -SEIYAMA[END_REF]. The first one attributes the role of the electron donor to the 'rooted' OH group, this one including lattice oxygen according to equation (12).

H 2 O + Sn + O lat ↔ (HO-Sn) + (O lat H) + + e - (12)
where Sn and O lat are tin and oxygen atoms in the lattice, (HO-Sn) is referred to as an isolated hydroxyl or OH group and (O lat H) + is the rooted one.

The second mechanism of Heiland and Kohl takes into account the possibility of the reaction between the hydrogen atom and the lattice oxygen, and the binding of the resulting hydroxyl group to the Sn atom. The resulting oxygen vacancy will produce additional electrons as shown in equation ( 13).

H 2 O + 2Sn + O lat ↔ 2(HO-Sn) + V O 2+ + 2e - ( 13
)
where V O 2+ is an oxygen vacancy on Sn. Morrison [START_REF] Morrison | The Chemical Physics of Surfaces[END_REF], as well as Henrich and Cox [START_REF] Henrich | The Surface Science of Metal Oxides[END_REF], consider an indirect effect. These interactions could be the interaction between either the hydroxyl group or the hydrogen atom originating from the water molecule with acid or basic group, which are also acceptor surface states. They supposed that their electronic affinity could change after the interaction. It could also be the influence of the co-adsorption of H 2 O on the adsorption of another adsorbate, which could be an electron acceptor. Henrich and Cox suggested that the pre-adsorbed oxygen could be displaced by H 2 O adsorption. In any of these mechanisms, the particular state of the surface plays a major role due to the fact that it considers that surface defects will increase the dissociative adsorption.

These hydroxyl groups act as donor states. This implies an increase in the conductivity of the sensitive layer.

Modulation of temperature

Under ambient conditions, the most abundant reactive gases present are oxygen and water vapor.

As shown before, the operating temperature affects the quantity and quality of adsorbed species on the surface of SnO 2 . These variations with the change of temperature affect the conductance and the interactions with gases. For example, the temperature affects the response of SnO 2 sensor to reducing gases according to the following processes:

 Dissociative adsorption of oxygen (molecular oxygen does not interact with reducing gases).

Generally, this process begins at T > 170-200 °C to form atomic oxygen atoms.

 Adsorption of reducing gases molecules and conversion into oxidized form. This process is influenced by the type of reducing gas; for example, detection of H 2 or CH 4 in contrast to CO, which requires dissociative adsorption of molecules.

 Desorption of reducing gas.

 Desorption of chemisorbed oxygen.

The first two processes determine the decrease in sensitivity at low operating temperatures. In addition, the last two determine the drop of the sensitivity at high temperatures. The competition between these processes determines the temperature of maximum sensitivity. It means that gases have specific combination of adsorption/desorption parameters and reaction rates can have different temperature profiles of sensor response. Therefore, these gases have to be selectively recognized [START_REF] Valleron | Exhaust Gas Sensor Based On Tin Dioxide For Automotive Application[END_REF]. Each gas has a specific reaction rate and specific sensitive temperature as shown in Figure 1234567. Thus, sensors operated at different temperatures will show a degree of selectivity [START_REF] Tournier | Selective filter for SnO 2 -based gas sensor: application to hydrogen trace detection[END_REF]. Therefore, by changing operation temperature it is possible to create conditions where the response to one gas will exceed considerably the response to other one.

1.6. Conductometric room temperature gas sensors

General description

As already mentioned, the semiconductor gas sensors are operated at temperatures above 200 °C in order to increase reaction kinetics and sensitivity. In addition to the fact that the installation of a heating circuit on the sensor induces technological constraints, it also has an increase in manufacturing and operating costs of the devices. The working at elevated temperature could affect the stability of these devices with time. This phenomenon is mainly due to the diffusion of the oxygen in the network and to the dilation-agglomeration of the grains. Operation at high temperature is also undesirable for safety reasons when detecting the combustible gases [START_REF] Wei | A novel SnO 2 gas sensor doped with carbon nanotubes operating at room temperature[END_REF][START_REF] Comini | UV light activation of tin oxide thin films for NO 2 sensing at low temperatures[END_REF].

Efforts have been made in recent years to develop new sensitive materials operating at lower temperatures or even at ambient temperature. These new Conductometric gas sensors are mostly made up of conductive polymers, nanostructured thin films (nanoparticles, nanowires, integration of carbon nanotubes), composite materials (combination of metal oxides), or metal oxide layers modified by the addition of various catalysts (doping elements).

Conductive polymer based gas sensors

The use of conducting polymers in the field of gas sensor is of high interest because they are operated at room temperature. Conductive polymers refer to macromolecules which have the property of carrying charges (electrons or holes). They possess a conjugate structure allowing the delocalization of the electrons along the macromolecular skeleton. This extended π-conjugated system of the conducting polymers has single and double bonds alternating along the polymer chain.

Recently, conducting polymers such as polyaniline (PANI), polypyrrole, polythiophene have been widely investigated as an active layer for the development of gas sensors operating at room temperature [38]. They are characterized by good conductivity, high sensitivity, short response time at room temperature and good mechanical properties. These improved characteristics allow facile fabrication of sensors, thus making them popular [START_REF] Bartlett | Conducting polymer gas sensors Part III: Results for four different polymers and five different vapours[END_REF]. However, conducting polymer based sensors have disadvantages such as lack of selectivity and low stability. Thus, the major challenge in conducting polymer-based sensors is enhancing their selectivity and stability.

Among these conducting polymers, PANI is the most investigated recently.

Polyaniline has been widely investigated as room temperature gas sensors. The structural properties of PANI can be modified conveniently by adding metals, metal-oxides, organic molecules, or carbon nanotubes [START_REF] Athawale | Nanocomposite of Pd-polyaniline as a selective methanol sensor[END_REF][START_REF] Sadek | A polyaniline/WO 3 nanofiber composite-based ZnO/64° YX LiNbO 3 SAW hydrogen gas sensor[END_REF][START_REF] Khuspe | Camphor sulfonic acid doped polyaniline-tin oxide hybrid nanocomposites: Synthesis, structural, morphological, optical and electrical transport properties[END_REF][START_REF] Yun | Improvement of NO gas sensing properties of polyaniline/MWCNT composite by photocatalytic effect of TiO 2[END_REF] which in many cases improve the selectivity and stability. The modification of PANI films by SnO 2 nanoparticles can produce a sensor highly selective and responsive to NH 3 [START_REF] Deshpande | Studies on tin oxide-intercalated polyaniline nanocomposite for ammonia gas sensing applications[END_REF]. Working at room temperature improves the reproducibility and stability of gas sensors. Table 1-2 lists some gas sensors based on PANI conductive polymer operating at ambient temperature. As shown, PANI can be used for the detection of various gases such as ammonia, hydrogen and LPG gases. Concerning the PANI detection mechanism, many explanations were proposed in the literature including acid-base and redox reactions [START_REF] Bai | Gas sensors based on conducting polymers[END_REF]. PANI can exist in different oxidation states depending on the pH. Thus, PANI can change its oxidation state upon interaction with acidic or basic gases. This variation of oxidation states affects the conductance of the film [START_REF] Lakard | Gas Sensors Based on Electrodeposited Polymers[END_REF]. Despite these advantages of PANI-based gas sensors, they still have very low sensitivity for the detection at low gas concentrations. Recently, one of the main challenges in disease diagnoses field is how we can detect the disease when we are still feeling healthy. Today, it is highly requested to have the capability of predicting healthy people who are going to develop a disease in the future using very simple and affordable device. This approach is relying on chemical biomarker or so-called volatile organic compounds (VOCs). These compounds basically are chemical analytes which are emitted as results of the disease evolvements in the human body into the blood stream. These analytes should be emitted outside the human body because they have high vapor pressure. They can do it either throughout the urine, feces, skin, or breath. The detection of these volatile disease biomarkers throughout the breath would be the most beneficial [START_REF] Di Natale | Solid-state gas sensors for breath analysis: A review[END_REF]. This type of diagnosis is characterized by the noninvasiveness of the sample collection [START_REF] Wang | Artificial Sensing Intelligence with Silicon Nanowires for Ultraselective Detection in the Gas Phase[END_REF]. Each type of disease has a unique gases fingerprint in the breath exactly as people have a unique fingerprint that discriminates a person from another person [START_REF] Shehada | Silicon Nanowire Sensors Enable Diagnosis of Patients via Exhaled Breath[END_REF]. Thus, breath analysis permits to know whether the patient is healthy or has a disease, as well as it can predict the healthy people who have high risk to get a disease in the future. This is a critical point because if the disease can be identified at very early stages, the survival rates can be increased by several folds [START_REF] Shehada | Ultrasensitive Silicon Nanowire for Real-World Gas Sensing: Noninvasive Diagnosis of Cancer from Breath Volatolome[END_REF]. For this mission, the gas sensor is supposed to interact selectively with biomarker gases if it present in the breath. This type of sensors could be installed a smart phone. Once the person talk over the phone, the smart phone will react to the content of the breath and it will analyze the results. This is a quite important approach because it allows every person to get a diagnosis of the diseases without going into the clinic.

Exhaled breath of the human contains water vapor, nitrogen, oxygen, carbon dioxide, carbon monoxide, ammonia, and a mixture of VOCs [START_REF] Barreto | 8-Isoprostane in Exhaled Breath Condensate and Exercise-Induced Bronchoconstriction in Asthmatic Children and Adolescents[END_REF][START_REF] Montuschi | Measurement of 8-Isoprostane in Exhaled Breath Condensate[END_REF]. These gases are expired in air saturated with water vapor. Normally, the breath content varies from each individual person qualitatively and quantitatively [START_REF] Mathew | Technologies for Clinical Diagnosis Using Expired Human Breath Analysis[END_REF]. Ammonia is one of the disease biomarker. Indeed, ammonia in humans is converted to urea in the liver and then passes to urine through the kidney, while unconverted ammonia is excreted in breath at 10 ppb for healthy subjects [START_REF] Capone | Solid state gas sensors: state of the art and future activities[END_REF]. The ammonia concentration in exhaled air increases in case of malfunctioning of liver and kidney reaching more than 1 ppm in presence of renal failure [START_REF] Dubois | Breath Ammonia Testing for Diagnosis of Hepatic Encephalopathy[END_REF][START_REF] Güntner | Selective sensing of NH 3 by Si-doped α-MoO 3 for breath analysis[END_REF].

Ammonia

Ammonia gas has basic character and it is lighter than air, colorless under ambient temperature and pressure conditions, emitting a pungent odor (its olfactory concentration limit is generally between 5 and 25 ppm). It is stable at room temperature (thermal decomposition at 450 °C-550 °C).

This gas is widely used in many sectors of the chemical industry, including petroleum and fuel chemistry, metal processing, and paper industries. Ammonia in the atmosphere comes mainly from agricultural activities (fertilizers and animal waste). It is also used as a refrigerant gas. It is also found in cigarette smoke and certain household products [68].

Although often present in the environment, ammonia is considered as toxic gas, which can have adverse health effects such as irritation of the ocular system. In addition, it causes severe lung damage. This compound is also harmful to our ecosystem. It can form explosive mixtures with air in the range of 15 to 28% by volume. The contact of ammonia with certain products such as mercury, halogens, calcium, and silver oxide is a source of fires and explosions. Because of its toxicology and chemical reactivity, the use of ammonia is submitted to a number of regulations.

For information purposes, in France, the short-term exposure limit value measured over a maximum of 15 min is set at 20 ppm (14 mg/m 3 ). In addition, the threshold toxic limit value put to protect workers from long-term, measured over the duration of a workstation for 8 h, is 10 ppm (7mg/m 3 ). Thus, ammonia gas sensors are needed for the early detection of possible leakages from such systems.

Detection of ammonia by metal oxide gas sensors

Many techniques were described in the literature for the detection of ammonia gas. Among all, metal oxide-based gas sensors are the most frequently used and investigated at high and at low temperatures. Metal-oxide, mostly based on SnO 2 sensors have been manufactured and tested in the largest quantities [START_REF] Sberveglieri | Recent developments in semiconducting thin-film gas sensors[END_REF][START_REF] Hübner | Tin oxide gas sensors: an analytical comparison of gas-sensitive and non-gas-sensitive thin films[END_REF]. Several examples of ammonia sensors reported in the literature are shown in Table 1-4. For the detection of ammonia gas, Metal oxides can be used as pure, composite and doped. Among all material present, SnO 2 is the most powerful for the detection of ammonia. Many techniques were applied to improve the sensing performance of SnO 2 to ammonia gas such as doping with another metal, SnO 2 /carbon nanotubes [START_REF] Van Hieu | Highly sensitive thin film NH 3 gas sensor operating at room temperature based on SnO 2 /MWCNTs composite[END_REF], and hybrid sensors like SnO 2 /polyaniline [START_REF] Kondawar | Conductive polyaniline-tin oxide nanocomposites for ammonia sensor[END_REF]. However, all the reported sensors are limited with respect to sensor selectivity and the ability to detect very low concentrations of NH 3 (in the range of ppb).

Therefore, the presented sensors do not fulfill needs for the detection of ammonia in the human breath. To develop a selective and sensitive ammonia gas sensor based on SnO 2 material, it is essential to investigate the sensing mechanism. , OH, O 2-, etc.) adsorbed on the surface of the metal oxide interact with reducing gases at high temperatures (> 150 °C.) which leads to an increase in the conductance of the film. Thus, in the presence of ammonia, the oxidation reactions ( 14), ( 15) and ( 16) can occur depending on the operating temperature where the oxygenated species are predominant at the surface.

NH 3 + s ↔ NH 3 -s (14) 2NH 3 -s + 3O --s → N 2 + 3H 2 O + 3e -+ 5s (15) 2NH 3 -s + 3O 2--s → N 2 + 3H 2 O + 6e -+ 5s (16)
The same mechanism has also been proposed by Wagh et al. [START_REF] Wagh | Modified zinc oxide thick film resistors as NH 3 gas sensor[END_REF]. Shao et al. [START_REF] Shao | Interaction Mechanisms of Ammonia and Tin Oxide: A Combined Analysis Using Single Nanowire Devices and DFT Calculations[END_REF] modeled the adsorption of ammonia on a surface of SnO 2 : the retained mechanism consists in an NH 3 dehydrogenation reaction chain to form N 2 as shown in equations ( 17) and [START_REF] Bicerano | Prediction of polymer Properties[END_REF].

NH 3 -s + O(lat) → NH 2 + H-O(lat) + s (17) NH 2 + NH 2 → N 2 H 4 → N 2 H 3 → N 2 H 2 → N 2 H → N 2 (18)
All the proposed mechanisms lead to the formation of N 2 at the end of the adsorption of ammonia at temperature above 150 °C. These interactions give electrons to the conduction band of metal oxide (equations ( 15) and ( 16)) which leads to the increase in the conductance of whole film.

However, at temperature less than 150 °C, the sensor response becomes in opposite sense (decrease in conductance) which makes the mechanism of interactions more complicated due to -H -H -H -H the presence of many types of adsorbed species on the surface in atomic and molecular forms [START_REF] Khun Khun | SnO 2 thick films for room temperature gas sensing applications[END_REF][START_REF] Ghosh | Room temperature ammonium gas sensing behavior of upright-standing ZnO nano-sheets[END_REF].

Interactions of ammonia with SnO 2 gas sensors at ambient temperature

As shown before, the interaction of ammonia with n-type semiconductors at temperature more than 150 °C like SnO 2 leads to increase in conductance of the whole film. In contrast, at ambient temperature, decrease in conductance was observed for SnO 2 , rhodium doped SnO 2 , WO 3 , and CeO 2 [START_REF] Wang | Enhanced Sensitivity and Stability of Room-Temperature NH3 Sensors Using Core-Shell CeO 2 Nanoparticles@Cross-linked PANI with p-n Heterojunctions[END_REF] upon exposure to ammonia. This means that another mechanism takes place, other than the one showed before. Research has been carried out in this direction, but the mechanisms are still barely known to date. Sipra Choudhury et al. [START_REF] Choudhury | Room temperature gas sensitivity of ultrathin SnO 2 films prepared from Langmuir-Blodgett film precursors[END_REF] have studied the sensitivity of ultra-thin films of SnO 2 to ammonia at room temperature. The electrical current of these films decreases upon exposure to ammonia gas (15-30 ppm) as reported in Figure 12345678. The electrical current is proportional to the conductance. This means that the conductance of SnO 2 decreases when it exposed to ammonia. This response is generated as a result of the decrease in carrier mobility during ammonia adsorption. Betty et al. [START_REF] Betty | Discerning specific gas sensing at room temperature by ultrathin SnO 2 films using impedance approach[END_REF], detected ammonia at room temperature by ultra-thin SnO 2 using electrical impedance spectroscopy. These sensors showed a rapid increase in their electrical resistance in the presence of ammonia.

Reversibility of detection suggests that the ammonia molecules adsorb and easily desorb on the surface of SnO 2 at room temperature. This is due to the low interaction energy between the hydrogen of ammonia and the surface oxygen of SnO 2 compared with N-H bond of ammonia itself (E N-H-O = 5 kcal.mol -1 for ammonia-SnO 2 , E N-H = 93 kcal.mol -1 for ammonia molecule).

According to these authors, the adsorbed ammonia can function as active diffusion centers, thus reducing the mobility of the free carriers and consequently increase the resistance. This is due to the fact that the mean free path of the free charge carriers is comparable to the grain size. Likely, Kamalpreet Khun Khun et al. [START_REF] Khun Khun | SnO 2 thick films for room temperature gas sensing applications[END_REF], have found that SnO 2 sensor exhibits an decrease in the electrical conductance in presence of ammonia at temperature between 25 to 200 °C. They supposed that there is compensation between two different reactions. The first reaction is as mentioned in equation ( 15) before, which leads to increase in sensor conductance. The second is the reaction of ammonia with molecular adsorbed oxygen ion (O 2 -), producing nitrogen monoxide gas (NO) according to equation (19). In presence of oxygen, NO can be easily transformed into NO 2 which is very good oxidizing agent. The reaction of NO 2 with SnO 2 at ambient temperature leads to the decrease in conductance of the sensor. Therefore, the second reaction is the predominant one because the conductance actually decreases when SnO 2 sensor exposed is to ammonia at ambient temperature.

4NH 3 + 5s-O 2 -→ 4NO + 6H 2 O + 5s + 5e - (19)
In summary, research on SnO 2 -based sensors operating at room temperature is still very limited.

Efforts are still needed to understand the mechanisms of detection as well as the correlation between the nature of the response and the concentration of ammonia at low temperatures. These interactions will be discussed more in details in the next chapters.

Time (s)

NH 3 gas

SnO 2 surface functionalization

Many possibilities exist to enhance the performance of SnO 2 gas sensors such as selectivity, sensitivity, response/recovery time and stability with time. Selectivity of the gas sensor to a specific target gas in the presence of a mixture of gases is necessary for many applications in which the gas origin is of interest. However, in most cases, the gases often produce very similar sensor responses except when reducing and oxidizing gases are being compared. For example, gases such as ethanol, carbon monoxide and methane have appreciable cross-sensitivity which hinders the development of gas sensor able to distinguish these species [START_REF] Moos | A Brief Overview on Automotive Exhaust Gas Sensors Based on Electroceramics[END_REF]. The techniques which were developed for improving the selectivity of SnO 2 gas sensors includes the control of the sensor operating temperature [START_REF] Tournier | Selective filter for SnO 2 -based gas sensor: application to hydrogen trace detection[END_REF], addition of gas filter [START_REF] Tournier | Selective filter for SnO 2 -based gas sensor: application to hydrogen trace detection[END_REF], small amount of noble metals [START_REF] Tian | A low temperature gas sensor based on Pd-functionalized mesoporous SnO 2 fibers for detecting trace formaldehyde[END_REF][START_REF] Cabot | Analysis of the noble metal catalytic additives introduced by impregnation of as obtained SnO 2 sol-gel nanocrystals for gas sensors[END_REF][START_REF] Trung | Effective decoration of Pd nanoparticles on the surface of SnO 2 nanowires for enhancement of CO gas-sensing performance[END_REF], oxides mixture [START_REF] Zeng | Enhanced gas sensing properties by SnO 2 nanosphere functionalized TiO 2 nanobelts[END_REF][START_REF] Van Hieu | Enhanced performance of SnO 2 nanowires ethanol sensor by functionalizing with La 2 O 3[END_REF], hybrid film of SnO 2 and organic materials [START_REF] Bai | Gas sensors based on conducting polymers[END_REF][START_REF] Geng | Characterization and gas sensitivity study of polyaniline/SnO 2 hybrid material prepared by hydrothermal route[END_REF] as discussed before.

Organic surface functionalization

Inorganic and organic compounds have been proposed as gas sensing materials, and have gained increasing development respectively due to some particular advantages. Inorganic compounds have generally high chemical and thermal stability that allows their application under different operating conditions.

Organic functionalization is the modification of the surface of a material by reactive end groups for further modification, protective layers, biological or gas sensors, catalysis, solar batteries, etc.

These modifications lead to change in physical and chemical properties of the surface and later change in sensing response. The attached organic groups act as chemical antennas on the surface of sensor. The design of organically modified surfaces is a rapidly expanding field of research in material science, in which the central purpose is access to materials possessing tunable properties in different areas. These modifications involve attaching organic polymers to metal oxide or grafting of monolayer onto the surface using appropriate functional molecules.

Organic compounds are described by a synthetic versatility and reactivity, which makes it possible to modulate the molecular structure of the sensing materials to enhance the selectivity toward a target analyte [START_REF] Nardis | Preparation and characterization of cobalt porphyrin modified tin dioxide films for sensor applications[END_REF]105]. As shown before, the organic sensing materials such as conducting polymers can show response to target analyte at ambient temperature, which has a suitable operating and attractive outlook [38,[START_REF] Huang | Polyaniline, a Novel Conducting Polymer Morphology and Chemistry of its Oxidation and Reduction in Aqueous Electrolytes[END_REF][START_REF] Joshi | ZnO-nanowires modified polypyrrole films as highly selective and sensitive chlorine sensors[END_REF][START_REF] Wang | Organic/inorganic hybrid sensors: A review[END_REF]. Nevertheless, there are many drawbacks for using single inorganic or single organic sensing materials. Gas sensors based on inorganic sensing layer have high operating temperature and low selectivity. As for organic sensing materials, the drawbacks are poor chemical stability, and mechanical strength that restrict their practical application. The addition of the advantages of organic and inorganic materials is with high interest. In the past decade, various organic/ inorganic hybrid sensors were synthesized by intercalation of inorganic particles in organic film, as for example, the intercalation of SnO 2 particles in conducting polymers like polyaniline [START_REF] Khuspe | Ammonia gas sensing properties of CSA doped PANi-SnO 2 nanohybrid thin films[END_REF][START_REF] Sadek | A Room Temperature Polyaniline/SnO 2 Nanofiber Composite Based Layered ZnO/64° YX LiNbO 3 SAW Hydrogen Gas Sensor[END_REF][START_REF] Murugan | Enhanced sensor functionality of in situ synthesized polyaniline-SnO 2 hybrids toward benzene and toluene vapors[END_REF][START_REF] Khuspe | Facile method of synthesis of polyaniline-SnO 2 hybrid nanocomposites: Microstructural, optical and electrical transport properties[END_REF]. It can be also the inverse by surface functionalization of SnO 2 film by polyaniline [START_REF] Li | Rectifying effect of soluble polyaniline films[END_REF]. These sensors have enhanced sensing performances to several toxic or harmful gases for example NH 3 , ethanol, methanol, H 2 S and acetone etc. Molecular modification is another way to organically modify metal oxide surface which provides a better control of the density and orientation of the organic component at the surface.

Molecular layers of organic molecules can interact with the substrate in two different ways, either by physisorption or covalent bond formation. Physisorbed molecules retain their base structure upon adsorption, whereas molecules can decompose and be covalently attached in the chemisorption process, for example, dissociation upon adsorption on a hydroxyl terminated oxide surface [START_REF] Helmy | Self-assembled monolayers supported on TiO 2 : comparison of C 18 H 37 SiX 3 (X= H, Cl, OCH 3 ), C 18 H 37 Si (CH 3 ) 2Cl, and C 18 H 37 PO (OH) 2[END_REF]. These molecules possess a head chemical functionality (ethoxysilanes) with high affinity to bind with metal oxide surface and a tail with desired chemical composition (e.g. amine group). The self-assembly of ethoxysilanes terminated with amine group, in particular 3aminopropyltriethoxysilane (APTES), is in the focus of this dissertation.

The following section of the thesis will focus on possible covalent surface modification strategies of SnO 2 sensor. Thick SnO 2 films produced by screen printing technique will be intended to be functionalized by organic film to modify their sensitive properties to gases like ammonia.

Covalent functionalization

Self-assembled monolayer on SnO 2 1.8.2.1.

Many attachment methods have been used to covalently modify the surface of metal oxide by organic self-assembled monolayers (SAMs). As shown in Figure 123456789, carboxylic acid, alkenes, alkynes, amines, phosphoric acid, catechol and silane groups can be used in functionalization [START_REF] Pujari | Covalent Surface Modification of Oxide Surfaces[END_REF]. Among all of these methods, the most commonly used method to covalently functionalize metal oxide surfaces is through the self-assembly of organofunctional alkoxysilane and chlorosilane molecules. Silane technique can be applied onto the majority of metal oxide materials. The interest of SAMs comes from the ability to form very dense, stable, and highly ordered structures. In addition, the molecules to be grafted can arrange upon adsorption reaction with the substrate (SnO 2 ). Depending on the end functional groups, SAMs can selectively interact with gas. In this case, metal oxide sensitivity may be changed with SAMs of organic molecules bearing the desired functional groups. The process to functionalize a metal oxide by silane products is known as silanization. Silanes are existing in different bases (alkoxysilane and chlorosilane) and different end functional groups as shown in Figure 12345678910. Figure 1-11 gives a schematic representation of the formation of APTES monolayer terminated with amine onto silicon surface. It should be noted that this chemistry is mostly investigated on SiO x surfaces; other hydroxylated oxides surfaces can be modified as well (e.g. SnO 2 [START_REF] Stambouli | Development and functionalisation of Sb doped SnO 2 thin films for DNA biochip applications[END_REF], Al 2 O 3 [START_REF] Wang | Silicon-based nanochannel glucose sensor[END_REF]).

Previously, many papers have reported the silanization by APTES on silicon nanowires-based sensor devices [START_REF] Chen | Silicon-based nanoelectronic field-effect pH sensor with local gate control[END_REF][START_REF] Lin | Control and detection of organosilane polarization on nanowire field-effect transistors[END_REF][START_REF] Engel | Supersensitive detection of explosives by silicon nanowire arrays[END_REF]. APTES functionalized silicon films were used as pH sensors, biosensors, and gas sensors. As in SiO x , functionalizing SnO 2 by organic molecules could be of special interest to improve sensitivity, selectivity, and electrical properties in gas sensor applications and others. Among all the silane products, APTES is the most used in silanization of metal oxide surfaces.

Silanization mechanism 1.8.2.2.

Normally, metal oxide surfaces are terminated with hydroxyl groups under atmospheric conditions as mentioned before. These -OH groups are the active sites where the condensation reactions can take place. The major pathway to covalently modify OH-terminated oxide surfaces via self-assembly is the use of condensation reactions. Silanes with reactive groups, such as, methoxy-, chloro-and ethoxysilanes (ethoxysilanes in the case of APTES), are converted to hydroxysilanes in contact with water. Then, condensation reaction between -OH groups of APTES and the -OH of SnO 2 surface takes place. The resulting silanol moieties can then react with each other via another condensation reaction producing siloxane bonds. Besides covalent linkage, the interaction of APTES with metal oxide film can be also in form of physisorption, hydrogen bonding, or electrostatic attachment [START_REF] Acres | Molecular Structure of 3-Aminopropyltriethoxysilane Layers Formed on Silanol-Terminated Silicon Surfaces[END_REF]. Thus, annealing after silanization is important to remove all the non-covalent bonded APTES.

The hydrolysis of chlorosilane or ethoxysilanes to hydroxysilanes is a key step in the silanization.

It controls the final quality and morphology of the SAMs. The incomplete hydrolysis results in incomplete monolayer, while excessive hydrolysis leads to polymerization of silane on the surface of metal oxide. In addition, the same effects are observed if the surface of metal oxide does not have sufficient amount of adsorbed water. In these cases, creation of OH groups on the surface is needed and it can be done by plasma activation or by immersing in NaOH solution for 2 h [START_REF] Serre | Fabrication of silicon nanowire networks for biological sensing[END_REF][START_REF] Stambouli | Semiconductor oxide based electrodes for the label-free electrical detection of DNA hybridization: Comparison between Sb doped SnO 2 and CdIn 2 O 4[END_REF].

To the best of our knowledge, the properties of APTES films such as thickness, orientation, and the density of the monolayer have essentially been investigated on silicon oxide [START_REF] Ben Haddada | Optimizing the immobilization of gold nanoparticles on functionalized silicon surfaces: amine-vs thiol-terminated silane[END_REF][START_REF] Herzer | Fabrication of patterned silane based selfassembled monolayers by photolithography and surface reactions on silicon-oxide substrates[END_REF][START_REF] Ulman | Formation and structure of self-assembled monolayers[END_REF]. However, only a few studies on the preparation of APTES layer deposited on thick SnO 2 film have been explored, although the development of low cost films is of high interest [START_REF] Le | A Label-Free Impedimetric DNA Sensor Based on a Nanoporous SnO 2 Film: Fabrication and Detection Performance[END_REF]. APTES modified SnO 2 could have a special interaction with gases or can served as a substrate for further modifications.

Silanization by 3-aminopropyltriethoxysilane 1.8.2.3.

Silanization is the most widely process used to modify surface properties of hydroxide terminated oxide substrates [START_REF] Ulman | Formation and structure of self-assembled monolayers[END_REF][START_REF] Silverman | Comparative Properties of Siloxane vs Phosphonate Monolayers on A Key Titanium Alloy[END_REF][START_REF] Sagiv | Organized monolayers by adsorption. 1. Formation and structure of oleophobic mixed monolayers on solid surfaces[END_REF][START_REF] Wasserman | The structure of self-assembled monolayers of alkylsiloxanes on silicon: a comparison of results from ellipsometry and low-angle x-ray reflectivity[END_REF][START_REF] Ohlhausen | Time-of-flight secondary ion mass spectrometry measurements of a fluorocarbon-based self-assembled monolayer on Si[END_REF][START_REF] Fadeev | Trialkylsilane Monolayers Covalently Attached to Silicon Surfaces: Wettability Studies Indicating that Molecular Topography Contributes to Contact Angle Hysteresis[END_REF]. 3-aminopropyltriethoxysilane (APTES) is commonly used to introduce terminal amine functionality to the surface. In most cases, APTES is used as intermediate step for further modifications such as coupling reaction with acyl chloride and carboxylic acid [START_REF] Tiller | A novel efficient enzyme-immobilization reaction on NH 2 polymers by means of L-ascorbic acid[END_REF][START_REF] Onclin | Engineering silicon oxide surfaces using Self-Assembled monolayers[END_REF][START_REF] Cook | Hydroxyapatite-coated titanium for orthopedic implant applications[END_REF] to attach a variety of materials such as DNA, proteins and polymers [START_REF] Stambouli | Development and functionalisation of Sb doped SnO 2 thin films for DNA biochip applications[END_REF][START_REF] Cecchetto | A silane pre-treatment for improving corrosion resistance performances of emeraldine base-coated aluminium samples in neutral environment[END_REF][START_REF] Aissaoui | Silane Layers on Silicon Surfaces: Mechanism of Interaction, Stability, and Influence on Protein Adsorption[END_REF]. The main advantage of an aminosilane monolayer on the surface of metal oxide is the rapid formation of covalent bonds between the oxide surface and the anchoring groups. This covalent linkage stabilizes the monolayer and facilitates further modifications with the conservation of monolayer integrity [START_REF] Pujari | Covalent Surface Modification of Oxide Surfaces[END_REF]. Furthermore, APTES has bulky alkoxy groups that improve the stability of the resulting monolayer as shown by Linford et al. increasing hydrolytic stability using bulky alkoxy side [START_REF] Zhang | Chemical Vapor Deposition of Three Aminosilanes on Silicon Dioxide: Surface Characterization, Stability, Effects of Silane Concentration, and Cyanine Dye Adsorption[END_REF]. One major drawback of this molecule is the possibility of polymerization, which can occur if the reaction conditions are not carefully controlled [START_REF] Howarter | Optimization of Silica Silanization by 3-Aminopropyltriethoxysilane[END_REF]. The functionalization of metal oxide surface by APTES can be carried out in liquid or in the vapor phase [START_REF] Le | A Label-Free Impedimetric DNA Sensor Based on a Nanoporous SnO 2 Film: Fabrication and Detection Performance[END_REF].

Liquid phase silanization 1.8.2.4.

The first silane-based self-assembled monolayer produced in liquid silanization was reported by Sagiv in 1980 [START_REF] Sagiv | Organized monolayers by adsorption. 1. Formation and structure of oleophobic mixed monolayers on solid surfaces[END_REF]. In general, a hydroxylated surface like SnO 2 is introduced into a solution of a silane dissolved in an organic solvent. Many factors affect the nature of monolayer such as APTES concentration, reaction time, reaction temperature, type of solvent, and water in the reaction solution. The study of silanization parameters was carried out mostly on silicon oxide [START_REF] Aissaoui | Silane Layers on Silicon Surfaces: Mechanism of Interaction, Stability, and Influence on Protein Adsorption[END_REF]. It has been shown that keeping the film in 50 mM of APTES dissolved in toluene for 12 h at room temperature is sufficient to have successful grafting. Water in the reaction mixture plays an essential role in the silanization process. When the water quantity is too low, incomplete silane layer formed; however, too much water can lead to polymerization of silane producing multilayers [START_REF] Fadeev | Trialkylsilane Monolayers Covalently Attached to Silicon Surfaces: Wettability Studies Indicating that Molecular Topography Contributes to Contact Angle Hysteresis[END_REF][START_REF] Wen | Postassembly Chemical Modification of a Highly Ordered Organosilane Multilayer: New Insights into the Structure, Bonding, and Dynamics of Self-Assembling Silane Monolayers[END_REF][START_REF] Gun | On the formation and structure of self-assembling monolayers: III. Time of formation, solvent retention, and release[END_REF]. Ethanol [START_REF] Zebda | Electrical resistivity dependence of semi-conductive oxide electrode on the label-free electrochemical detection of DNA[END_REF], toluene [START_REF] Tan | Preparation and characterization of APTES films on modification titanium by SAMs[END_REF], methanol [START_REF] Morrill | Imaging 3-aminopropyltriethoxysilane self-assembled monolayers on nanostructured titania and tin (IV) oxide nanowires using colloidal silver nanoparticles[END_REF], are among the solvents used in liquid silanization. Ethanol has high capacity to dissolve water, so it can be ideal in liquid silanization to have well covered SnO 2 . Thus, it could be ideal to carry out the silanization in hydrous solution. The reaction at room temperature and the ease of controlling the reaction parameters are the advantages of liquid silanization. In gas sensor application it is essential to have very dense layer of silane to inhibit the direct interaction of gases with substrate which can change the response. The interaction of gases with the surface of Conductometric gas sensors can increase or decrease the conductance of metal oxide film. Thus the effect of substrate (SnO 2 ) and the attached organic film could be opposite. In this case, the sensor response could be reduced or annulled by the compensation of two interactions.

Vapor phase silanization 1.8.2.5.

Vapor phase deposition of silane is another way of functionalization. It was first reported by

Haller in 1978 [START_REF] Haller | Covalently attached organic monolayers on semiconductor surfaces[END_REF]. He carried out the silanization by APTES on silicon and gallium arsenide in vapor phase. Vapor silanization is very interesting because it decreases the polymerization of APTES molecules and lead to monolayer formation [START_REF] Li | Elucidating the role of surface hydrolysis in preparing organosilane nanostructures via particle lithography[END_REF]. This high-quality film of APTES is needed for many applications like biosensors [START_REF] Le | A Label-Free Impedimetric DNA Sensor Based on a Nanoporous SnO 2 Film: Fabrication and Detection Performance[END_REF]. It is achieved by exposing the metal oxide to APTES vapor for several hours in closed system at temperature between 50 and 120 °C. This process is generally accomplished in dry atmosphere to prevent the polymerization of APTES. However, to start the reaction, water is essential to reduce the ethoxy groups to hydroxyl. Chidsey et al. [START_REF] Lowe | Deposition of Dense Siloxane Monolayers from Water and Trimethoxyorganosilane Vapor[END_REF] investigated a new method to add very little and controlled amount of water molecules to the reaction process from the dehydration of MgSO 4 •7H 2 O. Actually, the miss of experience of many chemists with gas-phase reaction, limits the use of vapor silanization.

In this thesis, the functionalization of thick SnO 2 film by APTES in vapor and liquid phases will be studied. The effect of synthesis parameters will be investigated in liquid phase silanization.

APTES modified films will be conserved as a reactive substrate for further modifications.

Functionalization of APTES modified SnO 2 by organic functional 1.8.2.6. groups

The interesting point of molecularly functionalized materials is the large number of chemical and structural modifications available. The modifications with organic functional groups having different polarities could change the sensor response to specific gases depending on their polarity [START_REF] Wang | Effect of Functional Groups on the Sensing Properties of Silicon Nanowires toward Volatile Compounds[END_REF]. These interactions could be dipole-dipole, induced dipole or diffusion between the molecular layers. Many parameters can be adapted in functionalization like chain length, backbone conformation, end functional groups (electron donating/withdrawing groups). It can be carried out starting with APTES combined with acyl chloride molecules bearing alkyl, benzene, acid, or ester functional groups (Figure 1-12). These devices were tested under some aliphatic alcohol and alkanes molecules. The functionalized field effect transistors have shown responses to a wide variety of volatile organic compounds like alcohols and alkanes etc. (see Figure 12345678910111213).

The electrostatic field of the molecular layer can generate charge carriers in the silicon field effect transistor interface. Adsorption of volatile organic compounds (VOCs) on the molecular layer can take place in two types: first type is the adsorption on the surface of the molecular layer, and the second type is the diffusion between the molecular layers. For the first type, the interactions could be dipole-dipole or induced dipole. If the tested gas and the end functional group of the molecular layer are polar the interactions are called dipole-dipole, while if one of them is non-polar, in this case the interaction is named induced dipole [START_REF] Wang | Artificial Sensing Intelligence with Silicon Nanowires for Ultraselective Detection in the Gas Phase[END_REF][START_REF] Wang | Effect of Functional Groups on the Sensing Properties of Silicon Nanowires toward Volatile Compounds[END_REF]. When VOCs adsorb on the molecular layer's surface, they are attached to the functional groups on the surface.

Thus, the attachment of gases on the molecular layer's surface is controlled by the properties of surface functional groups. For example, alkyl and benzene functional groups are electron donating groups. In this case, the gas negative pole bonds to the functionalized molecular layer's surface. Under these situations, the dipole moment orientation of the adsorbed VOCs is similar to the molecular layer. As a result, adsorption of gases induces change in the molecular layer's dipole moment. Furthermore, the changes in the dipole moment of the functionalized film lead to change in the electrical properties. Moreover, the type of molecular functional groups on the surface also controls the diffusion of VOCs in the molecular layer. For example, ester functional group has methyl groups at the end. In this case, the steric hindrance prevents the diffusion of VOCs in the molecular layer.

Functionalized silicon nanowire field effect transistors have exhibited response to various VOCs.

However, these field effect transistor sensors still lack of selectivity. These effects could be very interesting when they are applied on SnO 2 gas sensor. The principal advantage offered by organic materials is the promise for better chemical specificity. The organic-SnO 2 gas sensors could be expected to exhibit a specific response to a particular gas. Ichiro Matsubara et al. [START_REF] Matsubara | Organically hybridized SnO 2 gas sensors[END_REF] have reported an organically hybridized SnO 2 gas sensors. The functionalization was carried out by attaching aminosilane film first. Second, these amines groups react with another molecule which contains carboxylic acid for coupling reaction.

Normally, the reaction between amine group and carboxylic acid is carried out in presence of coupling reagent like 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. In addition, these molecules can be ended with either benzene, dimethyl benzene, or dihydroxyl benzene as shown in Figure 1234567891011121314. This type of response is related to the interactions of CO gas with the grafted organic component.

There are hydrogen bonds between the hydroxyl groups of organic layer and the SnO 2 surface.

These hydrogen bonds results in decrease of the depth of space-charge region of SnO 2 . As CO is a polar molecule, it can break these hydrogen bonds which in turn increase the resistance. The response of DHBA-APS-SnO 2 to H 2 gas was supposed to be related to typical oxidation reaction on the surface of SnO 2 . Thus, the functionalized sensor with DHBA shows selectivity to CO gas with respect to other reducing gas. However, this sensor cannot be selective in the presence of oxidizing gases because the response will be in the same direction (decrease in resistance). In addition, the sensor detection limit to CO gas is very high (30000 ppm) and the operating temperature is relatively elevated for organically modified sensor.

Conclusions

As a result of this bibliographic study, gas sensors can be used for the detection of many gases such as ammonia. Metal oxide materials have attracted the attention in gas sensors applications under atmospheric conditions due to their simplicity of use, large number of detectable gases and thus application fields, flexibility in production and low cost. Among many metal oxides, SnO 2 was chosen due to its unique physical and chemical properties such as large energy gap and environmental-friendliness. In addition, SnO 2 shows high sensitivity to many target compounds, but at high operating temperatures (300-500 °C, [START_REF] Dufour | Increasing the sensitivity and selectivity of metal oxide gas sensors by controlling the sensitive layer polarization[END_REF] thick film is poorly studied.

Our objective is the organic functionalization of SnO 2 to prepare sensitive sensor to low ammonia concentrations, with high selectivity. Up to our knowledge, an ambient temperature ammonia sensor based on molecular modification of SnO 2 has not been reported. Another objective is to reduce the power consumption by decreasing the operating temperature, since the sensor will be used later for smart devices and portable applications.

In the next work, SnO 2 sensitive layer will be functionalized with organic molecules to enhance the sensor performance to ammonia. This sensor should be operated at ambient temperature to prevent the organic layer from thermal degradation. It is important to investigate the consequences of working at ambient temperature with respect to the sensing mechanism of pure SnO 2 to compare it later with the modified one.

Materials and methods Chapter 2.

Introduction

This chapter will focus on the methods of fabrication and characterization of organically functionalized SnO 2 sensitive layer. The synthesis of the desired sensors is carried out by deposition of SnO 2 film on alumina substrate followed by molecular functionalization. The deposited SnO 2 film will be organically functionalized in two steps. The first step is the silanization by APTES. As shown before, APTES can be deposited using vapor or liquid phases.

The second step is the functionalization of APTES modified SnO 2 by molecules bearing different end functional groups. For these syntheses, it is essential to be sure that the functionalization on SnO 2 film is well achieved. Hence, the different characterization techniques used and their principle will be presented in this chapter. Finally the parameters of the electrical characterization under gases will be described.

Sensor fabrication

It is essential to know the different parts of conductometric gas sensors to study their performances. A typical metal oxide gas sensor is composed of sensitive layer, which can be made of SnO 2 , ZnO, etc. in form of thin film, thick film, nanowire, or pellet. It can be deposited over a substrate such as alumina (Al 2 O 3 ), glass ceramics, flexible, and silicon substrates. The sensitive layer is connected with gold electrodes in order to measure its electrical characteristics.

Heating the sensors is needed to favor the interactions of the target compound with the sensitive film. For that, the sensor can be self-heated by attaching a platinum resistor on the other side of the substrate or by an external heater. Such device generally operates in air in presence of humidity and target gases. The most important part is the sensing film composition. Moreover, the particles size and the thickness of the sensing layer play important role in sensing behavior. SnO 2 thick film is generally deposited on a substrate by screen printing or inkjet technologies.

For all the sensors used in this work, thick SnO 2 film was deposited on alumina substrate by screen printing. Thus, SnO 2 ink is needed for printing it in this process.

Ink preparation

SnO 2 ink should have acceptable rheological characteristics such as the viscosity and the ratio of solvent to powder. The optimized ink composition was reported before in our laboratory by Jean-Paul Viricelle et al. [1]. This ink was prepared by grinding 4 g of SnO 2 (Prolabo Company, sieved at 200 µm) with mortar and pestle for 5 min, and added to 1.7 g of organic binder (ESL® V400A) and then stirred with 25 drops of organic solvent (ESL® 404). The ink was homogenized 3 times (90:30 µm; 30:10 µm; 15:5 µm) by three roll mill EXAKT 80E/120E machine.

Deposition of sensing element

The sensor elements except the gold electrodes attached to SnO 2 which are made by sputtering, are deposited by screen printing technique. Screen printing has advantages over other deposition techniques such as low cost, and ease of use. In general, screen printing is a simple and an automated manufacturing technique which allows the production of very porous and robust chemical sensors with a good reproducibility. The thickness of the film can be adjusted by controlling the distance between screen and substrate or by deposition of several layers. In addition, screen printing permits to produce many sensors at the same time.

In this work, a semi-automatic screen printer (Aurel Model C890) was used. Thick-film screen made from a woven mesh of polymer is mounted under tension on a stainless steel frame. The mounted screen has open-mesh areas through which the desired pattern can be printed. The screen is held in position within a screen-printing machine (screen holder) at a distance of 0.3 mm from the surface of the substrate. The paste (ink) is poured onto the top surface of the stencil, and a rubber squeegee traverses the screen under pressure. This action brings the screen in contact with the substrate and forces the ink through the open areas. The demanded pattern is thus deposited onto the substrate as shown in Figure 2-1. For each sensor element a specific ink, annealing temperature and screen were used. These different parts with its properties are presented in Table 2-

Surface modification of SnO 2 sensor by APTES molecule

In this work, the silanization of SnO 2 by APTES was performed in vapor and liquid phases. The presence of hydroxyl group at the surface of SnO 2 is essential to allow the condensation reaction with the hydroxyl groups of APTES. Thus, the first step is the creation of hydroxyl groups on Vapor phase silanization 2.2.3.1.

The silanization of SnO 2 film by 3-aminopropyltriethoxysilane (Acros-Organics) in vapor phase has been described elsewhere [3]. First, the SnO 2 films were immersed for 5 min in acetone, ethanol and distilled water successively to remove contaminants, and then dried under argon gas.

Second, films were treated by air/oxygen plasma (air/O 2 pressure =0.4 torr, homemade machine) for 4 min to clean the sample surface from residual impurities and to create hydroxyl groups at the surface of SnO 2 . Third, the SnO 2 sample and APTES (liquid) were placed side by side in a Teflon holder as shown in Figure 234. The installation of SnO 2 samples and 150 µL of APTES was conducted in a dry atmosphere to avoid the humidity in the Teflon holder. Then, the Teflon pot was sealed and heated for 1 h at 80 °C. After 1 h silanization, the samples were rinsed with absolute ethanol. Finally, the films were annealed at 110 °C for 1 h to eliminate water trapped in the deposited APTES layer.

Modification of APTES by functional groups

The produced SnO 2 -APTES film acts as an intermediate step for further modification. The next step of functionalization is the reaction of amine groups of APTES with acyl chloride (acid chloride). Acyl chloride is a reactive group regarding the reaction with amine which leads to the formation of amide bond. This reaction is known as Schotten-Baumann reaction [4]. Schotten-Baumann reaction leads to productions of one equivalent acid. This acid protonates the unreacted amine groups of APTES which lead to the formation of salt and reduce the reaction yield. The addition of triethylamine base is to neutralize this acid and pushes the reaction forward the formation of amide. In our case, acyl chloride molecule contains alkyl, acid or ester end functional groups on the other side. The main advantage of this reaction is that the end functional group is conserved after the reaction. A schematic illustration of the second step functionalization is reported in Figure 23456.

Methods of characterization

Pure and functionalized SnO 2 films have been characterized by different physico-chemical analytical techniques, namely Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Total Attenuated Reflectance Fourier Transform Spectroscopy (ATR-FTIR), and X-ray Photoelectron Spectroscopy (XPS). An indirect method was also used to investigate the presence of APTES molecules on SnO 2 surface by measuring the contact angles. Alexa Fluor fluorescent molecules and citrated gold nanoparticles have also been implemented to test the reactivity of APTES film on SnO 2 . Electrical characterizations under gases were carried out to test the effect of functionalization on the sensors performance.

Scanning Electron Microscopy

The scanning electron microscopy (SEM) technique was used to characterize the morphology of elaborated SnO 2 thick films and the APTES modified SnO 2 after deposition of gold nanoparticles. This technique mainly exploits the detection of the emitted secondary electrons from the surface excited by electron beam. The emitted secondary electrons give information about the topology and the composition of the analyzed specimen. SEM observations were carried out at the LMGP laboratory in Grenoble by XL30 microscope (Philips, Eindhoven, Netherlands).

X-ray diffraction

X-ray diffraction allows to determine the atomic arrangement and the interatomic distances in the crystal lattices and to estimate the average particle size. XRD was used in this study to determine the average size particle of SnO 2 film. For that, Siemens D5000 Diffractometer having X-ray tube with anticathode of copper was used. The machine is of configuration . The diffracted X-ray was detected by an AR monochromator detector.

Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy

Infrared Spectroscopy is a very useful tool for characterization, since it provides information about the composition and the structure of materials. In addition, FTIR has high sensitivity, high resolution and quickness of measurement.

Basically, an IR spectrometer is composed of source, monochromator and receptor. Normally, if the vibrational frequency of the molecule equals the frequency of the electromagnetic radiation, absorption can take place. As an approximation, the larger the strength of the bond the higher the frequency of its original vibration. Furthermore, the higher the masses of the atoms attached to the bond the lower the wavenumber of the original vibration. In general, the band intensity is proportional to the number of groups and the polarity of the bonds.

The domain of the infrared (IR) electromagnetic spectrum can be divided into three regions according to wavelengths: far-IR between 10 and 400 cm -1 , mid-IR between 400 and 4000 cm -1 , and the near-IR between 4000 and 12500cm Recently, the most used IR technique to characterize the solid surfaces is Attenuated Total Reflection (ATR). ATR requires a crystal where the sample to be analyzed is deposed on it. ATR is operated by measuring the changes that occur in a totally internally reflected infrared beam, when the beam comes in contact with a sample. An infrared beam is focused to an optically dense crystal (diamond, Ge, ZnSe, etc.) with a high refractive index at a certain angle. In this case, it is possible to generate an evanescent wave that extends beyond the surface of the crystal into the sample held in contact with the crystal as shown in Figure 234567. This evanescent wave protrudes between 0.5 μm and 5 μm beyond the crystal surface and into the sample. The quality of the contact between the crystal and the sample plays an important role in the quality of the spectrum: bad contact could reduce the total attenuated reflection and leads to irregular baselines and to the observation of few vibrational bands [5]. In our case, diamond crystal was used with 1 mm × 1 mm in size which allows making good contact between the sample to be analyzed and the diamond. The evanescent wave will be attenuated or altered in regions of the infrared spectrum where the sample absorbs energy. The attenuated energy from each evanescent wave is reflected back to the IR beam, which then exits the opposite end of the diamond and is passed to the detector in the IR spectrometer. The system then can generate an infrared spectrum.

The spectra presented in this study were collected in the region of the middle infrared. This region corresponds on the one hand to the spectral fingerprints of the functional groups constituting the grafted molecules on the surface, and on the other hand to the transparency ranges of the ATR diamond crystal. ATR-spectra were recorded using a Golden Gate Diamond In this thesis, IR technique will be used for the characterization of the functionalized sensors by APTES and other functional groups. As mentioned before, APTES grafting can be carried out in vapor or liquid phase. It will be essential to be sure that the functionalization is well achieved before testing the sensors under gas. In addition, IR analysis will be used to compare the vapor and liquid silanization as well as the effect of synthesis parameters on the nature of the APTES film in liquid phase.

X-ray Photoelectron Spectroscopy

Working principle 2.3.4.1.

X-ray Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis

(ESCA) is extensively used to examine the chemical composition of material surfaces. XPS was developed in 1960 by Kai Siegbahn and his research group [7]. It can analyze surface of a material over a depth ranging from 3 nm to 10 nm. This method is non-destructive and applicable to all elements with high sensitivity except hydrogen and helium. The information obtained are the elemental chemical composition (with a detection threshold of the order of 0.3 atomic %) and the nature of the chemical forms of the elements present on the analyzed depth. Generally, the experiments are carried out at room temperature. XPS measurements are accomplished in ultrahigh vacuum atmosphere (10 -7 Pa) to avoid contamination of the sample and to prevent the interaction of gases with incident X-rays and emitted electrons. The photon source is an X-ray tube.

In XPS technique, the sample is irradiated with X-ray photons of energy hν causing the ionization of its atoms by photoelectric effect [8,9]. During the interaction of photons with the atom, part of their energy is used to eject photoelectron away from the atom. This part is the binding energy (BE). Any electron (core and valence) with a binding energy less than hν can be ejected into the vacuum with specific kinetic energy (KE). The measurement of the kinetic energy of the emitted photoelectrons makes it possible to determine their binding energy as shown in equations (20 and (21. Each atom has core electrons with the specific binding energy.

h = BE + KE +  (20) BE = h -KE - (21)
 is the work function induced by the analyzer, it is generally constant about 4~5 eV.

The photoelectron is the electron which is emitted from the lowest energy levels (Figure 2-9a).

When a core electron is removed, leaving vacancy, this is followed by an electronic rearrangement in which an electron with higher energy level (L 1 for example) occupies the vacant space on the inner layer. The released energy can be used for ejecting an Auger electron (Figure 2-9b). respect to its environment can be obtained, so it is possible to distinguish the different chemical bond on the surface [10]. Under these circumstances it will be possible to carry out qualitative and quantitative characterization of APTES after different silanization processes.

Quantitative analysis 2.3.4.2.

The intensity of the detected photoelectron peak of an element A (I A ) can be written as shown in equation [START_REF] Morrison | The Chemical Physics of Surfaces[END_REF].

I A = KT A N A  A  𝐴 𝑀 cos θ k A M (22)
with K constant depending on instrumental factors and analysis conditions (K is the same for all elements during the analysis of a given specimen),

T A the transmission function of the spectrometer at a kinetic energy corresponding to the photoelectron peak A, N A the atomic concentration per unit volume of the element x,  A the photoionization cross section of the core level A,  𝐴 𝑀 the inelastic mean free path for an electron A in the matrix M.

θ the escape angle of photoelectrons with respect to the normal of the surface of the analyzed specimen, and

k A M = exp (- d M  A M cos θ
) the attenuation factor of an electron of A in the matrix M, for example the attenuation of Sn 4d peak by grafted APTES on the surface.

In the next chapter, this equation will be used to calculate the concentration of APTES molecules on SnO 2 after silanization.

Experimental procedures used 2.3.4.3.

The analysis were performed with a Thermo VG Thetaprobe instrument with a focused monochromatic Al Kα source (hν=1486.7 eV, 400 μm spot size). Photoelectrons were analyzed using a concentric hemispherical analyzer operating in the constant ΔE mode. The energy scale was calibrated with sputter-cleaned pure reference samples of Au, Ag and Cu in order that the Au 4f7/2, Ag 3d5/2 and Cu 2p3/2 were positioned at a binding energies of respectively 84, 386.2 and 932.7 eV. Spectra were recorded in the binding energy range of 0-1200 eV with step size of 1 eV and pass energy of 300 eV for survey scans, step size of 0.1 eV and pass energy of 50 eV for narrow C 1s, O1s, N 1s, Si 2p, Sn 4d and Sn 3d scans. The two pass energies give energy resolution (width of the Ag 3d5/2 peak) measured on sputter clean silver samples of respectively 1 eV and 0.5 eV. During XPS experiments, charge compensation was achieved by irradiation of the sample with a system combining a diffuse beam of low energy electrons and low energy ions.

Optimum charge compensation was achieved in order to have the narrower-shape of the tail at the low binding energy of Sn 3d peak. Analysis of the different components of C 1s, O 1s, N 1s and Si 2p photoelectron peaks were carried out using synthetic line shapes consisting of a convolution product of a Gaussian function with 30% of a lorentzian function. To study APTES thermal stability, SnO 2 -APTES films were in-situ annealing at 200 °C, 300 °C, 400 °C and 500 °C, for 4 h in a preparation chamber connected to the spectrometer at a pressure of 5.10 -9 mbar .

As a conclusion, the aims from XPS analysis are to carry out qualitative and quantitative characterization of the different functionalized films. Qualitative analysis comprises the detection of different characteristic APTES peaks and the shape of the film. However, quantitative analysis will deal with the thickness and the concentration of APTES on SnO 2 . In addition, XPS will be used to study the thermal stability of the APTES deposited layer on SnO 2 under different operating temperature.

Contact angle measurements

The value of contact angle is a quantitative measurement using a drop of liquid (deionized water).

It reflects the ability of a liquid to spread on a surface by wettability. The method consists in measuring the angle value formed by the intersection of the liquid-solid interface and the liquidvapor interface (geometrically acquired by applying a tangent line from the contact point along the liquid-vapor interface in the droplet profile) as shown in Figure 2-10. In practice, a drop of distilled water is deposited using a syringe on the surface of the film to be characterized. The measurement of the contact angle allows to characterize the wettability of a surface and to verify presence of an organic film deposited on the surface of metal oxide. By attaching the organic film on SnO 2 , the contact angle is supposed to vary. The contact angle measurement was made at the LMGP laboratory in Grenoble using an optical instrument type Krüss equipped with a CCD camera connected to a graphic card that records the profile of the drop by contrast (Figure 234567891011).

The image processing software DSA10-Mk2 (Drop Shape Analysis system) makes possible to extract the contact angle of both sides from the recorded image.

h

Electrical characterizations

We have developed a home-made test bench at the "Ecole des Mines Saint-Etienne" as presented in Figure 2-15 to test the sensors under gases. In this test bench, two sensors are installed, each one is in an 80 cm 3 glass chamber under constant gas flow of 15 l/h. The test bench is provided with gas mass flow controllers (Brooks) which allows controlling the concentrations of different gases at same time by PC computer. LabVIEW program allows controlling the temperature of the sensors as well as the concentration of gases. In addition, the test bench is equipped with water bubbler to test the sensors under different relative humidity (RH) balanced with air at 20 °C. Thus, this system can generate a mixture of gases at different parameters like amount of oxygen, humidity, sensor temperature, and gases concentration. Conductance measurements were performed by electronic unit equipped with voltage divider circuit that permits measuring the conductance of the SnO 2 film as function of time or temperature. The most used technique for measuring the conductance of metal oxide film is voltage divider. It is a simple method to measure the conductance of surface as shown in Figure 2-16. In a simple electrical circuit, the fixed reference resistor (r) and the tested sensor (R) are connected in series with 1 volt generator (E). As the tested sensor and the reference resistor are connected in series so they have the same value of current. In this case, the current can be written as in equation ( 23) according to Ohm's law. This equation allows to calculate the resistance as in equation ( 24) of the tested sensor and of course its conductance (equation (25)). For example, with "r" equal 10 kΩ we can measure conductance values between 10 -6 and 10 -3 Ω -1 . This technique of measurement is included in the electronic card.

I = (E -U) R = U r (23) R = (E -U) U r (24)
𝐺 = 𝑈 (𝐸 -𝑈) × 𝑟 [START_REF] Henzler | Oberflächenphysik des Festkörpers[END_REF] Before injecting the target gas in the test chamber, the sensors were kept 10 h at 100 °C, and then stabilized under air flow for 5 h at 25 °C. Gas sensing properties were then measured to different concentrations of ammonia gas balanced with air with different relative humidities. The normalized conductance (G/G 0 where G 0 is the initial conductance measured under air with different RH without target gas, and G is the conductivity after 20 minutes of gas injection) is plotted to monitor the sensor response as well as to calculate the response and recovery time. In addition, the curve of relative response (G-G 0 )/G 0 versus ammonia concentration was plotted for different sensors to compare their sensitivity. Sensors selectivity was tested versus ethanol, carbon monoxide and acetone gases.

Conclusion

We synthesized SnO 2 sensor starting from commercial SnO 2 powder. Thick film of SnO 2 was deposited by screen printing. Surface morphology, thickness and composition of the produced SnO 2 film can be characterized by SEM, ATR-FTIR, XPS etc. Unfortunately, this type of sensors previously studied in PTSI-SPIN center, has shown low selectivity to ammonia gas with respect to redox gases and has high operating temperature. Functionalization was carried out by APTES as an intermediate molecule for other modification steps to enhance the sensors performance regarding ammonia detection. Silanization was achieved in vapor and liquid phases. The effect of different liquid silanization parameters of the final APTES film will be investigated by various characterization techniques in the next chapter. Furthermore, the thermal stability of grafted APTES film on SnO 2 will be tested by XPS analysis. Finally, it will be possible to carry out the electrical characterizations under various conditions with the developed test bench.

Physico-chemical characterization of SnO 2 before and after

Chapter 3. functionalization

Introduction

Before testing the effect of the functionalization on the sensor behavior, it is important to verify whether the functionalization is effective. The APTES silanization of thick SnO 2 film produced by screen printing is not described in the literature. It is necessary to study the effect of different synthesis parameters on final APTES film. Vapor and liquid silanization can be applied to attach APTES on SnO 2 surface. In the vapor silanization, the synthesis parameters were fixed as mentioned by Ming Hai Le et al [1]. In liquid, many parameters were investigated: water content, reaction time and APTES concentration. The presence and reactivity of grafted amine terminated film on SnO 2 were investigated using fluorescent molecules (Alexa Fluor®). In addition, APTES grafting was characterized using Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) and X-ray Photoelectron Spectrometry (XPS) techniques. These characterizations showed how synthesis parameters affect the amount of APTES films. An XPS model was provided to calculate the APTES film thickness in addition to the concentration of grafted APTES. Optimal liquid silanization parameters were determined in order to obtain a saturated SnO 2 surface with APTES molecules. In addition, the thermal stability of APTES on SnO 2 has been investigated.

SnO 2 -APTES film has been used as a substrate to attach ester, acid and alkyl functional groups.

These functional groups are supposed to interact selectively with gases. The characterization of ester, acid and alkyl modified SnO 2 -APTES was carried out by ATR-FTIR technique.

Characterization of pure SnO 2 sensing layer on alumina

The surface morphological analysis of SnO 2 thick films was performed by scanning electron microscopy (SEM). Figure 3-1 shows the cross section and top images of SnO 2 film deposited on alumina substrate by screen printing. Figure 3-1a illustrates the film with various thicknesses which are related to the threads of the screen printing mesh. The film has a thickness around 40 µm (Figure 3-1b). In addition, Figure 3-1c andd show that the film is very porous and contains SnO 2 particles with spherical shape and different sizes. The porosity of the film is due to the evaporation of the organic binder and the solvent during drying and annealing steps (700 °C, 10 h). These pores are very important since the molecules in the functionalization steps can deposit inside SnO 2 film in addition to its surface. These features make SnO 2 a good substrate for molecular modification. The screen printed film exhibit SnO 2 particles with sizes between 30 to 500 nm as present in Figure 3-1d. The effect of annealing on the particle size was studied by Xray diffraction technique (XRD). We have found that the average particle size of the initial powder and in the annealed film were almost the same, around 80 nm. Therefore, the annealing step does not affect the size of SnO 2 particles. Then, SnO 2 film acts as a substrate for molecular functionalization. As shown in chapter 1, the silanization reaction takes place between the hydroxyl groups of SnO 2 and the hydroxyl groups of silane molecule. So, hydroxyl groups on SnO 2 surface are necessary to perform functionalization by silane products like APTES. To check the present species on SnO 2 surface, ATR-FTIR was achieved.

ATR-FTIR spectrum in Figure 3-2 presents the absorption bands of SnO 2 film between 400 cm -1 and 4000 cm -1 . The most important FTIR information regarding pure SnO 2 are found as two intense broad peaks at 462 cm -1 and 571 cm -1 which correspond to Sn-OH and O-Sn-O bonds respectively. Another broad peak at 3480 cm -1 is related to the OH groups adsorbed on SnO 2 (Figure 3-2). In addition, the peak at 1627 cm -1 corresponds to the OH bond of adsorbed water molecules on SnO 2 surface. This peak confirms the presence of adsorbed water molecules at ambient temperature as shown in chapter 1. All the sensors (pure and modified) show similar features in the range between 1950 and 2380 cm -1 which is not exploitable as it corresponds to CO 2 gas contributions in ambient air. The spectrum of SnO 2 will be used later to identify the adventitious peaks after functionalization. SnO 2 film was also characterized by XPS in order to investigate if there are any impurities as well as the presence of hydroxyl groups.

Macroscopic checking of APTES grafting

In order to check, that silanization by APTES has occurred, wettability tests were carried out.

Indeed, APTES is supposed to create a hydrophobic film on SnO 2 which should increase the contact angle after silanization [3]. Contact angle on SnO 2 film before and after functionalization was found to be less than 5°. This low contact angle value is due to the fact that the SnO 2 film is very porous as shown by SEM images and very hydrophilic (before functionalization) because of the adsorbed water species. As reported in chapter 1, the silanization can also be established on

Chemical analyses of APTES grafting

The grafted APTES films on SnO 2 substrate were chemically characterized by ATR-FTIR and XPS. These techniques were used to study the structure of APTES on SnO 2 as well as the effect of different synthesis parameters.

Regarding the characterization of APTES film by Attenuated Total Reflectance-Fourier

Transform Infra-Red spectroscopy (ATR-FTIR), the most important signals corresponding to APTES were found between 800 and 1800 cm -1 as shown in Figure 3456789. The peak at 938 cm -1 is attributed to Sn-O-Si stretch [4]. The peak at 1070 cm -1 arises from the un-hydrolyzed ethoxy groups of APTES (-OCH 2 CH 3 ) [5]. The strong absorption peaks at 1029 and 1125 cm -1 are attributed to Si-O bond of polymerized APTES on the surface [6,7]. The vibrational mode around 1390 cm -1 is assigned to Si-C bond (-Si-CH 2 -) [7]. NH 2 vibrational signal is found at 1570 cm -1 which confirms the presence of amine end functional group of APTES after silanization. The two peaks present at 1486 and 1643 cm -1 correspond respectively to the symmetric and asymmetric -NH 3 + group indicating the protonation of amine group in air [8,9]. All these modes are being proportional to the quantity of the APTES on SnO 2 . [11]. Regarding only the peak at 531.8 eV, it can be hardly differentiated with Sn-OH peak at 531.9 eV (Figure 3-4). To conclude, one have also to consider the Si 2p peak located at 101.9 eV (figure 3-11d). This peak was assigned to Si atom bonded to oxygen and carbon. So, the whole XPS spectrum allows to confirm the presence of the different chemical bonds related to APTES molecules. Therefore, both the characterizations by ATR-FTIR and XPS confirm that the functionalization by APTES is achieved. In the next discussion, the synthesis parameters will be investigated to produce an optimum APTES modified SnO 2 .

Influence of synthesis parameters in silanization

Different parameters were explored for APTES grafting: vapor and liquid silanization with or without H 2 O, APTES concentrations and reaction times. Each of these parameters could affect the grafted APTES film quantitatively and qualitatively.

Sn-OH, Si-O

Comparison between vapor, liquid hydrous and liquid anhydrous silanization processes

The effect of different silanization ways were characterized by ATR-FTIR to check the effects on the produced SnO 2 -APTES film. Generally, the peaks corresponding to APTES are more or less significant depending on the synthesis procedures. As shown in Figure 3-12, APTES liquid deposition in presence of 5 vol% H 2 O (red curve) shows the most intense peaks for all the vibrational modes (938, 1029, 1125, 1390, 1496, and 1570 cm -1 ) which implies that more APTES molecules were grafted on the surface in this synthesis. As mentioned before, the intensity of these peaks is proportional to the amount materials (APTES). Furthermore, the observed peak at 1070 cm -1 in the case of vapor silanization (green curve) and liquid synthesis without H 2 O (blue curve) is linked to un-hydrolyzed ethoxy groups of APTES as mentioned earlier [5]. This peak has disappeared for the SnO 2 -APTES film synthesized with H 2 O. The water molecules initiate the silanization process by hydrolysis of ethoxy groups to create corresponding hydroxysilane (RSi(OH) 3 ) [12]. Hydrolysis of ethoxy groups of APTES pushes forward the condensation reaction between SnO 2 film and APTES [9,13]. From these results, one can notice that the presence of water in the synthesis solution increases the APTES grafting on SnO 2 with also more APTES molecules bonding with the surface and with APTES each other. Therefore, APTES is more stable on SnO 2 surface in presence of water during synthesis. The effect of water in the liquid silanization was also investigated by XPS. Indeed, the more grafted APTES there is, the higher N 1s, Si 2p and C 1s peaks are. For Sn 3d, if SnO 2 is covered by APTES, as XPS analyzes only the surface of the film, less Sn atoms can be seen.

Effect of APTES concentration in liquid hydrous phase

As shown in the previous discussions, more APTES molecules were grafted by hydrous liquid silanization than anhydrous liquid and vapor silanization. Thus, the synthesis parameters have been more deeply investigated for hydrous liquid silanization. In Figure 3 (green curve) while they increase with 50 mM (red curve), and then almost no change is noticed when the concentration is increased up to 500 mM (purple curve). Therefore, a very little amount of APTES was grafted on the surface with a concentration of 1 mM, while a concentration of 50 mM was sufficient to well cover SnO 2 with APTES molecules.

Effect of reaction time in liquid hydrous phase

With the same idea, to improve the synthesis parameters, reaction time was also investigated. As this peak is proportional to the amount of APTES attached on the surface [6], one can note that the increase of reaction time from 5 min to 4 h attaches a larger amount to the surface.

No increase in the amount of grafted APTES on the surface was observed after 4 h, since the signal intensity is the same between films immersed 4 and 5 h. Evidently, this steady state means that 4 h is sufficient to saturate the SnO 2 film with APTES.

Nanostructure of the APTES film on SnO 2

Nanostructure of the APTES film was determined from inelastic electron background analysis of the Sn 3p transition taking advantage of the weak sensitivity of this method to the roughness. "Analyze" program from QUASES software package developed by Tougaard [14] was used by means of the active substrate model, where the modelled signal originates from the SnO 2 substrate and is attenuated by an overlayer constituted of APTES molecules. The shape of the measured spectrum J(E) strongly depends on the morphology of the overlayer. A primary excitation spectrum F(E) is obtained after correction of the measured spectrum J(E) by an inelastic background calculated for a given coverage and thickness of the overlayer. These set of parameters can be adjusted using two criterions. The first one is that after background subtraction F(E) must be of zero intensity in an energy region beyond 30 eV below the primary peak energy.

As a second criterion, F(E) can be compared to the one determined from the analysis of spectrum on reference sample with well-known in depth concentration profile. Reference Sn 3p spectrum was recorded on a pure SnO 2 sample. After linear background subtraction and analyzer transmission function correction, the background of the model spectrum was adjusted to zero by analyzing the reference to the depth of 1000 Å that corresponds to the infinite thick homogeneous layer. The background was calculated using 'Universal cross section' formula [15] for the inelastic electron cross section and changing the "x-scale" parameter in the QUASES program from default value to 0.97. A value of 12.36 Å for the inelastic mean free path of Sn 3p electrons in SnO 2 was considered according to the TPP2M method [16]. The same procedure and parameters were used for analyzing SnO 2 covered with APTES molecules using "active substrate" model.

Results obtained after analysis of Sn 3p spectrum measured on SnO 2 samples with different methods of silanization are presented in the Figure 3-16. From these results, two main conclusions can be drawn. First, for the three methods of silanization (vapor, hydrous liquid, anhydrous liquid), the nanostructure of the grafted film consists in a monolayer of APTES molecules which does not cover entirely the surface. Second, the silanization procedures used can be classified as "high coverage" for the hydrous liquid method (SnO 2 surface coverage of 0.8) and "low coverage" for the two other methods for which a similar SnO 2 surface coverage around 0.5 was found.

Finally, it is worth noting that considering a theoretical length of the APTES molecule of 8 Å, the calculated overlayer thickness of 6 Å indicate a ~42° grafting angle of the APTES molecules with respect to the normal of the SnO 2 surface. However, due to the rugous surface of SnO 2 samples, even if the proposed methodology brings corrections to XPS spectrum, one must be cautious and consider relative values and differences between the analyzed samples, rather than the absolute values.

Determination of APTES concentration on SnO 2

As shown before, using electron inelastic background analysis, the silanization of SnO 2 with vapor, hydrous liquid and anhydrous liquid methods produce a monolayer film of APTES molecules which does not cover entirely SnO 2 . Owing to the difference in coverage rate according to the experimental silanization protocol, it seems interesting to determine the concentration of APTES molecules grafted on SnO 2 for each method. Considering one silicon atom for each APTES molecule, these concentrations were estimated calculating the silicon concentration per unit area using the ratio of the intensities of Si 2p and Sn 4d photoelectron peaks. The choice of these peaks, especially Sn 4d which is much less intense than Sn 3d, was motivated by the fact that several assumptions can be made due to the position at high kinetic energies of these transitions as we will see below. For the purpose of this calculation, we define Using the description in Figure 3-17, the intensities of Si 2p and Sn 4d can be calculated as shown in equations ( 1) and [START_REF] Barteau | Site requirements of reactions on oxide surfaces[END_REF].

I Si2p = KT Si2p N Si  Si2p  Si2p Si cos θ [1 -exp ( -d Si  Si2p Si cos θ )] × k Si2p C 3 NH 8 (26)
I Sn4d = KT Sn4d N Sn  Sn4d  Sn4d SnO 2 cos θ k Sn4d APTES (27)
(see chapter 2, equation 3 for parameters definition)

Concerning the inelastic mean free path, two methods were applied. The values of  Si2p Si and  𝑆𝑛4𝑑 𝑆𝑛𝑂 2 were estimated using the TPP2M method [16] which gives respectively 30.8 Å and 24.1 Å.

As regards the value of  Si2p Si , one may assumes that d Si <<  Si2p Si which allows to modified equation ( 1) and express the intensity of Si 2p as a function of the silicon atomic concentration per unit area in equation [START_REF] Gillot | Study of the oxidation kinetics of finelydivided magnetites. II -Influence of chromium substitution[END_REF].

I Si2p = KT Si2p n Si  Si2p k Si2p C 3 NH 8 (28)
With n Si = N Si d Si

The silicon surface concentration can be consequently expressed as a function of the ratio of the intensities of Si 2p and Sn 4d following equation [START_REF] Kuang | High-Sensitivity Humidity Sensor Based on a Single SnO2 Nanowire[END_REF].

n Si = I Si2p I Sn4d × T Sn4d N Sn  Sn4d  𝑆𝑛4𝑑 𝑆𝑛𝑂 2 cos θ T Si2p  Si2p × k Sn4d APTES k Si2p C 3 NH 8 (29)
For k Si2p C3NH8 and k Sn4d APTES , a method using quantitative structure-property relationships in organic materials [17] was used. Inelastic mean free paths were determined using expression [START_REF] Li | Water pre-adsorption effect on room temperature SnO 2 nanobelt ethanol sensitivity in oxygen-deficient conditions[END_REF].

(nm) = [ 3.117( 0 x V ) + 0.4207N rings N non-H ] ( E keV ⁄ ) 0.79 (30)
With

x 0 V = ∑ ( 1 √ δ (V) )
non-H atoms the zeroth-order valence connectivity index of the molecule, N rings the number of aromatic six-member rings in the molecule, N non-H the number of atoms in the molecule excluding hydrogen, E the kinetic energy of the electron travelling through the molecule and δ (V) the valence connectivity index which depends on the hybridization state and number of hydrogen atoms bonded to the atom considered. Values of δ (V) were taken from literature using

Bicerano work [START_REF] Bicerano | Prediction of polymer Properties[END_REF].

The values of inelastic mean free path () and associated attenuation factors (k), assuming a collecting angle θ of 50°, are presented in the Table 3-1. together and in range where the variation of the transmission function is weak. Following these assumptions, the silicon surface concentration n Si corresponding to the surface concentration of APTES molecules n APTES can be described in equation [START_REF] Heiland | Physical and Chemical Aspects of Oxidic Semiconductor Gas Sensors A2 -SEIYAMA[END_REF].

n APTES = n Si = I Si2p I Sn4d × N Sn  Sn4d  𝑆𝑛4𝑑 𝑆𝑛𝑂 2 cos θ  Si2p (31)
It is worth noting that the main uncertainty on the estimation of APTES surface concentration comes from the electron collecting angle θ due to the roughness of the SnO 2 substrates. However, results of atomic force microscopy measurement, not presented here, have shown comparable roughness for the three substrates used which allows comparing the three processes of silanization in terms of surface concentrations of grafted APTES molecules.

Calculations were carried out with Scofield cross section (relative sensitivity factor) of 0.817 for  Si2p , 2.7 for  Sn4d and using an atomic concentration per unit volume of Sn in SnO 2 of 2.77 x 10 22 atoms/cm 3 . Table 3-2 reports the calculated concentrations after the different synthesis ways of SnO 2 -APTES (anhydrous and hydrous liquid versus vapor silanization). anhydrous liquid phase 4.24×10 14 The SnO 2 -APTES carried out by liquid silanization in presence of 5 vol% H 2 O processes the highest grafted APTES concentration (𝑛 APTEs ). The presence of H 2 O hydrolyzes ethoxy groups of the APTES to silanol groups which enhances the condensation reaction with hydroxyl groups of SnO 2 and polymerization of APTES [7]. With vapor deposition and with liquid synthesis without water, the condensation is less favored due to the absence of water (𝑛 APTES =4.7×10 14 and 𝑛 APTES =4.24×10 14 , respectively). In addition, the liquid silanization without water shows lower APTES concentrations than the one in vapor, both are in the same range. As a conclusion, hydrous solution synthesis gives the higher silanization rate, superior to vapor or anhydrous synthesis.

Thermal stability of APTES on SnO 2

To study the thermal stability of APTES, samples were in-situ annealed at 110 °C (synthesis annealing temperature), 200 °C, 300 °C, and 400 °C for 4 h at a pressure of 5.10 -9 mbar in XPS device. The evolution of the intensities of N 1s, C 1s, Si 2p, and Sn 3d are presented in Figure 3-18. The decrease of the N 1s peak intensity with temperature can be explained by the partial thermal degradation of amine group between 110 °C and 400 °C, while at 500 °C no more amine groups are present on the surface. The evolution of the intensity of C 1s peak also suggests a decrease in the amount of carbon on the surface upon thermal treatment which is related to the degradation of different carbon bonds. After 4 h treatment at 500 °C, a presence of carbon at the surface was still detected. This residual carbon was assigned to Si-CH 2 bonds as the annealing treatments show no effects on the intensity of the Si 2p peak. The intensity of Sn 3d peak depends on the attenuating APTES overlayer. The increase of the intensity of Sn 3d peak with temperature is consequently in accordance with the progressive degradation of the APTES film as the temperature increases. However, Sn 3d is still less intense for SnO 2 -APTES heated at 500 °C than for pure SnO 2 . This could be related to the remaining silicon layer on SnO 2 metal oxide surface. Therefore, grafted APTES started to degrade at temperature above 110 °C.

Characterization of SnO 2 -APTES modified by different functional groups

Among the above reported synthesis parameters of APTES modified SnO 2 , the modified film obtained by liquid silanization in presence of 5 vol% H 2 O (4 h, 50 mM of APTES) was the most efficiently functionalized. Thus, this film was used for the second step of functionalization.

APTES modified SnO 2 offers an amine terminated substrate for further modification as reported before by Bin Wang et al [19]. Alkyl, acid and ester modified SnO 2 (red, orange and green curves respectively) FTIR spectra exhibit two peaks at 1547 cm -1 and 1645 cm -1 which correspond to N-H bending mode and carbonyl stretch mode of amide respectively. An additional broad peak between ~ 3000 and ~ 3600 cm -1 corresponds to N-H stretch of amide. These peaks confirm the success of the reaction between amine group of APTES and acyl chloride group. In addition, the peak at 1570 cm -1 of The characterization of second modification step was also achieved by XPS analysis. These modifications increase the quantity of carbon on the surface and add carbon-based functional groups such as alkyl, acid and ester. Therefore, it is possible to detect these modifications by analyzing the changes in C 1s XPS peak. The areas under the peaks of C-Si and C-N after the second step of functionalization are the same as for SnO 2 -APTES. This indicates that the APTES molecules were not dissociated during the second step. However, the area under the peak related to C-C/CH was 222 counts.eV.s -1 this increased after functionalization by alkyl, acid and ester as shown in Table 3- As mentioned before, tin dioxide material allows the detection of many gaseous compounds.

Nevertheless, the SnO 2 -based sensors suffer from two major drawbacks which constitute an obstacle to their development:

-Lack of selectivity because SnO 2 sensitive material has a high affinity with many gases.

-High operating temperature between 250 °C and 600 °C.

The aim of the molecular functionalization performed in this work was to cover these blocking points by elaboration of a sensitive and selective sensor operating at low temperature.

Functionalization was performed to optimize the interactions of sensors with ammonia gas.

Molecular modification passivates the surface states on SnO 2 sensors by molecular layer containing a specific functional group. These functional groups can interact specifically with gases depending on their polarities as shown by Hossam Haick et al. [1]. They reported a silicon nanowire sensor functionalized with nonpolar end functional group to improve the detection of nonpolar gas. Other silicon nanowire sensors were modified by polar end functional groups to improve the detection of polar gases.

In the present work, we focus on the change in sensitivity and selectivity of SnO 2 sensors to ammonia after functionalization. Three SnO 2 sensors were coated with molecules having mostly polar (functional) side groups which are amine (APTES), acid (COOH), and ester (COOCH 3 ) and one SnO 2 sensor was coated with molecules having mostly nonpolar side group which is alkyl (CH 3 ). The sensors are firstly functionalized by APTES followed by covalent attachment of alkyl, acid or ester end functional groups molecule. The detection performances of resulting sensors are investigated in regards of ammonia detection. The changes in the response of the modified sensors were compared with pure SnO 2 . Such sensor will be used for the detection of ammonia in the human breath which contains high amount of water vapor. For this reason, the effect of water vapor on the sensors performance was investigated. In addition, the selectivity was tested with respect to ethanol, acetone and carbon monoxide gases. Another objective is to reduce the power consumption by decreasing the operating temperature, since the sensor will be used later for smart devices and portable applications.

Conditioning of the sensor

Ester modified SnO 2 -APTES sensor has showed the best performance among all the functionalized sensors regarding the detection of ammonia as we will show later in this chapter.

This sensor is operated preferentially at 25 °C in air with 5%RH to be a tool for breath analysis.

The modification by ester end functional groups was carried out on SnO 2 -APTES synthesized by hydrous liquid (5 vol% H 2 O) silanization. This silanization produces a denser APTES film than the others (vapor and anhydrous liquid silanization) as shown in chapter 3. After this process, the films were annealed in the oven to desorb the unreacted molecules (not fixed) as well as the water from the film. The next step was the reaction of APTES film with acyl chloride products. Here, the annealing after the reaction was performed in-situ to control its effect on the response to ammonia. In addition, the needed time to clean the film from unreacted molecules and other physisorbed species in the film can be estimated. Annealing was carried out in-situ at 100 °C for different times. The effect of annealing was noticed on the stability of response to ammonia at 25 °C. For SnO 2 -APTES-ester without heating and heated for 5 min, decrease in conductance was observed. However, after heating 30 min, increase in conductance was noticed. In addition, the response value increases with increase of pre-heating time until 9.5 h. At this level, stable response and base line were remarked. In contrast, SnO 2 shows no changes in the response to 50 ppm of ammonia gas after heating for different times. SnO 2 shows decrease in conductance upon exposure to ammonia with and without pre-heating. Noise in the curve can be due to the limit of the electrical measurements. These results indicate that SnO 2 and SnO 2 -APTES-ester without or with short-pre-heating (5 min) give a negative response (conductance decrease) to ammonia. The fact that the response becomes positive (conductance increase) for SnO 2 -APTES-ester with longer heat treatment, but remain unchanged for SnO 2 , reveals that the treatment affects SnO 2 -APTES-ester surface by modifying reactive species. This process adjusts the polarity of the ester film on SnO 2 and changes the dipole moment as well as the response to ammonia. This indicates that the sensors should be heated at least 9.5 h to remove the adsorbed species from SnO 2 -APTES-ester surface and to obtain stable response. The interaction mechanism of ammonia with SnO 2 and SnO 2 -APTES-ester sensors will be studied in the following discussion.

The desorption of the trapped species including water, unreacted APTES or acyl chloride in the organic film was checked by ATR-FTIR.

Sensing measurements of the different functionalized SnO 2 sensors

The previous discussions have shown that SnO 2 sensors are well functionalized by APTES, APTES-alkyl, APTES-acid and APTES-ester. In addition, we have shown that the conditioning of functionalized sensor at 100 °C does not damage the grafted film. Now the test under gases can be achieved. For the entire test under gas shown in this section, the sensors were heated insitu for 10 h at 100 °C. The sensors operated at 25 °C in air with 5 %RH.

Response of the different sensors towards ammonia

The first part of the test under gases was to show the characteristic of the response of different sensors to ammonia gas. As shown in Figure 4-3, the sensors were tested to 100 ppm of ammonia balanced with 5% RH air at 25 °C. Figure 4-3 (red curve) shows that the conductance of pure SnO 2 decreases upon exposure to ammonia gas. This type of response has been found before by Kamalpreet Khun Khun et al. [2] at temperature between 25 to 200 °C. They have supposed that ammonia reacts with molecular adsorbed oxygen ion (O 2 -, created as shown in equation ( 32))

producing nitrogen monoxide gas (NO) according to equation [START_REF] Henrich | The Surface Science of Metal Oxides[END_REF]. In presence of oxygen, NO can be easily transformed into NO 2 especially at low temperature (equation ( 34)). The reaction of NO 2 with SnO 2 at ambient temperature causes the decrease of sensor conductance because NO 2 is a good oxidizing agent. NO 2 adsorbs on SnO 2 surface adsorption sites (s) and bring out electrons from the conduction band (equation ( 35)) [3]. Thus, the overall reaction of ammonia with SnO 2 at room temperature can be written as in equation [START_REF] Wei | A novel SnO 2 gas sensor doped with carbon nanotubes operating at room temperature[END_REF]. Therefore, this reaction confirms the obtained results because the conductance was actually decreased when SnO 2 sensor is exposed to ammonia. In addition, the abundant species adsorbed on SnO 2 surface at room temperature are the hydroxyl groups. This explains the measurable conductance base line under air balanced with 5%RH of pure SnO 2 [4].

O 2 + s + e -→ s-O 2 - ( 32
) 4NH 3 + 5s-O 2 -→ 4NO + 6H 2 O + 5s + 5e - (33)
2NO + O 2 ↔ 2NO 2 (equilibrium in air) (34)

NO 2 + s + e -→ s-NO 2 - ( 35
) 4NH 3 + 4s + 7O 2 + 4e -→ 4s-NO 2 -+ 6H 2 O ( 36
)
Where s is an adsorption site. APTES, and because of the hydrophobic nature of APTES film. Therefore, in the following discussion, the conventional mechanism of interaction of SnO 2 with gases cannot be taken in consideration. For example, the interactions with adsorbed oxygen species and the production of NO x cannot take place in this case. SnO 2 -APTES shows no response to ammonia (Figure 4-3).

This implies that no significant interactions occur between the grafted APTES and ammonia gas.

In term of polarity and other chemical properties like acidity, the amine and ammonia groups are almost the same, since amine is one of the derivatives of ammonia. In addition, this behavior indicates that the SnO 2 surface is well covered by APTES molecules because the negative response observed on pure SnO 2 is totally inhibited. These responses could be related to the different polarities of the attached end functional groups.

Ester and acid are electron withdrawing groups, while alkyl is mostly considered as nonpolar.

Ammonia molecule is a nucleophilic molecule (donating). Regarding the interactions between electron withdrawing groups from one side (ester and acid) and electron donating group (ammonia) in from the other side, dipole-dipole interaction is taking place. On the contrary, in the case of SnO 2 -APTES-alkyl, the interaction is of induced dipole type because ammonia is a polar molecule and alkyl end group is mostly nonpolar as presented in Figure 4-4. It was reported before by B. Wang et al. [5], that the dipole-dipole interaction is always stronger than induced dipole interaction. This can explain the difference in the response between SnO 2 -APTES-ester, SnO 2 -APTES-acid and SnO 2 -APTES-alkyl sensors to ammonia gas. These two types of interactions (i.e. dipole-dipole and induced-dipole) result in a modification in the dipole moment of the whole film. The variation in the molecular layer's dipole moment affects the electron mobility in SnO 2 film which modifies the conductance [6,7]. The exposure to ammonia leads to increase in electron mobility (proportional to conductance). The same behavior was found for a selection of polar and non-polar gases but on alkyl, acid and ester silicon oxide functionalized substrate [5]. Since the dipole-dipole interaction is stronger than the induced dipole interaction, adsorption of ammonia on ester and acid end groups lead to larger change in conductance than on alkyl one. But we can also mention that the interaction can also result from diffusion of the gas in the molecular layer. This type of interaction is favorable only for SnO 2 -APTES-alkyl. It is difficult for ammonia molecules to diffuse in the molecular layer of SnO 2 -APTES-ester as well as SnO 2 -APTES-acid, because of the steric hindrance induced by ester and acid end groups. This phenomenon can explain the response of SnO 2 -APTES-alkyl to ammonia in addition to the induced dipole interaction. According to the above discussion, the response of the molecular modified sensors does not obey the conventional mechanism of direct interaction with SnO 2 . The response comes mainly from the interaction of ammonia molecules with end function group of the attached layer. The response of SnO 2 -APTES-ester and SnO 2 -APTES-acid is generated from dipole-dipole interaction, while the response of SnO 2 -APTES-alkyl is produced from induced dipole interaction which has less significant effect (Figure 4-4).

Sensitivity

Regarding the different sensors sensitivity against ammonia concentrations, Figure 4-5 shows the relative responses versus ammonia concentrations of SnO To summarize this section, SnO 2 -APTES-ester and SnO 2 -APTES-acid sensors showed good sensitivity to ammonia gas in a range of concentration compatible with breath analysis applications (sub-ppm). These two sensors are studied in more details in the following parts. Upon exposure to ammonia gas the sensors conductance increases in the all cases (the two sensors, dry air, 5%RH, 26%RH, and 50%RH). As shown in Figure 4-6a and b, the response and recovery times decrease with the increase of the relative humidity percentage for the two sensors.

Focus on

In dry air and 5%RH, SnO 2 -APTES-ester (Figure 4-6a) shows almost the same response magnitude, 1.3 for 50 ppm of ammonia, which decreases to 1.14 and 1.04 in 26%RH and 50%RH respectively. Decrease in response was also observed for SnO 2 -APTES-acid (Figure 4-6b) to 50 ppm ammonia by addition of 5%RH (decrease from 1.68 to 1.59).This means that a small quantity of relative humidity does not affect the sensor response, while at elevated amount the response starts decrease significantly. A potential explanation is that the attached ester or acid films are saturated by water molecules or adsorbed hydroxyl groups at high RH. Hence, during exposure to ammonia, there is a limitation of response due to adsorption competition between water and ammonia. In these conditions, 5%RH at 25 °C, the response and recovery times (as defined in chapter 1) are around 98 s and 130 s respectively for the two sensors, which is quite noticeable for a tin oxide based sensors working at room temperature. For the future tests, the humidity will be kept at 5%RH.

Effect of operating temperature

In the most cases, the increase of the operating temperature of SnO 2 sensors increases the sensitivity to gases. In order to evaluate the influence of the temperature on functionalized sensors performance, acid and ester modified SnO 2 -APTES sensors were operated at different temperature (25 °C, 50 °C and 100 °C) and the response to ammonia was recorded. As shown in These changes in the response to ammonia at different operating temperature are generated from the effect of temperature on the dipole moment as reported by Vera Doron et al [8]. The increase in temperature changes the dipole moment value in the same sense as the effect of ammonia (increase in conductance, showed experimentally). A hypothesis can be drawn to explain the In contrast to most of sensors, in the case of SnO 2 -APTES-ester and SnO 2 -APTES-acid, the increase of temperature generates decrease of ammonia sensor response. So, interesting conclusion is that SnO 2 -APTES-ester and SnO 2 -APTES-acid sensors have to be operated close to room temperature (25 °C), without any power consumption for the sensitive detection of low ammonia concentrations.

Selectivity

Human breath contains a wide variety of volatile organic compounds which are polar or nonpolar, oxidant or reductant. It is well known that SnO 2 sensor which usually operates at temperature between 350-500 °C can give a response to most of these types of gases, unfortunately without distinction (selectivity) or even with compensating effect for oxidant/reducing gases [9]. Field effect transistors functionalized with ester end group have shown responses to a wide variety of volatile organic compounds like alcohols and alkanes [10].

This implies that these sensors also have a lack of selectivity. To check the selectivity to ammonia of the developed SnO 2 -APTES-ester and SnO 2 -APTES-acid, such sensors were tested with respect to ethanol, carbon monoxide and acetone gases. Polar molecule such as ammonia might selectively attach to the functional group of the molecular layer. It is likely that the adsorption process occurs through interaction between the nitrogen of ammonia and the end functional group of the molecular layer (alkyl, acid and ester). The difference in response of ester and acid modified SnO 2 -APTES sensors to ethanol and carbon monoxide gases is due to the fact that carboxylic acid is more polar than ester group. The increase in the polarity of the attached film increases the response to gases. In addition to the reaction with acid end functional group, these gas molecules can penetrate into the organic layer and interact with the amide group which is polar. But this is not the case for SnO 2 -APTES-ester because of the steric hindrance generated by the CH 3 group of ester. Regarding the response of acetone gas, the steric hindrance is generated by the CH 3 of acetone molecule. For this reason, no response to acetone gas was found for the two sensors as shown in Figure 4-10.

Stability and reproducibility

A common challenge in organically modified sensors is how to elaborate many sensors with good stability and reproducibility. The main parameter that can affect these properties is the amount of grafted molecule on each sensor, since the interactions occur with these molecules.

The influence of ammonia concentrations on sensor response of SnO 2 -APTES-ester and SnO 2 -APTES-acid was studied at the optimum conditions defined previously, 5%RH at 25 °C.

Effect of oxygen

As said in chapter 1, the metal oxide gas sensors are highly affected by the amount of oxygen. In addition, the exhaled breath of human has not always the same amount of oxygen because it depends on many reasons like the time of breath holding. This is a blocking point which limits the use of such sensors in breath analysis in addition to other factors (selectivity and operating temperature). As the interaction is not any more with adsorbed oxygen ions of the molecularly modified sensors, the oxygen quantity is expected to have no influence on the response to ammonia gas. Figure 4-13 shows the response to different ammonia concentrations at 25 °C either in 20% of oxygen or in inert gas (N 2 ). It can be clearly noticed that no significant change occur in the response to ammonia in presence and in absence of oxygen, at least for the duration of the experiment (20 min). These results confirm the above discussion regarding the sensing mechanism, which is based on the interaction of ammonia with attached molecular layer. These interactions modify the dipole moment of the molecular layer [5]. The mobility of electrons is affected by the change in dipole moment. By modifying the dipole moment, the conductance of the whole film variates. Thus, oxygen does not participate in the detection mechanism of ammonia by ester modified SnO 2 sensor. Therefore, oxygen has no impacts on the detection of ammonia in the exhaled breath by this sensor.

Conclusion

Molecularly modified SnO 2 thick films were produced by screen printing followed by functionalization using solution chemical processes. The functionalization processes were carried out first by grafting of 3-aminopropyltriethoxysilane followed by reaction with hexanoyl chloride, 1,4-butanedicarbonyl chloride, or methyl adiapoyl chloride. Then, tests under gases were performed. APTES modified SnO 2 did not show any significant sensitivity to ammonia between 0.2-100 ppm under 5%RH at 25 °C. The sensitivity to ammonia becomes significant from 30 ppm for alkyl modified sensor. For ester and acid modified sensors, fast response and recovery time to ammonia with a limit of detection estimated to 80 ppb at 5%RH, 25 °C is observed. In addition, these sensors show constant sensitivity between 0.2 and 10 ppm to SnO2-APTES-ester O2% ammonia (0.023 ppm -1 ). Ester and acid modified SnO 2 -APTES sensors have also shown increase of sensor response to ammonia with the decrease of operating temperature in presence of 5 %RH.

These interesting responses to ammonia gas are generated mostly from the interactions of gas with the end functional groups. Ester and acid functional groups interact with ammonia by dipole-dipole interactions. These interactions change the dipole moment of the film which produces an increase in the conductance of the whole film. Moreover, ester modified SnO 2 -APTES sensor is more selective to ammonia gas with respect to reducing gases like ethanol, carbon monoxide and acetone than the acid modified one. 5%RH does not affect the response of SnO 2 -APTES-ester to ammonia in contrast to SnO 2 -APTES-acid. However, relative humidity higher than 5% decreases the response to ammonia for the ester and acid modified SnO 2 -APTES.

These sensors are reproducible regarding the sensitivity tests from one sensor to another and are not affected by the oxygen content in the gas. By working at room temperature, ester modified sensor is a good candidate for breath analysis applications for the diagnosis of diseases related to ammonia gas biomarker. APTES grafted by liquid without water and vapor silanization were respectively 4.24×10 14 molecules.cm -2 and 4.7×10 14 molecules.cm -2 , while it is 9×10 14 molecules.cm -2 with the addition of 5 vol% H 2 O. In addition, the obtained results from this model were in good agreement with the ATR-FTIR characterization results. Therefore, all the used characterization techniques confirm that the liquid silanization in the presence of water leads to more APTES grafting onto SnO 2 indicating an almost 50% improvement. This film was used to attach the desired end functional groups on it. In this step, molecules containing acyl chloride group from one side and alkyl, acid or ester functional groups from the other side were used. For the second step functionalization, ATR-FTIR and XPS demonstrate that SnO 2 -APTES is well modified by ester, acid and alkyl end functional groups.

The tests under gases have reaveled that pure SnO 2 sensor and APTES modified SnO 2 do not display any significant sensitivity to ammonia between 0.2-100 ppm under 5%RH at 25 °C.

However, the sensitivity to ammonia begins to increase starting from 30 ppm for alkyl modified SnO 2 -APTES sensor. On the contrary, ester and acid modified SnO 2 -APTES sensors show constant sensitivity (0.023 ppm -1 ) to ammonia in the range of interest between 0.2 and 10 ppm. In addition, these sensors exhibit fast response and recovery time to ammonia with a limit of detection estimated to 80 ppb at 25 °C in air with 5%RH. These interesting responses to ammonia gas could be generated mostly from the dipole-dipole interactions of gas with the end functional groups. These interactions change the dipole moment of the film which produces an increase in the conductance of the whole film. Concerning the effect of water vapor, 5%RH does not affect the response of SnO 2 -APTES-ester to ammonia in contrast to SnO 2 -APTES-acid. The test under different operating temperatures (25, 50 and 100 °C) demonstrates that the ester and acid modified sensors work better at 25 °C. Moreover, ester modified SnO 2 -APTES sensor has shown better selectivity to ammonia gas with respect to reducing gases like ethanol, carbon monoxide and acetone than the acid modified one. The repetition of test on several ester and acid modified sensors have proved that these sensors are reproducible regarding the sensitivity to ammonia. Furthermore, ester modified sensors were not affected by the oxygen content in the mixture of gases. The presented results show that ester modified sensor is a good candidate for the detection of liver diseases from the exhaled breath. The working at room temperature makes this sensor convenient for smart portable applications.

Regarding the obtained results, the following perspectives could be investigated: One of the major challenges in the modern era is how we can detect the disease when we are still feeling healthy via noninvasive methods. Exhaled breath analysis is offering a simple and noninvasive tool for early diagnosis of diseases. Molecularly modified SnO 2 sensors seem to be promising devices for sensing polar gases such as ammonia. In this study, SnO 2 surface functionalization was performed in order to obtain sensitive and selective ammonia gas sensor that operates at room temperature. The first step of functionalization is the covalently attachment of 3-aminopropyltriethoxysilane (APTES) film on SnO 2 in vapor or liquid phases. Liquid phase silanization was achieved in hydrous or anhydrous APTES solution. The characterization performed by the Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS), show that more APTES were grafted by hydrous liquid phase silanization. This film was used as a substrate for further modifications. The second step was the functionalization of APTES modified SnO 2 with molecules having acyl chloride of different end functional groups molecules such as alkyl, acid and ester groups. Pure SnO 2 and APTES modified SnO 2 sensors did not show any significant sensitivity to ammonia (0.2-100 ppm) at 25 °C, while the sensitivity to ammonia gas started to increase from 30 ppm for alkyl modified sensor. On the contrary, acid and ester modified sensors are sensitive to ammonia between 0.2 and 10 ppm at room temperature. However, ester modified SnO 2 was more selective than acid modified sensor regarding the ethanol and carbon monoxide gases. These results imply that the response is generated by the attached acid and ester functional groups. Ammonia variates the attached molecular layer's dipole moment which leads to change in SnO 2 conductance. Working at ambient temperature is also one of the advantages of these sensors in addition to the selectivity to ammonia regarding other gases such as ethanol, carbon monoxide and acetone.

Figure 1 .

 1 Figure 1. Schematic illustration of ammonia cycle in human body of a) normal liver, b) in the case cirrhosis where the liver cannot degrade the entering ammonia. The excess of ammonia in the blood can be transferred to the brain and cause hepatic encephalopathy [2].

Figure 2 .

 2 Figure 2. The number of published papers in ScienceDirect on gas sensors throughout the years.

Figure 1 - 1 .

 11 Figure 1-1. Schematic diagram of the different parts of chemical sensor [1].

  2 has been chosen due to its unique physical and chemical properties such as wide band gap (3.6 eV), dielectric constant, accuracy or repeatability at the present stage of development, environmental-friendliness and synthesis easiness. Its electronic configuration is [kr] 4d 10 . SnO 2 is usually regarded as an oxygen-deficient n-type semiconductor. The mineral form of SnO 2 is called cassiterite, and this is the main source of tin. This oxide of tin is the most important raw material in tin chemistry. It crystallizes with the rutile structure, where the tin atoms are 6 coordinate and the oxygen atoms three coordinate, and the lattice parameters are a = b = 4.737Å and c = 3.185Å as shown in Figure 1-2.

Figure 1 - 2 .

 12 Figure 1-2. Unit cell of rutile SnO 2 [13].

Figure 1 - 3 .

 13 Figure 1-3. Simplified model illustrating the flat band (left) and the band banding upon exposure to O 2 (right) of n-type semiconductor metal oxides. The adsorption of oxygen as O -, leads to band bending. E C , E V , E F and S refer to energy of conducting band, valence band, the Fermi level, and surface vacancy respectively [6].

Figure 1 - 4 .

 14 Figure 1-4. Literature study of diverse oxygen species detected at different temperatures at SnO 2 material surface by IR (infrared analysis), TPD (temperature programed desorption), and EPR (electron paramagnetic resonance) [4,20].

Figure 1 - 5 .

 15 Figure 1-5. Potential barrier appearance at the grain boundaries [21,22], eVs is the potential barrier.

  of the sensitive layer, G 0 : Conductivity obtained under reference gas (air), eVs: Energy of the potential barrier, k B : Boltzmann constant, T: Absolute temperature.

Figure 1 - 6 .

 16 Figure 1-6. Literature study of water-related species formed at different temperatures on SnO 2 surface. Results were detected by IR and TPD [20].

Figure 1 - 7 .

 17 Figure 1-7. Temperature dependances of conductivity response of SnO 2 thick films to (1) C 2 H 5 OH (50 ppm), (2) H 2 (500 ppm), (3) CO (300 ppm), (4) CH 4 (1000 ppm), in dry synthetic air [35].

Figure 1 - 8 .

 18 Figure 1-8. Response curve of SnO 2 sensor to ammonia gas at room temperature [73]. The current is proportional to conductance.

Figure 1 - 9 .

 19 Figure 1-9. Different approaches towards surface functionalization of metal oxides surface [114].

Figure 1 - 10 .

 110 Figure 1-10. Some examples of silane products.

Figure 1 - 11 .

 111 Figure 1-11. Schematic representation of the silanization process by APTES on silicon substrate [120].

Figure 1 - 12 .

 112 Figure 1-12. Schematic illustration of APTES layer modified with different functional groups [147].

Figure 1 - 13 .

 113 Figure 1-13. The sensor response of APTES modified silicon nanowire field effect transistors, with and without modifications by various functional groups upon exposure to different gases at p a /p o = 0.02, (where p a and p o are the VOC's partial pressure and vapor pressure at a given temperature, respectively). ΔV th is the threshold voltage [147].

Figure 1 - 14 .

 114 Figure 1-14. Schematic illustrating the organically hybridized SnO 2 thin film sensors [148].

Figure 1 - 15 .

 115 Figure 1-15. The response curves of (a) DHBA-APS-SnO 2 and SnO 2 sensors to 3 vol.% CO balanced with air at 150 °C and (b) DHBA-APS-SnO 2 sensor to 3 vol.% H 2 , CO, and CH 4 balanced with air at 150 °C [148].

Figure 2 - 1 .

 21 Figure 2-1. Schematic representation of screen printing technique [2].

  1. For the synthesis of the used sensors, Pt-resistor to heat the sensitive layer was printed on the heating side of alumina substrate (Al 2 O 3 , purity of 96%). On the two faces of Al 2 O 3 substrate, screen printed gold electrodes were deposited as shown in Figure 2-2. A dielectric film was deposited on Pt-heater to prevent the interactions of Pt with gases. Finally, two successfully screen printed layers of SnO 2 are deposited on sputtered gold electrodes which are in turn (~0.6 µm thickness and 1 mm distance between the electrodes) deposited by sputtering on Al 2 O 3 substrate (sensing side). Each screen printed element was dried in an oven (MEMMERT UM/SM 100) to evaporate the solvent and annealed in muffle furnace (Thermolyne furnace 48000) to remove the organic binder according to Table 2-1. This furnace allows controlling the ramp (10 °C/min) and the dwell temperature and time (700 °C, 10 h for SnO 2 ). The image of fabricated sensor is shown in Figure 2-2.

Figure 2 - 2 .

 22 Figure 2-2. The two sides of SnO 2 sensor elaborated by screen printing.

SnO 2 Figure 2 - 3 .

 223 Figure 2-3. Vapor phase deposition was carried out at LMGP laboratory of Grenoble with standard synthesis parameters. Liquid phase deposition was developed at the Ecole des Mines of Saint Etienne by investigating the effects of silanization parameters.

Figure 2 - 3 .

 23 Figure 2-3. Theoretical schematic representation of the silanization process.

Figure 2 - 4 . 2 .

 242 Figure 2-4. Installation of the SnO 2 samples and APTES solution in the Teflon pot.

Figure 2 - 5 .

 25 Figure 2-5. Photograph of the experimental setup of sensors in liquid phase silanization process.

Figure 2 - 6 .

 26 Figure 2-6. Schematic illustration of SnO 2 -APTES-ester, SnO 2 -APTES-acid, and SnO 2 -APTES-alkyl synthesis steps. Practically, the SnO 2 -APTES sensors were immersed in a solution containing an acyl chloride product with 5 µL of triethylamine (Fluka) in 10 mL of chloroform as solvent. The different acyl chloride concentrations are 10 mM of hexanoyl chloride (98%, ALDRICH, alkyl: C 6 H 11 ClO), 50 mM of 1,4-butanedicarbonyl chloride (98%, Fluka, acid: C 6 H 8 Cl 2 O 2 ), or 10 mM of methyl adipoyl chloride (96%, Alfa Aesar, ester: C 7 H 11 ClO 3 ). These different solutions were kept for 12 h under stirring. This reaction was carried out in a setup as shown in Figure 2-5. The sensors were then rinsed with chloroform and dried under N 2 flow (sensors: SnO 2 -APTES-alkyl, SnO 2 -APTES-acid and SnO 2 -APTES-ester). This step of SnO 2 sensors functionalization leads to covalent attachment of alkyl, acid and ester end functional groups.

- 1 .

 1 According to these regions, different phenomena are observed in IR spectroscopy. During the reflection of the incident beam, a part of this beam penetrates into the medium of the lower refraction index before undergoing reflection. This penetration causes an evanescent wave. This evanescent wave can interact with the studied molecule. During the penetration to the second medium, a part of the incident light can be absorbed by tested sample and therefore the reflected ray has less energy than the incident ray. In this case, it becomes possible to detect an absorption spectrum. It is one of the techniques of infrared spectroscopy that has evolved mostly in recent years.

VERTEX 70 (

 70 Germany) spectrometer from Bruker with a DTGS detector operated at 4 cm -1 resolution in Saint Etienne (Figure2-8). 64 scans were collected in each run. Absorbance mode was used for the collection of all spectra.

Figure 2 - 7 .

 27 Figure 2-7. Schematic representation of ATR-FTIR detection principle [6].

Figure 2 - 8 .

 28 Figure 2-8. ATR-FTIR machine VERTEX 70 with diamond crystal.

Figure 2 - 9 .

 29 Figure 2-9. The diagram showing the process of photo-emission.

Figure 2 - 10 .

 210 Figure 2-10. Schematic representation of contact angle measurement.

Figure 2 1 .

 21 Figure 2-11. Krüss DSA10-Mk2 contact angle machine.

Figure 2 - 12 .SnO 2 -

 2122 Figure 2-12. Schematic representation of SnO 2 modified Alexa Fluor® 488 carboxylic acid, succinimidyl ester.

Figure 2 - 13 .

 213 Figure 2-13. Citrated gold nanoparticle.

Figure 2 - 14 .

 214 Figure 2-14. The functionalization steps of SnO 2 -APTES by citrated gold nanoparticle.

Figure 2 - 15 .

 215 Figure 2-15. Schematic diagram of the homemade test bench.

Figure 2 - 16 .

 216 Figure 2-16. Schematic representation of voltage divider circuit. E= fixed measurement voltage, U = measured voltage, R= sensor resistor, r = fixed reference resistor.

Figure 3 - 1 .

 31 Figure 3-1. SEM images of the cross section (a and b), and top SnO 2 film (c and d) on alumina substrate.

Figure 3 - 2 .

 32 Figure 3-2. ATR-FTIR spectrum of SnO 2 film between 400 and 4000 cm -1 and the zoom between 800 and 4000 cm -1 .

Figure 3 -

 3 Figure 3-4, O 1s peak is composed of two components at 530.3 eV, and 531.7 eV which correspond to Sn-O, and Sn-OH bonds respectively [2]. These hydroxyl groups on the surface of APTES will be the reactive sites for the silanization process by APTES. Resulting SnO 2 -APTES films will contain higher quantity of oxygen on the surface since each APTES molecule contains 3 oxygen atoms. Furthermore, functionalization by APTES introduces Sn-O-Si and Si-O-Si bonds. Therefore, the O 1s peak presented in Figure 3-4 will be compared with the peak after modification.

Figure 3 - 3 .

 33 Figure 3-3. XPS survey spectrum of SnO 2 film on Al 2 O 3 substrate.

Figure 3 - 4 .

 34 Figure 3-4. XPS narrow scan of O 1s peak for SnO 2 (X-ray incident angle is 41°).

Al 2 O

 2 3 surface. Thus, the wettability test was carried out on Al 2 O 3 substrate instead of SnO 2 .

Figure 3 -

 3 Figure 3-5 shows the change of the contact angle after silanization on Al 2 O 3 substrate. The contact angle was changed from 35° to 80° after silanization. It results from the modification of alumina by APTES molecules.

Figure 3 - 5 . 2 (

 352 Figure 3-5. Photographs of water droplets (wettability test) on alumina substrate before (a) and after (b) silanization. SnO 2 -APTES film will be further used as a substrate to attach molecule containing acyl chloride group. For these reasons, it is important to verify the effectiveness of grafting on SnO 2 . The reactivity of terminated amine groups of APTES with respect to coupling reactions was checked using fluorescent molecules such as Alexa Fluor molecules which have affinity with amine groups of APTES. Figure 3-6 shows the epifluorescence microscope images of SnO 2 (Figure 3-6a), and SnO 2 -APTES after silanization in vapor (Figure 3-6b) and liquid phase (synthesis parameters: 50 mM of hydrous APTES solution, immersed for 5 h, Figure 3-6c) modified by Alexa Fluor. As expected, for SnO 2 without modification by APTES, no fluorescence was observed (Figure 3-6a). On the contrary, the obtained fluorescence of SnO 2 -APTES films (Figure 3-6b and c) means that the Alexa Fluor molecules are attached to the surface by the grafted amine terminal part of APTES. The fluorescence intensity after silanization was measured by Fluorescence Spectrometer. The intensity of APTES modified SnO 2 in vapor phase was 3185 (arbitrary units), while it was 4541 in liquid silanization. Therefore, more reactive amine groups are present on the surface for the SnO 2 -APTES carried out in liquid phase.

Figure 3 - 6 .(

 36 Figure 3-6. Epifluorescence images after immersion in Alexa Fluor solution: (a) SnO 2 , (b) SnO 2 -APTES-vapor phase and (c) SnO 2 -APTES-liquid phase (5 vol% H 2 O, 50 mM of APTES, 5 h).Another method to check the presence and the reactivity of APTES film, as well as the distribution of APTES molecules on SnO 2 was carried out by using 20 nm citrated gold nanoparticles. The primary amine groups of APTES react with the citrate groups present on the surface of gold nanoparticles. This reaction leads to the physisorption of the gold nanoparticles on the surface of SnO 2 . As shown in Figure3-7, The SnO 2 -APTES sample has changed to grey, while the other sample (pure SnO 2 ) has remained white after 1.5 h reaction. The change in color is due to the attachment of gold nanoparticles on SnO 2 as shown in Figure 3-8. Gold nanoparticles on SnO 2 -APTES are clearly seen by SEM (Figure 3-8b), contrary to pure SnO 2 (Figure 3-8a). This ensured the presence of APTES on the surface of SnO 2 . Furthermore, gold nanoparticles are distributed in the entire tested zone which means that APTES is homogenously distributed over SnO 2 surface.

Figure 3 - 7 .

 37 Figure 3-7. The change in color of SnO 2 and SnO 2 -APTES after 1.5 h dipping in suspension of citrated gold nanoparticles.

Figure 3 - 8 .

 38 Figure 3-8. SEM images of pure and modified SnO 2 after 1.5 h dipping in suspension of citrated gold nanoparticles.

Figure 3 - 9 .Figure 3 -Figure 3 - 10 . 3 +[ 10 ]

 393310310 Figure 3-9. ATR-FTIR spectra of SnO 2 and SnO 2 -APTES (hydrous liquid silanization, 50 mM APTES, 5 h). XPS was carried out to show the elemental evolution after silanization process. The XPS survey scan spectrum of SnO 2 -APTES film obtained by hydrous liquid synthesis presented in Figure 3-10 shows the presence of Sn and O related to the SnO 2 substrate. These two peaks have less intensity comparing with pure SnO 2 (Figure 3-3). The grafting of APTES film decreases the intensity of the peaks related to SnO 2 (Sn and O) because of the formed film. In addition, this spectrum reveals the existence of N, C and Si atoms related to APTES molecules at the surface which confirms the silanization of SnO 2 . N, C, and Si peaks were found at 400, 285 and 102 eV respectively.

Figure 3 - 11 .

 311 Figure 3-11. XPS peak fitting of (a) C 1s, (b) N 1s, (c) O 1s, and (d) Si 2p of SnO 2 -APTES film.

Figure 3 -

 3 Figure 3-12. ATR-FTIR spectra of SnO 2 , and SnO 2 -APTES synthesized in liquid and vapor phase. Liquid phase silanization was carried in 50 mM APTES dissolved in hydrous or anhydrous ethanol for 5 h.

Figure 3 -

 3 13 illustrates the N 1s, Si 2p, Sn 3d and C 1s peaks of SnO 2 -APTES synthetized in hydrous and anhydrous APTES solution. N 1s, Si 2p, and C 1s peaks are more intense for the film synthesized in aqueous APTES solution than in a dry one. For Sn 3d peaks, the film prepared with water has lower peak intensity than the dry one. All these results imply that more APTES molecules are grafted on the surface in solution with water that in dry one as already shown by ATR-FTIR (Figure 3-12).

Figure 3 - 13 .

 313 Figure 3-13. XPS peaks of a) N 1s, b) Si 2p, c) Sn 3d, and d) C 1s for SnO 2 -APTES synthetized in solution with 5% by volume of H 2 O (red curve) and without H 2 O (blue curve) for 5 h and 50 mM of APTES.

  -14, three APTES concentrations (1 mM, 50 mM, 500 mM) were tested in ethanol solution with 5 vol% H 2 O for 5 h. Peaks intensities of Si-O-Si bonds at 1029 and 1125 cm -1 are low with 1 mM APTES solution

Figure 3 - 14 .

 314 Figure 3-14. Hydrous liquid phase silanization (5 h) with different APTES concentrations of 1, 50 and 500 mM in 5 mL ethanol.

Figure 3 -

 3 Figure 3-15 shows different reaction times of SnO 2 film in APTES solution (ethanol with 5 vol% H 2 O, 50 mM of APTES). The peak of Si-O-Si (1029 cm -1 ) rises with the increase of reaction time. As this peak is proportional to the amount of APTES attached on the surface [6], one can

Figure 3 - 15 .

 315 Figure 3-15. ATR-FTIR of SnO 2 and SnO 2 -APTES synthesized in various reaction times.

Figure 3 -

 3 Figure 3-16. (a) Modelisation of the electron inelastic background of Sn 3p using QUASES software [14], (b) (c) (d) schematic representation of the coverage and thickness of the APTES overlayer for the three methods used for the silanization of SnO 2 .

  three thicknesses: d C 3 NH 8 , d Si and d APTES . These parameters are presented in the Figure 3-17 and correspond to different portions of the APTES film.

Figure 3 - 17 .

 317 Figure 3-17. Schematic illustration of the different portions of the APTES film considered for the calculation of the APTES surface concentration using the ratio of Si 2p and Sn 4d intensities.

Figure 3 - 18 .

 318 Figure 3-18. Evolution of N 1s, C 1s, Si 2p and Sn 3d intensities as a function of temperature for 4 h annealing treatment of SnO 2 -APTES synthetized in vapor phase.

SnO 2 -

 2 APTES (blue curve), SnO 2 -APTES-alkyl (red curve), SnO 2 -APTES-acid (orange curve) and SnO 2 -APTES-ester (green curve) are presented in Figure 3-19. The IR scan was carried out between 800 and 4000 cm -1 .

Figure 3 - 19 .

 319 Figure 3-19. ATR-FTIR spectra of SnO 2 (black curve), SnO 2 -APTES (blue curve), SnO 2 -APTESalkyl (red curve), SnO 2 -APTES-acid (orange curve), and SnO 2 -APTES-ester (green curve) films.

SnO 2 -SnO 2 - 1 (

 221 APTES (blue curve) which relates to amine groups has totally disappeared. This confirms the formation of amide bond in place of amine. Asymmetrical C-H stretching mode of CH 3 for APTES-alkyl and for SnO 2 -APTES-ester appears at around 2940 cm -1 . The stretching peak of carbon double bounded to oxygen of ester group of SnO 2 -APTES-ester is found at 1734 cm -Figure3-19, green curve).

Figure 3 -

 3 20 shows C 1s and Sn 3d peaks of SnO 2 -APTES before and after modification by alkyl, acid and ester functional groups. As shown before (Figure 3-11a), the peaks present for SnO 2 -APTES (Figure 3-20a) at 282.2, 284.9 eV and 286.4 eV were assigned respectively to C-Si, C-C/CH and C-N chemical bonds. The peak intensities may be different from previous test because the experiment was conducted in different incident angle (41°). These three peaks were found also for alkyl, acid and ester modified SnO 2 -APTES at almost the same binding energy (Figure 3-20b, c, and d).

3 .

 3 The increase in area is proportional to the increase in the quantity of C-C and C-H bond after the second step of functionalization. In addition to these peaks, alkyl, acid and ester modified SnO 2 -APTES present a peak at around 288 eV which correspond to the formed amide bond (-N-COR; where R is the other part of molecule containing alkyl, acid or ester) by coupling reaction. Moreover, SnO 2 -APTES-ester exhibits a peak at 289.1 eV which is related to the oxygen double bounded to the carbon of ester group (-COOCH 3 , Figure3-20d). Furthermore, Sn 3d peaks decrease after functionalization by APTES, APTES-alkyl, APTES-acid and APTES-ester (Figure3-20e) compared to pure SnO 2 , because it is covered by an organic film, as shown previously for the first step resulting in SnO 2 -APTES formation. In addition, as the thickness of the organic film on SnO 2 increases, the intensity of Sn 3d peak decreases. Taking into account that XPS analyzes only 10 nm depth. For this reason, Sn 3d peak of APTES modified SnO 2 film was found with intermediate intensity between pure and alkyl, acid, and ester modified SnO 2 (Figure3-20e, blue curve).

Figure 3 -

 3 Figure 3-20. C 1s XPS spectra of SnO 2 -APTES (a), SnO 2 -APTES-alkyl (b), SnO 2 -APTES-acid (c), SnO 2 -APTES-ester (d). Sn 3d peaks of pure and modified SnO 2 by APTES, APTES-alkyl, APTESacid and APTES-ester (e).

Figure 4 -

 4 1 shows the normalized response of SnO 2 -APTES-ester (a) and SnO 2 (b) to ammonia at 25 °C, pre-heated at 100 °C under air with 5%RH for different times. SnO 2 -APTESester presents different responses to 50 ppm of ammonia depending of the time of pre-heating.

Figure 4 - 1 .

 41 Figure 4-1. Normalized conductance of SnO 2 -APTES-ester (a) and SnO 2 (b) sensor upon exposure to 50 ppm of ammonia at 25 °C just after the synthesis (without pre-heating) and after heating at 100 °C for 5 min, 30 min, 1.5 h, 4.5 h, and 9.5h. The test was carried out in air with 5 %RH.

Figure 4 -

 4 2 shows the ATR-FTIR spectra of SnO 2 -APTES-ester sensor before and after heating 10 h at 100 °C. With respect to APTES, the peaks related to Si-O-Sn and Si-O-Si bonds still exist after the thermal treatment with almost the same intensities. The little change in intensity after heating of the peak at 1029 cm -1 is due to the polymerization of unreacted hydroxysilane groups of APTES film. The other groups which are related to the second step of molecularly modifications such as amide, ester, CH 2 , and CH 3 are retained after thermal treatment. However, the intensity of these peaks are less than that without heating at 100 °C because of the desorption of unreacted molecules (acyl chloride). Therefore, the thermal treatment of the sensors does not affect the safety of the grafted organic film.

Figure 4 - 2 .

 42 Figure 4-2. ATR-FTIR spectra of SnO 2 -APTES-ester before (a) and after heating in-situ (in test bench) for 10 h in air with 5%RH (b).

Figure 4 - 3 .

 43 Figure 4-3. The sensor response of SnO 2 (G 0 =1.4×10 -5 ), SnO 2 -APTES (G 0 =7.9×10 -6 ), SnO 2 -APTESalkyl (G 0 =1.5×10 -5 ), and SnO 2 -APTES-ester (G 0 =9.5×10 -6 ) to 100 ppm ammonia gas balanced with humid air (5%RH) at 25 °C.From Figure4-3, it is observed that functionalized SnO 2 sensors do not respond in the same direction as pure SnO 2 . The next discussion is focused on explanations of the sensing mechanism of functionalized sensors to ammonia.

SnO 2 -

 2 APTES-alkyl, SnO 2 -APTES-acid and SnO 2 -APTES-ester exhibit increase in conductance upon exposure to 100 ppm of ammonia gas as shown in Figure4-3. However, the response of SnO 2 -APTES-ester and SnO 2 -APTES-acid are more important than for SnO 2 -APTES-alkyl.

Figure 4 - 4 .

 44 Figure 4-4. Schematic illustration of the interaction between ammonia and alkyl, acid and ester functionalized SnO 2 -APTES sensors.

SnO 2 -

 2 APTES-alkyl. These results confirm the above discussion which assumes that SnO 2 -APTES-ester and SnO 2 -APTES-acid have the strongest interactions among the functionalized sensors toward ammonia. The response at low concentrations and the standard deviation of the base line over 10 min were used to calculate the limit of detection (LOD) of the ester and acid modified SnO 2 sensors to ammonia gas which is around 80 ppb. This value confirms the potentiality of ammonia detection for breath analysis application, as concentrations to detect are in the range of ppb.

Figure 4 - 5 .

 45 Figure 4-5. Relative response of pure SnO 2 , SnO 2 -APTES, SnO 2 -APTES-alkyl, SnO 2 -APTES-acid, and SnO 2 -APTES-ester sensors versus ammonia concentrations balanced with 5% RH air at 25 °C.

Figure 4 - 6 .

 46 Figure 4-6. The sensor response curves of a) SnO 2 -APTES-ester and b) SnO 2 -APTES-acid sensors to 50 and 100 ppm of ammonia gas balanced with dry air, 5%RH, 26%RH, and 50%RH at 25 °C. To better understand the effect of water, Figure 4-7a and b show the response of SnO 2 -APTESester and SnO 2 -APTES-acid sensors respectively to different relative humidity percentage at 25 °C in dry air. SnO 2 -APTES-ester exhibits almost no change in conductance to 5%RH while SnO 2 -APTES-acid shows increase in conductance. These responses explain the fact that SnO 2 -

Figure 4 - 7 .

 47 Figure 4-7. The response of a) SnO 2 -APTES-ester and b) SnO 2 -APTES-acid to different percentage of relative humidities at 25 °C in dry air.

Figure 4 - 8 ,

 48 Figure 4-8, the response of SnO 2 -APTES-ester to ammonia decreases from 1.47 (25 °C) to 1.18 and 1.06 when the operating temperature is increased to 50 °C and 100 °C respectively (Figure 4-8a). The same behavior was found for SnO 2 -APTES-acid, the response to ammonia decreases from 1.42 (25 °C) to 1.17 and 1.02 when the operating temperature is increased to 50 °C and 100 °C respectively (Figure 4-8b). Therefore, the interaction of ammonia with ester or acid attached to SnO 2 is more significant at low temperature (25 °C) than at higher one (50 and 100 °C).

Figure 4 - 8 .

 48 Figure 4-8. The sensor response of SnO 2 -APTES-ester (a) and SnO 2 -APTES-acid (b) to 50 ppm of ammonia gas balanced with humid air (5%RH) at 25 °C, 50 °C, and 100 °C.

  decrease in the response to ammonia when the sensor operating temperature increases as demonstrated in Figure4-9. The dipole moment, illustrated as a vector in green, increases with the increase of operating temperature. The molecular layer reaches almost maximum dipole moment value at 100 °C (Figure4-9a). This increase in the dipole moment increases the conductance base line of the sensor. At 100 °C, the dipole moment reaches a maximum value due to temperature, before the injection of ammonia (Figure4-9). Hence, the dipole moment cannot change any more. Thus, very little response was observed to 50 ppm of ammonia at 100 °C. The same explanations can be applied on acid modified SnO 2 -APTES sensor concerning the effect of temperature on the response to ammonia. At 50 °C, the response to ammonia is less important that at 25 °C as shown in Figure4-8, because the dipole moment is already increased by the temperature.

Figure 4 - 9 .

 49 Figure 4-9. Schematics demonstration of SnO 2 -APTES-ester a) at different temperatures b) upon exposure to 50 ppm of ammonia at different operating temperatures.

Figure 4 - 10 .

 410 Figure 4-10. The sensor response of SnO 2 -APTES-ester and SnO 2 -APTES-acid upon exposure to 50 ppm of ammonia, ethanol, and acetone gases in humid air (5%RH) at 25 °C.

Figure 4 -

 4 Figure4-10 shows that the SnO 2 -APTES-ester sensor has almost no change in conductance upon exposure to 50 ppm of ethanol, carbon monoxide and acetone at 25 °C. This means that ester modified sensor is selective to ammonia, at least in regards of the three tested gases. This particular selectivity derives from the unique interactions of the grafted layer on SnO 2 with ammonia gas. As mentioned previously, interactions occur between the ester end group which is strongly electron withdrawing, and the ammonia molecule which is electron donnating. Hence, electrons are withdrawed by the attached ester end group from the ammonia molecules adsorbed on it during exposure. These interactions lead to the significant change in the dipole moment which in turn leads to the presented response of SnO 2 -APTES-ester sensor to ammonia. Others molecules like ethanol, carbon monoxide and acetone do not have this high affinity to donate the electrons to SnO 2 -APTES-ester, explaining negligable changes in conductance upon exposure to these gases.

Figure 4 -

 4 Figure 4-11 shows the change in conductance of SnO 2 -APTES-ester (Figure 4-11a and b) and SnO 2 -APTES-acid (Figure 4-11c and d) upon exposure to different concentrations of ammonia gas (0.2-100 ppm). Curves a) and c) present the response to low ammonia concentrations (0.2-5 ppm). These two curves were used to draw the sensitivity curve at low concentration presented in Figure 4-3.

Figure 4 - 11 .

 411 Figure 4-11. The change in conductance of SnO 2 -APTES-ester and SnO 2 -APTES-acid upon exposure to different concentrations of ammonia gas in humid air (5%RH) at 25 °C. a) and c) [NH 3 ] ranging from 0.2 to 5 ppm. b) and d) [NH 3 ] ranging from 5 to 100 ppm.The other curves (concentration higher than 5 ppm, Figure4-11b, and d) show the stability of the sensors with time even with different injections of ammonia. The stability of metal oxide is a challenge mostly for room temperature gas sensors. Present results also show the baseline stability with time. The drift is around 0.98% over 10 hours for the two sensors.

Figure 4 - 12 .

 412 Figure 4-12. Sensors response versus ammonia concentrations of SnO 2 -APTES-ester and SnO 2 -APTES-acid at 25 °C under 5%RH (magnification shows 0.2-10 ppm ammonia concentration range).

Figure 4 - 13 .

 413 Figure 4-13. The response of SnO 2 -APTES-ester sensor to ammonia in air or nitrogen at 25 °C with 5%RH.

École

  Sensitive and selective ammonia gas sensor based on molecularly functionalized tin dioxide working at room temperature Speciality: Chemical and process engineering Keywords: SnO 2 , Screen printing, Molecular functionalization, APTES, Acid, Ester, Ammonia, Selectivity, Room temperature detection, Exhaled breath analysis Abstract:

Table 1 -1. Classification of various chemical sensors.

 1

	Type	Measured variable	Sensor example
	Electrochemical sensors	Electromotive force	Electrochemical cell
	Calorimetric	Temperature	Pellistor
	Optical gas sensors	Absorption peak	Photoionization detector
	Gravimetric	Frequency	Quartz microbalance sensors
	Conductometric	Electrical conductivity	Metal oxide gas sensors

Table 1 -2. Different sensors based on pure and modified polyaniline material.

 1

	Sensitive material	Operating temperature (°C)	Detected gas	Reference
	polyaniline nanowires	25	NH 3 (25-500 ppm)	[47]
	polyaniline nanofibers	25	H 2	[48]
	polyaniline/TiO 2	25	LPG	[49]
	Polyaniline/palladium	25	H 2	[50]
	polyaniline	25	NH 3 (5-1000 ppm)	[51]
	polyaniline/SnO 2	25	NH 3 (100-500 ppm)	[44]

1.6.3. SnO 2 modified by other metal oxides gas sensors

	The operating temperature of the gas sensor can also be decreased by using a composite of metal
	oxides. This combination leads to a modification of the electronic structure of the composite
	oxides and consequently induces the formation of a high resistance between the grains of
	different types. The combination of metal oxides forms p-n type heterojunctions, for example in
	the case of the following composite oxides: CuO (p)/SnO 2 (n), ZnO (p)/SnO 2 (n) [52], and
	La 2 CuO 4 (p)/ZnO (n) [53]. Selim et al. [54] have fabricated a ZrO 2 -doped SnO 2 sensor for the
	detection of H 2 S at room temperature. Similarly, Patel et al. [55] have shown that thin films
	composed of a mixture of indium and tin oxide (In 2 O 3 / SnO 2 ) are sensitive to methanol at low
	temperature.
	1.6

.4. SnO 2 with addition of metals or carbon Several

  authors have shown that the addition of noble metals or carbon nanotubes to the metal oxide influences the operating temperature of the sensor as well as its selectivity. In this direction, Tadeev et al.[START_REF] Tadeev | Influence of Pd and Pt additives on the microstructural and electrical properties of SnO 2 -based sensors[END_REF] have shown that the addition of additives such as Pd and Pt to SnO 2 layer decreases the optimal CO detection temperature from 400 °C to 100 °C. Chaparadza et al.

	[57] found that SnO 2 doped with 0.1% of Sb showed a high Cl 2 response and selectivity with
	respect to compounds Br 2 , HCl, NO, NO 2 , CHCl 3 , NH 3 and H 2 at room temperature. This sensor
	has a relatively short response time (~ 60 s). Wei et al. [36] also revealed that the doping of the
	SnO 2 layers with very small amounts of the mono-walled carbon nanotubes (SWNT) improves
	sensitivity to NO 2 at room temperature.
	1.7

. Breath analysis application: focus on ammonia and SnO 2 1.7.1. Breath analysis

Table 1

 1

	illustrates the major contents of the human breath. Elevated
	levels of a number of these gases indicate a systemic disorders and extra pulmonary organ
	failures.

Table 1 -3. Major human breath content for health person [64].

 1

	Exhaled Breath	Concentration
	Nitrogen	78.04%
	Oxygen	16%
	Carbon dioxide	4%-5%
	Hydrogen	<5ppm
	Carbon monoxide	0-6 ppm
	Acetone and Ethanol	<1 ppm

Table 1 -4. Conductometric metal oxide sensors for the detection of ammonia.

 1

	Material	Analyte Temperature (°C) Concentration (ppm) Reference
	SnO 2 (ultrathin film)	NH 3	25	5-30	[73]
	SnO 2 (nanoparticles)	NH 3	25	50	[74]
	SnO 2 (ultrathin film)	NH 3	25	9	[75]
	ZnO	NH 3	25	40-160	[76]
	ZnO/Cr 2 O 3	NH 3	25	20	[77]
	WS 2	NH 3	25	1-10	[78]
	TeO 2	NH 3	25	10-100	[79]
	Sb doped SnO 2	NH 3	25	50	[80]
	SnO 2 /MWNTs	NH 3	25	60-800	[71]
	SnO 2 /SWNTs	NH 3	25	1-100	[81]
	SnO 2 /polyaniline	NH 3	25	100-500	[44]
	SnO 2 /polyaniline	NH 3	25	100	[82]
	TiO 2 /polyaniline	NH 3	25	20-100	[83]
	Camphor sulfonic acid doped	NH 3	25	20-100	[84]
	PANi-TiO 2
	Camphor sulfonic acid doped	NH 3	25	100	[85]
	PANi-SnO 2
	Polypyrrole-reduced	NH 3	25	10	[86]
	graphene oxide hybrid
	graphene oxide-silver	NH 3	25	15-100	[87]
	nanoparticles
	Pd/SnO 2 / reduced graphene	NH 3	25	5-300	[88]
	oxide
	SnO 2	NH 3	100, 400	500	[89,90]
	ZnO	NH 3	200	2-100	[91]
	TiO 2	NH 3	300, 350	43-156	[92]

.4. Interactions of ammonia with SnO 2 gas sensors at high temperature

  Numerous models have been proposed trying to explain the detection mechanism of ammonia by metal oxides based sensors[START_REF] Clifford | Characteristics of semiconductor gas sensors II. Transient response to temperature change[END_REF]. It is now well known that the interactions of gases with metal oxide sensitive layer lead to change in the conductivity of gas sensor. The oxygen species (O 2

-, O -

Table 2 -1. Different sensor elements deposition parameters.

 2

	Screen printed	Ink	Drying	Annealing	Screen mesh count
	element				(threads per square
					inch)
	Platinum heater	ESL 5545	100 °C, 10 min 980 °C, 1 h	325
	Gold electrodes	ESL 8880-G 100 °C, 10 min 850 °C, 1 h	-
	Dielectric film	ESL 4917	100 °C, 10 min 850 °C, 1 h	180
	Sensitive film (SnO 2 )	As-prepared	100 °C, 10 min 700 °C, 10 h	180

Table 3 -1. Values of inelastic mean free path λ and associated attenuation factor k calculated using the Cumpson [17] method for an electron with kinetic energy E travelling through a given matrix with thickness d.

 3

	Matrix	Peak, E Kinetic (eV)	d Matrix (Å)	 peak Matrix (nm)	k peak Matrix
	APTES	Sn 4d, 1461	8	6.01	0.81
	C 3 NH 8	Si 2p, 1384	~6	4.83	0.82

Table 3 -

 3 1, equation 4 can be simplified due to a ratio of the attenuation factors k Sn4d APTES and k Si2p C3NH8 close to unity. Moreover, the kinetic energies of Si 2p and Sn 4d are closed

Table 3 -2. Calculated concentrations of APTES grafted on SnO 2 obtained by different synthesis ways.

 3

	SnO 2 -APTES	𝑛 APTES (𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠/𝑐𝑚 2 )
	vapor phase	4.7×10 14
	hydrous liquid phase	9×10 14

  The second step of sensors modification is the attachment of a chemical function on SnO 2 -APTES, ended with alkyl, acid or ester groups. Furthermore, the reaction of APTES amine group with acyl chloride leads to the formation of amide bond and add more carbon on the surface of SnO 2 (CH 2 , CH 3 ). Thus, each modified SnO 2 -APTES is supposed to have amide and alkyl, acid or ester functional groups. Molecular characterization of the attached films on SnO 2 -APTES was carried out by ATR-FTIR. Spectra of SnO 2 (black curve),

Table 3 -3. The area under C-C/C-H component of C 1s peak found at 284.9 eV for the different functionalized SnO 2 sensors.

 3 Screen printed SnO 2 films exhibit particles size about 80 nm with random shapes and about 40 µm films thickness. These films were successfully functionalized by APTES via vapor and liquid phases. A comparison between the synthesis conditions was achieved. Liquid phase silanization with water clearly demonstrates more APTES grafting than vapor and liquid phase silanizations without water. Water during the liquid phase silanization enhances the polymerization of APTES on the surface. Interestingly, 4 h reaction and 50 mM were found to be sufficient to well cover SnO 2 film by APTES molecules. No larger APTES amounts or longer times are necessary. Furthermore, thermal treatment of APTES film on SnO 2 -APTES shows that the APTES start to degrade at temperature above 100 °C, while no notable change in the amount of silicon between 100 °C and 500 °C is observed. Amine groups have totally disappeared after heating the film at 500 °C. We have calculated the concentration of APTES by calculating the amount of Si on the surface. The number of Si atoms of SnO 2 -APTES synthesized by liquid without water and vapor silanization is respectively 4.24×10 14 atoms.cm -2 and 4.7×10 14 atoms.cm -2 , while it is 9×10 14 atoms.cm -2 with the addition of 5 vol% H 2 O. In addition, the concentrations of grafted APTES determined by XPS analyses are in good agreement with ATR-FTIR results which confirm that the liquid route in the presence of water leads to more APTES grafting onto SnO 2 with an almost 50% improvement. The optimum SnO 2 -APTES film was used to attach the desired end functional groups on it. ATR-FTIR and XPS prove that SnO 2 -APTES is well modified by ester, acid and alkyl end functional groups. The change in sensitivity and selectivity of these sensors to gases will be investigated in the next chapter.

	Sensor	Area of C-C/C-H peak (counts.eV.s -1 )
	SnO 2 -APTES	222
	SnO 2 -APTES-alkyl	309
	SnO 2 -APTES-acid	296
	SnO 2 -APTES-ester	325
	As a conclusion, FTIR and XPS analyses confirm that functionalization is effectively achieved on
	SnO 2 -APTES after reaction with acyl chloride products by showing the presence of ester, acid
	and alkyl molecules.
	3.10. Conclusion

  2 -APTES-ester and SnO 2 -APTES-acid in comparison with pure SnO 2 , SnO 2 -APTES and SnO 2 -APTES-alkyl sensors. Sensitivity is defined as the slope of the relative response curve versus ammonia concentrations, i.e., how large is the change in the sensor signal upon a certain change in ammonia concentration. Pure SnO 2 and SnO 2 -APTES sensors have almost no sensitivity to different ammonia concentrations. In addition, SnO 2 -APTES-alkyl gives no significant response between 0.2 ppm and 30 ppm, but its sensitivity starts to increase from 30 ppm of ammonia. It can be noticed that SnO 2 -APTES-ester and SnO 2 -APTES-acid exhibit constant sensitivity between 0.2 ppm and 10 ppm, around 0.023 ppm -1 . For higher concentration, the response is no more linear and the sensitivity continuously decreases down to nearly zero at 100 ppm of ammonia. This means that the sensors reach a saturation state. The saturation state can be interpreted as the state where attached layer reaches a maximum value of dipole moment. However, the sensitivity of SnO 2 -APTES-ester and SnO 2 -APTES-acid at concentrations higher than 30 ppm is still more significant than the sensitivity of

SnO 2 -APTES-ester and SnO 2 -APTES-acid sensors 4.4.1. Effect of humidity

  As known, human breath contains high amount of humidity (100 %RH at 37 °C). Water molecule presents high polarity, hence it can affect the response to ammonia gas. In order to check this effect, SnO 2 -APTES-ester and SnO 2 -APTES-acid were tested to ammonia gas under different amount of relative humidity ranging from 5 to 50% RH. Figure4-6shows the sensor responses of SnO 2 -APTES-ester and SnO 2 -APTES-acid to 50 and 100 ppm in dry and humid air at 25 °C.

  Capteur de gaz SnO 2 fonctionnalisé fonctionnant à température ambiante, sensible et sélectif pour la détection d'ammoniac Spécialité : Génie des procédés Mots clefs : SnO 2 , Sérigraphie, Fonctionnalisation moléculaire, APTES, Acide, Ester, Ammoniac, Sélectivité, Détection à température ambiante, Analyse d'haleineRésumé :Dans le domaine de la santé, l'un des défis actuel est de détecter une maladie de façon non invasive alors que nous nous sentons en «bonne santé». L'analyse de l'haleine expirée offre un outil simple et non invasif pour le diagnostic précoce des maladies. Les capteurs de gaz à base de SnO 2 modifiés de manière moléculaire semblent être des dispositifs prometteurs pour détecter les gaz polaires tels que l'ammoniac. Dans cette étude, la fonctionnalisation de la surface de SnO 2 a été réalisée afin d'obtenir un capteur de gaz sensible et sélectif à l'ammoniac, qui puisse fonctionner à température ambiante. La première étape de la fonctionnalisation est la fixation covalente d'un film de 3-aminopropyltriéthoxysilane (APTES) sur SnO 2 en phase vapeur ou liquide. La silanisation en phase liquide a été réalisée dans une solution APTES hydratée ou anhydre. Les caractérisations effectuées par Spectroscopie Infrarouge à Réflectance Totale Atténuée (ATR-FTIR) et Spectrométrie Photoélectronique X (XPS) montrent qu'une quantité plus importante d'APTES a été greffée par une silanisation en phase liquide hydratée. Ce film a donc été utilisé comme substrat pour la deuxième étape qui consiste à fonctionnaliser le SnO 2 modifié par APTES avec des molécules contenant du chlorure d'acyle avec différents groupes fonctionnels finaux, tel que des groupes alkyle, acide et ester. Les capteurs SnO 2 et SnO 2 -APTES n'ont pas montré de sensibilité significative à l'ammoniac (0,2-100 ppm) à 25 °C, tandis que la sensibilité à l'ammoniac a commencé à augmenter à partir de 30 ppm pour le capteur SnO 2 -APTES modifié par un alkyle. Au contraire, les capteurs modifiés par des acides et des esters sont sensibles à l'ammoniac entre 0,2 et 10 ppm à température ambiante. Cependant, le SnO 2 -APTES modifié par l'ester s'est révélé être plus sélectif que le capteur modifié par l'acide pour l'éthanol et le monoxyde de carbone. Ces résultats impliquent que la réponse est générée par les groupes fonctionnels acide et ester. L'ammoniac modifie le moment dipolaire de la couche moléculaire greffée, ce qui entraîne une modification de la conductance de SnO 2 . Le fonctionnement à température ambiante est l'un des avantages de ces capteurs, tout comme leur sélectivité à l'ammoniac en regard d'autres gaz tels que l'éthanol, le monoxyde de carbone et l'acétone.

	École Nationale Supérieure des Mines
	de Saint-Étienne
	NNT : 2017LYSEM030
	Mohamad HIJAZI
	TITRE DE LA THÈSE :

Remerciements

General conclusions and perspectives

The objective of this thesis was to elaborate a sensitive and selective ammonia gas sensors based on organically modified SnO 2 thick film. In this contribution, a first step of functionalization by organic functional groups was the attachment of APTES on SnO 2 which acts as a substrate in the second step. This step was achieved in vapor and liquid phases. Thus, a part of this work was devoted to the investigation and improvement of APTES grafting on SnO 2 film by studying the effect of different APTES synthesis parameters. The second step of functionalization was to attach alkyl, acid and ester functional groups on the fixed APTES. After physico-chemical characterizations of all the synthesized sensors, tests under different gases were carried out to investigate the effect of functionalization on SnO 2 sensor behavior. Ammonia was the target gas since the elaborated sensors are expected to be used later in the detection of liver diseases in the exhaled breath. The selectivity was tested regarding ethanol, carbon monoxide and acetone gases.

Furthermore, a sensing mechanism between ammonia and the different sensors was proposed in this study.

Major conclusions are reported thereafter.

Thick film of SnO 2 was deposited by screen printing. The deposited film exhibit a particle size of about 80 nm with random shapes and about 40 µm films thickness. APTES was deposited via liquid and vapor phases on SnO 2 films. The synthesis parameters in liquid phase silanization such as water content, reaction time and APTES concentration were investigated. Liquid phase silanization with water clearly demonstrates more APTES grafting than vapor and liquid phase silanization without water. Water during the liquid phase silanization enhances the polymerization of APTES on the surface. Interestingly, keeping SnO 2 film for 4 h under stirring in 50 mM APTES solution was found to be sufficient to efficiently cover it by APTES molecules. Furthermore, thermal treatment of APTES film on SnO 2 -APTES shows that the APTES start to degrade at temperature above 110 °C, while no notable change in the amount of silicon between 110 °C and 500 °C was observed. Amine groups had totally disappeared after heating the film at 500 °C. A model to calculate the concentration of APTES grafted after silanization was proposed.

This model shows that the silanization leads to attach APTES in form of self-assembled monolayer with different densities depending on the synthesis procedures. The concentrations of  Confirm the proposed sensing mechanism by in-situ Raman, AFM and DRIFT. These characterization techniques can be carried out in special cell under gas flow. Thus, the type of ammonia interactions with the functionalized sensor can be checked. Ammonia changes the polarity of the attached layer that can variate the intensity of Raman and IR peaks. In addition, adsorbed ammonia on the attached layer can be characterized. The change of dipole moment caused by ammonia could be detected by AFM.

 Test the effect of other gases present in human exhaled breath (amines and other gases) on ester modified SnO 2 sensor to simulate the performance of this sensor in real conditions. After this step, the sensors can be tested under real conditions for the detection of liver diseases.

 Develop a method for sampling of exhaled breath because the breath contains high amount of water vapor which can condense on the sensor since it is operated at room temperature. Thus, the sampling should include a way to reduce the amount of humidity in the sample to less than 5%RH. In addition, the sensor response to ammonia is affected by relative humidity superior to 5%.

 Integrate the functionalized sensor on plastic substrates because they are flexible and less expensive than alumina and silicon supports and in growing interest for portable device applications.