Ricardo Alvarado

Lara Karam

Redouane Dahmani

Antoine Lepicard

Florian Calzavara

Andréa Piarristeguy

Annie Pradel

Thierry Cardinal

Fréderic Adamietz

Evelyne Fargin

Matthieu Chazot

Kathleen Richardson

Luc Vellutini

Marc Dussauze

Patterning of the surface electrical potential on chalcogenide glasses by a thermo-electrical imprinting process

Keywords: Surface potential, Chalcogenide glasses, poling, glass structure

The development of novel sensing systems requires breakthroughs in the conception of multifunctional materials. In this sense, while extensive research has been dedicated to the individual tuning of the electrical or optical properties of different materials, the combination of both features would result in a promising field of research that would further extend opportunities for engineering novel function in sensor geometries. In the present work, we employed a highly attractive optical material for mid-infrared (MIR) sensing (chalcogenide glasses, ChG) and focused on the spatial control of its surface electrical potential via a thermoelectrical imprinting process. Different glass compositions based on the system Ge-Sb-S-Na were prepared by varying the sulfur stoichiometry and the sodium content. Each glass was thermally poled using electrodes with specific patterns, and subsequent structural modifications and surface electrical potential were then evaluated via Raman spectroscopy and Kelvin Probe Force Microscopy (KPFM). Raman cartographies show structural modifications attributed to alkali depletion following the patterns of the electrodes used for the imprinting process. Furthermore, KPFM measurements show clearly defined motifs on the electrical potential which are associated to charges implanted into the glass matrix. It was shown that the surface potential can vary in sign within an amplitude range of 10V and exhibit patterning at the micrometer scale. We observed that the efficiency of the surface Page 2 of 33

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The Journal of Physical Chemistry potential imprinted was dramatically impacted by the glass' sulfur and sodium content. Our results demonstrate for the first time the use of a one-step process, thermal poling, for large scale patterning of the surface potential of ChG creating a multi-functional material.

Introduction

In the context of lab-on-chip and other nanotechnological systems, control of a material's electrical functionality is a valuable lever that, if tunable, offers design flexibility to enhance device performance. The patterning of the electrical potential on the surface at micrometer or nanometer scale has been proven to be an effective strategy to assemble nanoparticles or nano-objects precisely at a desired location on a substrate through electrophoresis [START_REF] Mesquida | Attaching Silica Nanoparticles from Suspension onto Surface Charge Patterns Generated by a Conductive Atomic Force Microscope Tip[END_REF] . Xerography is a well-recognized technique that uses electrostatic forces to allow selective attachment of toner particles [START_REF] Schaffert | Xerography: A New Principle of Photography and Graphic Reproduction[END_REF] . Since its early development, the technique has evolved considerably such that one can now talk about micro [START_REF] Jacobs | Submicrometer Patterning of Charge in Thin-Film Electrets[END_REF] or even nanoxerography [START_REF] Palleau | Coulomb Force Directed Single and Binary Assembly of Nanoparticles from Aqueous Dispersions by AFM Nanoxerography[END_REF][START_REF] Jacobs | Approaching Nanoxerography: The Use of Electrostatic Forces to Position Nanoparticles with 100 Nm Scale Resolution[END_REF][START_REF] Ressier | How to Control AFM Nanoxerography for the Templated Monolayered Assembly of 2 Nm Colloidal Gold Nanoparticles[END_REF] . The electrical patterning of the substrate, an appropriate charge carrier or electret, can be realized through atomic force microscopy [START_REF] Mesquida | Attaching Silica Nanoparticles from Suspension onto Surface Charge Patterns Generated by a Conductive Atomic Force Microscope Tip[END_REF][START_REF] Palleau | Coulomb Force Directed Single and Binary Assembly of Nanoparticles from Aqueous Dispersions by AFM Nanoxerography[END_REF][START_REF] Ressier | How to Control AFM Nanoxerography for the Templated Monolayered Assembly of 2 Nm Colloidal Gold Nanoparticles[END_REF] , electron beam [START_REF] Fudouzi | Assembling 100 Nm Scale Particles by an Electrostatic Potential Field[END_REF] or focused ion beam [START_REF] Fudouzi | Site-Controlled Deposition of Microsized Particles Using an Electrostatic Assembly[END_REF] with a lateral resolution of about a hundred nanometers. The main limitation of these techniques is that they are time The Journal of Physical Chemistry consuming when large areas (centimeter square) are considered. To circumvent this problem parallel printing processes to pattern charges on the electret over sizes approaching a centimeter square, during a single process cycle have been developed [START_REF] Jacobs | Submicrometer Patterning of Charge in Thin-Film Electrets[END_REF][START_REF] Jacobs | Approaching Nanoxerography: The Use of Electrostatic Forces to Position Nanoparticles with 100 Nm Scale Resolution[END_REF][START_REF] Ma | Selective Discharge of Electrostatic Charges on Electrets Using a Patterned Hydrogel Stamp[END_REF][START_REF] Barry | Fringing Field Directed Assembly of Nanomaterials[END_REF] . This stamp-like process doesn't deteriorate the resolution and even lower resolution (to 50 nm) can be obtained when a localized fringing field [START_REF] Barry | Fringing Field Directed Assembly of Nanomaterials[END_REF] is induced to direct the nanoparticle assembly. Guiding the growth of nano-objects on specific sites is not the only possibility that offers surface potential patterning. Rapid and reversible changes induced by potential to alter the focal length of liquid lenses has been achieved by tuning the electrochemical desorption of self-assembled monolayers (SAMs) onto a metal surface [START_REF] Gorman | Control of the Shape of Liquid Lenses on a Modified Gold Surface Using an Applied Electrical Potential across a[END_REF] . It is also possible to induce dynamic changes in interfacial properties such as wettability by generating conformational transitions of single layered amphiphilic molecules deposited onto a metallic substrate: a change in the sign of the potential leads to a transition between hydrophilic (molecules are straight) and hydrophobic (molecules are bent) nature of the surface [START_REF] Lahann | A Reversibly Switching Surface[END_REF] .

Research efforts towards the development of components merging multiple functionalities are more and more essential. In this context, the combination of electrical and optical functionalities on a single material represents a great benefit. Optical glasses such as chalcogenides (ChG) are particularly interesting for sensing applications, as they are transparent in the MIR, the spectral region in which the organic molecules fingerprint can be found. In this study, we demonstrate for the first time the patterning of the surface electrical potential of Ge-Sb-S-Na glasses via a one-step imprinting process covering large areas at once (centimeter square scale): thermal poling. This process consists in applying a DC field to a glass heated at moderate temperature; the glass is brought back to ambient temperature with the DC field still on. This freezes the electrical constraint at the surface of the glass and induces several other modifications: depletion of cations [START_REF] Dussauze | Large Second-Harmonic Generation of Thermally Poled Sodium Borophosphate Glasses[END_REF] , structural rearrangements [START_REF] Dussauze | Structural Rearrangements and Second-Order Optical Response in the Space Charge Layer of Thermally Poled Sodium-Niobium Borophosphate Glasses[END_REF][START_REF] Dussauze | How Does Thermal Poling Affect the Structure of Soda-Lime Glass?[END_REF][START_REF] Redkov | How Does Thermal Poling Produce Interstitial Molecular Oxygen in Silicate Glasses?[END_REF] , modification of the surface reactivity [START_REF] Lepicard | Surface Reactivity Control of a Borosilicate Glass Using Thermal Poling[END_REF][START_REF] Lepicard | Micro-Structuring the Surface Reactivity of a Borosilicate Glass via Thermal Poling[END_REF][START_REF] Kamenskii | Effective Diffraction Gratings via Acidic Etching of Thermally Poled Glass[END_REF] , wettability change [START_REF] Lind | Modifying the Surface Wetting Behavior of Soda-Lime Silicate Glass Substrates through Thermal Poling[END_REF] , surface potential [START_REF] Yudistira | Electric Surface Potential and Frozen-in Field Direct Measurements in Thermally Poled Silica[END_REF] , changes of the optical properties (modification of the index [START_REF] Dussauze | Refractive Index Distribution in the Non-Linear Optical Layer of Thermally Poled Oxide Glasses[END_REF] and induced second order nonlinear response [START_REF] Myers | Large Second-Order Nonlinearity in Poled Fused Silica[END_REF][START_REF] Quiquempois | Model of Charge Migration during Thermal Poling in Silica Glasses: Evidence of a Voltage Threshold for the Onset of a Second-Order Nonlinearity[END_REF][START_REF] Dussauze | Thermal Poling of Optical Glasses: Mechanisms and Second-Order Optical Properties[END_REF][START_REF] Dussauze | Large Second Order Optical Nonlinearity in Thermally Poled Amorphous Niobium Borophosphate Films[END_REF][START_REF] Karam | Electrically Micro-Polarized Amorphous Sodo-Niobate Film Competing with Crystalline Lithium Niobate Second-Order Optical Response[END_REF] ) as well as the formation and patterning of glass-metal nanocomposites [START_REF] Deparis | Poling-Assisted Bleaching of Metal-Doped Nanocomposite Glass[END_REF][START_REF] Beresna | Poling-Assisted Fabrication of Plasmonic Nanocomposite Devices in Glass[END_REF][START_REF] Brunkov | Submicron-Resolved Relief Formation in Poled Glasses and Glass-Metal Nanocomposites[END_REF][START_REF] Chervinskii | Formation and 2D-Patterning of Silver Nanoisland Film Using Thermal Poling and out-Diffusion from Glass[END_REF] . The patterning of these changes has been realized using structured electrodes that act like stamps to control the glass' surface properties leading to the design of functional elements such as linear [START_REF] Kamenskii | Effective Diffraction Gratings via Acidic Etching of Thermally Poled Glass[END_REF] and nonlinear diffraction gratings [START_REF] Lepicard | Demonstration of Dimensional Control and Stabilization of Second Harmonic Electro-Optical Response in Chalcogenide Glasses[END_REF] or arrays of micro-lenses [START_REF] Lepicard | Long-Lived Monolithic Micro-Optics for Multispectral GRIN Applications[END_REF] . In this study, we present evidence of the viability in patterning of surface electrical potentials in glasses at the micrometer scale.

Through parallel studies illustrating the influence of the glass' sodium and sulfur content on these surface properties combined with Raman microscopy, we proposed a mechanistic explanation as to the origin of the surface potential in order to generate effective and stable patterns for future applications.

Experimental Methods

Synthesis of ChG glasses: ChG glasses were prepared using high purity elemental Ge, Sb and S (Alfa Aesar, 99.999%). Anhydrous sodium sulfide (purity unspecified) was used to incorporate sodium as doping agent. Based on the stoichiometric composition Ge 25 Sb 10 S 65 , different samples were prepared by varying either the S to Ge ratio (i.e. increasing the sulfur content) or by keeping the ratio between the other components constant and adding sodium. The nominal compositions of the different glasses are reported in table 1 as well as the name by which they will be referred to in the rest of the present work. Two series of glasses were synthetized: (i) A, B, C, D with constant sodium content to evidence the influence of sulfur and (ii) Na 0 , Na 2 , Na 5.5 with constant S to Ge ratio to discuss the influence of the sodium content. Raw materials were weighed out in a glove box under a nitrogen atmosphere before being inserted in a quartz ampule that was, then, put under vacuum (10 -2 mbar) and sealed using an oxygen-methane torch. A rocking furnace was employed to ensure the homogeneity of the mixture. The temperature was increased at a rate of 1 °C min -1 up to 850 °C and left at this value for 12 h for homogenization. After this period of time, the rocking of the furnace was stopped and the temperature was set to gradually decrease to 750 °C (quenching temperature). The sample was then quenched in water and annealed for 6 h, Vibrational spectroscopy: Raman spectra were acquired using an XploRA PLUS (Horiba) spectrometer with a laser source at 785 nm focused on the sample via a 50x objective lens.

Cartographies were acquired using a 100x (N.A. 0.9) objective lens on an area of 80 x 60 µm with a lateral resolution of ~2 µm.

Thermoelectrical imprinting: Electrical patterning was performed under a nitrogen atmosphere by pressing the ChG glass sample between two electrodes: a structured electrode used as a stamp located at the anode, and a silicon electrode at the cathode. A coverslip was used as a sacrificial layer in between the glass sample and the cathode. The structured electrode consists of a 100 nm thick Ti-Pt metallic films deposited on a silicate glass slide. The patterned electrode was prepared by a classical lithography process resulting in squared motifs (40x40 or 30x30 µm) leaving a 4 µm width electrically conductive Pt grid. The system was then heated to 210 °C, after reaching thermal stabilization, an electric field was applied at 375 V.min -1 up to 0.9 kV. The temperature and DC Page 7 of 33

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The Journal of Physical Chemistry bias were kept constant for 30 min, then, the system was cooled down to room temperature before removal of the electric field.

Characterization of surface electrical potential:

Measurements were performed at 100 nm from the surface by an AFM Dimensions Icon (Bruker) on PeakForce KPFM mode using a PFQNE-AL cantilever, over a 55 x 55 µm area and a resolution of 1024 points per line.

Results

Structural analysis of pristine glasses

Raman spectra of each pristine sample were acquired and normalized over their total integrated area (Figure 1). Different models to describe GeS 2 glass networks have been proposed in the literature [START_REF] Griffiths | Assignment of the Companion A1c Line in GeX(S, Se) 1-x Glasses[END_REF][START_REF] Bridenbaugh | Microscopic Origin of the Companion A 1 Raman Line in Glassy Ge (S, Se) 2[END_REF][START_REF] Kawamoto | Infrared and Raman Spectroscopic Studies on Short-Range Structure of Vitreous GeS 2[END_REF][START_REF] Jackson | Raman-Active Modes of a -GeSe 2 and a -GeS 2 : A First-Principles Study[END_REF] resulting in different assignations for the characteristic Raman bands associated with this network. [START_REF] Bérubé | Tailoring the Refractive Index of Ge-S Based Glass for 3D Embedded Waveguides Operating in the Mid-IR Region[END_REF][START_REF] Chazot | Multimodal Structural Characterization of Ge-S-I Glasses by Combination of DFT Calculation and IR and Polarized Raman Spectroscopy[END_REF][START_REF] Masselin | Spatially Resolved Raman Analysis of Laser Induced Refractive Index Variation in Chalcogenide Glass[END_REF] A recent article regarding multimodal structural characterization of Ge-S-I 39 as well as previously reported DFT calculations [START_REF] Masselin | Spatially Resolved Raman Analysis of Laser Induced Refractive Index Variation in Chalcogenide Glass[END_REF] were used for the assignations in this work. The spectra presented in Figure 1a show different contributions associated with the stretching modes of Ge-S bonds in corner sharing (CS) GeS 4 tetrahedra [START_REF] Chazot | Multimodal Structural Characterization of Ge-S-I Glasses by Combination of DFT Calculation and IR and Polarized Raman Spectroscopy[END_REF][START_REF] Masselin | Spatially Resolved Raman Analysis of Laser Induced Refractive Index Variation in Chalcogenide Glass[END_REF] around 340 cm -1 and 415 cm -1 as well as Sb-S stretching modes in SbS 3 pyramids [START_REF] Koudelka | Raman Spectra of Ge-Sb-S System Glasses in the S-Rich Region[END_REF] resulting in a small shoulder around 300 cm -1 . The most striking variation when the amount of sulfur increases (i.e. going from glass A to D) is the apparition of new bands at 152 cm -1 , 219 cm -1 and 474 cm -1 , they correspond to S-S bonds. [START_REF] Chazot | Multimodal Structural Characterization of Ge-S-I Glasses by Combination of DFT Calculation and IR and Polarized Raman Spectroscopy[END_REF][START_REF] Susarla | High-K Dielectric Sulfur-Selenium Alloys[END_REF][START_REF] Ward | Spectroscopy of Sulfur, Sulfur-Selenium, and Sulfur-Arsenic Mixtures[END_REF] The first two bands, present only when the sulfur content exceeds 70 at.%, are linked to the formation of sulfur rings (S 8 ) inside the glass matrix. [START_REF] Chazot | Multimodal Structural Characterization of Ge-S-I Glasses by Combination of DFT Calculation and IR and Polarized Raman Spectroscopy[END_REF][START_REF] Susarla | High-K Dielectric Sulfur-Selenium Alloys[END_REF] Another evolution when increasing the sulfur content, is the regular decrease of the contribution of the Ge related entities (340 and 415 cm - 1 ). This is consistent with the decrease of the Ge content in the composition when going from composition A to D. The Raman intensity of the modes related to Sb are not affected which is consistent with the fact the Sb content stays constant. More interestingly, the modes related to Page 8 of 33

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The Journal of Physical Chemistry either the Ge or Sb entities are observed at the same wavenumber, no frequency shift can be observed when the sulfur content increases. These observations suggest that the surplus of sulfur does not alter the glass forming units but rather the interconnections among them by establishing new homopolar S-S linkages. This results in the creation of S-S chains up to a given sulfur content (close to 70 at.%), passed this value, the increase of the sulfur content results in its organization into rings not linked to the other glass entities.

Let us focus on the effect of the sodium addition to the glass network (with fixed S to Ge ratio) on the basis of the Raman spectra presented in Figure 1b. Looking at the Raman response of samples Na 0 to Na 5.5 , it appears that only slight changes can be observed when the sodium content varies from 0 to 5.5 at.%. In order to highlight these spectral variations, the difference Raman spectrum is presented in Figure 1c. It corresponds to the Raman spectrum of a sodium rich glass (Na 5.5 ) subtracted to the one of the glass without sodium (Na 0 ). Multiple contributions from GeS 4 tetrahedra can be observed (333, 375, 408 and 445 cm -1 ) [START_REF] Masselin | Spatially Resolved Raman Analysis of Laser Induced Refractive Index Variation in Chalcogenide Glass[END_REF] . The peaks at ~333 cm -1 and ~408 cm - 1 can be ascribed to CS conformation [START_REF] Masselin | Spatially Resolved Raman Analysis of Laser Induced Refractive Index Variation in Chalcogenide Glass[END_REF] while the valleys at ~375 cm -1 and ~445 cm -1 are representative to an Edge Sharing (ES) arrangement [START_REF] Masselin | Spatially Resolved Raman Analysis of Laser Induced Refractive Index Variation in Chalcogenide Glass[END_REF] . The bands corresponding to the CS tetraedra appear positive in the Raman difference spectra (Na 0 minus Na 5.5 ) while those related to the ES are negative. This evolution denotes a modification of the interconnections of the germanate network with the addition of sodium as part of the CS tetrahedral units are substituted for the ES tetrahedra conformation. A peak at 304 cm -1 and a valley at around 264 cm -1 are present in the difference spectra and can be attributed to the contribution of antimony glass forming units. The addition of sodium in the vicinity of a SbS 3 pyramidal units can affect the strength of the Sb-S bonds. Therefore, the lower frequency of the band at 264 cm -1 for the sodium rich glasses (Na 5.5 ) as compared to 304 cm -1 without sodium (Na 0 ) could be linked to weaker Sb-S bonds as part of the unit electronegativity is now distributed within the sodium coordination shell.

Spatial distribution of sodium after thermoelectrical imprinting

The thermally poled glasses were studied by Raman spectroscopy to evidence the structural variations induced by the process. Based on previous studies, [START_REF] Lepicard | Demonstration of Dimensional Control and Stabilization of Second Harmonic Electro-Optical Response in Chalcogenide Glasses[END_REF][START_REF] Lepicard | Long-Lived Monolithic Micro-Optics for Multispectral GRIN Applications[END_REF] a spatial control of the sodium concentration through the use of a structured electrode is expected on the post-poling surface. In ). Such similarity suggests in the IC zone the structure approaches the structure of a glass without sodium. This confirms the sodium depletion under the conductive zones of the electrode previously reported. [START_REF] Dussauze | Large Second Order Optical Nonlinearity in Thermally Poled Amorphous Niobium Borophosphate Films[END_REF] A similar methodology has been applied to the glasses presenting different S/Ge ratio (with same initial sodium content). The resulting spectra and Raman difference spectra are presented Figure 3. The ones of glass A have been reproduced for comparison with two different sulfur contents (glass samples B and D). All the Raman difference spectra show the same evolution. For all S to Ge ratio, thermal poling has the same effect: the post-poling glass in the IC zone shows a structure similar to a pristine glass without sodium. This shows the flexibility of the glass matrix which is rearranged through poling whatever the initial composition. Figure 2c are presented the Raman cartographies associated with selected bands of the Raman difference spectrum showing the spatial distribution of the structural rearrangements. The same pattern is observed for the distribution of all bands demonstrating the spatial coherence of the spectral variations. Furthermore, the pattern of the electrode can be recognized in these cartographies confirming that, through the use of a structured electrode, thermal poling can be successfully employed as an imprinting process. One should also notice that the imprinted patterns do not correspond exactly to the electrode design. This is due to a gradient in sodium concentration from the edge of the electrode toward the center of the nonconductive square. Such effect was accurately described in a study linked to the formation of gradient of refractive indices (GRIN) in similar glasses. [START_REF] Dussauze | Large Second Order Optical Nonlinearity in Thermally Poled Amorphous Niobium Borophosphate Films[END_REF] Page 11 of 33

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Patterning of the surface electrical potential

The post-poling topography as well as the resulting surface potential magnitude and spatial distribution for all the studied glasses are presented Figure 4 Regarding the influence of the sodium content the first comment we have to make is that when no sodium is present in the initial composition (Na 0 ), the post-poling surface does not present any Page 14 of 33

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The Journal of Physical Chemistry topographic variation and no potential structuring is observed. For this reason, the measurements concerning this composition are not presented here. In Figure 5, can be found the post-poling topography and surface potential of glasses with higher sodium contents (Na 2 and Na 5.5 ). It appears that, for the imprinting process to be successful, the presence of alkali in the initial composition is necessary. Increasing the initial sodium content results in variation of the topography, the more the initial sodium content increases the stronger the topography changes are. The surface potential structuring can be observed for Na 2 but with higher sodium contents (Na 5.5 ), it tends to disappear.

Discussion

The main objective of this study is to engineer a spatially varying and stable surface electrical potential on a glass optical materials that could open new opportunities in the development of multifunctional approach in inorganic glass science. Charge implantation has been successfully realized in chalcogenide materials since the 70's with the development of xerography processes notably on amorphous selenium. [START_REF] Pfister | Electronic Properties of Chalcogenide Glasses and Their Use in Xerography[END_REF][START_REF] Mikla | Xerographic Spectroscopy of Gap States in Se-Rich Amorphous Semiconductors Review[END_REF] These works have demonstrated the potential of chalcogenide materials to sustain electrical or plasma processes for the fabrication of charged surfaces.

Nevertheless, in our case, the multifunctional properties targeted require novel objectives in terms of long term stability and micrometric spatial control. As compared to a xerography approach, the thermo-electrical imprinting process is considerably different because of the nature of the charge carriers as it involves necessarily alkali cations. The Raman mapping reported has evidenced the ability of this thermo-electrical process to control the sodium spatial distribution within the chalcogenide glassy matrix at the micrometer scale for all the glass compositions studied. This structural study also points out the similarity observed between spectral variations from glasses prepared with and without sodium addition (Figure 1) and the ones measured when the sodium is depleted from the glass matrix under the electrical constrain (Figure 2). It shows the flexibility of the glass network and its ability to rearrange under a high electric field at temperatures below its glass transition. Regarding the objective of surface electrical potential patterning, the presented results have also demonstrated that by using this imprinting poling process, stable surface charges can only be obtained with sodium doped glasses which denotes the importance of cationic charge carriers to modify the surface electrical properties for these germanate chalcogenide glasses.

To analyze the surface potential patterned for this series of glasses, one should consider the respective effects of: (i) the location of the trapped charges which have to be promoted close to the surface in order to influence the surface potential and (ii) the energy level of the traps which will determine the stability of the induced surface potential.

Concerning charged surface stabilities, let's remind that research studies in the domains of polarized or charged materials have demonstrated the importance of a material disorder state to enable a slow discharge [START_REF] Pfister | Dispersive (Non-Gaussian) Transient Transport in Disordered Solids[END_REF][START_REF] Yuan | SiO 2 Film Electret with High Surface Potential Stability[END_REF][START_REF] Andriesh | Dark Decay of Surface Potential in Vitreous As 2 S 3[END_REF][START_REF] Schnörer | Crossover from Dispersive to Nondispersive Transport in a Trap-Controlled Hopping Model[END_REF][START_REF] Godzik | Theory of dispersive transport in amorphous semiconductors[END_REF] . Depending on the trap energy distribution, charges can diffuse to reach a "sink" to be neutralized as described in a classical trapping model developed to describe electronic relaxation in amorphous semiconductors [START_REF] Pfister | Dispersive (Non-Gaussian) Transient Transport in Disordered Solids[END_REF][START_REF] Phillips | Stretched Exponential Relaxation in Molecular and Electronic Glasses[END_REF] . In addition, in reference to works done on Page 16 of 33

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The Journal of Physical Chemistry the photosensitivity or on thermal poling of chalcogenide glasses, it was shown that structural rearrangements induced by the optical or electrical polarization treatments are very favorable for the stability of charges as the modified structure considerably increases the energy barrier of the traps [START_REF] Shimakawa | Reversible Photoinduced Change of Photoconductivity in Amorphous Chalcogenide Films[END_REF][START_REF] Shoulders | Thermal Poling Behavior and SHG Stability in Arsenic-Germanium Sulfide Glasses[END_REF] .

To summarize the results of this study: (i) we have observed that the surface potential patterning is efficient only for the composition stoichiometric in sulfur, and (ii) if the sodium content increases, the electrical potential values decrease together with an enhancement of the surface topology variations. Both findings denote the importance of the glass local structure to achieve the targeted functional properties.

Concerning the effect of the sulfur content, it is not possible to conclude if the absence of surface potential observed for the rich-sulfur glasses can be linked to an implantation of trapped charges preferentially localized in the bulk or if it can be linked to a very poor stability of charges at the surface. Nevertheless, the structural study has confirmed a significant amount of S-S homopolar bonds and the presence of sulfur rings in the glassy structure, it induces a real change in the local homogeneity and polarizability of the structure which necessarily affect the distribution of trap energies and as a consequence the local transport/diffusion of charges. Under this assumption, one may expect an easier transport of the charge carriers in the poled matrix with the increase of homopolar bonds which could locally facilitate an electronic transport in the amorphous structure.

In any case, it denotes the key role of the glass local molecular structure to promote stable surface charges.

Focusing our analysis on the influence of the sodium content in the glass matrix, one should point out that when the amount of alkali increase the imprinting process induces non negligible surface topology variations (steps of 50 nm for 5.5 mol% of Na). This surface relief modification is expected from previous studies on poled ionic glasses [START_REF] Redkov | Nanoprofiling of Alkali-Silicate Glasses by Thermal Poling[END_REF] . It denotes the presence of matter displacement, volume contraction and glass density variations induced during the alkali-depleted layer formation. In such a case, we can note a clear decrease of the patterned surface potential amplitude when the amount of charge carriers involved increase imposing large modification of the physico-chemical properties of the glassy material surface.

Furthermore, one should evoke the spatial control of charge implantation managed by this imprinting process. In regards to a future electrical/optical multifunctional approach, a spatial control of the surface potential at the micrometer scale is essential in order to suite with the sizes of photonic waveguides for example. This study has demonstrated the potential of this polarization process to achieve such objectives. To explain the surface potential patterns obtained, one should focus on the electrode configuration. : a juxtaposition of conductive and non-conductive zones (as used in this study with a metallic thin film patterned by a lithography method). Such a juxtaposition of conductive and non-conductive area allows to promote in plane field components which can promote and control surface current direction. Such effect was recently reported for the electrooptical second harmonic generation response obtained for a similar glass system [START_REF] Lepicard | Demonstration of Dimensional Control and Stabilization of Second Harmonic Electro-Optical Response in Chalcogenide Glasses[END_REF] . These surface currents controlled by the electrode geometry might be at the origin of the sign control of the surface potential which allow accurate patterning at the micrometric scale, yet further studies will be required to explain this process in detail.

Finally, the question of the stability of the induced patterned surface potential with time is crucial for further integration in multifunctional systems. This temporal aspect wasn't the main objective of this work, however to justify the real interest of these polarized surfaces, is presented in figure 6 the surface potential measured on a sample right after the thermoelectrical treatment and 24 months later. The electrode used is constituted of a circular pattern. One can observe that, if the

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Figure 1 .

 1 Figure 1. Raman spectra of the pristine ChG samples: (a) A, B, C and D glasses series with

Figure 2a ,

 2a Figure 2a, are presented the Raman spectra from the post-poling surface of the A composition.

Figure 2 .Figure 3 .

 23 Figure 2. (a) Raman spectra from the thermally poled A glass, the black line corresponds to the

and Figure 5 .

 5 The Figure4is dedicated to the influence of the sulfur content with the series of glasses A, B, C and D (1 at. % of sodium) while in Figure5the emphasis is put on the sodium content with the Na 0 , Na 2 and Na 5.5 glasses.Let us focus first on theFigure 4 with the A, B, C and D glasses. Topography variations of a maximum of 50 nm amplitude can be observed. The main changes are located at the edges of the electrode; the rest of the surface is rather flat for all samples with even smaller changes as the sulfur content increases. Looking at the surface potential of the poled A glass sample, one can clearly recognize the pattern of the electrode resulting in negatively charged regions when IC with the electrode and positively charged regions when NC. The region IC, as the Raman study has shown, corresponds to a sodium-depleted region. The amplitude variation of surface potential is 9 V for this sample. Glass B only shows an amplitude variation of 1 V and ~0.2-0.4 V for C and D. Based on this observation the bigger the ratio S to Ge is (i.e. the more sulfur in the matrix), the lower the magnitude of the potential is: it is maximal for glass A (stoichiometric composition) and diminishes in B until the pattern can barely be observed for glasses C and D.

Figure 4 .

 4 Figure 4. AFM measured topography (top) and KPFM measured surface potential (bottom)

Figure 5 .

 5 Figure 5. AFM measured topography (top) and KPFM measured surface potential (bottom)

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 33 Paragon Plus EnvironmentThe Journal of Physical Chemistry amplitude variation of surface potential has significantly decreased (from 3 V to 1.6 V), the patterning of the surface potential is retained after two years. These preliminary results on the stability of the poled chalcogenide surface properties are encouraging. Some additional characterizations are actually in progress.

Figure 6 .Figure 1 .

 61 Figure 6. KPFM imaging of a ChG sample after thermal poling at after being poled (a) and after

Figure 2 .

 2 Figure 2. (a) Raman spectra from the thermally poled A glass, the black line corresponds to the region non in contact (NC) with the conductive zone and the red line in contact (IC). (b) Raman difference spectrum where the spectrum of the NC region has been subtracted to the IC region. (c) Spatial evolution of the Raman intensity of the different bands revealed by the Raman difference spectrum and the schematic representation of the glass units they are associated with.

Figure 3 .

 3 Figure 3. (Top) Raman spectra from the different thermally poled glasses A, B and D presenting different S/Ge ratio and same sodium content. The black lines correspond to the region not in contact (NC) with the conductive zone and the red line in contact (IC). (Bottom) Raman different spectra for each glass where the spectrum of the NC region has been subtracted from the spectrum of the IC region.

Figure 4 .

 4 Figure 4. AFM measured topography (top) and KPFM measured surface potential (bottom) images of the post-poling surface of glasses A, B, C and D i.e. increasing sulfur content with fixed sodium content. 178x84mm (150 x 150 DPI)

Figure 5 .

 5 Figure 5. AFM measured topography (top) and KPFM measured surface potential (bottom) images of the post-poling surface of glasses Na2 and Na5.5 i.e. increasing sodium content with other component's ratios constant. The results obtained for Na0 (no sodium) are not presented as no effect -topology or potentialwere observed in the absence of alkali cation in the initial composition.

Table 1 :

 1 Atomic composition of the different glasses of interest and the name they are referred to.

	Page 6 of 33

[START_REF] Lahann | A Reversibly Switching Surface[END_REF],

Acknowledgment

The authors gratefully acknowledge the financial support of: IdEx Bordeaux (Cluster of Excellence LAPHIA and the allocated grant referred to as ANR-10-IDEX-03-03) and the CNRS project EMERGENCE @INC2019. This project has received funding from the European Union's Horizon 202 research program under the Marie Skłodowska-Curie grant agreement No 823941 (FUNGLASS).

AUTHOR INFORMATION

Corresponding Author

* E-mail: marc.dussauze@u-bordeaux.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.


Céline Liaud

Maurice Millet

Stéphane Le Calvé
email: slecalve@unistra.fr

An analytical method coupling Accelerated solvent Extraction and HPLC-fluorescence for the quantification of Particle-bound PAHs in Indoor Air sampled with a 3-stages Cascade Impactor

Keywords: PAHs, Cascade Impactor, Particles, Indoor Air. Highlights

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Polycyclic Aromatic Hydrocarbons (PAHs) are widespread environmental contaminants mostly formed during incomplete combustion or pyrolysis of organic material but they can also be of biogenic origin [START_REF] Orecchio | How building an underwater pipeline connecting Libya to Sicilian coast is affecting environment: polycyclic aromatic hydrocarbons (PAHs) in sediments; monitoring the evolution of the shore approach area of the Gulf of Gela (Italy)[END_REF]. Their chemical structures contain 2 or more fused benzene rings arranged in linear, angular or in cluster structures. PAHs are ubiquitous in our environment and their emission sources are numerous: industrial processes, vehicle exhausts and domestic heating are among the major sources [START_REF] Fromme | Polycyclic aromatic hydrocarbons inside and outside of apartments in an urban area[END_REF]. More than 100 of them are nowadays referenced but the American Environmental Protection Agency (US-EPA) has listed 16 among the most hazardous. One of the most famous is the Benzo[a]Pyrene (B[a]P) due to its carcinogenic properties. Besides, the European ambient air legislation (directive n°2004/107/CE) targets this PAH and recommends an annual maximum guideline value of 1 ng m -3 . Because the PAHs structures are thermally stable, most of these compounds possess high boiling point and low vapor pressure favoring their association to particulate phase in the atmosphere. The lighter PAHs (MW ≤ 202 g mol -1 ) are preferentially in the gas phase whereas the heavier ones (MW ≥ 202 g mol -1 ) are associated to particles [START_REF] Kliucininkas | Indoor and outdoor concentrations of fine particles, particle-bound PAHs and volatile organic compounds in Kaunas, Lithuania[END_REF][START_REF] Krugly | Characterization of particulate and vapor phase polycyclic aromatic hydrocarbons in indoor and outdoor air of primary schools[END_REF][START_REF] Zhu | Pollution level, phase distribution and source analysis of polycyclic aromatic hydrocarbons in residential air in Hangzhou, China[END_REF].

Since their atmospheric concentration is relatively low (in the range of pg m -3 -ng m -3 ), and especially in indoor air [START_REF] Delgado-Saborit | Carcinogenic potential, levels and sources of polycyclic aromatic hydrocarbon mixtures in indoor and outdoor environments and their implications for air quality standards[END_REF][START_REF] Fischer | Trafficrelated differences in outdoor and indoor concentrations of particles and volatile organic compounds in Amsterdam[END_REF], the collection of airborne PAHs in vapor and particulate phases requires quite large volume of air to concentrate pollutants either on a sorbent material or on a filter. [START_REF] Pandey | A review of techniques for the determination of polycyclic aromatic hydrocarbons in air[END_REF] reviewed sampling flow rate, for high volume samplers, in the range 2-75 m 3 h -1 with sampling duration long enough to ensure a sufficient sampled volume of 150-900 m 3 [START_REF] Pandey | A review of techniques for the determination of polycyclic aromatic hydrocarbons in air[END_REF]. The most common sampling device is a high or low volume sampler composed of an adsorbent (XAD-2 or polyurethane foam) and a pre-filter for trapping the PAHs in the particulate phase by filtration of large air volume [START_REF] Li | Polycyclic aromatic hydrocarbons in residential air of ten Chicago area homes: Concentrations and influencing factors[END_REF][START_REF] Masih | Characteristics of polycyclic aromatic hydrocarbons in indoor and outdoor atmosphere in the North central part of India[END_REF][START_REF] Ohura | Polycyclic Aromatic Hydrocarbons in Indoor and Outdoor Environments and Factors Affecting Their Concentrations[END_REF]. However, in the case mentioned above, the particles size is not limited. In fact, particles cause adverse respiratory health effects related to their ability to enter the lungs and potentially carrying numerous toxic compounds with them. Particle size, especially fine and ultrafine particles, is strongly related to the bad effects on human health. That's why it is of interest to focus on particle aerodynamic diameter and particle number concentrations [START_REF] Buonanno | Health effects of daily airborne particle dose in children: Direct association between personal dose and respiratory health effects[END_REF][START_REF] Zanobetti | Fine particulate air pollution and its components in association with cause-specific emergency admissions[END_REF]. The use of Particulate Matter (PM) sampling heads upstream the filter gives access to several cut off sizes and allow to sample particles with aerodynamic diameter (Dae) usually below 10 µm, 2.5 µm or 1 µm [START_REF] Li | Polycyclic aromatic hydrocarbons in residential air of ten Chicago area homes: Concentrations and influencing factors[END_REF][START_REF] Castro | Polycyclic aromatic hydrocarbons in gas and particulate phases of indoor environments influenced by tobacco smoke: Levels, phase distributions, and health risks[END_REF][START_REF] Fromme | Polycyclic aromatic hydrocarbons (PAH) and diesel engine emission (elemental carbon) inside a car and a subway train[END_REF][START_REF] Jung | Effects of heating season on residential indoor and outdoor polycyclic aromatic hydrocarbons, black carbon, and particulate matter in an urban birth cohort[END_REF].

Other sampling systems are inertial samplers and consist to remove particles from an air stream by impaction of the particles depending on their inertia [START_REF] Zhu | Pollution level, phase distribution and source analysis of polycyclic aromatic hydrocarbons in residential air in Hangzhou, China[END_REF][START_REF] Lin | Size and composition effects of household particles on inflammation and endothelial dysfunction of human coronary artery endothelial cells[END_REF]. Particles samples collected from cascade impactors give access to more data because this device separates particles depending on their Dae.

Thus, physical and chemical data related to the particle sizes can be simultaneously determined to assess health effects. If further details are desired, chemical analyses may be achieved on the filters obtained in each size range. Nevertheless, despite the power of cascade impactors, to our knowledge only two a few studies were conducted in indoor air with this metrology tool to quantify particlebound PAHs [START_REF] Zhu | Pollution level, phase distribution and source analysis of polycyclic aromatic hydrocarbons in residential air in Hangzhou, China[END_REF][START_REF] Lin | Size and composition effects of household particles on inflammation and endothelial dysfunction of human coronary artery endothelial cells[END_REF]. These latter were performed using a 11-stages or 3-stages cascade impactors with Dae in the range 0.05-18 µm and 2.5-10 µm, respectively, which did not permit to easily compare the results on particles to those reported in the literature mentioning PM1, PM2.5 and PM10.

A third way to monitor particle-bound PAHs are Photoelectric Aerosol Sensors (PAS) which are analyzers using ionization principle to detect PAHs in air [START_REF] Junker | Particulate size distributions and pPAH -concentrations of ETS in a cafeteria before and after smoking ban[END_REF][START_REF] Junker | An assessment of indoor air contaminants in buildings with recreational activity[END_REF][START_REF] Wallace | Real-Time Monitoring of Particles, PAH, and CO in an Occupied Townhouse[END_REF]. However, PAS provides a measure of the total particle-bound PAHs concentration with no distinction between individual compounds. homes in Amsterdam, The Netherlands, were in the range 0.49 ng m -3 (0.15 ng m -3 -1.12 ng m -3 ) and 0.17 ng m -3 (0.03 ng m -3 -0.39 ng m -3 ) for high-traffic and low-traffic density, respectively [START_REF] Fischer | Trafficrelated differences in outdoor and indoor concentrations of particles and volatile organic compounds in Amsterdam[END_REF]

Several studies performed in

Materials and methods

Chemicals and materials

The overall analytical procedure, including calibration curves, was performed by using a

Cascade impactor theory

The impaction theory is based on the separation of particles from the air stream by the inertial forces. A cascade impactor is the result of the assembly of several impactors arranged in series in order to collect particles of decreasing size (Figure 1a). They generally operates to remove particles of a given size range from the sample flow, either collecting them for later analysis or removing them to avoid problems of instrument contamination [START_REF] Zhang | Indoor Air Quality Engineering[END_REF]. Different impactor designs exist such as round or rectangular impactors [START_REF] Marple | Fluid mechanics of the laminar flow aerosol impactor[END_REF][START_REF] Willeke | The influence of flow entry and collecting surface on the impaction efficiency of inertial impactors[END_REF] and this part summarizes theoretical considerations for the round impactor.

Each impactor stage is constituted of an orifice or nozzle which leads a high velocity air jet (U0), containing particles, towards a collection plate (Figure 1b) which causes the airstream to change direction abruptly following a sharp bend [START_REF] Marple | Fluid mechanics of the laminar flow aerosol impactor[END_REF][START_REF] Marple | Impactor design[END_REF]. Figure 1b shows the air streamlines that are deflected to a sharp bend because of the collection plate. Particles which are small enough follow the streamlines and remain suspended, while those that have too much inertia impact on the plate.

The operating principle of an impactor is derived from the dimensionless Stokes number (Stk) defined as the ratio between the stopping distance of a particle and the characteristic dimension of the nozzle. Thus, considering an air flow perpendicular to a cylinder with a radius R (case of a circular jet impactor), the Stokes number is:

𝑆𝑡𝑘 = 𝜏.𝑈 0 𝑅 ( 1
)
where U0 is the air velocity (cm s -1 ), R is the nozzle characteristic dimension, i.e. the radius (cm) and τ is the aerosol relaxation time (s) which is time until the aerosol direction is changing because of the Brownian motion effect.

However, relaxation time τ is the product between the particle mass m and its mobility B: 𝜏 = 𝑚. 𝐵 = (𝜌.

where dp is the particles' diameter, Cc is the slip correction factor which correct differences between coarse and little particles' behavior, µ is the viscosity (g cm -1 s -1 ) and ρ is the particles' density (g cm - 1 ).

The slip correction factor, Cc can be calculated from the mean free path of air λ, and the particle diameter [START_REF] Marjamäki | Performance evaluation of the electrical low-pressure impactor (ELPI)[END_REF]:

𝐶 𝑐 (𝑑 𝑝 ) = 1 + ( 𝜆 𝑑 𝑝 ) × (2,514 + 0,800𝑒 -0,55 𝑑 𝑝 𝜆 ) (3)
Therefore, the Stokes number, for a nozzle diameter W=R/2 is derived from ( 1) and ( 2

The Stokes number allows the characterization of the collection efficiency defined as the percentage of particles of a given size that are removed from the air flow by impaction. For a fixed impactor's geometry and flow condition, the Stokes number only depends on the particle properties and equation (4) shows that particle diameter is directly proportional to the square root of Stk. √Stk 50 is often used to determine the cut size of each impactor stage which represents the particle size with a collection efficiency of 50 %. For most of the circular jet impactors, the number Stk50, which corresponds to a collection efficiency of 50 %, takes values in the range 0.22-0.25 [START_REF] Boulaud | Évaluation du facteur de mise en suspension de contamination dans les installations nucléaires[END_REF][START_REF] De Boer | Characterization of inhalation aerosols: a critical evaluation of cascade impactor analysis and laser diffraction technique[END_REF]. Therefore, to collect particles with aerodynamic diameter even smaller it is necessary to increase U0 or decrease W, i.e. R. This is achieved by varying number and/or opening diameters from one stage to the other (see figure 1a).

As shown in figure 1c, the collection efficiency can be described with efficiency curves. And the collection efficiency increases for particles larger than the cut off size and decreases for smaller particles. In addition, the nondimensional Reynolds number (Re) which describe how turbulent or laminar is the flow has an impact on the collection efficiency. [START_REF] Marple | On fluid flow and aerosol impaction in inertial impactors[END_REF] show that high Reynolds numbers (until 3000) favor the development of sharp cut size impactor whereas for low values (below 300) the sharpness of cut size is less effective [START_REF] Willeke | The influence of flow entry and collecting surface on the impaction efficiency of inertial impactors[END_REF][START_REF] Marple | On fluid flow and aerosol impaction in inertial impactors[END_REF].

The Reynolds number for an impactor with round nozzles is defined as:

𝑅𝑒 = 𝜌.𝑈 0 .𝑊 𝜇 (5)
Even if the most important advantage of impaction technique is that aerosol parameters, size and composition, can be simultaneously established, one of the main problems stays the particle bounce effect which can occur when particles larger than the cut off diameter impact on the collection plate and then bounce and re-enter in the airstream [START_REF] Willeke | The influence of flow entry and collecting surface on the impaction efficiency of inertial impactors[END_REF]. This problem in cascade impactors is an interference because particles which have bounced can be collected on later stages and potentially contaminating the analysis. Some techniques used to reduce this effect include the use of sticky substrates [START_REF] Robache | Multi-element analysis by inductively coupled plasma optical emission spectrometry of airborne particulate matter collected witha lowpressure cascade impactor[END_REF] such as oil or porous substrates or by changing the sampling method by using a virtual impactor [START_REF] Zhang | Indoor Air Quality Engineering[END_REF].

In our study whose results are detailed elsewhere [START_REF] Liaud | Application of a 3-stages Cascade Impactor air sampling method to measure Particle-bound PAHs in 8 Indoor environments: Chemical analysis of the particles and their observation using Scanning Electron Microscopy[END_REF], glass fiber filter systems were chosen because they are not suffering from problems of particle bounce [START_REF] Willeke | The influence of flow entry and collecting surface on the impaction efficiency of inertial impactors[END_REF]. We decided to work with a 3stage cascade impactor operating at 0.5 m 3 h -1 . Collected fractions corresponded to particles with aerodynamic diameter (Dae) in the ranges Dae>10 µm, 10µm<Dae<2.5 µm, 2.5µm<Dae<1 µm and a final filter which permitted to collect particles with Dae < 1 µm.

Analytical procedure for sample treatment

The objective of our method is to use a cascade impactor as sampler which required to concentrate a lot the sample because this sampler, which separate particle size fractions, does not permit us to collect a lot of particles, especially in indoor air. As a consequence low LOD for the overall analytical procedure are needed. This latter consisted in a first step of extraction by means of an Accelerated Solvent Extractor (ASE 300, Dionex), a second step of concentration with a rotary evaporator and a final concentration step under a gentle nitrogen stream as described in figure 2.

All of the 3 steps were considered separately in order to evaluate the effect on PAHs losses of each step.

The overall analytical procedure was previously validated by recovery experiments using certified standard solutions of the 16 US-EPA PAHs as discussed later.

ASE extraction

Glass Fiber Filters (GFF) were placed in 34 mL stainless steel cells provided by Dionex (ASE 300). Fontainebleau sand was added in the cell to reduce dead space and spare solvent. In our study, the extraction temperature was selected based on other studies in the literature, i.e. temperature in the range 100 -150 °C [START_REF] He | Determination of Atmospheric Polycyclic Aromatic Hydrocarbons Using Accelerated Solvent Extraction[END_REF][START_REF] Percot | Use of beryllium-7 as a surrogate to determine the deposition of metal and polycyclic aromatic hydrocarbon through urban aerosols in Nantes, France[END_REF][START_REF] Popp | Application of accelerated solvent extraction followed by gas chromatography, high-performance liquid chromatography and gas chromatography-mass spectrometry for the determination of polycyclic aromatic hydrocarbons, chlorinated pesticides and polychlorinated dibenzo-p-dioxins and dibenzofurans in solid wastes[END_REF]. One ASE cycle consisted in a heating step of 6 min to a set point value of 120 °C and ASE pressure was fixed to 1500 psi (104 bars) for all extractions. At the end of a cycle, a nitrogen purge of 120 s was realized to dry all the tubing. The choice of solvent for extraction and the number of static extraction were optimized in this study as mentioned below. If necessary, the extracts were then carefully filtered through a PVDF filter (0.45 µm, Restek, France).

Before use, both GFF and Fontainebleau sand were pre-cleaned with acetonitrile to remove any residual PAHs. They were stored in aluminum foil until use. Analytical blank composed of blank filters and pre-cleaned sand were also analyzed and were taken into account for quantification of samples.

Concentration steps

After extraction, the extracts (approx. 70 mL) were then reduced to 1 mL using a rotary evaporator (Büchi) at 45 °C and 220 mbar. When necessary, a gentle stream of nitrogen was finally used to concentrate the extracts to approximately 100 -150 µL.

Sample analysis

For the analysis of particle samples, the HPLC-Fluorescence technique which is a classical analytical method to quantify PAHs in environmental matrixes [START_REF] Gratz | Interlaboratory comparison of HPLC-fluorescence detection and GC/MS: analysis of PAH compounds present in diesel exhaust[END_REF], was chosen.

Results & Discussion

Sample preparation

As shown in Table 2, many literature studies related to PAHs measurements mention an extraction step (solvent volume between 2-100 mL) then followed by a preliminary concentration step with rotary evaporator, then sometimes followed by a filtration and/or a purification step and finally a last concentration step (final volume in the range 10-4 000 mL) to enhance PAHs detection. Since our samples may contain small amounts of PAHs, it is essential to reduce the number of sample treatments. In addition, it is of interest to avoid large solvent volumes because they require long concentration step, which hence increase the risk of PAHs loss by evaporation during concentration step.

Three key steps of our method were evaluated separately in order to avoid any PAHs loss during the global sample preparation procedure described in Figure 2 water and the resulting PAHs solution of ~ 10µg L -1 was analyzed by HPLC. Figure 3a shows no significant PAHs loss within the experimental uncertainties during the primary concentration of the extracted solution by using a rotary evaporator.

Once this step of concentration by a rotary evaporator validated, the potential PAHs losses induced by the ASE extraction were also quantified. For this, as previously performed in other study [START_REF] Delgado-Saborit | Determination of atmospheric particulate-phase polycyclic aromatic hydrocarbons from low volume air samples[END_REF], filters were spiked with 20 µL of a solution containing 16 PAHs at 1 mg L -1 , i.e. with 20 ng of each PAH, before extraction under different ASE conditions. The extraction of blank filters spiked with certified standards was preferred instead of extracting reference materials [START_REF] Fromme | Polycyclic aromatic hydrocarbons inside and outside of apartments in an urban area[END_REF][START_REF] Masala | Improved efficiency of extraction of polycyclic aromatic hydrocarbons (PAHs) from the National Institute of Standards and Technology (NIST) Standard Reference Material Diesel Particulate Matter (SRM 2975) using accelerated solvent extraction[END_REF] for the same reasons cited by Delgado-Saborit et al. [START_REF] Delgado-Saborit | Determination of atmospheric particulate-phase polycyclic aromatic hydrocarbons from low volume air samples[END_REF] which mentioned, first that the matrix is different between certified reference materials (powder) and particle collected onto filters and then, that a certified material is more expensive that a PAHs standard solution. Consequently, the use of a standard solution has been chosen regarding the number of tests performed to optimize the method. In order to evaluate gave the same extraction efficiency for PAHs. However, dichloromethane contained many impurities and its use for extraction implied solvent exchange before HPLC analysis [START_REF] Zhu | Pollution level, phase distribution and source analysis of polycyclic aromatic hydrocarbons in residential air in Hangzhou, China[END_REF][START_REF] Ohura | Polycyclic Aromatic Hydrocarbons in Indoor and Outdoor Environments and Factors Affecting Their Concentrations[END_REF][START_REF] Cristale | Influence of sugarcane burning on indoor/outdoor PAH air pollution in Brazil[END_REF], so that acetonitrile was finally chosen. The extract of about 70 mL was then concentrated to 1 mL using the rotary evaporator before dilution with water and HPLC analysis. Only these latter results are presented in Figure 3b.

The final sample concentration step was evaluated with 1 mL of a solution containing 16 PAHs at 10 µg L -1 introduced in a vial and evaporated under a gentle Nitrogen stream to approximately 100-150 µL. The final volume was determined by weighing. This solution was diluted 10 times in a 50:50 v/v H2O: AcN mixture to obtain a final solution of about 10 µg L -1 which was analyzed by HPLC. The results reported in Figure 3c highlight no significant PAHs loss during the third step.

Consequently, Figure 3 shows no significant loss within the experimental uncertainties for the 3 steps investigated. However, small losses was observed for low molecular weight PAHs, i.e. PAHs containing 2 or 3 aromatic cycles such NAP, ACY, ACE, FLU, PHE and ANT. For these PAHs, the losses reached 2-8 % for the ASE extraction and varied in the range 1-5 % for the primary concentration step with rotary evaporator although they were almost nonexistent during the final concentration step using a nitrogen stream varying in the range 2-3 %. All these steps evaluated independently show that the overall PAHs loss, even for those with a low molecular weight, should not exceed more than a few percent. The overall recovery experiments of 92-99 % obtained in this study are slightly better than most of those found in the literature: 77.6-90.3 % for NAP and DB[a,h]A with 2 ASE cycles, respectively [START_REF] He | Determination of Atmospheric Polycyclic Aromatic Hydrocarbons Using Accelerated Solvent Extraction[END_REF], 73-88 % for NAP and IND using ultrasonic extraction [START_REF] Cristale | Influence of sugarcane burning on indoor/outdoor PAH air pollution in Brazil[END_REF],

78.8-96.5 % for NAP and PHE using ultrasonic extraction [START_REF] Zhu | Pollution level, phase distribution and source analysis of polycyclic aromatic hydrocarbons in residential air in Hangzhou, China[END_REF], 90.1-102.4 % for 1-methylanthracene and B[a]P using also ultrasonic extraction [START_REF] Ohura | Polycyclic Aromatic Hydrocarbons in Indoor and Outdoor Environments and Factors Affecting Their Concentrations[END_REF], and up to 102 % for DB[a,h]A with Soxhlet extraction [START_REF] Morville | Seasonal and diurnal variations of atmospheric PAH concentrations between rural, suburban and urban areas[END_REF]. Consequently, our sample preparation including both sample extraction and its concentration before HPLC analysis are validated and can be potentially applied to air particulate samples.

Calibrations

For the 16 studied PAHs, calibration curves were obtained by injecting 3 times the same concentrations, each calibration curves containing between 9 and 12 points. The calibration curves (see supplementary data) were plotted in the concentrations ranges of 0.05-45 µg L -1 for the most part of PAHs or alternatively in the range of 0.1(0.5)-92 µg L -1 for PAHs exhibiting a less intense response such as NAP, ACY, DB[a,h]A and IND. Calibration curves up to 27 µg L -1 for PAHs exhibiting a high responses such as B[k]F were achieved while calibration curve for ANT was in the range 0.01-18 µg L -1 because the response reached the saturation detector for upper concentrations. As shown in table 1, the calibrations were fully and remarkably linear (R 2 > 0.999) over the entire concentrations ranges investigated for all PAHs. Linearity was here determined by plotting the linear regression of the 9-12 calibration points repeated 3 times. This good linearity (R² ≥ 0,99) was also observed within the uncertainties for concentrations lower than 2.5 µg L -1 .

Repeatability, reproducibility and Limits Of Detection and Quantification

For each PAH and for each concentration, repeatability defined as the intra-day variation (VCintra) and reproducibility defined as the inter-day variation (VCinter) for a given concentration were expressed using the variation coefficient (VC). VCinter was determined by injecting the same concentration every week during 1 month. VC is derived by the response's standard deviation to average value ratio, converted to a percentage. VCinter, for an injected concentration of 1 µg L -1 , varied between 0.8 % for CHR, PHE and B[a]P and 7.3 % for ACY (UV detection). The accuracy was evaluated on 3 points; each one was injected 3 times. The method is considered accurate if the recalculated concentration is between 90 and 110 % of the real concentration. We just accept an accuracy of 20 % for B [g,h,i]P and IND because of their less intense response due to larger peaks.

Limits of Quantification (LOQ) were derived from the Food and Drug Administration (FDA) definition [START_REF]and Human Services, Food and Drug Administration, Guidance for Industry Bioanalytical Method Validation[END_REF] which define the LOQ of an individual analytical procedure as the lowest amount of analyte in a sample which can be quantitatively determined with suitable precision (VC ≤ 20 %) and accuracy (between 80 % and 120 % of the real concentration value). The Limit of Detection (LOD) is defined as the first point for which one of these 2 criteria is not respected. Every value was evaluated on the basis of three injections. The resulting LOQ reported in 1). Again, our values are similar to those found in literature with LOD between 0.076-15.6 pg [START_REF] Ohura | Polycyclic Aromatic Hydrocarbons in Indoor and Outdoor Environments and Factors Affecting Their Concentrations[END_REF], 0.53-29.1 pg [START_REF] Zhu | Sources and patterns of polycyclic aromatic hydrocarbons pollution in kitchen air, China[END_REF] or LOQ between 0.09 and 10 µg L -1 for B[a]A and ACY, respectively [START_REF] Castro | Polycyclic aromatic hydrocarbons in gas and particulate phases of indoor environments influenced by tobacco smoke: Levels, phase distributions, and health risks[END_REF]. LOD and LOQ corresponding to airborne concentrations with a sampling volume of 20 m 3 have been also derived and reported in Table 1. The comparison of these airborne concentrations with other values reported in the literature (Table 2) highlights the analytical method developed in this work presents lower LOD (in units of µg.m -3 ) if we consider all PAHs monitored. In addition, other methods that may be almost as sensitive for PAHs quantification were related to the global particulate phase or only one size fraction [START_REF] Ohura | Polycyclic Aromatic Hydrocarbons in Indoor and Outdoor Environments and Factors Affecting Their Concentrations[END_REF][START_REF] Fromme | Polycyclic aromatic hydrocarbons (PAH) and diesel engine emission (elemental carbon) inside a car and a subway train[END_REF][START_REF] Liu | Polycyclic Aromatic Hydrocarbons (PAHs) in Indoor and Outdoor Air of Hangzhou, China[END_REF][START_REF] Naumova | Gas/particle distribution of polycyclic aromatic hydrocarbons in coupled outdoor/indoor atmospheres[END_REF][START_REF] Naumova | Polycyclic Aromatic Hydrocarbons in the Indoor and Outdoor Air of Three Cities in the U.S[END_REF][START_REF] Li | Indoor characteristics of polycyclic aromatic hydrocarbons in the urban atmosphere of Taipei[END_REF].

Minimum air sampling volumes for PAHs collection using a 3-stages Cascade Impactor

The goal of this work was to determine what is the minimum sampling volume needed to detect PAHs in indoor air at each collection plate of our cascade impactor.

The analytical LOQ ( had a significant quantitative sample with 81 homes and 30 offices [START_REF] Delgado-Saborit | Carcinogenic potential, levels and sources of polycyclic aromatic hydrocarbon mixtures in indoor and outdoor environments and their implications for air quality standards[END_REF]. Considering the different PAHs, the geometric mean values ranged between 0.03 ng m -3 for DB[a,h]A and 0.36 ng m 3 for FLN in homes (see Table 3). In addition, these values were also representative of those found elsewhere in Europe. For instance, the concentrations of B[a]P in non-smoking area were as follows (in units of ng m -3 ): 0.09 (n.d-2.40) in homes and 0.09 (n.d-1.25) in offices [START_REF] Delgado-Saborit | Carcinogenic potential, levels and sources of polycyclic aromatic hydrocarbon mixtures in indoor and outdoor environments and their implications for air quality standards[END_REF], 0.1 in a cafeteria after a smoking ban [START_REF] Pey | An evaluation of mass, number concentration, chemical composition and types of particles in a cafeteria before and after the passage of an antismoking law[END_REF], 0.573 (0.131-1.510) [START_REF] Castro | Polycyclic aromatic hydrocarbons in gas and particulate phases of indoor environments influenced by tobacco smoke: Levels, phase distributions, and health risks[END_REF], 0.612 (0.107-1.02) [START_REF] Slezakova | Influence of tobacco smoke on carcinogenic PAH composition in indoor PM10 and PM2.5[END_REF], between 0.09 and 0.25 ng m -3 in a street-side non-smoker living room [START_REF] Fromme | Polycyclic aromatic hydrocarbons inside and outside of apartments in an urban area[END_REF], 0.17 ng m -3 (0.03-0.39) and 0.49 ng m -3 (0.15-1.12) in homes close to low and high-traffic density, respectively [START_REF] Fischer | Trafficrelated differences in outdoor and indoor concentrations of particles and volatile organic compounds in Amsterdam[END_REF].

Based on these calculations and assumptions, the minimum sampling air volume in homes reported in Table 3 varies between 0.09 and 1.57 m 3 except for DB[a,h]A where the value reach 4.70 [START_REF] Delgado-Saborit | Carcinogenic potential, levels and sources of polycyclic aromatic hydrocarbon mixtures in indoor and outdoor environments and their implications for air quality standards[END_REF][START_REF] Delgado-Saborit | Measurement of Personal Exposure to Volatile Organic Compounds and Particle Associated PAH in Three UK Regions[END_REF]. The calculated sampling air volume depends on the PAH and its LOQ but also on its average abundance in air. Note that these values could be 6 (1 mL:0.15 mL) times higher if the sample is maintained to 1 mL prior dilution and injection. The final concentration of the pre-concentrated extract down to 150 µL under a gentle Nitrogen stream is therefore an essential step which allows the reduction of the sampling volume.

Considering the use of a 3-stages Cascade Impactor to collect simultaneously PM > 10 µm, 2.5 µm < PM <10 µm, 1 µm < PM < 2.5 µm and PM<1 (and then to derive PM1, PM2.5 and PM10) for particulate individual PAHs quantification, it appears that the reasonable air sampling volume is one order magnitude higher than those calculated above. Consequently, 10 or 20 m 3 should be enough to determine with a good accuracy the concentrations of the 16 investigated PAHs in this study. At a flow rate of 0.5 m 3 h -1 , the duration of air sampling should be in the range 20-40 hours. This sampling air volume range is between 7.5 and 90 times lower than those of 150-900 m 3 recommended by [START_REF] Pandey | A review of techniques for the determination of polycyclic aromatic hydrocarbons in air[END_REF] for PAHs sampling in outdoor air.

Conclusion

This study proposes a sensitive and efficient method for quantification of particulate PAHs in ambient by coupling the particulate matter collection using a 3-stages Cascade Impactor and the classic analysis by HPLC-fluorescence (or UV in the case of Acenaphtylene). The development of this analytical method, and above all the sample treatment including its concentration to 100-150 µL followed by HPLC analysis, allows to quantify the 16 US-EPA PAHs related to the size particles (PM > 10 µm, 2.5 µm < PM <10 µm, 1 µm < PM < 2.5 µm and PM<1 with a sampling duration occurring for only 24 or 48 hours in relatively standard indoor environments such as homes and offices. This experimental approach allows to derive the individual PAHs concentrations in PM1, PM2.5 and PM10.

LOD and LOQ corresponding to the potential airborne concentrations (in units of µg m -3 ) with a sampling volume of 20 m 3 have been determined. These values compared to the literature ones highlight our analytical method coupling sampling, sample treatment and analysis, exhibit lower LOD.

This new sampling and analytical method has been implemented in different indoor environments and the results are detailed in another publication [START_REF] Liaud | Application of a 3-stages Cascade Impactor air sampling method to measure Particle-bound PAHs in 8 Indoor environments: Chemical analysis of the particles and their observation using Scanning Electron Microscopy[END_REF].

  An analytical method coupling accelerated solvent extraction and HPLC-fluorescence for the quantification of particle-bound PAHs in indoor air sampled with a 3-stages cascade impactor Céline Liaud, Maurice Millet, Stéphane Le Calvé To cite this version: Céline Liaud, Maurice Millet, Stéphane Le Calvé. An analytical method coupling accelerated solvent extraction and HPLC-fluorescence for the quantification of particle-bound PAHs in indoor air sampled with a 3-stages cascade impactor. Talanta, 2015, 131, pp.386-394. 10.1016/j.talanta.2014.05.027. hal-02308191

  European Environments reported particle-bound PAHs concentrations values in the range of pg m -3 -ng m -3 . Slezakova et al. (2009) [21] and Castro et al. (2011) [14] reported comparative values of PAHs concentrations bound to PM10 and PM2.5 in a smoking and a non-smoking site in Porto (Portugal). Castro et al. found B[a]P average levels of 1.33 ng m -3 (0.45 ng m -3 -2.18 ng m -3 ) in the smoking place and 0.573 ng m -3 (0.131 ng m -3 -1.510 ng m -3 ) in the non-smoking apartments used as reference. The same tendency was observed in Spain by Pey et al.(2013) who studied differences in PAHs concentrations in PM10 before and after a smoking ban in a cafeteria[START_REF] Pey | An evaluation of mass, number concentration, chemical composition and types of particles in a cafeteria before and after the passage of an antismoking law[END_REF].[START_REF] Fromme | Polycyclic aromatic hydrocarbons inside and outside of apartments in an urban area[END_REF] in Germany, also studied differences in particle-bound PAHs concentration in street-side living-rooms and found median values of 0.65 ng m -3 for B[a]P in smoker's living while median value was 0.25 ng m -3 in non-smoker place[START_REF] Fromme | Polycyclic aromatic hydrocarbons inside and outside of apartments in an urban area[END_REF]. Besides, Krugly et al. (2014) studied particulate phase PAHs in 5 schools located in urban, suburban and rural sites in Kaunas, Lithuania, and found total PAHs mean values in the range 20.3-131.1 ng m -3 [4]. Sangiorgi et al. (2013) also compared levels of PAHs in PM1 and PM2.5 in 4 Italian offices in Milan between the cold and the warm season [23]. Again, particle-bound B[a]P sampled with Harvard impactors in

  Extracts were analyzed using a High Performance Liquid Chromatography system (Thermo Electron Corporation, Spectra System) equipped with a Diode Array Detector (DAD, Thermo Finnigan, Spectra System UV6000LP) and a fluorescence detector (Thermo Scientific, Finnigan Surveyor FL Plus) using the external standard method. The analytical column was a C18 Pinnacle II PAH (Restek) 150 mm × 3.0 mm ID × 4 µm (particle size). Temperature oven was set to 30 °C.Acetonitrile and water was used as mobile phase with a flow rate of 1 mL min -1 . The elution gradient began with 50:50 (ACN: H2O), then went to 60:40 (ACN: H2O) in 7 min, then 70:30 (ACN: H2O) until 9 min, and linear gradient to 100 % of ACN was programmed until 20 min, with a final hold of 5 min. Initial conditions were reached in 1 min and maintained 5 min before next run. The overall run time was thus 30 min. Before HPLC analysis, the sample was diluted in Milli-Q water so that the sample was in the initial gradient conditions (50:50/ACN: H2O) allowing sharper peaks. Each compound (or group of PAHs compounds) was detected at its optimum emission/excitation wavelength: 270/330 nm (NAP, ACE, FLU), 250/370 nm (PHE), 250/400 nm (ANT), 270/440 nm (FLN), 270/400 nm (PYR), 270/390 nm (B[a]A, CHR), 290/430 nm (B[b]F, B[k]F, B[a]P, DB[a,h]A, B[g,h,i]P) and 305/500 nm (IND). ACY which is not fluorescent was quantified with the DAD at his optimum absorbance wavelength λ=229 nm.

  : 1) the accelerated solvent extraction of PAHs from the filter; 2) the primary concentration of the extract until 1 mL by means of a rotary evaporator at 45°C and 220 mbar; 3) the final concentration of the pre-concentrated extract to about 100-150 µL under a gentle Nitrogen stream. Each recovery experiment was realized in triplicates and the error bars reported in the figures 3a, b and c are the relative standard deviations.Recovery experiments were first evaluated for the second step by adding 20 µL of a solution containing 16 PAHs at 1 mg L -1 , i.e. 20 ng for each PAH, in a typical volume of 70 mL of acetonitrile which is an estimated volume obtained after a 3-static ASE extraction with 34 mL cells. The solution was then concentrated to about 1 mL by using a rotary evaporator at 45°C and 220 mbar, in order to obtain PAHs concentrations of ~ 20µg L -1 . The extract was then diluted by a factor 2 with milli-Q

  the efficiency of extraction and finally choose optimum conditions, solvent and static time parameters were tested. Two solvents were tested, dichloromethane and acetonitrile (AcN). Static durations tested were 5 and 10 min. The evaluation of the number of static necessary to achieve a quantitative extraction of the 16 PAHs was realized by extracting 3 consecutive times the same spiked filter, 3 × 5 min and 3 × 10 min, respectively. Our results show that extraction is almost quantitative after the first extraction, whatever its duration. The second static allows the collection of the rest of PAHs. The extraction yield reached about 100 % whatever the ASE extraction time so that 5 min was finally chosen. Consequently, the final ASE method chosen is based on 3 static steps of 5 min. Both solvents

m 3 .

 3 Similarly, these calculated values are in the range 0.13 -1.76 m 3 for the offices, when DB[a,h]A is still excluded. These estimated sampling volume are the same order of magnitude that those used by Delgado-Saborit et al. in their study, i.e. 1.44-4.32 m 3

Table 1

 1

	have been converted in mass

Table 1 )

 1 of individual PAHs were then used to evaluate the sampling volume of air required to quantify particulate PAHs. Even if detection limit is a key criterion to assess the sensitivity of an analytical instrument, different approaches to evaluate this value are available and show differences. Kim and Kim[START_REF] Kim | Ultimate Detectability of Volatile Organic Compounds: How Much Further Can We Reduce Their Ambient Air Sample Volumes for Analysis?[END_REF] evaluated for several Volatile Organic Compounds both LOD (determined from three times the standard deviation of background noise (n=7)) and the Method Detection Limit (MDL) (calculated following relevant U.S. EPA Guidelines (n=7)) and show that MDL are always higher than LOD, both values being below the LOQ value. Finally, the LOQ was used instead of LOD because the objective of our study is to determine the minimum sampling volume needed to quantify PAHs in indoor air at each collection plate of the cascade impactor.The detectable masses of PAHs were first calculated from the LOQ (in µg L -1 ) and the final volume in the HPLC vial of 300 µL (taken into account the water dilution) after the concentration steps of the sample concentration. These detectable masses (in units of ng) were then divided by the

concentrations of individual PAHs (in units of ng m -3 ) found in European indoor environments in order to calculate the minimum sampling air volume (in units of m 3 ) required to quantify PAHs with an acceptable accuracy. Several studies in Europe measured concentrations of particulate PAHs in indoor air

[START_REF] Fromme | Polycyclic aromatic hydrocarbons inside and outside of apartments in an urban area[END_REF][START_REF] Krugly | Characterization of particulate and vapor phase polycyclic aromatic hydrocarbons in indoor and outdoor air of primary schools[END_REF][START_REF] Delgado-Saborit | Carcinogenic potential, levels and sources of polycyclic aromatic hydrocarbon mixtures in indoor and outdoor environments and their implications for air quality standards[END_REF][START_REF] Fischer | Trafficrelated differences in outdoor and indoor concentrations of particles and volatile organic compounds in Amsterdam[END_REF][START_REF] Castro | Polycyclic aromatic hydrocarbons in gas and particulate phases of indoor environments influenced by tobacco smoke: Levels, phase distributions, and health risks[END_REF][START_REF] Fromme | Polycyclic aromatic hydrocarbons (PAH) and diesel engine emission (elemental carbon) inside a car and a subway train[END_REF][START_REF] Slezakova | Influence of tobacco smoke on carcinogenic PAH composition in indoor PM10 and PM2.5[END_REF][START_REF] Pey | An evaluation of mass, number concentration, chemical composition and types of particles in a cafeteria before and after the passage of an antismoking law[END_REF][START_REF] Sangiorgi | Indoor airborne particle sources and semi-volatile partitioning effect of outdoor fine PM in offices[END_REF]

. Among the latter, one conducted by Delgado-Saborit et al.

(2011)

Acknowledgment

Financial support for this work has been provided by the French Ministry of Environment and ADEME through the PRIMEQUAL 2 program (Project MERMAID). This work was also supported by the region of Alsace and the REseau Alsace de Laboratoires en Ingénierie et Sciences pour l'Environnement (REALISE).