Ghada M Abd El-Hafez

Nady H Mahmoud

Alain Walcarius

Amany M Fekry
email: amanym.fekry@gmail.com

Evaluation of the electrocatalytic properties of Tungsten electrode towards hydrogen evolution reaction in acidic solutions

Keywords: Tungsten electrode, HER, Electrochemical activity, EIS

Hydrogen has concerned interest universally as an environmentally nontoxic and renewable fuel. Electrocatalytic hydrogen evolution reaction (HER) is one of the utmost favorable methods for hydrogen creation on a vast scale; however, the high cost of Pt-based supplies, which demonstrate the highest activity for HER, forced investigators to look for cheaper electro-catalysts. Tungsten has been considered as an effective, active and low cost electrocatalyst for the hydrogen evolution reaction, mostly in alkaline media, and we have investigated here its behavior in acid electrolytes. HER has been studied utilizing linear polarization technique and electrochemical impedance spectroscopy (EIS). It happens on W at rather low overpotential (-0.32 V vs. NHE at 10 mA cm -2 , in 0.5 M H2SO4), yet more cathodic than the widely used Pt/C catalyst, but not so far from more sophisticated systems developed recently. The effect of acid concentration on the HER rate and the electrode stability was investigated. Cathodic transfer coefficient and exchange current density were calculated for the HER from Tafel curves obtained in H2SO4 solution at concentrations ranging from 0.1 to 3.0 M. EIS experiments were performed under both open circuit and/or cathodic polarization. It was found that the hydrogen evolution rate is relatively high under low overpotential, confirming that W is a possible applicant to substitute more expensive electrocatalysts usually used for the HER under acidic conditions. The process is economic and appropriate with no need for specific treatments, as supported by additional X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterization of the tungsten electrode surface.

Introduction

Hydrogen production from water electrolysis is a significant work owing to its importance as an alternative to hydrogen energy manufacturing field [START_REF] Mizuno | Sorption of hydrogen on and in hydrogen absorbing metals in electrochemical environment[END_REF][START_REF] Viswanathan | [END_REF][START_REF] Losiewicz | The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modified nickel electrodes[END_REF].

"Hydrogen economy" is a possible substitute to energy systems using fossil derived fuels, either for its useful environmental influence or as a substitute for rapidly exhausting fossil fuels [START_REF] Crabtree | The hydrogen economy[END_REF][START_REF] Tseng | A hydrogen economy: opportunities and challenges[END_REF]. HER has been studied on different electrode materials with the goal to find more efficient conditions for H2 production. The active materials for HER should be of large active surface area, highly intrinsic electrocatalytic activity, stable and of low cost [START_REF] Crabtree | The hydrogen economy[END_REF][START_REF] Navarro-Flores | Hydrogen evolution on nickel incorporated in three-dimensional conducting polymer layers[END_REF].

It is well recognized that noble metals like platinum, Pt, and palladium, Pd (especially Pt/C and Pd/C) have high efficient catalytic performance for the H2 production, good chemical stability, large surface area with low overpotential in acid media [START_REF] Sarkar | An overview on Pd-based electrocatalysts for the hydrogen evolution reaction[END_REF]. However, their high price reduces their utilization for industrial applications [START_REF] Huang | Facile synthesis of laminated porous WS2/C composite and its electrocatalysis for oxygen reduction reaction[END_REF][START_REF] Grigoriev | Pure hydrogen production by PEM electrolysis for hydrogen energy[END_REF]. Therefore, much effort has been exhausted to find costeffective, earth-abundant metal based compounds as a highly efficient HER catalyst [START_REF] Imran | Tungsten-molybdenum oxide nanowires/reduced graphene oxide nanocomposite with enhanced and durable performance for electrocatalytic hydrogen evolution reaction[END_REF].

Recently, non-precious metals like Ni [START_REF] Popczun | Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction[END_REF], Co [START_REF] Zhong | Recent advances of cobalt-based electrocatalysts for oxygen electrode reactions and hydrogen evolution reaction[END_REF], W [START_REF] Badawy | Electrochemical performance of tungsten electrode as cathode for hydrogen evolution in alkaline solutions[END_REF] or Mo [START_REF] Badawy | Hydrogen production on molybdenum in H2SO4 solutions[END_REF], and/or non-noble metal-based materials, such as transition metal sulfides, nitrides, phosphides, carbides and carbonaceous materials, which have been stated as efficient HER catalysts replacing noble metal catalysts [START_REF] Shi | Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction[END_REF][START_REF] Liu | Design, synthesis, and energyrelated applications of metal sulfides[END_REF][START_REF] Li | Nanostructured catalysts for electrochemical water splitting: current state and prospects[END_REF]. This is not only due to their chemical stability, but also because of their low cost and low HER overpotential [START_REF] Badawy | Electrochemical performance of tungsten electrode as cathode for hydrogen evolution in alkaline solutions[END_REF][START_REF] Badawy | Hydrogen production on molybdenum in H2SO4 solutions[END_REF]. In the tungsten family, more sophisticated W-based systems have been also reported for HER, including for instance tungsten carbide, tungsten oxide, tungsten sulfide or phosphide, tungsten-based alloys, or their multicomponent composites [START_REF] Zhang | Enhancing the hydrogen evolution reaction with Ni-W-TiO2 composites[END_REF][START_REF] Nayak | Highly Active Tungsten Oxide Nanoplate Electrocatalysts for the Hydrogen Evolution Reaction in Acidic and Near Neutral Electrolytes[END_REF][START_REF] Li | Local atomic structure modulations activate metal oxide as electrocatalyst for hydrogen evolution in acidic water[END_REF][START_REF] Fei | Tungsten-based porous thin-films for electrocatalytic hydrogen generation[END_REF][START_REF] Park | Electrochemical Reactivity of Chemically Roughened Tungsten Electrodes[END_REF][START_REF] Struck | Tungsten carbide cathodes for hydrogen production in acidic electrolytes[END_REF][START_REF] Zóltovski | Hydrogen evolution reaction on smooth tungsten carbide electrodes[END_REF][START_REF] Wondimu | Highly efficient and durable phosphine reduced iron-doped tungsten oxide/reduced graphene oxide nanocomposites for the hydrogen evolution reaction[END_REF][START_REF] Abbas | Novel strongly coupled tungsten-carbon-nitrogen complex for efficient hydrogen evolution reaction[END_REF][START_REF] Han | Nitrogen-doped tungsten carbide nanoarray as an efficient bifunctional electrocatalyst for water splitting in acid[END_REF][START_REF] Nsanzimana | An Earth-Abundant Tungsten-Nickel Alloy Electrocatalyst for Superior Hydrogen Evolution[END_REF][START_REF] Mcenaney | Electrocatalytic hydrogen evolution using amorphous tungsten phosphide nanoparticles[END_REF][START_REF] Wu | Integrated design and construction of WP/W nanorod array electrodes toward efficient hydrogen evolution reaction[END_REF]. Note that tungsten oxide can be also used as electrocatalyst for oxygen reduction [START_REF] Nsanzimana | An Earth-Abundant Tungsten-Nickel Alloy Electrocatalyst for Superior Hydrogen Evolution[END_REF]. Finally, one noticed also recent efforts directed towards the photocatalytic hydrogen generation using tungsten-based (nano) materials [START_REF] Liu | Synthesis of Nanosize Tungsten Oxide and Its Evaluation as an Electrocatalyst Support for Oxygen Reduction in Acid Media[END_REF][START_REF] Tahir | Nanostructured-based WO3 photocatalysts: recent development, activity enhancement, perspectives and applications for wastewater treatment[END_REF][START_REF] Tahir | Removal of acetylsalicylate and methyltheobromine from aqueous environment using nano-photocatalyst WO3-TiO2@g-C3N4 composite[END_REF][START_REF] Tahir | Carbon nanodots and rare metals (RM = La, Gd, Er) doped tungsten oxide nanostructures for photocatalytic dyes degradation and hydrogen production[END_REF][START_REF] Tahir | Construction of MoS 2 /CND-WO 3 Ternary Composite for Photocatalytic Hydrogen Evolution[END_REF][START_REF] Tahir | Enhanced photocatalytic performance of visible-light active graphene-WO 3 nanostructures for hydrogen production[END_REF][START_REF] Tahir | Role of MoSe 2 on nanostructures WO 3 -CNT performance for photocatalytic hydrogen evolution[END_REF][START_REF] Tahir | Role of europium on WO 3 performance under visible-light for photocatalytic activity[END_REF][START_REF] Tahir | Fabrication of heterogeneous photocatalysts for insight role of carbon nanofibre in hierarchical WO 3 /MoSe 2 composite for enhanced photocatalytic hydrogen generation[END_REF].

W was utilized successfully to produce H2 from KOH solutions at a reasonable rate and for a relatively long time [START_REF] Badawy | Electrochemical performance of tungsten electrode as cathode for hydrogen evolution in alkaline solutions[END_REF][START_REF] Borresen | Hydrogen evolution on RuxTi1-xO2 in 0.5 M H2SO4[END_REF]. This is due to its superior electronic conductivity [START_REF] Liu | Structural and electrochemical studies of tungsten carbide/carbon composites for hydrogen evolution[END_REF] that makes it significant candidate for electrocatalytic HER. The disadvantages of H2O alkaline electrolysis processes are mainly related to their low efficiency and high energy consumption [START_REF] Tuvic | A rotating tungsten disc electrode in concentrated strong alkaline solutions: an electroanalytical aspect[END_REF][START_REF] Kelly | Kinetics and Mechanism of the Hydrogen Evolution Reaction on Titanium in Acidic Media[END_REF].

Hence, W metal is inspected as a catalyst for the HER in this work in acid solution which provides a potential alternative to this issue. It is aimed to utilize it as an effective cathode for hydrogen evolution at low hydrogen overpotential in H2SO4 solution. This suggestion was done especially on tungsten owing to its low cost, superior electronic properties and easily available. The electrochemical and catalytic characteristics of this electrode were investigated here on the basis of potentiometry and linear scan voltammetry (LSV) measurements as well as electrochemical impedance spectroscopy (EIS). Also, composition and morphological or structural properties have been included on the basis of SEM, EDX, XRD and XPS measurements.

Experimental

The W rod (Sigma Aldrich of cross-section area of 0.2 cm 2 ) was embedded in epoxy resin and then polished using successive grades of emery papers down to 2000 grit then washed with triple distilled water and transferred quickly to the electrolytic cell. The W working electrode was held using a PTFE holder. Its electrochemical response was evaluated in H2SO4 medium (Sigma Aldrich) using the Voltalab PGZ 100 workstation, most often with a three-electrode cell as stated previously in Badawy et al. [START_REF] Badawy | Electrochemical performance of tungsten electrode as cathode for hydrogen evolution in alkaline solutions[END_REF] work. Triple distilled water was used.

The open circuit potential, OCP, was recorded for 1 h and all potential values are referred to the saturated calomel reference electrode (SCE, +0.241 V vs. NHE). All experiments were conducted at room temperature (25 ± 1 o C). Linear sweep voltammetry was performed at a scan rate of 5 mV s -1 . EIS tests were performed at OCP potential and at different overpotentials in the H2 evolution region with a 10 mV amplitude at frequency ranging from 1mHz to 100 kHz. The polarization and EIS tests were repeated three times. Pt electrode was used for comparison purposes.

Surface morphology and characterization was achieved utilizing the scanning electron microscope (SEM) (Model Quanta 250 Field Emission Gun) coupled with energy dispersive x-ray (EDX) Unit (FEI Company, Netherlands). XPS was collected on K-ALPHA (Themo Fisher Scientific, USA) with monochromatic Xray Al K-alpha radiation -10 to 1350 eV spt size 400 micro m at pressure 10-9 mbar with full spectrum pass energy 200 eV and at narrow spectrum 50 eV. XRD instrument was … Surface roughness was measured using the Nanosurf C3000 atomic force microscopy (AFM) apparatus.

Results and discussion

Preliminary characterization of W electrodes

The polished W rod electrode exhibited a flat surface (its roughness has been estimated from AFM imaging at about 45 nm), yet with noticeable polishing striations (see SEM micrograph on Fig. 1A), without any other detectable elements (as pointed out from EDX analysis, see Fig. 1B). XPS examination of its surface state (see the narrow scan corresponding to the W region on Fig. 1C) reveals the presence of two main contributions of W(4f) at binding energies of respectively 31.3 and 33.5 eV, which are characteristics of metallic W 0 , and two small peaks located at 35.6 and 37.8 eV generally attributed to tungsten oxidized to its hexavalent state (WO3) [START_REF] Tamboli | Studies on passivation behavior of tungsten in application to chemical mechanical polishing[END_REF][START_REF] Kamiura | Characterization of polycrystalline tungsten surfaces irradiated with nitrogen ions by X-ray photoelectron spectroscopy[END_REF][START_REF] Votta | Influence of Wet Cleaning on Tungsten Deposited with Different Techniques[END_REF]. The small amount of oxidized tungsten was even less after contacting a 0.5 M H2SO4 solution (i.e., the medium in which most of electrochemical measurements will be performed, as described below), which should be due to some wet etching of WO3 likely to occur in very acidic medium [START_REF] Votta | Influence of Wet Cleaning on Tungsten Deposited with Different Techniques[END_REF][START_REF] Reichman | The Electrochromic Process at WO 3 Electrodes Prepared by Vacuum Evaporation and Anodic Oxidation of W[END_REF]. The residual WO3 signature might be also due to the fact that the W sample was briefly exposed to air prior to XPS analysis, and might therefore not reflect exactly the surface state of the electrode in the electrolyte medium (as otherwise reported for air-sensitive materials [START_REF] Bogdanoff | Structural Transformation Identification of Sputtered Amorphous MoSx as an Efficient Hydrogen-Evolving Catalyst during Electrochemical Activation[END_REF]).

Both crude W electrode and W electrode treated with 0.5 M H2SO4 have been characterized by XRD. The results (Fig. 2) indicate the expected body-centered cubic phase characteristic of W [START_REF] Majumdar | Demonstration of Production of Tungsten Metal Powder and its Consolidation into Shapes[END_REF]. The same diffraction lines were observed for both crude and treated electrodes, with the typical reflections shown as (002) , (110), ( 200), ( 211) and (220), respectively located at 2 values of 29 o , 40 o , 58°,73 o and 87 o . No noticeable difference can be seen after W treatment in 0.5 M H2SO4, and no crystalline WO3 can be detected, suggesting that the small amount of oxide evidenced from XPS is either too low to be detected by XRD or amorphous.

Open circuit potential

The open-circuit potentials, EOCP, of W (Fig. 3a) were monitored over 1 h in aerated H2SO4 solution of various concentrations. EOCP was reached in ~ 30 minutes at all acid concentrations, It shifted positively at first few minutes and then a gradual increase of potential with time until a steady state value is obtained (the relatively steady values are in the range of 0 to 0.50 mV). This means a passivation and healing of W in aqueous media even with using a high acid concentration. By increasing the H2SO4 concentration from 0.1-0.5 M, there is no appreciable change in the steady state potential, Ess. At higher concentration of H2SO4 (> 0.5 M), the EOCP of the W electrode shifts toward more positive values indicating healing (cf. Fig. 3a). For comparison, the EOCP for Pt in 0.5 M H2SO4 solution was also outlined and the results of both Pt and W are set in Fig. 3b. The results indicate much more negative steady-state potential for W compared to Pt.

Electrocatalytic evolution of hydrogen on W electrode

The HER electrocatalytic activity for W electrode was estimated by the cathodic current-potential curves in different H2SO4 acid concentrations (Fig. 4a) and examined in comparison with commercial Pt electrode. The increase of H2SO4 concentration from 0.1 to 3.0 M resulted in different current densities related to the HER increasing from 95 to 592 mA cm -2 at -1.0 V and from 182 to 1470 mA cm -2 at -1.5 V. This means that the H2 production can be controlled by the electrolyte concentration. Fig. 4b illustrates the hydrogen evolution behavior of W, in comparison to Pt in 0.5 M H2SO4 solution, where the Pt catalyst exhibits the highest HER catalytic activity in the same solution, larger than that recorded on W electrode. These values are estimated from Fig. 4b, after enlargement of the current scale at 10 mA cm -2 are -0.32 V and -0.08 V vs. NHE (in 0.5 M H2SO4), respectively, for W and Pt electrodes. The steady state potentials of Pt and W are 485 and -2 mV, respectively, (cf. Fig. 3b). The potential at which H2 starts to evolve in the same solution on the two different electrodes is -310 and -596 mV for Pt and W, respectively (cf. Fig. 4b). Thus, the potential jump for HER on W amounts to -594 mV whereas that on Pt is -795 mV. So, W is an effective catalyst for H2 production in H2SO4 solutions. To compare between W and Pt according to HER rate using similar polarization circumstances, the current density on W at -1.0 V from the steady-state potential amounts to 210 mA cm -2 , i.e. only twice as less than on Pt (410 mA cm -2 ). The presented data were extracted from Fig. 4b at a concentration of 0.5 M H2SO4 only. It was found that the hydrogen evolution rate is relatively high under low overpotential (-1 V). This confirms well confirming that W is a possible applicant as low cost electrocatalyst for the HER under acidic conditions. At higher concentration the rate of hydrogen evolution on W will be higher (cf. Fig. 4a). It means that even for practical applications when one needs to work in higher acid concentrations, W can act effectively as a catalyst for the HER in acidic electrolytes, yet achieving 50% of the performance in terms of current density when compared to Pt electrode. A more detailed comparison to other electrocatalysts investigated in the past few years (Table 1) shows that the overpotential values for W from the present study remains quite high, but not that much in comparison to other systems that are often more sophisticated in terms of composition, (nano)structure or preparation methods, than a simple W rod as used here. Fig. 5 presents the polarization scans of the HER on W electrode at different H2SO4 concentrations fitted well with the Tafel equation [START_REF] Badawy | Electrochemical performance of tungsten electrode as cathode for hydrogen evolution in alkaline solutions[END_REF]. The kinetic parameters were derived by the extrapolation method and are summarized in Table 2. The slope of the cathodic Tafel line, b, is given by: b=2.3RT/(1-α)F, where it is inversely proportional to the cathodic transfer coefficient, (1-α). The exchange current density, io, is obtained from the intersection of Tafel lines.

Generally, to convert H + protons to H2 in acidic electrolytes, three elementary reaction steps are involved [START_REF] Conway | Behavior and Characterization of Kinetically Involved Chemisorbed Intermediates in Electrocatalysis of Gas Evolution Reactions[END_REF][START_REF] Gileadi | Electrode Kinetics for Chemists[END_REF], as follow.

Molecular H2 can be produced when reaction ( 1) is combined with reaction (2), or reaction [START_REF] Losiewicz | The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modified nickel electrodes[END_REF]. For materials that are good catalysts toward H2 production (HER), the Tafel reaction is the rate-determining step (rds) at low overpotentials, while at higher overpotentials, the Heyrovsky reaction becomes the rds [START_REF] Markovic | Temperature-Dependent Hydrogen Electrochemistry on Platinum Low-Index Single-Crystal Surfaces in Acid Solutions[END_REF]. The right reaction mechanism can't be simply known. As example, the Tafel slope for the HER on the Pt(110) was 28 mV/dec, indicating the reaction pathway to the Tafel-Volmer mechanism [START_REF] Vielstich | Handbook of fuel cells: fundamentals, technology, and applications[END_REF]. By knowing the evaluated values, we can assume that the route for HER on W (b = 25 mV dec -1 cf. Table 1) should follow Volmer-Tafel steps, where Tafel reaction is the rds. It is also evaluated that the exchange current density increases with the H2SO4 concentration. To know the actual meaning of the Tafel slope, the right value of the transfer coefficient α should be known. The transfer coefficient for the HER is given to be between 0 and 1 [START_REF] Bockris | Modern electrochemistry[END_REF].

The cathodic transfer coefficient decrease by a small value with increasing the H + concentrations with an increase of exchange current density io, leading to an overall effect of HER enhancement [START_REF] Macdonald | Impedance Spectroscopy[END_REF]. Comparison to other HER catalysts indicates that Tafel slope achieved here with W electrode is amongst the smallest values reported to date for other electrode materials (Table 1).

EIS investigations of W electrode.

Metallic catalysts are likely to be corroded, inducing a decrease in its performance when utilized in industrial applications (activity and lifetime drops). Hence, EIS is an appropriate technique to characterize such behavior [START_REF] Badawy | Electrochemical behavior and corrosion inhibition of Al, Al-6061 and Al-Cu in neutral aqueous solutions[END_REF][START_REF] Nady | Ni-Cu nano-crystalline alloys for efficient electrochemical hydrogen production in acid water[END_REF][START_REF] Ismail | Effect of Nickel Content on the Corrosion and Passivation of Copper-Nickel Alloys in Sodium Sulfate Solutions[END_REF][START_REF] Kucernak | Nickel phosphide: the effect of phosphorus content on hydrogen evolution activity and corrosion resistance in acidic medium[END_REF][START_REF] Badawy | Electrochemical behavior of Mg and some Mg alloys in aqueous solutions of different pH[END_REF][START_REF] Badawy | Cathodic Hydrogen Evolution in Acidic Solutions Using Electrodeposited Nano-crystalline Ni-Co Cathodes[END_REF]. The Bode plots of W electrode after 1 h immersion in an aerated solution of different H2SO4 concentrations were given in Fig. 6. They show a broad two time constants with a maximum phase angle of ~ 90 o demonstrating that the corrosion reaction is mostly controlled by a charge transfer process. The phase diagram shape or phase angle maximum does not vary with the H2SO4 concentration [START_REF] Badawy | Cathodic Hydrogen Evolution in Acidic Solutions Using Electrodeposited Nano-crystalline Ni-Co Cathodes[END_REF]100] ensuring the same reaction mechanism (Fig. 6). The data are fitted well utilizing an equivalent circuit model constituting a solution resistance, Rs in series with a two parallel combinations consisting of a resistor representing the charge transfer resistance of outer and inner layer, R1, R2, respectively, and a constant phase element (CPE), CPE1, CPE2, respectively, representing the double layer capacitance of the outer and inner layer (cf. Fig. 6 (inset)). The CPE is introduced to account for the nonideality of the surface [101-103] including α as an empirical parameter (0 < α < 1)

and f as the frequency in Hz [104,105]. The experimental results were fitted to the circuit inset in Fig. 6 and the calculated parameters are given in Table 3. The total charge transfer resistance (RT) (Table 3) was found to be increased with increasing the concentration of H2SO4 indicating a higher stability of W electrode at higher acidic solutions due to the formation of less soluble heteropolytungstates [100].

Fig. 7 presents the Bode and Nyquist plots of the W catalyst in H2SO4 solutions at various concentrations, as polarized catholically at a potential of -750 mV, where the HER is taking place at a measurable rate. Generally, the total impedance, Z, under cathodic polarization, is lower (cf. Fig. 7a) owing to the activation of the W surface due to the HER. The ϴmax value shifts toward lower values and the impedance, Z, value decreases in more concentrated H2SO4 solutions, indicating an increase in the rate of H2 generation (HER). There is no broadening of phase diagram meaning that the rate of hydrogen production is appreciably high. Only one depressed semicircle was observed in Nyquist plot for W catalyst at all H2SO4 concentrations (cf. Fig. 7b), which becomes smaller in more concentrated H2SO4 due to the fact that the adsorption process is facilitated and the charge transfer process controls the mechanism as the concentration increases. The data can be adjusted utilizing the simple Randles model (Fig. 7b)

and fitting values [106-110] are presented in Table 4. The charge-transfer resistance decreases as the acid concentrations increases. This confirms well the increase in the current density, i.e. the increase in HER rate. Fig. 8 presents the Bode and Nyquist plots performed after different intervals of electrode immersion in 0.5 M H2SO4 at a cathodic potential of -750 mV. RT increases due to a progressive adsorption of H + on the electrode surface [START_REF] Sarkar | An overview on Pd-based electrocatalysts for the hydrogen evolution reaction[END_REF] as given in Table 5. The resistance of the adsorbed layer increases with increasing immersion time indicating increased adsorption of hydrogen on the electrode surface [111].

Conclusions

This work has investigated the electrocatalytic properties of tungsten electrode in 0.5 M H2SO4 for hydrogen evolution reactions. Polarization and impedance results have shown that the hydrogen production occurs at a reasonable overpotential on W compared to other HER catalysts, yet more cathodic than the most efficient used to date (i.e., Pt/C or novel nanostructured catalysts), however, from cost comparison W is better economically. Tungsten catalyst exhibited high HER activity with an overpotential of -0.32 V vs. NHE (at 10 mA cm -2 , in 0.5 M H2SO4), large cathodic current, and a Tafel slope as small as 25 mV/decade, which makes them perspective for application in acidic electrolyzers for hydrogen production. Tables Table 1-Comparative performance of various HER electrocatalysts in acidic media (0.5 M H2SO4).

Electrode material

Tafel slope (mV dec -1 ) Overpotential (at 10 mA/cm 2 ) (mV)

A

  primary discharge step (Volmer reaction): M + H3O + + e -→ M-Hads + H2O (b =

Figures captions:Fig. 1 .

 1 Figures captions:

Fig. 2 .

 2 Fig. 2. XRD patterns for A) a crude W electrode (without polishing) and B) W electrode treated with 0.5 M H2SO4 (with polishing).

Fig. 3 .

 3 Fig. 3. (a) Variation of the open circuit potential with time for W electrode immersed in H2SO4 solutions of different concentrations at 25 o C. (b) Variation of the open-circuit potential with time for W and Pt electrodes in a stable natural aerated 0.5 M H2SO4 solution.

Fig. 4 .

 4 Fig. 4. (a) Influence of H2SO4 solution concentration on the catalytic activity of W electrode at 25 o C. (b) Cathodic polarization curves for HER on W and Pt in 0.5 M H2SO4 at 25 o C.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Cathodic Tafel lines for HER on W electrode immersed in aerated H2SO4 solution of different concentrations at 25 0 C.

Fig. 7 .

 7 Fig. 7. (a) Bode plots for W electrode at E= -750 mV vs SCE, for HER at different concentrations of H2SO4 solutions at 25 o C. (b) Nyquist plots for W electrode at E= -750 mV vs SCE, for HER at different concentrations of H2SO4 solutions at 25 o C.

Fig. 8 .

 8 Fig. 8. (a) Bode plots of W after 1 h of electrode immersion in stagnant naturally aerated 0.5 M H2SO4 solution at -750 mV and 25 o C. (b) Nyquist plots for W after 1 h of electrode immersion in stagnant naturally aerated 0.5 M H2SO4 solution at -750 mV and 25 o C.

Fig. 1 .

 1 Fig. 1.

Fig. 2

 2 Fig. 2.

  Fig.3

Fig. 4

 4 Fig. 4

Fig. 5

 5 Fig. 5

Fig. 6

 6 Fig. 6

Fig. 7

 7 Fig. 7

Fig. 8

 8 Fig. 8

Table 2 -

 2 Tafel parameters for the HER on W electrode in H2SO4 solution with different concentrations at 25 o C.

	Conc	io	-b	cα	1-αc	Onset pot.
	/M	/mA cm -2	/mVdec -1			/ V
	0.1	0.1168	0.02677	0.1668	0.8332	-0.5965
	0.3	0.1182	0.02535	0.1202	0.8798	0.5874 -
	0.5	0.1185	0.02552	0.1258	0.8742	0.5828 -
	1	0.1337	0.02592	0.1395	0.8605	-0.5783
	5	0.1345	0.02563	0.1296	0.8704	-0.5691

Table 3 -

 3 Equivalent circuit parameters for W electrode measured after 1 h of electrode immersion in H2SO4 solution with different concentrations at 25 o C.

	Conc.	Rs	RT	CT
	/M	/Ω cm 2	/kΩ cm 2	/µF cm 2
	0.1	5.94	22.27	129.6
	0.3	7.37	22.84	123.9
	0.5	12.00	24.71	108.1
	1.0	6.19	42.31	49.24
	3.0	3.59	69.63	26.68

Table 4 -

 4 Equivalent circuit parameters for W recorded after 1 h of electrode immersion in H2SO4 solution of different concentration under -750 mV cathodic polarizations at 25 0 C.

	Conc.	Rs	Rct	Cdl
	/M	/Ω cm 2	/Ω cm 2	/µFcm 2
	0.1	4.03	69.33	91.81
	0.3	1.51	26.61	75.34
	0.5	1.19	9.88	80.53
	1.0	0.625	7.84	64.18
	3.0	0.465	2.66	75.40

Table 5 -

 5 Equivalent circuit parameters for W electrode measured after different intervals of electrode immersion in 0.5 M H2SO4 solution under -750 mV at 25 0 C.

	Time	Rs	Rct	Cdl
	/min	/Ω cm 2	/ Ω cm 2	/µF cm 2
	5	1.15	14.19	70.88
	10	1.31	15.37	65.44
	25	1.48	16.96	75.05
	40	1.58	17.26	73.75
	60	1.45	13.84	72.68


Sherman L Z Jiokeng

Ignas K Tonle

Alain Walcarius

Amino-Attapulgite/mesoporous silica composite films generated by electroassisted self-assembly for the voltammetric determination of diclofenac

Keywords: Diclofenac, attapulgite clay, mesoporous silica, film electrode, voltammetric sensor

Diclofenac (DCF) is a common anti-inflammatory drug. In this work, we have developed two new modified electrodes based on attapulgite clay mineral for the determination of DCF. The first one is made of an amino-functionalized attapulgite modified glassy carbon electrode (GCE/Amino-AT) issued from the grafting of attapulgite particles with [3-(2aminoethylamino)propyl]trimethoxysilane. Voltammetric data indicate slightly easier oxidation of DCF on such modified electrode (irreversible anodic peak at +0.750 V vs.

Ag/AgCl i.e., about 85 mV lower than on bare GCE), and pH-dependent response resulting from interactions between the probe and the electrode material. The second one is an aminoattapulgite-mesoporous silica composite film generated by electro-assisted self-assembly of a surfactant-templated silica layer entrapping the amino-functionalized attapulgite particles on GCE pretreated by electrografting of [3-aminopropyl]triethoxysilane (GCE/APTES-Amino-AT-Silica), which was designed in order to avoid the loss of clay particles in solution over prolonged use of the modified electrode. After characterization of the film morphology, composition and permeability, respectively by scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX) and cyclic voltammetry (CV), the modified electrodes were applied to the detection of DCF by square wave voltammetry (SWV). Both electrodes gave rise to SWV peak currents varying linearly with DCF concentration in the range of 0.3 -20 µΜ (in phosphate buffer at pH 5.7). The detection limits were 0.204 μM on GCE/Amino-AT and 0.053 μM on GCE/APTES-Amino-AT-Silica. The proposed method was successfully applied to the determination of DCF in pharmaceuticals and water samples.

Introduction

Nonsteroidal anti-inflammatory drugs (NSAIDs) belong to a therapeutic class of drugs useful in the treatment of pains and rheumatoid arthritis [START_REF] Campanella | Determination of nonsteroidal anti-inflammatory drugs (NSAIDs) in milk and fresh cheese based on the inhibition of cyclooxygenase[END_REF][START_REF] Zhao | Electro-oxidation of diclofenac at boron doped diamond: Kinetics and mechanism[END_REF]. Pharmaceuticals become more and more present in the environment as new emerging pollutants. They are thus currently being considered ubiquitous contaminants in waters, soils and sediments, and can be prejudicial to both aquatic biota [START_REF] Santos | Ecotoxicological aspects related to the presence of pharmaceuticals in the aquatic environment[END_REF] and human health [START_REF] Collier | Pharmaceutical contaminants in potable water: potential concerns for pregnant women and children[END_REF]. With increasing regulatory pressures on the pharmaceutical industry, there is a growing need for robust sensors that could allow rapid and reliable determinations, and this is particularly the case in quality control analysis. One class of pharmaceuticals frequently found in water concerns the anti-inflammatory drugs, among which diclofenac (DCF, [o- (2,6-dichloroanilino]phenyl] acetate) can be considered as a representative-model. DCF is a non-steroidal, anti-inflammatory drug with analgesic and antipyretic properties, extensively used to relieve the symptoms of arthritis (i.e., inflammation, swelling, stiffness and joint pains) [START_REF] Quintino | Amperometric quantification of sodium metabisulfite in pharmaceutical formulations utilizing tetraruthenated porphyrin film modified electrodes and batch injection analysis[END_REF][START_REF] Iliescu | A Raman spectroscopic study of the diclofenac sodiumbetacyclodextrin interaction[END_REF].

DCF is likely to increase the blood pressure in patients with Shy-Drager syndrome and diabetes mellitus. In case of long-term use, it may also cause life-threatening heart or circulation problems such as heart attack and stroke, or serious effects on the stomach or intestines (such as bleeding or perforation) [START_REF] Goodarzian | Square wave voltammetric determination of diclofenac in liquid phase using a novel ionic liquid multiwall carbon nanotubes paste electrode[END_REF]. Therefore, it is vital to develop simple, fast, selective and cost-effective methods for the determination of trace amounts of DCF in pharmaceuticals and biological samples to prevent their potential human health and environment impacts. The quantification methods of DCF include mostly separation techniques such as gas chromatography-mass spectrometry [START_REF] Azzouz | Simultaneous Determination of 20 Pharmacologically Active Substances in Cow's Milk, Goat's Milk, and Human Breast Milk by Gas Chromatography Mass Spectrometry[END_REF][START_REF] Yilmaz | Determination of diclofenac in pharmaceutical preparations by voltammetry and gas chromatography methods[END_REF], liquid chromatography [START_REF] Damiani | Spectrofluorometric determination of diclofenac in tablets and ointments[END_REF][START_REF] Kramancheva | Spectrophotometric determination of diclofenac sodium in gel-ointment[END_REF][START_REF] Garcia | Flow-injection spectrophotometric determination of diclofenac sodium in pharmaceuticals and urine samples[END_REF][START_REF] Liu | Determination of diclofenac in rat bile and its interaction with cyclosporin A using on-line microdialysis coupled to liquid chromatography[END_REF], high-performance liquid chromatography (HPLC) [START_REF] Elkady | Simultaneous determination of diclofenac potassium and methocarbamol in ternary mixture with guaifenesin by reversed phase liquid chromatography[END_REF][START_REF] Kaphalia | Efficient high performance liquid chromatograph/ultraviolet method for determination of diclofenac and 4′-hydroxydiclofenac in rat serum[END_REF][START_REF] Lee | Simultaneous determination of aceclofenac and diclofenac in human plasma by narrowbore HPLC using column-switching[END_REF], as well as spectrophotometry [START_REF] Sastry | Spectrophotometric method for determination of diclofenac sodium in bulk samples and pharmaceutical preparations[END_REF][START_REF] Arcelloni | Highperformance liquid chromatographic determination of diclofenac in human plasma after solid-phase extraction[END_REF][START_REF] González | Determination of cyanocobalamin, betamethasone, and diclofenac sodium in pharmaceutical formulations, by high performance liquid chromatography[END_REF][START_REF] Birajdar | A RP-HPLC method for determiınatıon of diclofenac with rabeprazole in solid dosage form[END_REF] and spectrofluorometry [START_REF] Sastry | Spectrophotometric method for determination of diclofenac sodium in bulk samples and pharmaceutical preparations[END_REF][START_REF] Arcelloni | Highperformance liquid chromatographic determination of diclofenac in human plasma after solid-phase extraction[END_REF]. Most of them are time consuming, expensive, and often need complicated preconcentration as well as derivatization. By contrast, electrochemical techniques are rather fast and simple, and they can be implemented as portable and low cost devices. Thus, some electrochemical methods have been developed for the sensitive detection of DCF [START_REF] Yang | Enhanced oxidation of diclofenac sodium at a nanostructured electrochemical sensing film constructed by multi-wall carbon nanotubes-surfactant composite[END_REF][START_REF] Manea | Electrochemical determination of diclofenac sodium in aqueous solution on Cu-doped zeolite-expanded graphite-epoxy electrode[END_REF]. Various nanomaterials have been used as electrode modifier for the electroanalytical determination of DCF in pharmaceutical formulations and biological fluids, including carbon nanotubes [START_REF] Goodarzian | Square wave voltammetric determination of diclofenac in liquid phase using a novel ionic liquid multiwall carbon nanotubes paste electrode[END_REF][START_REF] Bayandori Moghaddam | Application of carbon nanotube-graphite mixture for the determination of diclofenac sodium in pharmaceutical and biological samples[END_REF], metal nanoparticles [START_REF] Vahedi | A fast and sensitive nanosensor based on MgO nanoparticle room-temperature ionic liquid carbon paste electrode for determination of methyldopa in pharmaceutical and patient human urine samples[END_REF][START_REF] Thiagarajan | Nano TiO2-PEDOT film for the simultaneous detection of ascorbic acid and diclofenac[END_REF] and other composites materials [START_REF] Manea | Electrochemical determination of diclofenac sodium in aqueous solution on Cu-doped zeolite-expanded graphite-epoxy electrode[END_REF][START_REF] Bayandori Moghaddam | Application of carbon nanotube-graphite mixture for the determination of diclofenac sodium in pharmaceutical and biological samples[END_REF][START_REF] Akbarian | Fabrication of a new electrocatalytic sensor for determination of diclofenac, morphine and mefenamic acid using synergic effect of NiO-SWCNT and 2, 4-dimethyl-N/-[1-(2, 3dihydroxyphenyl)methylidene]aniline[END_REF]. The interaction between the target and these materials mainly relies on the electrocatalytic activity of the latter.

The interest of nanomaterials in electroanalysis has prompted the development of organoclay modified electrodes, which are nowadays investigated as sensors [START_REF] Chen | Attapulgite with poly(methylene blue) composite film electrocatalytic determination of ascorbic acid[END_REF] or biosensors [START_REF] Maghear | A novel biosensor for acetaminophen detection with romanian clays and conductive polymeric films[END_REF] for pharmaceuticals. Organoclays are composite materials obtained by functionalizing pristine clay materials with selected organic compounds. They are commonly prepared by one of the following processes: intercalation, ion exchange, pillaring or surface grafting [START_REF] Park | Modification of external surface of laponite by silane grafting[END_REF][START_REF] Piscitelli | Sodium montmorillonite silylation: unexpected effect of the aminosilane chain length[END_REF][START_REF] Lagaly | Clay minerals organic interactions[END_REF]. As well, silica-based mesoporous materials are attractive electrode modifiers due to their mechanically-stable structure at the nanoscale (with long-range order, controlled pore size, high surface area and large adsorption capabilities) [START_REF] Pal | Effect of gold nanoparticles on the electrocatalytic reduction of oxygen by silica encapsulated cobalt phthalocyanine[END_REF][START_REF] Tarn | Mesoporous silica nanoparticle nanocarriers: biofunctionality and biocompatibility[END_REF][START_REF] Gupta | Gold nanoparticles impregnated mesoporous silica spheres for simultaneous and selective determination of uric acid and ascorbic acid[END_REF][START_REF] Walcarius | Mesoporous organosilica adsorbents: nanoengineered materials for removal of organic and inorganic pollutants[END_REF][START_REF] Gibson | Mesosilica materials and organic pollutant adsorption: part B removal from aqueous solutions[END_REF][START_REF] Urbanova | Vertically-aligned mesoporous silica films[END_REF]. These materials can be prepared in the form of thin films [START_REF] Innocenzi | Order-disorder in self-assembled mesostructured silica films: A concepts review[END_REF], or as inorganic-organic hybrids via appropriate surface modification [START_REF] Sanchez | Design, synthesis, and properties of inorganic and hybrid thin films having periodically organized nanoporosity[END_REF], extending thereby their properties. Various strategies have been developed to prepare nanostructured silica coatings onto electrode surfaces [40], and an attractive configuration is that made of hexagonally packed mesochannels aligned normal to the solid support [START_REF] Urbanova | Vertically-aligned mesoporous silica films[END_REF], as it ensures fast mass transport processes from the external solution. A versatile way to get such oriented mesoporous silica films is the electrochemically assisted selfassembly (EASA) method [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF][START_REF] Goux | Oriented mesoporous silica films obtained by electro-assisted self-assembly (EASA)[END_REF]. EASA is based on the use of cetyltrimethylammonium bromide (CTAB) as the surfactant template to generate materials with pore diameter ranging from 2 to 3 nm. In this method, the electrode is immersed into a prehydrolyzed sol formed by tetraethoxysilane (TEOS) and CTAB in moderately acidic medium (pH 3), and a suitable reductive potential is applied to generate hydroxyl ions locally at the electrode/solution interface. The local pH increase contributes to catalyze the condensation of precursors [START_REF] Shacham | Electrodeposition of methylated sol-gel films on conducting surfaces[END_REF][START_REF] Deepa | Electrochemically deposited sol-gel-derived silicate films as a viable alternative in thinfilm design[END_REF][START_REF] Collinson | Electrochemistry: an important tool to study and create new sol-gelderived materials[END_REF] and, in the same time, the formation of surfactant hemimicelles, which orients the growth of the mesoporous silica film in a direction perpendicular to the electrode surface by selfassembly [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF][START_REF] Goux | Oriented mesoporous silica films obtained by electro-assisted self-assembly (EASA)[END_REF]. EASA method can be also applied to prepare functionalized mesoporous silica layers, with great promise for the elaboration of sensitive electroanalytical devices [START_REF] Etienne | Oriented mesoporous organosilica films on electrode: a new class of nanomaterials for sensing[END_REF].

In addition, sol-gel electrogeneration is compatible with the encapsulation of biomolecules to build bioelectrocatalytic devices [START_REF] Nadzhafova | Direct electrochemistry of hemoglobin and glucose oxidase in electrodeposited sol-gel silica thin films on glassy carbon[END_REF][START_REF] Wang | Electrochemically assisted deposition of sol-gel bio-composite with co-immobilized dehydrogenase and diaphorase[END_REF], or can be used to generate clay-mesoporous silica composite films [START_REF] Maghear | Clay-mesoporous silica composite films generated by electro-assisted self-assembly[END_REF].

In this work, we have investigated the interest of amine-functionalized attapulgite clay as electrode modifier for diclofenac determination. We have also used the EASA method to generate amino-attapulgite-mesoporous silica composite films onto glassy carbon electrodes (pretreated by electrografting of 3-aminopropyltriethoxysilane, APTES, to ensure good adhesion of silica [START_REF] Nasir | Electrografting of 3aminopropyltriethoxysilane on a glassy carbon electrode for the improved adhesion of vertically oriented mesoporous silica thin films[END_REF]), which are likely to be more mechanically stable than those elaborated from only clay particles. After characterization of their physico-chemical features and permeability properties using selected redox probes (cationic, neutral and anionic), we demonstrated that this composite electrode can be used for preconcentration and sensitive detection of DCF in water and pharmaceutical samples. To the best of our knowledge, no electrochemical sensor of this type has been reported so far.

Experimental section

Chemicals and reagents

All chemicals and reagents (analytical grade) were used without further purification. [3-Aminopropyl]triethoxysilane (APTES, 99%), tetrabutylammonium tetrafluoroborate (Bu4NBF4, 99%), and acetonitrile (99.9%) used for the electrografting of APTES onto the glassy carbon surface were purchased from Sigma-Aldrich. Tetraethoxysilane (TEOS, 98%, Alfa Aesar) and cetyltrimethylammonium bromide (CTAB, 99%, Acros) were used for the synthesis of mesoporous silica film, from solutions prepared with ethanol (95-96% Merck), NaNO3 (99%, Fluka), and HCl (Riedel de Haen, 1 M solution). Hexaammineruthenium(III) chloride (Ru(NH3)6Cl3, 98%, Aldrich), ferrocene dimethanol (Fc(MeOH)2, Alfa Aesar) and potassium hexacyanoferrate(III) (K3Fe(CN)6, Fluka) served as redox probes for permeability characterization of the film modified electrodes. Diclofenac sodium (C14H10Cl2NNaO2, 98%, Sigma-Aldrich), and various electrolytes such as NaCl (99 %, Prolabo) or phosphate buffer solution (PB, 0.1 M, pH 5.6), as prepared from Na2HPO4.2H2O (98%, Fluka), and KH2PO4 (99%, Normapur), were used for square wave voltammetry analyses. High-purity water (18.2 MΩ cm) was obtained from a Purelab Option-Q from ELGA and used for all experiments.

Real samples (collected in Nancy, France) were made of mineral water, tap water and natural water from the strengbach watershed (referred as SB water). They were filtered using Durapore Membrane filters (HVLP 0.45µm, Millipore) and then stored at 4°C. Crude attapulgite (PFl-1 sample) was obtained from the Source Clay Minerals Repository (Purdue University, West Lafayette, USA). The ideal molecular formula of attapilgite is Mg5Si8O20(HO)2(OH2)4•4H2O. The mineral possesses surface reactive hydroxyl (-OH) groups and exchangeable cations [START_REF] Xue | Effective NH2-grafting on attapulgite surfaces for adsorption of reactive dyes[END_REF][START_REF] Gu | Attapulgite clay combined with Tris(2, 2'bipyridyl)ruthenium(II) for the enhancement of the electrogenerated chemiluminescence sensing[END_REF][START_REF] Frost | Dynamic and controlled rate thermal analysis of attapulgite[END_REF]. Its surface area ranges from 125 to 195 m 2 /g [START_REF] Duan | Nano-attapulgite functionalization by silane modification for preparation of covalently-integrated epoxy/TMPTMA nanocomposites[END_REF] and it has a good mechanical strength [START_REF] Xuefeng | Heavy metal adsorbents mercapto and amino functionalized palygorskite: preparation and characterization[END_REF]. In this work, a nanohybrid material was prepared by modifying the surface of attapulgite (AT) with grafting an amine terminated organosilane, namely ([3-(2-Aminoethylamino)propyl]trimethoxysilane, AEP-TMS, Aldrich, ≥ 98%), according to a previously published procedure [START_REF] Jiokeng | Sensitive stripping voltammetry detection of Pb(II) at a glassy carbon electrode modifed with an aminofunctionalized attapulgite[END_REF][START_REF] Tchieno | Electroanalytical application of amine-grafted attapulgite to the sensitive quantification of the bioactive compound mangiferin[END_REF]. The modified attapulgite (referred as Amino-AT) was used to build an electrochemical sensing device for DCF quantification.

Preparation of clay films and amino-attapulgite/mesoporous silica films on the GCE

Glassy carbon electrodes (GCE, surface area: 0.071 cm 2 ) were polished using alumina pastes of different particle sizes (5, 1 and 0.5 µm), then treated in 1:1 ethanol-water solution and sonicated for 10 min for removing any residual alumina particles. Thin film working electrodes were prepared as follow: 6 µL of either Amino-AT or AT dispersions (5 mg mL -1 ) were drop-coated onto the top of the GCE, which was dried at 50°C for 10 min in an oven.

Throughout the text, the modified electrodes are referred to as "GCE/AT" and "GCE/Amino-AT", respectively for GCE modified by pristine AT or Amino-AT; the optimal amount of AT or Amino-AT being 30 µg on the electrode surface.

Clay-mesoporous silica composite films on electrodes were prepared in two successive steps.

The first one involved the electrografting of APTES onto GCE via a known procedure in which aliphatic amines are electrochemically oxidized on the electrode surface [START_REF] Nasir | Electrografting of 3aminopropyltriethoxysilane on a glassy carbon electrode for the improved adhesion of vertically oriented mesoporous silica thin films[END_REF][START_REF] Adenier | Electrochemical oxidation of aliphatic amines and their attachment to carbon and metal surfaces[END_REF]. This was performed by dipping the electrode into a mixture of 1 mM APTES and 0.1 M Bu4NBF4 in acetonitrile and by scanning potentials in the +0.7 and +2.0 V range using cyclic voltammetry (scan rate: 100 mV s -1 ). Upon cycling, the obtained electrodes were rinsed with ethanol. An Amino-AT film (containing 30 µg of Amino-AT) was then formed by dropcoating onto the APTES grafted GCE, as previously reported [START_REF] Jiokeng | Sensitive stripping voltammetry detection of Pb(II) at a glassy carbon electrode modifed with an aminofunctionalized attapulgite[END_REF]. Finally, EASA was applied to generate a mesoporous silica film around the Amino-AT particles on GCE (the same electrodeposition protocol was used to modify a clay-free electrode for comparison purposes) [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF][START_REF] Goux | Oriented mesoporous silica films obtained by electro-assisted self-assembly (EASA)[END_REF][START_REF] Nasir | Electrografting of 3aminopropyltriethoxysilane on a glassy carbon electrode for the improved adhesion of vertically oriented mesoporous silica thin films[END_REF]. The sol used was prepared by mixing ethanol and aqueous NaNO3 (0.1 M) in a 1:1 v/v ratio. TEOS and CTAB were then added to this solution according to their previously optimized concentrations (respectively 100 and 32 mM) [START_REF] Goux | Oriented mesoporous silica films obtained by electro-assisted self-assembly (EASA)[END_REF]. Upon adjusting the sol pH to 3 with added HCl, sol hydrolysis was performed at room temperature, under stirring for 2.5 h. Film electrodeposition was achieved after immersion of the electrodes in this sol solution, to which a current density of -0.74 mA.cm -2 was applied for 30 s (a silver wire and a Pt wire were used as a pseudo-reference electrode and counter electrode, respectively). To ensure good cross-linking of the silica network, the electrodes were dried overnight at 130 °C.

The template was removed as previously described by using 0.1 M HCl in ethanol [START_REF] Etienne | Oriented mesoporous organosilica films on electrode: a new class of nanomaterials for sensing[END_REF]. The resulting composite films electrodes were denoted "GCE/APTES-Silica" (sample without clay) and "GCE/APTES-Amino-AT-Silica" (sample with amino-functionalized attapulgite).

Instrumentation and experimental procedures

All electrochemical experiments and galvanostatic depositions were carried out respectively with a µ-Autolab potentiostat and a PGSTAT 100 apparatus from Ecochemie (Metrohm, Switzerland). For electrochemical experiments (cyclic and square wave voltammetry), a conventional three electrode cell configuration was employed. The working electrodes were the film modified GCEs, while an Ag/AgCl/KCl 3 M reference electrode (Metrohm, Switzerland) and a platinum wire counter electrode completed the system. APTES electrografting was carried out with an EMStat2 potentiostat (PalmSens, The Netherlands), using a silver wire as a pseudo reference and a platinum wire as counter electrode. Cyclic voltammetry was performed in solutions containing 0.5 mM redox probes (in 0.1 M NaNO3).

It was used to characterize in a qualitative way the rate and effectiveness of accumulation/rejection processes at the film electrodes investigated here. Conditions for square wave voltammetry measurements were the following: 5 s equilibration time, 4 mV potential step, 20 mV amplitude, and 25 Hz frequency. The morphology of modified GCEs were investigated by scanning electron microscopy (SEM) along with energy-dispersive Xray spectroscopy (EDX) using a JEOL JCM-6000 apparatus (acceleration voltage of 15 kV).

The pH corresponding to the point of zero charge pHpzc of Amino-AT was measured by the so-called pH drift method [START_REF] Newcombe | Granular activated carbon: importance of surface properties in the adsorption of naturally occurring organics[END_REF]. To this end, 4 mL of 0.1 M NaCl solution was placed in a vessel, and its pH was adjusted to values ranging between 2 and 12, by adding either HCl or NaOH; then 5 mg of Amino-AT were added to the solution in the vessel which was sealed.

The final pH, reached after 48 h under stirring, was measured and plotted against the initial pH. The pH at which the curve crosses the graph pH(final)=pH(initial) was taken as the pHpzc of the Amino-AT. DCF is irreversibly oxidized on the three electrodes, but peak currents were found to increase when passing from GCE to GCE/AT and to GCE/Amino-AT. At the same time, peak potentials were shifted towards less anodic values: from +0.745 V on bare GCE to +0.660 V on GCE/AT and +0.615 V on GCE/Amino-AT. This indicates an ability of AT, and especially Amino-AT, to induce effective enhancement of the DCF oxidation signal and improved electron transfer rates, suggesting the existence of both accumulation and electrocatalytic effects originating from the grafted material towards the detection of DCF. Similar variations were observed at lower DCF concentrations and using square wave voltammetry (SWV), but actually the responses (i.e., peak potentials and peak currents) were strongly pH-dependent (Fig. 2A&B) and this was attributed to the acid-base properties of DCF, the surface charge of AT and Amino-AT, as well as the oxidation mechanism of DCF, as discussed hereafter. Fig. 2A shows that the oxidation peak potentials of DCF on GCE (curve 1), GCE/AT (curve 2)

Results and discussion

Preliminary voltammetric studies on binder-free attapulgite (AT) modified electrodes

and GCE/Amino-AT (curve 3) shifted negatively with increasing the solution pH, indicating that protons are involved in the reaction process at the electrode surface. The linear regression equations were Ep = -0.049pH + 0.995 (R = -0.994), Ep = -0.055pH + 0.949 (R = -0.994) and Ep = -0.056pH + 0.986 (R = -0.992), respectively on GCE, GCE/AT and GCE/Amino-AT, demonstrating that equal number of electrons and protons are involved in the oxidation of DCF. According to previously obtained data, it can be concluded that DCF oxidizes in aqueous medium by losing two electrons and two protons to give 5-hydroxy derivative (see scheme S1 in Supplementary Material) [START_REF] Arvand | MWCNTs/Cu(OH)2 nanoparticles/IL nanocomposite modified glassy carbon electrode as a voltammetric sensor for determination of the non-steroidal anti-inflammatory drug diclofenac[END_REF][START_REF] Goyal | Electrochemical investigations of diclofenac at edge plane pyrolytic graphite electrode and its determination in human urine[END_REF][START_REF] Goyal | The effect of modifying an edge-plane pyrolytic graphite electrode with single-wall carbon nanotubes on its use for sensing diclofenac[END_REF]. The variations of peak currents with pH are plotted in Fig. 2B. The oxidation of DCF occurs from its protonated form so that larger peak currents were observed on bare GCE (see curve 1 in Fig. 2B) at pH values lower than the pKa value (3.8) of DCF [START_REF] Ràfols | A comparison between different approaches to estimate the aqueous pKa values of several non-steroidal anti-inflammatory drugs[END_REF]. Almost the same variation was observed for GCE modified with AT (see curve 2 in Fig. 2B), except that slightly larger response was obtained due to some accumulation of DCF on the AT surface, most probably via hydrogen bonding to the surface hydroxyl groups of the clay. In acidic medium, electrostatic interactions between protonated DCF and the negatively charged AT surface (the point of zero charge (pHpzc) of attapulgite was estimated to about 6 by potentiometric titration experiment [START_REF] Chi | Impact of environmental conditions on the sorption behavior of Pb(II) onto attapulgite[END_REF]) are also possible. The behavior of GCE/amino-AT was totally different, showing poor response at pH lower than 3 and significant increase in peak currents at pH larger than 4 (see curve 3 in Fig. 2B). This increase is explained by the effective accumulation of negatively charged DCF (deprotonated at pH above 3.8) and positively charged Amino-AT surface (amino groups are protonated over the whole pH range of Fig. 2B). The point of zero charge (pHpzc) of Amino-AT was evaluated to 8.9 by the so-called pH drift method (see Fig. 2C). Amino-AT is thus always in its protonated form in the whole pH range 1-7 considered here (pH < pHpzc). Between pH 5 & 7, the SWV current response of GCE/Amino-AT was almost one order of magnitude larger than those obtained at GCE and GCE/AT. For further experiments with GCE/amino-AT, the accumulation/detection of DCF was performed at pH 5.7, an optimal value (Fig. 2B) corresponding also to that of unbuffered aqueous medium.

(1)

Ep (V)=-0.056pH+0.986 Ep (V)=-0.055pH+0.949 Peak potential / V pH Ep (V)=-0.049pH+0.995 1 2 3 4 5 6 7 0.0 6.0x10 -8 1.2x10 -7
1.8x10 -7

2.4x10 -7

3.0x10 -7

(2)

(

(1) The SWV response of DCF at an Amino-AT modified GCE was also influenced by the ionic strength (as investigated from solutions containing 30 to 100 mM NaCl, see Fig. S1 in Supplementary Material). Peak currents were first found to increase, reaching a maximum at 0.10 M of NaCl, and then decreasing continuously for larger the ionic strength values. Such slow decrease can be explained by somewhat impeded accumulation of the negatively charged DCF due to competition with the electrolyte anion (Cl -) for the binding sites of protonated Amino-AT, whereas lower intensities observed in more diluted solutions could be due to increased solution resistance. Finally, the possible effect of the nature of the electrolyte anion on the GCE/Amino-AT electrode response was also investigated using various electrolyte anions (Cl -, NO3 -, SO4 2-and H2PO4 -). No noticeable difference was observed, except for the phosphate medium. In that case, however, it was more related to a buffer effect, maintaining constant the fixed pH (i.e., at 5.7) better than the non-buffered NaCl solution for which DCF oxidation might induce some local pH increase at the electrode surface due to the associated consumption of protons (scheme 1). Such event would contribute to less protonated amino groups and therefore lesser preconcentration ability of the organoclay and smaller peak current intensity (compare curves (a) and (c) in Fig. S2 in Supplementary Material). A control experiment supporting this hypothesis/explanation is the more intense response observed when intentionally protonating all the amino groups of Amino-AT by HCl prior to SWV analysis in NaCl solution (see curve (b) in Fig. S2 in Supplementary Material).

Peak current / A pH (B)
In conclusion, DCF detection is the most sensitive when using GCE/Amino-AT electrode and the best conditions for detection are a phosphate buffer medium at pH 5.7 and ionic strength of 0.1 M. These conditions will be used afterwards.

Preparation and physico-chemical characterization of Amino-AT-silica composite films

The above results show that an GCE/Amino-AT electrode seems promising for the electrochemical sensing of DCF. However, the simple deposition of Amino-AT particles onto the GCE surface may lead to an organoclay film of poor mechanical stability (loss of clay particles upon prolonged use) so that we have proposed to electrogenerate a silica deposit around the clay particles in order to improve their durable adhesion to the GCE (as for montmorillonite clays [START_REF] Maghear | Clay-mesoporous silica composite films generated by electro-assisted self-assembly[END_REF]). To avoid any unwanted restricted access of the analyte to the electrode surface, a surfactant-templated silica film was generated and the surfactant molecules were removed afterwards to liberate the porosity.

Characterization by Scanning Electron Microscopy (SEM) and Energy-Dispersive Xray spectroscopy (EDX)

A random distribution of solid particles onto a solid electrode is expected to occur when it has been modified simply by depositing raw clay or organoclay samples [START_REF] Macha | Clays as architectural units at modified-electrodes[END_REF]. This fact was explored by using SEM analysis. Typical SEM micrographs of Amino-AT particles deposited by drop-coating onto the surface of GCE are shown on Figure 3, as respectively obtained before (Fig. 3a1) and after (Fig. 3a2) use in a stirred solution for 30 min. The surface of GCE/amino-AT is heterogeneous, made of small particles of about 10 µm size and bigger aggregates of more than 50 µm (Fig. 3a1), but the most important feature is that the quantity of Amino-AT onto the electrode surface decreases after prolonged use in the electrochemical cell (Fig. 3a2). This is most probably due to the poor adhesion of Amino-AT particles to the electrode surface. To improve the mechanical stability of the clay film on GCE, we propose to generate a mesoporous silica film by electro-assisted deposition (similarly as in [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF][START_REF] Goux | Oriented mesoporous silica films obtained by electro-assisted self-assembly (EASA)[END_REF]), with the hope that the silica coating will entrap the clay particles thus improving their durable attachment to the electrode surface. Because silica is not expected to bind to the carbon surface, a "molecular glue" was first generated by electrografting APTES onto the GCE surface prior to deposition of the mesoporous silica film by EASA [START_REF] Nasir | Electrografting of 3aminopropyltriethoxysilane on a glassy carbon electrode for the improved adhesion of vertically oriented mesoporous silica thin films[END_REF]; in that case, a uniform coating is obtained, yet with some silica particles on the top, in agreement to previously reported observations [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF][START_REF] Goux | Oriented mesoporous silica films obtained by electro-assisted self-assembly (EASA)[END_REF][START_REF] Etienne | Oriented mesoporous organosilica films on electrode: a new class of nanomaterials for sensing[END_REF][START_REF] Nasir | Electrografting of 3aminopropyltriethoxysilane on a glassy carbon electrode for the improved adhesion of vertically oriented mesoporous silica thin films[END_REF] (see Fig. 3b for a mesoporous silica film deposited onto bare GCE). EASA was then applied to the GCE/Amino-AT electrode. From what one knows from literature about EASA applied to an electrode covered with an assembly of microparticles (i.e., polystyrene beads [START_REF] Etienne | Electrochemical generation of thin silica films with hierarchical porosity[END_REF]), two successive steps are likely to occurs: a fast starting deposition around the solid particles (surface gelification is often faster than bulk one [START_REF] Brinker | Continuous formation of supported cubic and hexagonal mesoporous films by sol-gel dip coating[END_REF]), and a subsequent filling of the free interstitial spaces. It seems to be also the case here, with a mesoporous silica film electrogenerated around the Amino-AT particles and onto the GCE surface, which is mostly visible via the presence of the silica beads everywhere (see SEM micrograph on Fig. 3c1). More importantly the so-obtained Amino-ATmesoporous silica composite films were mechanically stable after prolonged use in solution (no noticeable loss of clay particles, see Fig. 3c2), confirming that both electrografted APTES and electrogenerated silica contribute to improve the adhesion of the amino-AT particles to the GCE surface. The GCE/APTES-Amino-AT-Silica film was also examined by EDX using the SEM microscope by focusing either on clay particles or on electrode surfaces free of clay (see Fig. S3 in Supplementary Material). The EDX spectrum showed that the modified electrode in spot (001) (i.e., with clay particle, see Fig. S3a) contained C, Na, N, O, Mg, Al and Si elements in relation to the presence of both Amino-AT and silica onto the electrode surface. In spot (002) (i.e., without clay particle, see Fig. S3b) only C, O and Si elements are evidenced, confirming the presence of a silica film also onto the clay-free electrode surface.

To further point out that Amino-AT film is mechanical stable on GCE/APTES-Amino-AT-Silica after electrogeneration of mesoporous silica film, GCE/APTES-Amino-AT-Silica films before and after prolonged uses (ca. 30 min) were also examined by EDX using the SEM microscope by focusing on full SEM images surface (see Fig. S4 in Supplementary Material).

The EDX spectrum of the electrode before (Fig. S4a) and after uses (Fig. S4b) showed that the mass percentage of O, Mg and Si elements are roughly the same, confirming the stability of electrode. Also, during the preparation of GCE/APTES-Amino-AT-Silica, it was necessary to extract the surfactant from the mesoporous silica using 0.1 M HCl in ethanol solution under moderately stirring for 2 hours while the GCE electrode was not subjected to stirring before use. Therefore, it can be reasonably admitted that the mesoporous silica allowed stabilizing Amino-AT particulate film not only during the detection phase but also during the development of the sensor. These results clearly confirm the presence of the mesoporous silica films on the modified electrode surface, acting as a stabilization coating for maintaining Amino-AT particles durably onto the GCE surface.

Permeability properties of the various films evaluated by cyclic voltammetry (CV)

The reactivity of an electrode coated with a film/material strongly depends on its permeability properties. This can be examined using redox probes in solution [START_REF] Etienne | Molecular transport into mesostructured silica thin films: electrochemical monitoring and comparison between P6m, P63/mmc, and Pm3n structures[END_REF]. Cyclic voltammetry was thus applied to characterize six electrodes (bare GCE, GCE/APTES-Silica before and after surfactant removal, GCE/APTES-Amino-AT-Silica before and after surfactant removal, and GCE/Amino-AT), using three relevant redox probes ([Fe(CN)6] 3-, Fc(MeOH)2 and [Ru(NH3)6] 3+ ), and the results are summarized in Fig. 4. The three probes were first analyzed at a bare GCE and gave the expected well defined reversible CV signals (see curves (1) in parts A-C of Fig. 4). When the APTES pre-coated GCE was covered by a thin mesoporous silica film, before template removal, the response to the ionic probes ([Fe(CN)6] 3-and [Ru(NH3)6] 3+ ) was suppressed (see curves [START_REF] Zhao | Electro-oxidation of diclofenac at boron doped diamond: Kinetics and mechanism[END_REF] in parts A&C of Fig. 4) because the surfactantcontaining silica film acts as a barrier to both negatively and positively charged species [START_REF] Vilà | Molecular sieving with vertically-aligned mesoporous silica films and electronic wiring through isolating nanochannels[END_REF].

This also proves the formation of a continuous film of silica on the whole electrode surface without cracking. By contrast, a pair of well-defined current peaks was observed for Fc(MeOH)2 (see curve [START_REF] Zhao | Electro-oxidation of diclofenac at boron doped diamond: Kinetics and mechanism[END_REF] in parts B of Fig. 4) because such neutral probe can permeate the CTAB surfactant phase and be detected on the underlying electrode surface [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF]. After surfactant removal, the mesoporous silica film modified electrodes displayed current responses for all three probes but with an obvious charge permselectivity (small response for [Fe(CN)6] 3-and large signals for [Ru(NH3)6] 3+ and Fc(MeOH)2, see curves (3) in parts A-C of Fig. 4). This is due to the negatively charged internal surface of ultrasmall channels (arising from the presence of silanolate groups), generating thereby some charge permselectivity [START_REF] Etienne | Molecular transport into mesostructured silica thin films: electrochemical monitoring and comparison between P6m, P63/mmc, and Pm3n structures[END_REF].

All these results agree well with those reported previously [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF][START_REF] Vilà | Molecular sieving with vertically-aligned mesoporous silica films and electronic wiring through isolating nanochannels[END_REF]. Now, let's consider the amino-attapulgite-mesoporous silica composite film modified GCE.

Before surfactant extraction, the results were almost the same as those obtained for the clayfree mesoporous silica film modified GCE, except that little permeability started to occur due to possible diffusion of the probes through clay aggregates incompletely covered with silica (see curves (4) in parts A-C of Fig. 4). After surfactant removal, the clay-silica composite film exhibited a dramatically different behavior as the mesoporous silica film alone in the presence of the ionic probes ([Fe(CN)6] 3-and [Ru(NH3)6] 3+ ) whereas almost no difference was observed for the neutral Fc(MeOH)2 species (compare curves ( 5) and ( 3) in parts A-C of Fig. 4). This is clearly due to electrostatic interactions induced by the Amino-AT particles present onto the electrode surface, which enable effective permeation to the negatively-charged [Fe(CN)6] 3-ions and consequently large peak currents (see curve [START_REF] Quintino | Amperometric quantification of sodium metabisulfite in pharmaceutical formulations utilizing tetraruthenated porphyrin film modified electrodes and batch injection analysis[END_REF] in Fig. 4A) thanks to electrostatic attractions from the protonated Amino-AT material, while repelling the cationic probe (see curve [START_REF] Quintino | Amperometric quantification of sodium metabisulfite in pharmaceutical formulations utilizing tetraruthenated porphyrin film modified electrodes and batch injection analysis[END_REF] in Fig. 4C showing the poor response to [Ru(NH3)6] 3+ ). The effective permeation to the negatively-charged [Fe(CN)6] 3-ions was even more pronounced when using a silica-free Amino-AT film electrode (GCE/Amino-AT), and even larger peak currents were observed (compare curves ( 6) and ( 5) in Fig. 4A), because of the absence of the mesoporous silica "barrier" likely to repel anions. On the other hand, the blocking effect towards [Ru(NH3)6] 3+ cations was no more observed in the absence of the silica binder (compare curves ( 6) and ( 5) in Fig. 4C) as this probe was likely to reach the electrode surface by diffusion across film (i.e., between clay particles) when using the GCE/Amino-AT electrode.

Again, no noticeable difference between GCE/Amino-AT and GCE/APTES-Amino-AT-Silica when using the neutral Fc(MeOH)2 probe (compare curves ( 6) and ( 5) in Fig. 4B) because it did not induce any electrostatic effect. This demonstrates that the charge selectivity of the protonated Amino-AT material remained effective in the amino-mesoporous silica composite film, enabling fast transport/accumulation of anions (and neutral molecules) while preventing the uptake of the positively-charged species. This is thus promising for the use of GCE/APTES-Amino-AT-Silica as an electrochemical sensor for DCF.

Factors affecting diclofenac preconcentration and detection

Influence of the amount of Amino-AT on the electrode

The accumulation capacity of the composite film on the GCE is expected to depend on the amount of Amino-AT in the dispersion used to modify the electrode surface. The anodic peak intensity was larger when increasing the amount of Amino-AT, and reaches a maximum value corresponding to 5 mg of Amino-AT (Fig. 5). Beyond this content, the current intensity decreases, the too thick films starting to act as a barrier to the detection of the analyte. 5 mg is then taken as the optimal amount of Amino-AT in the dispersion.

Peak current / A Amount of Amino-AT / µg Fig. 5. Effect of the amount of Amino-AT (in µg) on the SWV peak current of 5 µM DCF on GCE/Amino-AT. Accumulation time: 2 min in 0.1 M phosphate buffer pH 5.7. Experiments were performed in triplicate.

Effect of the presence of silica on the Amino-AT modified electrode response

Prior to the exploitation of the amino-attapulgite/mesoporous silica composite film as sensing material, the difference in its electrochemical response to DCF relative to that of silica-free GCE/Amino-AT should be examined. The results shown in Fig. 6 indicate only little sensitivity difference between GCE/Amino-AT-silica and GCE/Amino-AT electrodes.

Actually, the electrode modified with the Amino-AT-silica composite film was protonated because the extraction of surfactant from the mesoporous silica was made in acidic medium (EtOH/HCl 1:1). So, the most realistic comparison has to be made with respect to protonated GCE/Amino-AT (which contributes to facilitate the accumulation of negatively charged DCF); again, little difference in peak current intensity for DCF was observed (only a signal slightly lower in the presence of silica), confirming that the mesoporous silica binder does not affect significantly the accumulation ability of the amino-attapulgite material. Note that a small anodic shift in peak potentials is noticeable for protonated GCE/Amino-AT and GCE/APTES-Amino-AT-Silica (also protonated as a result of surfactant extraction in acidic ethanolic solution) electrodes, which might be due to some local pH increase around Amino-AT particles (in agreement with peak potentials variations reported on Fig. 2A). The potential shift is even slightly more marked in the presence of the mesoporous silica binder, suggesting possible lower electron transfer rates for DCF oxidation (that might result from electrostatic repulsions originating from the silica surface, which is negatively charged at pH > 4 [START_REF] Nasir | Mesoporous silica thin films for improved electrochemical detection of paraquat[END_REF]).

Influence of accumulation time

Figure 7 shows the dependence of SWV peak currents on the accumulation time for the analysis of 5 µM DCF at GCE/Amino-AT and GCE/APTES-Amino-AT-Silica electrodes. On both electrodes, the response was very fast, being at about 50% of its maximal value before applying any preconcentration step and then increasing gradually and almost linearly with the accumulation time up to 3-4 min, to reach a plateau when preconcentration was performed for longer periods. The plateau indicates that a saturation of binding sites and reaching a rather fast adsorptive equilibrium is probably imputable to low resistance to mass transport in such highly porous film electrodes. The threshold value of 3-4 min accumulation of DCF was thus used afterwards. GCE/APTES-Amino-AT-Silica electrode.

Calibration

The effect of concentration of diclofenac on peak current was studied at optimized conditions and the calibration curve was found to be linear in the concentration range from 0.3 to 10 µM on GCE/APTES-Amino-AT-silica (Fig. 8). The measurements were performed in triplicate and an average value of the current is reported, with error bars included in the inset. The linearity of peak current (Ip) vs. concentration was maintained up to 20 µM (with a sensitivity of 0.125 A M -1 , and correlation coefficient of 0.998) and then tended to level off when increasing DCF concentration up to 50 µM (see Fig. S5 in Supplementary Material). The sensitivity was almost the same (yet lower by ca. 10%) for GCE/Amino-AT electrode (0.111 A M -1 ), in agreement with the above observations (Fig. 6). The detection limit DL, is based on an analytical signal corresponding to the minimum detectable peak intensity. DL can be defined as the analyte concentration yielding such smallest detectable signal and is calculated from the standard deviation of the blank (σ) and the slope of the calibration line (b), according to the equation DL = 3σ/b [START_REF] Ghoneim | Ni, Co and Fe in water samples by differential pulse stripping voltammetry at a hanging mercury drop electrode[END_REF]. In this work, DL values were estimated to be 0.204 μM on GCE/Amino-AT and 0.053 μM on GCE/APTES-Amino-AT-silica, on the basis of a signal to noise ratio of 3. Relative standard deviations (RSD) of 5.68 % and 5.62 % for parallel detections of 5 μM DCF using six distinct electrodes were obtained for GCE/Amino-AT and GCE/APTES-Amino-AT-Silica, respectively, indicating a good reproducibility and repeatability for both modified electrodes. A definite advantage of the GCE/APTES-Amino-AT-Silica electrode is its better mechanical stability and thus repeatable SWV response upon successive measurements (Fig. 9), thanks to the physical entrapment of clay particles in the silica matrix avoiding their leaching in solution during the cleaning step of the electrode.

- The performance of the method proposed herein is comparable, and even better than some data already reported on the quantification of DCF using other modified electrodes [START_REF] Bayandori Moghaddam | Application of carbon nanotube-graphite mixture for the determination of diclofenac sodium in pharmaceutical and biological samples[END_REF][START_REF] Arvand | MWCNTs/Cu(OH)2 nanoparticles/IL nanocomposite modified glassy carbon electrode as a voltammetric sensor for determination of the non-steroidal anti-inflammatory drug diclofenac[END_REF][START_REF] Goyal | Electrochemical investigations of diclofenac at edge plane pyrolytic graphite electrode and its determination in human urine[END_REF][START_REF] Goyal | The effect of modifying an edge-plane pyrolytic graphite electrode with single-wall carbon nanotubes on its use for sensing diclofenac[END_REF][START_REF] Chethana | Voltammetric determination of diclofenac sodium using tyrosine-modified carbon paste electrode[END_REF][START_REF] Hajjizadeh | Electrocatalytic oxidation of some anti-inflammatory drugs on a nickel hydroxidemodified nickel electrode[END_REF][START_REF] Karuppiah | Carboxyl-functionalized graphene oxide-modified electrode for the electrochemical determination of nonsteroidal anti-inflammatory drug diclofenac[END_REF][START_REF] Afkhami | Gold nanoparticle/multi-walled carbon nanotube modified glassy carbon electrode as a sensitive voltammetric sensor for the determination of diclofenac sodium[END_REF], as shown in Table 1.

Analytical applications -real samples analysis

In order to demonstrate the ability of the modified electrodes for the determination of DCF in real samples, a pharmaceutical product and three water samples (Tap water, mineral water and SB water) were examined. The proposed method was first applied to the determination of DCF sodium in DCF gastro-resistant coated tablet (Voltaren®). The results are reported in Table 2. In this experiment, the concentration of diclofenac sodium was calculated using standard additions method. As shown, recovery values close to 100% were obtained with respect to the supplier specifications, indicating good confidence level of the modified electrodes. Tap water and mineral water were stored in a refrigerator immediately after collection (from the LCPME laboratory and the market, respectively). The natural water sample from the Strengbach Watershed (a granitic watershed located in NE of France, in the Vosges Mountains) referred as SB water was collected, filtered using a 0.45 μm filter and stored in a refrigerator. All water samples were conditioned with phosphate buffer (pH 5.7).

The solutions were transferred into the voltammetric cell to be analyzed without any further treatment. As shown in Table 3, a high recovery was obtained for all water samples spiked with DCF, suggesting that the two sensors are promising for practical detection of diclofenac.

Conclusion

Two new modified electrodes (GCE/Amino-AT and GCE/APTES-Amino-AT-Silica) have been characterized using cyclic voltammetry, SEM and EDX analyses, and then applied to the sensitive detection of diclofenac. The amino-AT-mesoporous silica composite films generated on glassy carbon electrode pretreated by electrografting of APTES was more mechanically stable after use than the Amino-AT modified GCE. Both were highly porous, exhibiting good accumulation ability for negatively charged species, owing to the positive charges present on the organoclay (protonated amino groups). Under the optimized conditions, the peak currents recorded by SWV were linear to DCF concentrations in the 0.3 to 20 µM range, with detection limits of 0.204 μM and 0.053 μM, respectively for GCE/Amino-AT and GCE/APTES-Amino-AT-Silica electrodes. The proposed sensors were successfully applied to the determination of DCF in pharmaceutical and water samples.

Figure 1

 1 Figure1illustrates typical cyclic voltammograms recorded in 100 µM DCF using bare GCE and GCE modified with attapulgite (AT) and AEP-TMS-grafted attapulgite (Amino-AT).

Fig. 1 .

 1 Fig. 1. Cyclic voltammograms of 100 µM DCF at the GCE (a, black), GCE/AT (b, red) and GCE/Amino-AT (c, blue) in 0.1 M NaCl pH 6. The scan rate was 50 mV•s -1 .

Fig. 2 .

 2 Fig. 2. Influence of pH on (A) the peak potential and (B) SWV Response (peak current) recorded in triplicate for the electrooxidation of 1 μM DCF at GCE (1, black), GCE/AT (2, red) and GCE/Amino-AT (3, blue). Accumulation time: 3 min in 0.1 M NaCl, with pH adjusted by adding HCl or NaOH. (C) pH drift method to obtain the pHpzc of Amino-AT.

Fig. 3 .

 3 Fig. 3. SEM micrographs of GCE covered with various films: Amino-AT particles before (a1) and after 30 min use in solution (a2), mesoporous silica (b), and Amino-AT-Silica before (c1) and after 30 min use in solution (c2).

Fig. 6 .

 6 Fig. 6. SWV curves obtained on (a) GCE/Amino-AT (black), (b) protonated GCE/Amino-AT (in a separate solution of 0.1 M HCl and washing with water prior to use) (red) and (c) GCE/APTES-Amino-AT-Silica (green). Detection was performed in triplicate in in 0.1 M phosphate buffer pH 5.7 containing 5 µM DCF. Accumulation time: 2 min. The inset shows peak current values measured after background subtraction.

Fig. 7 .

 7 Fig. 7. Influence of the accumulation time on SWV peak currents recorded in triplicate for the detection of 5 µM DCF (in 0.1 M phosphate buffer pH 5.7) using (a) GCE/Amino-AT and (b)

Fig. 8 .

 8 Fig. 8. Square-wave voltammograms recorded for DCF in phosphate buffer pH 5.7, in the 0.3 to 10 µM concentration range, using GCE/APTES-Amino-AT-Silica electrode. Inset: corresponding calibration curve obtained from SWV curves recorded in triplicates. Accumulation time: 3 min.

Fig. 9 .

 9 Fig. 9. SWV responses recorded in triplicate with (a) GCE/Amino-AT and (b) GCE/APTES-Amino-AT-Silica to successive preconcentration/detection of 5 µM DCF in phosphate buffer pH 5.7 after 2 min accumulation. A step of cleaning the surface of the modified electrode between each successive measurement has been applied under stirring for 15 min in (a) 0.1 M HCl and (b) ethanol solution (to remove the adsorbed DCF oxidation products).

( a )

 a All sample water samples were collected from Nancy, France. (b) Mean of three measurements (c) Relative standard deviation for n = 3.

Table 1 :

 1 Comparison of the performance of some DCF sensors based on modified electrodes.

	Modified electrode	Linear range	Detection	Sensitivity	Technique Reference
	configuration	(µM)	limit (µM)	(µA.µM -1 )
	IL/CNTPE	0.3 -750	0.09	0.0999	SWV	[7]
	MWCNTs-G/Ag electrode	0.14 -6.29	0.047	0.0008	SWV	[23]
	MWCNTs/Cu(OH)2/IL	0.18 -119	0.04	0.0147	DPV	[60]
	EPPG	0.010 -1	0.0062	69	SWV	[61]
	SWCNT/EPPGE	0.001 -0.5	0.00082	224.0	SWV	[62]
		0.025 -1.5	0.022.5	51.0
	TCPE	10 -140	3.28	0.1905	DPV	[72]
	NHMN electrode	196 -2650	31.7	0.0012	A	[73]
	GO-COOH/GCE	1.2-400	0.09	0.532	LSV	[74]
	AuNPs/MWCNTs/GCE	0.03-200	0.02	0.9329	SWV	[75]
	GCE/Amino-AT	0.3-20	0.204	0.139	SWV	This work
	GCE/APTES-Amino-AT-Silica 0.3-10	0.053	0.125	SWV	This work

SWV: Square Wave Voltammetry, DPV: Differential Pulse Voltammetry, A: amperometry LSV: Linear Sweep Voltammetry

Table 2 .

 2 Determination of DCF in pharmaceutical preparations using GCE/Amino-AT and GCE/APTES-Amino-AT-Silica.

	Electrode	Sample	Stated	Detected	Recovery (%)
			content	content
	GCE/Amino-AT	Voltaren® 25 mg 24.2 mg	96.8 (n=6)
	GCE/APTES-Amino-AT-Silica			25.9 mg	103.5 (n=5)

Table 3 .

 3 Determination of DCF in Water samples using GCE/Amino-AT and GCE/APTES-Amino-AT-Silica.

	Electrode	Type of water (a) Added (µM) Found (b) (µM) RSD (c) Recovery (%)
	GCE/Amino-AT	Tap water	3.00	2.81	1.69 % 93.7
		Mineral water	3.00	3.10	1.42 % 104
		SB water	3.00	3.21	4.01 % 107
	GCE/APTES-	Tap water	0.30	0.29	3.40 % 95.6
	Amino-AT-Silica	Mineral water	0.30	0.33	3.73 % 110
		SB water	0.30	0.35	2.51 % 117

Acknowledgments

The authors are grateful to the Agence Universitaire de la Francophonie (France) for a support through the programme Soutien aux Equipes de Recherche. Financial support from The World Academy of Sciences for the Advancement of Science in Developing Countries (TWAS grant no. 12-117 RG/CHE/AF/AC-G awarded to I.K. Tonle) is also acknowledged.

Dr Grégoire Herzog (Université de Lorraine, France) is thanked for facilitation in SEM and EDX analyses.

(2)

Current / A Potential / V vs Ag/AgCl