Deomila Basnig

Neus Vilá

Grégoire Herzog

Alain Walcarius
email: alain.walcarius@univ-lorraine.fr

Voltammetric behaviour of Cationic Redox Probes at Mesoporous Silica Film Electrodes

Keywords: Mesoporous silica film, modified electrode, redox cations, permeability, diffusion, accumulation

The effect of vertically aligned silica nanochannels on the electrochemical response of various cationic redox probes has been investigated. To this end, an oriented mesoporous silica thin film has been generated by electro-assisted self-assembly onto a fluoride-doped tin oxide (FTO) electrode, which was then used to characterize the voltammetric behaviour of redox cations of different nature, size and charge. Methylene Blue (MB + ), paraquat (PQ 2+ ), diquat (DQ 2+ ), ruthenium trisbipyridine (Ru(bpy)3 2+ ) and ruthenium hexamine (Ru(NH3)6 3+ ) were selected for that purpose. Multisweep cyclic voltammetry experiments showed significant accumulation of these cations thanks to favourable electrostatic interactions with the negatively-charge silica surface, but the enhancement of the votammetric signals compared to the bare FTO electrode was strongly dependent on the probe type, according the following trend: MB + > PQ 2+ > DQ 2+ > Ru(bpy)3 2+ > Ru(NH3)6 3+ . The accumulation processes were very fast owing to the vertical orientation of the mesopore channels ensuring fast diffusion from the solution to the electrode surface. Operating at various potential scan rates and different probe concentrations enabled to quantify the relative ratio between surface-confined and diffusion-controlled processes, the adsorption phenomena being clearly more prominent in diluted media and also accentuated at high potential scan rates. The mesoporous silica film electrodes exhibited higher sensitivities at lower concentrations, which can give benefits for sensor applications at trace levels.

Introduction

The transport of ions and molecules in nanochannels and through nanopores has been extensively studied [START_REF] Zharov | Surface-modified silica colloidal crystals: Nanoporous films and membranes with controlled ionic and molecular transport[END_REF][START_REF] Tagliazucchi | Transport mechanisms in nanopores and nanochannels: Can we mimic nature?[END_REF][START_REF] Kriel | Capillary Filling of Nanoscale Channels and Surface Structure[END_REF][START_REF] Zhu | Ion/Molecule Transportation in Nanopores and Nanochannels: From Critical Principles to Diverse Functions[END_REF][START_REF] Zhang | Wettability and Applications of Nanochannels[END_REF][START_REF] Zhang | Fundamental studies and practical applications of bioinspired smart solid-state nanopores and nanochannels[END_REF], notably because nanoporous membranes offer attractive features for applications in separation [START_REF] Yang | Molecular Filtration by Ultrathin and Highly Porous Silica Nanochannel Membranes: Permeability and Selectivity[END_REF][START_REF] Eygeris | Responsive Nanoporous Membranes with Size Selectivity and Charge Rejection from Self-Assembly of Polyelectrolyte "hairy" Nanoparticles[END_REF][START_REF] Feng | Highly Selective Vertically Aligned Nanopores in Sustainably Derived Polymer Membranes by Molecular Templating[END_REF][START_REF] Small | Nanoporous membranes with electrochemically switchable, chemically stabilized ionic selectivity[END_REF][START_REF] Yang | Diatom-Mimicking Ultrahigh-Flux Mesoporous Silica Thin Membrane with Straight-Through Channels for Selective Protein and Nanoparticle Separations[END_REF][START_REF] Wang | Infused-liquid-switchable porous nanofibrous membranes for multiphase liquid separation[END_REF], sensors [START_REF] Lintang | Effect of acidic aqueous alcohol solution on template sol-gel synthesis of phosphorescent hexagonal mesoporous silica film nanocomposite[END_REF][START_REF] Mo | NANOPOROUS MEMBRANE FOR BIOSENSING APPLICATIONS[END_REF][START_REF] Zhao | Carbon nanomaterial-enabled pesticide biosensors: Design strategy, biosensing mechanism, and practical application[END_REF], catalysis [START_REF] Liu | Parallel array of nanochannels grafted with polymer-brushes-stabilized Au nanoparticles for flow-through catalysis[END_REF][START_REF] Yang | Confined Pyrolysis within a Nanochannel to Form a Highly Efficient Single Iron Site Catalyst for Zn-Air Batteries[END_REF], or energy storage [START_REF] Yang | Confined Pyrolysis within a Nanochannel to Form a Highly Efficient Single Iron Site Catalyst for Zn-Air Batteries[END_REF].

Porous membranes made of nanochannels and nanopores are mostly prepared in the form of materials such as anodic aluminum oxide (AAO) [START_REF] Urbanova | Vertically-aligned Mesoporous Silica Films[END_REF][START_REF] Law | Nanoporous anodic alumina photonic crystals for optical chemo-and biosensing: Fundamentals, advances, and perspectives[END_REF][START_REF] Vázquez | Morphological, chemical surface, and diffusive transport characterizations of a nanoporous alumina membrane[END_REF], carbon nanotube (CNT) assemblies [START_REF] Holt | Fast mass transport through sub-2-nanometer carbon nanotubes[END_REF], polyethylene terephthalate (PET) nanochannels [START_REF] Powell | Electric-field-induced wetting and dewetting in single hydrophobic nanopores[END_REF], self-organized nanotubes [START_REF] Shimizu | Selforganized nanotube materials and their application in bioengineering[END_REF], zeolite nanoporous membranes [START_REF] Rangnekar | Zeolite membranes -a review and comparison with MOFs[END_REF][START_REF] Chang | Ordered Mesoporous Zeolite Thin Films with Perpendicular Reticular Nanochannels of Wafer Size Area[END_REF][START_REF] Woodtli | Indigo in the nanochannels of zeolite L: Towards a new type of colorant[END_REF], or boron nitride nanotube [START_REF] Siria | Giant osmotic energy conversion measured in a single transmembrane boron nitride nanotube[END_REF]. The nanochannels prepared from such materials are usually characterized by pore diameters above 10 nm, which is basically useful for confinement of large molecules and species such as metal, metal oxide, polymers, and ionic liquids [START_REF] Zhang | Wettability and Applications of Nanochannels[END_REF]. These are often subject to wettability issues, their mechanical and chemical stability needs to be improved, their pore size and density cannot always be accurately controlled, and vertical alignment of nanochannels may need complex processing [START_REF] Lin | Ultrathin Silica Membranes with Highly Ordered and Perpendicular Nanochannels for Precise and Fast Molecular Separation[END_REF]. Vertically-aligned nanoporous silica membranes made of a high density of hexagonallypacked mesopore channels with small and monodisperse diameters (in the ~2-3 nm range) [START_REF] Urbanova | Vertically-aligned Mesoporous Silica Films[END_REF][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] Giordano | Multi-layered, verticallyaligned and functionalized mesoporous silica films generated by sequential electrochemically assisted self-assembly[END_REF] can offer advantageous features, notably for the confinement and transport of small molecules and ions.

A versatile and fast way of preparing vertically-ordered silica nanochannels onto electrode surfaces is the use of the electrochemically assisted self-assembly (EASA) method, as developed in our group some time ago [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]. This method is likely to generate uniform thin films (with thicknesses ranging typically between 50 and 150 nm [START_REF] Goux | Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly (EASA)[END_REF], or even more in multilayer configurations [START_REF] Giordano | Multi-layered, verticallyaligned and functionalized mesoporous silica films generated by sequential electrochemically assisted self-assembly[END_REF]) with pore sizes adjustable between 2 and 3 nm [START_REF] Vilà | Molecular Sieving with Vertically Aligned Mesoporous Silica Films and Electronic Wiring through Isolating Nanochannels[END_REF], and the mesochannels can be further functionalized with a variety of organic moieties [START_REF] Vilà | Electrochemically Assisted Generation of Highly Ordered Azide-Functionalized Mesoporous Silica for Oriented Hybrid Films[END_REF][START_REF] Rafiee | Electrochemical fabrication of electroactive ordered mesoporous electrode[END_REF][START_REF] Vilà | Clickable Bifunctional and Vertically Aligned Mesoporous Silica Films[END_REF]. Other synthetic approaches include the Stöber-solution growth [START_REF] Teng | Highly Ordered Mesoporous Silica Films with Perpendicular Mesochannels by a Simple Stöber-Solution Growth Approach[END_REF], magnetically-induced orientation [START_REF] Yamauchi | Magnetically induced orientation of mesochannels in mesoporous silica films at 30 Tesla[END_REF], epitaxial growth [START_REF] Richman | Vertically oriented hexagonal mesoporous films formed through nanometre-scale epitaxy[END_REF], or via an oil-induced co-assembly process [START_REF] Kao | A General Method for Growing Large Area Mesoporous Silica Thin Films on Flat Substrates with Perpendicular Nanochannels[END_REF]. Such oriented nanochannel membranes are very promising for applications, especially in the fields of electrochemical sensing and separation at molecular level [START_REF] Walcarius | Mesoporous Materials-Based Electrochemical Sensors[END_REF][START_REF] Yan | Vertically ordered silica mesochannel films: electrochemistry and analytical applications[END_REF][START_REF] Walcarius | Silica-based electrochemical sensors and biosensors: Recent trends[END_REF][START_REF] Zhou | Silica Nanochannel Membranes for Electrochemical Analysis and Molecular Sieving: A Comprehensive Review[END_REF]. Their performance can be affected by various factors such as the mesopore size [START_REF] Vilà | Molecular Sieving with Vertically Aligned Mesoporous Silica Films and Electronic Wiring through Isolating Nanochannels[END_REF][START_REF] Etienne | Molecular Transport into Mesostructured Silica Thin Films: Electrochemical Monitoring and Comparison between p 6 m , P6 3 / mmc , and Pm 3 n Structures[END_REF], the mesochannel length/thickness [START_REF] Nasir | Critical Effect of Film Thickness on Preconcentration Electroanalysis with Oriented Mesoporous Silica Modified Electrodes[END_REF][START_REF] Singh | Effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel[END_REF], the solution pH and/or ionic strength [START_REF] Nasir | Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat[END_REF][START_REF] Vilà | pH-modulated ion transport and amplified redox response of Keggin-type polyoxometalates through verticallyoriented mesoporous silica nanochannels[END_REF] and the surface charge of the pores [START_REF] Zhou | Silica Nanochannel Membranes for Electrochemical Analysis and Molecular Sieving: A Comprehensive Review[END_REF][START_REF] Chen | Charge-Gated Transport of Proteins in Nanostructured Optical Films of Mesoporous Silica[END_REF]. They are also attractive hosts for inclusion of redox mediator species or adsorption/confinement of other catalysts, for applications in electrocatalytic or electrochemiluminescence sensing, for instance [START_REF] Del Valle | Electro-synthesis and characterization of polythiophene nano-wires/platinum nano-particles composite electrodes. Study of formic acid electro-catalytic oxidation[END_REF][START_REF] Del Valle | Electrosynthesis of polyquinone nanowires with dispersed platinum nnaoparticles towar formic acid oxidation[END_REF][START_REF] Ahoulou | Non-covalent Immobilization of Iron-triazole (Fe(Htrz) 3 ) Molecular Mediator in Mesoporous Silica Films for the Electrochemical Detection of Hydrogen Peroxide[END_REF][START_REF] Ding | Gold nanoparticles confined in vertically aligned silica nanochannels and their electrocatalytic activity toward ascorbic acid[END_REF][START_REF] Lu | Nanochannel-Confined Graphene Quantum Dots for Ultrasensitive Electrochemical Analysis of Complex Samples[END_REF][START_REF] Zhou | Two orders-of-magnitude enhancement in the electrochemiluminescence of Ru(bpy)3+[END_REF][START_REF] Zhang | Confined electrochemiluminescence in vertically ordered silica mesochannels for the imaging of hydrogen peroxide released from single cells[END_REF].

Vertically ordered silica mesochannel films exhibit molecular sieving properties and permselective ion transport (Scheme 1). The size selectivity of mesoporous silica membranes has been demonstrated using molecular probes with dimensions greater than the pore diameter [START_REF] Lin | Ultrathin Silica Membranes with Highly Ordered and Perpendicular Nanochannels for Precise and Fast Molecular Separation[END_REF][START_REF] Vilà | Molecular Sieving with Vertically Aligned Mesoporous Silica Films and Electronic Wiring through Isolating Nanochannels[END_REF][START_REF] Serrano | Mesoporous silica thin films for molecular sieving and electrode surface protection against biofouling[END_REF]. For example, on the basis of silica membranes with distinct pore sizes (i.e., 2.0 and 2.9 nm), it has been shown that a ferrocenyl dendrimer (critical size of about 2.4 nm) can only be detected at the underlying electrode surface by diffusion through the larger nanochannels (2.9 nm) while being totally excluded from the film made of narrower pores (2.0 nm) [START_REF] Vilà | Molecular Sieving with Vertically Aligned Mesoporous Silica Films and Electronic Wiring through Isolating Nanochannels[END_REF].

Cationic fluorophores of similar geometries but different sizes have been used to evidence possible permeation for the smaller ones (i.e., ruthenium trisbipyridine, Ru(bpy)3 2+ , and ruthenium trisphenanthroline, Ru(phen)3 2+ , with molecular size of 1.3 nm), whereas bigger species (ruthenium trisdiphenylphenanthroline, Ru(dpp)3 2+ , 2.0 nm in diameter) were excluded from the mesoporous silica membrane [START_REF] Lin | Ultrathin Silica Membranes with Highly Ordered and Perpendicular Nanochannels for Precise and Fast Molecular Separation[END_REF]. The size exclusion of small proteins (such as hemoglobin) from silica mesochannels was demonstrated and applied as protective barrier against biofouling [START_REF] Serrano | Mesoporous silica thin films for molecular sieving and electrode surface protection against biofouling[END_REF][START_REF] Sun | Anti-Biofouling Isoporous Silica-Micelle Membrane Enabling Drug Detection in Human Whole Blood[END_REF]. Note that similar molecular sieving effects have been also reported for non-silica mesoporous membranes (permeation of methylene blue, MB + (diameter, 1.4nm), and rejection of rhodamine 6G, R6G + , (diameter, 1.8nm), at vertically aligned nanopores in a polymer membrane having a pore size of 1.2-1.4 nm) [START_REF] Feng | Highly Selective Vertically Aligned Nanopores in Sustainably Derived Polymer Membranes by Molecular Templating[END_REF]. In addition, the vertically-ordered silica nanochannels are otherwise characterized by charge selectivity issues. At pH greater than 3, the silica surface is expected to be negatively charged owing to its isoelectric point of 2-3 [START_REF] Wu | Synthesis of mesoporous silica nanoparticles[END_REF][START_REF] Cloarec | pH driven addressing of silicon nanowires onto Si 3 N 4 /SiO 2 micro-patterned surfaces[END_REF]. As a consequence, when working in usual media (at moderately acidic or neutral pH values), the mesoporous silica films on electrodes acted as a permselective membrane, enabling an easy transport of positively-charged redox probes (such as ruthenium hexamine, Ru(NH3)6 3+ , for instance) while rejecting the anionic ones (as ferricyanide, Fe(CN)6 3-), resulting in large electrochemical signals for cations (sometimes larger than at bare electrode due to some accumulation) and low currents for anions (as a result of electrostatic repulsions preventing the negatively-charged redox probes to reach the underlying electrode surface) [START_REF] Etienne | Molecular Transport into Mesostructured Silica Thin Films: Electrochemical Monitoring and Comparison between p 6 m , P6 3 / mmc , and Pm 3 n Structures[END_REF][START_REF] Li | Differential pulse voltammetry detection of dopamine and ascorbic acid by permselective silica mesochannels vertically attached to the electrode surface[END_REF]. Such charge selectivity was similarly observed recently with polymer-based nanochannel membranes, exhibiting, e.g. selective permeation of methylene blue, MB + , over methyl orange, MO - [START_REF] Feng | Highly Selective Vertically Aligned Nanopores in Sustainably Derived Polymer Membranes by Molecular Templating[END_REF]. By contrast, small neutral molecules (such as ferrocene dimethanol or catechol) can freely diffuse through the mesoporous silica membranes and be detected onto the underlying electrode surface [START_REF] Nasir | Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat[END_REF][START_REF] Karman | Amplified Charge Transfer for Anionic Redox Probes through Oriented Mesoporous Silica Thin Films[END_REF], yet with some influence of the mesopore diameter on the transport rates [START_REF] Etienne | Electrochemical approaches for the fabrication and/or characterization of pure and hybrid templated mesoporous oxide thin films: a review[END_REF]. Attempts to modulate the permselective behaviour, especially to promote the detection of anionic redox species, have been reported in the literature, by means of turning the internal surface of nanochannels from negative to positive (by working in highly acidic media [START_REF] Vilà | pH-modulated ion transport and amplified redox response of Keggin-type polyoxometalates through verticallyoriented mesoporous silica nanochannels[END_REF], or by grafting positively-charged ammonium groups to the silica surface [START_REF] Li | Differential pulse voltammetry detection of dopamine and ascorbic acid by permselective silica mesochannels vertically attached to the electrode surface[END_REF][START_REF] Yan | Ionic Current Rectification by Laminated Bipolar Silica Isoporous Membrane[END_REF]), by operating in very high supporting electrolyte concentrations [START_REF] Goux | prussian blue electrodeposition within an oriented mesoporous silica film: preliminary observations[END_REF], or by using neutral redox mediators acting as electron shuttles for carrying charges from through the mesoporous film [START_REF] Karman | Amplified Charge Transfer for Anionic Redox Probes through Oriented Mesoporous Silica Thin Films[END_REF]. The transport of charged species through such regular and narrow channels bearing charges onto their internal surface was found to be highly depending on the ionic strength (defining the thickness of the electrical double layer at the silica surface as an interplay with the mesopore size) [START_REF] Nasir | Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat[END_REF][START_REF] Vilà | pH-modulated ion transport and amplified redox response of Keggin-type polyoxometalates through verticallyoriented mesoporous silica nanochannels[END_REF][START_REF] Li | Differential pulse voltammetry detection of dopamine and ascorbic acid by permselective silica mesochannels vertically attached to the electrode surface[END_REF]. Hydrophobic effects on mass transport processes have been also reported [START_REF] Guillemin | Electrogeneration of highly methylated mesoporous silica thin films with vertically-aligned mesochannels and electrochemical monitoring of mass transport issues[END_REF][START_REF] Lin | Gated Molecular Transport in Highly Ordered Heterogeneous Nanochannel Array Electrode[END_REF]. To date, however, no extensive studies on the factors affecting the accumulation and electrochemical response of redox cations at oriented mesoporous silica film electrode, depending on their characteristics (nature, charge, size), have been made. Here, we have thus investigated the voltammetric behaviour of various positivelycharged redox species at an fluorine-doped tin oxide (FTO) electrode covered with an oriented mesoporous silica film generated by EASA. The probes have been chosen for their different nature (organic or inorganic), charge (mono-, di-or tri-cationic) and size (yet, all of them being smaller than the nanochannels diameter to avoid any size-exclusion effects). These include paraquat (PQ 2+ , also called methylviologen), diquat (DQ 2+ ), methylene blue (MB + ), ruthenium hexamine (Ru(NH3)6 3+ ) and ruthenium tris-bipyridine (Ru(bpy)3 2+ ). Their confinement in the silica nanochannels was mainly monitored by multisweep cyclic voltammetry, in comparison to the bare FTO electrode and at various probe concentrations, as well as after open-circuit accumulation and subsequent voltammetric analysis. The extent of accumulation was strongly dependent on the probe type, as discussed in this paper.

Experimental section 2.1. Chemicals and Reagents

Tetraethoxysilane (TEOS, 98%, Alfa Aesar), cetyltrimethyammonium bromide (C16TAB, 99%, Acros), ethanol (95-96%, Merck), and HCl (1.0 M, Sigma-Aldrich) were used for the electrochemically assisted self-assembly (EASA) of mesoporous silica films on fluorinedoped-tin oxide (FTO) plates (surface resistivity 10-14  cm -2 , Delta Technologies).

Potassium ferrocyanide, (K4Fe(CN)6, 99.5%, Fluka), Ferrocenemethanol (C11H12FeO, 97%, Alfa Aesar) and hexaammineruthenium(III) chloride (Ru(NH3)6Cl3, 98%, Sigma-Aldrich), Paraquat (1-1′-dimethyl-4-4′-bipyridinium dichloride, 98%, Sigma-Aldrich), Diquat (C12H12Br2N2, H2O, 99%, Riedel-de Haën), Methylene Blue (C16H18ClN3S xH2O, Fluka), Ruthenium Trisbipyridine or Tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (C30H24Cl2N6Ru•H2O, 98%, Acros) were used as redox probes in the cyclic voltammetry experiments. Sodium nitrate (NaNO3, 99%, Fluka) was used as the supporting electrolyte.

Electrochemical Measurements

Cyclic voltammetry measurements were carried out using either an EMStat2 or a PalmSens potentiostat (PalmSens, Netherlands) in the absence of oxygen (achieved through N2 purging for 20 min). A three-electrode system using an Ag/AgCl (3M KCl) reference electrode (Metrohm) and a stainless steel rod as the counter electrode, and FTO plates as working electrode, modified with mesoporous silica thin films. The background electrolyte for the cyclic voltammetry experiments was at 70 mM NaNO3, as it was optimized in a previous study [START_REF] Nasir | Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat[END_REF].

Preparation of mesoporous silica thin films

The oriented mesoporous silica thin films were deposited electrochemically into FTO electrodes according to a previously reported procedure [START_REF] Walcarius | Electrochemically assisted selfassembly of mesoporous silica thin films[END_REF]. A starting sol solution containing 100 mM TEOS as the silica precursor, 32 mM surfactant as template, and NaNO3 (0.1 M) as supporting electrolyte was prepared in a water: ethanol (1:1) solution adjusted to a pH of 3 using 0.1 M HCl solution. The solution was hydrolysed for 2.5 h and used for electrochemical deposition of silica by applying -1.5 V for 25 s in FTO as a working electrode with a diameter of 0.8 cm, the reference electrode was a silver wire, and a stainless steel plate as a counter electrode. The modified FTO electrode surface was rinsed immediately with water and dried overnight at 130°C. The template was removed by solvent extraction, by immersion of the film in 0.1 M HCl / Ethanol solution for a minimum of 15 min. Mesoporous silica films with and without surfactants were further characterized by GISAXS, TEM and electrochemical techniques. The full characterization is described in the supplementary information (Figure S1 and associated text).

Grazing-incidence small-angle scaterring (GISAXS) experiments were carried out on "SAXSess mc²" intrument (Anton Paar) with GISAXS equiped "VarioStageXY". This instrument is attached to a ID 3003 laboratory X-ray generator (General Electric) equiped with a sealed X-ray tube (PANalytical, λ Cu, Kα = 0.1542 nm) operating at 40 kV and 50 mA.

Scattering of X-ray beam was registered by a CCD detector (Princeton Instruments, 2084 × 2084 pixels array with 24 × 24 μm 2 pixel size) at 299 mm distance from the sample. Using Winspec32 software (Princeton Instruments), the 2D image was integrated into onedimensional scattering intensities I(q) as a function of the magnitude of the scattering vector qz = (4π/λ) sin(θ), θ is the specular reflectance angle. The level of mesostructural order was evaluated by transmission electron microscopy (TEM) using a PhilipsCM20 microscope at an acceleration voltage of 200 kV. The samples were prepared by removing some pieces of the films, which were supported on a carbon-coated copper grid.

Results and Discussion

Accumulation of different cationic redox probes inside the mesochannels

The effect of oriented mesoporous silica thin film on FTO electrode was investigated by cyclic voltammetry in order to point out possible differences in the accumulation of cationic redox probes with different positive charges (ranging from +1 to +3), sizes, and molecular structure. The redox probes used in this study were paraquat (PQ 2+ ), diquat (DQ 2+ ), methylene blue (MB + ), ruthenium trisbipyridine (Ru(bpy)3 2+ ), and ruthenium hexamine (Ru(NH3)6 3+ ).

Cyclic voltammetry of the various probes was run 50 times to allow reaching steady-state current values (Figure S2). Figure 1 showed the 50 th cyclic voltammograms obtained at both bare (dashed curves) and mesoporous silica thin film electrodes (solid lines) for the five redox probes. Since the silica surface has an isoelectric point of 2 [START_REF] Etienne | Molecular Transport into Mesostructured Silica Thin Films: Electrochemical Monitoring and Comparison between p 6 m , P6 3 / mmc , and Pm 3 n Structures[END_REF][START_REF] Zhou | Two orders-of-magnitude enhancement in the electrochemiluminescence of Ru(bpy)32+ by vertically ordered silica mesochannels[END_REF][START_REF] Carrington | Electrochemical deposition of sol-gel films for enhanced chromium(VI) determination in aqueous solutions[END_REF][START_REF] Schindler | Die Acidität von Silanolgruppen. Vorläufige Mitteillung[END_REF], silanolate groups are present on the silica surfaces in pH conditions used here (unbuffered solutions with pH values around 6.5), leading to favourable electrostatic interactions with the cationic redox probes. The absence of an adsorption prepeak for all redox probes (Figure S2) suggests a weak type of adsorption on mesoporous silica [START_REF] Wopschall | Effects of Adsorption of Electroactive Species in Stationary Electrode Polarography[END_REF], with similar formal potentials for the adsorbed and the non-adsorbed redox couples [START_REF] Compton | UNDERSTANDING Voltammetry[END_REF]. For all redox probes investigated, the electrochemical signals were enhanced with mesoporous silica compared to the bare electrode (Figure 1), suggesting that all cationic probes were accumulated in the mesochannels of the silica films. However, the extent of the accumulation varied significantly from one probe to another. The highest current response was observed for MB + with a 13-fold increase of the peak current after accumulation whereas the signal for Ru(NH3)6 3+ was improved only by a factor 1.4. The order for the enhanced electrochemical signal of the cationic redox probes is the following: MB + > PQ 2+ > DQ 2+ , Ru(bpy)3 2+ > Ru(NH3)6 3+ . It seems therefore that multicationic species are less efficiently accumulated than the monocation, which can be due to the fact that they required more counteranions from the electrolyte to maintain charge balance [START_REF] Nasir | Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat[END_REF] (anion transport is restricted in such silica channels [START_REF] Vilà | pH-modulated ion transport and amplified redox response of Keggin-type polyoxometalates through verticallyoriented mesoporous silica nanochannels[END_REF]). At same charge, the size of the probe might have some effect (i.e., more effective accumulation of PQ 2+ having a kinetic diameter of 0.58 nm [START_REF] Park | Facile reduction of zeolite-encapsulated viologens with solvated electrons and selective dispersion of inter-and intramolecular dimers of propylene-bridged bisviologen radical cation[END_REF] than DQ 2+ with kinetic diameter of about 0.7 nm [START_REF] Hinterholzinger | Highly oriented surface-growth and covalent dye labeling of mesoporous metal-organic frameworks[END_REF]) but this is expected to be quite limited if considering that the bulkier cation (Ru(bpy)3 2+ , characterized by a size of 1.2-1.3 nm [START_REF] Turbeville | Zeolite-entrapped tris(2,2'bipyridene)ruthenium(2+): intermolecular structural and dynamic effects[END_REF]) led to enhancement values close to that of DQ 2+ (compare green and orange curves in Fig. 1). Figure 2 showed the evolution of the absolute peak current value of the forward scan as a function of the number of cycles for both the silica film electrode (part A) and the bare FTO electrode (part B). Peak currents at the bare electrode were constant (independent on the number of cycles) for the five redox probes, and much lower than those recorded for the film electrode, confirming that the accumulation behavior is due to the presence of mesoporous silica thin films. For MB + , PQ 2+ , DQ 2+ and Ru(bpy)3 2+ , the forward peak current rapidly increased in the first few cycles (10-20 cycles), before reaching a plateau. Even, the first cycle for each probe led to current values much larger than at bare FTO, with magnitudes of more than 50% of the maximal steady-state values, indicating quite fast accumulation processes (thanks to the vertical channels). The increase in current in the initial stage is due to transport-limited by diffusion accompanied by rapid adsorption onto the silica substrate [START_REF] Sakai | pHresponsive diblock copolymer micelles at the silica/aqueous solution interface: Adsorption kinetics and equilibrium studies[END_REF][START_REF] Geffroy | Kinetics of adsorption of polyvinylamine onto cellulose[END_REF][START_REF] Dijt | Kinetics of polymer adsorption in stagnation point flow[END_REF]. Once the full coverage of the silica surface is attained, redox probes will no longer adsorb as shown by the plateau in the curve. For Ru(NH3)6 3+ , the peak current remained more or less constant throughout the series of cycles, suggesting that its adsorption is limited, despite a charge of +3. Actually, a main difference between this probe and the four above ones is the absence of aromatic rings; accordingly, it seems that the presence of aromatic moieties in the cationic probes contributed to improve their binding to the silica surface, which might be due to non-covalent forces between electron deficient aromatic systems of the redox cations and silanolate anions (i.e., π-anion interactions [START_REF] Schottel | Anion-π interactions[END_REF]), in addition to the more classical electrostatic interactions.

Thin-layer versus semi-infinite diffusion

In order to investigate further the behabiour of various redox probes in mesoporous silica thin films, cyclic voltammetry studies were conducted at two different concentrations (20 and 1000 µM) after accumulation at open-circuit potential for 30 min. Control experiments showed that the peak current increased with accumulation time, before reaching a plateau (Figure S3). Figure 3 shows the cyclic voltammograms recorded for DQ 2+ at the two concentrations for scan rates varying from 10 to 100 mV s -1 . At the lowest concentration (Fig. 3A), the peak-to-peak separation was approximatively 0.03 V, indicating an electrochemical response mainly controlled by the redox molecules adsorbed on the silica and corresponding to thin layer diffusion. When the concentration raised to 1000 µM (Fig. 3B), the peak-to-peak separation increased to 0.09 V (at 10 mV s -1 ) suggesting a semi-infinite diffusion-controlled reaction. These observations were further supported by the scan rate studies. When the redox probe concentration was 20 µM, the peak current varied linealry with the scan rate (Fig. 3A, inset), confirming that the electrochemical response is dominated by adsorbed species. On the other hand, diffusion of redox probes is dominating the electrochemical response at the highest concentration as established by the linear variation of the peak current with the square root of the scan rate (Fig. 3B, inset). Similar behaviour was observed for the other redox probes (Figure S4). Logarithmic plots of current vs scan rates are shown in Figure 4 for the different redox probes.

A slope of 1 indicates that the electrochemical response is confined to a thin-layer behaviour whereas a slope of 0.5 is due to a semi-infinite diffusion process according to Randles-Sevčik equation [START_REF] Sims | Effects of thin-layer diffusion in the electrochemical detection of nicotine on basal plane pyrolytic graphite (BPPG) electrodes modified with layers of multi-walled carbon nanotubes (MWCNT-BPPG)[END_REF]. At the lower concentration of DQ 2+ and PQ 2+ , adsorbed species are dominating the electrochemical response as indicated by the slope of nearly 1 recorded. However, for the same concentration of Ru(NH3)6 3+ and Ru(bpy)3 2+ , a mixed behaviour, with both diffusion and thin-layer processes, was observed (slopes were measured at 0.85 and 0.80 for Ru(NH3)6 3+ and Ru(bpy)3 2+ , respectively). These slope values lower than unity (Figure 4A), supported that geometry of the cationic probes may play a role in the electrochemical behaviour observed in such confined channels. In the case of Ru(bpy)3 2+ , this is further demonstrated by the deviation from the linear behaviour observed when the scan rate increased. This differences of behaviour could be attributed to either an effect of the pseudo-diffusion of electrons or to the limitation of counter-ions diffusion in the mesochannels. When probe concentrations were increased, the slope values approached closer to 0.5, which indicated the transition from thin layer behaviour to semi-infinite diffusion. At these higher concentrations, there was no significant behaviour between the different redox probes (Figure 4B). In the case of mixed behaviour, both diffusion-controlled and thin layer-confined reactions contributed to the recorded current, which was assessed using the following equation [START_REF] Tian | Metal-Organic Frameworks Mediated Synthesis of One-Dimensional Molybdenum-Based/Carbon Composites for Enhanced Lithium Storage[END_REF]:

𝑖 = 𝑘 𝜈 + 𝑘 𝜈 /
(1).

In equation [START_REF] Zharov | Surface-modified silica colloidal crystals: Nanoporous films and membranes with controlled ionic and molecular transport[END_REF], i is the current measured at a given potential,  was the scan rate and k1 and k2

were empirical parameters representing the thin layer and the diffusion contributions, respectively. Normalising equation (1) led to equation 2:

/ = 𝑘 𝜈 / + 𝑘 (2)
.

The values for the parameters k1 and k2 were extracted from the plots of normalized current values vs the square root of potential scan rate. The thin-layer confined and semi-infinite diffusion contributions to the current were plotted for different scan rates and concentrations (Figure 5). At the low concentration values, the electrochemical response is dominated by the reaction confined in the thin layer and its contribution increased slightly with the scan rate (Figure 5A). When the redox probe concentration is higher, the contribution of semi-infinite diffusion became dominant (Figure 5B). However, a complete comparison on the effect of the thin-layer diffusion the different redox probes is difficult, because of pseudo-diffusion of electrons [START_REF] Compton | UNDERSTANDING Voltammetry[END_REF] and of the counter ion diffusion through the film [START_REF] Nasir | Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat[END_REF][START_REF] Karman | Indirect amperometric detection of nonredox ions using a ferrocene-functionalized and oriented mesoporous silica thin film electrode[END_REF]. Nevertheless, the dominating thin-layer behaviour for the lower concentration (i.e. 20 µM), was observed for the redox probes investigated (as shown in Figure S5).

Effect of redox probe concentration on its accumulation in the mesochannels

The concentration of redox probes was varied between 10 µM and 8 mM and the forward peak current of cyclic voltammograms obtained at both bare and mesoporous silica film electrodes was plotted as a function of concentration (Figure 6). A linear response was observed for low concentrations of PQ 2+ (10 -200 µM, R 2 = 0.999), but exhibited two other different linear ranges at higher concentrations: 300 -1000 µM (R 2 = 0.985) and 2000 -8000 µM (R 2 = 0.996). As the concentration increased, the sensitivity of the modified electrode towards the redox probe decreased as shown by the results in Table 1; unlike the sensitivity at the bare electrode, which remained relatively constant throughout the 10 -8000 µM concentration range.

At the lower concentration range (10 -200 µM), the sensitivity for PQ 2+ at the mesoporous silica film electrode was three times higher than at the bare electrode, while the ratio of sensitivity of modified / bare electrode dropped to 1.8-fold for the 300 -1000 µM. This decrease of sensitivity with the concentration observed at the mesoporous silica electrode is consistent with the fall of the contribution of thin-layer diffusion and the increasing contribution of semiinfinite diffusion. At the highest concentration (8000 µM), the signal recorded at the modified electrode was even lower than at bare FTO, which is ascribed to restricted mass transport through mesochannels containing such large amounts of PQ 2+ . For the lowest concentrations, improved sensitivities for the mesoporous silica film electrodes were also observed for Ru(NH3)6 3+ (1.5-fold improvement) and for Ru(bpy)3 2+ (3.2-fold improvement).

Conclusions

The aim of this study was to characterize the accumulation of cationic redox probes at electrodes modified with mesoporous silica films for the improvement of electrochemical sensors. The electrochemical signal of the cationic redox probes was enhanced in the following order: MB + > PQ 2+ > DQ 2+ , Ru(bpy)3 2+ > Ru(NH3)6 3+ . Monocationic species are accumulated more efficiently, probably due to the necessity of more counter ions to diffuse with multicharged species to maintain the charge balance. Scan rate studies showed that thin-layer diffusion dominated at the lower concentration for all redox probes, even for Ru(NH3)6 3+ , which accumulation was rather limited compared to the other probes. As the redox probe concentration increased, semi-infinite diffusion contribution increased, cancelling out the benefit of accumulation. Mesoporous silica film electrodes exhibited higher sensitivities than bare electrodes at lower concentrations, which can give benefits for sensor applications at trace levels.

Scheme 1 :

 1 Scheme 1: Illustration of charge and size selectivity at vertically aligned mesoporous silica film. The film thickness is generally 80 -150 nm with 2-3 nm pore diameters.

Figure 1 :

 1 Figure 1: Cyclic voltammograms of various redox probes recorded at bare FTO (dashed line) and at mesoporous silica film modified FTO (straight line) electrodes: (a) MB + ; (b) PQ 2+ ; (c) DQ 2+ ; (d) Ru(bpy)3 2+ ; (e) Ru(NH3)6 3+ ; the curves correspond to the 50 th cycle obtained after multiple potential scanning at 20 mV s -1 ; The concentration for all probes was 20 μM, in a supporting electrolyte solution made of 70 mM NaNO3.

Figure 2 :

 2 Figure 2: Forward peak current as a function of the number of cycles for the various cationic redox probes at (A) a mesoporous silica film and (B) a bare FTO electrode for MB + (a, blue); PQ 2+ (b, red); DQ 2+ (c, green); Ru(bpy)3 2+ (d, orange); and Ru(NH3)6 3+ (e, magenta). All the experimental conditions were the same as Figure S2.

Figure 3 :

 3 Figure 3: Cyclic voltammograms of (A) 20 μM and (B) 1000 µM DQ 2+ on mesoporous silica film with scan rates ranging from 10 -100 mV s -1 . Insets show the peak current as a function of (A) scan rate, and (B) square root of scan rate,  1/2 .

Figure 4 :

 4 Figure 4: Logarithmic plots of peak current vs scan rate at low concentration for PQ 2+ (a), DQ 2+ (b), Ru(NH3)6 3+ (c), and Ru(bpy)3 2+ (d) at (A) low (20 μM) and (B) high concentration (1000 μM for PQ 2+ , DQ 2+ and Ru(NH3)6 3+ , and 500 μM for Ru(bpy)3 2+ ). All parameters are the same as for Figure 3.

Figure 5 :

 5 Figure 5: Contribution of thin-layer and semi-infinite diffusions to the current at mesoporous silica thin film electrodes for DQ 2+ at (A) 20 and (B) 1000 µM. Scan rates were ranging from 0.01 to 0.1 V s -1 . Supporting electrolyte was 70 mM NaNO3. Similar graphs for the other redox probes are shown in supplementary information.

Figure 6 :

 6 Figure 6: (A) Cyclic voltammograms for 10 -1000 µM PQ 2+ at a mesoporous silica film electrode (50 th cycles); supporting electrolyte was 70 mM NaNO3;  = 20 mV s -1 . Inset shows cylic voltammograms in the 2000 -8000 μM concentration range. (B) Calibration curves of

Table 1 :

 1 Sensitivities for PQ2+ , Ru(bpy)3 2+ , and Ru(NH3)6 3+ at mesoporous silica films and bare FTO electrodes

	Electrode	Probe	Dynamic range / µM	Sensitivity cm -2 / A M -1	R 2
			10 -200	0.274	0.999
		PQ 2+	300 -1000	0.162	0.986
	Mesoporous silica film		2000 -8000	0.052	0.996
		Ru(bpy)3 2+	2 -20	0.192	0.991
		Ru(NH3)6 3+	10 -100	0.095	0.999
			10 -200	0.089	0.998
		PQ 2+	300 -1000	0.092	0.997
	Bare FTO electrode		2000 -8000	0.080	0.998
		Ru(bpy)3 2+	2 -20	0.065	0.977
		Ru(NH3)6 3+	10 -100	0.060	0.998

doi:10.1016/j.electacta.2017.01.126.

Acknowledgements

This work was supported partly by the French PIA project "Lorraine Université d'Excellence" (Reference N° ANR-15-IDEX-04-LUE). DB is grateful to CHED-PHILFRANCE and Campus France for sponsoring of her PhD. M Lionel Richaudeau is thanked for his help with the GISAXS measurements.


Samuel Ahoulou

Neus Vila
email: neus.vila@univ-lorraine.fr

Sébastien Pillet

Dominik Schaniel

Alain Walcarius
email: alain.walcarius@univ-lorraine.fr

Non-covalent immobilization of iron-triazole (Fe(Htrz) 3 ) molecular mediator in mesoporous silica films for the electrochemical detection of hydrogen peroxide

Keywords: Mesoporous silica, Thin film, Iron-triazole complex (Fe(Htrz)3), electrocatalytic detection, hydrogen peroxide

Mesoporous silica thin films encapsulating a molecular iron-triazole complex, Fe(Htrz)3 (Htrz = 1,2,4,-1H-triazole), have been generated by electrochemically assisted self-assembly (EASA) on indium-tin oxide (ITO) electrode. The obtained modified electrodes are characterized by well-defined voltammetric signals corresponding to the Fe II/III centers of the Fe(Htrz)3 species immobilized into the films, indicating fast electron transfer processes and stable operational stability. This is due to the presence of a high density of redox probes in the material (1.610 -4 mol g -1 Fe(Htrz)3 in the mesoporous silica film) enabling efficient charge transport by electron hopping. The mesoporous films are uniformly deposited over the whole electrode surface and they are characterized by a thickness of 110 nm and a wormlike mesostructure directed by the template role played by Fe(Htrz)3 species in the EASA process. These species are durably immobilized in the material (they are not removed by solvent extraction). The composite mesoporous material (denoted Fe(Htrz)3@SiO2) is then used for the electrocatalytic detection of hydrogen peroxide, which can be performed by amperometry at an applied potential of -0.4 V versus Ag/AgCl and by flow injection analysis. The organic-inorganic hybrid film electrode displays good sensitivity for H2O2 sensing over a dynamic range from 5 to 300 μM, with a detection limit estimated at 2 μM.

Introduction

Iron-based complexes have been the focus of great interest due to their attractiveness for applications in various fields, including catalysis, organic synthesis, optics and electronics, photo-and electrochemistry, or biology and (nano)medicine [1][2][3][4][5][6][7]. Among them, the iron-triazole complexes (formed upon reaction of Fe(II) with triazole ligands to give coordination polymers in solution) have received considerable attention notably for their optical, magnetic, and electronic characteristics [8][9][10] However, most of the metal-based catalysts may suffer from poisoning under the reaction conditions, resulting in their deactivation and loss of catalytic activity. Preparation of supported metal complexes could effectively solve this problem, which might be achieved via their dispersion in porous materials such as metal oxides, carbon or polymers. In particular, mesoporous matrices are an auspicious type of material as their porous structures with large, regular, and accessible channels make them potential candidates to act as a hosting support for several active chemical species [11][12][13]. For instance, mesoporous silica-based matrices have been exploited to accommodate Fe(II)-triazole complexes in the form of monolith or thin film nanocomposites [14][15][16]. To date, however, the electrochemical applications of such complexes remain limited in spite of their known electroactivity in solution [17].

On the other hand, ordered mesoporous silica materials coated on electrode surfaces are increasingly used for electrochemical sensing and biosensing purposes [18][19][20]. Of special interest are thin films offering pores accessible from the surface because they ensure fast mass transport of target species from the solution to the electrode surface, and consequently large voltammetric responses [21,22]. They can be prepared by evaporation-induced self-assembly [21,23] or by electrochemically-assisted self-assembly [24,25], this latter method enabling the generation of vertically oriented mesopore channels which are particularly suited to analytical applications [20,26]. Some attempts have been already made to immobilize redox mediators within mesoporous silica thin films on electrodes. A straightforward approach is the simple doping using redox-active species that are likely to interact with the silica surface via weak bonds (electrostatic interactions), as reported for [Ru(bpy)3] 2+ [27,28], but such weakly bound species can leach out of the film, resulting in rather poor operational stability in multisweep cyclic voltammetry, for instance [27]. More durable immobilization is achieved by covalent binding of the redox moieties onto [29][30][31][32] or into [33] the mesopore walls, but such groups physically attached to an insulating material as silica can be electrochemically accessible only via electron hopping between adjacent sites, resulting in effective charge propagation through the film in case of redox centers very close to each other (i.e., high content) [29,34]. Long-range charge transfer by electron hopping has been demonstrated for a vertically-aligned ferrocene-functionalized film [35], but the rate of charge transfer remains limited [34], except in special configuration [36]. Promising ways to promote charge transport in mesoporous silica films are via conducting polymers electropolymerized in the mesopore channels [37] or via the use of redox-active coordination polymers as template in the synthesis of the films [38] because these strategies ensure the electroactive moieties being very close to each other in the hybrid film. Here, we plan to use the iron-triazole complex, Fe(Htrz)3 (Htrz = 1,2,4,-1Htriazole), as coordination polymer template to generate Fe(Htrz)3-doped mesoporous silica films by electrochemicallyassisted self-assembly (EASA) on indium-tin oxide (ITO) electrode and its subsequent use for the electrocatalytic detection of hydrogen peroxide.

Detection of hydrogen peroxide is relevant to various domains (biological, pharmaceutical, medical and environmental fields, or food and textile industry), and electrochemical methods have been developed for that purpose [39]. They include protein-based electrochemical biosensors [40,41] and nonenzymatic sensors involving the use of either charge transfer mediators [42,[START_REF] Kumar | Handbook of Functional Nanomaterials[END_REF] or nanomaterials [START_REF] Kruusma | [END_REF][45][46], or a combination of both [47,48], or even nanocomposites (such as Pt nanoparticles and polyaniline hosted in mesoporous silica film [49]), in order to lower the high overpotentials usually associated to the electrochemical transformation of H2O2. Widely used mediators for H2O2 sensing are Prussian Blue and related derivatives [50] as well as organic or organometallic compounds [42], often deposited onto electrode surfaces as polymeric coatings or as covalently attached moieties [START_REF] Kumar | Handbook of Functional Nanomaterials[END_REF]51]. Free molecular organic redox mediators are much less used because the resulting devices may suffer from poor long-term stability.

Here we would like to exploit the durable immobilization of Fe(Htrz)3 complexes simply entrapped in a mesoporous silica film in the course of its generation by EASA on ITO electrodes to investigate their potential interest in electrocatalysis, using hydrogen peroxide as model analyte. After a brief physico-chemical characterization of the films, in order to point out the successful deposition of the mesoporous hybrid material encapsulating Fe(Htrz)3 species in an electroactive form, the modified electrode will be applied to H2O2 sensing by amperometry and flow injection analysis.

Experimental

Chemicals

Most chemicals and reagents were commercially available and used as received: tetraethoxysilane (TEOS, 98%, Alfa Aesar), cetyltrimethylammonium bromide (CTAB, 99%, Acros), ethanol (95-96%, Merck), HCl (37% Riedel de Haen), NaNO3 (99%, Fluka), H2O2 (30%, WWR Chemicals) and ferrocene dimethanol (FcMeOH, analytical grade, Alfa Aesar). The Fe(Htrz)3 coordination polymer was prepared as previously described [14], from the reaction between Fe(BF4)2 and 1,2,4,1H-triazole.

Electrode preparation

Indium-tin oxide (ITO) plates (surface resistivity 8-12 Ω, from Delta Technologies) or fluorine-doped tin oxide (FTO) plates (surface resistivity 7 Ω, from Solaronix) were used as supports for film growth. Fe(Htrz)3-containing mesoporous silica films were generated by adapting the previously reported EASA method [24,25]. A sol solution was first prepared by adding TEOS (100 mM), CTAB (32 mM) and the Fe(Htrz)3 complex (5 mM, optimized value [38]) to a 50:50 water:ethanol medium containing also 0.1 M NaNO3 as electrolyte. Its pH was adjusted to 3 by adding 0.1 M HCl and the sol was aged for 2.5h to ensure proper hydrolysis of the silica precursors. It was then transferred to the electrochemical cell mounted with an ITO plate at the bottom and the electrode potential was biased at a cathodic value of -1.3 V for 20 s (optimized parameters [25]) to initiate the self-assembly condensation process and concomitant growth of the mesoporous Fe(Htrz)3-silica hybrid coating (Fe(Htrz)3@SiO2). The electrode was then removed and thoroughly rinsed with water. It was aged overnight at 130°C to stabilize the obtained mesostructure. The films were washed with an ethanol solution containing 0.1 M HCl in order to remove traces of CTAB and unwanted silica aggregates that may have formed upon synthesis.

Apparatus

A μAutoLab III potentiostat (Eco Chemie, monitored by the GPES software) was used for all the electrochemical experiments, i.e., the film synthesis, its voltammetric characterization and H2O2 detection. A home-made electrochemical cell with a conical shape and a three electrode configuration was employed for preparation of the mesoporous films. The working electrode was placed at the bottom, the stainless steel counter electrode took place on the cell walls and an AgCl-coated silver wire served as pseudo-reference electrode. The setup was slightly modified for the electrochemical characterization of the Fe(Htrz)-functionalized films, using this time a Ag/AgCl reference electrode (Metrohm) and a platinum rod as counter electrode. Flow injection analysis (FIA) was performed in the amperometric mode (by applying a constant potential of -0.4 V unless specified otherwise), while injecting 100-µL aliquots of H2O2 at the desired concentration in the carrier solution (0.1 M NaNO3) flowing at a rate of 1 mL min -1 (optimized value). The film morphology was examined by scanning electron microscopy (SEM) using the JEOL JSM-840 apparatus, and its structure was observed by transmission electron microscopy (TEM) with the aid of a ACCEL ARM 200F microscope operating at an acceleration voltage of 200 kV. Fourier transform infrared spectroscopy (FTIR) measurements were carried out by using a Nicolet 8700 apparatus equipped with a diffuse reflectance accessory (Smart Collector).

Results and discussions

The electrochemical behavior of the Fe(Htrz)3 complex in aqueous medium is illustrated at Figure 1, showing an irreversible cyclic voltammogram with an ill-defined anodic signal around +1.0 V and a cathodic peak located at -0.25 V on scan reversal (see curve (a) in Fig. 1), which can be attributed to the Fe II/III redox system.

FIGURE 1

In the presence of H2O2, the intensity of the cathodic peak is increased (see curve (b) in Fig. 1), indicating that the Fe(Htrz)3 is indeed likely to act as a mediator for the electrocatalytic reduction of hydrogen peroxide, by decreasing the overpotential by more than 500 mV with respect to the direct reduction of H2O2 on ITO (compare to curve (c) in Fig. 1). This confirms the possible interest of the Fe(Htrz)3 complex for electrocatalytic purpose.

Characterization of the Fe(Htrz)3-containing mesoporous silica film

Figure 2 confirms the successful incorporation of the coordination polymer in the mesoporous silica film prepared by EASA in the presence of Fe(Htrz)3 in the starting sol. The observed voltammetric signals are well-defined and much more reversible than for the complex in solution (i.e., easier oxidation of Fe II into Fe III ), indicating a stabilizing effect of the silica matrix.

FIGURE 2

Indeed, the negatively-charged silica surface is likely to stabilize the Fe(Htrz)3 complex via electrostatic interactions, which are thought to be at the origin of the self-assembly process (driving force for mesoporous film formation). Interestingly, the reversible signals located at -0.35 V vs. Ag/AgCl remained well-defined up to rather high potential scan rates (above 1 V s -1 ), yet with cathodic-to-anodic peak potential differences increasing at higher speeds, and peak currents were directly proportional to the potential scan rate in the 0.1-4 V s -1 range (see bottom inset in Fig. 2). This indicates fast and surface-confined charge transfer reactions. Compared to the mesoporous silica films grafted with redox-active groups, which are characterized by rather slow electron transport kinetics and voltammetric responses usually controlled by diffusion (i.e., pseudo-diffusion of electrons, resulting in peak currents varying linearly with the square root of scan rate) [29,34], the present system is more efficient thanks to the very close proximity of redox sites in the coordination polymer enabling easy electron hopping. This is consistent with the high density of redox probes in the material (1.610 -4 mol g -1 Fe(Htrz)3 in the film, as estimated from the integration of peak currents in Fig. 2, giving a charge of 27 µC, corresponding to a surface concentration of 1.410 -9 mol cm -2 , and taking into account the electrode area, 0.196 cm 2 , the film diameter, 5 mm, its thickness, 110 nm, and density, 0.8 g cm -3 (as estimated from that of MCM-41 [52])).

Robust and durable voltammetric responses have been obtained (Fig. 3A), even at high scan rate (up to 5 V s -1 ), before and after treatment of the electrode in ethanol/HCl (a medium usually used for surfactant template extraction from mesoporous silica films [25]), and peak currents remained constant upon multiple successive voltammetric scans (data not shown). The good stability and durable immobilization of the Fe(Htrz)3 template into the film was further confirmed by permeability measurements using a redox probe in solution (ferrocene dimethanol, FcMeOH), showing the absence of any noticeable current response neither before nor after treatment in ethanol/HCl (Fig. 3B). This confirms that a crack-free film recovers well the whole ITO surface and that Fe(Htrz)3 species are not removed by solvent extraction (no possibility for the FcMeOH probe to cross the film and reach the electrode surface, contrary to what happened for the same kind of films pepared by EASA using a classical surfactant template [24]). Actually, for such Fe(Htrz)3@SiO2 film, almost no CTAB is incorporated during synthesis and the coordination polymer made of Fe(Htrz)3 species is playing itself the role of template which is strongly entrapped in the mesopore channels [38], explaining why the nanocomposite film is totally impermeable to external redox probes (Fig. 3B) and in that potential window the Fe(Htrz)3 complex is not electrochemically accessible.

FIGURE 3

Figure 4A gathers top SEM views of the film electrode, indicating a uniform morphology, yet with some particles or aggregates on top of the materials (as common for such deposits generated by EASA [24]), which are mostly eliminated after 15 min treatment in stirred solution (HCl/ethanol). In agreement to previous observations [38], TEM micrographs indicate the existence of a quite regular wormlike mesostructured (Fig. 4B) with mesochannels having the tendency to orient vertically (see inset in Fig. 4B) although not so overwhelming as for non-functionalized mesoporous silica films [24,25]. The film thickness is 110 nm, a constant value over the whole film area. FTIR spectra (Fig. 4C) confirm the expected composition of the hybrid film, with typical signatures for silica and the the iron-triazole complex, Fe(Htrz)3 (from the typical vibration bands of =C-H, -N-H, -C=N and -N=N groups [53,54]). Again, no noticeable difference can be observed before and after HCl/ethanol treatment, confirming that the iron-triazole coordination polymer acting as template is strongly embedded in the mesostructure and can be thus used as a durablyimmobilized redox catalyst, as shown hereafter for the amperometric detection of H2O2. Further characterization (XPS, diffraction patterns) can be found in our previous work [38].

FIGURE 4

Electrocatalytic detection of H2O2

Cyclic voltammetry was first used to evidence the effective electrocatalytic properties of the Fe(Htrz)3-based mesoporous silica film towards H2O2 reduction and to evaluate the most appropriate electrode support between two conductive oxides (Fig. 5). As shown, the addition of H2O2 in the medium leads to a noticeable increase of the cathodic signal in both cases, featuring marked electrocatalysis independently on the nature of the support ITO or FTO (fluorine-doped tin oxide). The intensity of the catalytic response is more important on FTO, but the signal is shifted towards a more cathodic value (by ca. 200 mV), as a result of larger overpotential for Fe(Htrz)3 on FTO (Fig. 5A) than on ITO (Fig. 5B). ITO will be thus used in further experiments as lowering overpotentials is often a prerequisite in electrochemical sensing. FIGURE 5 H2O2 detection was then performed by amperometry at constant potential by injecting aliquots of the analyte at increasing concentrations in 0.1 M NaNO3 aqueous solution under constant stirring. The effect of the applied potential on the amperometric response is illustrated in Figure 6A. Too low potential values (e.g., -0.2 V) do not allow reaching steady-state currents (they are decreasing with time as a result of too small interconversion of Fe II/III centers). More cathodic potentials (e.g., -0.4 V) enable to reach more intense and stable current values, proportional to the concentration of H2O2, while more cathodic values have to be avoided for reason of too large overpotentials. The value of -0.4 V is thus chosen in further experiments. In these conditions, a linear current response is obtained for H2O2 in the 5-300 µM concentration range (Fig. 6B), with a sensitivity of 1.9 A mol -1 cm -2 and a detection limit estimated at 2 µM (S/N = 3). The response time is very short (only 3 s to reach >90% of the steady-state currents), thanks to the fast electron transfer kinetics associated to the immobilized Fe(Htrz)3 catalyst (Fig. 2). By comparison, no detectable response can be seen when using bare ITO biased at the same potential of -0.4 V, confirming again the electrocatalytic properties of Fe(Htrz)3 species embedded into the mesoporous silica film. Good operational stability is also observed over prolonged use (hour timescale, see Fig. 6C), which agrees with the highly stable electrochemical response of a nanocomposite film bearing durably the iron-triazole complex in an active form.

FIGURE 6

In addition, the modified electrode was accommodated to an electrochemical flow cell in order to evaluate its possible use for the flow injection analysis (FIA) of H2O2. Typical results are illustrated on Figure 7. After optimization of the operational parameters (injected volume, solution flow rate), good repeatability can be achieved (i.e., 2.7 % standard deviation (n = 9) for the analysis of 30 µM H2O2, see Fig. 7A), along with a correct dynamic response upon successive analyses of increasing-decreasing concentrations (Fig. 7B), indicating the absence of memory effects. A linear calibration curve is obtained in the 5-300 µM H2O2 concentration range (Fig. 7C).

FIGURE 7

Finally, the feasibility of the proposed electrode for practical analysis was evaluated for the determination of H2O2 in a commercially available disinfectant (hydrogen peroxide, Mercurochrome, Juva Santé, Paris). After appropriate dilution (10 000 times) and measuring with the standard addition method, it gave rise to a value of 89  3 µM, consistent with that obtained with using the classical potassium permanganate titration (88  2 µM) [55] thus confirming the accuracy of the present electrochemical method.

Conclusion

In this work, we have successfully immobilized an iron-triazole complex, Fe(Htrz)3, in a mesoporous silica film generated by electrochemically assisted self-assembly. The effective and durable entrapment is ensured by the template role played by such coordination polymer in the course of film deposition, owing to favorable electrostatic interactions between the cationic complex and the silica surface. The encapsulated species exhibit remarkable electrochemical features, with reversible thin-layer behavior above 1 V s -1 , indicating of fast electron transfer processes occurring throughout the insulating silica film. The hybrid nanocomposite film electrode can be applied to the electrocatalytic detection of hydrogen peroxide, constituting the first use of Fe(Htrz)3 as redox mediator. Good sensitivity and operational stability have been achieved in both amperometry and flow injection analysis, thanks to the stable immobilization of Fe(Htrz)3 in an active form into the film onto the electrode surface. When operating at an applied potential of -0.4 V, H2O2 sensing was possible over a dynamic concentration range from 5 to 300 μM.

List of Figures

Fig. 1

 1 Fig. 1. Cyclic voltammograms recorded at 100 mV s -1 on bare ITO electrode in 0.1 M NaNO3 solution (at pH 3.5) containing 5 mM Fe(Htrz)3 (a) alone or (b) in the presence of 5 mM H2O2; (c) curve obtained for 5 mM H2O2 alone. Insert: Fe(Htrz)3.

  Fig. 1. Cyclic voltammograms recorded at 100 mV s -1 on bare ITO electrode in 0.1 M NaNO3 solution (at pH 3.5) containing 5 mM Fe(Htrz)3 (a) alone or (b) in the presence of 5 mM H2O2; (c) curve obtained for 5 mM H2O2 alone. Insert: Fe(Htrz)3.

Fig. 2 .

 2 Fig.2. Cyclic voltammograms recorded at various potential scan rates (0.05-4 V s -1 range and zoom on 0.1-1 V s -1 in top inset) in 0.1 M NaNO3 solution using a mesoporous silica film bearing Fe(Htrz)3 (top illustration); the variation of peak currents with potential scan rate is shown in bottom inset.

Fig. 3 .Fig. 4 .Fig. 5 .

 345 Fig. 3. (A) Cyclic voltammograms recorded at 5 V s -1 in 0.1 M NaNO3 solution using a mesoporous silica film bearing Fe(Htrz)3 (a) after or (b) before treatment in an ethanol solution containing 0.1 M HCl for 15 min. (B) Cyclic voltammograms recorded at 100 mV s -1 in 1 mM Fc(MeOH)2 (+0.1 M NaNO3) aqueous solution using the same electrode as in (A), also (a) after or (b) before treatment in HCl/ethanol; the dotted line (c) is the voltammogram recorded at bare ITO electrode.

Fig. 6 .

 6 Fig. 6. Amperometric responses recorded at ITO electrodes modified with mesoporous silica films bearing Fe(Htrz)3 to the addition of various concentrations of H2O2 (in a stirred aqueous solution of 0.1 M NaNO3); (A) Comparison of responses obtained for increasing H2O2 concentrations at applied potentials of respectively (a) -0.2 V and (b) -0.4 V: (B) comparison of (a) bare ITO and (b) the film electrode to additions (arrows) of 5, 10, 25, 50, 75, 100, 150, 200, 250 and 300 µM H2O2 (calibration data in inset); (C) Amperometric response to 100 µM H2O2 (addition at arrow) recorded for more than one hour.

Fig. 7 .

 7 Fig. 7. FIA responses recorded at ITO electrodes modified with mesoporous silica films bearing Fe(Htrz)3 to the injections of 100 µL aliquots of H2O2 (in an aqueous carrier solution of 0.1 M NaNO3 flowing at 1 mL min -1 ); (A) nine successive injections of 30 µM H2O2; (B) three successive injections of (a) 30, (b) 150 and (c) 300 µM H2O2; (C) Calibration curve.

Acknowledgements

We are grateful to the CNRS, the French PIA project "Lorraine Université d'Excellence" (reference ANR-15-IDEX-04-LUE), and the CPER (program SusChemProc), for financial supports. SA acknowledges a PhD fellow from the Université de Lorraine. We also thank Jaafar Ghanbaja for TEM imaging.