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Electrochimica Acta 94 (2013) 198– 205

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al h om epa ge: www.elsev ier .com/ locate /e lec tac ta

ydroquinone functionalized oriented MCM-41 mesochannelst the electrode surface

ohammad Rafieea,∗, Babak Karimia,∗, Samaneh Farrokhzadeha, Hojatollah Valib

Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 45137-6731, Gava Zang, Zanjan, IranDepartment of Earth & Planetary Sciences, McGill University, 3450 University St., Montreal, Quebec, H3A 2A7, Canada

r t i c l e i n f o

rticle history:eceived 2 December 2012eceived in revised form 17 January 2013ccepted 28 January 2013

a b s t r a c t

The ordered mesoporous silica (MCM-41) thin film has been electrodeposited at the electrode surface bythe method known as electrochemically assisted self assembly (EASA). The constructed structures withunique morphology have been functionalized with hydroquinone derivatives as an electron mediator.Three functionalization routes, one co-condensation and two grafting methods, have been examined for

vailable online 4 February 2013

eywords:esoporous silicaodified electrodeuinone/hydroquinonelectron propagation

the attachment of hydroquinone derivative to the electrode surface. For all electrodes the electroactivegroup immobilized successfully on deposited MCM-41 thin film and all of the constructed modifiedelectrodes show the adequate and relatively stable currents. The electrochemical behaviors of theseelectroactive porous electrodes and their performances in electrocatalytic oxidation of benzyl alcoholand hydrazine have been studied by electrochemical methods.

© 2013 Elsevier Ltd. All rights reserved.

. Introduction

The past thirty years have seen increasingly rapid advancesn the field of chemically modified electrodes (CME). The distin-uishing feature of CMEs is to endow the electrodes with thelectrical, chemical, electrochemical and other desirable properties1]. Among the CMEs the redox-active modified electrodes are theubject of much interest in the field of electrocatalysis and sensing2]. There are two possible main routes for the electrode modifica-ion with the electroactive groups; (a) the attachment or adsorptionf monolayer of redox active species on electrode surface andb) covering the electrode surface by multilayer redox containingrganic or inorganic films [3]. The use of monolayer modified elec-rodes suffers from limited number of mediator centers presentt the electrode surface and rapid loss of electrocatalytic activityue to possible loss of the mediator. In contrary, the use of multi-

ayer modified electrodes has the advantage that they compriseore mediator sites present at the electrode surface. However,

he major problems with this kind of application are more com-licated electron transfer and limitation of mass and/or coupled

harge transfer [4]. A considerable amount of literatures haveeen published on electron propagation through the films [5]. Onhe other hands the concept of nanoporous electrode with high

∗ Corresponding authors. Tel.: +98 2414153125; fax: +98 241 4153232.E-mail addresses: rafiee@iasbs.ac.ir, moh rafiee@yahoo.com (M. Rafiee),

arimi@iasbs.ac.ir (B. Karimi).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.01.147

surface area has attracted significant scientific attention due totheir diverse potential applications in attaining maximum elec-trode/electrolyte interface [6]. Many efforts have been made toimprove the microstructures and morphologies of electrode mate-rials and numerous porous materials have recently been introducedin the field of electrochemistry [6–8]. Among the porous mate-rials with high specific surface area, ordered mesoporous silicastructures are of particular importance because of possibility tointroduce a broad range of chemical functionalities within theirframeworks [9–12]. The most important method of electrode mod-ification with silica-based materials is the deposition of thin films ofprecursor by spin or deep coating, which often followed by evapora-tion induced self assembly (EISA) [9–12]. Although there are severalremarkable reports on the mechanism of electron transfer throughthe films there are still two challenges in this field. The first is thediffusion of substrate considering channel direction and second oneis the electron transfer due to insulating character of silica [13,14].The problem of substrate diffusion and channel direction has beenresolved interestingly by the outstanding work of Walcarius and hiscoworkers via electrochemical generation of ordered mesoporousfilm having hexagonally packed channel (MCM-41) perpendicularto the electrode surface and parallel to each other [10]. They havealso provided the quantitative studies on the mechanism of masstransfer through the designed channels [15–17]. Despite the unique

morphology, electrode accessibility and uniform thickness of thiselectrochemically derived mesoporous films, their impact as thesupport for electroactive groups at the electrode surface has beenunexplored to date. Furthermore it seems that the study of electron

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M. Rafiee et al. / Electroch

ransfer for supported electroactive groups on this structure mayonstitute a further step forward in designing novel sophisticatedystems.

. Experimental

.1. Chemicals and reagents

Tetraethoxysilane (TEOS, 99% Merck), (3-minopropyl)triethoxysilane (APTS 99% Sigma–Aldrich) ethanol96%, Merck), sodium nitrate (99%, Merck), hydrochloric acid36%, Prolabo) and cetyltrimethylammonium bromide (99%TAB, Merck), were used as received for films synthesis.,5-Dihydroxybenzoic acid (DHBA, Merck) and Hexaam-ineruthenium(III) chloride, (Ru(NH3)6Cl3, 98%, Sigma–Aldrich)ere used for electrochemical studies in solution phase.-Hydroxysuccinimide (NHS, 98% Sigma–Aldrich) N,N′-icyclohexylcarbodiimide (DCC, Sigma–Aldrich) and dryetrahydrofuran (THF) were used in preparation of silica pre-ursor. The mineral salts which have been used in preparation ofuffers were analytical grade. All aqueous solutions were preparedith high purity water from a Millipore milli-Q water purification

ystem.

.2. Apparatus

All electrochemical experiments performed using an Autolabotentiostat/galvanostat 101. Two types of Glassy Carbon (disk andlates) and a platinum electrode were used as working electrodes.he plate GC electrodes were used for film preparation and char-cterization. A Pt wire was used as a counter electrode and theorking electrode potentials were measured versus an Ag/AgCl

eference electrode (all electrodes from Azar Electrode). The pHf aqueous solution were measured and fixed using a Metrohm pHeter and the sol solution pH adjustment was performed usingacherey–Nagel indicator sticks. The synthetic products and the

lm characterization were performed using the following instru-ents: Elementar Analysen System GmbH-vario EL III Elementnalyzer for elemental analysis (CHN); TGA-PL 1500 for Thermalravimetric Analysis (TGA); Philips CM20 Microscope an Accelera-

ion Voltage of 200KV for Transmission Electron Microscopy (TEM),ruker Cector 22 FT-IR Spectrometer for Fourier transform infraredpectroscopy (FTIR) and Brucker 400 MHz for nucleic magnetic res-nance.

.3. Preparation of hydroquinone containing precursors

DHBA is activated by treatment with NHS in the presence ofCC in dry THF according to the reported procedure [18]. A mix-

ure of DHBA (1.54 g, 10 mmol), NHS (1.15 g, 10 mmol) and DCC2.06 g, 10 mmol) in 75 mL of dry THF was stirred under Ar at roomemperature for 54 h. The precipitated byproduct (is believed toe urea derivative of DCC) was removed by filtration. The solvent

n the yellow filtrate was partly evaporated under reduced pres-ure, and the resulting viscous liquid residue (containing smallrystals) was placed in refrigerator overnight and the obtained crys-als were washed with cold THF to obtain the pure activated ester,,2-dioxopyrrolidin-1-yl 2,2-dihydroxybenzoate (1), with 75% per-ent yield. 1 (0.251 g, 1 mmol) was added to a solution of APTS230 �l, 1 mmol) in 30 mL of dry THF [19]. The solution was stirrednder Ar at room temperature for 72 h. The solvent was evapo-ated partly under reduced pressure, and the residue was used as

btained (mixture of THF and desired product, 2,5-dihydroxy-N-(3-triethoxysilyl)propyl)benzamide which is denoted as 2). The ratiof precursor over THF was obtained by the integration of NMR peaksFigs. 3S–6S supplementary information].

Acta 94 (2013) 198– 205 199

1 (2,2-dioxopyrrolidin-1-yl 2,2-dihydroxybenzoate): 1H NMR(400 MHz, acetone D6): ıH = 3.00 (s, 4H), 7.01 (d, 1H), 7.26 (d-d, 1H),7.42 (s, 1H), 8.62 (Broad) and 9.16 (s, 1H); 13C NMR (100 MHz, ace-tone D6): ıC = 26.0, 110.8, 115.7, 119.4, 125.2, 149.9, 153.1, 161.6,171.0.

2 (2,5-dihydroxy-N-(3-(triethoxysilyl)propyl)benzamide): 1HNMR (400 MHz, acetone D6): ıH = 0.59 (t, 2H), 1,15 (t, 9H) 1.61(m, 2H), 3.26 (t, 2H), 3.77 (q, 6H), 6.72 (d, 1H), 6.85 (d–d, 1H),7.24 (s, 1H), 8.71 (s, 1H), 8.91 (s, 1H) and 10.96 (s, 1H); 13C NMR(100 MHz, acetone D6): ıC = 7.8, 18.6, 22.9, 42.1, 58.1, 113.6, 115.0,118.2, 121.6, 149.6, 153.0 and 169.0.

2.4. Initial sol preparation

A typical mixture consist of 9.1 mmol TEOS, 0.85 mmol APTS or2 (the ratio of functional group containing silica precursor shouldbe less than 10% over TEOS) 15 ml ethanol, 15 ml aqueous solutionof 0.1 M NaNO3, 1.0 mM HCl and 3.26 mmol cetyltrimethyl ammo-nium bromide (CTAB) were used for sol preparation. 10 mmol ofTEOS only, instead of mixture of silica precursors, was used toprepare pure MCM-41. It should be noted that CTAB and APTSshould be added under stirring and vigorous stirring respectively[16]. The sol was aged under stirring for 2.5 h at pH 3 prior toelectrodeposition.

2.5. Electro-assisted generation of the oriented MCM-41 thinfilms on glassy carbon electrodes

Oriented MCM-41 thin films were deposited under galvano-static conditions from the sol solutions. The electrode wasimmersed in the above mentioned precursor solutions and elec-trodeposition was achieved by applying a suitable cathodic current(−1.9 mA cm−2) for a defined period of time (typically 20 s). Theelectrode was rapidly removed from the solution and immediatelyrinsed with distilled water. The electrodeposited surfactant-templated film was then dried overnight in an oven at 130 ◦C.Extraction of the surfactant template was carried out in acidicethanol solutions under moderate stirring for 10 min [15,16].

2.6. Functionalization of ordered mesoporous silica modifiedelectrode

Hydroquinone derivatives were loaded into electrodepositedMCM-41 films at the electrode surface by incipient-wetnessimpregnation, which was performed by immersing of the unfunc-tionalized modified electrode in dry toluene solution containingvarious concentrations (5–200 mM) of dissolved 2 [20]. The dura-tions of immersions were 10–15 min. The electrodes were removedfrom the solution and dried in an oven at 100 ◦C overnight.

Hydroquinone groups were also loaded on electrodepositedaminopropyl containing MCM-41thin films via amide formationof 1 and aminopropyl groups. In a typical experiment the APTSmodified electrode was immersed in a dry THF solution (25 ml)containing 1 mmol of 1 and the solution was stirred under Ar atroom temperature for 30 h.

3. Result and discussion

3.1. Silica films formation and functionalization

Preparation of all MCM-41 thin films on glassy carbon (GC)

and platinum (Pt) electrodes was achieved by electro assisted selfassembly (EASA) in the presence of CTAB as structure directingagent and appropriate silica precursor composition under reductivegalvanostatic conditions [10]. The parent MCM-41 thin film was

200 M. Rafiee et al. / Electrochimica Acta 94 (2013) 198– 205

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eposited from a solution containing only TEOS as silica precursor.he aminopropyl functionalized MCM-41 thin film (APS) with theaximum APTS to TEOS ratio (10% APTS/TEOS in sol solution) was

lso deposited following the reported EASA procedure with slightodification [16]. The structure and composition of scratched films,

fter surfactant extraction, have been approved by transmissionlectron microscopy (TEM) and thermogravimetric analysis (TGA),espectively. The performance of film deposition was examined byoltammetric study of Fe(CN)6

3− and Ru(NH3)63+ as electroactive

harged probes. As expected, voltammetric currents and electrodeccessibility for both APS and MCM-41 films were negligible sinceost of the channels are blocked with the template molecules.owever a drastic increase in both voltammetric currents and thelectrode accessibility was observed upon extraction of CTAB. Itas found that Fe(CN)6

3− exhibit higher electrode accessibilityhan Ru(NH3)6

3+ through the electrode modified with APS films atH 4.5, whereas an opposite trend was observed by employing thelectrode coated with MCM-41 films under essentially the sameonditions. This behavior can be related to electrostatic interac-ions of these charged species to positively and negatively chargedmmonium and silanol groups at the channel walls in two electrodeystems, respectively [16].

In the next stage, two grafting methods have been separatelysed to further functionalize either MCM-41 or APS modified elec-rodes with the electro-active hydroquinone group. All of the

esired precursors have the hydroquinone group (Scheme 1) ashe electroactive moiety; therefore we use this expression for allf the synthesized and anchored groups. The MCM-41 modified

1.

electrode from the previous step was allowed to react with 2via incipient-wetness impregnation to give the correspondinghydroquinone functionalize electrode which was donated as SGAH(Scheme 1) [20]. Thereafter, the impact of concentration of 2 inthe functionalization solution on the electrochemical performanceof the resulting SGAH was examined. Although, the voltammetriccurrents of the SGAH electrode increase by increasing the concen-tration of 2 to 0.01 M, by further increase, the electrode responsedecrease and completely suppressed for the SGAH electrode thatprepared using 0.1 M of 2. This clearly shows that by employ-ing high concentration of 2 in the functionalization solution asignificant channel blockage can be most likely occurred. The max-imum functionalization and responses were attained using 0.01 M2 in functionalization solution. The amide formation (Scheme 1)between the activated ester of DHBA, 1, and the amine groupof previously co-condensed APS modified electrode (denoted asAGSH) was the second route for the functionalization of the desiredfilm and electrode surface [19]. Once again voltammetric analysesconfirm the presence of electroactive hydroquinone groups withadequate loading and comparable responses to those obtained bySGAH.

Finally EASA has been combined with co-condensation methodin the presence of hydroquinone functionalized alkyltriethoxysi-lane (2) as an electroactive precursor. Fig. 1 shows the cyclicvoltamograms of the modified electrode with the film that obtained

by EASA from a sol solution containing 2 and TEOS (denoted asCAPH), the molar ratio of 2 over TEOS was 0.09. Interestinglythe constructed modified electrode, CAPH in a blank supporting

M. Rafiee et al. / Electrochimica Acta 94 (2013) 198– 205 201

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ig. 1. (a) The CV of 1.0 mM DHBA in phosphate buffer solution on GC electrode,b) the CV of CAPH before and (c) after extraction of surfactant in blank phosphateuffer solution at pH 7.0, scan rates 100 mV s−1.

lectrolyte, shows one anodic and corresponding cathodic peakith the mid-peak potential (Emid) similar to dissolved DHBA,

mid = 0.17 V vs. Ag/AgCl, and �Ep = 0.12 mV before the templatextraction.

The electrode response and the difference of anodic and cathodiceak potentials improve considerably after surfactant extrac-ion, Emid = 0.16 V vs. Ag/AgCl and �Ep = 0.04 V. These observationspprove the incorporation of hydroquinone groups in the channelsf ordered mesoporous structure. It should be noted that the pres-nce of 2 with the relatively large functional group may be lead toisordered structures but fortunately the constructed CAPH films athe electrodes show well-ordered porous structure from the solu-ion containing less than 9% of 2 to TEOS. Fig. 2 displays some TEMmages of the film that prepared with 8.5% 2 to TEOS in solution onC electrodes and scratched before TEM analysis. Top view imagesf films clearly show the hexagonal packing of channels in film sames MCM-41 and APS films [Figs. 1S and 2S supplementary informa-ion].

The amounts of the incorporated hydorquinone moieties intohe framework of constructed electrodes was determined by ele-

ental analysis, which showed a good agreement with the loading

ig. 2. TEM (top view) images and corresponding FFT image of scratched CAPH silicahin film.

Fig. 3. Cyclic voltammograms of 1.0 mM Ru(NH3)6 in acetate buffer solution atpH 4.5 on (a) modified electrode with unfunctionalized ordered mesoporous silicafilm, (b) covered by CAPH, (c) AGSH and (d) SGAH silica films, scan rate: 100 mV s−1.

of organic groups estimated from thermogravimetric analysis(TGA) data. For example the weight loss, due to the breakdownof organic groups anchored on the MCM-41 thin films surface, isabout 19.2% [Figs. 7S, 8S and Table 1S supplementary information].Considering the well documented electrochemical characteristicsof hydroquinone groups, in the next stage, the performance ofchemical modifications and also the quality and reliability of theelectrodes were further studied in details by employing variouselectrochemical techniques [21].

3.2. Voltammetric study and electrochemical performance

One of the most challenging tasks in the multilayer modifiedelectrode concept is to explain and clarify the actual mechanismof electron transfer and charge transport processes, which areoperated through the electrochemically active films. However thecomplex nature of chemically modified porous as well as non-porous electrodes especially in amorphous film systems hindersthe easy elucidation of electrochemical behavior of the systemby employing conventional methods [5]. In this regards it seemsthat uniform and easy accessible structures of the aligned chan-nel in modified electrodes comprising an appropriate electroactivegroups provide a much simpler and reliable method to gaininsight into the better understanding of electron-charge transferphenomena at the surface of film modified electrodes. The voltam-metric studies of dissolved electroactive probes were applied againfor elucidation of electrode accessibility. Fig. 3 shows the cyclicvoltammograms of Ru(NH3)6

3+ at the surface of hydroquinonefunctionalized ordered mesoporous silica electrodes consideringthis fact that the signal of Ru(NH3)6

3+ does not have any overlapwith signal of the incorporated hydoquinone groups.

Our preliminary studies showed that the functionalized MCM-41 thin films modified electrodes displayed considerable electrodeaccessibility for the dissolved Ru(NH3)6

3+ and thus well-definedelectrode responses (A0 and C0 peaks) were observed. This observa-tion may be explained by the fact that the partial filling of pore sizeby pendant organic groups restricting the diffusion of electroactiveprobes into the mesochannels. The order of electrodes responses

are CAPH > AGSH > SGAH and the electrochemical responses of allelectrodes are less than those obtained with unfunctionalizedMCM-41 modified electrode. The much higher electrode of CAPHin comparison with those of AGSH and SGAH might also be due

202 M. Rafiee et al. / Electrochimica

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ig. 4. The CV of GC electrode covered by (a) CAPH, (b) AGSH and (c) SGAH in blankhosphate buffer solution at pH 7, scan rate 100 mV s−1.

he better distribution of electroactive groups with avoiding thehannel blockage in this electrode.

Voltammetric behavior of incorporated hydroquinone groupsnd the electrochemical responses of the electrodes were also stud-ed in more details. Fig. 4 demonstrates the voltammetric responsesf incorporated functional groups on porous silica structures in alank phosphate buffer solution at pH 7.

As can be clearly seen, the Emid of all incorporated hydroquinonere relatively the same to each other and provide a good adoptionith the Emid of dissolved DHBA. The order of mid-peak potential

re CAPH (0.16) < AGSH (0.19) < SGAH (0.20) which suggest that thelectrode with higher voltammetric responses undergoes electronransfer at lower overvoltage. The difference of anodic to cathodiceak potentials (�Ep) has also the same order and �Ep of CAPH0.04 V) is less than AGSH (0.15 mV) and SGAH (0.25 mV) at theame scan rate, 100 mV s−1.

The voltammetric studies of all modified electrodes havextended at various scan rates. The amounts of increase in the

aradaic currents are found to scale with the square root of the scanate, which is expected for a diffusion-controlled electrode process.nly the currents of CAPH electrode at very low scan rates deviate

ig. 5. The plot of anodic peak currents of CVs (a) versus scan rates, (b) versus square rocan rates, are shown on each curve, for CAPH in blank phosphate buffer solution at pH 7

Acta 94 (2013) 198– 205

and show a linear dependence to the scan rate (Fig. 5). To determinewhether the peaks are depend on diffusion in solution or not, somediagnostic criteria were employed [22]. The diffusion like behaviorof the electrode responses may be due to the presence of coupledelectron-proton transfer of hydroquinone moiety. Therefore, theeffects of solution pH in well-buffered and acidic solutions havebeen examined. The linear dependence of Emid and pH was similarto that of dissolved DHBA, a line with 59 mV/pH slope whereas thepeak current remained constant within the entire pH range. Hydro-dynamic voltammetry (RDE) have also been used and the obtainedvoltammograms of all constructed electrodes show the peaks, nota transport limited current plateau, whereas the peak currents areindependent of the electrode rotation rate [22].

Based on these results, the fast transportation of proton throughthe film is conceivable because of porous structure of film and elec-trode accessibility and thus rate determining step must be electrondiffusion process within the film. Moreover, the charge calculatedfrom the area enclosed the voltammograms of CAPH electrode wereplotted versus scan rate (Fig. 5c). The less consumption of chargeat higher scan rates, less experimental time scale, prove that theelectron transfer of anchored electroactive groups through the filmis a time dependent process and the whole groups do not undergoelectron transfer at high scan rates.

The loadings (in mol cm−2) of anchored redox centers wereestimated upon determination of charges under the system’svoltammetric peaks (oxidation) recorded at minimum scan rate,10 mV s−1. The charge under the hydroquinone peak shows theloading of 2.7 × 10−9 mol cm−2 in the film. There is also a quantita-tive description for the diffusion-like electron propagation throughthe film. The TEM analyses show that there are approximately76,000 channels per each �m2 with an average of 100 nm height[16,19]. Therefore, it is reasonable to speculate that each chan-nel has approximately the surface area of about 942 nm2 and 210anchored hydroquinone groups. Based on the above-mentionedcalculation, the average linear distance of grafted hydroquinoneson the surface of silica mesochannels has been estimated to bearound 2 nm; percolation of electrons through such a layer maynot be expected to be rapid and thus the diffusion-like behav-ior is related to diffusion of electron through the film [5]. In this

on Scholz–Lovric [23,24], formed by: (i) electrode surface at theend of each channel, (ii) the electrolyte, and (iii) the walls of silicachannels containing localized hydroquinone groups (Fig. 6). The

ot of scan rates and (c) the consumed charge for anodic currents of CVs at various.

M. Rafiee et al. / Electrochimica

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studied by cyclic voltammetry. The cyclic voltammograms of all

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ig. 6. Proposed model for the electron propagation through the aligned silicaesochannels.

lectron propagation take place only in z direction and charge-alancing proton can enter simply in the aligned channels [25].he flux of protons does not cause diffusion controlled limitation

n these aligned and accessible channels. There is also no flux inhannel walls because electron transfer proceeds only by anchoredroup on silica walls which are in contact with electrolyte solution.

ig. 7. The CVs of (a) Pt and (c) Pt-CAPH electrodes in the absence and (b) and (d) presenn phosphate buffer solution (e) in the absence and (f) presence of 1.0 mM Hydrazine, (g)

Acta 94 (2013) 198– 205 203

Among the grafted hydroquinone groups, those in close proximityof the electrode surface are able to initially participate in electrontransfer and therefore immediately oxidized as soon as the elec-trode were subjected to an external applied potential. The electrontransfer process was then proceeded through the successive elec-tron transfer from the next redox sites to these oxidized sites andthus to the electrode. As long as redox sites are still accessible andthe external potential is applied, electron hopping takes place in a“layer-by-layer” fashion to the electrode surface.

The pseudo diffusion coefficient of electrons, De, depends onthe electron rate transfer constant inside the material (k), the aver-age distance between redox sites (d) and the concentration of theredox probe in the film. c is an important parameter because itdirectly influences the other two parameters [5,22]. This modelhas been well-elaborated for a homogenous dispersion of redoxsites in mesoporous structures. The diffusion coefficient of electronthrough the film was obtained, according to the above discussedmodel, 6.1 × 10−10 cm2 s−1 and 8.9 × 10−11 cm2 s−1 for the surfaceconcentrations of 2.7 × 10−9 mol cm−2 and 1.9 × 10−9 mol cm−2

respectively [4,5,22–25]. With a rough estimation the diffusioncoefficient of electron transfer for AGSH and SGAH were alsoobtained 2.1 × 10−11 cm2 s−1 and 1.4 × 10−11 cm2 s−1 respectively.Unfortunately, owing to partially blockage of pore spaces by DHBAgroup, significant disordering in the pore structure of MCM-41 thinfilms and also synthetic limitations arising from the size of pendantorganic group; the use of higher concentration of 1 was actuallyimpossible. However, our electrochemical studies show that therewas a decrease in the rate of electron diffusion in the film, the extentof which was found to be dependent of the loading of redox centersin the modified films. Furthermore, within the relatively same load-ing window, among the described modified electrodes the AGSHand SGAH which prepared by means of grafting method resulted ineven slower diffusion than CAPH in which the redox groups wereuniformly distributed through the entire pore surface instead of inthe proximity of the mesochannel entrance.

Likewise the stability of the electrodes toward leaching was

modified electrodes recorded over time in a blank phosphatebuffered solution. The signal of SGAH modified electrode decreasesat the first hour to 60% of the initial values and thereafter shows

ce of 0.01 M BA in aqueous sodium acetate solution. The CVs of GC-CAPH electrodeCV of hydrazine at the surface of unmodified GC electrode, scan rate 25 mV s−1.

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airly stable currents over the day. The initial current fall may beue to leaching of weakly bonded or adsorbed electroactive silicarecursor. CAPH electrodes currents show no consistent decreaseut a slight and continuous decrease was observed for all the day.or example the current loose for both anodic and cathodic peaks isess than 12% after 5 h remaining in buffer solution and successiveotential scans. Finally it is also worth mentioning that the peakurrent only decreased by 3.7% of the initial value after 3 days forGSH electrode.

.3. Electrocatalytic activity of the electrodes

The quinone/hydroquinone redox couple, their electrochemicalnd electrocatalytic behavior are quite known in electrochem-stry [26–28]. Furthermore the above results confirm that theonstructed modified electrodes show very good electrochemicalesponses and solution accessibility as well. Therefore in the nexttage we were interested to study the performance of the desiredlectrodes for some electrocatalytic transformations. Fig. 7 showshe voltammetric responses of CAPH electrode in the absence andresence of hydrazine. Considerable increase of oxidative peakA1) current together with the decrease of the reductive peak (C1)urrents that appear in the presence of hydrazine, in phosphateuffer solution pH 7, is the characteristic of electrocatalytic or EC′

echanism. The diagnostic criteria of cyclic voltammograms at var-ous scan rates and concentrations of hydrazine support the EC′

echanism and electrocatalytic ability of these electrodes towardhe oxidation of hydrazine [19].

The quinone derivatives have been also used as mediator inome organic transformations and oxidation reactions. The cat-lytic ability of the incorporated DHBA as hydroquinone derivativeor the oxidation of alcohols was another subject of this study. Theoltammetric study of the functionalized MCM-41 thin films at theurface of glassy carbon electrode does not show an acceptableatalytic ability. All of the catalytic transformations that quinonect as mediator in them have been reported in the presence someo-catalysts [28]. A wide range of metallic, organometallic andnorganic co-catalysts are of interest and have been studied inhe presence of hydroquinone derivatives for the oxidation oflcohols [29]. Since the EASA can be simply applied on platinumlectrode; the catalytic activity of Pt as electrode material wastudied in this electrocatalytic system [16]. Interestingly the Ptodified electrode shows a catalytic current at the more positive

otentials than 0.3 V, vs. Ag/AgCl, in an acetate solution contain-ng benzylalcohol (BA). Electrocatalytic currents increase linearly

ith the rise of BA concentrations whereas the same voltammetricehavior was not observed at the surface of unmodified Pt elec-rode. These observations suggest electrocatalytic possibility forhe oxidation of BA also at the surface of constructed modifiedlectrodes.

. Conclusions

We have demonstrated that oriented MCM-41 thin film asn appropriate matrix for immobilization of discrete electroac-ive moieties yielding uniform thin film electrodes with definedexture and enhanced electrochemical activity. The electroactiveydroquinone groups anchored in silica structure with adequate

oading, appropriate electrochemical signal and good leaching sta-ility. The modified electrode which covered by thin film of silicaas also a good solution accessibility. The amounts of molecules

nvolved in the electron-transfer sequence depend on total amountf embedded guests, and potential sweeping rate. Electron hoppingan be proposed as a possible contribution to the mechanism ofharge propagation through the insulating silica matrix. The good

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[

Acta 94 (2013) 198– 205

electrochemical responses and ease of access to solution render theconstructed electrodes an appropriate catalytic activity.

Acknowledgements

This work was supported by the IASBS Research Council and IranNational Science Foundation (INSF).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.electacta.2013.01.147.

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