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aDepartment of Chemistry, Institute for Adv

P.O. Box 45137-66731, Gava Zang, Zanja

+98 241 4153232bDepartment of Earth & Planetary Sciences

Montreal, Quebec, H3A 2A7, Canada

† Electronic supplementary informatithermogravimetric and elemental analyslms and some optimizations. See DOI: 1

Cite this: Analyst, 2013, 138, 1740

Received 14th September 2012Accepted 23rd December 2012

DOI: 10.1039/c2an36325a

www.rsc.org/analyst

1740 | Analyst, 2013, 138, 1740–174

Electrochemical fabrication of electroactive orderedmesoporous electrode†

Mohammad Rafiee,*a Babak Karimi,a Yousef Abdossalami Asla and Hojatollah Valib

A novel and simple method for the electrochemical modification of ordered mesoporous silica is described.

A well-organized thin film of amine-functionalized ordered mesoporous silica has been deposited

electrochemically on the electrode surface. The resulting amine-functionalized electrodes were then

subjected to post-functionalization with catechol moieties through electrochemical generation of

reactive o-quinone followed by covalent bonding to the anchored amine groups inside the mesoporous

channels of silica to afford the corresponding modified electrodes bearing aminocatechol electroactive

groups. This simply obtained nanoporous modified electrode with adequate loading of electroactive

groups shows very good electrochemical responses.

Introduction

This year MCM-41 as the rst ordered mesoporous silicastructure achieves its 20th anniversary.1 Over the past twodecades, MCM-41 and other ordered mesoporous silica struc-tures have received considerable attention in the eld ofmaterial science and there is no exaggeration to say that theyhave become one of the pillars of this eld. Among severalimportant and interesting properties, their high specic surfacearea and pore volume render them attractive in versatileapplications in surface-related sciences such as catalysis,separation, electrochemistry and analytical chemistry over thistime period.2 The possibility to functionalize their surface OHgroups using either appropriately ne-tuned organic functionalgroups or commercially available groups is another signicantfeature of these materials that has been widely used to tailortheir properties toward specic purposes.3 However, function-alization of the mesoporous material involves multiple steps,which is a time-consuming process and can sometimes evendamage the ordered structure and textural properties of theoriginal materials.4 Therefore, despite considerable successesin this area, surface functionalization of ordered mesoporoussilica is the subject of vast investigations in order to attainsimpler, faster and more efficient methods.5

Furthermore, porous materials are among the most attrac-tive concepts for electrochemists in attaining the maximum

anced Studies in Basic Sciences (IASBS),

n, Iran. E-mail: raee@iasbs.ac.ir; Fax:

, McGill University, 3450 University St.,

on (ESI) available: IR spectrum,is and more TEM images of modied0.1039/c2an36325a

4

electrode/electrolyte interface.6 Along this line, many effortshave been directed towards the application of ordered meso-porous silica structures for electrode modication.7 But due tothe insulating character of silica materials, the electroactivefunctional groups are the main objectives of these applica-tions.8 Electrochemical techniques are mutually used in thestudy, characterization and even construction of porous mate-rials. One of the good examples of these studies is the electro-chemical (cathodic) generation of hydroxide ion which serves asa catalyst and is a necessary requirement for the formation ofhighly ordered mesoporous silica lms.9 Recently, Walcariusand co-workers reported the synthesis of an ordered meso-porous silica (MCM-41) thin lm with hexagonally packedchannels parallel to each other and perpendicular to the elec-trode surface, through an outstanding electrochemical co-condensation approach.10 They have also shown that thesealigned channels are accessible to the electrode surface andappropriate for electrochemical applications. Later on, theelectrochemical generation of methyl- and amine-functional-ized ordered mesoporous structures having similar morphologywas also successfully achieved using the same protocol.11

However, to the best of our knowledge there is currently noreport of employing the electrochemical methods in a domino-oxidation post-functionalization of ordered mesoporous mate-rials and/or modied electrodes. Herein we wish to explore thisnovel approach for the electrochemical functionalization ofordered mesoporous structure with electroactive groups at theelectrode surface.

ExperimentalChemicals

Tetraethoxysilane (TEOS, 99%), (3-aminopropyl)triethoxysilane(APTES 99%), cetyltrimethylammonium bromide (CTAB, 99%),

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HCl (36%, Merck), potassium nitrate and other salts that havebeen used for preparation of buffered solutions were reagentgrade from Merck and were used as received. Ethanol (95–96%,Pars alcohol) and catechol (98%, Aldrich) were used withoutfurther purication. All aqueous solutions were prepared withdistilled water (Aquatron A8000). The stock solutions of cate-chol were prepared daily.

Apparatus

All electrochemical experiments were performed using anAutolab potentiostat/galvanostat 101. Two types of glassycarbon electrode (disk and plate) were used as working elec-trodes. The plate GC electrodes were used for lm preparationand characterization. A Pt wire was used as a counter electrodeand the working electrode potentials were measured versus anAg/AgCl (KCl 3.0 M) reference electrode (all electrodes from AzarElectrode). The pH values of aqueous solutions were measuredand xed using a Metrohm pH meter and the sol solution pHadjustment was performed using Macherey-Nagel indicatorsticks. Also the lm characterization was performed using thefollowing instruments: Elementar Analysen System GmbH-varioEL Element Analyzer for elemental analysis (CHN); TGA-PL 1500for Thermal Gravimetric Analysis (TGA); Philips CM20 Micro-scope an acceleration voltage of 200 kV for Transmission Elec-tron Microscopy (TEM) and Bruker Vector 22 FT-IRSpectrometer for Fourier transform infrared spectroscopy(FTIR).

Pre-treatment of glassy carbon (GC) electrodes

The glassy carbon electrodes were polished using 0.3 and 0.05mm alumina slurry on wool polishing cloth and were rinsedcopiously thereaer. Before deposition of the silica structurethe activation of the glassy carbon electrode, in order to attainthe better adhesion of lm, was achieved by electrochemicalpre-treatment. A polished and clean GC electrode wasimmersed into 0.1 M phosphate buffer solution (pH ¼ 7.0) andits potential was held for 120 s at +1.55 V followed by 120 s at�1.55 V, and nally the potential was cycled between �1 V and+1 V (all of them vs. Ag/AgCl reference electrode) until a repro-ducible background cyclic voltammogram was obtained.

Initial sol preparation

A typical sol mixture consisted of 9.1 mmol tetraethoxysilane(TEOS), 0.85 mmol (3-aminopropyl)triethoxysilane (APTS),15 ml ethanol, 15 ml of an aqueous solution of 0.1 M NaNO3,and 1 mM HCl and 3.26 mmol cetyltrimethylammoniumbromide (CTAB). It should be noted that CTAB and APTS shouldbe added under stirring and vigorous stirring respectively.11 Thesol was aged under stirring for 2.5 h at pH 3 prior toelectrodeposition.

Electro-assisted generation of the organo-silica lms on glassycarbon electrodes

Mesoporous silica lms were deposited under galvanostaticconditions on the GC electrode from a precursor solution. The

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electrode was immersed in the precursor solution and electro-deposition was achieved by applying a suitable cathodic current(�1.9 mA cm�2 on the glassy carbon disk electrode) for adened period of time (typically 20 s). The electrode was rapidlyremoved from the solution and immediately rinsed withdistilled water. The electrodeposited surfactant-templated lmwas then dried overnight in an oven at 130 �C. Extraction of thesurfactant template was carried out in both aqueous andethanol solutions under moderate stirring for 10 min.10,11

Electrochemical functionalization of ordered mesoporoussilica-modied electrode

The electrochemical functionalization was performed by elec-trochemical oxidation of buffered solutions containing 1.0–10.0mM of catechol on an aminopropyl-functionalized silica-modied glassy carbon electrode (AFS-GC) under both poten-tiostatic (single and double steps) and potentiodynamicconditions. The 0.1 M buffered solutions were prepared basedon Kolthoff tables.12 The time of the potentiostatic experimentsand the numbers of cycles have been optimized for the desiredelectrochemical steps respectively in achieving the maximumelectrode responses. Further details of the optimization exper-iments have been described in the ESI.†13

Results and discussions

Preparation of amine-functionalized thin lms was achieved byemploying electro-assisted self-assembly (EASA) under galva-nostatic conditions on glassy carbon (GC) electrodes in thepresence of CTAB as structure-directing agent following thereported procedure.10,11 The GC electrodes had been pre-treatedelectrochemically to ensure better adhesion of the silica lm ontheir surfaces. The electrochemical techniques and APTS toTEOS ratio were optimized for the preparation of AFS-GC. Aerseveral screenings the maximum loading of functional aminegroups in the structure can be attained using 10% APS/TEOS insol solution, a feature that has also been highlighted previ-ously.11 In the next stage, we were very interested to investigatethe possibility of using a typical electrode reaction to furthermodify the resulting aminopropyl-functionalized electrode withan appropriate electroactive precursor. Highly efficient Michaeladdition of –NH2 groups to the electrochemically generatedo-quinone in situ has been well-applied in the selective synthesisof aminocatechol derivatives.14 We reasoned that by choosingan appropriate reaction condition, it might be possible toemploy the same approach to build a novel aminocatechol-functionalized mesoporous silica-based electrode. To validateour hypothesis, we carried out the electrochemical oxidation ofan aqueous solution of catechol using our AFS-GC.

The lm permeability and surface coverage were rstexamined by voltammetric study of Fe(CN)6

3�, Ru(NH3)63+

(Fig. 1S and 2S†, respectively) and catechol as electroactiveprobes in 0.1 M phosphate buffer solution (Fig. 1).

The voltammetric currents are very low for the AFS-GCelectrode before the extraction of CTAB as the structure-direct-ing agent and were enhanced considerably aer its extraction

Analyst, 2013, 138, 1740–1744 | 1741

Fig. 1 Cyclic voltammograms of 5.0 mM catechol in phosphate buffer solutionpH 6.0 on (a) bare GC electrode, (b) AFS-GC electrode before and (c) after CTABextraction. Scan rate: 100 mV s�1.

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(Fig. 1, 1S and 2S,† curves b and c). The catechol current wasproved to be more than both negatively Fe(CN)6

3� and positivelyRu(NH3)6

3+ charged species before the extraction of CTAB. Thisresult can be attributed to better diffusion of catechol as aneutral organic species through the micellar media.10 Also, theelectrode accessibility of catechol through the lm, aer thetemplate extraction, is less than Fe(CN)6

3� and more thanRu(NH3)6

3+ which is related to the electrostatic interactions ofthese charged species with positively charged ammoniumgroups under this condition of aqueous phosphate solutionpH ¼ 6. These results conrm the successful coating of theelectrode with an appropriately oriented aminopropyl-func-tionalized silica thin lm.

Moreover, the TEM images with different magnications(Fig. 3S and 4S†) of the obtained AFS lms are in good agree-ment with the previously reported results for the formation ofuniform and parallel mesochannels.11 Successful extraction ofCTAB as the template has been proved by IR spectroscopy and

Fig. 2 (I) Multi-cyclic voltammograms of 1.0 mM catechol in phosphate buffersolution, pH ¼ 7, on AFS-GC. The cycles from (a) to (k) are the first to 100th cyclewith 10 cycles interval. (II) 10 cyclic voltammograms of modified electrode inblank supporting electrolyte, pH 7, with 10 min interval for each cycle. Scan rate:100 mV s�1.

1742 | Analyst, 2013, 138, 1740–1744

elemental analysis (Fig. 5S†). Fig. 2I shows the multi-cyclicvoltammograms (CV) of 1.0 mM catechol in phosphate buffersolution pH 7.0 on the AFS-GC. In the rst cycle the CV showsonly one anodic peak (A1) in the positive-going scan and itscathodic counterpart (C1) in the negative-going one, a quasi-reversible two-electron electrode reaction with E1/2 ¼ 0.22 V vs.Ag/AgCl and DEP ¼ 60 mV.

During the next successive potential scans the new anodicpeak (A2) and its counterpart (C2) appear and their heightsincrease to reach a maximum value aer 100 cycles. Interest-ingly, we found that A2 and C2 remained unchanged when theelectrode was taken away from the catechol solution, waswashed thoroughly with deionized water and was immersedagain in a supporting electrolyte solution. In this way, themodied electrode displayed consistent currents in the form ofan adsorptive redox couple with E1/2 ¼ 0.07 V vs. Ag/AgCl andDEP¼ 6 mV in several consecutive runs (Fig. 2II). Therefore, it isreasonable to speculate that A2 and C2 can be related to anelectroactive functional group covalently anchored onto thesurface of the AFS-GC. However, the half-wave potential of theA2/C2 redox couple is remarkably (150 mV) less than catecholitself, suggesting the possibility of anchoring catechol onto thesurface of silica via a C–N bond.14 The relatively large negativeshi in the redox potential of the anchored group may beexplained in terms of the strong resonance electron-donatingproperty of the substituted amine groups.

Moreover, the functionalized lm exhibits a shoulder with apeak potential the same as catechol and which decreases, but isnot eliminated, by multiple potential scans in aqueous solution.The current fall at this potential is proportional to the adsorbedcatechol on the structure and the remaining current may be dueto the product of catechol polymerization during its oxidation.It is also worth mentioning that the current loss for the new A2

and C2 peaks is less than 6% aer remaining for 3 h in thesupporting electrolyte solution, phosphate buffer pH 7. Thisobservation clearly shows that the presented electrochemicallyfunctionalized modied group displays excellent stability underthe desired conditions. Based on these results the followingmechanism is proposed for silica lm functionalization at theelectrode surface (Scheme 1).

Scheme 1 Proposed mechanism for the electrochemical modification ofordered mesoporous film.

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The functionalized amine groups are in their ammoniumform aer surfactant extraction in acidic media, whereas thefavorable form for nucleophile addition is that of deprotonatedamine. The partial deprotonation (activation) of anchoredamine groups was simply achieved by either immersing theelectrode into a slightly basic acetate solution (even a phos-phate buffer solution with pH 7) for 10 min or by applying anegative potential, as a so deprotonating method, for 3 min.It was found that these pre-treatments resulted in 15% and10% improvements in the modied electrode responses,respectively. Our repeated experiments also exhibit that it isnecessary to use buffered phosphate solution (around pH 7) inorder to obtain the best functionalization and electroderesponses. Considerably higher and lower pH values than 7were found not to be benecial, presumably due to eitherdecomposition (dissolution) of the modied silica lm orextensive protonation of the functional amine groups, respec-tively (Fig. 6S†).

Both single- and double-step (anodic and cathodic) poten-tiostatic and also potentiodynamic methods have been exam-ined for electrochemical modication: the orders of signalstrength were obtained as follows for modied electrode:potentiodynamic > double-step potentiostatic > single-steppotentiostatic. The inuence of scan repetition, scan rate,catechol concentration and time of functionalization were alsostudied on the obtained electrode responses.13 From these datathe optimum condition of electrode modication was found tobe 50 potentiodynamic scans in the potential range between�0.3 V and +0.7 V vs. Ag/AgCl electrode in phosphate buffersolution, pH 7.2, and 5.0 mM catechol concentration.

In thin lm applications it is very important to control thehomogeneity and porosity to obtain desirable coatings.The TEM images of scratched lms, Fig. 3 and 16S,† show theuniform and well-dened structure that remains unchangedaer template extraction, activation and electrochemicalfunctionalization.

Also, the voltammetric study of catechol as a probe showsthat the electrode surface remains accessible to the solution forall lms aer all of the above-mentioned treatments (Fig. 11S†).Thermogravimetric (TGA) analysis of the scratched modiedlm allows information concerning the loading of amine oraminocatechol groups.

Fig. 3 Top-view TEM images of scratched aminocatechol-functionalized meso-porous silica film after surfactant extraction, activation and electrochemicalfunctionalization.

This journal is ª The Royal Society of Chemistry 2013

The weight loss, due to the breakdown of organic groupsanchored on the silica surface, is about 12.4 and 17.2% foramine- and aminocatechol-containing MCM-41, respectively.Considering the catechol and aminopropyl molecular weights,this indicates the bonding of 21% of amine groups to catechol.The rst result is in good agreement with the 10% of APTES inthe initial sol solution. The ratio of catechol to amine groupswas found also to be 20% based on the molar percentage ofcarbon and nitrogen in elemental analysis (Fig. 12S and 13S†).

Voltammetric studies of electroactive lms also provideunique information about the loading of catechol groups. Sincewith a good estimation all faradaic charges passing through theelectrode in our experiments could be related to the anchoredcatechol groups, the surface coverage of aminocatechol wascalculated to be 2.9 � 10�9 mol cm�2 for the geometric area ofthe electrode. TEM images also showed that there are approxi-mately 76 000 channels mm�2 with an average of 100 nm inheight and 3 nm in diameter. It means that each channel has asurface area of about 942 nm2 and thus contains 235 anchoredaminocatechol groups or 1 electroactive group per 4 nm2 ofchannel surface.

Voltammetric study of the functionalized lm (Fig. 4)demonstrates the adsorption-like character in spite of thenanometer size distance of the electroactive groups.15 This canbe explained by the possibility of charge propagation throughthe whole lm and electron transfer of all electroactive groupsdue to the nanometer size, less than 120 nm thickness, anduniform structure of the lm.

Moreover, the interphase characteristics of the aminopropylgroups and the relatively easy movement of the linked electro-active group for electron propagation, and in this particularcase the electroactive polymerization of catechols that accel-erate the electron percolation through the lm, should beconsidered. Further studies of electron propagation, more effi-cient electrochemical modications and electrocatalytic appli-cations of these structures are currently underway in ourresearch group.

The presence of catechol units in this permeable lmencouraged us to examine the performance of the desired

Fig. 4 (I) Cyclic voltammograms of aminocatechol-functionalized electrode inphosphate buffer solution pH 7; scan rates from (a) to (d) are 50, 100, 150 and250 mV s�1 respectively. (II) Variation of A2 anodic peak currents vs. scan rate.

Analyst, 2013, 138, 1740–1744 | 1743

Fig. 5 Cyclic voltammograms of aminocatechol-functionalized electrode inphosphate buffer solution pH 7; (a) in the absence, (b) in the presence of 2.0 mMNADH and (c) cyclic voltammograms of NADH at the surface of bare GC electrode.Scan rate 25 mV s�1.

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modied electrode in the electrocatalytic oxidation of NADH(the reduced form of nicotinamide adenine dinucleotide). Asdemonstrated in Fig. 5, in the presence of NADH, the height ofthe anodic peak increases and the cathodic peak disappears.These are in good agreement with the characteristics of theelectrocatalytic mechanism (EC0) and conrm the electro-catalytic performance of the electrode toward electrocatalytictransformations.16

Conclusion

The present study was designed to demonstrate the rstexample of the simple electrochemical functionalization ofordered mesoporous silica via covalent bonding in aqueoussolutions. This method is quite fast, modications were ach-ieved in less than half an hour, and the obtained modied lmshows adequate loading and electrochemical response. Inconclusion, the present results are signicant in at least twomajor aspects. In particular, the presence of catechol units as anefficient and pH-tunable electrocatalyst in the lm could ndpromising applications of these structures in sensors andelectrocatalytic transformations. In general, controlled poten-tial electrode reactions are known as the most efficient methodfor the in situ generation of reactive and even highly reactivespecies in chemical reactions.17 Thus, the present dominostrategy, as an interesting combination of two heterogeneousphenomena, can be extended for the fabrication of variousother appropriate electroactive groups to generate the corre-sponding novel, functionalized, ordered mesoporous silica thinlm. The proposed method can be extended for the function-alization of ordered mesoporous silica thin lms with specicproperties and applications.

Acknowledgements

This work was supported by the IASBS Research Council (Grantnumber: G2011IASBS126) and Iran National Science Founda-tion (INSF).

1744 | Analyst, 2013, 138, 1740–1744

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