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Journal of Electroanalytical Chemistry 610 (2007) 218–226

ElectroanalyticalChemistry

Self-assembled monolayers of a hydroquinone-terminatedalkanethiol onto gold surface. Interfacial electrochemistry and

Michael-addition reaction with glutathione

Mojtaba Shamsipur a,*, Sayed Habib Kazemi b, Abdolhamid Alizadeh a,Mir Fazlollah Mousavi b, Mark S. Workentin c

a Department of Chemistry & Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah, Iranb Department of Chemistry, Tarbiat Modares University, Tehran, Iran

c Department of Chemistry, The University of Western Ontario, London ON, Canada N6A 3NB

Received 14 May 2007; received in revised form 16 July 2007; accepted 25 July 2007Available online 3 August 2007

Abstract

An electroactive self-assembled monolayer (SAM) was fabricated by covalent attachment of a novel hydroquinone-terminateddodecanethiol onto the gold surface and its electrochemical behavior was investigated using cyclic voltammetry and electrochemicalimpedance spectroscopy. The capability of the designed SAM in immobilization of organic molecules onto the gold surface was studiedutilizing the Michael-addition as a model reaction. The results obtained from cyclic voltammetry, electrochemical impedance and grazingincidence Fourier transform infrared (GI-FTIR) spectroscopy revealed that, upon applying an anodic potential to the Au-SAM electrodesystem in the presence of glutathione, the electrochemically generated p-quinone participated in a Michael-addition reaction with gluta-thione and the corresponding Michael adduct was formed at the solid–liquid interface. The kinetic parameters were then derived for thisinterfacial Michael-addition reaction.� 2007 Elsevier B.V. All rights reserved.

Keywords: Self-assembled monolayers; Cyclic voltammetry; Impedance spectroscopy; Nanolayers; Michael-addition

1. Introduction

During the past two decades, the surface modificationhas been significantly studied, particularly to providewell-organized controllable media for investigation ofkinetics and mechanism of electron transfer phenomena.Among different methods, self-assembling of organothiolsonto the noble metal surfaces, especially gold, has attractedgreat attention from application prospects and fundamen-tal points of view [1–8]. The discovery of self-assembledmonolayers (SAMs), and in particular, the identification

0022-0728/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2007.07.014

* Corresponding author. Tel.: +98 21 88050528; fax: +98 21 66908030.E-mail address: mshamsipur@yahoo.com (M. Shamsipur).

of gold–thiol route has essentially improved the surfacechemistry and opened a new area of physically orientedgroups. The great flexibility of the concept of SAMsresulted in a wide choice of head group moieties whichcan be anchored to the substrate and this has led to a broadrange of applications of SAMs [9,10].

Exceptional stability, highly packed and ordered nature,excellent insulating power and other unique properties ofthe monolayers in nanodimension make them important touse as perspective scaffolds for a wide range of applications.These include determination of electron transfer quantities,understanding the reaction and electron transfer mecha-nisms, coating the surfaces from corrosion, and adding selec-tivity to the surface. Moreover, from the electrochemicalapplication point of view, self-assembled monolayers have

M. Shamsipur et al. / Journal of Electroanalytical Chemistry 610 (2007) 218–226 219

become one of the best approaches to design selective elec-trodes and sensors for analytical applications [11–16].This is possibly due to chemical and electrochemical inert-ness and simultaneously strong attachment onto conduct-ing surfaces of gold, and other noble metals, whilemaintaining the ability to functionalize the SAM electrodesurfaces [1–3].

The possibility to generate surfaces containing electroac-tive relevant functionalities is certainly one of the mostexciting properties of SAMs, due to their ability of bindingof specific functional head groups with variety of chemicaland biological species in solution. Some of the publishedapplications have been devoted to the use of these proper-ties for the detection and elucidation of the biological inter-actions and to study some important chemical reactions atthe solid–liquid interfaces in the nanometer scale [17]. Upto now, electrochemical methods, especially cyclic voltam-metry, have been used to evaluate these factors. In addi-tion, the conventional spectroscopic methods like FT-IRand new microscopic scanning methods like AFM andSTM have been used to confirm the voltammetric results[18–22].

Recently, Mrksich and co-workers have described aninteresting method to evaluate and elucidate the factorsthat uniquely affect the reactions of molecules confined atthe solid–liquid interfaces [23–26]. They have used theDiels-Alder reactions as a model to evaluate the corre-sponding parameters and elucidate the interfacial interac-tions. They employed conventional cyclic voltammetryand investigated the kinetics of Diels-Alder reaction usingthe reversible redox behavior of the head groups at SAMand determined the reaction rates, in the order of 10 s.

It should be noted that one of the promising electro-chemical techniques for extracting highly accurate informa-tion about the solid–liquid interface is electrochemicalimpedance spectroscopy. This is a powerful technique forthe evaluation of double layer parameters and a supple-mentary method to confirm the reactions occurring at theinterfaces using capacitive characteristics and charge trans-fer properties of SAM systems. This is feasible with respectto interpretation of the impedance results using properelectrical circuits [27–31]. Electrochemical impedance spec-troscopy has also been used to confirm the voltammetricobservations [27].

In the present study, 2-(12-mercaptododecyl)benzene-1,4-diol, as an electroactive probe molecule, was covalentlyattached onto the gold surface as a working solid–liquidinterface in order to perform electrochemical experimentsfor predictive interfacial Michael-addition reaction ofhydroquinone moiety of SAMs with the nucleophile gluta-thione and to elucidate the possible interactions and evalu-ate kinetic parameters of the predictive reaction at thenanostructured solid–liquid interface. To the best of ourknowledge, literature reports dealing with the Michael-addition reaction at the nanolayers, especially by usingelectrochemical techniques like impedance spectroscopy,are quite sparse [32].

2. Experimental

2.1. Reagents

All chemicals were analytical grade Sigma-Aldrich prod-ucts, except organic solvents, purchased from Merck. Allthe experiments were carried out at room temperature.Doubly distilled water was used throughout.

2.2. Apparatus

Electrochemical studies were carried out in a conven-tional three electrodes cell equipped with a Teflon stopperwith holes to hold electrodes in appropriate configurationsfor minimizing the solution resistance in electrochemicalimpedance spectroscopy. Electrochemical impedance mea-surements were done at open circuit potential (OCP) withproper redox potential as bias potential. These experimentswere carried out using an Autolab PG30 electrochemicalanalyzer system (Eco Chemie, B. V. Utrecht, The Nether-lands), supplied with FRA2 module boards, and frequen-cies were swept between 100 kHz to 10 mHz with anapplied sine wave amplitude of 5 mV rms as an excitationsignal and obtaining at least seven points per decade. Thesystem was run by a PC through FRA and GPES 4.9 soft-wares. The working electrode was a 2-mm diameter golddisc (Azar Electrode, Urmieh, Iran), a 2-cm2 area Pt elec-trode as a counter electrode and an Ag/AgCl sat’d KClelectrode as a reference electrode, with the double junctionfilled with a proper electrolyte solution in each case. Thegold electrode was polished before SAMs preparation inthree steps. The GI-FTIR spectra were recorded on PikeFT-IR instrument (Bruker Vertex 70).

2.3. Procedure

First, the electrode surface was polished on a polishingpaper of 0.05 lm alumina powder until a mirror likesurface was obtained. Then, it was sonicated in ethanolicsolution at least for 2 min and, finally, electropolishingwas done using consecutive cyclic voltammograms in0.5 mol l�1 sulfuric acid and potential scanning between�0.5 and 1.5 V vs. Ag/AgCl sat’d electrode until a typicalvoltammogram for gold was obtained. The SAMs prepara-tion was simply afforded by immersing a clean gold elec-trode in a ethanolic solution of hydroquinone-terminateddodecanethiol (4) containing appropriate amount of co-adsorbate thiol (i.e., a dodecanethiol (4)/co-adsorbate thiolmolar ratio of 2:5, evaluated based on the redox responseoptimization) for 24–48 h. The schematic chemical struc-ture and preparation of the thiol 4 are presented inFig. 1. Thiol 4 was synthesized according to the methodthat described in the experimental section and kept underinert atmosphere to prevent probable oxidation.

Electrochemical investigations for the designed SAMs inthe absence and presence of selected nucleophile were con-ducted in binary aqueous organic solvent mixtures (aceto-

OCH3

OCH3

S CH3

O

OCH3

OCH3

Br

OH

OH

SH

OH

OH

S CH3

O

OCH3

OCH3

Br Br+

n-BuLi

Acetone

dry THF

CH3COS-K+

BBr3 CH2Cl2

C2H5OHK2CO3

(1)

(2)

(3)

(4)

Fig. 1. Synthesis of hydroquinone-terminated electroactive probe mole-cule (4).

220 M. Shamsipur et al. / Journal of Electroanalytical Chemistry 610 (2007) 218–226

nitrile:water 20:80 v/v%) containing small amounts ofsodium acetate as supporting electrolyte.

Glutathione solution were prepared freshly and storedunder argon atmosphere at 4 �C before each experiment.It should be mentioned that throughout all experiments,the electrochemical cell was maintained under argon atmo-sphere. In all cases, the solution pH was adjusted to about6, at which glutathione is mainly in its protonated form.

2.4. Synthesis of the electroactive probe molecule

The hydroquinone-terminated dodecanethiol was syn-thesized following the modified procedure reported byHickman et al. [33], as shown in Fig. 1.

2.4.1. 2-(12-Bromododecyl)-1,4-dimethoxybenzene (1)

A solution of n-butyllithium (1.7 M in pentane,7.7 mmol) was slowly added to a dry THF (25 ml) solutionof 1,4-dimethoxybenzene (0.95 g, 6.9 mmol) in a nitrogen-purged Schlenk flask over 20 min. The mixture was stirredfor additional 60 min at room temperature and transferreddropwise via a canola to a second flask containing a dryTHF (10 ml) solution of 1,12-dibromododecane (6.8 g,20.7 mmol) at 0 �C. The contents were allowed to stir for30 min at 0 �C, and the reaction was allowed to warm up

to room temperature. The mixture was stirred for over-night at room temperature, diluted with 50 ml of dichloro-methane, poured into 25 ml of saturated NH4Cl solutionand washed with brine (30 ml). The solution was extractedwith CH2Cl2 (3 · 20 ml), and the organic solution waswashed with water and dried over MgSO4. The solventand unreacted dimethoxybenzene were evaporated in vac-uum, and the residue was purified by column chromatogra-phy using silica gel and 90:10 hexane: ethylacetate as eluantto yield 1 as colorless oil (1.35 g, 3.51 mmol) in 60% yield.1H NMR (CDCl3): dH 1.31–1.38 (br, 14H, aliphatic CH2),1.42 (quintet, 2H, –CH2CH2CH2Br), 1.6 (quintet, 2H,–CH2CH2Ph) 1.86 (quintet, 2H, –CH2CH2Br), 2.6 (t, 2H,–CH 2Ph), 3.42 (t, 2H, –CH2Br), 3.77 (s, 3H, –OCH3),3.79 (s, 3H, –OCH3), 6.72 (dd, 1H, Ar), 6.85 (dd, 2H,Ar). 13C NMR (CDCl3) 28.16, 28.65, 29.38, 29.42, 29.45,29.48, 29.52, 29.58, 29.82, 30.18, 32.56 (–CH2Ph), 33.58(–CH2Br), 55.40 (–OCH3), 55.80 (–OCH3), 110.40,111.15, 116.2, 132.25, 151.18, 153.15. MS (EI) m/z (%)M+ = 384 (72), 304.1 (5), 151 (75), 136 (28), 121 (29), 91(8), 76 (7). Exact mass (C20H33O2Br) calc. 384.1664; found384.1669.

2.4.2. S-12-(2,5-dimethoxyphenyl)dodecyl ethanethioate

(2)

To a 100 ml round bottom flask fitted with a condenserwere added 1 (1.0 g, 2.6 mmol), potassium thioacetate(0.59 g, 5.2 mmol) and 30 ml of acetone. The reaction mix-ture was heated to reflux and stirred for 6 h. The comple-tion of reaction was monitored by TLC and then, thesolvent was removed in vacuum, and water (10 ml) and tol-uene (20 ml) were added. After separation, the organicphase was washed with water (2 · 10 ml), dried overMgSO4, transferred to a round bottom flask via gravity fil-tration and concentrated to yield 0.74 g of 2 (yield = 75 %).1H NMR (CDCl3): dH 1.27–1.33 (br, 16H, aliphatic CH2),1.59 (quintet, 4H, –CH2CH2Ph and –CH2CH2S–), 2.33 (s,3H, –SCOCH3) 2.58 (t, 2H, –CH2S–), 2.86 (t, 2H,–CH2Ph), 3.76 (s, 3H, –OCH3), 3.78 (s, 3H, –OCH3),6.69 (dd, 1H, Ar), 6.75 (dd, 2H, Ar). 13C NMR (CDCl3)28.80, 29.10, 29.31, 29, 37, 29.40, 29.42, 29.43, 29.45,29.47, 29.50, 29.82, 30.18, 30.40, 55.70 (–OCH3), 55.90(–OCH3), 110.20, 111.15, 116.2, 132.20, 151.80, 153.50,196.10 (–SCOCH3).

2.4.3. S-12-(2,5-dihydroxyphenyl)dodecylethanethioate (3)

Compound 2 (0.7 g, 1.84 mmol) was dissolved in the drymethylene chloride (30 ml) in a round flask immersed in theice bath under nitrogen and temperature cooled down to�75 with dry ice/acetone mixture. A solution of BBr3

(1.0 M in CH2Cl2, 10 ml, 10.4 mmol) was added dropwiseto the solution of 2. The mixture was allowed to warmup to room temperature and stirred for additional 2 h.The solution was poured into 25 ml of saturated NH4Clsolution and was extracted with CH2Cl2 (3 · 20 ml), andthe organic phase was washed with water, brine and driedover MgSO4. The solvent was removed in vacuum, and the

M. Shamsipur et al. / Journal of Electroanalytical Chemistry 610 (2007) 218–226 221

residue was purified by column chromatography using sil-ica gel and hexane: diethyl ether (70/30) as eluant to yield3 as a white solid (0.56 g, 1.6 mmol) in 87% yield. 1HNMR (CDCl3): dH 1.24–1.34 (br, 16H, aliphatic CH2),1.58 (quintet, 4H, –CH2CH2Ph and –CH2CH2S–), 2.33(s, 3H, –SCOCH3) 2.55 (t, 2H, –CH2S–), 2.86 (t, 2H,–CH2Ph), 5.10 (br, 1H, –OH), 5.78 (br, 1H, –OH), 6.66(dd, 1H, Ar), 6.78 (dd, 2H, Ar). 13C NMR (CDCl3)28.71, 28.97, 29.22, 29.32, 29.34, 29.38, 29.39, 29.41,29.42, 29.45, 29.59, 29.98, 30.59, 113.20, 115.97, 116.77,130.08, 147.28, 149.28, 197.43 (–SCOCH3). IR (dropcaston NaCl): 3450, 3022, 2955, 2853, 1692, 1665, 1506,1455, 1354, 1195, 1132, 958, 808, 629 cm�1. MS (EI) m/z

(%) M+ = 352.1 (18), 310.1 (85), 278.2 (68), 149 (7), 123(100), 95 (8), 67 (10). Exact mass (C20H32O3S) calc.352.2061; found 352.2062.

2.4.4. 2-(12-Mercaptododecyl)benzene-1,4-diol (4)The compound S-12-(2,5-dihydroxyphenyl)dodecyl eth-

anethioate (3) (0.56 g, 1.6 mmol) was transferred to a100 ml round bottom flask, fitted with a reflux condenser,dissolved in 30 ml absolute ethanol and the solutiondegassed with nitrogen for 20 min after which time thesolution was charged with potassium carbonate (0.42 g,3.03 mmol) and heated to reflux for 12 h. After monitoringthe completion of reaction by TLC, ethanol was removedvia rotary evaporation to give white precipitate. Then,water (20 ml) and saturated ammonium chloride (30 ml)were added and the product was extracted by dichloro-methane (2 · 25 ml). The organic phase was washed withdistilled water (3 · 30 ml) and dried over MgSO4. Concen-tration yielded pure 4 as a white powder (0.4 g, 1.3 mmol)in 82% yield. 1H NMR (CDCl3): dH 1.25–1.32 (br, 16H,aliphatic CH2), 1.35, (t, 1H, -SH), 1.60 (quintet, 4H,–CH2CH2Ph and –CH2CH2S–), 2.58 (quintet, 2H,–CH2SH), 2.86 (t, 2H, –CH2Ph), 4.55 (br, 1H, –OH),4.71 (br, 1H, –OH), 6.56 (dd, 1H, Ar), 6.68 (dd, 2H, Ar).13C NMR (CDCl3) 24.65, 28.33, 29.02, 29.32, 29.34,29.44, 29.46, 29.50, 29.55, 29.62, 29.99, 33.98, 113.21,115.98, 116.77, 130.03, 147.28, 149.18. MS (EI) m/z (%)M+ = 310.1 (100), 278.2 (18), 123 (65), 95 (8), 67 (10).Exact mass (C18H30O2S) calc. 310.1956; found 310.1959.

3. Results and discussion

3.1. Mixed SAMs formation onto gold surface: Mappingpinholes

3.1.1. Cyclic voltammetry

The redox behavior of the self-assembled monolayersprepared by incorporation of 2-(12-mercaptododecyl)ben-zene-1,4-diol (4) onto the gold surface was first studiedby cyclic voltammetry. The electrochemical behavior ofsome similar monolayers has been reported previously[6,23–25]. Recently, Larsen and Gothelf have studied someimportant factors affecting the stability of SAMs chains,

and the reactivity of quinone moieties present which arecrucial in formation of SAMs and in performing some sub-sequent important interactions and reactions [6]. Theyinvestigated the use of different mixtures of simple chainalkanethiols, as co-adsorbates, and the target hydroqui-none-terminated alkanethiol to find maximum stability ofSAMs towards potential together with minimum peak sep-aration and broadening. They found that maximum lengthof co-adsorbate should be equal to that of the spacer ofquinone moiety. In this work, we also found that a mixtureof thiol derivative 4 and n-dodecanethiol (C12–SH), as a co-adsorbate, will give the best results.

Thus, to obtain the best SAMs, we optimized someimportant parameters including concentration of thiol 4in the mixed thiol solutions, conditioning time, and temper-ature during the process of SAMs formation. Conditioningtimes between 48 and 72 h were the optimum times for ourstudies. Also, the SAM prepared from a mixture of0.2 mmol l�1 thiol 4 and 0.5 mmol l�1 of n-dodecanethiol,in 20% acetonitrile-water solution found to give the bestresults. It should be mentioned that the effect of scan rateon the peak height found to result in a linear relationship,which is in good agreement with previous works [6]. Thisresult revealed the surfaced confined characteristics of theattached layer, and is a good evidence indicating that thethiol 4 has tethered onto the gold surface [6,34,35]. Inour studies, the peak separation in CV experiments was lar-ger than 100 mV, which is in good agreement with the workof Hong and Park [34]. This separation is due to the largealkyl group (i.e., C12).

To evaluate the surface coverage, C, of the gold elec-trode with the mixed thiols, the electrochemical desorptionexperiment was carried out in 0.5 mol l�1 of KOH solutionby scanning the potential from 0.1 to �1.4 V. Based on thedesorption experiment, the approximate C for the modifiedelectrode was calculated as 7.4 · 10�10 mol cm�2. This is ina satisfactory agreement with the C values reported in theworks of Larsen and Gothelf [6] and Hong and Park [34]and is close to the full hexagonal close packed surface cov-erage of long chain alkanethiols, reported by Yang et al.[35]. Based on the reductive desorption experiment ofSAMs from gold surface, the surface fractional coveragewas estimated to be very close to 1.

3.1.2. Ac-impedance spectroscopy

The alternating current impedance experiment isanother method which has been used frequently to evaluatethe surface coverage by thiols. Fig. 2 represents the com-plex plane plots for both the bare (inset) and SAMs mod-ified electrodes in the frequency range of 100 kHz to1 mHz. For the bare electrode, a very small semicircle(10–1 kHz) followed by a straight Warburg line, indicatinga diffusion controlled process, is observed in the presence of1 mmol l�1 of FeðCNÞ3�=4�

6 as a probe redox. In contrast,for the case of SAMs electrode, the semicircle diameter isdramatically increased as a result of barrier properties ofthe modified surface.

Fig. 2. Nyquist plots for both bare gold electrode (inset) and SAMsmodified electrode in the frequency range of 100 kHz to 1 mHz in anacetonitrile:water (20:80 w/w) mixture containing 0.1 mol l�1 sodiumacetate as supporting electrolyte, at 0.25 V vs. Ag/AgCl in the presence of1 mmol l�1 of FeðCNÞ3�=4�

6 probe.

222 M. Shamsipur et al. / Journal of Electroanalytical Chemistry 610 (2007) 218–226

Different methods have been utilized to analyze theimpedance data. A relatively simple one suggested byRubinstein and co-workers and extended by Fawcett andJanek [27] is based on the relationship between the magni-tude of the Rct and the coverage of the electrode, by assum-ing that the electron-transfer reactions occur only at barespots on the electrode surface and that the diffusion to pin-hole sites is planar. Considering the above assumptions,one may assume the following equation for the apparentfractional coverage of the electrode based on the Rct ofthe uncoated electrode [27]:

himpedance ¼ 1� Rbarect

RSAMct

� �ð1Þ

where h is the fractional coverage of the electrode and Rbarect

and RSAMct are the values of the charge-transfer resistance

before and after electrode modification. Since the Rct ischanged from less than 1 X to more than 200 kX (seeFig. 3), the fractional coverage is estimated as >0.999. Inaddition, one can use the mathematical calculation ofimpedance response of alkanethiol SAMs as an array ofsmall microelectrodes [30,31]. This approach considersthe analysis of pore size distribution in the SAMs. Using

Fig. 3. Nyquist plot for under investigating SAMs onto the gold electrodein a mixture of acetonitrile:water (20:80 w/w) solution containing0.1 mol l�1 sodium acetate as supporting electrolyte in the frequencyrange of 100 kHz to 1 mHz, experimental data are triangle points and thered solid line shows fitted data. Inset is a schematic representation ofequivalent circuit used to obtain electrochemical parameters of thiol 4

SAMs in the presence of 1 mmol l�1 of FeðCNÞ3�=4�6 probe.

this analysis method, two types of diffusional behavior isconsidered if h exceeds 0.9; one for the high frequency re-gion with no overlap between spherical diffusion to eachmicroelectrode and the second for the low frequency partof impedance data where the individual spherical diffusionlayers of each microelectrode will overlap [30,31].

To obtain the fractional surface coverage by using thisassumption, one can plot the real part of impedance, Z 0F,for the SAMs modified electrode against x�0.5 and evalu-ate the h value from the intercept of the high and low fre-quency domains (Randles plots) [30]. Our estimatedfractional coverage of >0.99, evaluated from the interceptof Randles plots, is in good agreement with that obtainedfrom the method proposed by Fawcett and Janek [27].

3.2. Interfacial characteristics of the prepared SAMs using

the ac-impedance method

After a SAM of long alkyl chain is attached onto thesurface of gold electrode, the charging current wasobserved to decrease drastically, by more than one orderof magnitude. Porter et al. reported this effect for variousalkyl chains [36]. It has been reported that, in general, formost linear alkanethiols containing 10 or more methylenegroups, the charging capacity will be in the range of 1–5 lF cm�2 [36,37]. The high blocking properties of SAMnanolayers could be assessed by performing an ac-imped-ance analysis and interpreting the results using a propor-tional equivalent circuit model and complex nonlinearleast square method (CNLS) [38].

Fig. 3 demonstrates the Nyquist diagram for the pre-pared SAMs on the gold electrode during the electrontransfer at the adsorbed layer. This experiment was carriedout at a bias potential of +0.25 V vs. the reference electrodeto monitor the electron transfer reactions through SAMs.A simple electrical equivalent circuit consisting of a parallelconnection between a constant phase element, CPE, andcharge transfer resistance through the thin film attachedonto the gold surface, in series with the extreme resistanceobserved at high frequency, mainly due to solution resis-tance, was used to fit the impedance data and to evaluatethe interfacial capacitance of the adsorbed layer. Basedon the above model, the interfacial capacitance of theadsorbed layer was evaluated as 2.25 lF cm�2. Thisparameter is in good agreement with the reported valueby Porter et al. [36]. A schematic representation of equiva-lent circuit used, is depicted in the inset of Fig. 3.

Furthermore, the large resistance for charge transferreaction is due to densely packed monolayer which slowsdown the rate of electron transfer considerably. Thedepressed semicircle is associated with the slow electrontransfer experienced by the hydroquinone moiety of theSAM nanolayer. A possible mechanism for this electrontransfer reaction is the redox reaction occurred duringthe conversion of hydroquinone moiety to p-benzoquinone.

The rate of electron transfer through SAMs nanolayer isvery slow considering the long alkyl chain which dramati-

M. Shamsipur et al. / Journal of Electroanalytical Chemistry 610 (2007) 218–226 223

cally reduces the electron transfer through a tunnelingmechanism. It has been reported by Hong and Park [34]that, as the alkyl chain becomes longer, the electron trans-fer should occur from a longer distance from the gold sur-face to the redox centers. Therefore, it will result in slowingthe overall electron transfer rate and also decreasing theobserved current (Itunneling � e�kx), and indicates that theheterogeneous electron transfer possesses some irreversibil-ity. The depressed semicircle is mainly associated with theslow electron transfer experienced by the probe ionsthrough the SAM nanolayer.

3.3. Interfacial reaction of the designed SAM, evidences and

kinetics

3.3.1. Electrochemical investigations

Reactions that occur at a solid–liquid interface can differsubstantially from the corresponding solution-phase reac-tions [26]. These differences, as well as the mechanistic fac-tors, will influence the interfacial reactions. The rates andproducts of interfacial reactions, for example, often showunexpected and substantial dependencies on the surfacestructure. This challenge has resulted in some difficultiesin applying standard approaches of physical organic chem-istry to interfacial reactions [24,39].

In this work, it is supposed that upon applying an ano-dic potential to the prepared SAM in a solution containingnucleophilic compounds like glutathione, the hydroqui-none moiety of SAM generates o-benzoquinone, and thisactive intermediate undergoes a subsequent Michael-addi-tion reaction with glutathione at the electrode-solution

S S S

HO

OH

S S S

O

O

- 2e- - 2H+

SH

HN

HO

HN

OH

H2N

O

O

O

O

+ H+

Au Au

H

Fig. 4. Schematic representation of the immobilization of glutathione onto moglutathione and the generated quinine under electrochemically activated surfa

interface (Fig. 4). This is followed by a reversible electro-chemical reduction and oxidation of hydroquinone/benzo-quinone moiety, which permitted us to study the reactionand to evaluate some kinetic parameters like the rate ofinterfacial reaction, from the corresponding cyclicvoltammograms.

The electrochemical techniques such as cyclic voltamme-try and ac-impedance spectroscopy were used to prove theinterfacial Michael-addition. Fig. 5 shows the resultingNyquist diagrams for the thiol 4 modified gold electrode,before and after Michael-addition reaction with glutathi-one. The electrochemical parameters changes observedfrom the impedance data before and after Michael-addi-tion for thiol 4 are shown in Table 1. The capacitancechange observed after addition of an appropriate amountof nucleophile to the solution in electrochemical cell, dueto the change in surface structure of monolayer, is a goodevidence to prove the occurrence of the reaction. Accordingto the EIS results obtained before and after addition of glu-tathione to the cell solution, by applying a proper biaspotential to the system to perform Michael-additionreaction, approximately 8% decrease in SAMs capacitancewas observed upon the adduct formation, which couldestablish the proportional increase in the film thicknessafter Michael-addition. The capacitance changes wellknown to be a function of SAM structure and the thiol 4

to co-adsorbate ratio. The observed decrease in capaci-tance is attributed to an increase in the chain length of thiol4 due to the reaction with glutathione.

In addition, cyclic voltammetry was used to monitor therate of reaction between glutathione and SAMs nanolayer.

S S S

O

OH

Au

S

HN

HO

HN

OH

2N

O

O

O

O

S S S

HO

OH

Au

S

HN

HO

HN

OH

H2N

O

O

O

O

H

dified gold surface through interfacial Michael-addition reaction betweence.

Fig. 5. Nyquist plots for thiol 4 SAMs in a mixture of acetonitrile:water(20:80 w/w) solution containing 0.1 mol l�1 sodium acetate as supportingelectrolyte, before and after the reaction of glutathione with the generatedquinine (Michael-addition) in a solution containing 1 l mol l�1 of gluta-thione at 0.25 V vs. Ag/AgCl, in the presence of 1 mmol l�1 ofFeðCNÞ3�=4�

6 probe. Experimental results presented with the points andsolid lines are fitted results with the equivalent circuit which presented inFig. 4.

Table 1Electrochemical parameters obtained from the CNLS fitting of impedancedata for thiol 4 SAMs

Parameter Rct

(kX)Cdl

(lF)Change in Cdl

Before Michael-addition 220 2.25 8.5% (which is equalto at least 8.5% innanolayer thickness)

After Michael-addition 1100 2.04

224 M. Shamsipur et al. / Journal of Electroanalytical Chemistry 610 (2007) 218–226

Considering the proposed mechanism by Gwalt and Mrks-ich [26], as used by Yousaf et al. [24], the following path-way is suggested to simply explain the Michael-additionreaction:

K rxn

Fig. 6. Typical cyclic voltammograms (one to sixth), for SAMs in amixture of acetonitrile:water (20:80 w/w) solution containing 0.1 mol l�1

sodium acetate as supporting electrolyte at 20 mV s�1 scan rate. Peak topeak separation was increased from one to sixth more than 50 mV.

Fig. 7. Corrected current density against time for the Michael-additionreaction obtained from the 20 successive voltammograms in a mixture ofacetonitrile:water (20:80 w/w) containing 1 lmol l�1 of glutathione.

Nucleophile (solutionl)

Kads

Nucleophile (ads)

Au- SC12-HQ

- 2e- -2H+

Au- SC12-Q + Nucleophile (ads) Michael product

This mechanism is a representation of the reversible oxida-tion of hydroquinone moiety to quinone followed by aninterfacial chemical nucleophilic addition which can beconsidered as an EC mechanism.

A control experiment has been performed to prove thatthe hydroquinone moiety of SAM is inactive toward anynucleophilic addition and this is the p-quinone moietywhich is capable of undergoing Michael-type nucleophilicadditions at the solid–liquid interface. To this end, theAu electrode modified with thiol 4 was immersed in an elec-trolyte solution containing 10 mmol l�1 of nucleophile atleast for 3 h without applying any potential. After that,the electrode dropped out of nucleophile solution andimmersed in the cell after rinsing thoroughly with waterand ethanol. No significant changes was observed duringthe potential cycling in the cyclic voltammetric experi-ments, clearly emphasizing that it is only the p-quinone

form of the thiol derivative that is capable of undergoingthe interfacial Michael-addition reaction. Typical cyclicvoltammograms of the reaction are presented in Fig. 6.

As is obvious from Fig. 6, the consecutive voltammo-grams show that the peak of oxidation and reduction ofbenzoquinone is diminished with time and also the peak-to-peak separation was increased due to kinetic limitations.It should be noted that no changes was observed for thevoltammograms of SAMs modified electrode in theabsence of nucleophile at least over 100 cycles, supportingthe fact that any decrease in the anodic and cathodic peaksis properly due to its reaction with the added nucleophiles.Plotting the normalized peak currents against time, ornumber of cycles, revealed a pseudo-first order mechanism.This is reasonable due to much higher concentration of thenucleophile as it compared with the benzoquinone moietyconcentration.

Fig. 7 represents the plot of corrected current densitiesagainst time of reaction. The observed decrease in peakcurrent could be fitted to an exponential decay to obtainthe pseudo-first order rate constant using the followingequation:

I t ¼ I f þ ðI0 � I tÞ expð�k0tÞ ð2Þ

where It is the peak current at time t, I0 is the initial peakcurrent before addition of nucleophile and If is the residual

Fig. 8. FT-IR spectra of thiol 4 SAMs, before and after Michael-additionreaction, recorded with the grazing angle mode.

M. Shamsipur et al. / Journal of Electroanalytical Chemistry 610 (2007) 218–226 225

non-faradaic (mainly capacitive) current. The satisfactoryfit of the experimental data to a pseudo-first order behaviorcould be interpreted by the fact that the reactivity of p-qui-none moiety is independent of the extent of reaction. Thepseudo-first order rate constant was evaluated as9 · 10�4 cm s�1. Complete investigation of the effect ofnucleophiles’ concentration on the mechanism and kineticsof the predictive reaction is under way.

3.3.2. Spectroscopic evidence

FT-IR spectroscopy in the mode of grazing angle is avaluable probing technique for surface studies especiallyfor the monolayer structures [15,36]. In order to have a bet-ter view about surface structure and the structural changesduring the interactions of diffused molecules from electro-lyte solution towards the SAMs surface with the functionalmoiety of SAMs, one should focus on characteristic IRregions which can tell about the packing and orientationof adsorbed layers. These studies could be performed toprove the reactions done at the SAMs surface [39]. In thepresent study, the FT-IR probing was carried out beforeand after interfacial Michael-reaction reaction and adductsformation. In order to make sure that the new peaks arerelated to the Michael adduct, after the specific time to pro-ceed the reaction, the electrode was removed from the cellsolution and rinsed thoroughly with doubly distilled waterto remove any physically absorbed species and after thatthe FT-IR spectra were recorded. Fig. 8 illustrates the typ-ical FT-IR spectra before and after Michael-addition reac-tion. As is obvious from Fig. 8, the IR absorption bandsobserved at 1415 and 1595 cm�1 are assigned to the C@Cskeletal stretching of the benzene ring of the 4 monolayer.Upon interfacial Michael-addition reaction of p-quinonewith glutathione, a new absorption band appeared at�1725 cm�1 which is attributed to the C@O stretching ofthe immobilized glutathione on surface and an alternateevidence of occurring the interfacial reaction.

4. Conclusions

Interface electrochemistry at nanoscale SAMs structuresas promising solid–liquid interfaces are attracting greatinterests, especially in the biological applications like inter-

actions of materials with the proteins, enzymes and otherbio-related compounds. It is also possible to transformthe chemical and biological activities to the electrical sig-nals, and detect and interpret the activities using the elec-trochemical techniques such as cyclic voltammetry andelectrochemical impedance spectroscopy. Elucidation ofthe kinetics and thermodynamics of chemical reactions willbe feasible using the mentioned techniques. It should benoted that this procedure introduces an important way tocontrol the surface behavior, due to having a dynamic sub-strate which can control over the surface reactivity. In thepresent study, a novel hydroquinone-terminated alkaneth-iol as an electroactive probe molecule was attached ontothe gold surface via self-assembly process and its electro-chemical behavior was investigated for the Michael-addi-tion reaction of the electrochemically generatedp-quinone and a glutathione. Some electrochemical andkinetic parameters have been evaluated and discussed.The electrochemical and spectroscopic methods were usedto prove the Michael-addition reaction. This study presentsa new horizon to control the reactivity over the nanostruc-ture solid–liquid interfaces which will become important inbiological and chemical interactions like drug deliveryusing gold nanoparticles capped with functional thiols. Itshould be mentioned that the effects of the concentrationand nature of the nucleophile on the kinetics of theMichael-addition reaction are under more investigations.

Acknowledgement

The support of this work by a research grant fromIran National Science Foundation (INSF) is gratefullyacknowledged.

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