The electrochemical study of w-mercaptohexanoic acid self-assembled monolayer modified gold electrode

Tu Yifeng , Sun Ru, Gu Renao
(College of Chemistry and Chemical Engineering, Suzhou University, Suzhou, 215006, China)

Received July 21, 2004.

Abstract An w-mercaptohexanoic acid self-assembled monolayer modified gold electrode was studied in this paper. The surface structure and properties were characterized by surface enhanced Raman scattering spectrum and electrochemical method. The experimental results showed that the double-layer capacitance of electrode decreased from 71m F/Cm² to 18m F/Cm² after forming a high coverage(96%) self-assembled monolayer. The electrons transfer associated with the Fe(CN)₆^3- /Fe(CN)₆^4- redox reaction is considered as an electron tunneling process, the apparent electron transfer rate constant is 2.47× 10^-5 cm/s.
Keywords w-Mercaptohexanoic acid; Self-assembled monolayer; Chemical modification; Electrochemical property

1. INTRODUCTION
The self-assembled monolayers(SAMs)^[1] have the remarkable characteristics of in situ spontaneous formation, thermodynamical steady, highly ordered, densely stacking and low defect. The sulfhydryl alkane compounds is a kind of priority species in this field due to their peculiar molecular structure^[2] and the reaction with Au^[3]. The bond energy of Au-S (184KJ/mol) is strong enough to ensure the selectivity of the adsorbing process. The characters of outer surface and the order degree of SAMs are mainly influenced by the length of alkyl chain and the property of end group -R^[4]. The ordered monolayer is formed if the carbon number is larger than nine. And the influence on the Au/S interface will be shielded by the carbon chain when it was longer than five. These are the important reasons of why the w-mercaptohexanoic acid(6-MHA) had been selected to carry on the researches in this paper.

2. EXPERIMENTS
2.1 Instruments and chemicals
A RDE₄ Potentiostat (Pine Corporation, USA) and a BAS-100A Electrochemical Analyzer (Bioanalytical Systems Inc., USA) were used in all of electrochemical studies. A HRD-1 Raman Spectrometer (Jobin Yvon Corporation, France) with a 40mW 488.0nm Argon Laser (A-240 Type) was used to characterize the surface properties.
    w-Mercaptohexanoic acid^[5] was synthesized in our laboratory. It had been identified by FT-IR and NMR. IR Wavenumbers(cm^-1): 733.0(C-S), 1709.1(C=O), 2573.2(S-H), 2858.7, 2935.9(both CH₂ stretch), d_H of ¹H-NMR(CDCl₃) (ppm): 1.328(t, SH), 10.850(s, COOH).
    Acetone, potassium ferricyanide, sulfuric acid and other reagents are all of analytical grade.
2.2 The preparation and modification of electrode
The surface of a gold electrode was polished and then washed with acetone and redistilled water in an ultrasonator. In 1.0mol/L H₂SO₄ solution, with a SCE reference electrode and a Pt auxiliary electrode, cyclic scan in range of -0.2V to1.2V to pretreat the electrode surface till a steady voltammogram was obtained. Then, cyclic scan in 0.1mol/L KCl solution with scanning rate of 500mV/S to obtain a rough electrode surface^[6]. The true surface area equals to 0.042cm² which was calculated from the result of coulometric electrolysis^[7]. After washed by redistilled water and acetone, the pretreated electrode was dipped into 0.1mol/L 6-MHA solution of acetone under the protection of pure nitrogen gas to bring about the self-assembling modification.
2.3 Study of the electrochemical properties of 6-MHA SAM
Cyclic voltammetric scanned from -0.2V to 0.8V (vs. SCE) for studying the double layer capacitance of electrodes, and use the Fe(CN)₆^3-/4- as the probe to study the  transfer process of the electrons by CV.

3. RESULTS AND DISCUSSION
3.1 Characterization of the SAM by Surface Enhanced Raman Scattering Spectrum(SERS)
It was proved by SERS(see Fig. 1) that the 6-MHA molecules were linked on the surface of gold electrode. The stretching vibration bands of S-H of 6-MHA(2574 cm^-1 on curve a) disappeared on the spectrum of 6-MHA SAM, and the C-S stretching vibration band(714cm^-1 on curve a, 807 cm^-1 on curve b) was enhanced. It could be concluded that the S-H bond in 6-MHA had been replaced by S-Au bond due to the strong affinity between S and Au. The other atoms of the carbon chain ranged away from electrode surface gradually, so the C-S stretching vibration was enhanced. The 6-MHA molecules linked on the surface of gold electrode by C-S bond, the -COOH groups were far apart from the surface.

Fig.1 (a) Raman spectrum of 6-MHA in liquid and (b) SERS spectrum of 6-MHA SAM on gold electrode

3.2 The change of double layer capacitance
The experimental results had proved that, when the exterior orientational arranged water molecules of electrode were replaced by organic molecules, the double layer capacitance would decrease on account of the larger distance between the charged layers and the smaller dielectric constant. The 6-MHA played the role in this SAM. The cyclic voltammetric curves in Fig.2 showed that the capacitance current of gold electrode decreased after modified by 6-MHA. According to the equation
C_d = i/vS (1)
    Here i is capacitance current; v is scan rate and S is the surface area of electrode. The double layer capacitance of SAM modified electrode was then calculated^[8] for 18 m F/cm², which is smaller than the bare gold electrode of 71 m F/cm².

Fig.2 The CV curves of (a) bare gold electrode and (b) SAM modified electrode in 1.0mol/L H₂SO₄ solution. Scan rate: 100mV/s; real surface area: 0.042cm²

3.3 Study of the electron transfer property of 6-MHA SAM
A conventional Fe(CN)₆^3-/Fe(CN)₆^4- couplet was selected as a probe to study the electron transfer property. Its cyclic voltammetric responses on a bare electrode and the SAM electrode were different obviously (see Fig.3). On the CV curve of the SAM electrode, the reduction and oxidation peak currents were both smaller than that on bare electrode and the difference of peak potentials was much larger than 60mV. It revealed that the electrochemical process on the SAM modified electrode was not diffusion controlled.

Fig.3 Cyclic voltammetric curves of a bare electrode(a) and the 6-MHA/Au(b) in 1.0mol/L KCl solution containing 1.0mmol/L Fe(CN)₆^3-. Scan rate: 100mV/s

    In the aforementioned supporting electrolyte solution, the peak current and peak potential of Fe(CN)₆^3- on SAM electrode responded upon the self-assembling degree that was controlled by immersion time of electrode in 6-MHA solution. The relative curves were showed in Fig.4 and Fig.5.

Fig.4 The response of cathodic peak current of Fe(CN)63- upon the self-assembling degree of the electrode.	Fig.5 Relation between peak potential difference and the immersion time

　From the above two curves, the self-assembling process could be divided into three periods. The first period (part a in Fig. 4) is a fast adsorption process for about thirty min. In this process, the peak currents decreased rapidly with the increase of dipping time due to the strongly affinitive action between S and Au atoms. The formation of S-Au bond brought about the adsorption of 6-MHA on gold electrode surface. This process would complete within about 30 minutes and obeyed the adsorption equation^[9]:
ln(1-q ) = -K_adct (2)
    Here q is the coverage rate of adsorptive molecules on electrode surface, c is the concentration in solution, t is the immersion time.
    The line in Fig.6 proves the aforementioned recount. The average adsorption rate of the electrode was calculated from the slope as 3.67×10^-3M^-1s^-1.

Fig.6 Relation between ln(1-q ) and the immersion time in first period of the adsorption process

    The second period (part b in Fig. 4) is a slower orientation and arrangement process of carbon chain. The decreasing of peak current and the enlarging of peak potential difference got slow down gradually. It needs 1.5 hours more. In this period, the molecules of 6-MHA had adopted an identical orientation and formed a fine arrangement.
    The third period is in equilibrium state. After immersing the gold electrode into 6-MHA’s acetone solution for two hours, the self-assembled process would achieve the equilibrium state. The peak current and peak potential difference did not change any more. The coverage of SAM was calculated for 96% in this state by the results of coulometry of bare electrode and SAMs modified electrode in Fe(CN)₆^3-/Fe(CN)₆^4- solution.
    From the previous discussion about the SAM, the mechanism of redox reaction of Fe(CN)₆^3-/Fe(CN)₆^4- could be considered that the diffusion controlled process was replaced by an electron transfer process after the formation of SAM. The 6-MHA SAM had obstructed the electron transfer process between the gold electrode and probe couplet. It can be explained with a tunneling process^[10] that the electrons passed through the SAM via a tunnel, the tunneling coefficient is 1.02± 0.2 per methylene. The apparent electron transfer rate constants^[11] have been calculated respectively for bare electrode of 9.13×10^-4 cm/s and 6-MHA SAM electrode of 2.47×10^-5 cm/s.

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