Electrochemical investigation on the film of L-cysteine self-assembled to nanoparticles on a gold electrode

doi:10.4236/jbpc.2012.31005

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Vol.3, No.1, 39-43 (2012) Journ al of Biophysical Chemistry

http://dx.doi.org/10.4236/jbpc.2012.31005

Electrochemical investigation on the film of

L-cysteine self-assembled to nanoparticles

on a gold electrode

Wenting Wang1, Chunming Wang1*, Xiaoquan Lu2*

1Department of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, China;

*Corresponding Author: [email protected]

2Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineer-

ing, Northwest Normal University, Lanzhou, China; *Corresponding Author: [email protected]

Received 27 October 2011; revised 10 December 2011; accepted 21 December 2011

ABSTRACT

The film contained L-cysteine and gold nanopar-

ticles were provided by self-assembled monola-

yers (SAMs) and potentiostatic electrodeposi-

tion technology on the gold electrode. Two

methods were used to study the film: In the first,

cyclic voltammetr y (CV) was used to inspect the

functional groups of the film and the same time

hydroquinone was chosen to be a probe mole-

cule in the based solution; secondly, based on

analytical technology of scanning electrochemi-

cal microscopy (SECM), the heterogeneous rate

constant (keff) bet ween solid phase (the modified

electrode) and liquid phase (K3Fe(CN)6) was ob-

tained. As a result, the better binary catalysis of

hydroquinone was demonstrated and the het-

erogeneous rate constant (keff) is the greater at 8

h for L-cysteine self-assembled monolayers (SAMs).

Keywords: S E CM; L-Cysteine; Hydroquinone;

Self-Assembled Monolayers; Heterogeneous Rate

Constant

1. INTRODUCTION

As well-known, L-cysteine of the favorable electro-

chemical activity is a common amino acid in the living

creature. Because of the special functional end groups,

such as -SH, -COOH and -NH2, it attracted many re-

searchers in different fields deeply. Nano-material has

been a hot topic recently. The specific surface area of the

modified electrode with nanoparticles by electrochemical

technology is larger than that of bare electrode [1]. The

new structure of L-cysteine on the Au (111) surface was

investigated by situ electrochemical scanning tunneling

microscopy [2,3]. Surface-enhanced Raman spectros-

copy (SERS) and surface-enhanced second-harmonic

generation (SESHG) were employed to investigate the

adsorption of L-cysteine on a polycrystalline silver elec-

trode [4]. What matters most was that self-assemble

monolayers (SAMs) was a simple and stable method in

the study of modified electrode, and it can obtain spon-

taneously high-order monolayer on gold via the thiol end

group [5-8]. The electrode modified directly with L-

cysteine SAMs can investigate various performances,

including the following several aspects, voltammetric

behavior of vitamin B2, selective response of dopamine in

ascorbic acid and electrochemical behavior of epineph-

rine [9-11]. Previously, the modified electrode with the

biochemical molecular L-cysteine was usually used to

catalyze the quinones in the water solution. However, the

research on the electrode modified with nanoparticles

and L-cysteine is seldom advanced. Moreover, hydro-

quinone, as electron carrier in photosynthesis and respi-

ration, plays a key role in biological energy metabolism.

The electrochemical process of quinone is usually invest-

tigated by cyclic voltammetry (CV) at glassy carbon

electrode [12,13], and electroactive quinone as terminal

group is immobilized on electrode to investigate the

well-know electron transport (ET) [14,15]. As an elec-

trochemical technology, scanning electrochemical mi-

croscopy (SECM) is a crucial tool to study electron

transfer (ET) at solid/liquid interface, liquid/liquid inter-

face, and it has several advantages over cyclic voltam-

metry (CV), such as allowing more informations to be

extracted from a single measurement [16-18].

Here, we first modified the gold electrode with the

gold nanoparticles by potentiostatic electrodeposition

and then applied self-assembly method to modify L-

cysteine on the gold nanoparticles. SECM was employed

to observe heterogeneous phase ET reaction between the

electrode and mediator. In the presence of hydroquinone

as a probe molecule, CV was used to investigate electro-

chemical response of the film, which consisted of gold

W . T. Wang et al. / Journal of Biophysical Chemistry 3 (2012) 39-43

nanoparticles and L-cysteine on the gold electrode,

2. MATERIAL AND METHODS

2.1. Chemicals

L-cysteine was received from the Factory of Caoyang

second middle school (Shanghai, China). Hydroquino-

neand HAuCl4·4H2O was from Fucheng Chemical Re-

gent Factory (Tianjin, China), Sinopharm Chemical Re-

gent Co., Ltd (Shanghai, China), respectively. All other

reagents were analytical-reagent grade, unless otherwise

specified. Solutions were prepared from water that had

been purified through an Ultra-pure water system Milli-

Q Plus (Millipore).

2.2. Preparation of Substrate and SAMs

A gold electrode (Diameter = 2 mm) was used as the

substrate for these experiments. The electrode was pol-

ished to a mirror finish using 0.05 μm alumina powder

before careful rinsing with deionized (DI) water, sonica-

tion in absolute ethanol and DI water for 10 min in turn,

and then dried with nitrogen gas. The gold electrode was

modified with gold nanoparticles by potentiostatic elec-

trodeposition at –0.2 V in 3 mM HAuCl4 solution [1].

SAMs was prepared with a solution of 1 mM L-cysteine

in phosphate-buffered solution (pH = 7).

2.3. Electrochemical Measurements

Electrochemical experiments were carried out using a

CHI 900 scanning electrochemical microscope (CH In-

struments Co. Ltd, Austin, USA) with a four-electrode

cell. The gold (or modified gold) electrode, a platinum

wire, and a KCl saturated Ag/AgCl electrode were used

as working, counter and reference electrode, respectively.

The SECM tip was a 25-m diameter Pt ultramicroelec-

trode (UME). Before each experiment, the tip was pol-

ished with 0.3-m alumina and rinsed with DI water.

3. RESULTS

3.1. Electrochemical Characterization of the

SAMs

Electrochemical means of cyclic voltammetry (CV)

have been performed to characterize the SAMs. Figure

1(A) showed CVs for 1 mM K3Fe(CN)6 containing 0.1

M KCl at a) a clean gold electrode, b) a gold electrode

modified with nano gold, c) L-cysteine self-assembled to

the nanogold electrode.

For the bare Au electrode, the typical CV response

shape of ferricyanide was performed. The peak separa-

tion of 70 mV showed a reversible voltammogram for

the redox couple, indicating a diffusion-limited or elec-

trochemical quasi-reversible one electron redox process.

(A)

(B)

Figure 1. (A) Cyclic voltammograms in 1 mM

K3Fe(CN)6 at scan rate 100 mV/s. Insert: Cyclic

voltammograms in 1 M H2SO4 at scan rate 50

mV/s. (B) Impedance spectra corresponding in 1

mM K3Fe(CN)6 with 0.1 M KCl as supporting

electrolyte. Insert: The equivalent circuit model

used to obtain equations for Zre and Zim. W.E is

the working electrode, R.E is the reference elec-

trode, and C.E is the counter electrode.

When the bare gold electrode was modified with gold

nanoparticles, the peak current increased obviously in the

solution of either 1 mM K3Fe(CN)6 or 1 M H2SO4. Com-

pared with the bare and nanogold electrode, the electrode

modified with gold nanoparticles and L-cysteine showed

the better reversible CV response in K3Fe(CN) 6. This

may be the protonation of -NH2 in L-cysteine and the

electrostatic interaction with K3Fe(CN)6 [19]. Figure 1(B)

showed the electrochemical impedance spectroscopy

(EIS) of Nyquist plots. Among them, a) represented the

EIS curve of the bare gold electrode, where had a straight

line at low-frequency and a small semicircle at a high-

frequency region. The phenomenon demonstrated that

the process was essentially diffusion-controlled for the

redox couple. The electrode modified with gold nanopar-

ticles was used as the working electrode in b). Different

from the above dates, c) indicated that L-cysteine had

been self-assembled on gold electrode modified with

gold nanoparticles previously.

W . T. Wang et al. / Journal of Biophysical Chemistry 3 (2012) 39-43 41

3.2. Heterogeneous Rate Constant (keff) by

Scanning Electrochemical Microscopy

(SECM)

Figure 2 shows the schematic diagram of the electro-

chemical cell and the SECM tip.

The 25 m diameter Pt ultramicroelectrode (UME),

which is made by our own laboratory, were used to in-

vestigate the modified gold electrode surface and record

the feedback approach curve at the different concentra-

tion of the mediator solution (K3Fe(CN)6). In the SECM

experiments, the modified electrode was used as the sub-

strate. Because the terminal groups contain -COOH and

-NH2, we modified electrode with COOH-terminated

alkanethiol for the distinction. It was found that the feed-

back approach curve of SAM with COOH-terminated

alkanethiol was negative from 0.5 V to 0.7 V (the sub-

strate potential (Es)) vs Ag/AgCl with 1 mM 6



Fe CN



in 0.1 M KCl, –0.1 V (UME tip potential) vs Ag/AgCl in

Figure 2 (Inser), which was consistent with the previous

report [4]. However, the feedback curve for L-cysteine

SAMs was just positive at 0.5 V (the substrate potential

(Es)) vs Ag/AgCl with 1 mM 6 in 0.1 M KCl,

–0.1 V (UME tip potential) vs Ag/AgCl, which was at-

tribute to the terminal group: -NH2 in the L-cysteine

SAMs on the modified electrode with the nano gold. For

this reason, we conjectured that -NH2 in the L-cysteine

SAM was the key role in the ET process between the

modified electrode and the mediator (K3Fe(CN)6). Table

1 showed the heterogeneous rate constant (keff) of the

modified gold electrode with the different assembling

time by L-cysteine.



Fe CN

Figure 2. Scheme of the modified electrode and the setup

by SECM. Insert: experimental approach curves.

Table 1. Heterogeneous rate constant (keff) of SAMs with L-

cysteine on modified electrode with gold nanoparticles.

Assembling time (h) 2 3 4 5 6 7 8

keff (10–3) cm/s 6.0 7.0 6.4 6.8 6.6 6.6 7.1

The heterogeneous rate constant (keff) was the greater

at 3 h and 8 h. During the initial stage of self-assembling,

the -SH of L-cysteine by Au-S bond attached to the gold

nanoparticles in major. The coverage of L-cysteine on

gold nanoparticles was relatively large at 3 h and Au-S

bond was the main factor. Subsequently, due to the con-

nection between -NH2 and the gold nanoparticles, the Au-

NH bond may replace the Au-S bond in a certain degree.

The film, including the gold nanoparticles and L-cysteine,

reached the new equilibrium at 8 h and performed the

greater heterogeneous rate constant (keff) once again.

3.3. Cyclic Voltammetry (CV) with

Hydroquinone as a Probe Molecule

Compared to the previous work, the research on the

self-assembled time was rarely mentioned. Here, at the

same modified condition, we paid the more attention on

the assembling time of SAMs.

Figure 3 showed the cyclic voltammograms of hy-

droquinone modified electrode at different assembling

time in acetate buffer solution (pH = 3.8). Obviously, in

the first stage (t = 2 h, 3 h, 4 h), the oxidation peak had a

negative shift and peak current increased at 3 h (Figure

3(A)). In the next stage (t = 5 h, 6 h, 7 h, 8 h), we found

that the better change at 8h and the oxidation peak had a

negative shift and peak current evidently increased (Fig-

ure 3(B)). In the further research, we have chosen the

three different assembling times, 3 h, 5 h, 8 h (Figure 3

(C)). It showed the oxidation peak position of 3h and 8h

were approximately same. However, the potential differ-

ence of 8h was the smaller than that of 3 h. Figure 4

showed the three possible SAMs and the reaction with

the hydroquinone, where L-cysteine was self-assembled

in the phosphate buffer solution (pH = 6.8).

Figure 3. Cyclic voltammograms of the modified gold elec-

rode at different assembling time.

W . T. Wang et al. / Journal of Biophysical Chemistry 3 (2012) 39-43

Figure 4. Three different possible SAM and the reaction of hydroquinone.

Because of the existence of the -COOH and -NH2, the

close to neutral solution was chosen for avoiding the

effect of pH. Moreover, we supposed that double SAMs

of L-cysteine may be formed at the appropriate pH con-

ditions, namely close to the neutral solution. As above

discussion, there were three kinds of the terminal groups

in SAMs of L-cysteine: -NH2,-COOH and -SH, and in

the assembling process, the three situations didn’t present

separately. The CVs of modified electrode in hydro-

quinone as probe molecule accorded with the heteroge-

neous rate constant (keff) between the film and the me-

diator solution. That strongly suggestted quinone could

be applied as the probe molecule, and further to learn the

film containing some special functional groups on the

electrode.

sis of CVs, it was found that L-cysteine performed the

better electrochemical response at 8 h (self-assembling

time) with hydroquinone as a probe molecule in acetate

buffer solution (pH = 3.8). The heterogeneous rate con-

stant (keff) obtained from the feedback approach curves of

SECM was aslo greater at 8h. In the low pH-value solu-

tion, the functional group which reacted with hydro-

quinone was mostly -NH2, and -COOH by the free hy-

drogen ion. We confirmed that what was the optimal as-

sembling time, and which functional group mainly par-

ticipated in the reaction. Moreover, the modified elec-

trode with the film performed the binary catalysis in

acetate buffer solution of hydroquinone.

5. ACKNOWLEDGEMENTS

3.4. Binary Catalysis of the Modified

Electrode This work was supported by the Natural Science Foundation of

China (No. 20775060, 20875077), the Key Project of Scientific Re-

search Base of education department (08zx-07) and the Key Laboratory

of Ploymer Materials of Gansu Province.

Figure 5 showed the cyclic voltammograms of hy-

droquinone in acetate buffer solution (pH = 3.8) at the

different modification on gold electrode. It was worth to

point out that the gold electrode, which was modified

with gold nanoparticles and then self-assembled with L-

cysteine, performed the preferable capability of catalysis

for hydroquinone. From a view point of catalysis, this

catalytic reaction should be called binary catalysis, that

meant both nano gold and L-cysteine participated the

binary catalysis reaction.

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