Electrochemical Detection of Catechol on Boron-doped Diamond

Electrode Modified with Au/TiO2 Nanorod Composite

Min Wei,a,b Yong Liu,c Zhong-Ze Gub,d and Zhong-Dong Liua*aCollege of Food Science and Technology, Henan University of Technology, Zhengzhou 450052, P. R. China

bState Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering,

Southeast University, Nanjing 210096, P. R. ChinacSchool of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, P. R. China

dSuzhou Key Laboratory of Environment and Biosafety, Research Institute of Southeast University in Suzhou,

Dushu Lake Higher Education Town, Suzhou 215123, P. R. China

Received November 1, 2010; Accepted March 7, 2011; Published Online March 25, 2011

Au/TiO2 nanorod composites with different ratios of [TiO2]:[Au] have been prepared by chemically

reducing AuCl4� on the positively charged TiO2 nanorods surface and used to modify boron-doped dia-

mond (BDD) electrodes. The electrochemical behaviors of catechol on the bare and different Au/TiO2

nanorod composites-modified BDD electrodes are studied. The cyclic voltammetric results indicate that

these different Au/TiO2 nanorod composites-modified BDD electrodes can enhance the electrocatalytic

activity toward catechol detection, as compared with the bare BDD electrode. Among these different con-

ditions, the Au/TiO2-BDD3 electrode (the ratio of [TiO2]:[Au] is 27:1) is the most choice for catechol de-

tection. The electrochemical response dependences of the Au/TiO2-BDD3 electrode on pH of solution and

the applied potential are studied. The detection limit of catechol is found to be about 1.4 × 10-6 M in a lin-

ear range from 5 × 10-6 M to 200 × 10-6 M on the Au/TiO2-BDD3 electrode.

Keywords: Boron-doped diamond; Au/TiO2 nanorod composites; Electrochemical sensor;

Catechol.

INTRODUCTION

The widespread application of catechol in various

fields such as photographic chemicals, pesticides and med-

icines may produce environmental pollution with high tox-

icity.1,2 In addition, catechol is concerned with neurotrans-

mission processes and its concentration in different body

fluids serves as a prognostic marker for several diseases

such as pheochromocytoma and neuroblastoma.3,4 Thus the

analysis of catechol is significant for environmental pro-

tection and biological detection. Among the various meth-

ods for catechol detection, the electrochemical methods

have more and more been widely attended because of their

simple procedure, fast response and inexpensive instru-

mentation.5,6

As the new electrode materials, the boron-doped dia-

mond (BDD) electrodes attracted much attention due to

their superiority to other electrodes in terms of high signal

to noise (S/N), long-term stability, high sensitivity and

good reproducibility.7-11

On the other hand, nanomaterial-modified electrodes

have been increasingly attended aiming to attain better per-

formance by enhancing the electrode conductivity, facili-

tating the electron transfer and improving the sensitivity

and selectivity.12-16 Moreover, due to their structure-, size-

dependent properties, the materials with various shapes

have distinct effects on electrode modification.17-19 The

metal/TiO2 composites with different nano-structure have

been reported to construct electrochemical sensors due to

their superior properties such as high active electrode sur-

face area, good biocompatibility, high electrocatalytic ac-

tivity, long-term chemical stability and interface-domi-

nated properties.20-23

In the present work, Au/TiO2 nanorod composites

have been prepared by chemically reducing negatively

charged AuCl4� on the positively charged TiO2 nanorods

surface. We utilized these Au/TiO2 nanorod composites to

516 Journal of the Chinese Chemical Society, 2011, 58, 516-521

* Corresponding author. E-mail: wei_min80@163.com

modify BDD electrode and studied its electrochemical cat-

alytic activity toward catechol detection, and further ex-

plored the response dependences and amperometric char-

acteristics including sensitivity, linear range and detection

limit.

RESULTS AND DISCUSSION

Characterization of the prepared Au/TiO2 nanorod

composites

Fig. 1 shows the representative TEM images of naked

TiO2 nanorods (a) and Au/TiO2 nanorod composites (b).

The naked TiO2 nanorods had smooth surfaces with an av-

erage diameter of ca. 60-80 nm, and the length of several

micrometers. The process of [AuCl4]� reduction gave the

dark spots and roughness on the surface of TiO2 nanorods,

which proved that Au have been grown on the TiO2 nano-

rods surface.

Electrochemical response on the different Au/TiO2

nanorod composites modified BDD electrodes

Fig. 2 shows the cyclic voltammograms (CVs) for 4.0

× 10-3 M K3Fe(CN)6 in 0.07 M PBS (pH 7) obtained on bare

and different modified BDD electrodes. Well-defined CVs

of K3Fe(CN)6 were obtained on these electrodes, indicating

nearly reversible electron transfer kinetics for these elec-

trode interfaces. Compared with those obtained on the bare

BDD electrode, the peak current responses increased and

the �Ep (peak-to-peak separations) reduced in varying de-

grees on the different modified BDD electrodes, which was

contributed to the interaction of Au nanoparticles and TiO2

nanorods. On the Au/TiO2-BDD1 electrode, when the ratio

of [TiO2]:[Au] is 9:1, the existence of nano-Au on TiO2

nanorods could promote electron transfer and accelerate

the reaction rate, and the oxidation peak current increased

from 46.72 �A obtained on bare BDD electrode to 57.13

�A obtained on Au/TiO2-BDD1 electrode, and the �Ep re-

duced from 318 mV obtained on bare BDD electrode to 249

mV obtained on Au/TiO2-BDD1 electrode. However, the

enhanced degree of current response gradually decreased

by increasing the concentration of TiO2 nanorods, which

may be ascribed that the semi-conductivity of TiO2 nano-

rods hindered the electron transmission and made interfa-

cial charge transfer more difficult on the electrode surface.

Electrochemical responses for 5 × 10-4 M catechol on

the bare and different modified BDD electrodes were in-

vestigated in 0.07 M PBS solution (pH 7). As shown in Fig.

3, on the bare BDD electrode, the oxidation of catechol and

reduction of the product occurred at +1.15 V and -0.3 V re-

spectively, indicating the sluggish electrocatalytic process.

However, the existence of Au/TiO2 nanorod composite on

the modified BDD electrodes could accelerate electron

transfer and enhance electrocatalytic activity toward cate-

chol detection. The current response increased and oxida-

tive peak potential of catechol shifted negatively in various

Detection of Catechol on Au/TiO2 Nanorod Modified BDD J. Chin. Chem. Soc., Vol. 58, No. 4, 2011 517

Fig. 1. TEM images of TiO2 nanorod (a) and Au/TiO2

nanorod (b).

Fig. 2. Cyclic voltammograms (CVs) for 4.0 × 10-3 M

K3Fe(CN)6 in 0.07 M PBS (pH 7) obtained on

the bare and different modified BDD elec-

trodes. Scan rate was 50 mV s-1.

extents on the different Au/TiO2 nanorod composite-modi-

fied BDD electrodes. Though the semi-conductivity of

TiO2 nanorods hindered the interfacial charge transfer on

the electrode surface to a certain degree by detecting the

electrochemical probe Fe(CN)63-, herein, the results from

Fig. 3 demonstrated that the existence of TiO2 nanorods

promote the electron transfer and enhance the current re-

sponse for the detection of catechol, which may be ascribed

that TiO2 can facilitate adsorption and enrichment of cate-

chol onto the electrode surface of by forming bidentate

groups in the form of bidentate mononuclear and binuclear

structures.28-30 The current responses increased and the oxi-

dation peak potential shifted in the negative direction as the

ratio of [TiO2]:[Au] increased from 9:1 to 27:1, and when

the ratio of [TiO2]:[Au] is increased further to 36:1, the en-

hancement of current response and the negative shift of ox-

idation peak potential decreased slightly. So, the Au/TiO2-

BDD3 electrode (the ratio of [TiO2]:[Au] is 27:1) was the

most choice for electrochemical sensor for catechol detec-

tion. The oxidation peak potential of catechol shifted from

1.149 V on the bare BDD electrode to 0.95 V on the

Au/TiO2-BDD3 electrode, i.e., the oxidation potential of

catechol shifted by 199 mV in the negative direction, and

the oxidation current increased by 20.25%, exhibiting evi-

dence for the electrocatalytic oxidation of catechol. These

results suggested that the optimum ratio of Au/TiO2 could

show high electrocatalytic activity and promote the detec-

tion of catechol. Here, TiO2 could adsorb and facilitate

catechol to reach the electrode surface, and Au possessing

good conductivity could act as nanoscale electrodes that

electrically communicate between TiO2 and bulk electrode

material,31 thus accelerate electron transfer efficiently on

the electrode surface, promote the catalytic activity and

enhance the electrochemical performance.

In addition, the point to emphasize here is the impor-

tance for purification of the prepared nanocomposites, and

the reason is as follows: Sodium borohydride used as re-

ducing reagent for Au preparation may produce boric acid

in acidic conditions, and boric acid acting as an electron-

pair acceptor can combine with catechol to form catechol-

boric acid complexes, which is also electroactive and elec-

trochemically oxidized, thus can influence the effect of

catechol detection.32-34

Optimization of the experimental conditions

The response dependence for 5 × 10-4 M catechol on

pH value in 0.07 M PBS solution on the Au/TiO2-BDD3

electrode was investigated. As shown in Fig. 4, the oxida-

518 J. Chin. Chem. Soc., Vol. 58, No. 4, 2011 Wei et al.

Fig. 3. CVs for 5 × 10-4 M catechol in 0.07 M PBS (pH

7) obtained on the bare and different modified

BDD electrodes. Scan rate was 50 mV s-1.

Fig. 4. (A) CVs for 5 × 10-4 M catechol and (B) plot of

oxidation potential and current vs. pH obtained

on Au/TiO2-BDD3 electrode in 0.07 M PBS

with different pH values. Scan rate was 50 mV

s-1.

tion peak potentials shifted in the negative direction as so-

lution pH increased in the beginning, and when pH was in-

creased from 7 to 7.5, the potential change was not obvi-

ously. The current responses increased as the pH changing

from 5 to 7, and then decreased slightly at pH 7.5. So the

optimum result occurred at pH 7.0. The results were basi-

cally in accordance with the literature,29,30 which studied

the optimum adsorption conditions of a variety of catechols

on the TiO2 surface.

The effect of different applied potential on the current

response was studied over the potential range from +0.7 V

to +1.05 V versus Ag/AgCl with 5 × 10-4 M catechol in 0.07

M PBS at pH 7.0. From the results shown in Fig. 5, it can be

seen that the signal current and the background current in-

creased as the applied potential increased. A maximum ra-

tio of signal-to-background current was obtained at +0.9 V

(as shown in curve c). When the applied potential was more

positive than +0.9 V, a higher signal current was achieved

(as shown in curve b), but the background current in-

creased more rapidly (as shown in curve a). So, a working

potential of +0.9 V was preferred for the subsequent am-

perometric experiments.

Amperometric response for the Au/TiO2-BDD3 elec-

trode

Fig. 6(A) illustrates a typical amperometric response

for the Au/TiO2-BDD3 electrode at +0.9 V (vs. Ag/AgCl)

after the addition of successive aliquots of catechol to the 5

mL, 0.07 M PBS solution at pH 7 under constant stirring. A

well-defined current response was observed with increas-

ing the concentration of catechol. It could be observed that

the Au/TiO2-BDD3 electrode responded rapidly within 5 s

to the oxidation of catechol when an aliquot of catechol

was added into the system. Such fast response was attrib-

uted to the rapid electron transfer between Au/TiO2 nano-

composites and the BDD electrode. The inset of Fig. 6

(curve B) shows the dependence of the steady-state current

on the catechol concentration. Linearity was observed

within the range 5 × 10-6 M to 200 × 10-6 M (r = 0.999). A

curvature was observed for higher concentration as a con-

sequence of slow surface fouling by the reaction products.

Sensitivity corresponding to the linear range for catechol

was 51.58 �A mM-1, which is higher than that obtained on

Horseradish peroxidase biosensor and tyrosinase biosen-

sors-modified electrodes.35-37 Moreover, the detection limit

of catechol was found to be about 1.4 �M according to the

formula 3sb/m criteria, which is lower than that obtained in

previous report,38,39 demonstrating that Au/TiO2-BDD3

electrode is a better choice for catechol detection.

The study of reproducibility and stability on the

Au/TiO2-BDD3 electrode

The reproducibility and stability of the Au/TiO2-

BDD3 electrode were studied. A relative standard devia-

tion of 4.1% was estimated from the slopes of the calibra-

tion plots at five freshly prepared Au/TiO2-BDD3 elec-

trodes. At a concentration of 5 × 10-4 M catechol, the Au/

TiO2-BDD3 electrode showed relative standard deviation

Detection of Catechol on Au/TiO2 Nanorod Modified BDD J. Chin. Chem. Soc., Vol. 58, No. 4, 2011 519

Fig. 5. Dependence of amperometric response on the

applied potential on the Au/TiO2-BDD3 elec-

trode in pH 7.0 PBS solution without (a) and

with (b) 5 × 10-4 M catechol. Inset: (c) Curve on

the ratio of the current of (b) to (a) vs. applied

potential.

Fig. 6. (A) Successive amperometric response and (B)

calibration curve (inset figure) for increasing

catechol concentration from 5 × 10-6 M to 275 ×

10-6 M on the Au/TiO2-BDD3 electrode in pH

7.0 PBS at +0.9 V (vs. Ag/AgCl).

of 4.7% examined for five determinations. The modified

electrode was stored in pH 7.0 PBS at 4 °C when it was not

in use. When the cyclic voltammogram was recorded once

each day, the response of catechol remained 89.7% of its

initial response after 30 days. A decrease of 3.9% ampero-

metric response was observed after 30 days. These results

demonstrated that the reproducibility and stability of the

Au/TiO2-BDD3 electrode were acceptable, and it was

suitable for the determination of catechol.

CONCLUSIONS

In this work, Au/TiO2 nanorod composites were pre-

pared by chemically reducing HAuCl4 on the TiO2 nano-

rods surface and used to modify BDD electrode. The re-

sults demonstrated that the Au/TiO2 nanorod composites-

modified BDD electrode showed the favorable electro-

catalytic activity toward the detection of catechol with fast

response, high sensitivity and low detection limit, as com-

pared with the bare BDD electrode.

EXPERIMENTAL

Chemicals and apparatus

Rutile TiO2 nanorods (acicular-type TiO2, FTL-200)

were obtained from Ishihara Sangyo Co., Ltd., Japan. All

other chemicals for the synthesis of nanocomposite and

Catechol were obtained from Wako Chemicals. Milli-Q

water (>18 M� cm) was used throughout the experiments.

The pH value of the solution was adjusted to about 1.5 with

hydrochloric acid (35-37%). The supporting electrolyte

was 0.07 M phosphate buffer solution (PBS) prepared with

Na2HPO4 and KH2PO4.

Electrochemical measurements were performed on a

potentiostat (HZ-5000, Hokuto Denko, Japan) with a three

electrode electrochemical cell. The geometric area of BDD

electrode was 0.07 cm2. A Ag/AgCl (saturated KCl) refer-

ence electrode and a Pt wire counter electrode were used.

All measurements were made at room temperature in solu-

tions deoxygenated with N2 for 10 min and maintained un-

der nitrogen atmosphere during measurement. The UV-vis-

ible absorption spectra were recorded on a Shimadzu

UV-2450 spectrophotometer equipped with an integrating

sphere. The prepared materials were characterized by TEM

and scanning electron microscopy (SEM, LE01530VP,

Zeiss, Germany).

Preparation of the BDD electrode15

BDD electrodes were prepared by a microwave-as-

sisted plasma chemical vapor deposition (CVD) technique

on silicon (100) wafers, using a commercial microwave

plasma reactor (ASTeX Corp., Woburn, MA) at 5 kW with

high purity hydrogen as the carrier gas. First, the silicon

substrates were hand-polished with diamond powder (0.5

�m) for nucleation, they were then rinsed with 2-propanol.

The carbon source was a mixture of acetone and methanol

(9:1, v/v). The boron source was B2O3, which was dis-

solved in the above-mentioned mixture at a B/C molar ratio

of 1:100. After a 10 h deposition process, a BDD film

thickness of ~ 40 �m was achieved.

Preparation of Au/TiO2 nanorod composites and

modification of BDD electrode

Au/TiO2 nanorod composites were prepared based on

the literatures.24-27 Firstly, 1 wt%, 0.6 mL HAuCl4 solution

was added into the dispersed TiO2 nanorods suspension (40

mL, pH 1.5) under stirring vigorously and keep stirred for

several hours to allow complete adsorption of negatively

charged [AuCl4]� ions onto the positively charged TiO2

nanorods surface. Reduction of [AuCl4]� was achieved by

the dropwise addition sodium borohydride (0.5 mg mL-1)

until a color change was observed. The Au coated on TiO2

nanorod surface serves as seeds for subsequent growth by

electroless gold plating. After being rinsed repeatedly with

deionized water, the Au-coated TiO2 nanorods dispersion

were diluted to 30 mL with water, then HAuCl4 (1 wt%, 0.1

mL) and hydroxylamine hydrochloride (0.04 M, 0.5 mL)

were added under stirring for 15 min to increase and stabi-

lize the amount of Au grown on the TiO2 nanorods. Scheme

I shows the procedure for preparation of the Au/TiO2 nano-

rod composites.

Four different Au/TiO2 nanorod composites were ob-

tained by the above-mentioned processes by keeping the

Au concentration constant while varying the TiO2 nano-

rods concentration. These four Au/TiO2 nanorod compos-

ites suspensions contained [TiO2]:[Au] ratios of 9:1, 18:1,

27:1, and 36:1.

Modification of BDD surfaces was performed simply

by the following processes. The as-grown BDD electrodes

520 J. Chin. Chem. Soc., Vol. 58, No. 4, 2011 Wei et al.

Scheme I Schematic illustration of the formation of

Au/TiO2 nanorod composite

were sonicated successively in 2-propanol and Milli-Q wa-

ter before use. The obtained Au/TiO2 nanorod composites

were washed repeatedly to remove the excess ions existed

in the soultion. Then 20 �l of four different Au/TiO2 nano-

rod composites were dropped onto the pretreated BDD

electrode surfaces and dried at room temperature to obtain

the Au/TiO2 nanorod composite-modified BDD1 electrode

(the ratio of [TiO2]:[Au] is 9:1, Au/TiO2-BDD1), the Au/

TiO2 nanorod composite-modified BDD2 electrode (the ra-

tio of [TiO2]:[Au] is 18:1, Au/TiO2-BDD2), the Au/TiO2

nanorod composite-modified BDD3 electrode (the ratio of

[TiO2]:[Au] is 27:1, Au/TiO2-BDD3), and the Au/TiO2

nanorod composite-modified BDD4 electrode (the ratio of

[TiO2]:[Au] is 36:1, Au/TiO2-BDD4), respectively.

ACKNOWLEDGEMENTS

This research was supported by National Basic Re-

search Program of China (Grant No. 2007CB936300), Na-

tional Natural Science Foundation of China (Grant No.

50925309, 31071606, 11079019), 333 Talent Project Foun-

dation of Jiangsu Province, open fund of State Key Labora-

tory of Bioelectronics of China, and Doctor foundation of

Henan University of Technology (2010BS019).

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Detection of Catechol on Au/TiO2 Nanorod Modified BDD J. Chin. Chem. Soc., Vol. 58, No. 4, 2011 521