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Materials Science and Enginee

Fabrication of TiO2 nanoparticles/surfactant polymer complex film on

glassy carbon electrode and its application to sensing trace dopamine

Shuai Yuan, Wanhua Chen, Shengshui HuT

Department of Chemistry, Wuhan University, Wuhan 430072, China

Received 14 July 2004; received in revised form 18 October 2004; accepted 18 December 2004

Available online 10 February 2005

Abstract

A novel method for the fabrication of a TiO2/Nafion nano-film on glassy carbon electrode (NTGCE) is described. In the presence of

dispersant, TiO2 nanoparticles were dispersed into water to give a homogeneous and stable suspension. After the solvent evaporation, a

porous and uniform TiO2 nano-film was obtained on the GCE surface. Further coated with Nafion, the complex film possesses remarkable

stability in aqueous solution. This nano-film was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM).

The prepared electrode showed excellent electrocatalytic behavior of dopamine and high concentration of ascorbic acid does not interfere

with the dopamine detection. Based on this, an electrochemical method is developed for the determination of dopamine with simplicity and

high sensitivity.

D 2005 Elsevier B.V. All rights reserved.

Keywords: TiO2 nanoparticles; Nafion; Dopamine; Chemically modified electrode; Detection

1. Introduction

Recently, multifarious nanomaterials have been applied

in analytical chemistry. Nano-TiO2, due to its unique

physical chemistry properties [1–3] and the physiochem-

ical inclination to selectively combine with some groups

of biomolecules, is now an attractive, biocompatiable and

environmentally benign material, widely used in tooth-

paste and cosmetics. In contrast to the broad application

in photochemistry [4–6], the studies of nano-TiO2 in

electrochemistry is not energetic. The major barriers

involve the low solubility of TiO2 nanoparticles and the

poor stability of the TiO2 film modified on electrodes.

Efforts have been made to obtain TiO2 nano-film on the

electrode surface, e.g. screen printing procedure [7], sol–

gel strategy [8], and dispersing nano-TiO2 with organic

solvent [9].

0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.msec.2004.12.004

T Corresponding author. Fax: +86 27 87684573.

E-mail address: sshu@whu.edu.cn (S. Hu).

Nafion, a surfactant perfluorosulfonated derivative of

Teflon is widely employed in electrochemistry. Nafion has

a structure of a hydrophobic fluorocarbon chain and

hydrophilic –SO3� groups, enables it to attract cations via

the ion-exchange model and to exclude anions by the

electrostatic action. The excellent capability of film

formation on a carbon electrode surface makes Nafion a

competent material to fabricate a charge-selective sensor.

Significant advantages are expected to combine the

electrocatalysis effect of TiO2 nanoparticles with the cation

selectivity of Nafion.

Dopamine is an important neurotransmitter in mammalian

central nervous systems and low levels of dopamine have

been found in patients with Parkinson’s disease [10].

Electrochemical method is one of the most favorable

techniques for the determination of dopamine because of

its low cost, high sensitivity and easy operation. One of the

major problems encountered in the electrochemical assay of

dopamine is the interference of ascorbic acid, which has a

similar structure (Fig. 1a,b) and an oxidation potential to

dopamine. The overlap of their responses often puzzles the

ring C 25 (2005) 479–485

Fig. 1. Molecular structures and electrochemical oxidation mechanisms of

(a) dopamine and (b) ascorbic acid, (c) working mechanism for the

NTGCE.

S. Yuan et al. / Materials Science and Engineering C 25 (2005) 479–485480

measurement of dopamine. Various methods involving

chemically modified electrode with functional films [11–

14], e.g. a negatively charged [11] and a positively charged

self-assembled monolayer [12], a ruthenium oxide pyro-

chlore [13] and bilayer-modified electrodes [14], have been

developed.

In this work, TiO2 nano-particles were homogeneously

dispersed into water with a small amount of dispersant via

ultrasonication. TiO2 nano-film modified glassy carbon

electrode was prepared by droplet evaporation of 5 AL of

this suspension. Covering the nano-TiO2 film with hydro-

phobic Nafion, the resulting complex coating possesses

remarkable stability in aqueous solution. The modified

electrodes were characterized by scanning electron micro-

scopy (SEM) and atomic force microscopy (AFM). NTGCE

exhibits an electrocatalytic behavior and a very sensitive

response to dopamine in the physiological pH. A survey of

the literature shows that the design of TiO2/Nafion complex

nano-film has not been reported.

In the physiological pH 7.4 phosphate buffer solution, the

dopamine cations are selectively absorbed onto the Nafion

covered TiO2 nano-film while ascorbic acid anions are

excluded (Fig. 1c). The anodic square-wave voltammetric

sweep of dopamine yielded a significantly increased single

peak with a detection limit as low as 9.5 nM. The complexly

modified electrode is more stable, sensitive, and selective to

dopamine when compared with bare GCE or with single-

coated GCEs. Based on this, an electrochemical method is

developed for the determination of trace dopamine.

2. Experimental details

2.1. Chemicals and reagents

Rutile style TiO2 nanoparticles (diameter of 30–60 nm,

specific surface area of 65 m2/g) was synthesized by Wuhan

University Silicone New Material Co. Ltd. and used without

further encapsulation. Nafion was purchased from Aldrich.

Prior to use, it was diluted to 0.1% solution with alcohol.

Dopamine was from Fluka. Ascorbic acid and all the other

compounds (Shanghai Reagent Company) used were ana-

lytical reagent grade and prepared with doubly distilled water.

2.2. Apparatus

Electrochemical detection was carried out using a CHI

660a electrochemistry working station (ChenHua Instru-

mental, Shanghai, China). Saturated calomel electrode

(SCE) reference electrode, platinum wire auxiliary elec-

trode, and glassy carbon working electrodes with or without

modification were employed.

The SEM was performed on an X-650 microscope

(HITACHI, Japan). For AFM imaging, samples were

analyzed using a Picoscan atomic force microscope (Molec-

ular Imaging, USA) in a contact mode with commercial

MAClever II tips (Molecular Imaging, USA), with a spring

constant of 0.95 N/m.

2.3. The preparation of nano-film electrode

TiO2 nanoparticles were dispersed in 0.1 wt.%

(NaPO3)6 aqueous solution by 15 min ultrasonic agitation

to give a 4 mg/mL stable homogenous suspension. Prior to

modification, the GCE was polished with 0.3, 0.05 Amalumina slurry to a mirror finish, then rinsed and sonicated

(1 min) in redistilled water. For the preparation of NTGCE,

5 AL nano-TiO2 colloid was dropped onto the GCE and

left to evaporate the water under ambient conditions. Then

10 AL Nafion solution of certain concentration was coated

and dried on the TiO2 nano-film. The single-coated

electrode was prepared by the alternative procedure

described above.

S. Yuan et al. / Materials Science and Engineering C 25 (2005) 479–485 481

2.4. Analytical procedure

Solutions of dopamine and ascorbic acid were prepared

daily and used directly under open air at room temperature.

0.1 M pH 7.4 Na2HPO4–NaH2PO4 buffer was used as the

supporting electrolyte. Before measurement, working elec-

trodes were preanodized at +2.0 V for 30 s in blank buffer

solution to improve the activity and reproducibility of GCE

[15]. The electrode was then transferred to the working

solution. After accumulating for a certain period, the potential

sweep from�0.3 to 0.8 V was performed after 5 s quiet time.

Deaeration was unnecessary since dissolved oxygen did not

interfere with the anodic voltammetric response of dopamine.

3. Results and discussion

3.1. SEM and AFM images

Fig. 2 shows the SEM images of the TiO2 nano-film (a)

and the nano-TiO2/Nafion complex film (b) on the GCE

Fig. 2. SEM images of (a) the nano-TiO2 film and (b) the nano-TiO2/

Nafion complex film on GCE surface.

surface. It is very clear that the TiO2 nano-film covering on

the GCE surface is even and compact. However, floppy

conglomerations were observed under the semi-transparent

Nafion coating. These conglomerations may form by the

corrugation of Nafion during the solvent evaporation. It has

been demonstrated previously that water content can change

the microstructure of Nafion and affect its conductivity [16].

The water-channels in the Nafion microstructure constricted

when the water content decreases. AFM was employed to

investigate the topographies of the corresponding GCE

surfaces. Fig. 3a illustrates the 3-dimensional image of the

TiO2 nano-film surface. As can be seen, the surface bestrewed

with little bores. Things were quite different in the case of

Nafion coated TiO2 nano-film (Fig. 3b). Irregular stacking

crimples relevant to the insoluble surface of Nafion can been

observed, which is consistent with the result of SEM. When

the complex film was dried in the shade after immersing into

dopamine aqueous solution (Fig. 3c), the stacking crimples

become stretched with the penetration of solvent inside to the

film and the conductivity therefore increased [16].

3.2. Enhancement effects of TiO2 nano-film and Nafion on

SWV response of dopamine

It is reported that the sensitivity of square-wave

voltammetry (SWV) of adsorbed species is proportional

to the degree of reversibility of the electrochemical

reaction [17]. An evident advantage of NTGCE in

dopamine detection with SWV mode is expected, since

it has been demonstrated in our previous work [18] that

dopamine gives a more reversible redox behavior at the

NTGCE than at bare GCE or at either the single-modified

GCEs. As can be seen in Fig. 4, obvious increases of the

peak currents at the modified electrodes (curves b, c, d)

are obtained compared with the bare GCE (curve a). The

enhancement of the peak current is most predominant at

NTGCE than any single-coated electrodes. The negative

shifts of the oxidation peak potential at GCEs modified

with TiO2 nano-film (curves c and d), in contrast to those

without TiO2 (curves a and b), suggest that both the

electron transfer and the mass transport between GCE and

the solution have been accelerated in the presence of TiO2

nano-film, while only the mass transport rate has been

improved by single Nafion.

3.3. The effect of nano-TiO2/Nafion Composition

To obtain better responses, the composition of the nano-

TiO2/Nafion film was optimized. The competent amount of

TiO2 suspension is 5 AL of 4 mg/mL in terms of the stability

of the nano-TiO2 aqueous suspension and the efficiency of

the TiO2 nano-film. The effect of Nafion concentration (10

AL) is described in Fig. 5. A decrease of the peak current was

observed with a Nafion concentration increase from 0.1% to

5%. Even though an increase in Nafion amount could

increase the ion-exchange capacity, an excessive thick film

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Fig. 3. AFM images of (a) TiO2 nano-film modified GCE; (b) NTGCE; (c)

after (b) had been immersed in aqueous solution of dopamine.

Fig. 4. Square-wave voltammograms of 8 AM dopamine at (a) a bare GCE;

(b) a Nafion coated GCE; (c) a TiO2 nano-film modified GCE; and (d) a

NTGCE. tp=30 s at open circuit, SW modulation amplitude=0.045 V,

modulation frequency=50 Hz, step height=0.004 V.

S. Yuan et al. / Materials Science and Engineering C 25 (2005) 479–485482

may block the mass transport of dopamine adsorbing onto

the TiO2 nano-film. Meanwhile, the diffusion of counterions,

which are necessary to maintain the electroneutrality during

the redox changes of the analyte, will be restricted due to the

thick film. However, decreasing the concentration of Nafion

lower than 0.1% also weakened the current response in two

aspects: the reproducibility and the tolerance. Since Nafion is

hydrophobic while TiO2 nano-film tends to be shattered after

dipping in aqueous samples for a long time, Nafion is

employed to immobilize the TiO2 nano-film clinging on the

GCE surface. A decrease of Nafion amount is obvious to

weaken the stability of the film. As a result, the tolerance of

the complex film for anionic interference becomes poor.

Thus, 0.1% Nafion solution and 4 mg/mL nano-TiO2

suspension were used to fabricate the complex film. Based

on this, a rough estimate of coverage for Titania on GCE is

0.283 mg/cm2 and the thickness of Nafion film is in the range

of 100–300 nm (the geometric area of glassy carbon is

0.07065 cm2).

Fig. 5. Effects of the coating Nafion concentration used in preparing the

NTGCE on the SWV response of dopamine.

Fig. 6. Cyclic voltammograms of dopamine at a NTGCE in 0.1 mold L�1 pH 7.4 phosphate buffer. Scan rates from the innermost to the outermost waves: 20,

40, 60, 80, 100, 120, 140, and 160 mVd s�1.

S. Yuan et al. / Materials Science and Engineering C 25 (2005) 479–485 483

3.4. The adsorption and preconcentration on the complex

film

The transport characteristics of dopamine at the

NTGCE were investigated with cyclic voltammetry. As

can be seen from the cyclic voltammograms in Fig. 6,

the anodic peak currents (Ipa) and the cathodal peak

currents (Ipc) of dopamine at the NTGCE were linearly

proportional to the scan rate, which indicates an

adsorption behavior [19]. When the NTGCE was trans-

ferred to the phosphate buffer solution after a measure-

ment experiment, similar voltammetric signal of

dopamine was observed with little changes, confirming

that the charge transfer for dopamine at NTGCE is an

adsorption-controlled procedure. The large specific sur-

face area of TiO2 nanoparticles and the cation affinity of

Nafion make it feasible for a preconcentration procedure

in sensing trace dopamine.

Table 1

Effects of Ep and tp at the NTGCE on the SWV response of dopamine

Varying Ep (tp fixed as 30 s) Varying tp (Ep fixed as �0.3 V)

Ep/V ip/AA tp/s ip/AA

+0.1 108.6 0 44.32

0.0 137.2 10 108.5

�0.1 147.5 20 167.0

�0.2 186.4 30a 206.8

�0.3 206.8 40 207.1

�0.4 161.5 50 214.9

�0.5 156.8 60 220.1

SW parameters: modulation amplitude=0.045 V, modulation frequency=50

Hz, step height=0.004 V.a ip=178.9 AA with 30 s preconcenrtation at open circuit.

The effects of preconcentration potential (Ep) and the

preconcentration time (tp) are listed in Table 1. The SWV

response increases clearly with Ep from +0.1 to �0.3 V and

then decreased from �0.3 to �0.5 V; an open circuit

preconcentration also obtained considerable enhancement,

which indicates that the electrostatic adsorption acts slightly

between modified electrode and dopamine. In this case,

dopamine selectively enters Nafion film mainly by the ion-

exchange model. As for tp, the peak current increased with

the tp prolonged and became lazy at tp=30 s.

3.5. Tolerance of ascorbic acid and application for sensing

dopamine

The tolerance of ascorbic acid for trace dopamine

determination is shown in Fig. 7, the SW voltammogram

presented almost no change in peak current for 8 AM

Fig. 7. SWV responses for 8 AM dopamine recorded at a NTGCE in 0.1 M

pH 7.4 phosphate buffer (a) in the absence of and (b) in the presence of 5

mM ascorbic acid.

Fig. 8. SWV responses for dopamine of various concentrations at NTGCE.

S. Yuan et al. / Materials Science and Engineering C 25 (2005) 479–485484

dopamine (curve a) regardless of the presence of 5 mM

ascorbic acid (curve b). An oxidation peak of ascorbic acid

at about �0.1 V began to appear, and resulted in a

regression of the response of dopamine when the ratio of

ascorbic acid increased continuously. The acceptable toler-

ance of ascorbic acid is, therefore, as high as 5 mM.

Fig. 8 shows the dependence of SWV peak current on the

concentration of dopamine. Under the selected conditions

(frequency, 50 Hz; modulation amplitude, 45 mV; pulse

increment, 4 mV), linear calibration curves are obtained

over the 2–8 and 8–20 AM ranges with slopes (AA/AM) and

correlation coefficients of 18.1, 0.9986 and 2.68, 0.9862,

respectively. It should be pointed out that the deflexion also

indicates and confirms the adsorption behavior of dopamine

on the NTGCE surface, which is attributed mainly to the

bio-affinity and the large specific surface area of TiO2

nanoparticles. The detection limit (S/N=3) is as low as 9.5

nM. To characterize the reproducibility of the NTGCE,

parallel measurements were performed at 8 AM dopamine,

10 successive processes showed a relative standard devia-

tion of 4.95%.

The proposed method was applied to the determination

of dopamine in a pharmaceutical products–dopamine

Table 2

Determination of dopamine in injection samples with the NTGCE

Sample Added

value (AM)

Recovery Result (mg mL�1) RSD

(%)(AM) (%) This

method

Reference

value

1 3.00 2.91 97

2 5.00 5.32 106.4 10.5 10 2.17

3 7.00 6.87 98.1

hydrochloride injection. The results are showed in Table

2. The values obtained by the standard addition method,

are in agreement with the reference value. Thus, this TiO2

nano-film/Nafion chemically modified electrode is appli-

cable to the detection of dopamine in commercial

samples.

4. Conclusion

A novel TiO2 nanoparticles/Nafion modified GCE is

developed and very sensitive and selective to sensing trace

dopamine over large amount of ascorbic acid. The

enhancement effects of the complex film toward dopamine

were mainly attributed to the combination of the electro-

catalytic behavior by TiO2 nano-film with the cation

affinity of Nafion. Determining dopamine using this

nano-film modified electrode possesses the following

advantages: simplicity, fast response, environmental benig-

nancy, high selectivity, and low detection limit. This

method has demonstrated its practical application for a

rapid and precise assay of trace dopamine in the

pharmaceutical product.

Acknowledgements

The support of this project by the National Natural

Science Foundation of China (No. 60171023 and

No.30370397) is gratefully acknowledged. The authors are

thankful for the technical assistance and helpful discussions

of Prof. J.C. Zhong on the nano-TiO2 synthesis and Dr. Z.X.

Lu on the AFM studies.

S. Yuan et al. / Materials Science and Engineering C 25 (2005) 479–485 485

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