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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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