Analytica Chimica Acta 545 (2005) 21–26

Electrochemical DNA biosensors based on platinum nanoparticles

combined carbon nanotubes

Ningning Zhu

a

, Zhu Chang

b

, Pingang He

b

,

∗

, Yuzhi Fang

b

,

∗

a

Department of Chemistry, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China

b

Department of Chemistry, East China Normal University, Shanghai 200062, China

Received 12 January 2005; received in revised form 5 April 2005; accepted 6 April 2005

Available online 24 May 2005

Abstract

Platinum nanoparticles were used in combination with multi-walled carbon nanotubes (MWCNTs) for fabricating sensitivity-enhanced

electrochemical DNA biosensor. Multi-walled carbon nanotubes and platinum nanoparticles were dispersed in Nafion, which were used to
fabricate the modification of the glassy carbon electrode (GCE) surface. Oligonucleotides with amino groups at the 5



end were covalently

linked onto carboxylic groups of MWCNTs on the electrode. The hybridization events were monitored by differential pulse voltammetry
(DPV) measurement of the intercalated daunomycin. Due to the ability of carbon nanotubes to promote electron-transfer reactions, the
high catalytic activities of platinum nanoparticles for chemical reactions, the sensitivity of presented electrochemical DNA biosensors was
remarkably improved. The detection limit of the method for target DNA was 1.0

× 10

−11

mol l

−1

.

© 2005 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotubes; Platinum nanoparticles; Electrochemical DNA biosensor

1. Introduction

The demands for innovative analytical device capable of

delivering the genetic information in a faster, simpler, and
cheaper manner at the sample source are becoming increas-
ingly important. DNA biosensors and high-density DNA ar-
rays can fit these demands

[1–3]

. Hybridization of nucleic

acids to their complementary sequences is the essence of
DNA biosensor and DNA chip technology. Detection of hy-
bridization on an electrode surface was commonly based on
detecting the signal changes from labeled-target DNA hy-
bridized with surface-bound DNA probes. Fluorescent de-
tection is successfully used, but bulky and expensive control
instrumentation hampers its wide application, which has also
encouraged the development of lower-cost detection tech-
niques. Electrochemical devices offer promising routes for
interfacing the DNA recognition and signal transduction el-
ements, and are uniquely qualified for meeting the size, cost,

∗

Corresponding authors. Fax: [phone].
E-mail address: [e-mail] (Y. Fang).

and power requirements of DNA diagnostics

[4,5]

. The use

of nanomaterials in electrical detection is relatively new and
offers unique opportunities for electrochemical transduction
of DNA sensing events.

The emergence of nanotechnology is opening new hori-

zons for the application of nanoparticles in analytical
chemistry. The unique physical and chemical properties of
nanoparticles offer excellent prospects for chemical and bi-
ological sensing

[6,7]

. One of the attractive applications for

bioanalytical is the catalytic property of noble metal nanopar-
ticles. Platinum nanoparticles have been an intensive research
subject for the design of electrodes

[8]

. Platinum films mod-

ified microelectrodes were shown to be excellent ampero-
metric sensors for H

2

O

2

in a wide range of concentrations

[9]

. Carbon nanotubes (CNTs) are of special interest due to

their unique electronic, metallic, and structural characteristic

[10]

. Among the diversified applications of CNTs, there has

been growing interest to use CNTs in biological devices ow-
ing to the ability of CNTs to promote electron-transfer reac-
tions with biomolecules

[11]

. CNTs have been suspended in

Nafion, which were used to modify the electrode for the devel-

0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.aca.2005.04.015

22

N. Zhu et al. / Analytica Chimica Acta 545 (2005) 21–26

opment of an amperometric biosensor for glucose

[12]

. CNTs

have also been used as carrier platforms for CdS nanoparticle
trace for enhanced electrical detection of DNA hybridization

[13]

. Fang and co-workers have also used MWCNTs-COOH

for DNA immobilization and electrochemical detection

[14]

.

Platinum nanoparticles and single-walled carbon nanotubes
(SWCNTs) were combined to modify a glassy carbon elec-
trode to improve their electro-activity for H

2

O

2

. The results

showed that the Pt nanoparticles/CNTs modified electrode
responds more sensitively to glucose than those modified by
platinum nanoparticles or CNTs alone

[15]

.

In this paper, we use platinum nanoparticles combined

MWCNTs for the modification of electrode and DNA hy-
bridization detection. MWCNTs were suspended in Nafion,
a perfluorosulfonated polymer, which also interacted with
platinum nanoparticles to form a network, which connected
platinum nanoparticles to the electrodes surface. Oligonu-
cleotides probe with an amino group at 5



end were cova-

lently linked onto the carboxyl groups of the MWCNTs in the
presence of the water-soluble coupling reagent l-ethyl-3-(3-
dimethylaminopropy-l)-carbodiimide (EDAC). Hybridiza-
tion was conducted by immersing the electrode immobi-
lized with DNA probe into the buffer solution containing its
complementary sequence. Then difference pulse voltamme-
try (DPV) measurement was performed using electro-active
daunomycin as an indicator. The results showed that the DNA
biosensors using MWCNTs combined platinum nanoparti-
cles respond more sensitively to target DNA than those based
on platinum nanoparticles or MWCNTs alone. The perfor-
mance of the DNA biosensors with respond to selectivity,
linear range, and sensitivity was discussed.

2. Experimental

2.1. Apparatus

The cyclic voltammetry (CV) and DPV measurements

were performed with a CHI 630 Electrochemical Analyzer
(CHI Instruments Inc., USA). The three-electrode system
consisted of a glassy carbon working electrode (effective area
7.07 mm

2

), an Ag/AgCl reference electrode (saturated KCl)

and a counter electrode made of platinum. All electrochem-
ical measurements were conducted in a 10 ml cell. A trans-
mission electron microscope (TEM) (Hitachi, Japan), a JB-1
stirring machine and a TDL-16B centrifuge were used.

2.2. Chemicals

All stock solutions were prepared with ultrapure wa-

ter from an Aquapro system. MWCNTs (with a diameter
of about 40–60 nm and length of around 1–10

␮m) with

carboxylic groups were obtained from Shenzhen Nanotech
Co. Ltd. (Shenzhen, China). Daunomycin hydrochloride
was obtained from Shanghai Institute for Drug Control and
used without further purification. Nafion-perfluorinated ion-

exchange resin (5 wt.%), sodium dodecyl sulfate (SDS) and
EDAC were purchased from Sigma (US). H

2

PtCl

6

·6H

2

O,

0.3 mol l

−1

, PBS solution (0.3 mol l

−1

NaCl + 0.1 mol l

−1

phosphate buffer, pH 7.3) and 0.1 mol l

−1

PBS solution

(0.1 mol l

−1

NaCl + 0.1 mol l

−1

phosphate buffer, pH 7.3)

were used. Other reagents were commercially available and
were all of analytical reagent grade.

24-Base synthetic oligonucleotides were purchased

from Shenggong Bioengineering Ltd. Company (Shanghai,
China):

• 24-base probe sequence: 5



-NH

2

-GAGCGGCGCAACAT-

TTCAGGTCGA-3



• its fully complementary sequence: 5



-TCGACCTGAAA-

TGTTGCGCCGCTC-3



• a non-complementary sequence: 5



-GAGCGGCGCAA-

CATTTCAGGTCGA-3



2.3. Preparation of platinum nanoparticles and
modification electrode

Platinum nanoparticles (Pt

nano

) were prepared according

to the literature

[16]

. Briefly, H

2

PtCl

6

·6H

2

O (4 ml 5% aque-

ous solution) was added to distilled water (340 ml) and heated
to 80

◦

C with stirring in a 500 ml flask. After adding 60 ml of

sodium citrate (1% aqueous solution), the resulting solution
was maintained at 80

± 0.5

◦

C for about 4 h. The course of

the reduction was followed by absorption spectroscopy, and
the end of the reaction was marked by the disappearance of
the absorption bands of PtCl

6

2

−

.

Glassy carbon electrode was carefully polished with pol-

ishing paper and subsequently with alumina until a mirror
finish was obtained. After 5 min of sonication to remove the
alumina residues, the electrode was immersed in concen-
trated H

2

SO

4

for 3 min followed by thorough rinsing with

water and ethanol. The electrode was then transferred to the
electrochemical cell for cleaning by cyclic voltammetry be-
tween

−0.5 and +1.2 V (versus Ag/AgCl) in 50 mM phos-

phate buffer, pH 7.2, until a stable CV profile was obtained.
The prepared electrode was dried and use immediately for
modification.

An amount of 2.0 mg of MWCNTs were dissolved in a

mixture of 100

␮l of Nafion and 900 ␮l of Pt

nano

solution

[15]

, as the stock solution. After about 40 min of sonication,

uniformly dispersed MWCNTs and Pt

nano

were formed. GCE

was then modified by a 5

␮l drop of MWCNTs/Pt

nano

, and

dried in air. After the electrode was thoroughly rinsed with
water, the MWCNTs/Pt

nano

-modified electrodes were pre-

pared.

2.4. Immobilization of ssDNA on
MWCNTs/Pt

nano

-modified GCE

The immobilization of oligonucleotides probe on the

MWCNTs/Pt

nano

-modified GCE was carried out as the fol-

lowing:

N. Zhu et al. / Analytica Chimica Acta 545 (2005) 21–26

23

Fig. 1. Schematic representation of the electrochemical detection of DNA hybridization based on platinum nanoparticles combined MWCNTs.

The MWCNTs/Pt

nano

-modified GCE was immersed in

a 2.25

× 10

−5

mol l

−1

oligonucleotide solution containing

0.1 mol l

−1

ED AC and 10 mM acetate buffer (pH 5.2) for

10 h with stirring at room temperature. Then the electrode
was washed with a 0.2% SDS phosphate buffer (pH 7.3)
for 5 min to remove the unbound DNA probes and prevent
the nonspecific binding. Thus the oligonucleotide probe
was immobilized through the formation of amide bonds
between the –COOH on the MWCNTs and –NH

2

of the

oligonucleotides at 5



end.

2.5. Hybridization and electrochemical detection

Hybridization reaction was carried out by immersing

the ssDNA probe captured MWCNTs/Pt

nano

/GCE into

a stirred hybridization solution (0.3 mol l

−1

PBS buffer)

containing different concentration of DNA target for 30 min
at 37

◦

C. After that, the electrode was rinsed three times

with a 0.2% SDS phosphate buffer (pH 7.3) to remove
the non-hybridized target DNA. The hybridized electrode
was placed in the stirred daunomycin (1.0

× 10

−5

mol l

−1

)

solution containing 0.1 mol l

−1

phosphate buffer for 5 min.

After that, the electrode was washed with 0.1 mol l

−1

phos-

phate buffer for 5 min to remove the physically adsorbed
molecules.

The electrochemical detection of hybridization was

performed in a 10 ml of electrochemical cell with hy-
bridized working electrode, an Ag/AgCl electrode and a
platinum wire counter electrode. The DPV measurements
were conducted from +0.70 to 0.0 V (versus Ag/AgCl)
in a 0.1 mol l

−1

phosphate buffer (pH 7.3). The peak

current related to the reduction of daunomycin at about
+0.44 V was taken as the electrochemical measurement
signal.

3. Results and discussion

The electrochemical DNA biosensor based on platinum

nanoparticles and MWCNTs for DNA hybridization de-
tection using daunomycin as indicator was illustrated as

Fig. 1

.

3.1. Electrochemical characteristics of
MWCNTs/Pt

nano

particles/GCE

CNTs are very hydrophobic, and most metals could not

easily adhere to CNTs. Pt nanoparticles can be deposited
on Nafion-modified CNTs due to charged interactions

[15]

.

TEM micrograph of CNTs in the presence of Pt nanoparti-
cles is shown in

Fig. 2

. As can be seen, platinum nanoclus-

ters agglomerate to some extent and disperse on the surface
of CNTs, indicating that Pt nanoparticles are deposited on
Nafion-modified CNTs.

Fig. 2. TEM micrograph of platinum nanoparticles combined MWCNTs.

24

N. Zhu et al. / Analytica Chimica Acta 545 (2005) 21–26

Fig. 3. Cyclic voltammograms of Pt

nano

/GCE (A), MWNTs/GCE (B), and

Pt

nano

/MWNTs/GCE (C) recorded in 0.3 mol l

−1

PBS blank solution (pH

7.2), scan rate: 0.1 V/s. Scan range:

−0.2 to 0.8 V (vs. Ag/AgCl).

Electrochemical characteristics of three types of elec-

trodes: GCE modified by MWCNTs, GCE modified by
Pt

nano

, and GCE modified by MWCNTs/Pt

nano

were con-

ducted in PBS buffer and electro-active daunomycin so-
lution by CV scans. As shown in

Fig. 3

, curve A

is the CV response of Pt

nano

/GCE in 0.3 mol l

−1

PBS,

curve B is that of MWCNTs/GCE and curve C is that
of MWCNTs/Pt

nano

/GCE. Compared with the Pt

nano

/GCE

(

Fig. 3

A) and MWCNTs/GCE (

Fig. 3

B), the back current of

the MWCNTs/Pt

nano

/GCE (

Fig. 3

C) is the largest. This shows

that the effective electrode surface of MWCNTs/Pt

nano

/GCE

is larger than that of Pt

nano

-modified GCE or MWCNTs-

modified GCE alone. After MWCNTs or MWCNTs/Pt

nano

is deposited on the GCE, a pair of stable redox wave ap-
pears, which is corresponding to the redox of carboxylic
group of MWCNTs

[17]

. So MWCNTs also provide an ac-

tive binding group for oligonucleotides conjugation.

Fig. 4

is the CV response of three types of electrodes in dauno-
mycin solution respectively. The GCE modified by MWC-
NTs + Pt

nano

(

Fig. 4

C) exhibits the highest electro-active sur-

face area according to the Randles–Sevcik equation

[19]

.

I

p

= 2.69

× 10

5

AD

l/2

n

3/2

γ

1/2

C, where n is the number of elec-

trons participating in the redox reaction, A the area of the elec-
trode (cm), D the diffusion coefficient of the molecule in solu-
tion (cm

2

s

−1

), C the concentration of the probe molecule in

the bulk solution (mol cm

−3

), and y the scan rate of the poten-

tial perturbation (V s

−1

). For the MWCNTs-modified GCE,

the peak current is still pronounced (

Fig. 4

B), but it exhibits a

decrease compared with that of MWCNTs/Pt

nano

/GCE, likely

due to the lack of Pt nanoparticles that were in contact with
MWCNTs. For the Pt

nano

/GCE, the current value (

Fig. 4

A)

was much smaller, which illustrated a decrease in the electro-
active surface area of the electrode

[15]

, mainly due to the lack

of CNTs that act as nanoconnectors between Pt nanoparticles
and the electrode. Nevertheless, the resultant current value
was significantly higher than that of the bare GCE (figure
not shown). With MWCNTs + Pt

nano

having a much larger

Fig. 4. Cyclic voltammograms of (A) Pt

nano

/GCE, (B) MWNTs/GCE, and

(C) Pt

nano

/MWNTs/GCE recorded in 0.1 mol l

−1

PBS blank solution (pH

7.2) containing 1

× 10

−5

mol l

−1

daunomycin, scan rate: 0.10 V/s. Scan

range: 0.0–0.7 V (vs. Ag/AgCl).

surface area, the quantities of ssDNA immobilized on the
electrode surface can be greatly enhanced, therefore the de-
tection limits of sequence-specific DNA in solution should
be greatly lowered. Such results show that Pt nanoparticles
were in contact with MWCNTs, which were also in electri-
cal contact with the GCE. enabling the use of the resulting
composite structure as electrode materials.

3.2. Optimum of DNA assay conditions

Pt nanoparticle solutions were used in combination with

Nafion for the solubilization of MWCNTs and then for
the modification of GCE. To optimize the concentration
of MWCNTs in a mixture of Pt nanoparticles and Nafion,
the amount was varied from 0.5 to 2.0 g 1

−1

. The optimum

concentration for the detection of DNA hybridization was
2.0 g1

−1

. The amount of MWCNTs/Pt

nano

-modified on the

GCE was also optimized for DNA immobilization and hy-
bridization detection. The experimental results showed that
the hybridization peak current increased with the increase
of the amount of MWCNTs/Pt

nano

, the optimum amount of

MWCNTs/Pt

nano

was 5.0

␮l. So, 5.0 ␮l of MWCNTs/Pt

nano

was used for the modification of GCE. The effect of the hy-
bridization time was investigated (

Fig. 5

). As the hybridiza-

tion time prolonged, the electrochemical signal rose gradu-
ally, and reached a constant value after 30 min, so 30 min was
used as hybridization time.

To prevent the nonspecific binding, ssDNA-modified

MWCNTs/Pt

nano

/GCE are pretreated with 0.2% SDS be-

fore the intercalator binding process. It has been reported
that the long aliphatic chains of SDS molecules can inter-
act attractively with the bases of ssDNA

[18]

, when dauno-

mycin molecules approach the SDS-treated single-stranded,
some of the SDS molecules on the ssDNA form micelles
that can be washed out in the subsequent rinsing step. The

N. Zhu et al. / Analytica Chimica Acta 545 (2005) 21–26

25

Fig. 5. Effect of the hybridization time on daunomycin DPV peak current.

experimental results show that after the ssDNA-modified
MWCNTs/Pt

nano

/GCE has been pretreated with 0.2% SDS,

the redox current of daunomycin decreases remarkably. That
is to say, this process can avoid the strong nonspecific in-
teractions between daunomycin and ssDNA, and the detec-
tion limit of DNA hybridization detection can be improved
greatly. After DNA hybridization, a DNA double helix forms.
The sugar-phosphate backbone lies on the outside and the
bases on the inside. When treated with SDS, the hydropho-
bic chains of SDS molecules hardly interact with the in-
ternally stacked bases. Therefore, daunomycin is relatively
freely intercalated into a double helix. With the increase of
the target DNA concentration, the peak current of intercalated
daunomycin increases. It is observed that SDS treatment does
not significantly change the electrochemical character of the
duplex-modified electrodes.

3.3. Electrochemical DNA hybridization detection based
on MWCNTs+Pt

nano

and daunomycin

Fig. 6

illustrates the difference in DPV responses be-

tween the oligonucleotides covalently immobilized on the
MWCNTs/Ptngno/GCE (

Fig. 6

A) and that on the MWC-

NTs/GCE in the absence of Pt nanoparticles (

Fig. 6

B) for

2.25

× 10

7

mol l

−1

of target-complementary sequence assay.

It shows that the use of MWCNTs/Pt

nanno

/GCE for the fabri-

cation of the DNA hybridization assays, the electrochemical
signals of daunomycin are two times larger than that in the
MWCNTs/GCE-based biosensor without Pt nanoparticles.
Such a result reconfirmed that Pt nanoparticles were in elec-
trical contact, through the MWCNTs with the glassy carbon
electrode, enabling the composite structure to be used as an
electrode for DNA hybridization detection.

The selectivity of the DNA hybridization assay was in-

vestigated with the probe/MWCNTs/Pt

nano

/GCE hybridized

with its complete complementary sequence and non-
complementary sequence in certain conditions. After hy-

Fig. 6. The DPV responses of daunomycin intercalating in the dsDNA
recorded in 0.1 mol l

−1

PBS. (A) Pt

nano

/MWNTs/GCE for probe caption and

complementary DNA detection (2.25

× 10

−7

mol l

−1

). (B) MWNTs/GCE

for probe caption and the same concentration of the complementary DNA
detection. Amplitude: 50 mV; pulse period: 0.2 s; pulse width: 60 ms.

bridization and daunomycin intercalates, DPV measurements
were conducted from +0.7 to 0.0 V. As shown in

Fig. 7

, a pro-

nounced increase in the current value of the daunomycin lo-
cated at +0.44 V is observed when hybridized with its comple-
mentary sequence; and negligible electrochemical response
is obtained when incubating with non-complementary se-
quence, which is similar to that of the blank measurement.
The results show that the MWCNTs/Pt

nano

-based hybridiza-

tion assay has high selectivity.

The analytical performance of the DNA sensor based on

MWCNTs/Pt

nano

was also explored by using the immobi-

lized probe to hybridize with the different concentrations
of the complementary sequence according to the described
procedure. The peak current in DPV response of inter-

Fig. 7. The DPV response of the daunomycin recorded for the
oligonucleotides/Pt

nano

+ MWNTs/GCE probe without hybridization (1),

hybridized with non-complementary oligonucleotides sequence (2), and hy-
bridized with complementary oligonucleotides sequence (3). Amplitude:
50 mV; pulse period: 0.2 s; pulse width: 60 ms.

26

N. Zhu et al. / Analytica Chimica Acta 545 (2005) 21–26

Fig. 8. (A) Differential pulse voltammograms for different target con-
centrations: (a) 2.25

× 10

5

pM; (b) 2.25

× 10

4

pM; (c) 2.25

× 10

3

pM; (d)

2.25

× 10

2

pM; (e) 2.25

× 10

1

pM; (f) 0.0 pM. (B) The resulting logarithmic

standard plot.

calated daunomycin increases with the increase of the
complementary target DNA concentrations. The average
currents of daunomycin are linear with the logarithmic
value of the complementary sequence concentration from
2.25

× 10

−7

to 2.25

× 10

−11

mol l

−1

with three independent

measurements (shown in

Fig. 8

). The regression equation

is y = 3.264 log x

− 3.375 (x is the concentration of comple-

mentary sequence, pM; y the peak current of daunomycin,
␮A), and the regression coefficient (R) of the linear curve is
0.9980. The detection limit of probe/MWCNTs/Pt

nano

/GCE

to its complementary sequence is 1.0

× 10

−11

mol l

−1

using

3s (s is the standard deviation of blank solution, n = 11). The
sensitivity of the electrochemical DNA biosensor is superior
to that based on carbon nanotubes alone

[14]

with the detec-

tion limit of 1.0

× 10

−10

mol l

−1

for the same target sequence

(24-base) detection, which is also competitive with that using
carbon-nanotubes loaded with CdS tags with the detection
limit of 40 pg ml

−1

(ca. 2

× 10

−12

mol l

−1

) for the analyzed

target sequence (60-base): 5



-TTCCCTAGCCCCCCCA-

GTGTGCAAGGGCAGTGAAGACTTGATTGTACAAAA-
TACGTTTTG-3



)

[13]

.

4. Conclusion

Pt nanoparticles were combined with Nafion-solubilized

MWCNTs, which were used for the electrode surface

modification. CNTs have the ability to promote electron-
transfer reactions and large surface area; platinum nanopar-
ticles possess the high catalytic activities for chemical
reactions, so the sensing signal for DNA hybridization
is greatly amplified. By minimizing the background sig-
nal, the sensitivity of the DNA biosensors is remarkably
improved.

Acknowledgements

Financial support from the National Nature Science Foun-

dation of China (NSFC, grants no. 29875008) and Shanghai
Municipal Education Commission (no. 04DB27) is gratefully
acknowledged.

References

[1] G. Ramsay, Nat. Biotechnol. 16 (1998) 40–44.
[2] J. Wang, D. Xu, A. Kawde, R. Polsky, Anal. Chem. 73 (2001)

5576–5581.

[3] J. Wang, Nucl. Acids Res. 28 (2000) 3011–3016.
[4] E. Palecek, M. Fojta, Anal. Chem. 73 (2001) 74A–83A.
[5] J. Wang, Anal. Chim. Acta 469 (2002) 63–71.
[6] F. Carusu, Adv. Mater. 13 (2001) 11–22.
[7] J. Storhoff, C.A. Mirkin, Chem. Rev. 99 (1999) 1849–1862.
[8] O. Antoine, Y. bultel, R. Durand, J. Electroanal. Chem. 499 (2001)

85–94.

[9] S. Evans, J. Elloit, I. Andrens, et al., Anal. Chem. 74 (2002)

1322–1326.

[10] T.W. Odom, J. Huang, P. Kim, J. Phys. Chem. B 104 (2000)

2794–2809.

[11] A. Guiseppi-Elie, C. Lei, R. Baughman, Nanotechnology 13 (2002)

559–564.

[12] J. Wang, M. Musamch, Y. Lin, J. Am. Chem. Soc. 125 (2003)

2408–2409.

[13] J. Wang, G.D. Liu, et al., Electrochem. Commun. 5 (12) (2003)

1000–1004.

[14] H. Cai, X. Cao, Y. Jiang, P. He, Y. Fang, Anal. Bioanal. Chem. 375

(2003) 287–293.

[15] S. Hrapovic, Y. Yliu, K. Male, J. Luong, Anal. Chem. 76 (2004)

1083–1088.

[16] A. Harriman, G.R. Millward, P. Neta, M.C. Richoux, J. Phys. Chem.

92 (1988) 1286–1290.

[17] H. Luo, Z. Shi, N. Li, Z. Gu, Q. Zhuang, Anal. Chem. 73 (2001)

915–920.

[18] P. Nokyoung, Jong Hoon H., Anal. Chem. 76 (2004) 900–

906.

[19] A.J. Bard, L.R. Faulkner, Electrochemical Methods-Fundamentals

and Applications, Wiley, New York, 2000.