Electrochimica Acta 123 (2014) 51–57
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical Biosensor consisted of conducting polymer layer on
gold nanodots patterned Indium Tin Oxide electrode for rapid and
simultaneous determination of purine bases
Waleed Ahmed El-Said a,b , Jeong-Woo Choi a,c,∗
a
Interdisciplinary Program of Integrated Biotechnology, Sogang University, #1 Shinsu-dong Mapo-gu, Seoul 121-742, Republic of Korea
Department of Chemistry, Faculty of Science, Assiut University, Assiut, 71516, Egypt
c
Department of Chemical & Biomolecular Engineering, Sogang University, #1 Shinsu-dong Mapo-gu, Seoul 121-742, Republic of Korea
b
a r t i c l e
i n f o
Article history:
Received 21 September 2013
Received in revised form
18 December 2013
Accepted 20 December 2013
Available online 15 January 2014
Keywords:
Biosensor
Adenine and Guanine
Differential pulse voltammetry
Conducting polymer nanostructures
Human serum
a b s t r a c t
The present work focused on the development of new simple method for fabrication of conducting
poly(4–aminothiophenol) nanostructures layered on gold nanodots patterned indium tin oxide (ITO)
electrode based on the self assembly of the monomer. This was followed by electrochemical polymerization of 4–aminothiophenol molecules. In addition, we studied the electrochemical catalytic activity
of the modified electrode towards a mixture of two protein bases (Adenine and Guanine). The modified
gold nanodots ITO electrode was fabricated based on thermal evaporation of pure gold metal onto ITO
surface through polystyrene monolayer. Then, a monolayer of 4–aminothiophenol was self–assembly
immobilized onto the gold nanodots array/ITO electrode. This was followed by electrochemical polymerization process based on cyclic voltammetry technique. The electrochemical behavior of the adenine
and guanine mixture at the modified electrode was investigated based on differential pulse voltammetry
technique. The results indicated that the polymer nanostructures modified gold nanodots/ITO electrode
exhibited an excellent electrocatalytic activity towards the oxidation of adenine and guanine with a
detection limit of 500 and 250 nM, respectively. Moreover, our finding demonstrated a linear relation
between the concentration of both adenine and guanine and their oxidation current peaks (R = 0.9953
and 0.9935, respectively). Finally, the modified electrode was successfully used to detect adenine and
guanine in human serum sample. Therefore, we proposed that this biosensor could have high sensitivity
for simultaneous determination of adenine and guanine in the related physiology process.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Adenine (A) and guanine (G) are two of the purine bases that
play a vital role as essential building blocks of nucleic acids. These
two bases are fundamental compounds in different biological systems, contribute in several processes such as energy transduction,
metabolic co-factors and cell signaling. Abnormal change of A and
G concentration is considered as indication for the deficiency of the
immunity system. Thus, there is great demand for the monitoring
of the variation of A and G concentration in living organisms.
Several techniques has been developed to determine the concentration of these purine bases in DNA such as HPLC and
isotope dilution mass spectrometry techniques [1]. Although
these techniques showed high sensitivity and selectivity, they are
∗ Corresponding author. Department of Chemical and Biomolecular Engineering,
Sogang University, Seoul, South Korea, Tel.: +82 2 705 8480; fax: +82 2 3273 0331.
E-mail addresses: [email protected] (W.A. El-Said), [email protected]
(J.-W. Choi).
0013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.12.144
time-consuming and require complicated instruments. Therefore,
they are unsuitable for real time determination field.
Recently, electrochemical methods for detection of purine bases
(A and G) have attracted attention due to their advantages including; fast response speed, cheap instrument, good sensitivity and
selectivity [2–4]. However, the direct oxidation of A and G at the
traditional bare electrodes has shown poor response with slow
electron transfer, in addition to higher over potential of their oxidation that leading to a low selectivity and sensitivity [5]. Hence,
numerous chemically modified electrodes such as carbon ionic liquid electrode and fullerene–C60–modified glassy carbon electrode
[6,7] had been developed to widen the potential range in which
oxidation of A and G occurs. While, these modified electrodes has
shown improvement in oxidation signals; however, most of them
has elicited complicated preparation process, high background current and leakage of modified electrodes. It is still a challenging to
develop stable electrode modified material with high sensitivity
and selectivity.
Different metal and semiconductor nanoparticles (that exhibit
unique optical, electrochemical and catalytic properties) have been
52
W.A. El-Said, J.-W. Choi / Electrochimica Acta 123 (2014) 51–57
widely used to improve the electrode’s surface area as well as
its sensitivity. Therefore, it enhanced their applications in electrochemical sensors and biosensors [8–13]. Furthermore, there is
growing interest in the use of gold (Au) nanoporous modified electrode due to the wide range of applications that require inertness,
conductivity and/or large surface area [14]. Also, nickel oxide or
zinc oxide nanoparticles modified electrodes exhibited high electrocatalytic activity towards different biological materials [15,16].
Recently, we have developed Au nanoporous layers modified
electrode to change the mass transport processes from planar diffusion to thin layer character [17,18]. That could shift the potential
under which the target redox active species from that required to
electrolyse the interfering species that facilitate the differentiation between species that oxidize or reduce at similar potentials
under planar diffusion conditions [19,20]. Moreover, conducting
polymers have a wide variety of applications such as sensors, cathode material of a lithium secondary battery, electrocatalysts and
microelectronic devices [21,22] due to their good electrochemical
properties. In addition, conducting polymers coating metal nanostructures have been of great interest due to the ability of them to
generate additional electro-catalytic sites as well as the probable
electronic interactions between metal nanostructures and groups
in the polymer [23,24]. Recently, modified Au nano-particles based
on self–assembly is promising for the construction of nano-devices
and nano-biosensors [25].
Self-assembly of 4–Aminothiophenol (ATP) onto bulk Au electrode has been used for selective determination of dopamine and
ascorbic acid [26]. However, the bulk electrodes bear disadvantages; slow electron transfer, low sensitivity, and poor selectivity
[27]. In the recent years, ATP has attracted significant attention in
producing assembly layer onto nanoparticles via covalent or electrostatic interactions [28]. The difference in reactivity between the
thiol and amine ends of ATP has been effectively utilized to design
molecular assemblies leading to unique morphologies and chemical treatments [29].
On the contrary, previous studies have reported fabrication of conducting polymer coating Au nanostructures based
on disperse Au nanoparticles into thin film such as PATP
(poly(4–aminothiophenol)) over the surface of different electrodes
[30–33]. Up to our knowledge, the present work is the first to report
fabrication of a newly polymer nanostructures modified electrode
based on thermal evaporation of pure gold metal onto ITO surface through monolayer of polystyrene. These Au nanodots acted
as template for uniform immobilization of ATP molecules based on
self–assembly of a monolayer of ATP onto Au nanodotos/ITO electrode. Due to the selectivity interaction between ATP molecules
and Au surface, ATP molecules could be attached solely to the
surface of Au nanodots. Thus, ATP will be converted into PATP
based on electro-chemical polymerization of the ATP film. The
electro-chemical oxidation behavior of A and G was investigated by
PATP/Au nanodots/ITO electrode. In summary, the performance of
the fabricated electrode such as; sensitivity, linear range and selectivity, has been evaluated and discussed. Also, the applicability of
this modified electrode has been demonstrated by determining G
and A bases in real samples.
2. Experimental
2.1. Materials
ATP, A, G, human serum (HS) and phosphate-buffered saline
(PBS) (pH 7.4, 10 mM) were purchased from Sigma (St. Louis,
MO, USA). The other chemicals used in this study were obtained
commercially as reagent grade. All solutions were prepared with
de-ionized water (DIW).
2.2. Instruments
Electrochemical polymerization of ATP into PATP and all electrochemical experiments were performed using a potentiostat
(CHI–660, CH Instruments, USA) controlled by general–purpose
electrochemical system software. A homemade three–electrode
system consisted of a nano–PATP/Au nanodots/ITO substrate as a
working electrode, a platinum wire as the auxiliary electrode and
Ag/AgCl as the reference electrode. In addition, the synthesis of
nano–PATP/Au nanodots/ITO was characterized by Fourier transform infrared spectrophotometer (FT–IR) using Nicolet is10 FT–IR
spectrometer (Thermo Scientific). The morphologies of Au nanodots/ITO as well as nano–PATP/Au nanodots/ITO were analyzed
by a scanning electron microscope (SEM) (ISI DS–130 C, Akashi Co.,
Tokyo, Japan).
2.3. Fabrication of Au nanodots/ITO substrate
Au nanodots modified ITO substrates have been fabricated by
using monolayer of polystyrene (PS) as an evaporation mask based
on the thermal deposition of Au layer onto ITO substrate. The PS
masks that has been prepared by the following method; 7 ␮L of
PS particles with a diameter of 500 nm (10 wt % aqueous solution) mixed with a surfactant (Triton–X and methanol 1:400 v/v)
in a ratio of 1: l was applied onto the ITO substrate. Different spin
speeds (500, 1000 and 1500 rpm) for 1 min were used. The substrate was then left to dry in the spin coater with a covered lid to
maintain a consistent drying ambient and evaporation rate [34].
Then, an Au layer (50 nm) was thermally evaporated through the
deposited PS nano-sphere mask. Fifty nm thickness of the Au layer
has been selected to fill the spaces between the PS nano-spheres
and to obtain the triangle nanostructures. This resulted in a uniform
enhancement of the electro-chemical activity. In addition, evaporation of thicker Au layer will not change the morphology of the
resulted triangle nano-structures. After dissolution of the PS mask
in chloroform, an array of ordered triangle Au nanostructures was
left onto the ITO substrate.
2.4. Fabrication of PATP/Au nanodots/ITO electrode
A monolayer of 4–aminothiophenol was self-assembled on Au
nanodots/ITO substrates by immersing the substrates in 2 mM
of 4-aminothiophenol ethanol solution for 2 hrs, then the fabricated ATP–modified substrates were rinsed thoroughly by DIW
and ethanol successively, then dried under N2 gas. Fabrication
of nano–PATP/Au nanodots/ITO was achieved by applying cyclic
voltammetry (CV) technique to ATP self–assembled monolayer in
0.5 M HClO4 aqueous solution from 0.0 V to 1.0 V for 15 cycles at
scan rate of 50 mV/sec.
3. Results and Discussion
3.1. Characterization of PATP/Au nanodots/ITO electrode
Herein, the uniform distribution of Au nanodots over ITO has
been used as an effective template to develop nano-PATP/Au nanodots/ITO substrates, because of the selectivity interaction between
ATP molecules and Au surface. ATP molecules could be attached
only on the surface of Au nanodots by covalent bonds between
thiols ends of ATP molecules and Au nanodots (Scheme 1). Different spin speeds were used to achieve a uniform monolayer over
large area of the substrate. Fig. 1a showed that multilayer film
was obtained by applying spin speed of 500 rpm; while, spin speed
of 1500 rpm resulted in non-compacted monolayer film (Fig. 1b).
Whilst, a closed-compacted monolayer film (Fig. 1c) was obtained
with spin speed of 1000 rpm speed.
W.A. El-Said, J.-W. Choi / Electrochimica Acta 123 (2014) 51–57
53
Scheme 1. Preparation of PATP nanostructures patterns gold modified Indium Tin Oxide.
The morphologies of Au nanodots/ITO and PATP/Au nanodots/ITO electrodes were characterized by SEM images. Fig. 2a
demonstrated that the Au nanostructures exhibit triangle shapes
of lateral dimensions about 80 nm in diameter. The modification
of these triangle Au nanodots with ATP was followed by electrochemical polymerization of ATP into PATP, resulted in coating of the
Au nanodots with a thin film of the PATP and the triangle nature
was changed to irregular structures (Fig. 2b). In addition, the CV
behavior of ATP/Au nanodots/ITO electrode in 0.5 M HClO4 solution demonstrated an anodic current peak at 0.683 V and cathodic
current peak at 0.385 V in the first cycle scan as shown in Fig. 2c.
Interestingly, we noticed that the anodic peak was decreased after
the first cycle scan with negative shift from 0.683 V to 0.67 V. In the
meantime, another anodic current peak at 0.46 V appeared then
increased gradually with the number of potential cycles (Fig. 2d).
These changes signified the formation of the electro-active monolayer PATP film. Furthermore, synthesis of PATP/Au nanodots/ITO
was also confirmed by using FT-IR (Fig. 3) in the range between
500-3500 cm−1 that showed a characteristic peaks at 832 cm−1
(C-H deformation vib. out of plane), 1166 cm−1 (the characteristic bonds of electron delocalization), 1498 cm−1 (characteristic for
bond of benznoid/amine) and 1590 cm−1 (characteristic for bonds
of quinoid/imine), which demonstrated the successful polymerization of ATP to form PATP/Au nanodots/ITO substrate
3.2. Electrochemical behavior of PATP/Au nanodots/ITO electrode
Fig. 4 illustrated the CV behavior of Au nanodots/ITO, ATP/Au
nanodots/ITO, PATP/bulk Au/ITO and PATP/Au nanodots/ITO electrodes in 5 mM Fe(CN)6 3−/4− . Au nanodots/ITO electrode showed
a couple of well–defined redox peaks at 0.28 and 0.17 V. While,
these redox peaks disappeared completely at ATP/Au nanodots/ITO
electrode, which could be attributed to the negatively charged
of the ATP monolayer that blocking the diffusion of Fe(CN)6 3−/4−
ions from solution to the electrode surface [35]. Nevertheless, after
electro-polymerization of ATP to PATP, the redox peak current
Fig. 1. SEM image of the PS mask on the ITO substrate spread at spin speed of (a) 500 rpm, (b) 1500 rpm and (c) 1000 rpm.
54
W.A. El-Said, J.-W. Choi / Electrochimica Acta 123 (2014) 51–57
Fig. 2. (a) SEM of Au nanodots/ITO substrate. (b) SEM of PATP/Au nanodots/ITO substrate, scale bar 500 nm. (c) The first scan cycle of CV behavior of ATP coated Au nanodots/ITO
electrode in 0.5 mol l−1 HClO4 . (d) Electro-polymerization of ATP coated Au nanodots/ITO electrode in 0.5 mol l−1 HClO4 , E vs (Ag/AgCl)/V, scan rate 50 mV/s.
increased significantly at PATP/Au nanodots/ITO compared with
ATP/Au nanodots/ITO, indicating that PATP can effectively increase
the electron transfer rate of Fe(CN)6 3−/4− . Moreover, the findings
demonstrated that PATP/Au nanodots/ITO electrode has shown significantly higher redox peak current compared with PATP coated
bulk Au/ITO.
Fig. 3. FT-IR of nano-PATP coating Au nanodots/ITO substrate.
3.3. Electrochemical oxidation behavior of adenine and guanine
The oxidation mechanism of G and A on the surface of three
different modified electrodes was investigated by DPV (Fig. 5). The
DPV behavior of the mixture of A (80 ␮M) and G (80 ␮M) in PBS
solution at the bare Au/ITO electrode (Fig. 5a) displayed a broad
Fig. 4. CV of 5 mmol l−1 Fe(CN)6 3−/4− at Au nanodots/ITO ( ), ATP/Au nanodots/ITO
), PATP/Au/ITO (•••) and PATP/Au nanodots/ITO ( • •) electrodes, in PBS (pH
(
7.4), E vs (Ag/AgCl)/V, scan rate 50 mV/s.
W.A. El-Said, J.-W. Choi / Electrochimica Acta 123 (2014) 51–57
55
Fig. 5. DPVs of (a) mixture of 80 ␮mol l−1 of A and G at different electrodes in PBS (pH 7.4) at bare Au nanodots/ITO ( ), ATP/Au nanodots/ITO (
), and PATP nanostructures/Au nanodots/ITO (•••) electrodes within potential range from 0.2 V to 1.6 V, (b) mixture of 80 ␮mol l−1 of A and G at PATP nanostructures/Au nanodots/ITO electrode
within potential range from 1.6 V to 0.2 V, (c) 80 ␮mol l−1 of A in different pH values at PATP nanostructures/Au nanodots/ITO electrode within potential range from 1.6 V to
) at PATP nanostructures/Au nanodots/ITO electrode within potential range from 1.6 V to 0.2 V,
0.2 V, (d) mixture of 80 ␮mol l−1 of A and G in PBS (pH 7.4) ( ) or in HS (
E vs (Ag/AgCl)/V.
overlapped peak with two small oxidation peaks at 810 and 900 mV,
so it was difficult to distinguish independent oxidation potential
peaks of A and G. In contrast, the DPV for a mixture of 80 ␮M of
A and G at ATP/Au nanodots/ITO (Fig. 5a) produced two anodic
peaks at 830 and 1210 mV for G and A, respectively. While, the
DPV response for the same bases mixture at PATP/Au nanodots/ITO
electrode showed that the oxidation peaks potential was shifted
negatively for G (780 mV) and positively for A (1320 mV) as shown
in (Fig. 5a). These results revealed that the peak to peak separation of A and G at PATP/Au nanodots/ITO (540 mV) was larger than
that at ATP/Au nanodots/ITO (380 mV). The large potential peak to
peak separation can effectively increase the selectivity of the simultaneous determination of A and G. This denoted that the uniform
distribution of PATP layer over Au nanodots could enhance the electrostatic accumulation of the positively charged G and A bases into
PAPT nanostructures. Thus, the fabricated modified electrode facilitated the electron transfer of G and A, resulted in increase of the
oxidation signals.
On the other hand, Fig. 5b displayed the DPV behaviors of G and
A at PATP/Au nanodots/ITO electrode by applying the potential in
the reverse direction (from 1.6 V to 0.2 V) to study the reversibility
of these oxidation processes. These results demonstrated that the
oxidation processes of G and A are irreversible processes. In addition, the results of the oxidation process of A at different pH values
(Fig. 5c) demonstrated the ability of the modified PATP/Au nanodots/ITO electrode to detect A in the acidic electrolyte (4.5 pH) and
up to the weak alkaline electrolyte (7.4 pH). However, electoactivity
for the modified PATP/Au nanodots/ITO electrode toward oxidation of A could not be observed at alkaline electrolyte (9 pH). This
electrochemical activity of PATP/Au nanodots/ITO at weak alkaline
electrolyte could be explained by the presence of acidic groups
on the ITO surface which appeared to act as acidic counter-ions
[21,36]. Moreover, Fig. 5d showed the DPV behaviors of the mixture of G and A in HS at PATP/Au nanodots/ITO electrode, which
demonstrated that G and A displayed the same behaviors for mixture of G and A in PBS. In addition, different concentration of A
in HS was determined and the recovery percentages were investigated (Table 1). These results indicated that PATP/Au nanodots/ITO
Table 1
Determination of A in human serum solution.
Samples
Added (␮M/L)
Found (␮M/L)
RSD (%)
Recovery (%)
1
2
3
4
5
0.2
0.4
0.6
0.8
1.0
0.19
0.41
0.57
0.79
0.98
2.3
2
1.7
2.8
2.1
96
101
96
99
98
56
W.A. El-Said, J.-W. Choi / Electrochimica Acta 123 (2014) 51–57
Fig. 6. DPVs for (a) simultaneous determination of G (20 ␮mol l−1 ) in PBS (pH 7.4) containing a different concentrations of A ((—) 0.5, ( •• ) 10, ( • •) 25, (•••) 50, (
)
75, ( ) 100 ␮mol l−1 ), E vs (Ag/AgCl)/V, scan rate 50 mV/s. Inset Figure: calibration plot of the oxidation peak current versus different concentration of A. (b) simultaneous
) 70, ( ) 90 ␮mol l−1 ), E vs
determination of A (50 ␮mol l−1 ) in PBS (pH, 7.4) containing a different concentrations of G ((—) 0.25, ( •• ) 10, ( • •) 20, (•••) 50, (
(Ag/AgCl)/V, scan rate 50 mV/s. Inset Figure: calibration plot of the oxidation peak current versus different concentration of G.
electrode responds well for the recovery of A in real samples with
high sensitivity without interferences.
3.4. Simultaneous determinations of guanine and adenine
Fig. 6a illustrated the DPV curves for various concentration of
A (500 nM–100 ␮M) in PBS containing 20 ␮M of G, which demonstrated that the Ipa was proportional to the concentration of A in the
range of 500 nM–100 ␮M (Fig. 6a inset) with no significant change
for the oxidation current peak of G. In addition, DPVs for a mixture of increasing concentration of G (250 nM–90 ␮M) with a fixed
concentration of A (50 ␮M) were presented (Fig. 6b). The anodic
peak current response to G concentration showed to increase linearly within range from 250 nM to 90 ␮M (Fig. 6b inset), with no
significant change in the oxidation current peak of A. These results
demonstrate that both G and A showed linear concentrations in the
range from 500 nM to 100 ␮M at PATP/Au nanodots/ITO electrode
by using DPV technique, with an experimental limit of detection
down to 500 nM for A and 250 nM for G. This detection limit was
lower than that was reported for graphene–Nafion composite film
modified glassy carbon electrode or [37,38].
Therefore, the modified PATP/Au nanodots/ITO electrode could
be applied for simultaneous determination of A and G. In addition,
the anodic peak current corresponding to the oxidation of A and
G in the presence of 1% human serum exhibited almost the same
anodic peak current for the same mixture in PBS solution. These
results indicated that PATP/Au nanodots/ITO electrode could be
suitable for the determination of G and A in real samples without
interferences.
3.5. Stability and reproducibility of PATP/Au nanodots/ITO
The efficiency of the modified PATP/Au nanodots/ITO electrode
for the detection of A was tested in phosphate buffer (pH 7.4) for
twenty days (Fig. 7). In the first 5 days, the current response showed
2% decrease of its initial response. Over the next 5 days, the current
response decreased by about 3% and in the following 10 days, the
decrease was 5%. The total decrease over the time period was 10%.
Thus, PATP/Au nanodots/ITO retains 90% of its original activity after
20 days and this continued to exhibit excellent response to A.
Fig. 7. Stability of PATP nanostructures/Au nanodots/ITO electrode toward the oxidation of A over 20 days.
4. Conclusions
In a nutshell, the current study reported an easy and rapid
method for fabrication of PATP/Au nanodots/ITO electrode for
simultaneous monitoring of the electro-chemical oxidation behavior of A and G purine bases. The results demonstrated that the
peak-to-peak separation of A and G obtained at the fabricated electrode was a little larger than ATP/Au nanodots/ITO electrode. In
addition, the modified electrode was successfully used to determine A and G concentration in human serum as a real sample,
presenting no interference with detection limits of 500 nM and
250 nM for A and G, respectively. Therefore, this modified electrode
is expected to be a promising technique for real sample biosensor
applications.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government
W.A. El-Said, J.-W. Choi / Electrochimica Acta 123 (2014) 51–57
(MSIP) (2009-0080860), by the Leading Foreign Research Institute
Recruitment Program through the National Research Foundation
of Korea(NRF) funded by the Ministry of Science, ICT & Future
Planning(MSIP) (2013K1A4A3055268) and by the Sogang University Research Grant of 2013(SRF-201314003.01) and Misr El-Kheir
Foundation (MEK).
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