Novel nanotextured microelectrodes: Electrodeposition-based fabrication and their application to ultrasensitive nucleic acid detection

Article abstract of DOI:10.1016/j.electacta.2010.12.065

Microelectrodes have become well-established tools in a wide range of analytical studies and applications because they exhibit many cardinal advantages bestowed by their micrometer-sized dimensions over conventional large-surface-area electrodes. Nanostru

Full text of DOI:10.1016/j.electacta.2010.12.065

Electrochimica Acta 56 (2011) 2832–2836
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Novel nanotextured microelectrodes: Electrodeposition-based fabrication
and their application to ultrasensitive nucleic acid detection
Xuping Sun∗, Yonglan Luo, Fang Liao, Wenbo Lu, Guohui Chang
Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, School of Chemistry and Chemical Industry, China West Normal University,
Nanchong 637002, Sichuan, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Microelectrodes have become well-established tools in a wide range of analytical studies and applica-
tions because they exhibit many cardinal advantages bestowed by their micrometer-sized dimensions
over conventional large-surface-area electrodes. Nanostructures display quite unique properties different
greatly from those of the bulk materials and are widely used in the field of electroanalysis. Nanotextured
microelectrodes (NTMEs) are expected to combine attributes of nanostructures with microelectrodes.
In this paper, electrodeposition technique, in combination with well-established microfabrication tech-
niques, is used for the first time to the controllable fabrication of novel Pd, Pt, and Au NTMEs. The use of
such NTMEs as a novel platform for ultrasensitive nucleic acid detection is also demonstrated.
© 2010 Elsevier Ltd. All rights reserved.
Received 16 October 2010
Received in revised form
15 December 2010
Accepted 20 December 2010
Available online 27 December 2010
Keywords:
Microelectrode
Nanotexture
Electrodeposition
Ultrasensitive nucleic acid detection
The miniaturization of electrochemical systems for applications
has received considerable attention of researchers for many years
and continues to be a major trend in the field of analytical and
bioanalytical chemistry [1], one of the main reasons for which
is the down-scaling in size will provide increased sensitivity [2].
However, miniaturization often causes decrease of both the rate
of electrochemical conversion and the signal-to-noise ratio. From
the point of view of miniaturization it should be noted that the
dimensions of electrodes can be reduced from the mm to the ␮m
scale without deteriorating the signal-to-noise characteristics [1].
Microelectrodes are just such kind of electrodes with character-
istic dimensions on the micrometer or sub-micrometer scale and
offers many cardinal advantages over conventional large-surface-
area electrodes such as faster double-layer charging, reduced ohmic
loss, high mass-transport rates, and high current intensity, etc.
[3,4]. Indeed, recent years have witnessed their rapid development
and such electrodes have become well-established tools in a wide
range of analytical studies and applications [5,6]. Up to date, many
approaches have been successfully used to fabricate microelec-
trodes, however, the development of a simple but versatile route
for the controllable fabrication of such electrodes is still a challenge
[7].
and shape, and as a result, size and shape-controlled synthesis
of such structures has always been a hot research topic [8]. It is
also well-established that nanoparticles-modified electrodes can
exhibit several unique advantages over conventional macroelec-
trodes such as enhancement of mass transport, catalysis, high
effective surface area and control over electrode microenvironment
[9], and the great applications of nanostructures for electroanalysis
have been largely reported [10]. For example, boron doped dia-
mond microelectrodes modified by electrodeposition of platinum
nanoparticles have been used for the oxidative determination of
As(III) at levels below 1 ppb [11]. So it is reasonable to believe that
nanotextured microelectrodes (NTMEs) will combine the merits of
both microelectrodes and nanostructures for electroanalysis and
especially bioelectroanalysis applications. However, to the best of
our knowledge, until now, no attention has been paid to the simple
fabrication of such kind of novel microelectrodes. In this paper, the
widely-used electrodeposition method [12], in combination with
well-established microfabrication techniques, is used for the first
time to develop a general approach for the fabrication of novel
NTMEs, carried out by electrodeposition of Pd, Pt, or Au starting
from the bottom of a 500-nanometer pore pre-fabricated by micro-
fabrication technology. The use of such NTMEs as a novel platform
for ultrasensitive nucleic acid detection is also demonstrated.
The 500-nm pore used for growing NTMEs by electrodeposi-
tion was fabricated using the following four-step procedures: (1)
A thick silicon dioxide insulating layer was thermally grown on
a silicon wafer. (2) Gold patterns were created on the substrate
using electron-beam assisted gold deposition and a standard lift-
Compared to their bulk counterparts, nanostructures exhibit
quite unique properties which are heavily depended on their size
∗
Corresponding author.
E-mail address: sun.xuping@hotmail.com (X. Sun).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.12.065

X. Sun et al. / Electrochimica Acta 56 (2011) 2832–2836
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Scheme 1. A schematic drawing to illustrate the fabrication process of the NTME.
Fig. 1. (a) Top-view and (b) 66◦ tilted-view SEM images of the Pd NTME thus fabricated, and (c) a local view the Pd NTMEs surface.
off process. (3) A silicon dioxide dielectric layer with a thickness
of 500 nm was then deposited on the gold pattern using plasma-
enhanced chemical vapor deposition (PECVD). (4) Openings about
500 nm in diameter were imprinted through the dielectric exposing
the underlying gold electrode. Electrodeposition and electrochem-
ical studies were performed using BAS Bioanalytical Systems (BAS
CV-50W analyzer) in a three-electrode configuration at room tem-
perature with the use of a Ag/AgCl electrode equipped with a Luggin
capillary as the reference electrode and a platinum wire as the
counter electrode. Pd NTMEs were fabricated by electrodeposition
from an aqueous solution containing 4.5 mM of the corresponding
metal salt solution (H2PdCl4) and 0.5 M of the supporting elec-
trolyte (HCl) at −100 mV for 200 s by single potential time base
mode, with the use of the 500 nm opening of gold substrate as
the working electrode. Scheme 1 shows a schematic drawing to
illustrate the fabrication process of the NTME.
25 mM sodium phosphate/25 mM NaCl (pH 7) containing 20 mM
MgCl2 in a humidity chamber at room temperature for 1.5 h, and
then rinsed in 25 mM sodium phosphate/25 mM NaCl (pH 7). The
chemisorption of DNA onto electrodes was confirmed by scanning
from 0 to 500 mV in a solution of 2 mM Fe(CN)63− in 25 mM sodium
phosphate (pH 7)/25 mM NaCl at a scan rate of 100 mV/s. Elec-
trodes were then rinsed in 25 mM sodium phosphate/25 mM NaCl
(pH 7) and electrocatalysis currents were measured as described
below. A solution of 10 pM target DNA in 25 mM sodium phos-
phate/25 mM NaCl (pH 7) containing 100 mM MgCl2 was then
introduced in a thermostatted humidity chamber at 37–40 ◦C and
1 h was used for the hybridization experiment. Electrocatalytic
current was then remeasured as described below. Electrocatalytic
3−
currents were measured using in solutions of 4 mM Fe(CN)6 and
3+
10 ␮M Ru(NH3)6 in 25 mM sodium phosphate/25 mM NaCl (pH
7) at a scan rate of 50 mV/s. Catalytic current change due to the
Scanning electron microscopy (SEM) measurements were made
on a HITACHI S-3400N microscope operated at an accelerating volt-
age of 10 kV. The ss-thiolated-DNA probes were immobilized on Pd
NTMEs. The NTMEs were exposed to a solution of 5 ␮M SH-DNA in
hybridization event (ꢀI) was calculated as follows:
Ifinal − Iinitial
ꢀI % =
× 100
Iinitial

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X. Sun et al. / Electrochimica Acta 56 (2011) 2832–2836
Fig. 2. Top-view SEM image of (a) Pt and (b) Au NTMEs obtained at −100 mV for 200 s with the use of HCl as supporting electrolyte, the corresponding 66◦ tilted-view SEM
image is shown in c and d, respectively (scale bar: 500 nm).
shaped microstructure about 4.2 and 2.2 ␮m and in diameter
and height, respectively, was formed at the potential applied.
A local view the electrode surface reveals its nano-roughed
nature, as shown in Fig. 1c. All these observations clearly indi-
where Ifinal and Iinitial correspond to current signal after and before
the hybridization, respectively.
Fig. 1a and b shows the top-view and 66◦ tilted-view SEM
images of the Pd NTME thus fabricated. It is seen that a cauliflower-
Fig. 3. (a) A schematic illustration of Ru(III)/Fe(III) electrocatalysis at a DNA-modified, nano-roughed Pd NTMEs. (b) Detection of target DNA through changes in catalytic
current measured by DPV technique. (c) Hybridization efficiency of target DNA at cauliflower and fractal Pd NTMEs. (d) CVs of these two Pd NTMEs after hybridization in
3+
solution of buffer only, Ru(NH3)6 in buffer, and catalytic solution in buffer.

X. Sun et al. / Electrochimica Acta 56 (2011) 2832–2836
2835
cate the formation of Pd microelectrode with nano-roughed
texture.
The versatile nature of our NTMEs system is further explored by
performing electrodeposition of Pt and Au. Fig. 2 shows top-view
SEM image of (a) Pt and (b) Au structures obtained at −100 mV for
250 s with the use of HCl as supporting electrolyte, and the corre-
sponding 66◦ tilted-view SEM image is also shown in Fig. 2c and d,
respectively. It is clearly seen that Pt and Au microelectrodes are
also formed. It is quite interestingly found that semispherical Pt
NTME with smooth surface was obtained. The diameter and height
of Pt NTMEs is measured to be about 6.1 and 3.8 ␮m, respectively.
However, the same deposition conditions give tree-like fractal Au
microstructure about 21 ␮m in diameter and 5.5 ␮m in height. A
close view of the Au NTMEs further indicates that the “leaf” of such
tree is nanoscale in size and triangular in shape. All the above obser-
vations indicate that the shape of the NTMEs can be affected by the
type of metal ions used.
The use of as-fabricated NTMEs as a novel kind of platform for
electrochemical DNA detection has also been preliminarily inves-
tigated by choosing Pd NTMEs as proof-of-concept demonstration.
For comparative study of hybridization efficiency of DNA at such
NTMEs, we tested two kinds of Pd NTMEs with different geome-
try, that is, cauliflower and fractal formed at deposition potential
of −100 mV and −250 mV, respectively, as a biosensing platform
using an electrocatalytic DNA detection method d. This label-free
system reports on the binding of a target DNA sequence to an immo-
bilized probe oligonucleotide using a catalytic reaction between
3+
two transition-metal ions, Ru(NH3)6 and Fe(CN)63−. The Ru(III)
electron acceptor is reduced at the electrode surface and then reox-
idized by excess Fe(III), which makes the electrochemical process
catalytic. The increased concentration of anionic phosphates at the
electrode surface that accompanies DNA hybridization increases
the local concentration of Ru(NH3)63+, and therefore produces large
changes in the electrocatalytic signal. This approach works with
sequences of varied composition and is thus widely applicable to
any target gene of interest. In the present study, a single-stranded
probe DNA was thiolated and covalently attached to Pd NTME
through a Pd–thiol bond [13]. Fig. 3a shows a schematic illustration
of Ru(III)/Fe(III) electrocatalysis at a DNA-modified, nanoroughed
Pd NTMEs. To study hybridization efficiency, each DNA-modified
Pd NTMEs was analyzed using Ru(III)/Fe(III) electrocatalysis before
and after the hybridization of target sequences. Fig. 3b shows the
detection of complementary target sequences through changes
in catalytic current measured by differential pulse voltamme-
try (DPV) technique at probe-modified cauliflower and fractal Pd
NTMEs due to the occurrence of hybridization event. Obviously,
current signal increases upon hybridization were observed for both
NTMEs (concentration of target DNA: 10 pM). Note that no sig-
nal change was observed for the probe-modified NTMEs toward
non-complementary target sequences, indicating that the change
in catalytic current in our present system is indeed due to the base
pairing between probe and its target, excluding the nonspecific
target binding. It was reported that the efficiency of an electrocat-
alytic process should be strongly influenced by the rate of diffusion
[11]. Given that these two NTMEs exhibit different nanotexturing
and thus provide different nanoenvironment to the DNA molecules
involved in this system, it is reasonable to conclude that differ-
ent diffusion behavior and rate of DNA towards the cauliflower
and fractal Pd NTMEs may be expected, and therefore, different
hybridization efficiency was observed. Another possible reason for
such difference may be due to the formation of the film of probe
DNA with different density film at such two different kinds of Pd
NTMEs. It is worthwhile pointing out that cauliflower Pd NTME
is expected to permit much more efficient target capture than
its fractal counterpart, leading to bigger change of current signal,
as shown in Fig. 3c. The relative standard deviation (RSD) of the
Fig. 4. Chronocoulometry data of (a) cauliflower and (b) fractal Pd NTMEs after
3+
hybridization in solution of buffer only, 10 ␮M Ru(NH3)6 in buffer, and catalytic
3+
3−
solution containing 10 ␮M Ru(NH3)6 and 4 mM Fe(CN)6 in buffer.
amperometric responses of probe-modified cauliflower and frac-
tal Pd NTMEs to 10 pM complementary target sequences are about
2.5% and 3.5% for 5 measurements, respectively, indicating the good
reproducibility of this detection system. Fig. 3d shows the cyclic
voltammetries (CVs) of these two Pd NTMEs after hybridization in
3+
solution of buffer only, Ru(NH3)6 in buffer, and catalytic solu-
tion in buffer, and the big change of current clearly confirms that
Fe(CN)63− also effectively catalyzes the electrochemical redox reac-
2+
tion of the Ru(NH3)63+/Ru(NH3)6 couple in the present system.
We have performed one control experiment by using one regular
planar electrode as the sensing platform at the same conditions
and found that the change of current signal after the hybridization
event is only about 5% of that obtained on cauliflower Pd NTME.
This observation clearly indicates that the NTME has the advantage
of increasing largely the detection sensitivity.
To further calculate the turnovers per Ru(NH3)63+, we did
chronocoulometry experiment at these two Pd NTMEs after
3+
hybridization in solution of buffer only, Ru(NH3)6 in buffer, and
catalytic solution in buffer (Fig. 4). When a cauliflower Pd NTMEs
was used, eighteen turnovers were observed per Ru(NH3)63+, how-

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X. Sun et al. / Electrochimica Acta 56 (2011) 2832–2836
ever, when a fractal Pd NTMEs was used under the same conditions,
only four turnovers were observed per Ru(NH3)63+. These results
suggest that better accessibility of the redox-active ions is provided
by the cauliflower Pd structure.
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