Electrodeposition and electrocatalytic properties of Pt/Ni-Co nanowires for non-enzymatic glucose detection

Article abstract of DOI:10.1016/j.jallcom.2012.10.186

A nanowire arrays system consisting of an ordered configuration of Pt, Ni and Co was constructed in single-bath solution through pulse electrodeposition. This structure was evaluated as a potential amperometric non-enzymatic sensor to detect glucose in alkaline solution. We observed a strong and fast amperometric response at low applied potential of 0.4 V vs. SCE over linear ranges of 0-0.2 mM and 0.2-8 mM glucose with sensitivities of 1125 and 333 μA mM^-1 cm^-2, respectively. We also observed a low detection limit for glucose of 1 μM. Correlation of the electronic and geometric modifications with the electrochemical performance characteristics enhanced catalytic activity of the electrode by applying the Ni and Co components to Pt nanowires structure. The electrode showed good analytical performance and high selectivity with no interference from other oxidable species, demonstrating its promise for developing an effective glucose sensor.

Full text of DOI:10.1016/j.jallcom.2012.10.186

Journal of Alloys and Compounds 554 (2013) 169–176
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Electrodeposition and electrocatalytic properties of Pt/Ni–Co nanowires
for non-enzymatic glucose detection
a,b
c
a,
⇑
a
d
Sahar Sadat Mahshid , Sara Mahshid , Abolghasem Dolati , Mohammad Ghorbani , Lixia Yang ,
b
b,
⇑
Shenglian Luo , Qingyun Cai
a
Materials Science and Engineering Department, Sharif University of Technology, Tehran 11155-9466, Iran
State Key Laboratory of Chemo/Biosensing and Chemometrics, Department of Chemistry, Hunan University, Changsha 410082, China
School of Engineering, Monash University Sunway Campus, Bandar Sunway 46150, Malaysia
b
c
d
School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2012
Received in revised form 26 October 2012
Accepted 29 October 2012
Available online 16 November 2012
A nanowire arrays system consisting of an ordered configuration of Pt, Ni and Co was constructed in
single-bath solution through pulse electrodeposition. This structure was evaluated as a potential amper-
ometric non-enzymatic sensor to detect glucose in alkaline solution. We observed a strong and fast
amperometric response at low applied potential of 0.4 V vs. SCE over linear ranges of 0–0.2 mM and
ꢀ1
ꢀ2
0.2–8 mM glucose with sensitivities of 1125 and 333
lA mM cm , respectively. We also observed a
low detection limit for glucose of 1 M. Correlation of the electronic and geometric modifications with
l
Keywords:
Nanowires
Electrodeposition
Non-enzymatic
Glucose oxidation
the electrochemical performance characteristics enhanced catalytic activity of the electrode by applying
the Ni and Co components to Pt nanowires structure. The electrode showed good analytical performance
and high selectivity with no interference from other oxidable species, demonstrating its promise for
developing an effective glucose sensor.
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
x
on the immobilization of glucose oxidase (GO ) on different types
of electrodes, are the focus of most previous studies on this subject,
due to their high sensitivity and selectivity [14–23]. However, an
important challenge in improving enzymatic glucose sensors is
the loss of enzyme activity that occurs during the immobilization;
and also the activity of enzyme is easily affected by temperature,
oxygen, pH, humidity, detergents, organic reagents and toxic
chemicals which accordingly affects their stability, sensitivity
and reproducibility [3,12]. Another limitation is interference from
oxidizable species in blood, such as ascorbic and uric acid, since
the electrode must be poised at at least 0.7 V vs. Ag/AgCl for the
electrochemical detection of hydrogen peroxide generated by
enzyme-catalyzed oxidation of glucose [11,12]. Therefore, the
development of non-enzymatic glucose biosensors has received
much attention because of their potential for higher sensitivity
and selectivity as well as their reduced susceptibility to fouling
by enzyme-aging and adsorbed intermediates. In this regard,
metallic platinum is one of the mostly researched noble metals
in the biosensor field because of its stability in electrochemical
reactions and its prominent catalytic activities. It has been re-
ported that sensitivity, selectivity and anti-fouling property may
be improved by increasing the ratio of nanoscopic surface area to
the geometric surface area (roughness factor) [24,19,25,26]. Simi-
larly, the electrochemical property of metallic nickel electrode,
nickel nanoparticles and nickel nanowire arrays have been well
Among one-dimensional structured materials, nanowires have
been extensively used as the backbones of nanoelectronic devices,
nanosensors, catalysts and information storage systems, not only
because of high surface to volume ratio but also their unique phys-
ical and electronic properties [1–5]. The properties of nanowire de-
vices depend strongly on the distribution, nanostructure and
orientation of individual wires within the sensor. In order to con-
trol the transfer of electrons through nanowires and thereby en-
hance their functionality, great efforts have been made to create
dense and aligned nanowire arrays [6]. Different metals such as
platinum, gold, and alloys containing Pt, Pb, Au, Pd and Rh have
been explored as electrode materials in the fields of catalysts, fuel
cells and biosensors including glucose sensors [7–11]. Glucose
sensing is extremely important for the treatment of diabetic pa-
tients, which accounts for 85% market share of all the biosensor
products. Since the risk for renal, retinal and neural complications
is directly related to the magnitude of chronic elevations of blood
glucose, the demand for a fast and reliable glucose sensor with
high sensitivity, good response, high stability and low cost have
drawn much attention [4,11–13]. Enzymatic glucose sensors, based
⇑
Corresponding authors. Tel.: +98 21 66165259; fax: +98 21 66165717
A. Dolati), tel./fax: +86 731 88821848 (Q. Cai).
(
E-mail addresses: dolati@sharif.edu (A. Dolati), qycai0001@hnu.cn (Q. Cai).
2
investigated; and nickel electrodes (Ni, NiO or Ni(OH) based
0
925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.10.186

1
70
S.S. Mahshid et al. / Journal of Alloys and Compounds 554 (2013) 169–176
2
electrodes due to the existence of the redox couple of Ni(OH) /NiO-
OH formed on the electrode surface in alkaline medium) have been
widely used as a non-enzymatic electrode for determining glucose
concentrations in alkaline media [27,28]. In addition, many mixed
valent cobalt compounds, such as oxides, complexes, and alloys,
exhibit electrocatalytic properties [21,29–33]; and their applica-
tion as biosensors based on immobilization of enzymes has been
widely studied for detection of dopamine, morphine, catechola-
mines, hydrazine, thiosulphate and glucose [34–37].
Therefore, employing metallic catalysts that combine Pt with
metals such as Ni and Co may improve the biosensor performance.
The study of Pt-based bimetallic catalysts has been started since
1
990s, and the use of Pt–Ni alloy in fuel cells has been reported
previously [38–42]. Recently, the use of Pt–Ni as glucose sensor
has been also well investigated and reported [26,43,44]. However,
to the authors’ knowledge, it has not been determined whether the
combination of such alloyed structures with other active metals
can improve the properties of the proposed electrode for glucose
detection.
Fig. 1. Linear voltammetry in Pt (5 mM) (curve 1); Co (0.2 M) (curve 2); Ni (0.77 M)/
Co (0.51 M) (curve 3) solutions in polycarbonate template representing the
potential deposition of Pt and Ni–Co respectively; scan rate: 10 mV/s.
In this work, the advantageous features of metallic platinum,
nickel and cobalt together with the one-dimensional nanostruc-
tured arrays in electrocatalytic oxidation of glucose, have been
combined to develop an enzyme-free glucose sensor based on
Pt/Ni–Co nanowires (NWs). The resulting electrode provides highly
sensitive, stable and fast amperometric glucose sensing while
effectively avoiding interference from ascorbic acid and uric acid.
higher percentage. It is while the Pt deposition started in more po-
sitive potential than ꢀ0.3 V vs. SCE (curve 1). Therefore, in a single-
bath solution, while at potentials less negative than ꢀ0.8 V vs. SCE
the chance for the deposition of Ni and Co beside Pt is low, at
potentials more negative than ꢀ0.8 V vs. SCE the chance for the
Ni–Co deposition extremely increases as the concentration of
2
+
2+
4+
Ni and Co ions is hundreds of times higher than that of Pt ions
in the solution. Increasing the deposition potential to more nega-
tive amount than ꢀ0.8 V vs. SCE, results in decrease in the amount
of Co and increase in the amount of Ni deposition [48,49]. In this
regard, the deposition potential of ꢀ1.5 V vs. SCE was chosen in a
way that the quite equal ratio of Ni to Co was achieved (Co to Ni
was 1.22). Since the Ni–Co alloys form solid solutions, this enables
alloys to be obtained with different proportions of the two metals
2
. Experimental methods
2.1. Synthesis of the Pt/Ni–Co nanowires electrode
A thin film of gold (ꢁ50 nm) was sputtered onto one face of a polycarbonate
template (PCT) to make the template conductive. The PCTs (200 nm pore diameter
and 6 lm thickness) were purchased from Whatman Ltd. In a typical experiment,
the membrane was attached gold-side down on a glassy carbon (GC – 4 mm diam-
eter) electrode surface and covered by a rubber O-ring. Pt, Ni and Co were electro-
deposited on the PCT by immersing the GC electrode in a mixture of 0.77 M NiSO
4
,
[
47]. On the other hand, the potential ꢀ0.35 V vs. SCE was chosen
0
.51 M CoSO , 5 mM H PtCl and 0.4 M H BO in a standard three-electrode cell
4
2
6
3
3
in the region where the deposition peak of Pt reached its maximum
and started a plateau region in which the rate of Pt deposition was
controlled by a diffusion process.
containing the GC working electrode, a Pt foil counter electrode and a SCE reference
electrode. The amperommetric pulse deposition was carried out with each pulse
consisting of two deposition steps of ꢀ1.5 and ꢀ0.35 V vs. SCE with deposition time
of 1 and 5 s respectively. Between each deposition step, a rest stage was designed
The procedure was designed as a two-step cathodic pulse po-
tential (ꢀ0.35 V vs. SCE, 5 s; ꢀ1.5 V vs. SCE, 1 s) which was fol-
lowed by a resting stage (0 V vs. SCE, 1 s) in each step, resulting
in an ordered structure consisted of regions with mainly Pt and re-
gions with mainly Ni and Co components.
(
ꢀ0.01 V vs. SCE for 1 s) to allow the ions travel near the deposition sites. All mea-
surements, including amperommetric pulse deposition, cyclic voltammetry and i-t
transients, were carried out using a CHI660B electrochemical working station (CH
Instruments. Inc., Austin, TX). After deposition, the PC template was dissolved by
2 2
immersing the electrode in dichloromethane (CH Cl ). The resulting Pt/Ni–Co
NWs electrode was washed with deionized water. Morphological characterization
of each sample was performed using a scanning electron microscope (SEM) (Hit-
achi, model S-4800, Japan) and a transmission electron microscope (TEM) (Tecnai,
model F20, America).
3.2. Characterization of the Pt/Ni–Co NWs electrode
The Pt/Ni–Co NW array electrode was generated by pulse elec-
trodeposition of Pt, Ni and Co, using two deposition steps of ꢀ1.5
and ꢀ0.35 V vs. SCE into the pores of a polycarbonate membrane.
Afterward, by chemical etching of the template, uniform nanowires
with an average diameter of about 200 nm and a constant length of
2
.2. Characterization of the Pt/Ni–Co nanowires electrode
The Pt/Ni–Co NWs electrode was tested for catalytic activity in the supporting
electrolyte containing 0.1 M pH 13 NaOH. Glucose (Gl) detection was performed by
the cyclic voltammetry and i-t transients methods using a CHI660B work station
(
CH Instruments. Inc., Austin, TX). Solutions of glucose were prepared daily using
almost 4 lm were created. Fig. 2a and b shows SEM images of the
double distilled water and then were directly used for the detection assays.
Pt/Ni–Co NWs, which indicate that the length of wires is almost
similar due to the diffusion-controlled deposition inside the pores
[50–52]. The density of the nanowires has been calculated the be
3
. Results and discussion
8
ꢀ2
about 3 ꢂ 10 cm by directly counting the number of wires on
the surface. The striations along wires (indicated by arrows in
Fig. 2a) represent each layer deposited in each pulse step which ap-
pear to form a platinum-rich- and a platinum-deficient zone in
alternating deposited layers. Fig. 2c and d shows the TEM image
of the Pt/Ni–Co NWs with diameter of about 200 nm having com-
pact layers upon each other. In addition, the periodic bumps along
the walls (indicated by arrows in Fig. 2a–c), result from changes in
the deposition rate as the deposition potential has periodically
changed in each deposition step, creating an ordered structure. It
3
.1. Fabrication of the Pt/Ni–Co NWs electrode
Fig. 1 shows the CVs of Pt (curve 1), Co (curve 2) and Ni/Co
curve 3) deposition on polycarbonate template (PCT) recorded in
(
the single bath. The deposition potential interval of each metal
was determined. Regarding to the anomalous co-deposition of Co
and Ni which has been studied previously [45–47], the co-deposi-
tion of Ni and Co started at ꢀ0.8 V vs. SCE (curves 3) in a way that
the less noble metal (Co) is preferentially deposited with much

S.S. Mahshid et al. / Journal of Alloys and Compounds 554 (2013) 169–176
171
a
b
c
d
Element Weight% Atomic%
C K
6.12
37.73
19.90
19.48
15.96
4.25
O K
4.30
Co K
Ni K
Pt M
Au M
12.94
10.61
58.91
7.12
2.68
e
Totals
100.00
Fig. 2. (a and b) SEM images of Pt/Ni–Co nanowires electrodeposited by pulse method, and (c) TEM image of the Pt/Ni–Co NWs. (d) Elemental analysis of Pt/Ni–Co NWs by
EDX spectrum. Arrows show the periodic bumps along nanowires.
is these alternating lighter and darker regions (shown by wide ar-
rows in Fig. 2d) that are indicative of the differences in composi-
tion of the layers. The darker region is probably platinum rich
while the ligher region is platinum deficient zone. The EDX analysis
of the Pt/Ni–Co NWs (Fig. 2d) indicates the presence of Ni, Co and
Pt in the proportions 10.61 wt.%, 12.94 wt.% and 58.91 wt.%,
respectively.
electrode surface and thus the electrode was electrochemically sta-
ble through the oxidation and reduction reactions.
In order to study the effect of Ni and Co in the Pt NWs electrode
on its catalytic activity, a series of voltammetry tests was per-
formed by changing the surface molar ratio of Ni/Co. Fig. 4 shows
the cyclic voltammetry measurements obtained for six electrodes:
(1) Pt NWs, (2) Pt/Co NWs, (3) Pt/20Ni-80Co NWs, (4) Pt/40Ni-
6
0Co NWs, (5) Pt/60Ni-40Co NWs and (6) Pt/Ni NWs in 0.1 M
3
.3. Electrochemical Properties of the Pt/Ni–Co NWs electrode
NaOH solution containing glucose. We first studied the catalytic
behavior of Pt NWs (Fig. 4a) in the wide range of ꢀ1 to 0.8 V vs.
SCE in alkaline solution. As indicated previously in the literature
[53–55], the voltammetry obtained in the absence of glucose
(curve 1 of Fig. 4a) showed the characteristic electrochemical re-
sponse of Pt in NaOH, including adsorption/desorption of hydrogen
The electrochemical properties of the proposed electrode were
investigated using cyclic voltammetry and i-t transients tech-
niques. Fig. 3a illustrates the cyclic voltammetry curves of the Pt/
Ni–Co NWs electrode in 0.1 M NaOH containing 2 mM glucose at
different scan rates. Both anodic and cathodic peak currents in-
crease in parallel with potential scan rate; and a linear relationship
between peak current and square root of the scan rate was found,
which indicated a diffusion controlled process (inset of Fig. 3a)
ꢀ
in the early stages of the scans, reversible adsorption of OH in the
middle stages of the scans and formation of oxide in the last stages
of the scans. A prominent peak marked the cathodic scans in this
case, might be due to the reduction of the oxide layer.
[
11,12]. In addition, a set of 20 successive cyclic voltammetry
When glucose is added to the NaOH media (curve 2 of Fig. 4a),
the electro-oxidation peak at ꢀ0.7 V vs. SCE in the positive-going
scan appears due to chemisorptions and dehydrogenation of glu-
cose at very low potential resulting in formation of gluconate
curves were recorded in 0.1 M NaOH containing 2 mM glucose to
study the anti-fouling behavior of the electrode. As shown in
Fig. 3b, no obvious change was found in the peak potential or the
current during the oxidation and reduction cycles, implying that
the reaction intermediates and products did not deposit on the
-
structure. The peak at ꢀ0.25 V vs. SCE results from OH adsorbed
on the catalyst surface by direct glucose oxidation, which on

1
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S.S. Mahshid et al. / Journal of Alloys and Compounds 554 (2013) 169–176
Fig. 4. CVs of (a) Pt NWs in the absence and presence of glucose in 0.1 M NaOH, (b)
Pt NWs, Pt/Co NWs and Pt/Ni NWs compared to Pt/Ni–Co NWs electrodes with
different molar ratio of Ni/Co in 0.1 M NaOH containing 2 mM glucose; inset:
comparison of Ni NWs and Pt/Ni NWs in the same solution; scan rate: 100 mV/s.
Fig. 3. CVs (a) of Pt/Ni–Co NWs in 0.1 M NaOH containing of 2 mM glucose at
different scan rates ranged from 0.01 to 0.2 V/s. Inset shows the linear relationship
between square root of scan rate and the oxidation (curve (A)) and reduction (curve
(
B)) peaks current, (b) upon 20 consecutive cycles in the same solution at scan rate
of 20 mV/s.
moved negatively following the addition of Co to the nanowires
2
+
3+
system, which can be attributed to the presence of Co /Co redox
couples. In addition, the current density is considerably larger than
previously reported [28], which indicates the presence of Pt as the
conductive catalytic element in the electrode. Addition of Ni to the
Pt/Co system results in a rise in the peak current as well as a posi-
tive shift in peak potential. The curves 3, 4 and 5 in Fig. 4b clearly
show such trend in catalytic activity of the electrode by changing
the molar ratio of Ni/Co. On the other hand, the Pt/Ni NWs elec-
trode has presented a pair of redox peaks in alkaline media at
0.55 and 0.25 V vs. SCE (curve 6 of Fig. 4b), which can be attributed
to the Pt–Ni redox couples 43In this regard, the molar ratio of
40Ni/60Co (curve 4) provides the best combination of Ni–Co to
Pt, since this electrode with this composition shows a pair of
well-defined redox peaks at 0.4 and 0.15 V vs. SCE with high peak
current response. The oxidation peak potential for this electrode is
located in the middle of the oxidation peaks of curves (2) and (6) as
reflecting the presence of Ni and Co in the system.
Many publications have reported that the Pt-based alloys pro-
vide higher catalytic activity than Pt alone. This can be explained
by a number of factors, including: changes in the physical structure
of Pt (the Pt–Pt bond distance and coordination number) and mod-
ification of the electronic structure of Pt (electron density in the 5d
Pt orbital and Pt electronegativity), surface oxide layers and redox
type processes involving the first row transition alloying element
[60–62,39]. Accordingly, the high current response of the Pt NWs
after addition of Ni and/or Co (inset of Fig. 4b) may be attributed
to the enhanced catalytic activity of Pt in the Ni and/or Co alloyed
system. The alloying atoms are in oxidized state having one or
hydrolysis produces gluconate. The peak 0.25 V vs. SCE is due to
glucose oxidation on the previously oxidized catalyst surface
[
56]. In this regard, a one-electron (per Pt atom) transfer reaction
occurs at potential less positive and then another one-electron
transfer reaction occurs coupled with place change reaction of
platinum and oxygen at higher potential. Glucose is oxidized on
platinum in the potential range where platinum oxide is formed
by reversible adsorption on platinum covering the hydrogen
adsorption sites [57–59].
In summary, glucose undergoes electrochemical oxidation on Pt
in three potential ranges; hydrogen adsorption, double layer, and
oxide formation regions [56]. Accordingly, addition of glucose re-
sults in a rise in the whole voltammetry curve of the Pt NWs elec-
trode in alkaline media since a variety of glucose reactions on
different types of Pt surfaces has occurred over a wide range of
potentials. However, no sharp peak has been detected on this curve
until Ni and/or Co is added as alloying components to the Pt-NWs
electrode. Fig. 4b shows the responses of PtNWs electrodes after
addition of Ni and/or Co with different molar ratio in 0.1 M NaOH
containing 2 mM glucose. As indicated in Fig. 4a, the oxidation rise
at 0.35 V vs. SCE (curve 1 of Fig. 4b) was attributed to glucose oxi-
dation on the oxidized Pt. In this case, the background current was
considerably high as a result of the high conductivity of the elec-
trode. As indicated in curve 2 of Fig. 4b, a similar trend has been
observed for the Pt/Co NWs electrode, with the oxidation and
reduction peaks shifting to lower potentials and appearing at 0.2
and 0.1 V vs. SCE, respectively. In this case, the oxidation peak

S.S. Mahshid et al. / Journal of Alloys and Compounds 554 (2013) 169–176
173
Fig. 6. Current–time responses of the Pt/Ni–Co NWs electrode upon successive
addition of 1 mM glucose at applied potential of 0.35 (curve 1), o.4 (curve 2), and
o.45 (curve 3) V vs. SCE.
Fig. 5. Pt/Ni–Co NWs in 0.1 M NaOH solution in the absence (curve 1) and presence
of 2 mM (curve 2); and 4 mM (curve 3) glucose; scan rate: 20 mV/s.
each step. Increasing the applied potential to 0.4 and 0.45 V vs. SCE
resulted in current responses shown in curves 2 and 3 of Fig. 6,
respectively. Since the response of the electrode was high and re-
mained constant after each rise for both potentials, the applied po-
tential of 0.4 V vs. SCE was selected for the rest of experiments.
The amperometric current responses of the Pt/Ni–Co NWs elec-
trode upon successive addition of glucose at different concentra-
tions are illustrated in Fig. 7. The oxidation reaction occurs very
more OH ligands attached, which can influence the OH adsorption
on the neighboring Pt atoms. In this regard, the OH species on the
surface hinder oxide formation on the neighboring Pt. As a result,
in contrast to various chemical states of Ni and/or Co, the Pt in
Pt/Ni, Pt/Co and Pt/Ni–Co alloys consist of elemental state of plat-
0
inum (Pt ), which provides more suitable sites for glucose decom-
position than PtO or PtO
activity of Pt in alloyed system with Ni and/or Co than Pt alone
60,61,39]. In addition, Ni and/or Co (hydro)oxides on the Pt/Ni,
2
[60], which can explain the higher
[
Pt/Co and Pt/Ni–Co NWs can promote glucose oxidation via a sur-
face redox process (reactions (1)-(3)) [11–13,28,60]:
NiO=CoO þ OHꢀ ! NiOðOHÞ=CoOðOHÞ þ eꢀ
ð1Þ
or
ꢀ
O þ eꢀ
NiðOHÞ =CoðOHÞ þ OH ! NiOðOHÞ=CoOðOHÞ þ H
2
ð2Þ
2
2
NiOðOHÞ=CoOðOHÞ þ glucose ! NiðOHÞ =CoðOHÞ
2
2
þ glucolactone
ð3Þ
The CVs were measured in the absence or presence of glucose in
order to study the electrocatalytic activity of the electrode. Fig. 5
shows the CVs of the Pt/Ni–Co NWs electrode in 0.1 M NaOH solu-
tion containing different concentrations of glucose. As indicated,
the Pt/Ni–Co NWs electrode exhibited a pair of redox peaks located
at 0.37 (anodic peak) and 0.23 V (cathodic peak) vs. SCE. As dis-
cussed above, these are mainly assigned to the redox couples form-
ing in the alkaline medium [11,28,34,56]. Following addition of
glucose (curves 2 and 3), enhancement of the anodic peak current
can be detected and the CV response increases proportionately to
the rise in the glucose concentration. It is due to the fact that glu-
cose oxidation in alkaline solution proceeds rapidly, due to the
-
high concentration of OH ions which are believed to be directly in-
volved in the reaction intermediates oxidation [63].
Regarding Fig. 4, the oxidation/reduction peaks on Pt/Ni–Co
NWs electrode (curve 4) were placed at lower potentials than that
of Pt/Ni NWs (curve 6). In our previous report [42], the applied po-
tential of Pt/Ni NWs electrode was reported 0.45 V vs. SCE. Accord-
ingly, the impact of applied potentials on the amperometric
response of the Pt/Ni–Co NWs electrode was investigated upon
successive additions of glucose at 1 mM at three different applied
potentials. The results are shown in Fig. 6 which illustrates the cur-
rent responses at 0.35 (curve 1), 0.4 (curve 2) and 0.45 (curve 3) V
vs. SCE. Curve 1 of Fig. 6 shows the response of the electrode at
Fig. 7. Current–time responses of the Pt/Ni–Co NWs electrode upon successive
addition of (a) 0.01 mM (curve 1), 0.1 mM (curve 2), and (b) 1 mM glucose; inset 1
of (a) and inset of (b): calibration curves for glucose concentration in the linear
ranges; inset 2 of (a): limit of detection of glucose for the Pt/Ni–Co NWs electrode
with the S/N = 3 in 0.1 M NaOH; applied potential: 0.4 V vs. SCE.
0
.35 V vs. SCE indicating that the current response at this applied
potential is high but it is not stable as it gradually decreases after

1
74
S.S. Mahshid et al. / Journal of Alloys and Compounds 554 (2013) 169–176
rapidly and the current reaches its steady state region within less
than 3 s depending on the glucose concentration. As indicated in
inset 1 of Fig. 7a, and inset of Fig. 7b, two linear ranges of
Table 1
Comparison of various non-enzymatic glucose sensors.
Glucose sensor
Potential
V (SCE)
Linear
range
(mM)
LOD
M)
References
0
–0.2 mM and 0.2–8 mM were observed for the electrode with
(
l
2
2
calculated sensitivities of 1125 (R = 0.99) and 333 (R = 0.99)
ꢀ1
ꢀ2
lA mM cm , respectively. Regarding the higher sensitivity of
Pt/Ni–Co NWs
0.4
0–0.2
1
Present
work
the proposed electrode for glucose concentrations ranging from 0
to 0.2 mM in comparison to that of Pt/Ni NWAs electrode [42] (will
be discussed in Fig. 8), lower amount of detection limit is expected
from the Pt/Ni–Co NWAs as well.
Considering the detection limit as 3 times of the noise level, the
limit of detection (LOD) for the proposed electrode was obtained at
applied potential 0.4 V vs. SCE and shown in inset 2 of Fig. 7a. The
0
.2–8
Ni powder modified electrode 0.4
0.0005–5
ꢀ11
2
8
[64]
[65]
Pt–Pb nanowire array
electrode
ꢀ0.2
Copper nanocluster/MWCNT
Nanoscale nickel hydroxide/
carbon ionic liquid
0.65
0.55
0.7–3.5
0.05–1
0.21
6
[66]
[12]
electrode
amount of 1
mentioned applied potential.
l
M was calculated for the LOD of the electrode at
Electrospun Ni NPs/carbon
nanofiber paste electrode
0.6
0.5–23
1
[13]
0.002–2.5
Fig. 8 compares the specific effects of Ni and Ni–Co on the
amperometric responses of the Pt/Ni NWs and Pt/Ni–Co NWs elec-
trodes. Insets of Fig. 8 shows the calibration curves in the linear
ranges of glucose concentrations in which the slopes show the sen-
sitivity of the sensor. As indicated previously [42], the Pt/Ni NWs
electrode showed very good response for glucose detection in alka-
line solution. In this case, modifying this electrode by addition of
Co to the system, resulted in not only a decrease in applied poten-
tial (from 0.45 V vs. SCE [42] to 0.4 V vs. SCE), but also a rise in sen-
sitivity of the electrode. Fig. 8a compares the response of the two
electrodes in low amount of glucose addition which indicates high-
er sensitivity of Pt/Ni–Co NWs in this range (inset: from 0.9195 to
Highly ordered Ni NWAs
CuO nanofibre
CuO nanostructures
Pt/Ni NWs
Pinecone-shaped Pt–Pb
nanostructures electrode
CuO/TiO2NT arrays electrode
PdNPs–graphene
nanohybrids-based
electrode
0.55
0.4
0.6
0.45
ꢀ0.1
0.0005–7
–
–
0.002–2
–12
0.1
–
4
1.5
8.4
[11]
[67]
[68]
[41]
[9]
0.5
0.7Ag/AgCl
–2
0.01–5
1
1
[69]
[70]
Pt nanoflowers electrode
Pt–Au nanocorals modified
electrode
0.03
0.4Ag/AgCl
1–16
48
3.2
[24]
[8]
ꢀ7.2
(alkaline)
ꢀ
22
(neutral)
28
1
.1255). This would result in lower limit of detection for the P/Ni–
Co NWs electrode as mentioned above (from 1.5 lM [42] to 1 lM).
Fig. 8b and inset also represents higher sensitivity of the Pt/Ni–Co
NWs electrode in wider range of glucose addition (inset: from
ꢀ1
ꢀ2
0
.3147 to 0.3332 lA mM cm ). Taken together, the Pt/Ni–Co
NWs electrode displays good sensitivity and linear calibration as
compared to the previous results in Table 1 [11–13,42,64–70].
3.4. Selectivity of the Pt/Ni–Co NWs electrode
One of the main analytical factors for a non-enzymatic biosen-
sor is the ability to discriminate interfering analytes such as uric
acid (UA) and ascorbic acid (AA). The normal physiological level
of glucose is about 3–7 mM, which is more than the concentration
of interfering species like UA (0.1 mM) and AA (0.1 mM) [1,2,39].
So, the amperometric responses of the proposed electrode toward
0
.1 mM UA and 0.1 mM AA was tested and compared with the re-
sponse of the electrode to 1 mM glucose addition at applied poten-
tials of 0.4 V vs. SCE (Fig. 9). The results indicate that the influence
of AA and UA on the glucose response is weak under the testing
conditions. In this case, the ratio of iG+I (response current of
1
G
mM glucose in the presence of 0.1 mM interferent) to i (re-
sponse current of 1 mM glucose) has been calculated as 1.04 for
AA and 1.01 for UA (Table 2) which also revealed that the response
of electrode toward glucose in presence of interfering species is
considerably high.
3.5. Stability and reproducibility of the Pt/Ni NWs electrode
The stability of the Pt/Ni–Co NWs electrode on the long time of
electrolysis was tested by cyclic voltammetry measurements be-
fore and after forty times of amperometric analysis as shown in
Fig. 10. The results reveal that after long time of electrolysis on
the electrode the current respond has kept almost 100% of its ini-
tial amount whilst the peak potential has slightly moved positively
by 0.03 V. The reproducibility and repeatability of the electrode
Fig. 8. Comparison of current–time responses of the Pt/Ni–Co NWs (curve 1;
applied potential: 0.4 V vs. SCE.) and Pt/Ni NWs (curve 2; applied potential: 0.45 V
vs. SCE.) electrodes upon successive addition of (a) 0.01 mM, and (b) 1 mM glucose
in 0.1 M NaOH; insets: calibration curves for glucose concentration in the linear
ranges.

S.S. Mahshid et al. / Journal of Alloys and Compounds 554 (2013) 169–176
175
plate. After chemical etching of the template, the Pt/Ni–Co NWs
showed high electrocatalytic activity for the electrooxidation of
glucose due to presence of an ordered system containing Ni, Co
and Pt. Enhancement of both potential and current response took
place due to (1) the presence of Ni/Co redox couples, (2) modifica-
tion of electronic and geometric of Pt structure in alloyed system,
and (3) inhibition of Pt from getting oxide by presence of Ni/Co
(
hydro)oxides. The resulting electrode, working at applied poten-
tial of 0.4 V vs. SCE, exhibited two ranges of linear response to glu-
cose from 0 to 0.2 mM and from 0.2 to 8 mM with sensitivity of
ꢀ1
ꢀ2
1
125 and 333
l
A mM cm , respectively. The electrode had a
detection limit of 1
l
M and detection of interfering species such
as ascorbic acid and uric acid was successfully avoided. Further-
more, high stability and reproducibility as well as simplicity of
preparation make this electrode a suitable device for detection of
glucose in biological samples.
Fig. 9. Current–time response of 1 mM glucose in 0.1 M NaOH in the absence and
presence of 0.1 mM AA and UA at Pt/Ni–Co NWs in 0.1 M NaOH; applied potential:
0
.4 V vs. SCE.
Acknowledgments
Funding for this work, provided by the program for Changjiang
Scholars and Innovative Research Team in University (PCSIRT), the
National Science Foundation of China under the Grant 20827006,
and the National Science Fund for Distinguished Young Scholars
under Grant No. 50725825, is gratefully acknowledged.
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