Application of a Cu-Co alloy dendrite on glucose and hydrogen peroxide sensors

Article abstract of DOI:10.1016/j.electacta.2011.11.066

A Cu-Co alloy dendrite with a hierarchical nanostructure was electrochemically synthesized, and it characterized using scanning electron microscope, energy dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray diffractometry, X-ray photoelectron spectroscopy, coulometry, and cyclic voltammetry. The catalytic characteristics of the dendrite were examined for its use with highly sensitive glucose and H₂O₂ sensors. The TOF-SIMS image shows that the Cu-Co alloy dendrite grew homogeneously in a hierarchical structure. The Co ²⁺ ions that formed on the alloy dendrite mainly contributed to glucose oxidation, and the Cu⁺ ion cooperated synergically with the Co²⁺ ion. However, Co³⁺ was mainly involved in the reduction of H₂O₂, while the reduction with Cu ²⁺ ion cooperated synergically. The major oxidation products of glucose on the dendrite was confirmed to be formate (12-electron oxidation product) through coulometry and product analysis with a HPLC-MS analyzer. The linear calibration plots for the glucose and H₂O₂ analyses were obtained between 0.5 μM to 14.0 mM (detection limit was 0.10 ± 0.05 μM) and 1.0 μM to 11.0 mM (detection limit was 0.75 ± 0.15 μM), respectively. The performance of the final sensor was evaluated with a human blood sample.

Full text of DOI:10.1016/j.electacta.2011.11.066

Electrochimica Acta 61 (2012) 36–43
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Application of a Cu–Co alloy dendrite on glucose and hydrogen peroxide sensors
Hui-Bog Noha, Kyung-Sun Leea, Pranjal Chandraa, Mi-Sook Wonb, Yoon-Bo Shima,∗,1
a Department of Chemistry and Institute of BioPhysio Sensor Technology, Pusan National University, Busan 609-735, South Korea
b Busan Center, Korea Basic Science Institute, Busan 609-735, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A Cu–Co alloy dendrite with a hierarchical nanostructure was electrochemically synthesized, and it
characterized using scanning electron microscope, energy dispersive X-ray spectroscopy, time-of-flight
secondary ion mass spectrometry (TOF-SIMS), X-ray diffractometry, X-ray photoelectron spectroscopy,
coulometry, and cyclic voltammetry. The catalytic characteristics of the dendrite were examined for its
use with highly sensitive glucose and H2O2 sensors. The TOF-SIMS image shows that the Cu–Co alloy
dendrite grew homogeneously in a hierarchical structure. The Co2+ ions that formed on the alloy den-
drite mainly contributed to glucose oxidation, and the Cu+ ion cooperated synergically with the Co2+
ion. However, Co3+ was mainly involved in the reduction of H2O2, while the reduction with Cu2+ ion
cooperated synergically. The major oxidation products of glucose on the dendrite was confirmed to
be formate (12-electron oxidation product) through coulometry and product analysis with a HPLC-
MS analyzer. The linear calibration plots for the glucose and H2O2 analyses were obtained between
0.5 ␮M to 14.0 mM (detection limit was 0.10 0.05 ␮M) and 1.0 ␮M to 11.0 mM (detection limit was
0.75 0.15 ␮M), respectively. The performance of the final sensor was evaluated with a human blood
sample.
Received 29 July 2011
Received in revised form
10 November 2011
Accepted 16 November 2011
Available online 9 December 2011
Keywords:
Cu–Co alloy dendrite
Catalytic reaction mechanism
Glucose sensor
H2O2 sensor
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
have been studied intensively, as their physical properties are quite
different from those of bulk materials [8,9]. The intrinsic proper-
Glucose detection is of great important in clinical diagnostics as
a diabetic marker [1] and in environmental monitoring [2]. Hence,
many researchers have studied glucose biosensors based on a form
of enzymatic catalyst, such as glucose oxidase [3]. However, there
are some disadvantages with these enzymatic biosensors, such as
insufficient long-term stability and a lack of reproducibility due
to the nature of the enzymes. Thus, it is essential to develop new
electrode materials for nonenzymatic glucose oxidations that can
be applied for the development of sensors.
Early studies of nonenzymatic electrochemical glucose oxida-
tions focused on the use of monometallic traditional electrode
materials, such as Pt [4] and Ni [5]. However, these electrode
materials resulted in two fundamental problems: low efficiency
and poisoning, due to chemisorbed intermediates [6]. Therefore,
the development of ideal materials with the abilities to catalyze
the oxidation of glucose and overcome the drawbacks mentioned
above is needed. Recently, enhanced catalytic performance has
been achieved by using nanoparticle materials composed of transi-
tion metals [7], such as Cu, Ni, or Pt. These nanostructured materials
ties of the metal nanostructures that have been investigated are
their sizes, shapes, morphologies, compositions, and crystallinities
[10,11].
To enlarge the active area for the catalytic reaction, dendritic
patterns are essential, and these patterns are observed when
using non-equilibrium conditions for metallurgic, inorganic, and
organic crystal growths [12]. During these growth, hierarchical
structures consisting of main stems and many side branches are
formed [13]. Metallic dendrites are an important class of materials,
and they are attractive due to their high surface-area-to-volume
ratios and their high degrees of connectivity. These properties are
useful for a number of applications, including catalysis [14,15].
Furthermore, monometallic dendrite materials, such as Ag [16],
Cu [17], and Pt [18], have been studied in multiple of fields.
However, in recent years, growing attention has been paid to
bimetallic dendrites such as Pd–Pt [19], Pd–Ag [20], and Pt–Pb
[21], because bimetals often exhibit better catalytic activities than
their corresponding monometallic counterparts. Furthermore, it
was recently reported that carbon electrode-supported bimetal-
lic Au–Ag alloy nanoparticles were effective for glucose oxidation,
and these results indicate that the design and preparation of cat-
alysts with bifunctional properties are important in achieving
high activities and selectivities for the electrocatalytic oxida-
tions of glucose [22]. Although bimetallic materials are important
∗
Corresponding author. Tel.: +82 51 510 2244; fax: +82 51 514 2430.
E-mail address: ybshim@pusan.ac.kr (Y.-B. Shim).
ISE Member.
1
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.11.066

H.-B. Noh et al. / Electrochimica Acta 61 (2012) 36–43
37
for biosensor issues, a few research groups have reported the
2.2. Preparation of the Cu, Co, Co//Cu and Cu//Co bilayer and
Cu–Co alloy dendrites
syntheses and catalytic applications of dendrite materials [23].
However, we are the first to report catalysts for the oxidation
of glucose and the reduction of H2O2 using a Cu–Co alloy den-
drite.
Prior to electrodeposition, the GC electrode was polished to a
mirror finish with 0.5 ␮m of alumina powder on a polishing cloth
and then rinsed with distilled water. The electrodes modified with
the Cu or Co dendrites were separately prepared in a 0.1 M Na2SO4
solution containing 0.01 M CuCl2 or 0.01 M CoCl2·6H2O, respec-
tively. The Cu–Co alloy dendrite was prepared by electrodeposition
in a 0.1 M Na2SO4 solution containing 0.01 M CuCl2 and 0.01 M
CoCl2·6H2O. The applied potential in this sample was −0.8 V with
an electrolysis time of 600.0 s. The mixed solution was purged with
nitrogen gas for 30 min before the electrodeposition process. For
the bilayer dendrites a two-step process was utilized. With the
Cu//Co bilayer dendrite, the Cu dendrite was modified using the
same method as above. Next, 0.01 M CoCl2·6H2O was added into the
solution, and the Co dendrite was formed on the Cu dendrite. The
Co//Cu bilayer dendrite was formed by a similar method but utilized
the solutions containing single metal ions through the alternative
formation of Cu or Co. All of the electrodes were washed several
times with distilled 0.1 M HCl, acetone, ethanol, and water.
In the present study, the catalytic properties of Cu, Co, and
Cu–Co alloy dendrites, which were electrochemically synthesized
for the oxidation of glucose and the reduction of H2O2, were inves-
tigated. Monometallic bilayer dendrites were also studied, such
as a Co//Cu or Cu//Co bilayer, to confirm the roles of Co and Cu
in the catalytic processes. The surface morphologies and crystal
structures of the dendrites were characterized using a scanning
electron microscope (SEM), energy dispersive X-ray spectroscopy
(EDX), time-of-flight secondary ion mass spectrometry (TOF-SIMS),
and X-ray diffractometry (XRD) experiments. X-ray photoelectron
spectroscopy (XPS), coulometry, cyclic voltammetry (CV), and HPLC
(high-performance liquid chromatography)-ESI (electrospray ion-
ization) mass analyses were employed to evaluate the reaction
mechanisms of the dendrites. In addition, the Cu–Co alloy dendrite
was used as a catalytic electrode in the oxidation of glucose and
in the reduction of H2O2. Finally, the proposed sensor was suc-
cessfully tested in a real human blood sample for the detection of
glucose.
3. Results and discussion
3.1. Preparation of the Cu–Co alloy dendrite
2. Experimental
The Cu–Co alloy dendrite was prepared on a GC electrode by
a controlled potential method by applying the optimized poten-
tial of −0.80 V for 600.0 s. The applied potential was optimized by
varying the potential between −0.60 and −1.20 V and evaluating
the resulting SEM images obtained for each potential (data not
shown). When the potential was less negative than −0.60 V, the
amount of alloy formed was minimal, and the alloy had a parti-
cle form. However, excessive particle aggregation occurred within
the dendrite structure when the potential was more negative than
−1.00 V. Interestingly, an increase in the reduction current as the
electrodeposition time increased indicated that the Cu–Co alloy
dendrite was growing on the electrode surface. In this case, the
increase in reduction current was likely caused by the following
reactions: Cu2+ + 2e− ꢀ Cu (E◦ = 0.34 V vs. SHE) and Co2+ + 2e− ꢀ Co
(E◦ = −0.28 V vs. SHE).
2.1. Materials
CuCl2 (99.999%), CoCl2·6H2O (98%), Na2SO4 (99.99 + %),
NaOH (98%), d-(+)-glucose, ascorbic acid (AA), uric acid (UA),
acetaminophen (AP), dopamine, l-cysteine, and hydrogen per-
oxide were purchased from Sigma–Aldrich Chemical Co. (USA).
All aqueous solutions were prepared in doubly distilled water,
which was obtained from
a Milli-Q water purifying system
(18 Mꢀ cm). All other reagents used were commercially avail-
able extra pure grade. The electrodepositions of the Cu, Co, and
Cu–Co materials were performed by a Kosentech Model KST-P1
(S. Korea). Glassy carbon (GC) (dia. 3.0 mm), Ag/AgCl (sat. KCl),
and Pt wire were used as the working, reference, and counter
electrodes, respectively. SEM images were obtained using a Cam-
bridge Stereoscan 240. XRD (using an X’pert PRO MRD system
model) was used to determine the Cu–Co metal phases of each
deposition. XPS experiments under vacuum conditions were
performed using a VG Scientific ESCALAB 250 XPS spectrometer
with a monochromatic Al K␣ source, which included charge
compensation. TOF-SIMS experiments were performed with a
TOF-SIMS 5 (ION TOF GmbH, Germany) using a bismuth liquid
3.2. Morphology of the dendrite surface
The morphology of deposited dendrite obtained using SEM
shows that the microstructure of the Cu–Co alloy dendrite had
the typical multi-branched tree form. Fig. 1A reveals the main and
side branches observed, which are clearly distinguishable from the
tip splitting of the side branches. The branches developed at the
crystal apexes during the dendrite growth and penetrated the dif-
fusion layer of the solution to support further growth. To determine
the content ratio of Cu and Co in the dendrite, EDX analyses were
performed at different points along the surfaces of the dendrite
stem and branches (Fig. 1B). From the EDX results, the atomic
metal content ratio of Cu:Co is 48.8:51.2 in the full scale analysis
(Fig. 1B(a)). However, as shown in Fig. 1B(b) and (c), the Co con-
tent increased from 49.3 to 50.7 and the Co content decreased from
54.9 to 45.1 from the stem to the branch in the point scale anal-
yses. However, it is also known that an appropriate concentration
gradient of the metal ions is necessary for the successful growth
of a well-defined dendrite [24]. Therefore, to obtain further infor-
mation on the distribution of Cu and Co during dendrite growth,
mass spectra and images were acquired using a TOF-SIMS in posi-
tive mode. Fig. 1C shows the TOF-SIMS images of the Cu–Co alloy
dendrite with its (a) Cu3+ (m/z 190.80), (b) Co2+ (m/z 117.84), and (c)
+
metal ion source. The primary analysis beam was a 25.0 keV Bi3
beam that was operated in the Burst Alignment mode with a
10.0 kHz repetition rate and a pulse width of 100.0 ns. The target
current was <0.2 pA when using a Faraday cup by the grounded
sample holder, and the spot size was estimated to be ∼300 nm.
The mass range was 0–800 amu, with unit mass resolution, and
a 256.0 × 256.0 pixel image resolution was used. The analysis
area (10.0 ␮m × 10.0 ␮m) was a randomly raster image of the
primary ions, and the primary ion dose was maintained below
1012 ions cm−2 to ensure static SIMS conditions. Positive ion
spectra were internally calibrated using H+, H2+, CH3+, C2H3
,
+
+
and C3H4 signals. A HPLC (Agilent 1200 series, Agilent, USA),
combined with packed columns (SUPELCOGEL C-610H, Agilent,
USA) and a ESI-MS detector (HCT Basci System, Bruker, Germany),
was employed to detect the products generated by electroly-
sis. A phosphate solution (0.1%) was used as the eluent for the
HPLC.

38
H.-B. Noh et al. / Electrochimica Acta 61 (2012) 36–43
Fig. 1. (A) SEM image of the Cu–Co alloy dendrite (magnification: (a) 6000× and (b) 30,000×). (B) EDX spectra of the Cu–Co alloy dendrite (a) at full scale, (b) at the stem,
+
+
+
+
and (c) at the branch. (C) TOF-SIMS images of the Cu–Co alloy dendrite (a) Cu3 ion, (b) Co2 ion, and (c) two-color overlay of the Cu3 (red) and Co2 (green) ions (field of
view, 10.0 ␮m × 10.0 ␮m). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
overlaid Cu3+ (red) and Co2+ (green) ions components. The analysis
an isolated vacuum state. Fig. 2B shows XPS spectra obtained for
(a) Cu2p3/2 and (b) Co2p3/2 from the their corresponding dendrites
after various experiments using different conditions: spectrum (I)
was obtained for the pure dendrite, which was synthesized at
−0.80 V for 600.0 s; (II) was obtained for the dendrite after oxi-
dation at +0.70 V in a 0.1 M NaOH solution; (III) was obtained
for the dendrite after oxidation at +0.70 V in a 0.1 M NaOH solu-
tion containing 10.0 mM glucose; and (IV) was obtained for the
dendrite after reduction at −0.60 V in a PBS containing 10.0 mM
H2O2. As shown in each figure, the pure dendrite displayed peaks
at 932.0 (a(I)) and 778.2 eV (b(I)), corresponding to Cu0 and Co0,
respectively [25,26], and indicating that both Cu and Co existed
in their reduced metallic state. As shown in (a(II)), the peak for
Cu0 appeared at 932.0 eV with new broad peaks at approximately
932.5 and 934.6 eV, which corresponds to the formation of Cu+ (at
932.5 eV) and Cu2+ (at 934.6 eV), and the additional broad peaks
appear around 940.0–943.5 eV due to the formation of Cu2+. In this
case, the peak intensity of Cu+ at 932.5 eV is larger than that of
Cu2+ at 934.6 eV, indicating that there is a greater quantity of Cu+
species compared with that of Cu2+ formed during the oxidation of
the dendrite in the blank NaOH solution. In the case of Co (b(II)), a
major peak appears at 778.2 eV, corresponding to Co0, with small
peaks at 781.1 and 780.1 eV, which are due to the formation of the
Co2+ and Co3+ species. Furthermore, additional small peaks around
784.9 and 787.0 eV, due to the Co2+ and Co3+ species, were present.
In this case, the peak corresponding to Co2+ (781.1 eV) is higher
than that of Co3+ (780.1 eV), indicating a greater quantity of Co2+
compared to Co3+ formed during oxidation in the blank NaOH solu-
tion. When 10.0 mM glucose is oxidized at the dendrite in the NaOH
area is 10.0 ␮m × 10.0 ␮m, and the brighter colors on the TOF-SIMS
+
images correspond to the higher intensities of each species. Cu3
+
and Co2 were chosen due to their obvious signals and these TOF-
SIMS images presented similar topographies and morphologies to
those observed in the SEM images. We observe that each of them
is formed simultaneously in a dendrite alloy shape. Additionally,
Cu and Co are quite close and grow interlaced and complementary.
Overall, these homogeneous distributions of Cu and Co in these
porous alloy dendrites are expected to promote catalytic oxidation
of glucose and reduction of H2O2.
3.3. Characterizations of surface elements
Fig. 2A shows the XRD pattern obtained for the Cu–Co alloy
dendrite. Three well-defined main peaks were observed at approx-
imately 43, 50, and 73 of 2ꢁ◦ with distinct side peaks at 44 and
51 of 2ꢁ◦, which indicate a crystallized structure. All of the index
peaks were identified as face-centered cubic (fcc) and appear at
diffraction angles that correspond to electrodeposited fcc copper
and fcc cobalt [24]. The peak intensities of Cu are higher than the
intensities of Co, which are analogous to the EDX results. Overall,
the Cu–Co diffraction peaks are very clearly distinguishable, which
indicate that the alloy is formed on the electrode successfully.
To confirm the oxidation states of the dendrite surfaces accord-
ing to the experimental condition, XPS measurements were
performed immediately upon the dendrite electrodeposition in a
N2 purged solution and after the various reactions; each sample
under a N2 atmosphere was transferred into the XPS chamber under

H.-B. Noh et al. / Electrochimica Acta 61 (2012) 36–43
39
Fig. 2. (A) XRD spectra of the Cu–Co alloy dendrite. (B) XPS spectra of the (a) Cu2p3/2 and (b) Co2p3/2 peaks for the Cu–Co alloy dendrite: (I) bare dendrite, after oxidation in
(II) a 0.1 M NaOH solution and (III) a glucose solution and (IV) after reduction in a H2O2 solution.
solution, the Cu+ peak intensity is the same as the Cu0 intensity in
the spectrum, as shown (a(III)); therefore, the oxidized Cu and Co
ions are more abundant than the Cu and Co metals. Moreover, the
dendrite surface is oxidized more readily to the Cu+ state than in
the absence of glucose. In the hydrogen peroxide reduction reac-
tion, the XPS spectra were obtained when the dendrite was used as
the electrode for the reduction reactions with and without hydro-
gen peroxide at −0.60 V. In the absence of hydrogen peroxide in
the blank solution (PBS), only peaks that corresponded to Cu0 at
932.0 and Co0 at 778.2 eV were observed, which were similar to
the spectrum (B(I)). However, after applying the reduction poten-
tial of −0.60 V on the dendrite electrode in PBS containing 10.0 mM
H2O2, the peaks for the oxidized Cu and Co ions appeared, along
with major Cu and Co metal peaks, as shown in spectrum (B(IV)).
In this case, however, the intensity of the Cu2+ peak (934.6 eV) was
higher than that of Cu+ (932.5 eV), and the intensity of the Co3+
peak (780.1 eV) was higher than that of Co2+ (781.1 eV). This result
revealed the relative peak intensity of the oxidized ions formed
on the dendrite surface during the reduction of hydrogen perox-
ide compared with that of the glucose oxidation, which showed
that the Cu and Co ions formed on the dendrite were involved in
the catalytic redox reactions of both glucose and hydrogen perox-
ide. In the case of the glucose oxidation, the surface shows greater
quantities of Cu+ and Co2+ compared with Cu2+ and Co3+ formed
after the oxidation of glucose. However, higher quantities of Cu2+
and Co3+ than that of Cu+ and Co2+ are formed after the reduc-
tion reaction of H2O2. From these results, it can be assumed that
the Cu2+ and Co3+ ions generated by the oxidation of the den-
drite surface at +0.70 V in a glucose solution are reduced to Cu+
and Co2+ by the catalytic oxidation of glucose. The Cu+ and Co2+
ions generated by the hydrogen peroxide solution are oxidized
to Cu2+ and Co3+ by the catalytic reduction reaction of hydrogen
peroxide at −0.60 V. However, the dendrite surface reveals there
is a presence of Cu+, Cu2+, Co2+, and Co3+, which might be found
in the forms of Cu2O, CuO, Cu(OH)2, CoO, Co(OH)2, Co2O3, and
CoOOH.
3.4. Electrocatalytic oxidation of glucose on the dendrite
electrodes
To estimate the mechanism of glucose oxidation at the Cu–Co
alloy dendrite electrode, the number of electrons participating in
the glucose oxidation was determined by employing controlled
potential coulometry and product analysis with an HPLC-ESI MS
analyzer. The coulometric experiment was performed in a 5.0 mL
blank (0.1 M NaOH) and separately with a 1.0 ␮M glucose solution.
The potential for glucose oxidation was +0.65 V, which was applied
for 100 min. In both cases, the Q values were maintained at −19.7
and −25.5 mC for 100 min each. Coulometry shows that glucose
oxidized to specified products by transferring of 12.02 0.15 elec-
trons to the Cu–Co alloy dendrite. For the HPLC-ESI MS system, the
anodic products of glucose at the Cu, Co, and Cu–Co dendrite elec-
trodes were analyzed. The Cu dendrite electrode oxidized glucose
to formate in a 97.0% conversion (major product), which is also a 12-
electron transfer product. Specifically, this datum is consistent with
the previous report of conventional Cu electrode glucose oxidation
where formate was formed without side products [27]. Otherwise,
Cu–Co dendrite oxidized glucose to formate in a 90.4% conver-
sion with other minor products such as oxalate (1.4%, 18-electron
oxidation product (EOP)), glyoxatlylate (<1%, 12-EOP), glycolate
(3.2%, 6-EOP), glucalate (1.9%, 6-EOP), 2-keto-gluconate (<1%, 4-
EOP), glycelate (1.2%, 4-EOP), and gluconolactone (<1%, 2-EOP). In
the case of the glucose oxidation on the Co dendrite electrode,
formate was formed, achieving a 75.8% conversion, together with
larger amount of the side-products, such as oxalate (5.1%), glyox-
atlylate (2.0%), glycolate (6.1%), glucalate (3.2%), 2-keto-gluconate
(2.0%), glycelate (3.0%), and gluconolactone (2.8%). Overall, the

40
H.-B. Noh et al. / Electrochimica Acta 61 (2012) 36–43
Fig. 3. CVs recorded for the (a) bare GC, (b) Cu, (c) Co, and (d) Cu–Co alloy dendrite electrodes in a blank solution (dotted line), (A) 10.0 mM glucose in a 0.1 M NaOH solution
(solid line), and (B) 10.0 mM H2O2 in a 0.1 M PBS (pH 7.0) (solid line). Scan rate = 50.0 mV/s.
glucose oxidation at the Cu–Co alloy dendrite electrode involved
a 12-electron reaction, in which the major product was formate.
solution. CVs in Fig. 3A with the solid line were recorded for the
Cu, Co, and Cu–Co alloy dendrite electrodes in a 0.1 M NaOH solu-
tion containing 10.0 mM glucose. The catalytic oxidation peak of
glucose to gluconic acid at the Cu dendrite electrode was observed
at +0.51 V, and the peak current was determined to be 105.9 ␮A. In
the case of the Co dendrite, the peak current for glucose oxidation
was observed at +0.56 V, which was more positive than that of Cu,
and the catalytic peak current was determined to be 111.7 ␮A. In
comparison, the oxidation peak of glucose at the Cu–Co alloy den-
drite electrode was observed at +0.57 V, which is more positive than
that for the mere Cu or Co dendrite electrodes. The catalytic peak
current of the glucose oxidation recorded for the Cu–Co alloy den-
drite electrode was determined to be 1110.0 ␮A, which was 10.5
and 9.9 times higher than that of the Cu or Co dendrite electrodes,
respectively. All of the peaks correspond to the oxidations of Cu+
to Cu2+ and of Co2+ to Co3+ involved in the catalytic oxidations of
glucose at the electrodes in a pH 13.0 solution. The following is a
possible catalytic mechanism for the glucose oxidation with these
3.5. Voltammetric behaviors of the glucose oxidation and H2O2
reduction on the Cu–Co alloy dendrite
To evaluate the catalytic properties of the dendrite in the oxi-
dation of glucose, cyclic voltammograms (CVs) were recorded for
the bare GC, Cu, Co, and Cu–Co alloy dendrites in a 0.1 M NaOH
solution (dotted line) and in a solution containing 10.0 mM glucose
(solid line) (Fig. 3A). In the blank solution, no redox peaks were
observed for (a) the bare GC electrode. However, the CVs recorded
for (b) the Cu dendrite electrode showed the redox peaks (Epc/Epa
)
at +0.28/+0.37 V while (c) the Co dendrite electrode showed redox
peaks at +0.14/+0.18 V. These peaks correspond to the redox cou-
ples of Cu and Co themselves. However, as shown in Fig. 3A(d)
by the dotted line, the redox peaks recorded for the Cu–Co alloy
dendrite electrode were approximately +0.16/+0.20 V in the blank

H.-B. Noh et al. / Electrochimica Acta 61 (2012) 36–43
41
Fig. 4. Optimizations of experimental parameters: the effects of (a) pH, (b) applied potential, and (c) temperature on the response peak current for (A) 10.0 mM glucose and
(B) 10.0 mM H2O2 when obtained with the Cu–Co alloy dendrite electrode from the chronoamperometric experiments.
conditions: Cu+ and Co2+ are first oxidized electrochemically to
Cu2+ and Co3+, respectively, and then react with glucose, which
results in the conversion of glucose to its products and the regen-
eration of the catalyst [28–30].
3.6. Optimization of the experimental parameters for analysis
The experimental parameters for glucose sensing with the
Cu–Co alloy dendrite electrode were optimized in terms of pH,
applied potential, and temperature (Fig. 4A). The pH effect of the
media on the analytical sensitivity was studied with a broad pH
range. The response current increased as the pH of the medium
changed from 5.0 to 13.0 (Fig. 4A(a)). This increase was possibly
due to the higher catalytic activity of the Cu–Co alloy dendrite
electrode for the glucose oxidation at the higher pH levels. It is
also known that at a high alkaline pH, more metal oxide forms at
the electrode surface, and it can effectively catalyze the oxidation
of glucose. In addition, higher oxidized species, such as CuOOH,
form at higher pH levels and act significantly on the glucose oxi-
dation [33]. Thus, pH 13.0 was chosen as the optimum pH, and
subsequent experiments were performed out at pH 13.0. Next, the
effect of the applied potential on the chronoamperometric current
response was studied for the glucose oxidation with the Cu–Co
alloy dendrite electrode (Fig. 4A(b)). When the applied potential
was stepped from +0.20 to +0.70 V in the 0.1 M NaOH solution con-
taining 10.0 mM glucose, the current response increased. However,
as the applied potential reached +0.65 V, the maximum response
observed due to the complete oxidation of glucose was achieved
at this potential using the Cu–Co alloy dendrite electrode. The cur-
rent response remained unchanged with application of potential
over +0.70 V. Therefore, the Cu–Co alloy dendrite electrode was
polarized at +0.65 V vs. Ag/AgCl in the chronoamperometric exper-
iments. To determine the effect of temperature on glucose sensing,
a temperature range of 10.0–50.0 ◦C was investigated (Fig. 4A(c)).
The response of the current for the glucose oxidation gradually
increased as the temperature increased from 10.0 to 35.0 ◦C, and
the temperature showed the maximum response at 35.0 ◦C.
To investigate the catalytic potential of the dendrite electrode
for the reduction of H2O2, CVs were recorded for the bare GC, Cu,
Co, and Cu–Co alloy dendrite electrodes in a 0.1 M PBS (pH 7.0, dot-
ted line) and PBS containing 10.0 mM H2O2 (solid line) (Fig. 3B). In
the blank buffer solution, no redox peaks were observed for (a)
the bare GC electrode. Meanwhile, the CVs recorded for (b) the
Cu dendrite electrode showed cathodic peaks at −0.16 V and (c)
the Co dendrite electrode showed peaks at −0.43 V, which corre-
sponded to the reduction reactions of Cu2+ to Cu+ and Co3+ to Co2+
,
respectively. As shown in Fig. 3B(d) by the dotted line, the cathodic
peak in the CV recorded for the Cu–Co alloy dendrite electrode is at
−0.22 V in the blank solution. CVs (solid line) recorded for dendrite
electrodes in the H2O2 solution are shown in Fig. 3B. The catalytic
reduction peak of H2O2 to H2O at the Cu dendrite electrode was
observed at −0.23 V, and the peak current was 258.7 ␮A. In the
case of the Co dendrite, the H2O2 reduction peak was observed
at −0.39 V, which is more negative than that of Cu, and the cat-
alytic peak current was 233.5 ␮A. Comparatively, the catalytic peak
current of H2O2 recorded for the Cu–Co alloy dendrite electrode is
673.4 ␮A at −0.31 V, which is 2.6 and 2.9 times higher, respectively,
than those of the pure Cu and Co dendrite electrodes.
In the catalytic process for the reduction of H2O2, Cu2+ and Co3+
are reduced electrochemically to Cu+ and Co2+, respectively, which
then reduce H2O2 to H2O [31,32]. As shown in the present study, the
Cu–Co alloy dendrite shows a huge catalytic ability in both glucose
oxidation and H2O2 reduction compared to the traditional Cu and
Co electrodes.
However, to confirm each role of Co and Cu for the catalytic
processes, we overlapped the monometallic dendrites of Cu and Co
through different formations in to two bilayers. The Cu//Co bilayer
electrode in which Co is exposed toward the solution, showed
higher catalytic effects for glucose oxidation and H2O2 reduction
than the Cu exposed electrode (Co//Cu bilayer dendrite). Further-
more, the glucose oxidation and H2O2 reduction peak currents at
the Cu//Co bilayer dendrite electrode were twice and 1.3 times
larger, respectively, than at the Co//Cu bilayer dendrite electrode.
This result reveals that Co is more effectively involved in the cat-
alytic reaction than Cu.
The experimental parameters for the electro-reduction of H2O2
to H2O with the Cu–Co alloy dendrite electrode were also optimized
in terms of pH, applied potential, and temperature (Fig. 4B). The
effect of pH on the reduction of H2O2 was studied over the pH range
of 5.8–8.0 in a 10.0 mM H2O2 solution. Fig. 4B(a) shows that the
reduction current gradually increased from pH 5.8 to 7.0 following
which the current decreased rapidly as the pH increased over 7.0.
The maximum response current was observed at a pH of 7.0, pos-
sibly due to the high catalytic activity of the Cu–Co alloy dendrite
electrode at this pH. Thus, the optimum pH was chosen to be 7.0.

42
H.-B. Noh et al. / Electrochimica Acta 61 (2012) 36–43
Fig. 5. Chronoamperometric responses obtained for the Cu–Co alloy dendrite elec-
trode in (A) 0.1 M NaOH with glucose spikes (applied potential: +0.65 V) and in (B)
0.1 M PBS after successive spikes with H2O2 (applied potential: −0.40 V). Inset: Cali-
bration plots for the concentrations of (A) glucose (0.5 ␮M to 14.0 mM) and (B) H2O2
(1.0 ␮M to 11.0 mM) under optimized conditions.
Fig. 6. (A) Chronoamperometric responses for glucose and interference effects of
1.0 mM each of AA, UA, AP, dopamine, and l-cysteine. (B) Calibration plot using the
standard addition method for the determination of saccharide content in a human
blood sample.
The effect of the applied potential on the reduction of 10.0 mM H2O2
was also studied with chronoamperometry. The current response
increased as the applied potential varied from −0.10 V to more neg-
ative values (Fig. 4B(b)). The maximum response was observed at
−0.40 V, and the application of a more negative potential up to
−0.45 V did not increase the current response due to the complete
reduction of H2O2 at this potential. Thus, the Cu–Co alloy dendrite
electrode was polarized at −0.40 V in the subsequent amperomet-
ric experiments. Fig. 4B(c) shows the effects of temperature for the
reduction of H2O2 in the range of 10.0–50.0 ◦C. The response current
was found to increase as the temperature rose from 10.0 to 35.0 ◦C,
and it then began to decrease for temperatures greater than 35.0 ◦C.
Thus, the optimum temperature was considered to be 35.0 ◦C.
was 5.0 s, which is quick and ideal for glucose detection in medical
diagnosis.
Fig. 5B represents the typical current response curve using
the dendrite electrode during successive additions of H2O2. The
steady-state response was obtained at a potential of −0.40 V
vs. Ag/AgCl after adding various aliquots of H2O2 into PBS (pH
7.0). Based on this experiment, the calibration plot was obtained,
and the current response showed a linear relationship according
to the H2O2 concentrations. The dynamic range of the cali-
bration plot for H2O2 was between 1.0 ␮M and 11.0 mM, and
the linear regression equation was expressed as follows: ꢂIp
(mA) = (0.002 0.0001) + (0.090 0.004) [C] (mM), with a correla-
tion coefficient of 0.999. The detection limit of H2O2 is determined
to be 0.75 0.15 ␮M (RSD < 5%) with the Cu–Co dendrite electrode
and based on the five-time measurements performed for the stan-
dard deviation of the blank noise (95% confidence level, k = 3, n = 5).
These results clearly indicate that the Cu–Co dendrite has catalytic
activities toward the oxidation of glucose and the reduction of
H2O2.
The stability of the Cu–Co dendrite electrode was examined
by measuring responses for longer than three months. The Cu–Co
dendrite electrode was stored in a vacuum, and the ampero-
metric response recorded was stable; no significant decrease in
the signal for glucose detection was observed over three months
(RSD < 5%). The sensor retained 95% of its original response up to
three months, after which it gradually decreased, likely due to
electrode degradation. These results indicate that the Cu–Co den-
drite electrode exhibited not only high sensitivity, but also good
stability.
3.7. Calibration plots of glucose and H2O2
Fig. 5A shows the typical current response curve observed
during additions of varying amounts of glucose in
a 0.1 M
NaOH solution. In this case, the applied potential was set
to +0.65 V for the electro-oxidation of glucose by the Cu–Co
dendrite electrode. The dynamic range of the calibration plot
for glucose was between 0.5 ␮M and 14.0 mM. Additionally,
the linear regression equation was expressed as follows: ꢂIp
(mA) = (0.00678 0.0003) + (0.09858 0.004) [C] (mM), with a cor-
relation coefficient of 0.999. The detection limit (DL) of glucose was
estimated to be 0.1 0.05 ␮M (RSD < 5%) by the Cu–Co dendrite
electrode based on the five-time measurements performed for the
standard deviation of the blank noise (95% confidence level, k = 3,
n = 5). The catalytic response of the sensor for glucose detection

H.-B. Noh et al. / Electrochimica Acta 61 (2012) 36–43
43
3.8. Interference effect and real sample analysis
Acknowledgment
In order to assess the possibility of interference, the chronoam-
perometric response of the Cu–Co dendrite sensor was measured
for ascorbic acid (AA), uric acid (UA), acetaminophen (AP),
dopamine, and l-cysteine at a concentration of 1.0 mM (Fig. 6A).
In the amperometric responses in Fig. 6A, which were magnified
10 times, it is observed that AA, UA, AP, dopamine, and l-cysteine
showed an approximately 0.5% response compared with the glu-
cose signal. This result shows less interference than that the copper
film electrode was used in glucose detection [34]. Overall, the
response of the Cu–Co dendrite sensor was very selective, and it
could be extended for use in glucose detection in medical diagnosis.
The proposed Cu–Co sensor was examined for its ability to
detect the saccharide concentration in a diabetic patient’s blood
sample. The blood sample was carefully collected immediately
before the measurements, and the diabetic patient’s blood sam-
ple was obtained from the Department of Internal Medicine and
Division of Endocrinology and Metabolism, Pusan National Univer-
sity Hospital (S. Korea). The blood sample was diluted 40 times
using 0.1 NaOH as reported earlier [35], and the standard addi-
tion method was followed. The glucose concentration in the sample
was determined from a standard addition curve (Fig. 6B). For the
diluted sample, the concentration was 0.22 0.01 mM. Thus, com-
pared to the results obtained from the blood glucose meters at
CareSensors Co. (S. Korea) (8.8 0.07 mM), the glucose concen-
tration in the blood sample was found to be 8.8 0.50 mM. The
results obtained using our method showed good agreement with
the results obtained using the blood glucose meters.
This work was supported by NRF grants funded by the MEST,
South Korea (Grant No. 20090079376).
References
[1] A. Heller, B. Feldman, Chem. Rev. 108 (2008) 2482.
[2] U. Bilitewski, A.P.F. Turner, Biosensors for Environmental Monitoring, Harwood
Academic Publishers, Amsterdam, Netherlands, 2000.
[3] P.N. Bartlett, Bioelectrochemistry: Fundamentals. Experimental Techniques
and Applications, John Wiley and Sons Ltd., Chichester, England, 2008.
[4] S. Park, T.D. Chung, H.C. Kim, Anal. Chem. 75 (2003) 3046.
[5] T. Watanabe, Y. Einaga, Biosens. Bioelectron. 24 (2009) 2684.
[6] L. Meng, J. Jin, G. Yang, T. Lu, H. Zhang, C. Cai, Anal. Chem. 81 (2009) 7271.
[7] J.O’M. Bochris, S.U.M. Khan, Surface Electrochemistry, Plenum Press, New York,
USA, 1993.
[8] A.P. Alivisatos, Science 271 (1996) 933.
[9] X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos,
Nature 404 (2000) 59.
[10] R. Jin, Y.C. Cao, E. Hao, G.S. Metraux, G.C. Schatz, C.A. Mirkin, Nature 425 (2003)
487.
[11] D. Golberg, P.M.F.J. Costa, M. Mitome, S. Hampel, D. Haase, C. Mueller, A. Leon-
hardt, Y. Bando, Adv. Mater. 19 (2007) 1937.
[12] K.A. Dick, K. Deppert, M.W. Larsson, T. Martensson, W. Seifert, L.R. Wallenberg,
L. Samuelson, Nat. Mater. 3 (2004) 380.
[13] P.K. Galenko, V.A. Zhuravlev, Physics of Dendrites: Computational Experiments,
World Scientific, Singapore, 1994.
[14] M. Sanles-Sobrido, M.A. Correa-Duarte, S. Carregal-Romero, B. Rodriguez-
Gonzalez, R.A. Alvarez-Puebla, P. Herves, L.M. Liz-Marzan, Chem. Mater. 21
(2009) 1531.
[15] K.D. Hermanson, S.O. Lumsdon, J.P. Williams, E.W. Kaler, O.D. Velev, Science
294 (2001) 1082.
[16] A. Gutes, C. Carraro, R. Maboudian, J. Am. Chem. Soc. 132 (2010) 1476.
[17] S. Sahoo, S. Husale, B. Colwill, T.M. Lu, S. Nayak, P.M. Ajayan, ACS Nano 3 (2009)
3935.
[18] J.K. Kawasaki, C.B. Arnold, Nano Lett. 11 (2011) 781.
[19] B. Lim, M. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X. Lu, Y. Zhu, Y. Xia, Science
324 (2009) 1302.
4. Conclusions
[20] J. Huang, S. Vongehr, S. Tang, H. Lu, X. Meng, J. Phys. Chem. C 114 (2010) 15005.
[21] J. Wang, R.M. Asmussen, B. Adams, D.F. Thomas, A. Chen, Chem. Mater. 21 (2009)
1716.
[22] M. Tominaga, T. Shimazoe, M. Nagashima, I. Taniguchi, J. Electroanal. Chem.
615 (2008) 51.
[23] R. Qiu, H.G. Cha, H.B. Noh, Y.B. Shim, X.L. Zhang, R. Qiao, D. Zhang, Y.I. Kim, U.
Pal, Y.S. Kang, J. Phys. Chem. C 113 (2009) 15891.
[24] A.K. Kar, P. Morrow, X.T. Tang, T.C. Parker, H. Li, J.Y. Dai, M. Shima, G.C. Wang,
Nanotechnology 18 (2007) 295702.
[25] V. Bansal, H. Jani, J.D. Plessis, P.J. Coloe, S.K. Bhargava, Adv. Mater. 20 (2008)
717.
[26] X. Wang, J. Xu, B. Zhang, H. Yu, J. Wang, X. Zhang, J. Yu, Q. Li, Adv. Mater. 18
(2006) 2476.
[27] M. Tominaga, Y. Taema, I. Taniguchi, J. Electroanal. Chem. 624 (2008) 1.
[28] J.R. Rao, in: G. Milazzo, M. Blank (Eds.), Bioelectrochemistry. I. Biological Redox
Reactions, Plenum Press, New York, 1983.
[29] M.Z. Luo, R.P. Baldwin, J. Electroanal. Chem. 387 (1995) 87.
[30] S. Kerzenmacher, J. Ducree, R. Zengerlea, F.V. Stettena, J. Power Sources 182
(2008) 1.
A
Cu–Co alloy dendrite electrode was electrochemically
prepared and provided a homogenous crystal structure. This homo-
geneous distribution of Cu and Co in a porous alloy dendrite was
expected to cause highly effective catalytic redox reactions of glu-
cose and H2O2. Therefore, when much higher catalytic anodic and
cathodic peak currents were observed for the glucose and H2O2
reactions using the Cu–Co alloy dendrite electrode than the Cu and
Co dendrite electrodes, the addition of Co metal to the Cu alloy
was assumed to allow for excellent catalytic activity. Furthermore,
the Cu–Co alloy dendrite electrode exhibited dynamic ranges of
0.5 ␮M to 14.0 mM and 1.0 ␮M to 11.0 mM for the glucose oxida-
tion (DL: 0.10 0.05 ␮M) and H2O2 reduction (DL: 0.75 0.15 ␮M)
reactions, respectively. The long-term stability of the sensor was
found to be satisfactory (approximately 3.0 months), and it showed
excellent selectivity. The developed glucose sensor was also applied
in a real human blood sample analysis, and satisfactory results were
obtained. Overall, the results show that a Cu–Co dendrite material
can be widely used as a long-term stable catalyst for glucose and
H2O2 sensors.
[31] V.L.N. Diasa, E.N. Fernandesa, L.M.S.D. Silvaa, E.P. Marquesa, J. Zhang, A.L.B.
Marquesa, J. Power Sources 142 (2005) 10.
[32] K.L. Stewart, A.A. Gewirth, Langmuir 33 (2007) 9911.
[33] Y. Xie, C.O. Huber, Anal. Chem. 63 (1991) 1714.
[34] F. Sun, L. Li, P. Liu, Y. Lian, Electroanalysis 23 (2011) 395.
[35] M. Soejima, K. Egashira, H. Kawano, A. Kawaguchi, K. Sagawa, Y. Koda, J. Mol.
Diagn. 13 (2011) 334.