# Fabrication and Characterization of ZnO Nanowire Arrays with an Investigation into Electrochemical Sensing Capabilities

Authors: Jessica Weber; Sathyaharish Jeedigunta; Ashok Kumar
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

ZnO nanowire arrays were grown on a Si (100) substrate using the vapor-liquid-solid (VLS) method. ZnO nanowires were characterized by XRD, SEM, bright field TEM, and EDS. They were found to have a preferential orientation along thec-axis. The as-prepared sample was functionalized with glucose oxidase by physical adsorption. FTIR was taken before and after functionalization to verify the presence of the attached enzyme. Electrochemical measurements were performed on the nanowire array by differential pulse voltammetry in the range of −0.6 to 0.4 V. The nanoarray sensor displayed high sensitivity to glucose in the range of 1.0 ×10−4 to 1.0 ×10−2 mol L−1.


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## Body

## 1. Introduction

One-dimensional metal-oxide nanostructures have gained
prominence after the immense interest developed in the synthesis of carbon
nanotubes and its wide range of applications [1].  Metal oxides such as SnO2  [2], TiO2  [3], In2O3 [4], ITO  [5], Ga2O3  [6], and ZnO  [7] have been synthesized into nanowires, nanorods, nanobelts, and
nanohelices. Due to their excellent electronic and optical properties, they are
widely found in transparent electronic devices [8], flat panel displays [9],
field emitters [10], electrochemical sensors, and toxic gas sensors [11]. As a
biocompatible semiconducting material, ZnO is being actively investigated for
biosensor applications
[12–14].Miniaturization
is one ongoing important development in biosensor technology.  Miniaturization, however, may result in low
current because of the decreased amount of immobilized enzyme onto the
available active area. It has already been reported that nanostructures can
enhance the sensitivity of a biosensor by one to two orders of magnitude, due
to the large surface area per unit volume ratio, which allows the
immobilization of a larger amount of the  enzyme. Since the development of the
first glucose sensor enzyme electrode performance, stability and selectivity have
been a main thrust for further research [15]. The incorporation of biomolecules
into carbon nanotubes (CNTs) and metal oxide nanowires is achieved through
various methods of immobilization such as covalent linkage [16], entrapment [17],
cross-linking with glutaraldehyde [18], microencapsulation [19], and adsorption
[20–22]. Adsorption
is one of the more common schemes of immobilization because it is a method that
requires minimal preparation. In this work, prolonged exposure of glucose
oxidase to ZnO nanowires has resulted in enzyme immobilization through
nonspecific adsorption of the enzyme on the sidewalls of the nanowires. This
letter reports on the synthesis and characterization of ZnO nanowires by vapor-liquid-solid
(VLS) mechanism and its application as an electrode for glucose measurement
without any additional protective coating.


## 2. Methods and Materials

For the growth of ZnO nanowires, ZnO nanopowder (99.999%,
Sigma ~50–70 nm grain size)
and graphite nanopowder (99.99%, Sigma ~70 nm) in 1 : 1 ratio were mixed to form
a homogenous source weighing 300 mg. For the amperometric glucose detection,
glucose oxidase (GOX, EC 1.1.3.4, type II fromAspergillus niger, 47 200 U/g), D-(+)-glucose (purity 99.5%), and
potassium phosphate were purchased from Sigma-Aldrich, St. Louis, Mo, USA. Phosphate buffer electrolyte solutions
(PBSs) with various pHs were prepared from standard stock solutions of KH2PO4 and K2HPO4. All
solutions were prepared with deionized water.A high temperature furnace (Lindberg/Blue) was used for the growth of ZnO
nanowires.  As synthesized products were
characterized by X-ray diffraction with Cu-Kα radiation (Philips X’pert Pro
diffractometer), field emission scanning electron microscopy (FE-SEM, Hitachi
S-800), and high-resolution transmission electron microscopy (FEI Tecnai F30, HR-TEM).  TEM specimens were prepared by
ultrasonicating the ZnO nanowires in methanol and dispersing a drop of solution
on a carbon-coated copper grid. Chemical compositional analysis was carried out
by EDX coupled with the HR-TEM system.Electrochemical experiments were performed using a Princeton
Applied Research PARSTAT 2263 advanced electrochemical analyzer. All
electrochemical measurements were executed in a standard three-electrode system
at room temperature. The modified zinc oxide sample acted as the working
electrode, with an Ag/AgCl (3 M KCl) reference electrode, and a platinum wire
(CH Instruments, Tex, USA)
counter electrode. All potentials given in this paper are relative to the
Ag/AgCl electrode. The pH of the glucose solutions was measured with a Fisher
Scientific AB15 pH meter. FTIR studies were performed on a
Perkin-Elmer Spectrum One
FT-IR Spectrometer.For the fabrication of a glucose sensor, we have initially
grown an array of ZnO nanowires on Si (100) via thermal evaporation, with the
use of a gold catalyst. Freshly prepared ZnO source powder and substrates were
loaded in two different alumina boats in the high-temperature and low-temperature
zones of the vacuum furnace, respectively.  The furnace was initially evacuated to a
pressure of10−3 Torr and argon was then passed at a constant flow
rate of 500 sccm.  The temperature of the furnace was approximately raised to
900C°–950C°. The
substrates were unloaded after the furnace was cooled to room temperature. The zinc oxide nanowire array was then
functionalized with the enzyme glucose oxidase. Approximately 15 IU of GOX was
applied onto the nanowire surface via physical adsorption. The newly
constructed electrode was allowed to dry over 24 hours at room temperature
prior to use.


## 3. Results and Discussion

The X-ray diffraction pattern of the
as-grown ZnO products is shown in Figure1. All the visible peaks are indexed
to a wurtzite (hexagonal) structure of ZnO with lattice constants of a=0.3250 nm and c=0.5205 nm, respectively [23].
A small shift was observed in the peaks of ZnO
nanowires when compared to ZnO bulk. This might be due to the thermal stresses
developed at the time of growth. In addition, Au (111) and Au (200) peaks were
also detected from the XRD pattern. The high intensity of (002) peak of ZnO
nanowires shows that the
preferential growth direction is along the
c-axis.Figure 1
(upper curve) X-ray diffraction spectrum of an array of ZnO nanotubular structures and (lower curve) bulk ZnO, respectively.The surface morphology of the patterned sample
can be observed in the SEM images (see Figure2). The ZnO nanowires have a
typical length of 0.5–2 μm and a diameter of
40–120 nm. Figure 3
shows the TEM image of a pair of nanowires and inset shows the electron
diffraction pattern of the wires. It is clearly shown from the electron
diffraction pattern that the one-dimensional nanowires were single crystal and
grown along [0001].  A representative
energy dispersive X-ray (EDX) spectrum was performed near the tip of the ZnO
nanowire as indicated by the arrow shown in Figure 3(c). The peaks associated
with Zn, O, Au, Cu, and C are seen in the EDX spectrum, where the peak
corresponding to Au confirms that the tips of the nanowires were encapsulated
with a gold particle of diameter ~52 nm (see Figure 3(c)) and the copper and
carbon signatures are from the carbon-coated copper TEM grid.(a) Low magnification top-view SEM image of patterned ZnO nanotubes. (b–d) Side
view of patterned- and aligned-ZnO nanotubes from lower to higher magnification
(clockwise direction).
(a)(b)(c)(d)(a) Bright field TEM image of a pair of ZnO nanotubes, inset shows electron
diffraction pattern on a pair of ZnO nanotubes along the zone axis [21−1−0]. (b) TEM image of a ZnO nanotube with an Au
particle at the end. (c) Shows high-resolution image of the end of the ZnO
nanotube. (d) EDS spectrum recorded near the catalyst particle indicated by arrow.
(a)(b)(c)(d)The as-grown ZnO nanowires on silicon substrate were analyzed
by Fourier transform infrared (FTIR) spectroscopy before and after
functionalization with GOX (see Figure4). The absorption peak at about 1000 cm−1 can be interpreted as the Si-O-Zn vibrational mode [24]. GOX is seen through
the presence of the primary amine group. The N-H bending is observed at 1600 cm−1 while the N-H stretch due to asymmetric and symmetric vibrations occurs at
3400 cm−1 and 3300 cm−1, respectively. The activity of the
enzyme glucose oxidase is affected by the pH of the glucose solution. The pH
dependence of the sensor was evaluated at 5 mM glucose solutions in the range
of pH 6 to 9 (see Figure 5).  An optimal
peak current of the sensor was displayed at pH 6.5. Considering that the pH of
human blood is about 7.4, the amperometric experiments were performed at pH
7.0.  Figure 6 shows the cyclic
voltammograms of the ZnO-GOX electrode in PBS at a pH of 7.0 and at room
temperature. The inset shows the plot of peak current versus the square root of
the scan rate. The plot is nearly
linear with less than 3% error from 50 to 400 mV s−1. The decrease in current response with successive increase in
scan rate indicates that the electrode reaction is diffusion controlled. The direct pulse voltammetry (DPV) response
of the sensor to successive increments of glucose is shown in Figure 7(a).  These results were obtained  with a scan rate
of 0.020 mV/s, step height of 2 mV, and a potential sweep between  −0.6 and 0.4 V. The well-defined peaks occur at approximately −0.05 V, showing that the
enzyme is active at this potential. This data displays a linear relationship of
current to the corresponding glucose concentration. The calibration response
curve (see Figure 7(b)) shows a linear trend in the range of  1.0×10−4  to   1.0×10−2 mol L−1 glucose with an r-value of 0.9903 and less than 5%
error.Figure 4
FTIR spectra of (lower curve) ZnO nanowires and
(upper curve) glucose oxidase entrapped-ZnO nanowires, onto Si substrate.Figure 5
Current response of ZnO-based glucose sensor in PBS with increasing pH
containing5.0×10−3 mol L−1 glucose.Figure 6
Cyclic voltammograms of ZnO-GOX/Si in PBS (pH 7.0) containing5.0×10−3 mol L−1 glucose at a scan rate of (a)
50 mV s−1, (b) 100 mV s−1, (c) 200 mV s−1, (d) 300 mV s−1,  and (e) 400 mV s−1.  Inset plot: relationship between scan rate
and response current of ZnO-GOX/Si in PBS (pH 7.0) containing 5.0×10−3 mol L−1 glucose.(a) DPV response of ZnO-GOX/Si in PBS
(pH 7.0) at (a)1×10−4 mol L−1, (b) 5×10−4 mol L−1, (c)  1×10−3 mol L−1, (d) 5×10−3 mol L−1,  and (e)  1×10−2 mol L−1 glucose. (b) Linear calibration curve of
ZnO-GOX biosensor.
(a)(b)


## 4. Conclusions

The successful fabrication of a highly selective
ZnO nanowire-based amperometric glucose biosensor has been achieved. The ZnO
electrodes were synthesized on Si (100) substrates by VLS mechanism.  High-density ZnO nanowires with a large
surface area are found to have a preferential growth direction along [0001]  axis. No additional protective coating has been utilized during the electrode
preparation. The sensor functioned in the range of1.0×10−4 to  1.0×10−2 mol L−1 glucose. The biosafe nature of ZnO
and successful immobilization of glucose oxidase onto the electrode surface
leads to a new novel approach to biosensor construction and applications.



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*Source: 638523-2009-02-02.xml*