# Studies on Calcium Ion Selectivity of ZnO Nanowire Sensors Using Ionophore Membrane Coatings

Authors: M. H. Asif; O. Nur; M. Willander; M. Yakovleva; B. Danielsson
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

Zinc oxide nanorods with 100 nm diameter and 900 nm length were grown on the surface of a silver wire (0.25 mm in diameter) with the aim to produce electrochemical nanosensors. It is shown that the ZnO nanorods exhibit aCa2+-dependent electrochemical potentiometric behavior in an aqueous solution. The potential difference was found to be linear over a large logarithmic concentration range (1 μM to 0.1 M) using Ag/AgCl as a reference electrode and the response time was less than one minute. In order to adapt the sensors for calcium ion measurements in biological fluids with sufficient selectivity and stability, plastic membrane coatings containing ionophores were applied. These functionalized ZnO nanorods sensors showed a high sensitivity (26.55 mV/decade) and good stability.


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## Body

## 1. Introduction

Calcium is essential in the living cells; it has important
functions such as regulation of enzyme activity, neuronal activity, muscle
contraction, and vesicle exocytosis. In this letter, we present measurements
showing potentiometric sensitivity of functionalized ZnO nanorods of Ca2+ in water solution. The advantages of ZnO nanorod sensors are their small size,
being biosafe, possesses polar surface, and many other properties that
facilitate chemical sensing. The detection of biological and chemical species
is central to many areas of healthcare and life sciences [1]. Of major
importance to detection is the signal transduction associated with selective
recognition of a biological or chemical species of interest. Nanostructures,
such as nanowires [2–7] and
nanocrystals [7–12], offer new and sometimes unique opportunities in this rich
and interdisciplinary area of science and technology. The diameters of these
nanostructures are comparable to the size of the biological and chemical
species being sensed, which intuitively makes them represent excellent primary
transducers for producing electrical signals.ZnO
nanorods, nanowires, and nanotubes have recently attracted considerable
attention for the detection  of chemical and biological species
[13–18]. Among a variety
of nanosensor systems, the nanostructured electrochemical wire approach
employed in this study is one which offers high sensitivity and real-time
detection [19]. ZnO nanostructures have unique advantages including high surface
to volume ratio, nontoxicity, chemical stability, electrochemical activity, and
high electron communication features, which make them one of the most promising
materials for chemical and biological applications [20].The focus of the current study is the fabrication and demonstration of ZnO nanorods
on a relatively thin silver wire suitable for the sensing of calcium ions in
aqueous chemical solutions. Silver wire with grown ZnO nanorods has proven to
be a convenient and practical choice as we have demonstrated before in our
intracellular pH nanosensor developed earlier [19]. Our main efforts have been
directed toward the construction of a sensor having potentiometric properties.
These ZnO nanorods are highly sensitive to detect and monitor calcium ions with
potential applicability to in vivo measurements during
biological processes in which Ca2+ is important for activating
biological processes such as muscle contraction, protein secretion, as well as cell
death and development [21].


## 2. Experimental Details

ZnO
nanorods were grown on the surface of 0.25 mm thick silver wire by using a low-temperature
growth technique as described below. For the optimization steps the silver wire
(0.25 mm in diameter) was cut into small pieces of 5 cm in length. The wires
were dipped into the seed solution to facilitate aligned growth. The wires were
then dried in air for one minute. ZnO nanorods were grown in 150 mL of aqueous
solution of 0.025 M zinc nitrate(Zn(NO3)2)  and 0.025 M
hexamethylenetetramine (HMT, C6H12N4) in a
conventional flask. The reaction temperature was kept at 95°C.SEM
images of the ZnO nanowires grown on the silver wires were made with a field emission
scanning electron microscope (JEOL JSM-6335F Scanning Electron Microscope)
revealed that the diameter of the nanowires was 100–150 nm and the
length was 900–1000 nm as shown
in Figure1. The nanorods were rather uniform in size.Figure 1
SEM
images of ZnO nanorods on silver wire that is 0.25 mm in diameter. The nanorods
are 100 to 150 nm in diameter and 900–1000 nm long.The
ZnO layer on the silver wires was coated with ionophore membrane by a manual
procedure. Powdered PVC, 120 mg was dissolved in 5 mL tetrahydrofuran together
with 10 mg of a plasticizer (dibutyl phthalate, DBP) and 10 mg of Ca2+-specific
ionophore (DB18C6). All chemicals were from Sigma-Aldrich-Fluka.
The ZnO-coated wires were dipped
two times into and in  between the ionophore solution, and the solvent was
allowed to evaporate. After this the probes were conditioned in
10 mM CaCl2 solution.The
electrochemical potential of the Ca2+-probe was measured with an
Orion model 420 A+ pH-meter (Thermo Electron Corporation) versus an Ag/AgCl
reference electrode. The ZnO-coated silver wire was mounted in a three-way
junction for chromatography (Upchurch) using a tubing coupling screw and a
small O-ring to make a fluid-tight seal around the wire. One of the remaining
ports of the T-coupling was connected to a peristaltic pump and the other port
served as outlet and was connected with the reference electrode. Thus a flow
cell was formed with low dead volume allowing easy and quick change of sample
solution. The response of the Ca2+-probe was checked with
CaCl2 solutions with pH around 7 of all concentrations.


## 3. Results and Discussion

The
potentiometric response of the Ca2+-electrode was studied in aqueous
solutions of CaCl2 with concentration ranging from 1 μM to 0.1 M. The construction of a
two-electrode electrochemical potential cell is as follows:Reference
electrode| reference electrolyte solution ∥  test electrolyte solution |
indicator electrode.The
electrochemical cell voltage (electromotive force) changes when the composition
of the test electrolyte is changed. These changes can be related to the
concentration of ions in the test solution via a calibration procedure. The
actual electrochemical potential cell can be described by the following diagram:(1)Ag|ZnO|CaCl2∥Cl−|AgCl|Ag. Figure 2 shows a
typical induced voltage of our potentiometric sensor for different
concentrations of Ca2+ ions. As clearly seen it presents a linear
dependence, which implies that such sensor configuration can provide a large
dynamic range.Figure 2
Calibration curve showing the electrochemical potential difference, for the
ZnO nanorod as potentiometric electrode
with Ag/AgCl reference electrode versus logarithmic concentration
range for Ca2+ change for buffer solution.Polymeric
membranes are mainly made of polymer, which can selectively transfer certain
chemical species over others. Therefore, membranes are the key component of all
potentiometric ion sensors [22–24]. In fact, the
vast majority of membranes used commercially are polymer-based. Analogous to biological
ion channels, in analytical technology there are the so called ionophores and
neutral carriers incorporated into synthetic membranes or biomolecule membranes
in order to achieve the desired selectivity or detection of ionic species in
complex samples.DB18C6
is a coplanar, symmetrical, and polyether with a highly charged cavity [25] with
diameter of 4 Å and can accommodate only the nonsolvated  metal cation. The calcium ion being solvated in aqueous medium has a diameter of 7 Å, which is too large to
be accommodated in the cavity. High charge on the oxygen atoms of DB18C6 turns
the oxygen atoms into strong donors [26]. Calcium
ions are also strong
acids. Consequently there should be reasonably good interaction between oxygen
atoms and calcium ions.It
is expected that the covalent functionalization is a chemical process in which
a strong bond is formed between the nanostructured material and the biological
and chemical species. In most cases, some previous chemical modification of the
surface is necessary to create active groups that are necessary for the binding
of biological and chemical species [27].The
emf values of the Ca2+-electrode system obtained with CaCl2 solutions in water ranging from 1 μM to 0.1 M are plotted against the
logarithmic concentration of Ca2+ as shown in
Figure 3. The diagram
includes three experiments showing good reproducibility and linearity. Here,
the same ZnO sensor electrode was used for the three experiments to check the
reproducibility. The ZnO sensor electrode was carefully washed with 18 MΩ water after each reading to remove the
Ca2+ ions from the surface of the electrode. These calibration curves
are for PVC membrane-coated ZnO electrodes with DB18C6 as ionophore. The
coating increases the stability considerably as well as the selectivity. The
results show that the electrode is highly sensitive to calcium ions with a slope
around 26.5 mV/decade. The response time is fast. It takes less than one minute
to obtain a stable signal as shown in Figure 4. The morphology of the
functionalized ZnO sensor electrode was investigated after measurements. The
results are shown in Figure 5. The ZnO nanorods were not dissolved but affected
after measurements. As clearly seen in Figure 5(b), it shows shorter and thinner nanorods as
compared to Figure 5(a). This was important to investigate as it is known that
ZnO nanostructures can dissolve in many different aqueous solutions of
different pH values. This result was expected because the functionalization
provided protection for the surface of the nanowires.Figure 3
Calibration
curves from three different experiments using the same sensor
electrode
showing the electrochemical potential difference at different Ca2+
ion
concentrations for the ZnO nanorod electrode with an Ag/AgCl reference  electrode.Figure 4
Stability curve for ZnO
nanorod sensor at a Ca2+ concentration of 0.001 mM.Scanning
electron microscopy images of the ZnO nanorods grown on silver
wire using low temperature growth.  (a) Nanorods on silver wire coated with membrane before measurement. (b) SEM of the
same sensor after measurements.
(a)(b)


## 4. Conclusion

We have studied the use of
ZnO nanorods as electrochemical nanosensor for Ca2+ in
water solutions. A convenient sensor design was realized by growing the ZnO
nanorods on a thin silver wire that could be readily inserted into a low-volume
flow cell. Good performance in stability and selectivity was achieved by
coating the sensor surface with a plastic ionophore membrane. The potential difference
was linear over a wide logarithmic concentration range (1 μM to 0.1 M). These
results demonstrated the capability of performing biologically relevant
measurements inside a solution of CaCl2 using functionalized ZnO
nanorods.



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*Source: 701813-2009-01-08.xml*