Electrochemical biosensors-based-on-zno-nanostructures-to-measure-intracellular-metal-ions-and-glucose-2155-9872.s7-003

Open AccessResearch Article
Analytical & Bioanalytical
Techniques
Electrochemical Biosensors Based on ZnO Nanostructures to Measure
Intracellular Metal Ions and Glucose
Muhammad H. Asif1,2
*, Fredrik Elinder3
and Magnus Willander2
1
Materials Research, Department of Physics, COMSATS Institute of information Technology, Lahore-54000, Pakistan
2
Department of Science and Technology, Campus Norrköping, Linköping University, SE-601 74 Norrköping, Sweden
3
Department of Clinical and Experimental Medicine, Division of Cell Biology, Linköping University, SE- 581 85 Linköping, Sweden
Biosensing
Keywords: ZnO nanorods; Human adipocytes; Frog oocytes; Poten-
tiometric; Metal ions; Glucose; Membranes; Enzymes; Functionaliza-
tion
Introduction
Nanotechnology is attractive in the research community to
create and manipulate materials at the nanoscale to fabricate unique
metal-oxide nanostructures with interesting properties. Metal-oxide
nanostructures (e.g. nanorods, nanowires, and nanotubes) have been
explored for many biochemical and biomedical applications (e.g.
biosensing, biological separation, molecular imaging, biomarker,
and anticancer therapy) because of their interesting properties and
functions. Particularly their high surface/volume ratio, surface tailor
ability, biosafety, and biocompatibility open many new possibilities
for biomedicine. At present the application of nanostructure in
advancement of electrochemical biosensors for the intracellular
environment is an active research area. A metal-oxide nanostructure
material with exciting surface-charge properties provides a fascinating
platform for interfacing biorecognition elements with transducer for
signal amplification. These nanostructures display appealing nano-
morphological, functional, biocompatible nontoxic and catalytic
properties. They show higher electron-transfer kinetics and durable
adsorption capability, providing appropriate microenvironments
for the immobilization of biomolecules and enhanced biosensing
characteristics.
Zinc oxide (ZnO) is one of the best metal-oxide semiconductors,
and it is relatively biosafe and biocompatible which is suitable for
intracellular sensor/transducer applications [1-5]. The dimensions of
ZnO nanostructure can be engineered to the size of the biochemical
species being sensed which make the ZnO an excellent primary
transducer for producing electrical signals. All these facts make ZnO
nanostructures suitable candidates for intracellular measurements
[6]. The focus of this review is the fabrication and demonstration of
functionalized ZnO nanorod-based potentiometric sensors suitable
for intracellular free metal-ions and glucose. Our main effort has
been directed towards the construction of borosilicate glass capillary
tips (0.7µm in diameter) selective for metal ions (Ca2+
, Mg2+
, K+
and
Na+
) and glucose, capable of penetrating the cell membrane, as well
as the optimization of the electrochemical potential properties. Glass
tips with grown ZnO nanorods have proven to be a convenient and
practical choice as we will demonstrate in this review.
Biological role of intracellular free metal ions
Metal ions play fundamental roles in life processes by acting as
cofactors in enzymes, as osmotic regulators, as current carriers and
consequently as factors in information processing, and as integrators
and stabilizers of proteins and lipids [7]. Na/K-ATPases charge the
cellular battery by pumping monovalent Na+
and K+
across the cellular
membrane – Na+
to the outside of the cell and K+
to the inside. This
generates an electric potential across the resting membrane; opening
of passive ion channels release the energy, which can be used to
transmit fast electric impulses in the form of action potentials. Divalent
cations, such as Ca2+
, Mg2+
and Zn2+
, normally play a regulatory role in
modifying ion channels, receptors and enzymes [8]. The regulation of
the metal ion concentration is strictly controlled and failures to do so
lead to disease and death.
The intracellular K+
concentration is high, generating a negative
*Corresponding author: Muhammad H. Asif, Materials Research, Department of
Physics, COMSATS institute of Information Technology- Lahore-54000, Pakistan,
E-mail: muhas43@gmail.com, muhammadasif@ciitlahore.edu.pk
Received July 01, 2011; Accepted September 26, 2011; Published September
30, 2011
Citation: Asif MH, Elinder F, Willander M (2011) Electrochemical Biosensors
Based on ZnO Nanostructures to Measure Intracellular Metal Ions and Glucose. J
Anal Bioanal Tech S7:003. doi:10.4172/2155-9872.S7-003
Copyright: © 2011 Asif MH, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
Abstract
Zinc oxide (ZnO) nanostructures have attracted much interest for intracellular electrochemical measurements
because of its large surface area, and its biocompatible properties. To design intracellular biosensors for metal
ions and glucose, we grew ZnO nanorods on the tip of borosilicate glass capillaries (0.7µm in diameter) and
characterized the nano-scale structure with field-emission scanning electron microscopy and high-resolution
transmission electron microscopy. The ZnO nanorods were functionalized accordingly for intracellular free metal
ions or glucose measurements. Selectivity was achieved by using a metal-ion selective plastic membrane or glucose
oxidase enzyme for glucose measurements. These functionalized ZnO nanorods showed high sensitivity and good
biocompatibility for intracellular environments. Human adipocytes and frog oocytes were used for determinations of
intracellular free metal ions and glucose concentrations. In this review, we discuss the simple and direct approach
for intracellular measurements using ZnO nanostructure-based potentiometric biosensors for clinical and non-
clinical applications. The performance of ZnO nanostructure-based intracellular sensor can be improved through
engineering of morphology, effective surface area, functionality, and adsorption/desorption capability. This study
paves the way to find applications in biomedicine by using this simple and miniaturized biosensing device.
Asif et al. J Anal Bioanal Tech 2011, S7
http://dx.doi.org/10.4172/2155-9872.S7-003
ISSN:2155-9872 JABT, an open access journalJ Anal Bioanal Tech

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resting membrane potential, and fluctuations in the K+
concentration
alters the membrane potential and consequently, affects excitability
and other physiological functions of nerve and muscle [9]. The
intracellular concentration of Na+
is low and it may act as a second
messenger in different cell types. Na+
regulates different transporters
and receptors in renal cells and neurons, and it regulates the activation
of both Na+
and K+
channels, thereby regulating the reabsorption of salt
and water in the kidneys and neuronal excitability [10-15]. Na+
and K+
also play fundamental roles in a number of less explored mechanisms.
Intracellular K+
participates in cell growth, maintenance of cell volume,
DNA, and protein synthesis, enzymatic activity, acid-base balance, and
programmed cell death (apoptosis) [16]. Reduced intracellular K+
and
subsequent cell shrinkage are early events in apoptosis. The reduced K+
concentration release caspase inhibition, resulting in DNA degradation
and eventually cell death [17-20]. Intracellular Na+
is also important in
the early stages of apoptotic processes in cell lines, and may be involved
in intracellular signalling during apoptosis [17,21-24].
Mg2+
is the most abundant intracellular divalent cation in living
cells, where it plays a major biological role in its involvement with
phosphate compounds (e.g. ATP) and phosphate metabolism, binding
in chlorophyll pigments, and acting as a critical cofactor in numerous
enzymatic reactions [7]. Intracellular Mg2+
also affects ion channels, for
instance by gating inward-rectifier K channels [25]. Ca2+
is essential in
living cells [26] – it has important roles in regulating enzyme activity,
neuronal activity, muscle contraction, vesicle exocytosis, cell death,
development, and plastic changes (i.e. LTP, long-term potentiation;
[27]) at the synapse. Ca2+
binds to synaptic proteins to trigger synaptic
release, it binds to intracellular proteins in muscles to trigger muscle
contraction, and it binds to ion channels to regulate their activity [8].
To be able to accurately measure the concentrations and alterations
of specific metal ions is therefore of great importance in cell biological
research.There are, however, situations when real-time monitoring
in single cells with probes inserted in living cells is the preferred
configuration. In our studies of (bio) sensors based on probes covered
by ZnO nanorods, we have found that it is possible to make sufficiently
small probes to measure free metal ions inside large cells [1-4].
Glucose in health and disease
Glucose is one of the main products of photosynthesis and serves as
thehumanbody’sprimarysourceofenergy.Duringhypoxicconditions,
as in hard working muscles, glucose conversion in glycolysis is the only
means of generating cellular adenosine triphosphate (ATP). While
most tissues and cells can oxidize fatty acids for their energy supply, the
brain is critically dependent on a constant glucose supply. Glucose also
serves as a metabolic intermediate in the synthesis of more complex
molecules such as fats.In the research community, it has become one
of the most targeted analyte in bioanalysis whether it is in pharmacy,
medicine or biotechnology. When glucose levels in the bloodstream are
not properly regulated, diseases such as diabetes can develop [6].
In fact, there are not many intracellular biosensors reported in
literature. Abe et al. (1992) [28] described a glucose microsensor
with a tip diameter of 2µm that could provide reliable and accurate
measurements of glucose during few hours. As an alternative to direct
measurements in cells, metabolite concentrations can be assessed by
indirect, non-invasive measurements using various techniques, such as
NMR that gives average concentrations on samples containing several
cells [29]. Other techniques, like those employing fluorescently labeled
glucose molecules, may permit real time measurements in single living
cells [30]. A precise and fast measurement of glucose is obviously
significant to diagnosis and handling of the different forms of diabetes,
as well as in research to understand the diabetes diseases and to develop
new therapies [5].
Electrochemical miniaturization for intracellular measure-
ments
In recent years the research community has been working to
miniaturize the materials for intracellular measurements in biological
and biomedical applications. There have been great progresses in the
application of metal-oxide nanomaterials such as ZnO, CuO, Fe2
O3
in intracellular biosensors [1-5,31]. Semiconductors, such as ZnO
and SnO2
have high resistivity that provides alternative template for
gas sensors [32]. Nanomaterial-based biosensors, which represent the
integration of material science, molecular engineering, chemistry and
biotechnology, can significantly advancement in the sensitivity and
selectivity for biomolecule detection, and also hold the capability of
detecting atoms and molecules [33-35]. The continuous progress in
synthesizing and fabrication of controlled materials on the submicron
and nanometer scale results from novel advanced functional materials
with tailored properties. When the bulk materials scale down to a
nanoscale, most of the materials reveal novel properties that cannot
be deduced from their bulk behavior. For the fabrication of an efficient
and fast biosensor, the choice of substrate for dispersing the sensing
material decides the sensor performance.
The ZnO nanomaterials are applied to biosensors because of their
unique physical, chemical, mechanical, and optical properties which
markedly enhance the sensitivity and specificity of detection [1-5,36].
The interesting property of ZnO nanostructure may offer excellent sce-
narios for interfacing biological recognition events with electronic sig-
nal transduction. All these facts described above open up for possible
sensitive extra- and intracellular measurements. Metal-oxide nano-
structures (nanorods or nanotubes) have been widely used in immobi-
lization of polymeric membranes and enzymes for bioanalytical appli-
cations. These can play an important role on electrical, optical, electro-
chemical and magnetic transducers [37]. ZnO nanostructure can act as
excellent material for immobilization of enzymes and rapid electron-
transfer agent for the fabrication of efficient biosensors due to the wide
direct band gap [38,39].The resulting biosensors could be used to detect
the glucose level of serum samples with high sensitivity. Nanostructure
metal oxides display appealing nano-morphological, functional, bio-
compatible, and catalytic properties. These nanostructures also show
enhanced electron-transfer kinetics and strong adsorption capability,
providing suitable microenvironments for the immobilization of bio-
molecule and improved biosensing characteristics [40]. Miniaturiza-
tion of sensors based on nanostructure is very important since a large
surface area-to-volume ratio leads to a short diffusion distance of the
analyte towards the electrode surface, thereby providing an improved
signal-to-noise ratio, faster response time, enhanced analytical perfor-
mance and increased sensitivity. Nanostructure-based biosensors are
expected to find new manufacture of miniaturized, smart biosensing
device. In this review, we focus on new methods for the fabrication of
electrochemical sensors with desired properties for health care, intra-
cellular environment, and confining different biomolecule onto closely
spaced one dimensional nanostructure based on ZnO nanorods [6].
Fundamentals of the ZnO nanostructure
ZnO is a relatively biosafe, biocompatible and highly atomic ionic-
ity in the crystal which can be used for biomedical applications (e.g.,
bio sensing, biological separation, molecular imaging, and/or antican-
cer therapy). ZnO nanorods display appealing nano-morphological,
Biosensing
Citation: Asif MH, Elinder F, Willander M (2011) Electrochemical Biosensors Based on ZnO Nanostructures to Measure Intracellular Metal Ions and
Glucose. J Anal Bioanal Tech S7:003. doi:10.4172/2155-9872.S7-003

of 9
functional, non-toxic and catalytic properties. ZnO is a polar semicon-
ductor with two crystallographic planes with opposite polarity and dif-
ferent surface relaxation energies, which leads to a higher growth rate
along the c-axis. The crystal structures formed by ZnO are wurtzite,
zinc blende, and rocksalt. The most stable phase of ZnO is the wurtzite
structure in which the sub-lattices of cation (Zn2+
) and anions (O2-
)
are interconnected tetrahedrally. A diverse group of nanostructures
has been obtained from ZnO, and this is probably the richest family
of nanostructures among the entire family of one dimensional nano-
structure [41]. The ZnO nanorods have large surface area-to-volume
ratio which makes them attractive for biochemical sensing applica-
tions. They have the ability to control their nucleation sites which
makes them candidates for high density sensor arrays [42], and they
have alternating planes composed of hexagonal with neutral surface
to have equal possibility of contribution to the ions with ionicity of
around 60%. A literature survey reveals that ZnO nanorods show n-
type semiconducting property and their electrical transport is highly
dependent on the adsorption/desorption nature of chemical species
[43]. The diameters of these nanostructures can be comparable to the
size of the biological and chemical species being sensed, which makes
them represent excellent primary transducers for producing electrical
signals [1-5]. All these facts make them suitable for extra and intracel-
lular ion measurements. The sensor in this study was used to detect
and monitor real changes in metal ions and glucose using changes in
the electrochemical potential at the single cell/ZnO nanorod surface
interface. In the past it was demonstrated that the pH inside cells can
be monitored without damaging the cell by the use of our grown ZnO
nanowires [31,44]. If ZnO nanorods are lost in the body or in a blood
vessel, it can be dissolved in the biofluid into ions that can be absorbed
by the body and become part of the nutrition; Zn2+
are needed by each
one of us every day. Finally the slow solubility and high compatibility of
ZnO in biofluids is required for its application in biology [41].
Growth and characterization of ZnO nanostructure
A simple and direct technique was used to grow the nanostructures
at desired substrate and positions with suitable arrangements such
as nanorod arrays on tip of borosilicate glass capillaries (0.7µm in
diameter) for intracellular measurements. The ZnO nanorods grow
easily and normally self-organized with a preferential growth direction
along the c-axis (0001). A diverse group of ZnO nanostructures,
such as nanorods, nanowires, nanobelts, nanocombs, nanosprings,
nanorings, nanoflakes, nanoflowers, and nanotubes, can be grown by a
variety of different methods. There are many techniques to grow ZnO
nanostructure; low-temperature growth methods use temperature
below 100o
C and high-temperature growth methods use temperatures
around 1000o
C.
In this article we will briefly review the most commonly used and
well developed methods for the growth of ZnO nanorods on the tip of
borosilicate glass capillaries (0.7µm in diameter) for intracellular mea-
surements. The low-temperature approach used is based on the aque-
ous chemical growth (ACG) method [45,46] and is relatively easy and
attractive because the nanostructure can be grown on a variety of sub-
strates. The ZnO nanorods were grown in 150mL of an aqueous solu-
tion of 0.025M Zn(NO3
)2
and 0.025M hexamethylenetetramine (HMT,
C6
H12
N4
) in a conventional flask. The reaction temperature was kept
to 95o
C [1-5]. Before the samples were placed inside the solution for
growth, they were coated with a ZnO seed layer by using a technique
develop by Green et al. [47]. The seed layer plays an important role to
achieve vertically aligned ZnO nanostructures. The whole procedure
was repeated three times in order to ensure a uniform ZnO seed layer.
Then the substrate was annealed at 110o
C to solidify the seed layer. This
seed solution was used to facilitate aligned nanorod growth. After the
growth the samples were cleaned in de-ionized water and left to dry in
air inside a closed beaker [1-5].
Two types of microscopy techniques were used to investigate the
morphology, crystal structure, and chemical composition of ZnO
nanorods/nanowires: 1) A field-emission scanning electron micro-
scope (FESEM), model JEOL JSM-6335F, and 2) a high resolution
transmission electron microscopy (HRTEM) using a Tecnai G2 UT
instrument operated at 200 kV with 0.19nm point resolution. Figure
1 shows the FESEM images revealing that the diameter of the nano-
rods was 100-150nm and that the length was 900 – 1000nm. The ZnO
nanorods cover 3mm of the silver-coated film on the glass capillary tip
which was used as sensing electrode for intracellular measurement. The
appearing nanostructure has a rod like shape with a hexagonal cross
section and primarily aligned along the perpendicular direction [1-5].
The morphology of the functionalized ZnO nanorod decorated tip was
checked in FESEM after the intracellular measurements at different
magnifications as shown in figure 2 which shows that some compo-
nents from the cell and the cell membrane adhere to the probe. Possibly
this contamination occurs when the probe is pulled out from the cell.
Figure 3 shows the HRTEM images of ZnO nanorods grown by the
ACG method on the surface of a glass tip. The specimen was made by
scraping the nanorods onto a copper grid with a carbon film. Chemical
composition microanalysis using the energy dispersive X-Ray (EDX)
demonstrates that the ZnO nanorods consists only of O and Zn with-
out other metal impurities as catalysts (see figure 3 (b)). The additional
peak at 8keV is the Cu Kα from the copper grid. A HRTEM image and a
selected area electron diffraction (SAED) pattern are shown in figure 3
(c) with the inset revealing that the nanorod axis is in the [0001] direc-
tion. The rods contain some defects such as surface steps and stacking
faults shown by arrows in figures 3 (d, e) [3].
Functionalization of ZnO nanostructure
The nanostructure materials, used in biosensors, are usually cov-
ered with a thin layer of biomaterials (i.e. membrane or enzyme) that
has many functions, including diffusion control, reduction of interfer-
ence and mechanical protection of the sensing probe. There are dif-
ferent types of polymers and enzymes commercially available which
are commonly used to functionalize the interface due to their suitable
physical and chemical properties especially for selectivity. Biosensors
with polymer membranes have been successfully applied in many fields
such as the monitoring of food production, environmental pollution
and pathological specimens. Polymeric membranes are mainly made
of polymers, which can selectively transfer certain chemical species.
Therefore, membranes are the key components of all potentiometric
ion sensors [48-51]. Analogous to biological ion channels, in analyti-
cal 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. Solvent polymeric membrane are commercially
available and routinely used for the selective detection of several metal
ions such as K+
, Na+
, Ca2+
etc. in complex biological matrices [6].To
measure the intracellular free metal ion selectively, the ZnO nanorod
layer on the tip of borosilicate glass capillaries were coated with poly-
meric membrane incorporated with ionophore by a manual procedure.
For optimization, powdered polyvinyl chloride (PVC) was dissolved
in tetrahydrofuran (THF) together with the plasticizer dibutylphtalate
(DBP) and metal ion-specific ionophores. After preparing the solution,
the ZnO-coated glass tip was dipped twice into the resultant solution
Biosensing

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Figure 1: SEM images of ZnO nanorods on the surface of glass capillary tip at different magnifications.
Figure 2: FESEM images showing the working electrode after intracellular measurements at different magnifications [5].
Biosensing

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until a thin film of the membrane was attached to the surface of the
ZnO-coated glass tip and then let them to dry at room temperature
[1-4].
For glucose measurements, the enzyme glucose oxidase (GOD) is
the most commonly used enzyme. Among other glucose-specific en-
zymes, GOD is usually preferred since it is highly stable and specific
and also cheap. Nanostructure have exceptional prospects for interfac-
ing biological recognition events with electronic signal transduction
in which active enzymes sites are coupled directly with the nanostruc-
ture-based electrode resulting in direct electron transfer between the
enzyme and nanostructure with enhanced biosensing properties. The
nanostructure-based electrodes provide a biocompatible electro-active
surface for enzyme immobilization with enhanced confirmation, orien-
tation and biological activity [52]. For the measurements, the enzyme
GOD was electrostatically immobilised by dipping the tip of a boro-
silicate glass capillary with well-aligned ZnO nanorods into 2 ml of an
enzyme solution (5 mg/ml prepared in 10mM PBS containing 1.5mM
Na2
HPO4
, 8mM KH2
PO4
, 0.138M NaCl, and 2.7mM KCl pH 7.4) for 15
minutes at room temperature and then drying it in air for more than
20 minutes. GOD was electrostatically immobilised because there is a
large difference in the isoelectric points of ZnO (IEP = 9.5) and GOD
(IEP ~ 4.2).All enzyme electrodes were stored in dry condition at 4o
C
when not in use. All chemicals were from Sigma-Aldrich-Fluka [5].
Quantification of intracellular free metal ions
For the quantification of intracellular free metal ions Ca2+
, Mg2+
,
Na+
, and K+
we used functionalized ZnO nanorods covered by an iono-
phore-containing membrane selective for specific ions. The functional-
ized ZnO nanorod-based ion-selective microelectrode was calibrated
versus the Ag/AgCl reference electrode in an aqueous solution with a
wide concentration range of metal ions. The output electromotive force
(emf) change when the composition of the test electrolyte is changed.
The actual electrochemical potential cell can be described by the dia-
gram below [1-4]:
Ag |ZnO | aqueous solution || Cl-
|AgCl | Ag
Figure 4 shows a typical induced voltage measured by our potentio-
metric sensor for different known concentrations of Ca2+
in the range
from 100nM to 10mM and shows that this Ca2+
dependence is linear
and has sensitivity equal to 29.67mV/decade at around 23°C (inset Fig-
ure 4). This linear dependence implies that such sensor configuration
can provide a large dynamical range [1].
The ZnO nanorod-grown tip was used to measure the free concen-
tration of the intracellular Ca2+
in single human adipocytes. The Ca2+
-
selective microelectrode, mounted on a micromanipulator, was moved
into position in the same plane as the cells. The two microelectrodes
were then gently micro manipulated a short way into the cell. The ex-
perimental setup for intracellular potentiometric sensor is shown in
figure 5. Once the two microelectrodes were inside the cell, that is iso-
lated from the buffer solution surroundings, an electrochemical poten-
tial difference was detected and identified the activity of Ca2+
. The in-
tracellular Ca2+
concentration was 123 ± 23nM, corresponding closely
with the intracellular concentration determined after loading human
adipocytes with the Ca2+
-binding fluorophore fura-2 [53]. In another
set of experiments we used the nanosensor to measure intracellular
Ca2+
concentration in single frog oocytes. The intracellular Ca2+
con-
centration was 250 ± 50nM which is also close to previously reported
values [54]. When the Ca2+
-selective microelectrode was removed from
the cells after measurements, it was monitored by microscopy. Figure
2 shows SEM images at different magnification from the Ca2+
-selective
microelectrode after measurements. The ZnO nanorods were not dis-
Figure 3: (a, d, e) HRTEM images; (b) EDX analysis; and (c) SAED pattern from typical ZnO nanorod [3].
Biosensing

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solved. This result was expected because the functionalization provided
protection for the surface of the nanorods.
To test the sensitivity of our constructed biosensor we performed
measurements while changing the Ca2+
concentration in the buffer
solution around the cell. The Ca2+
was varied from 100nM to 10mM.
These measurements were performed for two cases; the first was for a
configuration where half of the working functionalized ZnO nanorods
were inserted inside the cell and the other half was in contact with the
surrounding buffer solution. The second configuration was when all
the functionalized ZnO nanorods were inserted inside the cell. Signifi-
cantly, the functionalized ZnO nanorods exhibited a stepwise decrease
in the induced electrochemical potential only in the first case of the
configuration. Once the functionalized ZnO nanorods working elec-
trode was totally inserted inside the cell, (i.e. isolated from the buffer
solution surrounding the cell), the electrochemical potential difference
signal detected was stable (corresponding to the actual intracellular
Ca2+
concentration) even when the Ca2+
concentration was varied in
the buffer solution. This implies that the constructed electrode has a
high sensitivity. In addition, this observation confirms that the values
of the potentiometric response when the whole sensing electrode was
inserted inside the cell membrane is the signal corresponding to the
value of the Ca2+
concentration inside the cell [1].
We have also used functionalized hexagonal ZnO nanorods, grown
on silver-coated capillary glass tip, as a selective intracellular sensor
for other free metal ions (Na+
, K+
, Mg2+
) in single human adipocytes
and frog oocytes. The functionalization was achieved through the use
of ion-selective PVC polymeric membrane. The potential difference
was found to be linear over a wide concentration range of interest. The
measured intracellular metal-ion concentrations using the ZnO nano-
rods sensor in human fat cells or in frog egg cells were consistent with
values reported in the literature. These results demonstrate the capabil-
ity to perform biological relevant measurements inside living cells. The
ZnO nanorod-based metal ion microelectrodes thus hold promise for
minimally invasive dynamic analyses of single cells [1-4].
Quantification of intracellular glucose
The determination of the intracellular glucose concentration with
functionalized ZnO nanorod-coated microelectrodes was performed
in two types of cells, human adipocytes and frog oocytes. In the hu-
man body, the hormone insulin only stimulates glucose transport into
muscle and fat cells. However, insulin has also been found to affect glu-
cose uptake in oocytes from the frog Xenopuslaevis [55,56]. The large
size of these cells makes it possible to microinject specific reagents that
interrupt or activate signal transmission to glucose. The response of
the electrochemical potential difference of the ZnO nanorods to the
changes in buffer electrolyte glucose was measured for the range of
500nM to 1mM and shows that the glucose dependence is linear and
has sensitivity equal to 42.5 mV/decade (Figure 6) The intracellular
glucose concentration in the human adipocytes was 50 ± 15µM (n =
5), which can be compared with the 70µM intracellular concentration
determined by nuclear magnetic resonance spectroscopy in rat muscle
tissue in the presence of a high, 10mM, and extracellular glucose con-
centration [29]. For the frog oocytes, it was 125 ± 23µM (n = 5). When
we achieved a stable potential for the intracellular measurement, 10nM
insulin was added to extracellular solution. After few minutes, the in-
sulin increased the glucose concentration in the human adipocyte from
50 ± 15 to 125 ± 15µM and the glucose concentration in the frog oo-
cytes increased from 125 ± 23µM to 250 ± 19µM. Figure 7 a,b showed
the output response with respect to time for intracellularly positioned
electrodes when insulin is applied to the extracellular solution [5]. The
reported values of the glucose concentration in human adipocytes and
frog oocytes using our functionalized ZnO nanorods sensor was con-
sistent with values of glucose concentration reported in the literature
survey [5].
Conclusions
We have developed and studied a simple and robust biosensing
technique, based on ZnO nanorods, for intracellular biological mi-
y= -29.674x+ 98.072
-40
-20
0
20
40
60
80
100
120
140
160
-2 -1 0 1 2 3 4 5
Log [aCa
2+
]
EMF(mV)
Buffer Solution
Linear fitting
Figure 4: Sensitivity diagram showing the potentiometric response versus time
as the concentrations of Ca2+
solution is changed in the buffer solution sur-
rounding the cell. The Ca2+
concentration was varied from 100nM to 10mM.
The inset is a typical calibration curve showing the electrochemical potential
difference versus the Ca2+
concentration using the functionalized ZnO nano-
rods as working electrode and an Ag/AgCl as reference electrode [1].
Figure 5: Experimental setup for the intracellular potentiometric measure-
ments [6].
Biosensing

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croenvironments. We have used functionalized ZnO nanorod-based
electrochemical microelectrodes to measure intracellular free metal
ions and glucose concentration. A convenient sensor design was real-
ized by growing the ZnO nanorods on a silver-coated capillary glass tip
that could be readily inserted into a small-volume cell. For the glucose
measurements the glucose enzyme GOD was immobilized on the ZnO
nanorod-covered microelectrode. For the metal-ion measurements the
ZnO nanorod-covered microelectrode was coated by an ionophore-
containing plastic membrane. The measured intracellular free metal
ions and glucose concentrations using our ZnO nanorod-covered sen-
sors in human fat cells or in frog egg cells were consistent with values
reported in the literature [1-6].
Outlook
The main theme of this research work was to develop and study
a simple and robust sensing technique for intra-cellular biological
microenvironments. Among a variety of biosensor based on
nanostructure system, ZnO nanostructure electrochemical probe is
one that offers high sensitivity, direct, and real time detection. Some of
the drawbacks and future tasks are given below.
1. There is possibility of characterizing the dynamic nature of glucose
transport by in vivo measurements.
2. One can engineer the diameter of these nanostructures
comparable to the size of the biological and chemical species being
sensed, which intuitively could be excellent primary transducers
for producing electrical signals.
3. Researchers can focus on biological mechanism, correlation with
sensors to body system and continuous measurements of different
ions simultaneously.
4. The sensors described in the article could be applied to live system
such as mice or others to understand the role of sensors also there
is possibility to detect bio fluids from selective part inside the cells.
5. One can overcome the selectivity difficulty by further modification
for commercial point of view and for in vivo measurements. Also
possibility to estimate the ion in and out fluxes using intracellular
selective probe.
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This article was originally published in a special issue, Biosensing handled
by Editor(s). Dr. Dr. Michael J. Serpe, University of Alberta, Canada
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