Highly sensitive electrochemical determination of nitric oxide using fused spherical gold nanoparticles modified ITO electrode

Article abstract of DOI:10.1016/j.electacta.2010.01.084

This paper describes the highly sensitive electrochemical determination of nitric oxide (NO) using the fused spherical gold nanoparticles (FAuNPs) modified ITO electrode. The FAuNPs were self-assembled on a (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film, which was preassembled on ITO electrode. The attachment of FAuNPs on MPTS sol-gel film was confirmed by UV-vis absorption spectroscopy, atomic force microscopy (AFM) and cyclic voltammetry (CV). The AFM image shows that the AuNPs retain their fused morphology after immobilized on MPTS sol-gel film. The FAuNPs modified ITO electrode shows an excellent electrocatalytic activity towards the oxidation of NO. Using FAuNPs modified electrode, the detection of 12 nM NO was achieved for the first time by amperometry method. Further, the current response was increased linearly with increasing NO concentration in the range of 1.2 × 10^-8 to 7 × 10^-4 M and the detection limit was found to be 3.1 × 10^-10 M (S/N = 3). The FAuNPs modified ITO electrode displays an excellent selectivity towards the determination of 12 nM NO even in the presence of 1000-fold excess common interfering agents.

Full text of DOI:10.1016/j.electacta.2010.01.084

Electrochimica Acta 55 (2010) 3497–3503
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Highly sensitive electrochemical determination of nitric oxide
using fused spherical gold nanoparticles modified
ITO electrode
∗
P. Kannan, S. Abraham John
Department of Chemistry, Gandhigram Rural University, Gandhigram 624 302, Dindigul, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper describes the highly sensitive electrochemical determination of nitric oxide (NO) using the
fused spherical gold nanoparticles (FAuNPs) modified ITO electrode. The FAuNPs were self-assembled on
a (3-mercaptopropyl)-trimethoxysilane (MPTS) sol–gel film, which was preassembled on ITO electrode.
The attachment of FAuNPs on MPTS sol–gel film was confirmed by UV–vis absorption spectroscopy,
atomic force microscopy (AFM) and cyclic voltammetry (CV). The AFM image shows that the AuNPs
retain their fused morphology after immobilized on MPTS sol–gel film. The FAuNPs modified ITO elec-
trode shows an excellent electrocatalytic activity towards the oxidation of NO. Using FAuNPs modified
electrode, the detection of 12 nM NO was achieved for the first time by amperometry method. Further,
Received 14 December 2009
Received in revised form 12 January 2010
Accepted 21 January 2010
Available online 29 January 2010
Keywords:
FAuNPs
MPTS sol–gel film
ITO electrode
Amperometry
Nitric oxide
−
8
the current response was increased linearly with increasing NO concentration in the range of 1.2 × 10
−
4
−10
to 7 × 10 M and the detection limit was found to be 3.1 × 10
M (S/N = 3). The FAuNPs modified ITO
electrode displays an excellent selectivity towards the determination of 12 nM NO even in the presence
of 1000-fold excess common interfering agents.
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
fascinating electrocatalytic properties [19–25]. Among the differ-
ent AuNPs modified electrodes for the determination of NO, the
AuNPs modified on indium tin oxide (ITO) electrode has several
advantages over carbon based electrodes [26,27]. Since ITO can
be used in a wide potential window and possesses stable elec-
trochemical property, it can be served as an excellent working
electrode substrate for the fabrication of electrochemical sensors
[28]. Only few reports are available in the literature related to
the electrochemical determination of NO using AuNPs modified
ITO electrodes [29–31]. For instance, Caruso and co-workers pre-
pared polyelectrolyte (PE)/AuNPs hybrid films by incorporating
4-(dimethylamino) pyridine-stabilized AuNPs into PE multilayers
preassembled on ITO electrode and used for the electrochemi-
cal determination of NO [29]. Zhang and Oyama reported that
AuNP arrays directly grown on nanostructured ITO electrode can
be used for the determination of NO [30]. While Marken and co-
workers demonstrated that TiO2 nanoparticles deposited with a
Nafion polyelectrolyte showed the electrocatalytic activity towards
NO [31]. However, the sensitivity obtained at these modified elec-
trodes for NO is relatively poor. Thus, the main objective of the
present investigation is to improve the sensitivity of NO detection
by employing the fused gold nanoparticles (FAuNPs) modified ITO
electrode. The FAuNPs modified ITO electrode showed excellent
electrocatalytic activity towards NO and a detection limit of 12 nM
NO was achieved by amperometry method.
Huge research interest in the chemical and physiological func-
tions of nitric oxide (NO) has been aroused since it was involved
in the endothelium-derived relaxing factor (EDRF) in cardiovascu-
lar systems [1–5]. NO plays an important role in various important
physiological processes, serving as a neurotransmitter [6], and an
immune system mediator [7]. It can also act as a marker for tis-
sue damage [8]. Further, NO regulates the blood pressure via its
vasodilating activity and inhibits platelet aggregation, leukocyte
adherence, and vascular muscle cell proliferation [1,2,9]. It is an
active molecule with a short half-life time of ∼6 s in physiological
−
−
solution, and can be easily oxidized by O2 to form NO2 or NO
3
ions [10], which makes NO detection difficult. Thus, the accurate
measurement of NO is very important to unravel the action of this
key molecule.
Several modified electrodes have been reported for the determi-
nation of NO including porphyrin [11,12] and phthalocyanine [13],
vinylterpyridine complexes [14], multi-walled carbon nanotubes
[
15] and gold nanoparticles (AuNPs) [16–18]. In recent years, AuNPs
modified electrodes have received much attention owing to their
∗ Corresponding author. Tel.: +91 451 245 2371; fax: +91 451 245 3071.
E-mail address: abrajohn@yahoo.co.in (S.A. John).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.01.084

3
498
P. Kannan, S.A. John / Electrochimica Acta 55 (2010) 3497–3503
Scheme 1. Schematic representation for the immobilization of FAuNPs on MPTS sol–gel film modified ITO electrode.
2
. Experimental
silane molecules on the ITO electrode surface. Finally, the ITO/MPTS
modified ITO electrode was immersed in a colloidal solution of
FAuNPs (TAA–AuNPs) for 6 h (Scheme 1). The FAuNPs were strongly
bonded with –SH functional groups of MPTS sol–gel film. This elec-
trode was referred as ITO/MPTS/FAuNPs electrode.
2
.1. Chemicals
Hydrogen
3-mercaptopropyl)-trimethoxysilane (MPTS) and 2-mercapto-
-methyl-5-thiazoleacetic acid (TAA) were purchased from
Sigma–Aldrich and were used as received. Indium tin oxide
ITO) plates were purchased from Asahi Beer Optical Ltd., Japan.
tetrachloroaurate
trihydrate
(HAuCl ·3H O),
4
2
(
4
2.4. Instrumentation
(
UV–vis spectra were recorded with a Perkin-Elmer Lambda
Sodium nitrite (NaNO ) was purchased from Merck (India). 0.2 M
phosphate buffer (PB) solution (pH 2) was prepared by using
Na HPO –NaH PO and phosphoric acid. All other chemicals were
of analytical reagent grade and were used as received without any
further purification. Double distilled water was used to prepare all
the experimental solutions in the present work.
2
35 Spectrophotometer. The atomic force microscopy (AFM) image
of FAuNPs was obtained from digital instrument, Nanoscope IV,
Veeco. The electrochemical measurements were carried out with
2
4
2
4
CHI electrochemical workstation Model 643B, Austin, TX, USA. The
ITO electrode with an exposed geometric area of ca. 0.24 cm2 was
employed as a working electrode; a platinum wire electrode as
counter electrode and an Ag/AgCl (saturated NaCl) electrode as a
reference electrode. All the electrochemical measurements were
performed at room temperature in the presence of nitrogen gas
atmosphere.
2
.2. Synthesis of TAA–AuNPs
All glassware was thoroughly cleaned with freshly prepared
aquaregia (3:1; HCl/HNO ) and rinsed comprehensively with dou-
3
ble distilled water prior to use. We have prepared the fused
TAA–AuNPs based on our recent report [32]. TAA–AuNPs were pre-
3. Results and discussion
pared by adding 0.50 ml of HAuCl ·3H O and 0.25 ml of TAA to
4
2
3.1. Spectral and AFM characterization of FAuNPs modified ITO
substrate
4
4.25 ml of water in a round bottom flask with constant stirring.
To this solution, 5 ml of (0.125%) NaBH4 was added and the stirring
was continued for another 30 min. The color of the solution turns
The FAuNPs on MPTS sol–gel film modified ITO electrode
was characterized by UV–vis absorption spectroscopy and AFM.
Fig. 1 shows the UV–vis spectra obtained for bare ITO, ITO/MPTS
and ITO/MPTS/FAuNPs modified plates. No absorption band was
observed for both bare ITO (curve a) and ITO/MPTS (curve
b) plates in the region of 400–1000 nm. On the other hand,
ITO/MPTS/FAuNPs plate showed an absorption band at 550 nm, cor-
responding to the surface plasmon resonance (SPR) band of AuNPs
wine-red immediately after the final addition of NaBH , indicating
4
◦
the formation of AuNPs. The prepared AuNPs were kept at 4 C and
remained stable for several months. The UV–vis spectrum of col-
loidal TAA–AuNPs showed an absorption band at 508 nm and the
HR-TEM images of TAA–AuNPs showed that most of the nanopar-
ticles were in fused spherical shape with a size range of ∼7 nm
diameters (Fig. S1; supporting information).
(curve c). The appearance of a SPR band at 550 nm for FAuNPs on
2
.3. Preparation of MPTS sol–gel and FAuNPs/MPTS/ITO electrode
MPTS modified ITO plate confirmed that they were successfully
immobilized on the MPTS sol–gel film. The thiol group of MPTS
has a strong affinity to FAuNPs and is able to chemisorb on the sur-
face of AuNPs through the cleavage of the S–H bond. Since the thiol
groups are distributed throughout the MPTS sol–gel film, it is likely
that the FAuNPs were self-assembled both inside and on the surface
of the MPTS sol–gel film. The observed 40 nm red shift of SPR band
in contrast to colloidal solution is likely due to the electromagnetic
interaction between the assembled FAuNPs and the ITO substrate
[34]. Further, the size and surface morphology of the FAuNPs on
the MPTS sol–gel film ITO plate were examined by AFM. Fig. 2A
shows the AFM image obtained for the FAuNPs on MPTS modified
The MPTS sol–gel was prepared by dissolving MPTS, methanol,
and water (as 0.1 M HCl) in a molar ratio of 1:3:3 and stirring the
mixture vigorously for 30 min [33]. The ITO electrodes (exposed
geometry area 0.24 cm ) were sonicated for 10 min in each of the
following solvents: water, acetone, ethanol in an ultrasonic bath.
Then, the electrodes were rinsed with double distilled water and
dried with a stream of nitrogen gas. To obtain the –SH functional-
ized ITO surfaces, the pretreated ITO electrode was immersed into
MPTS for 20 min. Then the MPTS/ITO electrode was washed with
methanol followed by water to remove the physically adsorbed
2

P. Kannan, S.A. John / Electrochimica Acta 55 (2010) 3497–3503
3499
Fig. 1. UV–vis spectra obtained for (a) bare ITO, (b) ITO/MPTS and (c) FAuNPs on
MPTS modified ITO plates.
Fig. 3. CVs obtained for (a) bare ITO, (b) ITO/MPTS and (c) ITO/MPTS/FAuNPs mod-
−
1
ified electrodes in 0.2 M PB solution (pH 2) at a scan rate of 50 mV s
.
ITO plate. It showed that most of the nanoparticles are fused spher-
ical in shape. Further, the nanoparticles are uniformly distributed
throughout the MPTS sol–gel film. The three-dimensional view of
AFM image clearly showed that the immobilized AuNPs were fused
spherical shape and were not aggregated (Fig. 2B). These results
indicated that the TAA capped AuNPs retain their fused spherical
morphology on MPTS sol–gel film modified ITO surface.
other hand, the FAuNPs modified ITO electrode shows an oxidation
peak at 1.15 V and a corresponding reduction peak at 0.1 V (curve
c). The redox peaks are attributed to the oxidation and subsequent
reduction of surface gold oxide (AuOx) formation of the FAuNPs, as
a result of the positive potential polarization of the FAuNPs modi-
fied ITO electrode. The amount of Au oxide exposed to the solution
was calculated by integrating the charge under the Au oxide reduc-
tion peak and it was found to be 63.84 ␮C without subtracting the
roughness factor of 1.95 [35]. We have calculated the particle cover-
age (ꢀp) of FAuNPs on the MPTS sol–gel film modified ITO electrode
using Eq. (1). The particle coverage is defined as the ratio between
the electrochemically accessible AuNPs area and the geometric area
of the ITO electrode in contact with the electrolyte solution [36]. The
particle coverage of FAuNPs modified ITO electrode was calculated
by using the charge involved in the reduction of electrochemically
formed Au oxide from the CV recorded, between the potential win-
dow from 0.5 to −0.2 V in 0.2 M PB solution, by assuming the charge
3.2. Electrochemical characterization of FAuNPs modified ITO
electrode
The attachment of AuNPs on ITO electrode was also confirmed
by cyclic voltammetry (CV). CVs obtained for a bare ITO, ITO/MPTS
and ITO/MPTS/FAuNPs modified electrodes in 0.2 M PB solution (pH
2
) are shown in Fig. 3. No electrochemical response was observed
for both the bare and MPTS sol–gel film modified ITO electrodes in
the potential region from −0.20 to 1.5 V (curves a and b). On the
Fig. 2. AFM images obtained for FAuNPs on MPTS sol–gel film modified ITO plate (A) 2D and (B) 3D views.

3
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P. Kannan, S.A. John / Electrochimica Acta 55 (2010) 3497–3503
Ret can be used to describe the interface properties of the elec-
trode [40]. The EIS measurements in this work were carried out
in a background solution of 0.2 M PB solution (pH 7.2) contain-
ing 1 mM K [Fe(CN) ] as a redox probe using a frequency range
3
6
between 0.01 Hz and 100 kHz. The EIS–Nyquist plots obtained for
bare ITO, ITO/MPTS and ITO/MPTMS/FAuNPs modified electrodes
are shown in Fig. 5. The EIS includes a semicircle portion at higher
frequencies, corresponding to the electron-transfer (Ret) limiting
process, and a linear part at lower frequencies, resulting from the
diffusion limiting step of the electrochemical process. It is observed
that a semicircle of about 2750 ꢁ (curve (a)) diameter with an
almost straight tail line is observed for the bare ITO, demonstrating
low-electron-transfer resistance to the redox probe dissolved in the
electrolyte solution. It also can be seen that a much greater semi-
circle diameter appeared for MPTS sol–gel film (Ret = 42,000 ꢁ),
modified ITO electrode (inset, Fig. 5), suggesting that the resis-
tance of the MPTS sol–gel film was increased significantly when
compared to bare ITO electrode. The self-assembly of MPTS sol–gel
film on the electrode surface generated an insulating layer on the
electrode that functioned as a barrier to the interfacial electron
transfer. Interestingly, the Ret value of FAuNPs modified ITO elec-
trode enormously decreased to about 600 ꢁ (curve (b)) after the
immobilization of FAuNPs on the MPTS sol–gel film. The decrease
of Ret at FAuNPs modified ITO electrode is due to the homogeneous
Fig. 4. CVs obtained for (a) bare ITO, (b) ITO/MPTS and (c) ITO/MPTS/FAuNPs modi-
distribution of the FAuNPs within the MPTS sol–gel film and thus
fied electrodes in 0.2 M PB solution (pH 7.2) containing 1 mM K3[Fe(CN)6] at a scan
−
1
3−/4−
and
rate of 50 mV s
.
accelerated the electron transfer between the [Fe(CN)6]
ITO electrode. The results extracted from EIS measurements con-
nected with each step of the electrode modification are in good
agreement with the results obtained from CVs (Fig. 4). It has been
shown that if electron tunneling across an insulating barrier is
implicated in the electron-transfer process, the redox current at any
potential should decrease exponentially with the barrier thickness
according to the following expression [41]:
density for the reduction peak of the Au oxide is 723 ␮C/cm2 [36].
Au oxide reduction charge (␮C)/723 (␮C/cm2)
ꢀp =
× 100 (1)
ITO geometric area (cm2)
The particle coverage of FAuNPs on MPTS modified ITO elec-
trode was found to be 36.79%. Previously, the particle coverage of
5–30% was reported for citrate-stabilized gold nanoparticles (C-
−
ˇd
I = I0e
(2)
2
AuNPs) immobilized on amine functionalized ITO substrate [36]
while 10% particle coverage was reported for C-AuNPs immobilized
on a monolayer of 1-mercapto-11-aminoundecane on a polycrys-
talline Au electrode surface [37]. In the present case, the FAuNPs
showed relatively higher particle coverage on the MPTS modified
ITO electrode when compared to reported papers [35–37].
where I0 is the current measured at the bare electrode, ˇ is the
potential independent electron tunneling coefficient, and d is the
3−/4−
Further, the [Fe(CN )]
redox probe was used to access the
6
electronic communication between the immobilized FAuNPs and
the electrode [38]. Fig. 4 shows the CVs of bare ITO, ITO/MPTS and
ITO/MPTS/FAuNPs modified electrodes in 0.2 M PB solution (pH 7.2)
containing 1 mM K [Fe(CN) ]. The bare ITO electrode shows a redox
3
6
3−/4−
response for [Fe(CN)6]
with a peak separation of 330 mV indi-
cating the characteristics of a diffusion-controlled redox process
curve a), whereas its redox reaction was blocked completely at
(
the ITO/MPTS electrode, indicating that MPTS forms a relatively
compact three-dimensional monolayer on ITO electrode (curve b).
However, after the attachment of FAuNPs on the MPTS sol–gel film,
not only the peak currents were increased but also the peak separa-
tion (220 mV) was decreased (curve c) when compared to bare ITO
electrode [39]. This reveals that the FAuNPs electrode improved
3−/4−
the electron transfer between [Fe(CN )]
and the electrode
6
surface. Here, the FAuNPs can act as “electron antennae” and effec-
tively facilitating the electron transfer on the MPTS sol–gel film.
These results once again confirmed the successful immobilization
of FAuNPs on the MPTS sol–gel film.
The electrochemical impedance spectrum (EIS) is an efficient
tool for studying the interface properties of surface modified
electrodes. The electron-transfer resistance (Ret), the semicircle
diameter at the higher frequencies in the Nyquist plot of impedance
spectroscopy, controls the interfacial electron-transfer rate of the
redox probe between the solution and the electrode. Therefore,
Fig. 5. EIS–Nyquist plots obtained for bare ITO (curve a), ITO/MPTS (inset) and
ITO/MPTS/FAuNPs (curve b) modified electrodes in 0.2 M PB solution (pH 7.2) con-
taining 1 mM K3[Fe(CN)6]. The frequency range for impedance measurements was
from 0.01 to 100,000 Hz.

P. Kannan, S.A. John / Electrochimica Acta 55 (2010) 3497–3503
3501
thickness of the monolayer. For an electrochemical reaction at equi-
librium, Eq. (3) can be obtained using Eq. (2):
k = k0e−
ˇd
(3)
where k0 and k are the electron-transfer rate constants at the
bare and ITO/MPTS modified electrodes. Heterogeneous electron-
transfer rate constant (ket) at the bare and modified electrodes can
be calculated using Eq. (4) [42]:
RT
ket =
(4)
2
2
n F ARCTC0
where R is the gas constant, T is temperature (K), F is the Faraday
2
constant, A is the electrode area (cm ), R is the charge transfer
CT
resistance, C0 is the concentration of the redox couple in the bulk
−
3
3
of solution (1 × 10 mol/cm ), and n is the number of transferred
electrons per molecule of the redox probe (n) one in the case of
3−/4−
[
Fe(CN)6]
redox couple. The apparent charge transfer resis-
tance values were obtained from fittings of the EIS spectra. The
heterogeneous electron-transfer rate constant, ket, estimated using
−
7
−1
Eq. (4) was found to be 4.0323 × 10 cm s for bare ITO electrode,
−8
−1
2
.6402 × 10 cm s for MPTS sol–gel film modified ITO electrode
−6
−1
and 1.8481 × 10 cm s for FAuNPs modified ITO electrode. The
highest ket value obtained at FAuNPs modified electrode indicated
that the fastest electron transfer takes place at the FAuNPs modified
electrode.
3.3. Electrocatalytic oxidation of NO at FAuNPs modified electrode
The prime objective of the present study is to utilize the FAuNPs
modified ITO electrode for the determination of NO. Sodium nitrite
NaNO ) was used as a precursor to produce NO in acidic solution.
(
2
It has been well established that NaNO2 can generate free NO by
the disproportionation reaction in acidic solution (pH ≤4) (Eqs. (5)
and (6)) [43]. Addition of a known amount of NaNO2 into the bulk
electrolyte solution at pH ≤4 generates a series of concentrations
of NO.
Fig. 6. (A) CVs obtained for 0.50 mM NO at (a) bare ITO, (b) ITO/MPTS and (c)
ITO/MPTS/FAuNPs modified electrodes in 0.2 M PB solution (pH 2). CV obtained at
ITO/MPTS/FAuNPs modified electrode in the absence of NO (d) in 0.2 M PB solution
2
NaNO + H SO → 2HONO + Na SO
(5)
(6)
2
2
4
2
4
HONO → H+ + NO − + 2NO + H O
3
3
2
−
1
(
pH 2). Scan rate = 50 mV s . (B) CVs obtained for 0.50 mM NO at ITO/MPTS/FAuNPs
modified electrode in 0.2 M PB solution (pH 2) after 1st (solid line), 5th (dotted line)
Fig. 6A shows the CVs obtained for bare ITO, ITO/MPTS and
ITO/MPTS/FAuNPs modified electrodes in 0.2 M PB solution (pH
−
1
and 9th (dashed line) continuous potential cycles at a scan rate of 50 mV s
.
2
) containing 0.5 mM NO. The bare ITO electrode does not show
after nine repeated potential cycles indicating that the oxidation of
NO was highly stable at FAuNPs modified ITO electrode.
any oxidation wave for NO (curve a, dotted line), Similar response
was also observed at ITO/MPTS modified electrode (curve b) due
to the formation of a compact MPTS sol–gel film on ITO electrode.
On the other hand, FAuNPs modified electrode showed the oxida-
tion peak with higher oxidation current for NO at 0.87 V (curve c).
When compared to bare and MPTS sol–gel modified ITO electrodes
3.4. Effect of scan rate
Fig. 7 shows the effect of scan rate on the electrochemical oxi-
dation of NO at ITO/MPTS/FAuNPs modified electrode in 0.2 M PB
solution (pH 2) containing 0.5 mM NO. The oxidation peak cur-
rents of NO increased linearly with the scan rates in the range of
5–100 mV/s with a correlation coefficient of 0.9984 (inset, Fig. 7).
This suggests that the oxidation of NO at FAuNPs modified ITO
electrode is diffusion-controlled process. We presumed that the
oxidation mechanism of NO oxidation at FAuNPs modified ITO elec-
(
curves a and b) and the NO oxidation current was 15 times higher
at FAuNPs modified electrode (curve c). The FAuNPs modified ITO
electrode does not show any oxidation response in the absence
of NO (curve d, dashed line). These results indicated that the pres-
ence of nanostructured FAuNPs modified ITO electrode in the MPTS
sol–gel film mediates the electrochemical oxidation of NO. Since
the FAuNPs attached on the MPTS sol–gel film are in good electri-
cal communication with each other, an efficient electron-transfer
process is observed at the FAuNPs modified ITO electrode in the
absence of any other mediator in the film. The fused gold nanostruc-
tures provide a large surface area with specific interaction towards
the substrate, resulting an improved electron-transfer kinetics and
higher enhancement in the oxidation of NO. Further, the oxida-
tion peak of NO at FAuNPs modified ITO electrode was found to
be highly stable. Fig. 6B shows the CVs recorded for the oxidation
of NO after every three continuous potential cycles. It can be seen
from Fig. 6B, the oxidation potential of NO remained stable even
+
trode as the NO in solution first loses an electron to form NO at
+
the FAuNPs modified electrode. The formed NO can then be further
oxidized to form other more stable nitrogen products [44].
3.5. Amperometric determination of NO
Amperometric method was used to examine the sensitivity of
FAuNPs modified ITO electrode towards the detection of NO in
acidic solution (pH 2). Fig. 8A shows the amperometric i–t curve
obtained for NO at FAuNPs modified ITO electrode in a homoge-

3
502
P. Kannan, S.A. John / Electrochimica Acta 55 (2010) 3497–3503
Fig. 7. CVs obtained for 0.50 mM NO at ITO/MPTS/FAuNPs modified electrode in
− −1
1
0
5
.2 M PB solution (pH 2) at different scan rates: (a) 5 mV s ; (b) 25 mV s ; (c)
−
1
−1
−1
0 mV s ; (d) 75 mV s and (e) 100 mV s . Inset shows the square root of the
scan rate vs. NO peak current.
neously stirred 0.2 M PB solution (pH 2) at an applied potential
of 1.0 V. The FAuNPs modified ITO electrode shows the initial cur-
rent response due to 12 nM NO and further addition of 12 nM NO
in each step with a sample interval of 50 s, the current response
increases and a steady state current response was attained within
2
s. The amperometric current response was increased linearly with
increasing NO concentration on the addition of 12 nM NO in each
step (inset, Fig. 8A) with a correlation coefficient of 0.9989. We
have estimated 30 nA for 100 nM NO at FAuNPs modified ITO elec-
trode. Further, the amperometric current response was increased
Fig. 8. (A) Amperometric i–t curve for the determination of NO at FAuNPs modified
ITO electrode in a homogeneously stirred solution of 0.2 M PB solution (pH 2). Each
addition increases the concentration of 12 nM of NO, Eapp = 1.0 V. (B) Amperomet-
ric i–t curve for the determination of (a–d) 12 nM NO in the presence of various
interferents such as 12 ␮M each (e) ascorbic acid, (f) uric acid, (g) dopamine, (h)
−
8
linearly with increasing NO concentration in the range of 1.2 × 10
−4
to 7 × 10 M with a correlation coefficient of 0.9986 and the detec-
+
+
2+
2+
−
10
dl-cysteine, (i–l) 12 nM NO, (m) Na , (n) K , (o) Mg , (p) Ni and (q–t) 12 nM NO
at FAuNPs modified ITO electrode in a homogeneously stirred 0.2 M PB solution (pH
tion limit was found to be 3.1 × 10
M (S/N = 3). The dynamic
range and the lowest detection limit of the FAuNPs modified ITO
electrode towards NO oxidation were compared with the recently
reported ITO and other modified electrodes [15–18,29,30,45,46]
and are given in Table 1. As can be seen from Table 1, the present
modified electrode showed the lowest detection limit for the NO
M). We have also studied the determination of NO in
the presence of common physiological interferents such as ascorbic
acid, uric acid, urea and cysteine and common metal ions such as
Na , K , Mg and Cu . Fig. 8B shows the amperometric i–t curve
obtained for NO at FAuNPs modified ITO electrode in the presence
of several interferences in a homogeneously stirred 0.2 M PB solu-
tion. The increased initial current response was due to the addition
of 12 nM NO (a) and further addition of 12 nM NO in each step with a
sample interval of 50 s, the current response increases and a steady
2). Eapp = 1.0 V.
state current response was attained within 2 s (b–d). After four
steps, the addition of 12 ␮M of each ascorbic acid (e), uric acid (f),
urea (g) and cysteine (h), separately with a sample interval of 50 s
to the same solution no change in current response was observed.
However, addition of 12 nM NO to the same 0.2 M PB solution (pH
2) solution, the current response was again increased (i–l). After
−10
(
3.1 × 10
+
+
2+
2+
four steps, the addition of 12 ␮M Na (m), K+ (n), Mg (o) and
+
2+
2+
Cu (p), separately with a sample interval of 50 s to the same solu-
tion caused no change in current response. While 12 nM of NO was
added to the same 0.2 M PB solution (pH 2) solution, an increase
in current response was observed and it is similar to the current
Table 1
Linear range and detection limit of different modified electrodes for the determination of NO.
S. No.
Electrode
Linear range
Detection limit
Reference
−
−
7
−4
−8
−8
1
2
3
4
5
6
7
8
9
Multi-walled carbon nanotubes modified electrode
Mono-/bi-layer Au nanoparticle films modified electrode
2 × 10 to 1.5 × 10
M
8.0 × 10
2.7 × 10
M
M
15
16
17
18
29
30
45
46
8
−5
−4
5 × 10 to 1 × 10
M
−
5
−9
Nafion polymer/gold nanostructures film modified GC electrode
Electrodeposition of Pt–Fe(III) nanoparticle on glassy carbon electrode
Polyelectrolyte(PE)/gold nanoparticle hybrid films modified electrode
Gold nanoparticle arrays directly grown on nanostructured indium tin oxide electrodes
Carbon fiber covered with nickel porphyrin layer
5 × 10 to 5 × 10
M
1 × 10
M
−
5
8
−4
−8
−5
8.4 × 10 to 7.8 × 10
M
1.8 × 10
M
−
−4
−4
5 × 10 to 5 × 10
M
M
1 × 10
M
−
6
−7
1 × 10 to 5 × 10
6.5 × 10
2–3 × 10
−8
M
M
M
−
9
–
−
7
−5
−4
Nano-Au colloid and Nafion film modified Pt electrode as a microsensor
Fused gold nanoparticles modified electrode
1 × 10 to 4 × 10
M
5.0 × 10
3.1 × 10
−
8
−10
M
1.2 × 10 to 7 × 10
M
This work

P. Kannan, S.A. John / Electrochimica Acta 55 (2010) 3497–3503
3503
response observed in the earlier steps (q–t). Further, we have stud-
ied the effect of NO2 as an interferent for the determination of NO.
Inorganic and Physical Chemistry, Indian Institute of Science, Ban-
galore for AFM measurements.
−
It was found that no interference effect was noted even in the pres-
−
ence of 12 ␮M NO2 . It is likely that the negatively charged TAA
Appendix A. Supplementary data
molecules present on the surface of the AuNPs repel the negatively
−
charged NO2 ions while allow the NO molecules. This may be the
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2010.01.084.
possible reason for the observed selective determination of NO at
FAuNPs modified electrode in the presence of NO2 ions. The above
−
results indicate that the determination of 12 nM NO is possible even
in the presence of 1000-fold excess of common interferences.
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P K thanks the Council of Scientific and Industrial Research
(
CSIR), New Delhi (File No. 09/715 (0009)/2008-EMR-I) for the
[
[
[
financial support in the form of Senior Research Fellowship. Finan-
cial support from the Department of Science and Technology, New
Delhi, under Nano Mission (No. SR/NM/NS-28/2008) is gratefully
acknowledged. We thank Professor S. Sampath, Department of