Reduced graphene oxide-gold nanorod composite material stabilized in silicate sol-gel matrix for nitric oxide sensor

Article abstract of DOI:10.1039/c4ra04859h

A facile synthetic method for the preparation of reduced graphene oxide-gold nanorods embedded in an amine functionalized silicate sol-gel matrix (RGO-Au-TPDT NRs) composite in an aqueous medium and its applications towards the electrochemical sensing of nitric oxide (NO) are reported. The electrochemical characteristics of the RGO-Au-TPDT NRs modified electrode are studied by using cyclic voltammetry. The electrocatalysis and amperometric sensing of NO are studied at physiological pH by using the RGO-Au-TPDT NRs modified electrode. The RGO-Au-TPDT NRs modified electrode shows excellent electrocatalytic activity towards the oxidation of NO due to the synergistic catalytic effect of the RGO-Au-TPDT NRs composite material. The amperometric current is increased linearly while increasing the NO concentration in the range between 10 and 140 nM and the lowest detection limit is estimated as 6.5 nM. The GC-RGO-Au-TPDT NRs modified electrode is simple to prepare and shows fast amperometric response towards NO detection. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04859h

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Reduced graphene oxide–gold nanorod composite
material stabilized in silicate sol–gel matrix for
nitric oxide sensor†
Cite this: RSC Adv., 2014, 4, 33541
*
Subramaniam Jayabal, Perumal Viswanathan and Ramasamy Ramaraj
A facile synthetic method for the preparation of reduced graphene oxide–gold nanorods embedded in an
amine functionalized silicate sol–gel matrix (RGO–Au–TPDT NRs) composite in an aqueous medium and its
applications towards the electrochemical sensing of nitric oxide (NO) are reported. The electrochemical
characteristics of the RGO–Au–TPDT NRs modified electrode are studied by using cyclic voltammetry.
The electrocatalysis and amperometric sensing of NO are studied at physiological pH by using the RGO–
Au–TPDT NRs modified electrode. The RGO–Au–TPDT NRs modified electrode shows excellent
electrocatalytic activity towards the oxidation of NO due to the synergistic catalytic effect of the RGO–
Au–TPDT NRs composite material. The amperometric current is increased linearly while increasing the
NO concentration in the range between 10 and 140 nM and the lowest detection limit is estimated as
6.5 nM. The GC–RGO–Au–TPDT NRs modified electrode is simple to prepare and shows fast
amperometric response towards NO detection.
Received 22nd May 2014
Accepted 17th July 2014
DOI: 10.1039/c4ra04859h
www.rsc.org/advances
amine functionalized graphene–gold nanocomposite material
was employed as a biosensor for H2O2.22
Introduction
Nitric oxide (NO) has been shown to be involved in regu-
lating neuronal excitability, synaptic transmission, functioning
of neuronal networks, learning and memory mechanisms.23–25
The excess or deciency of NO results in various pathological
conditions such as tumor angiogenesis,26 atherosclerosis,27
parkinson's disease28 and diabetes.29 In recent years, the
determination of NO has become an important research subject
in the eld of biology, medicine and pharmacy.30–33 The deter-
mination of submicromolar concentration of NO at physiolog-
ical pH is difficult owing to its short life time (ꢀ5 s) and its rapid
Gold nanorods (Au NRs) are elongated nanostructures with
unique optical properties, which depend on their shape anis-
ometry.1–7 The unique properties of Au NRs nd potential
applications in imaging, therapy and sensing.1–5 The silica-
coated Au nanoparticles possess large pore size, high surface
area, good stability and biocompatibility and hence they nd
applications in the eld of bioimaging, drug delivery, catalysis
and sensors.8–11 Graphene has attracted much attention in
recent years due to their attractive properties such as large
surface area, high electrical conductivity, wide electrochemical
detection window and chemical inertness.12–15 Most of the
chemically synthesized graphene materials contain principal
element oxygen and they are called reduced graphene oxide
(RGO).16 The RGO–Au nanocomposites have showed synergistic
effect towards electrocatalysis and sensor applications.17–21 The
ꢁ
conversion to NOx by oxygen and superoxides present in bio-
uids34–36 and therefore the development of NO sensor is very
important. The most commonly used methods for the detection
of NO are chemiluminescence,37 UV-visible spectroscopy,38
uorescence,39 electron paramagnetic resonance40 and electro-
chemistry.35,36 Among these methods, the electrochemical
technique is advantageous due to its sensitivity, selectivity,
response time and low-cost fabrication.34–36
School of Chemistry, Centre for Photoelectrochemistry, Madurai Kamaraj University,
Madurai-625021, India. E-mail: ramarajr@yahoo.com; Tel: +91-452-2459084
† Electronic supplementary information (ESI) available: Fig. S1 shows diffuse
reectance spectra of GC–RGO–Au–TPDT NRs at different conditions. Fig. S2
shows SAED patterns of GC–RGO–Au–TPDT NRs. Fig. S3 shows EDS of
GC–RGO–Au–TPDT NRs. Fig. S4 shows cyclic voltammograms recorded for bare
GC (a) and GC–RGO–Au–TPDT NRs (b) electrodes in 0.1 M H2SO4. Fig. S5 shows
cyclic voltammograms obtained for 10 mM NO at GC–RGO–Au–TPDT NRs
modied electrode in 0.1 M PBS (pH 7.2) at different scan rates and inset shows
the corresponding calibration plot. Fig. S6 shows amperometric i–t curve
response time for 10 nM NO addition to RGO–Au–TPDT NRs modied
electrode. See DOI: 10.1039/c4ra04859h
In the present work, we have prepared reduced graphene
oxide–gold nanorods embedded in amine functionalized sili-
cate sol–gel matrix (RGO–Au–TPDT NRs) composite material for
the rst time and the prepared composite is characterized by
absorption spectroscopy, diffuse reectance spectroscopy,
transmission electron microscopy (TEM), X-ray diffraction
analysis (XRD) and cyclic voltammetry. The RGO–Au–TPDT NRs
modied electrode is used to construct amperometric sensor
for the detection of NO at physiological pH. The electrocatalytic
oxidation of NO is studied at the RGO–Au–TPDT NRs modied
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electrode and the results are compared with Au–TPDT NRs
modied electrode. The encapsulation of reduced graphene
oxide–gold nanorods in amine functionalized silicate sol–gel
provides an effective support matrix to prepare the modied
electrode for the amperometric sensing of NO.
Preparation of modied electrodes
The GC electrode (diameter ¼ 3 mm, CH Instruments) was twice
polished using alumina powder (0.05 mm) and sonicated in
distilled water for 3 min. The cleaned GC electrode was dried in
air for 5 min at room temperature. A known volume of the
solution containing RGO–Au–TPDT NRs or Au–TPDT NRs was
coated on the surface of the GC electrode and allowed to dry at
room temperature for 2 h (represented as GC–RGO–Au–TPDT
NRs or GC–Au–TPDT NRs). The dried electrodes were then kept
in water for 10 min and then used for electrochemical experi-
ments. Nitrogen gas was bubbled into the electrolyte solution
for 30 min before each experiment.
Experimental
Chemicals
Cetyltrimethylammonium bromide (CTAB) was purchased from
Sigma. N1-[3-(trimethoxysilyl)propyl]diethylene triamine (TPDT)
and hydrogen tetrachloroaurate(III) hydrate were purchased
from Aldrich. Graphite powder was supplied by Alfa Aesar. All
other chemicals, unless otherwise specied, used in the present
work were of analytical grade. All the glasswares were thor-
oughly cleaned with aqua regia (1 : 3 (v/v) HNO3–HCl) (Caution:
Aqua regia is a powerful oxidizing agent and it should be
handled with extreme care) and rinsed extensively with doubly
distilled water before use.
Preparation of nitric oxide solution
The saturated nitric oxide in phosphate buffer solution (PBS)
was prepared using the reported procedure.34,43,44 In brief, all
glasswares and PBS solution were purged with nitrogen gas for
30 min prior to preparation. Then, 2 M of sulfuric acid was
added drop-wise to a saturated sodium nitrite solution, leading
to the production of nitrogen oxide gas through dispropor-
tionation reaction of nitrite ions in the acidic solution. The NO
gas produced was bubbled through 5% (w/v) pyrogallol solution
in saturated potassium hydroxide and 10% (w/v) potassium
hydroxide in order to remove other forms of nitrogen oxides.
Finally, NO gas was collected in PBS solution and stored under
nitrogen. The NO standard solutions were prepared by making
successive dilutions of the saturated NO solution. For all
experiments, freshly prepared NO standard solutions were used
and were kept in a tightly closed rubber glass ask and light-free
septum with wrapped black foils. The concentration of satu-
Preparation of GO
Graphite oxide (GO) was prepared from graphite powder by the
modied Hummers method.41,42 In a typical synthesis, 1 g of
graphite was added into 23 mL of 98% H2SO4, followed by
stirring at room temperature for 24 h. To this solution, 100 mg
of NaNO3 was added and stirred for 30 min. Then, the mixture
was kept below 5 ꢂC in an ice bath and 3 g of KMnO4 was slowly
added to the mixture. Aer heating to 35–40 ꢂC, the mixture was
stirred for another 30 min. Subsequently, 46 mL of water was
added into the mixture for a period of 25 min and nally 140 mL
of water and 10 mL of H2O2 were added to the mixture to stop
the reaction. The unexploited graphite in the resulting mixture
was removed by centrifugation and the as-synthesized GO was
dispersed as individual sheets in distilled water at a concen-
tration of 0.5 mg mLꢁ1 by using ultrasonication and used for
further studies.
rated NO solution at 20 C was reported as 1.8 mM.43,44
ꢂ
Characterization and electrochemical studies
The absorption spectra were recorded using an Agilent 8453
diode array spectrophotometer. The diffuse reectance spectra
of Au–TPDT NRs coated on GC electrode surface were recorded
using Ocean Optics, Jaz spectrometer. The TEM images were
recorded in JEOL JEM 2100 high resolution transmission elec-
tron microscopy (HRTEM) equipped with energy dispersive
spectrum (EDS) with an operating voltage of 200 kV. The Au–
Preparation of RGO–Au–TPDT NRs
The Au NRs with an aspect ratio of ꢀ3.5 were prepared by using TPDT NRs solutions were centrifuged at 9000 rpm for 10 min to
seed mediated growth method reported by Nikoobakht and El- remove excess amounts of CTAB before the TEM analysis. The
Sayed.7 The excess of CTAB present in the Au NRs solution was sample for HRTEM measurement was prepared by placing a
removed by centrifuging the solution at 9000 rpm for 10 min drop of sample solution on a carbon-coated copper grid and
and then redispersed in water. 1 mg of GO was dispersed in 2 dried in air at 25 ꢂC. The powder X-ray diffraction (XRD) analysis
mL of water and the mixture was ultrasonicated for 30 min to was carried out using X'Pert PANalytical X-ray diffractometer
˚
obtain a homogeneous suspension. A 2 mL of centrifuged Au with monochromatized Cu Ka radiation (l ¼ 1.5406 A). The
NRs solution ([Au] ¼ 0.5 mM) was mixed with the above solution sample for XRD was prepared by casting the RGO–Au–TPDT
containing 1 mg of GO in water and nally 50 mL of 1 M TPDT NRs solution on a glass plate and allowed to dry at 25 ꢂC.
silane solution was added. The resulting solution was further Electrochemical measurements were performed using a three-
allowed to complete the reaction for 24 h. The color of the electrode cell with a glassy carbon (GC) working electrode
solution was changed from brownish yellow to black which (electrode area ¼ 0.07 cm2), a Pt wire counter electrode and an
indicates the formation of RGO–Au–TPDT NRs composite. The Ag/AgCl reference electrode at 25 ꢂC. The electrochemical
obtained RGO–Au–TPDT NRs composite were used for electro- experiments were conducted using a CHI760D Electrochemical
catalysis and electrochemical sensor studies.
Workstation, CH Instruments, USA. For all the electrochemical
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experiments, the electrolyte solutions were deaerated with groups in the graphene oxide and gets adsorbed on the surface
nitrogen gas before each experiment.
of RGO and Au NRs to form a stable RGO–Au–TPDT NRs.50,51
Aer the addition of TPDT silane to the solution, the color of the
solution was changed from brownish yellow to black aer 24 h,
indicating the reduction of graphene oxide to RGO.
Results and discussion
Optical characterization
The diffuse reectance spectra of the RGO–Au–TPDT NRs
modied glassy carbon electrodes (GC–RGO–Au–TPDT NRs)
were recorded in wet and dry conditions (Fig. S1†). When the
GC–RGO–Au–TPDT NRs modied electrode was wetted with
distilled water, the RGO–Au–TPDT NRs lm underwent swelling
in water and the LSPR band was appeared at 689 nm
(Fig. S1(a)†). When the GC–RGO–Au–TPDT NRs modied elec-
trode was allowed to dry in air at room temperature for 30 min,
the LSPR band was appeared at 748 nm (Fig. S1(b)†) with a red
shi of 59 nm when compared to the wet GC–RGO–Au–TPDT
NRs modied electrode (Fig. S1(a)†). This observation reveals
that the air-dried lm brought about a change in the dielectric
environment. It conrms that the environment for the RGO–
Au–TPDT NRs in the swelled lm is inuenced by the solvent
trapped inside the silicate sol–gel lm and the amount of
hydration that exists in the silicate lm. The appearance of the
transverse and longitudinal plasmon bands of Au NRs indicates
that they are not aggregated in the RGO and TPDT silicate sol–
gel lms.
The absorption spectra of the Au NRs, Au–TPDT NRs and RGO–
Au–TPDT NRs showed that the optical properties of Au NRs
were inuenced by the presence of TPDT sol–gel silicate matrix
and RGO (Fig. 1). The as-prepared Au NRs (Fig. 1(a)) show
transverse plasmon band at 520 nm and the longitudinal
surface plasmon resonance (LSPR) band at 780 nm, whereas the
Au–TPDT NRs (Fig. 1(b)) exhibit a transverse plasmon band at
518 nm and the LSPR band at 750 nm. A signicant 30 nm blue
shi in the LSPR band of the Au–TPDT NRs was observed due to
the change in the local refractive index and dielectric of the
medium surrounding the Au NRs aer the dispersion of Au NRs
in TPDT silicate sol–gel matrix.45,46 The absorption spectrum of
RGO–Au–TPDT NRs (Fig. 1(c)) shows the transverse plasmon
band at 507 nm and the LSPR band at 684 nm. The signicant
blue shi in the longitudinal and transverse bands of the RGO–
Au–TPDT NRs was observed due to the change in the local
environment surrounding the Au NRs aer the dispersion of
RGO into the Au–TPDT NRs.45,46 The RGO–Au–TPDT NRs
composite material was prepared by changing the volume of
Au–TPDT NRs solution by adding 1, 2 and 3 mL to the RGO
solution. However, the prepared composite material was not
stable upon the addition of 1 mL of Au–TPDT NRs and aggre-
gation of Au–TPDT NRs was observed upon the addition of 3 mL
of Au–TPDT NRs. The prepared composite material RGO–Au–
TPDT NRs was found to be stable and well dispersed only upon
the addition of 2 mL of Au–TPDT NRs to the GO solution. The
prepared RGO–Au–TPDT NRs composite material was stable for
more than a month at room temperature.
XRD studies
Fig. 2 shows the XRD patterns of the graphite (a), GO (b) and
RGO–Au–TPDT NRs (inset). The XRD pattern of graphite
(Fig. 2(a)) shows a diffraction peak at 26.3 (2q) and the XRD
pattern of GO (Fig. 2(b)) shows a diffraction peak at 10.7 (2q),
corresponding to the C(002) interlayer d-spacing of 0.82 nm.
The diffraction peak observed at 26.3 (2q) for graphite was
completely disappeared in the XRD pattern of GO, indicating
the total oxidation of graphite to GO.52 The C(002) interlayer d-
spacing of GO (0.82 nm) is larger than the graphite (0.38 nm,
26.3 (2q)) due to the intercalated water molecules between
layers.53 Fig. 2 (inset) shows that the diffraction peak observed at
10.7 (2q) for GO was disappeared and a very broad peak around
23.3 (2q) was observed for the RGO–Au–TPDT NRs, indicating
the reduction of graphene oxide to RGO by TPDT silane.52 The
The amine groups present in the TPDT silane acts as a
reducing agent for the reduction of graphene oxide leading to
the formation of RGO and also acts as a stabilizer for the Au
NRs. It was reported that the amine groups act as a reducing
agent and also it stabilizes the metallic nanoparticles.47–49 The
amine groups in the TPDT silicate reduce the surface oxygen
Fig. 1 Normalized absorption spectra of Au NRs (a), Au–TPDT NRs (b)
and RGO–Au–TPDT NRs (c). The spectra were normalized at the LSPR Fig. 2 XRD patterns of graphite (a) and GO (b). Inset: XRD pattern of
maximum.
RGO–Au–TPDT NRs.
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diffraction peak observed for RGO–Au–TPDT NRs (Fig. 2 (inset)) modied electrode and exhibits the electrochemical behavior
is different from both graphite and GO (Fig. 2(a) and (b)) which due to the electrochemical contact between the RGO–Au–TPDT
indicates the formation of RGO.
NRs and the GC electrode surface.
The electroactive surface area of the modied electrodes
were determined by recording the chronoamperogram for 3 mM
Fe(CN)6 as a redox probe in 0.1 M KCl by using the Cottrell
TEM studies
4ꢁ
The TEM images of Au NRs and RGO–Au–TPDT NRs are shown
in Fig. 3. The Au NRs showed an average length of 41 nm and a
breadth of 11.5 nm with an aspect ratio of ꢀ3.6 (Fig. 3(A)). The
TEM images of RGO–Au–TPDT NRs showed an average length of
49 nm and a breadth of 15 nm with an aspect ratio of ꢀ3.3
(Fig. 3(B) and (C)). The TEM images of RGO–Au–TPDT NRs
(Fig. 3(B) and (C)) indicate that the Au NRs are well dispersed in
the RGO and TPDT silicate sol–gel matrix. The SAED pattern of
the RGO–Au–TPDT NRs shows well-dened six-fold-symmetry
diffraction pattern, conrming the crystalline nature of the
obtained RGO,52,54 which indicates the formation of RGO. The
SAED pattern of RGO–Au–TPDT NRs (Fig. S2†) shows ring
patterns that are assigned to the (111) and (222) planes of Au.
The EDS of the RGO–Au–TPDT NRs (Fig. S3†) shows the pres-
ence of Au and Si in the solution.
equation.58 The electrochemically active area was calculated as
0.103 cm2 for the bare GC electrode, 0.25 cm2 for the GC–Au–
TPDT NRs electrode and 0.276 cm2 for the GC–RGO–Au–TPDT
NRs electrode.
Electrochemical impedance spectroscopic studies
Fig. 4 shows the Nyquist plots of electrochemical impedance
spectra recorded at their corresponding open circuit potential
for bare GC (a) GC–Au–TPDT NRs (b) and GC–RGO–Au–TPDT
NRs (c). The Nyquist plot shows semicircle at higher
frequencies corresponding to the electron-transfer-limited
process and the linear portion at lower frequencies corre-
sponding to the diffusion-limited process.59–61 The Nyquist
plot observed for bare GC electrode (Fig. 4(a)) showed a large
semicircle with an electron-transfer resistance (Rct) of 1493 U.
When the Au–TPDT NRs was coated on the GC electrode (GC–
Au–TPDT NRs), the electron-transfer resistance was decreased
to 339 U (Fig. 4(b)), which indicates an increase in the electron-
transfer process at the modied electrode interface, due to the
presence of Au NRs in the silicate sol–gel matrix (GC–Au–TPDT
NRs). When the RGO–Au–TPDT NRs was coated on the GC
electrode (GC–RGO–Au–TPDT NRs), the electron-transfer
resistance was signicantly decreased to 68 U (Fig. 4(b)),
which indicates the highly conductive nature of RGO in the
RGO–Au–TPDT NRs composites material at the modied
electrode. These results show that the Au NRs are well
dispersed in the RGO–TPDT silicate matrix coated on the GC
electrode and facilitate improved interfacial electron transfer
process at the modied electrode.
Electrochemical characterization
The cyclic voltammograms were recorded for the bare GC and
GC–RGO–Au–TPDT NRs modied electrodes in 0.1 M H2SO4
(Fig. S4†). In the absence of RGO–Au–TPDT NRs at the GC
electrode, the bare GC electrode did not show any peaks
(Fig. S4(a)†). The cyclic voltammogram of the RGO–Au–TPDT
NRs modied electrode showed the characteristic oxidation
peak at 0.95 V and its corresponding reduction peak were
observed at 0.45 V for the Au (Fig. S4(b)†).55–57 This result
conrms the presence of Au NRs in the RGO–Au–TPDT NRs
Fig. 4 Nyquist plots of electrochemical impedance spectra obtained
for bare GC (a) GC–Au–TPDT NRs (b) and GC–RGO–Au–TPDT NRs
(c) modified electrodes. Inset: enlarged plot of EIS obtained for GC–
RGO–Au–TPDT NRs modified electrode. The redox analyte was 2.5
mM of K3Fe(CN)6–K4Fe(CN)6 (1 : 1) in 0.1 M PBS and 0.1 M KCl. The
Fig. 3 TEM images of Au NRs (A) and RGO–Au–TPDT NRs (B and C) at open circuit potential was chosen as the electrode potential and the
different magnifications and SAED pattern of RGO–Au–TPDT NRs (D). frequency range was 0.1 Hz to 100 kHz.
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Electrocatalytic oxidation of NO at RGO–Au–TPDT NRs
modied electrode
The electrocatalytic oxidation of NO was carried out at the RGO–
Au–TPDT NRs modied electrode in the presence of 0.1 M PBS
(pH 7.2). Fig. 5 shows the cyclic voltammograms recorded at the
GC–RGO–Au–TPDT NRs both in the absence (a) and in the
presence of NO (e), bare GC (b), GC–RGO–TPDT (c) and GC–Au–
TPDT NRs (d) electrodes. In the absence of NO, no oxidation
peak was observed due to NO at the GC–RGO–Au–TPDT NRs
modied electrode (Fig. 5(a)). When NO was introduced into the
cell solution, an oxidation peak was observed at 0.815 V with a
large oxidation current at the GC–RGO–Au–TPDT NRs electrode
(Fig. 5(e)). The bare GC, GC–RGO–TPDT and GC–Au–TPDT NRs
electrodes showed less intense oxidation peaks for NO at 0.983,
0.955 and 0.839 V, respectively. A higher catalytic current was
observed with a positive shi of ꢀ170 mV in the oxidation
potential for the electrocatalytic oxidation of NO at the GC–
RGO–Au–TPDT NRs modied electrode (Fig. 5(e)) when
compared to the bare GC (Fig. 5(b)) and GC–Au–TPDT NRs
modied electrodes (Fig. 5(c)). These results revealed that the
RGO–Au–TPDT NRs modied electrode shows higher electro-
catalytic activity towards NO oxidation at the modied electrode
due to the synergistic electrocatalytic effect of RGO and Au NRs
present in the RGO–Au–TPDT NRs composite and also due to
the higher surface area of RGO–Au–TPDT NRs composite. The
swelled TPDT silicate sol–gel matrix will further enhance the
preconcentration of analyte at the modied electrode resulting
in the enhanced electrocatalytic activity. The Scheme 1 shows
the schematic representation of the electrocatalytic oxidation of
NO at the GC–RGO–Au–TPDT NRs modied electrode.
Fig. 6 Linear sweep voltammograms recorded at GC–RGO–Au–
TPDT NRs modified electrode during the successive addition of 2 mM
NO in pH 7.2 (0.1 M PBS) at a scan rate of 50 mV sꢁ1. Inset: corre-
sponding calibration plot.
Scheme 1 Schematic illustration of electrocatalytic oxidation of NO at
the GC–RGO–Au–TPDT NRs modified electrode. (P-Products).
Fig. 6 shows the linear sweep voltammograms recorded for
different concentrations of NO at the GC–RGO–Au–TPDT NRs
modied electrode in 0.1 M PBS (pH 7.2). The GC–RGO–Au–
TPDT NRs modied electrode showed an anodic peak at 0.818 V
and the peak current were increased with increasing the
concentration of NO. The plot of anodic peak current against
concentration of NO showed linear response (Fig. 6 (inset)).
The effect of scan rate on the oxidation of NO was studied at
the GC–RGO–Au–TPDT NRs modied electrode (Fig. S5†). The
oxidation peak current due to NO increased linearly with
increasing the scan rate. The plot of anodic peak current against
the square root of scan rate showed linear relation (Fig. S5†
(inset)) owing to the diffusion controlled electron transfer
process at the modied electrode.
Fig. 5 Cyclic voltammograms recorded at GC–RGO–Au–TPDT NRs Fig. 7 Amperometric i–t curve obtained for NO at GC–RGO–Au–
in the absence of NO (a) and in the presence of 10 mM NO at bare GC TPDT NRs modified electrode during the successive addition of 10 nM
(b), GC–RGO–TPDT (c), GC–Au–TPDT NRs (d) and GC–RGO–Au– NO to a stirred solution of 0.1 M PBS (pH 7.2) at an applied potential of
TPDT NRs (e) electrodes in 0.1 M PBS (pH 7) at a scan rate of 50 mV sꢁ1
.
0.8 V. Inset: corresponding calibration plot.
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Table 1 Different parameters observed for the GC–RGO–Au–TPDT NRs modified electrode towards electrochemical sensing of NO
Paper
Correlation
Response
Analytical method
Sensitivity
Linear range
LOD
coefficient
time
Amperometric i–t curve
0.598 nA/nM
10–140 nM
6.5 nM
0.99
2 s
modied electrode (133 nM)34 and hemoglobin–Au nano-
particles–graphene modied BPG electrode (12 nM).63 The LOD
observed for the present modied electrode is slightly higher
than that of the previously reported modied electrode with
1,8,15,22-tetraaminophthalocyanatocobalt(II)30 and polymer
membrane stabilized Au nanostructures modied electrode24
using nitrite ions as a precursor for NO in acidic medium. The
amperometric experiment showed that the GC–RGO–Au–TPDT
NRs modied electrode is an excellent sensor for the electro-
chemical sensing of NO.
The amperometric determination of NO in the presence of
common physiological interferents such as glucose (b), urea (c),
oxalate (d) and NaCl (e) was studied at the GC–RGO–Au–TPDT
NRs modied electrode in 0.1 M PBS (pH 7.2) and the results are
shown in Fig. 8. The addition of each 10 nM NO to the cell
solution showed amperometric current and further addition of
each 100 mM of glucose (b), urea (c) oxalate (d) and NaCl (e)
separately at an interval of 30 s to the solution did not change
the amperometric i–t response of NO. The addition of 10 nM NO
aer the addition of interferences clearly showed the ampero-
metric response due to NO (Fig. 8). These observations clearly
reveal that the above interferents do not interfere during the
determination of NO and further show that the selective
determination of NO is possible at the GC–RGO–Au–TPDT NRs
modied electrode even in the presence of 1000-fold excess
physiological interferents.
The GC–RGO–Au–TPDT NRs modied electrode was
prepared repeatedly and the cyclic voltammograms was recor-
ded for 10 mM NO in 0.1 M PBS (pH 7.2). It was found that the
oxidation current for NO remained the same with a relative
standard deviation of 2.4% (n ¼ 3) indicating stability of the
modied electrode. Aer each experiment, the GC–RGO–Au–
TPDT NRs modied electrode was kept in 0.1 M PBS at room
temperature and the cyclic voltammogram was recorded for 10
mM NO in 0.1 M PBS (pH 7.2) and a ꢀ15% decrease in the peak
current was noticed aer one week. The observations show that
GC–RGO–Au–TPDT NRs modied electrode was stable and
reproducible towards the detection of NO.
Amperometric detection of NO at GC–RGO–Au–TPDT NRs
modied electrode
The GC–RGO–Au–TPDT NRs modied electrode was used as
amperometric sensor for the detection of NO at pH 7.2. The
amperometric i–t responses were recorded for the detection of
NO at the GC–RGO–Au–TPDT NRs modied electrode at an
applied potential of 0.8 V upon each addition of 10 nM NO to
the stirred solution of 0.1 M PBS. The typical amperometric
response for the detection of NO and the corresponding cali-
bration plot obtained at the GC–RGO–Au–TPDT NRs modied
electrode upon each addition of 10 nM NO are shown in Fig. 7. A
linear relationship with a correlation coefficient of 0.99 (n ¼ 14)
for the regression equation I (nA) ¼ 0.59 + 1.52C (nM) was
obtained for the NO concentration in the range from 10 nM to
140 nM (Fig. 7 (inset)). The response time was found to be ꢀ2 s
(Fig. S6†), which is lower than the physiological lifetime (ꢀ5 s)
of nitric oxide and this, indicates the fast electron transfer
process at the GC–RGO–Au–TPDT NRs modied electrode. The
limit of detection (LOD) and limit of quantication (LOQ) for
NO detection were calculated using the standard deviation of y-
intercept (SD) and the slope of the regression line (S) (LOD ¼
3.3(SD/S) and LOQ ¼ 10(SD/S)).49,62 The LOD and LOQ values for
the NO were estimated as 6.5 nM and 20 nM, respectively at the
GC–RGO–Au–TPDT NRs modied electrode. The different
sensor parameters observed for the GC–RGO–Au–TPDT NRs
modied electrode towards NO sensing is summarized in
Table 1. This LOD value observed in the present study was
found to be lower than that of the previously reported LODs at
the electrochemically reduced graphene oxide–Au nanoparticles
Conclusion
The reduced graphene oxide–gold nanorods embedded in
functionalized silicate sol–gel matrix (RGO–Au–TPDT NRs) were
synthesized and characterized. The preparation of RGO–Au–
TPDT NRs is very simple and stable for more than a month and
it requires environment benign solvent medium at room
temperature. The longitudinal plasmon resonance band of the
RGO–Au–TPDT NRs was blue-shied when the RGO was
dispersed in Au–TPDT NRs (RGO–Au–TPDT NRs). The cyclic
Fig. 8 Amperometric i–t curve obtained at GC–RGO–Au–TPDT NRs
modified electrode in a stirred solution of 0.1 M PBS (pH 7.2) at each
addition of 10 nM NO (a) at a regular time interval of 30 s in the
presence of common physiological interferents such as each 10 mM
addition of glucose (b), urea (c), oxalate (d) and NaCl (e). The applied
potential was 0.8 V.
33546 | RSC Adv., 2014, 4, 33541–33548
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RSC Advances
voltammetric studies of the GC–RGO–Au–TPDT NRs modied 15 W. Wen, W. Chen, Q.-Q. Ren, X.-Y. Hu, H.-Y. Xiong,
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X.-H. Zhang, S.-F. Wang and Y.-D. Zhao, Sens. Actuators, B,
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enabled the fabrication of a simple amperometric sensor for NO 16 M. A. Tabrizi and Z. Zand, Electroanalysis, 2014, 26, 171.
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activity towards the detection of NO due to the synergistic effect 18 W. Bai, H. Huang, Y. Li, H. Zhang, B. Liang, R. Guo, L. Du
of RGO and Au NRs. The LOD for NO was estimated as 6.5 nM at
the GC–RGO–Au–TPDT NRs modied electrode. The ampero- 19 N. Zhou, J. Li, H. Chen, C. Liao and L. Chen, Analyst, 2013,
metric sensor for NO prepared using RGO–Au–TPDT NRs 138, 1091.
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TPDT silicate sol–gel matrix nanocomposites material (RGO–
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Acknowledgements
RR acknowledges the Science and Engineering Research Board
(SERB), New Delhi for nancial support. SJ is a recipient of
Senior Research Fellowship under UGC-BSR scheme. The TEM
images were recorded at PSG Institute of Advanced Studies,
Coimbatore for which the authors thank Dr Anuratha M. Ashok
and Mr T. Vijayaraghavan for their timely help.
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