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

Abstract

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.

---

Introduction

Gold nanorods (Au NRs) are elongated nanostructures with unique optical properties, which depend on their shape anisometry.^1–7 The unique properties of Au NRs find 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 find applications in the field 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 amine functionalized graphene–gold nanocomposite material was employed as a biosensor for H₂O₂.²²

Nitric oxide (NO) has been shown to be involved in regulating neuronal excitability, synaptic transmission, functioning of neuronal networks, learning and memory mechanisms.^23–25 The excess or deficiency 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 field of biology, medicine and pharmacy.^30–33 The determination of submicromolar concentration of NO at physiological pH is difficult owing to its short life time (∼5 s) and its rapid conversion to NO_x⁻ by oxygen and superoxides present in biofluids^34–36 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 fluorescence,³⁹ electron paramagnetic resonance⁴⁰ and electrochemistry.^35,36 Among these methods, the electrochemical technique is advantageous due to its sensitivity, selectivity, response time and low-cost fabrication.^34–36

In the present work, we have prepared reduced graphene oxide–gold nanorods embedded in amine functionalized silicate sol–gel matrix (RGO–Au–TPDT NRs) composite material for the first time and the prepared composite is characterized by absorption spectroscopy, diffuse reflectance spectroscopy, transmission electron microscopy (TEM), X-ray diffraction analysis (XRD) and cyclic voltammetry. The RGO–Au–TPDT NRs modified 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 modified electrode and the results are compared with Au–TPDT NRs modified electrode. The encapsulation of reduced graphene oxide–gold nanorods in amine functionalized silicate sol–gel provides an effective support matrix to prepare the modified electrode for the amperometric sensing of NO.

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 specified, used in the present work were of analytical grade. All the glasswares were thoroughly 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 GO

Graphite oxide (GO) was prepared from graphite powder by the modified 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 °C in an ice bath and 3 g of KMnO₄ was slowly added to the mixture. After heating to 35–40 °C, 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 finally 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 concentration of 0.5 mg mL⁻¹ by using ultrasonication and used for further studies.

Preparation of RGO–Au–TPDT NRs

The Au NRs with an aspect ratio of ∼3.5 were prepared by using seed mediated growth method reported by Nikoobakht and El-Sayed.⁷ The excess of CTAB present in the Au NRs solution was removed by centrifuging the solution at 9000 rpm for 10 min and then redispersed in water. 1 mg of GO was dispersed in 2 mL of water and the mixture was ultrasonicated for 30 min to obtain a homogeneous suspension. A 2 mL of centrifuged Au NRs solution ([Au] = 0.5 mM) was mixed with the above solution containing 1 mg of GO in water and finally 50 μL of 1 M TPDT silane solution was added. The resulting solution was further allowed to complete the reaction for 24 h. The color of the solution was changed from brownish yellow to black which indicates the formation of RGO–Au–TPDT NRs composite. The obtained RGO–Au–TPDT NRs composite were used for electrocatalysis and electrochemical sensor studies.

Preparation of modified electrodes

The GC electrode (diameter = 3 mm, CH Instruments) was twice polished using alumina powder (0.05 μm) 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 experiments. Nitrogen gas was bubbled into the electrolyte solution for 30 min before each experiment.

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 disproportionation 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 flask and light-free septum with wrapped black foils. The concentration of saturated NO solution at 20 °C was reported as 1.8 mM.^43,44

Characterization and electrochemical studies

The absorption spectra were recorded using an Agilent 8453 diode array spectrophotometer. The diffuse reflectance 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 electron microscopy (HRTEM) equipped with energy dispersive spectrum (EDS) with an operating voltage of 200 kV. The Au–TPDT NRs solutions were centrifuged at 9000 rpm for 10 min to remove excess amounts of CTAB before the TEM analysis. The sample for HRTEM measurement was prepared by placing a drop of sample solution on a carbon-coated copper grid and dried in air at 25 °C. The powder X-ray diffraction (XRD) analysis was carried out using X'Pert PANalytical X-ray diffractometer with monochromatized Cu Kα radiation (λ = 1.5406 Å). The sample for XRD was prepared by casting the RGO–Au–TPDT NRs solution on a glass plate and allowed to dry at 25 °C. Electrochemical measurements were performed using a three-electrode cell with a glassy carbon (GC) working electrode (electrode area = 0.07 cm²), a Pt wire counter electrode and an Ag/AgCl reference electrode at 25 °C. The electrochemical experiments were conducted using a CHI760D Electrochemical Workstation, CH Instruments, USA. For all the electrochemical experiments, the electrolyte solutions were deaerated with nitrogen gas before each experiment.

Results and discussion

Optical characterization

The absorption spectra of the Au NRs, Au–TPDT NRs and RGO–Au–TPDT NRs showed that the optical properties of Au NRs were influenced 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 significant 30 nm blue shift 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 after 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 significant blue shift 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 after 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 aggregation 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.

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 maximum.

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 groups in the graphene oxide and gets adsorbed on the surface of RGO and Au NRs to form a stable RGO–Au–TPDT NRs.^50,51 After the addition of TPDT silane to the solution, the color of the solution was changed from brownish yellow to black after 24 h, indicating the reduction of graphene oxide to RGO.

The diffuse reflectance spectra of the RGO–Au–TPDT NRs modified glassy carbon electrodes (GC–RGO–Au–TPDT NRs) were recorded in wet and dry conditions (Fig. S1†). When the GC–RGO–Au–TPDT NRs modified electrode was wetted with distilled water, the RGO–Au–TPDT NRs film underwent swelling in water and the LSPR band was appeared at 689 nm (Fig. S1(a)†). When the GC–RGO–Au–TPDT NRs modified electrode 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 shift of 59 nm when compared to the wet GC–RGO–Au–TPDT NRs modified electrode (Fig. S1(a)†). This observation reveals that the air-dried film brought about a change in the dielectric environment. It confirms that the environment for the RGO–Au–TPDT NRs in the swelled film is influenced by the solvent trapped inside the silicate sol–gel film and the amount of hydration that exists in the silicate film. 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 films.

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 (2θ) and the XRD pattern of GO (Fig. 2(b)) shows a diffraction peak at 10.7 (2θ), corresponding to the C(002) interlayer d-spacing of 0.82 nm. The diffraction peak observed at 26.3 (2θ) 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 (2θ)) due to the intercalated water molecules between layers.⁵³ Fig. 2 (inset) shows that the diffraction peak observed at 10.7 (2θ) for GO was disappeared and a very broad peak around 23.3 (2θ) was observed for the RGO–Au–TPDT NRs, indicating the reduction of graphene oxide to RGO by TPDT silane.⁵² The diffraction peak observed for RGO–Au–TPDT NRs (Fig. 2 (inset)) is different from both graphite and GO (Fig. 2(a) and (b)) which indicates the formation of RGO.

Fig. 2 XRD patterns of graphite (a) and GO (b). Inset: XRD pattern of RGO–Au–TPDT NRs.

TEM studies

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-defined six-fold-symmetry diffraction pattern, confirming 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 presence of Au and Si in the solution.

Fig. 3 TEM images of Au NRs (A) and RGO–Au–TPDT NRs (B and C) at different magnifications and SAED pattern of RGO–Au–TPDT NRs (D).

Electrochemical characterization

The cyclic voltammograms were recorded for the bare GC and GC–RGO–Au–TPDT NRs modified 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 modified 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 confirms the presence of Au NRs in the RGO–Au–TPDT NRs modified electrode and exhibits the electrochemical behavior due to the electrochemical contact between the RGO–Au–TPDT NRs and the GC electrode surface.

The electroactive surface area of the modified electrodes were determined by recording the chronoamperogram for 3 mM Fe(CN)₆⁴⁻ as a redox probe in 0.1 M KCl by using the Cottrell 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 corresponding 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 (R_ct) of 1493 Ω. When the Au–TPDT NRs was coated on the GC electrode (GC–Au–TPDT NRs), the electron-transfer resistance was decreased to 339 Ω (Fig. 4(b)), which indicates an increase in the electron-transfer process at the modified 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 significantly decreased to 68 Ω (Fig. 4(b)), which indicates the highly conductive nature of RGO in the RGO–Au–TPDT NRs composites material at the modified 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 modified electrode.

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 open circuit potential was chosen as the electrode potential and the frequency range was 0.1 Hz to 100 kHz.

Electrocatalytic oxidation of NO at RGO–Au–TPDT NRs modified electrode

The electrocatalytic oxidation of NO was carried out at the RGO–Au–TPDT NRs modified 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 modified 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 shift of ∼170 mV in the oxidation potential for the electrocatalytic oxidation of NO at the GC–RGO–Au–TPDT NRs modified electrode (Fig. 5(e)) when compared to the bare GC (Fig. 5(b)) and GC–Au–TPDT NRs modified electrodes (Fig. 5(c)). These results revealed that the RGO–Au–TPDT NRs modified electrode shows higher electrocatalytic activity towards NO oxidation at the modified 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 modified 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 modified electrode.

Fig. 5 Cyclic voltammograms recorded at GC–RGO–Au–TPDT NRs in the absence of NO (a) and in the presence of 10 μM NO at bare GC (b), GC–RGO–TPDT (c), GC–Au–TPDT NRs (d) and GC–RGO–Au–TPDT NRs (e) electrodes in 0.1 M PBS (pH 7) at a scan rate of 50 mV s⁻¹.

Fig. 6 Linear sweep voltammograms recorded at GC–RGO–Au–TPDT NRs modified electrode during the successive addition of 2 μM NO in pH 7.2 (0.1 M PBS) at a scan rate of 50 mV s⁻¹. Inset: corresponding 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 modified electrode in 0.1 M PBS (pH 7.2). The GC–RGO–Au–TPDT NRs modified 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 modified 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 modified electrode.

Amperometric detection of NO at GC–RGO–Au–TPDT NRs modified electrode

The GC–RGO–Au–TPDT NRs modified 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 modified 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 calibration plot obtained at the GC–RGO–Au–TPDT NRs modified electrode upon each addition of 10 nM NO are shown in Fig. 7. A linear relationship with a correlation coefficient of 0.99 (n = 14) for the regression equation I (nA) = 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 modified electrode. The limit of detection (LOD) and limit of quantification (LOQ) for NO detection were calculated using the standard deviation of y-intercept (SD) and the slope of the regression line (S) (LOD = 3.3(SD/S) and LOQ = 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 modified electrode. The different sensor parameters observed for the GC–RGO–Au–TPDT NRs modified 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 modified electrode (133 nM)³⁴ and hemoglobin–Au nanoparticles–graphene modified BPG electrode (12 nM).⁶³ The LOD observed for the present modified electrode is slightly higher than that of the previously reported modified electrode with 1,8,15,22-tetraaminophthalocyanatocobalt(II)³⁰ and polymer membrane stabilized Au nanostructures modified electrode²⁴ using nitrite ions as a precursor for NO in acidic medium. The amperometric experiment showed that the GC–RGO–Au–TPDT NRs modified electrode is an excellent sensor for the electrochemical sensing of NO.

Fig. 7 Amperometric i–t curve obtained for NO at GC–RGO–Au–TPDT NRs modified electrode during the successive addition of 10 nM NO to a stirred solution of 0.1 M PBS (pH 7.2) at an applied potential of 0.8 V. Inset: corresponding calibration plot.

Table 1 Different parameters observed for the GC–RGO–Au–TPDT NRs modified electrode towards electrochemical sensing of NO

Analytical method	Sensitivity	Linear range	LOD	Correlation coefficient	Response time
Amperometric i–t curve	0.598 nA/nM	10–140 nM	6.5 nM	0.99	2 s

---

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 modified 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 μM 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 after the addition of interferences clearly showed the amperometric 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 modified electrode even in the presence of 1000-fold excess physiological interferents.

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 μM addition of glucose (b), urea (c), oxalate (d) and NaCl (e). The applied potential was 0.8 V.

The GC–RGO–Au–TPDT NRs modified electrode was prepared repeatedly and the cyclic voltammograms was recorded for 10 μM 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 = 3) indicating stability of the modified electrode. After each experiment, the GC–RGO–Au–TPDT NRs modified electrode was kept in 0.1 M PBS at room temperature and the cyclic voltammogram was recorded for 10 μM NO in 0.1 M PBS (pH 7.2) and a ∼15% decrease in the peak current was noticed after one week. The observations show that GC–RGO–Au–TPDT NRs modified electrode was stable and reproducible towards the detection of NO.

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-shifted when the RGO was dispersed in Au–TPDT NRs (RGO–Au–TPDT NRs). The cyclic voltammetric studies of the GC–RGO–Au–TPDT NRs modified electrode showed good electrical communication between the RGO–Au–TPDT NRs film and at the electrode surface that enabled the fabrication of a simple amperometric sensor for NO detection. The RGO–Au–TPDT NRs showed enhanced catalytic activity towards the detection of NO due to the synergistic effect of RGO and Au NRs. The LOD for NO was estimated as 6.5 nM at the GC–RGO–Au–TPDT NRs modified electrode. The amperometric sensor for NO prepared using RGO–Au–TPDT NRs showed selectivity in the presence of physiological interferents, linear range and good stability. The GC–RGO–Au–TPDT NRs modified electrode can be used for the determination of 10 nM NO in the presence of 1000-fold excess concentration of physiological interferences. To the best of our knowledge, this is the first report that deals about the introduction of Au NRs in RGO–TPDT silicate sol–gel matrix nanocomposites material (RGO–Au–TPDT NRs) for electrochemical sensor application.

Acknowledgements

RR acknowledges the Science and Engineering Research Board (SERB), New Delhi for financial 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.

References

1. X. Huang, S. Neretina and M. A. El-Sayed, Adv. Mater., 2009, 21, 4880.
2. J. Perez-Juste, I. Pastoriza-Santos, L. M. Liz-Marzan and P. Mulvaney, Coord. Chem. Rev., 2005, 249, 1870.
3. A. M. Alkilany, L. B. Thompson, S. P. Boulos, P. N. Sisco and C. J. Murphy, Adv. Drug Delivery Rev., 2012, 64, 190.
4. C. Wang, Z. Ma, T. Wang and Z. Su, Adv. Funct. Mater., 2006, 16, 1673.
5. H. Chen, L. Shao, Q. Li and J. Wang, Chem. Soc. Rev., 2013, 42, 2679.
6. N. R. Jana, L. Gearheart and C. J. Murphy, J. Phys. Chem. B, 2001, 105, 4065.
7. B. Nikoobakht and M. A. El-Sayed, Chem. Mater., 2003, 15, 1957.
8. S. Liu and M.-Y. Han, Chem.–Asian J., 2010, 5, 36.
9. J. Kim, J. E. Lee, J. Lee, J. H. Yu, B. C. Kim, K. An, Y. Hwang, C.-H. Shin, J.-G. Park, J. Kim and T. Hyeon, J. Am. Chem. Soc., 2006, 128, 688.
10. J. Kim, H. S. Kim, N. Lee, T. Kim, H. Kim, T. Yu, I. C. Song, W. K. Moon and T. Hyeon, Angew. Chem., Int. Ed., 2008, 47, 8438.
11. G. Wang, Z. Chen, W. Wang, B. Yan and L. Chen, Analyst, 2011, 136, 174.
12. D. Chen, L. Tang and J. Li, Chem. Soc. Rev., 2010, 39, 3157.
13. D. A. C. Brownson, D. K. Kampouris and C. E. Banks, Chem. Soc. Rev., 2012, 41, 6944.
14. M. Pumera, Chem. Soc. Rev., 2010, 39, 4146.
15. W. Wen, W. Chen, Q.-Q. Ren, X.-Y. Hu, H.-Y. Xiong, X.-H. Zhang, S.-F. Wang and Y.-D. Zhao, Sens. Actuators, B, 2012, 166–167, 444.
16. M. A. Tabrizi and Z. Zand, Electroanalysis, 2014, 26, 171.
17. L. Li, H. Lu and L. Deng, Talanta, 2013, 113, 1.
18. W. Bai, H. Huang, Y. Li, H. Zhang, B. Liang, R. Guo, L. Du and Z. Zhang, Electrochim. Acta, 2014, 117, 322.
19. N. Zhou, J. Li, H. Chen, C. Liao and L. Chen, Analyst, 2013, 138, 1091.
20. P. Zhang, X. Zhang, S. Zhang, X. Lu, Q. Li, Z. Su and G. Wei, J. Mater. Chem. B, 2013, 1, 6525.
21. X. Liu, L. Xie and H. Li, J. Electroanal. Chem., 2012, 682, 158.
22. K.-J. Huang, D.-J. Niu, X. Liu, Z.-W. Wu, Y. Fan, Y.-F. Chang and Y.-Y. Wu, Electrochim. Acta, 2011, 56, 2947.
23. L. J. Ignarro, G. M. Buga, K. S. Wood, R. E. Byrns and G. Chaudhuri, Proc. Natl. Acad. Sci. U. S. A., 1987, 84, 9265.
24. S. Thangavel and R. Ramaraj, J. Phys. Chem. C, 2008, 112, 19825.
25. F. O. Brown, N. J. Finnerty and J. P. Lowry, Analyst, 2009, 134, 2012.
26. P. Thejass and G. Kuttan, Nitric Oxide: Biology and Chemistry, 2007, 16, 247.
27. C. Napoli and L. J. Ignarro, Nitric Oxide: Biology and Chemistry, 2001, 5, 88.
28. R. Kavya, R. Saluja, S. Singh and M. Dikshit, Nitric Oxide: Biology and Chemistry, 2006, 15, 280.
29. O. Traub and R. Vanbibber, West. J. Med., 1995, 162, 439.
30. A. Sivanesan and S. A. John, Electroanalysis, 2010, 22, 639.
31. J.-M. Zen, A. S. Kumar and H.-F. Wang, Analyst, 2000, 125, 2169.
32. F. Bedioui and N. Villeneuve, Electroanalysis, 2003, 15, 5.
33. T. Xu, N. Scafa, L.-P. Xu, L. Su, C. Li, S. Zhou, Y. Liu and X. Zhang, Electroanalysis, 2014, 26, 449.
34. S. L. Ting, C. X. Guo, K. C. Leong, D.-H. Kim, C. M. Li and P. Chen, Electrochim. Acta, 2013, 111, 441.
35. B. J. Privett, J. H. Shin and M. H. Schoenfisch, Chem. Soc. Rev., 2010, 39, 1925.
36. Y. M. Liu, C. Punckt, M. A. Pope, A. Gelperin and I. A. Aksay, ACS Appl. Mater. Interfaces, 2013, 5, 12624.
37. P. Y. Wu, J. Wang, C. He, X. L. Zhang, Y. T. Wang, T. Liu and C. Y. Duan, Adv. Funct. Mater., 2012, 22, 1698.
38. L. A. Ridnour, J. E. Sim, M. A. Hayward, D. A. Wink, S. M. Martin, G. R. Buettner and D. R. Spitz, Anal. Biochem., 2000, 281, 223.
39. X. Ye, S. S. Rubakhin and J. V. Sweedler, Analyst, 2008, 133, 423.
40. E. M. Hetrick and M. H. Schoenfisch, Annu. Rev. Anal. Chem., 2009, 2, 409.
41. Y. Zhang, S. Liu, L. Wang, X. Qin, J. Tian, W. Lu, G. Chang and X. Sun, RSC Adv., 2012, 2, 538.
42. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
43. S. Mesaros, S. Grunfeld, A. Mesarosova, D. Bustin and T. Malinski, Anal. Chim. Acta, 1997, 339, 265.
44. C. M. Li, J. Zang, D. Zhan, W. Chen, C. Q. Sun, A. L. Teo, Y. T. Chua, V. S. Lee and S. M. Moochhala, Electroanalysis, 2006, 18, 713.
45. C. Wu and Q.-H. Xu, Langmuir, 2009, 25, 9441.
46. S. Jayabal, R. Sathiyamurthi and R. Ramaraj, J. Mater. Chem. A, 2014, 2, 8918.
47. J. D. S. Newman and G. J. Blanchard, Langmuir, 2006, 22, 5882.
48. G. Maduraiveeran and R. Ramaraj, Anal. Chem., 2009, 81, 7552.
49. S. Jayabal and R. Ramaraj, Electrochim. Acta, 2013, 88, 51.
50. H. Yang, F. Li, C. Shan, D. Han, Q. Zhang, L. Niu and A. Ivaska, J. Mater. Chem., 2009, 19, 4632.
51. H. Yang, C. Shan, F. Li, D. Han, Q. Zhang and L. Niu, Chem. Commun., 2009, 3880.
52. H.-L. Guo, X.-F. Wang, Q.-Y. Qian, F.-B. Wang and X.-H. Xia, ACS Nano, 2009, 3, 2653.
53. H.-K. Jeong, Y. P. Lee, R. J. W. E. Lahaye, M.-H. Park, K. H. An, I. J. Kim, C.-W. Yang Park, R. S. Ruoff and Y. H. Lee, J. Am. Chem. Soc., 2008, 130, 1362.
54. S. Zhang, Y. Shao, H. Liao, M. H. Engelhard, G. Yin and Y. Lin, ACS Nano, 2011, 5, 1785.
55. X. Dai, G. G. Wildgoose, C. Salter, A. Crossley and R. G. Compton, Anal. Chem., 2006, 78, 6102.
56. S. Jayabal and R. Ramaraj, Appl. Catal., A, 2014, 470, 369.
57. S. Bharathi, M. Nogami and O. Lev, Langmuir, 2001, 17, 2602.
58. A. J. Bard and L. R. Faulkner, Electrochemical Methods–Fundamentals and Applications, John Wiley and Sons, New York, 2nd edn, 2000.
59. I. I. Suni, Trends Anal. Chem., 2008, 27, 604.
60. G. Maduraiveeran and R. Ramaraj, J. Electroanal. Chem., 2007, 608, 52.
61. X. Liu, L. Xie and H. Li, J. Electroanal. Chem., 2012, 682, 158.
62. S. S. Razola, B. L. Ruiz, N. M. Diez, H. B. Mark and J.-M. Kauffmann, Biosens. Bioelectron., 2002, 17, 921.
63. M.-Q. Xu, J.-F. Wu and G.-C. Zhao, Sensors, 2013, 13, 7492.

---

Footnote

† Electronic supplementary information (ESI) available: Fig. S1 shows diffuse reflectance 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 μM NO at GC–RGO–Au–TPDT NRs modified 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 modified electrode. See DOI: 10.1039/c4ra04859h

---