New Disposable Nitric Oxide Sensor Fabrication Using GaN Nanowires.

Gallium nitride (GaN) nanowires anchored on the surface of cost-effective pencil graphite electrodes (PGEs) have been developed as a new disposable nitric oxide (NO) sensor through a hydrothermal method followed by annealing treatment. The as-obtained nanomaterials were examined by field emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and EIS. Concurrently, the electrocatalytic performance has been analyzed using cyclic voltammetry and amperometric measurements. The experimental results exhibit good electrochemical sensing performance toward the generated NO in NO2 - with a wide linear detection range of 1.0 μM to 1.0 mM with a correlation coefficient of 0.999 and a detection limit of 0.180 μM. In addition, the GaN nanowire-modified PGE surface showed high selectivity for the detection of NO as compared to other relevant biomolecules. This confirms that the PGE/GaN nanowire is a new promising electrochemical sensor for the sensitive detection of NO.

Introduction

Detailed investigations on the evaluation of the characteristics of nanomaterials over the past few decades have emerged in the better design and development of device performances including the electrochemical sensing platform for the detection of biomolecules. Among the various wurtzite crystalline structure-based electrode materials, gallium nitride (GaN) is a wide and direct band gap semiconducting material that is of paramount research interest for a range of electronic and optoelectronic applications owing to its long-term chemical stability, nontoxicity, and biocompatibility. Hence, GaN is identified as a potential candidate for biosensing applications.[1−3] The unique structure of GaN nanowires has a large surface to volume ratio, which provides a higher surface conductivity compared to the conventional planar GaN.[4] Recently, we have reported on the synthesis of GaN nanoparticles by the hydrothermal method.[5] There have been many attempts at sensing (l-cysteine, DNA molecules) biomolecules using GaN nanowires through electrochemical detection methods.[6,7]

Nitric oxide (NO) is one of the most important biomolecules in nature, which plays a key role in the regulation of blood pressure for many physiological processes including cardiovascular systems, wound healing, angiogenesis, platelet aggregation, immune responses, vasodilation, inflammation, and neurotransmission.[8−13] However, the detection of NO in biological systems is significant with challenges because of its short half-life (<10 s), wide concentration range (picomolar to micromolar), and complexity of other molecules.[14−16] In general, numerous analytical techniques such as chemiluminescence, absorbance, fluorescence, and electron paramagnetic resonance have been using expensive instruments. Detection of NO by the electrochemical detection method is simple, rapid, and of relatively low cost and high sensitivity. Hence, nanomaterial-based electrodes are used, which offer a better platform for the sensitive detection of NO.[16,17]

The design of an electrochemical sensor, based on carbon electrodes, has attracted wide attention in the recent years.[18,19] Among different kinds of carbon-based electrodes such as carbon paste, glassy carbon, carbon nanotube, ordered mesoporous, normal graphite electrodes modified with various redox mediators are used in the electroanalytical field. Especially, the pencil graphite electrode (PGE) has enormous advantages such as low cost, ease of availability, adjustable active surface area, ease of technology for surface modification, and miniaturization.[20,21] Modified PGE-based electrochemical sensing platforms have attracted considerable attention in the field of analytical chemistry, which can improve the detection of biomolecules; for example, electrochemical detection of adenine based on copper nanoparticles (CuNPs)-modified PGE,[22] detection of glucose by immobilization of graphene oxide on the graphene-modified PGE,[23] and DNA-based biosensor by immobilization of MWCNT-PDDA/DNA-modified PGE.[24] The electroactivity of PGEs depends on surface pretreatment using electrochemical procedures,[25,26] To the best of our knowledge, this is the first report on the preparation of GaN nanowires on PGE and its use for NO-sensing. In this report, we demonstrate a novel approach for the preparation of GaN nanowire growth on PGE via hydrothermal method. The resulting nanowires of GaN/PGE showed good analytical performance for NO owing to its higher electrocatalytic activity and improved active sites for NO detection than pristine PGE. The PGE surface could be efficiently covered with electroactive sites of GaN nanowires for the electrocatalytic oxidation of NO detection with a wide linear range, good detection limit, and remarkable sensitivity and selectivity.

Results and Discussion

The morphological and structural features of the PGE and GaN/PGE nanowires were studied using field emission scanning electron microscopy (FE-SEM) and are shown in Figure . The obtained morphological features are seen in Figure i–ii for PGE and GaN/PGE nanowire samples, respectively. The FE-SEM images in Figure i show the pristine PGE at different magnifications. Figure i(b,c) shows the layer by layer pattern that can be clearly observed in the pristine PGE. Figure ii shows the GaN/PEG samples at different magnifications, which clearly exhibit the nanowire-like structure that have been grown using hydrothermal method. These observations are similar to the reports published on GaN nanowires.[27,28] The GaN nanowires coated on the PGE substrate are of an average size of 32 ± 5 nm. The hydrothermally grown GaN nanowire has improved the active sites through large surface to volume ratio yield with a higher surface electrical conductivity. Furthermore, the elemental analysis was investigated using energy-dispersive X-ray analysis (EDAX), which confirmed carbon on PGE and C, Ga, and N on GaN/PGE and is presented in Figure d–h. The results confirm the growth of GaN nanowires on PGE and its suitability for the detection of NO.

Figure 1

Different magnification FE-SEM images of (i) (a–c) PGE, (ii) (e,f) GaN/PGE nanowires, and (d–h) EDAX spectra of PGE and GaN/PGE nanowires.

High-resolution transmission electron microscopy (HR-TEM) provides additional information on the crystal structure and morphology of GaN nanowires. Figure a,b displays the low-magnification HR-TEM images of GaN nanowires with dimensions of 31.4 and 35.9 nm. Recently, low-dimensional nanostructures (nanorods and nanowires) realized through a catalyst-free method have been reported.[29,30] The well-resolved lattice fringe observed in Figure c reveals that the GaN nanowires grown on PGE are highly single crystalline in nature. Figure d shows the selected area electron diffraction (SAED) pattern of single GaN nanowires. The SAED pattern results also confirm the crystalline quality of GaN nanowires. The structural confirmation has clearly showcased the hexagonal GaN nanowires shown in Figures S1 and S2 (Supporting Information).

Figure 2

HR-TEM images of (a,b) GaN nanowires at different magnifications, (c) HR-TEM image of the GaN nanowire, and (d) SAED pattern of the GaN nanowire.

Figure 3

(a) XPS survey spectrum of PGE, (b) XPS survey spectrum of GaN/PEG nanowires, (c) XPS spectrum of the O 1s region of PGE and GaN/PGE nanowires, (d,e) XPS Spectrum of the C 1s region of PGE and GaN/PGE, (f) XPS spectrum of the Ga 2p region of GaN/PGE nanowires, and (g) XPS spectrum of the N 1s region of GaN/PGE nanowires.

X-ray photoelectron spectroscopy (XPS) measurement was carried out for investigations on the chemical composition and on the chemical states of PGE and GaN/PGE nanowires. Figure a,b shows the XPS survey spectra of the GaN/PGE nanowires and PGE, indicating the presence of the following elements: Ga, N, C, and O. Figure C indicates the O 1s center peak at 529.8 eV; the element O 1s arises from the surface pollution of the PGE and GaN/PGE nanowires. Figure d,e is the gauss fit of the C 1s spectrum, ranging from 282 to 288 eV. The major peak in the XPS spectra was at 284.2 eV, which is assigned to the graphitic region created by the sp2 carbon atoms (C1), which confirms that the C atoms are arranged in a honeycomb lattice. The peak in the XPS spectra observed at 285.7 eV corresponding to the sp3 C–O bonds were found as the only form of carbon–oxygen functionalities on the PGE and GaN/PGE nanowires.[31] The XPS spectra of the Ga 2p and N 1s core level regions of the sample have been clearly observed and are shown in Figure f,g. The XPS peaks of GaN/PGE nanowires obtained at 1021, 1044, and 398 eV corresponding to the Ga 2p and N 1s core levels are shown in Figure f,g. The results are in good agreement with those previously reported for GaN nanoparticles[32] and confirms the formation of the GaN/PGE nanowire.

Figure 4

(A) Effect of applied potential using chronoamperometric curves observed at the GaN/PGE modified electrode in 1 μM NO2–. (B) Chronoamperometric responses recorded at (a) PGE and (b) GaN/PGE modified electrode for 1 μM of NO2– in 0.1 M PBS (pH 2.5) at an applied potential of 0.85 V.

The effect of applied potential for amperometric response of the GaN nanowire-modified PGE is shown in Figure A. Over the potential range of 0.80–0.90 V, the highest current response of NO2– was achieved at 0.85 V for GaN nanowire-modified PGE. This observation indicates higher electrocatalytic ability of GaN nanowire-modified PGE toward the detection of NO. Therefore, 0.85 V was chosen as the applied potential for all the other amperometric experiments. The cyclic voltammetry (CV) analysis implies the large surface area and high electrical conductivity of GaN/PGE nanowire shown in Figure S3A (Supporting Information). Figure S3B realizes the good electrical conductivity and accelerates the good interfacial electron transfer for the sensor applications (Supporting Information).

The electrochemical properties of PGE (a) and PGE on GaN nanowire (b) was probed through amperometric measurement in 0.1 M PBS (pH 2.5) at an applied potential of 0.85 V. Figure B reveals that the GaN nanowire on PGE has significantly enhanced amperometric signal response toward the NO detection compared to the PGE. This confirms that GaN nanowire on PGE can provide a better electrocatalytic activity because of its large surface area and high electrical conductivity.

The electrochemical sensing performance of GaN/PGE toward NO2– in studied upon addition of NO2– in 0.1 M PBS (pH 2.5) under the applied potential of 0.85 V. Figure A presents the chronoamperometric responses at different concentrations of NO2– in 0.1 M PBS (pH 2.5), the electrochemical signal responses dramatically increases with increasing NO2– concentration. The nanomaterial of the GaN/PGE nanowire shows a linear relationship between the chronoamperometric signal responses of NO2– concentration in the range of 1.0 μM to 1.0 mM with a correlation coefficient of 0.999 and a detection limit of 0.180 μM.

Figure 5

(A) Chronoamperometric response recorded at the GaN/PGE nanowire for the successive addition of NO2– (1 to 1000 μM) in 0.1 M PBS (pH 2.5) at an applied potential of 0.85 V. (B) Chronoamperometric responses of the GaN/PGE nanowire upon the successive addition of NO2–, DA, AA, GLU, UA, and NO2.

To test the anti-interference ability is one of the crucial requirements for the practical sensor under the applied potential of 0.85 V. Figure B shows the amperometric response achieved by successive addition to 10 μM NO2–, 100 μM DA, 100 μM AA, 100 μM GLU, 100 μM UA, and 20 μM NO2– in 0.1 M PBS (pH 2.5). No obvious amperometric signal responses were observed with the coexisting electroactive species such as DA, AA, GLU, and UA. The results indicate that the GaN nanowire on PGE has high selectivity toward the detection of NO. The comparison of the sensor materials, electrolyte, linear range, detection limit, analytical techniques, and applied potentials of the GaN nanowire-modified PGE with other reported electrochemical sensors for the detection of NO is given in Table .

Table 1

Comparison of Some Recently Reported Electrochemical Sensor for the Detection of NOa

electrode	electrolyte	linear range (μM)	detection limit (μM)	analytical technique	applied potentials (V)	references
GCE/rGO-Co3O4@Pt	pH 2.5	10–650	1.73	amperometry	0.84	(33)
GCE/ErGO-AuNPs	pH 7.4	up to 3.38	0.13	amperometry	0.80	(34)
GCE/PAM-SDS-Cyt c	pH 7.0	0.8–95	0.1	amperometry	–0.65	(35)
GCE/rGO-NF@Au8	pH 2.5	1.0–10	0.5	amperometry	0.80	(36)
GCE/DNA-Cyt c	pH 5.0	0.6–8	0.1	amperometry	–0.65	(37)
GCE/ErGO	pH 7.0	0.7–78	0.2	amperometry	1.05	(38)
GCE/rGO-Ag	pH 2.5	10–220	2.8	amperometry	0.96	(39)
AuE/l-cys-Cyt c	pH 7.0	0.7–10	0.3	amperometry	0.82	(40)
GaN/PGE nanowire	pH 2.5	1.0–1000	0.18	amperometry	0.85	this work

CO3O4@Pt: cobalt oxide nanocube@platinum; rGO: reduced graphene oxide; ErGO: electrochemically reduced grapheneoxide; AuNPs: gold nanoparticles; PAM: polyacrylamide; SDS: sodium dodecyl sulphate; Cyt c: cytochrome c: NF@Au8: Nafion@gold nanoparticles; DNA: deoxyribonucleic acid; Ag: silver nanoparticles; AuE: gold electrode; l-cys: l-cysteine; PGE: pencil graphite electrode; GaN: GaN gallium nitride nanowire.

Conclusions

In summary, we report on the growth of the GaN/PGE nanowire using the hydrothermal method followed by annealing treatment. PGE is efficiently covered with GaN nanowires and is confirmed using Raman spectroscopy, FE-SEM, HR-TEM, and XPS. The GaN/PGE nanowires showed the higher electrocatalytic activity toward NO, which is dramatically lower than conventional planar GaN structures owing to its specific large surface to volume ratio yield with a higher surface conductivity. The results demonstrated provide the features of wide linear range, low detection limit, remarkable sensitivity and selectivity compared to the other reports in literature. The GaN/PGE nanowire provides a simple, fast, enzyme-free, and cost-effective method to detect NO and can be further applied to electrochemical detection of real samples.

Experimental Section

Materials

Gallium (III) oxide (Ga2O3), hydrochloric acid (HCl), ammonium hydroxide (NH4OH), disodium phosphate (Na2HPO4), monosodium phosphate (NaH2PO4), phosphoric acid (H3PO4), K3[Fe(CN)6], H2O2 (30%), glucose, acetaminophen, ascorbic acid and uric acid were purchased from sigma Aldrich; PGEs (0.7 mm in diameter, 6 cm in length, and geometric area of 31.19). The working electrodes were covered with Teflon and the electrical contact with the working electrode was made by joining a copper wire in one end.[26] Phosphate buffer solution (0.1 M PBS) pH 2.5 was prepared by mixing a standard stock solution of 0.1 M Na2HPO4 and NaH2PO4 in deionized water and adjusting the pH with 1.0 M H3PO4.

Characterization

An electrochemical cell connected to a three-electrode system with a GaN nanowire-modified pencil graphite working electrode (PGE), Ag/AgCl (3 M NaCl) reference electrode, and platinum wire auxiliary electrode were used. CV and electrochemical impedance spectroscopy (EIS) were carried out in a CHI 660C electrochemical instrumentation. EIS data were measured in the frequency range of 0.1 Hz and 100 kHz at an applied dc potential 0.25 V (the formal potential of a [Fe(CN)6]3– redox couple) and ac amplitude of 5 mV. EIS measurement was taken in the presence of 5 mM [Fe(CN)6]3– solution containing 0.1 mM KCl. Morphological studies of the PGE and GaN nanowire on PGE surfaces were conducted using FE-SEM (Carl Zeiss Supra 55VP). Structural studies of the GaN nanowire on PGE were carried out using HR-TEM (JEOL 3010 with a UHR pole piece). Raman spectra were recorded using a diode pumped solid state laser source of 532 nm [WITec alpha300RA (WITec GmbH, Ulm, Germany)]. XPS analysis was carried out using [Oxford instruments (Germany)] an aluminum source (Al Kα radiation hν = 1486.7 eV).

Growth of GaN Nanowires

GaN nanowires were realized using the hydrothermal method followed by annealing treatment. 0.1 M Ga2O3 was used as the starting material. As Ga2O3 is insoluble in water, initially HCl was added to dissolve Ga2O3 and then 40 mL of distilled water was added to the solution. The obtained solution was stirred for 2 h. Subsequently, a sufficient amount of NH4OH was added to the Ga2O3 solution and maintained at a pH of 6. Finally, the solution was transferred to a Teflon-lined autoclave inside which PGEs were placed. The autoclave was kept inside a hot air oven and maintained at a temperature of 453 K for 15 h. The gallium oxide-coated PGE was kept for nitrification in a quartz reactor under constant ammonia flow at the rate of 0.8 standard liters per minute (slm) and at a temperature of 1223 K for 5 h. The overall growth process is schematically shown in Figure .

Figure 6

Schematic on the growth of the GaN nanowire and the electrochemical NO detection.