Non-Enzymatic Glucose Sensor Based on the Novel Flower Like Morphology of Nickel Oxide ()
1. Introduction
Due to increase in the number of diabetes patients, the accurate and sensitive detection of glucose has drawn the attention of researchers towards the simple and cheap fabrication of glucose sensors [1]. Updike and Hicks for the first time developed glucose oxidase immobilized amperometric glucose sensor [2]. Afterwards electrochemical biosensors, using glucose oxidase as biorecognition material, have been largely studied and examined [3-5]. Although the glucose sensors using glucose oxidase have several merits such as high sensitivity and selectivity, however the catalytic activity of glucose oxidase is easily reduced by variations in pH, temperature, humidity and by the interference offered by other substances. Recently, non-enzymatic glucose sensors are becoming more popular due to their acceptable reproducibility, stability, low costs and simplicity. Several metal oxides nanostructures based non-enzymatic glucose sensors have been reported such as NiO [6-8], MnO2 [9], Co3O4 [10], CuO [11]. Still more work is required in the fabrication of non-enzymatic glucose sensors for improving their sensitivity and selectivity.
The nickel oxide (NiO) nanomaterial with homogenous size and well defined dispersion is highly demanded for the various applications such as designing ceramic, magnetic, electrochromic and heterogeneous catalytic materials [12]. Different synthetic routes have been proposed for the fabrication of NiO nanomaterial including sol-gel [13], surfactant mediated synthesis [14], thermal decomposition [15] and polymer matrix assisted preparation [16]. Some of the above mentioned growth routes are facing problems in the uniform size and well dispersion of NiO nanoparticles.
The hydrothermal method for the growth of various nanomaterials is also known as super-heated hydrothermal solution based method. Hydrothermal method is considered environmentally benign and it can be used for the growth of various nanomaterials even in strong solvents in the closed system. Moreover, hydrothermal method is inexpensive, simple and yields high purity of desired product, thus it is a highly suitable method from industrial point of view. To date, several methods have been proposed for the synthesis of NiO nanoparticles and nanorods, but few reports have been published for the preparation of porous nanosheets [17,18]. Nickel oxide is the material which can easily be grown by hydrothermal method with high yield on the substrate.
In this work, novel NiO nanoflowers have been synthesized by the hydrothermal method and successfully applied for the development of enzyme free glucose sensor by cyclic voltammetry and amperometric measurements. The sensor electrode is very stable, reproducible, repeatable, sensitive and selective.
2. Experimental Section
2.1. Used Materials
Nickel nitrate hexahydrate, hexamethylenetetramine, 25% ammonia, nitric acid d-glucose, ascorbic acid, uric acid and dopamine were purchased from Sigma Aldrich Sweden. All other chemical used were of analytical standard.
2.2. The Synthesis of NiO Nanostructures on the Gold Coated Glass Substrate
The coating of gold layer on the glass substrate was done according to our previous work [19]. The synthesis of NiO was carried out in two steps. Firstly nickel hydroxide Ni(OH)2 was grown the gold coated glass substrate by hydrothermal growth approach using the equimolar 0.1 M solution of nickel hexahydrate and hexamethylenetetramine. Nickel hydroxide was grown in alkaline medium of pH 11 and the pH of growth solution was adjusted by using few drops f 25% ammonia and 0.1 M nitric acid. The gold electrodes were cleaned in the isopropanol in the ultrasonic bath for 10 - 15 minutes, then were washed with the deionized water and dried with the nitrogen gas at room temperature. Afterwards, the gold electrodes were affixed in the Teflon sample holder and vertically dipped in the growth solution and left in the preheated oven at 98˚C for 4 - 6 hours. Secondly, after the synthesis of nickel hydroxide the conversion of nickel hydroxide into NiO was performed by annealing the nickel hydroxide grown samples at 450˚C for 2 - 3 hours.
Scanning electron microscopy and X-ray differaction techniques were used for the morphological and crystal studies of prepared NiO nanostructures respectively. The prepared NiO nanostructures were used for the development of enzyme free glucose sensor and the electrochemical measurements were performed by the potentiostat/galvanostat FRA 2 µ Autolab type (III) model. The electrochemical cell used in the proposed work was a three electrode system, NiO nanostructures were used as the working electrode, platinum wire as counter electrode and silver-silver chloride wire as reference electrode. The glucose solution was prepared in the deionized water and all measurements were carried out in 0.1 M NaOH solution with successive addition of glucose solution.
3. Results and Discussions
3.1. The Structural Study of as Grown NiO Nanostructures on the Gold Coated Glass Substrate
The morphology of as prepared NiO nanostructures was investigated by using the scanning electron microscopy and the obtained nanostructures are shown in Figure 1. It can be seen from the Figure 1(a) that NiO nanostructures are composed of nanoflowers that are synthesized in alkaline pH medium [11]. The nanostructures resemble a rose flower like morphology with average thickness of 10 - 20 nm. Figure 1(b) shows the high magnification image of nanoflowers and it can be observed that nanostructures are highly dense, uniform and covered the whole substrate area. Energy dispersive X-ray analysis has shown the pure composition of NiO nanostructures presented in this study as shown in Figure 1(c). For the analysis of crystalline array of NiO nanostructures, X-ray differaction technique was used and the measured XRD spectrum is shown in Figure 2. The observed crystal planes include (012), (110) and (113) which are the characteristic peaks of NiO and supported by JCPDS card no: (47-1049). One of the NiO peak is merged with the very intense peak of gold due gold coating on the glass substrate.
3.2. The Cyclic Voltametric and Amperometric Measurement of the Proposed Enzyme Free Glucose Sensor Using NiO Nanostructures
The electro catalytic activity of NiO flower-like nanostructured electrode taword the oxidation of glucose was studied by cyclic voltammetry (CV). Figure 3 shows the CV response of the bare gold electrode and NiO grown gold electrode in the different concentrations of glucose in 0.10 M NaOH solution at the scan rate of 10 mV/s. As it can be seen from this Figure a clear oxidation wave is recorded; this wave is increasing wit the concentration of the glucose. Likewise, the oxidative potential was slightly shifted positively. Figure 4 shows the influence of scan rate, between 10 and 100 mV/s, on the current response. As it can be observed from this Figure 4(a) the oxidative current peak is enhanced with the raise of scan rate; the plot of the current vs the square root of the scan rate showed a linear behaviour indicating that the process is diffusion controlled.
Figure 5 shows the amperometric response of the NiO nanostructures based sensor electrode measured at + 0.5 V in 0.1 M NaOH solution. Upon the addition of different concentrations of glucose, it can be found that NiO