Mesoporous Nickel Oxide (NiO) Nanopetals for Ultrasensitive Glucose Sensing
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Glucose sensing properties of mesoporous well-aligned, dense nickel oxide (NiO) nanostructures (NSs) in nanopetals (NPs) shape grown hydrothermally on the FTO-coated glass substrate has been demonstrated. The structural study based investigations of NiO-NPs has been carried out by X-ray diffraction (XRD), electron and atomic force microscopies, energy dispersive X-ray (EDX), and X-ray photospectroscopy (XPS). Brunauer–Emmett–Teller (BET) measurements, employed for surface analysis, suggest NiO’s suitability for surface activity based glucose sensing applications. The glucose sensor, which immobilized glucose on NiO-NPs@FTO electrode, shows detection of wide range of glucose concentrations with good linearity and high sensitivity of 3.9 μA/μM/cm2 at 0.5 V operating potential. Detection limit of as low as 1 μΜ and a fast response time of less than 1 s was observed. The glucose sensor electrode possesses good anti-interference ability, stability, repeatability & reproducibility and shows inert behavior toward ascorbic acid (AA), uric acid (UA) and dopamine acid (DA) making it a perfect non-enzymatic glucose sensor.
KeywordsNiO nanopetals Electrochemical Sensing Glucose
Diabetes, a chronic disease in which glucose level increases in blood and if undiagnosed and untreated, can be very hazardous for health and eventually may lead to death [1, 2]. Different therapy regimes in the management of diabetes include drugs’ dose adjustment according to the level of glucose in the blood as a result of compromised insulin level, main cause of the disease. Hence, accurate and reliable glucose sensor to sense the level in the blood is the most important parameter in managing diabetes. Generally, glucose sensor works on the use of an enzyme, glucose oxidase (GOx), which converts glucose into gluconic acid and H2O2 [3, 4, 5, 6, 7]. The concentration of glucose is determined by monitoring the number of electrons flowing through electrode for the formation of hydrogen in the form of peroxide . In enzymatic biosensors, quantitative sensing is done by controlling the potential and measuring the current as a result of substance (to be sensed) reacting with the active area of the material (acting as sensor) on the working electrode. Enzymatic glucose sensors, working on the same principle, display high sensitivity to glucose. Limitations with these sensors include their shorter life span, the environmental conditions such as temperature, pH value, and toxicity of the chemical used. To address these issues, many metal oxide-based non-enzymatic glucose sensors have been developed in recent time [9, 10, 11, 12, 13, 14]. The sensing mechanism of these non-enzymatic glucose sensors is based on oxidation of glucose, by metal-oxide ion near the surface of the electrode, to gluconolactone. In electrochemical sensing, cyclic voltammetry (CV) proves to be an efficient technique due to its high sensitivity at low detection limits, accurate quantitative analysis, and fast and clear characterization [15, 16]. These oxide-based glucose sensors certainly have potential to be used in real diagnosis and need further study.
There are increasing interests on fabrication of electrodes with low-cost metal-oxide materials, such as NiO, CuO, TiO2, ZnO, and composites which can show high sensitivity toward glucose by improving electro-catalytic activity [17, 18, 19, 20, 21, 22, 23, 24]. When it comes to reaction-based sensing, nanomaterials could be of interest as they can provide more surface area for reaction and hence better sensing. In recent times, a variety of materials in nanostructured form have shown great potential in sensing, electronics, and optoelectronics [25, 26, 27]. Established fact about nanostructures is the capability of tailoring a physical property by changing its size and/or morphology which gives the versatility to the nanomaterials to be used in diverse applications. Hence, for sensors also design of electrodes surface is one of the key parameters. Amongst plenty, Ni-based nanomaterials exhibit remarkable properties, such as catalysis [28, 29, 30] and high sensitivity due to large surface-to-volume ratio. An economic yet sensitive glucose sensor can be a reality with NiO nanostructure-based sensors by appropriately designing the device and synthesizing the material. In this paper, a working electrode consisting of petal-like NiO nanostructures for glucose sensing via electrochemical study has been fabricated to be used as the active compound. Fluorene-doped tin oxide (FTO)-coated conducting glass substrate has been used to grow the NiO nanostructures (NSs) by hydrothermal technique.
Nickel nitrate precursor mixed with potassium persulfate in the presence of less amount of ammonium solution has been used for the alignment during the preparation of these NiO NSs. After 5 h of continuous heating at 150 °C, deposited film was rinsed with deionized water and dried in air. Subsequently, the NiO-NSs film was annealed at 250 °C for 2 h. Uniform and well-aligned NiO NSs were obtained on the conducting surface of FTO-coated glass. The microstructure of the film was investigated by a XRD (Rigaku SmartLab X-ray diffractometer using monochromatic Cu-Kα radiation λ = 1.54 Å) along with electron microscopy (Supra55 Zeiss). Energy dispersive X-ray spectroscopy (Oxford Instrument) and X-ray photoelectron spectrometer (ESCA System, SPECS GmbH, Germany) with Al Kα radiation (1486.6 eV) have been used for the elemental confirmation. Atomic force microscopy has been performed on a Bruker (MultiMode 8-HR) machine, and analysis of high-resolution nanostructures were carried out using WSxM software . For glucose sensing with NiO-NSs, appropriate electrochemical measurements have been performed using Keithley 2450-EC electrochemical work station. Brunauer–Emmett–Teller (BET) method was also employed on Autosorb iQ, version 1.11 (Quantachrome Instruments) for surface analysis.
Results and Discussion
As mentioned earlier, basis of the sensing mechanism is the reactivity of glucose with NiO thus needing higher surface areas, which should be analyzed before investigating the sensing properties. The specific surface area and other parameters, like type of isotherm, average pore size, and total pore volume have been obtain by the N2 adsorption/desorption using BET method. Figure 2d reveals type IV isotherm and type-H3 hysteresis when measured at 77 K with the relative pressure range of 0.025 ≤ P/P0 ≤ 1.00 . The measured surface area, estimated by BET and Langmuir methods in the P/P 0 range of 0.05–0.30, is found to be 114.936 m2/g and pore size distribution around 3.7 nm. This indicates NiO NPs are mesoporous with relatively uniform pore size distribution. The total pore volume in the sample is found to be 0.267 cm3/g as estimated at a relative pressure (P/P0) of 0.99.
During CV measurement, Ni2+ oxidizes into Ni3+ by aqueous electrolytic solution present in the cell at NiO-NPs@FTO electrode (reaction 1). Oxidized Ni3+ works as catalyst for glucose and oxidizes glucose by reducing itself (reaction 2). On oxidation, glucose converts to gluconolactone which consequently gets converted immediately to gluconic acid (reaction 3) and this compound reacts with water molecules to form gluconate and hydronium ions (reaction 4). These ions near the surface of working electrode result in increased current as detectable signal with a very good specific sensitivity of 3.9 μA/μM/cm2.
In order to further support the “glucose-doping” induced enhancement in electric conductivity, electrochemical impedance spectroscopy (EIS) of NiO NP-fabricated working electrode has been measured with and without glucose (Fig. 3d). A single depressed semicircle in the high-frequency region and an inclined line in the low frequency region can be seen in the Nyquist (cole-cole) plot in Fig. 3d. Generally, the high-frequency semicircle shows the electrochemical reaction impedance between the glucose present in the electrolytic solution and NiO nanostructure interface, whereas inclined line in the lower frequency region shows the active material (NiO) and conducting electrode interface impedance . Effect of glucose on the cole-cole plot in Fig. 3d is clearly distinguishable, and thus, the same measurement can be utilized to sense the presence of glucose. This clearly exhibits the glucose sensing property of the material which is nanopetal shaped NiO NSs.
Comparative study of analytical performance of the NiO-NPs@FTO-fabricated glucose sensor
Type of electrode
Detection potential (V)
Ti/TiO2 nanotube arrays/Ni
Wang et al. 
Yang et al. 
Ni nanoparticles loaded MWCNT
Nie et al. 
Ni nanoparticles loaded carbon nanofibers
Liu et al. 
3D porous Ni nano-network
Niu et al. 
In this work
In summary, an excellent glucose sensing behavior with improved sensitivity has been achieved by using an electrode with hydrothermally grown highly dense, aligned NiO nanostructures (NSs), with high surface to volume ratio. The NiO NSs, grown by the simple technique, show better glucose sensing capabilities in terms of stability and sensitivity as compared to its counterparts grown by some others technique. The proposed sensor electrode demonstrates wide range of detection of glucose concentrations with high-specific sensitivity of 3.9 μA/μM/cm2 and a fast response time of less than 1 s. In addition to this, it shows inert response to the other enzymes present with glucose like ascorbic acid, folic acid, and uric acid, which makes it efficient non-enzymatic glucose sensor. All these obtained results indicate that the proposed glucose sensor can be an efficient analytical tool for the monitoring of glucose concentrations in drugs, human serum, and can be used in biomedical-related applications.
Authors acknowledge financial support from the Department of Science and Technology, Govt. of India. Authors are thankful to SIC facility provided by IIT Indore and Mr. Kinny Pandey for his assistance. Authors are thankful to Dr. U. Deshpande (UGC-DAE CSR Indore) for XPS analysis. Authors acknowledge Dr. J. Jayabalan and Dr. Rama Chari (RRCAT, Indore) for useful discussion and providing AFM facility. One of the authors (SM) is also thankful to MHRD, Govt. of India for providing fellowships.
This study is based on the work support from the Department of Science and Technology, Govt. of India and support by the MHRD for providing the fellowship.
SM planned and performed the experiments, analyzed the data, and drafted the manuscript. PY helped in editing the manuscript. The whole project was planned under the direction of RK who conceived the idea and designed the experiment and lead the research work. RK and PRS revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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