Plasma assisted surface treatment of solution deposited CuO:NiO mixed oxides thin film for nonenzymatic glucose detection

Abstract

The amount of people with diabetes mellitus is rapidly increasing with approximately 4.6 million South African sufferers in 2019. Self-management of the illness which includes daily blood glucose concentration tests is costly, stressful, and painful. The most used method for detecting glucose concentration involves an enzymatic sensor. Non-enzymatic sensors, which use metals to directly oxidise glucose, have the potential to replace expensive and complicated enzymatic sensors. Numerous studies have shown that metals, alloys and bimetallic, metal oxides and composites, and carbon-based compounds can be used as catalytic materials for glucose oxidation. Literature suggests that Cu, which is found in natural abundance, has good catalytic properties and stability, and is relatively inexpensive. Furthermore, copper oxide has superior electrocatalytic properties compared to pristine copper. The multivalent states of copper oxide enhance recovery of the electrocatalyst of the electrode surface. However, compared to other metal oxides such as NiOx and CoxOx, copper oxide has a lower conductivity, decreased electroactivity, and limited surface area. Electrocatalytic properties are more tuneable in multi-element nanostructures in comparison to pristine nanostructure. Therefore, the electroactivity of copper oxide is improved by the addition of nickel oxide. Plasma assisted nitrogen doping is used to improve the exposed surface area by etching the surface of the electrode and induces partial conversion of copper (II) oxide into copper (I) oxide. This would be favourable for copper oxide due to the nature of multivalent states which enhances recovery of the electrocatalyst, enhances lifespan of the sensor and introduces a different element into the multielement structure. To synthesise an electrode, nanostructures are fabricated through various techniques and cast onto a conductive substrate with a binder. Adhesion of the nanostructures to the conductive substrate presents a challenge and the binder used then reduces the electrical conductivity of the electrode. Direct growth of nanostructures onto the conductive substrate can be achieved through hydrothermal techniques or plasma sputtering. Challenges involved in these methods include controlling film thickness and homogeneity and high capital costs. Previously, a low-cost solution deposition-based technique had been used to deposit Co3O4 thin films. However, the biggest drawback was controlling the phase and morphology of the deposited film. The interest in this work is therefore to deposit (a) CuO:NiO thin films and (b) induce phase transformation of CuO to Cu2O by plasma assisted nitrogen doping. It was rationally designed to utilise the mixed oxidation sate of CuO/Cu2O together with the NiO doping for enhanced electrochemical activity towards glucose oxidation. It will be shown that the as-developed plasma-assisted nitrogen doped mixed oxide (N-CuO/Cu2O:NiO) thin film has excellent glucose sensing abilities with very high selectivity and an ultrafast response time. The N-CuO/Cu2O:NiO sensor was synthesised by making a precursor solution of copper oleate and nickel nitrate. The precursor was then spin coated onto the conductive substrate followed by calcination resulting in mixed oxides of CuO:NiO. Thereafter, the electrodes underwent plasma assisted nitrogen doping. Physical characterisation was performed on the developed electrodes using XRD, XPS, SEM, EDS, and Hall Effect measurements. Electrochemical characterisation was used to compare the glucose sensing abilities of the developed sensors with a pristine CuO sensor. The pristine host structure of the developed sensor consisted of CuO. Furthermore, NiO nanostructures were present within the host structure of CuO. The SEM results showed that NiO was present in the form of cube-like structures bonded with the CuO. Nitrogen was doped into the electrode though plasma assisted nitrogen doping and induced phase conversion of CuO to Cu2O. Electrochemical glucose testing showed that the as-developed sensor (labelled as N-CuO/Cu2O:NiO) showed an ultra-fast response time of 2.5 s with high sensitivity (1131 μA/mM.cm2). The linear range of the sensor was calculated to be up to 2.74 mM of glucose and excellent selectivity towards glucose at an applied potential of +0.67 V vs Ag/AgCl in 0.1 M NaOH electrolyte solution. The limit of detection was 20 μM for the N-CuO/Cu2O:NiO sensor. The N-CuO/Cu2O:NiO have smaller Tafel slope compared to pristine CuO and CuO:NiO mixed oxides. Enhanced electrochemical performance of the N-CuO/Cu2O:NiO originates from the improved electronic properties of the thin film.