Non-Enzymatic Paper-based Urea Sensor with Hierarchical NiO Catalysts

Abstract

Urea has attracted attention because of its various potential applications, such as hydrogen production, fuel cells, fertilizers, and electrochemical sensors. Furthermore, as an end-product of human metabolism, urea is essential for analyzing various metabolic disorders, such as liver disease and renal function. In order to overcome the stability issue associated with enzyme-based sensors, a non-enzymatic nickel-based oxide catalyst was explored to fabricate a highly efficient urea sensor. The research aims to develop non-enzymatic catalysts (NiO) for use in urea electrochemical sensors and to increase sensitivity through synthesis (hierarchical structure), process (electrophoretic deposition), and flexible electrodes (cellulose paper). First, a hierarchical NiO catalyst by a one-pot hydrothermal method was synthesized as a non-enzymatic catalyst. By introducing the SDBS soft template, the synthesis process became simpler by eliminating the removal process of a hard template. The hierarchical structure, in which 2D building block nanosheets comprise 3D hollow structures, could be achieved by the SDBS soft template during hydrothermal synthesis. The morphological modification of the NiO catalyst leads to a highly sensitive electrochemical urea sensor. Second, the modification of morphology through the catalyst loading method was investigated by electrophoretic deposition (EPD). For EPDs, direct current (DC) electric fields are commonly used. When alternating current (AC) was introduced to the EPD process, however, the deposition behavior was modified; therefore, the current response toward urea was increased over DC-EPD electrodes. A more environmentally friendly catalyst loading process was also achieved by using water solvent with AC EPD as well as maintaining a modified packing behavior. Third, as a substrate modification, the paper substrate was developed. The paper has the advantage of high surface area, cost, biocompatibility, and disposability, which makes it an ideal substrate material for use as point-of-care equipment. Having a complex cellulose network in the paper, it can be used to further enhance the electrochemical reaction. As the paper lacks electron conductivity, the mixed solution of CNT and NiO was used by means of the capillary property of the paper. The various design of paper electrodes were considered, and the optimization of electrode configuration was carried out to increase the current response toward urea. In the dissertation, three approaches were investigated to develop a highly sensitive non-enzymatic urea sensor: hierarchical catalyst synthesis by hydrothermal method, modified loading method by AC-EPD, and cellulose networks on paper as a substrate.