Non-Enzyme Urea Sensing with Ag Covered ZnO with Different Morphologies on Carbon Papers
Authors: Yoon Jaesik , Lee Doohee , Lee Eunji , Woo Sung Pil , Yoon Young Soo , Wang Yi , Kim Dong-Joo 
Published: February 2020

Urea is the nitrogenous waste substance of metabolism in the human body, and abnormal urea levels are associated with a variety of diseases such as renal failure, dehydration, and hepatic issues. For example, urea production and renal processing of urea, along with the causes of increased and reduced plasma/serum urea concentration are a major concern in physiological topic [1]. Thus, urea from the human urine and measurement of its concentration have been investigated for clinical application in the analysis of organic function [1-2]. Moreover, urea has attracted attention as a hydrogen source for fuel cells because of its stability, non-toxicity, and non-flammable properties [3].
Recently, many studies on non-enzyme urea sensing have been conducted due to the stability problems of enzymes. Non-enzymatic materials, e.g. Ni-based catalysts with peculiar structure [4] and 2-D graphene [5], have shown promising properties in the field of electrochemical sensors because of their unique properties such as remarkable surface area, good conductivity, and wide electrochemical window. Among them, approaches to high active surface areas are important in addition to exploration of various catalysts such as metal oxides, noble metals, and nickel-based catalysts due to its relation to the electro-catalytic activity towards urea oxidations. High surface area of the catalyst is critical for an excellent sensitivity, a low detection limit and a fast response time [6]. Although previous attempts have been made to have a high surface area for the urea detection, there is limited range of pH condition and Ni nanostructures for oxidation. As a result, ZnO or semiconducting metal oxide with a large surface to volume ratio and good crystallinity at nanoscale can be a good candidate for urea sensing [7].
In this work, covered Ag catalysts deposited on synthesized ZnO nanorods and nanoflakes on carbon substrates were prepared using low-temperature process for systematic investigation of surface area effect on non-enzymatic catalytic urea oxidation. The morphologies and structural properties of ZnO nanorods and nanoflakes with covered Ag were analyzed by SEM and XRD, and the quantity of the silver catalyst was measured by EDS. Thereafter electrochemical measurements of two catalysts have been conducted on a three-electrode cell in 1M urea containing 0.33 M urea. Electrochemical impedance spectroscopy (EIS) was also investigated for comparing the electron-transfer resistance. The surface effects of Ag/ZnO nanorods and nanoflakes electrodes for urea sensing were studied in terms of sensitivity and selectivity. Detailed discussion on the comparison of Ag/ZnO nanorods and nanoflakes surface effect will be given.
References

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Acknowledgement
This work was supported by Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD), the Ocean University of China-Auburn University (OUC-AU) Grants program, and the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20158520000210).