Non-Enzymatic Urea Biosensor Based on Silver-Nickel Oxyhydroxide Nanorods Composite Electrode
Authors: Yoon Jaesik , Lee Doohee , Lee Eunji , Yoon Young Soo , Wang Yi , Kim Dong-Joo 
Published: February 2020

Urea is an end-product of the metabolism of nitrogenous compounds, which can be detected in the blood as well as urine [1]. Ammonia catabolized by protein metabolism is converted into urea by liver and kidneys [2]. Consequently, the concentration of urea in the urine or blood is a suitable biomarker that monitors liver and kidney function. If a sensor platform can immediately determine the level of urea in the biological fluid, it will be valuable for many patients when estimating the concentration of urea in the blood.
Investigation on urea biosensors has been mainly guided out the advancement of enzyme-based catalysts [3]. However, the limitations such as denaturation of enzyme and mobilization to the catalysts lead to the lack of stability. Consequently, the improvement of non-enzyme-based catalysts can work the constraints mentioned above, thereby allowing the progression of urea biosensors. Many non-enzymatic catalysts have been investigated to catch urea molecules, including noble metals, transition metals, and metal hydroxide [4]-[6]. With them, the Ni-based catalysts are the most encouraging suggestion, because it gives excellent electrocatalytic efficiency, biocompatibility, nontoxicity, and high electron transfer. [7]. Although nickel-based catalysts are admitted being an excellent facilitator for the urea electrooxidation, published investigations are few in the nickel-based catalysts for biosensor applications. The inadequacy of research in the biosensor area is related to the mechanism of nickel-based catalysts in alkaline medium. Ni(OH)2 (Ni2+) is electrochemically oxidized to NiOOH (Ni3+) before electrocatalytic urea oxidation. However, since the pH of human blood is neutral (7.35-7.45), it is necessary to develop catalysts as a biomarker for urea electrooxidation which does not depend on pH. Therefore, the nickel-based catalysts intrinsically involving Ni3+ ion will more efficiently improve the electrooxidation performance of the urea.
In this study, A composite catalyst based on silver-nickel oxide hydroxide nanorods (Ag-NiOOH) was synthesized for non-enzymatic urea detection. The Ag-NiOOH was characterized by energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction techniques. The morphology and the structure of the Ag-NiOOH were investigated using scanning electron microscopy. The Ag-NiOOH coated carbon paper was employed to fabricate a new electrochemical sensor for urea detection. The Ag-NiOOH/carbon paper electrode showed a very high sensitivity of 177 μAmM−1cm−2, a low detection limit of 5 μM and a response time of 1.5 s. Detailed discussion on the performance of Ag-NiOOH and the role of inherent Ni3+ ions as a catalyst in urea biosensor will be given.
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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) and the Ocean University of China-Auburn University (OUC-AU) Grants program.