A Disposable Paper-Based Urea Sensor Using Metal Oxides
Authors: Lee Doohee , Yoon Jaesik , Lee Eunji , Yoon Young Soo , Kim Dong-Joo 
Published: May 2020

The level of urea in urine can be a crucial biomarker to detect kidney failure, dehydration, renal function, and hepatic failure. [1] Thus, rapid and accurate sensing of urea is essential in a diagnostic clinic, especially for point-of-care testing devices (POCT). Paper-based analytical devices have been demonstrated as POCT due to its disposability, portability, low cost, and environmental-friendly quality. [2] In order to measure urea, electrochemical biosensors based on the enzyme were explored, but the application of enzyme has its limitations due to low shelf-life, complicated immobilization procedure, high cost, and complicate operating condition which is not fit to POCT application. [3] To overcome the limitations of enzyme, non-enzymatic urea sensor have attracted from researchers due to advantages such as good stability, re-usability, high sensitivity, redox flexibility, simplicity, and low cost. [4]
As a non-enzymatic catalyst, nano metal oxides such as NiO, SnO2, and Fe2O3, exhibited catalytic activity on urea by the formation of the redox couple on the sensor surface.[5] [6] [7] To utilize non-enzymatic catalysts with comparable properties of enzyme, the development to increase reaction surface area has been emphasized by using nanomaterial, hierarchical structure, and support matrix.[8] [9] However, those processes involve complicate fabrication steps and often need additional post-process to remove the template leading to poisoning of the catalyst. As a potential approach for template-free process, paper can be used for an electrochemical sensor substrate. Paper has a complex fiber matrix that can provide pore pathways for the electrolyte to infiltrate inside, and by distributing catalyst uniformly within the paper, which formsmore reaction sites. Therefore, enhancement of performance and simplification of the fabrication can be achieved with the use of paper.
In this study, the simple, disposable paper-based device was developed for urea electro-oxidation by using paper matrix as support. The paper-based electrode was fabricated by a screen printing method with filter paper, and different combinations of nano-metal oxides, NiO, SnO2, and Fe2O3. The metal oxide catalysts were loaded on the working electrode area and compared the performance. Structure and morphology of paper-based electrode were characterized by XRD and SEM. Investigation of selectivity and LOD was conducted by potentiostat. A detailed description of the fabrication of paper-based electrode and sensing performances will be presented.
References
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[3] Riklin, Azalla, et al. "Improving enzyme–electrode contacts by redox modification of cofactors." Nature 376.6542 (1995): 672.
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[7] Das, Gautam, et al. "NiO-Fe2O3 based graphene aerogel as urea electrooxidation catalyst." Electrochimica Acta 237 (2017): 171-176.
[8] Wu, Mao-Sung, Ren-Yu Ji, and Yo-Ru Zheng. “Nickel Hydroxide Electrode with a Monolayer of Nanocup Arrays as an Effective Electrocatalyst for Enhanced Electrolysis of Urea.” Electrochimica Acta 144 (October 2014): 194–99.
[9] Liang, Yanhui, et al. "Enhanced electrooxidation of urea using NiMoO4· xH2O nanosheet arrays on Ni foam as anode." Electrochimica Acta 153 (2015): 456-460.
Acknowledgement
This work was partially 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.