Recently, wearable and flexible electrochemical sensors have attracted tremendous attention due to health monitoring and diagnostics for humans [1]. Such wearable devices will be able to provide low-cost, easy-to-use, and mass production for in-situ detection or point-of-care applications. Urea is an essential biomarker for the diagnosis of healthy kidneys because urea, a protein metabolite, is formed by the function of kidneys and is excreted into the urine. An abnormal concentration of urea can indicate potential kidney dysfunction. Although most urea sensors are based on the enzyme, the electrochemical sensors to use the nonenzymatic urea oxidation can provide higher stability, biocompatibility, and nontoxicity if nonenzymatic urea sensors address the hurdles such as high conductivity, a narrow range of urea reactions, and limited catalytic activities [2-4]. In addition, a sweat-based urea sensor may show good anti-interference performances for urea detection as the urea concentration in the sweat is 3.6 times higher than that of serum [5].

Herein, we report a sensitive and selective nonenzymatic urea sensor [6]. Flexible and wearable platforms were fabricated using electrospinning polystyrene (PS) containing carbon nanotube as a conductive component. For nonenzymatic catalysts, Ni-Cu alloys were co-sputtered on the PS substrate and transformed to a Ni-Cu oxyhydroxide form by cyclic voltammetry treatment. Such an electrode structure with porous PS is believed to provide sufficient active sites, easy access to reactants, and adequate water wettability for effective charge transfer. The fabricated sensor presented an excellent linear response of 2.00-30.00 mM with a high sensitivity of 10.72 μAmM^–1 cm^–2 and 4.67 μM LoD for urea detection in neutral pH solution and artificial sweat. The dynamic bending test via 200 times and up to 150° angle bending showed excellent mechanical tolerance where bending deformation of the sensor platform does not affect electrochemical performance.

References

1. Bariya, Mallika, et al. “Wearable sweat sensors.” Nature Electronics 1, 160-171 (2018).
2. Debata, Suryakanti, et al. "Design of CdV₂O₄-V₆O₁₃ micro flowers for non-enzymatic electrochemical detection of urea." In AIP Conference Proceedings, vol. 2115, no. 1, p. 030058. AIP Publishing LLC (2019).
3. Yoon, Jaesik, et al. “Silver-Nanoparticle-Decorated NiOOH Nanorods for Electrocatalytic Urea Sensing.” ACS Applied Nano Materials 3, 7651-7658 (2020).
4. Yoon, Jaesik, et al. “Ag/ZnO Catalysts with Different ZnO Nanostructures for Non‐enzymatic Detection of Urea.” Electroanalysis 31, 17-21 (2019).
5. Thudichum, J. L. W. "On the analysis of urea in urine for clinical purposes." British medical journal 1, 38, 788 (1857).
6. Yoon, Jaesik, et al. “Flexible Electrochemical Sensor Based on NiCu(OOH) for Monitoring Urea in Human Sweat.” Journal of Electrochemical Society 168, 117510 (2021).