Wearable and flexible electrochemical sweat analysis for monitoring health-related urea concentrations with high sensitivity and selectivity is highly required for individual medical care and disease diagnosis. Herein, we report a sensitive and selective sweat sensor based on a flexible NiCu(OOH)/polystyrene (PS) electrode to detect a urea biomarker. The non-enzymatic sensor was fabricated using electrospinning PS containing carbon nanotube as a conductive component and co-sputtering Ni-Cu alloys as a catalyst. The flexible PS provided a porous structure, leading to sufficient active sites, easy access to reactants, and adequate water wettability for effective charge transfer. The sputtered Ni-Cu alloys deposited on the PS were transformed to a Ni-Cu oxyhydroxide form by cyclic voltammetry treatment, managing the detection of the urea molecule in a neutral pH environment. This urea sensor displayed an excellent linear response with a sensitivity of 10.72 μAmM–1cm–2 toward a physiologically appropriate linear range of 2.00–30.00 mM, and negligible interferences from co-existing common species. Furthermore, bending tests demonstrated excellent mechanical tolerance where electrochemical performance was not affected under 200 cycles and 150° bending. The flexible electrochemical urea sensor platform can provide noninvasive monitoring of urea levels in sweat fluids, ensuring clinical diagnosis for biomedical applications.

We investigated CO oxidation behavior of doped cerium oxide fibers. Electrospinning technique was used to fabricate the inorganic fibers after burning off polymer component at 600 °C in air. Cu, Ni, Co, Mn, Fe, and La were doped at 10 and 30 mol% by dissolving metal salts into the polymeric electrospinning solution. 10 mol% Cu-doped ceria fiber showed excellent catalytic activity for low temperature CO oxidation with 50% CO conversion at just 52 °C. This 10 mol% Cu-doped sample showed unexpected regeneration behavior under simple ambient air annealing at 400 °C. From the CO oxidation behavior of the 12 samples, we conclude that absolute oxygen vacancy concentration estimated by Raman spectroscopy is not a good indicator for low temperature CO oxidation catalysts unless extra care is taken such that the Raman signal reflects oxide surface status. The experimental trend over the six dopants showed limited agreement with theoretically calculated oxygen vacancy formation energy in the literature.

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 Bariya, Mallika, et al. “Wearable sweat sensors.” Nature Electronics 1, 160-171 (2018). 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). Yoon, Jaesik, et al. “Silver-Nanoparticle-Decorated NiOOH Nanorods for Electrocatalytic Urea Sensing.” ACS Applied Nano Materials 3, 7651-7658 (2020). Yoon, Jaesik, et al. “Ag/ZnO Catalysts with Different ZnO Nanostructures for Non‐enzymatic Detection of Urea.” Electroanalysis 31, 17-21 (2019). Thudichum, J. L. W. "On the analysis of urea in urine for clinical purposes." British medical journal 1, 38, 788 (1857). Yoon, Jaesik, et al. “Flexible Electrochemical Sensor Based on NiCu(OOH) for Monitoring Urea in Human Sweat.” Journal of Electrochemical Society 168, 117510 (2021).