Carbon-coated UiO-66(Zr)@MoS2 for the electrochemical measurement of urea in sweat

This study reports a novel electrochemical sensor based on a metal-organic framework (MOF) derivative to measure urea in sweat. The MOF structure, UiO-66(Zr), was greenly synthesized using a solvent-free thermal method, functionalized with molybdenum disulfide (MoS 2 ), and then thermally treated to form carbon-coated UiO-66(Zr)@MoS 2 . The MOFs were characterized by X-ray diffraction, an N 2 adsorption instrument, scanning electron microscopy, and an X-ray photoelectron spectrometer. Among the prepared MOFs, the carbon-coated UiO-66(Zr)@MoS 2 showed many crystal defects, providing more active sites required to improve the electrocatalytic activity, and high carbon content to enhance the electrical conductivity of the stainless steel (SS) electrode. The electrocatalytic activity of the MOFs was evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and square wave voltammetry (SWV). CV studies showed poor catalytic activity of bare SS electrode and its modifiers, UiO-66(Zr), carbon-coated UiO-66(Zr), and UiO-66(Zr)@MoS₂, towards urea (0.1 mM) oxidation. In contrast, the carbon-coated UiO-66@MoS₂ modifier exhibited a urea oxidation peak with a current value of 5 μA, confirming its high electrocatalytic activity. Compared to CV, SWV exhibited a higher oxidation peak current (7.5 μA). Additionally, the EIS showed a 38.4 % reduction in the electron transfer resistance after SS electrode modification with the carbon-coated UiO-66(Zr)@MoS₂. The SWV sensor exhibited lower detection and quantification limits of 0.4 and 1.4 μM, respectively. Additionally, it showed high selectivity in the presence of possible interfering substances, such as glucose, lactic acid, ascorbic acid, cortisol, and sodium chloride. Moreover, its efficacy for measuring urea in artificial sweat samples was proved with acceptable recovery values.