Urea detection in phosphate buffer and artificial urine

A simplified kinetic model for the quantitative analysis of a potentiometric, pH-based urea biosensor is presented. The device was an electrolyte–insulator–semiconductor ca- pacitor (EISCAP) with a pH-sensitive Ta2O5 gate functionalized by a polyallylamine hy- drochloride (PAH)/urease bilayer. Within the steady-state approximation, the kinetic equations yielded an implicit algebraic relation linking the bulk urea concentration to the local pH at the sensor surface. Numerical solution of this equation, combined with a fitting routine, provides the apparent Michaelis–Menten constant (K ) and the normalized max- imum reaction rate (k ). Validation against the literature data confirmed the reliability of the approach. Experimental results were then analyzed in both phosphate buffer (PBS) and artificial urine (AU), covering urea concentrations of 0.1–50 mM. The fitted parame- ters showed comparable K values of 10.9 mM (PBS) and 32.4 mM (AU), but strongly different k values: 2.2 10 (PBS) versus 8.6 10 (AU). The three-order reduc- tion in AU was attributed to the inhibitory effects inherent to complex biological fluids. These findings highlight the importance of the model-based quantitative analysis of EISCAP biosensors, enabling the accurate characterization of immobilized enzyme layers and guiding optimization for applications in realistic sample matrices.