Integrated urea sensor module based on poly(3-methylthiophene)-modified p-type porous silicon substrate

Integrated three-electrode system module composed of a porous silicon (PS)-based sensing electrode, an Ag/AgCl thin film reference electrode (RE_tf), and a Pt thin film counter electrode (CE) was fabricated for monitoring urea level of artificially-prepared body fluid. After thermal evaporation of the 200 nm thick Ag on the Ti-underlayered planar p-type silicon (p-Si) substrate, the Ag Film was oxidized in a FeCl₃ solution to obtain the Ag/AgCl RE_tf. Multi-layered RE_tf was clearly shown from results of the auger electron spectroscopy (AES) depth profile. The nernstian slope of the RE_tf also showed good reproducibility. The PS layer was formed by electrochemical anodization with applying constant current to the p-Si substrate in an ethanolic HF solution and the macro PS (2 μm diameter and 10 μm depth) was obtained. The electrochemical active area (A_ea) of the PS-based Pt thin film electrode was determined from the cyclovoltammetric result of redox reactions of K₃Fe(CN)₆ on the electrode surface and compared with the A_ea of the planar silicon (PLS)-based Pt film substrate. After electropolymerization of the conductive poly(3-methylthiophene) (P3MT) on the PS-based Pt thin film, urease (isoelectric point ≈ 4.1) molecules were electrostatically doped into the P3MT film by applying positive bias. Amperometric calibration curves for both PS- and PLS-based sensing electrodes were compared in the range of 0.1–125 mM urea concentrations and the cross-sectional scanning electron microscopy (SEM) image of the PS-based sensing electrode was also shown.