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

Abstract

Integrated three-electrode system module composed of a porous silicon (PS)-based sensing electrode, an Ag/AgCl thin film reference electrode (REtf), 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 REtf. Multi-layered REtf was clearly shown from results of the auger electron spectroscopy (AES) depth profile. The nernstian slope of the REtf 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 mu m diameter and 10 mu m 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 a parts per thousand 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.