Ion sensing properties of vanadium/tungsten mixed oxides

Vanadium/tungsten mixed oxide (V₂O₅/WO₃) sensing membranes were deposited on glassy carbon substrates and used as the H⁺ sensor of the extended gate field effect transistor (EGFET) device. X-ray diffractograms indicated a decrease of the interplanar spacing of V₂O₅ after the insertion of WO₃ revealing that the lamellar structure is under compressive stress. The crystallinity increases with increasing WO₃ molar ratio. The film is not homogeneous, with more WO₃ material sitting at the surface. This influences the response of pH sensors using the EGFET configuration. The maximum sensitivity of 68 mV pH⁻¹ was obtained for the sample with 5% WO₃ molar ratio. For higher WO₃ molar ratios, the behavior is not linear. It can be concluded that V₂O₅ dominates for acidic solutions while WO₃ dominates for basic solutions. Therefore, the mixed oxide with low amount of WO₃ is the main candidate for further use as biosensor.