Toward selective electrochemical “E-tongue”: Potentiometric DO sensor based on sub-micron ZnO–RuO 2 sensing electrode

Abstract

Planar dissolved oxygen (DO) sensors based on thick-film ZnO–RuO 2 sensing electrodes (SEs) with different mol% of ZnO were prepared on the alumina substrates using a screen-printing method and their structural and electrochemical properties were closely studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), electrochemical impedance spectroscopy (EIS) and energy-dispersive spectroscopy (EDS) techniques. Structural and electrochemical properties of ZnO–RuO 2–SEs have been investigated. Interference testing ascertained that the DO sensor based on sub-micron ZnO–RuO 2-SE is insensitive to the presence of various dissolved ions including Cl −, Li +, SO 4 2−, NO 3−, Ca 2+, PO 4 3−, Mg 2+, Na + and K + within a concentration range of 10 −7 to 10 −1 mol/L for DO measurement from 0.5 to 8.0 ppm in the test solution at a temperature range of 11–30 °C. These dissolved salts had practically no effect on the sensor's output potential difference response, whereas Br − ions had some effects at concentration more than 10 −3 mol/L. The relationship between DO and the sensor's potential difference was found to be relatively linear with the maximum sensitivity of −50.6 mV per decade was achieved at 20 mol% ZnO at 7.35 pH. The response and recovery time to pH changes for the planar device based on 20 mol% ZnO–RuO 2-SE was found to be 10 and 25 s, respectively, at a temperature of 25 °C, whereas the response time to DO changes at the same temperature was about 60 s. It has also been demonstrated that the appropriate amounts of mixed ZnO and RuO 2 nanostructures not only improves the structure of developed SE but also enhances the device's sensing performance.