Toward selective electrochemical “E-tongue”: Potentiometric DO sensor based on sub-micron ZnO–RuO2 sensing electrode

Planar dissolved oxygen (DO) sensors based on thick-film ZnO–RuO2 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–RuO2–SEs have been investigated. Interference testing ascertained that the DO sensor based on sub-micron ZnO–RuO2-SE is insensitive to the presence of various dissolved ions including Cl−, Li+, SO42−, NO3−, Ca2+, PO43−, Mg2+, 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–RuO2-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 RuO2 nanostructures not only improves the structure of developed SE but also enhances the device's sensing performance.

► We examine ZnO-doped RuO2 sensing electrode of DO sensor.
► Study of ZnO-RuO2 confirmed the development of high surface-to-volume ratio.
► Developed sensing electrode is insensitive to the presence of various dissolved salts.
► 20 mol% ZnO-doped RuO2 sensing electrode enables maximum DO sensitivity.
► We conclude that DO sensor based on ZnO–RuO2 electrode can work at 11–30 °C.