Effects of Electrodeposition Conditions and Protocol on the Properties of Iridium Oxide pH Sensor Electrodes

The properties of iridium oxide pH sensors produced by electrochemically induced deposition on gold electrodes were examined as a function of the composition of the deposition solution, as well as the electrochemical deposition protocol. The composition of the Ir(IV) deposition solutions, which included oxalate or ethylene diamine tetraacetic acid complexing agent or no complexing agent, had no effect on the slope of the calibration curves. The slope of the calibration curves was shown to increase from ca. 49 to 76 mV/pH unit with the fractional coverage of gold substrates with iridium oxide. Increasing the film thickness beyond the full coverage did not further increase the slope of the calibration curves but resulted in a progressive increase of their intercept values. The method of deposition, which involved a constant current, single potential pulse, alternating potential pulse, or cyclic potential protocol, affected the maximum rate of pH response as well as the capacitance of the iridium oxide sensors. The latter two properties of the sensors were investigated using a microelectrochemical time-of-flight method with galvanostatic proton generation and potentiometric sensing. The alternating potential pulse and cyclic potential methods produced films of smaller rate of pH response and of smaller capacitance relative to the iridium oxide films of the same thickness produced by the other two methods. This is likely due to a smaller microscopic porosity of the films prepared by the potential pulse and cyclic potential methods. The maximum rate of pH response obtained with 50 nm thick iridium oxide sensors varied from ca. 7 to 23 V/s. The specific capacitance of the iridium oxide films varied from ca. 900 to 9000 F/cm(3).