NH4+-selective electrode with superhydrophobic solid contact for actual wastewater monitoring

Abstract

Ion-selective electrodes (ISEs) have significant superiority in monitoring water samples and various functional materials have been explored as solid contacts (SCs) to improve the detection reliability. However, water layer usually destroys the interfaces among electron collector, SCs and ion-selective membranes, especially in monitoring complicated actual wastewater, resulting in instability or loss of function. Here we demonstrate that surface hydrophobization of SCs is a feasible approach to minimize adverse effect of water layer on unstable SCs adhered to electron collector, ensuring long-term reliability of ISEs in actual wastewater monitoring. Though NH4+-ISE with MoOx @polyaniline composite SC has low standard deviations (SDs) of the standard potential (E0) measured in NH4+ standard solutions, Nernstian slopes are dramatically reduced from 58.66 to 27.65 mV/dec after 16-day actual wastewater monitoring. Accordingly, measured NH4+ concentrations of actual wastewater have a high degree of inaccuracy up to ca. 850%. In contrast, the ISE with a superhydrophobic SC has a slope closer to the ideal Nernstian slope, lower potential drift and E0 SDs, and higher resistance against external interferences, due to water layer inhibition of the SC. Importantly, superhydrophobic SC-ISE shows a near-Nernstian response in steady state and accurate NH4+ concentration determination during 30-day actual wastewater monitoring.