Electrochemical sensing mechanism of ammonium ions over an Ag/TiO2 composite electrode modified by hematite

Here, we demonstrate a new electrochemical sensing mechanism of ammonium ions (NH4+) involving a two-electron oxygen reduction reaction (ORR) and a hydrazine reaction. The NH4+ are electrooxidized to hydrazine by H2O2 derived from the ORR over a self-supporting Ag/TiO2 nanotube array composite electrode modified by hematite (Ag/Fe2O3/TNTs). The Ag/Fe2O3/TNT sensor exhibits a high sensitivity of 1876 mu A mM(-1) cm(-2) with a detection limit of 0.18 mu M under non-alkaline conditions, a short response time of 3 s, good reproducibility, and fine selectivity among various interferents, and is also successfully used in real water bodies to display high accuracy. Furthermore, this new mechanism has a certain universality in a range of Ag (main catalyst)/transition metal oxide (cocatalyst)/TNT sensing systems. This work offers a new design basis for the urgently needed electrochemical ammonia nitrogen sensors.