Electrochemical sensing mechanism of ammonium ions over an Ag/TiO 2 composite electrode modified by hematite.

Here, we demonstrate a new electrochemical sensing mechanism of ammonium ions (NH 4 + ) involving a two-electron oxygen reduction reaction (ORR) and a hydrazine reaction. The NH 4 + are electrooxidized to hydrazine by H 2 O 2 derived from the ORR over a self-supporting Ag/TiO 2 nanotube array composite electrode modified by hematite (Ag/Fe 2 O 3 /TNTs). The Ag/Fe 2 O 3 /TNT sensor exhibits a high sensitivity of 1876 µA mM -1 cm -2 with a detection limit of 0.18 µM 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.