A High Accuracy QCM Based Sensing System for in Water Ammonia Monitoring

Ammonia is among the most popular nutrients for plants and water ecosystems although in, it is also one of the most common water pollutants such that it is used as an important indicator of its quality and, more general, of the health of the global marine ecosystem. In this regards, monitoring of ammonia in water, even in low concentration, arouses ever increasing interest in scientific community, increasingly involved in the environmental protection. In this paper we investigate the feasibility of an in-liquid ammonia detection system based on a quartz crystal microbalance (QCM) and an oscillator circuit. The QCM surfaces have been functionalized with a self-assembled monolayer (SAM) of 3-mercaptopropionic acid in order to exploit the electrostatic attraction between the gold (Au) quartz electrodes surface, covered by the deprotonated carboxylic groups, and the NH4+ ions in aqueous solution. Preliminary measurements have been performed and the quartz resonance frequency and the motional resistance have been simultaneously acquired in real time when the quartz surface is exposed to liquid solutions of ammonia at different concentrations. Experimental results show satisfactory ammonia sensing properties proving the formation of a strong bond between the functionalizing layer and the NH4+ ions even at the lower tested concentration with an effective change of the physical properties of the sensing layer related to the molecular bond.