Aqueous Ammonia Sensor with Neuromorphic Detection

Abstract

A hybrid inorganic–organic neuromorphic sensor utilizing a thin film zinc oxide (ZnO) detector with organic neuromorphic pre-processing is developed to quantify ammonia in aqueous environments, including biological analytes. Impedimetric ZnO sensor, connected to an organic somatic circuit, reliably and accurately detects changes in electrical impedance to measure and quantify variations in the concentration of ammonia. The sensing mechanism of the ZnO thin film sensor is hypothesized to be the cause of the decrease in resistance of a solution with an increase in ammonia concentration. It is found that the surface oxide of the ZnO layer reacts with even very low concentrations of ammonia ($N{H}_3$), leading to changes in resistivity. This makes the sensor capable of detecting ammonia in a range of concentrations between 0.0001 and 0.1 M. A neuromorphic circuit converts the analog change of ammonia concentration expressed as a change of sensor impedance to the digitized frequency of spikes. Detecting such a low ammonia concentration is critical for environmental monitoring and medical diagnosis. The digitized nature of neuromorphic signal pre-processing makes it more resilient for signal transmission in the presence of noise and serves as a demonstration of “smart sensing.”