Carbon dioxide sensors based on a surface acoustic wave device with a graphene–nickel–l-alanine multilayer film

Abstract

Surface acoustic wave (SAW) sensors containing graphene–nickel (Ni)–L-alanine composite sensing films for carbon dioxide (CO₂) detection were investigated. ST-cut quartz SAW resonators were modified with L-alanine, Ni nanoparticles and graphene by electrodeposition. The presence of graphene markedly enhanced the CO₂ sensing properties of the sensor. The performance of the sensor containing a graphene–Ni–L-alanine composite film depended both on the pH of the solution used to deposit the L-alanine sensitive layer and operation temperature. The CO₂ sensing mechanism of the SAW sensor is based on the adsorption of CO₂ and H₂O gas molecules by graphene, the catalytic reaction of Ni nanoparticles, and the reaction between L-alanine and CO₂ gas molecules; that is, the three materials in the sensitive layer have a synergistic effect. From the analysis of changes in acoustic signals, exposure of the sensor to CO₂ not only changed the conductivity of the film but also produced an additional capacitance, which ultimately changed the equivalent capacitance of the sensor.