PH measurements with ZvO based Surface Acoustic Wave Resonator

This paper investigated pH measurements using ZnO based Surface Acoustic Wave Resonator (SAW). The resonant frequency of the SAW decreased as pH changed from 7 to 2 (acid region) or from 7 to 12 (alkaline region). The detection limits were 0.03 and 0.02 pH change, respectively, which were comparable to commercial pH meters. The interaction between hydronium (H₃O⁺) or hydroxide (OH^-) and ZnO was proposed to be responsible for the frequency drop. Both hydronium and hydroxide can increase the conductivity of the ZnO film, resulting in the resonant frequency decrease due to the acoustoelectric effect.