Piezoelectric pH sensors: AT-cut quartz resonators with amphoteric polymer films

Piezoelectric AT-cut quartz resonators immersed in aqueous media, coated with cross-linked films of amphoteric polymer 1, exhibit large frequency changes when the pH is changed in the vicinity of the isoelectric point of the polymer film. The unique pH-sensing capabilities of the coated resonators, combined with their robustness, ease of fabrication and low cost, provide a convenient approach for the measurement of threshold pH changes.