Piezoelectric sensor for detection of organic amines in aqueous phase based on a polysiloxane coating incorporating acidic functional groups

The resonant frequency of piezoelectric quartz crystal microbalances (QCMs), coated with a polysiloxane film containing acidic functional groups was studied when the pH of aqueous solutions in contact with coatings was changed. The Δf vs. pH curves for the polymer coating indicated that the pKa values were about 7~8 (Ka is the ionization constant) and the transition region was around 3~4 pH units. The sorption characteristics of the crystal coated with polymer film were reported. These coated crystals were shown to behave as selective detectors for a range of organic amines in dilute aqueous solutions. The lower limits for reliable measurement using 10 MHz crystals coated with 15 kHz loading of OV-225 acid polymer were estimated to be at the ppm levels for primary amines and triethylamine in a pH 7.14 buffer solution and a water-ethanol (vv = 964) solution. Linear responses were obtained in the concentration ranges of 10–270 ppm and 10–250 ppm for the pH 7.14 buffer and the water-ethanol solution, respectively.