A salt-independent pH sensor

A polymer swelling-based pH sensor is described that operates independently of background salt concentration. The pH sensor is comprised of two magnetoelastic thick-film elements; one is coated with the pH and salt concentration sensitive mass-changing polymer poly(acrylic acid-co-isooctylacrylate) (pAA-IOA), while the other is coated with poly(3-sulfopropyl methacrylate-co-isooctylacrylate) (pSPMA-IOA), a polyelectrolyte gel the mass of which is salt-dependent but pH-independent. The magnetoelastic elements mechanically vibrate at a resonant frequency fr, inversely dependent upon the mass of the attached polymer layer. Cross-correlation between the frequency response of the two sensor elements is used to eliminate the effect of salt concentration on the pH measurements, which are linear between pH 5.4 and 7.2, with a slope of 310 Hz/pH (0.6% fr/pH).