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 magnet oelastic thick-film elements: one is coated with the pH and salt concentrat ion sensitive mass-changing polymer poly(acrylic acid-co-isooctylacrylate) (pAA-IOA), while the other is coated with poly(3-sulfopropyl methacrylate-c o-isooctylacrylate) (pSPMA-IOA), a polvelectrolyte gel the mass of which is salt-dependent but pl-l-independent. The magnetoelastic elements mechanica lly vibrate at a resonant frequency f(r), inversely dependent upon the mass of the attached polymer layer. Cross-correlation between the frequency res ponse of the two sensor elements is used to eliminate the effect of salt co ncentration on the pH measurements, which are linear between pH 5.4 and 7.2 . with a slope of 3 10 Hz/pH (0.6% f(r)/pH). (C) 2001 Elsevier Science B.V. All rights reserved.