A pH sensor based on force generated by pH-dependent polymer swelling.
We have demonstrated the feasibility of a new type of pH sensor by combining a bead of porous lightly crosslinked diethanolamine derivatized poly(vinylbenzyl chloride) with a strain gauge, i.e. a pressure sensitive resistor. The polymer bead is toughened with Kraton G1652, a styrene-ethylene,butylene-styrene triblock copolymer. The sensor is constructed so that the shrunken form of the bead is held in contact with the strain gauge with a small force. Increases in the hydrogen ion concentration protonate the diethanolamine introducing a positive charge onto the polymer backbone. This results in an electrostatic swelling force that causes the polymer to swell. This is detected as a change in strain gauge resistance that is readout via a Wheatstone bridge. When the pH of 0.10 M buffers is changed from 10 to 4, the response time is 390 s for a bead that is 0.25 mm in diameter in the shrunken state. The response varies with the square of the bead radius. The magnitude of the response is highly correlated with the penetration modulus, a measure of the extent to which the bead resists deformation when subjected to an external force. The response to pH appears to be shifted by the application of pressure in the sensor. This instrumentally simple approach to sensing has the potential to be stable and long-lived if the polymer bead can undergo a large number of swelling/shrinking cycles without changing mechanical properties.