Surface and potentiometric properties of a SO2 sensor based on a hydrogel coated pH-FET

A field effect transistor (FET) sensor for sulfur dioxide was developed based on a pH-sensitive FET. The gate surface of a the FET was coated with a thin film of hydrogel containing a hydrogensulfite electrolyte. The sulfur dioxide analyte diffuses into the reagent phase through a Teflon membrane covering the hydrogel film and produces a pH change that is monitored by the pH-FET. The sensor exhibited very good linearity (r = 0.9948) in the range of 2.4 x 10(-7) to 2.25 x 10(-4) M, a sensitivity of 23 mV per decade of sulfur dioxide concentration and a response time of 2-3 min. The hydrogel reagent phase was characterized using surface sensitive techniques such as Xray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS and SEM-EDAX analyses indicated the absence of the hydrogensulfite electrolyte on the surface of the hydrogel. SEM experiments on the reagent phase provided evidence that the NaHSO3 electrolyte was embedded in the bulk of the hydrogel membrane. This observation can be related to the non-instantaneous response of the sensor. (C) 2001 Elsevier Science B.V. All rights reserved.