Surface and potentiometric properties of a SO 2 sensor based on a hydrogel coated pH-FET

Abstract

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 2.4×10 −7 to 2.25×10 −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 X-ray 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 NaHSO 3 electrolyte was embedded in the bulk of the hydrogel membrane. This observation can be related to the non-instantaneous response of the sensor.