Photocurable ISFET for anionic surfactants. Monitoring of photodegradation processes

The preparation of a new ion-selective field-effect transistor (ISFET) based on a photocurable membrane sensitive to anionic surfactants is described. The membrane is formed by an urethane–acrylate matrix with 2-cyanophenyl octyl ether as the plasticiser. When compared to conventional ion-selective electrodes, the prepared ISFETs do not show significant differences in sensitivity and reproducibility (P=0.05). When calibrating with dodecylbenzenesulfonate (DBS−) the prepared ISFETs show a nernstian behaviour, with a slope of 57.5 mV per decade. The linear working range is 1.0×10−3 to 3.0×10−6 M DBS− and the detection limit is 1.2×10−6 M. The response times were below 0.7 min in all cases (95% of the step change). As the application, photodegradation processes using titanium dioxide dispersions, were monitored for two common anionic surfactants: DBS−, being aromatic, and the more alkylic dodecylsulfate, DS−. The determination of surfactant concentration was performed following a standard addition methodology, using ISFETs as the sensors, and without any previous separation stages. The degradation kinetics in both cases are first-order processes, with half-life times (t0.5) of 31.5 min for DBS− and 52.0 min for DS−.