Caesium (Cs +) cations are active radioisotope 137 Cs can be released in nuclear incidents and find its way into the water supply, where it is harmful to humans and animals drinking it. We here report a water-gated thin film transistor (WGTFT) which allows the detection of Cs + in drinking water at very low concentrations. The transistor channel is formed from spray pyrolysed tin dioxide, SnO 2 which gives WGTFTs with near-zero initial threshold. When the WGTFT is sensitised with a plasticised PVC membrane containing the Cs +-selective zeolite 'mordenite', it displays a threshold shift when exposed to drinking water samples carrying traces of Cs +. The response characteristic is given by the Langmuir adsorption isotherm instead of the Nikolsky-Eisenman law commonly found for ion-sensitive WGTFTs sensitised with organic ionophores. We find a complex stability constant K = (3.9 +/-0.4) x 10 9 L / mole and a limit-of detection (LoD) of 33 pM. Our LoD is far lower than the Cs + potability limit of 7.5 nM, which cannot be met by organicsensitised membranes where LoD is typically in the order of 100 nM or more.
Some materials that are active heterogeneous catalysts for the breakdown of non-ionic aromatic solutes in water are found to act as potentiometric sensitizers for same solutes.As an example, here the aromatic water pollutant, benzyl alcohol, was sensed with a limitof-detection (LoD) below its potability limit of 19 M. Our findings are rationalized on the grounds that both catalysis and sensing rely on adhesion of analyte/substrate on the sensitizer / catalyst. Specifically, a set of powdered transition metal doped zeolites and related frameworks that catalyze the oxidation of waterborne aromatic pollutants were dispersed in phase transfer matrices. Matrices were introduced into water-gated thin film transistors (WGTFTs) that act as potentiometric transducers. Potentiometric sensing of non-ionic waterborne pollutants is limited to molecules with a 'free' molecular dipole, i.e. a dipole that is not locked in the molecular plane. The present work establishes an application for catalysts beyond catalysis itself. The use of catalysts as sensitizers is recommended for wider uptake and in reverse, to screen candidate catalysts.
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