Sub-nanomolar detection of Cesium with water-gated transistor

Caesium (Cs + ) cations are rare in nature but the β - 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 NikolskyEisenman 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.