Sensing harmful ions in water by using water-gated thin-film transistor sensors

This thesis demonstrates the development of a worthy approach to detect harmful waterborne analytes in significantly low concentrations in water by using water-gated thin-film transistors (WGTFTs) as potentiometric ion sensors. The successful gating of a thin-film transistor with water to be the gate medium in 2010 paved the way for a new sensor technology, WGTFTs, to detect waterborne analytes in water gate media. WGTFT sensors have been applied to detect waterborne analytes (e.g. K+, Na+ and Ca2+) with the help of organic macrocyclic ionophores (sensitisers) incorporated within the membrane in WGTFT’s architecture. The response characteristics of such WGTFT sensors underwent the Nikolsky-Eisenman (modified Nernstian) law, with a limit-of-detection (LoD) in the range of 100 nM – 1 µM. This limit is insufficient to monitor drinking water against harmful or toxic ions. Accordingly, ion-exchange sorbents such as zeolites and resin were exploited in this work to sensitise the WGTFTs. These sorbents were embedded within a PVC membrane as a sensitiser and located in the transistor’s gate medium, which separated the sample solution (containing an analyte ion) and a reference solution (free of analyte ion) in the composition of WGTFT sensors.
Radioactive- 137Cs is rare in nature but finds its way into water supplies and then into humans and animals due to nuclear accidents. The sensitisation of the WGTFT sensor with Cs+-selective mordenite zeolite as an ion-exchange ionophore enabled the detection of Cs+ in very low concentrations. The response of such Cs+- WGTFT sensor followed the Langmuir adsorption isotherm with high stability constant and an exceedingly low LoD (33 pM). Such response characteristics enabled us to determine very low LoD. The LoD of a Cs+- WGTFT sensor is much lower than the potability limit of Cs+ in drinking water (7.5 nM), which has not been obtained by organic macrocyclic sensitisers.
Pb2+ and Cu2+ are common drinking water pollutants deposited in water resulting from the use of these metals in manufacturing water pipes. To detect these cations in low concentrations, clinoptilolite zeolite was used to sensitise the WGTFT in a similar manner used in previous Cs+- WGTFT sensors. The threshold shifted in response to increasing Pb2+ or Cu2+ concentrations following the Langmuir-Freundlich characteristic. Hence, the LoDs were much lower than the action levels of the lead-and-copper rule recommended by the Environmental Protection Agency for drinking water. Such WGTFT sensors achieved respective Pb2+ and Cu2+ LoDs of 0.9 nM and 14 nM against 72 nM and 20.5 µM as action levels with high selectivity for these metal cations, even with the presence of other interfering cations in the water sample. Therefore, these features qualify clinoptilolite-sensitised WGTFTs for the monitoring of the lead-and-copper rule.
WGTFTs sensitised with another class of ion-exchange sorbents, La- loading of PurometTM MTS9501 resin, demonstrated excellent response to fluoride (F-) anion in a low dynamic range, following the Langmuir-Freundlich adsorption isotherm. This process enabled picomolar LoD and extremely low c1/2 concentration. A successful routine was suggested to restrict the interferant from co-solutes. Although, the LoD of F- was much below practical requirements, this work provides a template for further studies using resins as sensitisers in applications where an extremely low LoD is crucial.