Abstract
We introduce fluoride-selective anion exchange resin sorbents as sensitisers into membranes for water-gated field effect transistors (WGTFTs). Sorbents were prepared via metal (La or Al)-loading of a commercial macroporous aminophosphonic acid resin, PurometTM MTS9501, and were filled into a plasticised poly(vinyl chloride) (PVC) phase transfer membrane. We found a potentiometric response (membrane potential leading to WGTFT threshold shift) to fluoride following a Langmuir–Freundlich (LF) adsorption isotherm with saturated membrane potential up to ~480 mV, extremely low characteristic concentration c1/2 = 1/K, and picomolar limit of detection (LoD), even though ion exchange did not build up charge on the resin. La-loading gave a superior response compared to Al-loading. Membrane potential characteristics were distinctly different from charge accumulating sensitisers (e.g., organic macrocycles) but similar to the Cs+ (cation) selective ion-exchanging zeolite mineral ‘mordenite’. We propose a mechanism for the observed threshold shift and investigate interference from co-solutes. Strong interference from carbonate was brought under control by ‘diluting’ metal loading in the resin. This work sets a template for future studies using an entirely new ‘family’ of sensitisers in applications where very low limit of detection is essential such as for ions of arsenic, mercury, copper, palladium, and gold.
Highlights
In recent years, water-gated thin-film transistors (WGTFTs) have been developed into a novel potentiometric transducer for the sensing of waterborne ions
We have provided a second example of an ion-exchanging rather than charge-accumulating sorbent as sensitiser in the phase transfer membrane of a water-gated field effect transistor (WGTFT) for potentiometric ion sensing
The fluoride-selective sorbent used here was derived by metal loading (La or Al) of a commercial macroporous aminophosphate (AMP) resin, ‘PurometTM MTS9501‘ to activate it for ion-exchange with fluoride
Summary
Water-gated thin-film transistors (WGTFTs) have been developed into a novel potentiometric transducer for the sensing of waterborne ions. Macrocycles selectively ‘accumulate’ ions (usually cations) in their central cavity They build up charge and potential at the membrane/water interface. The Nernst law is logarithmic with ion concentration, c, without lower cut-off This leads to a formal divergence in the limit c → 0 that is not practically observed. We found a membrane potential in response to increasing analyte concentration following a Langmuir or Langmuir–Freundlich isotherm (to be introduced below), instead of a Nikolsky–Eisenman law. The origin of the membrane potential remained unclear, : ion exchange does not accumulate a net charge on the membrane This sets such membranes apart from membranes that are sensitised (e.g., with ion-selective organic macrocycles [1,2,3]), which give a Nikolsky–Eisenman membrane potential as a result of charge accumulation when exposed to a ‘target’ ion
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