Ionic liquid-based dye: A “Dyed plasticizer” for rapid and highly sensitive anion optodes based on a plasticized PVC membrane

Abstract

Abstract An ionic liquid-based dye was synthesized and directly used as a plasticizer in a plasticized poly(vinyl chloride) (PVC) membrane-based anion optode. In contrast to conventional PVC-based optode membranes, the optode membrane presented can contain a very high concentration (∼296.1 mmol/kg) of dye molecules (for conventional membranes, the concentration is typically 5–50 mmol/kg), which allowed the preparation of a very thin membrane (∼0.8 μm) exhibiting a rapid response time (6.5 s), with sufficient sensitivity (1.2 Absorbance unit (AU)) and excellent response reversibility. We prepared the ionic liquid-based dye using trihexyltetradecylphosphonium (P66614) as a lipophilic monocation and naphtholphthalein (NP) as a dianion. The prepared ionic liquid-based dye contained approximately 40% dye relative to the formula [P66614]2[NP], as confirmed by proton NMR measurements and elemental analysis. Optimized PVC content of the plasticized PVC membrane, without leaching of dye, was 10 wt%. Under these conditions, surprisingly, the membrane exhibited fully reversible responses to acidic and basic solutions, even though the dye was not lipophilized using long alkyl chains, something that has been considered an indispensable chemical modification for dyes used in typical ion-selective optodes. A comparison with conventional plasticized PVC membranes, using nitrophenyl octyl ether (NPOE) as a typical plasticizer and a lipophilic derivative of NP as a lipophilic dye, was carried out in terms of the response time and sensitivity, the results of which demonstrated the advantages of the present approach. Finally, as the simplest application, a plasticized PVC membrane based on an ionic liquid-based dye was successfully applied to coextraction-based anion sensing.