Lipophilic Fluorescent Dye Liquids: Förster Resonance Energy Transfer-Based Fluorescence Amplification for Ion Selective Optical Sensors Based on a Solvent Polymeric Membrane.

Optical sensors based on solvent polymeric membranes have the potential to measure analytes present in an aqueous solution through the development of a tailored method for a specific target. However, limits in the concentrations of the component dyes have prevented improvements in sensitivity. We propose a Förster resonance energy transfer (FRET)-based fluorescence amplification system for ion-selective optical sensors using a highly fluorescent liquid material composed of a lipophilic phosphonium cation and a pyrene modifying sulfonate anion ([P66614][HP-SO3]), as both the plasticizer and donor, in addition to a combination of the lipophilic phosphonium cation and the fluorescein dodecyl ester anion ([P66614][12-FL]) as the fluorescent sensing dye acceptor. For ion extraction-based sensing, the donor and acceptor were retained in the plasticized PVC membrane with negligible leaching upon exposure to acidic and basic aqueous solutions. Systematic investigation of the donor and acceptor ratios clarified the effect of the amplification factor and the sensitivity of the sensor. At an acceptor doping level of 0.5 mol % (vs donor), an approximately 22-fold higher sensitivity was obtained compared to that of a conventional PVC membrane optical sensor. During ion measurement based on the coextraction of protons and anions, selectivity following the Hofmeister order was observed, which was controlled by the addition of ionophores. The proposed FRET system based on a lipophilic fluorescent liquid material has the potential to significantly improve the sensitivities of optical sensors using solvent polymeric membranes with high selectivities for various target analytes.