Förster resonance energy transfer-based anion-responsive nanoemulsion optodes: the importance of fluorescent dye liquid lipophilicity and ionophore–dye interaction for stable and background-free anion response

Abstract

In this study, we fabricated and evaluated a highly sensitive Förster Resonance Energy Transfer (FRET)-based anion-responsive nanoemulsion optode (FRET-NE optode) using an originally developed lipophilic fluorescent dye liquid. When the donor (D) dye liquid, designed and synthesized for a previously developed poly(vinyl) chloride (PVC) film optode, was directly applied to the NE optode, it was found that the D dye liquid leaked into the aqueous phase upon anion response due to insufficient lipophilicity. To address this issue, we designed and synthesized a new D dye liquid incorporating a lipophilic alkyl chain. This modification successfully prevented leakage and enabled the construction of a stable FRET-NE optode. However, a new challenge emerged: the fabricated FRET-NE optode exhibited a significant background signal upon mixing with buffer solution. This was attributed to protonation of the acceptor (A) dye near the organic–aqueous interface. Upon addition of an anion ionophore, hydrogen-bonding interactions between the A dye and the anion ionophore suppressed protonation near the organic–aqueous interface and effectively suppressed the background signal. This paper highlights the critical importance of D dye liquid lipophilicity in FRET-NE optode fabrication and demonstrates that hydrogen-bonding interactions between the A dye and the anion ionophore are effective in suppressing background signals. The optimized FRET-NE optode composition exhibited approximately 10-fold higher sensitivity compared to conventional non-FRET sensors. These findings suggest that the application of FRET using fluorescent dye liquids holds great promise for dramatically enhancing the sensitivity of conventional NE optodes.