Ion-Selective Optical Nanosensors Based on Solvatochromic Dyes of Different Lipophilicity: From Bulk Partitioning to Interfacial Accumulation.

The sensing mechanism of fluorescent ion-selective nanosensors incorporating solvatochromic dyes (SDs), with K+ as model ion, is shown to change as a function of dye lipophilicity. Water-soluble SDs obey bulk partitioning principles where the sensor response directly depends on the lipophilicity of the SD and exhibits an influence on the phase volume ratio of nanosensors to aqueous solution (dilution effect). A lipophilization of the SDs is shown to overcome these limitations. An interfacial accumulation mechanism is proposed and confirmed with Förster resonance energy transfer (FRET) with a ratiometric near-infrared fluorescence FRET pair. This work lays the foundation for operationally more robust ion-selective nanosensors incorporating SDs.