Surface-Doped Polystyrene Microsensors Containing Lipophilic Solvatochromic Dye Transducers.

Ion-selective optical microsensors based on surface-modified polystyrene (PS) beads with positively charged lipophilic solvatochromic dye (SD) on the surface were studied with K+ as model ion. Water-soluble SDs are expelled into the aqueous phase by ion exchange with the cationic analyte of interest, resulting in a detectable color change. In contrast, lipophilic SDs are more attractive, as they appear to remain anchored to the surface after expulsion from the sensing phase. This transfers just the ionic chromophore functionality into the aqueous phase and allows the system to act as a reversible, truly self-contained sensor. In this work this mechanism was evaluated with ζ-potential measurements on microsensor suspensions. It indeed provides experimental evidence for the mechanism of SD transfer, as a reversal of the ζ-potential of the PS microsensors was observed for higher potassium concentrations with valinomycin-doped microspheres. For a discriminated ion such as sodium, the ζ-potential change occurs at much higher electrolyte concentrations, in agreement with sensor selectivity. Undoped microspheres showed no apparent dependence of ζ-potential on electrolyte concentration. The study also shows that the effective range of microsensor surface charge is tunable and depends on the concentration of the SD on the coating phase.