We developed novel, fluorescent, water-soluble, pH-sensitive 2-(4-hydroxystyryl)-indolenine dyes St1–St3. Dyes St1 and St2 were synthesized by condensation of 4‑hydroxy benzaldehydes A1 and A2 with indolenine Ind while St3 was obtained by an alkaline hydrolysis of cyanine QCy7.
The absorption and emission spectra of these dyes were investigated in buffer solutions vs. pH. In the acidic media these dyes absorb in a short-wavelength region (~410–430 nm). St1 exhibits a moderate fluorescence at ~530 nm while St2 and St3 are not fluorescent. In the alkaline media the absorption band decreases and a new, longer-wavelength absorption band appear at ~490–530 nm and a bright fluorescence arises at ~550–560 nm. Based on the obtained absorption/emission spectra, the pKa values were calculated and found to increase in the order St2 (4.67/4.61) < St3 (5.93/5.83) < St1 (6.96/7.10).
The developed fluorescence dyes can be utilized as pH indicators and sensing probes for analytical, bioanalytical and diagnostic applications.
This work was supported by the Israel Science Foundation (ISF), project 810/18 and the KAMEA program (Leonid Patsenker).