A pH-responsive fluorescent sensor based on a new pyranoxanthylium salt.

Stimuli-responsive pigments are very interesting for several technological applications, such as food and cosmetic colorants, color-based sensors and fluorescence probes, among others. In this work, the synthesis of a new xanthylium-derived pigment was developed and the chemical and photophysical properties in hydroalcoholic solution at different pH values were investigated by UV-Vis and fluorescence spectroscopy. The UV-Vis titration of 3,6,8-trihydroxy-11-methylpyranoxanthylium has shown four different colored chemical species (AH+, A, A- and A2-) in hydroalcoholic solution in a pH range between 1 and 12 with the thermodynamic acidic constants of pKa1 = 4.80 ± 0.03, pKa2 = 6.51 ± 0.05 and pKa3 = 8.64 ± 0.01. Regarding fluorescence properties, this dye revealed an interesting pH-dependent emission behavior. In fact, the anionic quinoidal base A- predominant at pH range between 5 and 9 should be mainly responsible for the pronounced fluorescence intensity observed at λex 467 nm/λem 510 nm pair (maximum at pH 7.5). This set of new insights make this dye useful as a potential "off-on-off" pH-responsive fluorescent probe for biological applications. A pyranoxanthylium dye was developed and revealed a selective fluorescence emission between 5 < pH < 9, being maximum at pH 7.5, which make it very interesting as a pH-responsive "off-on-off" fluorescent probe for biomedical applications.