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A pentamethine cyanine dye has been synthesised that is fluorescent when protonated, becoming non-fluorescent upon proton abstraction. The probe has a pKa of 7.5, with observable changes in the fluorescent emission properties across a pH range of 6.0–8.0, therefore providing a useful probe for studying pH in biological media.
Briggs et al. (Sat,) studied this question.