New pyrazoline-based pH fluorescent probes have been synthesized and characterized. These probes
respond to acidic pH with short response time (less than 1 min), high selectivity and sensitivity.
They have good reversibility and nearly no interference from common metal ions. These probes are
suitable for strong acid conditions and can quantitatively detect pH value based on equilibrium
equation, pH = pKa −
rlog(Ia − Ix )/(Ix − Ib )], with very good linear relationship. Moreover, using 1 H NMR analysis,
we verified
that the protonation of the nitrogen in thiazole weakened the thiazole ring’s ability to donate
electrons to pyrazoline moiety and changed the intramolecular charge transfer that was actually the
mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. More
importantly, these
probes can image strong acidity in Saccharomyces cerevisiae.