New fluorescent pH probes for acid conditions

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 = pK(a) - [log(I-a - I-x)/(I-x - I-b)], with very good linear relationship. Moreover, using H-1 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. (C) 2014 Elsevier B.V. All rights reserved.