A new fluorescent pH probe for extremely acidic conditions

A novel turn-off fluorescent probe based on coumarin and imidazole moiety for extremely acidic conditions was designed and developed. The probe with pK(a) =2.1 is able to respond to very low pH value (below 3.5) with high sensitivity relying on fluorescence quenching at 460 nm in fluorescence spectra or the ratios of absorbance maximum at 380 nm to that at 450 nm in UV-vis spectra. It can quantitatively detect pH value based on equilibrium equation, pH = pK(a) - log[(I-x - I-b)/(I-a - I-x)]. It had very short response time that was less than 1 min, good reversibility and nearly no interference from common metal ions. Moreover, using H-1 NMR analysis and theoretical calculation of molecular orbital, we verified that a two-step protonation process of two N atoms of the probe leaded to photoinduced electron transfer (PET), which was actually the mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. Furthermore, the probe was also applied to imaging strong acidity in bacteria, E.coli and had good effect. This work illustrates that the new probe could be a practical and ideal pH indicator for strongly acidic conditions with good biological significance. (C) 2014 Elsevier B.V. All rights reserved.