A new fluorescent pH probe for extremely acidic conditions

Abstract

A novel turn-off fluorescent probe based on coumarin and imidazole moiety for extremely acidic conditions was designed and developed. The probe with pKa=2.1 is able to respond to very low pH value (below 3.5) with high sensitivity relying on fluorescence quenching at 460nm in fluorescence spectra or the ratios of absorbance maximum at 380nm to that at 450nm in UV–vis spectra. It can quantitatively detect pH value based on equilibrium equation, pH=pKa−log[(Ix−Ib)/(Ia−Ix)]. It had very short response time that was less than 1min, good reversibility and nearly no interference from common metal ions. Moreover, using 1H 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.