Lysosomes-targeting near-infrared fluorescent probe for the detection of pH in living cells.

A new near-infrared fluorescent probe Qcy-OH with a turn-on mechanism based on a acceptor-donor-acceptor (ADA) π-electron system that can undergo an internal charge transfer to form new fluorochromes was developed for monitoring pH fluctuations in biological systems. With the pH value increases, Qcy-OH exhibited a strong pH-sensitive response from 1.1 to 9.0 (pKa = 4.41) and a good linear response from pH 3.0 to 5.5 in aqueous solution and in living cells. In addition, based on the phenolic and ketonic structure interconversions of the fluorescent backbone, Qcy-OH showed rapidly and reversibly response to pH with high selectivity. Because the introduce of two benzothiazolium units in the backbone of the probe as the electron-withdrawing group to enhance the selectivity for intracellular lysosomes, the probe had been applied successfully for tracking lysosomal pH changes and the fluorescence changes showed a good linear enhancement from pH 3.0 to 7.4 in living cells. We believe that this sulfur-driving lysosomes-targeting ability of the probe affords a guarantee for achieving long-term monitoring of lysosomal pH biology by the elimination of harmful protonating effects of the probe.