A structure optimized fluorescent probe for highly sensitive monitoring drug induced lysosomal pH value changes

Lysosomes generally maintain the weak acidic microenvironment, to ensure highly efficient activity and functions of hydrolytic enzymes and proteins. Aberrations of the lysosomal pH may result in cellular functional changes and influence human physiology, possibly causing serious diseases. Small-molecular fluorescent probes based imaging techniques capable of providing information on target locations are considerably appreciated. Herein, by reducing the size of the typical lysosome targetable group 4-(2-aminoethyl) morpholine, we rationally designed a rhodamine analogue probe Ly-HN2AM with N-Aminomorpholine as the ring-closed switch and the lysosome targetable moiety for visualizing lysosomal pH changes. With the benefit of constructing multi-pentacyclic intramolecular hydrogen bond when binding with the H⁺, Ly-HN2AM gives a highly sensitive response towards pH values ranging from 4.79 to 6.07, with a remarkable higher pK_a 5.35 over the typical 4-(2-aminoethyl) morpholine modified probes. The new probe was successfully applied to visualize pH value changes in lysosome-associated physiological and pathological processes with excellent photostability and low cytotoxicity, indicating the potential applications of lysosome specific bioimaging.