A “turn-on-and-off” pH sensitive BODIPY fluorescent probe for imaging E. coli cells

Abstract

We report fluorescent turn-on-and-off pH BODIPY probes which show a drastic shift in pK_a values upon introducing electron withdrawing substituents on the meso-phenyl ring. The sensors utilize 4-amino- and 4-hydroxyl groups as proton receptors and work by intramolecular photoinduced electron transfer (PET) under acidic pH conditions. The acid sensing mechanism is proven by ¹H NMR, IR spectroscopic methods and electrochemical studies. Unlike previous reports, the pH response of the –OH group in compound 3 is due to the formation of –OH₂⁺. Compounds 1 and 4 are highly selective to H⁺ over other metal ions and displayed good photostability and reversibility. The utility of acid sensitive behavior of 1 and 4 to visualize extreme acidity is also demonstrated using the model organism E. coli.