Reversible Near-Infrared pH Probes Based on Benzo[<i>a</i>]phenoxazine

Abstract

Several benzo­[<i>a</i>]­phenoxazine derivatives containing substituted N-aromatic groups are evaluated for their pH-dependent absorption and emission properties. Among the compounds exhibiting optical responses under near-neutral and subacid pH conditions, benzo­[<i>a</i>]­phenoxazine derivatives with an electron-withdrawing aromatic group attached to nitrogen of the imino group show potential application as near-infrared pH sensors. Three water-soluble pH probes based on benzo­[<i>a</i>]­phenoxazine with different pyridinium structures are designed and synthesized. Their reversible pH-dependent emissions in buffer solution containing 0.1% dimethyl sulfoxide (DMSO) locate in 625–850 nm with the fluorescent enhancement of 8.2–40.1 times, and their calculated p<i>K</i>_a values are 2.7, 5.8, and 7.1, respectively. A composite probe containing the three benzo­[<i>a</i>]­phenoxazines shows a linear pH–emission relationship in the range of pH 1.9–8.0. Real-time detection of intracellular pH using an in vitro assay with HeLa cells is also reported