<i>N</i>‑Pyridineium-2-yl Darrow Red Analogue: Unique Near-Infrared Lysosome-Biomarker for the Detection of Cancer Cells

The lysosome-targetable OFF–ON type pH sensor that does not emit at pH = 4.0 is adopted for the selective detection of cancer cells, and the acidity difference of lysosomes in cancer and normal cells is verified. Three pH probes based on Darrow Red derivatives were designed and prepared that were demonstrated to be lysosome-specific biomarkers with inducible emission at 580–850 nm by the comparable in cellular imaging assays using HeLa, KB, and V79 cells. Of these, a pyridineium-2-yl Darrow Red analogue with a p<i>K</i>_a of 2.4 was found to be a lysosome tracker for cancer cells, it is a unique pH sensor for the optical identification and distinction of cancer cells from normal cells and has potential application as a fluorescent biomaker of cancer cells in <i>in vitro</i> assays

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

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