A near-infrared fluorescent probe with a substantial Stokes shift designed for the detection and imaging of β-galactosidase within living cells and animals.

β-Galactosidase serves as a pivotal biomarker for both cancer and cellular aging. The advancement of fluorescent sensors for tracking β-galactosidase activity is imperative in the realm of cancer diagnosis. We have designed a near-infrared fluorescent probe (PTA-gal) for the detection of β-galactosidase in living systems with large Stokes shifts. PTA-gal exhibits remarkable sensitivity and selectivity in detecting β-galactosidase, producing near-infrared fluorescent signals with a remarkably low detection limit (2.2 × 10 U/mL) and a high quantum yield (0.30). Moreover, PTA-gal demonstrates biocompatibility and can effectively detect β-galactosidase in cancer cells as well as within living animals.