Live-cell imaging of β-galactosidase based on a quinoline-malononitrile-derived AIE probe

Human β-galactosidase (β-gal) is a lysosomal enzyme responsible for the hydrolysis of glycoconjugates, which is overexpressed in primary ovarian cancers and regarded as a biomarker. Therefore, it is of great importance to develop fluorescent probes activatable by endogenous β-gal with strong signal-to-background contrast to facilitate ovarian cancer diagnosis. Many of the available probes suffer from small Stokes shifts, poor cellular permeability, easy diffusion away from living cells, and self-quenching when accumulated inside the lysosome, asking for the development of new turn-on probes suitable for non-invasive imaging with living cells.

The quinoline-malononitrile derivative (SDC) with aggregation-induced emission (AIE) property and large Stokes shift (∼160 nm) was conjugated with the galactose moiety via a self-immolating linker, giving SDB. Upon cleavage of the galactose by β-gal, strong red emission from SDB was detected via AIE of SDC. To improve the cell permeability, SDA containing the fully acetylated galactose was constructed, whose acetyl groups can be removed by intracellular esterases upon cell entry, resulting in unblocked SDB. Live-cell imaging demonstrated that SDA selectively distinguishes ovarian cancer cell lines (OVCAR3 and SKOV3) from others.

The AIE property of quinoline-malononitrile derived SDA allows in situ capture of β-gal activities with minimal signal diffusion, useful for long-term tracking of β-gal in living cells and promising for early cancer diagnosis.