Development of fluorescent probes for on-site sensing and long-term tracking of specific biomarkers is particularly desirable for the early detection of diseases. However, available small-molecule probes tend to facilely diffuse across the cell membrane or remain at the activation site but always suffer from the aggregation-caused quenching (ACQ) effect. Here we report an enzyme-activatable aggregationinduced emission (AIE) probe QM-bgal, which is composed of a hydrophilic b-galactosidase (b-gal)triggered galactose moiety and a hydrophobic AIE-active fluorophore QM-OH. The probe is virtually non-emissive in aqueous media, but when activated by b-gal, specific enzymatic turnover would liberate hydrophobic AIE luminogen (AIEgen) QM-OH, and then highly fluorescent nanoaggregates are in situ generated as a result of the AIE process, allowing for on-site sensing of endogenous b-gal activity in living cells. Notably, taking advantage of the improved intracellular retention of nanoaggregates, we further exemplify QM-bgal for long-term ($12 h) visualization of b-gal-overexpressing ovarian cancer cells with high fidelity, which is essential for biomedicine and diagnostics. Thus, this enzyme-activatable AIE probe not only is a potent tool for elucidating the roles of b-gal in biological systems, but also offers an enzyme-regulated liberation strategy to exploit multifunctional probes for preclinical applications.Scheme 1 Schematic illustration of an enzyme-regulated liberation strategy for on-site sensing and long-term tracking.Scheme 2 Enzyme-activatable probes for b-gal activity sensing.This journal is

Various squaraine dyes have been developed for biological imaging. Nevertheless, squaraine dyes with emission in the second window (NIR-II, 1000–1700 nm) have few reports largely due to the short of a simple and universal design strategy. In this contribution, molecular engineering strategy is explored to develop squaraine dyes with NIR-II emission. First, NIR-I squaraine dye SQ2 is constructed by the ethyl-grafted 1,8-naphtholactam as donor units and square acid as acceptor unit in a donor–acceptor–donor (D–A–D) structure. To red-shift the fluorescence emission into NIR-II window, malonitrile, as a forceful electron-withdrawing group, is introduced to strengthen square acid acceptor. As a result, the fluorescence spectrum of acceptor-engineered squaraine dye SQ1 exhibits a significant red-shift into NIR-II window. To translate NIR-II fluorophores SQ1 into effective theranostic agents, fibronectin-targeting SQ1 nanoprobe was constructed and showed excellent NIR-II imaging performance in angiography and tumor imaging, including lung metastatic foci in deep tissue. Furthermore, SQ1 nanoprobe can be used for photoacoustic imaging and photothermal ablation of tumors. This research demonstrates that the donor–acceptor engineering strategy is feasible and effective to develop NIR-II squaraine dyes.