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Molecular probes activated by a single enzyme have been extensively used in bioimaging and disease diagnosis; however, imaging and identification in an accurate manner remains a challenge for such probes. Here, based on the specificity of enzyme recognition, we engineered a "double-locked" and enzyme-activated molecular probe (NML) for accurate bioimaging and hepatopathy differentiation. Triggered by the successive reactions with leucine aminopeptidase (LAP, first "key") and monoamine oxidase (MAO, second "key"), the emissive fluorophore (NF) was released. NML can be activated only in the presence of both LAP and MAO and can be silenced when either enzyme is inhibited. Benefiting from the "double-locked" strategy, NML showed higher accuracy for imaging of drug-induced liver injury (DILI) than the "single-locked" probe. With serum testing, NML showed significant differences in mouse models of both CCl4-induced liver cirrhosis and DILI. Significantly, NML can be applied to accurately distinguish serum samples from clinical patients with different hepatopathies. Our smart molecular probe may hold great potential for hepatopathy diagnosis and clinical transformation.

The complex environment of living organisms significantly challenges the selectivity of classic small-molecule fluorescent probes for bioimaging. Due to their predesigned topological structure and engineered internal pore surface, covalent organic frameworks (COFs) have the ability to filter out coexisting interference components and help to achieve accurate biosensing. Herein, we propose an effective interference-resistant strategy by creating a COF-based hybrid probe that combines the respective advantages of COFs and small-molecule probes. As a proof of concept, a two-photon fluorescent COF nanoprobe, namely TpASH-NPHS, is developed for targeting hydrogen sulfide (H2S) as a model analyte. TpASH-NPHS exhibits limited cytotoxicity, excellent photostability and long-term bioimaging capability. More importantly, compared with the small-molecule probe, TpASH-NPHS achieves accurate detection without the interference from intracellular enzymes. This allows us to monitor the levels of endogenous H2S in a mouse model of cirrhosis.

Mitophagy induced by hypoxia plays an important role in regulating cellular homeostasis via the removal of dysfunctional mitochondria in the lysosomal degradation pathway, which results in physiological changes in the mitochondria, such as the pH, polarity and viscosity. However, the lack of an effective method for imaging of both the hypoxic microenvironment and the resulting variable mitochondria limits the visualization of hypoxia-induced mitophagy. Based on the specific mitochondrial pH changes during the hypoxia-induced mitophagy process, we have reported a near-infrared fluorescent probe (NIR-HMA) for real-time simultaneous visualization of the hypoxic microenvironment and the subsequent mitophagy process in live cells. NIR-HMA selectively accumulated in the hypoxic mitochondria in the NIR-MAO form, emitting at 710 nm, and then transformed into NIR-MAOH, emitting at 675 nm, in the acidified mitochondria-containing autolysosomes. Importantly, by smartly tethering the hypoxia-responsive group to the hydroxyl group of the NIR-fluorochrome, which shows ratiometric pH changes, NIR-HMA can differentiate between different levels of the hypoxic microenvironment and mitophagy. Furthermore, using NIR-HMA, we could track the complete mitophagy process from the mitochondria to the autolysosomes and visualize mitophagy caused only by hypoxia both in cancer cells and normal cells. Finally, NIR-HMA was applied to investigate the role that mitophagy plays in the hypoxic microenvironment via the cycling hypoxia-reoxygenation model. We observed a decreased fluorescence ratio after reoxygenation and a further increased mitophagy level after hypoxia was induced again, suggesting that mitophagy might be a self-protective process that allows cells to adapt to hypoxia. Our work may provide an attractive way for real-time visualization of relevant physiological processes in hypoxic microenvironments.

A two photon-excited fluorescent supramolecular nanoplatform is first designed for quantitative analysis with host molecules, sensing probes and an internal reference., Fluorescence quantitative analyses for vital biomolecules are in great demand in biomedical science owing to their unique detection advantages with rapid, sensitive, non-damaging and specific identification. However, available fluorescence strategies for quantitative detection are usually hard to design and achieve. Inspired by supramolecular chemistry, a two-photon-excited fluorescent supramolecular nanoplatform (TPSNP) was designed for quantitative analysis with three parts: host molecules (β-CD polymers), a guest fluorophore of sensing probes (Np–Ad) and a guest internal reference (NpRh–Ad). In this strategy, the TPSNP possesses the merits of (i) improved water-solubility and biocompatibility; (ii) increased tissue penetration depth for bioimaging by two-photon excitation; (iii) quantitative and tunable assembly of functional guest molecules to obtain optimized detection conditions; (iv) a common approach to avoid the limitation of complicated design by adjustment of sensing probes; and (v) accurate quantitative analysis by virtue of reference molecules. As a proof-of-concept, we utilized the two-photon fluorescent probe NHS–Ad-based TPSNP-1 to realize accurate quantitative analysis of hydrogen sulfide (H2S), with high sensitivity and good selectivity in live cells, deep tissues and ex vivo-dissected organs, suggesting that the TPSNP is an ideal quantitative indicator for clinical samples. What’s more, TPSNP will pave the way for designing and preparing advanced supramolecular sensors for biosensing and biomedicine.