Xian-Jun Liu

A novel fluorescent probe for sensitive detection and imaging of hydrazine in living cells.

A turn-on fluorescent probe (Naphsulf-O) for hydrazine was developed by protecting the hydroxy group of the fluorophore 6-acetyl-2-hydroxynaphthalene via O-4-nitrobenzenesulfonylation, where 4-nitrobenzene was used as a fluorescence quenching moiety as well as an electrophile. Upon nucleophilic aromatic substitution (NAS) reaction of hydrazine toward the probe, the protecting group was removed and fluorophore was released. The probe exhibits a large Stokes shift, excellent selectivity and high sensitivity for hydrazine detection in aqueous solution with a detection limit of 0.716 ppb (22nM), which is of great importance in both environmental and biological system. Furthermore, it was successfully applied to imaging of hydrazine in living cells.

A novel mitochondria-targeted near-infrared fluorescence probe for ultrafast and ratiometric detection of SO2 derivatives in live cells

A novel mitochondria-targeted ratiometric near-infrared fluorescence probe NDMBT for Sulfur dioxide (SO2) derivatives was constructed based on the SO2 derivatives-triggered Michael addition reaction. It displayed ultrafast response time (within 10s), large hypsochromic shift (260nm), high photostability, excellent selectivity and high sensitivity in aqueous media with a detection limit of 43nM. More importantly, it was successfully applied to imaging of the enzymatically generated SO2 derivatives in mitochondria of live cells.

An azidocoumarin-based fluorescent probe for imaging lysosomal hydrogen sulfide in living cells

A commercially available fluorescent hydrogen sulfide (H2S) probe 7-azido-4-methylcoumarin (AzMC) was developed into its lysosome-targeted counterpart Lyso-C via a four-step synthetic approach. Lyso-C displayed fast response (within 5 min), excellent sensitivity (with a detection limit of 37 nM) and high selectivity toward H2S. More importantly, Lyso-C was successfully applied to imaging lysosomal H2S and showed potential capability to quantitatively detect H2S in living cells.

An activatable fluorescent probe with an ultrafast response and large Stokes shift for live cell bioimaging of hypochlorous acid

A novel activatable fluorescent probe for hypochlorous acid (HOCl) imaging, has been developed based on HOCl-triggered aldehyde recovery reaction. This probe features with a long emission wavelength because of the use of intramolecular charge transfer (ICT) effect in the fluorophore. Moreover, this probe displayed ultrafast response (within 1 s), good selectivity and high sensitivity (detection limit of 50 nm) toward HOCl with excellent independency upon pH of the assay system. Live cells imaging studies demonstrate that the probe can be applied successfully to detect exogenous and endogenous HOCl.

Activatable Fluorescence Probe via Self-Immolative Intramolecular Cyclization for Histone Deacetylase Imaging in Live Cells and Tissues

Histone deacetylases (HDACs) play essential roles in transcription regulation and are valuable theranostic targets. However, there are no activatable fluorescent probes for imaging of HDAC activity in live cells. Here, we develop for the first time a novel activatable two-photon fluorescence probe that enables in situ imaging of HDAC activity in living cells and tissues. The probe is designed by conjugating an acetyl-lysine mimic substrate to a masked aldehyde-containing fluorophore via a cyanoester linker. Upon deacetylation by HDAC, the probe undergoes a rapid self-immolative intramolecular cyclization reaction, producing a cyanohydrin intermediate that is spontaneously rapidly decomposed into the highly fluorescent aldehyde-containing two-photon fluorophore. The probe is shown to exhibit high sensitivity, high specificity, and fast response for HDAC detection in vitro. Imaging studies reveal that the probe is able to directly visualize and monitor HDAC activity in living cells. Moreover, the probe is demonstrated to have the capability of two-photon imaging of HDAC activity in deep tissue slices up to 130 μm. This activatable fluorescent probe affords a useful tool for evaluating HDAC activity and screening HDAC-targeting drugs in both live cell and tissue assays.

Mitochondrion-Targeting, Environment-Sensitive Red Fluorescent Probe for Highly Sensitive Detection and Imaging of Vicinal Dithiol-Containing Proteins

Mitochondrial vicinal dithiol-containing proteins (VDPs) are key regulators in cellular redox homeostasis and useful markers for diagnostics of redox-dependent diseases. Current probes fail to target mitochondrial VDPs and show limited sensitivity and response rate. We develop a novel fluorescent probe using an engineered benzoxadiazole fluorophore that allows selective targeting of mitochondria and exhibits highly sensitive environment responsiveness. This probe is almost nonfluorescent in aqueous media, while delivering intense fluorescence upon binding to VDPs via a cyclic dithiaarsane ligand. The fluorescence probe is shown to have rapid response within 30 s and high sensitivity for detecting reduced bovine serum albumin (rBSA) in the concentration range from 0 to 0.1 μM with a detection limit of 2 nM. To our knowledge, this is the first fluorescence probe for VDPs which exhibits deep red emission, instantaneous response, high turn-on fluorescence ratio, and specific mitochondrial localization. It may provide a new tool for in situ monitoring mitochondrial VDPs.

Development of large Stokes shift, near-infrared fluorescence probe for rapid and bioorthogonal imaging of nitroxyl (HNO) in living cells

Nitroxyl (HNO), as an electron reduced and protonated form of nitric oxide, is emerging as a potential diagnostic and therapeutic biomarker. It is still of great interest to develop probes of desirable properties to study its biological functions. Here we develop a near infrared fluorescence probe for detecting and visualizing exogenous and endogenous HNO in living cells. The probe is designed by coupling a HNO-responsive moiety, diphenylphosphinobenzoyl group, with a near infrared fluorophore with large of Stokes shift via an ester linker. The probe was initially nonfluorescent. HNO-catalyzed oxidation reaction generates an aza-ylide, which intramolecularly attacks the carbonyl carbon, liberating the initial fluorophore with activated fluorescence signals. The probe is proportional to the concentrations of HNO in the range of 2.0-80 μM with a limit of detection of 0.05 μM. Furthermore, the probe also exhibits high selectivity and fast response (reaching plateau within 600 s) towards HNO in vitro. Moreover, imaging studies reveal that the probe is capable of detecting exogenous HNO with dose-dependent fluorescence signals. Its ability to image endogenous HNO without or with induction is also demonstrated in living cells. This turn-on fluorescence probe provides a useful tool for studying HNO in living cells.

A naphthalene-based fluorescent probe for ratiometric imaging of lysosomal hydrogen sulfide in living cells

Hydrogen sulfide is an important gasotransmitter that exhibits various functions in physiological processes. We present a ratiometric fluorescence probe (SN-N3) for H2S by functionalizing naphthalene imide with 4-(azidomethyl)benzene as a H2S recognition moiety and morpholine moiety as a lysosomal targeting unit. After reaction with H2S, the azido moiety is reduced to amine group, and the probe releases the self-immolative linker and regenerates the fluorophore with internal charge transfer effect. SN-N3 is responsive to H2S in a ratiometric mode, exhibiting excellent sensitivity and high selectivity. The probe is demonstrated to be localized in lysosomes with high specificity. More importantly, SN-N3 is successfully demonstrated to image lysosomal H2S in a ratiometric manner. Our design provides a novel tool to image H2S in living cells that would hold great potential in exploring various H2S-related physiological and pathological cellular processes.

Engineering Organelle-Specific Molecular Viscosimeters Using Aggregation-Induced Emission Luminogens for Live Cell Imaging

Subcellular viscosity is essential for cell functions and may indicate its physiological status. We screen two fluorescent probes by engineering tetraphenylethene (TPE) for measuring viscosity in mitochondria and lysosomes, respectively. These two probes are only weakly emissive in nonviscous medium and the emission signals are greatly enhanced in viscous medium due to the restriction of intramolecular motion. The presence of pyridium has endowed one probe with mitochondrial specificity, while the presence of indole ring has granted the other probe with lysosome-targeting ability. Their optical properties are characterized in vitro and their applications in imaging viscosity variations in mitochondria and lysosomes are also demonstrated in living cells under different stimulated processes. In addition, an increase in both mitochondrial and lysosomal viscosity during mitophagy was revealed for the first time with our probes. To our knowledge, this is the first time that TPE is engineered to be fluorescent molecular viscosimeters that possess desirable aqueous solubility, red-shifted emission, and organelle specificity.