Design,Synthesis And Properties Of Deep-Red Fluorescent Probes For Sulfur-Containing Compounds

In recent years,small-organic-fluorescent probes have attracted wide attention owing to their advantages of simple synthesis,low toxicity,low detection limit,and high spatiotemporal resolution.Among them,deep-red fluorescent probe is favored because of its longer emission wavelength,smaller photodamage,deeper tissue penetration and lower background interference.It has become a powerful tool for the fast visualization of analyte substances.As an important industrial chemical,thiophenol is mainly used in the organic synthesis of medicine and pesticide preparations,but it is highly toxic to organisms and extremely detrimental to the environment.Long-term exposure to the environment containing thiophenol can lead to respiratory diseases.Under physiological condition,thiophenol can be oxidized into disulphide with concomitant generation of harmful reactive oxygen species?ROS?such as superoxide radicals and hydrogen peroxide,and the generated ROS can cause significant oxidative damage to erythrocyte cells.Therefore,monitoring the thiophenol content in the environment and organisms is of great significance for rectifying environmental pollution and treating human diseases.Biothiols,as an essential reactive sulfur species?RSS?,are mainly including cysteine?Cys?,homocysteine?Hcy?,glutathione?GSH?three amino acids.They have a similar chemical structure:an amino group?–NH₂?and a nucleophilic sulfhydryl group?–SH?,perform important actions in maintaining redox homeostasis and signal transduction in living organisms.Therefore,tracking the dynamic fluctuations of biothiols has an important biological significance for clarifying their intricate interrelationships.As an important reactive species,sulfur dioxide exists with the form of HSO₃^?/SO₃^2?in organism,can be produced via the oxidation of hydrogen sulfide or sulfur-containing amino acids.It plays vital roles in important physiological processes such as vasodilation,muscle relaxation,information transmission,and participates actively in regulating the redox status as an antioxidant against reactive oxygen species?ROS?.Mitochondria,as vital organelle provide energy for the body is the main source of intracellular reactive oxygen/sulfur species?ROS/RSS?.Studies have shown that reactive sulfur species?RSS?is closely related to tumor cells,and endogenous SO₂ is produced by mitochondrial aspartate aminotransferase 2,therefore,developing an accurate and effective mitochondrial SO₂ research method has a positive effect on understanding the occurrence of cancer.In the first part of the paper,the research group developed a far-red fluorescent probe?FRP-Thio?based on merocyanine core for the specific and visual detection of thiophenol.The introduction of malononitrile groups enhances the intramolecular charge transfer?ICT?effect of the probe,increases the emission wavelength to the deep-red region,can reduce the photo damage or background autofluorescence interference in bioimaging.Thiophenol can undergo a nucleophilic substitution reaction with the probe to expose the hydroxyl group to open the ICT effect to restore fluorescence in solution of DMF-H₂O,which lead to strong deep-red fluorescence increase at 645 nm?50-fold?with a low detection limit of 9 nM,response time was approximately 30 minutes.The probe has been successfully applied to the visualization of thiophenol in SH-SY5Y cells because of its good biocompatibility.In the second part of the paper,a two-photon,deep-red fluorescent probe?Cou-DNBS?through connecting 2,4-dinitrobenzenesulfonyl chloride?DNBS?to construct a sulfonamide structure for the detection of biothiols has been developed.Our group increases the rigidity of the compound through fusing additional aromatic rings to red-shift the emission wavelength of the iminocoumarin fluorescent dye?Cou-NH?to the deep-red region.In the DMF-H₂O solution,the thiol undergoes a nucleophilic substitution reaction with the probe,the sulfonamide structure is cleaved,the imine group is exposed,and the fluorescence is restored.At the excitation wavelength of 470 nm,the probe Cou-DNBS showed strong orange fluorescence at617 nm.The probe Cou-DNBS has good biocompatibility,rapid response?10 s?,and low detection limit?Cys is 2.5 nM,Hcy is 1.7 nM,GSH is 0.84 nM?.In addition,the probe Cou-DNBS has been successfully used in monitoring self-healing condition of oxidative damage in living He La cells and endogenous biothiol in mouse model.In the third part of the paper,the research group connected the pyranium ring structure with coumarin dye,which has high fluorescence quantum yield to realize the specific detection of sulfur dioxide use the tunability of the fluorescence generation/quenching of the pyranium structure.At the same time,because the pyranium is positively charged,the mitochondria are localized.In EtOH-H₂O solution,HSO₃^?and the probe?Cou-Pyranium?undergo Michael addition reaction to form Cou-Pyranium-SO₃H,the conjugation of the probe is reduced,and the emission wavelength is blue shifted from 690 nm to 570-610 nm.The probe has a fast response speed?1min?,high sensitivity,and low detection limit,after testing and calculation,the detection limit reached 0.075?M.