Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species.

Recent studies revealed that the increased level of hypochlorous acid (HOCl) may be deemed to be one of the signs of chronic inflammatory joint disease. Accordingly, the development of effective methods for rapid and accurate detection or monitoring of HOCl in vivo is of great significance for further understanding the role of HOCl in rheumatoid arthritis (RA). Herein, a ratiometric near-infrared (NIR) fluorescent probe (PTA) was reported for the detection and monitoring of HOCl. In the presence of HOCl, the electron-rich S atom and C=C double bond of probe PTA were oxidized in sequence, resulting in the significant hypochromatic shift and decline of absorption spectra. Simultaneously, the intramolecular charge transfer (ICT) process of PTA is inhibited, causing the intrinsic fluorescence emission of PTA shift from 680 to 550 nm. PTA-based test paper strips were successfully prepared and applied to determinate HOCl in actual water samples by “naked eye” colorimetric method. PTA features NIR emission, large Stokes shift (200 nm), low cytotoxicity, high sensitivity (33.9 nM), and short response time (45 s), which enable it to be successfully utilized for imaging endogenous and exogenous HOCl in living zebrafish and mice. More importantly, PTA shows remarkable effectiveness for the monitoring of HOCl-mediated treatment response to RA. Consequently, PTA provides a new approach to further understand the role of HOCl in RA and evaluate the drug treatment efficiency of RA. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/smo.20220007

A ratiometric near‐infrared fluorescent probe for the detection and monitoring of hypochlorous acid in rheumatoid arthritis model and real water samples

CD-based ratiometric fluorescence probes are of great significance for visual detection, but accomplishing this goal is still a particularly challenging task. Herein, nitrogen-doped carbon dots (NCDs) with bright yellow fluorescence were easily manufactured via a one-pot hydrothermal method for visual detection of hypochlorite (ClO-) and thiosulfate (S2O32-) under UV light irradiation. The as-prepared NCDs demonstrate favorable water solubility, excellent biocompatibility, superior optical properties and low cytotoxicity. Strikingly, the fluorescence of the NCDs could be quenched with ClO-. Based on these results, an original fluorescent nanoprobe was constructed for the highly discriminating recognition of ClO- by oxidation of the amino groups on their surface to nitro groups. The assay covered the ranges from 0.067 to 19.33 μM and 24 to 98 μM with a limit of detection (S/N = 3) of as low as 0.013 μM. Remarkably, a growing peak appears at 537 nm and the emission at 492 nm shrinks with the introduction of S2O32-, which demonstrates ratiometric fluorescence emission characteristics (F537nm/F492nm) in the range of 6.6-100 μM with a limit of detection (S/N = 3) of as low as 0.78 μM. In addition, the fluorescence color of the NCDs also changes (yellow-green-blue) after adding various ClO- concentrations. The fluorescence color of the NCDs-ClO- also changes (blue-green-yellow) after adding various S2O32- concentrations. This excellent ratiometric fluorescence probe was successfully further used for nuclear imaging. Accordingly, an easy-to-prepare paper-based sensor to identify ClO- and S2O32- was fabricated, which demonstrated their adaptability for in situ on-site testing. This research further opens up new opportunities for the development of efficient yellow fluorescent probes based on NCDs nanomaterials for visual detection, biomarking, and biomedical optical imaging.

One-pot synthesis of efficient multifunctional nitrogen-doped carbon dots with efficient yellow fluorescence emission for detection of hypochlorite and thiosulfate.

3-Aminophenylboronic acid–functionalized molybdenum disulfide quantum dots for fluorescent determination of hypochlorite

In this work, we reported a fluorescent probe Fur-SH, a derivative of benzofuranone, which was used to detect H2S in living cells and zebrafish. Based on the three structural characteristics of the probe, the effects of different structural modifications on the optical properties of the fluorophore were compared. Then, the fluorophore Fur-OH was synthesized by modifying diethylamino group with benzofuranone as the main skeleton. With 2,4-dinitrofluorobenzene as the recognition group and diethylamino as the electron donor, the push–pull electron effect occurred with nitro group, which led to fluorescence quenching, and an openable fluorescent probe Fur-SH was formed. The probe Fur-SH (λex = 510 nm; λem = 570 nm) had the advantages of smaller full width at half maxima, rapid response (5 min) and wide pH window. The quantitative properties of the probe were excellent, reaching saturation at 50 equivalents of substrate. The probe Fur-SH showed high sensitivity to H2S, with LOD of 48.9 nM and LOQ of 50 nM. At present, the probe Fur-SH had been applied to fluorescence imaging of MCF-7 cells and zebrafish. By comparing the effects of different structures on the optical properties of fluorophores, this work was expected to be helpful to the development of fluorescent probes in the future. 

A structural optimized fluorescent probe for monitoring hydrogen sulfide in cells and zebrafish

A dual functional fluorescent probe based on naphthalimide for detecting Cu2+ and pH and its applications

Hydrazine is toxic and reductive, and excessive intake will cause irreversible harm to animals and plants. It is necessary to develop an effective method to detect N2H4. Therefore, in this paper, a novel near-infrared fluorescent probe XCCM was designed and synthesized with xanthene as the fluorophore for the detection of N2H4. The probe XCCM is very sensitive to N2H4 (LOD = 13.3 nM), fast response (20 s) and has a very large Stokes shift value (231 nm). In addition, it has been successfully applied to detect N2H4 in natural environment (actual water sample, soil and food) and biological system (HepG2 cells). 

A novel near-infrared fluorescence probe for detecting N2H4 and its application in natural environment and biological system

A simple and efficient fluorescent probe based on 1,8-naphthalimide – Ebselen for selectively detecting H2S in living cells

A Dual Functional Fluorescent Probe Based on Phenothiazine for Detecting Hg2+ and ClO- and its Applications.

A turn-on colorimetric dual-response fluorescent probe for rapid detecting and its application in bio-imaging and real water samples

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