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.

A phenothiazine-linked pyridine acetonitrile with a cyanoethylene bond PZ-PY has been synthesized by Knoevenagel condensation reaction and characterized by spectroscopic and analytical techniques. The structure of PZ-PY was confirmed through single-crystal X-ray diffraction studies; revealing its monoclinic crystal system with a P21/c space group. The absorption maximum and emission spectrum of the PZ-PY receptor were observed at 431 and 608 nm respectively. Cyanide anion is one of the most prevalent chemicals in the environment, its toxic nature poses a constant threat to human daily life. The PZ-PY receptor's recognition of cyanide ions is associated with remarkable photophysical properties. The binding stoichiometry of PZ-PY-CN- was confirmed by Job's plot, B-H studies, 1H NMR titration, TD-DFT, and ESI-mass spectroscopy. The relatively low limit of detection was determined to be 8.79 × 10 -8 M in a linear range between 0 and 120 µL. PZ-PY receptor exhibits an aggregation-caused quenching (ACQ) nature with the increase in water fraction; it is stable between pH 2 to 11. Moreover, the practical applicability of PZ-PY was investigated through the test strip, solid-state, bioimaging (A547 cell line), detection of cyanide in diverse water samples (tap water, distilled water, and lake water), and food samples (sprouted potatoes, bitter almonds, and cassava). 

Phenothiazine-derived fluorescent chemosensor: A versatile platform enabling swift cyanide ion detection and its multifaceted utility in paper strips, environmental water, food samples and living cells

In the present work, a simple and highly selective colorimetric chemosensor (CS-02), was designed and synthesized from 5(4)-amino-4(5) -(amino carbonyl)-imidazole hydrochloride and 5-(4-chloro phenyl) furan-2-carbaldehyde. The formation of the CS-02 (E)-5-amino-N-((5-(4-chlorophenyl) furan-2-yl) methylene)-1H-imidazole-4-carboxamide was confirmed by 1HNMR and Mass spectral techniques. The receptor CS-02 was found to show a colorimetric response of the colour change from Light yellow to Cyber yellow towards CN− ion, due to de-protonation and intra-ligand charge transfer. The receptor CS-02 interacts with cyanide in a 1:1 stoichiometric manner with a detection limit of 10.39 × 10−5 M. Receptor CS-02 could be recyclable by treatment with TFA. The sensing behaviour of the receptor was further enunciated by computational studies and it's also be efficiently used to detect trace amounts of cyanide present in Real samples such as tobacco, flax seeds, apple seeds, cigarette and Indian cigarette. Moreover, a visible colorimetric kit such as a test paper strip would be used in the CS-02 for practical applications. 

The development of a simple imidazole-based probe for the selective detection of cyanide ion in real samples

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

New dual-responsive fluorescent sensor for hypochlorite and cyanide sensing and its imaging application in live cells and zebrafish.

A near-infrared fluorescent probe based on phenothiazine for rapid detecting of CN and ClO

A fluorescent probe based on phenothiazine for detection of ClO- with naked-eye color change properties.

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

In order to specifically detect Ni2+ and ClO−, a double functional s-triazine-phenothiazine fluorescent probe (BPTD) was synthesized, in which the multiple nitrogen atoms in s-triazine and hydrazone connecting arm were used for recognizing Ni2+, sulfur atom in phenothiazine was used as the reaction site for recognizing ClO−. BPTD has excellent optical properties: a large Stokes shift (131 nm) and good stability in a wide pH range (3.00 ∼ 12.00). Compared to the two fluorophores (s-triazine and phenothiazine), fluorescence emission peak of BPTD produces a large red-shift. The spectral experimental results show that BPTD can detect Ni2+ (5 ∼ 80 μM) and ClO− (10 ∼ 40 μM) with high selectivity and strong anti-interference. Compared with the reported fluorescent probes for detection of Ni2+, BPTD can detect Ni2+ more sensitively (response time 2.5 min, red shift 26 nm, the limit of detection (LOD) 98 nM), and simultaneously achieve the goal of highly sensitive detection of ClO− (response time 20 s, blue shift 18 nm, LOD 96 nM). BPTD can be applied to qualitative and quantitative detection of Ni2+ and ClO− in actual water samples, and made into portable test paper for rapid detection of Ni2+ and ClO−. 

A novel fluorescent probe based on phenothiazine for recognition of Ni2+ and ClO− 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

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