Following carbon monoxide and nitric oxide, hydrogen sulfide (H₂S) is found to be the third endogenous gasotransmitter, which provides the regulation significance of physiological and pathological processes in the cardiovascular and nervous systems. Therefore, the selective recognition and detection of H₂S are of importance. Fluorescent probe method, among biological detection technologies, is an indispensable technique for the biological species analysis, and that is highlighted by its good selectivity, high sensitivity, noninvasive detection, and real-time monitoring in situ. Recently, the development of fluorescent probes for intracellular H₂S detection has been becoming one of the hot topics. Herein, the progress during the last three years of fluorescent molecular probes based on the small molecules for H₂S detection are reviewed. These fluorescent probes are classified and concluded according to the different types of chemical reaction with H₂S, and then arranged specifically by the fluorophores in different type. The design concepts of molecular structures, detection mechanism and biological applications of these probes are introduced. In addition, the relationship between molecular structures and properties while testing are elucidated. Finally, the challenge and application prospects for the development of hydrogen sulfide fluorescent probes are also discussed.

Contents
1 Introduction
2 Chemical strategies to design H₂S fluorescent probes
2.1 Detection of H₂S via reduction reactions
2.2 Detection of H₂S via nucleophilic addition reactions
2.3 Detection of H₂S via copper-sulfide precipitation
2.4 Detection of H₂S via thiolysis reactions
2.5 Detection of H₂S via redox reactions
3 Fluorescent probes based on the reduction reaction
3.1 H₂S probes employ coumarin dyes as fluorophore
3.2 H₂S probes employ naphthalimide dyes as fluorophore
3.3 H₂S probes employ rhodamine dyes as fluorophore
3.4 H₂S probes employ near-infrared fluorescent (including two-photon) dyes as fluorophore
3.5 H₂S probes employ other dyes as fluorophore
4 Fluorescent probes based on the nucleophilic addition
4.1 H₂S probes employ BODIPY dyes as fluorophore
4.2 H₂S probes employ fluorescein dyes as fluorophore
4.3 H₂S probes employ benzothiazole dyes as fluorophore
4.4 H₂S probes employ cyanine dyes as fluorophore
4.5 H₂S probes employ other dyes as fluorophore
5 Fluorescent probes based on the Cu-S precipitation
5.1 H₂S probes employ fluorescein dyes as fluorophore
5.2 H₂S probes employ cyanine dyes as fluorophore
5.3 H₂S probes employ other dyes as fluorophore
6 Fluorescent probes based on the thiolysis reaction
7 Fluorescent probes based on mimics selenium enzyme
8 Fluorescent probes for other active sulfur species
8.1 Fluorescent probes for bisulfite anion
8.2 Fluorescent probes for sulfane sulfurs
9 Conclusion