Fluorescent probes for hydrogen polysulfides (H₂S_n, n > 1): from design rationale to applications

Hydrogen polysulfides (H₂S_n, n > 1) being active members of the reactive sulphur species family are gaining much research interest these days due to their involvement in signal transduction, anti-cancer activity and cytoprotection. The importance of H₂S_n particularly in human health makes it desirable to explore selective and sensitive techniques for their detection. Out of the several techniques and tools available, fluorescence spectroscopy seems advantageous due to its high sensitivity, non-destructive nature and applicability to living systems. Remarkable progress has been made in this field and different approaches have been adopted for the selective detection of H₂S_nin vitro as well as in vivo. The present review discusses the importance of H₂S_n in living systems and highlights recent advances in the design of fluorescent probes for the detection of H₂S_n. It offers a critical overview of the designing strategies adopted for H₂S_n detection with emphasis on various ways for achieving selectivity over other reactive sulphur species (RSS) and their applications in biological systems. Further, the fundamental challenges and future prospects in this field are also discussed.