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

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.

Comment on the Definition and Labeling of pK50

We propose a more rigorous definition for the recently introduced concept of pK50. The value of pK50 should be associated not with a "functional group", as originally postulated, but instead with an atom. The proposed clarification is meant to improve the interpretation and labeling of pK50.

Theoretical Investigation on the ESIPT Process and Detection Mechanism for Dual-Proton Type Fluorescent Probe

Recently, a new fluorescent probe AE-Phoswas reported to detect the activity of alkaline phosphatases (ALP) in different living cell lines. Here, we present an in-depth computational analysis of the mechanism and source of the fluorescence of the AE-Phos probe. There is an intermediate product (AE-OH-Phos) in the experiment as well as a different configuration of products that may emit fluorescence. It is essential to investigate the origin of fluorescence and the detection mechanism of the probe, which could help us eliminate the interference of other substances (including an intermediate product and possible isomers) on fluorescence during the experiment. According to the change of geometric parameters and Infrared spectra, we deduce that the dual intramolecular hydrogen bonds of salicylaldehyde azine (SA) were enhanced at the excited state, while AE-OH-Phos was attenuated. Considering the complex ESIPT behavior of the dual proton-type probe, the potential energy surfaces were further discussed. It can be concluded that the single proton transfer structure of SA (SA-SPT) is the most stable form. Both the concerted double proton transfer process and stepwise single proton transfer process of SA were forbidden. The fluorescence for SA was 438 nm, while that of SA-SPT was 521 nm, which agrees with the experimentally measured fluorescence wavelength (536 nm). The conclusion that single proton transfer occurs in SA is once again verified. In addition, the distribution of electron-hole and relative index was analyzed to investigate the intrinsic mechanism for the fluorescence quenching of the probe and the intermediate product. The identification of the origin of fluorescence sheds light on the design and use of dual-proton type fluorescent probes in the future.

Cyanide Boosting Copper Catalysis: A Mild Approach to Fluorescent Benzazole Derivatives from Nonemissive Schiff Bases in Biological Media

An application of nucleophilic cyclization and oxidation of nonemissive Schiff bases via cyanide boosting copper catalysis to synthesize fluorescent benzazole derivatives in high conversion yield is disclosed. This approach is highlighted by broad substrate scope, fast reaction time, and mild conditions and can efficiently proceed in living cells or Arabidopsis root tissues. Furthermore, this methodology can be applied for selective detection of Cu²⁺ and CN^-.

A novel colorimetric and far-red emission ratiometric fluorescent probe for the highly selective and ultrafast detection of hypochlorite in water and its application in bioimaging

Hypochlorous acid (HOCl)/hypochlorite (OCl⁻), an important reactive oxygen species, plays a number of important roles in various physiological processes.

A red-emitting fluorescent probe for hydrogen sulfide in living cells with a large Stokes shift

An azido-based fluorescent probe was developed for the sensitive and selective detection of H₂S with a red emission and a large Stokes shift. The probe was successfully applied to detect H₂S both in aqueous solution and in living cells.

A novel isophorone-based red-emitting fluorescent probe for selective detection of sulfide anions in water for in vivo imaging

A new red-emitting fluorescent probe for detection of sulfide anions in living cells (MCF-7 cells) and zebrafish was developed.

A new ESIPT-based fluorescent probe for highly selective and sensitive detection of hydrogen sulfide and its application in live-cell imaging

A new ESIPT-based fluorescent probe for the sensitive detection of hydrogen sulfide has been reasonably designed and developed.