Naphthalimide-Based Turn-On Fluorosensor for Aqueous Sulfide Ions for Staining in Living Cells

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.

A novel fluorescent probe based on naphthimide for H2S identification and application

In view of the superior chemical activity of selenoether bond (-Se-) and the excellent optical properties of naphthimide, a novel fluorescent probe (NapSe) with near-rectangular structure, which contains double naphthimide fluorophores linked by selenoether bond, is designed for specific fluorescence detection of hydrogen sulfide (H2S). NapSe has excellent optical properties: super large Stokes Shift (190 nm) and good stability in a wide pH range. The selectivity of NapSe fluorescence detection of H2S is high, and displays excellent "turn-on" phenomenon and strong anti-interference. And the fluorescence intensity increased obviously, reaching 42 times. The time response of probe NapSe is very rapid (3 min) compared with other fluorescence probes that respond to H2S. It shows high sensitivity by calculating the detection limit (LOD) as low as 5.4 μM. Notably, the identification of H2S by probe NapSe has been successfully applied to the detection of test paper and the detection of exogenous and endogenous fluorescence imaging of MCF-7 breast cancer cells.

Current Advances in Diazoles-based Chemosensors for CN- and FDetection

Advances in molecular probes have recently intensified because they are valuable tools in studying species of interest for human health, the environment, and industry. Among these species, cyanide (CN-) and fluoride (F-) stand out as hazardous and toxic ions in trace amounts. Thus, there is a significant interest in probes design for their detection with diverse diazoles (pyrazole and imidazole) used for this purpose. These diazole derivatives are known as functional molecules because of their known synthetic versatility and applicability, as they exhibit essential photophysical properties with helpful recognition centers. This review provides an overview of the recent progress (2017-2021) in diazole-based sensors for CN- and F- detection, using the azolic ring as a signaling or recognition unit. The discussion focuses on the mechanism of the action described for recognizing the anion, the structure of the probes with the best synthetic simplicity, detection limits (LODs), application, and selectivity. In this context, the analysis involves probes for cyanide sensing first, then probes for fluoride sensing, and ultimately, dual probes that allow both species recognition.

Multifunctional peptide-based fluorescent chemosensor for detection of Hg2+ , Cu2+ and S2- ions

A novel multifunctional fluorescent peptide sensor based on pentapeptide dansyl-Gly-His-Gly-Gly-Trp-COOH (D-P5) was designed and synthesized efficiently using Fmoc solid-phase peptide synthesis (SPPS). This fluorescent peptide sensor shows selective and sensitive responses to Hg²⁺ and Cu²⁺ among 17 metal ions and six anions studied in N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid (HEPES) buffer solution. The peptide probe differentiates Hg²⁺ and Cu²⁺ ions by a 'turn-on' response to Hg²⁺ and a 'turn-off' response to Cu²⁺ . Upon addition of Hg²⁺ or Cu²⁺ ions, the sensor displayed an apparent color change that was visible under an ultraviolet lamp to the naked eye. The limits of detection (LOD) of DP-5 were 25.0 nM for Hg²⁺ and 85.0 nM for Cu²⁺ ; the detection limits for Cu²⁺ were much lower than the drinking water maximum contaminant levels set out by the United States Environmental Protection Agency (USEPA). It is noteworthy that both D-P5-Hg and D-P5-Cu systems were also used to detect S^2- successfully based on the formation of ternary complexes. The LODs of D-P5-Hg and D-P5-Cu systems for S^2- were 217.0 nM and 380.0 nM, respectively. Furthermore, the binding stoichiometry, binding affinity and pH sensitivity of the probe for Hg²⁺ and Cu²⁺ were investigated. This study gives new possibilities for using a short fluorescent peptide sensor for multifunctional detection, especially for anions.

Highly Sensitive Ratiometric Chemosensor and Biomarker for Cyanide Ions in the Aqueous Medium

A newly designed cyanide-selective chemosensor based on chromone containing benzothiazole groups [3-(2,3-dihydro-benzothiazol-2-yl)-chromen-4-one (DBTC)] was synthesized and structurally characterized by physico-chemical, spectroscopic, and single-crystal X-ray diffraction analyses. The compound DBTC can detect cyanide anions based on nucleophilic addition as low as 5.76 nM in dimethyl sulfoxide-N-(2-hydroxyethyl)piperazine-N'-ethanesulfonic acid buffer (20 mM, pH 7.4) (v/v = 1:3). The binding mode between receptor DBTC and cyanide nucleophile has also been demonstrated by experimental studies using various spectroscopic tools and theoretical studies, and the experimental work has also been verified by characterizing one supporting compound of similar probable structure of the final product formed between DBTC and cyanide ion (DBTC-CN compound) by single-crystal X-ray analysis for detailed structural analyses. In theoretical study, density functional theory procedures have been used to calculate the molecular structure and the calculation of the Fukui function for evaluation of the electrophilic properties of each individual acceptor atom. Furthermore, the efficacy of the probe (DBTC) to detect the distribution of CN^- ions in living cells has been checked by acquiring the fluorescence image using a confocal microscope. Notably, the paper strips with DBTC were prepared, and these could serve as efficient and suitable CN^- test kits successfully.