New fluorescent zinc complexes: towards specific sensors for hydrogen sulfide in solution

Fluorescent half-salen phenoxy-imine zinc complexes to reveal exogenous and endogenous H2S

In this contribution, the synthesis and the reactivity with HS- of a family of half-salen Zn complexes are reported. We provide evidence that HS- binds the zinc center of all the complexes under investigation. DFT and CCSD(T) calculations were performed to model the reactivity of these complexes with HS-. We successfully applied a homoleptic zinc complex bearing a Schiff-based ligand with pyridine pendant arms as a probe for the monitoring of exogenous and endogenous H2S levels in live 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 purine-based fluorescent probe for H2S detection and imaging of cells

Hydrogen sulfide (H2S) is a gas with a toxic odor that plays an irreplaceable role in physiological activities within the mammalian body. Therefore, it is important to do the distribution and quantitative detection of H2S in mammalian cells. In this paper, a fluorescence probe (EDPH) based on purine scaffold was designed and synthesized with high sensitivity and good selectivity. H2S induced ether bond breakage in EDPH, resulting in a significant redshift of the absorption band (from 370 nm to 500 nm) with a Stokes shift of 130 nm. After the addition of H2S, the fluorescence intensity of EDPH showed a good linear correlation with the concentration of H2S, which enabled the quantitative detection of H2S with a low limit of detection (41 nM). Finally, the EDPH was applied to the cellular Hele, and the probe has good cellularity imaging capability for the detection of H2S in living systems.

Advances and Opportunities in H2S Measurement in Chemical Biology

Hydrogen sulfide (H2S) is an important biological mediator across all kingdoms of life and plays intertwined roles in various disciplines, ranging from geochemical cycles to industrial processes. A common need across these broad disciplines is the ability to detect and measure H2S in complex sample environments. This Perspective focuses on key advances and opportunities for H2S detection and quantification that are relevant to chemical biology. Specifically, we focus on methods for H2S detection and quantification most commonly used in biological samples, including activity-based H2S probes, the methylene blue assay, the monobromobimane assay, and H2S-sensitive electrode measurements. Our goal is to help simplify what at first may seem to be an overwhelming array of detection and measurement choices, to articulate the strengths and limitations of individual techniques, and to highlight key unmet needs and opportunities in the field.

Sol-Gel Dipping Devices for H2S Visualization

In this contribution we report the synthesis and full characterization, via a combination of different spectroscopies (e.g., ¹H NMR, UV-vis, fluorescence, MALDI), of a new family of fluorescent zinc complexes with extended π-conjugated systems, with the final aim of setting up higher performance H₂S sensing devices. Immobilization of the systems into a polymeric matrix for use in a solid-state portable device was also explored. The results provided proof-of-principle that the title complexes could be successfully implemented in a fast, simple and cost-effective H₂S sensing device.

Insight on Mercapto-Coumarins: Synthesis and Reactivity

Mercapto (or sulfanyl)-coumarins are heterocycles of great interest in the development of valuable active structures in material and biological domains. They represent a highly exploitable class of compounds that open many possibilities for further chemical transformations. The present review aims to draw focus toward the synthetic applicability of various forms of mercapto-coumarins and their representations in pharmaceuticals and industries. This work covers the literature issued from 1970 to 2021.

Comparative Study of Nitro and Azide Functionalized Zn(II) based Coordination Polymers as Fluorescent Turn-on Probes for Rapid and Selective Detection of H2S in Living Cells

Selective Detection of H 2 S in cellular system using fluorescent CPs/MOFs is of great scientific interest due to their outstanding aqueous stability, biocompatibility and real-time detection ability.  Fabrication of such materials using complete biologically essential elements and applying them as an efficient biosensors is still quite challenging.  In this context, we present two newly synthesized CPs containing biologically essential metal ion (Zn) and nitro/azido functional group on the framework to sense extracellular and intracellular H 2 S by reducing into respective amines.  The CP- 1 containing the azide group acted as an efficient fluorencent turn-on probe with lowest detection limit (7.2 µM) and shortest response time (30 sec) among the Zn-based probes reported till date.  Moreover, CP-1 exhibited green luminescence in live cells after imaging very low concentration of H 2 S, while the nitro analogue, CP-2, couldn't detect the target analyte due to it's framework disruption.

Imidazo-pyridine-based zinc(II) complexes as fluorescent hydrogen sulfide probes

In this work we explore the interaction of HS^- with a family of fluorescent zinc complexes. In particular we selected a family of complexes with N,O-bidentate ligands aiming at assessing whether the zinc-chelating ligand plays a role in influencing the reactivity of HS^- with the complexes under investigation. Different experiments, performed by diverse spectroscopic techniques, provide evidence that HS^- binds the zinc center of all the complexes included in this study. The results highlight the potential of the devised systems to be used as HS^-/H₂S fluorescent sensors via a coordinative-based approach. To shed light on the species formed in solution when HS^-/H₂S interacts with the title complexes and aiming to rationalize the photophysical properties of the sensing constructs, we performed a computational analysis based on the time dependent density functional theory (TD-DFT). Preliminary bio-imaging experiments were also performed and the results indicate the potential of this class of compounds as probes for the detection of H₂S in living cells.

The contribution of metalloporphyrin complexes in molecular sensing and in sustainable polymerization processes: a new and unique perspective

This review highlights the recent developments in the field of metalloporphyrins as optical probes for biologically relevant molecules, such as nitric oxide (NO) and hydrogen sulfide (H2S), and as catalysts for the preparation of sustainable polymers such as polyesters, by the ring-opening polymerization (ROP) of cyclic esters and the ring-opening co-polymerization (ROCOP) of epoxides and anhydrides, and polycarbonates by the chemical fixation of carbon dioxide (CO2). The great potential of porphyrins is mainly due to the possibility of making various synthetic modifications to the porphyrin ring, such as modifying the coordinated metal, peripheral substituents, or even the molecular skeleton. Due to the strict structure-property relationships, one can use porphyrinoids in several different applications such as, for instance, activation of molecular oxygen or catalysis of photosynthetic processes. These possibilities broaden the application of porphyrins in several different fields of research, further mimicking what nature does. In this context, here, we want to provide evidence for the great flexibility of metalloporphyrins by presenting an overview of results obtained by us and others in the research fields we are currently involved in. More specifically, we report a survey of our most significant achievements regarding their use as optical probes in the context of the results reported in the literature from other research groups, and of the use of porphyrin metal(iii) complexes as catalysts for sustainable polymerization processes. As for the optical probe section, in addition to the metalloporphyrins synthesized ad hoc in the laboratory, the present work also covers the natural proteins containing a porphyrin core.

Fluorescent salen-type Zn(II) Complexes As Probes for Detecting Hydrogen Sulfide and Its Anion: Bioimaging Applications

In this work, we investigate the mode of interaction of a family of fluorescent zinc complexes with HS- and H2S. Different experiments, performed by diverse spectroscopic techniques, provide evidence that HS- binds the zinc center of all the complexes under investigation. Treatment with neutral H2S exhibits a markedly different reactivity which indicates selectivity for HS- over H2S of the systems under investigation. Striking color changes, visible to the naked eye, occur when treating the systems with HS- or by an H2S flow. Accordingly, also the fluorescence is modulated by the presence of HS-, with the possible formation of multiple adducts. The results highlight the potential of the devised systems to be implemented as HS-/H2S colorimetric and fluorescent sensors. Bioimaging experiments indicate the potential of using this class of compounds as probes for the detection of H2S in living cells.

Small-molecule fluorescent probes for imaging gaseous signaling molecules: current progress and future implications

Endogenous gaseous signaling molecules including nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) have been demonstrated to perform significant physiological and pharmacological functions and are associated with various diseases in biological systems. In order to obtain a deeper insight into their roles and mechanisms of action, it is desirable to develop novel techniques for effectively detecting gaseous signaling molecules. Small-molecule fluorescent probes have been proven to be a powerful approach for the detection and imaging of biological messengers by virtue of their non-invasiveness, high selectivity, and real-time in situ detection capability. Based on the intrinsic properties of gaseous signaling molecules, numerous fluorescent probes have been constructed to satisfy various demands. In this perspective, we summarize the recent advances in the field of fluorescent probes for the detection of NO, CO and H2S and illustrate the design strategies and application examples of these probes. Moreover, we also emphasize the challenges and development directions of gasotransmitter-responsive fluorescent probes, hoping to provide a general implication for future research.

Hydrosulfide complexes of the transition elements: diverse roles in bioinorganic, cluster, coordination, and organometallic chemistry

Molecules containing transition metal hydrosulfide linkages are diverse, spanning a variety of elements, coordination environments, and redox states, and carrying out multiple roles across several fields of chemistry.

A high-efficiency salamo-based copper(ii) complex double-channel fluorescent probe

In this paper, a salamo-based copper(ii) complex probe L-Cu2+ was synthesized, which combined with copper(ii) ions to form L-Cu2+ for the detection of S2- and had a good fluorescence chemical response. Through spectral analysis, we found that S2- could be identified with high sensitivity and selectivity in the presence of various anions and could be used for the detection of S2- by the naked eye. With the addition of S2-, the solution color changed from colorless to bright yellow. UV absorption, fluorescence and other characterization methods were carried out, and the mechanism of action was determined. In addition, we performed a visual inspection of H2S gas, and the probe L-Cu2+ could detect S2- in the gas molecules, revealing its potential application value in biology and medicine.

Reversible Detection and Quantification of Hydrogen Sulfide by Fluorescence Using the Hemoglobin I from Lucina pectinata

A new detection system for the endogenous gaseous transmitter and environmental pollutant hydrogen sulfide is presented. It is based on the modulation of the fluorescence spectrum of a coumarin dye by the absorption spectrum of the recombinant hemoglobin I from clam Lucina pectinata upon coordination of the analyte. While we establish that the reported affinity of rHbI for H₂S has been overestimated, the association of the protein with an appropriate fluorophore allows fast, easy, and reversible detection and quantification of hydrogen sulfide in buffer as well as biological fluids such as human plasma, with a quantification limit around 200 nM at pH 7.4.

The Drug Developments of Hydrogen Sulfide on Cardiovascular Disease

The recognition of hydrogen sulfide (H₂S) has been evolved from a toxic gas to a physiological mediator, exhibiting properties similar to NO and CO. On the one hand, H₂S is produced from L-cysteine by enzymes of cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), 3-mercaptopyruvate sulfurtransferase (3MST) in combination with aspartate aminotransferase (AAT) (also called as cysteine aminotransferase, CAT); on the other hand, H₂S is produced from D-cysteine by enzymes of D-amino acid oxidase (DAO). Besides sulfide salt, several sulfide-releasing compounds have been synthesized, including organosulfur compounds, Lawesson's reagent and analogs, and plant-derived natural products. Based on garlic extractions, we synthesized S-propargyl-L-cysteine (SPRC) and its analogs to contribute our endeavors on drug development of sulfide-containing compounds. A multitude of evidences has presented H₂S is widely involved in the roles of physiological and pathological process, including hypertension, atherosclerosis, angiogenesis, and myocardial infarcts. This review summarizes current sulfide compounds, available H₂S measurements, and potential molecular mechanisms involved in cardioprotections to help researchers develop further applications and therapeutically drugs.

Chemically reversible binding of H2S to a zinc porphyrin complex: towards implementation of a reversible sensor via a "coordinative-based approach"

Binding of hydrogen sulfide (H₂S) to a zinc porphyrin complex and the stabilization of the related zinc hydrosulfido adduct are explored. High-resolution MALDI Fourier transform ion cyclotron resonance mass spectrometry (HR MALDI-FT-ICR) and ¹H NMR experiments provide evidence that HS^- coordination occurs at the zinc centre. The coordination of HS^- occurs in a reversible manner and modulates fluorescence emission of a tetra(N-methylpyridyl)porphine zinc complex (TMPyPZn). The results highlight the potential of TMPyPZn and related systems for the implementation of fast and simple H₂S sensors via a coordinative-based approach.

Interaction of monohydrogensulfide with a family of fluorescent pyridoxal-based Zn(ii) receptors

We studied the reactivity of HS⁻ with a family of fluorescent zinc complexes. In the case of complexes 1 and 3, we have evidence that the interaction with HS⁻ results in the displacement of the coordinated ligand from the Zn center. For complex 2, our data points to the coordination of HS⁻ to the metal center likely assisted by hydrogen bondings with the OH of the pyridoxal moiety.

Detection of sulfide ion and gaseous H2S using a series of pyridine-2,6-dicarboxamide based scaffolds

This work presents a series of pyridine-2,6-dicarboxamide based scaffolds with different appendages and their roles as chemosensors for the selective detection of S²⁻ ion, as well as gaseous H₂S, in primarily aqueous media.

Nanocrystalline cellulose mediated seed-growth for ultra-robust colorimetric detection of hydrogen sulfide

We describe an ultra-stable, ultra-robust, straightforward and low-cost approach for the colorimetric detection of H₂S with nanocrystalline cellulose (NCC) based on the reaction of H₂S with lead acetate. The presence of NCC not only mediates the seed growth of a PbS/NCC complex, but also acts as a stabilizer protecting PbS from precipitation. This stable system is so robust that it can be used to quantitatively detect H₂S even after two-year storage.

One-pot fabrication of FRET-based fluorescent probe for detecting copper ion and sulfide anion in 100% aqueous media

The design of effective tools for detecting copper ion (Cu²⁺) and sulfide anion (S^2-) is of great importance due to the abnormal level of Cu²⁺ and S^2- has been associated with an increase in risk of many diseases. Herein, we report on the fabrication of fluorescence resonance energy transfer (FRET) based fluorescent probe PF (PEI-FITC) for detecting Cu²⁺ and S^2- in 100% aqueous media via a facile one-pot method by covalent linking fluorescein isothiocyanate (FITC) with branched-polyethylenimine (b-PEI). PF could selectively coordinate with Cu²⁺ among 10 metal ions to form PF-Cu²⁺ complex, resulting in fluorescence quenching through FRET mechanism. Furthermore, the in situ generated PF-Cu²⁺ complex can be used to selectively detect S^2- based on the displacement approach, resulting in an off-on type sensing. There is no obvious interference from other anions, such as Cl^-, NO₃^-, ClO₄^-, SO₄^2-, HCO₃^-, CO₃^2-, Br^-, HPO₄^2-, F^- and S₂O₃^2-. In addition, PF was successfully used to determine Cu²⁺ and S^2- in human serum and tap water samples. Therefore, the FRET-based probe PF may provide a new method for selective detection of multifarious analysts in biological and environmental applications, and even hold promise for application in more complicated systems.

Investigation of a sensing approach based on a rapid reduction of azide to selectively measure bioavailability of H2S

A new reaction-based sensor (AHS) was synthesized for quantitative detection of H2S. AHS showed a high selectivity and sensitivity toward H2S over other thio-containing molecules, or reducing reagents with high abundance in living cells. In the presence of H2S, significant fluorescence enhancement (17-fold) was observed due to the reduction of the azide on AHS. The absorption (362 nm) and fluorescence emission (557 nm) of reduced AHS showed a highly linear correlation to H2S level, which were used to measure concentration of H2S in the range of 0-100 μM.

Stabilization of a Zn(ii) hydrosulfido complex utilizing a hydrogen-bond accepting ligand

Inclusion of a hydrogen bond accepting motif in the secondary coordination sphere of a pyridinediimine ligand enables formation of a stable Zn–SH adduct. We report here reversible coordination of HS⁻ to Zn(didpa)Cl₂ to form [Zn(didpa)Cl₂SH]⁻, which is stabilized by an intramolecular hydrogen bond.

Single molecular multianalyte signaling of sulfide and azide ions by a nitrobenzoxadiazole-based probe

Sulfide- and azide-selective multianalyte optical signaling using nitrobenzoxadiazole-pivalate was realized by regioselective cleavage of the probe under the same conditions.

Alizarin red S–zinc(ii) fluorescent ensemble for selective detection of hydrogen sulphide and assay with an H2S donor

A new Alizarin Red S based fluorescent ensemble ARS–Zn(ii) for the selective detection of H₂S is reported. It exhibited H₂S selectivity and also detected H₂S in serum under physiological conditions. The calculated detection limit of H₂S was 92 nM.

Selective turn-on fluorescent probes for imaging hydrogen sulfide in living cells

Hydrogen sulfide (H(2)S) is an important biological messenger but few biologically-compatible methods are available for its detection. Here we report two bright fluorescent probes that are selective for H(2)S over cysteine, glutathione and other reactive sulfur, nitrogen, and oxygen species. Both probes are demonstrated to detect H(2)S in live cells.