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 D-π-A-based near-infrared fluorescent probe with large Stokes shift for the detection of cysteine in vivo

D-π-A dyes are an ideal strategy for building near-infrared fluorescent probes that have a large Stokes shift due to their excellent properties of adjustable emission wavelength and Stokes shift. Developing a near-infrared (NIR) fluorescent probe (JTPQ-Cys) capable of detecting cysteine (Cys) was the aim of this study. In JTPQ-Cys, julolidine served as the electron donor (D) and quinoline as the electron acceptor (A), with 3,4-ethylenedioxythiophene as the π-bridge. The π-conjugation and vibrational/rotational activity of the molecule were increased by the introduction of 3,4-ethylenedioxythiophene, causing the molecule to exhibit NIR emission and a large Stokes shift. When JTPQ-Cys was used to detect Cys, a clear fluorescence turn-on signal was observed at 741 nm, together with a Stokes shift of 268 nm. The limit of detection of JTPQ-Cys for Cys is 24 nM. Moreover, JTPQ-Cys has been utilized successfully for imaging studies of Cys in cells and zebrafish because it has good photostability, low cytotoxicity, and a high signal-to-noise ratio. Overall, our findings demonstrate the potential of JTPQ-Cys to be one of the best choices for detecting Cys in biological systems, and JTPQ is an ideal fluorophore to construct fluorescence dyes for bioimaging.

A highly selective red-emitting fluorescent probe and its micro-nano-assembly for imaging endogenous peroxynitrite (ONOO-) in living cells

Endogenous peroxynitrite plays a very important role in the regulation of life activities. However, validated tools for ONOO^- tests are currently insufficient. We designed a fluorescent probe TPA-F-NO₂ with a low fluorescence background in water based on the D-π-A structure for the imaging of endogenous ONOO^- in living cells. TPA-F-NO₂ can realize the naked eye detection of ONOO^- due to the obvious color change. TPA-F-NO₂ has the advantages of large stokes shift, high signal-to-noise ratio, high selectivity and sensitivity. The quantitative detection can be achieved in the range of 0-14 μM ONOO^-. Due to its solvatochromic characteristics, TPA-F-NO₂ has the potential to be used in OLEDs and other fields. In addition, 4-methylumbelliferone has a wide range of anticancer effects as an inhibitor of hyaluronic acid. We prepared TPA-MU-NPs by assembling TPA-F-NO₂ and 4-methylumbelliferone. It also endows TPA-MU-NPs with ONOO^- imaging function and anti-proliferation effect on breast cancer cells and other cells. This 'probe-drug' assembly strategy provides ideas for the design and optimization of dual-functional probes.

Small-molecule fluorescent probes based on covalent assembly strategy for chemoselective bioimaging

In this review, we comprehensively summarize the recent progress in the development of small molecular fluorescent probes based on the covalent assembly principle. The challenges and perspective in this field are also presented.

Analytical Methods for Detection of Gasotransmitter Hydrogen Sulfide Released from Live Cells

Hydrogen sulfide (H₂S) plays an important role in mammals as a signaling molecule. Recently, abnormal H₂S concentration has been associated with several pathophysiological states, such as diabetes mellitus, hypertension, Alzheimer's disease, and Parkinson's disease. As regulating H₂S concentration can be a very prominent way of developing new drugs, many researchers have paid great attention to H₂S research. To understand the role of H₂S in pathophysiology and develop H₂S-based therapies, it is necessary to measure the exact concentration of H₂S within biological systems. But, H₂S is volatile and can be easily oxidized. Besides, the active sites for several biological effects of H₂S are inside the cell. Therefore, there is a need for the development of new methods for the accurate and reliable detection of H₂S within live cells. This review provides a summary of recent developments in H₂S detection methods for live cell analysis.

Mitochondria-Targeted Red-Emission Fluorescent Probe for Ultrafast Detection of H2S in Food and Its Bioimaging Application

Hydrogen sulfide (H₂S) contributes to human health and prolongs the storage time of postharvest fruits and vegetables. At the same time, H₂S can cause a negative impact on some foodstuffs and beverages, so an efficient probe to detect H₂S is needed. Herein, a fluorescent turn-on responding probe SPy-DNs for H₂S detection has been designed and synthesized. SPy-DNs exhibited a red emission (608 nm), large Stokes shift (111 nm), and a detection limit of a nanomolar level (356 nM) in a dimethylformamide/phosphate-buffered saline (DMF/PBS) (1:1, v/v, 10 mM, pH 7.4) solution. SPy-DNs can detect H₂S with ultrafast response within 4 s, which is faster than the response of other reported probes. In addition, the applicability of SPy-DNs to detect H₂S has been determined in the actual water samples, targeted mitochondria, and imaged H₂S in living cells. Moreover, SPy-DNs was successfully used as a tool to judge H₂S levels in beer, which indicates that SPy-DNs possesses the advantage of rapid detection of H₂S in foodstuffs.

Simple NIR-Emitting ESIPT Fluorescent Probe for Thiophenol with a Remarkable Stokes Shift and Its Application

Thiophenol as a highly toxic compound can harm the environment and living organisms and thus demands effective detection. In this work, we presented a near-infrared fluorescent probe (DAPH-DNP) for detecting thiophenol according to the ESIPT mechanism using 2,4-dinitrophenyl group as a recognition unit. This probe displayed specificity toward thiophenol over other related analytes. Meanwhile, there was good linearity between the relative fluorescence intensity of DAPH-DNP and the concentration of thiophenol in the range of 0-80 μM. This probe also showed a low detection limit of 3.8 × 10^-8 and a marked Stokes shift (192 nm). Further, this probe could be used for monitoring thiophenol in environmental water samples and imaging thiophenol in living cells, which indicated that this probe had a real application in the environment and living organisms.

A highly sensitive ratiometric fluorescent probe for imaging endogenous hydrogen sulfide in cells

A novel fluorescent probe (QL-N3) has high potential to detect the concentration of endogenous hydrogen sulfide in cells.

Deeper insight into protease-sensitive "covalent-assembly" fluorescent probes for practical biosensing applications

We report a rational and systematic study devoted to the structural optimisation of a novel class of protease-sensitive fluorescent probes that we recently reported (S. Debieu and A. Romieu, Org. Biomol. Chem., 2017, 15, 2575-2584), based on the "covalent-assembly" strategy and using the targeted enzyme penicillin G acylase as a model protease to build a fluorescent pyronin dye by triggering a biocompatible domino cyclisation-aromatisation reaction. The aim is to identify ad hoc probe candidate(s) that might combine fast/reliable fluorogenic "turn-on" response, full stability in complex biological media and ability to release a second molecule of interest (drug or second fluorescent reporter), for applications in disease diagnosis and therapy. We base our strategy on screening a set of active methylene compounds (C-nucleophiles) to convert the parent probe to various pyronin caged precursors bearing Michael acceptor moieties of differing reactivities. In vitro stability and fluorescent enzymatic assays combined with HPLC-fluorescence analyses provide data useful for defining the most appropriate structural features for these fluorogenic scaffolds depending on the specifications inherent to biological application (from biosensing to theranostics) for which they will be used.

Endoplasmic reticulum targeted fluorescent probe for the detection of hydrogen sulfide based on a twist-blockage strategy

Hydrogen sulfide (H₂S) is very important for humans and is involved in many physiological processes. Here, we designed and reported a new naked-eye colorimetric fluorescent probe Z1 for detecting H₂S in absolute HEPES solution. The fluorescence intensity, after the reaction of the probe and H₂S, is about 32 times that of the probe alone. When the concentration of H₂S is 0-100 μM, the detection limit (DL) is rather low at about 0.15 μM (3σ/slope). The response mechanism is based on the leaving of the 2,4-dinitrobenzene moiety, followed by intramolecular cyclization to give a fluorescent iminocoumarin-benzothiazole group. Moreover, Z1 was applied to endogenous and exogenous H₂S imaging in living cells. The high overlap coefficient proved that probe Z1 has good ER-tracker localization in living cells.

Dual-Function Fluorescent Probe for Detection of Hydrogen Sulfide and Water Content in Dimethyl Sulfoxide

To detect hydrogen sulfide (H2S) and water content in dimethyl sulfoxide, the fluorescent probe (Probe 1) was used, as it not only detects H2S but also detects the water content. After H2S was added into Probe 1, the intensity of fluorescence increased and was up to 1300 times. In case the H2S concentration was in the range 0-20 μM, it was able to be detected by Probe 1, and the limit of detection was 0.851 nM. When Probe 1 and H2S underwent a reaction, the solution color had some changes. These colors changed in terms of the concentration changes of H2S, ranging from colorless to yellow. The Probe 1 test paper only needed to be exposed to hydrogen sulfide gas for 20 s for the color change to occur. Besides, Probe 1-H2S was used to detect water content in dimethyl sulfoxide which ranged from 0 to 100%. The color change of the solution was opposite to that of H2S, ranging from yellow to colorless.

An instantaneous fluorescent probe for detecting hydrogen sulfide in biological systems

An instantaneous (within seconds) fluorescent probe for detecting H₂S was developed and successfully used for H₂S imaging in living cells and zebrafish.

A reversible and turn-on type fluorescence behaviour of hydrogen sulfide via a redox cycle between selenoxide and selenide

Upon treatment with H₂S in MeCN–PBS, the fluorescence dormant selenoxides of dibenzobarrelene- and benzobarrelene-based 1-seleno-1,3-butadiene derivatives are rapidly converted to strongly fluorescent selenides.

A red-emitting fluorescent probe for the detection of Hg2+ in aqueous medium, living cells and organisms with a large Stokes shift

A vinethene-based fluorescent probe has been developed for the sensitive and selective detection of Hg²⁺ with a low detection limit, a red emission and a large Stokes shift. The probe has been successfully applied to detect Hg²⁺ in aqueous solutions, living cells and organisms.