A ratiometric fluorescent probe for gasotransmitter hydrogen sulfide based on a coumarin-benzopyrylium platform

Simple turn-on fluorescent probe for ultrafast and highly selective detection of hydrogen sulfide in aqueous solutions

5H-Benzimidazo[1,2-c]quinazoline-6-thione (BI-QT), was synthesized as a benzimidazole-based probe to detect H2S. BI-QT exhibits a fluorescent "turn-on" response in DMSO/H2O (9:1, HEPES 10 mM, pH 7.4) upon the addition of H2S. The BI-QT probe can determine micromolar (0-600 µM) H2S concentrations in aqueous systems, with a detection limit of 1.12 µM. Interestingly, BI-QT exhibited an ultrafast response to H2S, with maximum intensity achieved almost instantly when exposed to H2S. BI-QT is largely unaffected by pH and responds reliably over the wide 4-11 pH range, which highlights its applicability to various physiological scenarios. UV-vis, fluorescence, and 1H NMR spectroscopic analyses investigated the sensing mechanism. The practicality of the probe was demonstrated using water samples and living cells.

Analysis of optical properties and response mechanism of H2S fluorescent probe based on rhodamine derivatives

H2S plays a crucial role in numerous physiological and pathological processes. In this project, a new fluorescent probe, SG-H2S, for the detection of H2S, was developed by introducing the recognition group 2,4-dinitrophenyl ether. The combination of rhodamine derivatives can produce both colorimetric reactions and fluorescence reactions. Compared with the current H2S probes, the main advantages of SG-H2S are its wide pH range (5-9), fast response (30 min), and high selectivity in competitive species (including biological mercaptan). The probe SG-H2S has low cytotoxicity and has been successfully applied to imaging in MCF-7 cells, HeLa cells, and BALB/c nude mice. We hope that SG-H2S will provide a vital method for the field of biology.

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.

Sensitive Detection of Various Forms of Hydrogen Sulfide via Highly Selective Naphthalimide-Based Fluorescent Probe

Hydrogen sulfide (H2S) is an important gasotransmitter, but only a few methods are available for real-time detection. Fluorescent probes are attractive tools for biological applications because of their high sensitivity, convenience, rapid implementation, noninvasive monitoring capability, and simplicity in fluorescent imaging of living cells and tissues. Herein, we report on a pro-fluorescent probe, NAP-Py-N3 based on naphthalimide derivative, which was found to show high selectivity toward H2S over various other analytes, including biothiols, making it feasible to detect H2S. After reaction with H2S, this probe showed rapid and significant turn-on green fluorescent enhancement at 553 nm (about 54-fold, k2 = 9.62 M-1s-1), high sensitivity (LOD: 15.5 nM), significant Stokes shift (118 nm), and it was found that the fluorescence quantum yield of fluorescence product can reach 0.36. Moreover, the probe has also been successfully applied to detect the gaseous H2S and to confirm the presence of H2S released from modern organic donors, which in recent years have been commonly used to investigate the role of H2S in biological systems. All the results indicate that this probe is excellent and highly valuable.

Ratiometric Near-Infrared Fluorescence Liposome Nanoprobe for H2S Detection In Vivo

Accurate detection of H₂S is crucial to understanding the occurrence and development of H₂S-related diseases. However, the accurate and sensitive detection of H₂S in vivo still faces great challenges due to the characteristics of H₂S diffusion and short half-life. Herein, we report a H₂S-activatable ratiometric near-infrared (NIR) fluorescence liposome nanoprobe HS-CG by the thin-film hydration method. HS-CG shows "always on" fluorescence signal at 816 nm and low fluorescence signal at 728 nm; the NIR fluorescence ratio between 728 and 816 nm (F₇₂₈/F₈₁₆) is low. Upon reaction with H₂S, the fluorescence at 728 nm could be more rapidly turned on due to strong electrostatic interaction between enriched HS^- and positively charged 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC) doped in the liposome nanoprobe HS-CG, resulting in a large enhancement of F₇₂₈/F₈₁₆, which allows for sensitive visualization of the tumor H₂S levels in vivo. This study demonstrates that this strategy of electrostatic adsorption between HS^- and positively charged molecules provides a new way to enhance the reaction rate of the probe and H₂S, thus serving as an effective platform for improving the sensitivity of imaging.

Rational Design of a Dual-Channel Fluorescent Probe for the Simultaneous Imaging of Hypochlorous Acid and Peroxynitrite in Living Organisms

Hypochlorous acid (HOCl) and peroxynitrite (ONOO^-) are two important highly reactive oxygen/nitrogen species, which commonly coexist in biosystems and play pivotal roles in many physiological and pathological processes. To investigate their function and correlations, it is urgently needed to construct chemical tools that can track the production of HOCl and ONOO^- in biological systems with distinct fluorescence signals. Here, we found that the coumarin fluorescence of coumarin-benzopyrylium (CB) hydrazides (spirocyclic form) is dim, and their fluorescence properties are controlled by their benzopyran moiety via an intramolecular photo-induced electron transfer (PET) process. Based on this mechanism, we report the development of a fluorescent probe CB2-H for the simultaneous detection of HOCl and ONOO^-. ONOO^- can selectively oxidize the hydrazide group of CB2-H to afford the parent dye CB2 (Abs_max/Em_max = 631/669 nm). In the case of HOCl, it undergoes an electrophilic attack on the benzopyran moiety of CB2-H to give a chlorinated product CB2-H-Cl, which inhibits the PET process within the probe and thus affords a turn-on fluorescence response at the coumarin channel (Abs_max/Em_max = 407/468 nm). Due to the marked differences in absorption/emission wavelengths between the HOCl and ONOO^- products, CB2-H enables the concurrent detection of HOCl and ONOO^- at two independent channels without spectral cross-interference. CB2-H has been applied for dual-channel fluorescence imaging of endogenously produced HOCl and ONOO^- in living cells and zebrafish under different stimulants. The present probe provides a useful tool for further exploring the distribution and correlation of HOCl and ONOO^- in more biosystems.

An ESIPT-based fluorescent turn-on probe with isothiocyanate for detecting hydrogen sulfide in environmental and biological systems

A probe with an isothiocyanate group was synthesized and evaluated for its H₂S sensing ability. Upon addition of H₂S, the probe exhibited ratiometric properties during absorption with a red-shift. The probe exhibited fluorescent off-on responses towards H₂S via the ESIPT process, due to the conversion of isocyanate into amine. UV-vis, fluorescence, and ¹H NMR spectroscopic analyses were performed to investigate the sensing mechanism. The probe has a large Stokes shift, short response time, and low detection limit. It can be used to estimate H₂S levels within the range of 0-36 nM. The practical applicability of the probe was demonstrated using water samples and living cells.

Natural flavylium-inspired far-red to NIR-II dyes and their applications as fluorescent probes for biomedical sensing

Fluorescent probes that emit in the far-red (600-700 nm), first near-infrared (NIR-I, 700-900 nm), and second NIR (NIR-II, 900-1700 nm) regions possess unique advantages, including low photodamage and deep penetration into biological samples. Notably, NIR-II optical imaging can achieve tissue penetration as deep as 5-20 mm, which is critical for biomedical sensing and clinical applications. Much research has focused on developing far-red to NIR-II dyes to meet the needs of modern biomedicine. Flavylium compounds are natural colorants found in many flowers and fruits. Flavylium-inspired dyes are ideal platforms for constructing fluorescent probes because of their far-red to NIR emissions, high quantum yields, high molar extinction coefficients, and good water solubilities. The synthetic and structural diversities of flavylium dyes also enable NIR-II probe development, which markedly advance the field of NIR-II in vivo imaging. In the last decade, there have been huge developments in flavylium-inspired dyes and their applications as far-red to NIR fluorescent probes for biomedical applications. In this review, we highlight the optical properties of representative flavylium dyes, design strategies, sensing mechanisms, and applications as fluorescent probes for detecting and visualizing important biomedical species and events. This review will prompt further research not only on flavylium dyes, but also into all far-red to NIR fluorophores and fluorescent probes. Moreover, this interest will hopefully spillover into applications related to complex biological systems and clinical treatments, ranging in focus from the sub-organelle to whole-animal levels.

A novel coumarin-benzopyrylium based near-infrared fluorescent probe for Hg2+ and its practical applications

A novel colorimetric and ratiometric NIR fluorescent probe for Hg²⁺ was designed and synthesized based on a coumarin-benzopyrylium platform. The ring-open form of the probe exhibits NIR absorption (670 nm) and emission (720 nm). The probe shows high sensitivity, high selectivity and rapid response (30 s) to Hg²⁺. The detection limit was as low as 6.94 nM. The probe shows high stability in a wide pH range from 2 to 10. Moreover, the probe can be employed for detecting Hg²⁺ in food and biological samples.

An ESIPT-based fluorescent probe with fast-response for detection of hydrogen sulfide in mitochondria

Excited-state intramolecular proton transfer (ESIPT) has recently received considerable attention due to its dual fluorescent changes and large Stokes shift. Hydrogen sulfide (H₂S) is a gas signal molecule that plays important roles in modulating the functions of different systems. Herein, by modifying 2-(2́-hydroxyphenyl) benzothiazole (HBT) scaffold, a novel near-infrared mitochondria-targeted fluorescent probe HBTP-H₂S has been rationally designed based on excited-state intramolecular proton transfer (ESIPT) effect. The nucleophilic addition reaction of the H₂S with probe HBTP-H₂S caused the break of the conjugated skeleton, resulting the shifting of maximum emission peak from 658 nm to 470 nm. HBTP-H₂S showed fast-response response time, good selectivity and a large Stokes shift (188 nm) toward H₂S. Most importantly, inspired by the inherent advantages of the probe, HBTP-H₂S was successfully employed to monitor mitochondrial H₂S in HepG2 cells.

An indirect electrochemical method for aqueous sulfide determination in freshwaters using a palladium chelate as a selective sensor

Sulfide anion is a highly toxic and corrosive compound and its presence above the threshold concentrations (i.e. μmol L^-1) in freshwaters may indicate environmental pollution. Besides, the increase in sulfide concentration results in modifications of the organoleptic proprieties of water and air. Many analytical methodologies have been designed for aqueous sulfide quantification, however, due to the high reactivity and instability of sulfide, the pursue of a simple, sensitive, selective, and portable analytical method is still a current demand. In this study, an indirect electrochemical method for the determination of sulfide based on its interaction with a palladium complex - bis(2-aminobenzoate) palladium(II) - acting as a selective chemosensor is described. The reaction leads to the demasking of the electroactive ligand 2-aminobenzoic acid (i.e. anthranilic acid) and square wave voltammetry is employed to monitor its concentration using a glassy carbon electrode (GCE). Experimental conditions were optimized and the reaction was performed in Britton-Robinson (BR) buffer at pH 5 for 4 min, providing the higher magnitude of the analytical signal. A linear relation (r² > 0.99) from 3 to 30 μmol L^-1 of sulfide was obtained with a limit of detection of 0.10 μmol L^-1. Recovery experiments using freshwater samples spiked with sulfide revealed overall satisfactory results for the limit concentration levels permitted by regulatory agencies. Therefore, the proposed methodology shows advantages in terms of portability, selectivity, sensitivity, low-cost, and easiness-to-use enabling monitoring of sulfide in a variety of waters.

Cyano Derivatives of 7-Aminoquinoline That Are Highly Emissive in Water: Potential for Sensing Applications

6-Cyano-7-aminoquinoline (6CN-7AQ) and 3-cyano-7-aminoquinoline (3CN-7AQ) were synthesized and found to exhibit intense emission with quantum yield as high as 63 % and 85 %, respectively, in water. Conversely, their derivatives 6-cyano-7-azidoquinoline (6CN-7N₃ Q) and 3-cyano-7-azidoquinoline (3CN-7N₃ Q) show virtually no emission, which makes them suitable to be used as recognition agents in azide reactions based on fluorescence recovery. Moreover, conjugation of 6CN-7AQ with a hydrophobic biomembrane-penetration peptide PFVYLI renders a nearly non-emissive 6CN-7AQ-PFVYLI composite, which can be digested by proteinase K, recovering the highly emissive 6CN-7AQ with ∼200-fold enhancement. The result provides an effective early confirmation for RT-qPCR in viral detection.

A dual-functional colorimetric and fluorescent peptide-based probe for sequential detection of Cu2+ and S2- in 100% aqueous buffered solutions and living cells

Highly sensitive and selectivite detection of copper ions (Cu²⁺) and hydrogen sulfide (H₂S) have become important research topics due to the potential harmful impacts of these chemicals to human health and the environment. In this study, we report the synthesis of a dual-functional peptide-based probe L (FITC-AhxSerSerHis), designed to mimic a copper-sulfur metalloprotein, and capable of continuous detection of Cu²⁺ and S^2- based on colorimetric and fluorescent methods. The new probe L displayed excellent "turn off" fluorescence response and good selectivity for Cu²⁺ ions via a modification of the tripeptide and fluorescein isothiocyanate group, and produced an obvious color change visible to the naked eye. Furthermore, as an excitable probe, the L-Cu complex could continuously detect S^2- with high selectivity and sensitivity in 100% aqueous buffered solutions. The detection limits for fluorescence titration measurements, calculated using the equation 3σ/k, were 76.7 nM (Cu²⁺) and 27.2 nM (S^2-), which were well below U.S. EPA safety levels. In addition, L could be cycled to alternately detect Cu²⁺ and S^2-, thereby making it a promising reversible probe. Moreover, L was successfully applied to monitoring Cu²⁺ and S^2- in live RKO cells through fluorescence imaging, exhibiting low cytotoxicity and good cell permeability.

Environmentally Sensitive Color-Shifting Fluorophores for Bioimaging

We introduce color-shifting fluorophores that reversibly switch between a green and red fluorescent form through intramolecular spirocyclization. The equilibrium of the spirocyclization is environmentally sensitive and can be directly measured by determining the ratio of red to green fluorescence, thereby enabling the generation of ratiometric fluorescent probes and biosensors. Specifically, we developed a ratiometric biosensor for imaging calcium ions (Ca2+ ) in living cells, ratiometric probes for different proteins, and a bioassay for the quantification of nicotinamide adenine dinucleotide phosphate.

Design, synthesis and evaluation of a novel fluorescent probe to accurately detect H2S in hepatocytes and natural waters

Design and synthesis of fluorescent probe with fast response, excellent water solubility and good hepatocyte-targeting capacity to detect hydrogen sulfide (H₂S) in hepatocytes and water samples is of great significance. Here, a novel fluorescent probe QL-Gal-N₃ for detection of H₂S was designed and synthesized based on H₂S-mediated azide reduction strategy. This sensor demonstrated low toxicity, fast response (within 1 min), high selectivity and low detection limit (as low as 126 nM in water) for the detection of H₂S. HeLa, A549 and HepG-2 cells were chosen to investigate the hepatocyte-targeting ability of QL-Gal-N₃ respectively. The results indicated that the specific recognition of ASGPR over-expressed in hepatocytes by galactose group was an important reason for the good targeting ability of probe QL-Gal-N₃. Furthermore, due to the introduction of glycosyl moiety, the water solubility of fluorescent probe was enhanced obviously. It was successfully applied for the detection of H₂S in environmental water samples including river water, tap water, lake water and waste water.

Construction of a novel cell-trappable fluorescent probe for hydrogen sulfide (H2S) and its bio-imaging application

Fluorescence detection of H₂S in living organisms is greatly advantageous because it is nondestructive and can be used for in situ analysis. We have constructed a novel rhodamine analogue dye (Rho630) by extending the conjugated system of rhodamine to create a novel cell-trappable H₂S fluorescent probe Rho630-AM-H₂S with red light emission. Its application for H₂S fluorescence detection in living HeLa cells and zebrafish was investigated. As expected, Rho630-AM-H₂S showed a huge fluorescence turn-on response of about 20-fold at 630 nm and good selectivity toward H₂S in solution. An MTT assay demonstrated that the probe showed negligible cytotoxicity in the concentrations typically used in fluorescence imaging experiments. Cell imaging experiments revealed that compared with compound 4 without cell-trappable unit modification, Rho630-AM-H₂S exhibited remarkably enhanced cell penetration ability, as an enormous fluorescence signal increase was observed at the red channel within 5 min after Rho630-AM-H₂S was incubated with HeLa cells. Finally, the probe Rho630-AM-H₂S was used to detect H₂S in living HeLa cells and zebrafish with great fluorescence enhancement in the red channel. Graphical abstract.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

A sequential and reversibility fluorescent pentapeptide probe for Cu(II) ions and hydrogen sulfide detections and its application in two different living cells imaging

In this study, we report a sequential and reversibility fluorescent probe (DP5) based on pentapeptide conjugated with dansyl groups using the solid phase peptide synthesis (SPPS) technology. DP5 showed immediate "turn off" response toward Cu²⁺ ions at an excitation wavelength of 330 nm with detection limits of 23.5 nM. The 2:1 binding ratio between DP5 and Cu²⁺ were confirmed using Job's plot method and fluorescence titration study, and DP5-Cu complex was observed with an association constant of 6.76 × 10⁸ M^-2. As designed, DP5-Cu complex as a promising analytical probe exhibited highly selective for H₂S detection in aqueous solutions. The detection limit for H₂S was obtained to be 17.2 nM, and lower than EPA and WHO guidelines. In addition, the reversibility and cyclicity were imparted to the DP5 during the detection of Cu²⁺ and H₂S, and cycle effect is very good. Furthermore, DP5 displayed better biocompatibility and low biotoxicity, and sequential fluorescence "on-off-on" responses of DP5 to Cu²⁺ and H₂S were successfully applied in two different living cells.

HS- induced thiolysis reaction to regulate UV- and fluorescence spectra change and their bioimaging in living cells

The design and synthesis of efficient probes for the detection of hydrogen sulfide is essential to explore the physiological role of such signaling molecules. A new probe，CDP containing 2-[(2,4-dinitrophenyl)thio]benzoic acid as responsive moiety was developed for H₂S. After cleavage of responsive moiety occurs through a H₂S-triggered nucleophilic reaction, the fluorescence of CDP was switched on. Up to 20-fold fluorescence enhancement toward H₂S was observed and the detection limit was calculated to be as low as 307 nM. Moreover, this probe CDP was simple with good selectivity, high sensitivity and low cytotoxicity, which enabled it to detect H₂S in solutions and exogenous/endogenous H₂S in HepG-2 cells respectively.

A ratiometric fluorescent chemosensor for the convenient monitoring of hydrogen sulfide concentration by the dual fluorescence fluctuation mode of two distinct emission bands in living cells and zebrafish

An interplaying ICT-FRET-based chemosensor with two response sites having differentiated sensitivities to H₂S was engineered to conveniently determine the concentration of H₂S by the dual fluorescence fluctuation mode of two distinct emission bands.

A novel 3-hydroxychromone fluorescent probe for hydrogen sulfide based on an excited-state intramolecular proton transfer mechanism

A novel fluorescent probe based on an excited-state intramolecular proton transfer mechanism can detect H₂S with high sensitivity and high selectivity.

A simple but effective fluorescent probe for the detection of bisulfite

A simple but effective fluoresecent probe for the detection of bisulfite based on Michael addition reaction.

Single fluorescent probe for reversibly detecting copper ions and cysteine in a pure water system

In this work, we have engineered a novel fluorescent probe PI, which remarkably can reversible detect copper ion and cysteine in pure water system for the first time.

Real-time detection of hypochlorite in tap water and biological samples by a colorimetric, ratiometric and near-infrared fluorescent turn-on probe

A highly selective and sensitive ratiometric and NIR fluorescent turn-on probe was reported for real-time detection of HClO.

Construction of a turn-on probe for fast detection of H2S in living cells based on a novel H2S trap group with an electron rich dye

A fluorescent probeANRbased on a novel H₂S trap group was synthesized for discriminating detection of H₂S in living cells.