A multifunctional two-photon fluorescent probe for detecting H2S in wastewater and GSH in vivo

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.

Dual-channel luminescent Ir(III) complex for detection of GSH and Hcy/Cys in cells

Glutathione (GSH), homocysteine (Hcy) and cysteine (Cys) play important roles in many physiological processes. However, due to their structural and functional similarities, it is still a challenge to develop a probe that can differentiate between GSH and Hcy/Cys simultaneously. In this work, a luminescent probe Ir-NBD was designed and synthesized, which emit weakly due to the presence of photo induced electron transfer (PET) interaction. When it reacted with the three biothiols, NBD dissociated and luminescence of Ir-OH was enhanced in the near-infrared (NIR) region due to the disappearance of the PET effect. On the other hand, the products obtained from the reaction of GSH with NBD were hardly luminescent, but the products from the reaction of Hcy/Cys with NBD could undergo an intramolecular rearrangement, resulting in an enhanced luminescence of the solution in the visible region. Ir-NBD enabled highly selective and sensitive detection of GSH and Cys/Hcy in a relatively short time (15 min). The two luminescent colors were clearly differentiated without spectral interference and the detection limit reached 1.32 μM (GSH), 0.42 μM (Hcy) and 0.51 μM (Cys), respectively. Ir-NBD also had low cytotoxicity, it realized the simultaneous detection of GSH and Hcy/Cys by dual-channel luminescence, and also provided ideas for the design of multifunctional luminescent probes.

Strongly emitting, centrosymmetric, ladder-type bis-coumarins with crankshaft architecture

Quadrupolar bis-coumarins bearing dialkylamino groups, prepared by a double Pechmann reaction and subsequent oxidation, strongly emit yellow-orange light. Comparison with non-substituted analogs reveals that, the photophysical properties of the conjugated bis-coumarins are controlled both by the dialkylamino substituents and by the π-system. Analogous but non-conjugated bis-coumarins emit blue light both in solution and in crystalline state. Unusually fast oxidation process in the crystalline state is responsible for the presence of two bands in their solid-state emission. Two-center, charge-transfer transition from an orbital delocalized on the entire molecule to the central benzene ring is responsible for photophysical properties.

A fluorous-tag-assisted fluorescent probe for simple and selective detection of hydrogen sulfide: application for turbid dyeing solutions

Accurate hydrogen sulfide (H2S) detection has attracted much attention because its toxicity may affect aquatic environments and human health. However, recognizing H2S levels by conventional fluorescent probes in turbid wastewater has been challenging because the opaque environment interferes with their photophysical properties. To overcome this limitation, a fluorous-tagging strategy can be used for the development of fluorescent sensors to detect H2S in turbid solutions. The use of fluorescent probe assisted with fluorous-tag allowed for easy isolation of the probe using polytetrafluoroethylene (PTFE) material, while disturbing species were eliminated through a simple aqueous wash. This approach enabled the fluorescent probe to effectively quantify H2S, even in opaque solutions containing organic dyes that could interfere with fluorescence emission.

Polarized, V-Shaped, and Conjoined Biscoumarins: From Lack of Dipole Moment Alignment to High Brightness

Eleven conjoined coumarins possessing a chromeno[3,4-c]chromene-6,7-dione skeleton have been synthesized via the reaction of electron-rich phenols with esters of coumarin-3-carboxylic acids, catalyzed by either Lewis acids or 4-dimethylaminopyridine. Furthermore, Michael-type addition to angular benzo[f]coumarins is possible, leading to conjugated helical systems. Arrangement of the electron-donating amino groups at diverse positions on this heterocyclic skeleton makes it possible to obtain π-expanded coumarins with emission either sensitive to, or entirely independent of, solvent polarity with large Stokes shifts. Computational studies have provided a rationale for moderate solvatochromic effects unveiling the lack of collinearity of the dipole moments in the ground and excited states. Depending on the functional groups present, the obtained dyes are highly polarized with dipole moments of ∼14 D in the ground state and ∼20–25 D in the excited state. Strong emission in nonpolar solvents, in spite of the inclusion of a NO₂ group, is rationalized by the fact that the intramolecular charge transfer introduced into these molecules is strong enough to suppress intersystem crossing yet weak enough to prevent the formation of dark twisted intramolecular charge transfer states. Photochemical transformation of the dye possessing a chromeno[3,4-c]pyridine-4,5-dione scaffold led to the formation of a spirocyclic benzo[g]coumarin.

Development and Challenge of Fluorescent Probes for Bioimaging Applications: From Visualization to Diagnosis

Fluorescent probes have been used widely in bioimaging, including biological substance detection, cell imaging, in vivo biochemical reaction process tracking, and disease biomarker monitoring, and have gradually occupied an indispensable position. Compared with traditional biological imaging technologies, such as positron emission tomography (PET) and nuclear magnetic resonance imaging (MRI), the attractive advantages of fluorescent probes, such as real-time imaging, in-depth visualization, and less damage to biological samples, have made them increasingly popular. Among them, ultraviolet-visible (UV-vis) fluorescent probes still occupy the mainstream in the field of fluorescent probes due to the advantages of available structure, simple synthesis, strong versatility, and wide application. In recent years, fluorescent probes have become an indispensable tool for bioimaging and have greatly promoted the development of diagnostics. In this review, we focus on the structure, design strategies, advantages, representative probes and latest discoveries in application fields of UV-visible fluorescent probes developed in the past 3-5 years based on several fluorophores. We look forward to future development trends of fluorescent probes from the perspective of bioimaging and diagnostics. This comprehensive review may facilitate the development of more powerful fluorescent sensors for broad and exciting applications in the future.

A reversible coumarin-based sensor for intracellular monitoring cysteine level changes during Cu2+-induced redox imbalance

Biological thiols are crucial small molecule amino acids widely existing in cells, which play indispensable roles in maintaining redox homeostasis of living systems. Owing to their abnormal levels have close relation with many diseases, thus, developing more convenient, rapid and practical in-vivo detection tools is imminent. Herein, a reversible coumarin-based probe (HNA) was successfully constructed through a simple two-step synthesis. HNA can detect Cys/Hcy with high response speed and desirable selectivity based on Michael addition recognition mechanism. Free HNA has an orange emission at 580 nm, but after addition of Cys/Hcy, the conjugated structure of probe HNA was destroyed by the attack of sulfhydryl, resulting in a new green emission at 507 nm. Further, HNA has been applied to monitor Cys/Hcy in HeLa cells and zebrafish. Notably, HNA has also been successfully applied for real-time tracing Cys levels changes in living cells and zebrafish during the imbalance in redox status caused by copper (II). This provides a new strategy for studying the process of oxidative stress in cells.

A coumarin-based fluorescence sensor for rapid discrimination of cysteine/homocysteine and glutathione under dual excitation wavelengths

Biological thiols (Cys, Hcy and GSH) are crucial biomolecules in living cells and play indispensable roles in maintaining the redox homeostasis of organisms. But due to their similar molecular structure, the development of effective tools for distinguishing two or three of them remains a great difficulty. Herein, we constructed a sensitive sensor (CB) by connecting the bifunctional fluorescent reagent with coumarin derivatives for simultaneous recognition of these three thiols through different pathways. Free CB had no fluorescence; however, with gradual addition of thiols, the chlorine unit was replaced by sulfhydryl. Furthermore, the intramolecular rearrangement occurred between the amino and sulfhydryl groups of Cys/Hcy and yellow fluorescence was observed at 570 nm. However, GSH with a large structure could not undergo intramolecular rearrangement, and green fluorescence was excited at 505 nm. In this way, Cys/Hcy and GSH can be detected distinctively. Under dual excitation wavelengths, CB exhibited high selectivity and fast response to the three thiols. Furthermore, CB was successfully applied to imaging endogenous and exogenous thiols in living cells and zebrafish, providing us with a reliable tool for thiols recognition.

A multifunctional fluorescent probe for visualizing H2S in wastewater with portable smartphone via fluorescent paper strip and sensing GSH in vivo

In this paper, a bifunctional tri-site fluorescent probe was designed for the first time not only for visualization and quantitative analysis of sensing H₂S in wastewater by coupling paper strip and smartphone (Color recognizer, Xiyi Technology) but also for sensitively monitoring GSH in living cells, which relied on different emission channels and the pH of solutions. The recognition properties of GH towards H₂S/GSH were satisfactorily demonstrated through fluorescence, UV-vis, ¹H NMR and DFT calculations. More importantly, integrated with the paper strip, portable smartphone-sensing platform with a color recognizer app would accomplish cost-effective and rapid assays for colorimetric water quality testing, which displayed huge application potential in fields of environmental monitoring.

Fluorescent probes based on nucleophilic aromatic substitution reactions for reactive sulfur and selenium species: Recent progress, applications, and design strategies

Reactive sulfur species (RSS) and reactive selenium species (RSeS) are important substances for the maintenance of physiological balance. Imbalance of RSS and RSeS is closely related to a series of human diseases, so it is considered to be an important biomarker in early diagnosis, treatment, and stage monitoring. Fast and accurate quantitative analysis of different RSS and RSeS in complex biological systems may promote the development of personalized diagnosis and treatment in the future. One way to explore the physiological function of various types of RSS and RSeS in vivo is to detect them at the molecular level, and one of the most effective methods for this is to use fluorescent probes. Nucleophilic aromatic substitution (SNAr) reactions are commonly exploited as a detection mechanism for RSS and RSeS in fluorescent probes. In this review, we cover recent progress in fluorescent probes for RSS and RSeS based on SNAr reactions, and discuss their response mechanisms, properties, and applications. Benzenesulfonate, phenyl-O ether, phenyl-S ether, phenyl-Se ether, 7-nitro-2,1,3-benzoxadiazole (NBD), benzoate, and selenium-nitrogen bonds are all good detection groups. Moreover, based on an integration of different reports, we propose the design and synthesis of RSS- and RSeS-selective probes based on SNAr reactions, current challenges, and future research directions, considering the selection of active sites, the effect of substituents on the benzene ring, and the introduction of other functional groups.

A colorimetric and near-infrared fluorescent probe for detection of hydrogen sulfide and its real multiple applications

A novel colorimetric and near-infrared fluorescent probe (L) with D-π-A structure derived from 4-diethylaminosalicylaldehyde and 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile has been developed. Probe L displays highly selective and sensitive recognition for H₂S over various anions with a large Stokes shift (96 nm) in DMF/H₂O (3/7, v/v, PBS-HCl 10 mM, pH = 7.4). A naked-eye observable color change of L solution from colorless to bluish-purple occurred on treatment with H₂S. The potential applications of probe L were evaluated and the results show that probe L can detect H₂S vapor and H₂S in real water, red wine and living cells.

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.

Fast response near-infrared fluorescent probe for hydrogen sulfide in natural waters

Rapid and sensitive detection of hydrogen sulfide (H₂S) is of great importance for the environmental monitoring. Near-infrared fluorescent probes are recently developed for the sensitive H₂S detection thanks to their low background interference, while often hampered by relatively long response time (around 30 min). In this work, we reported a fast response (within 5 min), highly sensitive near-infrared (NIR) fluorescent probe (DCM-OCN) for H₂S. The rapid nucleophilic reaction between cyanate moiety and H₂S endowed fast response of the NIR probe. The influence of experimental parameters (including CTAB concentration, reaction time, pH value etc.), interference study, and possible mechanism were investigated in detail. The fluorescence increment was linear with H₂S concentration in 1-10 μM with a detection limit of 0.28 μM (3σ). The probe was successfully applied to environmental water samples including river water, tap water, lake water, mineral water and artificial wastewater.