Recent advances of small-molecule fluorescent probes for detecting biological hydrogen sulfide

An AND-Gate Photoacoustic Probe for Cys and H2S Precise Photoacoustic Sensing in Localized Tumors

Photoacoustic (PA) tomography has shown many promising aspects in noninvasive and precise imaging of deep-localized biomarkers. However, these traditional single-locked PA probes always face challenges in precise PA imaging with high specificity. Here, we report a novel AND-gate photoacoustic probe, BAE, to improve tumor imaging accuracy via the combination of two tumor-associated biomarkers, cysteine (Cys) and hydrogen sulfide (H2S). Only when Cys and H2S are concurrently introduced into the detection system does the absorption of BAE red-shift from the initial 680 to 810 nm, thereby showing a 5.29-fold enhancement in its PA signal at 810 nm. The good specificity of BAE is proven, since an obvious PA signal could be observed only in the solution containing both Cys and H2S and was not affected by other reactive sulfur species. After being taken up by tumors with the assistance of a nanomicelle, the AND-gate PA probe BAE was applied for dynamic real-time monitoring of Cys and H2S in vivo, achieving precise identification of tumors. This AND-gate PA probe provides a potential technical tool for precise sensing analysis of deep-seated diseases.

A novel coumarin-fluorescein-based fluorescent probe for ultrafast and visual detection of H2S in a Parkinson's disease model

Hydrogen sulfide (H2S) has a crucial impact on diverse biological processes and has been shown to be related to various diseases. Many probes have been developed to detect intracellular H2S by fluorescent imaging. However, the development of rapid, highly selective and sensitive H2S probes remains a challenge. Herein, two fluorogenic probes, CNS and FCS, are designed and synthesized for the ultrafast detection of H2S with fluorescein and coumarin fluorophores. The results show that both probes can be applied to monitor and image endogenous H2S in cervical cancer HeLa cells and live zebrafish, and FCS shows a higher sensitivity, selectivity and fluorescence intensity. We then further applied FCS in a Parkinson's disease Drosophila model, and the results show that FCS can precisely indicate the level of H2S in the Parkinson's disease model. Thus, FCS will likely to be applied for the early diagnosis of Parkinson's disease.

A novel polymer-based probe for fluorescently ratiometric sensing of hydrogen sulfide with multiple applications

Hydrogen sulfide (H2S) as an endogenous signaling molecule, plays an irreplaceable role in many important physiological activities. It is also closely related to sewage treatment, wine quality evaluation, and food spoilage. Herein, we have successfully synthesized a novel polymer-based probe P1 for fluorescently ratiometric sensing of H2S with a high selectivity and sensitivity. By virtue of ring-opening metathesis polymerization (ROMP), P1 was prepared with the disulfide bond linked coumarin-norbornene dyad NB-SS-COU as energy donor, the aggregation-induced emission (AIE) fluorophore anchored norbornene NB-TPE as energy receptor, and the polyethylene glycol (PEG) attached norbornene NB-PEG as a hydrophilic chain. At the 400 nm excitation, P1 displays a bright red emission at 615 nm due to the efficient fluorescence resonance energy transfer (FRET) from energy donor COU to energy acceptor TPE. Upon addition of H2S, it shows strong COU-based blue emission at 473 nm for cleavage of the disulfide bond. We also constructed a smartphone sensing platform to conduct visual quantitative detection of H2S by calculating the B/R (blue/red) emission ratio values. Moreover, P1 can be successfully employed in evaluating the level fluctuations of endogenous and exogenous H2S in living cells, testing water samples/wine samples, and monitoring food freshness.

Construction of a novel chitosan-based macromolecular nanoprobe for specific fluorescent detection of H2S in live animals

H2S is one of the signal molecules in live organisms and a poisonous gas, which is closely related to our life. The traditional synthetic small molecular organic probes often have the disadvantages of low biocompatibility. In this paper, a fluorescent nanoprobe for detecting H2S in live organisms was constructed based on chitosan. The structure of CH-CN was characterized by infrared spectroscopy, nuclear magnetic resonance, x-ray photoelectron spectroscopy (XPS), XRD and scanning electron microscope (SEM). In the presence of Na2S, the fluorescence intensity at 560 nm was significantly enhanced, and showed high selectivity and sensitivity toward H2S. Based on the good fluorescence response of CH-CN, the probe was also successfully applied to H2S imaging in HepG2 cells and zebrafish. These experimental results indicate that the probe has lower cytotoxicity and excellent stability. The present research shows a typical example of construction of chitosan-based macromolecular fluorescent materials and their bio-imaging application.

Imidazoles as Serotonin Receptor Modulators for Treatment of Depression: Structural Insights and Structure-Activity Relationship Studies

Serotoninergic signaling is identified as a crucial player in psychiatric disorders (notably depression), presenting it as a significant therapeutic target for treating such conditions. Inhibitors of serotoninergic signaling (especially selective serotonin reuptake inhibitors (SSRI) or serotonin and norepinephrine reuptake inhibitors (SNRI)) are prominently selected as first-line therapy for the treatment of depression, which benefits via increasing low serotonin levels and norepinephrine by blocking serotonin/norepinephrine reuptake and thereby increasing activity. While developing newer heterocyclic scaffolds to target/modulate the serotonergic systems, imidazole-bearing pharmacophores have emerged. The imidazole-derived pharmacophore already demonstrated unique structural characteristics and an electron-rich environment, ultimately resulting in a diverse range of bioactivities. Therefore, the current manuscript discloses such a specific modification and structural activity relationship (SAR) of attempted derivatization in terms of the serotonergic efficacy of the resultant inhibitor. We also featured a landscape of imidazole-based development, focusing on SAR studies against the serotoninergic system to target depression. This study covers the recent advancements in synthetic methodologies for imidazole derivatives and the development of new molecules having antidepressant activity via modulating serotonergic systems, along with their SAR studies. The focus of the study is to provide structural insights into imidazole-based derivatives as serotonergic system modulators for the treatment of depression.

Synthesis and applications of a corrole-based dual-responsive fluorescent probe for separate detection of hydrazine and hydrogen sulfide

Here, a corrole-based dual-responsive fluorescent probe DPC-DNBS was rationally designed and synthesized for the separate detection of hydrazine (N₂H₄) and hydrogen sulfide (H₂S) with high selectivity and sensitivity. The probe DPC-DNBS is intrinsically none fluorescent due to PET effect, however, addition of increasing amount of N₂H₄ or H₂S to DPC-DNBS turned on an excellent NIR fluorescence centered at 652 nm and thereby provided a colorimetric signaling behavior. The sensing mechanism was verified by HRMS, ¹H NMR and the DFT calculations. Common metal ions and anions do not interfere with the interactions of DPC-DNBS with N₂H₄ or H₂S. Furthermore, the presence of N₂H₄ does not affect the detection of H₂S; however, the presence of H₂S interferes with the detection of N₂H₄. Hence, quantitative detection of N₂H₄ must occur in an H₂S-free environment. The probe DPC-DNBS displayed some fascinating merits in separate detection of these two analytes, including large Stokes shift (233 nm), fast response (15 min for N₂H₄, 30 s for H₂S), low detection limit (90 nM for N₂H₄, 38 nM for H₂S), wide pH range (6-12) and outstanding biological compatibility. Significantly, DPC-DNBS was utilized to detect hydrazine in real water, soil and food samples. And its favorable performances for separate detection N₂H₄ and H₂S were successfully demonstrated in HeLa cells and zebrafish, indicating its value of practical application in biology.

A highly selective and easily acquisitive near-infrared fluorescent probe for detection and imaging of hydrogen sulfide in cells

Hydrogen sulfide (H₂S) plays a substantial role as a messenger in the physiological and pathological processes of many diseases. Recently, the fluorescence probe of H₂S based on organic dye has attracted great attention. However, the emission of many probes is in the UV-vis region (400-600 nm), so it has the disadvantages of shallow tissue penetration and more vulnerable to spontaneous fluorescence interference. Although several H₂S probes have been developed that emit more than 650 nm, there is a complex structure difficult to synthesize or unstable in storage. Aimed at simply structural and easily synthesized H₂S fluorescent probes with emission wavelength more than 650 nm, a novel near-infrared (NIR) probe (NIR-H₂S) here was rationally designed with 4-(2-carboxyphenyl)-7-(diethylamino)-2-(4-hydroxystyryl)chromenylium (NIR-OH) as a fluorescent dye and 2,4-dinitrophenyl moiety as a recognition group. Addition of H₂S, the "turn-on" NIR fluorescence response at 736 nm of NIR-H₂S was displayed, accompanied by a visual colour change from purple to green when excited at 686 nm. As an easily acquisitive H₂S probe, NIR-H₂S has been successfully applied to cell imaging for H₂S detection with the advantages such as long fluorescence emission, low toxicity, high sensitivity and strong selectivity.

A water-soluble NIR fluorescent probe capable of rapid response and selective detection of hydrogen sulfide in food samples and living cells

DDAO (1,3-Dichloro-7-hydroxy-9,9-dimethyl-2(9H)-acridone) is a near-infrared (NIR) fluorophore that has received increasing attention in recent years, exhibiting near-infrared emission at 658 nm, low pKa (∼5.0), good water solubility and high quantum yield (Φ = 0.39). The reported DDAO-based fluorescent probes can be applied to biological imaging ofenzymes and other substances in vivo with high sensitivity and selectivity. Herein, using -OCN as the detection group, a novel NIR H₂S fluorescent probe DDAO-CN based on DDAO was designed and synthesized. In PBS buffer (10 mM, pH 7.4), probe DDAO-CN displayed specific selection, short response time (within 10 s) and low detection limit (4.3 nM) towards to H₂S under the catalysis of CTAB. At the same time, the probe is able to sense H₂S gas produced by food spoilage via the fluorescent test strip loaded with DDAO-CN. Moreover, since the probe has optimal pH range (6.0-9.0), it has been successfully used for bioimaging H₂S in the HeLa cells with low cytotoxicity.

Comprehensive Insights into Medicinal Research on Imidazole-Based Supramolecular Complexes

The electron-rich five-membered aromatic aza-heterocyclic imidazole, which contains two nitrogen atoms, is an important functional fragment widely present in a large number of biomolecules and medicinal drugs; its unique structure is beneficial to easily bind with various inorganic or organic ions and molecules through noncovalent interactions to form a variety of supramolecular complexes with broad medicinal potential, which is being paid an increasing amount of attention regarding more and more contributions to imidazole-based supramolecular complexes for possible medicinal application. This work gives systematical and comprehensive insights into medicinal research on imidazole-based supramolecular complexes, including anticancer, antibacterial, antifungal, antiparasitic, antidiabetic, antihypertensive, and anti-inflammatory aspects as well as ion receptors, imaging agents, and pathologic probes. The new trend of the foreseeable research in the near future toward imidazole-based supramolecular medicinal chemistry is also prospected. It is hoped that this work provides beneficial help for the rational design of imidazole-based drug molecules and supramolecular medicinal agents and more effective diagnostic agents and pathological probes.

A ruthenium nitrosyl complex-based highly selective colorimetric sensor for biological H2S and H2S-NO cross-talk regulated release of NO

A ruthenium nitrosyl complex (1·NO) and 1·NO incorporated phospholipid-based liposomes (Lip-1·NO) were reported for highly selective colorimetric detection of H₂S. The probe 1·NO "cross-talks" with H₂S and releases nitric oxide (NO) in the process. The detection limit for H₂S was found to be 0.31 μM and 0.45 μM in the cases of 1·NO and Lip-1·NO, respectively. The DAF-FM DA assay has been performed to confirm the H₂S-induced NO release from 1·NO and Lip-1·NO. The sensing of H₂S was also verified by ESI-MS and FT-IR spectroscopy. It was also observed that external stimuli, H₂S and light worked in an almost similar way to release NO as observed by UV-Vis spectroscopy. A molecular logic gate operation "OR" was applied to the probe 1·NO in combination with inputs 'light' and 'H₂S' to give the output 'NO release'. Hence, the probe 1·NO performs the dual work of sensing H₂S with a colorimetric response, releasing NO upon cross-talk between NO and H₂S.

A Fast-Response AIE-Active Ratiometric Fluorescent Probe for the Detection of Carboxylesterase

Hepatocellular carcinoma (HCC) is associated with a high mortality rate worldwide. The therapeutic outcomes can be significantly improved if diagnosis and treatment are initiated earlier in the disease process. Recently, the carboxylesterase (CaE) activity/level in human plasma was reported to be a novel serological biomarker candidate for HCC. In this article, we fabricated a new fluorescent probe with AIE characteristics for the rapid detection of CaE with a more reliable ratiometric response mode. The TCFISE probe showed high sensitivity (LOD: 93.0 μU/mL) and selectivity toward CaE. Furthermore, the good pH stability, superior resistance against photobleaching, and low cytotoxicity highlight the high potential of the TCFISE probe for application in the monitoring of CaE activity in complex biological samples and in live cells, tissues, and animals.

A fast-responsive fluorescent probe based on a styrylcoumarin dye for visualizing hydrogen sulfide in living MCF-7 cells and zebrafish

As a vital antioxidant molecule, H₂S can make an important contribution to regulating blood vessels and inhibiting apoptosis when present at an appropriate concentration. Higher levels of H₂S can interfere with the physiological responses of the respiratory system and central nervous system carried out by mammalian cells. This is associated with many illnesses, such as diabetes, mental decline, cardiovascular diseases, and cancer. Therefore, the accurate measurement of H₂S in organisms and the environment is of great significance for in-depth studies of the pathogenesis of related diseases. In this contribution, a new coumarin-carbazole-based fluorescent probe, COZ-DNBS, showing a rapid response and large Stokes shift was rationally devised and applied to effectively sense H₂S in vivo and in vitro. Upon using the probe COZ-DNBS, the established fluorescent platform could detect H₂S with excellent selectivity, showing 62-fold fluorescence enhancement, a fast-response time (<1 min), high sensitivity (38.6 nM), a large Stokes shift (173 nm), and bright-yellow emission. Importantly, the probe COZ-DNBS works well for monitoring levels of H₂S in realistic samples, living MCF-7 cells, and zebrafish, showing that COZ-DNBS is a promising signaling tool for H₂S detection in biosystems.

The probe COZ-DNBS displayed excellent selectivity, a fast response, high sensitivity, a large Stokes shift, and bright-yellow emission in response to H₂S.

Fluorescent probes and functional materials for biomedical applications

Due to their simplicity in preparation, sensitivity and selectivity, fluorescent probes have become the analytical tool of choice in a wide range of research and industrial fields, facilitating the rapid detection of chemical substances of interest as well as the study of important physiological and pathological processes at the cellular level. In addition, many long-wavelength fluorescent probes developed have also proven applicable for in vivo biomedical applications including fluorescence-guided disease diagnosis and theranostics (e.g., fluorogenic prodrugs). Impressive progresses have been made in the development of sensing agents and materials for the detection of ions, organic small molecules, and biomacromolecules including enzymes, DNAs/RNAs, lipids, and carbohydrates that play crucial roles in biological and disease-relevant events. Here, we highlight examples of fluorescent probes and functional materials for biological applications selected from the special issues “Fluorescent Probes” and “Molecular Sensors and Logic Gates” recently published in this journal, offering insights into the future development of powerful fluorescence-based chemical tools for basic biological studies and clinical translation.