Simultaneous detection of Cys/Hcy and H2S through distinct fluorescence channels

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 Novel DNBS-based Fluorescent Probe for the Detection of H2S in Cells and on Test Strips

Hydrogen sulfide (H2S) plays a key role in the physiology and pathology of organisms, and H2S in the environment is easily absorbed and harmful to health. It is of great significance to develop a probe with good selectivity, high sensitivity and good stability that can detect hydrogen sulfide inside and outside organisms. In this work, we designed a novel "turn-on" fluorescent probe CIM-SDB for the detection of H2S. The probe CIM-SDB used indene-carbazole as the fluorophore and 2,4-dinitrobenzenesulfonyl as the recognition site. The probe CIM-SDB exhibited high selectivity and sensitivity to H2S (detection limit as low as 123 nM). Moreover, the probe CIM-SDB was successfully applied to the detection of intracellular exogenous and endogenous H2S, and the test strips prepared by the probe CIM-SDB could realize the convenient and rapid detection of H2S.

A Chalcone-based Fluorescence Probe for H2S Detecting Utilizing ESIPT Coupled ICT Mechanism

The accurate and effective identification of hydrogen sulfide holds great significance for environmental monitoring. Azide-binding fluorescent probes are powerful tools for hydrogen sulfide detection. We combined the 2'-Hydroxychalcone scaffold with azide moiety to construct probe Chal-N3, the electron-withdrawing azide moiety was utilized to block the ESIPT process of 2'-Hydroxychalcone and quenches the fluorescence. The fluorescent probe was triggered with the addition of hydrogen sulfide, accompanied by great fluorescence intensity enhancement with a large Stokes shift. With excellent fluorescence properties including high sensitivity, specificity selectivity, and wider pH range tolerance, the probe was successfully applied to natural water samples.

An overview on recent advances of reversible fluorescent probes and their biological applications

Due to the simplicity and low detection limit, fluorescent probes are widely used in both analytical sensing and optical imaging. Compared to conventional fluorescent probes, reversibility endows the reversible fluorescent probe outstanding advantages and special properties, making reversible fluorescent probes with capable of quantitative, repetitive or circulatory. Reversible fluorescent probes can also monitor the concentration dynamics of target analytes in real time, such as metal ions, proteins and enzymes, as well as intracellular redox processes, which have been widely applied in various fields. This review summarized the types and excellent properties of reversible fluorescent probes designed and developed in recent years. It also summarized the applications of reversible fluorescent probe in fluorescence imaging, biological testing, monitoring redox cycles, and proposed the remaining challenges and future development directions of the reversible fluorescent probe. This review provided comprehensive overview of reversible fluorescent probe, which may provide valuable references for the design and fabrication of the reversible fluorescent probe.

A dual-response NIR fluorescent probe for separately and continuously recognizing H2S and Cys with different fluorescence signals and its applications

Given the significant interactions between hydrogen sulfide (H2S) and cysteine (Cys) in organisms, a dual-site multi-purpose fluorescent probe (Cy-NP) for H2S and Cys was synthesized. Cy-NP is composed of two fluorophores: naphthalimide that emits in the visible region of 500-600 nm, and cyanine dye that emits in the NIR region of 700-800 nm. Cy-NP showed admirable sensitivity and selectivity for identifying H2S and Cys by fluorescent signals with limits of detection as low as 0.15 μM and 1.4 μM, respectively. Furthermore, other biological thiols (especially GSH and Hcy) showed no positive response to Cy-NP compared with H2S and Cys. The chemical mechanism of Cy-NP with H2S and Cys in DMF/PBS (1/1, v/v, pH = 7.4) solution was verified by HRMS and DFT calculations. Further, Cy-NP was successfully applied to monitor H2S released in raw meat and adapted to detect H2S and Cys in MCF-7 cells independently and continuously.

Fluorescent probe for highly selective detection of cysteine in living cells

Cys play an important physiological role in the human body. Abnormal Cys concentration can cause many diseases. Therefore, it is of great significance to detect Cys with high selectivity and sensitivity in vivo. Because homocysteine (Hcy) and glutathione (GSH) have similar reactivity and structure to cysteine, few fluorescent probes have been reported to be specific and efficient for cysteine. In this study, we designed and synthesized an organic small molecule fluorescent probe ZHJ-X based on cyanobiphenyl, which can be used to specifically recognize cysteine. The probe ZHJ-X exhibits specific selectivity for cysteine, high sensitivity, short reaction response time, good anti-interference ability, and has a low detection limit of 3.8 × 10^-6 M. The probe ZHJ-X was successfully applied to the visualization of Cys in living cells and had great application prospects in cell imaging and detection.

Facile synthesis of non-modified yellow emission silicon quantum dots and their visualization of hydrogen sulfide in living cells and onion tissues

Yellow fluorescent silicon quantum dots (y-SiQDs) with 22.2% fluorescence quantum yield were synthesized by a simple hydrothermal method using 3-glycidoxypropyl triethoxysilane (GOTS) and m-aminophenol. The excitation wavelength is 550 nm with an emission wavelength of 574 nm, which effectively avoids the interference of biological autofluorescence. Notably, the synthesis approach does not require any post-modification and the y-SiQDs can be directly used for hydrogen sulfide (H₂S) quantification due to static quenching. It exhibits high sensitivity and excellent selectivity for H₂S with a 0.2-10 μM (R² = 0.9953) linear range and detection limit of 54 nM. y-SiQDs have excellent stability and biocompatibility and can be used for H₂S imaging in living cells and onion tissues.

A highly selective colorimetric and fluorescent probe Eu(tdl)2abp for H2S sensing: Application in live cell imaging and natural water

Using 4-([2,2': 6', 2'- terpyridin] -4'-yl) -N, N-dimethylaniline (tdl) as auxiliary ligand and 6-azido-2,2'-bipyridine (abp) as recognition ligand, a europium complex fluorescent probe Eu(4-([2,2': 6', 2'-terpyridin] -4' -yl) -N, N-dimethylaniline)₂-6-azido-2,2'-bipyridine Eu(tdl)₂abp for efficient and specific recognition of hydrogen sulfide (H₂S) was successfully synthesized and characterized by NMR and MS. Eu(tdl)₂abp represented "on-off" fluorescence signals for H₂S and its color changes could be identified with naked eyes. Eu(tdl)₂abp had short response time (2 min) to H₂S, high selectivity and good anti-interference, large stokes shift (207 nm). In various samples, when H₂S existed, the azide group was reduced to amine group, resulting in closed fluorescence signal, and the fluorescence intensity reached the degree of quenching without being affected by other interference. At the same time, there was a good linear relationship between relative fluorescence intensity and H₂S concentration with the detection limit (LOD) of 0.64 μM. The sensing mechanism of Eu(tdl)₂abp to detect H₂S was characterized by ¹H NMR and HR-MS. Eu(tdl)₂abp was used with success for the sensitive detection of H₂S in natural water and living cells.

Dual Response Site Fluorescent Probe for Highly Sensitive Detection of Cys/Hcy and GSH In Vivo through Two Different Emission Channels

Much research has demonstrated that metabolic imbalances of biothiols are closely associated with the emergence of different types of disease. In view of the significant effect of biothiols, quantitative evaluation and discrimination of intracellular Cys/Hcy and GSH in complex biological environments is very important. In this study, probe CDS-NBD, synthesized by attaching 2,4-dinitrobenzenesulfonate (DNBS, site 1) and nitrobenzoxadiazole (NBD, site 2) as the highly sensitive and selective dual response site for thiols onto the coumarin derivative 7-hydroxycoumarin-4-acetic acid, exhibited large separation of the emission wavelengths, fast response, notable fluorescence enhancement, excellent sensitivity and selectivity to Cys/Hcy and GSH over other biological species. Additionally, CDS-NBD could make a distinction between two different fluorescent signals, GSH (an obvious blue fluorescence) and Cys/Hcy (a mixed blue-green fluorescence). Further study on imaging of Cys/Hcy and GSH in vivo by employing probe CDS-NBD could also be successfully achieved.

Specific chiroptical sensing of cysteine via ultrasound-assisted formation of disulfide bonds in aqueous solution

Cysteine (Cys) can serve as a biomarker to indicate diseases or disorders, and its chiral sensing has attracted increasing attention. Herein, we established an ultrasound-facilitated chiral sensing method for Cys using 4-chloro-7-nitro-1,2,3-benzoxadiazole (NBD-Cl) and electronic circular dichroism (ECD) spectroscopy. The formation of chiral disulfide bonds induced degenerate exciton coupling between two NBD chromophores, resulting in intense Cotton effects (CEs) of the sensing product. The anisotropy factor (g) was linearly correlated with the enantiomeric excess of Cys across the visible region (400-500 nm), and other natural amino acids or biothiols did not interfere with the detection. This ultrasound-promoted efficient and specific chiral sensing method of Cys has potential for application in the diagnosis of related diseases.

A bifunctional fluorescent probe based on PET & ICT for simultaneously recognizing Cys and H2S in living cells

Most reported probes that respond to Cysteine (Cys) and Hydrogen sulfide (H₂S) can only identify one analyte, or they were interfered with homocysteine (Hcy) and glutathione (GSH) when recognizing Cys and H₂S. In addition, nitrobenzoxadiazole (NBD) ether, as one of thiols recognition sites, inevitably encounters the situation that Cys, GSH and H₂S cannot be distinguished on the same channel at the cellular level. In this work, by introducing NBD ether and NBD amine, we constructed a bifunctional fluorescent probe NJB for dual-site response to Cys and H₂S via PET & ICT processes. NJB has wonderful selectivity for identifying Cys and HS^-, with limits of detection as low as 58.4 nM and 81.1 nM, respectively. Interestingly, NJB has been successfully applied to detect Cys and HS^- in MCF-7 cells. Therefore, the probe that serves as a great tool for inquiring the physiological and pathological functions of Cys and H₂S in living cells is promising.

A turn-on fluorescent probe via substitution-rearrangement for highly sensitive and discriminative detection of cysteine and its imaging in living cells

Biothiols play an important role in many physiological and pathological processes, especially in the occurrence of oxidative stress caused by abnormal cysteine (Cys) concentration. Therefore, it is particularly critical to develop a method that can specifically identify Cys to avoid interference from other biological analytes. However, most Cys-specific fluorescent probes are difficult to distinguish between homocysteine (Hcy) and glutathione (GSH). In this work, to avoid the interference of Hcy and GSH, we developed a fluorescent probe triarylimidazole-naphthalimide-piperazine-sulfonyl benzoxadiazole (TNP-SBD-Cl) based on fluorescence resonance energy transfer (FRET) on platform of naphthalimide-sulfonyl benzoxadiazole (SBD), the main SBD 4-chlorine groups have mild reactivity to undergo substitution and rearrangement to distinguish Hcy and GSH. The TNP-SBD-Cl response to Cys would turn on FRET and generate a new yellow fluorescence with a large Stokes shift (157 nm), and with excellent selectivity and low detection limit (0.87 μM). Moreover, TNP-SBD-Cl can be used to monitor Cys in living HeLa cells with low cytotoxicity, suggesting that it has markedly diagnostic significance in physiological and pathological processes.

Rational design of a fluorescent probe and its applications of imaging and distinguishing between exogenous and endogenous H2S in living cells

Hydrogen sulfide (H₂S), a recognized environmental pollutant, comes from a wide range of sources. For example, H₂S will be produced in the process of plant protein corruption, the decomposition of domestic sewage and garbage, food processing (wine brewing), etc. and once the concentration is too high, it will cause significant damage of environment and human body. Besides H₂S is an important gas signal molecule in vivo, which can be transferred through lipid membrane. Its existence level is closely related to many diseases. If we can "visually" trace the transmembrane transmission of hydrogen sulfide, it will be very helpful for the study of oxidative stress processes, cell protection, signal transduction and related diseases closely related to H₂S. Although some probes can detect H₂S in environment, cytoplasm and organelles, there are few reports on the release and internalization of H₂S. In this work, we report a H₂S fluorescence probe that can retain on the cell membrane, named PCM. The probe PCM can not only detect endogenous and exogenous H₂S, but also distinguish them, this provides a general strategy for the construction of probes to detect other biomarkers. In addition, PCM has been successfully applied to the detection of endogenous and exogenous H₂S in zebrafish, which has the potential to become a new chemical tool and provide help for the research of H₂S-related diseases.

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.

Plasmonic Gold Nanomaterials as Photoacoustic Signal Resonant Enhancers for Cysteine Detection

The development of photoacoustic systems is important for the real-time detection of cysteine (Cys), a biothiol in biological systems that serves as a significant biomarker for human health. Advanced photoacoustic (PA) signals with colloidal plasmonic Au nanomaterials rely on the efficient conversion of light to energy waves under moderately pulsed laser irradiation. In this study, we synthesized Cys-capped Au nanorods (Au@Cys NRs) and Cys-capped Au nanoparticles (Au@Cys NPs) through a conjugate of three Cys concentrations (10, 100, and 1000 μM). These plasmonic Au nanomaterials can be used as a PA resonance reagent due to their maximum localized surface plasmon resonance (LSPR) absorption bands at 650 nm and 520 nm in Au NRs and Au NPs, respectively. Subsequently, the PA signals were noticeably increased proportionally to the concentrations in the Au@Cys NRs and Au@Cys NPs under 658 nm and 520 nm laser irradiation, respectively, according to our portable photoacoustic system. Furthermore, PA signal amplitudes in Cys detection are boosted by ~233.01% with Au@Cys NRs and ~102.84% with Au@Cys NPs enhancement, compared to free Cys, according to ultrasound transducers at frequencies of 3 MHz.

NBD-based synthetic probes for sensing small molecules and proteins: design, sensing mechanisms and biological applications

Compounds with a nitrobenzoxadiazole (NBD) skeleton exhibit prominent useful properties including environmental sensitivity, high reactivity toward amines and biothiols (including H2S) accompanied by distinct colorimetric and fluorescent changes, fluorescence-quenching ability, and small size, all of which facilitate biomolecular sensing and self-assembly. Amines are important biological nucleophiles, and the unique activity of NBD ethers with amines has allowed for site-specific protein labelling and for the detection of enzyme activities. Both H2S and biothiols are involved in a wide range of physiological processes in mammals, and misregulation of these small molecules is associated with numerous diseases including cancers. In this review, we focus on NBD-based synthetic probes as advanced chemical tools for biomolecular sensing. Specifically, we discuss the sensing mechanisms and selectivity of the probes, the design strategies for multi-reactable multi-quenching probes, and the associated biological applications of these important constructs. We also highlight self-assembled NBD-based probes and outline future directions for NBD-based chemosensors. We hope that this comprehensive review will facilitate the development of future probes for investigating and understanding different biological processes and aid the development of potential theranostic agents.

A novel rosamine-based fluorescent probe for the rapid and selective detection of cysteine in BSA, water, milk, cabbage, radish, apple, and pear

A novel fluorescent probe (RA), based on the rosamine skeleton bearing acrylate group, has been reasonably designed and prepared, which employed an addition-cyclization-elimination sequence reaction mechanism to detect cysteine. RA displayed rapid response to cysteine within 1.5 min, and exhibited satisfactory selectivity for cysteine over H₂S, glutathione (Glu), and homocysteine (Hcy), due to the formation of seven-membered lactam favored kinetically. Fluorescence ratio was utilized to detect cysteine from 6.0 to 20.0 μM with a detection limit of 0.29 μM. More, RA was used to monitor cysteine in BSA, water, milk, milk powder, cabbage, radish, apple, and pear.

A novel fast-responsive fluorescent probe based on 1,3,5-triazine for endogenous H2S detection with large Stokes shift and its application in cell imaging

A novel fast-responsive fluorescent probe TzAr-H2S based on 1,3,5-triazine was constructed to monitor endogenous H₂S. The probe TzAr-H2S can quickly (only 20 s) detect H₂S with good selectivity, large Stokes shift (100 nm) and low cytotoxicity.

Novel turn-on fluorescence sensor for detection and imaging of endogenous H2S induced by sodium nitroprusside

Currently, fluorescence analysis method has a good application in the detection and imaging of biomarkers and has become an important analytical method. Although there are many fluorescent probes for detecting hydrogen sulfide(H₂S), they are mostly based on fluorophores which already existed, such as 1,8-naphthalimide, coumarin, rhodamine and their derivatives. Here, a new type of fluorescent molecule (BOTD) was synthesized and applied to the detection of H₂S. The probe BOTD could quickly and sensitively detect H₂S and turn on fluorescence. Moreover, the probe BOTD was successfully applied to the detection of exogenous and endogenous H₂S in living cells, and may be expected to become a research tool for studying H₂S-induced drugs.

A new chloro-substituted dicyanoisophorone-based near-infrared fluorophore with a larger Stokes shift and its application for detecting cysteine in cells and in vivo

Many dicyanoisophorone-based fluorophores with an optical hydroxyl group have been explored to meet different imaging needs along with the rapid and wide development of molecular fluorescence bioimaging in recent years.