Advancements in the development of fluorescent chemosensors based on CN bond isomerization/modulation mechanistic approaches

The CN bond isomerization/modulation as a fluorescence signalling mechanism was explored by studying the photophysical properties of conformationally restricted molecules. From the beginning, the CN bond isomerization method has attracted the attention of researchers owing to its simplicity, high selectivity, and sensitivity in fluorescence evaluation. Continuous developments in the field of sensing using CN bond-containing compounds have been achieved via the customization of the isomerization process around the CN bond in numerous ways, and the results were obtained in the form of specific discrete photophysical changes. CN isomerization causes significant fluorescence enhancement in response to detected metal cations and other reactive species (Cys, Hys, ClO-, etc.) straightforwardly and effectively. This review sheds light on the process of CN bond isomerization/modulation as a signalling mechanism depending on fluorescence changes via conformational restriction. In addition, CN bond isomerization-based fluorescent sensors have yet to be well reviewed, although several fluorescent sensors based on this signalling mechanism have been reported. Therefore, CN-based fluorescent sensors are summarized in this review.

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.

Development of fluorescent probes with specific recognition moiety for hydrogen polysulfide

Hydrogen polysulfide (H2Sn, n > 1) is an important component of reactive sulfur species (RSS), which is an important substance for maintaining the redox balance in cells. However, limited recognition moieties are available for hydrogen polysulfide probe design. In this study, we have constructed a small library containing several organic molecules to explore a new specific recognition moiety for H2Sn fluorescent probe design. To validate the discovery, two fluorescent probes, 7 and BCC, were further developed based on coumarin and its derivative. The probes exhibited desirable specificity for H2Sn monitoring, which can be used for detecting H2Sn in solution and cells. The new specific recognition moiety for H2Sn fluorescent probe design discovered in this work has certain guiding significance for development of H2Sn probes exploring biological roles in the future.

Molecular engineering of benzenesulfonyl analogs for visual hydrogen polysulfide fluorescent probes based on Nile red skeleton

Hydrogen polysulfide (H₂S_n, n > 1) has a valuable function in various aspects of biological regulation. Therefore, it is of great significance to realize the visual monitoring of H₂S_n levels in vivo. Herein, a series of fluorescent probes NR-BS were constructed by changing types and positions of substituents on the benzene ring of benzenesulfonyl. Among them, probe NR-BS4 was optimized due to its wide linear range (0 ∼ 350 μM) and little interference from biothiols. In addition, NR-BS4 has a broad pH tolerance range (pH = 4 ∼ 10) and high sensitivity (0.140 μM). In addition, the PET mechanism of probe NR-BS4 and H₂S_n was demonstrated by DFT calculations and LC-MS. The intracellular imaging studies indicate that NR-BS4 can be successfully devoted to monitor the levels of exogenous and endogenous H₂S_n in vivo.

A New Ratiometric Fluorescent Probe Based on BODIPY for Highly Selective Detection of Hydrogen Sulfide

Hydrogen sulfide (H₂S) as small molecular signal messenger plays key functions in numerous biological processes. The imaging detection of intracellular hydrogen sulfide is of great significance. In this work, a ratiometric fluorescent probe BH based on an asymmetric BODIPY dye for detection of H₂S was designed and synthesized. After the interaction with hydrogen sulfide, probe display colorimetric and ratiometric fluorescence response, with its maximum emission fluorescence wavelength red-shifted from 542 nm to 594 nm, which is attributed to the sequential nucleophilic reaction of H₂S leading to enhanced molecular conjugation after ring formation of the BODIPY skeleton. A special response mechanism has been fully investigated by NMR titration and MS, so that the probe has excellent detection selectivity. Furthermore, probe BH has low cytotoxicity and fluorescence imaging experiments indicate that it can be used to monitor hydrogen sulfide in living cells.

Detection of sulfane sulfur species in biological systems

Sulfane sulfur species such as hydropersulfides (RSSH), polysulfides (RSnR), and hydrogen polysulfides (H2Sn) are critically involved in sulfur-mediated redox signaling, but their detailed mechanisms of action need further clarification. Therefore, there is a need to develop selective and sensitive sulfane sulfur detection methods to gauge a better understanding of their functions. This review summarizes current detection methods that include cyanolysis, chemical derivatization and mass spectrometry, proteomic analysis, fluorescent probes, and resonance synchronous/Raman spectroscopic methods. The design principles, advantages, applications, and limitations of each method are discussed, along with suggested directions for future research on these methods. The development of robust detection methods for sulfane sulfur species will help to elucidate their mechanisms and functions in biological systems.

A FRET-based ratiometric fluorescent probe for hydrogen polysulfide detection in living cells and zebrafish

Hydrogen polysulfide (H₂S_n, n > 1) is an important active sulfur molecule (RSS) in organisms, which have been considered to be involved in redox signaling and cytoprotective processes. In this work, in order to quickly and accurately detect H₂S_n in biosystems, 2-fluoro-5-nitrobenzoic ester was used as the response moiety for H₂S_n, and the FRET strategy was adopted to effectively connect the donor (6-hydroxy-2-naphthoic acid) and acceptor (4-substituted-1,8-naphthalimide) to construct a new ratiometric H₂S_n fluorescent probe NPNA-H₂S_n. NPNA-H₂S_n exhibited a more than ∼ 8.0-fold ratio enhancement towards H₂S_n at I₄₅₀/I₅₂₆ and a very high sensitivity with a very low detection limit of 40.3 nM. Impressive, NPNA-H₂S_n was further used for fluorescence imaging of H₂S_n in living cells and zebrafish, which showed high-clear ratiometric images. Therefore, we have demonstrated that NPNA-H₂S_n could be applied for ratiometric images of endogenous H₂S_n in living biosystems and provide a powerful molecular tool for evaluating the physiological and pathological functions of H₂S_n.

New strategy for detection of hydrogen peroxide based on bi-nucleophilic reaction

Two novel fluorescent probes based on 7-hydroxy-4-methyl-coumarin, FAA-MC-OH (2-fluoro-4-nitro-phenylacetyl hydroxyl coumarin) and FBA-MC-OH (2-fluoro-4-nitro-benzoyl hydroxyl coumarin) are first synthesized, and spectral studies confirm that both the probes display highly selective and sensitive to H₂O₂, especially FBA-MC-OH has a shorter response time. Moreover, it is worth noting that the reaction mechanism is based on bi-nucleophilic substitution instead of oxidation or hydrolysis, which is different from previous reported probes'.

A FRET-based ratiometric fluorescent probe for highly selective detection of hydrogen polysulfides based on a coumarin-rhodol derivative

In modern biology, hydrogen polysulfides (H₂S_n, n > 1) are members of reactive sulfur species (RSS), with anti-oxidation, cell protection and redox signals in tissues and organs. Therefore, it is crucial to develop a method to monitor the changes of H₂S_n level in organisms. We designed and synthesized a ratiometric fluorescent probe for highly selective detection of H₂S_n based on the fluorescence resonance energy transfer (FRET) process. In this work, a coumarin derivative was chosen as an energy donor, a rhodol derivative was used as an energy acceptor and a 2-fluoro-5-nitrobenzoate group was applied as a recognition unit for H₂S_n. In the absence of H₂S_n, the rhodol receptor existed in the non-fluorescent spirolactone state and FRET process was disabled. In the presence of H₂S_n, the closed spirolactone form was converted to a conjugated fluorescent xanthenes form to invoke the occurrence of FRET which resulted in a 77 nm red-shift of fluorescence emission from 460 nm to 537 nm. The ratio value of the fluorescence intensity between 537 nm and 460 nm (I_537nm/I_460nm) of the probe exhibited a good linear relationship toward H₂S_n in the range of 3.0 × 10^-6-1.0 × 10^-4 mol·L^-1, and the detection limit was estimated to be 8.0 × 10^-7 mol·L^-1. In addition, the ratiometric fluorescent probe showed high specificity for H₂S_n over other biologically related species. Moreover, the probe displayed little cell toxicity and had been successfully used to the confocal imaging of H₂S_n in HepG2 cells by dual emission channels.

A rapid and sensitive fluorescent probe for detecting hydrogen polysulfides in living cells and zebra fish

Hydrogen polysulfides (H₂S_n, n>1) plays crucial roles in many biological processes, while it remains a challenge for rapid and selective detection of H₂S_n. We designed and synthesized a turn-on fluorescent probe (JCCF) for detecting H₂S_n based on a new julolidine-coumarinocoumarin scaffold. H₂S_n could trigger a dramatic fluorescence enhancement (52-fold) with a fast response time and a low detection limit of 98.3 nM (S/N = 3). Moreover, JCCF was successfully applied to image H₂S_n in living cells and zebra fish with low cytotoxicity.

Bioapplications of small molecule Aza-BODIPY: from rational structural design to in vivo investigations

This review highlights the empirical design guidelines and photophysical property manipulation of Aza-BODIPY dyes and the latest advances in their bioapplications.

A BODIPY-based ratiometric probe for sensing and imaging hydrogen polysulfides in living cells

Hydrogen polysulfides (H₂S_n, n > 1) have attracted increasing attention in biological systems due to its redox signaling effect. To illustrate the process of the physiological and pathological roles played by H₂S_n, accurate detection is highly desired. In this work, we report a BODIPY-based fluorescent probe (BDP-PHS) for ratiometric H₂S_n sensing. BDP-PHS shows higher sensitivity and selectivity ratiometric response toward H₂S_n than various biological related species. Moreover, BDP-PHS has been successfully applied in imaging of H₂S_n in living cells.

Visualization of hydrogen polysulfides in living cells and in vivo via a near-infrared fluorescent probe

Hydrogen polysulfides (H₂S_n, n > 1), as the oxidized forms of H₂S, have attracted increasing attention these years due to their involvement in signaling transduction and cytoprotective processes. It is necessary to detect H₂S_n in living systems for the study of their functions. In this work, we report a BODIPY-based near-infrared emitting fluorescence probe NIR-PHS1, with "turn-on" response, rapid response rate (within 10 min), outstanding selectivity and excellent sensitivity (detection limit = 12 nM) response towards H₂S_n. The probe was successfully applied to the visualizing of endogenous H₂S_n in living cells. Moreover, it can be used for near-infrared in vivo imaging of H₂S_n in living mice. Therefore, NIR-PHS1 could be a potential imaging tool to study the biological roles of H₂S_n in living systems.

A new turn-on fluorescent probe with ultra-large fluorescence enhancement for detection of hydrogen polysulfides based on dual quenching strategy

Based on dual quenching strategy (ESIPT inhibited quenching and PET quenching), we have developed a new turn-on fluorescent probe 1. Combining 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2) as the fluorophore and 2-fluoro-5-nitro-benzoic as the recognition moiety, probe 1 had feature of notable large Stokes shift, highly sensitivity and selective for monitoring H₂S_n with remarkable fluorescence enhancement (328-fold) response at 534 nm. Probe 1 exhibited excellent performance in the quantitative detection of H₂S_n with a 137 nm Stokes shift and a low detection limit of 26 nM in solution. Finally, probe 1 was successfully utilized to image H₂S_n in living A549 cells and zebrafish.

Inorganic hydrogen polysulfides: chemistry, chemical biology and detection

Recent studies suggest that inorganic hydrogen polysulfides (H2 Sn , n ≥ 2) play important regulatory roles in redox biology. Modulation of their cellular levels could have potential therapeutic value. This review article focuses on our current understanding of the biosynthesis, biofunctions, fundamental physical/chemical properties, detection methods and delivery techniques of H2 Sn . LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

A Promising Family of Fluorescent Water-Soluble aza-BODIPY Dyes for in Vivo Molecular Imaging

A new family of water-soluble and bioconjugatable aza-BODIPY fluorophores was designed and synthesized using a boron- functionalization strategy. These dissymmetric bis-ammonium aza-BODIPY dyes present optimal properties for a fluorescent probe; i.e., they are highly water-soluble, very stable in physiological medium; they do not aggregate in PBS, possess high quantum yield; and finally, they can be easily bioconjugated to antibodies. Preliminary in vitro and in vivo studies were performed for one of these fluorophores to image PD-L1 (Programmed Death-Ligand 1), highlighting the high potential of these new probes for future in vivo optical imaging studies.

True one cell chemical analysis: a review

The constantly growing field of True One Cell (TOC) analysis has provided important information on the direct chemical composition of various cells and cellular components.

A resorufin-based fluorescent probe for imaging polysulfides in living cells

Nowadays H₂S_n have attracted ever-increasing attention in the field of biomedical research. Herein, we report a resorufin-based “turn-on” probe for H₂S_n sensing in vitro and in living cells.

A reversible fluorescent probe based on C[double bond, length as m-dash]N isomerization for the selective detection of formaldehyde in living cells and in vivo

Formaldehyde (FA) is an endogenously produced reactive carbonyl species (RCS) through biological metabolic processes whose concentration is closely related to human health and disease. Noninvasive and real-time detection of FA concentration in organisms is very important for revealing the physiological and pathological functions of FA. Herein, we design and synthesize a reversible fluorescent probe BOD-NH₂ for the detection of FA in living cells and in vivo. The probe is composed of two moieties: the BODIPY fluorophore and the primary amino group response unit. The probe undergoes an intracellular aldimine condensation reaction with FA and forms imine (C[double bond, length as m-dash]N) which will result in C[double bond, length as m-dash]N isomerization and rotation to turn-off the fluorescence of the probe. It is important that the probe can show a reversible response to FA. The probe BOD-NH₂ has been successfully applied for detecting and imaging FA in the cytoplasm of living cells. BOD-NH₂ is capable of detecting fluctuations in the levels of endogenous and exogenous FA in different types of living cells. The probe can be used to visualize the FA concentration in fresh hippocampus and the probe can further qualitatively evaluate the FA concentrations in ex vivo-dissected organs. Moreover, BOD-NH₂ can also be used for imaging in mice. The above applications make our new probe a potential chemical tool for the study of physiological and pathological functions of FA in cells and in vivo.

A highly selective fluorescent probe for the detection of hypochlorous acid in tap water and living cells

A turn-on fluorescent probe (DAME) for sensing hypochlorous acid (HClO) with excellent selectivity was presented. The fluorescent probe was composed of coumarin derivative as the fluorophore and dimethylcarbamothioic chloride group with a sulfide moiety as modulator. Additionally, the sulfide moiety would be oxidized by HClO, and then free dye of coumarin derivate was released and exhibited significant fluorescence. In addition, the probe could respond to HClO in solutions within 60 s and the limit of detection was down to 34.75 nM. Moreover, the probe was used for the detection of HClO in tap water through the home-made test paper. And confocal images confirmed that probe DAME could be used for recognizing HClO in living cells.

Imaging of intracellular sulfane sulfur expression changes under hypoxic stress via a selenium-containing near-infrared fluorescent probe

Hypoxia is a significant global issue affecting the health of organisms. Oxygen homeostasis is critical for mammalian cell survival and cellular activities. Hypoxic stress can lead to cell injury and death, which contributes to many diseases. Sulfane sulfur is involved in crucial roles in physiological processes of maintaining intracellular redox state and ameliorating oxidative damage. Therefore, real-time imaging of changes in sulfane sulfur levels is important for understanding their biofunctions in cells. In this study, we develop a new near-infrared (NIR) fluorescent probe BD-diSeH for imaging of sulfane sulfur changes in cells and in vivo under hypoxic stress. The probe includes two moieties: an NIR azo-BODIPY fluorophore equipped with a strong nucleophilic phenylselenol group (-SeH). The probe is capable of tracing dynamic changes of endogenous sulfane sulfur based on a fast and spontaneous intramolecular cyclization reaction. The probe has been successfully used for imaging sulfane sulfur in 3D-multicellular spheroid and mouse hippocampus under hypoxic stress. The overall levels of sulfane sulfur are affected by the degree and length of hypoxic stress. The results reveal a close relationship between sulfane sulfur and hypoxia in living cells and in vivo, allowing better understanding of physiological and pathological processes involving sulfane sulfur. Moreover, to investigate the effects of environmental hypoxia on aquatic animals, this probe has been applied for sulfane sulfur detection in hypoxic zebrafish.

A fluorescent τ-probe: quantitative imaging of ultra-trace endogenous hydrogen polysulfide in cells and in vivo

Hydrogen sulfide (H2S) has been recognized as an important endogenous gasotransmitter associated with biological signaling transduction. However, recent biological studies implied that the H2S-related cellular signaling might actually be mediated by hydrogen polysulfides (H2S n , n > 1), not H2S itself. Unraveling such a mystery strongly demanded the quantification of endogenous H2S n in living systems. However, endogenous H2S n has been undetectable thus far, due to its extremely low concentration within cells. Herein, we demonstrated a strategy to detect ultra-trace endogenous H2S nvia a fluorescent τ-probe, through changes of fluorescence lifetime instead of fluorescence intensity. This τ-probe exhibited an ultrasensitive response to H2S n , bringing about the lowest value of the detection limit (2 nM) and a lower limit of quantification (10 nM) to date. With such merits, we quantified and mapped endogenous H2S n within cells and zebrafish. The quantitative information about endogenous H2S n in cells and in vivo may have a significant implication for future research on the role of H2S n in biology. The methodology of the τ-probe established here might provide a general insight into the design and application of any fluorescent probes, beyond the limit of utilizing fluorescence intensity.

Fluorogenic NIR-probes based on 1,2,4,5-tetrazine substituted BF2-azadipyrromethenes

A series of 1,2,4,5-tetrazine integrated near infrared (NIR) fluorophores based on the BF₂ azadipyrromethene (NIR-AZA) class has been synthesised and their ability to modulate emission from low to high in response to Diels-Alder cycloaditions has been assessed. Substituents on the tetrazine component of the probe (Cl, OMe, p-NO₂C₆H₄O) were seen to strongly influence quantum yields, fluorescence enhancement factors, and rates of cycloadditions. Cycloadditions between tetrazine-NIR-AZA constructs and a strained alkyne substrate were seen to be highly efficient in organic or aqueous solutions and in gels with high fluorescence enhancements of up to 48-fold observed. Real-time demonstration of the cycloaddition mediated fluorogenic property was achieved by imaging the "turn-on" reaction within a continous flow micro-reactor. Preliminary evidence indicates that excited state quenching involves a photoinduced electron transfer.

Polysulfide-triggered fluorescent indicator suitable for super-resolution microscopy and application in imaging

A new physiologically benign and cell membrane permeable BODIPY based molecular probe, MB-Sn, specifically senses intracellular hydrogen polysulfides (H₂S_n, n > 1) localized in the endoplasmic reticulum.

Novel π-extended hybrid xanthene dyes with two spirolactone rings for optoelectronic and biological applications

Two hybrid xanthene dyes that can operate as half-subtractors in methanol and can target mitochondria have been developed.

An isophorone-based far-red emitting ratiometric fluorescent probe for selective sensing and imaging of polysulfides

An isophorone-based far-red emitting fluorescent probe (RPHS1) for selective ratiometric sensing and imaging of H₂S_n in living cells was reported.

Fluorescent Probes and Selective Inhibitors for Biological Studies of Hydrogen Sulfide- and Polysulfide-Mediated Signaling

SIGNIFICANCE

Hydrogen sulfide (H₂S) plays roles in many physiological processes, including relaxation of vascular smooth muscles, mediation of neurotransmission, inhibition of insulin signaling, and regulation of inflammation. Also, hydropersulfide (R-S-SH) and polysulfide (-S-S_n-S-) have recently been identified as reactive sulfur species (RSS) that regulate the bioactivities of multiple proteins via S-sulfhydration of cysteine residues (protein Cys-SSH) and show cytoprotection. Chemical tools such as fluorescent probes and selective inhibitors are needed to establish in detail the physiological roles of H₂S and polysulfide. Recent Advances: Although many fluorescent probes for H₂S are available, fluorescent probes for hydropersulfide and polysulfide have only recently been developed and used to detect these sulfur species in living cells.

CRITICAL ISSUES

In this review, we summarize recent progress in developing chemical tools for the study of H₂S, hydropersulfide, and polysulfide, covering fluorescent probes based on various design strategies and selective inhibitors of H₂S- and polysulfide-producing enzymes (cystathionine γ-lyase, cystathionine β-synthase, and 3-mercaptopyruvate sulfurtransferase), and we summarize their applications in biological studies.

FUTURE DIRECTIONS

Despite recent progress, the precise biological functions of H₂S, hydropersulfide, and polysulfide remain to be fully established. Fluorescent probes and selective inhibitors are effective chemical tools to study the physiological roles of these sulfur molecules in living cells and tissues. Therefore, further development of a broad range of practical fluorescent probes and selective inhibitors as tools for studies of RSS biology is currently attracting great interest. Antioxid. Redox Signal. 27, 669-683.

Systematic in vitro assessment of responses of roGFP2-based probes to physiologically relevant oxidant species

The genetically encoded probes roGFP2-Orp1 and Grx1-roGFP2 have been designed to be selectively oxidized by hydrogen peroxide (H₂O₂) and glutathione disulfide (GSSG), respectively. Both probes have demonstrated such selectivity in a broad variety of systems and conditions. In this study, we systematically compared the in vitro response of roGFP2, roGFP2-Orp1 and Grx1-roGFP2 to increasing amounts of various oxidant species that may also occur in biological settings. We conclude that the previously established oxidant selectivity is highly robust and likely to be maintained under most physiological conditions. Yet, we also find that hypochlorous acid, known to be produced in the phagocyte respiratory burst, can lead to non-selective oxidation of roGFP2-based probes at concentrations ≥2µM, in vitro. Further, we confirm that polysulfides trigger direct roGFP2 responses. A side-by-side comparison of all three probes can be used to reveal micromolar amounts of hypochlorous acid or polysulfides.

Quantitative determination of polysulfide in albumins, plasma proteins and biological fluid samples using a novel combined assays approach

Hydrogen sulfide (H₂S) signaling involves polysulfide (RSS_nSR') formation on various proteins. However, the current lack of sensitive polysulfide detection assays poses methodological challenges for understanding sulfane sulfur homeostasis and signaling. We developed a novel combined assay by modifying Sulfide Antioxidant Buffer (SAOB) to produce an "Elimination Method of Sulfide from Polysulfide" (EMSP) treatment solution that liberates sulfide, followed with methylene blue (MB) sulfide detection assay. The combined EMSP-MB sulfide detection assay performed on low molecular weight sulfur species showed that sulfide was produced from trisulfide compounds such as glutathione trisulfide and diallyl trisulfide, but not from the thiol compounds such as cysteine, cystine and glutathione. In the case of plasma proteins, this novel combined detection assay revealed that approximately 14.7, 1.7, 3.9, 3.7 sulfide mol/mol released from human serum albumin, α₁-anti-trypsin, α₁-acid glycoprotein and ovalbumin, respectively, suggesting that serum albumin is a major pool of polysulfide in human blood circulation. Taken together with the results of albumins of different species, the liberated sulfide has a good correlation with cysteine instead of methionine, indicating the site of incorporation of polysulfide is cysteine. With this novel sulfide detention assay, approximately 8,000, 120 and 1100 μM of polysulfide concentrations was quantitated in human healthy plasma, saliva and tear, respectively. Our promising polysulfide specific detection assay can be a very important tool because quantitative determination of polysulfide sheds light on the functional consequence of protein-bound cysteine polysulfide and expands the research area of reactive oxygen to reactive polysulfide species.

Fluorescent probes for hydrogen polysulfides (H2Sn, n > 1): from design rationale to applications

Hydrogen polysulfides (H₂S_n, n > 1) are gaining much research interest due to their involvement in signaling and cytoprotection. The present review highlights recent advances in the design of fluorescent probes for the detection of H₂S_n along with the fundamental challenges and future prospects in this field.

A near-infrared fluorescence off–on probe for sensitive imaging of hydrogen polysulfides in living cells and mice in vivo

A new near-infrared fluorescence off–on probe with phenyl 2-(benzoylthio)benzoate as the recognition moiety is developed and applied in imaging H₂S_n in living cells and mice in vivo.

Fluorescent chemical probes for accurate tumor diagnosis and targeting therapy

This review focuses on small molecular ligand-targeted fluorescent imaging probes and fluorescent theranostics, including their design strategies and applications in clinical tumor treatment.

Ratiometric Two-Photon Fluorescent Probe for in Vivo Hydrogen Polysulfides Detection and Imaging during Lipopolysaccharide-Induced Acute Organs Injury

Acute organ injury observed during sepsis, caused by an uncontrolled release of inflammatory mediators, such as lipopolysaccharide (LPS), is quite fatal. The development of efficient methods for early diagnosis of sepsis and LPS-induced acute organ injury in living systems is of great biomedical importance. In living systems, cystathionine γ-lyase (CSE) can be overexpressed due to LPS, and H₂S_n can be formed by CSE-mediated cysteine metabolism. Thus, acute organ injury during sepsis may be correlated with H₂S_n levels, making accurate detection of H₂S_n in living systems of great physiological and pathological significance. In this work, our previously reported fluorescent platform was employed to design and synthesize a FRET-based ratiometric two-photon (TP) fluorescent probe TPR-S, producing a large emission shift in the presence of H₂S_n. In this work, a naphthalene derivative two-photon fluorophore was chosen as the energy donor; a rhodol derivative fluorophore served as the acceptor. The 2-fluoro-5-nitrobenzoate group of probe TPR-S reacted with H₂S_n and was selectively removed to release the fluorophore, resulting in a fluorescent signal decrease at 448 nm and enhancement at 541 nm. The ratio value of the fluorescence intensity between 541 and 448 nm (I_541 nm/I_448 nm) varied from 0.13 to 8.12 (∼62-fold), with the H₂S_n concentration changing from 0 to 1 mM. The detection limit of the probe was 0.7 μM. Moreover, the probe was applied for imaging H₂S_n in living cells, tissues, and organs of LPS-induced acute organ injury, which demonstrated its practical application in complex biosystems as a potential method to achieve early diagnosis of LPS-induced acute organ injury.

Quantification of cysteine hydropersulfide with a ratiometric near-infrared fluorescent probe based on selenium-sulfur exchange reaction

Cysteine hydropersulfide (Cys-SSH) plays primary roles in the synthesis of sulfur-containing cofactors, regulation of cellular signaling, activation or inactivation of enzyme activities, and modulation of cellular redox milieu. However, its biofunctions need to be further addressed due to the fact that many issues remain to be clarified. Herein, we conceive a novel ratiometric near-infrared fluorescent probe Cy-Dise for the sensitive and selective detection of Cys-SSH in living cells and in vivo for the first time. Cy-Dise is composed of three moieties: bis(2-hydroxyethyl) diselenide, heptamethine cyanine, and d-galactose. Cy-Dise exhibits a satisfactory linear ratio response to Cys-SSH via a selenium-sulfur exchange reaction in the range of 0-12 μM Cys-SSH. The experimental detection limit is determined to be 0.12 μM. The results of ratio imaging analyses confirm the qualitative and quantitative detection capabilities of Cy-Dise in HepG2 cells, HL-7702 cells, and primary hepatocytes. The level changes of Cys-SSH in cells stimulated by some related reagents are also observed. The probe is also suitable for deep tissue ratio imaging. Organ targeting tests with Cy-Dise in normal Spraque-Dawley (SD) rats and Walker-256 tumor SD rats verify its predominant localization in the liver. The probe is promising for revealing the roles of Cys-SSH in physiological and pathological processes.

Highly Selective and Sensitive One- and Two-Photon Ratiometric Fluorescent Probe for Intracellular Hydrogen Polysulfide Sensing

Hydrogen polysulfide (H2Sn) has attracted increasing attention due to the fact that it is actually the key signaling molecule rather than hydrogen sulfide (H2S). Therefore, developing a sensitive and accurate assay to investigate the biosynthetic pathways of H2Sn is of physiological and pathological significance. In this work, based on the commonly used two-photon fluorophore, 1,8-naphthalimide, a new probe, NRT-HP, has been designed and synthesized that displayed both one- and two-photon ratiometric fluorescence changes toward H2Sn via H2Sn-mediated benzodithiolone formation. NRT-HP exhibits excellent pH stability, high selectivity and low detection limit (0.1 μM) in aqueous media. Furthermore, two-photon fluorescence microscopy experiments have demonstrated that NRT-HP could be used for the H2Sn detection in live cells as well as tissue slices.

Azadipyrromethenes: from traditional dye chemistry to leading edge applications

The journey of azadipyrromethenes from accidental dye chemistry to a compound class with widely applicable near infrared photophysical properties is documented.

A flavylium-based turn-on fluorescent probe for imaging hydrogen polysulfides in living cells

A flavylium-based turn-on fluorescent probe for imaging of hydrogen polysulfides in living cells has been developed.

Long-wavelength analyte-sensitive luminescent probes and optical (bio)sensors

Long-wavelength luminescent probes and sensors become increasingly popular. They offer the advantage of lower levels of autofluorescence in most biological probes. Due to high penetration depth and low scattering of red and NIR light such probes potentially enable in vivo measurements in tissues and some of them have already reached a high level of reliability required for such applications. This review focuses on the recent progress in development and application of long-wavelength analyte-sensitive probes which can operate both reversibly and irreversibly. Photophysical properties, sensing mechanisms, advantages and limitations of individual probes are discussed.

A near-infrared fluorescent probe for the selective detection of HNO in living cells and in vivo

We present a near-infrared fluorescent probe for the detection of nitroxyl (HNO) in living cells and in mice.