Visualizing Hydrogen Sulfide in Mitochondria and Lysosome of Living Cells and in Tumors of Living Mice with Positively Charged Fluorescent Chemosensors

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.

Targeting sub-cellular organelles for boosting precision photodynamic therapy

Among various cancer treatment methods, photodynamic therapy has received significant attention due to its non-invasiveness and high efficiency in inhibiting tumour growth. Recently, specific organelle targeting photosensitizers have received increasing interest due to their precise accumulation and ability to trigger organelle-mediated cell death signalling pathways, which greatly reduces the drug dosage, minimizes toxicity, avoids multidrug resistance, and prevents recurrence. In this review, recent advances and representative photosensitizers used in targeted photodynamic therapy on organelles, specifically including the endoplasmic reticulum, Golgi apparatus, mitochondria, nucleus, and lysosomes, have been comprehensively reviewed with a focus on organelle structure and organelle-mediated cell death signalling pathways. Furthermore, a perspective on future research and potential challenges in precision photodynamic therapy has been presented at the end.

Recent Developments On Activatable Turn-On Fluorogenic Donors of Hydrogen Sulfide (H2S)

Hydrogen sulfide (H2S) is considered the third member of the gasotransmitter family, along with nitric oxide (NO) and carbon monoxide (CO). Besides its role in physiological and pathophysiological conditions, the promising therapeutic potential of this small-molecule makes it advantageous for various pharmaceutical applications. The endogenous production of H2S at a lower concentration is crucial in maintaining redox balance and cellular homeostasis, and the dysregulation leads to various disease states. In the event of H2S deficiency, the exogenous donation of H2S could help maintain the optimal cellular concentration of H2S and cellular homeostasis. Over the last several years, researchers have developed numerous small-molecule non-fluorogenic organosulfur compounds as H2S donors and investigated their pharmacological potentials. However, reports on stimuli-responsive turn-on fluorogenic donors of H2S have appeared recently. Interestingly, the fluorogenic H2S donors offer additional advantages with the non-invasive real-time monitoring of the H2S release utilizing the simultaneous turn-on fluorogenic processes. The review summarizes the recent developments in turn-on fluorogenic donors of H2S and the potential biological applications that have developed over the years.

Concurrent Subcellular Delivery of Hydrogen Sulfide and a Payload with Near-Infrared Light

Hydrogen sulfide (H2S) is a gaseous signaling molecule, exerting crucial regulatory functions in organelles and cellular environments. H2S exhibits high therapeutic potential and synergistic effects with other drugs, and its potency is notably enhanced through organelle-specific targeting. Yet, the navigation of light-activated H2S donors to specific organelles remains absent. Here, we report the first organelle-specific photocage that simultaneously delivers H2S and a payload with subcellular precision to mitochondria of live human cells using tissue-penetrating near-infrared light as a trigger. The fluorogenic payload enables real-time monitoring of the process, and we demonstrate the concurrent uncaging in mitochondria through a combination of fluorescence microscopy and mitochondria-specific fluorescent probes. We anticipate that these photocages will permit the precise delivery of H2S-drug combinations with exceptional spatiotemporal control, thereby driving the harnessing of known synergistic effects and the discovery of novel therapeutic strategies.

Accelerated Activity-Based Sensing by Fluorogenic Reporter Engineering Enables to Rapidly Determine Unstable Analyte

Accurate detection of labile analytes through activity based fluorogenic sensing is meaningful but remains a challenge because of nonrapid reaction kinetic. Herein, we present a signaling reporter engineering strategy to accelerate azoreduction reaction by positively charged fluorophore promoted unstable anion recognition for rapidly sensing sodium dithionite (Na2S2O4), a kind of widespread used but harmful inorganic reducing agent. Its quick decomposition often impedes application reliability of traditional fluorogenic probes in real samples because of their slow responses. In this work, four azo-based probes with different charged fluorophores (positive, zwitterionic, neutral, and negative) were synthesized and compared. Among of them, with sequestration effect of positively charged anthocyanin fluorophore for dithionite anion via electrostatic attraction, the cationic probe Azo-Pos displayed ultrafast fluorogenic response (∼2 s) with the fastest response kinetic (kpos' = 0.373 s-1) that is better than other charged ones (kzwi' = 0.031 s-1, kneu' = 0.013 s-1, kneg' = 0.003 s-1). Azo-Pos was demonstrated to be capable to directly detect labile Na2S2O4 in food samples and visualize the presence of Na2S2O4 in living systems in a timely fashion. This new probe has potential as a robust tool to fluorescently monitor excessive food additives and biological invasion of harmful Na2S2O4. Moreover, our proposed accelerating strategy would be versatile to develop more activity-based sensing probes for quickly detecting other unstable analytes of interest.

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 dual-responsive fluorescent probe based on cyanine and naphthalimide units for detecting HClO and H2S in living cells

Hypochlorous acid (HClO) and hydrogen sulfide (H2S) play very important roles in both physiological and pathological processes, however, the methods for simultaneously detecting HClO and H2S were rarely reported. Here, a dual-responsive fluorescent probe (CyNa-N3) based on cyanine and naphthalimide dyes was synthesized and investigated. The fluorescence probe showed better sensitivity, high selectivity response to HClO and H2S by red emission and green emission bands, and the limits of detection were 0.17 µM and 0.15 µM respectively. MS (Mass Spectrum) and 1H NMR (Nuclear Magnetic Resonance) confirmed the sensing mechanism of CyNa-N3 detected HClO and H2S, the calculation of density functional theory (DFT) further explained the internal mechanism of spectral change of CyNa-N3. Moreover, CyNa-N3 was successfully applied to image HClO and H2S in living cells, which is beneficial for more efficient application in biological imaging.

Chemical Strategies Toward Prodrugs and Fluorescent Probes for Gasotransmitters

Three gaseous molecules are widely accepted as important gasotransmitters in mammalian cells, namely NO, CO and H2S. Due to the pharmacological effects observed in preclinical studies, these three gasotransmitters represent promising drug candidates for clinical translation. Fluorescent probes of the gasotransmitters are also in high demand; however, the mechanisms of actions or the roles played by gasotransmitters under both physiological and pathological conditions remain to be answered. In order to bring these challenges to the attention of both chemists and biologists working in this field, we herein summarize the chemical strategies used for the design of both probes and prodrugs of these three gasotransmitters.

Specific imaging of intracellular hydrogen sulfide by a positively charged NIR fluorescent probe

The poor water solubility of traditional activatable organic molecular probes usually limits their detection ability in physiological environment. In this work, a positively charged H2S probe was designed, which exhibited a significantly enhanced responsiveness to H2S in the aggregated state due to the increased positive charge density on the aggregate surface. Under physiological conditions, the probe could be activated by H2S with specificity and sensitivity to release near-infrared fluorescence signal. Moreover, endogenous H2S levels in living cells were successfully monitored by using this probe. We expect that this probe can provide a new strategy for the design of activatable probes to break the limitation of poor water solubility of conventional organic molecular probes.

Molecular Engineering Design of Enhanced Donor-Acceptor Therapeutic Reagent for Efficient Image-Guided Photodynamic Therapy

The greatest barrier to the further development and clinical application of tumor image-guided photodynamic therapy (PDT), is the inconsistency between the fluorescence intensity and singlet oxygen generation yield of the photosensitizer under light excitation. Herein, a novel donor-acceptor (D-A) system is designed from the point of molecular selection by wrapping a classical porphyrin molecule (5,10,15,20-tetraphenylphorphyrin, H2 TPP) as an acceptor into conjugated polymer (Poly[N,N'-bis(4-butylpheny)-N,N'-bis(phenyl)benzidine], ADS254BE) as a donor through fluorescence resonance energy transfer (FRET) mechanism, which exhibits bright red emission centered at 650 nm (quantum yield, 0.12), relatively large Stoke shift of 276 nm, enhanced singlet oxygen generation rate of 0.73, and excellent photostability. The investigations on distribution and killing effect of nanomaterials in cancer cells reveal that ADS254BE/H2 TPP NPs can accumulate in the cytoplasm for imaging while simultaneously producing a large amount of singlet oxygen to remarkably kill cancer cells, which can be used for real-time image-guided PDT. In the xenograft tumor model, real-time imaging and long-term tracing in tumor tissue with ADS254BE/H2 TPP NPs disclose that the growth of lung cancer in mice can be effectively inhibited during in situ imaging. From the standpoint of molecular engineering design, this work provides a feasible strategy for novel D-A systems to improve the development of image-guided PDT.

Advances and Opportunities in H2S Measurement in Chemical Biology

Hydrogen sulfide (H2S) is an important biological mediator across all kingdoms of life and plays intertwined roles in various disciplines, ranging from geochemical cycles to industrial processes. A common need across these broad disciplines is the ability to detect and measure H2S in complex sample environments. This Perspective focuses on key advances and opportunities for H2S detection and quantification that are relevant to chemical biology. Specifically, we focus on methods for H2S detection and quantification most commonly used in biological samples, including activity-based H2S probes, the methylene blue assay, the monobromobimane assay, and H2S-sensitive electrode measurements. Our goal is to help simplify what at first may seem to be an overwhelming array of detection and measurement choices, to articulate the strengths and limitations of individual techniques, and to highlight key unmet needs and opportunities in the field.

Organelle-targeting ratiometric fluorescent probes: design principles, detection mechanisms, bio-applications, and challenges

Biological species, including reactive oxygen species (ROS), reactive sulfur species (RSS), reactive nitrogen species (RNS), F-, Pd2+, Cu2+, Hg2+, and others, are crucial for the healthy functioning of cells in living organisms. However, their aberrant concentration can result in various serious diseases. Therefore, it is essential to monitor biological species in cellular organelles such as the cell membrane, mitochondria, lysosome, endoplasmic reticulum, Golgi apparatus, and nucleus. Among various fluorescent probes for species detection within the organelles, ratiometric fluorescent probes have drawn special attention as a potential way to get beyond the drawbacks of intensity-based probes. This method depends on measuring the intensity change of two emission bands (caused by an analyte), which produces an efficient internal referencing that increases the detection's sensitivity. This review article discusses the literature publications (from 2015 to 2022) on organelle-targeting ratiometric fluorescent probes, the general strategies, the detecting mechanisms, the broad scope, and the challenges currently faced by fluorescent probes.

Recent progress in the development of fluorescent probes for imaging pathological oxidative stress

Oxidative stress is closely related to the physiopathology of numerous diseases. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are direct participants and important biomarkers of oxidative stress. A comprehensive understanding of their changes can help us evaluate disease pathogenesis and progression and facilitate early diagnosis and drug development. In recent years, fluorescent probes have been developed for real-time monitoring of ROS, RNS and RSS levels in vitro and in vivo. In this review, conventional design strategies of fluorescent probes for ROS, RNS, and RSS detection are discussed from three aspects: fluorophores, linkers, and recognition groups. We introduce representative fluorescent probes for ROS, RNS, and RSS detection in cells, physiological/pathological processes (e.g., Inflammation, Drug Induced Organ Injury and Ischemia/Reperfusion Injury etc.), and specific diseases (e.g., neurodegenerative diseases, epilepsy, depression, diabetes and cancer, etc.). We then highlight the achievements, current challenges, and prospects for fluorescent probes in the pathophysiology of oxidative stress-related diseases.

Observing hydrogen sulfide in the endoplasmic reticulum of cancer cells and zebrafish by using an activity-based fluorescent probe

A crucial endogenous signaling chemical, hydrogen sulfide, is involved in many physiological actions. In this work, we created the fluorescent probe ER-Nap-NBD using a naphthalimide fluorophore as the signal reporter, a 7-nitro-1,2,3-benzoxadiazole amine as the responsive moiety, and a sulfonamide part for endoplasmic reticulum targeting. ER-Nap-NBD could be detected the H₂S levels in solution and in living systems (cells and zebrafish).

Three near-infrared and lysosome-targeting probes for photodynamic therapy (PDT)

Lysosome, an organelle which contains a number of hydrolases and hydrogen ions, plays a crucial role in cellular survival and apoptosis. If selectively destroy lysosomes membrane, inner hydrolases and hydrogen ions will leak and induce cell death. In this work, three lysosome-targeting fluorescent probes (HCL 1-3, heptamethine cyanine lysosomal-targeting probe) were designed, synthesized and developed for photodynamic therapy. Piperazine and N, N-dimethyl structures made HCL 1-3 have good lysosome targeting ability while Pearson's correlation coefficients reached 0.85, 0.87 and 0.78. It can be concluded from MTT test, HCL 1-3 have high photo cytotoxicity and low dark cytotoxicity from MTT test. Calcein/PI staining assays also supported cytotoxicity of HCL 1-3 under light conditions. In vivo experiments, HCL 2 accumulated in tumor and a strong fluorescence signal was observed at 12 h post injection. All results showed that our experiments provide help and new ideas for cyanine dyes in cancer treatment.

H2S Sensors: Synthesis, Optical Properties, and Selected Biomedical Applications under Visible and NIR Light

Hydrogen sulfide (H₂S) is an essential signaling gas within the cell, and its endogenous levels are correlated with various health diseases such as Alzheimer's disease, diabetes, Down's syndrome, and cardiovascular disease. Because it plays such diverse biological functions, being able to detect H₂S quickly and accurately in vivo is an area of heightened scientific interest. Using probes that fluoresce in the near-infrared (NIR) region is an effective and convenient method of detecting H₂S. This approach allows for compounds of high sensitivity and selectivity to be developed while minimizing cytotoxicity. Herein, we report a review on the synthesis, mechanisms, optical properties, and selected biomedical applications of H₂S sensors.

A biotin-guided near-infrared fluorescent probe for imaging hydrogen sulfide and differentiating cancer cells

Imaging cancer specific biomarkers with near-infrared (NIR) fluorescent probes can help inaccurate diagnosis. Hydrogen sulfide (H₂S) has been reported to be involved in many physiological and pathological processes and is considered as one of the key gasotransmitters during the development of cancer. To achieve specific H₂S detection in cancer cells, we reported a biotin-guided NIR fluorescent sensor P1 targeting a cancer cell surface biomarker, based on the H₂S-specific thiolysis of the NBD-amine-hemicyanine conjugate. The probe showed a fast turn-on signal at 754 nm upon H₂S activation and good selectivity towards H₂S over millimolar levels of other biothiols. We successfully employed P1 to image endogenous H₂S and demonstrated its tumor-targeting ability in live cells. P1 could differentiate multiple cancer cells with various levels of H₂S from normal cells, indicating its potential for cancer imaging.

Organelle-targeted therapies: a comprehensive review on system design for enabling precision oncology

Cancer is a major threat to human health. Among various treatment methods, precision therapy has received significant attention since the inception, due to its ability to efficiently inhibit tumor growth, while curtailing common shortcomings from conventional cancer treatment, leading towards enhanced survival rates. Particularly, organelle-targeted strategies enable precise accumulation of therapeutic agents in organelles, locally triggering organelle-mediated cell death signals which can greatly reduce the therapeutic threshold dosage and minimize side-effects. In this review, we comprehensively discuss history and recent advances in targeted therapies on organelles, specifically including nucleus, mitochondria, lysosomes and endoplasmic reticulum, while focusing on organelle structures, organelle-mediated cell death signal pathways, and design guidelines of organelle-targeted nanomedicines based on intervention mechanisms. Furthermore, a perspective on future research and clinical opportunities and potential challenges in precision oncology is presented. Through demonstrating recent developments in organelle-targeted therapies, we believe this article can further stimulate broader interests in multidisciplinary research and technology development for enabling advanced organelle-targeted nanomedicines and their corresponding clinic translations.

Biosynthesis-mediated Ni-Fe-Cu LDH-to-sulfides transformation enabling sensitive detection of endogenous hydrogen sulfide with dual-readout signals

Hydrogen sulfide (H₂S) is a vital endogenous gas signal molecule undertaking numerous physiological functions such as biological regulation and cytoprotection. Herein, we developed an electrochemical (EC) and photothermal (PT) dual-readout signals method for H₂S detection based on a novel biosynthesis-mediated Ni-Fe-Cu LDH-to-sulfides transformation strategy. Interestingly, the Cu²⁺-based Ni-Fe LDH (Ni-Fe-Cu LDH) can act as the Cu²⁺ source to react with H₂S, resulting in the in-situ generation of Cu_xS on Ni-Fe-Cu LDH surfaces. Because of the EC signal and intrinsic near-infrared (NIR) PT conversion ability of Cu_xS under 808 nm laser irradiation, the obtained Cu_xS@Ni-Fe-Cu LDH is applied to stimulate EC signal and temperature readout. By this means, a dual-readout signal mode is established for H₂S detection. Under the optimum conditions, this combination of EC and PT methods displays a wide linear range for H₂S to 0.1 μM-90 μM and 50 μM-400 μM, respectively, with a low detection limit of 0.09 μM. In addition, the practicality of Ni-Fe-Cu LDH is verified by determination of endogenous H₂S in living cells. This work not only provides a promising application for H₂S diagnosis but also exhibits the new characteristic of Ni-Fe-Cu LDH nanomaterials as signal transduction tags.

Vascular smooth muscle cell-derived hydrogen sulfide promotes atherosclerotic plaque stability via TFEB (transcription factor EB)-mediated autophagy

Vascular smooth muscle cells (VSMCs) contribute to plaque stability. VSMCs are also a major source of CTH (cystathionine gamma-lyase)-hydrogen sulfide (H₂S), a protective gasotransmitter in atherosclerosis. However, the role of VSMC endogenous CTH-H₂S in pathogenesis of plaque stability and the mechanism are unknown. In human carotid plaques, CTH expression in ACTA2⁺ cells was dramatically downregulated in lesion areas in comparison to non-lesion areas. Intraplaque CTH expression was positively correlated with collagen content, whereas there was a negative correlation with CD68⁺ and necrotic core area, resulting in a rigorous correlation with vulnerability index (r = -0.9033). Deletion of Cth in VSMCs exacerbated plaque vulnerability, and were associated with VSMC autophagy decline, all of which were rescued by H₂S donor. In ox-LDL treated VSMCs, cth deletion reduced collagen and heightened apoptosis association with autophagy reduction, and vice versa. For the mechanism, CTH-H₂S mediated VSMC autophagosome formation, autolysosome formation and lysosome function, in part by activation of TFEB, a master regulator for autophagy. Interference with TFEB blocked CTH-H₂S effects on VSMCs collagen and apoptosis. Next, we demonstrated that CTH-H₂S sulfhydrated TFEB at Cys212 site, facilitating its nuclear translocation, and then promoting transcription of its target genes such as ATG9A, LAPTM5 or LDLRAP1. Conclusively, CTH-H₂S increases VSMC autophagy by sulfhydration and activation of TFEB, promotes collagen secretion and inhibits apoptosis, thereby attenuating atherogenesis and plaque vulnerability. CTH-H₂S may act as a warning biomarker for vulnerable plaque.Abbreviations ATG9A: autophagy related 9A; CTH: cystathionine gamma-lyase; CQ: chloroquine; HASMCs: human aortic smooth muscle cells; H₂S: hydrogen sulfide; LAMP1: lysosomal associated membrane protein 1; LAPTM5: lysosomal protein transmembrane 5; NaHS: sodium hydrosulfide hydrate; ox-LDL: oxidized-low density lipoprotein; PPG: DL- propagylglycine; TFEB: transcription factor EB; 3-MA: 3-methyladenine; VSMCs: vascular smooth muscle cells.

Subcellular Delivery of Hydrogen Sulfide Using Small Molecule Donors Impacts Organelle Stress

Hydrogen sulfide (H₂S) is an endogenously produced gaseous signaling molecule with important roles in regulating organelle function and stress. Because of its high reactivity, targeted delivery of H₂S using small molecule H₂S donors has garnered significant interest to minimize off-target effects. Although mitochondrially targeted H₂S donors, such as AP39, have been reported previously and exhibit significantly higher potency than nontargeted donors, the expansion of targeted H₂S delivery to other subcellular organelles remains largely absent. To fill this key unmet need, we report a library of organelle-targeted H₂S donors that localize H₂S delivery to specific subcellular organelles, including the Golgi apparatus, lysosome, endoplasmic reticulum, and mitochondria. We measured H₂S production in vitro from each donor, confirmed the localization of H₂S delivery using organelle-specific H₂S responsive fluorescent probes, and demonstrated enhanced potency of these targeted H₂S donors in providing protection against organelle-specific stress. We anticipate this class of targeted H₂S donors will enable future studies of subcellular roles of H₂S and the pathways by which H₂S alleviates subcellular organelle stress.

A hemicyanidin-based NIR fluorescent probe for detection of H2S and imaging study in cells and mice

The selective detection of hydrogen sulfide in physiological and pathological processes has gained substantial attention in recent years. However, the real-time detection of hydrogen sulfide remains an elusive goal. In this work, a new type of hemicyanidin-based fluorescent "turn-on" probe NTR-HS (Ex = 680 nm, Em = 760 nm) was developed to detected H₂S in a very short time (3 min). The fluorescence quantum yield is 0.15 and accompanied with a noticeable color change from violet to blue that can be used to detect H₂S in the range 1.04 × 10^-7-4 × 10^-5 M with a limit of detection of 1.04 x 10^-7 M. The NTR-HS probe was also used for imaging of endogenous hydrogen sulfide and mitochondrial localization in HCT116 and HeLa cells. The detection mechanism was studied through fluorescence, UV-Vis, NMR, and mass analysis. Notably, the probe was successfully used to imaging H₂S in mice and locating hydrogen sulfide in the large intestine of mice.

A super sensitive fluorescent probe for imaging endogenous hydrogen sulfide in living cells

Hydrogen sulfide (H₂S) that typically performs biphasic biological functions in organisms plays an opposite role at the concentrations above or below normal level of the organism. Therefore, it is significant to develop a fluorescent probe with high sensitivity and selectivity and rapid response for the detection of hydrogen sulfide in vivo. The work describes the design and biological applications of a novel turn-on fluorescence probe SS-N₃ in which the quinoline quaternary ammonium salt derivative is introduced as the fluorescent skeleton and azide is employed as the detection group of H₂S. The probe SS-N₃ shows strong fluorescence at 610 nm, as the azide group is reduced to an amino group by H₂S. The probe SS-N₃ shows high selectivity to H₂S than other anions and some biological mercaptans, and strong anti-interference capacity. In addition, the probe SS-N₃ exhibits little cytotoxicity, improved photostability and large Stokes shift (135 nm), as well as can be effectively used as an indicator of H₂S level in living cells.

Development of an activatable hydrogen sulfide-specific two-photon fluorescent probe for bioimaging in an air pouch inflammation model

Inflammation caused by traumatic, ischemic, infectious, autoimmune or toxic injury may further trigger cancer and even death. Overexpression of hydrogen sulfide (H₂S) in vivo has been identified as a biomarker for various types of inflammation. Identification-responsive fluorescence imaging probes have broad application prospects for in vivo diagnosis of inflammation. However, it is a challenge to design an imaging probe that concurrently responds to the target molecules to improve the sensitivity and specificity of inflammation detection. Herein, we designed and synthesized an activatable two-photon fluorescent probe to detect H₂S. Fl-H2S had high selectivity, excellent photostable signals and low detection limit for recognizing H₂S. In addition, Fl-H2S showed excellent two-photon fluorescence properties in cell and liver tissue visualization experiments, with a penetration depth of up to 126 μm in liver tissue. Most importantly, the unique probe Fl-H2S was the first probe to monitor H₂S levels in a mouse air pouch inflammation model by fluorescence imaging technology. We expect Fl-H2S to become an effective tool for longitudinal monitoring of inflammation, diagnosis of inflammation and prediction of underlying pathogenesis of related diseases by detecting H₂S.

In vivo detection of hydrogen sulfide in the brain of live mouse: application in neuroinflammation models

PURPOSE

Hydrogen sulfide (H₂S) plays important roles in brain pathophysiology. However, nuclear imaging probes for the in vivo detection of brain H₂S in living animals have not been developed. Here, we report the first nuclear imaging probe that enables in vivo imaging of endogenous H₂S in the brain of live mice.

METHODS

Utilizing a bis(thiosemicarbazone) backbone, a fluorescent ATSM-FITC conjugate was synthesized. Its copper complex, Cu(ATSM-FITC) was thoroughly tested as a biosensor for H₂S. The same ATSM-FITC ligand was quantitatively labeled with [⁶⁴Cu]CuCl₂ to obtain a radioactive [⁶⁴Cu][Cu(ATSM-FITC)] imaging probe. Biodistribution and positron emission tomography (PET) imaging studies were performed in healthy mice and neuroinflammation models.

RESULTS

The Cu(ATSM-FITC) complex reacts instantly with H₂S to release CuS and becomes fluorescent. It showed excellent reactivity, sensitivity, and selectivity to H₂S. Endogenous H₂S levels in living cells were successfully detected by fluorescence microscopy. Exceptionally high brain uptake of [⁶⁴Cu][Cu(ATSM-FITC)] (> 9% ID/g) was observed in biodistribution and PET imaging studies. Subtle changes in brain H₂S concentrations in live mice were accurately detected by quantitative PET imaging. Due to its dual modality feature, increased H₂S levels in neuroinflammation models were characterized at the subcellular level by fluorescence imaging and at the whole-body scale by PET imaging.

CONCLUSION

Our biosensor can be readily utilized to study brain H₂S function in live animal models and shows great potential as a novel imaging agent for diagnosing brain diseases.

Dinitrobenzene ether reactive turn-on fluorescence probes for the selective detection of H2S

Two novel fluorescent probes, namely, 3-(2,4-dinitrophenoxy)-2-(4-(diphenylamino)phenyl)-4H-chromen-4-one (P1) and 3-(2,4-dinitrophenoxy)-2-(pyren-1-yl)-4H-chromen-4-one (P2), were designed and synthesized here. The probes (P1 and P2) were found to be highly selective and sensitive toward hydrogen sulfide (H₂S) in the presence of a wide range of anions. The new probes (P1 and P2) were fully characterized by analytical, NMR spectroscopy (¹H and ¹³C), and ESI mass spectrometry. The sensing capability of chemodosimeters (P1 and P2) toward H₂S was confirmed by fluorescence studies. The 'turn-on' fluorescence was used to calculate the detection limit of probes (LOD), which were found to be 2.4 and 1.2 μM for P1 and P2, respectively. Moreover, the probes were tested for their cytotoxicity against HeLa cells using the MTT assay and found to be non-cytotoxic in nature; hence, the probes P1 and P2 were successfully utilized to visualize H₂S in the living cells.

A phase-transfer catalyst-based nanoreactor for accelerated hydrogen sulfide bio-imaging

Hydrogen sulfide (H₂S) is an important signaling molecule in various biological processes; however, its real-time monitoring in living cells is hampered by long detection time for current fluorescent probes. To overcome this challenge, we designed a phase-transfer catalyst (PTC) approach to accelerate the reaction between the probe and the analyte by conjugating common fluorescent probes - mostly hydrophobic small molecules - with an amphiphilic PEG-PPG-PEG polymer, enabling the controllable assembly of H₂S nanoprobes in an aqueous solution. The PEG block helps to establish a PTC microenvironment that endows the assembled nanoprobes with a significantly reduced detection time (3-10 min; versus 20-60 min for small-molecule probes). Based on this approach, we synthesised two nanoprobes of different wavelengths, DS-Blue-nano and DN-Green-nano, which can sensitively detect H₂S in living macrophage cells with bright fluorescence starting at as early as 7 min and reaching stability at 15 min. These data suggest PTC-based nanoprobes as a new and generic approach for constructing sensitive fluorescent probes for the real-time imaging of H₂S, and perhaps other molecules in future, under biological conditions.

Mitochondria-targeted phosphorescent cyclometalated iridium(III) complex for bioimaging of H2S

The selective visualization of H₂S in mitochondria is still a challenge, but it correlates closely with mitochondrial damage and some related diseases. In this work, a cyclometalated iridium complex Ir-DNB, [Ir(ppy)₂(N^N)](PF₆) (ppy = 2-phenylpyridine, N^N = (4'-methyl-[2,2'-bipyridin]-4-yl)methyl 2-((2,4-dinitrophenyl) thio)benzoate) has been explored for the detection of mitochondrial H₂S. Adding H₂S to a solution of complex Ir-DNB results in a clearly luminescence enhancement, and displays high selectivity and sensitivity. Moreover, this complex displays negligible toxicity and good mitochondrial localization to HeLa cells, and has also been successfully used for endogenous and exogenous H₂S imaging in vitro and in vivo.

Recent progress in developing fluorescent probes for imaging cell metabolites

Cellular metabolites play a crucial role in promoting and regulating cellular activities, but it has been difficult to monitor these cellular metabolites in living cells and in real time. Over the past decades, iterative development and improvements of fluorescent probes have been made, resulting in the effective monitoring of metabolites. In this review, we highlight recent progress in the use of fluorescent probes for tracking some key metabolites, such as adenosine triphosphate, cyclic adenosine monophosphate, cyclic guanosine 5'-monophosphate, Nicotinamide adenine dinucleotide (NADH), reactive oxygen species, sugar, carbon monoxide, and nitric oxide for both whole cell and subcellular imaging.

A novel fluorescent BODIPY-based probe for detection of Cu2+ and H2S based on displacement approach

A new BODIPY-based fluorescent probe (BC-DPA) was prepared by a simple method for Cu²⁺ detection in aqueous media and living cells. BC-DPA displayed excellent selectivity toward Cu²⁺via fluorescence "turn-off" mode when a mononuclear Cu(Ⅱ) complex is formed. The corresponding BC-DPA-Cu(Ⅱ) complex, whose structure was characterized by X-ray crystallography, has Cu(Ⅱ) in a distorted octahedral geometry. On the basis of the displacement approach, the fluorescence of BC-DPA-Cu²⁺ was recovered in the presence of S^2-, which allowed the system to act as a sensitive "turn-on" sensor for hydrogen sulfide. Furthermore, BC-DPA exhibited noticeable permeability and low cytotoxicity, making it a useful tool to detect Cu²⁺ in biosystems.

A near-infrared fluorogenic probe with fast response for detecting sodium dithionite in living cells

Developing a reliable fluorescence probe is crucial for accurately monitoring sodium dithionite (Na₂S₂O₄, SDT) in biosystems, but the current reported azo-based ones suffers from short excitation/emission wavelengths and relative slow response speed. To address this issue, we herein present a novel near-infrared emissive fluorescence probe for SDT, namely DCM-MQ, consisting of a dicyanomethylene-benzopyran fluorogenic reporter and a 1-methylquinolinium as recognition moiety. On the basis of the specific reduction mechanism, DCM-MQ exhibited a rapid colorimetric and fluorescent recognition for SDT (less than 3 s) with large Stokes shift (112 nm) and high sensitivity (detection limit was 19 nM). The fluorescence imaging results demonstrate that DCM-MQ is competent for monitoring SDT in living systems.

A spiropyran functionalized fluorescent probe for mitochondria targeting and imaging of endogenous hydrogen sulfide in living cells

A turn-on spiropyran functionalized fluorescein derivative (FMC) is developed for targeting HS- in mitochondria. FMC exhibits very weak fluorescence at 525 nm under the excitation of 470 nm in aqueous solution due to its colorless spiropyran form; upon addition of HS-, a strong fluorescence enhancement by 6.4-fold is observed with spirocycle-opened merocyanine form and rapid trapping kinetics for HS-. FMC has good biocompatibility and high selectivity towards HS- with a detection limit of 88.2 nM and is very sensitive among the reported H2S fluorescent probes. Moreover, the significant colocalization of FMC with Mito Tracker® Deep Red FM in human laryngeal epidermoid carcinoma (HEp-2) cells and the Pearson correlation coefficient of 0.87 together demonstrate that FMC can target and image the endogenous H2S in the mitochondria of living cells.

Aggregation Enhanced Responsiveness of Rationally Designed Probes to Hydrogen Sulfide for Targeted Cancer Imaging

Activatable molecular probes hold great promise for targeted cancer imaging. However, the hydrophobic nature of most conventional probes makes them generate precipitated agglomerate in aqueous media, thereby annihilating their responsiveness to analytes and precluding their practical applications for bioimaging. This study reports the development of two small molecular probes with unprecedented aggregation enhanced responsiveness to H₂S for in vivo imaging of H₂S-rich cancers. The subtle modulation of the equilibrium between hydrophilicity and lipophilicity by N-methylpyridinium endows these designed probes with the capability of spontaneously self-assembling into nanoprobes under physiological conditions. Such probes in an aggregated state, rather than a molecular dissolved state, show NIR fluorescence light up and photoacoustic signals turn on upon H₂S specific activation, allowing in vivo visualization and differentiation of cancers based on differences in H₂S content. Thus, our study presents an effective design strategy which should pave the way to molecular design of optimized probes for precision cancer diagnostics.

The biothiol-triggered organotrisulfide-based self-immolative fluorogenic donors of hydrogen sulfide enable lysosomal trafficking

Biothiol-reactive organotrisulfide-based self-immolative fluorogenic donors of H2S are rationally designed for the efficient monitoring of intracellular and lysosomal trafficking of H2S with a concomitant turn-on fluorescence. The non-toxic nature of the donors with a sustained release of H2S will certainly be helpful for their biomedical applications in the future.

Ratiometric detection and imaging of hydrogen sulfide in mitochondria based on a cyanine/naphthalimide hybrid fluorescent probe

A novel fluorescent probe (L1) for ratiometric detection and imaging of H₂S in mitochondria was developed by combining a H₂S-sensitive naphthalimide fluorophore and a mitochondria targeting cyanine fluorophore.

A highly sensitive ratiometric fluorescent probe for imaging endogenous hydrogen sulfide in cells

A novel fluorescent probe (QL-N3) has high potential to detect the concentration of endogenous hydrogen sulfide in cells.

Two isomeric and distinguishable H2S fluorescence probes for monitoring spoilage of eggs and visualizing exogenous and endogenous H2S in living cells

Accurate fabrication of fluorescence probes to efficiently monitor and detect H₂S levels in the fields of foodstuffs and physiology is crucial.

An electron-deficiency-based framework for NIR-II fluorescence probes

Increasing the electron withdrawing ability of substituents in monochlorinated BODIPY could vary the emission from the NIR-I to NIR-II region together with enhanced response rate, indicative of a promising approach for activatable NIR-II probes.

Endoplasmic reticulum targeted fluorescent probe for the detection of hydrogen sulfide based on a twist-blockage strategy

Hydrogen sulfide (H₂S) is very important for humans and is involved in many physiological processes. Here, we designed and reported a new naked-eye colorimetric fluorescent probe Z1 for detecting H₂S in absolute HEPES solution. The fluorescence intensity, after the reaction of the probe and H₂S, is about 32 times that of the probe alone. When the concentration of H₂S is 0-100 μM, the detection limit (DL) is rather low at about 0.15 μM (3σ/slope). The response mechanism is based on the leaving of the 2,4-dinitrobenzene moiety, followed by intramolecular cyclization to give a fluorescent iminocoumarin-benzothiazole group. Moreover, Z1 was applied to endogenous and exogenous H₂S imaging in living cells. The high overlap coefficient proved that probe Z1 has good ER-tracker localization in living cells.

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.

Hydrogen sulfide lowers hyperhomocysteinemia dependent on cystathionine γ lyase S-sulfhydration in ApoE-knockout atherosclerotic mice

BACKGROUND AND PURPOSE

Hydrogen sulfide donors can block the cardiovascular injury of hyperhomocysteinemia. H₂ S also lowers serum homocysteine in rats with mild hyperhomocysteinemia, but the pharmacological mechanism is unknown. The present study investigated the mechanism(s) involved.

EXPERIMENTAL APPROACH

ApoE-knockout mice were fed a Paigen diet and L-methionine in drinking water for 16 weeks to create a mouse model of atherosclerosis with hyperhomocysteinemia. H₂ S donors (NaHS and GYY4137) were administered by intraperitoneal injection. We also assayed the H₂ S produced (by methylene blue assay and mito-HS [H₂ S fluorescence probe]), cystathionine γ lyase (CSE) mRNA and protein expression, and CSE sulfhydration and nitrosylation and its activity.

KEY RESULTS

H₂ S donor treatment significantly lowered atherosclerotic plaque area, macrophage infiltration, and serum homocysteine level in the mouse model of atherosclerosis with co-existing hyperhomocysteinemia. mRNA and protein levels of CSE, a key enzyme catalyzing homocysteine trans-sulfuration, were down-regulated with hyperhomocysteinemia, and CSE catalytic activity was inhibited. All these effects were reversed with H₂ S donor treatment. Hyperhomocysteinemia induced CSE nitrosylation, whereas H₂ S sulfhydrated CSE at the same cysteine residues. Nitrosylated CSE decreased and sulfhydrated CSE increased its catalytic and binding activities towards L-homocysteine. Mutation of C252, C255, C307, and C310 residues in CSE abolished CSE nitrosylation or sulfhydration and prevented its binding to L-homocysteine.

CONCLUSIONS AND IMPLICATIONS

Sulfhydration or nitrosylation of CSE represents a yin/yang regulation of catalysis or binding to L-homocysteine. H₂ S donor treatment enhanced CSE sulfhydration, thus lowering serum L-homocysteine, which contributed in part to the anti-atherosclerosis effects in ApoE-knockout mice with hyperhomocysteinemia.

Fluorescent probes for organelle-targeted bioactive species imaging

The dynamic fluctuations of bioactive species in living cells are associated with numerous physiological and pathological phenomena. The emergence of organelle-targeted fluorescent probes has significantly facilitated our understanding on the biological functions of these species. This review describes the design, applications, challenges and potential directions of organelle-targeted bioactive species probes.

Bioactive species, including reactive oxygen species (ROS, including O₂˙^–, H₂O₂, HOCl, ¹O₂, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO^–, NO, NO₂, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H₂S, H₂S_n, SO₂ derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.