A dual response near-infrared fluorescent probe for hydrogen polysulfides and superoxide anion detection in cells and in vivo

Study of the Energy Crossing Between Excited States Affected by the Electronegativity of Substituents for Three 4-Azido-1,8-naphthalimide Derivatives

Rapid detection of H2S is crucial for human physiological health and natural ecosystems. In this study, the fluorescent sensing mechanisms of three 4-azido-1,8-naphthalimide-based fluorescent probes to monitor H2S were theoretically investigated by density functional theory and time-dependent density functional theory. The potential energy curve of the charge transfer (CT) state has a crossover with that of the locally excited (LE) state proved by the constructed linear interpolating internal coordinate pathway. Thus, the transform takes place from the LE state to the CT state causing the fluorescence quenching of the probes from a nonradiative transition process of the CT state. The distance between the Franck-Condon point and the minimal energy conical intersection becomes larger with the increase of the electronegativity of substituents on the 1,8-naphthalimide fluorophore. In addition, the degree of charge separation is closely related to the energy difference between the CT and the LE states which are also essentially affected by the electronegativity of the substituents. Since the electronegativity of the substituents has proved important for the probes, our work lays a certain theoretical foundation for the design and synthesis of more sensitive 4-azido-1,8-naphthalimide-based fluorescent probes.

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Advancements in Small Molecule Fluorescent Probes for Superoxide Anion Detection: A Review

Superoxide anion (O2•-), a significant reactive oxygen species (ROS) within biological systems, plays a widespread role in cellular function regulation and is closely linked to the onset and progression of numerous diseases. To unveil the pathological implications of O2•- in these diseases, the development of effective monitoring techniques within biological systems is imperative. Small molecule fluorescent probes have garnered considerable attention due to their advantages: simplicity in operation, heightened sensitivity, exceptional selectivity, and direct applicability in monitoring living cells, tissues, and animals. In the past few years, few reports have focused on small molecule fluorescence probes for the detection of O2•-. In this small review, we systematically summarize the design and application of O2•- responsive small molecule fluorescent probes. In addition, we present the limitations of the current detection of O2•- and suggest the construction of new fluorescent imaging probes to indicate O2•- in living cells and in vivo.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Oxidation of Hydrogen Sulfide to Polysulfide and Thiosulfate by a Carbon Nanozyme: Therapeutic Implications with an Emphasis on Down Syndrome

Hydrogen sulfide (H2 S) is a noxious, potentially poisonous, but necessary gas produced from sulfur metabolism in humans. In Down Syndrome (DS), the production of H2 S is elevated and associated with degraded mitochondrial function. Therefore, removing H2 S from the body as a stable oxide could be an approach to reducing the deleterious effects of H2 S in DS. In this report we describe the catalytic oxidation of hydrogen sulfide (H2 S) to polysulfides (HS2+n - ) and thiosulfate (S2 O3 2- ) by poly(ethylene glycol) hydrophilic carbon clusters (PEG-HCCs) and poly(ethylene glycol) oxidized activated charcoal (PEG-OACs), examples of oxidized carbon nanozymes (OCNs). We show that OCNs oxidize H2 S to polysulfides and S2 O3 2- in a dose-dependent manner. The reaction is dependent on O2 and the presence of quinone groups on the OCNs. In DS donor lymphocytes we found that OCNs increased polysulfide production, proliferation, and afforded protection against additional toxic levels of H2 S compared to untreated DS lymphocytes. Finally, in Dp16 and Ts65DN murine models of DS, we found that OCNs restored osteoclast differentiation. This new action suggests potential facile translation into the clinic for conditions involving excess H2 S exemplified by DS.

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.

Developing a novel benzothiazole-based red-emitting probe for intravital imaging of superoxide anion

Superoxide anion (O2•-), the first generated reactive oxygen species (ROS), is a critical player in cellular signaling network and redox homeostasis. Imaging of O2•-, particularly in vivo, is of concern for further understanding its roles in pathophysiological and pharmacological events. Herein, we designed a novel probe, (E)-4-(5-(2-(benzo[d]thiazol-2-yl)-2-cyanovinyl)furan-2-yl)phenyl trifluoromethane-sulfonate (BFTF), by modifying hydroxyphenyl benzothiazole (a widely used dye scaffold) which includes insertion of both an acrylonitrile unit and a furan ring to extend the total π-conjugation system and to enhance push-pull intramolecular charge transfer process, and utilization of trifluoromethanesulfonate as the response unit. Toward O2•-, the probe features near-infrared fluorescent emission (685 nm), large Stokes shift (135 nm), and deep tissue penetration (300 μm). With its help, we successfully mapped preferential generation of O2•- in HepG2 cells over L02 cells, as well as in A549 over BEAS-2B cells by β-lapachone (an anticancer agent that generates O2•-), and more importantly, visualized overproduction of O2•- in living mice with liver injury induced by acetaminophen (a well-known analgesic and antipyretic drug).

A new lysosome-targeted Cys probe and its application in biology and food samples

Cysteine (Cys) is a sulfur-containing amino acid that plays an important role in living systems. The most common way to supplement the body with exogenous Cys is through the consumption of Cys-rich foods. Therefore, it is important to detect and analyze Cys in living systems and food samples. However, most of the Cys fluorescent probes developed so far are limited to the detection of the cellular environment only, and very few probes can take into account the detection of Cys in plant roots and food samples. In this paper, a novel fluorescent probe LN-NCS targeting the detection of Cys in lysosomes was designed and synthesized by modifying the naphthalimide fluorophore. The probe LN-NCS has a large Stokes shift (140 nm), low cytotoxicity, low detection limit (16.3 nM), and high selectivity, and probe LN-NCS reacts with Cys to produce the compound LN-NH2 with good fluorescence quantum yield (Ф = 0.81). Probe LN-NCS can be used to detect Cys in cells, zebrafish, plant roots, food samples, and environmental water samples. In addition, by modeling cellular inflammation, we have demonstrated that probe LN-NCS can detect changes in Cys concentration induced by cellular inflammation, providing a potential tool to better study the cellular inflammatory environment.

Development of a "double reaction" type-based fluorescent probe for the imaging of superoxide anion in living cells

Superoxide anion (O2•-) is an important ROS in living systems, and rapid and in situ detection of O2•- is critical for the in-depth study of its roles in the closely related diseases. Herein, we present a "double reaction" type-based fluorescent probe (BZT) for the imaging of O2•- in living cells. BZT employed a triflate group as a recognition site for O2•-. In response to O2•-, the probe BZT underwent double chemical reactions, including the nucleophilic reaction between O2•- and triflate, and the cyclization reaction through the other nucleophilic reaction between hydroxyl and cyano group. BZT could show high sensitivity and selectivity to O2•-. Biological imaging experiments demonstrated that the probe BZT could be successfully applied to detect the exogenous and endogenous O2•- in living cells, and the results suggested that rutin could efficiently scavenge the endogenous O2•- induced by rotenone. We expected that the developed probe could provide a valuable tool to investigate the pathological roles of O2•- in relevant diseases.

Monitoring nutrients in plants with genetically encoded sensors: achievements and perspectives

Understanding mechanisms of nutrient allocation in organisms requires precise knowledge of the spatiotemporal dynamics of small molecules in vivo. Genetically encoded sensors are powerful tools for studying nutrient distribution and dynamics, as they enable minimally invasive monitoring of nutrient steady-state levels in situ. Numerous types of genetically encoded sensors for nutrients have been designed and applied in mammalian cells and fungi. However, to date, their application for visualizing changing nutrient levels in planta remains limited. Systematic sensor-based approaches could provide the quantitative, kinetic information on tissue-specific, cellular, and subcellular distributions and dynamics of nutrients in situ that is needed for the development of theoretical nutrient flux models that form the basis for future crop engineering. Here, we review various approaches that can be used to measure nutrients in planta with an overview over conventional techniques, as well as genetically encoded sensors currently available for nutrient monitoring, and discuss their strengths and limitations. We provide a list of currently available sensors and summarize approaches for their application at the level of cellular compartments and organelles. When used in combination with bioassays on intact organisms and precise, yet destructive analytical methods, the spatiotemporal resolution of sensors offers the prospect of a holistic understanding of nutrient flux in plants.

Organelle-Targeted Fluorescent Probes for Sulfane Sulfur Species

Sulfane sulfurs, which include hydropersulfides (RSSH), hydrogen polysulfides (H2Sn, n > 1), and polysulfides (RSnR, n > 2), play important roles in cellular redox biology and are closely linked to hydrogen sulfide (H2S) signaling. While most studies on sulfane sulfur detection have focused on sulfane sulfurs in the whole cell, increasing the recognition of the effects of reactive sulfur species on the functions of various subcellular organelles has emerged. This has driven a need for organelle-targeted detection methods. However, the detection of sulfane sulfurs, particularly of RSSH and H2Sn, in biological systems is still a challenge due to their low endogenous concentrations and instabilities. In this review, we summarize the development and design of organelle-targeted fluorescent sulfane sulfur probes, examine their organelle-targeting strategies and choices of fluorophores (e.g., ratiometric, near-infrared, etc.), and discuss their mechanisms and ability to detect endogenous and exogenous sulfane sulfur species. We also present the advantages and limitations of the probes and propose directions for future work on this topic.

Fluorescent probes for biomolecule detection under environmental stress

The use of fluorescent probes in visible detection has been developed over the last several decades. Biomolecules are essential in the biological processes of organisms, and their distribution and concentration are largely influenced by environmental factors. Significant advances have occurred in the applications of fluorescent probes for the detection of the dynamic localization and quantity of biomolecules during various environmental stress-induced physiological and pathological processes. Herein, we summarize representative examples of small molecule-based fluorescent probes that provide bimolecular information when the organism is under environmental stress. The discussion includes strategies for the design of smart small-molecule fluorescent probes, in addition to their applications in biomolecule imaging under environmental stresses, such as hypoxia, ischemia-reperfusion, hyperthermia/hypothermia, organic/inorganic chemical exposure, oxidative/reductive stress, high glucose stimulation, and drug treatment-induced toxicity. We believe that comprehensive insight into the beneficial applications of fluorescent probes in biomolecule detection under environmental stress should enable the further development and effective application of fluorescent probes in the biochemical and biomedical fields.

A Polymeric Nanobeacon for Monitoring the Fluctuation of Hydrogen Polysulfides during Fertilization and Embryonic Development

Fertilization and early embryonic development as the beginning of a new life are key biological events. Hydrogen polysulfide (H₂ S_n ) plays important roles during physiological regulation, such as antioxidation-protection. However, no report has studied in situ H₂ S_n fluctuation during early embryonic development because of the low abundance of H₂ S_n and inadequate sensitivity of probes. We herein construct a polymeric nanobeacon from a H₂ S_n -responsive polymer and fluorophores, which is capable of detecting H₂ S_n selectively and of signal amplification. Taking the zebrafish as a model, the polymeric nanobeacon revealed that the H₂ S_n level was significantly elevated after fertilization due to the activation of cell multiplication, suppressed partially during embryonic development, and finally kept steady up to zebrafish emergence. This strategy is generally accessible for biomarkers by altering the responsive unit and significant for facilitating biological analysis during life development.

BODIPY-based rapid response fluorescence probe for sensing and bioimaging endogenous superoxide anion in living cells

The superoxide anion radical (O₂^-), a pernicious ROS in living cells, has long been recognized as an important cell signaling molecule involved in numerous physiological and pathological processes, including innate immunity and metabolic homeostasis. Here, we developed a new bodipy-based fluorescent probe for monitoring O₂^- based on the selective cleavage of phosphate bond in BODIPY-T by O₂^-, producing a high-brightness fluorescent BODIPY-COOH. The probe exhibits excellent selectivity for O₂^- with little interference from other ROS species. Fluorescence imaging of RAW264.7 cells also demonstrated successful detection of endogenous O₂^- changes in living cells, indicating that BODIPY-T is a potential probe for the diagnosis and study of the corresponding diseases.

Fluorescent Molecular Logic Gates Driven by Temperature and by Protons in Solution and on Solid

Temperature-driven fluorescent NOT logic is demonstrated by exploiting predissociation in a 1,3,5-trisubstituted Δ² -pyrazoline on its own and when grafted onto silica microparticles. Related Δ² -pyrazolines become proton-driven YES and NOT logic gates on the basis of fluorescent photoinduced electron transfer (PET) switches. Additional PASS 1 and YES+PASS 1 logic gates on silica are also demonstrated within the same family. Beside these small-molecule systems, a polymeric molecular thermometer based on a benzofurazan-derivatized N-isopropylacrylamide copolymer is attached to silica to produce temperature-driven fluorescent YES logic.

Optically superior fluorescent probes for selective imaging of cells, tumors, and reactive chemical species

Fluorescent chemical probes have become powerful tools to study biological events in living cells. They provide a great opportunity to quantitatively and qualitatively analyze the physiological and biochemical properties of living cells in real time. The ability of researchers to manipulate these probes for a desired specific purpose has turned many heads in the scientific community. Despite a slow start, fluorescent probe research has seen exponential growth over the last decade in the world. This change required some adventurous and creative scientists from different fields-like biology, medicine, and chemistry-to come together to facilitate the constant expansion of this field. This review article introduces some fundamental concepts related to fluorescent probe designing and development. It also summarizes various fluorescent probes with superior optical properties used in fields like cell biology, cellular imaging, medical research, and cancer diagnosis. It is hoped that this article will encourage more young and creative scientists to contribute their talents to this field.

Dual-locked spectroscopic probes for sensing and therapy

Optical imaging probes allow us to detect and uncover the physiological and pathological functions of an analyte of interest at the molecular level in a non-invasive, longitudinal manner. By virtue of simplicity, low cost, high sensitivity, adaptation to automated analysis, capacity for spatially resolved imaging and diverse signal output modes, optical imaging probes have been widely applied in biology, physiology, pharmacology and medicine. To build a reliable and practically/clinically relevant probe, the design process often encompasses multidisciplinary themes, including chemistry, biology and medicine. Within the repertoire of probes, dual-locked systems are particularly interesting as a result of their ability to offer enhanced specificity and multiplex detection. In addition, chemiluminescence is a low-background, excitation-free optical modality and, thus, can be integrated into dual-locked systems, permitting crosstalk-free fluorescent and chemiluminescent detection of two distinct biomarkers. For many researchers, these dual-locked systems remain a 'black box'. Therefore, this Review aims to offer a 'beginner's guide' to such dual-locked systems, providing simple explanations on how they work, what they can do and where they have been applied, in order to help readers develop a deeper understanding of this rich area of research.

Responsive mechanism of 2-fluoro-5-nitrobenzoate based two-photon fluorescent probes for H2Sn detection: A theoretical perspective

Two-photon fluorescent probes with large two-photon absorption (TPA) cross sections have shown wide applications in biomedical domain. However, both the species and amounts of high efficient probes are far from meeting the requirements, one main reason is that the relationship between the molecular structures and the responsive mechanisms are not clear and theoretical framework in this field is not perfect. In this work, the photophysical properties including one- and two-photon absorption and emission of three newly synthesized fluorescent probes for hydrogen polysulfide (H₂S_n) detection are investigated by density functional theory and time-dependent density functional theory with the polarizable continuum model in different solvents. Results indicate that the enhanced fluorescent intensity and enlarged TPA cross section can be found when the probes reacted with H₂S_n. Moreover, the OPA intensity is largest and its fluorescent intensity is largely enhanced when detecting H₂S_n for Pro2, this verifies its superior performance in the detection of H₂S_n than Pro1 and Pro 3. Furthermore, the inner mechanism for the increase of TPA cross section is revealed, the responsive mechanisms for photo induced electron transfer (PET) and fluorescence resonance energy transfer (FRET) processes are revealed through analyzing the energies and distributions of frontier orbitals. Our calculations provide theoretical perspectives for experimental measurements and could sever as a useful reference for developing advanced probes in biomedical fields.

A semi-naphthorhodafluor-based red-emitting fluorescent probe for tracking of hydrogen polysulfide in living cells and zebrafish

Hydrogen polysulfides (H₂S_n, n ≥ 2) is recently regarded as a potential signaling molecule which shows a higher efficiency than hydrogen sulfides (H₂S) in regulating enzymes and ion channels. However, the development of specific fluorescent probes for H₂S_n with long-wavelength emission (>600 nm) are still rare. In this work, a semi-naphthorhodafluor-based red-emitting fluorescent probe SNARF-H₂S_n containing a phenyl 2-(benzoylthio) benzoate responsive unit was constructed. SNARF-H₂S_n was capable of selectively detecting H₂S_n over other reactive sulfur species. Treatment with H₂S_n would result in a > 1000-fold fluorescence enhancement within 10 min. SNARF-H₂S_n showed a low limit of detection down to 6.7 nM, and further enabled to visualize exogenous/endogenous H₂S_n in living A549 cells and zebrafish.

A near-infrared fluorescent probe that can image endogenous hydrogen polysulfides in vivo in tumour-bearing mice

Hydrogen polysulfides (H₂S_n, n > 1), which are important reactive sulfur species, play crucial roles in H₂S-related bioactivities, including antioxidation, cytoprotection, activation of ion channels, transcription factor functions and tumour suppression. Monitoring H₂S_nin vivo is of significant interest for exploring the physiological roles of H₂S_n and the exact mechanisms of H₂S_n-related diseases. Herein, we conceive a novel near-infrared fluorescent probe, NIR-CPS, that is used to detect H₂S_n in living cells and in vivo. With the advantages of high sensitivity, good selectivity and a remarkably large Stokes shift (100 nm), NIR-CPS was successfully applied in visualizing H₂S_n in living cells and mice. More importantly, NIR-CPS monitored H₂S_n stimulated by lipopolysaccharide in tumour-bearing mice. These results demonstrate that the NIR-CPS probe is a potentially powerful tool for the detection of H₂S_nin vivo, thus providing a valuable approach in H₂S_n-related medical research.

Detection Technologies for Reactive Oxygen Species: Fluorescence and Electrochemical Methods and Their Applications

Reactive oxygen species (ROS) have been found in plants, mammals, and natural environmental processes. The presence of ROS in mammals has been linked to the development of severe diseases, such as diabetes, cancer, tumors, and several neurodegenerative conditions. The most common ROS involved in human health are superoxide (O2•-), hydrogen peroxide (H2O2), and hydroxyl radicals (•OH). Organic and inorganic molecules have been integrated with various methods to detect and monitor ROS for understanding the effect of their presence and concentration on diseases caused by oxidative stress. Among several techniques, fluorescence and electrochemical methods have been recently developed and employed for the detection of ROS. This literature review intends to critically discuss the development of these techniques to date, as well as their application for in vitro and in vivo ROS detection regarding free-radical-related diseases. Moreover, important insights into and further steps for using fluorescence and electrochemical methods in the detection of ROS are presented.

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

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

Fluorescent probe for the imaging of superoxide and peroxynitrite during drug-induced liver injury

Drug-induced liver injury (DILI) is an important cause of potentially fatal liver disease. Herein, we report the development of a molecular probe (LW-OTf) for the detection and imaging of two biomarkers involved in DILI. Initially, primary reactive oxygen species (ROS) superoxide (O₂˙^-) selectively activates a near-infrared fluorescence (NIRF) output by generating fluorophore LW-OH. The C[double bond, length as m-dash]C linker of this hemicyanine fluorophore is subsequently oxidized by reactive nitrogen species (RNS) peroxynitrite (ONOO^-), resulting in cleavage to release xanthene derivative LW-XTD, detected using two-photon excitation fluorescence (TPEF). An alternative fluorescence pathway can occur through cleavage of LW-OTf by ONOO^- to non-fluorescent LW-XTD-OTf, which can react further with the second analyte O₂˙^- to produce the same LW-XTD fluorescent species. By combining NIRF and TPEF, LW-OTf is capable of differential and simultaneous detection of ROS and RNS in DILI using two optically orthogonal channels. Probe LW-OTf could be used to detect O₂˙^- or O₂˙^- and ONOO^- in lysosomes stimulated by 2-methoxyestradiol (2-ME) or 2-ME and SIN-1 respectively. In addition, we were able to monitor the chemoprotective effects of tert-butylhydroxyanisole (BHA) against acetaminophen (APAP) toxicity in living HL-7702 cells. More importantly, TPEF and NIRF imaging confirmed an increase in levels of both O₂˙^- and ONOO^- in mouse livers during APAP-induced DILI (confirmed by hematoxylin and eosin (H&E) staining).

Protein-Enhanced NIR-IIb Emission of Indocyanine Green for Functional Bioimaging

Fluorescence imaging performed in the 1500-1700 nm spectral range (labeled near-infrared IIb, NIR-IIb) promises high imaging contrast and spatial resolution for its little photon scattering effect and minimum autofluorescence. Though inorganic and organic probes have been developed for NIR-IIb bioimaging, most are in the preclinical stage, hampering further clinical application. Herein, we showed that indocyanine green (ICG), a US Food and Drug Administration (FDA)-approved agent, exhibited a remarkable amount of NIR-IIb emission when dissolved into different protein solutions, including human serum albumin, rat bile, and fetal bovine serum. We performed fluorescence imaging in the NIR-IIb window to visualize structures of the lymph system, extrahepatic biliary tract, and cerebrovascular. The results demonstrated that proteins promoted NIR-IIb emission of ICG in vivo and that NIR-IIb imaging with ICG preserved a higher signal-to-background ratio and spatial resolution compared with the conventional NIR-II fluorescence imaging. Our findings confirm that NIR-IIb fluorescence imaging can be successfully performed using the clinically approved agent ICG. Further clinical application in the NIR-IIb region would hopefully be carried out with appropriate ICG-protein solutions.

A near-infrared fluorescent probe for observing thionitrous acid-mediated hydrogen polysulfides formation and fluctuation in cells and in vivo under hypoxia stress

Hydrogen polysulfides (H₂S_n, n>1) as important intracellular reactive sulfur species (RSS) are believe to be responsible for cellular redox regulation. Lots of researches about H₂S_n focusing on their formation, detection and bio-function in signalling regulation are spring up but with poor understanding, especially for biosynthesis and bio-function remain complicated and confusing. Recent studies reveal that thionitrous acid (HSNO) as potential intermediate linked signalling molecules of nitrogenous and sulphureous during biotic redox regulation. However, there are limited evidences for supporting the interrelation and bioeffect between HSNO and H₂S_n. Herein, we have successfully designed a near-infrared (NIR) fluorescent probe ((2-fluoro-5-nitrobenzoyl)oxy)-Benzo[e]cyanine (BCy-FN) for detection H₂S_n and for the first time observing HSNO-mediated H₂S_n generation in cells and in vivo. The probe is harvested from fluorophore BCy-Keto and 2-fluoro-5-nitrobenzoic acid in one step, featuring mitochondria localization. The unique Enol-Keto tautomerization of fluorophore enables the probe becomes more sensitive and has powerful application. Hypoxia model has been constructed and powerfully interpreted the pretreatment of HSNO for zebrafish hypoxia process effectively improves H₂S_n levels and defends the hypoxia induced brain damage. We believe the present studies will help environmentalist and biologist for better understanding of biosynthesis and bio-function in HSNO-mediated H₂S_n formation process under hypoxia stress.

Engineering a highly selective probe for ratiometric imaging of H2S n and revealing its signaling pathway in fatty liver disease

Hydrogen polysulfides (H2S n , n > 1) have continuously been proved to act as important signal mediators in many physiological processes. However, the physiological role of H2S n and their signaling pathways in complex diseases, such as the most common liver disease, nonalcoholic fatty liver disease (NAFLD), have not been elucidated due to lack of suitable tools for selective detection of intracellular H2S n . Herein, we adopted a general and practical strategy including recognition site screening, construction of a ratiometric probe and self-assembly of nanoparticles, to significantly improve the probes' selectivity, photostability and biocompatibility. The ratiometric probe PPG-Np-RhPhCO selectively responds to H2S n , avoiding interaction with biothiol and persulfide. Moreover, this probe was applied to image H2S n in NAFLD for the first time and reveal the H2S n generation pathways in the cell model of drug-treated NAFLD. The pathway of H2S n revealed by PPG-Np-RhPhCO provides significant insights into the roles of H2S n in NAFLD and future drug development.

The binding mechanism of nitroreductase fluorescent probe: Active pocket deformation and intramolecular hydrogen bonds

Nitroreductase (NTR), a member of the flavoenzyme family, could react with nicotinamide adenine dinucleotide by reducing nitro to amino at hypoxic tumor, which can be monitored by some fluorescent probes in vivo. Here, molecular docking and molecular dynamics simulation techniques were used to explore the molecular mechanisms between NTR and probes. The results showed that formation of hydrogen bond in 1F5V-13 between A@His215 and B@Ser41 with 74.53% occupancy might be the main reason for the decrease of probe fluorescence emission in experiment. Moreover, Probe 16 was rotated by nearly 60 degrees with respect to the position of other probes in protein binding pocket, deforming the protein active pocket, changing the hydrogen bond formation, which leads to the fluorescence performance of 16 with electron donor and electron acceptor groups was superior to other probes in experiment. The deformation of protein active pocket and the formation of intramolecular hydrogen bonds revealed the difference in performance of NTR fluorescent probe at molecular level, which provide theoretical guidance for latter design of fluorescent probes with better performance.

An imidazo[1,5-α]pyridine-derivated fluorescence sensor for rapid and selective detection of sulfite

Sulfur-containing species are essential in the composition and the metabolism of the organisms, thus developing a full set of implements to cover all of them is still a favorable choice. Herein, we chose imidazo [1,5-α]pyridine moiety as the basic fluorophore for the detection of sulfite, and preliminarily completed the toolset since biothiols (GSH, Cys, Hcy), H₂S, and PhSH could be detected by sensors based on the same backbone. The designed sensor, IPD-SFT, with structural novelty and large Stokes shift (130 nm), indicated the most attractive advantages of remarkably rapid response period (within 1 min) and high selectivity for sulfite from all the sulfur-containing species. Other practical properties included high sensitivity (LOD = 50 nM) and wide pH adaptability (5.0-11.0). Furthermore, IPD-SFT could monitor both exogenous and endogenous sulfite. It not only raised a potential tool for sulfite detection, but also preliminarily completed the toolset for all the sulfur-containing species. The development of such toolsets might reveal the sulfur-containing metabolism and corresponding physiology and pathological procedures.

A pocket-escaping design to prevent the common interference with near-infrared fluorescent probes in vivo

Near-infrared (NIR) fluorescent probes are among the most attractive chemical tools for biomedical imaging. However, their in vivo applications are hindered by albumin binding, generating unspecific fluorescence that masks the specific signal from the analyte. Here, combining experimental and docking methods, we elucidate that the reason for this problem is an acceptor (A) group-mediated capture of the dyes into hydrophobic pockets of albumin. This pocket-capturing phenomenon commonly applies to dyes designed under the twisted intramolecular charge-transfer (TICT) principle and, therefore, represents a generic but previously unidentified backdoor problem. Accordingly, we create a new A group that avoids being trapped into the albumin pockets (pocket-escaping) and thereby construct a NIR probe, BNLBN, which effectively prevents this backdoor problem with increased imaging accuracy for liver fibrosis in vivo. Overall, our study explains and overcomes a fundamental problem for the in vivo application of a broad class of bioimaging tools.

In Situ Fluorescent and Photoacoustic Imaging of Golgi pH to Elucidate the Function of Transmembrane Protein 165

Golgi pH homeostasis affects many different biological processes, including glycosylation. Recent studies have demonstrated that transmembrane protein 165 (TMEM165) deficiency leads to Golgi glycosylation abnormalities by disturbing Golgi pH homeostasis. However, due to the lack of specific tools to measure Golgi pH in situ, evidence for TMEM165 involvement in H⁺ transport in the Golgi apparatus is still absent. Herein, the photoacoustic and fluorescent dual-mode probe CPH was developed for ratiometric detection of Golgi pH. CPH was proved to accumulate in the Golgi apparatus and reversibly image Golgi pH in real-time with high sensitivity in cells. Furthermore, we found that the absence of TMEM165 influenced H⁺ equilibrium and caused Golgi apparatus acidification. Our work provides strong evidence that TMEM165 regulates Golgi pH homeostasis. Moreover, we believe that CPH has the potential to be a practical tool to monitor Golgi pH in various biological processes.

Incorporating Thiourea into Fluorescent Probes: A Reliable Strategy for Mitochondrion-Targeted Imaging and Superoxide Anion Tracking in Living Cells

Three aggregation-induced emission active fluorescent compounds, TPA-Pyr-Octane, TPA-Pyr-Br, and TPA-Pyr-Thiourea (TPA = triphenylamine pyridinium), are synthesized; their tiny differences in chemical structures result in a huge difference in cell-imaging applications. Especially, incorporating thiourea into fluorescent probes is found as a reliable strategy for mitochondrion-targeted imaging and superoxide anion tracking in living cells, which is possibly due to the presence of hydrogen bonding between thiourea and mitochondrion proteins. This finding is very useful for the design of biosensors and delivery carriers in disease treatment.

The role of reactive oxygen species in tumor treatment

Reactive oxygen species (ROS) are by-products of aerobic metabolism and can also act as signaling molecules to participate in multiple regulation of biological and physiological processes. The occurrence, growth and metastasis of tumors, and even the apoptosis, necrosis and autophagy of tumor cells are all closely related to ROS. However, ROS levels in the body are usually maintained at a stable status. ROS produced by oxidative stress can cause damage to cell lipids, protein and DNA. In recent years, ROS have achieved satisfactory results on the treatment of tumors. Therefore, this review summarizes some research results of tumor treatments from the perspective of ROS in recent years, and analyzes how to achieve the mechanism of inhibition and treatment of tumors by ROS or how to affect the tumor microenvironment by influencing ROS. At the same time, the detection methods of ROS, problems encountered in the research process and solutions are also summarized. The purpose of this review is to provide a clearer understanding of the ROS role in tumor treatment, so that researchers might have more inspiration and thoughts for cancer prevention and treatment in the next stage.

This review provides a clear understanding of the ROS role in tumor treatment and some thoughts for potential cancer prevention.

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.

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.