A mitochondria-localized two-photon fluorescent probe for ratiometric imaging of hydrogen peroxide in live tissue

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.

Recent Advances in Mitochondria-Localized Luminescent Ruthenium(II) Metallodrugs as Anticancer Agents

Presently, the most effective way to transport drugs specifically to mitochondria inside the cells is of pharmacophoric interest, as mitochondria are recognized as one of the most important targets for new drug design in cancer diagnosis. To date, there are many reviews covering the photophysical, photochemical, and anticancer properties of ruthenium(II) based metallodrugs owing to their high interest in biological applications. There are, however, no reviews specifically covering the mitochondria-localized luminescent Ru(II) complexes and their subsequent mitochondria-mediated anticancer activities. Therefore, this review describes the physicochemical basis for the mitochondrial accumulation of ruthenium complexes, their synthetic strategies to localize and monitor the mitochondria in living cells, and their related underlying anticancer results. Finally, we review the related areas from previous works describing the mitochondria-localized ruthenium complexes for the treatment of cancer-related diseases. Along with this, we also deliberate the perspectives and future directions for emerging more bifunctional Ru(II) complexes that can target, image, and kill tumors more efficiently in comparison with the existing mitochondria-targeted cancer therapeutics.

A highly effective "naked eye" colorimetric and fluorimetric curcumin-based fluorescent sensor for specific and sensitive detection of H2O2in vivo and in vitro

Hydrogen peroxide (H₂O₂) is involved in many important tasks in normal cell metabolism and signaling. However, abnormal levels of H₂O₂ are associated with the occurrence of several diseases. Therefore, it is important to develop a new method for the detection of H₂O₂in vivo and in vitro. A turn-off sensor, 2,2-difluoro-4,6-bis(3-methoxy-4-((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)styryl)-2H-1,3,2-dioxaborine (DFCB), based on curcumin was developed for the detection of H₂O₂. The DFCB, an orange-emitting sensor, was constructed by employing 2,2-difluoro-4,6-bis(4-hydroxy-3-methoxystyryl)-2H-1,3,2-dioxaborine (DFC) as the main carrier, and 2-(4-bromomethylphenyl)-4,4,5,5-tetramethyl-1,3,2-doxaborolane as the recognition site. The recognition group on the DFCB sensor could be completely cleaved by H₂O₂ to generate the intermediate DFC, which would lead to a colorimetric change from bright orange to light blue accompanying by a significantly quenched fluorescence, which could be seen by the naked eye. This sensor exhibited a highly specific fluorescence response to H₂O₂, in preference to other relevant species, with an excellent anti-interference performance. The sensor DFCB also possessed some advantages including a wide pH response range (6-11), a broad linear range (0-300 μM), and a low detection limit (1.31 μM). The sensing mechanism of the DFCB sensor for H₂O₂ was verified by HRMS analysis, ¹H-NMR titration and DFT calculations. In addition, the use of the DFCB sensor was compatible with the fluorescence imaging of H₂O₂ in living cells and zebrafish.

Activity-Based Sensing for Chemistry-Enabled Biology: Illuminating Principles, Probes, and Prospects for Boronate Reagents for Studying Hydrogen Peroxide

Activity-based sensing (ABS) offers a general approach that exploits chemical reactivity as a method for selective detection and manipulation of biological analytes. Here, we illustrate the value of this chemical platform to enable new biological discovery through a case study in the design and application of ABS reagents for studying hydrogen peroxide (H2O2), a major type of reactive oxygen species (ROS) that regulates a diverse array of vital cellular signaling processes to sustain life. Specifically, we summarize advances in the use of activity-based boronate probes for the detection of H2O2 featuring high molecular selectivity over other ROS, with an emphasis on tailoring designs in chemical structure to promote new biological principles of redox signaling.

Mitochondria-Targeted Luminescent Organotin(IV) Complexes: Synthesis, Photophysical Characterization, and Live Cell Imaging

Five fluorescent ONO donor-based organotin(IV) complexes, [Sn^IV(L^1-5)Ph₂] (1-5), were synthesized by the one-pot reaction method and fully characterized spectroscopically including the single-crystal X-ray diffraction studies of 2-4. Detailed photophysical characterization of all compounds was performed. All the compounds exhibited high luminescent properties with a quantum yield of 17-53%. Additionally, the results of cellular permeability analysis suggest that they are lipophilic and easily absorbed by cells. Confocal microscopy was used to examine the live cell imaging capability of 1-5, and the results show that the compounds are mostly internalized in mitochondria and exhibit negligible cytotoxicity at imaging concentration. Also, 1-5 exhibited high photostability as compared to the commercial dye and can be used in long-term real-time tracking of cell organelles. Also, it is found that the probes (1-5) are highly tolerable during the changes in mitochondrial morphology. Thus, this kind of low-toxic organotin-based fluorescent probe can assist in imaging of mitochondria within living cells and tracking changes in their morphology.

Triarylborane-triphenylamine based luminophore for the mitochondria targeted live cell imaging and colorimetric detection of aqueous fluoride

Bioimaging of subcellular organelles such as mitochondria is crucial for detecting physiological abnormalities induced by fluctuations in the levels of various analytes. Herein, we report the design and synthesis of two novel water-soluble cationic Lewis acid triarylborane-triarylamine conjugates 1 and 2. The optical characteristics of 1 and 2 and their precursor compounds BTPA-NMe2 and BTPA-2NMe2 were evaluated, which show similar absorption and fluorescence spectra, with 1 and 2 exhibiting higher quantum yields of 0.73 and 0.64, respectively, than those of the precursors BTPA-NMe2 and BTPA-2NMe2, indicating the partial disruption of the ICT process and the activation of alternative emission bands in 1 and 2. The live cell imaging ability of compound 2 was examined in HeLa cells using a confocal microscope. Moreover, mitochondrial internalisation using compound 2 was effective and it was found to have high photostability under UV light conditions. Furthermore, compound 2 demonstrated an evident colorimetric response with a colour change to dark yellow in aqueous environments, indicating that it could be used for anion sensing. The spectral changes were observed in UV-visible and fluorescence titration experiments, which were strongly supported by DFT calculations. In short, compound 2 synthesized by us can be exclusively utilized for the selective localization of mitochondria with less cytotoxicity and shows excellent colorimetric response to aqueous inorganic fluoride at levels as low as 0.1 ppm with high selectivity.

Optically Triggering and Monitoring Single-Cell-Level Metabolism Using Ormosil-Decorated Ultrathin Fibers

The integration of biological components and artificial devices requires a bio-machine interface that can simultaneously trigger and monitor the activities in biosystems. Herein, we use an organically modified silicate (ormosil) composite coating containing a light-responsive nanocapsule and a fluorescent bioprobe for reactive oxygen species (ROS) to decorate ultrathin optical fibers, namely, ormosil-decorated ultrathin fibers (OD-UFs), and demonstrate that these OD-UFs can optically trigger and monitor the intracellular metabolism activities in living cells. The sizes and shapes of UF tips were finely controlled to match the dimension and mechanical properties of living cells. The increased elasticity of the ormosil coating of OD-UFs reduces possible mechanical damage during the cell membrane penetration. The light-responsive nanocapsule was physically absorbed on the surface of the ormosil coating and could release a stimulant to trigger the metabolism activities in cells upon the guided laser through OD-UFs. The fluorescent bioprobe was covalently linked with the ormosil matrix for monitoring the intracellular ROS generation, which was verified by the in vitro experiments on the microdroplets of a hydrogen peroxide solution. Finally, we found that the living cells could maintain most of their viability after being inserted with OD-UFs, and the intracellular metabolism activities were successfully triggered and monitored at the single-cell level. The OD-UF provides a new platform for the investigation of intracellular behaviors for drug stimulations and represents a new proof of concept for a bio-machine interface based on the optical and chemical activities of organic functional molecules.

Acetyl group assisted rapid intramolecular recognition of hydrogen peroxide: A novel promising approach for efficient hydrogen peroxide probe

As a vital biomolecule, hydrogen peroxide (H₂O₂) is involved in many physiological and pathological processes. Therefore, it is important to detect H₂O₂ in vivo conveniently and efficiently. In this paper, we report a new method of nucleophilic addition of H₂O₂ to the acetyl group to promote the rapid intramolecular reaction, which can be used to develop an efficient H₂O₂ probe. Based on this unique auxiliary recognition part, a fluorescent probe for H₂O₂ detection was designed and synthesized. This probe has the advantages of high sensitivity (limits of detection 7.0 × 10^-8 M or even lower.), fast response (within 3 min) and large Stokes shift (225 nm), which not only can monitor exogenous and endogenous H₂O₂ in cells but also successfully achieves the change of endogenous H₂O₂ level caused by drug sexual organ injury in zebrafish.

A unique red-emitting molecular rotor for high-fidelity visualizing and long-term tracking mitochondria

Visualizing and tracking mitochondrial changes is the key to understand the processes of diseases related to mitochondria, which is meaningful to physiology, pathology, and pharmacology. So, a great deal of mitochondrial probes was designed and synthesized according to the principle that probes with a positive charge can target mitochondria through mitochondrial membrane potential (MMP). However, these traditional mitochondrial probes are not able to visualize and track mitochondrial changes, because their targeting abilities depend on high MMP. Once MMP decreases, they will leak from mitochondria. Herein, we designed and synthesized a red-emitting molecule rotor (SQ, sensitive to viscosity) that could visualize mitochondria with high-fidelity. The rotor was able to firmly immobilize in mitochondrial inner membrane through the cooperation of MMP and the high viscosity property of mitochondrial membrane, and it could still stain mitochondria with long-term regardless of MMP changes. Hence, the probe is able to real-time image and distinguish four kinds of mitochondria with high-fidelity in muscle tissues. In addition, SQ can monitor mitochondrial autophagy in real time. These results demonstrate that SQ is a powerful tool for high-fidelity visualizing and long-term tracking mitochondria in vitro and in vivo.

Recent Advances in Hydrogen Peroxide Responsive Organoborons for Biological and Biomedical Applications

Hydrogen peroxide is the most stable reactive oxygen species generated endogenously, participating in numerous physiological processes and abnormal pathological conditions. Mounting evidence suggests that a higher level of H₂ O₂ exists in various disease conditions. Thus, H₂ O₂ functions as an ideal target for site-specific bioimaging and therapeutic targeting. The unique reactivity of organoborons with H₂ O₂ provides a method for developing chemoselective molecules for biological and biomedical applications. This review highlights the design and application of boron-derived molecules for H₂ O₂ detection, and the utility of boron moieties toward masking reactive compounds leading to the development of metal prochelators and prodrugs for selectively delivering an active species at the target sites with elevated H₂ O₂ levels. Additionally, the emergence of H₂ O₂ -responsive theranostic agents consisting of both therapeutic and diagnostic moieties in one integrated system are discussed. The purpose of this review is to provide a better understanding of the role of boron-derived molecules toward biological and pharmacological applications.

Revealing the signaling regulation of hydrogen peroxide to cell pyroptosis using a ratiometric fluorescent probe in living cells

A ratiometric fluorescent probe with a large emission shift was developed for the accurate measurement of hydrogen peroxide (H2O2) in sophisticated pyroptosis signaling pathways. The results reported here demonstrate that H2O2, as a principal member of ROS, is a critical upstream signaling molecule in regulating pyroptosis.

Patented AIE materials for biomedical applications

In recent years aggregation induced emission (AIE) concept has attracted researcher's interest worldwide. Several organic building blocks are developed as AIE materials. This chapter discusses the patented AIE material and their utilization related in biological, medicinal and biotechnology fields. It is demonstrated that AIE chromophores such as tetraphenylethylene (TPE) as well as other AIE building blocks became important fluorescent emissive bioactive materials. Such emissive materials are widely employed as bioprobes for the detection of mitochondria, cellular imaging and tracking, protein carrier detection of S-phase DNA, detection of d-glucose, visualization of cancer treatment, drug screening, image-guided therapy, bacterial imaging, photodynamic therapy and drug screening. Such AIE materials upon imaging in cellular environment displays significant enhancement of fluorescence emission. Such patented AIE chromophores has a great potential for bioimaging and biomedical applications. In this chapter we compile some patented representative examples to explore their bioimaging/medicinal imaging applications since lot of new inventions are reported every day.

A biotin-guided two-photon fluorescent probe for detection of hydrogen peroxide in cancer cells ferroptosis process

Hydrogen peroxide (H₂O₂) plays a vital role in organism due to its strong oxidizability, especially in resisting the invasion of pathogens. Cancer cells have abnormal concentrations of hydrogen peroxide due to their disordered reproduction. In complex biological systems, however, conventional fluorescent probes based solely on their fluorescent response to abnormal H₂O₂ overexpression in cancer cells are not enough to distinguish cancer cells from other unhealthy or immune cells. Therefore, it is necessary to develop other methods to allow the probe to selectively enter the cancer cells and perform fluorescence imaging of the hydrogen peroxide in the cancer cells. Herein, we developed a biotin-guided, two-photon fluorescent probe (BT-HP) for sensitive detection of H₂O₂ in cancer cells. Through the study on the properties of the probe, it was found that the probe can selectively enter cancer cells. The depth penetration imaging of H₂O₂ in cancer cells and tumor tissues by two-photon microscope proves the potential of the probe BT-HP as a tumor targeting H₂O₂ biosensor. The probe was further applied to detect hydrogen peroxide in cancer cells during the ferroptosis process.

Rh(iii)-Catalysed synthesis of cinnolinium and fluoranthenium salts using C-H activation/annulation reactions: organelle specific mitochondrial staining applications

The construction of a novel class of indazolo[2,1-a]cinnolin-7-ium and diazabenzofluoranthenium salts was developed by using Rh(iii)-catalyzed C-H activation/annulation reactions with 2-phenyl-2H-indazole, and internal alkynes, which resulted in structurally important polycyclic heteroaromatic compounds (PHAs). This reaction uses mild reaction conditions and has a high efficiency, low catalyst loading, and wide substrate scope. The overall catalytic process involves C-H activation followed by C-C/C-N bond formation. Furthermore, the synthesised cinnolinium/fluoranthenium salts exhibit potential fluorescence properties and 5i was targeted in particular for specific mitochondrial staining in order to investigate cancer cell lines.

Molecular Design toward Heavy-Atom-free Photosensitizers Based on the C═S Bond and their Dual Functions in Hypoxia Photodynamic Cancer Therapy and ClO- Detection

In this article, we designed and synthesized the thionated NpImidazole derivatives BS and NS, new heavy-atom-free photosensitizers, which efficiently generate a triplet excited state with high singlet oxygen quantum yield. The introduction of the C═S bond to the NpImidazole core is essential for increasing spin-orbit coupling (SOC). The fluorescence emission of BS and NS was quenched at standard ambient temperature, accompanied with the increase in the ISC process from the singlet states to triplet excited states via thionation. BS and NS showed negligible dark cytotoxicity against HeLa cells in working concentration. In contrast, BS and NS rapidly induced cell death under blue light irradiation both under normoxia and hypoxia conditions. Our current study demonstrates that the C═S group can play an important role in type I ROS generation of PSs, which are unprecedented in the previous reports. Finally, the photophysical changes were assigned to the oxidative desulfurization of the C═S group of BS and NS to the C═O group of the corresponding BO and NO via hypochlorite. The combined results demonstrated the dual function of BS and NS as a fluorescent imaging agent for ClO^- and an anti-cancer therapeutic by PDT that showed the potential strategy for "one-for-all" and multifunctional agents.

A pyridine-Si-rhodamine-based near-infrared fluorescent probe for visualizing reactive oxygen species in living cells

A lysosomal-targeted near infrared (NIR) fluorescent probe for reactive oxygen species (ROS) was developed with highly sensitive ability. The different responding activity toward H₂O₂, OH, and HClO were investigated. Meanwhile, the probe has been successfully applied in detecting and imaging reactive oxygen species both in cells and in vivo.

Mitochondria and lysosome-targetable fluorescent probes for hydrogen peroxide

Hydrogen peroxide (H₂O₂), as a key member of the reactive oxygen species (ROS), has a certain regulatory effect on many physiological processes, such as cell proliferation, differentiation and migration. However, abnormal production of H₂O₂ can cause diseases including cancer, Alzheimer's disease, cardiovascular disease, and so on. Therefore, it is important to detect changes in H₂O₂ at the subcellular level. In recent years, many fluorescent probes for H₂O₂ have been developed and used in living cells. In this review, we introduce some typical fluorescent probes for H₂O₂ with mitochondrial and lysosomal targeting. This review contains targeting strategies, detection mechanisms, optical characteristics and cell imaging of these probes.

Reactive Oxygen Species (ROS)-Responsive Prodrugs, Probes, and Theranostic Prodrugs: Applications in the ROS-Related Diseases

Elevated levels of reactive oxygen species (ROS) have commonly been implicated in a variety of diseases, including cancer, inflammation, and neurodegenerative diseases. In light of significant differences in ROS levels between the nonpathogenic and pathological tissues, an increasing number of ROS-responsive prodrugs, probes, and theranostic prodrugs have been developed for the targeted treatment and precise diagnosis of ROS-related diseases. This review will summarize and provide insight into recent advances in ROS-responsive prodrugs, fluorescent probes, and theranostic prodrugs, with applications to different ROS-related diseases and various subcellular organelle-targetable and disease-targetable features. The ROS-responsive moieties, the self-immolative linkers, and the typical activation mechanism for the ROS-responsive release are also summarized and discussed.

Boronate-Based Probes for Biological Oxidants: A Novel Class of Molecular Tools for Redox Biology

Boronate-based molecular probes are emerging as one of the most effective tools for detection and quantitation of peroxynitrite and hydroperoxides. This review discusses the chemical reactivity of boronate compounds in the context of their use for detection of biological oxidants, and presents examples of the practical use of those probes in selected chemical, enzymatic, and biological systems. The particular reactivity of boronates toward nucleophilic oxidants makes them a distinct class of probes for redox biology studies. We focus on the recent progress in the design and application of boronate-based probes in redox studies and perspectives for further developments.

Ratiometric fluorescence imaging of Golgi H2O2 reveals a correlation between Golgi oxidative stress and hypertension

Golgi oxidative stress is significantly associated with the occurrence and progression of hypertension. Notably, the concentration of hydrogen peroxide (H2O2) is directly proportional to the degree of Golgi oxidative stress. Therefore, based on a novel Golgi-targeting phenylsulfonamide group, we developed a two-photon (TP) fluorescent probe, Np-Golgi, for in situ H2O2 ratiometric imaging in living systems. The phenylsulfonamide moiety effectively assists Np-Golgi in the precise location of Golgi apparatus. In addition, the raw material of phenylsulfonamide is easily available, and chemical modification is easily implemented. By application of Np-Golgi, we explored the generation of H2O2 during Golgi oxidative stress, and also successfully revealed increases on the levels of Golgi H2O2 in the kidneys of mice with hypertension. This work provides an ideal tool to monitor Golgi oxidative stress for the first time and novel drug targets for the future treatment of hypertension.

Research progress in the development of organic small molecule fluorescent probes for detecting H2O2

Hydrogen peroxide (H2O2), as an important signaling molecule during biological metabolism, is a key member of the reactive oxygen species (ROS) family. The excess of H2O2 will lead to oxidative stress, which is a crucial factor in the production of various ROS-related diseases. In order to study the diverse biological roles of H2O2 in cells and animal tissues, many methods have been developed to detect H2O2. Recently, fluorescence imaging has attracted more and more attention because of its high sensitivity, simple operation, experimental feasibility, and real-time online monitoring. Based on the response group, this study will review the research progress on hydrogen peroxide and summarizes the mechanisms, actualities and prospects of fluorescent probes for H2O2.

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.

Ionic Pyridinium-Oxazole Dyads: Design, Synthesis, and Application in Mitochondrial Imaging

We recently developed an oxidative intramolecular 1,2-amino-oxygenation reaction, combining gold(I)/gold(III) catalysis, for accessing structurally unique ionic pyridinium-oxazole dyads (PODs) with tunable emission wavelengths. On further investigation, these fluorophores turned out to be potential biomarkers; in particular, the one containing -NMe₂ functionality (NMe₂-POD) was highly selective for mitochondrial imaging. Of note, because of mitochondria's involvement in early-stage apoptosis and degenerative conditions, tracking the dynamics of mitochondrial morphology with such imaging technology has attracted much interest. Along this line, we wanted to build a library of such PODs which are potential mitochondria trackers. However, Au/Selecfluor, our first-generation catalyst system, suffers from undesired fluorination of electronically rich PODs resulting in an inseparable mixture (1:1) of the PODs and their fluorinated derivatives. In our attempt to search for a better alternative to circumvent this issue, we developed a second-generation approach for the synthesis of PODs by employing Cu(II)/PhI(OAC)₂-mediated oxidative 1,2-amino-oxygenation of alkynes. Thes newly synthesized PODs exhibit tunable emissions as well as excellent quantum efficiency up to 0.96. Further, this powerful process gives rapid access to a library of NMe₂-PODs which are potential mitochondrial imaging agents. Out of the library, the randomly chosen POD-3g was studied for cell-imaging experiments which showed high mitochondrial specificity, superior photostability, and appreciable tolerance to microenvironment changes with respect to commercially available MitoTracker green.

Selectively light-up hydrogen peroxide in hypoxic cancer cells with a novel fluorescent probe

A novel fluorescent turn-on probe (HCyHP) was developed in a simple two-step synthesis for monitoring of exogenous and endogenous H2O2 levels in biological samples and hypoxic cancer diagnosis.

A two-photon fluorescent probe for basal formaldehyde imaging in zebrafish and visualization of mitochondrial damage induced by FA stress

A two-photon fluorescence probe Mito-FA-FP can monitor mitochondrial morphology change and image endogenous FA in vivo.

Redox Signaling by Reactive Electrophiles and Oxidants

The concept of cell signaling in the context of nonenzyme-assisted protein modifications by reactive electrophilic and oxidative species, broadly known as redox signaling, is a uniquely complex topic that has been approached from numerous different and multidisciplinary angles. Our Review reflects on five aspects critical for understanding how nature harnesses these noncanonical post-translational modifications to coordinate distinct cellular activities: (1) specific players and their generation, (2) physicochemical properties, (3) mechanisms of action, (4) methods of interrogation, and (5) functional roles in health and disease. Emphasis is primarily placed on the latest progress in the field, but several aspects of classical work likely forgotten/lost are also recollected. For researchers with interests in getting into the field, our Review is anticipated to function as a primer. For the expert, we aim to stimulate thought and discussion about fundamentals of redox signaling mechanisms and nuances of specificity/selectivity and timing in this sophisticated yet fascinating arena at the crossroads of chemistry and biology.

Challenges and Opportunities for Small-Molecule Fluorescent Probes in Redox Biology Applications

SIGNIFICANCE

The concentrations of reactive oxygen/nitrogen species (ROS/RNS) are critical to various biochemical processes. Small-molecule fluorescent probes have been widely used to detect and/or quantify ROS/RNS in many redox biology studies and serve as an important complementary to protein-based sensors with unique applications. Recent Advances: New sensing reactions have emerged in probe development, allowing more selective and quantitative detection of ROS/RNS, especially in live cells. Improvements have been made in sensing reactions, fluorophores, and bioavailability of probe molecules.

CRITICAL ISSUES

In this review, we will not only summarize redox-related small-molecule fluorescent probes but also lay out the challenges of designing probes to help redox biologists independently evaluate the quality of reported small-molecule fluorescent probes, especially in the chemistry literature. We specifically highlight the advantages of reversibility in sensing reactions and its applications in ratiometric probe design for quantitative measurements in living cells. In addition, we compare the advantages and disadvantages of small-molecule probes and protein-based probes.

FUTURE DIRECTIONS

The low physiological relevant concentrations of most ROS/RNS call for new sensing reactions with better selectivity, kinetics, and reversibility; fluorophores with high quantum yield, wide wavelength coverage, and Stokes shifts; and structural design with good aqueous solubility, membrane permeability, low protein interference, and organelle specificity. Antioxid. Redox Signal. 29, 518-540.

Versatile Histochemical Approach to Detection of Hydrogen Peroxide in Cells and Tissues Based on Puromycin Staining

Hydrogen peroxide (H2O2) is a central reactive oxygen species (ROS) that contributes to diseases from obesity to cancer to neurodegeneration but is also emerging as an important signaling molecule. We now report a versatile histochemical approach for detection of H2O2 that can be employed across a broad range of cell and tissue specimens in both healthy and disease states. We have developed a first-generation H2O2-responsive analogue named Peroxymycin-1, which is based on the classic cell-staining molecule puromycin and enables covalent staining of biological samples and retains its signal after fixation. H2O2-mediated boronate cleavage uncages the puromycin aminonucleoside, which leaves a permanent and dose-dependent mark on treated biological specimens that can be detected with high sensitivity and precision through a standard immunofluorescence assay. Peroxymycin-1 is selective and sensitive enough to image both exogenous and endogenous changes in cellular H2O2 levels and can be exploited to profile resting H2O2 levels across a panel of cell lines to distinguish metastatic, invasive cancer cells from less invasive cancer and nontumorigenic counterparts, based on correlations with ROS status. Moreover, we establish that Peroxymycin-1 is an effective histochemical probe for in vivo H2O2 analysis, as shown through identification of aberrant elevations in H2O2 levels in liver tissues in a murine model of nonalcoholic fatty liver disease, thus demonstrating the potential of this approach for studying disease states and progression associated with H2O2. This work provides design principles that should enable development of a broader range of histochemical probes for biological use that operate via activity-based sensing.

Boronate-Based Fluorescence Probes for the Detection of Hydrogen Peroxide

In this work, we synthesized a series of boronate ester fluorescence probes (E)-4,4,5,5-tetramethyl-2-(4-styrylphenyl)-1,3,2-dioxaborolane (STBPin), (E)-N,N-dimethyl-4-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)styryl)aniline (DSTBPin), (E)-4-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)styryl)benzonitrile (CSTBPin), (E)-2-(4-(4-methoxystyryl)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (MSTBPin), (E)-N,N-dimethyl-4-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)styryl)naphthalen-1-amine (NDSTBPin), and N,N-dimethyl-4-(2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)oxazol-5-yl)aniline (DAPOX-BPin) for the detection of hydrogen peroxide (H2O2). DSTBPin and MSTBPin displayed an "Off-On" fluorescence response towards H2O2, owing to the loss of the intramolecular charge transfer (ICT) excited state. Whereas, CSTBPin displayed a decrease in fluorescence intensity in the presence of H2O2 owing to the introduction of an ICT excited state. STBPin, on the other hand, produced a small fluorescence decrease, indicating the importance of an electron-withdrawing or electron-donating group in these systems. Unfortunately, the longer wavelength probe, NDSTBPin, displayed a decrease in fluorescence intensity. Oxazole-based probe DAPOX-BPin produced a "turn-on" response. Regrettably, DAPOX-BPin required large concentrations of H2O2 (>3 mm) to produce noticeable changes in fluorescence intensity and, therefore, no change in fluorescence was observed in the cell imaging experiments.

A fast-response two-photon fluorescent probe for imaging endogenous H2O2 in living cells and tissues

As a second messenger, hydrogen peroxide plays significant roles in numerous physiological and pathological processes and is related to various diseases including inflammatory disease, diabetes, neurodegenerative disorders, cardiovascular disease and Alzheimer's disease. Two-photon (TP) fluorescent probes reported for the detection of endogenous H₂O₂ are rare and most have drawbacks such as slow response and low sensitivity. In this report, we demonstrate a simple H₂O₂-specific TP fluorescent probe (TX-HP) containing a two-photon dye 6-hydroxy-2,3,4,4a-tetrahydro-1H-xanthen-1-one (TX) on the modulation of the ICT process. The probe exhibits a rapid fluorescent response to H₂O₂ in 9min with both high sensitivity and selectivity. The probe can detect exogenous H₂O₂ in living cells. Furthermore, the probe is successfully utilized for imaging H₂O₂ in liver tissues.

Bright and two-photon active red fluorescent dyes that selectively move back and forth between the mitochondria and nucleus upon changing the mitochondrial membrane potential

Pyrene-based two-photon active and bright red emitters that localize between the mitochondria and nucleus in response to changes in the mitochondrial membrane potential.

A Two-Photon Ratiometric Fluorescent Probe for Imaging of Hydrogen Peroxide Levels in Rat Organ Tissues

Hydrogen peroxide (H2O2) is important in the regulation of a variety of biological processes and is involved in various diseases. Quantitative measurement of H2O2 levels at the subcellular level is important for understanding its positive and negative effects on biological processes. Herein, a two-photon ratiometric fluorescent probe (SHP-Cyto) with a boronate-based carbamate leaving group as the H2O2 reactive trigger and 6-(benzo[d]thiazol-2'-yl)-2-(N,N-dimethylamino) naphthalene (BTDAN) as the fluorophore was synthesized and examined for its ability to detect cytosolic H2O2 in situ. This probe, based on the specific reaction between boronate and H2O2, displayed a fluorescent color change (455 to 528 nm) in response to H2O2 in the presence of diverse reactive oxygen species in a physiological medium. In addition, ratiometric two-photon microscopy (TPM) images with SHP-Cyto revealed that H2O2 levels gradually increased from brain to kidney, skin, heart, lung, and then liver tissues. SHP-Cyto was successfully applied to the imaging of endogenously produced cytosolic H2O2 levels in live cells and various rat organs by using TPM.

A ratiometric fluorescent hydrogen peroxide chemosensor manipulated by an ICT-activated FRET mechanism and its bioimaging application in living cells and zebrafish

A ratiometric fluorescent chemosensor manipulated by an ICT-activated FRET mechanism was engineered for monitoring hydrogen peroxide in living cells and zebrafish.

Mitochondria-Targeted Triphenylphosphonium-Based Compounds: Syntheses, Mechanisms of Action, and Therapeutic and Diagnostic Applications

Mitochondria are recognized as one of the most important targets for new drug design in cancer, cardiovascular, and neurological diseases. Currently, the most effective way to deliver drugs specifically to mitochondria is by covalent linking a lipophilic cation such as an alkyltriphenylphosphonium moiety to a pharmacophore of interest. Other delocalized lipophilic cations, such as rhodamine, natural and synthetic mitochondria-targeting peptides, and nanoparticle vehicles, have also been used for mitochondrial delivery of small molecules. Depending on the approach used, and the cell and mitochondrial membrane potentials, more than 1000-fold higher mitochondrial concentration can be achieved. Mitochondrial targeting has been developed to study mitochondrial physiology and dysfunction and the interaction between mitochondria and other subcellular organelles and for treatment of a variety of diseases such as neurodegeneration and cancer. In this Review, we discuss efforts to target small-molecule compounds to mitochondria for probing mitochondria function, as diagnostic tools and potential therapeutics. We describe the physicochemical basis for mitochondrial accumulation of lipophilic cations, synthetic chemistry strategies to target compounds to mitochondria, mitochondrial probes, and sensors, and examples of mitochondrial targeting of bioactive compounds. Finally, we review published attempts to apply mitochondria-targeted agents for the treatment of cancer and neurodegenerative diseases.