Rational design of fluorescein-based fluorescence probes. Mechanism-based design of a maximum fluorescence probe for singlet oxygen

Dual-emissive polystyrene@zeolitic imidazolate framework-8 composite for ratiometric detection of singlet oxygen

The co-encapsulation of EuPS and AnC in a ZIF-8 host leads to the formation of a dual-emissive composite for fluorescent ratiometric sensing of singlet oxygen.

Rational design of a fast and selective near-infrared fluorescent probe for targeted monitoring of endogenous nitric oxide

We developed a fast and selective near-infrared (NIR) fluorescent probe for the targeted tracing of endogenous NO which possesses vital features including a significant turn-on NIR response, high specificity, and a fast response.

Design of a β-diketonate–Eu3+ complex-based time-gated luminescence probe for visualizing mitochondrial singlet oxygen

A β-diketonate–Eu³⁺ complex-based time-gated luminescence probe was developed for highly sensitive monitoring of mitochondrial singlet oxygen during the photodynamic therapy process.

Application of oxygen tolerant PET-RAFT to polymerization-induced self-assembly

The inhibitory effects of molecular oxygen in PET-RAFT polymerization can be overcome by the addition of singlet oxygen quenchers. This oxygen tolerant approach is compatible with a range of organic solvents and can be used to synthesize nanoparticles according to a PISA process.

A highly sensitive and selective visible-light excitable luminescent probe for singlet oxygen based on a dinuclear ruthenium complex

A water-soluble dinuclear Ru(ii) complex acts as visible-light excitable luminescent probes for ¹O₂ with low detection limits of 2.7–3.2 nM and exhibits high selectivity over other reactive oxygen species in both neutral and basic media.

Synthesis and fluorescence properties of six fluorescein-nitroxide radical hybrid-compounds

Six fluorescein-nitroxide radical hybrid-compounds (2ab, 3ab, 4, and 5) were synthesized by the condensation of 5- or 6-carboxy-fluorescein and 4-amino-TEMPO (2ab), 5- or 6-aminofluorescein and 4-carboxy-TEMPO (3ab), and fluorescein and 4-carboxy-TEMPO (4), or by reaction of the 3-hydroxyl group of fluorescein with DPROXYL-3-ylmethyl methanesulfonate (5). Fluorescence intensities (around 520nm) after reduction of the radical increased to 1.43-, 1.38-, and 1.61-folds for 2a, 2b and 3b respectively; 3a alone exhibited a decrease in intensity on reduction. Since 4 was readily solvolyzed in PBS or even methanol to afford fluorescein and 4-carboxy-TEMPO, its fluorescence change could not be measured. Hybrid compound 5 containing an ether-linkage between the fluorescein phenol and 3-hydroxymethyl-DPROXYL hydroxyl centers, was stable and on reduction, showed a maximum increase (3.21-fold) in relative fluorescence intensity in PBS (pH5.0), despite its remarkably low absolute fluorescence intensity.

Use of fluorescent probes for ROS to tease apart Type I and Type II photochemical pathways in photodynamic therapy

Photodynamic therapy involves the excitation of a non-toxic dye by harmless visible light to produce a long-lived triplet state that can interact with molecular oxygen to produce reactive oxygen species (ROS), which can damage biomolecules and kill cells. ROS produced by electron transfer (Type 1) include superoxide, hydrogen peroxide and hydroxyl radical (HO), while singlet oxygen (1O2) is produced by energy transfer. Diverse methods exist to distinguish between these two pathways, some of which are more specific or more sensitive than others. In this review we cover the use of two fluorescence probes: singlet oxygen sensor green (SOSG) detects 1O2; and 4-hydroxyphenyl-fluorescein (HPF) that detects HO. Interesting data was collected concerning the photochemical pathways of functionalized fullerenes compared to tetrapyrroles, stable synthetic bacteriochlorins with and without central metals, phenothiazinium dyes interacting with inorganic salts such as azide.

Cold sensitivity of TRPA1 is unveiled by the prolyl hydroxylation blockade-induced sensitization to ROS

Mammalian transient receptor potential ankyrin 1 (TRPA1) is a polymodal nociceptor that plays an important role in pain generation, but its role as a cold nociceptor is still controversial. Here, we propose that TRPA1 can sense noxious cold via transduction of reactive oxygen species (ROS) signalling. We show that inhibiting hydroxylation of a proline residue within the N-terminal ankyrin repeat of human TRPA1 by mutation or using a prolyl hydroxylase (PHD) inhibitor potentiates the cold sensitivity of TRPA1 in the presence of hydrogen peroxide. Inhibiting PHD in mice triggers mouse TRPA1 sensitization sufficiently to sense cold-evoked ROS, which causes cold hypersensitivity. Furthermore, this phenomenon underlies the acute cold hypersensitivity induced by the chemotherapeutic agent oxaliplatin or its metabolite oxalate. Thus, our findings provide evidence that blocking prolyl hydroxylation reveals TRPA1 sensitization to ROS, which enables TRPA1 to convert ROS signalling into cold sensitivity.

The transient receptor potential ankyrin 1 (TRPA1) is a cation channel that is involved in nociceptive pain sensing. Here, the authors show that hydroxylation of a proline in the N terminus of TRPA1 renders it sensitive to reactive oxygen species resulting from noxious cold.

Activatable Water-Soluble Probes Enhance Tumor Imaging by Responding to Dysregulated pH and Exhibiting High Tumor-to-Liver Fluorescence Emission Contrast

Dysregulated pH has been recognized as a universal tumor microenvironment signature that can delineate tumors from normal tissues. Existing fluorescent probes that activate in response to pH are hindered by either fast clearance (in the case of small molecules) or high liver background emission (in the case of large particles). There remains a need to design water-soluble, long circulating, pH-responsive nanoprobes with high tumor-to-liver contrast. Herein, we report a modular chemical strategy to create acidic pH-sensitive and water-soluble fluorescent probes for high in vivo tumor detection and minimal liver activation. A combination of a modified Knoevenagel reaction and PEGylation yielded a series of NIR BODIPY fluorophores with tunable pKas, high quantum yield, and optimal orbital energies to enable photoinduced electron transfer (PeT) activation in response to pH. After intravenous administration, Probe 5c localized to tumors and provided excellent tumor-to-liver contrast (apparent T/L = 3) because it minimally activates in the liver. This phenomenon was further confirmed by direct ex vivo imaging experiments on harvested organs. Because no targeting ligands were required, we believe that this report introduces a versatile strategy to directly synthesize soluble probes with broad potential utility including fluorescence-based image-guided surgery, cancer diagnosis, and theranostic nanomedicine.

Turn on Fluorescent Probes for Selective Targeting of Aldehydes

Two different classes of fluorescent dyes were prepared as a turn off/on sensor system for aldehydes. Amino derivatives of a boron dipyrromethene (BDP) fluorophore and a xanthene-derived fluorophore (rosamine) were prepared. Model compounds of their product with an aldehyde were prepared using salicylaldehyde. Both amino boron dipyrromethene and rosamine derivatives are almost non-fluorescent in polar and apolar solvent. However, imine formation with salicylaldehyde on each fluorophore increases the fluorescence quantum yield by almost a factor of 10 (from 0.05 to 0.4). These fluorophores are therefore suitable candidates for development of fluorescence-based sensors for aldehydes.

Antioxidant Activity/Capacity Measurement. 2. Hydrogen Atom Transfer (HAT)-Based, Mixed-Mode (Electron Transfer (ET)/HAT), and Lipid Peroxidation Assays

Measuring the antioxidant activity/capacity levels of food extracts and biological fluids is useful for determining the nutritional value of foodstuffs and for the diagnosis, treatment, and follow-up of numerous oxidative stress-related diseases. Biologically, antioxidants play their health-beneficial roles via transferring a hydrogen (H) atom or an electron (e(-)) to reactive species, thereby deactivating them. Antioxidant activity assays imitate this action; that is, antioxidants are measured by their H atom transfer (HAT) or e(-) transfer (ET) to probe molecules. Antioxidant activity/capacity can be monitored by a wide variety of assays with different mechanisms, including HAT, ET, and mixed-mode (ET/HAT) assays, generally without distinct boundaries between them. Understanding the principal mechanisms, advantages, and disadvantages of the measurement assays is important for proper selection of method for valid evaluation of antioxidant properties in desired applications. This work provides a general and up-to-date overview of HAT-based, mixed-mode (ET/HAT), and lipid peroxidation assays available for measuring antioxidant activity/capacity and the chemistry behind them, including a critical evaluation of their advantages and drawbacks.

X-ray induced singlet oxygen generation by nanoparticle-photosensitizer conjugates for photodynamic therapy: determination of singlet oxygen quantum yield

Singlet oxygen is a primary cytotoxic agent in photodynamic therapy. We show that CeF₃ nanoparticles, pure as well as conjugated through electrostatic interaction with the photosensitizer verteporfin, are able to generate singlet oxygen as a result of UV light and 8 keV X-ray irradiation. The X-ray stimulated singlet oxygen quantum yield was determined to be 0.79 ± 0.05 for the conjugate with 31 verteporfin molecules per CeF₃ nanoparticle, the highest conjugation level used. From this result we estimate the singlet oxygen dose generated from CeF₃-verteporfin conjugates for a therapeutic dose of 60 Gy of ionizing radiation at energies of 6 MeV and 30 keV to be (1.2 ± 0.7) × 10⁸ and (2.0 ± 0.1) × 10⁹ singlet oxygen molecules per cell, respectively. These are comparable with cytotoxic doses of 5 × 10⁷–2 × 10⁹ singlet oxygen molecules per cell reported in the literature for photodynamic therapy using light activation. We confirmed that the CeF3-VP conjugates enhanced cell killing with 6 MeV radiation. This work confirms the feasibility of using X- or γ- ray activated nanoparticle-photosensitizer conjugates, either to supplement the radiation treatment of cancer, or as an independent treatment modality.

Fluorescence probe for the convenient and sensitive detection of ascorbic acid

Ascorbic acid is an important antioxidant that plays an essential role in the biosynthesis of numerous bioactive substances. The detection of ascorbic acid has traditionally been achieved using high-performance liquid chromatography and absorption spectrophotometry assays. However, the development of fluorescence probes for this purpose is highly desired because they provide a much more convenient and highly sensitive technique for the detection of this material. OFF-ON-type fluorescent probes have been developed for the detection of non-fluorescent compounds. Photo-induced electron transfer and fluorescence resonance energy transfer are the two main fluorescence quenching mechanisms for the detection of ascorbic acid, and several fluorescence probes have been reported based on redox-responsive metals and quantum dots. Profluorescent nitroxide compounds have also been developed as non-metal organic fluorescence probes for ascorbic acid. These nitroxide systems have a stable unpaired electron and can therefore react with ascorbic acid and a strong fluorescence quencher. Furthermore, recent synthetic advances have allowed for the synthesis of α-substituted nitroxides with varying levels of reactivity towards ascorbic acid. In this review, we have discussed the design strategies used for the preparation of fluorescent probes for ascorbic acid, with particular emphasis on profluorescent nitroxides, which are unique radical-based redox-active fluorescent probes.

A singlet oxygen photosensitizer enables photoluminescent monitoring of singlet oxygen doses

Dichromophoric molecular dyad enables photosensitization and detection of sinlget oxygen.

BODIPY catalyzed amide synthesis promoted by BHT and air under visible light

A novel and efficient protocol for the synthesis of amides is reported via a BODIPY catalyzed oxidative amidation of aromatic aldehydes under visible light, with broad substrate scope and mild reaction conditions. Mechanistic studies reveal that dioxygen could be activated through both an ET and SET pathway to form the active peroxide intermediates.

Fluorescence probe for the convenient and sensitive detection of ascorbic acid

Ascorbic acid is an important antioxidant that plays an essential role in the biosynthesis of numerous bioactive substances. The detection of ascorbic acid has traditionally been achieved using high-performance liquid chromatography and absorption spectrophotometry assays. However, the development of fluorescence probes for this purpose is highly desired because they provide a much more convenient and highly sensitive technique for the detection of this material. OFF-ON-type fluorescent probes have been developed for the detection of non-fluorescent compounds. Photo-induced electron transfer and fluorescence resonance energy transfer are the two main fluorescence quenching mechanisms for the detection of ascorbic acid, and several fluorescence probes have been reported based on redox-responsive metals and quantum dots. Profluorescent nitroxide compounds have also been developed as non-metal organic fluorescence probes for ascorbic acid. These nitroxide systems have a stable unpaired electron and can therefore react with ascorbic acid and a strong fluorescence quencher. Furthermore, recent synthetic advances have allowed for the synthesis of α-substituted nitroxides with varying levels of reactivity towards ascorbic acid. In this review, we have discussed the design strategies used for the preparation of fluorescent probes for ascorbic acid, with particular emphasis on profluorescent nitroxides, which are unique radical-based redox-active fluorescent probes.

Palladium-Based Nanomaterials: A Platform to Produce Reactive Oxygen Species for Catalyzing Oxidation Reactions

Oxidation reactions by molecular oxygen (O2 ) over palladium (Pd)-based nanomaterials are a series of processes crucial to the synthesis of fine chemicals. In the past decades, investigations of related catalytic materials have mainly been focused on the synthesis of Pd-based nanomaterials from the angle of tailoring their surface structures, compositions and supporting materials, in efforts to improve their activities in organic reactions. From the perspective of rational materials design, it is imperative to address the fundamental issues associated with catalyst performance, one of which should be oxygen activation by Pd-based nanomaterials. Here, the fundamentals that account for the transformation from O2 to reactive oxygen species over Pd, with a focus on singlet O2 and its analogue, are introduced. Methods for detecting and differentiating species are also presented to facilitate future fundamental research. Key factors for tuning the oxygen activation efficiencies of catalytic materials are then outlined, and recent developments in Pd-catalyzed oxygen-related organic reactions are summarized in alignment with each key factor. To close, we discuss the challenges and opportunities for photocatalysis research at this unique intersection as well as the potential impact on other research fields.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Design strategy for photoinduced electron transfer-based small-molecule fluorescent probes of biomacromolecules

As the cardinal support of innumerable biological processes, biomacromolecules such as proteins, nucleic acids and polysaccharides are of importance to living systems. The key to understanding biological processes is to realize the role of these biomacromolecules in thte localization, distribution, conformation and interaction with other molecules. With the current development and adaptation of fluorescent technologies in biomedical and pharmaceutical fields, the fluorescence imaging (FLI) approach of using small-molecule fluorescent probes is becoming an up-to-the-minute method for the detection and monitoring of these imperative biomolecules in life sciences. However, conventional small-molecule fluorescent probes may provide undesirable results because of their intrinsic deficiencies such as low signal-to-noise ratio (SNR) and false-positive errors. Recently, small-molecule fluorescent probes with a photoinduced electron transfer (PET) "on/off" switch for biomacromolecules have been thoroughly considered. When recognized by the biomacromolecules, these probes turn on/off the PET switch and change the fluorescence intensity to present a high SNR result. It should be emphasized that these PET-based fluorescent probes could be advantageous for understanding the pathogenesis of various diseases caused by abnormal expression of biomacromolecules. The discussion of this successful strategy involved in this review will be a valuable guide for the further development of new PET-based small-molecule fluorescent probes for biomacromolecules.

Discovery, understanding, and bioapplication of organic fluorophore: a case study with an indolizine-based novel fluorophore, Seoul-Fluor

Owing to its high sensitivity and great applicability, the fluorescence phenomenon has been considered as an inevitable research tool in the modern scientific fields of chemistry, biology, materials science, biomedical science, and their interfaces. Many strategies have been pursued to understand and manipulate the photophysical properties of fluorescent materials, but the scientific community has been focused on the repeated application of existing organic fluorophores or the identification of unique fluorescence properties in a trial-and-error basis without systematic studies. Moreover, recent studies are emphasizing the necessity of deeper understanding about the structure-photophysical property relationship of organic fluorophores for the development of better fluorescent probes. Herein, we provide an overview of a novel fluorescent molecular framework, Seoul-Fluor, which can be rationally engineered to furnish a wide variety of fluorophores in terms of the photophysical properties. Seoul-Fluor is built on an indolizine-based fluorescent platform with three different positions to introduce various substituents: R(1) and R(2) substituents for electronic perturbation; R(3) substituent as a functional handle for bioconjugation. Over the past decade, we have demonstrated that the Seoul-Fluor system has (i) tunable and predictable emission wavelength covering a full visible-color range; (ii) controllable quantum yield via photoinduced electron transfer phenomenon; and (iii) environment-sensitive fluorogenic properties that can be modified through intramolecular charge transfer processes. We convincingly demonstrated the prediction of photophysical properties, that is, emission wavelength and quantum yield, through the construction of a systematic set of analogues and the subsequent analysis of their photophysical properties without the highly sophisticated theoretical support. Guided by quantifiable parameters such as the Hammett substituent constants or energy levels of the molecular orbitals, this unique organic fluorophore can serve as a versatile molecular platform for the development of novel fluorescent switchable biosensors and fluorogenic bioprobes. In this Account, we will discuss the discovery and recent progress made on Seoul-Fluor, the rational design of Seoul-Fluor-based bioprobes, and their practical applications to specific biological processes that are facilitated by systematic studies of the structure-photophysical property relationships.

Synthetic fluorescent probes for studying copper in biological systems

The potent redox activity of copper is required for sustaining life. Mismanagement of its cellular pools, however, can result in oxidative stress and damage connected to aging, neurodegenerative diseases, and metabolic disorders. Therefore, copper homeostasis is tightly regulated by cells and tissues. Whereas copper and other transition metal ions are commonly thought of as static cofactors buried within protein active sites, emerging data points to the presence of additional loosely bound, labile pools that can participate in dynamic signalling pathways. Against this backdrop, we review advances in sensing labile copper pools and understanding their functions using synthetic fluorescent indicators. Following brief introductions to cellular copper homeostasis and considerations in sensor design, we survey available fluorescent copper probes and evaluate their properties in the context of their utility as effective biological screening tools. We emphasize the need for combined chemical and biological evaluation of these reagents, as well as the value of complementing probe data with other techniques for characterizing the different pools of metal ions in biological systems. This holistic approach will maximize the exciting opportunities for these and related chemical technologies in the study and discovery of novel biology of metals.

Detection of the halogenating activity of heme peroxidases in leukocytes by aminophenyl fluorescein

The formation of hypochlorous and hypobromous acids by heme peroxidases is a key property of certain immune cells. These products are not only involved in defense against pathogenic microorganisms and in regulation of inflammatory processes, but contribute also to tissue damage in certain pathologies. After a short introduction about experimental approaches for the assessment of the halogenating activity in vitro and in cell suspensions, we are focusing on novel applications of fluorescent dye systems to detect the formation of hypochlorous acid (HOCl) in leukocytes. Special attention is directed to properties and applications of the non-fluorescent dye aminophenyl fluorescein that is converted by HOCl, HOBr, and other strong oxidants to fluorescein. This dye allows the detection of the halogenating activity in samples containing free myeloperoxidase and eosinophil peroxidase as well as in intact granulocytes using fluorescence spectroscopy and flow cytometry, respectively.

The antioxidant mechanism of nitroxide TEMPO: scavenging with glutathionyl radicals

A rhodamine-nitroxide probe (R-NO˙) was introduced to probe glutathionyl radicals (GS˙) with high sensitivity and selectivity.

The singlet excited state of BODIPY promoted aerobic cross-dehydrogenative-coupling reactions under visible light

We disclose for the first time that the singlet excited state (¹PS*) of BODIPY rather than triplet excited state (³PS*) can drive C–H bond activation to form C–C and C–P bonds smoothly upon irradiation by visible light.

Silicon-substituted xanthene dyes and their applications in bioimaging

Silicon-substituted xanthene structures are excellent platforms for far-red to near-infrared fluorophores. Fluorescent probes based on these xanthene dyes are reviewed with some illustrative applications.

Protein-encapsulated bilirubin: paving the way to a useful probe for singlet oxygen

Oxygen- and singlet-oxygen-dependent parameters that characterize the behavior of bilirubin encapsulated in a protein have been quantified.

Fluorescence-labeled bis-benzamidines as fluorogenic DNA minor-groove binders: photophysics and binding dynamics

In recent decades there has been great interest in the design of highly sensitive sequence-specific DNA binders. The eligibility of the binder depends on the magnitude of the fluorescence increase upon binding, related to its photophysics, and on its affinity and specificity, which is, in turn, determined by the dynamics of the binding process. Therefore, progress in the design of DNA binders requires both thorough photophysical studies and precise determination of the association and dissociation rate constants involved. We have studied two bis-benzamidine (BBA) derivatives labeled by linkers of various lengths with the dye Oregon Green (OG). These fluorogenic binders show a dramatic fluorescence enhancement upon binding to the minor groove of double-stranded (ds) DNA, as well as significant improvement in their sequence specificity versus the parent BBA, although with decreased affinity constants. Detailed photophysical analysis shows that static and dynamic quenching of the OG fluorescence by BBA through photoinduced electron transfer is suppressed upon insertion of BBA into the minor groove of DNA. Fluorescence correlation spectroscopy yields precise dynamic rate constants that prove that the association process of these fluorogenic binders to dsDNA is very similar to that of BBA alone and that their lower affinity is mainly a consequence of their weaker attachment to the minor groove and the resultant faster dissociation process. The conclusions of this study will allow us to go one step further in the design of new DNA binders with tunable fluorescence and binding properties.