Construction of an upconversion luminescence composite nanoprobe for ratiometric single particle imaging detection of hydrogen peroxide in food

Hydrogen peroxide (H2O2) is associated with diseases and food safety. Thus, it is essential to achieve sensitive and efficient detection of H2O2. Herein, a ratiometric luminescence composite nanoprobe was designed for single particle imaging sensing of H2O2 by combining NaYbF4:Er@NaYbF4:Tm@NaGdF4:Yb upconversion nanoparticles (UCNPs) with cyanine dye IR-628. High brightness NaYbF4:Er@NaYbF4:Tm@NaGdF4:Yb UCNPs with double-peak emission (665 and 812 nm) were synthesised. Cyanine dye IR-628 with an absorption peak at 628 nm was synthesised and served as a recognition unit. More importantly, on the basis of the upconversion luminescence total internal reflection imaging technique, we developed a ratiometric single particle imaging quantitative analysis for sensing H2O2 with a limit of quantitation of 5 nM. This ratiometric single particle imaging method not only greatly eliminates the influence of the probe concentration and instrumental and environmental factors, but also reduces the dosage of the reagent used and improves the sensitivity of detection.

Dual-stimuli responsive theranostic agents based on small molecules

Stimuli-responsive theranostic agents represent a class of molecules that integrate therapeutic and diagnostic functions, offering the capability to respond to disease-associated biomarkers. Dual-stimuli responsive agents, particularly those based on small molecules, have shown considerable promise for precise imaging-guided therapeutic applications. In this Highlight, we summarize the progress of dual-stimuli responsive theranostic agents based on small molecules, for diagnostic and therapeutic studies in biological systems. The Highlight focuses on comparing different responsive groups and chemical structures of these dual-stimuli responsive theranostic agents towards different biomarkers. The potential future directions of the agents for further applications in biological systems are also discussed.

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 progress on near-infrared fluorescence heptamethine cyanine dye-based molecules and nanoparticles for tumor imaging and treatment

Recenly, near-infrared fluorescence heptamethine cyanine dyes have shown satisfactory values in bioengineering, biology, and pharmacy especially in cancer diagnosis and treatment, owing to their excellent fluorescence property and biocompatibility. In order to achieve broad application prospects, diverse structures, and chemical properties of heptamethine cyanine dyes have been designed to develop novel functional molecules and nanoparticles over the past decade. For fluorescence and photoacoustic tumor imaging properties, heptamethine cyanine dyes are equipped with good photothermal performance and reactive oxygen species production properties under near-infrared light irradiation, thus holding great promise in photodynamic and/or photothermal cancer therapies. This review offers a comprehensive scope of the structures, comparisons, and applications of heptamethine cyanine dyes-based molecules as well as nanoparticles in tumor treatment and imaging in current years. Therefore, this review may drive the development and innovation of heptamethine cyanine dyes, significantly offering opportunities for improving tumor imaging and treatment in a precise noninvasive manner. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Unraveling the Chemistry of meso-Cl Tricarbocyanine Dyes in Conjugation Reactions for the Creation of Peptide Bonds

Tricarbocyanine dyes have become popular tools in life sciences and medicine. Their near-infrared (NIR) fluorescence makes them ideal agents for imaging of thick specimens or in vivo imaging, e.g., in fluorescence-guided surgery. Among other types of cyanine dyes, meso-Cl tricarbocyanine dyes have received a surge of interest, as it emerged that their high reactivity makes them inherently tumor-targeting. As such, significant research efforts have focused on conjugating these to functional moieties. However, the syntheses generally suffer from low yields. Hereby, we report on the reaction of meso-Cl dyes with a small selection of coupling reagents to give the corresponding keto-polymethines, potentially explaining low yields and the prevalence of monofunctionalized cyanine conjugates in the current state of the art of functional near-infrared dyes. We present the synthesis and isolation of the first keto-polymethine-based conjugate and present preliminary investigation in the prostate cancer cell lines PC3 and DU145 by confocal microscopy and discuss changes to optical properties in biological media.

Graphene frameworks-confined synthesis of 2D-layered NiCoP for the electrochemical sensing of H2O2 at lower overpotential

A new 2D-layered nickel cobalt phosphide nanosheet confined by 3D graphene frameworks (denoted as NiCoP/GFs) is in situ controllably synthesized as a highly efficient and durable electrocatalyst, which is obtained from the transformation of corresponding NiCo layer double hydroxides and GFs. Hydrogen peroxide (H₂O₂) is selected as a demonstration to study the electrochemical sensing performance of the NiCoP/GFs. Benefiting from 2D morphology of NiCoP and network structure of GFs, NiCoP/GFs exhibits remarkable electroactivity toward H₂O₂ at a relatively low overpotential of approximately - 0.3 V (vs sat. Ag/AgCl) in 0.01 M phosphate-buffered saline solution (PBS, pH = 7.4). The NiCoP/GFs-based H₂O₂ electrochemical sensor achieves a high sensitivity of ∼4398 μA mM^-1 cm^-2, a low detection limit of 0.028 ± 0.006 μM, and desirable selectivity. In addition, the sensor can sensitively detect H₂O₂ from living cancer cells. This study not merely broadens the synthesis methods of transition metal phosphide-based nanocrystals but the NiCoP/GFs also has broad prospects in diverse electrochemistry fields. We have reported a controllable synthesis of 2D nickel cobalt phosphide nanosheet confined by graphene frameworks (denoted as NiCoP/GFs) as a greatly efficient and durable electrocatalyst. The NiCoP/GFs exhibits remarkable electroactivity toward detection of H₂O₂ at a relatively low overpotential of approximately -0.3 V. Density functional theory (DFT) calculations further prove that regulation of the electronic structure of NiCoP by GFs lowers the adsorption free energy of *OOH intermediates, and thus contributes to the greatly improved the electrocatalytic performance of NiCoP/GFs toward H₂O₂ reduction. The developed NiCoP/GFs can be applied as excellent electrode materials for efficient electrochemical sensing of H₂O₂.

Red and Near-Infrared Fluorescent Probe for Distinguishing Cysteine and Homocysteine through Single-Wavelength Excitation with Distinctly Dual Emissions

Small-molecule biothiols, including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), participate in various pathological and physiological processes. It is still a challenge to simultaneously distinguish Cys and Hcy because of their similar structures and reactivities, as well as the interference from the high intramolecular concentration of GSH. Herein, a novel fluorescent probe, CySI, based on cyanine and thioester was developed to differentiate Cys and Hcy through a single-wavelength excitation and two distinctly separated emission channels. The probe exhibited a turn-on fluorescence response to Cys at both 625 nm (the red channel) and 740 nm (the near-infrared channel) but only showed fluorescence turn-on to Hcy at 740 nm (the near-infrared channel) and no fluorescent response to GSH. With the aid of built-in self-calibration of single excitation and dual emissions, simultaneous discriminative determinations of Cys and Hcy were realized through red and near-infrared channels. CySI exhibited excellent selectivity toward Cys and Hcy with a fast response. This probe was further exploited to visualize exogenous Cys and Hcy in cells through dual emission channels under one excitation. Moreover, it could efficiently target mitochondria and was applied to monitor the endogenous Cys fluctuations independently in mitochondria through the red emission channel.

A Glutathione Activatable Photosensitizer for Combined Photodynamic and Gas Therapy under Red Light Irradiation

Although photodynamic therapy (PDT) is a promising approach for cancer therapy, most existing photosensitizers lack selectivity for tumor cells and the overexpressed glutathione (GSH) in tumor cells reduces the PDT efficiency. Therefore, designing photosensitizers that can be selectively activated within tumor cells and combine PDT with other therapeutic modalities represents a route for precise and efficient anticancer treatment. Herein, an organic activatable photosensitizer, CyI-DNBS, bearing 2,4-dinitrobenzenesulfonate (DNBS) as the cage group is reported. CyI-DNBS can be uptaken by cancer cells after which the cage group is selectively removed by the intracellular GSH, resulting in the generation of SO₂ for gas therapy. The reaction also releases the activated photosensitizer, CyI-OH, that can produce singlet oxygen (¹ O₂ ) under red light irradiation. Therefore, CyI-DNBS targets cancer cells for both photodynamic and SO₂ gas therapy treatments. The activatable photosensitizer provides a new approach for PDT and SO₂ gas synergistic therapy and demonstrates excellent anticancer effect in vivo.

A yellow-emitting carbon quantum dot-based fluorescent logic gate for the continuous detection of Au3+ and biothiols

Yellow-emitting carbon quantum dots, named Y-CQDs, were synthesized from O-phenylenediamine and ethylene glycol via a one-pot hydrothermal method. A fluorescent IMPLICATION logic gate for the continuous and "on-off-on" detection of Au³⁺ and biothiols in tap water at the nanoscale level was constructed based on these QDs. It showed promise in real sample detection and also as a fluorescent ink.

Enantiospecific Detection of D-Amino Acid through Synergistic Upconversion Energy Transfer

D-amino acids (DAAs) are indispensable in regulating diverse metabolic pathways. Selective and sensitive detection of DAAs is crucial for understanding the complexity of metabolic processes and managing associated diseases. However, current DAA detection strategies mainly rely on bulky instrumentation or electrochemical probes, limiting their cellular and animal applications. Here we report an enzyme-coupled nanoprobe that can detect enantiospecific DAAs through synergistic energy transfer. This nanoprobe offers near-infrared upconversion capability, a wide dynamic detection range, and a detection limit of 2.2 μM, providing a versatile platform for in vivo noninvasive detection of DAAs with high enantioselectivity. These results potentially allow real-time monitoring of biomolecular handedness in living animals, as well as developing antipsychotic treatment strategies.

Mitochondria-Targeting and Reversible Near-Infrared Emissive Iridium(III) Probe for in vivo ONOO-/GSH Redox Cycles Monitoring

Peroxynitrite (ONOO^-) and glutathione (GSH), two unique reactive species, play an essential regulating role in the oxidation and antioxidation in the living body and are closely associated with various physiological and pathological processes, like cancer, cardiovascular disorders, diabetes, inflammation, Alzheimer's disease, and hepatotoxicity. Thus, it is crucial to study mitochondria ONOO^-/GSH redox cycles by an effective molecular tool. In this work, a mitochondria-targeting and redox-reversible near-infrared (NIR) phosphorescent iridium complex, Ir-diol, has been synthesized and used for the detection and imaging of a cellular redox state by visualizing endogenous ONOO^-/GSH content. Ir-diol shows excellent photophysical properties, including NIR emission (the maximum emissive wavelength for 704 nm, approximately) and high phosphorescent quantum yield (Φ = 0.136) and exhibits high sensitivity and selectivity toward ONOO^-/GSH redox cycles in aqueous solution and living cells. Therefore, these features, combined with low cytotoxicity and excellent cell permeability, enable probe Ir-diol to monitor the changes of the intracellular ONOO^-/GSH level induced by drug both in vitro and in vivo.

Glutathione-decorated fluorescent carbon quantum dots for sensitive and selective detection of levodopa

Levodopa has been a standard drug for treating Parkinson's disease since the 1960s, but it has caused many side effects such as wearing-off, motor fluctuation, and dystonia. In this work, we developed glutathione-conjugated carbon quantum dots (GSH-CQDs) as a novel fluorescent sensor for sensitive and selective detection of levodopa. The GSH-CQDs were prepared by EDC/NHS coupling reaction of glutathione (GSH) with amine-functionalized CQDs (N-CQDs) synthesized using meta-phenylenediamine and ethylenediamine. The synthesized GSH-CQDs emitted bright green fluorescence with a high quantum yield (QY) of 22.42 ± 6.88%. However, upon the addition of levodopa to GSH-CQDs under alkaline conditions, the fluorescence of GSH-CQDs was quenched. Since levodopa is converted to dopaquinone in an alkaline environment, it is presumed that thiol groups of GHS-CQDs form covalent bonds with dopaquinone, causing fluorescence quenching through photoinduced electron transfer. Therefore, as the concentration of levodopa increased, the fluorescence intensity of GSH-CQDs was gradually decreased. Under optimal conditions, a linear response was observed in the range of 0.05-1 μM, and limit of detection (LOD) was determined to be 0.057 μM. The GSH-CQDs exhibited high specificity to levodopa over other non-target biological substances, quinone derivatives, and Parkinson's medications. Furthermore, the capability of this GSH-CQDs sensor for monitoring levodopa in human serum were validated with excellent precision and recovery rates of 100.20-103.33%.

Real-Time Evaluation of Hydrogen Peroxide Injuries in Pulmonary Fibrosis Mice Models with a Mitochondria-Targeted Near-Infrared Fluorescent Probe

Pulmonary fibrosis is a fatal chronic lung disease, leading to poor prognosis and high mortality. Accumulating evidence suggests that oxidative stress characterized by excessive production of hydrogen peroxide (H₂O₂) is an important molecular mechanism causing pulmonary fibrosis. We conceive a new type of mitochondria-targeted near-infrared fluorescent probe Mito-Bor to investigate changes in the level of endogenous H₂O₂ in living cells and mice models with pulmonary fibrosis. In the design strategy of the Mito-Bor probe, we selected azo-BODIPY as the fluorophore owing to its near-infrared fluorescence, strong photochemical stability, and low biological toxicity. Under physiological conditions, the response moiety 4-bromomethylphenylboronic acid pinacol ester could easily detect H₂O₂, and turn the fluorescence switch on. The modification of the lipophilic triphenylphosphine cation on the fluorophore would allow the probe to easily pass through the phospholipid bilayer of cells, and the internal positive charge could contribute to the selectivity of the mitochondria accumulation. The Mito-Bor probe provides high selectivity, low limit of detection, high biocompatibility, and excellent photostability. It can be used to detect changes in the level of H₂O₂ in living cells and in vivo. Therefore, the probe is applied to investigate the fluctuation of the H₂O₂ level during the process of inducing pulmonary fibrosis in cells, with changes in its fluorescence intensity correlating with the concentration of H₂O₂ and indicating the level of oxidative stress in fibroblasts. Conversely, pulmonary fibrosis can be modulated by adjusting the level of H₂O₂ in cells. A further study in mice models of bleomycin-induced pulmonary fibrosis confirms that NADPH oxidase 4 (NOX4) acts as a "button" to regulate H₂O₂ levels. The direct inhibition of NOX4 can significantly reduce the level of H₂O₂, which can delay the progression of lung fibrosis. These results provide an innovative way for the clinical treatment of pulmonary fibrosis.

Sensing mechanism of fluorogenic urea with fluoride in solvent media: A new fluorescence quenching mechanism

The interaction of 1-Phenyl-3-(pyren-1-yl) urea (LH) and fluoride anion (F^-) with a unique ON¹-OFF-ON² fluorescent response has been investigated by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The hydrogen-bonding dynamics and photophysical properties of the complex LH-F, as well as its isolated receptor LH and anion form L-H₁, have been studied in detail. We demonstrate that the intermolecular hydrogen bond (N-H…F) of the complex LH-F is greatly enhanced in the electronically excited state. The nonradiative deactivation via electron transfer and internal conversion rather than excited-state intramolecular proton transfer (ESIPT) can be facilitated by the excited state hydrogen bond strengthening. The results have been cross-validated by molecular structure, electronic spectra, frontier molecular orbitals, and infrared spectra as well as hydrogen bond binding energy. These results indicate that the current calculations completely reproduce the experimental results and provide compelling evidence for the sensing mechanism of LH for F^-.

A highly selective AIEgen fluorescent probe for visualizing Cys in living cells and C. elegans

As essential biological thiols in organisms, Cys, Hcy, and GSH are closely related to each other, and they can be involved in various pathological processes if expression levels are abnormal.

Research progress of near-infrared fluorescence probes based on indole heptamethine cyanine dyes in vivo and in vitro

Near-infrared (NIR) fluorescence imaging is a noninvasive technique that provides numerous advantages for the real-time in vivo monitoring of biological information in living subjects without the use of ionizing radiation. Near-infrared fluorescent (NIRF) dyes are widely used as fluorescent imaging probes. These fluorescent dyes remarkably decrease the interference caused by the self-absorption of substances and autofluorescence, increase detection selectivity and sensitivity, and reduce damage to the human body. Thus, they are beneficial for bioassays. Indole heptamethine cyanine dyes are widely investigated in the field of near-infrared fluorescence imaging. They are mainly composed of indole heterocyclics, heptamethine chains, and N-substituent side chains. With indole heptamethine cyanine dyes as the parent, introducing reactive groups to the parent compounds or changing their structures can make fluorescent probes have different functions like labeling protein and tumor, detecting intracellular metal cations, which has become the hotspot in the field of fluorescence imaging of biological research. Therefore, this study reviewed the applications of indole heptamethine cyanine fluorescent probes to metal cation detection, pH, molecules, tumor imaging, and protein in vivo. The distribution, imaging results, and metabolism of the probes in vivo and in vitro were described. The biological application trends and existing problems of fluorescent probes were discussed.

A Novel Ratiometric Cationic Iridium(III) Complex Phosphorescent Probe for Hydrogen Peroxide

A novel cationic iridium(III) complex phosphorescent probe (Ir-BE) containing aromatic boronate ester moiety as recognition unit was designed and synthesized. The probe exhibits ratiometric emission variations (I₄₉₀/I₅₅₀) with the color change from yellow to green only toward H₂O₂ with a broad pH range of 4 - 13. Different from other aromatic boronate-based probes, Ir-BE possesses larger Stokes shift (320 nm) and exhibits good linear correlation (R² = 0.998) between the emission ratio (I₄₉₀/I₅₅₀) and the H₂O₂ concentration in a range of 0 - 300 μM. The detection limit was as low as 0.21 μM. Furthermore, the real water sample studied further proved Ir-BE has excellent selectivity for the quantitative detection of H₂O₂.

A novel bimacrocyclic polyamine-based fluorescent probe for sensitive detection of Hg2+ and glutathione in human serum

Detection of glutathione in human serum is of great importance for clinical diagnosis of various diseases, such as AIDS, diabetes mellitus, Alzheimer disease and cancer. In this work, a new water-soluble bismacrocyclic polyamine-derived compound, namely L, which contains two molecules of 4-nitro-1,2,3-benzoxa-diazole as the fluorophores, was designed and prepared. The experiments of selectivity of L toward metal ions showed it could rapidly and sensitively detect Hg²⁺ with a detection limit of 27 nM. Furthermore, the cell imaging and co-staining experiments in HeLa cells demonstrated that the L-Hg²⁺ probe had selectivity for the Golgi apparatus to a certain degree. Moreover, it had excellent selectivity for biothiols, especially for glutathione. Finally, the probe was successfully applied to sensitively detect glutathione (GSH) in human serum and fetal bovine serum.

Synthesis, characterization, and supramolecular architectures of two distinct classes of probes for the visualization of endogenously generated hypochlorite ions in response to cellular activity

Two distinct classes of compounds, (E)-2-(((3-amino-4-nitrophenyl) imino) methyl)-5-(diethylamino) phenol (SB) and 5-(diethylamino)-2-(5-nitro-1H-benzo[d]imidazol-2-yl) phenol (IM) were synthesized. SB, a bright red colored compound was crystallized in acetonitrile as a triclinic crystal system while IM, yellow colored compound crystallized as a monoclinic crystal system in dimethylformamide by vapor diffusion of diethylether. These compounds were characterized using spectroscopic techniques (IR, UV-visible, ¹H, and ¹³C NMR), and X-ray crystallography. SB and IM displayed classical and non-classical H-bonding involving C-H…O and π…π interactions. These compounds detected hypochlorite ions in aqueous DMSO (1: 9, v/v, HEPES buffer, pH 7.4), and detection was visible via color changes by naked eye. We also performed UV-visible and fluorescence titrations, showing detection limits of 8.82 × 10^-7 M for SB and 2.44 × 10^-7 M for IM. The fluorometric responses from SB and IM were also studied against different ROS and anions. DFT calculations were performed to strengthen the proposed sensing mechanisms of both SB and IM. Hypochlorite, which is endogenously generated by myeloperoxidase in endosomes, was specifically visualized using SB and IM in lipopolysaccharide-treated RAW264.7 cells. These probes were also used to image the generation of hypochlorite by RAW264.7 cells during phagocytosis of non-fluorescent polystyrene beads.

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.

Evaluation of Glutathione S-Transferase Inhibition Effects on Idiopathic Pulmonary Fibrosis Therapy with a Near-Infrared Fluorescent Probe in Cell and Mice Models

Idiopathic pulmonary fibrosis (IPF) is a lung-limited and progressive fibrotic disease. The early diagnosis and therapies of IPF are still full of clinical challenges. Glutathione S-transferase (GSTs) plays significant roles in promoting the formation of pulmonary fibrosis. Herein, we report a fluorescent probe (Cy-GST) for the detection of GSTs concentration fluctuations in cells and in mice models. The probe can selectively and sensitively respond to GSTs with an "off-on" type fluorescence switch. Our results demonstrated that the level of intracellular GSTs increase in the pulmonary fibrosis cells and mice models. And the IPF patients hold high levels of GSTs concentrations. Thus, GSTs are likely to play important roles in pulmonary fibrosis. The inhibitor of GSTs TLK117 can reduce the severity of pulmonary fibrosis. The synergistic treatment of TLK117 and pirfenidone have better therapeutic effects than only using pirfenidone in pulmonary fibrosis mice models. The level of GSTs in IPF may be a new potential marker for IPF diagnosis. And the inhibition of GSTs may be a new therapeutic strategy for IPF treatment.

Fluorogenic probes for disease-relevant enzymes

Traditional biochemical methods for enzyme detection are mainly based on antibody-based immunoassays, which lack the ability to monitor the spatiotemporal distribution and, in particular, the in situ activity of enzymes in live cells and in vivo. In this review, we comprehensively summarize recent progress that has been made in the development of small-molecule as well as material-based fluorogenic probes for sensitive detection of the activities of enzymes that are related to a number of human diseases. The principles utilized to design these probes as well as their applications are reviewed. Specific attention is given to fluorogenic probes that have been developed for analysis of the activities of enzymes including oxidases and reductases, those that act on biomacromolecules including DNAs, proteins/peptides/amino acids, carbohydrates and lipids, and those that are responsible for translational modifications. We envision that this review will serve as an ideal reference for practitioners as well as beginners in relevant research fields.

Dye-Assembled Upconversion Nanocomposite for Luminescence Ratiometric in Vivo Bioimaging of Copper Ions

Overdose of Cu²⁺ is associated with multiple diseases, such as Wilson disease, Parkinson disease, and Alzheimer disease. Therefore, detections of Cu²⁺ in vivo and in vitro are meaningful. Near-infrared fluorescent probes are commonly applied for the detection of Cu²⁺ in real time. With mono detection signal provided, these probes are not persuasive when applied to complicated biological environments, such as cells and animals. In this report, we conjugated the organic fluorescent probe CYDAC₁₆ with the UCNPs (lanthanide-doped upconversion nanoparticles) to develop a ratiometric luminescent probe for Cu²⁺. The composite material UCNPs-CYDAC₁₆ provides a ratiometric signal based on an upconversion luminescence 660 and 800 nm. With good sensitivity and selectivity for Cu²⁺, it works well in vivo and in vitro for the detection of Cu²⁺. We believe it is promising in further biological applications.

Lysosome-specific sensing and imaging of pH variations in vitro and in vivo utilizing a near-infrared boron complex

A NIR lysosome-targeting boron complex has been developed based on hemicyanine for monitoring pH variations in vitro and in vivo.

A reversible and turn-on type fluorescence behaviour of hydrogen sulfide via a redox cycle between selenoxide and selenide

Upon treatment with H₂S in MeCN–PBS, the fluorescence dormant selenoxides of dibenzobarrelene- and benzobarrelene-based 1-seleno-1,3-butadiene derivatives are rapidly converted to strongly fluorescent selenides.

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.

Easy-to-Use Colorimetric Cyanine Probe for the Detection of Cu2+ in Wilson's Disease

Copper(II) is one of the essential metal elements in human body, which can accumulate in many organs and finally excrete in urine. Excessive load of Cu²⁺ can cause liver cirrhosis, kidney dysfunction, and many neurological symptoms in the case of Wilson's disease (WD). Therefore, the selective and efficient detection of Cu²⁺ is of great importance. Although various fluorescent probes have been reported for the detection of Cu²⁺, an efficient and capable probe is still rare for patients' self-use on a routine basis. In this study, we developed an easy-to-use probe CY1 based on UV-vis-near-infrared absorption changes with excellent sensitivity and selectivity for Cu²⁺. The mechanism of oxidation of CY1 by Cu²⁺ was first explored. We demonstrated the role of the probe in the quantitative detection of Cu²⁺ in urine from WD patients and showed that it has great potential for clinical applications.

Keto-enol-based modification on piperlongumine to generate a potent Cu(II) ionophore that triggers redox imbalance and death of HepG2 cells

Altered redox status including higher levels of copper in cancer cells than in normal cells inspired many researchers to develop copper ionophores targeting this status. We have recently found that flavon-3-ol (3-HF) works as a potent Cu(II) ionophore by virtue of its keto-enol moiety. To further emphasize the significance of this moiety for developing Cu(II) ionophores, we herein designed a β-diketo analog of piperlongumine, PL-I, characterized by the presence of high proportion of the keto-enol form in dimethylsulfoxide and chloroform, and identified its keto-enol structure by NMR and theoretical calculations. Benefiting from deprotonation of its enolic hydroxyl group, this molecule is capable of facilitating the transport of Cu(II) through cellular membranes to disrupt redox homeostasis of human hepatoma HepG2 cells and trigger their death.

A near-infrared fluorescent probe based on chloroacetate modified naphthofluorescein for selectively detecting cysteine/homocysteine and its application in living cells

We have prepared a near-infrared (NIR) turn-on fluorescent probe (NFC) based on chloroacetate modified naphthofluorescein for specific detection of cysteine (Cys) and homocysteine (Hcy) over glutathione (GSH) and other amino acids (AAs) with the detection limits of 0.30 μM and 0.42 μM, respectively. The fluorescence intensity of the naphthofluorescein (NF) chromophore is modulated by an internal charge transfer (ICT) process. The probe NFC is readily available and weakly fluorescent, but of observably enhanced fluorescence after reacting with Cys or Hcy. We assumed and then demonstrated that the fluorescence off-on process involves a conjugate nucleophilic substitution/cyclization sequence. Furthermore, the probe has been successfully applied for detecting the total content of Cys and Hcy in human plasma and imaging in living cells with low toxicity.

A BODIPY-based two-photon fluorescent probe validates tyrosinase activity in live cells

Herein, we report rational design, synthesis, and application of a two-photon fluorescent probe (Tyro-1) for tracking intracellular tyrosinase activity. The chemoselective detection of tyrosinase is precluded from interference of other competitive omnipresent oxidizing entities in cellular milieu. The probe showed 12.5-fold fluorescence enhancement at λ_em = 450 nm in the presence of tyrosinase. The nontoxic probe Tyro-1 provides information about H₂O₂-mediated upregulation of tyrosinase through cellular imaging. Its two-photon imaging ability makes it a noninvasive tool for validating the expression of tyrosinase in the live cells.

Quinone-based fluorophores for imaging biological processes

Quinones are privileged chemical structures playing crucial roles as redox and alkylating agents in a wide range of processes in cells. The broad functional array of quinones has prompted the development of new chemical approaches, including C-H bond activation and asymmetric reactions, to generate probes for examining their activity by means of fluorescence imaging. This tutorial review covers recent advances in the design, synthesis and applications of quinone-based fluorescent agents for visualizing specific processes in multiple biological systems, from cells to tissues and complex organisms in vivo.

Introduction

Primordial life forms on earth comprised oxygen-sensitive organisms: the anaerobic fermenters and cyanobacteria, which released oxygen as a metabolic by-product, causing the oxygen levels in the atmosphere to rise Benzie (Eur J Nutr 39:53–61, 2000 [1]), Halliwell (Free Radic Res 31:261–272, 1999 [2]).

A novel fluorescent probe for imaging the process of HOCl oxidation and Cys/Hcy reduction in living cells

A new on-off-on fluorescent probe, CMOS, based on coumarin was developed to detect the process of hypochlorous acid (HOCl) oxidative stress and cysteine/homocysteine (Cys/Hcy) reduction. The probe exhibited a fast response, good sensitivity and selectivity. Moreover, it was applied for monitoring the redox process in living cells.

A new on–off–on fluorescent probe, CMOS, was designed and applied to detect the process of HOCl oxidation and Cys/Hcy reduction.

A novel mitochondria-targeted two-photon fluorescent probe for dynamic and reversible detection of the redox cycles between peroxynitrite and glutathione

Redox homeostasis is important for maintenance of normal physiological functions within cells. Redox state of cells is primarily a consequence of precise balance between levels of reducing equivalents and reactive oxygen species. Redox homeostasis between peroxynitrite (ONOO^-) and glutathione (GSH) is closely associated with physiological and pathological processes, such as prolonged relaxation in vascular tissues and smooth muscle preparations, attenuation of hepatic necrosis, and activation of matrix metalloproteinase-2. We report a two-photon fluorescent probe (TP-Se) based on water-soluble carbazole-based compound, which integrates with organic selenium, to monitor changes in ONOO^-/GSH levels in cells. This probe can reversibly respond to ONOO^- and GSH and exhibits high selectivity, sensitivity, and mitochondrial targeting. The probe was successfully applied to visualize changes in redox cycles during ONOO^- outbreak and antioxidant GSH repair in cells. The probe will lead to significant development on redox events involved in cellular redox regulation.