Mitochondria-Accessing Ratiometric Fluorescent Probe for Imaging Endogenous Superoxide Anion in Live Cells and Daphnia magna

Cyclodextrin-Assisted l-Cysteine-Capped Copper Nanoclusters: Rapid Synthesis, Enhanced Photoluminescence, and Small Molecule Interactions in Complex Biological Matrices

The rapid synthesis of stable copper nanoclusters has long been challenging. To address this, here we report the synthesis of cysteine-capped copper nanoclusters (Cys-Cu NCs) in just 30 min under ambient aqueous conditions. The incorporation of γ-cyclodextrin (γ-CD) enhanced the stability and immediately amplified the photoluminescence of the nanoclusters by triggering aggregation-induced emission (AIE), increasing their quantum yield from 0.15 to 0.24. The remarkable photoluminescence of γ-CD-assisted Cys-Cu NCs (γ-CD-Cys-Cu NCs) was selectively quenched by protoporphyrin IX (PPIX), enabling ultrasensitive detection with an exceptionally low limit of 70 pM. Stern-Volmer analysis revealed the underlying interaction mechanisms between γ-CD-Cys-Cu NCs and PPIX. This precise detection of PPIX is critical for diagnosing and monitoring porphyrias and other heme-related disorders. The method demonstrated excellent PPIX recovery in complex biological matrices, such as human serum and artificial urine, across a broad PPIX concentration range (0.5-10 µM), highlighting its applicability in real-world systems. Additionally, the nanoclusters exhibited strong sensitivity to reactive oxygen species (ROS), underscoring their potential for oxidative stress monitoring. These findings position γ-CD-Cys-Cu NCs as a versatile, cost-effective, diagnostic tool for clinical and biomedical applications.

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

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

Green Emissive Molybdenum Nanoclusters for Selective and Sensitive Detection of Hydroxyl Radical in Water Samples

In this work, Cassia tora (C. tora) have been used as a template to synthesize green fluorescent C. tora molybdenum nanoclusters (C. tora-MoNCs) through a green chemistry approach. These C. tora-MoNCs showed a quantum yield (QY) of 7.72% and exhibited a significant emission peak at 498 nm when excited at 380 nm. The as-prepared C. tora-MoNCs had an average size of 3.48 ± 0.80 nm and showed different surface functionality. The as-synthesized C. tora-MoNCs were successfully identified the hydroxyl radical (•OH) via a fluorescence quenching mechanism. Also, fluorescence lifetime and Stern-Volmer proved that after the addition of •OH radicals it was quenched the fluorescence intensity via a static quenching mechanism. The limit of detection is 9.13 nM, and this approach was successfully utilized for sensing •OH radicals in water samples with a good recovery rate.

Designing superoxide-responsive near-infrared afterglow materials for enhanced arthritis imaging

Here we report an afterglow material, APT/TTQNPs. This material can be activated by superoxide anions (O2˙-) to emit NIR afterglow. APT/TTQNPs possess high selectivity, high sensitivity, and high imaging signal-to-noise ratio to O2˙-. We successfully use APT/TTQNPs for imaging LPS-induced arthritis in mice.

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.

Superoxide-responsive quinone methide precursors (QMP-SOs) to study superoxide biology by proximity labeling and chemoproteomics

Superoxide is a reactive oxygen species (ROS) with complex roles in biological systems. It can contribute to the development of serious diseases, from aging to cancers and neurodegenerative disorders. However, it can also serve as a signaling molecule for important life processes. Monitoring superoxide levels and identifying proteins regulated by superoxide are crucial to enhancing our understanding of this growing field of redox biology and signaling. Given the high reactivity and very short lifetime of superoxide compared to other ROS in biological systems, proteins redox-modified by superoxide should be in close proximity to where superoxide is generated endogenously, i.e. superoxide hotspots. This inspires us to develop superoxide-specific quinone methide-based precursors, QMP-SOs, for proximity labeling of proteins within/near superoxide hotspots to image superoxide and profile proteins associated with superoxide biology by chemoproteomics. QMP-SOs specifically react with superoxide to generate an electrophilic quinone methide intermediate, which subsequently reacts with nucleophilic amino acids to induce a covalent tag on proteins, as revealed by liquid chromatography-mass spectrometry (LC-MS) and shotgun MS experiments. The alkyne handle on the covalent tag enables installation of fluorophores onto the tagged proteins for fluorescence imaging of superoxide in cells under oxidative stress. By establishing a chemoproteomics platform, QMP-SO-TMT, we identify DJ-1 and DLDH as proteins associated with superoxide biology in liver cancer cells treated with menadione. This work should provide insights into the crosstalk between essential cellular events and superoxide redox biology, as well as the design principles of quinone methide-based probes to study redox biology through proximity labeling and chemoproteomics.

Novel mycophenolic acid precursor-based fluorescent probe for intracellular H2O2 detection in living cells and Daphnia magna and Zebrafish model systems

Innovative for the scientific community and attracting attention in the extensive biomedical field are novel compact organic chemosensing systems built upon unique core molecular frameworks. These systems may demonstrate customized responses and may be adaptable to analytes, showing promise for potential in vivo applications. Our recent investigation focuses on a precursor of Mycophenolic acid, resulting in the development of LBM (LOD = 13 nM) - a specialized probe selective for H2O2. This paper details the synthesis, characterization, and thorough biological assessments of LBM. Notably, we conducted experiments involving living cells, daphnia, and zebrafish models, utilizing microscopy techniques to determine probe nontoxicity and discern distinct patterns of probe localization. Localization involved the distribution of the probe in the Zebrafish model within the gut, esophagus, and muscles of the antennae.

Sleep deprivation boosts O2·- levels in the brains of mice as visualized by a Golgi apparatus-targeted ratiometric fluorescence nanosensor

Sleep deprivation (SD) is highly prevalent in the modern technological world. Emerging evidence shows that sleep deprivation is associated with oxidative stress. At the organelle level, the Golgi apparatus actively participates in the stress response. In this study, to determine whether SD and Golgi apparatus stress are correlated, we rationally designed and fabricated a novel Golgi apparatus-targeted ratiometric nanoprobe called Golgi dots for O2·- detection. This probe exhibits high sensitivity and selectivity in cells and brain slices of sleep-deprived mice. Golgi dots can be readily synthesized by coprecipitation of Golgi-F127, an amphiphilic polymer F127 modified with a Golgi apparatus targeting moiety, caffeic acid (CA), the responsive unit for O2·-, and red emissive carbon nanodots (CDs), which act as the reference signal. The fluorescence emission spectrum of the developed nanoprobe showed an intense peak at 674 nm, accompanied by a shoulder peak at 485 nm. As O2·- was gradually added, the fluorescence at 485 nm continuously increased; in contrast, the emission intensity at 674 nm assigned to the CDs remained constant, resulting in the ratiometric sensing of O2·-. The present ratiometric nanoprobe showed high selectivity for O2·- monitoring due to the specific recognition of O2·- by CA. Moreover, the Golgi dots exhibited good linearity with respect to the O2·- concentration within 5 to 40 μM, and the limit of detection (LOD) was ~ 0.13 μM. Additionally, the Golgi dots showed low cytotoxicity and an ability to target the Golgi apparatus. Inspired by these excellent properties, we then applied the Golgi dots to successfully monitor exogenous and endogenous O2·- levels within the Golgi apparatus. Importantly, with the help of Golgi dots, we determined that SD substantially elevated O2·- levels in the brain.

Construction of a super large Stokes shift near-infrared fluorescent probe for detection and imaging of superoxide anion in living cells, zebrafish and mice

As one of the major reactive oxygen species (ROS), superoxide anion (O2•-) is engaged in maintaining redox homeostasis in the cell microenvironment. To identify the pathological roles in related disorders caused by abnormal expression of O2•-, it is of great significance to monitor and track the fluctuation of O2•- concentration in vivo. However, the low concentration of O2•- and the interference caused by tissue autofluorescence make the development of an ideal detection methodology full of challenges. Herein, a "Turn-On" chemical response near-infrared (NIR) fluorescence probe Dcm-Cu-OTf for O2•- detection in inflamed models, was constructed by conjugating the NIR fluorophore (dicyanisophorone derivative) with an O2•- sensing moiety (trifluoromethanesulfonate). Dcm-Cu-OTf exerted about 140-fold fluorescence enhancement after reacting 200 μM O2•- with an excellent limited of detection (LOD) as low as 149 nM. Additionally, Dcm-Cu-OTf exhibited a super large Stokes shift (260 nm) and high selectivity over other bio-analytes in stimulated conditions. Importantly, Dcm-Cu-OTf showed low toxicity and enabled imaging of the generation of O2•- in the Lipopolysaccharide (LPS)-stimulated HeLa cells, zebrafish, and LPS-induced inflamed mice. The present study provided a potential and reliable detection tool to inspect the physiological and pathological progress of O2•- in living biosystems.

An FRET-based and ER-targeting fluorescent probe for tracking superoxide anion (O2•-) in the hippocampus of the depressive mouse

Exploration of the pathway for the excessive generation of O2•- in hippocampus during depression is critical for the study on molecular mechanism of depression, and is currently still inconclusive. Herein, we put forward a hypothesis that depression increases the generation of O2•- in hippocampus by triggering ER stress, and verified this hypothesis by constructing an FRET-based ER-targeting fluorescent probe (ER-CRh) which can provide ratiometric detection of O2•- with high sensitivity and selectivity. The probe ER-CRh showed desirable ER-targeting capability, and could detect the endogenous O2•- in the ER of the hippocampal neuronal cells experiencing ER stress. Fluorescence imaging indicates that ER-CRh possesses the capability to penetrate the blood-brain barrier in mouse, and depression could promote the production of endogenous O2•- in hippocampus. Western blotting analysis reveals that the proteins GRP78 and CHOP from the hippocampus of depressive mouse show an up-regulated expression, and it suggests depression causes ER stress in hippocampal neurons. These findings prove our hypothesis, and could conduce to develop safe and effective antidepressants by the protection and repair of hippocampal neurons.

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

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

Development of a Golgi-targeted superoxide anion fluorescent probe for elucidating protein GOLPH3 function in myocardial ischemia-reperfusion injury

Superoxide anion (O₂^•-) is an important reactive oxygen species (ROS) and participates in various physiological and pathological processes in the organism. The O₂^•- burst induced by ischemia-reperfusion (I/R) is associated with cardiovascular disease and promotes the cell apoptosis. In this work, a turn-on type Golgi-targeting fluorescent probe Gol-Cou-O₂^•- was rationally designed for sensitive and selective detection of O₂^•-. The minimum detection limit concentration for O₂^•- was about 3.9 × 10^-7 M in aqueous solution. Gol-Cou-O₂^•- showed excellent capacity of detecting exogenous and endogenous O₂^•- in living cells and zebrafish, and was also used to capture the up-regulated O₂^•- level during the duration of I/R process in cardiomyocytes. Golgi Phosphoprotein 3 (GOLPH3) is a potential Golgi stress marker protein and plays a key role in cells apoptosis during I/R. The fluorescence imaging and flow cytometry assay results indicated that silencing GOLPH3 through siRNA could give rise to the down-regulated O₂^•- level and alleviation of apoptosis in I/R myocardial cells. Thus, development of Gol-Cou-O₂^•- provides a diagnostic tool for myocardial oxidative stress injury and distinct insights on roles of GOLPH3 in myocardial I/R injury.

Fluorescent Probes as a Tool in Diagnostic and Drug Delivery Systems

Over the last few years, the development of fluorescent probes has received considerable attention. Fluorescence signaling allows noninvasive and harmless real-time imaging with great spectral resolution in living objects, which is extremely useful for modern biomedical applications. This review presents the basic photophysical principles and strategies for the rational design of fluorescent probes as visualization agents in medical diagnosis and drug delivery systems. Common photophysical phenomena, such as Intramolecular Charge Transfer (ICT), Twisted Intramolecular Charge Transfer (TICT), Photoinduced Electron Transfer (PET), Excited-State Intramolecular Proton Transfer (ESIPT), Fluorescent Resonance Energy Transfer (FRET), and Aggregation-Induced Emission (AIE), are described as platforms for fluorescence sensing and imaging in vivo and in vitro. The presented examples are focused on the visualization of pH, biologically important cations and anions, reactive oxygen species (ROS), viscosity, biomolecules, and enzymes that find application for diagnostic purposes. The general strategies regarding fluorescence probes as molecular logic devices and fluorescence–drug conjugates for theranostic and drug delivery systems are discussed. This work could be of help for researchers working in the field of fluorescence sensing compounds, molecular logic gates, and drug delivery.

Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

Fluorescent probes for biomolecule detection under environmental stress

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

Vanadium nitride@carbon nanofiber composite: Synthesis, cascade enzyme mimics and its sensitive and selective colorimetric sensing of superoxide anion

Nanozymes featuring with favorable activity, good stability and easy scale-up production, are promising to replace natural enzymes for various applications. However, it remains a challenge to explore the cascade reactions of multi-enzyme mimics, aiming at synergistic catalysis for various applications. Herein, vanadium nitride nanoparticles deposited on carbon nanofibers (VN@CNFs) composite was facilely prepared by typical electrospinning route with subsequently ammonia reduction process. The nanocomposite showed excellent peroxidase (POD)-like and superoxide dismutase (SOD)-like activities. Additionally, their catalytic mechanisms were systematically researched. Coupling of SOD-like with POD-like as cascade enzyme, a selective and sensitive colorimetric detection of superoxide anion (O₂^•-) was explored, which has two linear parts, 0.05-30 μM and 30-250 μM O₂^•- with the LOD of 0.0167 μM (S/N = 3). The as-proposed method was applicable to practical samples detection with satisfactory accuracy and recovery. Therefore, the VN@CNFs composite shows great prospect in biosensing, superoxide anion removal and biocatalysis.

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

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

A naphthimide-based ratiometric fluorescent probe for selective and visual detection of phosgene in solution and the gas phase

As a colorless, highly toxic and widely used chemical reagent, phosgene poses a potentially serious threat to public health and environmental safety. Therefore, there is an urgent need to develop a simple and sensitive method for detecting phosgene. In this work, a ratiometric fluorescent probe (NED) for phosgene was developed by utilizing 4-substituted 1,8-naphthimide unit as the fluorophore and ethylenediamine as the recognition moiety. The probe NED undergoes intramolecular cyclization reaction with phosgene, resulting in a remarkable ratiometric fluorescence response. The probe NED displays high sensitivity (LOD = 4.9 nM), excellent ratiometric fluorescence signal, and high selectivity toward phosgene over other relevant analytes. In addition, paper test strip capable of visually detecting gaseous phosgene has also been fabricated.

Activity-based NIR fluorescent probes based on the versatile hemicyanine scaffold: design strategy, biomedical applications, and outlook

The discovery of a near-infrared (NIR, 650-900 nm) fluorescent chromophore hemicyanine dye with high structural tailorability is of great significance in the field of detection, bioimaging, and medical therapeutic applications. It exhibits many outstanding advantages including absorption and emission in the NIR region, tunable spectral properties, high photostability as well as a large Stokes shift. These properties are superior to those of conventional fluorogens, such as coumarin, fluorescein, naphthalimides, rhodamine, and cyanine. Researchers have made remarkable progress in developing activity-based multifunctional fluorescent probes based on hemicyanine skeletons for monitoring vital biomolecules in living systems through the output of fluorescence/photoacoustic signals, and integration of diagnosis and treatment of diseases using chemotherapy or photothermal/photodynamic therapy or combination therapy. These achievements prompted researchers to develop more smart fluorescent probes using a hemicyanine fluorogen as a template. In this review, we begin by describing the brief history of the discovery of hemicyanine dyes, synthetic approaches, and design strategies for activity-based functional fluorescent probes. Then, many selected hemicyanine-based probes that can detect ions, small biomolecules, overexpressed enzymes and diagnostic reagents for diseases are systematically highlighted. Finally, potential drawbacks and the outlook for future investigation and clinical medicine transformation of hemicyanine-based activatable functional probes are also discussed.

A new mitochondria-targeted fluorescent probe for exogenous and endogenous superoxide anion imaging in living cells and pneumonia tissue

Schematic illustration of in situ detection for superoxide anions by Mito-YX.

Detection strategies for superoxide anion: A review

Reactive oxygen species (ROS) play an essential role in regulating various physiological functions of living organisms. Superoxide anion (O₂^-.), one kind of ROS, is the single-electron reduction product of oxygen molecules, which mainly exists in plants and animals, and is closely related to many inflammatory diseases. In the field of biomedicine, with the deepening understanding of superoxide anion, more and more detection methods have been developed. This review mainly introduces the detection techniques for superoxide anion in recent years.

Rational design of near-infrared fluorescent probes for superoxide anion radical: Enhancement of self-stability and sensitivity by self-immolative linker

Fluorescent imaging of cellular superoxide anion radical (O₂^•-) is of great significance to investigate reactive oxygen species-related pathophysiological processes and drug metabolism. However, the application of this technique is far away from maximum partially due to the lack of suitable probes. In this work, we propose a new strategy for design of near-infrared (NIR) O₂^•- fluorescent probes in which p-cresol is used as a self-immolative linker to conjugate the NIR fluorophore DDAO (9H-1,3-Dichloro-7-hydroxy-9,9-dimethylacridine-2-one) with the O₂^•--sensing group (i.e., trifluoromethanesulfonate). The introduction of self-immolative linker effectively increases the self-stability of these probes under physiological conditions. Importantly, the electron-withdrawing halogen substituents on the linker greatly enhance the sensitivity of the probes to O₂^•-. As such, the representative probe DLS4 exhibits high self-stability over a broad range of pHs (5.0-8.5), high selectivity as well as excellent sensitivity to O₂^•- with a detection limit (LOD) of 7.3 nM and 720-fold fluorescence enhancement upon reaction with O₂^•-. Moreover, DLS4 enables imaging of O₂^•- generation in PMA-stimulated RAW 264.7 cells and HeLa cells, and the fluorescence intensities are proportional to the PMA concentrations. In addition, the doxorubicin-induced cytotoxicity of H9c2 cells was also evaluated using DLS4. The present study provides a novel strategy for molecular design of small-molecule O₂^•- fluorescent probes and the resulting probes show great potential as reliable tools to study the development and progression of O₂^•--related diseases and drug metabolism in various systems.

Precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity with a targetable and activatable fluorescent probe

An activatable and mitochondrial-targetable fluorescent probe was developed. This designed probe showed ratiometric fluorescence and light-up near-infrared emission responsiveness to nitroreductase, achieving precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity.

Dual-emission copper nanoclusters-based ratiometric fluorescent probe for intracellular detection of hydroxyl and superoxide anion species

A fluorescent nanoprobe based on copper nanoclusters (CuNCs) has been developed for ratiometric detection of hydroxyl radicals (^•OH) and superoxide anion radicals (O₂^•-). Two differently luminescent CuNCs, namely cyan-emissive poly(methacrylic acid)-protected copper nanoclusters (PCuNCs) and orange-emissive bovine serum albumin-protected CuNCs (BCuNCs), were conjugated to obtain a hybrid, dual-emission nanoprobe (PCuNCs-BCuNCs) with the corresponding peaks at 445 nm and 652 nm at an excitation wavelength of 360 nm. In particular, the fluorescence peak at 445 nm gradually enhanced with the incremental addition of ^•OH and O₂^•-. However, the fluorescence emission at 652 nm was greatly quenched in the presence of ^•OH, while in case of O₂^•-, the fluorescence intensity remained constant. The differential response of the PCuNCs-BCuNCs towards ^•OH and O₂^•- formed the basis of ratiometric detection. Under optimal conditions, the PCuNCs-BCuNCs exhibited good sensitivity and linearity towards ^•OH and O₂^•- with limits of detection of 0.15 μM and 1.8 μM, respectively. Moreover, the nanoprobe exhibited high selectivity for ^•OH and O₂^•- over other potential ROS interferences. Besides, PCuNCs-BCuNCs were eventually applied for qualitative and quantitative ratiometric assessment of intracellular ^•OH and O₂^•- in L-132 cells. Therefore, this strategy unveils a new potential for copper nanocluster-based sensing of ROS.

Advances in Imaging Reactive Oxygen Species

Reactive oxygen species (ROS) play a pivotal role in many cellular processes and can be either beneficial or harmful. The design of ROS-sensitive fluorophores has allowed for imaging of specific activity and has helped elucidate mechanisms of action for ROS. Understanding the oxidative role of ROS in the many roles it plays allows us to understand the human body. This review provides a concise overview of modern advances in the field of ROS imaging. Indeed, much has been learned about the role of ROS throughout the years; however, it has recently been shown that using nanoparticles, rather than individual small organic fluorophores, for ROS imaging can further our understanding of ROS.

A highly responsive, sensitive NIR fluorescent probe for imaging of superoxide anion in mitochondria of oral cancer cells

Superoxide anion (O₂^•-) is an important biomarker for reactive oxygen species (ROS) generated through physiological and pathological processes. However, due to the short half-life of O₂^•- and high autofluorescence of cell tissues, in situ real-time tracking and monitoring of endogenous O₂^•- can be difficult. In this paper, a fluorescent probe IFP-O₂ was developed to detect endogenous O₂^•- in cells. The probe could instantaneously react with O₂^•- to produce fluorescence off-on effect; its detection limit was as low as 10 nM. Cell experiments also showed that the probe had low toxicity and mitochondrial targeting ability. The article presents, for the first time, a probe that can be employed to measure endogenous O₂^•- in oral cancer Cal-27 cells and is a promising tool for monitoring and evaluating apoptosis.

Detection of Lipase Activity in Cells by a Fluorescent Probe Based on Formation of Self-Assembled Micelles

Reliable and sensitive detection of lipase activity is essential for the early diagnosis and monitoring of acute pancreatitis or progression of digestive diseases. However, the available fluorescent probes for detection of lipase activity are only implemented in a hexane-water two-phase system due to the nature of heterogeneous catalysis of lipase, thus limiting their applications in direct imaging of lipase activity in living cells and tissues. Here we designed and synthesized a "turn on" fluorescent probe CPP based on self-assembled micelles for hydrolysis of lipase. The CPP probe exhibits high selectivity and excellent sensitivity for the detection of lipase in such a homogeneous system and is successfully applied for monitoring lipase activity in pancreatic AR42J cells, tissues, and serums. Taken together, the fluorescent CPP probe not only provides a tool for diagnostic potential in pancreatic disease but also demonstrates an application potential for micelle self-assembly-based development of biological probes.

Time-Resolved Fluorescence Detection of Superoxide Anions Based on an Enzyme-Integrated Lanthanide Coordination Polymer Composite

In this work, we proposed a new strategy of fabricating time-resolved fluorescent nanoprobes by using an enzyme-integrated lanthanide coordination polymer (CP) composite for the detection of superoxide anions (O₂^•-). This CP composite was constructed with terbium ions (Tb³⁺) as a metal node, adenosine triphosphate (ATP) as a bridge ligand, and carboxyphenylboronic acid (CPBA) as a sensitizer in which superoxide dismutase (SOD) was encapsulated by a self-adaptive inclusion process. The as-prepared SOD@ATP/Tb-CPBA displays both catalytic and fluorescence properties. Benefiting from the shielding effect of ATP/Tb CP, greatly enhanced catalytic activity and stability against harsh environments can be obtained in the loaded SOD. Meanwhile, the loaded SOD can remove the water molecules on the coordination sphere of Tb³⁺, leading to a significant increase in the fluorescence intensity and lifetime of SOD@ATP/Tb-CPBA. However, upon the addition of O₂^•-, the fluorescence of SOD@ATP/Tb-CPBA was quenched significantly. This is because SOD can convert O₂^•- into H₂O₂ to induce the deboronation of CPBA, resulting in an intramolecular charge transfer process. On this basis, by taking advantage of Tb³⁺ in long lifetime emission, a time-resolved fluorescence method was developed for the detection of O₂^•-, and satisfactory results have been achieved in both buffered aqueous solutions and serum samples. We believe that the presented study will open up a new avenue to develop enzyme-involved fluorescent nanoprobes.

Visualization of endogenous β-galactosidase activity in living cells and zebrafish with a turn-on near-infrared fluorescent probe

β-Galactosidase (β-gal) is an important biomarker for primary ovarian cancers. Developing noninvasive bioimaging probes for studying the activity of β-gal is highly desirable for cancer diagnosis. Herein, a turn-on near-infrared (NIR) fluorescent probe， 2-((6-(((2S, 3R, 4S, 5R, 6R)-3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran -2-yl)oxy)-2,3-dihydro-1H-xanthen-4-yl)methylene)malononitrile named DXM-βgal, was rationally designed based on enzymatic reaction for the detection of β-gal activity both in vitro and in vivo. Upon incubating with β-gal, DXM-βgal displayed a significant fluorescence enhancement at 640 nm, accompanying by a color change of solution color from red to purple. DXM-βgal exhibited high selectivity and sensitively to β-gal with low limit of detection (2.92 × 10^-4 U mL^-1). Besides, based on its advantages of long-wavelength emission and excellent biocompatibility, DXM-βgal was successfully applied to imaging β-gal in living cells and zebrafish. Given these prominent properties, we believe that DXM-βgal will be a potential tool for investigating β-gal activity in biomedical research.

Electrochemiluminescence Biosensor for Nucleolin Imaging in a Single Tumor Cell Combined with Synergetic Therapy of Tumor

Nucleolin, a nuclear biological multifunctional protein, plays significant roles in modulating the proliferation, survival, and apoptosis of tumor cells. Different from the traditional electrochemiluminescence (ECL) method, a new ECL biosensor was built to perform ECL imaging of nucleolin in a single HeLa cell with high sensitivity and throughput. Briefly, mesoporous silica nanoparticles (MSN) loaded with doxorubicin (DOX) and phorbol 12-myristate 13-acetate (PMA) were used as drug carriers and could be specifically opened by nucleolin in a HeLa cell. PMA then induced the HeLa cell to produce reactive oxygen species (ROS) and realized ECL imaging of nucleolin. After that, ROS could damage DNA and proteins of the tumor cell and DOX could induce the apoptosis of HeLa cells by inhibiting genetic material, nucleic acid, synthesis. HeLa cells were then efficiently killed by DOX and ROS in a synergetic pathway. Herein, a new ECL biosensor for ECL imaging of nucleolin in a single HeLa cell and synergetic tumor therapy was built.

A FRET-based ratiometric two-photon fluorescent probe for superoxide anion detection and imaging in living cells and tissues

The superoxide anion (O2˙-) plays a crucial role in several physiological processes and many human diseases. Developing new methods for O2˙- detection in biological systems is very important. A FRET-based two-photon (TP) fluorescent probe with a ratiometric signal, TFR-O, was developed. A naphthalene derivative based TP fluorescent group was selected as the energy donor group, and a rhodol fluorescent group was chosen as the energy acceptor; the trifluoromethanesulfonate group was chosen as the recognition moiety. After reacting with O2˙-, the recognition moiety was removed and the fluorophore was released, leading to a fluorescence intensity decrease at the wavelength of 425 nm and a significant enhancement of the fluorescence intensity at 550 nm. The fluorescence intensity ratio between 550 and 425 nm (I550/I425) varied from 0.15 to 6.72, with the O2˙- concentration increasing from 0 to 50 μM. The detection limit of the TFR-O was 83 nM. Moreover, TFR-O was applied for detecting and imaging O2˙- in cells and liver tissues.

An ultrasensitive electrochemical sensor based on cotton carbon fiber composites for the determination of superoxide anion release from cells

A sensor is described for determination of superoxide anion (O₂˙^-). The electrode consists of nitrogen-doped cotton carbon fiber (NCFs) modified with silver nanoparticles (AgNPs) which have excellent catalytic capability. The resulting sensor, best operated at working potentials around -0.5 V (vs. SCE), can detect O₂˙^- over an extraordinarily wide range that covers 10 orders of magnitude, and the detection limit is 2.32 ± 0.07 fM. The electrode enables the release of O₂˙^- from living cells under normal or under oxidative stress conditions to be determined. The ability to scavenge the superoxide anions of antioxidants was also investigated. In the authors' perception, the method represents a viable tool for studying diseases related to oxidative stress. Graphical abstract Schematic presentation of the construction of an electrochemical sensor based on Nitrogen-doped cotton carbon fiber and silver nanoparticles. It can be used for the direct detection of superoxide anions released from Glioma cells (U87) under normal or under oxidative stress conditions.