Simultaneous Fluorescence Visualization of Endoplasmic Reticulum Superoxide Anion and Polarity in Myocardial Cells and Tissue

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

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

A naphthalimide-based fluorescent platform for endoplasmic reticulum targeted imaging

A series of naphthalimide dyes (TRNATR, MOTNAMOT, MPNAMP, TYNATY, PNAP and IZNAIZ) were designed and synthesized by altering the side chains of the naphthalimide. Without the need for ER-targeting groups, the first five dyes were found to specifically target the ER, likely due to their well-suited lipophilic properties. Furthermore, TRNATR and TYNATY were proven effective for studying ER stress, showing promise in tracking ER autophagy in living cells triggered by tunicamycin and nutritional starvation.

Visualization of polarity changes in endoplasmic reticulum (ER) autophagy and rheumatoid arthritis mice with near-infrared ER-targeted fluorescent probe

The crucial cellular activities for maintaining normal cell functions heavily rely on the polarity of the endoplasmic reticulum (ER). Understanding how the polarity shifts, particularly in the context of ER autophagy (ER-phagy), holds significant promise for advancing knowledge of disorders associated with ER stress. Herein, a polarity-sensitive fluorescent probe CDI was easily synthesized from the condensation reaction of coumarin and dicyanoisophorone. CDI was composed of coumarin as the electron-donating moiety (D), ethylene and phenyl ring as the π-conjugation bridge, and malononitrile as the electron-accepting moiety (A), forming a typical D-π-A molecular configuration that recognition in the near-infrared (NIR) region. The findings suggested that as the polarity increased, the fluorescence intensity of CDI decreased, and it was accompanied by a redshift of emission wavelength at the excitation wavelength of 524 nm, shifting from 641 nm to 721 nm. Significantly, CDI exhibited a notable ability to effectively target ER and enabled real-time monitoring of ER-phagy induced by starvation or drugs. Most importantly, alterations in polarity can be discerned through in vivo imaging in mice model of rheumatoid arthritis (RA). CDI has been proven effective in evaluating the therapeutic efficacy of drugs for RA. ER fluorescent probe CDI can be optically activated in lysosomes, providing a sensitive tool for studying ER-phagy in biology and diseases.

Endoplasmic reticulum targeted fluorescent probe for real-time monitoring the viscosity changes induced by calcium homeostasis disruption

The endoplasmic reticulum (ER) acts as the major storage site for calcium ions, which are messenger ions for intracellular signaling. Disruption of calcium ion homeostasis can significantly affect the viscosity, polarity and pH of the ER. However, it is still unclear the relationship between the viscosity changes in ER and the imbalance of calcium ion homeostasis. Herein, we developed a novel fluorescent probe, named TPA, for monitoring viscosity changes that specifically targets the endoplasmic reticulum rather than mitochondria or lysosomes. TPA probe displayed good stability, as well as high responsiveness and selectivity to viscosity. The fluorescence intensity of TPA was significantly enhanced with the increased concentration or incubation time of the stimulating agents(i.e., tunicamycin), showing high responsiveness to the viscosity changes in ER. Furthermore, the TPA probe successfully demonstrated that an increase in intracellular calcium ion concentration leads to an increase in ER viscosity, whereas a decrease in calcium ion concentration leads to a decrease viscosity in ER. Both in vitro and in vivo experiments demonstrated that TPA probe successfully detected the viscosity changes in ER, especially the effects of calcium ion homeostasis disruption on ER. Overall, the TPA probe represents an efficient method for studying the relationship between calcium ion homeostasis and ER viscosity.

Fluorescence and Lifetime Imaging of Endoplasmic Reticulum Polarity Change During Ferroptosis

As a new form of regulated cell death, ferroptosis is closely related to various diseases. Tracing ferroptosis related biological behavior is helpful to better understand this process and its related biology. Considering that ferroptosis is featured with remarkable lipid peroxidation which can easily change the membranes' compositions and structures, it is potential to detect intracellular environmental changes for direct assessment of ferroptosis. In view of the close relationship between endoplasmic reticulum (ER) and ferroptosis, we designed an ER-targeted and polarity-sensitive fluorescent probe SBD-CH, which has superior photostability and can respond to polarity with high selectivity without the affection of viscosity. SBD-CH can monitor the trend of ER polarity during ferroptosis by confocal laser scanning microscopy (CLSM), and analyze the distribution of polarity in ferroptosis by fluorescence lifetime imaging microscopy (FLIM). During Erastin induced ferroptosis, the polarity of ER in HT-1080 cells increased and the polarity distribution in ER was more dispersed. Our work provides an effective strategy for evaluating the process of ferroptosis by monitoring the changes of ER polarity.

Activatable fluorescent probes for early diagnosis and evaluation of liver injury

With the increase in people's living standards, the number of patients suffering from liver injury keeps on increasing. Traditional diagnostic methods can no longer meet the needs of early and accurate diagnosis due to their limitations in application. However, fluorescent probes based on different fluorophores and nanomaterials have been gradually lighting up medical research due to their unique properties, such as high specificity and non-invasiveness. In addition, accurate identification of the different types of liver injury biomarkers can significantly improve the level of early diagnosis. Therefore, this review reviews the fluorescent probes used in the detection of biomarkers of liver injury over recent years and briefly summarizes the corresponding biomarkers of different types of liver injury. Impressively, this review also lists the structures and the response mechanisms of the different probes, and concludes with an outlook, suggesting directions in which improvements can be made. Finally, we hope that this review will contribute to the further development of fluorescent probes for the early diagnosis and assessment of liver injury.

Natural Enzyme-Inspired Design of the Single-Atom Cu Nanozyme as Dual-Enzyme Mimics for Distinguishing Total Antioxidant Capacity and the Ascorbic Acid Level

Although various oxidase mimetic or peroxidase (POD) mimetic nanozymes have been extensively studied, their poor substrate selectivity significantly inhibits their practical applications. Nanozymes with specific biomolecules as substrates, especially ascorbic acid oxidase (AAO) mimetic nanozymes with ascorbic acid (AA) as a substrate, have scarcely been studied. Herein, inspired by the multi-Cu atom sites and the redox electron transfer pathway of Cu2+/Cu+ in the natural AAO, atomically dispersed Cu sites immobilized on N-doped porous carbon (Cu-N/C) are artificially designed to simulate the function of natural AAO. Compared with their natural counterparts, the Cu-N/C catalysts exhibited higher catalytic efficiency and superior stability. Combined theoretical calculation and experimental characterizations reveal that the Cu-N/C nanozymes could catalyze the AA oxidation through a 2e- oxygen reduction pathway with H2O2 as the product. Moreover, the Cu-N/C nanozymes also possess high POD activity. As a proof-of-concept application, Cu-N/C can simultaneously realize AA detection in fluorescent mode based on its AAO activity and total antioxidant capacity detection in colorimetric mode utilizing its POD activity.

De Novo Design of Near-Infrared Fluorescent Agents Activated by Peroxynitrite and Glutathione-Responsive Imaging for Diabetic Liver Disease

Diabetes and its complications, such as diabetes liver disease, is a major problem puzzling people's health. The detection of redox states in its pathological process can effectively help us gain a deeper understanding of the disease. The pair of oxidation-reduction substances peroxynitrite (ONOO- ) and glutathione (GSH) is considered to be closely related to their occurrence and development. Thus, direct visualization of ONOO- and GSH in diabetes liver disease is critical to evaluate the disease at the molecular level. Herein, two activatable agents NTCF-ONOO- and NTCF-GSH are prepared for selectively detecting ONOO- and GSH through protection and deprotection strategies based on hydroxyl and amino groups of near-infrared fluorophore. Fluorescence imaging of exogenous and endogenous ONOO- and GSH changes in living cells and in vivo is observed. The ONOO- and GSH level in the diabetes liver disease cellular model are visualized and the possible redox imbalance mechanism related to the oxidized (NAD+ ) and reduced (NADH) nicotinamide adenine dinucleotides is explored in this process. Moreover, these probes can sensitively recognize ONOO- and GSH in the process of oxidative stress resulting from streptozotocin and streptozotocin/acetaminophen-induced complex diabetic liver disease in vivo. In addition, they can be applied for monitoring the clinical serum sample related with diabetic patients.

An endoplasmic reticulum-specific ratiometric fluorescent probe for imaging esterase in living cells

Esterase is primarily distributed in the endoplasmic reticulum (ER) and often overexpressed in cancer cells. Therefore, the detection of esterase in ER is significant for monitoring the metabolic process of various esters and evaluating the efficacy of chemotherapeutic prodrugs. However, only few fluorescent probes can detect esterase in the ER due to the lack of ER-specificity. More seriously, these probes are often limited by low pearson's colocalization coefficient and one single wavelength emission. To solve those problems, an ER-specific ratiometric fluorescent probe (ER-EST) is designed for detecting esterase in living cells. The ER-EST shows a ratiometric and red-shifted emission (125 nm) from 435 to 560 nm after hydrolysis by esterase. The fluorescence intensity ratio of ER-EST displays quantitative response to the esterase activity (0-0.5 U/mL) with low detection limit of 1.8 × 10^-4 U/mL. Importantly, the ER-EST with good biocompatibility and excellent ER-targeted ability was successfully employed to ratiometric image the endogenous endoplasmic reticulum esterase in living cells.

Organic Fluorescent Probes for Monitoring Micro-Environments in Living Cells and Tissues

As a vital parameter in living cells and tissues, the micro-environment is crucial for the living organisms. Significantly, organelles require proper micro-environment to achieve normal physiological processes, and the micro-environment in organelles can reflect the state of organelles in living cells. Moreover, some abnormal micro-environments in organelles are closely related to organelle dysfunction and disease development. So, visualizing and monitoring the variation of micro-environments in organelles is helpful for physiologists and pathologists to study the mechanisms of the relative diseases. Recently, a large variety of fluorescent probes was developed to study the micro-environments in living cells and tissues. However, the systematic and comprehensive reviews on the organelle micro-environment in living cells and tissues have rarely been published, which may hinder the research progress in the field of organic fluorescent probes. In this review, we will summarize the organic fluorescent probes for monitoring the microenvironment, such as viscosity, pH values, polarity, and temperature. Further, diverse organelles (mitochondria, lysosome, endoplasmic reticulum, cell membrane) about microenvironments will be displayed. In this process, the fluorescent probes about the "off-on" and ratiometric category (the diverse fluorescence emission) will be discussed. Moreover, the molecular designing, chemical synthesis, fluorescent mechanism, and the bio-applications of these organic fluorescent probes in cells and tissues will also be discussed. Significantly, the merits and defects of current microenvironment-sensitive probes are outlined and discussed, and the development tendency and challenges for this kind of probe are presented. In brief, this review mainly summarizes some typical examples and highlights the progress of organic fluorescent probes for monitoring micro-environments in living cells and tissues in recent research. We anticipate that this review will deepen the understanding of microenvironment in cells and tissues and facilitate the studies and development of physiology and pathology.

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.

A heat shock protein-inhibiting molecular photothermal agent for mild-temperature photothermal therapy

A heat shock protein-inhibiting photothermal agent (PTA) with endoplasmic reticulum targeting was synthesized to reduce the thermal resistance and enhance the effect of mild-temperature photothermal therapy (PTT).

AIEgen-Peptide Bioprobes for the Imaging of Organelles

Organelles are important subsystems of cells. The damage and inactivation of organelles are closely related to the occurrence of diseases. Organelles’ functional activity can be observed by fluorescence molecular tools. Nowadays, a series of aggregation-induced emission (AIE) bioprobes with organelles-targeting ability have emerged, showing great potential in visualizing the interactions between probes and different organelles. Among them, AIE luminogen (AIEgen)-based peptide bioprobes have attracted more and more attention from researchers due to their good biocompatibility and photostability and abundant diversity. In this review, we summarize the progress of AIEgen-peptide bioprobes in targeting organelles, including the cell membrane, nucleus, mitochondria, lysosomes and endoplasmic reticulum, in recent years. The structural characteristics and biological applications of these bioprobes are discussed, and the development prospect of this field is forecasted. It is hoped that this review will provide guidance for the development of AIEgen-peptide bioprobes at the organelles level and provide a reference for related biomedical research.

Recent Progress in Fluorescent Probes for Diabetes Visualization and Drug Therapy

Diabetes has become one of the most prevalent endocrine and metabolic diseases that threaten human health, and it is accompanied by serious complications. Therefore, it is vital and pressing to develop novel strategies or tools for prewarning and therapy of diabetes and its complications. Fluorescent probes have been widely applied in the detection of diabetes due to the fact of their attractive advantages. In this report, we comprehensively summarize the recent progress and development of fluorescent probes in detecting the changes in the various biomolecules in diabetes and its complications. We also discuss the design of fluorescent probes for monitoring diabetes in detail. We expect this review will provide new ideas for the development of fluorescent probes suitable for the prewarning and therapy of diabetes in future clinical transformation and application.

A ratiometric fluorescence probe for imaging endoplasmic reticulum (ER) hypochlorous acid in living cells undergoing excited state intramolecular proton transfer

Hypochlorous acid (HOCl), one of the most important ROS in living organisms, appears to serve an important role in the immune system in vivo. Endoplasmic reticulum (ER), the largest organelle in cells, manages many biological processes connected to vital activities. To better obtain insight into the relationship of ER stress and HOCl level, a ratiometric fluorescent probe RHE, based on rhodamine combined with HBT and ER-targeting group, was designed and synthesized for HOCl detection in the ER. Probe RHE shows a large stokes shift about 155 nm, which is derived to ESIPT principle. In addition, probe RHE exhibited excellent properties such as fast response (<80 s), high sensitivity with a low detection limit (40 nM), high selectivity and anti-interference. Moreover, probe RHE displayed an excellent ER-targeting ability and had been successfully applied for detection of exogenous and endogenous HOCl in HepG2 cells.

Synergistic ferroptosis-gemcitabine chemotherapy of the gemcitabine loaded carbonaceous nanozymes to enhance the treatment and magnetic resonance imaging monitoring of pancreatic cancer

Pancreatic adenocarcinoma (PDAC) is one of the deadliest cancers, and it is resistant to most conventional antineoplastic therapies. To address this challenge, gemcitabine (Gem)-loaded carbonaceous nanoparticles (MFC-Gem) as nanozymes and a theranostic platform were fabricated and used for MR-guided ferroptosis-chemo synergetic therapy of PDAC. As a biocompatible carrier, MFC-Gem nanoparticles are regarded as peroxidase-like and glutathione peroxidase-like nanozymes that promote ferroptosis therapy by effectively generating ROS and consuming GSH. Meanwhile, the combination of MnFe2O4 and Gem can markedly enhance synergetic therapy by both ferroptosis and Gem chemotherapy. MFC-Gem has higher magnetic susceptibility and was used for simultaneous magnetic resonance imaging (MRI) monitoring of the PDAC treatment. In conclusion, these salient features unequivocally indicate that this biocompatible nanotheranostic system has cooperative and enhancing chemotherapy effects for anti-PDAC therapy with simultaneous MRI monitoring. STATEMENT OF SIGNIFICANCE: Pancreatic adenocarcinoma (PDAC) is one of the deadliest cancers, and it is resistant to most conventional antineoplastic therapies. To address this challenge, gemcitabine (Gem)-loaded carbonaceous nanoparticles (MFC-Gem) as nanozymes and a theranostic platform were fabricated and used for MR-guided ferroptosis-chemo synergetic therapy of PDAC. i) MFC nanoparticles are regarded as peroxidase-like and glutathione peroxidase-like nanozymes that enhance ferroptosis therapy by effectively generating ROS and consuming GSH. ii) The combination of MnFe2O4 and Gem can markedly enhance synergetic therapy by both ferroptosis and Gem chemotherapy. iii) MFC-Gem has higher magnetic susceptibility and was used for simultaneous magnetic resonance imaging (MRI) monitoring of the PDAC treatment.

Recent progress in small-molecule fluorescent probes for endoplasmic reticulum imaging in biological systems

Endoplasmic reticulum (ER) is an indispensable organelle in eukaryotic cells involved in protein synthesis and processing, as well as calcium storage and release. Therefore, maintaining the quality of ER is of great importance for cellular homeostasis. Aberrant fluctuations of bioactive species in the ER will result in homeostasis disequilibrium and further cause ER stress, which has evolved to contribute to the pathogenesis of various diseases. Therefore, the real-time monitoring of various bioactive species in the ER is of high priority to ascertain the mysterious roles of ER, which will contribute to unveiling the corresponding mechanism of organism disturbances. Recently, fluorescence imaging has emerged as a robust technique for the direct visualization of molecular events due to its outstanding sensitivity, high temporal-spatial resolution and noninvasive nature. In this review, we comprehensively summarize the recent progress in design strategies, bioimaging applications, potential directions and challenges of ER-targetable small-molecular fluorescent probes.

Small molecule based fluorescent chemosensors for imaging the microenvironment within specific cellular regions

The microenvironment (local environment), including viscosity, temperature, polarity, hypoxia, and acidic-basic status (pH), plays indispensable roles in cellular processes. Significantly, organelles require an appropriate microenvironment to perform their specific physiological functions, and disruption of the microenvironmental homeostasis could lead to malfunctions of organelles, resulting in disorder and disease development. Consequently, monitoring the microenvironment within specific organelles is vital to understand organelle-related physiopathology. Over the past few years, many fluorescent probes have been developed to help reveal variations in the microenvironment within specific cellular regions. Given that a comprehensive understanding of the microenvironment in a particular cellular region is of great significance for further exploration of life events, a thorough summary of this topic is urgently required. However, there has not been a comprehensive and critical review published recently on small-molecule fluorescent chemosensors for the cellular microenvironment. With this review, we summarize the recent progress since 2015 towards small-molecule based fluorescent probes for imaging the microenvironment within specific cellular regions, including the mitochondria, lysosomes, lipid drops, endoplasmic reticulum, golgi, nucleus, cytoplasmic matrix and cell membrane. Further classifications at the suborganelle level, according to detection of microenvironmental factors by probes, including polarity, viscosity, temperature, pH and hypoxia, are presented. Notably, in each category, design principles, chemical synthesis, recognition mechanism, fluorescent signals, and bio-imaging applications are summarized and compared. In addition, the limitations of the current microenvironment-sensitive probes are analyzed and the prospects for future developments are outlined. In a nutshell, this review comprehensively summarizes and highlights recent progress towards small molecule based fluorescent probes for sensing and imaging the microenvironment within specific cellular regions since 2015. We anticipate that this summary will facilitate a deeper understanding of the topic and encourage research directed towards the development of probes for the detection of cellular microenvironments.

A novel ER-targeted two-photon fluorescent probe for monitoring abnormal concentrations of HClO in diabetic mice

Diabetes is closely related to the presence of excess HClO induced by endoplasmic reticulum stress. Thus, a novel two-photon fluorescent probe was designed and synthesized for the detection of HClO in the endoplasmic reticulum. Significantly, it has been verified that high glucose can indeed induce oxidative stress of the endoplasmic reticulum and produce excessive HClO. Moreover, the probe has also been successfully used in tissue imaging of diabetic mice.

Research on endoplasmic reticulum-targeting fluorescent probes and endoplasmic reticulum stress-mediated nanoanticancer strategies: A review

Subcellular localization of organelles can achieve accurate drug delivery and maximize drug efficacy. As the largest organelle in eukaryotic cells, the endoplasmic reticulum (ER) plays an important role in protein synthesis, folding, and posttranslational modification; lipid biosynthesis; and calcium homeostasis. Observing the changes in various metal ions, active substances, and the microenvironment in the ER is crucial for diagnosing and treating many diseases, including cancer. Excessive endoplasmic reticulum stress (ERS) can have a killing effect on malignant cells and can mediate cell apoptosis, proper modulation of ERS can provide new perspectives for the treatment of many diseases, including cancer. Therefore, the ER is used as a new anticancer target in cancer treatment. This review discusses ER-targeting fluorescent probes and ERS-mediated nanoanticancer strategies.

Trapping endoplasmic reticulum with amphiphilic AIE-active sensor via specific interaction of ATP-sensitive potassium (KATP)

The current aggregation-induced emission luminogens (AIEgens) sometimes suffer from poor targeting selectivity due to undesirable aggregation in the hydrophilic biosystem with 'always-on' fluorescence or unspecific aggregation in the lipophilic organelle with prematurely activated fluorescence. Herein, we report an unprecedented 'amphiphilic AIEgen' sensor QM-SO3-ER based on the AIE building block of quinoline-malononitrile (QM). The introduced hydrophilic sulfonate group can well control the specific solubility in a hydrophilic system with desirable initial 'fluorescence-off' state. Moreover, the incorporated p-toluenesulfonamide group plays two roles: enhancing the lipophilic dispersity, and behaving as binding receptor to the adenosine triphosphate (ATP)-sensitive potassium (KATP) on the endoplasmic reticulum (ER) membrane to generate the docking assay confinement effect with targetable AIE signal. The amphiphilic AIEgen has for the first time settled down the predicament of unexpected 'always-on' fluorescence in the aqueous system and the untargetable aggregation signal in the lipophilic organelle before binding to ER, thus successfully overcoming the bottleneck of AIEgens' targetability.

Organelle-Targeted Photosensitizers for Precision Photodynamic Therapy

Subcellular organelles are the cornerstones of cells, and destroying them will cause cell dysfunction and even death. Therefore, realizing precise organelle targeting of photosensitizers (PSs) can help reduce PS dosage, minimize side effects, avoid drug resistance, and enhance therapeutic efficacy in photodynamic therapy (PDT). Organelle-targeted PSs provide a new paradigm for the construction of the next generation of PSs and may provide implementable strategies for future precision medicine. In this Review, the recent targeting strategies of different organelles and the corresponding design principles of molecular and nanostructured PSs are summarized and discussed. The current challenges and opportunities in organelle-targeted PDT are also presented.

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.

Recent advances in dioxaborine-based fluorescent materials for bioimaging applications

Fluorescent materials are continuously contributing to important advances in the field of bioimaging. Among these materials, dioxaborine-based fluorescent materials (DBFM) are arousing growing interest. Due to their rigid structures conferred by a cyclic boron complex, DBFM possess appealing photophysical properties including high extinction coefficients and quantum yields as well as emission in the near infrared, enhanced photostability and high two-photon absorption. We herein discuss the recent advances of DBFM that found use in bioimaging applications. This review covers the development of fluorescent molecular probes for biomolecules (DNA, proteins), small molecules (cysteine, H₂O₂, oxygen), ions and the environment (polarity, viscosity) as well as polymers and nanomaterials used in bioimaging. This review aims at providing a comprehensive and critical insight on DBFM by highlighting the assets of these promising materials in bioimaging but also by pointing out their limitations that would require further developments.

Cellular Protein Quality Control in Diabetic Cardiomyopathy: From Bench to Bedside

Heart failure is a serious comorbidity and the most common cause of mortality in diabetes patients. Diabetic cardiomyopathy (DCM) features impaired cellular structure and function, culminating in heart failure; however, there is a dearth of specific clinical therapy for treating DCM. Protein homeostasis is pivotal for the maintenance of cellular viability under physiological and pathological conditions, particularly in the irreplaceable cardiomyocytes; therefore, it is tightly regulated by a protein quality control (PQC) system. Three evolutionarily conserved molecular processes, the unfolded protein response (UPR), the ubiquitin-proteasome system (UPS), and autophagy, enhance protein turnover and preserve protein homeostasis by suppressing protein translation, degrading misfolded or unfolded proteins in cytosol or organelles, disposing of damaged and toxic proteins, recycling essential amino acids, and eliminating insoluble protein aggregates. In response to increased cellular protein demand under pathological insults, including the diabetic condition, a coordinated PQC system retains cardiac protein homeostasis and heart performance, on the contrary, inappropriate PQC function exaggerates cardiac proteotoxicity with subsequent heart dysfunction. Further investigation of the PQC mechanisms in diabetes propels a more comprehensive understanding of the molecular pathogenesis of DCM and opens new prospective treatment strategies for heart disease and heart failure in diabetes patients. In this review, the function and regulation of cardiac PQC machinery in diabetes mellitus, and the therapeutic potential for the diabetic heart are discussed.

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.

Imaging of endoplasmic reticulum superoxide anion fluctuation in a liver injury model by a selective two-photon fluorescent probe

An endoplasmic reticulum-targeted two-photon probe is reported with excellent sensitivity and selectivity for visualizing the O₂˙⁻ level in a liver injury model.

An endoplasmic reticulum-targeting and ratiometric fluorescent probe for hypochlorous acid in living cells based on a 1,8-naphthalimide derivative

A novel reticulum-targeting and ratiometric fluorescent probe for determining hypochlorous acid has been developed.

The photophysical properties and imaging application of a new polarity-sensitive fluorescent probe

We develop a new polarity-sensitive fluorescent probe that displays weak fluorescence in low-polarity solvents and intense fluorescence in high-polarity solvents.

An endoplasmic reticulum-targeting fluorescent probe for imaging ˙OH in living cells

A new ER-targeting fluorescent probe for ˙OH is developed and applied to imaging ˙OH generation as well as lipid droplet formation in ER stress.

A highly selective and sensitive red-emitting fluorescent probe for visualization of endogenous peroxynitrite in living cells and zebrafish

Peroxynitrite (ONOO-) has been proven to participate in various physiological and pathological processes, and may also be a contributing factor in many diseases. In view of this, there is a need to develop detection tools for unambiguously tracking a small amount of endogenous ONOO- to reveal its exact mechanisms. In this paper, a colorimetric and red-emitting fluorescent probe Red-PN, based on a rhodamine-type fluorophore and hydrazide reactive site is described. The probe Red-PN possesses the advantages of rapid response (within 5 s), visual color change (from colorless to pink), preeminent sensitivity (detection limit = 4.3 nM) and selectivity. Because of these outstanding performances, it was possible to accurately detect endogenous ONOO-. It was encouraging that the probe Red-PN could be used effectively for tracking the relatively low levels of endogenous and exogenous ONOO- in living cells and zebrafish. Thus, it is envisioned that the probe Red-PN would have promising prospects in applications for imaging ONOO- in a variety of biological settings.

Fluorescent probes for organelle-targeted bioactive species imaging

The dynamic fluctuations of bioactive species in living cells are associated with numerous physiological and pathological phenomena. The emergence of organelle-targeted fluorescent probes has significantly facilitated our understanding on the biological functions of these species. This review describes the design, applications, challenges and potential directions of organelle-targeted bioactive species probes.

Bioactive species, including reactive oxygen species (ROS, including O₂˙^–, H₂O₂, HOCl, ¹O₂, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO^–, NO, NO₂, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H₂S, H₂S_n, SO₂ derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.

Sensitive Water-Soluble Fluorescent Probe Based on Umpolung and Aggregation-Induced Emission Strategies for Selective Detection of Hg2+ in Living Cells and Zebrafish

Using Hg²⁺-induced umpolung reaction and aggregation induced emission (AIE), we have rationally developed a water-soluble fluorescent probe 2,2'-(((4-(4,5-bis(4-methoxyphenyl)-1-phenyl-1H-imidazol-2-yl)phenyl)methylene)bis(sulfanediyl))diethanol (MPIPBS) for Hg²⁺ detection. MPIPBS was found to have high selectivity and sensitivity toward Hg²⁺ detection. The mechanism of MPIPBS response to Hg²⁺ was verified by ¹H NMR titration, HPLC, and HRMS spectroscopy. The detection limit was examined to be 1.45 nM, which is lower than most reported probes for Hg²⁺. Taking advantage of excellent optical properties of AIEgen, a paper based sensor for Hg²⁺ detection was fabricated by immobilizing the MPIPBS on Waterman test paper. Meanwhile, MPIPBS showed satisfactory analytical performance in real water and urine samples. Further, thanks to the high water solubility, cell membrane permeability and low cytotoxicity, MPIPBS was further used to detect Hg²⁺ both in living cells and zebrafish. We anticipate that the prepared probe was available to detect Hg²⁺ in environment and biosamples.

Two-photon imaging of the endoplasmic reticulum thiol flux in the brains of mice with depression phenotypes

Depression is a common mental illness with high morbidity and mortality. Mounting evidence suggests that an imbalance of the oxidant-antioxidant defence system is strongly correlated with depression and the dysfunction of the endoplasmic reticulum (ER) is strongly related to the oxidative stress. Therefore, as vital and abundant antioxidants in the ER, biothiols may contribute to the etiology of depression. However, ideal two-photon (TP) fluorescent probes for in vivo imaging of ER-associated thiols in the brains of mice with depression phenotypes are still lacking. Hence, we describe a fluorescent probe (ER-SH) to visualize thiols in living systems. ER-SH displays high sensitivity, excellent ER-targeting ability, outstanding TP properties and low cytotoxicity. Using this ER-SH probe, we succeeded in revealing an increase in the endogenous thiol levels under ER stress induced by DTT. Significantly, TP in vivo imaging showed for the first time that the thiol levels are reduced in brains of mice with depression phenotypes. Collectively, this work can assist in further understanding the molecular mechanism of depression and offers a crucial dimension for diagnosis and anti-depression treatments.

A high-energy-state biomimetic enzyme of oxygen-deficient MnTiO3 nanodiscs for sensitive electrochemical sensing of the superoxide anion

A high-energy-state biomimetic enzyme for the superoxide anion is presented by inducing surface oxygen defects in MnTiO₃ nanodiscs.