N-Heterocyclic Carbene Boranes as Reactive Oxygen Species-Responsive Materials: Application to the Two-Photon Imaging of Hypochlorous Acid in Living Cells and Tissues

Ratiometric Near-Infrared Fluorescent Probe Monitors Ferroptosis in HCC Cells by Imaging HClO in Mitochondria

Hypochlorous acid (HClO) is a typical endogenous ROS produced mainly in mitochondria, and it has strong oxidative properties. Abnormal HClO levels lead to mitochondrial dysfunction, strongly associated with various diseases. It has been shown that HClO shows traces of overexpression in cells of both ferroptosis and hepatocellular carcinoma (HCC). Therefore, visualization of HClO levels during ferroptosis of HCC is important to explore its physiological and pathological roles. So far, there has been no report on the visualization of HClO in ferroptosis of HCC. Thus, we present a ratiometric near-infrared (NIR) fluorescent probe Mito-Rh-S which visualized for the first time the fluctuation of HClO in mitochondria during ferroptosis of HCC. Mito-Rh-S has an ultrafast response rate (2 s) and large emission shift (115 nm). Mito-Rh-S was constructed based on the PET sensing mechanism and thus has a high signal-to-noise ratio. The cell experiments of Mito-Rh-S demonstrated that Fe2+- and erastin-induced ferroptosis in HepG2 cells resulted in elevated levels of mitochondrial HClO and that high concentration levels of Fe2+ and erastin cause severe mitochondrial damage and oxidative stress and had the potential to kill HepG2 cells. By regulating the erastin concentration, erastin induction time, and treatment of the ferroptosis model, Mito-Rh-S can accurately detect the fluctuation of mitochondrial HClO levels during ferroptosis in HCC.

Application of phosphorus-bridged rigid, bent bis(NHCs) as dipodal ligands in main group and transition metal chemistry

Phosphorus-bridged rigid, bent bis(N-heterocyclic) carbenes have not been reported, so far, despite having structural features that could make them interesting ligands in coordination and main group element chemistry. In previous reports, we had demonstrated that tuning of σ3- and σ4-phosphorus environments in planarised bis(NHCs) affects electronic properties and can provide additional coordination sites. Herein, we report on first examples of synthesis and conversion of 1,4-diphosphabarrelene-related compounds into rigid bent, doubly P-bridged bis(NHCs). The formation of main group element adducts with substrates from group 13, 14 and 15 illustrates opportunities to access novel scaffolds and to create nonplanar branching points. DFT calculations reveal the new bis(NHCs) to be good candidates as novel soft/hard ligands with up to four coordination sites. The synthesis of a dinuclear Fe(CO)4 complex is demonstrated. The thermal retro-[4 + 2] cycloaddition was theoretically and experimentally explored for a variety of ionic and zwitterionic 1,4-diphosphabarrelenes, and the generation and trapping of a dinuclear Fe(0) bis(NHC) complex with a tricyclic 1σ2,4 σ2-diphosphinine scaffold is presented.

Accurate construction of NIR probe for visualizing HClO fluctuations in type I, type II diabetes and diabetic liver disease assisted by theoretical calculation

Hypochlorous acid (HClO) is a key signaling molecule which involved in various pathological and physiological processes and the immune system. It had been proved that excess HClO in the organisms was closely associated with diabetes. In this paper, we constructed a series of BODIPY-based fluorophores modified with olefinic bond. With the assistance of theoretical calculations, the optimized near-infrared (NIR) dye BDP-ENE-S-Me, which possessed the longest wavelength (690 nm) and the best stability, was screened and synthesized. Based on BDP-ENE-S-Me, we further introduced N, N-dimethylcarbamate group to construct a NIR fluorescent probe BDP-ENE-ClO. BDP-ENE-ClO displayed excellent selectivity and sensitivity with a low detection limit (49 nM) towards HClO. Besides, the probe was successfully applied in monitoring concentration fluctuations of HClO in vitro and in vivo caused by various stimuli. Most importantly, the over-production of HClO in the type I, type II diabetes and diabetic liver disease mice models could be visualized and assessed precisely with the assistance of BDP-ENE-ClO. By comparing fluorescent intensity of diabetic mice models with that of diabetic liver disease mice models, the probe was competent to assess the progression of diabetes.

HClO imaging in vivo and drug-damaged liver tissues by a large Stokes shift fluorescent probe

Drug-induced liver injury (DILI), as a classic acute inflammation, has attracted widespread concern due to its unpredictability and severity. Among the various reactive oxygen species, HClO has been used as a marker for the detection of DILI process. Thus, we designed and synthesized a "turn-on" fluorescent probe FBC-DS by modifying 3'-formyl-4'-hydroxy-[1,1'-biphenyl]-4-carbonitrile (FBC-OH) with N, N-dimethylthiocarbamate group for sensitively sensing HClO. Probe FBC-DS showed a low detection limit (65 nM), fast response time (30 s), an enormous Stokes shift (183 nm) and 85-fold fluorescence enhancement at 508 nm in the detection of HClO. Probe FBC-DS could monitor exogenous and endogenous HClO in living HeLa cells, HepG2 cells and zebrafish. In addition, probe FBC-DS has been successfully utilized in biological vectors for imaging acetaminophen (APAP)-induced endogenous HClO. Moreover, DILI caused by APAP is evaluated by probe FBC-DS through imaging over-expression of endogenous HClO in the mice liver injury models. All in all, we have every reason to believe that probe FBC-DS can be a potential tool to study the complex biological relationship between HClO and drug-induced liver injury.

Polysiloxane-based hyperbranched fluorescent probe for dynamic visualization of HClO in lysosomes and vivo

As a type of reactive oxygen species, hypochlorous acid (HClO) is associated with inducing oxidative stress in lysosomes. Once its concentration is abnormal, it may lead to lysosomal rupture and subsequent apoptosis. Meanwhile, this may provide new inspiration for cancer treatment. Therefore, it is crucial to visualize HClO in lysosomes at the biological level. So far, numerous fluorescent probes have emerged to identify HClO. However, fluorescent probes that combine low biotoxicity with lysosome-targetable properties are scarce. In this paper, hyperbranched polysiloxanes were modified by embedding perylenetetracarboxylic anhydride red fluorescent cores with naphthalimide derivative green fluorophores to synthesize novel fluorescent probe (PMEA-1). PMEA-1 was a lysosome-targetable fluorescent probe with unique dual emission, high biosafety, and good response speed. PMEA-1 exhibited excellent sensitivity and responsiveness to HClO in PBS solution and could dynamically visualize HClO fluctuations in cells and zebrafish. Simultaneously, PMEA-1 also had monitoring ability for HClO produced in the process of cellular ferroptosis. In addition, the bioimaging results indicated that PMEA-1 was capable of accumulating within the lysosomes. We anticipate that PMEA-1 will broaden the application of silicon-based fluorescent probes in the field of fluorescence imaging.

A Biocompatible Probe for the Detection of Neutrophil Elastase Free from the Interference of Structural Changes and Its Application to Ratiometric Photoacoustic Imaging In Vivo

Neutrophil elastase (NE) plays a key role in chronic inflammation and acute responses to infection and injury. Effective disease interventions thus call for precise identification of NE to aid the clinical treatment of such diseases. However, the detection process suffers from the interference of structural changes of NE. Herein, we introduce a molecular probe with high biocompatibility to overcome the interference, which was achieved by combining theoretical calculations and experimental studies, that permits highly specific and sensitive detection of NE in cells and in vivo. The upregulated NE accumulation was specifically measured in inflammation by ratiometric photoacoustic and near-infrared fluorescence imaging, providing a new method for developing more specific fluorogenic probes for other enzymes.

A novel AIRE-based fluorescent ratiometric probe with endoplasmic reticulum-targeting ability for detection of hypochlorite and bioimaging

Hypochlorite (ClO^-) plays an important role in the human immune defense system, but high concentrations of ClO^- in the endoplasmic reticulum (ER) damage cellular proteins, causing ER stress, cell death, and various diseases. Herein, we developed a simple hydrazone probe (1) featuring aggregation-induced ratiometric emission, which would quickly (within 20 s) and sensitively (detection limit of 15.4 μM) respond to ClO^- in an almost pure aqueous solution via a fluorescent ratiometric output. Furthermore, the probe was employed to track the level of ClO^- in the ER of HeLa cells and zebrafish.

Porous organic polycarbene nanotrap for efficient and selective gold stripping from electronic waste

The role of N-heterocyclic carbene, a well-known reactive site, in chemical catalysis has long been studied. However, its unique binding and electron-donating properties have barely been explored in other research areas, such as metal capture. Herein, we report the design and preparation of a poly(ionic liquid)-derived porous organic polycarbene adsorbent with superior gold-capturing capability. With carbene sites in the porous network as the "nanotrap", it exhibits an ultrahigh gold recovery capacity of 2.09 g/g. In-depth exploration of a complex metal ion environment in an electronic waste-extraction solution indicates that the polycarbene adsorbent possesses a significant gold recovery efficiency of 99.8%. X-ray photoelectron spectroscopy along with nuclear magnetic resonance spectroscopy reveals that the high performance of the polycarbene adsorbent results from the formation of robust metal-carbene bonds plus the ability to reduce nearby gold ions into nanoparticles. Density functional theory calculations indicate that energetically favourable multinuclear Au binding enhances adsorption as clusters. Life cycle assessment and cost analysis indicate that the synthesis of polycarbene adsorbents has potential for application in industrial-scale productions. These results reveal the potential to apply carbene chemistry to materials science and highlight porous organic polycarbene as a promising new material for precious metal recovery.

A Hydroxytricyanopyrrole-Based Fluorescent Probe for Sensitive and Selective Detection of Hypochlorous Acid

Hypochlorous acid (HOCl) is a reactive substance that reacts with most biomolecules and is essential in physiological and pathological processes. Abnormally elevated HOCl levels may cause inflammation and other disease responses. To further understand its key role in inflammation, HOCl must be detected in situ. Here, we designed a hydroxytricyanopyrrole-based small-molecule fluorescent probe (HTCP-NTC) to monitor and identify trace amounts of HOCl in biological systems. In the presence of HOCl, HTCP-NTC released hydroxyl groups that emit strong fluorescence covering a wide wavelength range from the visible to near-infrared region owing to the resumption of the intramolecular charge transfer process. Additionally, HTCP-NTC demonstrated a 202-fold fluorescence enhancement accompanied by a large Stokes shift and a low detection limit (21.7 nM). Furthermore, HTCP-NTC provided a rapid response to HOCl within 18 s, allowing real-time monitoring of intracellular HOCl. HTCP-NTC exhibited rapid kinetics and biocompatibility, allowing effective monitoring of the exogenous and endogenous HOCl fluctuations in living cells. Finally, based on fluorescence imaging, HTCP-NTC is a potential method for understanding the relationship between inflammation and HOCl.

Imidazolium-Modified Bispyrene-Based Fluorescent Aggregates for Discrimination of Multiple Anions in Aqueous Solution

A great number of anions exist in biological systems and natural environment, and are highly relevant to human health and environment quality. It is necessary to develop simple and effective sensors to differentiate and identify those similar or different anions. Here, an imidazolium-modified bispyrene-based fluorescent amphiphilic probe DPyDIM was synthesized and its aggregates were applied to detect and discriminate various anions. The fluorescent aggregates exhibit ratiometric responses to different types of anions. Moreover, the ratiometric responses to different types of anions are featured with multiple-wavelength cross-reactivity. The collection of fluorescence variation at four typical wavelengths can generate distinct recognition patterns to specific anions. The heat map and principal component analysis results verify that this single fluorescent sensor system can effectively and sensitively identify 16 kinds of anions that belong to phosphorus-containing, sulfur-containing anions, and anionic surfactants. The cross-reactive sensing of the amphiphilic fluorescent aggregates was attributed to the different influences on the aggregation behaviors of the probes by different anions. The present work provides a promising strategy for effective detection and discrimination of multiple anions by employing dynamic fluorescent aggregates as a sensing platform.

Simultaneous Detection of Hypochlorite and Singlet Oxygen by a Thiocoumarin-Based Ratiometric Fluorescent Probe

The development of fluorescent probes derived from thiocarbonyl compounds for reactive oxygen species has been actively pursued in recent years. However, a better understanding of the optical response behaviors of thiocarbonyl compounds toward reactive oxygen species remains a challenge. Along with this, further studies to overcome the limitation of a single emission channel and aggregation-caused quenching features of thiocarbonyl-based fluorescent probes are highly desirable. Due to the important role of hypochlorite and singlet oxygen in biological processes and their common coexistence in living systems with frequent intertransformations, the design of a fluorescent probe that can recognize both hypochlorite and singlet oxygen is of great interest. Herein, a thiocarbonyl-based ratiometric fluorescent probe (Fcoum-S) for simultaneous detection of hypochlorite and singlet oxygen in aqueous solution and living cells was designed and synthesized. Upon the addition of hypochlorite in Fcoum-S solution (phosphate-buffered saline, 10 mM, pH 7.4, 10% acetonitrile), a ratiometric fluorescence response was observed via a specific hypochlorite-promoted desulfurization reaction with a good linear relationship between the ratio of fluorescence intensities at 526 and 602 nm (I _526nm/I _602nm) and the hypochlorite concentrations (a low detection limit of 0.15 μM). Furthermore, upon green light irradiation, Fcoum-S was efficiently desulfurized to its oxo analogue (Fcoum-O) by in situ generated singlet oxygen, leading to a significant change in fluorescence. Fcoum-S could work well in an aqueous medium owing to the high reactivity of the thiocarbonyl group and the aggregation-induced emission characteristics. More importantly, Fcoum-S could target mitochondria and was successfully utilized for fluorescence imaging of mitochondrial hypochlorite/singlet oxygen in live cells. This work provides a molecular design guideline for further exploring thioketone derivatives as fluorescent probes.

A specifically triggered turn-on fluorescent probe platform and its visual imaging of HClO in cells, arthritis and tumors

Understanding disease-related processes at the molecular level is of great importance for the prevention and treatment of diseases. However, due to the lack of effective analytical tools, it is challenging to gain insight into the relationships between a specific bioactive molecule and the associated disease. Herein, a rapid turn-on resorufin-based fluorescent probe platform utilizing the HClO-specific oxidative cleavage of the amide was constructed, allowing the visualization of HClO in vitro and in vivo. These probes could quickly respond to HClO (< 50 s) with high selectivity and sensitivity (12-153 nM). The probe REClO-6 had the fastest response (30 s) and the highest sensitivity (12 nM), and was successfully used for the imaging of endogenous and exogenous HClO in cells and zebrafish. Notably, it was also successfully applied to the imaging of HClO in mouse arthritis and solid tumors. This study provided a rapid imaging analysis tool, which would be used to investigate the relationship between HClO and the disease-related physiological processes.

A coumarin-based reversible two-photon fluorescence probe for imaging glutathione near N-methyl-D-aspartate (NMDA) receptors

Glutathione (GSH) is known to play a key role in the modulation of the redox environment in N-methyl-d-aspartate (NMDA) receptors. Coumarin derivative 1 bearing cyanoacrylamide and ifenprodil moieties was synthesized and reported to monitor GSH near NMDA receptors. The cyanoacrylamide moiety allows probe 1 to monitor GSH reversibly at pH 7.4 and the ifenprodil group acts as a directing group for NMDA receptors. Two-photon fluorescence microscopy allows probe 1 to successfully sense endogenous GSH in neuronal cells and hippocampal tissues with excitation at 750 nm. Furthermore, the addition of H₂O₂ and GSH induced a decrease and an increase in fluorescence emission. Probe 1 can serve as a potential practical imaging tool to get important information on GSH in the brain.

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.

Two-photon ratiometric fluorescent probe for imaging of hypochlorous acid in acute lung injury and its remediation effect

Acute lung injury (ALI) is a pulmonary inflammatory disease with high morbidity and mortality rates. However, owing to the unknown etiology and rapid progression of the disease, the diagnosis of ALI is full of challenges with no effective treatment. Since the inflammatory response and oxidative stress played vital roles in the development of ALI, we herein developed the largest emission cross-shift (△λ = 145 nm) two-photon ratiometric fluorescent probe of TPRS-HOCl with high selectivity and short response time toward hypochlorous acid (HOCl) for exploring the relevance between the degree of ALI and HOCl concentration in the development process of the disease. In addition, the inhibition effect of HOCl during different treatment periods was also evaluated. Moreover, the tendency of imaging results was basically in accordance with that of hematoxylin and eosin (H&E) staining and the treatment effect became better in the early stage when using N-acetylcysteine (NAC), demonstrating the sensitivity of TPRS-HOCl toward ALI response. Thus, TPRS-HOCl has great potential to diagnose ALI in the early stage and guide for effective treatment.

Two-photon ESIPT-based fluorescent probe using 4-hydroxyisoindoline-1,3-dione for the detection of peroxynitrite

Excited-state intramolecular proton transfer (ESIPT)-based fluorophores with two-photon excitation fluorescence (TPEF) are rare. Our aim with this research was to develop ESIPT-based fluorophores exhibiting TPEF. Herein, we used 4-hydroxyisoindoline-1,3-dione as a scaffold to develop a two-photon fluorescent probe BHID-Bpin, for the detection of peroxynitrite (ONOO^-). BHID-Bpin exhibits excellent selectivity, sensitivity, and fast response towards ONOO^- in PBS buffer solution (10 mM, pH = 7.40). Additionally, BHID-Bpin displays high photo-stability under two-photon irradiation at 750 nm. Furthermore, the probe can image endogenous ONOO^- in HeLa cells and exogenous ONOO^- in rat hippocampal slices at a depth of 110 μm.

Stress response decay with aging visualized using a dual-channel logic-based fluorescent probe

Diagnosing aging for preventative intervention generally relies on the tracking of aging biomarkers in the resting state. However, the static marker levels are insufficient to fully evaluate aging, particularly given that the stress response capacity (SRC) decay is currently viewed as a critical feature of aging. Therefore, we have developed a dual-channel fluorescent probe ROKS capable of the logic-based visualization of thiophenol (stressor) and HOCl (thiophenol-activated stress response product) in vivo, which provides a new strategy from the time dimension to precisely assess the SRC of individuals under stress using the dual-channel fluorescence ratio. Using ROKS we observed that the SRC of live cells decayed with senescence, and that a higher SRC was found for young vs. aged Caenorhabditis elegans. As such, our study offers a promising strategy for the fluorescence-guided diagnosis of aging and paves the way for accurate evaluation of the efficacy of anti-aging drugs.

Rather than tracking aging using the resting state, ROKS, an optical probe, was developed for evaluating the degree of aging dynamically by precisely monitoring the stress response of individuals under stress.

Multiple rapid-responsive probes towards hypochlorite detection based on dioxetane luminophore derivatives

In recent years, various methods for detecting exogenous and endogenous hypochlorite have been studied, considering its essential role as a biomolecule. However, the existing technologies still pose obstacles such as their invasiveness, high costs, and complicated operation. In the current study, we developed a glow-type chemiluminescent probe, hypochlorite chemiluminescence probe (HCCL)-1, based on the scaffold of Schaap's 1,2-dioxetane luminophores. To better explore the physiological and pathological functions of hypochlorite, we modified the luminophore scaffold of HCCL-1 to develop several probes, including HCCL-2, HCCL-3, and HCCL-4, which amplify the response signal of hypochlorite. By comparing the luminescent intensities of the four probes using the IVIS® system, we determined that HCCL-2 with a limit of detection of 0.166 μM has enhanced sensitivity and selectivity for tracking hypochlorite both in vitro and in vivo.

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Strategies for the design towards glow-type hypochlorite chemiluminescent probes.

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Methods to modify the hypochlorite luminophore scaffold.

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Applications of probes for the detection and imaging of hypochlorite in vitro and in vivo.

Highly selective two-photon fluorescent off-on probes for imaging tyrosinase activity in living cells and tissues

A coumarin-based two-photon (TP) fluorescent off-on probe has been developed for detecting tyrosinase activity. High selectivity, sensitivity and biocompatibility enable the probes to successfully image tyrosinase activity in live cells and tissues using TP microscopy.

Hypochlorite-Activated Fluorescence Emission and Antibacterial Activities of Imidazole Derivatives for Biological Applications

The ability to detect hypochlorite (HOCl/ClO-) in vivo is of great importance to identify and visualize infection. Here, we report the use of imidazoline-2-thione (R 1 SR 2 ) probes, which act to both sense ClO- and kill bacteria. The N2C=S moieties can recognize ClO- among various typical reactive oxygen species (ROS) and turn into imidazolium moieties (R 1 IR 2 ) via desulfurization. This was observed through UV-vis absorption and fluorescence emission spectroscopy, with a high fluorescence emission quantum yield (ՓF = 43-99%) and large Stokes shift (∆v∼115 nm). Furthermore, the DIM probe, which was prepared by treating the DSM probe with ClO-, also displayed antibacterial efficacy toward not only Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) but also methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum ß-lactamase-producing Escherichia coli (ESBL-EC), that is, antibiotic-resistant bacteria. These results suggest that the DSM probe has great potential to carry out the dual roles of a fluorogenic probe and killer of bacteria.

Rational Design of Meso-Phosphino-Substituted BODIPY Probes for Imaging Hypochlorite in Living Cells and Mice

Meso-phosphino-substituted BODIPY probes were developed for concise and rapid detection of hypochlorite (ClO^-). Interestingly, the probe BP gave a turn-on fluorescence response by shutting the photoinduced electron transfer (PET) effect and extending the coplanar conjugated π-system. In contrast, the probe TMBP showed a colorimetric response toward ClO^-. The key role of the steric repulsions was revealed to be for altering the electronic distribution of the BODIPY core, resulting in these obviously different responses. Finally, the probe BP, with high selectivity and sensitivity toward ClO^- (LOD = 1.9 nM; response time, <15 s), was further employed in imaging the variations of exogenous and endogenous hypochlorite (ClO^-) in living RAW 264.7 cells and mouse inflammation models. If wisely utilized, this strategy with meso-phosphino BODIPY dyes may serve as a powerful platform for the preparation of novel chemosensors.

A Sensitive and Selective Fluorescent Probe for Real-Time Detection and Imaging of Hypochlorous Acid in Living Cells

Hypochlorous acid (HClO), a reactive oxygen species, plays an essential role in the processes of physiology and pathology via reacting with most biological molecules. The abnormal level of HClO may cause inflammation, especially arthritis. To further understand its key role in inflammation, in situ detection of HClO is necessary. Herein, a water-soluble small molecule fluorescent probe (HDI-HClO) is employed to monitor and identify trace amounts of HClO in the biological system. In the presence of HClO, the probe releases a hydroxyl group emitting strong fluorescence because of the restoration of the intramolecular charge transfer process. Furthermore, this probe displays a 150-fold fluorescence enhancement accompanied by a large Stokes shift and a lower detection limit (8.3 nM). Moreover, the probe can make a rapid response to HClO within 8 s, which provides the possibility of real-time monitoring of intracellular HClO. Based on the advantages of rapid dynamics, good water solubility, and excellent biocompatibility, this probe could effectively monitor the fluctuations of exogenous and endogenous HClO in living cells. The fluorescence imaging of HDI-HClO indicated that it is an excellent potential approach for comprehending the relationship between inflammation and HClO.

Radical Borylative Cyclization of Isocyanoarenes with N-Heterocyclic Carbene Borane: Synthesis of Borylated Aza-arenes

Borylated aza-arenes are of great importance in the area of organic synthesis. A radical borylative cyclization of isocyanoarenes with N-heterocyclic carbene borane (NHC-BH₃) under metal-free conditions was developed. The reaction allows the efficient assembly of several types of borylated aza-arenes (phenanthridines, benzothiazoles, etc.), which are difficult to access using alternative methods. Mild reaction conditions, a good functional-group tolerance, and generally good efficiencies were observed. The utility of these products is demonstrated, and the mechanism is discussed.

Preparation of Yellow Fluorescent N,O-CDs and its Application in Detection of ClO. J Fluoresc 2021;31:659-666. [PMID: 33534115 DOI: 10.1007/s10895-021-02686-4]

Accurate and efficient detection of ClO^- was extremely important due to the harm of ROS in the environment and organism. In this paper, yellow fluorescent N,O-CDs were successfully prepared by the solvothermal method. The microscopic size of the N,O-CDs was approximately spherical with an average particle size of 4.8 ± 0.8 nm. The fluorescence quantum yield in ethanol solution was calculated as 10.5 % using fluorescein as the standard reference. The as-fabricated N,O-CDs had high sensitivity and low detection limit (7.5 µM) for quantitatively detecting ClO^- with a linear range from 0.07 mM to 0.16 mM. The probe not only shows good selectivity and anti-interference to metal ions, anions and amino acids but also has excellent light stability and thermal stability. Also, a wide selection range for pH was demonstrated.

The development of a highly selective fluorescent probe for the rapid detection of HClO in living cells and zebrafish

A new turn-on fluorescent probe, BM-HA, for the rapid detection of HClO in living cells and zebrafish is proposed, and DFT/TDDFT calculations provide insights into the optical properties of the BM-HA probe.

Two-photon small-molecule fluorescence-based agents for sensing, imaging, and therapy within biological systems

In this tutorial review, we will explore recent advances for the design, construction and application of two-photon excited fluorescence (TPEF)-based small-molecule probes.

Design Strategies and Applications of ROS-Responsive Phenylborate Ester-Based Nanomedicine

Reactive oxygen species (ROS)-responsive nanomedicine has been extensively developed to improve the therapeutic effects while reducing the systemic toxicity. ROS, as important biological metabolites and signaling molecules, are known to overexpress in most of tumors and inflammations. Among various ROS-sensitive moieties, phenylborate ester (PBAE) with easy modifiable structure and excellent biocompatibility, represents one of the most ROS-sensitive structures. To harness it as a switch, the past several years had witnessed a booming of ROS-sensitive PBAE-based nanomedicine for various medical purposes. Much of the efforts were devoted to exploiting the potential in the management of antitumor and anti-inflammation. This review first summarizes the design strategies of PBAE in the construction of nanomedicine, with PBAE acting as not only the ROS-responsive unit, but also the roles of hydrophobic backbone or bridging segment in the macromolecular structures. The ROS-responsive mechanisms are then briefly discussed. Afterward, we focus on the introduction of the state-of-the-art research on ROS-responsive PBAE-based nanomedicine for antitumor and anti-inflammation applications. The conclusion and future perspectives of ROS-responsive nanomedicine are also provided.

A visible-light-excitable mitochondria-targeted europium complex probe for hypochlorous acid and its application to time-gated luminescence bioimaging

Development of fluorescent/luminescent probes for rapid, selective and sensitive detection of reactive oxygen species (ROS) is of great significance for understanding the roles of ROS in pathophysiological processes. In the present research, a visible-light-excitable Eu³⁺ complex-based probe, Eu(L)₃(DPBT), is designed and synthesized for the time-gated luminescence (TGL) determination of hypochlorous acid (HClO) in vitro and in vivo. The proposed probe exhibits a rapid, selective and sensitive TGL response to HClO, and excellent localization of mitochondria in living cells with low cytotoxicity. These features allow the probe to be used for the TGL sensing and imaging of HClO formation in mimic inflammatory cells at a subcellular level, as well as in endotoxin-induced liver injury and rheumatoid arthritis in live mice. In addition, by immobilizing the probe in the PEG hydrogel, the smart sensor films with rapid response to HClO were prepared, and successfully used for the real-time monitoring of HClO generation in mouse wounds, in order to distinguish the infected wounds from acute ones. Overall, this study provides a useful tool for the clinical monitoring and treatment of wound diseases.

A novel nopinone-based fluorescent probe for colorimetric and ratiometric detection of hypochlorite and its applications in water samples and living cells

A novel fluorescent probe (OFN) based on nopinone for the detection of hypochlorite has been developed. The probe OFN exhibited a colorimetric and ratiometric response to hypochlorite with good selectivity, high sensitivity (the low detection limit is 0.136 μM) and fast response time (30 s). In response to ClO-, an obvious change was observed in both the fluorescence and absorption spectra, followed by the visible color change from colorless to yellow and the fluorescence color change from yellow to green. The sensing mechanism confirmed that the oxime group of OFN was oxidized to the aldehyde group, which was proved by HRMS and 1H NMR. What is more, the probe was used not only to detect the concentration of ClO- in water samples but also for monitoring ClO- in living cells.

Revisiting imidazolium receptors for the recognition of anions: highlighted research during 2010-2019

Imidazolium based receptors selectively recognize anions, and have received more and more attention. In 2006 and 2010, we reviewed the mechanism and progress of imidazolium salt recognition of anions, respectively. In the past ten years, new developments have emerged in this area, including some new imidazolium motifs and the identification of a wider variety of biological anions. In this review, we discuss the progress of imidazolium receptors for the recognition of anions in the period of 2010-2019 and highlight the trends in this area. We first classify receptors based on motifs, including some newly emerging receptors, as well as new advances in existing receptor types at this stage. Then we discuss separately according to the types of anions, including ATP, GTP, DNA and RNA.

A pyrene-based ratiometric fluorescent probe with a large Stokes shift for selective detection of hydrogen peroxide in living cells

Hydrogen peroxide (H₂O₂) plays a significant role in regulating a variety of biological processes. Dysregulation of H₂O₂ can lead to various diseases. Although numerous fluorescent imaging probes for H₂O₂ have been reported, the development of H₂O₂ ratiometric fluorescent probe with large Stokes shift remains rather limited. Such probes have shown distinct advantages, such as minimized interference from environment and improved signal-to noise ratio. In this work, we reported a new pyrene-based compound Py-VPB as H₂O₂ fluorescent probe in vitro. The probe demonstrated ratiometric detection behavior, large Stokes shift and large emission shift. In addition, the probe showed high sensitivity and selectivity towards H₂O₂ in vitro. Based on these excellent properties, we successfully applied Py-VPB to the visualization of exogenous and endogenous H₂O₂ in living cells. Cell imaging study also showed that our probe was localized in the mitochondria. We envision that the probe can provide a useful tool for unmasking the biological roles of mitochondrial H₂O₂ in living systems.

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The first pyrene-based fluorescent probe for H₂O₂ detection with ratiometric readout was presented.

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The probe has shown prominent properties in detecting H₂O₂, such as high sensitivity & selectivity and large Stokes shift.

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The probe was successfully applied to visualizing exogenous and endogenous H₂O₂ in living cells.

Molecular Fluorescent Probes for Imaging and Evaluation of Hypochlorite Fluctuations during Diagnosis and Therapy of Osteoarthritis in Cells and in a Mouse Model

The early diagnosis of osteoarthritis (OA) can halt or delay the progression of the disease, and it is essentially beneficial to its treatment. However, biomarkers with sufficient sensitivity for dynamically identifying early OA are still yet to be determined. The overproduced hypochlorous acid (HOCl) has been proposed as an obvious symptom in early OA. Herein, based on the oxidation reaction of the sulfur atom in phenothiazine into sulfoxide, we design and synthesize a phenothiazine-derived coumarin fluorescent probe PDC for the detection of ClO^- in cells and in an OA mouse model. The probe PDC exhibits excellent selectivity and sensitivity for ClO^- detection with a limit of detection as low as 16.1 nM. Taking advantage of the probe PDC, we visualize and evaluate the level changes of ClO^- in macrophage cells, which is stimulated by various inflammatory factors. The anti-inflammatory and therapeutic effects of selenocysteine and methotrexate in inflamed cells are also confirmed. Finally, with in vivo imaging of ClO^- concentration changes in OA BALB/c mouse models, we successfully inspected the relationship between OA phenotypes and the burst of ClO^-. We suggest that abnormal changes in HOCl concentration may be considered as a new biomarker for the early OA diagnosis.