In-Situ Imaging of Azoreductase Activity in the Acute and Chronic Ulcerative Colitis Mice by a Near-Infrared Fluorescent Probe

Fast-Responsive HClO-Activated Near-Infrared Fluorescent Probe for In Vivo Diagnosis of Inflammatory Bowel Disease and Ex Vivo Optical Fecal Analysis

Inflammatory bowel disease (IBD) is an idiopathic intestinal inflammatory disease, whose etiology is intimately related to the overproduction of hypochlorous acid (HClO). Optical monitoring of HClO in the living body favors real-time diagnosis of inflammatory diseases. However, HClO-activated near-infrared (NIR) fluorescent probes with rapid response and high inflammatory cell uptake are still lacking. Herein, we report an activatable acceptor-π-acceptor (A-π-A)-type NIR fluorescent probe (Cy-DM) bearing two d-mannosamine groups for the sensitive detection of HClO in early IBD and stool testing. Once reacted with HClO, nonfluorescent Cy-DM could be turned on within 2 s by generating a donor-π-acceptor (D-π-A) structure due to the enhanced intramolecular charge transfer mechanism, showing intense NIR fluorescence emission at 700 nm and a large Stokes shift of 115 nm. Moreover, it was able to sensitively and selectively image exogenous and endogenous HClO in the lysosomes of living cells with a detection limit of 0.84 μM. More importantly, because of the d-mannosamine modification, Cy-DM was efficiently taken up by inflammatory cells in the intestine after intravenous administration, allowing noninvasive visualization of endogenous HClO in a lipopolysaccharide-induced IBD mouse model with a high fluorescence contrast of 6.8/1. In addition, water-soluble Cy-DM has also been successfully applied in ex vivo optical fecal analysis, exhibiting a 3.4-fold higher fluorescence intensity in the feces excreted by IBD mice. We believe that Cy-DM is promising as an invaluable tool for rapid diagnosis of HClO-related diseases as well as stool testing.

Designing a Hypoxia-Activated Sensing Platform Using an Azo Group-Triggered Reaction with the Formation of Silicon Nanoparticles

Hypoxia is known as a specific signal of various diseases, such as liver fibrosis. We designed a hypoxia-sensitive fluorometric approach that cleaved the azo bond (N═N) in the presence of hypoxia-controlled agents (sodium dithionite and azoreductase). 4-(2-Pyridylazo) resorcinol (Py-N═N-RC) bears a desirable hypoxia-responsive linker (N═N), and its azo bond breakup can only occur in the presence of sodium dithionite and azoreductase and leads to the release of 2,4-dihydroxyaniline, which can react with 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane to generate yellow fluorescent silicon nanoparticles. This approach exhibited high selectivity and sensitivity toward both sodium dithionite and azoreductase over other potential interferences. The mouse liver microsome, which is known to contain azoreductase, was applied and confirmed the feasibility of the designed platform. Py-N═N-RC is expected to be a practical substrate for hypoxia-related biological analyses. Furthermore, silicon nanoparticles were successfully applied for Hela cell imaging owing to their negligible cytotoxicity and superb biocompatibility.

Development of a hypoxia-activated red-emission fluorescent probe for in vivo tumor microenvironment imaging and anti-tumor therapy

Hypoxia, a significant feature of the tumor microenvironment, is closely associated with tumor growth, metastasis, and drug resistance. In the field of tumor microenvironment analysis, accurately imaging and quantifying hypoxia - a critical factor associated with tumor progression, metastasis, and resistance to therapy - remains a significant challenge. Herein, a hypoxia-activated red-emission fluorescent probe, ODP, for in vivo imaging of hypoxia in the tumor microenvironment is presented. Among various imaging methods, optical imaging is particularly convenient due to its rapid response and high sensitivity. The ODP probe specifically targets nitroreductase (AzoR), an enzyme highly expressed in hypoxic cells, playing a vital role by catalyzing the cleavage of azo bonds. The optical properties of ODP exhibited excellent performance in terms of fluorescence enhancement, fluorescence lifetime (0.81 ns), and detection limit (0.86 µM) in response to SDT. Cell imaging experiments showed that ODP could effectively detect and image intracellular hypoxia and the imaging capability of ODP was studied under various conditions including cell migration, antioxidant treatment, and different incubation times. Through comprehensive in vitro and in vivo experiments, including cellular imaging and mouse tumor models, this work demonstrates the efficacy of ODP in accurately detecting and imaging hypoxia. Moreover, ODP's potential in inducing apoptosis in cancer cells offers a promising avenue for integrating diagnostic and therapeutic strategies in cancer treatment. This innovative approach not only contributes to the understanding and assessment of tumor hypoxia but also opens new possibilities for targeted cancer therapy.

Molecular Engineering of Activatable NIR-II Hemicyanine Reporters for Early Diagnosis and Prognostic Assessment of Inflammatory Bowel Disease

Molecular imaging in the second near-infrared window (NIR-II) provides high-fidelity visualization of biopathological events in deep tissue. However, most NIR-II probes produce "always-on" output and demonstrate poor signal specificity toward biomarkers. Herein, we report a series of hemicyanine reporters (HBCs) with tunable emission to NIR-II window (715-1188 nm) and structurally amenable to constructing activatable probes. Such manipulation of emission wavelengths relies on rational molecular engineering by integrating benz[c,d]indolium, benzo[b]xanthonium, and thiophene moieties to a conventional hemicyanine skeleton. In particular, HBC4 and HBC5 possess bright and record long emission over 1050 nm, enabling improved tissue penetration depth and superior signal to background ratio for intestinal tract mapping than NIR-I fluorophore HC1. An activatable inflammatory reporter (AIR-PE) is further constructed for pH-triggered site-specific release in colon. Due to minimized background interference, oral gavage of AIR-PE allows clear delineation of irritated intestines and assessment of therapeutic responses in a mouse model of inflammatory bowel disease (IBD) through real-time NIRF-II imaging. Benefiting from its high fecal clearance efficiency (>90%), AIR-PE can also detect IBD and evaluate the effectiveness of colitis treatments via in vitro optical fecalysis, which outperforms typical clinical assays including fecal occult blood testing and histological examination. This study thus presents NIR-II molecular scaffolds that are not only applicable to developing versatile activatable probes for early diagnosis and prognostic monitoring of deeply seated diseases but also hold promise for future clinical translations.

Visual monitoring of biocatalytic processes using small molecular fluorescent probes: strategies-mechanisms-applications

Real-time monitoring of biocatalytic-based processes is significantly improved and simplified when they can be visualized. Visual monitoring can be achieved by integrating a fluorescent unit with the biocatalyst. Herein, we outline the design strategies of fluorescent probes for monitoring biocatalysis: (1) probes for monitoring biocatalytic transfer: γ-glutamine is linked to the fluorophore as both a recognition group and for intramolecular charge transfer (ICT) inhibition; the probe is initially in an off state and is activated via the transfer of the γ-glutamine group and the release of the free amino group, which results in restoration of the "Donor-π-Acceptor" (D-π-A) system and fluorescence recovery. (2) Probes for monitoring biocatalytic oxidation: a propylamine is connected to the fluorophore as a recognition group, which cages the hydroxyl group, leading to the inhibition of ICT; propylamine is oxidized and subsequently β-elimination occurs, resulting in exposure of the hydroxyl group and fluorescence recovery. (3) Probes for monitoring biocatalytic reduction: a nitro group attached to a fluorophore as a fluorescence quenching group, this is converted to an amino group by catalytic reduction, resulting in fluorescence recovery. (4) Probes for monitoring biocatalytic hydrolysis: β-D-galactopyranoside or phosphate acts as a recognition group attached to hydroxyl groups of the fluorophore; the subsequent biocatalytic hydrolysis reaction releases the hydroxyl group resulting in fluorescence recovery. Following these 4 mechanisms, fluorophores including cyanine, coumarin, rhodamine, and Nile-red, have been used to develop systems for monitoring biocatalytic reactions. We anticipate that these strategies will result in systems able to rapidly diagnose and facilitate the treatment of serious diseases.

New Near-Infrared Fluorescence Imaging Platform with Large Stokes Shift for Carboxylesterase 2 Detection in Thyroid Cancer and Inflammatory Diseases Diagnosis

Development of new near-infrared fluorophores is one of the eternal themes in the field of biosensing and biological imaging. In this work, we constructed a novel fluorophore platform MOR by replacing methylindole of hemicyanine fluorophore (CyR) with benzoxazole to acquire better fluorescence characteristics. Based on the platform, a near infrared (NIR) fluorescent probe MOR-CES2 was synthesized for the specific "off-on" response to carboxylesterase 2 (CES2). The probe exhibited excellent properties including near-infrared emission (735 nm), large Stokes shift (105 nm), high sensitivity (LOD, 0.3 ng/mL), and rapid response (15 min). The successful application of MOR-CES2 in biological imaging of CES2 in mice with thyroid cancer and inflammatory bowel disease demonstrated that the probe could identify cancer cells and tissues and sensitively respond to inflammation. The results proved the potency of MOR-CES2 as an efficient imaging tool to assist in the surgical resection of CES2-related tumors.

Visualization of an Endogenous Mitochondrial Azoreductase Activity under Normoxic Conditions Using a Naphthalimide Azo-Based Fluorogenic Probe

In this paper, we demonstrate the existence of an endogenous mitochondrial azoreductase (AzoR) activity that can induce the cleavage of N═N double bonds of azobenzene compounds under normoxic conditions. To this end, 100% OFF-ON azo-based fluorogenic probes derived from 4-amino-1,8-naphthalimide fluorophores were synthesized and evaluated. The in vitro study conducted with other endogenous reducing agents of the cell, including reductases, demonstrated both the efficacy and the selectivity of the probe for AzoR. Confocal experiments with the probe revealed an AzoR activity in the mitochondria of living cells under normal oxygenation conditions, and we were able to demonstrate that this endogenous AzoR activity appears to be expressed at different levels across different cell lines. This discovery provides crucial information for our understanding of the biochemical processes occurring within the mitochondria. It thus contributes to a better understanding of its function, which is implicated in numerous pathologies.

Application of a Specific and Sensitive NQO1 Turn-On Near-Infrared Fluorescence Probe for Live Cancer Cell and Xenografted Tumor Imaging in Nude Mice

Sensitive activity stains for enzymes selectively expressed in human cancers offer valuable tools for imaging with wide applications in experimental, diagnostic, and therapeutic settings. The scant expression of the antioxidant enzyme NQO1 in normal tissues and its great abundance in malignant counterparts due to the increased redox stress and hypoxia is one such example. Previously, we described a potent nontoxic probe that remains nonfluorescent but releases an intense fluorogenic compound after intracellular cleavage by NQO1 catalysis. This infrared probe with a 644 nm emission has excellent tissue penetrating ability and low background absorption. Described here are methods (fluorescence microscopy, flow cytometry, and in vivo animal imaging) to rapidly image NQO1 activity in hypoxic and non-hypoxic cancer cells and tumors developed in live mouse xenograft models. The specificity of the dye for NQO1 in all three procedures was verified, and the methods should be useful for both in vitro and in vivo studies.

An oral ratiometric NIR-II fluorescent probe for reliable monitoring of gastrointestinal diseases in vivo

Early monitoring of gastrointestinal diseases via orally delivered NIR-II ratiometric fluorescent probes represents a promising noninvasive diagnostic modality, but is challenging due to the limitation of harsh digestive environment. Here, we report a single-component NIR-II ratiometric molecular nanoprobe (LC-1250 NP) to monitor gastrointestinal disease with high specificity to its biomarker H₂O₂ via oral administration. LC-1250 NP displays stable fluorescence in the channel of 1250 long-pass (F_1250LP) before and after the gastrointestinal disease detection as the reference, while it presents significantly enhanced fluorescence signal in the response channel of 1150 nm short-pass (F_1150SP) in diseased gastrointestinal environment due to the intramolecular cyclization of LC-1250 molecules activated by H₂O₂. The fluorescence ratio (F_1150SP/F_1250LP) increases linearly with the concentration of H₂O₂ with a low detection limit of 20 nM. Therefore, when delivered orally, LC-1250 NP can accurately map the diseased areas and surmount the false-positive interference from biological heterogeneity by NIR-II ratiometric fluorescence imaging, providing sensitive and reliable evaluation for the progress of gastroenteritis.

Small-molecule photoswitches for fluorescence bioimaging: engineering and applications

Fluorescence microscopy has revolutionised our understanding of biological systems, enabling the visualisation of biomolecular structures and dynamics in complex systems. The possibility to reversibly control the optical or biochemical properties of fluorophores can unlock advanced applications ranging from super-resolution microscopy to the design of multi-stimuli responsive and functional biosensors. In this Highlight, we review recent progress in small-molecule photoswitches applied to biological imaging with an emphasis on molecular engineering strategies and promising applications, while underlining the main challenges in their design and implementation.

Caspase-1-responsive fluorescence biosensors for monitoring endogenous inflammasome activation

The activation of inflammasome leads to secretion of inflammatory factors and cell pyroptosis that are critical in the pathogenesis of various chronic and acute inflammatory diseases. Recruitment and activation of caspase-1 is a marker of inflammasome activation. However, there is still lack of real-time and efficient methods to detect the activation of inflammasome, especially in vivo. Herein, we developed two activatable caspase-1-responsive fluorescence biosensors, WEHD-HCy and YVAD-HCy, to specifically monitor the activation of inflammasome in vivo. Our in vitro study demonstrated that WEHD-HCy and YVAD-HCy can sensitively and specifically respond to caspase-1 activation. Moreover, these biosensors can efficiency and specifically activated in the common inflammatory disease model, including inflammatory bowel disease, Salmonella infection, and acute arthritis. In particular, WEHD-HCy is more advantageous than YVAD-HCy to specifically image of caspase-1 activity both in vitro and in vivo. These caspase-1-responsive fluorescence biosensors provide an efficient, rapid, and in situ tool for monitoring inflammasome activation, and have the potential to be suitable for clinical diagnosis of various inflammatory diseases associated with inflammasome activation.

Orally Administrable Aggregation-Induced Emission-Based Bionic Probe for Imaging and Ameliorating Dextran Sulfate Sodium-Induced Inflammatory Bowel Diseases

As macrophage infiltration is significantly related to the progression of inflammatory bowel disease (IBD), monitoring the macrophages is a valuable strategy for IBD diagnosis. However, owing to the harsh physiological environment of the gastrointestinal tract and enzymatic degradation, the development of orally administrable imaging probes for tracking macrophages remains a considerable challenge. Accordingly, herein, an orally administrable aggregation-induced emission biomimetic probe (HBTTPIP/β-glucan particles [GPs]) is developed for tracing macrophages; HBTTPIP/GPs can diagnose and alleviate dextran sulfate sodium (DSS)-induced colonic inflammation and self-report the treatment efficiency. The fluorophore HBTTPIP can effectively aggregate in GPs, restricting intramolecular rotation and activating the fluorescence of HBTTPIP. After being orally administrated, HBTTPIP/GPs are phagocytosed by intestinal macrophages, which then migrate to colonic lesions, enabling non-invasive monitoring of the severity of IBD via in vivo fluorescence imaging. Notably, oral HBTTPIP/GPs ameliorate DSS-induced IBD by inhibiting the expressions of pro-inflammatory factors and improving colonic mucosal barrier function. Furthermore, these HBTTPIP/GPs realize self-feedback of the therapeutic effects of GPs on DSS-induced colitis. The oral biomimetic probe HBTTPIP/GPs reported herein provide a novel theranostic platform for IBD, integrating non-invasive diagnosis of IBD in situ and the corresponding treatment.

Naphthalimide-Based Fluorescent Probe for Profiling of Aldehydes during Oxidation of Unsaturated Lipids

A lipophilic naphthalimide hydrazine fluorescent probe was successfully developed in this study for profiling aldehyde oxidation products. Dodecyl amine was applied to afford lipophilicity of the fluorescent probe for lipids. Investigation of fluorescence properties of the probe and condensation products with typical aldehydes including MDA and hexanal revealed significant enhancement of fluorescence intensity after condensation due to the inhibition of photo-induced electron transfer. MDA and hexanal could be differentiated by the probe through emission of different fluorescence colors (blue, MDA; green, hexanal). Eight major oxidation components including seven aldehydes were detected by the fluorescent probe coupled with high-performance liquid chromatography-mass spectrometry during aerobic oxidation of typical unsaturated lipids. Formation of these aldehyde oxidation products was rationalized through the radical oxidation mechanism. Detection of representative aldehyde products demonstrated the generality in the application of this fluorescent probe for profiling of aldehydes after lipid oxidation.

Lactose azocalixarene drug delivery system for the treatment of multidrug-resistant pseudomonas aeruginosa infected diabetic ulcer

Diabetic wound is one of the most intractable chronic wounds that is prone to bacterial infection. Hypoxia is an important feature in its microenvironment. However, it is challenging for antimicrobial therapy to directly apply the existing hypoxia-responsive drug delivery systems due to the active targeting deficiency and the biofilm obstacle. Herein, we customizes a hypoxia-responsive carrier, lactose-modified azocalix[4]arene (LacAC4A) with the ability to actively target and inhibit biofilm. By loading ciprofloxacin (Cip), the resultant supramolecular nanoformulation Cip@LacAC4A demonstrates enhanced antibacterial efficacy resulting from both the increased drug accumulation and the controlled release at the site of infection. When applied on diabetic wounds together with multidrug-resistant Pseudomonas aeruginosa infection in vivo, Cip@LacAC4A induces definitely less inflammatory infiltration than free Cip, which translates into high wound healing performance. Importantly, such design principle provides a direction for developing antimicrobial drug delivery systems.

Recent Advances in the Enzyme-Activatable Organic Fluorescent Probes for Tumor Imaging and Therapy

The exploration of advanced probes for cancer diagnosis and treatment is of high importance in fundamental research and clinical practice. In comparison with the traditional "always-on" probes, the emerging activatable probes enjoy advantages in promoted accuracy for tumor theranostics by specifically releasing or activating fluorophores at the targeting sites. The main designing principle for these probes is to incorporate responsive groups that can specifically react with the biomarkers (e. g., enzymes) involved in tumorigenesis and progression, realizing the controlled activation in tumors. In this review, we summarize the latest advances in the molecular design and biomedical application of enzyme-responsive organic fluorescent probes. Particularly, the fluorophores can be endowed with ability of generating reactive oxygen species (ROS) to afford the photosensitizers, highlighting the potential of these probes in simultaneous tumor imaging and therapy with rational design. We hope that this review could inspire more research interests in the development of tumor-targeting theranostic probes for advanced biological studies.

Near-Infrared Carbonized Polymer Dots for NIR-II Bioimaging

Carbon dots (CDs) or carbonized polymer dots (CPDs) are an emerging class of optical materials that have exceptional applications in optoelectronic devices, catalysis, detection, and bioimaging. Although cell studies of CPDs have produced impressive results, in vivo imaging requires available CPDs to fluoresce in the near-infrared-II (NIR-II) window (1000-1700 nm). Here, a two-step bottom-up strategy is developed to synthesize NIR-CPDs that provide bright emissions in both NIR-I and NIR-II transparent imaging windows. The designed strategy includes a hydrothermal reaction to form a stable carbon core with aldehyde groups, followed by the Knoevenagel reaction to tether the molecular emission centers. This procedure is labor-saving, cost-efficient, and produces a high yield. The NIR-CPDs enable high-performance NIR-II angiography and real-time imaging of the disease degree of colitis noninvasively. This technology may therefore provide a next-generation synthesis strategy for CPDs with rational molecular engineering that can accurately tune the absorption/emission properties of NIR-emissive CPDs.

Metabolically Specific In Situ Fluorescent Visualization of Bacterial Infection on Wound Tissues

The ability to effectively detect bacterial infection in human tissues is important for the timely treatment of the infection. However, traditional techniques fail to visualize bacterial species adhered to host cells in situ in a target-specific manner. Dihydropteroate synthase (DHPS) exclusively exists in bacterial species and metabolically converts p-aminobenzoic acid (PABA) to folic acid (FA). By targeting this bacterium-specific metabolism, we have developed a fluorescent imaging probe, PABA-DCM, based on the conjugation of PABA with a long-wavelength fluorophore, dicyanomethylene 4H-pyran (DCM). We confirmed that the probe can be used in the synthetic pathway of a broad spectrum of Gram-positive and negative bacteria, resulting in a significantly extended retention time in bacterial over mammalian cells. We validated that DHPS catalytically introduces a dihydropteridine group to the amino end of the PABA motif of PABA-DCM, and the resulting adduct leads to an increase in the FA levels of bacteria. We also constructed a hydrogel dressing containing PABA-DCM and graphene oxide (GO), termed PABA-DCM@GO, that achieves target-specific fluorescence visualization of bacterial infection on the wounded tissues of mice. Our research paves the way for the development of fluorescent imaging agents that target species-conserved metabolic pathways of microorganisms for the in situ monitoring of infections in human tissues.

Cucurbit[7]uril Complexation of Near-Infrared Fluorescent Azobenzene-Cyanine Conjugates

Two new azobenzene heptamethine cyanine conjugates exist as dispersed monomeric molecules in methanol solution and exhibit near-infrared (NIR) cyanine absorption and fluorescence. Both conjugates form non-emissive cyanine H-aggregates in water, but the addition of cucurbit[7]uril (CB7) induces dye deaggregation and a large increase in cyanine NIR fluorescence emission intensity. CB7 encapsulates the protonated azonium tautomer of the 4-(N,N-dimethylamino)azobenzene component of each azobenzene-cyanine conjugate and produces a distinctive new absorption band at 534 nm. The complex is quite hydrophilic, which suggests that CB7 can be used as a supramolecular additive to solubilize this new family of NIR azobenzene-cyanine conjugates for future biomedical applications. Since many azobenzene compounds are themselves potential drug candidates or theranostic agents, it should be possible to formulate many of them as CB7 inclusion complexes with improved solubility, stability, and pharmaceutical profile.

Recent Progresses in Small Molecule Fluorescence and Photoacoustic Dual-modal Probes for the Detection of Bioactive Molecules in Vivo

Intracellular bioactive molecules are essential for the maintenance of homeostasis in living organisms. Abnormal levels of them are closely related to the occurrence and development of some diseases. Hence, the direct and accurate visualization of these bioactive molecules is of vital importance for exploring their pathological roles. However, the low-content, short-lived, and widely distributed properties of bioactive molecules impede the comprehensive analysis of them dramatically. Fluorescent and photoacoustic dual-mode imaging technology provides a new solution to the above issue. Specifically, the combination of fluorescence and photoacoustic, which possesses the advantages of high resolution and in-depth tissue analysis, enables a more in-depth and systematic exploration of the pathogenic mechanisms of bioactive molecules. Moreover, due to the structural tailorability of small molecule probes, numerous small molecule dual-mode probes have been developed to meet the demand for real-time tracking and visualization of bioactive molecules in living cells or in vivo. Hence, in this review, we briefly summarize the key advances in small molecule fluorescence and photoacoustic dual-modal probes within recent years (2015-2021). A particular focus is placed on the design strategies and biological applications of probes for the detection of various bioactive molecules in vivo . Furthermore, the challenges and further prospects in this hot field are highlighted.

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.

Engineering Molecular Probes for In Vivo Near-Infrared Fluorescence/Photoacoustic Duplex Imaging of Human Neutrophil Elastase

Early detection of human neutrophil elastase (HNE), the potential biomarker of lung cancer, is crucial for the accurate diagnosis and evaluation of lung cancer. Currently, little progress of HNE-activated probes has been made for in vivo imaging. Herein, assisted by probe-active pocket match engineering, we synthesized a series of near-infrared fluorescence (NIRF) and photoacoustic (PA) duplex imaging probes by conjugating diverse fluorinated amide chains onto hemi-cyanine. Finally, we identified that probe 2 (denoted as LET-8), with the pentafluoroethyl group, is a superior probe to detect HNE with the best selectivity as well as good response ability and thus successfully realized NIRF/PA duplex imaging of HNE activity both in vitro and in vivo.

A salicylaldehyde benzoyl hydrazone based near-infrared probe for copper(ii) and its bioimaging applications

Developing highly sensitive and selective methods for Cu²⁺ detection in living systems is of great significance in clinical copper-related disease diagnosis. In this work, a near infrared (NIR) fluorescent probe, CySBH, with a salicylaldehyde benzoyl hydrazone group as a selective and sensitive receptor for Cu²⁺ was designed and synthesized. The specific coordination of the salicylaldehyde benzoyl hydrazone group in CySBH with Cu²⁺ can induce a distinct quench of the fluorescence intensity, allowing for real-time tracking of Cu²⁺. We have demonstrated that CySBH could rapidly recognize Cu²⁺ with good selectivity and high sensitivity. Moreover, on the basis of low cell cytotoxicity, the probe was used to visualize Cu²⁺ in two cell lines by fluorescence imaging. Furthermore, CySBH can also be used to monitor Cu²⁺in vivo due to its NIR emission properties. These overall results illustrate that the NIR fluorescent probe CySBH provides a novel approach for the selective and sensitive monitoring of Cu²⁺ in living systems.

A near infrared fluorescent probe utilizing the salicylaldehyde benzoyl hydrazone group as the Cu²⁺ receptor was developed and used to selectively and sensitively monitor Cu²⁺ in living systems.

A pH-activated fluorescent probe via transformation of azo and hydrazone forms for lysosomal pH imaging

A pyridone-based hydrazone probe Sth-NH is established for monitoring the lower lysosomal pH of cancer cells via a turn-on fluorescence mode.

Aggregation-Induced Emission (AIE) Nanoparticles-Assisted NIR-II Fluorescence Imaging-Guided Diagnosis and Surgery for Inflammatory Bowel Disease (IBD)

The incidence of inflammatory bowel diseases (IBD), including Crohn's diseases and ulcerative colitis, is increasing by time and showing a trend of younger age. Precise diagnosis and effective treatments for IBD have attracted growing attention in recent years. However, diagnosing and locating inflammatory lesions remain a great challenge for IBD. In this study, assisted by a kind of aggregation-induced emission (AIE) nanoprobes (BPN-BBTD nanoparticles [NPs]), the second near-infrared (NIR-II) fluorescence imaging is first utilized to accurately trace inflammatory lesions, monitor inflammation severity and detect the response to the drug intervention in IBD mouse models. Through the advantages of high signal-to-background ratio (SBR) and sharp spatial resolution of bio-imaging in NIR-II region, the NIR-II fluorescence imaging-guided surgery can help to achieve a complete resection of severe inflammatory bowls and a secure surgical anastomosis. In addition, with the help of NIR-II fluorescence wide-field microscopy, the distribution of BPN-BBTD NPs can be directly detected in tissue level and found to be mainly accumulated in mucosa and submucosa layers. This study highlights that AIE NPs-assisted NIR-II fluorescence imaging hold a great potential value for future diagnosis and imaging-guided surgery in IBD.

A H2O2-activatable nanoprobe for diagnosing interstitial cystitis and liver ischemia-reperfusion injury via multispectral optoacoustic tomography and NIR-II fluorescent imaging

Developing high-quality NIR-II fluorophores (emission in 1000-1700 nm) for in vivo imaging is of great significance. Benzothiadiazole-core fluorophores are an important class of NIR-II dyes, yet ongoing limitations such as aggregation-caused quenching in aqueous milieu and non-activatable response are still major obstacles for their biological applications. Here, we devise an activatable nanoprobe to address these limitations. A molecular probe named BTPE-NO₂ is synthesized by linking a benzothiadiazole core with two tetraphenylene groups serving as hydrophobic molecular rotors, followed by incorporating two nitrophenyloxoacetamide units at both ends of the core as recognition moieties and fluorescence quenchers. An FDA-approved amphiphilic polymer Pluronic F127 is then employed to encapsulate the molecular BTPE-NO₂ to render the nanoprobe BTPE-NO₂@F127. The pathological levels of H₂O₂ in the disease sites cleave the nitrophenyloxoacetamide groups and activate the probe, thereby generating strong fluorescent emission (950~1200 nm) and ultrasound signal for multi-mode imaging of inflammatory diseases. The nanoprobe can therefore function as a robust tool for detecting and imaging the disease sites with NIR-II fluorescent and multispectral optoacoustic tomography (MSOT) imaging. Moreover, the three-dimensional MSOT images can be obtained for visualizing and locating the disease foci.

An NIR-Fluorophore-Based Theranostic for Selective Initiation of Tumor Pyroptosis-Induced Immunotherapy

Pyroptosis is an inflammatory form of programmed cell death that can effectively eliminate malignant cells and boost anticancer immunity. However, most of the current pyroptosis inducers lack cell selectivity, which may cause severe side effects for cancer therapy. In this work, for the first time, the authors discovered that the commonly used near-infrared (NIR) fluorogenic hemicyanine (CyNH₂ ) induces pyrolysis to kill cancer cells and boost antitumor immunity. Cancer cells overexpressing the NAD(P)H: quinone oxidoreductase isozyme 1 (NQO1)-responsive theranostic (NCyNH₂ ) are designed for selective cell pyroptosis and are nonfluorescent with low toxicity before activation. In the presence of NQO1, the fluorescence of CyNH₂ is restored and can selectively initiate pyroptosis of cancer cells and further lead to systemic antitumor immunity activation for solid tumor therapy. Thus, this fluorogenic NIR dye may represent a novel theranostic agent for the selective initiation of tumor pyroptosis.

Flavylium-Based Hypoxia-Responsive Probe for Cancer Cell Imaging

A hypoxia-responsive probe based on a flavylium dye containing an azo group (AZO-Flav) was synthesized to detect hypoxic conditions via a reductase-catalyzed reaction in cancer cells. In in vitro enzymatic investigation, the azo group of AZO-Flav was reduced by a reductase in the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH) followed by fragmentation to generate a fluorescent molecule, Flav-NH2. The response of AZO-Flav to the reductase was as fast as 2 min with a limit of detection (LOD) of 0.4 μM. Moreover, AZO-Flav displayed high enzyme specificity even in the presence of high concentrations of biological interferences, such as reducing agents and biothiols. Therefore, AZO-Flav was tested to detect hypoxic and normoxic environments in cancer cells (HepG2). Compared to the normal condition, the fluorescence intensity in hypoxic conditions increased about 10-fold after 15 min. Prolonged incubation showed a 26-fold higher fluorescent intensity after 60 min. In addition, the fluorescence signal under hypoxia can be suppressed by an electron transport process inhibitor, diphenyliodonium chloride (DPIC), suggesting that reductases take part in the azo group reduction of AZO-Flav in a hypoxic environment. Therefore, this probe showed great potential application toward in vivo hypoxia detection.

Direct Readout Hypoxia Tumor Suppression In Vivo through NIR-Theranostic Activation

Urgency in finding a suitable therapy in tumor hypoxia strives to develop hypoxia-targeted activatable theranostic. A strategic theranostic prodrug (Azo-M) has been synthesized. Its azo-linker scission under the hypoxia condition has released an near-infrared (NIR)-reporter to determine the extent of chemotherapeutic (melphalan analogue) activation. Under an artificial hypoxia condition, a large shift from 520 to 590 nm in UV absorption was observed in Azo-M. Alongside, the emission maxima had appeared at 625 nm under the said condition. The Azo-M post-incubated HeLa cells have shown upregulation of various apoptotic factors under oxygen deprivation (3%) condition. Azo-M has shown antiproliferative activity under hypoxia conditions in various cancer cells. An ex-vivo biodistribution study indicated that theranostic Azo-M only activated in tumor tissue and to some extent in the liver. The therapeutic activity study in vivo indicated that Azo-M effectively reduced the tumor size and volume (about 2-fold) without the change of bodyweight of mice. The theranostic Azo-M can be a cornerstone to suppress tumor hypoxia and tracking its extent of suppression.

Hemicyanine-Based Near-Infrared Activatable Probes for Imaging and Diagnosis of Diseases

Molecular activatable probes with near-infrared (NIR) fluorescence play a critical role in in vivo imaging of biomarkers for drug screening and disease diagnosis. With structural diversity and high fluorescence quantum yields, hemicyanine dyes have emerged as a versatile scaffold for the construction of activatable optical probes. This Review presents a survey of hemicyanine-based NIR activatable probes (HNAPs) for in vivo imaging and early diagnosis of diseases. The molecular design principles of HNAPs towards activatable optical signaling against various biomarkers are discussed with a focus on their broad applications in the detection of diseases including inflammation, acute organ failure, skin diseases, intestinal diseases, and cancer. This progress not only proves the unique value of HNAPs in preclinical research but also highlights their high translational potential in clinical diagnosis.

Advances in Application of Azobenzene as a Trigger in Biomedicine: Molecular Design and Spontaneous Assembly

Azobenzene is a well-known derivative of stimulus-responsive molecular switches and has shown superior performance as a functional material in biomedical applications. The results of multiple studies have led to the development of light/hypoxia-responsive azobenzene for biomedical use. In recent years, long-wavelength-responsive azobenzene has been developed. Matching the longer wavelength absorption and hypoxia-response characteristics of the azobenzene switch unit to the bio-optical window results in a large and effective stimulus response. In addition, azobenzene has been used as a hypoxia-sensitive connector via biological cleavage under appropriate stimulus conditions. This has resulted in on/off state switching of properties such as pharmacology and fluorescence activity. Herein, recent advances in the design and fabrication of azobenzene as a trigger in biomedicine are summarized.

Trends in small organic fluorescent scaffolds for detection of oxidoreductase

Oxidoreductases are diverse class of enzymes engaged in modulating the redox homeostasis and cellular signaling cascades. Abnormal expression of oxidoreductases including thioredoxin reductase, azoreductase, cytochrome oxidoreductase, tyrosinase and monoamine oxidase leads to the initiation of numerous disorders. Thus, enzymes are the promising biomarkers of the diseased cells and their accurate detection has utmost significance for clinical diagnosis. The detection method must be extremely selective, sensitive easy to use, long self-life, mass manufacturable and disposable. Fluorescence assay approach has been developed potential substitute to conventional techniques used in enzyme's quantification. The fluorescent probes possess excellent stability, high spatiotemporal ratio and reproducibility represent applications in real sample analysis. Therefore, the enzymatic transformations have been monitored by small activatable organic fluorescent probes. These probes are generally integrated with enzyme's substrate/inhibitors to improve their binding affinity toward the enzyme's catalytic site. As the recognition unit bio catalyzed, the signaling unit produces the readout signals and provides novel insights to understand the biochemical reactions for diagnosis and development of point of care devices. Several structural modifications are required in fluorogenic scaffolds to tune the selectivity for a particular enzyme. Hence, the fluorescent probes with their structural features and enzymatic reaction mechanism of oxidoreductase are the key points discussed in this review. The basic strategies to detect each enzyme are discussed. The selectivity, sensitivity and real-time applications are critically compared. The kinetic parameters and futuristic opportunities are present, which would be enormous benefits for chemists and biologists to understand the facts to design and develop unique fluorophore molecules for clinical applications.

Recent progress in fluorescent probes for bacteria

Food fermentation, antibiotics, and pollutant degradation are closely related to bacteria. Bacteria play an irreplaceable role in life. However, some bacteria seriously threaten human health and cause large-scale infectious diseases. Therefore, there is a pressing need to develop strategies to accurately monitor bacteria. Technology based on molecular probes and fluorescence imaging is noninvasive, results in little damage, and has high specificity and sensitivity, so it has been widely applied in the detection of bacteria. In this review, we summarize the recent progress in bacterial detection using fluorescence. In particular, we generalize the mechanisms commonly used to design organic fluorescent probes for detecting and imaging bacteria. Moreover, a perspective regarding fluorescent probes for bacterial detection is discussed.

Highly Sensitive D-A-D-Type Near-Infrared Fluorescent Probe for Nitric Oxide Real-Time Imaging in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a common gastrointestinal inflammatory disease, affecting a huge number of people worldwide with increasing morbidity each year. Although the etiology of IBD has not been fully elucidated, it is understood to be closely related to upregulation of the production of NO. Herein, we first report a donor-acceptor-donor (D-A-D)-type near-infrared (NIR) fluorescent probe LS-NO for real-time detection of NO in IBD by harnessing the enhanced intramolecular charge transfer mechanism. LS-NO exhibited good water solubility, high photostability, and excellent NIR absorbance and emission at 700 and 750/800 nm, respectively. Moreover, it was able to sensitively and specifically detect exogenous and endogenous NO in the lysosomes of living cells. Notably, LS-NO enabled to noninvasively visualize NO generation in a lipopolysaccharide-induced IBD mouse model for 30 h, showing a two- to threefold higher NIR fluorescence intensity in the intestines and feces of IBD mice than normal mice. This work demonstrates that LS-NO is promising as a diagnosis agent for real-time detection of NO in IBD and may promote inflammatory stool examination simultaneously.

Ebselen-Agents for Sensing, Imaging and Labeling: Facile and Full-Featured Application in Biochemical Analysis

Phenyl-1,2-benzoselenazol-3(2H)-one (ebselen) is a classical mimic of glutathione peroxidase (GPx). Thioredoxin interaction endows ebselen attractive biological functions, such as antioxidation and anti-infection, as well as versatile therapeutic usage. Accordingly, application of ebselen analogues in biosensing, chemical labeling, imaging analysis, disease pathology, drug development, clinical treatment, etc. have been widely developed, in which mercaptans, reactive oxygen species, reactive sulfur species, peptides, and proteins were involved. Herein, focusing on the application of ebselen-agents in biochemistry, we have made a systematic summary and comprehensive review. First, we summarized both the classical and the innovative methods for preparing ebselen-agents to present the synthetic strategies. Then we discussed the full functional applicability of ebselen analogues in three fields of biochemical analysis including the fluorescence sensing and bioimaging, derivatization for high throughput fluorescence analysis, and the labeling gents for proteomics. Finally, we discussed the current challenges and perspectives for ebselen-agents as analytical tools in biological research. By presenting the multifunctional applicability of ebselen, we hope this review could appeal researchers to design the ebselen-related biomaterials for biochemical analysis.

Fluorophore-Dapagliflozin Dyad for Detecting Diabetic Liver/Kidney Damages via Fluorescent Imaging and Treating Diabetes via Inhibiting SGLT2

Type II diabetes is a prevalent disease; if left untreated, it could cause serious complications including liver and kidney damages. Hence, early diagnosis for these damages and effective treatment of diabetes are of high importance. Herein, a fluorophore-dapagliflozin dyad (DX-B-DA) has been developed as a theranostic system that can be triggered by intrahepatic/intrarenal reactive oxygen species (ROS) to concomitantly release a near-infrared (NIR) fluorescent dye (DX) and a SGLT2 inhibitor dapagliflozin (DA). In this dyad (DX-B-DA), the NIR fluorophore (DX) and the drug DA were covalently linked through a boronate ester bond which serves as the fluorescence quencher as well as the ROS-responsive moiety that can be cleaved by pathological levels of ROS in diabetics. The in vitro experiments indicate that, in the absence of hydrogen peroxide, the dyad is weakly emissive and keeps its drug moiety in an inactive state, while upon responding to hydrogen peroxide, the dyad simultaneously releases the NIR dye and the drug DA, suggesting that it can serve as an activatable probe for detecting and imaging diabetic liver/kidney damages as well as a prodrug for diabetes treatment upon being triggered by ROS. The dyad was then injected in mouse model of type II diabetes, and it is found that the dyad can not only offer visualized diagnosis for diabetes-induced liver/kidney damages but also exhibit high efficacy in treating type II diabetes and consequently ameliorating diabetic liver/kidney damages.

A near-infrared fluorescent probe for hydrogen polysulfides detection with a large Stokes shift

Due to the crucial biological roles of hydrogen polysulfides (H₂S_n) in living systems, the selective determination of H₂S_n is essential. In this work, we reported a turn-on fluorescent probe, JSI-Sn, for H₂S_n with high sensitivity and good selectivity. Probe JSI-Sn displayed a near-infrared emission (λ_max^Em = 701 nm) and a large Stokes shift (123 nm) in the presence of H₂S_n in solution. Using probe JSI-Sn as an indispensable tool, the monitoring of intracellular H₂S_n in living cells and zebrafish were realized.

A biopolymer-based and inflammation-responsive nanodrug for rheumatoid arthritis treatment via inhibiting JAK-STAT and JNK signalling pathways

Rheumatoid arthritis (RA) is a common chronic autoimmune disease associated with progressive disability, systemic complications, and poor prognosis. The improved understanding of the roles of immune signaling pathway inhibitors has shed light on designing new and more effective approaches for RA treatment. In this work, an inflammation-responsive and molecularly targeted drug system has been developed for RA therapy. The drug carrier was synthesized by covalently grafting hydrophobic cholesterol (Chol) molecules onto a hydrophilic chondroitin sulfate (CS) chain via the inflammation-responsive diselenide bonds (SeSe). The resultant amphiphilic polymer CSSeSeChol readily forms nanoparticles (NPs) and encapsulates two kinase inhibitors tofacitinib and SP600125 in aqueous media. Upon administration into the RA mouse model, the nanodrug accumulates in RA lesions and releases the inhibitors for regulating the JAK-STAT and JNK pathways. As a result, the nanodrug exhibits satisfactory efficacy in RA treatment by suppressing the expression of relevant pro-inflammatory cytokines, blocking the activation of osteoclasts and providing protection for cartilage and joints.

Detecting Basal Myeloperoxidase Activity in Living Systems with a Near-Infrared Emissive "Turn-On" Probe

Detecting myeloperoxidase (MPO) activity in living organisms is important because MPO contributes to the pathogenesis of many diseases such as rheumatoid arthritis and other inflammatory diseases, artherosclerosis, neurodegenerative disease, and some cancers. However, rapid and effective methods for the detection of basal MPO activity in living systems have not yet been reported. Herein, we report a near-infrared (NIR) emissive "turn-on" probe FD-301 that can specifically bind to MPO and accurately measure MPO activity in living cells and in vivo via a rapid response to initial hypochlorous acid (HOCl), produced by MPO. Notably, FD-301 could detect the basal level of MPO activity in human promyelocytic leukemia cells (HL-60) and could discriminate between MPO high-expression and low-expression cells. Furthermore, FD-301 was successfully applied to in vivo imaging of MPO in MPO-dependent diseases, such as arthritis and inflammatory bowel disease.

Activatable Nanocomposite Probe for Preoperative Location and Intraoperative Navigation for Orthotopic Hepatic Tumor Resection via MSOT and Aggregation-Induced Near-IR-I/II Fluorescence Imaging

The precise location of tumor and completeness of surgical resection are critical to successful tumor surgery; thus, the method capable of preoperatively locating a tumor site and intraoperatively determining tumor margins would be highly ideal. Herein, an activatable nanocomposite probe was developed for preoperatively locating orthotopic hepatic tumor via multispectral optoacoustic tomography imaging and for intraoperative navigation via near-IR-1 (NIR-I) and NIR-II fluorescence imaging. The molecular probe comprises an electronic donor, an acceptor, and a recognition moiety and forms the nanocomposite probe with bovine serum albumin. The probe specifically responds to nitroreductase overexpressed in tumor cells, which transforms the aromatic nitro group into an electron-donating amino group and thus activates the probe. The activated probe with the aggregation-induced emission feature generates strong NIR-I/NIR-II fluorescence and optoacoustic signals for dual-mode imaging. Owing to the in situ response toward nitroreductase in tumor cells in the hepatic region, the probe is found capable of detecting early stage orthotopic liver tumors. Furthermore, with the nanocomposite probe, we can obtain the 3D MSOT images to accurately locate orthotopic liver tumors preoperatively and the NIR-I/NIR-II fluorescence images to provide intraoperative guidance for tumor resection surgery.

In Vivo Imaging of Hypoxia Associated with Inflammatory Bowel Disease by a Cytoplasmic Protein-Powered Fluorescence Cascade Amplifier

Accurate and sensitive imaging of hypoxia associated with inflammatory bowel disease (IBD) is significant for the precise diagnosis and treatment of this disease, but it remains a challenge for traditional hypoxia-activatable fluorescence probes because of a more moderate hypoxic state during IBD than under other pathological conditions. To address this issue, herein, we designed a hypoxia-activatable and cytoplasmic protein-powered fluorescence cascade amplifier, named HCFA, to image hypoxia associated with IBD in vivo. In our design, a 4-aminobenzoic acid (azo)-modified mesoporous silica nanoparticle (MSN) was used as a container to load black hole quencher 2 (BHQ2) and cytoplasmic protein-binding squarylium dye (SQ); then, the β-cyclodextrin polymer (β-CDP) combined with azo through a host-guest interaction to form HCFA. Upon passive stagnation in the inflamed tissue of IBD, the azo band would be cleaved under a hypoxic microenvironment, and SQ was released to activate the fluorescence of HCFA. Moreover, the unconstrained SQ can bind with cytoplasmic protein to exhibit drastic fluorescence intensity enhancement, realizing the fluorescence signal amplification for imaging of hypoxia. When one takes advantage of the large load capacity of MSN and the unique property of SQ, HCFA can sense oxygen levels in the range of 0% to 10%. Meanwhile, the fluorescence imaging results demonstrate that HCFA can sensitively distinguish different levels of cellular hypoxia and monitor the variations of hypoxia in vivo, highlighting HCFA as a promising tool for the detection of hypoxia associated with IBD.