Recognition Moieties of Small Molecular Fluorescent Probes for Bioimaging of Enzymes

Monitoring intracellular pH using a hemicyanine-based ratiometric fluorescent probe

Monitoring intracellular pH using ratiometric fluorescent probes can provide further insights into various biological processes including many diseases. Although ratiometric fluorescent probes with dual emission can efficiently exclude interferences (probe concentration, instrumental efficiency, and environmental conditions) compared with traditional off-on fluorescent probes, development of pH-responsive fluorescent probes with dual emission remains relatively unexplored and challenging. Herein we reported a new hemicyanine-based ratiometric fluorescent probe 1 with a hydroxyl group. The probe 1 exhibits dual emission and shows a real-time and selective fluorescence response to micro-environmental pH conditions in a range of 6.0 ∼ 8.0. Further studies revealed that 1 could exclusively enter and accumulate into mitochondria and monitor the pH micro-environmental conditions through fluorescence imaging in HepG2 cells. We suggest that this probe might be used as a probe to elucidate the role of pH in many physiological processes.

Rational Design of Organelle-Targeted Fluorescent Probes: Insights from Artificial Intelligence

Monitoring the physiological changes of organelles is essential for understanding the local biological information of cells and for improving the diagnosis and therapy of diseases. Currently, fluorescent probes are considered as the most powerful tools for imaging and have been widely applied in biomedical fields. However, the expected targeting effects of these probes are often inconsistent with the real experiments. The design of fluorescent probes mainly depends on the empirical knowledge of researchers, which was inhibited by limited chemical space and low efficiency. Herein, we proposed a novel multilevel framework for the prediction of organelle-targeted fluorescent probes by employing advanced artificial intelligence algorithms. In this way, not only the targeting mechanism could be interpreted beyond intuitions but also a quick evaluation method could be established for the rational design. Furthermore, the targeting and imaging powers of the optimized and synthesized probes based on this methodology were verified by quantitative calculation and experiments.

De Novo Design of a Highly Selective Nonpeptide Fluorogenic Probe for Chymotrypsin Activity Sensing in a Living System

Chymotrypsin, an extensively known proteolytic enzyme, plays a substantial role in maintaining physiological functions, including protein digestion, immune response, and tissue repair. To date, intense attention has been focused on the invention of efficient and sensitive chemical tools for chymotrypsin activity measurement. Among them, the "nonpeptide"-based chymotrypsin probe design strategy utilizing the esterase activity of chymotrypsin has been well-developed due to its low cost and high atom-economy feature. However, the ester-bond-based nature of these probes make them possibly vulnerable to esterases and active chemicals. These defects strictly restricted the application of the previously reported probes, especially for imaging in living systems. Therefore, to acquire fluorogenic probes with sufficient stability and specificity for chymotrypsin sensing in a complicated biological environment, a more stable skeleton for nonpeptide-based chymotrypsin probe construction is urgently needed. Herein, a novel nonpeptide-based fluorogenic probe for specific chymotrypsin activity sensing was designed and synthesized by the substitution of an ester-based linker with a heptafluorobutylamide moiety. The acquired probe, named TMBIHF, showed high selectivity toward various enzymes and reactive chemicals, while it retained high sensitivity and catalytic efficiency toward chymotrypsin. Moreover, TMBIHF was successfully applied for monitoring chymotrypsin activity and pancreas development in live zebrafish, specific sensing of exogenous and endogenous chymotrypsin in nude mice, and visualizing chymotrypsin-like activity-dependent cellular apoptosis, thus providing an alternative and reliable way for chymotrypsin-targeted biosensor or prodrug construction.

Monitoring Cell Plasma Membrane Polarity by a NIR Fluorescence Probe with Unexpected Cell Plasma Membrane-Targeting Ability

The cell plasma membrane, the natural barrier of a cell, plays critical roles in a mass of cell physiological and pathological processes. Therefore, revealing and monitoring the local status of the cell plasma membrane are of great significance. Herein, using a near-infrared (NIR) fluorescence probe BTCy, microenvironmental polarity in the cell plasma membrane was in situ monitored. BTCy showed sensitive and selective fluorescence decrease response at 706 nm with the increase of polarity as its polarity-responsive D-π-A structure. Most importantly, BTCy showed unexpected cell plasma membrane-targeting ability, probably due to its amphiphilic structure. With BTCy, the distinguishing imaging of cancer and normal cells was done, in which cancer cells exhibited significantly stronger signals due to their lower cell plasma membrane polarity. In addition, with the imaging of BTCy, the ferroptosis process was revealed with no significant cell plasma membrane polarity variation for the first time. Furthermore, BTCy was employed for in vivo imaging of tumor tissue in the 4T1-tumor-bearing mice. The polarity-responsive and cell plasma membrane-targeting properties of BTCy make it a useful tool for monitoring cell plasma membrane polarity variation, providing an efficient and simple method for tumor diagnosis.

HClO-triggered interventional probe enabled early detection and intervention of atherosclerosis

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of early atherosclerosis (AS). Targeting foam cell formation can be a promising approach for the early detection and prevention of AS. However, only a few studies have actually examined foam cells in vivo, and most methods combined nanotechnology with angiography, which is complex and could cause further damage to the endothelium. Herein, based on methylene blue, a biosafe NIR dye approved by the FDA, an interventional probe (HMB-NA@Mp) triggered by hypochlorous acid (HClO) was designed for imaging foam cells easily, safely, and effectively in the early stage of AS. Here, encapsulation of the probe by foam cells targeted platelet membrane (Mp) increased probe targeting and reduced toxicity. Cell and animal experimental results showed that the probe could accumulate at the lesion site and significantly enhance fluorescence in the early AS model group. Remarkably, at the same time, it could also release the metabolite niacin, which played a role in inhibiting atherosclerosis. Thus, HMB-NA@Mp is expected to be a powerful means for the early detection and timely intervention of early AS in the absence of clinical symptoms.

Bioimaging of Lysosomes with a BODIPY pH-Dependent Fluorescent Probe

Fluorescence-based probes represent a powerful tool for noninvasive imaging of living systems in real time and with a high temporal and spatial resolution. Amongst several known fluorophores, 3-difluoroborodipyrromethene (BODIPY) derivatives have become a cornerstone for innovative fluorescent labelling applications, mainly due to their advantageous features including their facile synthesis, structural versatility and exceptional photophysical properties. In this context, we report a BODIPY-based fluorescent probe for imaging of lysosomes in living cells. The BODIPY derivative displayed a remarkable fluorescence enhancement at low pH values with a pK_a* of 3.1. In vitro studies by confocal microscopy in HeLa cells demonstrated that the compound was able to permeate cell membrane and selectively label lysosome whilst remaining innocuous to the cell culture at the maximum concentration tested. Herein, the BODIPY derivative holds the promise of investigating lysosomal dynamics and function in living cells through fluorescence imaging.

Ratiometric detection and imaging of endogenous alkaline phosphatase activity by fluorescein-coumarin-based fluorescence probe

Alkaline phosphatase (ALP) is a type of enzyme that widely exists in various tissues of the human body; it plays an important role in regulating many cell functions. The development of a sensitive and accurate tool to detect the changes of ALP activity in organisms can contribute to research in the fields of biochemistry, cytology, clinical medicine, etc. In this paper, a small organic molecule-based ratiometric fluorescent probe (FCP) was designed based on the hydroxyl electron-donating group in fluorescein-coumarin protected by the phosphate group. ALP can trigger the fluorescence change through the enzyme-catalyzed cleavage of phosphoryl ester groups, and the ratio of ALP can be measured at wavelengths of 465 nm and 530 nm. The probe had high selectivity and sensitivity to ALP, and the detection limit measured under the optimal conditions in an aqueous medium reached 0.006 mU/mL. The ALP activity of human serum samples was determined using the probe and found to be in good agreement with that measured using commercial ALP kits. Finally, the probe was also successfully applied to image ALP in living hepatocytes with good selectivity and sensitivity.

Persistent and Efficient Multimodal Imaging for Tyrosinase Based on Two-Photon Excited Fluorescent and Room-Temperature Phosphorescent Probes

Tyrosinase is crucial to regulate the metabolism of phenol derivatives, playing an important role in the biosynthesis of melanin pigments, whereas an abnormal level of tyrosinase would lead to severe diseases. It is rather necessary to develop a sensitive and selective imaging tool to assess the level of tyrosinase in vivo. We thoroughly researched the luminous mechanism of the existing TPTYR probe and provided design strategies to improve its two-photon excited fluorescence properties. The designed probes benza2-TPTYR and product benza2-TPTYR-coumarin have large two-photon absorption cross sections at the NIR spectral region (41 GM/706 nm, 71 GM/852 nm), while benza2-TPTYR-coumarin possesses easily distinguishable spectrum in the visible region and a high fluorescence efficiency (Φ_F = 0.27). What is more, novel two-photon excited multimodal imaging based on the pure organic small molecule benza1-TPTYR-coumarin (61 GM/936 nm) is proposed first, simultaneously possessing strong instantaneous fluorescent (563.79 nm) and persistent room-temperature phosphorescent emissions (767.68 nm, 0.54 ms).

Precise Monitoring and Assessing Treatment Response of Sepsis-Induced Acute Lung Hypoxia with a Nitroreductase-Activated Golgi-Targetable Fluorescent Probe

Sepsis-induced acute lung injury (ALI) is mostly attributed to an outbreak of reactive oxygen species (ROS), which makes leukocytes infiltrate into the lung and results in lung hypoxia. Nitroreductase (NTR) is significantly upregulated under hypoxia, which is commonly regarded as a potential biomarker for assessing sepsis-induced acute lung hypoxia. Increasing evidence shows that NTR in the Golgi apparatus could be induced in sepsis-induced ALI. Meanwhile, the prolyl hydroxylase (PHD) inhibitor (dimethyloxalylglycine, DMOG) attenuated sepsis-induced ALI through further increasing the level of Golgi NTR by improving hypoxia inducible factor-1α (HIF-1α) activity, but as yet, no Golgi-targetable probe has been developed for monitoring and assessing treatment response of sepsis-induced ALI. Herein, we report a Golgi-targetable probe, Gol-NTR, for monitoring and assessing treatment response of sepsis-induced ALI through mapping the generation of NTR. The probe displayed high sensitivity with a low detection limit of 54.8 ng/mL and good selectivity to NTR. In addition, due to the excellent characteristics of Golgi-targetable, Gol-NTR was successfully applied in mapping the change of Golgi NTR in cells and zebrafish caused by various stimuli. Most importantly, the production of Golgi NTR in the sepsis-induced ALI and the PHD inhibitor (DMOG) against sepsis-induced ALI were visualized and precisely assessed for the first time with the assistance of Gol-NTR. The results demonstrated the practicability of Gol-NTR for the precise monitoring and assessing of the personalized treatment response of sepsis-induced ALI.

Orthogonally-tunable and ER-targeting fluorophores detect avian influenza virus early infection

Cell-based assays can monitor virus infection at a single-cell level with high sensitivity and cost-efficiency. For this purpose, it is crucial to develop molecular probes that respond selectively to physiological changes in live cells. We report stimuli-responsive light-emitters built on a T-shaped benzimidazole platform, and consecutive borylation reactions to produce a library of homologs displaying systematic changes in fluorescence quantum yield and environmental sensitivity. We find that certain fluorophores localize selectively at the endoplasmic reticulum, and interact with proteins involved in the stress signaling pathways. Notably, the mono-borylated compound responds selectively to the stress conditions by enhancing fluorescence, and detects avian influenza virus infection at the single-cell level. Our findings demonstrate the unprecedented practical utility of the stress-responsive molecular probes to differentiate cellular states for early diagnosis.

Methods to detect and distinguish the early stage of viral infection often involve complicated and time-consuming protocols. Here, the authors disclose a class of fluorescent molecules that enable fast detection of avian influenza virus infection by selectively localizing at the endoplasmic reticulum in the cell.

Monitoring mitochondrial nitroreductase activity in tumors and a hind-limb model of ischemia in mice using a novel activatable NIR fluorescent probe

We report a mitochondria-targeted nitroreductase (NTR)-activated near-infrared fluorescent probe: CS-NO2. Overexpressed NTR in mitochondria was measured with high sensitivity. More importantly, the probe CS-NO2 successfully monitored NTR activity in solid tumors and a hind-limb model of ischemia in mice. This novel finding indicates the promising function of our probe for the diagnosis of solid tumors and hypoxia-associated diseases.

Recent advances in small-molecule fluorescent probes for studying ferroptosis

Ferroptosis is an iron-dependent, non-apoptotic form of programmed cell death driven by excessive lipid peroxidation (LPO). Mounting evidence suggests that the unique modality of cell death is involved in the development and progression of several diseases including cancer, cardiovascular diseases (CVDs), neurodegenerative disorders, etc. However, the pathogenesis and signalling pathways of ferroptosis are not fully understood, possibly due to the lack of robust tools for the highly selective and sensitive imaging of ferroptosis analytes in complex living systems. Up to now, various small-molecule fluorescent probes have been applied as promising chemosensors for studying ferroptosis through tracking the biomolecules or microenvironment-related parameters in vitro and in vivo. In this review, we comprehensively reviewed the recent development of small-molecule fluorescent probes for studying ferroptosis, with a focus on the analytes, design strategies and bioimaging applications. We also provided new insights to overcome the major challenges in this emerging field.

Recent development of small-molecule fluorescent probes based on phenothiazine and its derivates

Fluorescence probes, as analytical tools with the ability to perform rapid and sensitive detection of target analytes, have made outstanding contributions to environmental analysis and bioassays. Considering the expanding developments in these areas, fluorophores play a key role in the de-sign of fluorescence probes. Compared to classical fluorophores, phenothiazines with elec-tron-rich characteristics have been widely applied to construct electron donor-acceptor dyes, which exhibit outstanding performance in both fluorimetric and colorimetric analysis. In addition, these probes also exhibit the pronounced ability in both solution and solid-state, achieving portable detection for environmental analysis. In this review, we summarize recent advances in the performance of phenothiazine-based fluorescent probes for detecting various analytes, especially in cations, anions, ROS/RSS, enzyme and other small molecules. The general design rules, response mechanisms and practical applications of the probes are analyzed, followed by a discussion of exiting challenges and future research perspectives. It is hoped that this review will provide a few strategies for the development of phenothiazine-based fluorescent probes.

Simultaneous Monitoring of HOCl and Viscosity with Drug-Induced Pyroptosis in Live Cells and Acute Lung Injury

Pyroptosis is a newly identified form of cell death that is closely correlated with many diseases. Recent studies have indicated that the inflammation in pyroptosis would accelerate the generation of reactive oxygen species (ROS). In addition, intracellular viscosity is another key microenvironmental parameter that reflects many physiological and pathological states in the early stage, hypochlorous acid (HOCl), as an important ROS, also plays significant roles in a variety of pathologies. However, the fluctuation of viscosity and HOCl in the process of pyroptosis is still unknown. Herein, we present a dual-responsive fluorescent probe (Lyso-VH) for simultaneously detecting viscosity and HOCl. Lyso-VH was successfully used to image the fluctuation of HOCl and viscosity in the lysosome of three kinds of cells with dependent and independent channels. Moreover, Lyso-VH can be employed to investigate the changes of HOCl and viscosity during the process of pyroptosis in living cells and acute lung injury (ALI). Thus, this work can not only serve as a powerful tool to simultaneously visualize the fluctuation of HOCl and viscosity in lysosomes, but also provide a new insight into drug-induced pyroptosis in living cells and acute lung injury.

A ratiometric fluorescence probe for selective and sensitive detection of leucine aminopeptidase in lysosome

We designed a novel ratiometric fluorescent probe P1-Leu with a donor-acceptor-donor fluorophore for the detection of leucine aminopeptidase in lysosomes. P1-Leu exhibits a lower detection limit than the ratiometric donor-π-acceptor probe, due to the low ratiometric background. Besides, P1-Leu has good lysosome-targeting ability and realizes the distinction of LAP levels in different cells.

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.

Endoplasmic Reticulum-Targeting Near-Infrared Fluorescent Probe for CYP2J2 Activity and Its Imaging Application in Endoplasmic Reticulum Stress and Tumor

CYP2J2 as an endoplasmic reticulum (ER)-expressed vital cytochrome P450 isoform participates in the metabolism of endogenous polyunsaturated fatty acids. Its abnormal expression and function are closely related to the progress of cancer and cardiovascular diseases. Herein, an ER-targeting near-infrared (NIR) fluorescent probe ER-BnXPI was developed for monitoring CYP2J2 activity, which possessed a high selectivity and sensitivity toward CYP2J2 among various CYP450 isoforms and exhibited excellent subcellular localization for ER. Then, the CYP2J2 variation behavior under the ER stress model was imaged by ER-BnXPI in living cells and successfully used for the in vivo imaging in different tumors that well distinguished tumor tissues from para-cancerous tissues. All these findings fully demonstrated that ER-BnXPI could be used as a promising tool for exploring the physiological function of CYP2J2 and provided some novel approach for the diagnosis and therapy of CYP2J2-related vascular inflammation and cancer.

In Vivo Monitoring of Hydrogen Polysulfide via a NIR-Excitable Reversible Fluorescent Probe Based on Upconversion Luminescence Resonance Energy Transfer

Hydrogen polysulfide (H₂S_n), derived from hydrogen sulfide (H₂S), has attracted increasing attention, which is suggested to be the actual signal molecule instead of H₂S in physiological and pathological processes. Reversible detection of H₂S_n through a NIR-excitable fluorescence probe is an effective means to understand its functions but is quite challenging. Herein, we reported a NIR-excitable ratiometric nanoprobe for the reversible detection of H₂S_n based on luminescence resonance energy transfer principle with upconversion nanoparticles as the energy donor and an organic molecule, SiR1, as the energy acceptor and reversible recognition unit of H₂S_n. The as-prepared nanoprobe exhibited high selectivity and fast response for the reversible detection of H₂S_n, which can monitor the formation and consumption of endogenous H₂S_n in living cells. Because of the reduced autofluorescence by NIR excitation, it was successfully applied for tracking the fluctuation of H₂S_n concentration of mice in physiological and pathological processes including inflammation and liver injury.

Recent Advances of Fluorescence Probes for Imaging of Ferroptosis Process

Ferroptosis is an iron−dependent form of regulated cell death. It has attracted more and more research interests since it was found because of its potential physiological and pathological roles. In recent years, many efforts have been made for the developments and applications of selective fluorescence probes for real−time and in situ tracking of bioactive species during ferroptosis process, which is necessary and significant to further study the modulation mechanisms and pathological functions of ferroptosis. In this review, we will focus on summarizing the newly developed fluorescence probes that have been applied for ferroptosis imaging in the recent years, and comprehensively discussing their design strategies, including the probes for iron, reactive oxygen species, biothiols and intracellular microenvironmental factors.

Ultrasensitive Fibroblast Activation Protein-α-Activated Fluorogenic Probe Enables Selective Imaging and Killing of Melanoma In Vivo

Melanoma is a malignant cancer with a high risk of metastasis and continued increase in death rates over the past decades, and its prognosis is highly related to the disease's stage, while early detection and treatment of melanoma are significant to the improvement of its therapy outcome. Different from the traditional methods for disease diagnosis, enzyme-activated fluorescent probes were developed rapidly due to their high sensitivity and temporal-spatial ratio and have been widely applied in tumor detection, surgical navigation, and cancer-related research. Fibroblast activation protein-α (FAPα), a serine-type cell surface protease that plays important roles in cell invasion and extracellular matrix degradation, is widely involved in tumor progression such as malignant melanoma, so developing a FAPα activity-based molecular tool would be of great potential for the early diagnosis and therapy of melanoma. However, few fluorescent probes targeting FAPα have been applied in melanoma-related studies, and thus, the construction of FAPα activity-based fluorescent probes for melanoma detection is in urgent need. By incorporating the selective recognition unit with a red-emission fluorophore, cresyl violet, we herein report an ultrasensitive (limit of detection = 5.3 ng/mL) fluorogenic probe for FAPα activity sensing, named CV-FAP; the acquired probe showed a significantly higher binding affinity (15.7-fold) and overall catalytic efficiency (2.6-fold) when compared with those of the best reported FAPα probes. The good performance of CV-FAP made it possible to discriminate malignant melanoma cells and tumor-bearing mice from normal cells and mice with high contrast. More importantly, CV-FAP showed significant antitumor activity toward melanoma in cultured cells and tumor-bearing nude mice (over 95% inhibited tumor growth) with good safety, which made it an ideal theranostic agent for melanoma.

Application of Fluorescent Probes in Reactive Oxygen Species Disease Model

Reactive oxygen species (ROS) play an important role in living activities as signaling molecules that regulate the living activities of organisms. There are many types of ROS, mainly including hydrogen peroxide (H2O2), hypochlorous acid (HOCl), hydroxyl radical (•OH), peroxyl radical (ROO•), singlet oxygen (1O2), peroxynitrite (ONOO-) and superoxide anion radical (O2-•) etc. Existing studies have shown that changes in ROS levels are closely associated with the development of many diseases, such as inflammation, cancer, cardiovascular disease, and neurodegenerative damage. Small molecule fluorescent probes have been widely used in biology, pathology and medical diagnosis due to their advantages of noninvasive, high sensitivity and in vivo real-time detection. It is extremely important to better apply small-molecule fluorescent probes to detect ROS levels in organisms to achieve early diagnosis of diseases and assessment of therapeutic conditions. This work focuses on summarizing the representative applications of some fluorescent probes in ROS disease models in recent years. This article focuses on summarizing the construction methods of various ROS-related disease models, and classifying and analyzing the basic ideas and methods of fluorescent probes applied to disease models according to the characteristics of various diseases.

Insight into the spatial interaction of D-π-A bridge derived cyanines and nitroreductase for fluorescent cancer hypoxia detection

Nitroreductase (NTR) detection in tumor is critical because NTR level is correlated with hypoxia degree and cancer prognosis. With the feature of high sensitivity and selectivity, fluorescence organic probes for NTR detection exhibited a promising future for tumor hypoxia detection. However, the discovery and design of such probes have been impeded due to the lack of the understanding of spatial match and mismatch of these probes with NTR. Here, we have developed two new nitrophenyl-functionalized trimethincyanine (Cy3) probes with para- or meta- positions of nitro-group in phenyl ring. Para-nitrophenyl substituted Cy3 (pNP-Cy3) exhibited a remarkable response to NTR (20-fold fluorescence enhancement) with good selectivity and sensitivity. Experimental and theoretical analysis verified that the substituent position of nitro group on phenyl ring of dyes altered the spatial arrangement of nitro-substituent group, thereby modulated the spatial match and mismatch between Cy3 dyes and binding domain of NTR, and consequently led to a different fluorescent turn-on response. In tumor-bearing mice model, hypoxia status of A549 xenografted tumor of mice was successfully delineated by using pNP-Cy3. These results may provide a clue for designing new cyanine-derived NTR probe to monitor NTR-overexpressed hypoxia cancer cells.

An effective approach to develop targetable and responsive fluorescent probes for imaging of organelles based on cresyl violet scaffold

Fluorescent probes combined with confocal microscopy are recognized as a powerful tool for imaging living cells and even organelles due to their high sensitivity and resolution. However, many of analyte-activatable and organelle-targetable fluorescent probes are developed via tedious attempts, and a relatively predictable method to design such probes is still lacking. Herein, we put forward an effective synthetic strategy to construct both targetable and responsive probes for organelles based on the cresyl violet scaffold. The approach allows access to a variety of organelle-targeting fluorescent probes for an analyte of interest via introducing the corresponding targeting and recognition groups to the 5- and 9-positions of cresyl violet, respectively. The potency of the approach is exemplified by its application to develop four cresyl violet-based fluorophores with different organelle-targeting groups, and a mitochondrion-targeting ratiometric probe capable of imaging Pd⁰ in living cells.

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.

Multifunctional Fluorescent Probe for Simultaneously Detecting Microviscosity, Micropolarity, and Carboxylesterases and Its Application in Bioimaging

Based on OR logic gate, we proposed a smart near-infrared (NIR) fluorescent probe, named VPCPP, for simultaneously monitoring local microviscosity, micropolarity, and carboxylesterases (CEs) in living cells through blue and red channels. This proposed probe was capable of distinguishing cancer cells from normal cells and had good potential for identifying living liver cell lines. Furthermore, the fluctuations of the three analytes of interest in different cell status was successfully explored. Particularly, facilitated with high-content analysis (HCA) and VPCPP, a simple and efficient high-throughput screening (HTS) platform was first constructed for screening antitumor drugs and studying their effect on the analytes. For the first time, we found that sorafenib-induced ferroptosis led to an increase in the microviscosity and up-regulation of CEs at the same time. Additionally, the procedure that aristolochic acid (AA) induced the overexpression of CEs was verified. Besides, VPCPP was utilized for imaging the variations of the two microenvironment parameters and CEs in the inflammation model. Finally, VPCPP was able to image the tumor ex vivo and in vivo through two channels and one channel separately, as well as to visualize the kidneys and liver ex vivo with dual emissions, which indicated that the probe had great potential for imaging applications such as medical diagnosis, preclinical research, and imaging-guided surgery.

Molecular fluorescent probes for liver tumor imaging

Liver cancer is a malignant tumor with both high morbidity and mortality. Traditional treatment method is mainly based on hepatectomy for liver tumor. However, it is difficult to accurately distinguish the tumor tissue and its boundary with the naked eye and palpation, leading to an ambiguous resection result, finally causes high recurrence of liver cancer. Molecular fluorescent probes possess lots of advantages, such as non-invasive, high sensitivity, and real-time imaging have been extensively studied in liver cancer imaging and therapy. In this minireview, we briefly introduce the recent developments of always on and activatable fluorescent probes in the liver cancer image and therapy. Future potential challenges of the fluorescent probes for liver tumor are also discussed. We expect that this minireview would improve the fluorescent probes development for real clinical application of liver cancer disease.

An easily available endoplasmic reticulum targeting near-infrared fluorescent probe for esterase imaging in vitro and in vivo

Here, we report an easily available endoplasmic reticulum-targeting near-infrared fluorescent probe (ER-CE), which can detect esterase in the endoplasmic reticulum and monitor the changes in the esterase amount in tumors in mice in real time. These results indicate that ER-CE is expected to become a powerful analysis tool for the research of endoplasmic reticulum esterase-related diseases.

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.

Polarity-Sensitive Fluorescent Probe for Reflecting the Packing Degree of Bacterial Membrane Lipids

The maintenance of an intact membrane structure is of great importance for bacteria to execute various biological functions. However, chemical probes for monitoring the dynamic changes of bacterial membranes are barely reported. Herein, we, for the first time, report a novel polarity-sensitive probe for reflecting the packing degree of bacterial membrane lipids. Specifically, we synthesize a membrane-targeting fluorescent probe (TICT-lipid) that possesses both twist intramolecular charge transfer and aggregation-induced emission properties. TICT-lipid exhibits sensitive responses to the minute difference in the packing degree of membrane lipids, facilitating rapid differentiation of Gram-negative and Gram-positive bacteria. Interestingly, in the presence of membrane-disrupting antibiotics, the localization of TICT-lipid shifts from the outer membrane to the cell membrane by outputting blue-shifted and enhanced emission, making the mechanism of action of antibiotics clearly visible. TICT-lipid is a polarity-sensitive fluorescent probe, holding great promise in the study of membrane-related bacterial processes and antibiotic screening.

MicroPET imaging of bacterial infection with nitroreductase-specific responsive 18F-labelled nitrogen mustard analogues

PURPOSE

Bacterial infection and antibiotic resistance are serious threats to human health. This study aimed to develop two novel radiotracers, ¹⁸F-NTRP and ¹⁸F-NCRP, that possess a specific nitroreductase (NTR) response to image deep-seated bacterial infections using positron emission tomography (PET). This method can distinguish infection from sterile inflammation.

METHODS

¹⁸F-NTRP and ¹⁸F-NCRP were synthesized via a one-step method; all the steps usually involved in tracer radiosynthesis were successfully adapted in the All-In-One automated module. After the physiochemical properties of ¹⁸F-NTRP and ¹⁸F-NCRP were characterized, their specificity and selectivity for NTR were verified in E. coli and S. aureus. The ex vivo biodistribution of the tracers was evaluated in normal mice. MicroPET-CT imaging was performed in mouse models of bacterial infection and inflammation after the administration of ¹⁸F-NTRP or ¹⁸F-NCRP.

RESULTS

Fully automated radiosynthesis of ¹⁸F-NTRP and ¹⁸F-NCRP was achieved within 90-110 min with overall decay-uncorrected, isolated radiochemical yields of 21.24 ± 4.25% and 11.3 ± 3.78%, respectively. The molar activities of ¹⁸F-NTRP and ¹⁸F-NCRP were 320 ± 40 GBq/μmol and 275 ± 33 GBq/µmol, respectively. In addition, ¹⁸F-NTRP and ¹⁸F-NCRP exhibited high selectivity and specificity for NTR response. PET-CT imaging in bacteria-infected mouse models with ¹⁸F-NTRP or ¹⁸F-NCRP showed significant radioactivity uptake in either E. coli- or S. aureus-infected muscles. The uptake for E. coli-infected muscles, 2.4 ± 0.2%ID/g with ¹⁸F-NTRP and 4.05 ± 0.49%ID/g with ¹⁸F-NCRP, was up to three times greater than that for uninfected control muscles. Furthermore, for both ¹⁸F-NTRP and ¹⁸F-NCRP, the uptake in bacterial infection was 2.6 times higher than that in sterile inflammation, allowing an effective distinction of infection from inflammation.

CONCLUSION

¹⁸F-NTRP and ¹⁸F-NCRP are worth further investigation to verify their potential clinical application for distinguishing bacterial infection from sterile inflammation via their specific NTR responsiveness.

Development of an esterase fluorescent probe based on naphthalimide-benzothiazole conjugation and its applications for qualitative detection of esterase in orlistat-treated biosamples

Esterase is a large hydrolysis family, and widely distributed in many kinds of cells. It is responsible for multiple physiological and pathological functions including metabolism, gene expression. While abnormality of esterase is associated with many pathological activities in obesity, Wolman's disease, and cancer. Thereby, it is essential to design an effective tool for esterase in situ detection in biological systems. Herein, a novel fluorescent probe Y-1 for monitoring esterase in living cells was rationally designed. Probe Y-1 was synthesized by the conjugation between an acetylation of 4-hydroxy naphthalimide and benzothiazole group. Benzothiazole moiety is a typical Excited-state intramolecular proton transfer (ESIPT) controller. Acetate group was selected as the responsive site and ESIPT initiator. As the acetate group could block the ESIPT effect, the probe emits no fluorescence under the excitation of 455 nm. When binding with esterase, Y-1 shows distinct fluorescence with the peak at 560 nm with short time when ESIPT is on. Y-1 displays high sensitivity (LOD is 0.216 × 10^-3 U/mL), fast response (within 5 min), high selectivity and photostability towards esterase. Furthermore, the %RSD (relative standard deviation) of within-day and day-to-day precision was no more than 13.0% and the accuracy ranged from -6.5 to -12.3%. Kinetics performance of Y-1 indicates that esterase has high affinity and hydrolysis to Y-1. For biological applications, our probe is a time-dependent visualizing esterase in living HepG2 and CoLo205 cells within 15 min. After the treatment of orlistat (1 and 5 μM) for inhibiting the activity of esterase, the bright fluorescence has also been detected using our probe. Furthermore, it has been successful in monitoring the esterase in zebrafish, the data were consistent with cellular phenomena. Therefore, all these findings indicate that the robust probe Y-1 is a useful qualitative tool for detecting esterase in biological systems.

A NIR fluorescent probe for Vanin-1 and its applications in imaging, kidney injury diagnosis, and the development of inhibitor

Vanin-1 is an amidohydrolase that catalyses the conversion of pantetheine into the amino-thiol cysteamine and pantothenic acid (coenzyme A precursor), which plays a vital role in multiple physiological and pathological processes. In this study, an enzyme-activated near-infrared (NIR) fluorescent probe (DDAV) has been constructed for sensitively detecting Vanin-1 activity in complicated biosamples on the basis of its catalytic characteristics. DDAV exhibited a high selectivity and sensitivity toward Vanin-1 and was successfully applied to the early diagnosis of kidney injury in cisplatin-induced kidney injury model. In addition, DDAV could serve as a visual tool for in situ imaging endogenous Vanin-1 in vivo. More importantly, Enterococcus faecalis 20247 which possessed high expression of Vanin-1 was screened out from intestinal bacteria using DDAV, provided useful guidance for the rational use of NSAIDs in clinic. Finally, oleuropein as a potent natural inhibitor for Vanin-1 was discovered from herbal medicines library using a high-throughput screening method using DDAV, which held great promise for clinical therapy of inflammatory bowel disease.

An enzyme cascade fluorescence-based assay for the quantification of phenylalanine in serum

An enzyme cascade fluorescence assay for phenylalanine quantification was established by the combination of phenylalanine dehydrogenase and nitroreductase.

Activatable fluorescent probes for in situ imaging of enzymes

As the main biomarkers of most diseases, enzymes play fundamental but extremely critical roles in biosystems. High-resolution studies of enzymes using activatable in situ fluorescence imaging may help to better elucidate their dynamics in living systems. Currently, most activatable probes can realize changeable imaging of enzymes but inevitably tend to diffuse away from the original active site of the enzyme and even translocate out of cells, seriously impairing in situ high-resolution observation of the enzymes. In situ fluorescence imaging of enzymes can be realized by labelling probes or antibodies with always-on signals that fail to enable activatable imaging of enzymes. Thus, fluorescent probes with both "activatable" and "in situ" properties will enable high-resolution studies of enzymes in living systems. In this tutorial review, we summarize the existing methods ranging from design strategies to bioimaging applications that could be used to develop activatable fluorescent probes for in situ imaging of enzymes. It is expected that this tutorial review will promote the new methods generated to design such probes for better deciphering enzymes in complex biosystems and further extend the application of these methods to other fields of enzymes.

An activator-induced quencher-detachment-based turn-on probe with a cationic substrate moiety for acetylcholinesterase

We report a choline ester-grafted turn-on fluorescence probe to detect acetylcholinesterase (AChE) in living cells. The AChE-mediated hydrolysis of the choline ester moiety producing carboxylate initiates the activation of the Cy5 fluorophore quenched by an intramolecular nucleophilic mercapto group. The probe has the advantages of high AChE affinity and low cytotoxicity.