Fluorogenic probes for disease-relevant enzymes

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

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

Review on the recent progress in the development of fluorescent probes targeting enzymes

Enzymes are very important for biological processes in a living being, performing similar or multiple tasks in and out of cells, tissues and other organisms at a particular location. The abnormal activity of particular enzyme usually caused serious diseases such as Alzheimer's disease, Parkinson's disease, cancers, diabetes, cardiovascular diseases, arthritis etc. Hence, nondestructive and real-time visualization for certain enzyme is very important for understanding the biological issues, as well as the drug administration and drug metabolism. Fluorescent cellular probe-based enzyme detectionin vitroandin vivohas become broad interest for human disease diagnostics and therapeutics. This review highlights the recent findings and designs of highly sensitive and selective fluorescent cellular probes targeting enzymes for quantitative analysis and bioimaging.

Rapid Two-Photon Fluorescence Imaging of Monoamine Oxidase B for Diagnosis of Early-Stage Liver Fibrosis in Mice

Liver fibrosis could induce cirrhosis and liver cancer, causing serious damages to liver function and even death. Early diagnosis of fibrosis is extremely requisite for optimizing treatment schedule to improve cure rate. In early-stage fibrosis, overexpressed monoamine oxidase B (MAO-B) can serve as a biomarker, which greatly contributes to the diagnosis of early liver fibrosis. However, there is still a lack of desired strategy to precisely monitor MAO-B in situ. In this work, we established a two-photon fluorescence imaging method for in vivo detection of MAO-B activity counting on a simply prepared probe, BiPhAA. The BiPhAA could be activated by MAO-B within 10 min and fluoresced brightly. To our knowledge, this BiPhAA-based imaging platform for MAO-B is more rapid than other current detection methods. Furthermore, BiPhAA allowed the dynamic observation of endogenous MAO-B level changes in hepatic stellate cells (LX-2). Through two-photon fluorescence imaging, we observed six times higher fluorescence brightness in the liver tissue of fibrosis mice than that of normal mice, thus successfully distinguishing mice with liver fibrosis from normal mice. Our work offers a simple, fast, and highly sensitive approach for imaging MAO-B in situ and paves a way to the diagnosis of early liver fibrosis with accuracy.

A lysosome-targeted probe for the real-time detection of hypobromous acid in living human cancer cells

We reported here a naphthalimide-based fluorescent probe LysOBr that localizes in the lysosome in live cells. LysOBr exhibits excellent HOBr selectivity and desirable optical properties. It can quantitatively detect lysosomal HOBr at 0-20 μM, with a detection limit of 243 nM. The short (4 s) response time allows real-time HOBr detection and imaging, as shown with studies in live HeLa cancer cells. It is thus the most rapidly responsive HOBr probe to date, among the most selective ones, and the first probe that is lysosome-specific with a "turn-on" signal. The probe structure is modular, and convenient structural modification should lead to other organelle-specific probes.

Recent progress in natural product-based inhibitor screening with enzymatic fluorescent probes

Drug discovery is a complex process in which many challenges need to be overcome, from the discovery of a drug candidate to ensuring the efficacy and safety of the candidate in humans. Modern analytical methods allow tens of thousands of drug candidates to be screened for their inhibition of specific enzymes or receptors. In recent years, fluorescent probes have been used for the detection and diagnosis of human pathogens as well as high-throughput screening. This review focuses on recent progress in organic small-molecule based enzyme-activated fluorescent probes for screening of inhibitors from natural products. The contents include the construction of fluorescent probes, working mechanism and the process of inhibitor screening. The progress suggests that fluorescent probes are a vital and rapidly growing technology for inhibitor screening of enzymes, in particular, inhibitor screening in situ.

Precipitated Fluorophore-Based Molecular Probe for In Situ Imaging of Aminopeptidase N in Living Cells and Tumors

Aminopeptidase N (APN) is capable of cleaving N-terminal amino acids from peptides with alanine in the N-terminal position and plays a key role in the growth, migration, and metastasis of cancer. However, reliable in situ information is hard to be obtained with the current APN-responsive molecular probes because the released fluorophores are cytoplasmic soluble and thus rapidly depart from the enzymatic reaction sites and spread out all over the cytoplasm. Here, we report a de novo precipitated fluorophore, HBPQ, which is completely insoluble in water and shows strong yellow solid emission when excited with a 405 nm laser. Owing to the controllable solid fluorescence of HBPQ by the protection-deprotection of phenolic hydroxyl, we further utilized HBPQ to design an APN-responsive fluorogenic probe (HBPQ-A) for the imaging of intracellular APN. Importantly, HBPQ-A can not only perform in situ imaging of APN in different organelles (e.g., lysosomes, mitochondria, endoplasmic reticula, and so forth) but also display a stable and indiffusible fluorescent signal for reliable mapping of the distribution of APN in living cells. In addition, through real-time imaging of APN in 4T1 tumors, we found that the fluorescent signal with high fidelity generated by HBPQ-A could remain constant even after 12 h, which further confirmed its diffusion-resistant ability and long-term reliable imaging ability. We believe that the precipitated fluorophore may have great potential for long-term in situ imaging.

Dual-Colored Fluorescence Imaging of Mitochondrial HNO and Golgi-HNO in Mice with DILI

Drug-induced liver injury (DILI) is the most common reason for the post-marketing withdrawal of drugs. Poor understanding of the mechanisms of DILI presents a large challenge in clinical diagnosis. Previous evidences indicate a potential relationship between reactive nitrogen species (RNS) and DILI. Hence, we developed two specific probes, Golgi-HNO and Mito-HNO, for the multicolored and simultaneous in situ imaging of nitroxyl (HNO) in the Golgi apparatus and mitochondria, respectively. We discovered a significant rise in HNO levels in the livers of mice with DILI, which means that for the first time, we revealed a positive correlation between HNO levels and DILI. Based on changes in the HNO level, we also successfully explored the extent of liver damage induced by an anticarcinogen, bleomycin. In addition, we uncovered catalase was involved in HNO synthesis, which is the unprecedented function of catalase. These findings demonstrate that HNO is an ideal biomarker for DILI diagnosis, and Golgi-HNO and Mito-HNO are ideal fluorescent probes to study in situ HNO changes in various physiological and biochemical processes.

Norcyanine-Carbamates Are Versatile Near-Infrared Fluorogenic Probes

Fluorogenic probes in the near-infrared (NIR) region have the potential to provide stimuli-dependent information in living organisms. Here, we describe a new class of fluorogenic probes based on the heptamethine cyanine scaffold, the most broadly used NIR chromophore. These compounds result from modification of heptamethine norcyanines with stimuli-responsive carbamate linkers. The resulting cyanine carbamates (CyBams) exhibit exceptional turn-ON ratios (∼170×) due to dual requirements for NIR emission: carbamate cleavage through 1,6-elimination and chromophore protonation. Illustrating their utility in complex in vivo settings, a γ-glutamate substituted CyBam was applied to imaging γ-glutamyl transpeptidase (GGT) activity in a metastatic model of ovarian cancer. Overall, CyBams have significant potential to extend the reach of fluorogenic strategies to intact tissue and live animal imaging applications.

Dual-Mechanism Quenched Fluorogenic Probe Provides Selective and Rapid Detection of Cathepsin L Activity*

Cathepsin L (CTL) is a cysteine protease demonstrating upregulated activity in many disease states. Overlapping substrate specificity makes selective detection of CTL activity difficult to parse from that of its close homologue CTV and the ubiquitous CTB. Current probes of CTL activity have limited applications due to either poor contrast or extra assay steps required to achieve selectivity. We have developed a fluorogenic probe, CTLAP, that displays good selectivity for CTL over CTB and CTV while exhibiting low background fluorescence attributed to dual quenching mechanisms. CTLAP achieves optimum CTL selectivity in the first 10 min of incubation, thus suggesting that it is amenable for rapid detection of CTL, even in the presence of competing cathepsins.

Multidimensional Digital Bioassay Platform Based on an Air-Sealed Femtoliter Reactor Array Device

Single-molecule experiments have been helping us to get deeper inside biological phenomena by illuminating how individual molecules actually work. Digital bioassay, in which analyte molecules are individually confined in small compartments to be analyzed, is an emerging technology in single-molecule biology and applies to various biological entities (e.g., cells and virus particles). However, digital bioassay is not compatible with multiconditional and multiparametric assays, hindering in-depth understanding of analytes. This is because current digital bioassay lacks a repeatable solution-exchange system that keeps analytes inside compartments. To address this challenge, we developed a digital bioassay platform with easy solution exchanges, called multidimensional (MD) digital bioassay. We immobilized single analytes in arrayed femtoliter (10^-15 L) reactors and sealed them with airflow. The solution in each reactor was stable and showed no cross-talk via solution leakage for more than 2 h, and over 30 rounds of perfect solution exchanges were successfully performed. With multiconditional assays based on our system, we could quantitatively determine inhibitor sensitivities of single influenza A virus particles and single alkaline phosphatase (ALP) molecules, which has never been achieved with conventional digital bioassays. Further, we demonstrated that ALPs from two origins can be precisely distinguished by a single-molecule multiparametric assay with our system, which was also difficult with conventional digital bioassays. Thus, MD digital bioassay is a versatile platform to gain in-depth insight into biological entities in unprecedented resolution.

Selective sensing of PPi by fluorogenic Al(III)-probe complex in aqueous medium

A newly designed Schiff base probe JN has been synthesized. It is highly selective and sensitive towards Al³⁺ in nearly 100% aqueous medium by exhibiting a dramatic "turn-on" fluorescence response at 495 nm (λ_ex = 450 nm). The sensing mechanism of JN towards Al³⁺ ions was proposed as the combination of PET, ICT, ESIPT, and CHEF processes according to spectra studies and theory calculations. The in situ generated mononuclear Al(III) complex JN-Al³⁺ could sequentially detect PPi ions by turn-off fluorescence response. The selectivity and sensitivity of the JN-Al³⁺ complex towards PPi ions are based on demetalization process. Interestingly, the fact that Al³⁺ can bind with 1, 2, or 3 PPi has been revealed by HRMS study. The probes JN and JN-Al³⁺ complexes were able to capture Al³⁺ and PPi ions, respectively, as demonstrated by fluorescence imaging of the adult zebrafish and onion inner epidermal cells samples.

Revealing the redox status in endoplasmic reticulum by a selenium fluorescence probe

As an important organelle, the endoplasmic reticulum (ER) participates in the synthesis and secretion of various proteins, glycogen, lipids and cholesterol in eukaryotic cells. In this work, an endoplasmic reticulum-targeted reversible fluorescent probe (ER-Se) was designed and synthesized. The probe, based on a selenide group, shows high sensitivity and good selectivity toward HClO (LOD = 0.85 μM). In addition, the probe has reversible capability towards HClO/GSH. Most importantly, co-location experiment results indicated that the probe exhibited a great ability to target the endoplasmic reticulum. Furthermore, the probe was successfully applied to detect exogenous and endogenous HClO in ER and monitored the redox status changes during ER stress.

A 1,8-naphthalimide-based lysosome-targeting dual-analyte fluorescent probe for the detection of pH and palladium in biological samples

Fluorescent probes containing 1,8-naphthalimide dyes have been used to detect biomolecules in the environmental and biological fields. However, most of the probes only exhibit single fluorescent output to one analyte, making them insufficient for detection of more analytes. Herein, we developed a novel 1,8-naphthalimide-based lysosome-targeting dual-analyte sensitive fluorescent probe (DPPP) for the detection of pH and palladium (Pd⁰) using two different emissive channels. The probe showed high selectivity, large Stokes shifts (Δλ ≥ 100 nm) and enhanced response to pH, with blue emission at 485 nm via a morpholine group, and responsive to Pd⁰ concentration, with yellow emission at 545 nm via an allylcarbamate group. The effect of DPPP was successfully observed for sensitive visualizing pH and Pd⁰ concentration in the lysosome of HeLa cells and zebrafish using fluorescence microscopy. This work provides guidance for the design of dual-analyte fluorescent probes.

Recent Progress in Fluorescent Sensors for Drug-Induced Liver Injury Assessment

Drug-induced liver injury (DILI) is a persistent concern in drug discovery and clinical medicine. The current clinical methods to assay DILI by analyzing the enzymes in serum are still not optimal. Recent studies showed that fluorescent sensors would be efficient tools for detecting the concentration and distribution of DILI indicators with high sensitivity and specificity, in real-time, in situ, and with low damage to biosamples, as well as diagnosing DILI. This review focuses on the assessment of DILI, introduces the current mechanisms of DILI, and summarizes the design strategies of fluorescent sensors for DILI indicators, including ions, small molecules, and related enzymes. Some challenges for developing DILI diagnostic fluorescent sensors are put forward. We believe that these design strategies and challenges to evaluate DILI will inspire chemists and give them opportunities to further develop other fluorescent sensors for accurate diagnoses and therapies for other diseases.

An activity-based fluorescent sensor for the detection of the phenol sulfotransferase SULT1A1 in living cells

Human phenol sulfotransferases mediate the transfer of a sulfuryl moiety from the activated sulfate donor PAPS to hydroxy-containing substrates, altering substrate solubility and charge to affect phase II metabolism and cell signaling. Here, we present the development, computational modeling, in vitro enzymology, and biological application of STS-3, an activity-based fluorescent sensor for the SULT1A1 isoform.

Luminescence approaches for the rapid detection of disease-related receptor proteins using transition metal-based probes

Protein biomarkers, particularly abnormally expressed receptor proteins, have been proved to be one of the crucial biomarkers for the rapid assessment, diagnosis, prognosis and prediction of specific human diseases. Transition metal based strategies in particular possess delightful strengths in the in-field and real-time visualization of receptor proteins owing to their unique photophysical properties. In this review, we highlight recent advances in the development of detection methods for receptor protein biomarkers using transition metal based approaches, particularly those employing transition metal complexes. We first discuss the strengths and weaknesses of various strategies used for protein biomarker monitoring in live cells. We then describe the principles of the various sensing platforms and their application for receptor protein detection. Finally, we discuss the challenges and future inspirations in this specific field.

Design, synthesis and evaluation of enzyme-responsive fluorogenic probes based on pyridine-flanked diketopyrrolopyrrole dyes

The ever-growing demand for fluorogenic dyes usable in the rapid construction of analyte-responsive fluorescent probes, has recently contributed to a revival of interest in the chemistry of diketopyrrolopyrrole (DPP) pigments. In this context, we have explored the potential of symmetrical and unsymmetrical DPP derivatives bearing two or one 4-pyridyl substituents acting as optically tunable group(s). The unique fluorogenic behavior of these molecules, closely linked to N-substitution/charge state of their pyridine unit (i.e., neutral pyridine or cationic pyridinium), has been used to design DPP-based fluorescent probes for detection of hypoxia-related redox enzymes and penicillin G acylase (PGA). In this paper, we describe synthesis, spectral characterization and bioanalytical validations of these probes. Dramatic differences in terms of aqueous stability and enzymatic fluorescence activation were observed. This systematic study enables to delineate the scope of application of pyridine-flanked DPP fluorophores in the field of enzyme biosensing.

Visualizing the Underlying Signaling Pathway Related to Nitric Oxide and Glutathione in Cardiovascular Disease Therapy by a Sequentially Activated Fluorescent Probe

Clarifying the signaling pathway associated with nitric oxide (NO) and glutathione (GSH) in cardiovascular disease therapy is important for understanding its physiological and pathological processes but is challenging due to the lack of efficient analytical techniques. Herein, we report a BODIPY-based fluorescent probe for recognition of NO and GSH in sequence with high sensitivity and selectivity. The probe exhibits turn-on fluorescence triggered by NO, followed by red-shifted emission in the presence of GSH. The sequentially activated mechanism allows the visualization of NO-induced GSH upregulation in drug-treated endothelial cells and zebrafish for the first time, revealing a signal pathway during the therapy. We hope that it can be used as a convenient and efficient tool for the study of the interplay between NO and GSH and for the screening of effective drugs for cardiovascular disease therapy.

Advances in Activity-Based Sensing Probes for Isoform-Selective Imaging of Enzymatic Activity

Until recently, there were no generalizable methods for assessing the effects of post-translational regulation on enzymatic activity. Activity-based sensing (ABS) has emerged as a powerful approach for monitoring small-molecule and enzyme activities within living systems. Initial examples of ABS were applied for measuring general enzymatic activity; however, a recent focus has been placed on increasing the selectivity to monitor a single enzyme or isoform. The highest degree of selectivity is required for differentiating between isoforms, where the targets display significant structural similarities as a result of a gene duplication or alternative splicing. This Minireview highlights key examples of small-molecule isoform-selective probes with a focus on the relevance of isoform differentiation, design strategies to achieve selectivity, and applications in basic biology or in the clinic.

A three-channel fluorescent probe to image mitochondrial stress

Dual-recognition probes based on one reacting site inevitably encounter competition problems. Here, NPClA, a two-photon fluorescent probe based on a dual-site response for SO2/HOCl, was developed and applied in imaging mitochondrial stress.

Substituted meso-vinyl-BODIPY as thiol-selective fluorogenic probes for sensing unfolded proteins in the endoplasmic reticulum

A new type of thiol probes based on the meso-vinyl-BODIPY (VB) scaffold were developed. The monochloro-substituted VB1Cl exhibited the largest fluorescence enhancement (>200-fold) as well as high selectivity upon biological thiol sensing. VB1Cl was successfully applied for reporting the protein unfolding process under ER stress in living cells.

Crystal structure and fluorescence study of (μ-N-[(3,5-dimethyl-1H-pyrrol-2-yl)methyl-idene]-N-{4-[(3,5-dimethyl-1H-pyrrol-2-yl)methyl-idene-aza-nium-yl]phen-yl}aza-nium)bis-[di-fluorido-boron(IV)]

The title mol-ecule, C20H20B2F4N4, assumes a planar conformation with all atoms apart from the F atoms lying on the symmetry plane. Each boron atom is four-coordinated by two fluorine atoms, a pyrrole N atom and an imine N atom. Both imine CH=N groups adopt a trans conformation. In the crystal, the mol-ecules self-assemble into a pillar structure through C-H⋯F hydrogen bonds and π-π inter-actions. The UV-vis spectrum and fluorescence spectra of the title compound are also reported.

A novel quinoline-based turn-on fluorescent probe for the highly selective detection of Al (III) and its bioimaging in living cells, plants tissues and zebrafish

A novel quinoline fluorescent probe QNP ((E)-N'-(5-chloro-2-hydroxybenzylidene) quinoline-2-carbohydrazide) for detection of Al3+ ion was designed, synthesized and characterized. QNP displayed a high fluorescence enhancement in the presence of Al3+ ion in DMF:PBS (99:1, v/v) solution and the detection limit was as low as 1.25 μM with high selectivity and excellent sensitivity from 0 to 3 μM. The sensing ability of QNP towards Al3+ ion is attributed to the synergistic effect of PET and ICT. Furthermore, the binding stoichiometry between QNP and Al3+ ion is of 1:1 by Job's plot and mass spectrum, and the calculated binding constant is 4.29 × 108 M-1. The detection of Al3+ ion in water samples illustrates that QNP could be applied to the detection of practical samples in the environment. Bioimaging experiments on Hela cells, zebrafish and soybean root tissues demonstrate that it has potential application to investigate biological processes involving Al3+ ion within living cells. A quinoline-based turn-on fluorescence probe for the detection of Al3+ and its bioimaging in living cells, plant, and zebrafish.

Construction of a red emission fluorescent protein chromophore-based probe for detection of carboxylesterase 1 and carbamate pesticide in culture cells

Designing fluorescent probe for detecting carboxylesterase 1 is remains challenging. Herein, a red emission human carboxylesterase 1 (CES1) probe (CAE-FP) was synthesized based on fluorescent protein chromophore. Probe CAE-FP can specific detect human CES1 with high selectively. The fluorescence quantum yield was calucated as 0.19. The carboxylic acid ester in CAE-FP could be easily hydrolyzed by CES1 under physiological conditions, and this process could induce the obvious fluorescence signal in red emission region. The detection limit of CES1 was calculated as 84.5 ng/mL. Due to the biological detoxification mechanism of carboxylesterase and the obvious inhibitory effect of pesticides on its activity, CAE-FP was applied to detect carbamate pesticide and have achieved good application results. Since fluorescent protein chromophore has excellent biocompatibility, probe CAE-FP with good cell membrane permeable and was successfully applied to monitor the real activities of CES1 in living cells. In summary, this is one of the few reported fluorescent probes that can specific detect the real-time activity of CES1 in biological samples. Besides, we first applied the fluorescent protein chromophore to construct the specific target enzyme probe. This work would contribute to further investigate CES1-associated physiological and pathological processe.

Fluorescent probes based on nucleophilic aromatic substitution reactions for reactive sulfur and selenium species: Recent progress, applications, and design strategies

Reactive sulfur species (RSS) and reactive selenium species (RSeS) are important substances for the maintenance of physiological balance. Imbalance of RSS and RSeS is closely related to a series of human diseases, so it is considered to be an important biomarker in early diagnosis, treatment, and stage monitoring. Fast and accurate quantitative analysis of different RSS and RSeS in complex biological systems may promote the development of personalized diagnosis and treatment in the future. One way to explore the physiological function of various types of RSS and RSeS in vivo is to detect them at the molecular level, and one of the most effective methods for this is to use fluorescent probes. Nucleophilic aromatic substitution (SNAr) reactions are commonly exploited as a detection mechanism for RSS and RSeS in fluorescent probes. In this review, we cover recent progress in fluorescent probes for RSS and RSeS based on SNAr reactions, and discuss their response mechanisms, properties, and applications. Benzenesulfonate, phenyl-O ether, phenyl-S ether, phenyl-Se ether, 7-nitro-2,1,3-benzoxadiazole (NBD), benzoate, and selenium-nitrogen bonds are all good detection groups. Moreover, based on an integration of different reports, we propose the design and synthesis of RSS- and RSeS-selective probes based on SNAr reactions, current challenges, and future research directions, considering the selection of active sites, the effect of substituents on the benzene ring, and the introduction of other functional groups.

Indication of Dynamic Peroxynitrite Fluctuations in the Rat Epilepsy Model with a Near-Infrared Two-Photon Fluorescent Probe

Epilepsy is a chronic neurodegenerative disease that has seriously threatened human health. Accumulating evidence reveals that the pathological progression of epilepsy is closely related to peroxynitrite (ONOO^-). Unfortunately, understanding the physiological roles of ONOO^- in epilepsy is still challenging due to the lack of powerful imaging probes for the determination of the level of fluctuations of ONOO^- in the epileptic brain. Herein, a near-infrared (NIR) two-photon (TP) fluorescent probe [dicyanomethylene-4H-pyran (DCM)-ONOO] is presented to trace ONOO^- in living cells and in kainate (KA)-induced rat epilepsy models with satisfactory sensitivity and selectivity. The probe is composed of a NIR TP DCM fluorophore and a recognition moiety diphenylphosphinamide. The phosphoramide bond of the probe is interrupted after reacting with ONOO^- for 10 min, and then, the released amino groups emit strong fluorescence due to the restoration of the intramolecular charge transfer process. The probe can effectively detect the changes of endogenous ONOO^- with excellent temporal and spatial resolution in living cells and in rat epileptic brain. The imaging results demonstrate that the increasing level of ONOO^- is closely associated with epilepsy and severe neuronal damage in the brain under KA stimulation. In addition, the low-dose resveratrol can effectively inhibit ONOO^- overexpression and further relieve neuronal damage. With the assistance of TP fluorescence imaging in the epileptic brain tissue, we hypothesize that the abnormal levels of ONOO^- may serve as a potential indicator for the diagnosis of epilepsy. The TP fluorescence imaging based on DCM-ONOO provides a great potential approach for understanding the epilepsy pathology and diagnosis.

Fluorescent probes for bioimaging of potential biomarkers in Parkinson's disease

This review comprehensively summarizes various types of fluorescent probes for PD and their applications for detection of various PD biomarkers.

Nitroreductase-responsive polymeric micelles based on 4-nitrobenzyl and AIE moieties for intracellular doxorubicin release

An amphiphilic polymer TNP demonstrated the formation of a nitroreductase-responsive DOX delivery nanoplatform with high sensitivity and selectivity.

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

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

Light-up photoluminescence sensing of a nerve agent simulant by a bis-porphyrin–salen–UO2 complex

A luminescent bis-porphyrin–salen–UO₂ complex, showing a significant fluorescence light-up response upon reacting with DMMP (a simulant of nerve agents), is reported. The fluorescence change of this complex by excitation at 365 nm can be clearly observed with the naked eye, and this complex was successfully employed to construct a test paper to detect nerve agents.

The exposure of a nerve agent simulant to a fluorogenic sensor results in a significant increase in fluorescence response, allowing the construction of a paper test for the naked-eye detection of DMMP.

Discovery of a highly efficient nitroaryl group for detection of nitroreductase and imaging of hypoxic tumor cells

Hypoxia is a pathological hallmark of solid tumors. Detection of hypoxia is therefore of great interest for tumor diagnosis and treatment. As a well-established biomarker of hypoxia, nitroreductase (NTR) has been widely exploited in the development of hypoxia-responsive fluorescent probes on the basis of its enzymatic activity to reduce nitroaryl groups. However, studies on the relationship between the nitroaryl structure and the probe performance for optimal probe design are still rare. Here we report a comparative investigation of nitroaryl groups and identification of the optimal nitroaryl structure for developing new fluorescent probes with extremely high efficiency in the detection of NTR and the imaging of hypoxic tumor cells. Specifically, we synthesized a series of resorufin-based fluorescent probes containing different nitroaryl groups, compared their fluorescence responses to NTR, and identified 2-nitro-N-methyl-imidazolyl as the optimal nitroaryl group that is much more efficient than the most widely used 4-nitrophenyl for NTR detection. The structure-performance relationship was then studied by theoretical molecular docking, revealing the unique features of 2-nitro-N-methyl-imidazolyl in binding and reaction with NTR. We further incorporated the 2-nitro-N-methyl-imidazolyl group into a near-infrared (NIR) hemicyanine fluorophore and developed a NIR fluorescent probe NFP-7 for the detection of NTR and hypoxic tumor cells. NFP-7 exhibits a strong fluorescence increase toward NTR in vitro with an ultrafast (within 40 seconds to fluorescence maximum) and ultrasensitive (0.2 ng mL-1 detection limit) response. NFP-7 has also been demonstrated for imaging the degree of hypoxia in live tumor cells and, more importantly, in a murine tumor model. Our study provides important insights into hypoxia probe development and new tools for hypoxia imaging.

A ratiometric fluorescent probe for monitoring pH fluctuations during autophagy in living cells

We present a ratiometric fluorescent probe for monitoring pH featuring superb photostability and chemostability.

Emerging point-of-care biosensors for rapid diagnosis of COVID-19: current progress, challenges, and future prospects

Coronavirus disease 2019 (COVID-19) pandemic is currently a serious global health threat. While conventional laboratory tests such as quantitative real-time polymerase chain reaction (qPCR), serology tests, and chest computerized tomography (CT) scan allow diagnosis of COVID-19, these tests are time-consuming and laborious, and are limited in resource-limited settings or developing countries. Point-of-care (POC) biosensors such as chip-based and paper-based biosensors are typically rapid, portable, cost-effective, and user-friendly, which can be used for COVID-19 in remote settings. The escalating demand for rapid diagnosis of COVID-19 presents a strong need for a timely and comprehensive review on the POC biosensors for COVID-19 that meet ASSURED criteria: Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment-free, and Deliverable to end users. In the present review, we discuss the importance of rapid and early diagnosis of COVID-19 and pathogenesis of COVID-19 along with the key diagnostic biomarkers. We critically review the most recent advances in POC biosensors which show great promise for the detection of COVID-19 based on three main categories: chip-based biosensors, paper-based biosensors, and other biosensors. We subsequently discuss the key benefits of these biosensors and their use for the detection of antigen, antibody, and viral nucleic acids. The commercial POC biosensors for COVID-19 are critically compared. Finally, we discuss the key challenges and future perspectives of developing emerging POC biosensors for COVID-19. This review would be very useful for guiding strategies for developing and commercializing rapid POC tests to manage the spread of infections.Graphical abstract.

Recent Progress in Small Spirocyclic, Xanthene-Based Fluorescent Probes

The use of fluorescent probes in a multitude of applications is still an expanding field. This review covers the recent progress made in small molecular, spirocyclic xanthene-based probes containing different heteroatoms (e.g., oxygen, silicon, carbon) in position 10'. After a short introduction, we will focus on applications like the interaction of probes with enzymes and targeted labeling of organelles and proteins, detection of small molecules, as well as their use in therapeutics or diagnostics and super-resolution microscopy. Furthermore, the last part will summarize recent advances in the synthesis and understanding of their structure-behavior relationship including novel computational approaches.

DNA Nanomachines for Identifying Cancer Biomarkers in Body Fluids and Cells

Identifying molecular biomarkers promises to significantly improve the accuracy in cancer diagnosis at its early stage. DNA nanomachines, which are designable and switchable nanostructures made of DNA, show broad potential to detect tumor biomarkers with noninvasive, inexpensive, highly sensitive, and highly specific advantages. This Feature summarizes the recent DNA nanomachine-based platforms for the early detection of cancer biomarkers, both from body fluids and in cells.

A reaction-based ratiometric fluorescent probe for mercury ion detection in aqueous solution

Mercury ions are crucially harmful to ecosystem and human being due to their characters of bioaccumulation and difficulty of biochemical degradation. Therefore, development of mercury ion detection methods has attracted increasing interests recently. In this study, we successfully synthesized a hydroxyphenylbenzothiazole (HBT)-based fluorescent probe HBT-Hg in an extremely simple manner for mercuric ions detection. The spectral studies revealed that the probe HBT-Hg could react with Hg²⁺ selectively and sensitively in PBS buffer (10 mM, pH = 7.40), showing ratiometric fluorescent changes from blue to light green. The response mechanism of the probe HBT-Hg and Hg²⁺ was finally confirmed by HPLC analysis, viz., the probe HBT-Hg converted to its precursor compound 1. Finally, the probe HBT-Hg was successfully applied in monitoring Hg²⁺ in living A549 cells.