Specific Fluorescent Probe Based on "Protect-Deprotect" To Visualize the Norepinephrine Signaling Pathway and Drug Intervention Tracers

Fluorescent probes for targeting the Golgi apparatus: design strategies and applications

The Golgi apparatus is an essential organelle constructed by the stacking of flattened vesicles, that is widely distributed in eukaryotic cells and is dynamically regulated during cell cycles. It is a central station which is responsible for collecting, processing, sorting, transporting, and secreting some important proteins/enzymes from the endoplasmic reticulum to intra- and extra-cellular destinations. Golgi-specific fluorescent probes provide powerful non-invasive tools for the real-time and in situ visualization of the temporal and spatial fluctuations of bioactive species. Over recent years, more and more Golgi-targeting probes have been developed, which are essential for the evaluation of diseases including cancer. However, when compared with systems that target other important organelles (e.g. lysosomes and mitochondria), Golgi-targeting strategies are still in their infancy, therefore it is important to develop more Golgi-targeting probes. This review systematically summarizes the currently reported Golgi-specific fluorescent probes, and highlights the design strategies, mechanisms, and biological uses of these probes, we have structured the review based on the different targeting groups. In addition, we highlight the future challenges and opportunities in the development of Golgi-specific imaging agents and therapeutic systems.

A dual-response fluorescent probe for norepinephrine and viscosity and its application in depression research

Depression is a serious mental disease that causes grievous harm to human health and quality of life. The vesicular exocytosis of noradrenaline (NE), rather than its intrinsic intracellular concentration, is more associated with depression. Based on the reports on exocytosis of NE, it is reasonable to assume that the viscosity of cells has an important effect on the release of NE. Herein, a dual-response fluorescent probe (RHO-DCO-NE) for detecting NE and viscosity was designed and synthesized. The probe can simultaneously detect NE concentration and viscosity level with negligible crosstalk between the two channels. We utilized the probe to study the effect of viscosity changes on the NE release of PC12 and the corticosterone-induced PC12 cells. The experiment data revealed that the decrease in viscosity level can accelerate the release of NE of depression cell models. The finding provides new insight into the study of the pathological mechanisms of depression.

Activatable NIR Fluorescence Probe for Epinephrine Detection and Bioimaging Based on Anionic Heptamethine Cyanine

Epinephrine (EP) is an essential catecholamine in the human body. Currently, most EP detection methods are not suitable for in vivo detection due to material limitations. An organic small molecule fluorescent probe based on a chemical cascade reaction for the detection of EP was designed. Anionic heptamethine cyanine dye was selected as a fluorescent dye because of its NIR fluorescence emission with excellent biocompatibility. The secondary amine of EP nucleophilically attacks the carbonate of the probe with its stronger nucleophilicity and further undergoes intramolecular nucleophilic cyclization to release the fluorophore. Other substances containing only primary amines or no β-OH lack reaction competitiveness due to their weaker nucleophilicity or inability to undergo further cyclization. The fluorescence recovery of the probe was linearly related to the EP concentration of 2-75 μmol/L. The detection limit was 0.4 μmol/L. The recovery rate was 94.78-111.32%. Finally, we successfully achieved bioimaging of EP in living cells and EP analogue in nematodes.

A ratiometric fluorescent probe revealing the abnormality of acetylated tau by visualizing polarity in Alzheimer's disease

Abnormal neuronal polarity leads to early deficits in Alzheimer's disease (AD) by affecting the function of axons. Precise and rapid evaluation of polarity changes is very important for the early prevention and diagnosis of AD. However, due to the limitations of existing detection methods, the mechanism related to how neuronal polarity changes in AD is unclear. Herein, we reported a ratiometric fluorescent probe characterized by neutral molecule to disclose the polarity changes in nerve cells and the brain of APP/PS1 mice. Cy7-K showed a sensitive and selective ratiometric fluorescence response to polarity. Remarkably, unlike conventional intramolecular charge transfer fluorescent probes, the fluorescence quantum yield of Cy7-K in highly polar solvents is higher than that in low polar solvents due to the transition of neutral quinones to aromatic zwitterions. Using the ratiometric fluorescence imaging, we found that beta-amyloid protein (Aβ) inhibits the expression of histone deacetylase 6, thereby increasing the amount of acetylated Tau protein (AC-Tau) and ultimately enhancing cell polarity. There was a high correlation between polarity and AC-Tau. Furthermore, Cy7-K penetrated the blood-brain barrier to image the polarity of different brain regions and confirmed that APP/PS1 mice had higher polarity than Wild-type mice. The probe Cy7-K will be a promising tool for assessing the progression of AD development by monitoring polarity.

Accurate detection depression cell model with a dual-locked fluorescence probe in response to noradrenaline and HClO

Due to the serious harm of depression to human health and quality of life, an accurate diagnosis of depression is warranted. For the complex etiology of depression, a single biomarker diagnostic method often leads to misdiagnosis. As noradrenaline and HClO are closely related to depression, a "dual-locked" fluorescence probe R-NE-HClO for diagnosing of depression through the simultaneous detection of noradrenaline and HClO was designed and synthesized. Fluorescence of R-NE-HClO can only be restored in the presence of both noradrenaline and HClO. The probe demonstrates excellent selectivity for noradrenaline and HClO and low cytotoxicity in cell imaging experiments. It is to be observed that we successfully applied the probe to accurately detect depressed cells which provides a possible tool for diagnosing depression.

A Near-Infrared Fluorogenic Probe for Rapid, Specific, and Ultrasensitive Detection of Sphingosine in Living Cells and In Vivo

Sphingosine (Sph) plays important roles in various complex biological processes. Abnormalities in Sph metabolism can result in various diseases, including neurodegenerative disorders. However, due to the lack of rapid and accurate detection methods, understanding sph metabolic in related diseases is limited. Herein, a series of near-infrared fluorogenic probes DMS-X (X = 2F, F, Cl, Br, and I) are designed and synthesized. The fast oxazolidinone ring formation enables the DMS-2F to detect Sph selectively and ultrasensitively, and the detection limit reaches 9.33 ± 0.41 nm. Moreover, it is demonstrated that DMS-2F exhibited a dose- and time-dependent response to Sph and can detect sph in living cells. Importantly, for the first time, the changes in Sph levels induced by Aβ42 oligomers and H2 O2 are assessed through a fluorescent imaging approach, and further validated the physiological processes by which Aβ42 oligomers and reactive oxygen species (ROS)-induce changes in intracellular Sph levels. Additionally, the distribution of Sph in living zebrafish is successfully mapped by in vivo imaging of a zebrafish model. This work provides a simple and efficient method for probing Sph in living cells and in vivo, which will facilitate investigation into the metabolic process of Sph and the connection between Sph and disease pathologies.

Synthesis of a Near-Infrared Fluorescent Probe for Imaging Catecholamines via a Tandem Nucleophilic Aromatic Substitution

A near-infrared (NIR) fluorescent probe NS667 was developed using a novel synthetic strategy by integrating an electron-rich 1,2,3,4-tetrahydroquinoxaline (THQ) into the scaffold from NS510, which binds to catecholamines with high affinity. The fluorophore core was constructed with a tandem nucleophilic aromatic substitution. Upon binding to catecholamines, the fluorescence of this probe shifted, with the emission in the NIR region. Live cell imaging results demonstrate that NS667 can effectively image norepinephrine in chromaffin cells with shifted fluorescence, which highlights the potential of the probe for neuroimaging in tissues.

Sub-100 ms Level Ultrafast Detection and Near-Infrared Ratiometric Fluorescence Imaging of Norepinephrine in Live Neurons and Brains

Norepinephrine (NE) is a key neurotransmitter in the central and sympathetic nervous systems, whose content fluctuates dynamically and rapidly in various brain regions during different physiological and pathophysiological processes. However, it remains a great challenge to directly visualize and precisely quantify the transient NE dynamics in living systems with high accuracy, specificity, sensitivity, and, in particular, high temporal resolution. Herein, we developed a series of small-molecular probes that can specifically detect NE through a sequential nucleophilic substitution-cyclization reaction, accompanied by a ratiometric near-infrared fluorescence response, within an impressively short time down to 60 ms, which is 3 orders of magnitude faster than that of present small-molecular probes. A unique water-promoted intermolecular proton transfer mechanism is disclosed, which dramatically boosted the recognition kinetics by ∼680 times. Benefiting from these excellent features, we quantitatively imaged the transient endogenous NE dynamics under external stimuli at the single living neuron level and further revealed the close correlations between NE fluctuations and Parkinson's disease pathology at the level of acute brain slices and live mouse brains in vivo.

Progress in the application of molecular imaging in psychiatric disorders

Psychiatric disorders have always attracted a lot of attention from researchers due to the difficulties in their diagnoses and treatments. Molecular imaging, as an emerging technology, has played an important role in the researchers of various diseases. In recent years, molecular imaging techniques including magnetic resonance spectroscopy, nuclear medicine imaging, and fluorescence imaging have been widely used in the study of psychiatric disorders. This review will briefly summarize the progression of molecular imaging in psychiatric disorders.

A dual-channel fluorescent nanoprobe for accurate cancer diagnosis by sequential detection of adenosine triphosphate and sulfur dioxide

Cancer is one of the major diseases that seriously endanger the health of all mankind. Accurate diagnosis of early cancer is the most promising way to reduce cancer harm and improve patient survival. However, many developed fluorescent probes for cancer imaging only have the function of identifying one marker, which cannot meet the needs of accurate diagnosis. Here, a fluorescent nanoprobe (CPH@ZIF-90) utilizing ZIF-90 to encapsulate SO₂-sensitive dye (CPH) is synthesized for the sequential detection of ATP and SO₂. The nanoprobe first interacts with ATP to release CPH, thus increasing the fluorescence at 685 nm and realizing the near-infrared (NIR) fluorescence detection of ATP. Then, SO₂ acts on the released CPH through nucleophilic addition, affecting the π-conjugated structure of CPH and resulting in enhanced fluorescence at 580 nm. CPH@ZIF-90 exhibits satisfactory sensitivity and selectivity for sequential detection of ATP and SO₂. Excitedly, CPH@ZIF-90 can sequentially image the endogenous ATP and SO₂ in cells, showing sensitive fluorescence changes in dual channels (red and green). Due to the NIR emission properties of CPH@ZIF-90 and its ability to enrich in tumor, it is applied to monitor ATP and SO₂ in mice and distinguish normal mice from tumor mice. The ability of CPH@ZIF-90 to sequentially detect two cancer-related biomarkers makes it provide meaningful assistance in accurate early diagnosis of cancer.

Activity-based bioluminescence probes for in vivo sensing applications

Bioluminescence imaging is a highly sensitive technique commonly used for various in vivo applications. Recent efforts to expand the utility of this modality have led to the development of a suite of activity-based sensing (ABS) probes for bioluminescence imaging by 'caging' of luciferin and its structural analogs. The ability to selectively detect a given biomarker has presented researchers with many exciting opportunities to study both health and disease states in animal models. Here, we highlight recent (2021-2023) bioluminescence-based ABS probes with an emphasis on probe design and in vivo validation experiments.

Reversible colorimetric and NIR fluorescent probe for sensing SO2/H2O2 in living cells and food samples

Preservative sulfur dioxide (SO₂) and bleach hydrogen peroxide (H₂O₂) were widely used in the food industry, at the same time, they were also a redox pair in biological systems. Therefore, the reversible sensing SO₂/H₂O₂ was of great significance in food safety and biology. In this paper, a colorimetric and NIR fluorescent dual channels response probe (DCA-Bba) for SO₂/H₂O₂ based on chromene-barbiturate was developed. DCA-Bba exhibited a rapid and sensitive recognition of SO₂, and the adduct DCA-Bba-HSO₃^- could detect H₂O₂ in PBS (with 10 % DMSO, v/v, pH 7.4) solution. The reversible response of DCA-Bba was implemented by HSO₃^- involved 1,4-addition and H₂O₂ induced elimination reaction. DCA-Bba showed a strong red fluorescence based on the intramolecular charge transfer (ICT) process, after the recognition of SO₂, the fluorescence of the adduct was quenched based on the photoinduced electron transfer (PET) process. And importantly, DCA-Bba had been applied for imaging SO₂/H₂O₂ redox cycles in living cells, as well as could detect the levels of SO₂ in white sugar, biscuit, Chinese liquor and red wine samples.

Two-photon fluorescence imaging and specifically biosensing of norepinephrine on a 100-ms timescale

Norepinephrine (NE) is a key neurotransmitter in the central nervous system of organisms; however, specifically tracking the transient NE dynamics with high spatiotemporal resolution in living systems remains a great challenge. Herein, we develop a small molecular fluorescent probe that can precisely anchor on neuronal cytomembranes and specifically respond to NE on a 100-ms timescale. A unique dual acceleration mechanism of molecular-folding and water-bridging is disclosed, which boosts the reaction kinetics by ˃10⁵ and ˃10³ times, respectively. Benefiting from its excellent spatiotemporal resolution, the probe is applied to monitor NE dynamics at the single-neuron level, thereby, successfully snapshotting the fast fluctuation of NE levels at neuronal cytomembranes within 2 s. Moreover, two-photon fluorescence imaging of acute brain tissue slices reveals a close correlation between downregulated NE levels and Alzheimer's disease pathology as well as antioxidant therapy.

Intracerebral Fluorescence-Photoacoustic Dual-Mode Imaging for Precise Diagnosis and Drug Intervention Tracing in Depression

Unravelling the pathophysiology of depression is a unique challenge. Depression is closely associated with reduced norepinephrine (NE) levels; therefore, developing bioimaging probes to visualize NE levels in the brain is a key to elucidating the pathophysiological process of depression. However, because NE is similar in structure and chemical properties to two other catecholamine neurotransmitters, epinephrine and dopamine, designing an NE-specific multimodal bioimaging probe is a difficult task. In this work, we designed and synthesized the first near-infrared fluorescent-photoacoustic (PA) dual-modality imaging probe for NE (FPNE). The β-hydroxyethylamine of NE was shown to react via nucleophilic substitution and intramolecular nucleophilic cyclization, resulting in the cleavage of a carbonic ester bond in the probe molecule and release of a merocyanine molecule (IR-720). This process changed the color of the reaction solution from blue-purple to green, and the absorption peak was red-shifted from 585 to 720 nm. Under light excitation at 720 nm, linear relationships between the concentration of NE and both the PA response and the fluorescence signal intensity were observed. Thus, the use of intracerebral in situ visualization for diagnosis of depression and monitoring of drug interventions was achieved in a mouse model by fluorescence and PA imaging of brain regions after administration of FPNE by tail-vein injection.

Early diagnosis of heterotopic ossification with a NIR fluorescent probe by targeting type II collagen

Heterotopic ossification (HO) is a devastating sequela in which the pathologic extracellular matrix of the cartilage and bone forms abnormally in soft tissues following traumatic injuries or orthopaedic surgeries. Early treatment is essential for inhibiting the progression of HO but is currently limited by the absence of sensitive and specific early diagnosis. Herein, this study exploits the enrichment of type II collagen (Col2a1) in the HO cartilage formation stage towards developing a near-infrared (NIR) probe for early HO diagnosis, namely WL-808 by conjugating a Col2a1-binding peptide (WYRGRL) and a cyanine dye (IR-808). WL-808 exhibits high specificity for targeting the cartilage in vitro and in vivo with no apparent cytotoxicity. The NIR signal of WL-808 can be detected in the HO cartilage lesions as early as 1 week after injury when micro-CT cannot show any ectopic bone formation. Moreover, the probe is rarely distributed in the normal knee articular cartilage in vivo via the intravenous administration method. Taken together, WL-808 demonstrates great potential in early HO diagnosis under NIR imaging, facilitating early HO prophylaxis and treatment in the clinic.

Fast-Specific Fluorescent Probes to Visualize Norepinephrine Signaling Pathways and Its Flux in the Epileptic Mice Brain

Norepinephrine (NE) is synthesized in the locus coeruleus and widely projected throughout the brain and spinal cord. It regulates various actions and consciousness linked to a variety of neurological diseases. A "hunting-shooting" strategy was proposed in this work to improve the specificity and response rate of an NE fluorescent probe: 2-(cyclohex-2-en-1-ylidene)malononitrile derivatives were chosen as a fluorophore. To create a dual-site probe, an aldehyde group was added to the ortho of the ester group (or benzene sulfonate). Because of its excellent electrophilic activity, the aldehyde group could rapidly "hunt" the amino group and then form an intramolecular five-membered ring via the nucleophilic reaction with the β-hydroxyl group. The -NH- in the five-membered ring "shoots" the adjacent ester group, releasing the fluorophore and allowing for rapid and specific NE detection. The NE release and reuptake ″emetic″-″swallow″ transient process is captured and visualized under the action of the primary NE receptor drug. Furthermore, by introducing halogen into the fluorophore to lengthen the absorption wavelength, improve lipid solubility, and adjust the pK_a appropriately, the probe successfully penetrated the blood-brain barrier (BBB). In situ synchronous probe imaging was used to detect the NE level in the brains of epileptic and normal mice, and abnormal expression of NE in the brain was discovered during epilepsy. Brain anatomy was used to examine the distribution and level changes of NE in various brain regions before and after epilepsy. This research provides useful tools and a theoretical foundation for diagnosing and treating central nervous system diseases early.

High-performance fluorescence probe for fast and specific visualization of norepinephrine in vivo and depression-like mice

Norepinephrine (NE), as an important neurotransmitter, is closely associated with the pathogenesis of anxiety and depressive disorders. Effective monitoring of NE fluctuation aids in the diagnosis of depression and the therapeutic assessment of the antidepressant intervention. The construction of novel fluorescent probes with high specificity towards NE for imaging in depression models is still in demand urgently. In this work, a novel resorufin-based red-emitting fluorescent probe for real-time tracking NE was developed. NE can significantly increase the fluorescence of probe LNE by triggering deprotection of carbonothioate ligand via nucleophilic substitution. The probe LNE demonstrated significant NE selectivity and sensitivity over other analytes in vitro. In addition, probe LNE showed a fast response time (<10 min), and the change in fluorescence signal was positively linked with NE concentration, which could be utilized to track the dysregulation of NE in vivo. More importantly, this powerful probe was successfully employed for real-time visual and imaging of NE in living cells and depression-like behavior animals.

Real-Time Monitoring of Neurotransmitters in the Brain of Living Animals

Neurotransmitters, as important chemical small molecules, perform the function of neural signal transmission from cell to cell. Excess concentrations of neurotransmitters are often closely associated with brain diseases, such as Alzheimer's disease, depression, schizophrenia, and Parkinson's disease. On the other hand, the release of neurotransmitters under the induced stimulation indicates the occurrence of reward-related behaviors, including food and drug addiction. Therefore, to understand the physiological and pathological functions of neurotransmitters, especially in complex environments of the living brain, it is urgent to develop effective tools to monitor their dynamics with high sensitivity and specificity. Over the past 30 years, significant advances in electrochemical sensors and optical probes have brought new possibilities for studying neurons and neural circuits by monitoring the changes in neurotransmitters. This Review focuses on the progress in the construction of sensors for in vivo analysis of neurotransmitters in the brain and summarizes current attempts to address key issues in the development of sensors with high selectivity, sensitivity, and stability. Combined with the latest advances in technologies and methods, several strategies for sensor construction are provided for recording chemical signal changes in the complex environment of the brain.

Simultaneous Sensing of H2S and ATP with a Two-Photon Fluorescent Probe in Alzheimer's Disease: toward Understanding Why H2S Regulates Glutamate-Induced ATP Dysregulation

Energy deprivation and reduced levels of hydrogen sulfide (H₂S) in the brain is closely associated with Alzheimer's disease (AD). However, there is currently no fluorescent probe for precise exploration of both H₂S and adenosine triphosphate (ATP) to directly demonstrate their relationship and their dynamic pattern changes. Herein, we developed a two-photon fluorescent probe, named AD-3, to simultaneously image endogenous H₂S and ATP from two emission channels of fluorescent signals in live rat brains with AD. The probe achieved excellent selectivity and good detection linearity for H₂S in the 0-100 μM concentration range and ATP in the 2-5 mM concentration range, respectively, with a detection limit of 0.19 μM for H₂S and 0.01 mM for ATP. Fluorescence imaging in live cells reveals that such probe could successfully apply for simultaneous imaging and accurate quantification of H₂S and ATP in neuronal cells. Further using real-time quantitative polymerase chain reaction and Western blots, we confirmed that H₂S regulates ATP synthesis by acting on cytochrome C, cytochrome oxidase subunit 3 of complex IV, and protein 6 of complex I in the mitochondrial respiratory chain. Subsequently, we constructed a high-throughput screening platform based on AD-3 probe to rapidly screen the potential anti-AD drugs to control glutamate-stimulated oxidative stress associated with abnormal H₂S and ATP levels. Significantly, AD-3 probe was found capable of imaging of H₂S and ATP in APP/PS1 mice, and the concentration of H₂S and ATP in the AD mouse brain was found to be lower than that in wild-type mice.

A ratiometric ESIPT fluorescent probe for detection of anticancer-associated H2O2 level in vitro and in vivo

ROS is a significant factor in the cancer treatment mechanism. The monitoring anticancer-associated H₂O₂ level plays a vital role in the anticancer mechanistic exploration in pathology and physiology. Herein we synthesized a ratiometric fluorescent probe (HBQ-L) to detect and image H₂O₂ based on excited-state intramolecular proton transfer. HBQ-L had a high sensitivity (231-fold) with a low detection limit (28.5 nM) for monitoring H₂O₂ in solution. HBQ-L showed good mitochondrial-targeting and successfully detected both exo-/endogenous H₂O₂ in A549 cells. Furthermore, HBQ-L was used to ratiometric monitor H₂O₂ level in anticancer reagent DOX-treated cells or zebrafish. Importantly, it was employed to access the monitoring H₂O₂ in the A549 tumor-bearing mice.

An Activatable 19 F MRI Molecular Probe for Sensing and Imaging of Norepinephrine

Norepinephrine (NE), acting as both a neurotransmitter and hormone, plays a significant role in regulating the action of the brain and body. Many studies have demonstrated a strong correlation between mental disorders and aberrant NE levels. Therefore, it is of urgent demand to develop in vivo analytical methods of NE for diagnostic assessment and mechanistic investigations of mental diseases. Herein, we report a ¹⁹F MRI probe (NRFP) for sensing and imaging NE, which is constructed by conjugating a gadolinium chelate to a fluorine‐containing moiety through a NE‐responsive aromatic thiocarbonate linkage. The capacity and specificity of NRFP for detecting NE is validated with in vitro detecting/imaging experiments. Furthermore, the feasibility of NRFP for visualizing NE in animals is illustrated by ex vivo and in vivo imaging experiments, demonstrating the promising potential of NRFP for selective detection and specific imaging of NE in deep tissues of living subjects.

A Portable Sensing Platform Using an Upconversion-Based Nanosensor for Visual Quantitative Monitoring of Mesna

Mesna is an important regional antidote for protecting the urinary system of chemotherapy patients, which requires monitoring its level in real time to ensure the curative effect. The fluorescence method is a powerful tool in real-time detection with the advantages of fast response and visualization. However, the background interference limits its application in biological sensing. Here, we developed a portable sensing platform using an upconversion-based nanosensor for visual quantitative monitoring of mesna in real-time/on-site conditions. The nanosensor was constructed by upconversion nanoparticles (UCNPs) and ethyl violet (EV), in which the UCNPs emitted red and green light, while EV quenched the green light due to the inner filter effect (IFE). The reaction of mesna with EV caused its fading and broke the IFE process, leading to the recovery of green light. By the fluorescence and colorimetric chromaticity variations, the nanosensor achieved a dual-readout detection for mesna with low limits of detection (LODs) of 26 and 48 nM, respectively. Furthermore, a highly compatible sensing platform was fabricated for facile determination of mesna with an LOD of 56 nM, realizing visual quantitative monitoring of the mesna level to ensure the curative effect and providing a new strategy for point-of-care testing of drugs in clinical settings.

A Liquid Interfacial SERS Platform on a Nanoparticle Array Stabilized by Rigid Probes for the Quantification of Norepinephrine in Rat Brain Microdialysates

For the reliable determination of trace chemicals in the brain, we created a SERS platform based on a functionalized AuNPs array formed at a liquid/liquid interface in a uniform fashion over a large substrate area through ternary regulations for real-time quantification of trace norepinephrine (NE). The rigid molecule, 4-(thiophen-3-ylethynyl)-benzaldehyde (RP1) was designed and co-assembled at AuNPs with 4-mercaptophenylboronic acid (MPBA) to chemically define NE via dual recognition. Meanwhile, the rigid structure assembly of RP1 and MPBA efficiently fixed the interparticle gap, guaranteeing reproducible SERS analysis. Furthermore, the Raman peak of C≡C group in the silent region was taken as a response element to further improve the accuracy. Combined with microdialysis, this SERS platform was developed for in-the-field testing of NE in rat brain microdialysates following anxiety.

Fluorescent probes for the detection of disease-associated biomarkers

Fluorescent probes have emerged as indispensable chemical tools to the field of chemical biology and medicine. The ability to detect intracellular species and monitor physiological processes has not only advanced our knowledge in biology but has provided new approaches towards disease diagnosis. In this review, we detail the design criteria and strategies for some recently reported fluorescent probes that can detect a wide range of biologically important species in cells and in vivo. In doing so, we highlight the importance of each biological species and their role in biological systems and for disease progression. We then discuss the current problems and challenges of existing technologies and provide our perspective on the future directions of the research area. Overall, we hope this review will provide inspiration for researchers and prove as useful guide for the development of the next generation of fluorescent probes.

Mitochondria-Targeted Fluorescent Turn-On Probe for Rapid Detection of Bisulfite/Sulfite in Water and Food Samples

Bisulfite (HSO₃^-)/Sulfite (SO₃^2-) is widely used as a food additive, but excessive use often leads to serious consequences, so the detection of HSO₃^-/SO₃^2- is of great importance. In this paper, a novel 1,4-diethylpiperazine-modified coumarin-benzopyran derivative (probe QLP) has been synthesized and characterized. In PBS (10 mM, pH = 7.4), QLP displays good selectivity and is sensitive for HSO₃^-/SO₃^2- over various analytes with fluorescent "OFF-ON" rapid responding (2 min), long-wavelength emission (600 nm), and a detection limit of 177 nM. With the treatment of HSO₃^-/SO₃^2-, the color of the QLP solution obviously changes from blue-green to yellow, and the fluorescent color of QLP changes from colorless to amaranth. The fluorescence-enhanced mechanism is qualitatively evaluated by density functional theory calculations using the CAM-B3LYP/6-31G (d) method, which reveals that the photoinduced electron transfer leads to the fluorescence emission of the QLP-SO₃H adduct. Importantly, nontoxic QLP can be used to detect HSO₃^-/SO₃^2- in sugar, natural water samples, and living cells and localized to the mitochondria and monitor the mitochondrial HSO₃^-/SO₃^2- level.

A Bioluminescent Probe for Detecting Norepinephrine in Vivo

As a neurotransmitter, norepinephrine (NE) is critical for psychiatric conditions, neurodegenerative diseases, and pheochromocytoma. A real-time and noninvasive method for the detection of NE as a tracer to investigate the NE-relevant disease treatment process is urgently desirable. Herein, we successfully developed a turn-on NE bioluminescent probe (NBP), which was grounded on p-toluenethiol deprotectrf by nucleophilic substitution. Compared with other analytes, the NBP exhibited high sensitivity and selectivity in vitro. More importantly, the NBP provides a promising strategy for in vivo imaging of NE in living animals with noninvasive visualization and real-time features.

A NIR fluorescent probe tracing norepinephrine exocytosis and depression occurrence at the cellular level

A NIR fluorescent probe tracing norepinephrine exocytosis and depression occurrence at the cellular level revealed that norepinephrine exocytosis rather than the inherent intracellular concentration was related with depression.

An activatable and tumor-targeting NIR fluorescent probe for imaging of histone deacetylase 6 in cancer cells and in vivo

An activatable and tumor-targeting near-infrared (NIR) fluorescent probe CyAc-RGD was synthesized for the imaging of histone deacetylase 6 (HDAC6). The probe exhibited higher sensitivity and specificity for HDAC6 detection in cancer cells. Moreover, CyAc-RGD demonstrated good tumor-targeting ability and realized HDAC6 imaging in vivo.

3D two-photon brain imaging reveals dihydroartemisinin exerts antiepileptic effects by modulating iron homeostasis

Imbalanced iron homeostasis plays a crucial role in neurological diseases, yet direct imaging evidence revealing the distribution of active ferrous iron (Fe²⁺) in the living brain remains scarce. Here, we present a near-infrared excited two-photon fluorescent probe (FeP) for imaging changes of Fe²⁺ flux in the living epileptic mouse brain. In vivo 3D two-photon brain imaging with FeP directly revealed abnormal elevation of Fe²⁺ in the epileptic mouse brain. Moreover, we found that dihydroartemisinin (DHA), a lead compound discovered through probe-based high-throughput screening, plays a critical role in modulating iron homeostasis. In addition, we revealed that DHA might exert its antiepileptic effects by modulating iron homeostasis in the brain and finally inhibiting ferroptosis. This work provides a reliable chemical tool for assessing the status of ferrous iron in the living epileptic mouse brain and may aid the rapid discovery of antiepileptic drug candidates.

Simultaneous Monitoring of Intracellular Temperature and Norepinephrine Variation by Fluorescent Probes during Norepinephrine Reuptake

A long-standing challenge has been the simultaneous sensing of intracellular temperature and norepinephrine (NE) variations to explore signaling pathways and depression pathogeny. Here, we designed a fluorescent probe using poly(N-isopropylacrylamide) and 1-[4-(7-nitro-benzo [1,2,5]oxadiazol-4-yl)-piperazin-1-yl]-propenone (PNIPAm-AANBD) and (E)-1-(4-boronobenzyl)-2-(2-(1,3-dioxo-1H,3H-benzo[de]isochromen-6-yl)vinyl)pyridin-1-ium bromide (PHE) for simultaneously measuring the temperature and NE with high selectivity. The fluorescence intensity of the PNIPAm-AANBD moiety exhibited a good response to temperature changes. The PHE moiety could selectively sense NE due to the naphthalic anhydride group in PHE, which formed naphthalimide upon bonding with the primary amino group of NE. The hydroxyl-terminated ligand recognized the phenolic hydroxyl group of NE through the formation of hydrogen bonds. Using the proposed fluorescent probe, variations in the intracellular temperature and NE during NE reuptake could be simultaneously measured. It was first discovered that with the inhibition of antidepressant drugs, the intracellular temperature increased by 1.2-2.1 °C, and the NE reuptake decreased by about 21.5 μM. The measured variations in intracellular temperature and NE during neurotransmitter reuptake can shed light on the underlying mechanism of neurotransmitter signaling pathways, which may facilitate the treatment of depression.

Thiolate-Assisted Route for Constructing Chalcogen Quantum Dots with Photoinduced Fluorescence Enhancement

Despite great efforts in the development of diverse nanomaterials, a general route to synthesize metal-free chalcogen quantum dots (QDs) is still lacking. Moreover, the modification of chalcogen QDs is a bottleneck that severely hinders their applications. Herein, we develop a facile method to construct different chalcogen QDs (including S QDs, Se QDs, and Te QDs) with the assistance of thiolates. In addition to stabilizing chalcogen QDs, the thiolates also endow the chalcogen QDs with favorable modifiability. Different from most dyes whose fluorescence is quenched after short-term light irradiation, the prepared chalcogen QDs have significantly enhanced fluorescence emission under continuous light irradiation. Taking advantage of the distinctive photoinduced fluorescence enhancement property, long-time cell imaging with superb performance is realized using the chalcogen QDs. It is envisioned that the chalcogen QDs show promising potential as fluorescent materials in different fields beyond bioimaging.

Accurate Fluorescence Diagnosis of Cancer Based on Sequential Detection of Hydrogen Sulfide and pH

Cancer ranks as a leading cause of death in every country of the world. However, if they are discovered early, a lot of cancers can be prevented or cured. Discovering and monitoring cancer markers are the main methods for early diagnosis of cancer. To date, many fluorescent probes designed and used for early cancer diagnosis can only react with a single marker, which always causes insufficient accuracy in complex systems. Herein, a novel near-infrared (NIR) fluorescent probe (CyO-DNP) for the sequential detection of H₂S and H⁺ is synthesized. In this probe, a heptamethine dye is selected as the fluorophore and a 2,4-dinitrophenyl (DNP) ether is chosen as recognition group. In the presence of H₂S, CyO-DNP is transformed into CyO, which exhibits an intense fluorescence at 663 nm. Then, H⁺ induces the protonation of CyO to obtain CyOH, and the final fluorescence emission at 793 nm significantly enhances. Owing to the low cytotoxicity and the NIR fluorescence emission, CyO-DNP can sequentially monitor endogenous H₂S and H⁺ in cancer cells and image exogenous and endogenous H₂S and H⁺ in mice. It is worth mentioning that CyO-DNP can effectively avoid the false positive signal caused by the liver and kidney and discriminate normal mice and tumor mice accurately. For all we know, CyO-DNP is the first fluorescent probe for early accurate diagnosis of cancer by sequentially detecting H₂S and H⁺.

NIR-II cell endocytosis-activated fluorescent probes for in vivo high-contrast bioimaging diagnostics

Fluorescence probes have great potential to empower bioimaging, precision clinical diagnostics and surgery. However, current probes are limited to in vivo high-contrast diagnostics, due to the substantial background interference from tissue scattering and nonspecific activation in blood and normal tissues. Here, we developed a kind of cell endocytosis-activated fluorescence (CEAF) probe, which consists of a hydrophilic polymer unit and an acid pH-sensitive small-molecule fluorescent moiety that operates in the "tissue-transparent" second near-infrared (NIR-II) window. The CEAF probe stably presents in the form of quenched nanoaggregates in water and blood, and can be selectively activated and retained in lysosomes through cell endocytosis, driven by a synergetic mechanism of disaggregation and protonation. In vivo imaging of tumor and inflammation with a passive-targeting and affinity-tagged CEAF probe, respectively, yields highly specific signals with target-to-background ratios over 15 and prolonged observation time up to 35 hours, enabling positive implications for surgical, diagnostic and fundamental biomedical studies.

An ultrasensitive ratiometric fluorescent probe for the detection of Hg2+ and its application in cell and zebrafish

Mercury is a highly toxic metal element, and the accumulation of mercury in the human body can cause great harm, including but not limited to brain damage, kidney damage and behavioral disorders. Therefore, an effective way to detect mercury ions in the environment is urgently needed. In this study, a novel fluorescent probe (CP-Hg) was synthesized with coumarin as the fluorophore and propanethiol as the recognition receptor. The probe was characterized with high sensitivity (detection limit is approximately 0.5 nM) and selectivity. Note that the probe can react with mercury ions with a distinct color change. In addition, it has been proved to have low toxicity and successfully applied to detect mercury in water samples, macrophages and zebrafish model.