Fluorescent Probes with Multiple Binding Sites for the Discrimination of Cys, Hcy, and GSH

In situ and dynamic SERS monitoring of glutathione levels during cellular ferroptosis metabolism

Ferroptosis is a non-apoptotic cell death regulated by iron-dependent lipid peroxidation. Glutathione (GSH), a key antioxidant against oxidative damage, is involved in one of the most important metabolic pathways of ferroptosis. Herein, an excellent plasmonic nanoprobe was developed for highly sensitive, in situ, dynamic real-time monitoring of intracellular GSH levels during ferroptosis. A nanoprobe was prepared by functionalizing gold nanoparticles (AuNPs) with the probe molecule crystal violet (CV). The fluctuation in the SERS signal intensity of CV via the competitive displacement reaction can be used to detect GSH. The advantages of the plasmonic nanoprobe including low-cost production techniques, outstanding stability and biocompatibility, high specificity and sensitivity towards GSH with a detection limit of 0.05 μM. It enables real-time dynamic monitoring of GSH levels in living cells during erastin-induced ferroptosis. This approach is expected to provide important theoretical support for elucidating the GSH-related ferroptosis metabolic mechanism and advancing our understanding of ferroptosis-based cancer therapy. Overview of the workflow of sensing principle for highly sensitive, in situ and dynamic tracking of intracellular GSH levels during drug-triggered ferroptosis process.

Functional Nucleic Acid Probes Based on Two-Photon for Biosensing

Functional nucleic acid (FNA) probes have been widely used in environmental monitoring, food analysis, clinical diagnosis, and biological imaging because of their easy synthesis, functional modification, flexible design, and stable properties. However, most FNA probes are designed based on one-photon (OP) in the ultraviolet or visible regions, and the effectiveness of these OP-based FNA probes may be hindered by certain factors, such as their potential for photodamage and limited light tissue penetration. Two-photon (TP) is characterized by the nonlinear absorption of two relatively low-energy photons of near-infrared (NIR) light with the resulting emission of high-energy ultraviolet or visible light. TP-based FNA probes have excellent properties, including lower tissue self-absorption and autofluorescence, reduced photodamage and photobleaching, and higher spatial resolution, making them more advantageous than the conventional OP-based FNA probes in biomedical sensing. In this review, we summarize the recent advances of TP-excited and -activated FNA probes and detail their applications in biomolecular detection. In addition, we also share our views on the highlights and limitations of TP-based FNA probes. The ultimate goal is to provide design approaches for the development of high-performance TP-based FNA probes, thereby promoting their biological applications.

Reactive Oxygen Species: A Crosslink between Plant and Human Eukaryotic Cell Systems

Reactive oxygen species (ROS) are important regulating factors that play a dual role in plant and human cells. As the first messenger response in organisms, ROS coordinate signals in growth, development, and metabolic activity pathways. They also can act as an alarm mechanism, triggering cellular responses to harmful stimuli. However, excess ROS cause oxidative stress-related damage and oxidize organic substances, leading to cellular malfunctions. This review summarizes the current research status and mechanisms of ROS in plant and human eukaryotic cells, highlighting the differences and similarities between the two and elucidating their interactions with other reactive substances and ROS. Based on the similar regulatory and metabolic ROS pathways in the two kingdoms, this review proposes future developments that can provide opportunities to develop novel strategies for treating human diseases or creating greater agricultural value.

Tumor-Targeting Probe for Dual-Modal Imaging of Cysteine In Vivo

Cysteine (Cys) is a crucial biological thiol that has a vital function in preserving redox homeostasis in organisms. Studies have shown that Cys is closely related to the development of cancer. Thus, it is necessary to design an efficient method to detect Cys for an effective cancer diagnosis. In this work, a novel tumor-targeting probe (Bio-Cy-S) for dual-modal (NIR fluorescence and photoacoustic) Cys detection is designed. The probe exhibits high selectivity and sensitivity toward Cys. After reaction with Cys, both NIR fluorescence and photoacoustic signals are activated. Bio-Cy-S has been applied for the dual-modal detection of Cys levels in living cells, and it can be used to distinguish normal cells from cancer cells by different Cys levels. In addition, the probe is capable of facilitating dual-modal imaging for monitoring changes in Cys levels in tumor-bearing mice. More importantly, the excellent tumor-targeting ability of the probe greatly improves the signal-to-noise ratio of imaging. To the best of our knowledge, this is the first Cys probe to combine targeting and dual-modal imaging performance for cancer diagnosis.

Glutathione displacement assay based on a fluorescent Au(I) complex

Glutathione (GSH) is an essential molecule that plays a pivotal role in maintaining intracellular redox homeostasis, as well as other physiological processes. However, the chemical mechanisms underlying the GSH-induced processes remain insufficiently understood due to the lack of appropriate detection tools. Fluorescence GSH imaging can serve as a useful principle for the rapid, convenient, and non-destructive detection of GSH in living organisms. In this study, we developed a fluorescent GSH probe based on a linear, homoleptic Au(I) complex with two 1,3-diphenylbenzimidazolium carbene ligands. The Au(I) complex produced a fluorescence turn-on response to GSH. Fluorescence GSH signaling was characterized with a short response time of a few seconds. The rapid response was attributed to the displacement of the carbene ligand with GSH, which involved a labile inner-sphere coordination interaction. Finally, we demonstrated the biological utility of our GSH probe by unambiguously discriminating between different GSH levels in normal and senescent preadipocytes.

Microperoxidase-11 functionalized nanozyme with enhanced peroxidase-mimicking activities for visual detection of cysteine

Various nanomaterials with peroxidase activity (nanozyme) have been designed for bio catalysis and biosensing, however, most of them need further design and modification of probe molecules for the specific binding reaction with targets. This results in a decrease in catalysis activity and hinders them to be perfect alternatives to natural enzyme in biosensing. In this work, an enhanced nanozyme was synthesized by functionalizing natural microperoxidase-11 (MP-11) on a hybrid graphene oxide-gold (GO-Au) material. The designed nanozyme showed an enhanced catalysis activity and realized a robust and efficient colorimetric detection of cysteine based on specific binding reaction between active iron center from MP-11 and thiol in cysteine. The enhanced properties show promising applications of complex nanozyme and provides a great opportunity for developing efficient sensing systems.

Sensitive and discriminative detection of cysteine by a Nile red-based NIR fluorescence probe

Cysteine (Cys) is a significant biological mercaptan that achieves key roles in several important physiological processes, such as reversible redox homeostasis in living organisms. Abnormal levels of Cys in the human body are directly related to many diseases. In this work, we constructed a sensitive sensor (Cys-NR) by connecting a Cys recognition group to a Nile red derivative. Due to photo-induced electron transfer (PET), the Cys-NR probe showed little fluorescence at 650 nm. With the addition of Cys to the assay solution, the chlorine unit of the probe was substituted by the thiol group of Cys. Further, the amino and sulfhydryl groups in cysteine underwent an intramolecular rearrangement, which led to the Cys-NR probe water solution turning from colorless to pink with an enhancement in fluorescence. The red fluorescence at 650 nm increased about 20 times. Based on the turn-on signal, a selective Cys detection method is developed. The probe signal is not affected by various potential interferences or other competing biothiols and the limit of detection (LOD) is determined to be 0.44 μM. In addition, the probe is further employed for imaging of Cys in live cells, revealing good biological imaging ability that could provide a new way of intracellular Cys detection.

General Strategy for Specific Fluorescence Imaging of Homocysteine in Living Cells and In Vivo

The aberrantly changed level of homocysteine (Hcy) triggers a variety of pathological symptoms and subsequently Hcy-related diseases. Direct and selective visualization of Hcy in biological systems is pivotal to understanding the pathological functions of Hcy at the molecular level. Herein, a general strategy was developed for the specific fluorescence imaging of Hcy through the combination of dual-binding sites and the introduction of a nitro group at the 6-position of the 7-diethylaminocoumarin fluorophore. Also, a series of novel fluorescent probes were exploited for monitoring Hcy with excellent selectivity, high sensitivity, and far-red/near-infrared fluorescence emission. Furthermore, fluorescence imaging of endogenous Hcy dynamics in living cells and in vivo was achieved, providing direct and solid evidence for the increasement of endogenous Hcy in type 2 diabetes mellitus and Alzheimer's disease. This research will greatly advance the development and understanding of the molecular nexus between the Hcy metabolism cascade and the root causes of diseases related to Hcy.

A fluorescent probe based on Cu(II) complex induced catalysis for repetitive detection of cysteine

Real-time imaging and monitoring of biothiols in living cells are essential for understanding pathophysiological processes. However, the design of the fluorescent probe that has accurate and repeatable real-time monitoring capabilities for these targets is highly challenging. In this study, we prepared a fluorescent sensor, Lc-NBD-Cu(II), which contains a N1, N1, N2-tris-(pyridin-2-ylmethyl) ethane-1,2-diamine as a Cu(II) chelating unit and a 7-nitrobenz-2-oxa-1,3-diazole fluorophore to detect Cysteine (Cys). Emission changes promoted by addition of Cys to this probe are distinctive and correspond to a range of processes including Cys induced loss of Cu(II) from Lc-NBD-Cu(II) to form Lc-NBD, Cu(I) oxidation to reform Cu(II), Cys oxidation to form Cys-Cys, Cu(II) binding to Lc-NBD to reform Lc-NBD-Cu(II), and competitive binding of Cu(II) to Cys-Cys. The study also shows that Lc-NBD-Cu(II) maintains high stability during the sensing process and that it can be utilized over a number of detection cycles. Finally, the findings show that Lc-NBD-Cu(II) can be utilized to repetitively sense Cys in living HeLa cells.

Recent Progress in the Rational Design of Biothiol-Responsive Fluorescent Probes

Biothiols such as cysteine, homocysteine, and glutathione play significant roles in important biological activities, and their abnormal concentrations have been found to be closely associated with certain diseases, making their detection a critical task. To this end, fluorescent probes have become increasingly popular due to their numerous advantages, including easy handling, desirable spatiotemporal resolution, high sensitivity, fast response, and favorable biocompatibility. As a result, intensive research has been conducted to create fluorescent probes for the detection and imaging of biothiols. This brief review summarizes recent advances in the field of biothiol-responsive fluorescent probes, with an emphasis on rational probe design, including the reaction mechanism, discriminating detection, reversible detection, and specific detection. Furthermore, the challenges and prospects of fluorescence probes for biothiols are also outlined.

Carbon dots as specific fluorescent sensors for Hg2+ and glutathione imaging

Nitrogen-doped carbon dots (NCDs) have been constructed in which coal washing wastewater is used as carbon precursor, tryptophan is added for nitrogen doping and surface functional together with polyethylene glycol. The nitrogen doping and surface functional with electron rich groups resulted in excellent fluorescent properties regarding stability, reversibility, printability with high quantum yield which not only enable the NCDs as fluorescent ink for advanced message encryption, but also realize specific on-off-on fluorescent sensing of Hg2+ and GSH as solution, hydrogel and filter paper sensors. The NCDs had a linear range of 0.01-100 μM and a detection limit of 6.27 nM (RSD 0.33%) for Hg2+ and the NCDs@Hg2+ had a linear range of 0.01-60 μM and a detection limit of 3.53 nM (RSD 1.53%) for GSH in sensing studies with aqueous solutions. In addition, with the low cytotoxicity and good biocompatibility NCDs have been successfully used for imaging Hg2+ and GSH in living MG-63 cells. The presented NCDs recycle waste coal washing water into worthwhile material which can be implemented as promising anti-counterfeiting and message encryption candidates as well as effective Hg2+ and GSH sensing, tracking and removing tools in complicated environmental and biological systems.

Covalent organic framework-MnO2 nanoparticle composites for shape-selective sensing of bithiols

Covalent organic frameworks (COFs) for detecting biological macromolecules in water or biological environments are generally challenging. In this work, a composite material IEP-MnO₂ is obtained by combining manganese dioxide (MnO₂) nanocrystals and a fluorescent COF (IEP), which is synthesized by using 2,4,6-tris(4-aminophenyl)-s-triazine and 2,5-dimethoxyterephthalaldehyde. By the addition of biothiols, such as glutathione, cysteine or homocysteine with different sizes, the fluorescence emission spectra of IEP-MnO₂ changed ("turn-on" or "turn-off") via different mechanisms. The fluorescence emission of IEP-MnO₂ increased in the presence of GSH by the elimination of the FRET (Förster resonance energy transfer) effect between MnO₂ and IEP. Surprisingly, due to the formation of a hydrogen bond between Cys/Hcy and IEP, the fluorescence quenching for IEP-MnO₂ + Cys/Hcy may be explained via the photoelectron transfer (PET) process, which endows IEP-MnO₂ with specificity in distinguishing the detection of GSH and Cys/Hcy compared to other MnO₂ complex materials. Therefore, IEP-MnO₂ was used to detect GSH and Cys in human whole blood and serum, respectively. The limit of detection for GSH in whole blood and Cys in human serum was calculated to be 25.58 μM and 4.43 μM, which indicates that IEP-MnO₂ can be used to investigate some diseases related to GSH and Cys concentration. Moreover, the research expands the application of covalent organic frameworks in the fluorescence sensing field.

Excited-state intramolecular proton transfer (ESIPT)-based fluorescent probes for biomarker detection: design, mechanism, and application

Biomarkers are essential in biology, physiology, and pharmacology; thus, their detection is of extensive importance. Fluorescent probes provide effective tools for detecting biomarkers exactly. Excited state intramolecular proton transfer (ESIPT), one of the significant photophysical processes that possesses specific photoisomerization between Keto and Enol forms, can effectively avoid annoying interference from the background with a large Stokes shift. Hence, ESIPT is an excellent choice for biomarker monitoring. Based on the ESIPT process, abundant probes were designed and synthesized using three major design methods. In this review, we conclude probes for 14 kinds of biomarkers based on ESIPT explored in the past five years, summarize these general design methods, and highlight their application for biomarker detection in vitro or in vivo.

Radical nucleophilic substitution/cyclization: A novel strategy for selective and ultrafast fluorescence imaging of cysteine levels in ferroptosis process

Diphenylisoindolo[2,1-a]quinoline can be used to detect cysteine among homocysteine, glutathione, and other 19 natural amino acids. Unlike other reported probes, the response mechanism involves sulfhydryl radical nucleophilic substitution and cyclization, and thus the differences in ring-formation kinetics enable high selectivity. After treated with Cys, the response process was completed rapidly and the maximum fluorescence intensity (at 496 nm) was reached extremely fast (<1 s) when excited at 380 nm in MeCN-PBS buffer (10.0 mM, pH = 7.4, 3:7 (v/v)). The quantum yield after the reaction was increased almost 7 times to be 0.02 from 0.003. Fluorescence intensity displayed a good quantitative linear relationship in the range 1-10 μM Cys with a detection limit of 270 nM. Furthermore, the probe was demonstrated for real-time monitoring of intracellular cysteine levels within HepG2 cells in ferroptosis process.

Reversible Modulation of Aptamer-Ligand Binding in RNA Light-Up Aptamers Containing G-Quadruplex Using Chemical Stimuli

Regulating a system in equilibrium transiently to out-of-equilibrium by using certain stimuli is the strategy used by natural biomolecules to function. Herein, we showed that the interaction of synthetic RNA aptamers, having a G-quadruplex core structure, with their corresponding ligands could be regulated from their equilibrium state to non-equilibrium state in a reversible manner using simple chemical stimuli (Ag⁺ and cysteine). The approach would be useful for designing aptamer regulators that work in a dynamic nucleic acid network, where a strict control on aptamer-ligand interaction is needed. In addition, to the best of our knowledge, this is the first report which shows that RNA G-quadruplexes can be disrupted by the addition of silver ions. This would be useful not only in designing RNA-based sensors or regulators but would also be useful for understanding the role of metal ions in RNA folding and catalysis.

A "turn-on" inverse opal photonic crystal fluorescent sensing film for detection of cysteine and its bioimaging of living cells

A "turn-on" inverse opal photonic crystal fluorescent sensing film infiltrated with a coumarin derivative is reported for the reliable and accurate detection of cysteine in human serum and fluorescence imaging of living cells. The coumarin derivative containing allyl ester specifically reacts with cysteine by ammonolysis to generate a fluorescent product whose emission wavelength is at ~ 535 nm, providing a selective fluorescence detection for cysteine. The emitted fluorescence is significantly enhanced due to the slow photon effect derived from the photonic crystal film. This is because the emission wavelength is overlapped with the blue-band edge of the photonic stopband of the selected inverse opal film. The fluorescence enhancement effect endows the prepared inverse opal film with highly sensitive detection with a limit of detection of 3.23 × 10^-9 mol/L and a wide linear detection range of 1 × 10^-7 - 1 × 10^-3 mol/L. A fast response within 30 s toward cysteine is also achieved due to the three-dimensional interconnected macroporous structure with a high-specific surface area of the inverse opal film. The prepared inverse opal fluorescent sensing film has been successfully applied to the detection of cysteine in human serum and bioimaging of living cells. In the diluted human serum, the recoveries for the detection of cysteine were 97.92 - 107.20%, and the relative standard deviations were 2.61-9.04%, demonstrating the potential applicability of the inverse opal fluorescent sensing film to real sample analysis. The method may provide a universal strategy for constructing various photonic crystal fluorescent sensing films by using different fluorescent probes.

Near-Infrared Probes for Biothiols (Cysteine, Homocysteine, and Glutathione): A Comprehensive Review

Biothiols (cysteine, homocysteine, and glutathione) are an important class of compounds with a free thiol group. These biothiols plays an important role in several metabolic processes in living bodies when present in optimum concentration. Researchers have developed several probes for the detection and quantification of biothiols that can absorb in UV, visible, and near-infrared (NIR) regions of the electromagnetic spectrum. Among them, NIR organic probes have attracted significant attention due to their application in in vivo and in vitro imaging. In this review, we have summarized probes for these biothiols, which could work in the NIR region, and discussed their sensing mechanism and potential applications. Along with focusing on the pros and cons of the reported probes we have classified them according to the fluorophore used and summarized their photophysical and sensing properties (emission, response time, limit of detection).

An indanone-based fluorescent probe for detection and imaging of Cys/Hcy in living cells

Effective detection of Cys and Hcy plays an important role in the diagnosis of diseases. In this work, a novel indanone-based fluorescent probe INIAc-CN for sensitively and effectively detecting Cys and Hcy was developed. The probe exhibited weak fluorescence, but obvious fluorescent enhancement after reacted with Cys/Hcy. Moreover, the good anti-interference and low cytotoxicity of the probe made it successfully applied for monitoring Cys and Hcy of in living cells.

Dual-Channel Recognition of Human Serum Albumin and Glutathione by Fluorescent Probes with Site-Dependent Responsive Features

Design of chemical probes with high specificity and responses are particularly intriguing. In this work, a fluorescent probe (M-OH-SO₃) with dual-channel spectral responses toward human serum albumin (HSA) is presented. By employing dinitrobenzenesulfonate as a recognition site as well as a fluorescence quencher, probe M-OH-SO₃ displayed weak fluorescence, which, nevertheless, exhibits extensive yellow (575 nm) and red (660 nm) fluorescence emissions toward HSA under excitations at 400 and 500 nm, respectively. Interestingly, M-OH-SO₃ displayed the best performance toward HSA with distinctly higher selectivity than that of its counterparts M-SO₃, M-H-SO₃, and M-F-SO₃, which were prepared simply by modulating the functional group at the ortho position of the dicyanoisophorone core. Molecular docking results revealed that M-OH-SO₃ possesses the lowest binding energy among the tested derivatives and accordingly the strongest binding affinity. Probe M-OH-SO₃ showed a good linear relationship toward HSA in a range of 0.5-18 μM with a limit of detection of 35 nM. Cell imaging results demonstrated that probe M-OH-SO₃ could visualize the variation HSA levels in hepatocarcinoma cells. In addition, probe M-OH-SO₃ could also be employed for the recognition of glutathione through the cleavage of the dinitrobenzenesulfonate group along with an enhancement of emission at 575 nm. The site-dependent properties inspired a novel paradigm for design of fluorescent probes with optimized selectivity and responses.

Miniaturized Chemical Tags for Optical Imaging

Recent advances in optical bioimaging have prompted the need for minimal chemical reporters that can retain the molecular recognition properties and activity profiles of biomolecules. As a result, several methodologies to reduce the size of fluorescent and Raman labels to a few atoms (e.g., single aryl fluorophores, Raman-active triple bonds and isotopes) and embed them into building blocks (e.g., amino acids, nucleobases, sugars) to construct native-like supramolecular structures have been described. The integration of small optical reporters into biomolecules has also led to smart molecular entities that were previously inaccessible in an expedite manner. In this article, we review recent chemical approaches to synthesize miniaturized optical tags as well as some of their multiple applications in biological imaging.

A unique water soluble probe for measuring the cardiac marker homocysteine and its clinical validation

A series of copper(II) compounds 1-4 were synthesized and developed as fluorogenic probes to measure the cardiac marker homocysteine (Hcy) without any interference from other bioanalytes prevalent in human blood plasma including, cysteine and glutathione. UV-vis and EPR studies have provided confirmatory evidence for reduction-induced-emission-enhancement of the probe, which is responsible for the observed "off-to-on" behaviour towards Hcy. Water solubility, remarkable fluorescence enhancement (55-111 fold), and low detection ability (nearly 2.5 μM) make the probe suitable for clinical testing of cardiac samples. Investigation of 1 against a few reductive interferents testifies its specificity for Hcy. Results from clinical examination of cardiac samples by 1 when combined with the outcome of the reliability testing involving a clinically approved commercial immunoassay kit, validates the prospect of the molecular probe for direct measurement of Hcy in human plasma, which is unprecedented.

Real-time, sensitive and simultaneous detection of GSH and Cys/Hcy by 8-substituted phenylselenium BODIPYs: a structure-activity relationship

Real-time and sensitive detection of biothiols is the key to biomedical research and clinical diagnosis. It is necessary to develop a highly sensitive and selective fluorescent probe for the detection of biothiols. In this paper, we have developed a series of meso-arylselenium BODIPY probes for the rapid and sensitive detection of biothiols and the dual-channel discrimination of GSH and Cys/Hcy. A structure-activity relationship was established from five p-substituted phenylselenium (R = NO₂, F, H, OCH₃ or N(CH₂CH₂)₂O) BODIPYs. Compared with most reported fluorescent probes, such as meso-BODIPY sulfur ethers, these probes display much lower LODs (∼nM levels) and more rapid responses, which are ascribed to the higher fluorescence efficiencies of the sensing products (Φ_f = 0.48 for GSH, 0.18 for Cys and 0.14 for Hcy) and the introduction of arylselenium, which is more active than arylthiol. Among them, the best sensing performance is that of probe 2a (R = NO₂); therefore, a structure-activity relationship of these fluorescent probes was also obtained. The excellent sensing performance was further revealed in the detection of GSH and Cys/Hcy in live cells.

Nanocellulose as a promising substrate for advanced sensors and their applications

Nanocellulose has broad and promising applications owing to its low density, large specific surface area, high mechanical strength, modifiability, renewability. Recently, nanocellulose has been widely used to fabricate flexible, durable and environmental-friendly sensor substrates. In this contribution, the construction and characteristics of nanocellulose-based sensors are comprehensively reviewed. Various nanocellulose-based sensors are summarized and divided into colorimetric, fluorescent, electronic, electrochemical and SERS types according to the sensing mechanism. This review also introduces the applications of nanocellulose-based sensors in the fields of biomedicine, environmental monitoring, food safety, and wearable devices.

Fluorescent-based nanosensors for selective detection of a wide range of biological macromolecules: A comprehensive review

Thanks to their unique attributes, such as good sensitivity, selectivity, high surface-to-volume ratio, and versatile optical and electronic properties, fluorescent-based bioprobes have been used to create highly sensitive nanobiosensors to detect various biological and chemical agents. These sensors are superior to other analytical instrumentation techniques like gas chromatography, high-performance liquid chromatography, and capillary electrophoresis for being biodegradable, eco-friendly, and more economical, operational, and cost-effective. Moreover, several reports have also highlighted their application in the early detection of biomarkers associated with drug-induced organ damage such as liver, kidney, or lungs. In the present work, we comprehensively overviewed the electrochemical sensors that employ nanomaterials (nanoparticles/colloids or quantum dots, carbon dots, or nanoscaled metal-organic frameworks, etc.) to detect a variety of biological macromolecules based on fluorescent emission spectra. In addition, the most important mechanisms and methods to sense amino acids, protein, peptides, enzymes, carbohydrates, neurotransmitters, nucleic acids, vitamins, ions, metals, and electrolytes, blood gases, drugs (i.e., anti-inflammatory agents and antibiotics), toxins, alkaloids, antioxidants, cancer biomarkers, urinary metabolites (i.e., urea, uric acid, and creatinine), and pathogenic microorganisms were outlined and compared in terms of their selectivity and sensitivity. Altogether, the small dimensions and capability of these nanosensors for sensitive, label-free, real-time sensing of chemical, biological, and pharmaceutical agents could be used in array-based screening and in-vitro or in-vivo diagnostics. Although fluorescent nanoprobes are widely applied in determining biological macromolecules, unfortunately, they present many challenges and limitations. Efforts must be made to minimize such limitations in utilizing such nanobiosensors with an emphasis on their commercial developments. We believe that the current review can foster the wider incorporation of nanomedicine and will be of particular interest to researchers working on fluorescence technology, material chemistry, coordination polymers, and related research areas.

Selective, Rapid, and Ratiometric Fluorescence Sensing of Homocysteine in Live Neurons via a Reaction-Kinetics/Sequence-Differentiation Strategy Based on a Small Molecular Probe

Small molecular biothiols, including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play essential roles in maintaining the redox homeostasis of biological systems, the disorders of which are closely associated with neuropathology. To date, many probes have been developed to identify Cys and GSH; however, due to the relatively low content and the high structural homology with Cys, there is still a lack of effective strategies to design probes enabling Hcy detection in physiological environments with high selectivity, high sensitivity, and rapid response. Herein, we developed a reaction-kinetics/sequence-differentiation strategy based on a dual-binding-site boron-dipyrrin (BODIPY) fluorophore, which was able to selectively distinguish Hcy from Cys and GSH within 50 s though a ratiometric fluorescence response mode. Benefiting from these features, the probe is capable of real-time imaging and quantitative analysis of intracellular Hcy in living neurons. Moreover, results of the disease-model experiments at the cellular level indicated a gradual increase of the Hcy level in neurons during the processes of aggregated amyloid-β (Aβ) peptide or ischemia treatment, which would further promote the neuron apoptosis. These findings provide the first direct experimental evidence for the impact of Alzheimer's disease and ischemic stroke on the Hcy metabolism of brain neurons and the associated neuron injury.

Photoacoustic/Fluorescence Dual-Modality Probe for Biothiol Discrimination and Tumor Diagnosis in Cells and Mice

Developing probes to simultaneously detect and discriminate biothiols is important, yet challenging. Activatable photoacoustic (PA) probes for discriminating biothiols in vivo are still lacking, and this hinders the diagnosis of thiol-related diseases. Herein we present the first PA and fluorescence dual-modality probe MB-NBD for discriminating different biothiol species. The probe has the advantages of both fluorescence imaging and PA imaging (high sensitivity and deep penetration) with distinct signal patterns toward hydrogen sulfide (H₂S), cysteine/homocysteine (Cys/Hcy), and glutathione (GSH) treatment. The biothiol-activated product of MB-NBD exhibits enhancements in near-infrared fluorescence (NIRF) at 690 nm and absorbance/PA at 664 nm upon fast reaction, allowing it to selectively detect biothiol species over other reactive species. On the other hand, MB-NBD displays characteristic absorbance enhancement at 547 nm toward H₂S, rendering specific detection of H₂S. In addition, the specific enhancements in absorbance/PA at 470 nm and fluorescence at 550 nm toward Cys/Hcy treatment endows the probe with the capability of selectively detecting Cys/Hcy. Furthermore, MB-NBD is able to discriminate Cys and GSH by fluorescent imaging in live-cell and ratiometric PA imaging in mice experiments. MB-NBD has been successfully used to diagnose tumors by dual-channel ratiometric PA imaging.

A Yellow Fluorescence Probe for the Detection of Oxidized Glutathione and Biological Imaging

It is well-known that the ratio of reduced l-glutathione (GSH) to oxidized l-glutathione (GSSG) is a vital biomarker for monitoring overall cellular health, thus detecting the intracellular concentration of glutathione is of great significance. Recently, an increasing number of reports have published various methods for GSH detection, but studies on the detection of GSSG are still rare. Here, we report a kind of new yellow fluorescent carbon dots (CDs) for the detection of GSSG through a fluorescence "off-on" process. Because the surface is rich in amino groups, the CDs show a positive potential. When the concentration of GSSG was continuously increased, the CDs' fluorescence dropped sharply, while the fluorescence gradually recovered after the addition of sodium sulfide. The phenomenon of fluorescence quenching is linear with the concentration of the quencher (GSSG)(0-200 μM), and 0.18 μM is calculated as the detection limit. More interestingly, as a fluorescent probe, the CDs can be further used for fluorescence imaging in living cells and zebrafish.

Noncovalent Dual-Locked Near-Infrared Fluorescent Probe for Precise Imaging of Tumor via Hypoxia/Glutathione Activation

Stimulus-responsive fluorescent probes have broad applications in the early detection and treatment of tumors and thus promote the personalized treatment of tumors and improve patient survival. Among the repertoires of probes, dual-locked near-infrared (NIR) fluorescent probes are of great significance due to their improved specificity and multiplex detection in tumor imaging but remain to be explored. In this work, a facile noncovalent strategy for constructing dual-locked probes was proposed. A glutathione (GSH)-activatable single-locked probe CySS (first lock) was preloaded into a hypoxia-responsive molecular container CF3C4A (second lock) through a host-guest interaction to form the dual-locked probe CF3C4A-CySS. Under physiological conditions, CF3C4A-CySS binds strongly to avoid undesired leakage in normal tissues. We have proven that CF3C4A-CySS can be activated and "turn on" its NIR fluorescent signal under the dual key stimulation of hypoxia and GSH in the tumor microenvironment, which enables precise tumor imaging with enhanced accuracy and specificity. Both in vitro and in vivo results indicated the superiority of CF3C4A-CySS in tumor imaging. This work not only provides an effective tool for tumor imaging but also proposes a promising strategy for dual-locked imaging agent construction.

Aberrant Expression of Mitochondrial SAM Transporter SLC25A26 Impairs Oocyte Maturation and Early Development in Mice

The immature germinal vesicle (GV) oocytes proceed through metaphase I (MI) division, extrude the first polar body, and become mature metaphase II (MII) oocytes for fertilization which is followed by preimplantation and postimplantation development until birth. Slc25a26 is the gene encoding S-adenosylmethionine carrier (SAMC), a member of the mitochondrial carrier family. Its major function is to catalyze the uptake of S-adenosylmethionine (SAM) from cytosol into mitochondria, which is the only known mitochondrial SAM transporter. In the present study, we demonstrated that excessive SLC25A26 accumulation in mouse oocytes mimicked naturally aged oocytes and resulted in lower oocyte quality with decreased maturation rate and increased reactive oxygen species (ROS) by impairing mitochondrial function. Increased level of Slc25a26 gene impacted gene expression in mouse oocytes such as mt-Cytb which regulates mitochondrial respiratory chain. Furthermore, increased level of Slc25a26 gene in fertilized oocytes slightly compromised blastocyst formation, and Slc25a26 knockout mice displayed embryonic lethality around 10.5 dpc. Taken together, our results showed that Slc25a26 gene plays a critical role in oocyte maturation and early mouse development.

Red and Near-Infrared Fluorescent Probe for Distinguishing Cysteine and Homocysteine through Single-Wavelength Excitation with Distinctly Dual Emissions

Small-molecule biothiols, including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), participate in various pathological and physiological processes. It is still a challenge to simultaneously distinguish Cys and Hcy because of their similar structures and reactivities, as well as the interference from the high intramolecular concentration of GSH. Herein, a novel fluorescent probe, CySI, based on cyanine and thioester was developed to differentiate Cys and Hcy through a single-wavelength excitation and two distinctly separated emission channels. The probe exhibited a turn-on fluorescence response to Cys at both 625 nm (the red channel) and 740 nm (the near-infrared channel) but only showed fluorescence turn-on to Hcy at 740 nm (the near-infrared channel) and no fluorescent response to GSH. With the aid of built-in self-calibration of single excitation and dual emissions, simultaneous discriminative determinations of Cys and Hcy were realized through red and near-infrared channels. CySI exhibited excellent selectivity toward Cys and Hcy with a fast response. This probe was further exploited to visualize exogenous Cys and Hcy in cells through dual emission channels under one excitation. Moreover, it could efficiently target mitochondria and was applied to monitor the endogenous Cys fluctuations independently in mitochondria through the red emission channel.

Dual-molecular targeted NIR II probe with enhanced response for head and neck squamous cell carcinoma imaging

In this research, a fluorescent probe of 7-(diethylamine) coumarin derivatives with multiple binding sites to detect biothiols in tumor cell with strong NIR II luminescencein vivowas synthesized. The biothiols include cysteine (Cys) and glutathione (GSH) in tumor cells, and the tumor-response luminescence was proved by the cell experiment. Importantly, the monolayer functional phospholipid (DSPE-PEG) coating and aggregation induced emission (AIE) dye of TPE modification made the probe have good stability and biocompatibility with little luminescence quenching in aqueous phase, which was proved byin vitroandin vivoexperiments. The final aqueous NIR II probe combined with bevacizumab (for VEGF recognition in the cancer cells) and Capmatinib (for Met protein recognition in the cancer cells) has stronger targeted imaging on head and neck squamous cell carcinoma (HNSCC) cancer with intravenous injection. This GSH/Cys detection in the tumor cell and strong dual-molecular NIR II bioimagingin vivomay provide new strategy to tumor detection.

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

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

Highly selective and sensitive detection of glutathione over cysteine and homocysteine with a turn-on fluorescent biosensor based on cysteamine-stabilized CdTe quantum dots

In this work, cysteamine (CA) stabilized CdTe quantum dots (QDs) (CA-CdTe QDs) and sodium citrate stabilized gold nanoparticles (AuNPs) were prepared. Because of the strong electrostatic interaction and spectral overlap of emission spectrum of CA-CdTe QDs and absorption spectrum of AuNPs, a highly effective fluorescence resonance energy transfer (FRET) system was formed and the fluorescence of CA-CdTe QDs was strongly quenched. The synthesized CA-CdTe and AuNPs were self-assembled to large clusters due to the electrostatic attraction and the fluorescence of CA-CdTe was sharply quenched as a result of FRET. Under the optimum pH of 5.5, the positive GSH could assemble with negative AuNPs through electrostatic interaction and destroy the FRET system of CA-CdTe and AuNPs, due to the red shift of absorption wavelength of AuNPs caused by aggregation. The fluorescence of CA-CdTe recovered, and the recovered fluorescence efficiency shows a linear function against the GSH concentrations from 6.7 nM to 0.40 μM, with a detecting limit of 3.3 nM. The quenched emission of CA-CdTe could be recovered attributed to the aggregation of AuNPs by GSH. Under optimal conditions, the sensing system was successfully applied in the detection of GSH in real human blood plasma samples with a recovery of 99.5-102.3%, showing a promising future for the highly sensitive and selective GSH detection in the human blood plasma samples.

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.

Platinum Crosslinked Carbon Dot@TiO2-x p-n Junctions for Relapse-Free Sonodynamic Tumor Eradication via High-Yield ROS and GSH Depletion

Sonodynamic therapy as a promising noninvasive modality is being developed for tumor therapy, but there is a lack of next-generation sonosensitizers that can generate full ROS at high yields and simultaneously deplete elevated levels of glutathione (GSH) in tumor cells. Semiconductor p-n junctions are engineered as high-efficacy sonosensitizers for sonodynamic tumor eradication using pyridine N-doped carbon dots (N-CDs) as a p-type semiconductor and oxygen-deficient TiO_2-x nanosheets as a n-type semiconductor. The rate constants of ¹ O₂ and •OH generation by ultrasound-excited N-CD@TiO_2-x p-n junctions are 4.3 and 4.5 times higher than those of TiO₂ , respectively. A Z-scheme carrier migration mechanism in the p-n junction achieving the rapid spatial separation of the ultrasound-generated electron-hole pairs for enhanced full ROS production is proposed. GSH-cleavable, Pt-crosslinked, N-doped CD fluorescent probes to detect the presence of intracellular GSH are also constructed. A GSH-responsive, p-n junction platform (Pt/N-CD@TiO_2-x ) with integrated GSH detection, GSH depletion, and enhanced sonodynamic performance is then assembled. Malignant tumors are completely eradicated without relapse via intravenous administration of low-dose Pt/N-CD@TiO_2-x under ultrasound irradiation. This work substantiates the great potential of biocompatible, GSH-responsive p-n junctions as next-generation sonosensitizers via p-n junction-enhanced ROS generation and metal ion oxidation of intracellular GSH.