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

A novel fluorescent probe utilizing Michael addition for the rapid detection of sulfur dioxide derivatives in food

Sulphur dioxide (SO2) and its derivatives (SO32-/HSO3-) are extensively utilized in food processing for their roles as bleaching agents, preservatives, and antimicrobials. Research has shown a strong association between high sulfur dioxide intake and various diseases. Consequently, developing rapid and accurate analytical techniques to monitor sulfur dioxide and its derivatives in food is of great value. This work designed a fluorescent probe ZR-I with a D-π-A structure using triphenylamine derivatives as electron donors. The probe ZR-I demonstrated high sensitivity and selectivity towards SO32-/HSO3-, along with strong anti-interference ability. The probe has effectively quantified SO32-/HSO3- in environmental and food samples, achieving a recovery rate of 97.65-104.15 %. Additionally, in situ tracking and imaging of SO32-/HSO3- in HepG2 cells and zebrafish were successfully achieved using ZR-I, which demonstrated high contrast and high sensitivity. To enhance the convenience and practicality of detection, a smartphone-based portable sensing platform has been developed, providing an efficient and practical tool for food safety assessment.

Self-supplying of hydrogen peroxide nanozyme-based colorimetric sensing array as electronic tongue for biothiol detection and disease discrimination

Developing a highly reliably and sensitive nanozyme-based colorimetric sensor array for biothiols analysis is critical owing to they play an essential role in diagnosing disease. The required procedure of introducing hydrogen peroxide (H2O2) directly into the colorimetric reaction systems in traditional biothiols array analysis limits its applicability due to its poor stability and inhibition in biomolecular activity by using the high-concentration H2O2. Herein, we carried out a "green" and convenient approach to propose for the biothiol detection and disease discrimination through nanozymes-based colorimetric sensor technique without adding the high-concentration H2O2 for the first time. The copper peroxide nanodots (CPNs) and graphene oxide (GO) modified CPNs (GO@CPNs) are as sensing units to release H2O2 and Cu2+ under acidic conditions, which triggered a Fenton-like reaction, generating hydroxyl radical (•OH) to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) accompanied by a change in TMB color from colorless to blue. Due to the synergistic effect of Cu2+ and GO, GO@CPNs showed increased the activity of peroxidase-like compared to CPNs. Therefore, the catalytic abilities of nanozymes-based colorimetric sensing array were inhibited to different degrees by different biothiols (i.e., glutathione (GSH), cysteine (Cys) and homocysteine (Hcy)) with a detection limit of 50 nM, which could be precisely distinguished by using pattern recognition method. Besides, the detection of a single biothiol at different concentrations and mixtures of biothiols has also been achieved. Moreover, the real biological samples (cells and human serum) can be accurately discriminated through array method, which demonstrated its potential application of medical diagnosis.

Simultaneous Quantitative Detection of Cysteine and Homocysteine Labeled by 1-Pyrenecarboxaldehyde Using MALDI-TOF MS

Cysteine (Cys) and homocysteine (Hcy) are two important reducing agents in living organisms and play crucial roles in many physiological processes. The quantitative analysis of Cys and Hcy holds significance in exploring the functions of biothiols in biological. In this work, 1-pyrenecarboxaldehyde (1-py) with high derivatization efficiency and ionization efficiency was used for quantitative analysis of Cys and Hcy by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS). After 1-py derivatization, the detection limit of Cys and Hcy can reach as low as 250 amol/L. Without internal standards, the simultaneous quantitative detection of Cys and Hcy was achieved by analyzing the proportion of peak intensities of derivative products to total compounds. The linear quantitative ranges for Cys and Hcy were over the concentrations from 5 to 2500 μM. Moreover, the specific hydrogen loss of the derivatized products was observed in MALDI-TOF detection, and the potential fragment pathway and nitrogen protonation mechanism were demonstrated through density functional theory (DFT) calculations. Finally, this method was successfully applied to the quantification of Cys and Hcy in HepG2 cell lysate, offering a rapid and highly sensitive approach for the quantitative analysis of Cys and Hcy using MALDI-TOF MS.

Developing an aurone-based colorimetric fluorescent probe for fast cysteine sensing in foods, test strips and biological imaging

Cysteine (Cys) is integral to both industrial applications and biological processes. In this study, a novel colorimetric fluorescent sensor APA, defined by its intramolecular charge transfer (ICT) properties, was optimized to effectively discriminate Cys from other structurally similar compounds such as homocysteine (Hcy) and glutathione (GSH). We present the aurone-incorporated fluorescent sensor APA, which features a 4-dimethylaminocinnamaldehyde group conjugated to the aurone scaffold and facilitates selective detection of Cys with a limit of detection (LOD) of 25.7 nM. Compared to previous studies, sensor APA exhibits near-infrared properties, a reduced reaction time of just 2 min, and a significant Stokes shift of 190 nm. Notably, APA has been successfully employed for visual imaging of Cys in test strips and quantitative detection in various food samples in real-time (including garlic, carrot, tomato, onion, green pepper, cauliflower, daikon, lotus root, apple, pear, milk powder, bread, and biscuits). Furthermore, APA has proven effective for colorimetric imaging of both endogenous and exogenous Cys in A549 cells as well as zebrafish and mice models demonstrating its practical biological applications. Overall, our findings highlight the potential of APA as one of the most promising designs for sensing Cys within the food industry and biological systems. Additionally, APA-OH serves as an ideal fluorophore for constructing fluorescence sensors aimed at bioimaging.

Activatable Photosensitizers: From Fundamental Principles to Advanced Designs

Photodynamic therapy (PDT) is a promising treatment that uses light to excite photosensitizers in target tissue, producing reactive oxygen species and localized cell death. It is recognized as a minimally invasive, clinically approved cancer therapy with additional preclinical applications in arthritis, atherosclerosis, and infection control. A hallmark of ideal PDT is delivering disease-specific cytotoxicity while sparing healthy tissue. However, conventional photosensitizers often suffer from non-specific photoactivation, causing off-target toxicity. Activatable photosensitizers (aPS) have emerged as more precise alternatives, offering controlled activation. Unlike traditional photosensitizers, they remain inert and photoinactive during circulation and off-target accumulation, minimizing collateral damage. These photosensitizers are designed to "turn on" in response to disease-specific biostimuli, enhancing therapeutic selectivity and reducing off-target effects. This review explores the principles of aPS, including quenching mechanisms stemming from activatable fluorescent probes and applied to activatable photosensitizers (RET, PeT, ICT, ACQ, AIE), as well as pathological biostimuli (pH, enzymes, redox conditions, cellular internalization), and bioresponsive constructs enabling quenching and activation. We also provide a critical assessment of unresolved challenges in aPS development, including limitations in targeting precision, selectivity under real-world conditions, and potential solutions to persistent issues (dual-lock, targeting moieties, biorthogonal chemistry and artificial receptors). Additionally, it provides an in-depth discussion of essential research design considerations needed to develop translationally relevant aPS with improved therapeutic outcomes and specificity.

A novel ratiometric BenzoBODIPY-Based fluorescent probe for the detection and imaging of Cysteine in living cells and zebrafish models

Cysteine (Cys) plays a critical role in various biological processes, including protein synthesis, cellular signaling, and antioxidant defense. However, precise detection of Cys in biological systems remains challenging due to interference from similar thiols such as homocysteine (Hcy) and glutathione (GSH). In this study, we report the synthesis and bioimaging of a novel ratio-type fluorescent probe based on the benzoBODIPY fluorophore, designed for the ratiometric detection of Cys. The probe operates through an intramolecular charge transfer (ICT) mechanism, where the reaction with Cys triggers a substitution reaction with 4-mercaptopyridine, followed by a Smiles rearrangement. This results in a shift from red to yellow-green fluorescence, providing a sensitive and specific method for the quantitative detection of Cys. The probe demonstrates excellent selectivity, with significantly lower responses to Hcy and GSH, and has been successfully applied in bioimaging experiments in HeLa cells and zebrafish models, highlighting its potential for diagnosing and treating Cys-related diseases.

Precision Imaging of Biothiols in Live Cells and Treatment Evaluation during the Development of Liver Injury via a Near-Infrared Fluorescent Probe

In this study, a biothiol-sensitive near-infrared (NIR) fluorescent sensor, BDP-CYS, based on a coumarin-hemicyanine skeleton, was designed and developed based on thiol-halogen SNAr nucleophilic substitution. BDP-CYS was successfully implemented to ratiometrically monitor endogenous and exogenous Cys, Hcy, and GSH in living cells as well as to distinguish between normal and cancer cells. Furthermore, the probe was utilized to detect changes of biothiols in drug-induced hepatotoxicity and evaluate the treatment effectiveness of diabetes-associated liver injury in vivo. The advantages of BDP-CYS's Cys, Hcy, and GSH include practical sensitivity, high selectivity, rapidity of reaction, and stability across a range of pH and temperature conditions, thus introducing a new tool to better understand the roles of biothiols in oxidative stress.

Smart Probes for Ultrasensitive and Highly Selective Sensing of Homocysteine over Cysteine Based on Multi-Cooperative Effects by Using Gold Nanoparticles

Homocysteine (Hcy) is a biothiol that plays a vital role in many physiological processes and is involved in a variety of diseases. However, it is significantly difficult to discriminate Hcy from cysteine (Cys) due to their similar chemical structures (only one methylene difference) and reactivity. In this study, a novel nanosensor was proposed to discriminate Hcy from Cys with multi-cooperative effects by using gold nanoparticles (AuNPs). The discrimination effect for Hcy originates from the interaction difference of the hydrogen bonding, steric hindrance, and carbon chain length in Hcy and Cys with AuNPs. Under the best conditions, this nanosensor has two unique advantages. Firstly, the sensor exhibits high sensitivity with detection limits of 0.1 μM through naked-eye determination and 0.008 μM through UV-vis spectroscopy analysis. Secondly, the sensor showed superior selectivity for Hcy over the other 16 natural amino acids (biothiol-containing Cys and glutathione (GSH)), and it is the first time to clearly distinguish Hcy from Cys (the Cys concentration is 40 times higher than Hcy). Furthermore, the system was further employed to detect Hcy in human serum, and the result was in agreement with that tested by clinicians via enzymatic assays, with acceptable recovery.

Cancer-specific dual-factor cascade recognition fluorescent probe for imaging and tumor diagnosis

Herein, we developed a dual-factor recognition activatable probe, S1-F, which accurately lights up Bcl-2 and GSH dual-overexpressed cancer tissue with a superior tumor-to-normal tissue (T/N) ratio, highlighting the potential for this probe to be used in the visual diagnosis and imaging-guided surgery of cancer.

A Redox-Active Copper Complex for Orthogonal Detection of Homocysteine Involving Fluorescence and Electrochemical Techniques

The present work reports the synthesis, characterization, and excited state photo-physical studies of two copper(II) compounds, 1 & 2, which show interference-free emission with homocysteine (Hcy). Cu(II) complexes offer an orthogonal detection strategy involving fluorescence and electrochemical methods, paving the way for improved point-of-care diagnostics and early cardiovascular diseases intervention. The reduction-induced emission enhancement (RIEE) of Cu complexes facilitates the fluorescence measurement of Hcy at physiological pH. The fluorogenic redox-active 1 and 2 are deposited onto gold electrode surfaces to construct the electrochemical sensors 1@Au and 2@Au, respectively. Under specific alkaline conditions, a distinct and selective redox peak at 0.6 V (vs Ag/AgCl) emerges for 1@Au upon interaction with homocysteine. Further, square wave voltammetry confirms the non-interference of its congener (cysteine) even at high concentrations (200 µM) while detecting Hcy (5-100 µM), demonstrating its potential for real-world applications. The fabricated 1@Au exhibits excellent sensitivity of 31.88 µA/µM, with an impressive detection limit of 2.26 nM, and a limit of quantification of 6.85 nM toward Hcy. The analytical applicability of the 1@Au is validated by quantifying Hcy levels in human blood plasma samples. The results highlighted the feasibility of the proposed technique as a rapid and portable monitoring of Hcy in diagnosing cardiovascular diseases (CVDs).

A mitochondria-targeted iridium(III) complex-based sensor for endogenous GSH detection in living cells

Glutathione (GSH) plays an important role in maintaining redox homeostasis in biological systems. Development of reliable glutathione sensors is of great significance to better understand the role of biomolecules in living cells and organisms. Based on the advantages of the photophysical properties of iridium complexes, we proposed a "turn-on" phosphorescent sensor. Ir-DNFB has the characteristics of a large Stokes shift, high sensitivity for GSH detection, low cytotoxicity, and extremely short response time, and can specifically analyze glutathione in living cells and highly target endogenous glutathione in mitochondria. The N-H group on the imidazole ring of Ir-DNFB could form a new electrostatic interaction with the α-carboxyl group on the glutamate moiety of glutathione. The nucleophilic attack reaction was regulated by the sulfhydryl group on GSH, following which the ether bond linking the 2,4-dinitrobenzene to probe Ir-DNFB was broken, accompanied with a phosphorescence enhancement. Most importantly, the process of recognizing glutathione was not affected by other amino acids. Overall, this work provided a very useful tool for rapidly distinguishing between normal, inflammatory, and progressive tumor cells.

A steric hindrance-regulated probe with single excitation dual emissions for self-adaptive detection of biothiols and H2S in human urine samples and living cells

Sulfur-containing small molecules, mainly including cysteine (Cys), homocysteine (Hcy), glutathione (GSH), and hydrogen sulfide (H2S), are crucial biomarkers, and their levels in different body locations (living cells, tissues, blood, urine, saliva, etc.) are inconsistent and constantly changing. Therefore, it is highly meaningful and challenging to synchronously and accurately detect them in complex multi-component samples without mutual interference. In this work, we propose a steric hindrance-regulated probe, NBD-2FDCI, with single excitation dual emissions to achieve self-adaptive detection of four analytes. This probe was meticulously designed and constructed from a pKa-tuned 2FDCI fluorophore and a thiol-specific recognition moiety NBD. Except for 661 nm fluorescence for indicating the total biothiols and H2S, Cys and Hcy could trigger an additional 550 nm fluorescence. Utilizing the distinctive responses, the probe NBD-2FDCI exhibited exclusive linear ranges for GSH, Cys/Hcy, and H2S to avoid high-level component interference. Thus, the probe was then applied for sulfur compound measurements in urine samples, indicating metabolic disorder of Cys and H2S in bladder cancer patients. Moreover, adaptive imaging of probe NBD-2FDCI in cells was performed with the results being consistent with in vitro testing. In a word, spatial hindrance strategy-guided probes may exhibit broader prospects in the detection of similar components in complex samples.

Scandium Oxide Mimic Enzymes toward Highly Sensitive Colorimetric Detection of Glutathione

Glutathione (GSH) is an important biochemical substance in the body, and efficient detection plays an important role in evaluating human health. This work develops a simple and efficient method for GSH detection by biomimetic enzyme-catalyzed colorimetric sensing. A novel scandium oxide crystal is prepared by a one-step pyrolysis method. The crystal structure, microstructure, and surface potential of scandium oxide crystals are analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and potentiometry. Furthermore, the scandium oxide crystal can catalyze the oxidation of the colorimetric substrate TMB in the presence of H2O2, exhibiting characteristics of peroxidase-like activity. Its catalytic effect is influenced by the concentration, temperature, pH, and reaction time, showing a certain degree of dependency. The Michaelis constants of scandium oxide crystal are 0.073 and 0.301 mM for H2O2 and TMB, respectively. Finally, an enzymatic colorimetric sensing method based on the scandium oxide crystal is established for the detection of GSH levels in human serum. The corresponding detection limit and linear range are 0.21 and 0.5-50 μM, respectively. This work provides new insights for developing detection materials and sensing methods toward GSH analysis.

A terphenyl derivative-based colorimetric-fluorimetric probe for the detection of lysine and arginine and their bioimaging

Lysine (Lys) and arginine (Arg) play a crucial role in the human diets, medical diagnostics, and functional biomaterial synthesis. The imbalance of intake for these two amino acids causes various diseases. Although many analytical techniques have been reported for the detection of amino acids, there are still some issues such as the need for bulky instruments, professional operators, sensitivity to be improved, and real-time detection. Here, a novel colorimetric-fluorimetric probe based on a terphenyl derivative (TPT) has been synthesized for the precise detection of Lys and Arg. In the EtOH-H2O solution of TPT, the NH2 group at the chain end of Lys/Arg undergoes a nucleophilic addition reaction with CN groups of benzothiazole groups of the probe TPT, which breaks the initial long-conjugated system of the probe molecule. As a result, blue shifts can be observed for both UV-vis absorption spectra and fluorescence spectra, accompanying with color changes of the TPT solution. The UV-vis absorption peak of TPT solution shifts from ∼410 nm to ∼325 nm, and the solution color changes from light-yellow to colorless. The fluorescence emission shifts from ∼580 nm to ∼470 nm and the bright-yellow TPT solution changes to blue under the irradiation of 365 nm UV light. For colorimetric method, the limits of detection (LoD) are 0.82 μM and 0.90 μM for Lys and Arg, respectively. For fluorimetric method, they are 2.02 nM and 1.62 nM for Lys and Arg, respectively. In addition, TPT has good selectivity and anti-interference for Lys and Arg. The synthesized probe TPT has been successfully used for the precise detection of Lys and Arg in drugs and for fluorescence imaging of living cells. This work demonstrates that terphenyl-based derivatives are promising organic probes for the detection of Lys and Arg, providing a new way for designing other amino acids probes.

Lanthanide-Assisted Function Tailoring of the HOF-Based Logic Gate Sensor Array for Biothiol Detection and Disease Discrimination

The advancement of lanthanide fingerprint sensors characterized by targeted emission responses and low self-fluorescence interference for the detection of biothiols is of considerable importance for the early diagnosis and treatment of cancer. Herein, the lanthanide "personality function tailoring" HOF composite sensor array is designed for the specific discrimination of biothiols (GSH, Cys, and Hcy) based on the activation of various luminescent molecules, such as r-AuNCs/luminol via HOF surface proximity. Lumi-HOF@Ce serves as a versatile platform for catalyzing the oxidation of o-phenylenediamine (OPD) to generate yellow fluorescent oligomers, accompanied by the fluorescence attenuation of luminol. HOF@Tb functions as a confinement interface that gathers gold nanoclusters (r-AuNCs) with red fluorescence, facilitating an aggregation-induced emission enhancement (AIEE). The fluorescence properties of AuNCs are subsequently impacted to varying degrees by the Au(I)-thiolate motifs from biothiol rooted in an enhanced ligand-metal charge transfer (LMCT) process. Additionally, the catalytic activity of Lumi-HOF@Ce, which exhibits oxidase-like properties, can be inhibited by different biothiols to varying extents. The five-channel fluorescent array demonstrates exceptional discrimination of biothiol fingerprints, aided by machine learning algorithms. Feature-tailored lanthanide HOF sensor arrays achieve sensitive identification with nearly 100% accuracy in classifying clinical liver cancer samples versus normal samples, using a logic gate strategy. The current strategy of lanthanide function tailoring boosts the suitability of biosensing applications.

An activatable near-infrared fluorescent probe with large Stokes shift for visualizing peroxynitrite in Alzheimer's disease models

Alzheimer's disease (AD), characterized by its incurable nature and prevalence among the elderly, has remained a focal point in medical research. Increasing evidence suggests that peroxynitrite (ONOO-) serves as a crucial biomarker for the diagnosis of AD. In this study, we present a novel, easily available, high-yield, and cost-effective near-infrared (NIR) fluorescent probe, CDCI-ONOO. This probe utilizes a coumarin-dicyanoisophorone conjugate as the fluorophore and diphenylphosphinic chloride as the recognition site, enabling the detection of ONOO- both in vitro and in vivo. Upon interaction with ONOO-, CDCI-ONOO exhibits a distinct maximum emission peak at 715 nm with a substantial Stokes shift of 184 nm. The probe demonstrates excellent selectivity and sensitivity (LOD = 144 nM), along with noticeable colorimetric and fluorescence changes after the reaction. Comprehensive analyses using high-performance liquid chromatography (HPLC), high-resolution mass spectrometry (HRMS), and density functional theory (DFT) calculations confirm that the reaction with ONOO- restores the initially quenched Intramolecular Charge Transfer (ICT), resulting in the formation of CDCI-OH, a product that emitting fluorescence in the near-infrared region. Furthermore, we demonstrated the successful application of CDCI-ONOO for ONOO- detection in neuronal cells and imaging of ONOO- in the brains of mice. These findings underscore the potential of CDCI-ONOO as a near-infrared fluorescent probe for in vivo ONOO- detection, offering a significant avenue for advancing our understanding of AD pathology and diagnosis.

An all-in-one smartphone-assisted ratiometric fluorescent device for visual and quantitative detection of glutathione

The daily consumption of foods abundant in Glutathione (GSH) can be supplemented to maintain the homeostasis of GSH in human health and alleviate pathologies resulting from abnormal GSH levels. The fluorescence-based visual determination of GSH has gradually attracted the attention of researchers due to its robust performance and versatile implementation. However, the current GSH visual strategy primarily relies on variations in fluorescence intensity at a single emission wavelength, which poses challenges for naked-eye and portable readout, as well as distorted signals caused by complex matrix effects in real samples. Herein, a ratiometric fluorescence sensor based on carbon dots (CDs) combined with an all-in-one 3D-printed smartphone-based device was successfully developed for low-cost, visual and rapid detection of GSH without the need for an external excitation light source. The ratiometric fluorescent materials were synthesized by conjugating blue carbon dots (B-CDs) with yellow carbon dots (Y-CDs) through the utilization of selected Cu2+ ions. The resulting mechanism demonstrated that the coordination interaction between Cu2+ and residual aromatic amino groups in Y-CDs (Y-CDs-Cu2+) contributed to a newly emitted peak at 580 nm, thereby inducing fluorescence resonance energy transfer from B-CDs to Y-CDs-Cu2+. A linear correlation was found between GSH concentrations and R/B values in the range of 10-100 μM, with a limit of detection observed at 4.8 μM. By utilizing this portable device in combination with RGB analysis, the quantitative detection of GSH can be achieved even in complex food matrices such as tomatoes and grapes. The universality of this all-in-one device was further validated by pre-spraying CDs onto a paper strip for visual measurement of GSH. This work offers a portable, visual, and accessible approach to evaluating food safety and nutrition, thereby demonstrating significant economic value and holding profound implications for human health.

A lysosome-targeted fluorescent probe with large Stokes shift for visualizing biothiols in vivo and in vitro

Lysosomal biothiols play critical roles in numerous cellular processes and diseases. Researching an effective method for real-time labeling biothiols in lysosomes is of great significance and urgency, as it could provide essential information for the diagnosis of relevant diseases. In this study, we developed a lysosome-targeted fluorescent probe (LY-DCM-P) with a large Stokes shift of 150 nm for the sensitive and selective detection of biothiols in vivo and in vitro. Additionally, LY-DCM-P showed low cytotoxicity and excellent lysosome-targeted ability. The probe was successfully employed to monitor fluctuations in lysosomal biothiols in various living systems, enabling enormous potential to accurately monitor the occurrence and progress of biothiol-related diseases.

Simultaneous Quantitation of Multiple Biological Thiols Using Reactive Ionization and Derivatization with Charged Mass Tags

The biologically important thiols (cysteine, homocysteine, N-acetyl cysteine, and glutathione) are key species in redox homeostasis, and there is a clinical need to measure them rapidly, accurately, and simultaneously at low levels in complex biofluids. The solution to the challenge presented here is based on a new derivatizing reagent that combines a thiol-selective unit to optimize the chemical transformation and a precharged pyridinium unit chosen to maximize sensitivity in mass spectrometry. Derivatization is performed simultaneously with ionization ("reactive ionization"), and mass spectrometry is used to record and characterize the thiol reaction products. The method is applicable over the concentration range from 1 μM to 10 mM and is demonstrated for 25 blood serum, 1 plasma, and 3 types of tissue samples. The experiment is characterized by limited sample preparation (<4 min) and short analysis time (<1 min). High precision and accuracy (both better than 8%) are validated using independent HPLC-MS analysis. Cystine-cysteine redox homeostasis can be monitored by introducing an additional reduction step, and the accuracy and precision of these results are also validated by HPLC-MS.

A near-infrared hepatocyte-targeting probe based on Tricyanofuran to detect cysteine in vivo: Design, synthesis and evaluation

In this work, a near-infrared hepatocyte-targeting fluorescence probe TCF-Gal-Cys was developed. The TCF-Gal-Cys exhibited a low detection limit, good sensitivity and selectivity toward Cys. The responsive mechanism of TCF-Gal-Cys was proposed that the acrylate of TCF-Gal-Cys was subsequently attacked by the thiol group and the amino group of Cys, releasing a strong near-infrared fluorescent group. TCF-Gal-Cys displayed a good hepatocyte-targeting capacity and could specifically distinguish hepatocytes from A549, Hela, SGC-7901 cells because the galactose group of TCF-Gal-Cys can be recognized by HepG2 cells overexpressing ASGPR. The TCF-Gal-Cys has achieved excellently imaging performance to Cys in the zebrafish, so TCF-Gal-Cys has potential to be an effective tool to in real time monitor Cys-related diseases in vitro and in vivo.

A fluorescence probe for monitoring toxic hypochlorous acid in biosystems and environmental waters with a broad pH adaptation

Hypochlorous acid (HOCl) is commonly utilized in various daily applications. As a toxic reactive oxygen species (ROS) that generated from the industrial and pharmaceutical aspects, it plays a critical role in the mass cycle of environmental system and various biological processes. Understanding the complicated roles of HOCl in environment and biosystems requires the development of precise and efficient detection methods. Thus, in this work, a novel fluorescent probe, MQ-ClO, has been designed to detect hypochlorous acid by utilizing hypochlorous acid-triggered oxidative intramolecular cyclization. This probe exhibits rapid and sensitive response, with a detection limit as low as 35 nM. More importantly, the probe is capable of functioning under highly acidic or basic conditions, exhibiting a wide pH range adaptability from pH 2-11. Furthermore, MQ-ClO has been effectively used to detect HOCl in real water samples. Besides, the low toxicity of MQ-ClO enables its practical application in monitoring endogenous/exogenous HOCl levels in living cells as well as zebrafish.

Eu/Tb-MOF as fluorescence sensors for the detection homocysteine in human serum performance and mechanistic investigation

Abnormal homocysteine (Hcy) levels in human serum have been associated with serious or vital diseases, making the reliable and easy detection of Hcy important to clinical analysis and biological study. In this work, five phosphorescent Ir(C^N)2(N^N) complexes (Irn) having aldehyde group were synthesized as probes (C^N and N^N denoted ligands). A discussion was conducted on their molecular structure, electronic structure, photophysical parameters, and Hcy sensing ability, revealing the correlations between their molecular structures and performances. Irn emission was enhanced (by ∼ two folds) and blue-shifted (by 100 nm) after meeting Hcy (free state), via a cyclization reaction between the -CHO group (from Irn) and Hcy. In addition, using RE(BTC) as a supporting material (RE = Tb and Eu), the Ir(III) probe was loaded onto a supporting material of RE(BTC) (H3BTC = 1, 3, 5-benzenetricarboxylic acid). The emission color was changed by increasing Hcy concentration. Straight working curves were obtained with LOD (limit of detection) of 1.9 μM and a response time of ∼200 s. The novelty of this work was the combination of Irn with RE(BTC), which offered enhanced and blue-shifted emission upon Hcy via a cyclization reaction. This demonstrated a high level of sensitivity towards homocysteine detection.

An activatable red emitting fluorescent probe for monitoring vicinal dithiol protein fluctuations in a stroke model

Vicinal dithiol proteins (VDPs) facilitate cellular redox homeostasis, modulate protein synthesis and participate in post-translational modifications through the dynamic equilibrium of dithiol and disulfide bonds. Herein, an activatable red emitting fluorescent probe, VDP-red, is developed for detecting VDPs. With the aid of this probe, we have discovered for the first time a reduction in the levels of reduced VDPs in a stroke mouse model. This work provides a fresh viewpoint for understanding stroke mechanisms.

Mitochondria-Targeted NIR Ratiometric and Colorimetric Fluorescent Probe for Biothiols Based on a Thiol-Chromene Click Reaction

In this work, a mitochondria-targeted NIR ratiometric and colorimetric fluorescent probe 1 was tactfully designed and synthesized by a novel design strategy of modifying chromene to pyridine for the first time. 1 exhibited a maximum absorption peak at 508 nm and a maximum fluorescence emission peak at 650 nm. Under the stimulus of biothiols (cysteine (Cys), homocysteine (Hcy), and glutathione (GSH)), the maximum absorption and fluorescence emission peaks of 1 blue-shifted to 448 and 541 nm, respectively, along with color changes from red to yellow under visible light and from red to green under a 365 nm ultraviolet (UV) lamp, which can be ascribed to the click reaction of biothiols with the α,β-unsaturated ketone of the chromene moiety with pyran ring-opening, phenol formation, and 1,6-elimination of the p-hydroxybenzyl moiety. 1 detected biothiols (Cys, GSH, and Hcy) with high sensitivity (LODs of 29, 23, and 16 nM for Cys, GSH, and Hcy, respectively), excellent selectivity, and fast response. Moreover, 1 can target mitochondria and image the fluctuation of intracellular biothiols by dual-emission channels.

A ratiometric fluorescent probe with a large Stokes shift for rapid and sensitive detection of Hg2+ in environmental water samples and its applications in living cells and zebrafish

Detection of highly toxic mercury ions (Hg2+) in actual environmental and biological samples is of significant importance for protecting environment and human health. In this paper, a new ratiometric fluorescent probe BTIA was designed and synthesized from 3-pinone based on Internal Charge Transfer (ICT) mechanism. BTIA could selectively recognize Hg2+ over other competitive analytes with short reaction time (5 s), distinct ratiometric response, strong anti-interference ability, large Stokes shift (200 nm), and low detection limit (2.36 × 10-7 M). Furthermore, BTIA was applicable for detecting Hg2+ in actual water samples and it also performed an excellent imaging capability in living RAW264.7 cells, zebrafish and onion tissue.

A zero-background fluorescent probe for sensing and imaging of glutathione via the "covalent-assembly" approach

Developing selective and sensitive fluorescent probes for the detection of glutathione (GSH) concentration and intracellular distribution is of great significance for early diagnosis and treatment of diseases such as liver injury and cancer since GSH plays irreplaceable roles in regulating intracellular redox homeostasis. Herein, we present a new fluorescent probe that can be specifically activated by GSH through the conjugate addition and hydrolysis induced covalent-assembly approach for achieving zero-background interference fluorescence off-on sensing. Besides, the probe exhibited prominent selectivity and sensitivity, a low detection limit and cytotoxicity, thus successfully realizing specific real-time monitoring and tracking of GSH levels in living cells. As a consequence, this work might provide a potentially promising candidate for validating the function of GSH in various physiological and pathological processes, which is beneficial for early diagnosis and therapeutics of related diseases.

A dual-functional NIR fluorescence probe for detecting hypochlorous acid and bisulfite in biosystem

BACKGROUND

Bisulfite (HSO3-) serves as a bleaching agent, antioxidant, antimicrobial, and regulator of enzymatic reactions in biosystem. However, abnormal levels of bisulfite can be detrimental to health. Hypochlorous acid (HOCl), which acts as bioactive small molecules, is crucial for maintaining normal biological functions in living organisms. Disruption of its equilibrium can lead to oxidative stress and various diseases. Therefore, it's essential to monitor the fluctuations of HOCl and HSO3- at cellular and in vivo levels to study their physiological and pathological functions.

RESULTS

This study constructed a novel NIR bifunctional colorimetric fluorescent probe using thienocoumarin-indanedione structures to identify hypochlorite (ClO-) and bisulfite (HSO3-). By using CSO-IO to recognize HSO3- and HOCl, two distinct products were generated, displaying green and blue fluorescence, respectively. This property effectively allows for the simultaneous dual-functional detection of HSO3- (LOD: 113 nM) and HOCl (LOD: 43 nM).

SIGNIFICANCE

In this work, the biocompatible molecule CSO-IO has been effectively designed to detect HOCl/HSO3- in living cells and zebrafish. As a result, the dual-functional fluorescent probe has the potential to be utilized as a molecular tool to detect HSO3- derived compounds and HOCl simultaneously within the complex biological system.

Recent advances on nanomaterial-based glutathione sensors

Glutathione (GSH) is one of the most common thiol-containing molecules discovered in biological systems, and it plays an important role in many cellular functions, where changes in physiological glutathione levels contribute to the progress of a variety of diseases. Molecular imaging employing fluorescent probes is thought to be a sensitive technique for online fluorescence detection of GSH. Although various molecular probes for (intracellular) GSH sensing have been reported, some aspects remain unanswered, such as quantitative intracellular analysis, dynamic monitoring, and compatibility with biological environment. Some of these drawbacks can be overcome by sensors based on nanostructured materials, that have attracted considerable attention owing to their exceptional properties, including a large surface area, heightened electro-catalytic activity, and robust mechanical resilience, for which they have become integral components in the development of highly sensitive chemo- and biosensors. Additionally, engineered nanomaterials have demonstrated significant promise in enhancing the precision of disease diagnosis and refining treatment specificity. The aim of this review is to investigate recent advancements in fabricated nanomaterials tailored for detecting GSH. Specifically, it examines various material categories, encompassing carbon, polymeric, quantum dots (QDs), covalent organic frameworks (COFs), metal-organic frameworks (MOFs), metal-based, and silicon-based nanomaterials, applied in the fabrication of chemo- and biosensors. The fabrication of nano-biosensors, mechanisms, and methodologies employed for GSH detection utilizing these fabricated nanomaterials will also be elucidated. Remarkably, there is a noticeable absence of existing reviews specifically dedicated to the nanomaterials for GSH detection since they are not comprehensive in the case of nano-fabrication, mechanisms and methodologies of detection, as well as applications in various biological environments. This research gap presents an opportune moment to thoroughly assess the potential of nanomaterial-based approaches in advancing GSH detection methodologies.

Old drug, new use: The thalidomide-based fluorescent probe for cysteine detection and imaging in living cells

Thalidomide, as a high-profile cereblon (CRBN) ligand, has attracted much attention because of its ability to target protein degradation. In this study, we are committed to developing a new fluorescent probe THD-1 based on thalidomide, aiming at improving the performance of cysteine fluorescent probe in optical properties and biocompatibility. The experimental results showed that THD-1, as a cysteine fluorescent probe, owned the characteristics of obvious colorimetric change, fast response time, good selectivity and high sensitivity. The mechanism of THD-1 sensing cysteine was further verified to ensure its reliability and effectiveness. It was also worth mentioning that THD-1 was successfully applied to the biological imaging of cysteine in living A549 cells, which highlighted its value in practical application. Overall, thalidomide, as a clinically approved drug, not only enriches the fluorescent skeleton library, but also paves a new way for the further development of fluorescent probes.

Synthesis and performance of a nanosensing platform for homocysteine detection: A series of iridium(III) complexes containing aldehyde group as probe and MOF as supporting substrate

The low cost and simple detection method for Hcy (homocysteine) is highly desired in analytical and biological fields since Hcy has been regarded as a bio-marker for multiple diseases. In this work, five Ir(C^N)2(N^N)+ compounds having -CHO group in their C^N or N^N ligand were synthesized and tried for Hcy sensing. Electron-donating groups such as -NH2 and -CH3 were incorporated into the C^N or N^N ligand. Their geometric structure, electronic structure, and optical parameters (with or without Hcy) were analyzed and compared carefully to explore their Hcy sensing potential. The sensing mechanism was revealed by NMR titration and theoretical simulation as a cyclization reaction between the -CHO group and Hcy. The optimal compounds, which showed increased emission quantum yield (2.5-fold) and emission blue-shift (by ∼ 100 nm) upon Hcy, were then covalently grafted into a porous host bio-MOF-1. Linear working plots were fitted, with good selectivity, LOD of 0.15 μM, and response time of 33 s. The novelty of this work was the eye-sensitive emission color change of this nanosensing platform from red (without Hcy) to green (with Hcy).

Efficient turn-on fluorescent probe cooperated by cascade response for disclosing the fluctuation of cysteine in cells

BACKGROUND

The research on cysteine (Cys) determination is deemed as a hot topic, since it has been reported to be connected with various physiological processes and disease prediction. However, existing Cys-responding probes may expose some defects such as long reaction time, disappointing photostability, and suboptimal sensitivity. Under such a circumstance, our team has proposed an efficient fluorescent probe with novel sensing mechanism to perfectly cope with the above-mentioned drawbacks.

RESULTS

A novel cascade reaction-based probe 9-(2,2-dicyanovinyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-yl acrylate (DPQA) has been synthesized for the first time. Undergoing addition-cleavage and cyclization-rearrangement processes, DPQA reacts with Cys to generate an iminocoumarin product with relucent green fluorescence, namely 11-imino-2,3,6,7-tetrahydro-1H,5H,11H-pyrano[2,3-f]pyrido[3,2,1-ij]quinoline-10-carbonitrile (IMC-J), and the relative fluorescence quantum yield (Φf) soars from 0.007 to 0.793. Utilizing such a mechanism, DPQA shows a superb turn-on signal (172-fold), low detection limit (4.1 nM), and wide detection range (5-6000 nM) toward Cys detection. Encouraged by the admirable sensing performance of DPQA, bioimaging of endogenous Cys has been attempted in HeLa cells with satisfactory results. Moreover, cell model of H2O2-induced oxidative stress has been established and the Cys fluctuation during this process has been inspected, elucidating how living cells confront with the eruption of reactive oxygen species (ROS) storm.

SIGNIFICANCE

The probe DPQA with such an intriguing cascade responding process for Cys detection has been endowed with many merits, such as fast reaction and superior sensitivity, conducive to improving responsiveness and rendering it more suitable for further applications. Thereby, we expect that the DPQA would be an efficient tool for detecting Cys fluctuation in living cells of different physiological processes.

Phosphorescent iridium (III) complex with covalent organic frameworks as scaffolds for highly selective and sensitive detection of homocysteine

Introduction: Developing a convenient and cost-effective platform for detecting homocysteine (Hcy) is of great interest as Hcy has been found to be a biomarker for Alzheimer's disease, gastric cancer, and other diseases. Methods: In this study, we synthesized five phosphorescent Ir(C∧N)2(N∧N)+ compounds (Irn, n = 1-5) with various substituents (-CHO or -CHO/-NH2), which were then doped into a covalent organic framework (COF) host via covalent bonding. Results and Discussion: The resulting optimal composites (denoted as Ir4/5@EBCOF) with -CHO/-NH2 substituents not only overcame the self-quenching issue of the bare Ir4/5 complexes but also showed rapid, highly selective, and sensitive detection of Hcy, with a limit of detection (LOD) of 0.23 μM and reaction time of 88 s. The sensing mechanism was revealed as the unique cyclization reaction between Ir(III) and Hcy that forms a six-membered ring. During the process, the color changes in the composites can be observed visually. It is expected that these phosphorescent Iridium (III) complexes with COFs will have the potential to serve as promising platforms for detecting thiols.

Development of Polymethine Dyes for NIR-II Fluorescence Imaging and Therapy

Fluorescence imaging in the second near-infrared window (NIR-II) is burgeoning because of its higher imaging fidelity in monitoring physiological and pathological processes than clinical visible/the second near-infrared window fluorescence imaging. Notably, the imaging fidelity is heavily dependent on fluorescence agents. So far, indocyanine green, one of the polymethine dyes, with good biocompatibility and renal clearance is the only dye approved by the Food and Drug Administration, but it shows relatively low NIR-II brightness. Importantly, tremendous efforts are devoted to synthesizing polymethine dyes for imaging preclinically and clinically. They have shown feasibility in the customization of structure and properties to fulfill various needs in imaging and therapy. Herein, a timely update on NIR-II polymethine dyes, with a special focus on molecular design strategies for fluorescent, photoacoustic, and multimodal imaging, is offered. Furthermore, the progress of polymethine dyes in sensing pathological biomarkers and even reporting drug release is illustrated. Moreover, the NIR-II fluorescence imaging-guided therapies with polymethine dyes are summarized regarding chemo-, photothermal, photodynamic, and multimodal approaches. In addition, artificial intelligence is pointed out for its potential to expedite dye development. This comprehensive review will inspire interest among a wide audience and offer a handbook for people with an interest in NIR-II polymethine dyes.

A ratiometric fluorescent probe based on a novel fluorophore with high selectivity for imaging cysteine in living cells

As a biothiol, cysteine (Cys) is essential to both physiological and pathological processes and has been associated with many diseases, including neurological disorders, rheumatoid arthritis, and renal dysfunction. Therefore, the development of a high-performance probe for detecting Cys levels can help prevent and diagnose disease. In this study, a ratiometric fluorescent probe based on a novel fluorophore was developed for detecting Cys, and it showed high specificity and a rapid response time toward Cys. This probe demonstrates excellent biocompatibility and has been utilized effectively for the imaging of Cys in living cells.

N-Aryl-DABCO Salts as an Unprecedented Sensing Platform for the Detection of Thiols and Selenols

Quaternary N-aryl-DABCO salts were introduced for the first time as a highly selective sensing platform for thiols and selenols. By employing this platform, a highly sensitive coumarin based "off-on" fluorescent probe was designed and synthesized. The probe possesses a good solubility in water, low background fluorescence, and, most importantly, demonstrates high selectivity to aryl thiols and selenols over their aliphatic counterparts and other common nucleophiles. A dramatic increase in fluorescence intensity is achieved through the selective cleavage of the quaternized DABCO-ring, yielding a piperazine derivatives with a high fluorescence quantum yield (~72 %). Moreover, stability of the probe to the most used reducing agents DTT and TCEP was demonstrated. The limits of detection for p-thiocresol and phenyl selenide were evaluated to be 22 nM and 6 nM, respectively.

A single mitochondria-targetable fluorescent probe for visualizing cysteine and glutathione in ferroptosis of myocardial ischemia/reperfusion injury

Ferroptosis plays an important role in the early stage of myocardial ischemia/reperfusion (MI/R) injury, which is closely associated with the antioxidant damage of mitochondrial cysteine (Cys)/glutathione (GSH)/glutathione peroxidase 4 (GPX4) axis. Visualization of Cys and GSH in mitochondria is meaningful to value ferroptosis and further contributes to understanding and preventing MI/R injury. Herein a mitochondria-targetable thiols fluorescent probe (MTTP) was designed and synthesized based on sulfonyl benzoxadiazole (SBD) chromophore with a triphenylphosphine unit as the mitochondria-targeted functional group. Cys and GSH can be differentiated by MTTP with two distinguishable emission bands (583 nm and 520 nm) through the controllable aromatic substitution-rearrangement reaction. Importantly, MTTP is capable of monitoring ferroptosis and its inhibition by measuring mitochondrial Cys and GSH. MTTP was also employed to non-invasively detect ferroptosis during oxygen and glucose deprivation/reoxygenation (OGD/R)-induced MI/R injury in H9C2 cells. In a word, MTTP provides a visual tool that can simultaneously detect Cys and GSH to monitor ferroptosis processes during MI/R injury, which helps for more deeper understanding of the role of ferroptosis in MI/R injury-related diseases.

Treatment evaluation of Rheumatoid arthritis by in situ fluorescence imaging of the Golgi cysteine

Rheumatoid arthritis (RA) is a long-term systemic inflammatory disease that causes severe joint pain. Golgi stress caused by redox imbalance significantly involves in acute and chronic inflammatory diseases, in which cysteine (Cys), as a representative reducing agent, may be an effective biomarker for RA. Hence, in order to achieve RA early detection and drugs evaluation, based on our previous work about innovative Golgi-targeting group, we established a phenylsulfonamide-modified fluorescence probe, Golgi-Cys, for the selective fluorescence imaging of Cys in Golgi apparatus in vivo. By application of Golgi-Cys, the Cys changes under Golgi stress in cells were elucidated. More importantly, we found that the probe can be effectively utilized for the RA detection and treatment evaluation in situ.

Exploring Efficient Dual-Phase Emissive Fluorophores with High Mobility by Integrating a Rigid Donor and Flexible Acceptor

Developing luminogens with a high emission efficiency in both single-molecule and aggregate states, as well as high mobility, shows promise for advancing the iteration and update of organic optoelectronic materials. However, achieving a delicate balance between the plane configuration of luminophores and the strong exciton interactions of aggregates is a formidable task from the molecular design perspective. This dilemma was overcome by integrating a rigid donor and flexible acceptor to establish donor-acceptor (D-A) type emitters. The π-conjugate-extended donor ensures the substantial planarity of these molecules, allowing strong emission in solution with photoluminescence quantum yield values of 86% and 75%. Furthermore, the restricted molecular motion of the aggregation-induced emission moiety and the formation of J-aggregates reduce the quenching effect, leading to a high emissive efficiency of 85% and 91% in the aggregate state. The mildly distorted D-A geometry builds moderate electrostatic interaction, resulting in high mobility with μM,h of 7.12 × 10-5 and 3.27 × 10-4 cm2/V s. Additionally, an improved synthesized procedure for terminal E-configured acrylonitrile with metal-free and concise reaction conditions is presented. The successful application of the synthesized compounds in organic light-emitting diode devices demonstrates the practicability of the molecular design strategy with connecting a rigid donor and flexible acceptor.

A Rationally Designed Prodrug for the Fluorogenic Labeling of Albumin and Theranostic Effects on Drug-Induced Liver Injury

The development of small-molecular fluorogenic tools for the chemo-selective labeling of proteins in live cells is important for the evaluation of intracellular redox homeostasis. Dynamic imaging of human serum albumin (HSA), an antioxidant protein under oxidative stress with concomitant release of antioxidant drugs to maintain redox homeostasis, affords potential opportunities for disease diagnosis and treatment. In this work, we developed a nonfluorogenic prodrug named TPA-NAC, by introducing N-acetyl-l-cysteine (NAC) into a conjugated acceptor skeleton. Through combined thiol and amino addition, coupling with HSA results in fluorescence turn-on and drug release. It was reasoned that the restricted intramolecular motion of the probe under an HSA microenvironment after covalent bonding inhibited the nonradiative transitions. Furthermore, the biocompatibility and photochemical properties of TPA-NAC enabled it to image exogenous and endogenous HSA in living cells in a wash-free manner. Additionally, the released drug evoked upregulation of superoxide dismutase (SOD), which synergistically eliminated reactive oxygen species in a drug-induced liver injury model. This study provides insights into the design of new theranostic fluorescent prodrugs for chemo-selective protein labeling and disease treatments.

N,Si co-doped GQDs: Facile green preparation and application in visual identifying dihydroxybenzene isomers and selective quantification of catechol, hydroquinone and antioxidants

A green economical procedure for preparing N,Si co-doped graphene quantum dots (N,Si-GQDs) using waste toners and ethylene diamine was reported, which not only minimizes waste and promotes recycling but also offers an alternative method for producing N,Si-GQDs. At a pH of 8.5, hydroquinone and catechol underwent oxidation in the presence of air, resulting in the formation of diquinones, specifically p-phenyldiquinone and o-phenyldiquinone. Resorcinol, on the other hand, was converted into monoquinone. The interaction between diquinones and N,Si-GQDs caused a linear fluorescence quenching effect when catechol and hydroquinone were present. However, this effect was minimal in the case of resorcinol. Furthermore, the antioxidants glutathione (GSH) and ascorbic acid (AA) were observed to disrupt the redox equilibrium of catechol and o-phenyldiquinone, leading to the activation of fluorescence. Conversely, hydroquinone and p-phenyldiquinone, due to the highly stable and symmetrical structure of p-phenyldiquinone, did not exhibit this fluorescence activation. Based on the described "Off-On" sensor system, it was possible to visually identify dihydroxybenzene isomers and selectively quantify catechol and hydroquinone in environmental samples, as well as GSH and AA in human serum. The method detection limits were 0.93, 1.35, 2.34, and 1.37 μM for catechol, hydroquinone, GSH, and AA, respectively. In conclusion, the presented procedure offers several advantages, including environmental friendliness, cost-effectiveness, and a means of recycling waste toners. It also demonstrates the successful synthesis of N,Si-GQDs, as well as the potential for their application in the "Off-On" sensor system for the detection and quantification of various analytes.

A D-π-A-based near-infrared fluorescent probe with large Stokes shift for the detection of cysteine in vivo

D-π-A dyes are an ideal strategy for building near-infrared fluorescent probes that have a large Stokes shift due to their excellent properties of adjustable emission wavelength and Stokes shift. Developing a near-infrared (NIR) fluorescent probe (JTPQ-Cys) capable of detecting cysteine (Cys) was the aim of this study. In JTPQ-Cys, julolidine served as the electron donor (D) and quinoline as the electron acceptor (A), with 3,4-ethylenedioxythiophene as the π-bridge. The π-conjugation and vibrational/rotational activity of the molecule were increased by the introduction of 3,4-ethylenedioxythiophene, causing the molecule to exhibit NIR emission and a large Stokes shift. When JTPQ-Cys was used to detect Cys, a clear fluorescence turn-on signal was observed at 741 nm, together with a Stokes shift of 268 nm. The limit of detection of JTPQ-Cys for Cys is 24 nM. Moreover, JTPQ-Cys has been utilized successfully for imaging studies of Cys in cells and zebrafish because it has good photostability, low cytotoxicity, and a high signal-to-noise ratio. Overall, our findings demonstrate the potential of JTPQ-Cys to be one of the best choices for detecting Cys in biological systems, and JTPQ is an ideal fluorophore to construct fluorescence dyes for bioimaging.

Cysteine and homocysteine can be exploited by GPX4 in ferroptosis inhibition independent of GSH synthesis

Ferroptosis is inhibited by glutathione peroxidase 4 (GPX4), an antioxidant enzyme that uses reduced glutathione (GSH) as a cofactor to detoxify lipid hydroperoxides. As a selenoprotein, the core function of GPX4 is the thiol-dependent redox reaction. In addition to GSH, other small molecules such as cysteine and homocysteine also contain thiols; yet, whether GPX4 can exploit cysteine and homocysteine to directly detoxify lipid hydroperoxides and inhibit ferroptosis has not been addressed. In this study, we found that cysteine and homocysteine inhibit ferroptosis in a GPX4-dependent manner. However, cysteine inhibits ferroptosis independent of GSH synthesis, and homocysteine inhibits ferroptosis through non-cysteine and non-GSH pathway. Furthermore, we used molecular docking and GPX4 activity analysis to study the binding patterns and affinity between GPX4 and GSH, cysteine, and homocysteine. We found that besides GSH, cysteine and homocysteine are also able to serve as substrates for GPX4 though the affinities of GPX4 with cysteine and homocysteine are lower than that with GSH. Importantly, GPX family and the GSH synthetase pathway might be asynchronously evolved. When GSH synthetase is absent, for example in Flexibacter, the fGPX exhibits higher affinity with cysteine and homocysteine than GSH. Taken together, the present study provided the understanding of the role of thiol-dependent redox systems in protecting cells from ferroptosis and propose that GSH might be a substitute for cysteine or homocysteine to be used as a cofactor for GPX4 during the evolution of aerobic metabolism.

An extra-large Stokes shift near-infrared fluorescent probe for specific detection and imaging of cysteine

Cysteine (Cys) plays a crucial role in numerous physiological and pathological processes. Therefore, it is imperative to design a highly selective and sensitive near-infrared (NIR) fluorescent probe to monitor Cys. In this study, we have developed a novel NIR fluorescent probe XA based on Xanthene hybrid tetrahydro-acridine salt dye for specifically tracking of Cys, where a chlorine-substituted tetrahydro-acridine acts as a high Cys-reactive site and water-soluble group. Probe XA exhibits a remarkable "turn-on" NIR emission (830 nm) with an extra-large Stokes shift (305 nm) for monitoring Cys. It also has a high selectivity, rapid response time (6 min) and high sensitivity (LOD as 0.5 μM). We fully characterized and discussed the sensing mechanism of XA toward Cys using HPLC and MS spectrums, as well as quantum theory calculations. Furthermore, the excellent properties of NIR fluorescent detection allow this novel probe to successfully monitor fluctuations of exogenous and endogenous Cys concentration levels in living cells and in vivo.

Applications of Pyrrole and Pyridine-based Heterocycles in Cancer Diagnosis and Treatment

BACKGROUND

The escalation of cancer worldwide is one of the major causes of economy burden and loss of human resources. According to the American Cancer Society, there will be 1,958,310 new cancer cases and 609,820 projected cancer deaths in 2023 in the United States. It is projected that by 2040, the burden of global cancer is expected to rise to 29.5 million per year, causing a death toll of 16.4 million. The hemostasis regulation by cellular protein synthesis and their targeted degradation is required for normal cell growth. The imbalance in hemostasis causes unbridled growth in cells and results in cancer. The DNA of cells needs to be targeted by chemotherapeutic agents for cancer treatment, but at the same time, their efficacy and toxicity also need to be considered for successful treatment.

OBJECTIVE

The objective of this study is to review the published work on pyrrole and pyridine, which have been prominent in the diagnosis and possess anticancer activity, to obtain some novel lead molecules of improved cancer therapeutic.

METHODS

A literature search was carried out using different search engines, like Sci-finder, Elsevier, ScienceDirect, RSC etc., for small molecules based on pyrrole and pyridine helpful in diagnosis and inducing apoptosis in cancer cells. The research findings on the application of these compounds from 2018-2023 were reviewed on a variety of cell lines, such as breast cancer, liver cancer, epithelial cancer, etc. Results: In this review, the published small molecules, pyrrole and pyridine and their derivatives, which have roles in the diagnosis and treatment of cancers, were discussed to provide some insight into the structural features responsible for diagnosis and treatment. The analogues with the chromeno-furo-pyridine skeleton showed the highest anticancer activity against breast cancer. The compound 5-amino-N-(1-(pyridin-4- yl)ethylidene)-1H-pyrazole-4-carbohydrazides was highly potent against HEPG2 cancer cell. Redaporfin is used for the treatment of cholangiocarcinoma, biliary tract cancer, cisplatin-resistant head and neck squamous cell carcinoma, and pigmentation melanoma, and it is in clinical trials for phase II. These structural features present a high potential for designing novel anticancer agents for diagnosis and drug development.

CONCLUSION

Therefore, the N- and C-substituted pyrrole and pyridine-based novel privileged small Nheterocyclic scaffolds are potential molecules used in the diagnosis and treatment of cancer. This review discusses the reports on the synthesis of such molecules during 2018-2023. The review mainly discusses various diagnostic techniques for cancer, which employ pyrrole and pyridine heterocyclic scaffolds. Furthermore, the anticancer activity of N- and C-substituted pyrrole and pyridine-based scaffolds has been described, which works against different cancer cell lines, such as MCF-7, A549, A2780, HepG2, MDA-MB-231, K562, HT- 29, Caco-2 cells, Hela, Huh-7, WSU-DLCL2, HCT-116, HBL-100, H23, HCC827, SKOV3, etc. This review will help the researchers to obtain a critical insight into the structural aspects of pyrrole and pyridine-based scaffolds useful in cancer diagnosis as well as treatment and design pathways to develop novel drugs in the future.

A ratiometric fluorescent probe for the detection of biological thiols based on a new supramolecular design

A new ratiometric fluorescent probe is designed and prepared based on the concept of supramolecular encapsulation and dye competition. This supramolecular probe is based on two commercially-available dyes, one common guest and a simple-to-synthesize host. Fluorescence spectroscopy confirms that the supramolecular probe is capable of detecting thiols quantitatively with a broad linear region in phosphate buffered saline or fetal bovine serum. Mechanistic study shows a reaction between thiol specie and the guest to alter the distribution of encapsulated dyes. The supramolecular probes are demonstrated to quantitatively detect extracellular biological thiols by plate reader, which shows it keeps its effectiveness in complex buffered systems.

A simple indanone-based red emission fluorescent probe for the rapid detection of cysteine in vitro and in vivo

Cysteine is a vital biothiols that plays an important role in numerous physiological and pathological processes. The development of simple molecule tools for detection and analysis Cys in subcellar environment is significant for further exploring their pathophysiological. In this work, a simple but activated fluorescent probe AMIA was constructed with a donor-π-accepter (D- π -A) structure, which using an indanone as the electron-withdrawing unit acting as the fluorophore, dimethylamino group attached to the position 4 of the benzene ring as the electron-donating, two double bonds as the linker group, and the acryloyl ester group as the trigger and response unit. This probe AMIA was exhibited highly selective and sensitive response to Cys over other amino acids and ions under physiological conditions. It was found that AMIA showed a red turn-on fluorescence response at 630 nm towards Cys with a large stroke shift of 170 nm and a very low detection limit of 26.3 nM. HRMS, 1H NMR and TD-DFT calculation further confirmed that the response mechanism is the Cys triggered the addition-cyclization reaction between AMIA' acryloyl group and Cys' sulfhydryl and amino unit, leading to the release of a red fluorescent dye AMIA-OH, which can be identified by naked eyes. Furthermore, AMIA was successfully applied for simultaneous determination of Cys in living cells and zebrafish with lower cytotoxicity and good cell permeability. We hope that this novel indanone-based probe AMIA will provide a new reference for visualized Cys in other complex biological system.

Sensitively detecting endogenous homocysteine in human serum and cardiomyocytes with a specific fluorescent probe

The elevated level of homocysteine (Hcy) in circulating blood is generally regarded as a risk factor for a variety of diseases including acute myocardial infarction (AMI), but there is no clear answer to whether circulating Hcy can be used for AMI diagnosis. To address it, here we have designed a tetraazacycle-based fluorescent probe for sensitive detection of endogenous Hcy in AMI patients' serum and cardiomyocytes, showing a perfect selectivity over other biothiols (e.g. Cys and GSH). It mainly relies on the formation of a stable six-membered ring structure when this probe responds to Hcy, which is accompanied by a weakening of photoinduced electron transfer (PET) that induces a sharp increase in the fluorescence emission. In this way, Hcy can be probed in biofluids with high sensitivity. We then employed this fluorescent sensor to statistically analyze the levels of Hcy in human circulating blood, indicating a big difference between AMI patients and the healthy participants. To tell whether such a difference is applicable to AMI diagnosis, we further compare the expression levels of Hcy in cardiomyocytes and other tissue cells. It reveals a lower level of endogenous Hcy in cardiomyocytes, implying no direct relationship between the elevated Hcy and cardiomyocyte damage. This observation suggests that Hcy in circulating blood cannot be utilized as a potential biomarker for AMI diagnosis, although it is proven as a risk factor for this disease.

A Reversible Dual-Channel Near-Infrared Flavonoid Probe for in Vivo Tracking Glutathione Dynamics in Living Mice

Glutathione (GSH) plays a critical role in various biological processes maintaining oxidative homeostasis. However, current reversible probe fluorescence emission is usually in the visible region, making it difficult to monitor glutathione levels in deep tissues and in vivo. Here, we developed a reversible near-infrared fluorescence probe, Flav-N, for real-time tracking of GSH in cells and tissues, which undergoes fast and reversible Michael addition reactions with biothiols. This Flav-N probe showed a rapid and reversible response with GSH at a time of less than 5 s (k = 1286 M-1S-1, t 1/2 = 729 ms). Notably, the dynamic changes in the ratio of Flav-N emission intensity at 505 and 728 nm were able to provide real-time feedback on the fluctuation of GSH concentration. We demonstrated that Flav-N enables the performance of fast and reversible imaging of intracellular GSH changes. Importantly, in light of the near-infrared emission and rapid response ability, Flav-N was successfully applied to track GSH dynamics in living mice. This reversible near-infrared NIR probe realizes advances in deep insight into the function of endogenous GSH.

Atomic electronegativity-dependent intramolecular hydrogen bond and fluorescence characteristics of novel scaffold-based fluorophore: a TD-DFT study

In this work, fluorescent properties and excited-state intramolecular proton transfer (ESIPT) processes of 2,5-bis(benzo[d]thiazol-2-yl)phenol (BTP) and its derivatives (BOP and BSeP) with different heteroatom atoms (O and Se) have been systematically explored by the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated absorption and fluorescence emission peaks agree well with the experimental values in acetonitrile. From the data of structures, topological parameters, reduced density gradient analyses, and infrared (IR) vibrational frequencies, the intramolecular hydrogen bonds (IHBs) of BTP and its derivatives are enhanced upon light-excitation. The potential energy curves show that the ESIPT process occurs in BTP and its derivatives after surmounting 0.167-0.306 eV energy barrier. The strength of intramolecular hydrogen bond, HOMO-LUMO energy gap, and red-shifted value of absorption and fluorescence emission wavelengths are dependent on the electron-withdrawing ability of heteroatom from O to S and Se. We believe that this work can pave the way for developing a new ESIPT-based fluorophore with better luminescent properties.

Selection of regioselective DNA aptamer for detection of homocysteine in nondeproteinized human plasma

Small molecule-binding aptamers often suffer from high cross reactivity to structure analogues in biological samples, limiting their value for clinical diagnosis. Herein, we present a method to overcome this issue, by performing binding-inhibited organic reaction-based regioselective selection of aptamers against homocysteine (Hcy), which is a marker for diagnosing many disorders including stroke and Alzheimer's. This approach has led to isolation of a DNA aptamer that binds to the alkane thiol chain of Hcy with exceptional specificity against cysteine. It also binds with oxidized Hcy at weaker affinity. Using this new aptamer, we produced a reusable fluorescent optical fiber aptasensor for direct and validated detection of both free and total Hcy in nondeproteinized patient plasma in the diagnostic concentration range. The binding site-specific aptamer selection and optical-fiber-sensing strategy can expand the practical utility of aptamers in clinical diagnosis.