Folic acid incorporated nitrogen-doped carbon dots as a turn-on fluorescence probe for homocysteine detection

This study describes the development of a low-cost fluorescence assay for detecting homocysteine (Hcy) without the interference of cysteine and glutathione using carbon quantum dots. Herein nitrogen-doped carbon quantum dots (NCDs) were synthesized from citric acid as the carbon source and urea as the dopant using a one-pot microwave-assisted method. The obtained NCDs were incorporated with folic acid (FA) by the direct ex situ addition method and were used as a fluorescence probe to detect Hcy. The probe exhibited a fluorescence turn-on response with increased Hcy concentration up to 50 μM with a limit of detection of 2.276 μM. The point of care detection of Hcy using the probe was also tested with a paper-based assay strip.

Lysosome-Targeting Fluorescence Sensor for Sequential Detection and Imaging of Cu2+ and Homocysteine in Living Cells

A conjugated polymer-based fluorescence sensor, namely, PTNPy, was constructed on the basis of a polythiophene scaffold coupled with dimethylpyridylamine (DPA) groups in side chains for the consecutive detection and quantification of Cu²⁺ and Hcy in a perfect aqueous medium. A dramatic fluorescence quenching of PTNPy by the addition of Cu²⁺ was observed in Tris-HCl buffer solution (2 mM, pH 7.4), demonstrating a quick (<1 min) and highly selective response to Cu²⁺ with a low limit of detection of 6.79 nM. Subsequently, the Cu²⁺-quenched fluorescence of PTNPy can be completely recovered by homocysteine (Hcy), showing excellent selectivity to Hcy over other competitive species such as cysteine and glutathione. Thanks to the low cytotoxicity and lysosomal targeting ability of PTNPy, it was further applied as an optical sensor for the sequential imaging of Cu²⁺ and Hcy in HeLa cells. More importantly, Hcy concentration was linearly related to the fluorescence intensity of PTNPy in living cells, demonstrating huge potential for real-time monitoring the fluctuation of Hcy levels in living cells.

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.

Coumarin-based fluorescent probe with 4-phenylselenium as the active site for multi-channel discrimination of biothiols

Biological mercaptans, also known as biothiols, play their own roles in a number of important physiological processes, and the abnormal levels of biothiols are closely associated with a variety of...

Direct Quantification and Visualization of Homocysteine, Cysteine, and Glutathione in Alzheimer's and Parkinson's Disease Model Tissues

Alzheimer's disease (AD) and Parkinson's disease (PD) are chronic neurodegenerative diseases with high morbidity and mortality. Homocysteine (Hcy), cysteine (Cys), and glutathione (GSH) are closely related to AD and PD. However, the dynamics of Hcy, Cys, and GSH in the brain tissues and the potential pathogenesis between Cys/Hcy/GSH with AD and PD remain unclear. Herein, a novel fluorescent probe 1 with multiple binding sites was rationally designed and exploited for the direct quantification of serum total Hcy and Cys along with superior optical properties. Importantly, differentiation and simultaneity fluorescence imaging of Cys, Hcy, and GSH dynamics were achieved in living cells, tissues, and mouse models of AD and PD with this probe, providing direct evidences for the relationship between Hcy/Cys/GSH and AD/PD for the first time. In addition, pathogenesis studies demonstrated that elevated Hcy and Cys levels are closely related to imbalanced redox homeostasis, increased amyloid aggregates, and nerve cell cytotoxicity. These findings will greatly promote the understanding of the functions of Hcy/Cys/GSH in Alzheimer's and Parkinson's diseases, demonstrating clinical promise for the early diagnosis and prevention of AD and PD.

The chronological evolution of small organic molecular fluorescent probes for thiols

Abnormal concentrations of biothiols such as cysteine, homocysteine and glutathione are associated with various major diseases. In biological systems, the structural similarity and functional distinction of these three small molecular thiols has not only required rigorous molecular design of the fluorescent probes used to detect each thiol specifically, but it has also inspired scientists to uncover the ambiguous biological relationships between these bio-thiols. In this minireview, we will discuss the evolution of small organic molecular fluorescent probes for the detection of thiols over the past 60 years, highlighting the potent methodologies used in the design of thiol probes and their particular applications in the semi-quantification of cellular thiols and real-time labelling. At the same time, the present challenges that limit their further application will be discussed. We hope that this minireview will promote future research to enable deeper insight into the crucial role of thiols in biological systems.

The chronological evolution of small organic molecular fluorescent probes for thiols: from separation dependency analysis to cellular specific analysis, what's next?

Fluorescent probe for sensitive discrimination of Hcy and Cys/GSH in living cells via dual-emission

Abnormal levels of Cys, Hcy and GSH are associated with various diseases, thus monitoring biothiols is of great significance. In this work, a dual-emission responsive near-infrared fluorescent probe NIR-NBD for detecting Hcy and Cys/GSH was developed based on the conjugation of a dicyanoisophorone based fluorophore (NIR-OH) and 7-nitrobenzofurazan (NBD). To our surprise, the addition of Hcy induced significant fluorescence enhancement at both 549 and 697 nm; while Cys/GSH resulted in major fluorescence emission at 697 nm. The detection limit was determined to be 33.2 nM for Cys, 33.5 nM for Hcy, and 34.4 nM for GSH. Therefore, the probe can be used for discriminative detection of Hcy and Cys/GSH. Moreover, fluorescence imaging of HeLa cells indicated that the probe was cell membrane permeable and could be used for visualizing Hcy and Cys/GSH in living cells.

A dual-response fluorescent probe for the discrimination of cysteine from glutathione and homocysteine

A highly selective and sensitive turn-on fluorescent BODIPY-based probe for the simultaneous and selective detection of Cys and Hcy/GSH from dual emission channels was developed. The spatial steric hindrance of the methyl groups at 1- and 7-positions in BODIPY skeleton prevented intramolecular displacement of sulfur with amino group of Hcy but not of Cys. GSH molecular skeleton is larger and amino is far away from sulfydryl group, and the product of the reaction of probe with GSH canstay in thiol phase. Therefore, the probe was successfully applied to the detection of Cys from GSH/Hcy. The confocal microscopy experiments implied that this probe is a promising candidate for imaging of Cys and Hcy/GSH in Hela cells.

A coumarin-based dual optical probe for homocysteine with rapid response time, high sensitivity and selectivity

In this study, a new coumarin-based fluorescent and chromogenic dual channel probe (DC) was used for the selective detection of homocysteine (Hcy) over other amino acids, especially for cysteine (Cys) and glutathione (GSH). When Hcy is present in the solution, the remarkable fluorescence enhancement and obvious blue shift in UV-vis spectra can be observed. In addition, the color change from purple to yellow can be observed clearly by unaided eyes. This probe DC has fast response time, excellent sensitivity and selectivity to Hcy. A linear relationship exists between the ratio of emissions at 486 and 625 nm, and Hcy can be detected in a wide concentration range (0-200 μM). The signal-to-background ratio of fluorescence at 486 nm can reach 8.4, and the detection limit is calculated to be 3.5 µM. The response mechanism is proved to be the Michael addition reaction by Hcy. Preliminary results on cell imaging enable the practical application of Hcy tracing in living cells.

A new fluorescence turn-on probe for biothiols based on photoinduced electron transfer and its application in living cells

A new biothiol-selective fluorescent probe 1 based on photoinduced electron transfer (PET) mechanism was designed and synthesized. The UV-Vis absorption and fluorescent emission properties of probe 1 towards various analytes were studied in detail. The probe exhibited a large stokes shift (~200nm) after reacted with biothiols and could selectively detect cysteine (Cys) in dimethyl sulfoxide (DMSO)/H2O solution (9:1, v/v, 10mM phosphate buffer saline, pH3.5) over glutathione (GSH), homocysteine (Hcy) and other analytes with a detection limit of 0.117μM. In addition, probe 1 responded well to GSH, Hcy and Cys in the same above solution with pH5.5 and got the detection limits of 0.151μM, 0.128μM and 0.037μM, respectively. Probe 1 was of very low cytotoxicity and successfully applied for imaging of thiols in living cells.

Fluorescein Tri-Aldehyde Promotes the Selective Detection of Homocysteine

Elevated homocysteine levels are a well-known independent risk factor for cardiovascular disease. To date, relatively few selective fluorescent probes for homocysteine detection have been reported. The lack of sensing reagents and remaining challenges largely derive from issues of sensitivity and/or selectivity. For example, homocysteine is a structural homologue of the more abundant (ca, 20-25 fold) aminothiol cysteine, differing only by an additional methylene group side chain. Fluorescein tri-aldehyde, described herein, has been designed and synthesized as a sensitive and selective fluorophore for the detection of homocysteine in human plasma samples. It responds to analytes selectively via a photoinduced electron transfer (PET) inhibition process that is modulated by predictable analyte-dye product hybridization and ionization states. Mulliken population analysis of fluorescein tri-aldehyde and its reaction products reveals that the characteristic formation of multiple cationic of homocysteine-derived heterocycles leads to enhanced relative negative charge build up on the proximal phenolate oxygen of the fluorophore as a contributing factor to selective emission enhancement.

A fluorescent probe for relay recognition of homocysteine and Group IIIA ions including Ga(iii)

Relay fluorescence recognition of homocysteine and gallium ions was realized by sequential chemodosimeter and chemosensor approaches for the first time.

A simple excited-state intramolecular proton transfer probe based on a new strategy of thiol–azide reaction for the selective sensing of cysteine and glutathione

A simple azido-substituted fluorescent sensorAHBObased on the mechanism of nucleophilic substitution–rearrangement reactions showed selective turn-on response to cysteine (Cys) and glutathione (GSH).

The structural characterization and biological activity of sulfamethoxazolyl-azo-p-cresol, its copper(ii) complex and their theoretical studies

Sulfonamide-azophenol and its copper(ii) complex exhibit antimicrobial activity and interaction with DNA. Molecular docking was used to determine the mechanism of drug action.

Acid-Activatable Michael-Type Fluorescent Probes for Thiols and for Labeling Lysosomes in Live Cells

A Michael addition is usually taken as a base-catalyzed reaction. Most fluorescent probes have been designed to detect thiols in slightly alkaline solutions (pH 7-9). The sensing reactions of almost all Michael-type fluorescent probes for thiols are faster in a high pH solution than in a low pH solution. In this work, we synthesized a series of 7-substituted 2-(quinolin-2-ylmethylene)malonic acids (QMAs, substituents: NEt2, OH, H, Cl, or NO2) and their ethyl esters (QMEs) as Michael-type fluorescent probes for thiols. The sensing reactions of QMAs and QMEs occur in distinct pH ranges, pH < 7 for QMAs and pH > 7 for QMEs. On the basis of experimental and theoretic studies, we have clarified the distinct pH effects on the sensing reactivity between QMAs and QMEs and demonstrated that two QMAs (NEt2, OH) are highly sensitive and selective fluorescent probes for thiols in acidic solutions (pH < 7) and promising dyes that can label lysosomes in live cells.

A water soluble glucopyranosyl conjugate as a selective and reactive probe for cysteine in a buffer and its application to living cells

A water soluble and biocompatible glucopyranosyl conjugate (L) has been synthesized, characterized and shown to be selective to Cys among the naturally occurring amino acids. TheLshows green fluorescence upon reaction with –SH containing molecules present in biological cells and hence provides cell imaging.

Heavy atom quenched coumarin probes for sensitive and selective detection of biothiols in living cells

Fluorescence coumarin sensors for biothiols were developed based on the quenching mechanism of the heavy atom effect.