Visual Assay of Glutathione in Vegetables and Fruits Using Quantum Dot Ratiometric Hybrid Probes

Facile electrochemiluminescence sensing platform based on Gd2O3:Eu3+ nanocrystals for organophosphorus pesticides detection in vegetable samples

In this work, europium ion-doped gadolinium trioxide nanocrystals (Gd2O3:Eu3+ NCs) were successfully synthesized and applied to construct an electrochemiluminescence (ECL) sensor. Compared with pure Gd2O3, the doping of Eu3+ ions caused enhanced ECL intensity and more stable signals. Based on the excellent ECL performance of Gd2O3:Eu3+ NCs, we constructed a new ECL sensing platform for the detection of organophosphorus pesticides (OPs). The ECL sensor showed a good linear relationship in the concentration range of 1 nM to 1 pM, with a limit of detection of 0.12 pM (S/N = 3) for dichlorvos (DDVP). In addition, the constructed ECL sensor was applied for the detection of DDVP in vegetable samples, and good recoveries were obtained. The results indicated that the ECL sensor exhibited fantastic performance properties and had good application prospects in OPs detection.

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

A dummy molecularly imprinted ratiometric fluorescence nanosensor for the sensitive detection of guanidyl-microcystins in environmental water

An effective strategy is proposed to construct a highly sensitive ratiometric fluorescence sensing platform for microcystins (MCs) based on a dummy molecularly imprinted polymer using metformin as a template. The imprinted nanohybrids of carbon dots (CDs) combined with fluorescein isothiocyanate (FITC) are synthesized (CDs-FITC-SiO₂@MIP), in which the CDs and FITC serve as assisted response signals and reference enhancement signals, respectively. Metformin can be used as a dummy template for MCs due to its partially similar molecular fragments to MCs that can form a specific recognition site cavity. MCs can simultaneously induce an obvious fluorescence quenching effect for the CDs and a reference fluorescence enhancement for FITC-SiO₂, enabling ratiometric fluorescence detection of MCs. Thus, CDs-FITC-SiO₂@MIP used as a signal probe has favorable sensitivity, stability, and selectivity. More importantly, a good linear relationship between the fluorescence intensity ratio (I_620/450) and the concentration of MCs in the range of 0.5-500 μg L^-1 is obtained with a LOD of 0.013 μg L^-1 and 0.022 μg L^-1 for MC-RR and MC-LR, respectively, under the optimum conditions. This method has great application potential in water quality monitoring by using CDs-FITC-SiO₂@MIP as a promising candidate for monitoring MCs in complex systems.

In-situ formation of MnO2 nanoparticles on Ru@SiO2 nanospheres as a fluorescent probe for sensitive and rapid detection of glutathione

Glutathione (GSH)-switched fluorescent assays have appealed much attention due to rapid signal changes of fluorescent probes. However, exposure to exterior environment of fluorescent probe causes photobleaching and premature leakage, leading to low sensitivity and poor photostability. Herein, luminescent SiO₂ nanoparticles encapsulated with Ru(bpy)₃²⁺ (Ru@SiO₂) were designed and synthesized as fluorescent probe to construct a GSH-switched fluorescent assay. The encapsulation of Ru(bpy)₃²⁺ in the SiO₂ nanoparticles could effectively prevent the leakage of Ru(bpy)₃²⁺ molecules, improving the photostability of probe. The fluorescence of Ru@SiO₂ nanoparticles was quenched by coating MnO₂ nanoparticles on Ru@SiO₂ surface (Ru@SiO₂@MnO₂ nanocomposites) through an in situ growth approach, which reduced background of the assay. The MnO₂ nanoparticles not only further inhibited the leakage of Ru(bpy)₃²⁺ molecules, but also could serve as a recognition unit of GSH. In the presence of GSH, the MnO₂ nanoparticles on the surface of Ru@SiO₂ nanoparticles were reduced to Mn²⁺, resulting the fluorescence recovery of Ru@SiO₂ nanoparticles. Thus, a signal-on fluorescent strategy was constructed for GSH detection. The assay displayed good analytical performance for GSH detection with a low detection limit of 16.2 nM due to excellent fluorescence quenching ability of MnO₂ nanoparticles and special role of Ru@SiO₂ nanoparticles to block probe leakage. The proposed assay was also applied to measure GSH levels in human serum samples. This work paves a new way to detect GSH with high sensitivity.

Iron porphyrin-based porous organic polymer with high peroxidase-like activity as colorimetric sensor for glutathione and ascorbic acid assay

A new iron porphyrin-based organic polymer (Fe-POP) was synthesized through the William ether reaction. The as-prepared Fe-POP presented high chemical stability, wide pore distribution, high iron content, and strong affinity with 3,3',5,5'-tetramethylbenzidine (TMB) and hydrogen peroxide (H₂O₂), which contributed to its excellent peroxidase-mimicking performance. In the presence of H₂O₂, Fe-POP could catalyze the transparent TMB into blue ox-TMB, which could be easily distinguished by the naked eyes. Moreover, glutathione (GSH) and ascorbic acid (AA) could convert blue ox-TMB into colorless TMB due to the inhibitory effect of GSH/AA to the catalytic oxidation of TMB. Based on this phenomenon, a rapid and sensitive colorimetric method for the assay of H₂O₂, GSH, and AA was developed using Fe-POP as sensor. The detection limits of H₂O₂, GSH, and AA  were 1.37, 0.44, and 0.33 μM, respectively. Finally, the colorimetric method based on Fe-POP was used to evaluate the GSH and AA content in real samples, which provided the guidance for GSH and AA supplements in our daily diet, suggesting the significant potential of Fe-POP in practical applications.

Simultaneous quantification of glutathione, glutathione disulfide and glutathione-S-sulfonate in grape and wine using LC-MS/MS

A fast, sensitive and reproducible method using LC-MS/MS for simultaneous quantification of glutathione (GSH), glutathione disulfide (GSSG) and glutathione-S-sulfonate (GSSO₃H) was developed, optimised and applied in analysis of grape juice and wine samples. The results show that only GSH (10-60 mg·L^-1) and GSSG (2-11 mg·L^-1) are found in grape juice when SO₂ is not added. GSSO₃H was detected in must samples treated with SO₂ but only at a low concentration (<1 mg L^-1). In the wine samples, the dominant form of glutathione was GSSO₃H (5-11 mg L^-1), followed by GSH (0-5 mg L^-1) and GSSG (0-6 mg L^-1), underscoring the importance of GSSO₃H quantification. GSSO₃H formation in wine was correlated with the total SO₂ level in the wine. We believe this is the first report on GSSO₃H quantification in wine.

Intelligent multicolor nano-sensor based on nontoxic dual fluoroprobe and MOFs for colorful consecutive detection of Hg2+ and cysteine

Ultrasensitive detection of Hg²⁺ in aquatic ecosystems is of great significance due to its high toxicity and ubiquity in water. Herein, using a one-step in-situ synthesis method, blue fluorescent carbon dots (CDs), red fluorescent InP/ZnS quantum dots (InPQDs), and MOFs (ZIF-8) integrated multicolor nano-sensor CDs/InPQDs@ZIF-8 was constructed for consecutive visual detection of Hg²⁺ and Cys. The InPQDs can act as the response unit for Hg²⁺ and Cys, with the limit of detection (LOD) of 8.68 and 37.96 nM, respectively. Significantly, the low detection limit combines with good specificity and accuracy of the nano-sensor meet the requirement for the safety monitoring and control of Hg²⁺ in drinking and environmental water. Moreover, the color recognition and processing software installed on smart phone can realize the real-time and rapid sensing of Hg²⁺ and Cys. A logic gate circuit was also devised, providing the possibilities for the application of the nano-sensor in the field of intelligent devices. As far as we know, this was the first example to apply InPQDs to the continuous multicolor visual detection of Hg²⁺ and Cys, which provided reference for the construction of environmentally-friendly dual emission fluorescent sensors for hazardous substance monitoring.

Biomass carbon dots derived from Wedelia trilobata for the direct detection of glutathione and their imaging application in living cells

The development of long-wavelength emission carbon dots as a label-free sensing platform for the direct detection of glutathione (GSH) is still of great challenge. Herein, novel rose-red fluorescence carbon dots (wCDs) with a long-wavelength emission of 654 nm were successfully synthesized by a one-step microwave-solvothermal treatment of biomass Wedelia trilobata. The biomass-based wCDs could sensitively respond to copper ions (Cu2+) with a limit of detection (LOD) of 0.22 μM, which could be utilized as the fluorescence turn off-on sensor for the detection of GSH through recovering their fluorescence quenched by Cu2+. Moreover, the label-free wCDs with abundant active functional groups could be used to construct a direct quenching platform for the sensitive and selective detection of GSH by the dynamic quenching effect within 20 s without further modification, displaying a good linear range of 0-3.0 mM with a LOD of 35 μM that fits well with the GSH content in living cells (0.5-10 mM). Additionally, confocal imaging of living cells indicates that as-prepared wCDs with high photobleaching resistance and favorable biocompatibility possess the capacity for real-time monitoring of the variations of intracellular Cu2+ and GSH levels, and distinguishing cancerous cells from normal cells owing to the overexpression of GSH in cancerous cells. This study opens a new vision for developing an efficient and sensitive fluorescent sensor for the detection of GSH, and demonstrates the great prospects for wCDs in ion/molecule tracking, bioimaging and cancer diagnosis.

Bright-yellow-emissive nitrogen-doped carbon nanodots as a fluorescent nanoprobe for the straightforward detection of glutathione in food samples

In this work, a novel yellow-emissive nanoprobe was for the first time developed for the fast detection of glutathione (GSH) based on nitrogen-doped carbon nanodots (N-CNDs) prepared via hydrothermal heating of o-Phenylenediamine. The N-CNDs and GSH could form non-fluorescent complex via static interaction, resulting in the fluorescence quenching of N-CNDs. Under optimal conditions, the N-CNDs served as a fluorescent nanoprobe for GSH sensing in a straightforward way with high selectivity and sensitivity. Two good linear responses were found for GSH detection in concentration ranges of 0.1-1.0 μM and 1.0-220.0 μM, respectively. The corresponding detection limits are as low as 0.059 μM and 5.54 μM, respectively. Meanwhile, the proposed sensing system was successfully applied for GSH determination in vegetable and fruit samples with high accuracy. This work highlights the detection of GSH in a simple, fast, cost-effective, selective and ultrasensitive way, which paves a new way for other food quality monitoring.

Dual-emission carbon dots-stabilized copper nanoclusters for ratiometric and visual detection of Cr2O72- ions and Cd2+ ions

The pollution of heavy metal increases greatly accompanying by the development of industries. So, it is very important to build up a quick and reliable technique for detection of heavy metal ions. In this work, we developed a simple and convenient method for ratiometric and visual detection of Cr₂O₇^2- ions and Cd²⁺ ions. We utilized glutathione as raw material to prepare cyan-emitting carbon dots (GSH@CDs). The GSH@CDs were further used as the template to prepare carbon dots-stabilized copper nanoclusters (GSH@CDs-Cu NCs) that displayed two well-separated emission peaks respectively at 450 nm and 750 nm. The GSH@CDs-Cu NCs can be applied for the ratiometric and visual detection of Cr₂O₇^2- and Cd²⁺ ions based on the fluorescence quenching or enhancement of GSH@CDs-Cu NCs at 750 nm. A linear range of 2-40 μmol L^-1 with a detection limit of 0.9 μmol L^-1 for Cr₂O₇^2- ions, and a linear range of 0-20 μmol L^-1 with a detection limit of 0.6 μmol L^-1 for Cd²⁺ ions were achieved based on this method. The fluorescent test strips for Cr₂O₇^2- ions were successfully prepared based on GSH@CDs-Cu NCs. Moreover, the GSH@CDs-Cu NCs were successfully applied to determine Cr₂O₇^2- and Cd²⁺ ions in real samples with promising results.

Controllable and robust dual-emissive quantum dot nanohybrids as inner filter-based ratiometric probes for visualizable melamine detection

The ratiometric fluorescence technique is of great interest due to its visualization characteristics. The construction of a reliable fluorescent ratiometric nanoprobe for high-sensitivity visual quantification is highly sought after but it is limited by poor stability and controllability. Herein, we report a robust dual-emissive quantum dot nanohybrid with precise color tunability and demonstrate its potential as a two-signal-change ratiometric probe for visual detection. A novel assembly strategy was developed for spatially implanting hydrophobic green and red quantum dots (QDs) into a silica scaffold to form a dual-emissive hierarchical fluorescent silica nanohybrid. The fluorescence intensity ratio and color of the nanohybrid were precisely tailored by altering the amounts of green and red QDs. Particularly, after the alkylsilane-mediated phase transfer and exterior silica shell growth, the nanohybrid exhibited the well-preserved fluorescence features of the original QDs and robust optical/colloid stability. An inner filter-based ratiometric nanoprobe for the visual determination of melamine was ultimately devised by combining the spectra-overlapped two-colored fluorescent nanohybrid with analyte-specific gold nanoparticles. Furthermore, based on the reversible fluorescence signal changes in two-colored QDs induced by melamine, a logic gate strategy for melamine monitoring was constructed. The newly developed fluorescent ratiometric nanoprobe shows great prospects for the visual and quantitative determination of analytes in a complex biological matrix.