Switch-on fluorescent strategy based on crystal violet-functionalized CdTe quantum dots for detecting L-cysteine and glutathione in water and urine

A red-emitting Europium(III) complex as a luminescent probe with large Stokes shift for the sequential determination of Cu2+ and biothiols in real samples

In this work, a novel Eu³⁺-DTPA-bis(AMC) complex with red luminescence was designed and synthesized for sequential detection of Cu²⁺ and biothiols (Cys/Hcy/GSH) based on the displacement strategy with the good selectivity, high sensitivity, and large Stokes shift (288 nm). The possible detection mechanism was verified by UV-vis, the high-resolution mass spectrometry, and the fluorescence decay curve. The experimental parameters, including the solution pH, the incubation time, the concentration ratio of Eu³⁺-DTPA-bis(AMC) to Cu²⁺ and biothiols concentration, were optimized. Under the optimal conditions, it shows a good linear relationship between the concentration (0-10 μM) of Cu²⁺ and the fluorescence intensity of Eu³⁺-DTPA-bis(AMC), with a low detection limit of 0.065 μM. The linear range and the limit of detection of the Eu³⁺-DTPA-bis(AMC)/Cu²⁺ system for Cys/Hcy/GSH were 2.5-22.5/5-45/5-50 μM and 0.11/0.07/0.05 μM, respectively. Surprisingly, the high or low concentration of Eu³⁺-DTPA-bis(AMC)/Cu²⁺ can significantly affect the selectivity of the sensing system to biothiols (Cys/GSH/Hcy). When the concentration of the Eu³⁺-DTPA-bis(AMC)/Cu²⁺ system is 10.0 μΜ, it could recognize biothiols (Cys/GSH/Hcy) from other substances, but when the concentration is as low as 3.3 μM, it could further specifically distinguished Cys from Hcy/GSH. Owing to the high anti-interference characteristics, accuracy and specificity, the sensing system was well applied to the cascade detection of Cu²⁺ in actual environmental samples and Cys in biological and food samples, including FBS, urine, milk, beverage, fresh juice with the satisfactory recoveries from 96.20 to 106.80 %.

Co-N-C single-atom nanozymes with oxidase-like activity for highly sensitive detection of biothiols

Biothiol detection is of great importance for clinical disease diagnosis. Previous nanozyme-based colorimetric sensors for biothiol detection showed unsatisfactory catalytic activity, which led to a high detection limit. Therefore, developing new nanozymes with the high catalytic activity for biothiol detection is extremely necessary. Recently, single-atom nanozymes (SAzymes) have attracted much attention in biosensing due to their 100% atom utilization and excellent catalytic activity. Most previous works focus on the peroxidase-like activity of Fe-based SAzymes by using unstable and destructive H₂O₂ as the oxidant. It is essential to develop new SAzymes with high oxidase-like activity for biosensing to break through the limitation. Herein, Co-N-C SAzymes with high oxidase-like activity are explored. Furthermore, Co-N-C SAzymes are used as a biosensor for colorimetric detection of biothiols (GSH/Cys) based on the inhibition of thiols toward the oxidase-like activity of Co-N-C SAzymes, which showed high sensitivity with a low detection limit of 0.07 µM for GSH and 0.06 µM for Cys. Besides, the method showed good reproducibility and high selectivity against other amino acids. This work offers new insights using Co-N-C SAzymes in the biosensing field.

Self-assembled nanoplatforms with ZIF-8 as a framework for FRET-based glutathione sensing in biological samples

A nanoprobe was constructed by embedding QDs and a rhodamine B derivative (RBD) into ZIF-8. Then, the ultraviolet absorption of RBD that reacted with glutathione can overlap with the emission spectrum of the QDs, causing FRET-based glutathione sensing.

Fluorescent reversible regulation of cysteamine-capped ZnSe quantum dots successively induced by photoinduced electron transfer of herring sperm DNA and intercalation binding of ethidium bromide

A fluorescent reversible regulation was studied by fluorescence spectra, ultraviolet-visible spectra in the combination of molecular docking, which based on the photoinduced electron transfer(PET) from hsDNA (herring sperm DNA) to CA (cysteamine)-capped ZnSe QDs (quantum dots) and intercalation of ethidium bromide (EB) into hsDNA. It was proven that the QDs bound with the adding hsDNA by electrostatic force and formed 1:1 hsDNA-QDs complexes, leading to the PET from hsDNA to QDs, and consequently the fluorescence quenching of the QDs; with EB being added in the complex solution, it bound with hsDNA by intercalation interaction and caused hsDNA releasing from hsDNA-QDs complex with forming 2.5:1 EB-hsDNA complex, leading to the recovery of fluorescence, based on the greater binding constant (1.74 × 106 L·mol-1) of hsDNA with the embedded EB comparing to that of QDs with the captured hsDNA (4.25 × 104 L·mol-1). A good linear relationship existed between the fluorescence recovery yield and the EB concentrations under the range of 1.0-12.0 × 10-6 mol·L-1 with bare interference of related substances. This work provided some useful insights into the study of binding mechanism between DNAs with their intercalators and fluorescence bi-direction regulation, and showed great potential for the determination of trace EB.

Novel synthesis of Mn: ZnSe@ZnS core-shell quantum dots based on photoinduced fluorescence enhancement

A novel Type-I Mn: ZnSe@ZnS core-shell quantum dots (QDs) was reported through a two-step procedure by using low-cost inorganic salts and naturalbiomacromolecule as raw materials. Based on a designed structure of L-cysteine-capped Mn: ZnSe QDs in aqueous media with the controllable surface, Mn: ZnSe@ZnS core-shell QDs were formed due to photoactive ions and defect curing under continuous constant light. The influences of experimental variables, including synthesis conditions of Mn: ZnSe QDs, different types and affecting factors of photo irradiation had been systematically investigated. Under the effect of photoinduced fluorescence enhancement, the photoluminescence (PL) intensity increases significantly by about 5-10 times after 1-3 h of UV irradiation. The position of the fluorescence peak was red-shifted by about 17 nm, emitting orange-red fluorescence. The photoluminescence quantum yield (PL QY) was markedly improved (up to 35%). The structure and morphology of Mn: ZnSe@ZnS core-shell QDs were also confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) in detail. The mechanism of photoinduced fluorescence enhancement was attributed to L-cysteine allowed to release S^2- to form a ZnS shell, and the passivated surface non-radiative relaxation centers of Mn: ZnSe@ZnS QDs was successfully synthesized with highuniform size, excellent photoluminescence performance, and good stability, all ofwhichmakethemgood potential candidates for white LEDs, and biological labels.

Nanozyme based on CoFe2O4 modified with MoS2 for colorimetric determination of cysteine and glutathione

A nanozyme based on CoFe₂O₄ modified with MoS₂ was constructed for colorimetric determination of cysteine (Cys) and glutathione (GSH). Firstly, ferrite CoFe₂O₄ is synthesized, and it is then modified by MoS₂ to form a flower-like polymer (MoS₂@CoFe₂O₄). In the presence of H₂O₂, a redox interaction takes place, and the resulting hydroxyl promoted a colorimetric conversion from colorless to blue in the presence of 3,3',5,5'-tetramethylbenzidine (TMB). However, once Cys or GSH is added, they are capable to compete with the interaction of the hydroxyl with TMB, resulting in an inhibition of the colorimetric conversion. The colorimetric distinction is sensitive to the amount of target. The results obtained proved that the catalytic efficiency of MoS₂@CoFe₂O₄ is 4.4-fold and 1.8-fold to that of MoS₂ and CoFe₂O₄. Meanwhile, the K_m values to TMB and H₂O₂ are 0.067 and 0.048 mM, respectively, which are 6.5-fold and 77-fold, respectively smaller than those of natural peroxidase such as HPR. This indicates that the MoS₂@CoFe₂O₄ possesses a favorable interaction affinity. Additionally, the colorimetric distinction caused by the competition between TMB and cysteine or glutathione is obvious. The signal responses to cysteine and glutathione are linear in the range 0.5~15 μM and 0.5~35 μM, and the LODs are 0.10 and 0.21 μM, respectively. In practical assay of Cys in serum, the RSD of the sample tests is 4.6%, and the recoveries for the spiked assays are 95.3% and 96.0% with the RSD of 2.1% and 4.2%, respectively.

Fluorescence biosensor based on silicon quantum dots and 5,5'-dithiobis-(2-nitrobenzoic acid) for thiols in living cells

An efficient and stable fluorescent sensor is described for the detection and imaging of thiols. It is making use of silicon quantum dots (SiQDs) which can be rapidly prepared. They were characterized by transmission electron microscopy, X-ray power diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectrometry. The SiQDs have an absorption maximum at 300 nm and displayed blue-green fluorescence with excitation/emission maxima at 410/480 nm. A mixture of SiQDs and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) exhibits strong fluorescence emission which however is quenched within 30 s of incubation with thiols. This is assumed to be due to an inner filter effect caused by the reaction of DTNB and thiols. The following thiols were tested: cysteine, homocysteine, and glutathione. The sensor has a linear response in the 3-100 μM thiol concentration range, and the LODs are between 0.80 and 0.96 μM. The sensor displays low cytotoxicity and was applied to fluorescence imaging of MCF-7 cells and Hela cells where it demonstrated excellent biocompatibility.

A Fluorescence Inner-Filter Effect Based Sensing Platform for Turn-On Detection of Glutathione in Human Serum

A novel turn-on fluorescence assay was developed for the rapid detection of glutathione (GSH) based on the inner-filter effect (IFE) and redox reaction. Molybdenum disulfide quantum dots (MoS₂ QDs), which have stable fluorescent properties, were synthesized with hydrothermal method. Manganese dioxide nanosheets (MnO₂ NSs) were prepared by exfoliating the bulk δ-MnO₂ material in bovine serum albumin (BSA) aqueous solution. The morphology structures of the prepared nanoparticles were characterized by transmission electron microscope (TEM). Studies have shown that the fluorescence of MoS₂ QDs could be quenched in the presence of MnO₂ NSs as a result of the IFE, and is recovered after the addition of GSH to dissolve the MnO₂ NSs. The fluorescence intensity showed a good linear relationship with the GSH concentration in the range 20⁻2500 μM, the limit of detection was 1.0 μM. The detection method was applied to the analysis of GSH in human serum samples. This simple, rapid, and cost-effective method has great potential in analyzing GSH and in disease diagnosis.

Fluorescent probes for detecting cysteine

Cysteine plays a crucial role in physiological processes. Therefore, it is necessary to develop a method for detecting cysteine. Fluorimetry has the advantages of convenient detection, short response time, high sensitivity and good selectivity. In this review, fluorescent probes that detect cysteine over the past three years are summarized based on structural features of fluorophores such as coumarin, BODIPY, rhodamine, fluorescein, CDs, QDs, etc and reaction groups including acrylate, aldehyde, halogen, 7-nitrobenzofurazan, etc. Then, effects of different combinations between fluorophores and response groups on probe properties and detection performances are discussed.