Switch on fluorescence mode for determination of l-cysteine with carbon quantum dots and Au nanoparticles as a probe

Turn -on FRET-based cysteine sensor by sulfur-doped carbon dots and Au nanoparticles decorated WS2 nanosheet

Cysteine is an essential biothiol that plays various functions in the human body. Decrease or exceeding of this excellent antioxidant from the expected range will lead to so many problems. Thus, appropriate sensing of it would be of great importance. Sulfur-doped carbon Dots(S-CDs) owe excellent fluorescence emission. Therefore, designing a Fluorescence resonance energy transfer (FRET) system between S-CDs as donor and Au nanoparticles (AuNPs) decorated tungsten disulfide nanosheet (WS2 NSs) would be a perfect strategy for cysteine detection. Excitation at 340 nm will give the maximum quantum yield of S-CDs (21%) and fluorescence emission peak at 460 nm. In the presence of cysteine, the FRET mechanism inhibited through the affinity of cysteine's functional groups (-SH and -NH2) toward AuNPs and S-CDs fluorescence emission was recovered. To find the best efficiency of the system, optimization of pH, temperature, and time was investigated. Here the linear range of 3-275 µM and limit of detection of 0.01 µM was obtained. Finally, the fluorescence method was applied to the analysis of cysteine in human blood serum, which poses the potential of rapid and sensitive sensing. It can detect both lower and higher amounts of serum cysteine.

Citrate functionalized gold nanoparticles assisted micro extraction of L-cysteine in milk and water samples using Fourier transform infrared spectroscopy

This paper describes the sensing application of citrate functionalized gold nanoparticles (AuNPs) employing for the determination of L-cysteine in food and water samples. It is established with diffuse reflectance Fourier transform infrared (DRS-FTIR) spectroscopic analysis. The disappearance of the thiol (-SH) band in the FTIR spectra and the shift in the peaks of the amino group (NH₃⁺) and carboxylate group (-COO^-) indicated the Au-S interaction and the aggregation of the NPs. The signal intensity of L-cysteine was enhanced due to hot-spots formed by the aggregation of AuNPs producing the effective absorption of electromagnetic radiation in the IR region for molecular vibration. The relationship between AuNPs and L-cysteine was theoretically investigated by the Density Function Theory (DFT) based on LANL2DZ with the aid of the Gaussian 09 (C.01) software. Interaction between AuNPs and L-cysteine molecules resulted to a shift to higher wavelengths in the plasmon bands, further verified by transmission electron microscopes (TEM), which have indicated random aggregated particles. Further dynamic light scattering (DLS) measurements showed a relatively high degree of polydispersity confirming the aggregation of the particles. Under optimized conditions, the calibration curve showed a good linearity range from 20 to 150 μg mL^-1 with a correlation coefficient (R²) 0.990. The limit of detection and quantification were 1.04 and 3.44 μg mL^-1, respectively by DRS-FTIR. This modified AuNPs sample was used successfully in milk and water samples with adequate results to determine L-cysteine.

A novel colorimetric sensor for naked-eye detection of cysteine and Hg2+ based on "on-off" strategy using Co/Zn-grafted mesoporous silica nanoparticles

In an attempt to explore the significance of inorganic mimetic enzymes as sensors, this study introduces a naked-eye analytical sensing platform for the detection of L-cysteine (cys), mercury ions (Hg²⁺) based on (turn off/turn-on) catalytic activity of zinc and cobalt grafted mesoporous silica nanoparticles (MSNs). To this end, Zn-MSN and Co/Zn-MSN catalysts were synthesized and characterized using XRD, FT-IR, FESEM, TEM, and nitrogen adsorption-desorption methods. Then, using the intrinsic peroxidase-like activity of as-synthesized samples, the oxidation reactions of the chromogenic substrate (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS)) was designed using H₂O₂, which produced green colored cation radical of ABTS. Considering the high peroxidase-like activity of Co/Zn-MSN in comparison to Zn-MSN, it was employed to detect cys and then Hg²⁺. The results indicated that the strong interaction between cys and Co/Zn-MSN was proved by a limit of detection (LOD) down to 0.24 nM and the linear relationship from 0.8-50 nM (turn off). Given the fact that Hg²⁺ has a high-affinity tendency to combine with cys, we were suggested a novel colorimetric path for sensing of Hg²⁺ in the presence of cys (turn on). Based on this method, LOD was found 0.17 nM with the linear range of 0.57-50 nM. Taken together, results showed that the as-prepared catalysts are superior to other nanoparticles as a sensor to measure the target molecules in biological monitoring and clinical diagnostics.

Glutathione-decorated fluorescent carbon quantum dots for sensitive and selective detection of levodopa

Levodopa has been a standard drug for treating Parkinson's disease since the 1960s, but it has caused many side effects such as wearing-off, motor fluctuation, and dystonia. In this work, we developed glutathione-conjugated carbon quantum dots (GSH-CQDs) as a novel fluorescent sensor for sensitive and selective detection of levodopa. The GSH-CQDs were prepared by EDC/NHS coupling reaction of glutathione (GSH) with amine-functionalized CQDs (N-CQDs) synthesized using meta-phenylenediamine and ethylenediamine. The synthesized GSH-CQDs emitted bright green fluorescence with a high quantum yield (QY) of 22.42 ± 6.88%. However, upon the addition of levodopa to GSH-CQDs under alkaline conditions, the fluorescence of GSH-CQDs was quenched. Since levodopa is converted to dopaquinone in an alkaline environment, it is presumed that thiol groups of GHS-CQDs form covalent bonds with dopaquinone, causing fluorescence quenching through photoinduced electron transfer. Therefore, as the concentration of levodopa increased, the fluorescence intensity of GSH-CQDs was gradually decreased. Under optimal conditions, a linear response was observed in the range of 0.05-1 μM, and limit of detection (LOD) was determined to be 0.057 μM. The GSH-CQDs exhibited high specificity to levodopa over other non-target biological substances, quinone derivatives, and Parkinson's medications. Furthermore, the capability of this GSH-CQDs sensor for monitoring levodopa in human serum were validated with excellent precision and recovery rates of 100.20-103.33%.

Polymer Electrochemiluminescence Featuring Thermally Activated Delayed Fluorescence

Electrochemiluminescence (ECL) based on conjugated polymers or oligomers is persistently being pursued owing to its huge application scope ranging from ultra-sensitive bioanalysis to ultra-resolution imaging and spectroscopy. Because of the theoretical limit in radiative exciton generation yield (typically ∼25 %) of those polymers or oligomers, the corresponding ECL efficiency is still limited, which hampers its ECL performance and its related applications. Herein, we report ECL based on a thermally activated delayed fluorescence (TADF) polymer scaffold, which is characteristic of all-exciton harvesting in the ECL process, and thus potentially capable of achieving ∼100 % ECL efficiency. These desired properties of the TADF polymer ECL is attributed to a fast and efficient up-conversion process from non-radiative triplet to radiative singlet states under thermal activation, which is absent in conventional fluorescent polymers/oligomers, such as F8BT. In this study, various ECL modes, including annihilation or co-reactant mode using TPrA or S₂ O₈ ^2- as co-reactant, are confirmed for our model TADF polymer ECL system, which was different from fluorescent polymer ECL counterpart. Furthermore, solid-state ECL sensing on L-cysteine (an important marker of disease) is also evaluated by using the model TADF polymer. Ultralow detection limit in combination with high sensitivity and good specificity are achieved for this model system, indicative of a high potential of the TADF polymer scaffold for applications in the broad field of ECL.

Colorimetric determination of cysteine based on inhibition of GSH-Au/Pt NCs as peroxidase mimic

In this work, we reported a facile and highly sensitive strategy for colorimetric detection of cysteine (Cys) based on the inhibition of catalytic activity of bimetallic nanoclusters induced by Cys. Glutathione-modified gold-platinum nanoclusters (GSH-Au/Pt NCs) with different Au/Pt molar ratios were prepared via one-pot approach and utilized as peroxidase mimics to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂. It has been found that Cys could inhibit the peroxidase-like activity of GSH-Au/Pt NCs efficiently, which leads to a decrease of the absorption intensity of the system at 652 nm with a fading of the blue color. These findings provide a worthy method for visualization and quantitative detection of Cys with different concentrations in the range from 0.5 to 30 μM, and the detection limit is 0.154 μM. Moreover, this method displays a promising application in colorimetric analysis of Cys in urine samples.

Colorimetric determination of L-cysteine in milk samples with surface functionalized silver nanoparticles

A simple, selective and sensitive method is proposed for determination of cysteine (Cys) in milk samples using ionic liquid functionalized silver nanoparticles (ILs-AgNPs) as a colorimetric probe. ILs-AgNPs was synthesized by simple reduction method using silver nitrate as a precursor and sodium borohydride as a reducing agent and functionalized with ILs to prevent particles from self-aggregation. The sensing mechanism has been dependent on the color change of ILs-AgNPs and red shift of absorption band from 395 nm to 560 nm in the visible region, which is found proportional to the concentration of target analyte in sample. ILs-AgNPs was characterized in absence and presence of Cys by UV-vis, Fourier transform-infrared (FTIR) spectroscopy, transmission electron microscope (TEM) and dynamic light scattering (DLS). The linear range was acquired in the range of 0-100 ng mL^-1, with correlation coefficient (R²) of 0.996 and limit of detection (LOD) of 4.0 nM. The binding mechanism and interactions between Cys and ILs-AgNPs was confirmed by calculating the binding constant and thermodynamic parameters such as enthalpy (∆H), entropy (∆S) and Gibb's free energy (∆G). The use of ILs-AgNPs exhibited high colorimetric selectivity for Cys in milk samples in presence of other amino acids. This proposed strategy possessed the advantages of simplicity and selectivity, hence is applied for analysis of Cys in milk samples.

Selective and Sensitive ZnO Quantum Dots Based Fluorescent Biosensor for Detection of Cysteine

In the present article, a novel and effective ZnO quantum dots-based fluorescent probe has been developed for the detection of cysteine in different solutions. Firstly, melamine-based fluorescent pre-probe was successfully synthesized via condensation reaction and, then ZnO quantum dots (QDs) were homogenously dispersed into this solution. This fluorescent probe was used for the detection of cysteine in different solutions such as bovine serum albumin and tap water. ZnO QDs were characterized using XRD, nano-particle size analyzer, and FE-SEM techniques. The size of the ZnO QDs was calculated as 28.03±9.86 nm, and 31.95±10.02 nm from Scherrer's equation and nano-particle size analyzer, respectively. The developed fluorescent probe was exhibited a highly selective and sensitive response to the detection of cysteine. Also, the proposed fluorescent probe has a larger Stokes shift value (236 nm). The limit of detection and linear range of ZnO QDs-based fluorescent biosensor were found as 0.642 μM and 0.1-600 μM, respectively. ZnO quantum dot-based fluorescent sensor for L-cysteine.

Gold nanoparticles in situ generated on carbon dots grafted paper: application in enantioselective fluorescence sensing of d-alanine

The proposed PCD/AuNPs possesses good biocompatibility and was applied to detect d-Ala in serum, simulated gastric fluid, and cancer cells.

Formation of a Photoelectrochemical Z-Scheme Structure with Inorganic/Organic Hybrid Materials for Evaluation of Receptor Protein Expression on the Membrane of Cancer Cells

Quantitative analysis of receptor protein expression is essential to give new insights into tumor-related research. Benefitting from their high sensitivity and low background, photoelectrochemical (PEC) platforms are considered as powerful tools for evaluating the expression of receptor proteins. Herein, to reduce the cytotoxicity and facilitate the subsequent assembly, l-cysteine-modified Ag-ZnIn₂S₄ quantum dots (l-Cys AZIS QDs) are prepared and PEC responses under the irradiation of long wavelength light are obtained. To further improve the PEC behavior, iron phthalocyanine (FePc) is employed to form a Z-scheme structure with l-Cys AZIS QDs. The Z-scheme structure based on l-Cys AZIS QDs/FePc hybrid materials exhibits high photo-to-electric conversion efficiency and can be excited with near-infrared range light. Because hyaluronic acid linked to photoactive materials can recognize CD44 expressed on the membrane of cancer cells, cancer cells are immobilized onto l-Cys AZIS QDs/FePc hybrid materials, inducing a decrease of the photocurrent intensity. Consequently, a PEC cytosensor is constructed to quantify cancer cells expressing CD44. The PEC analytical platform is able to determine A549 cells in the range of 2 × 10² to 4.5 × 10⁶ cells/mL, and a detection limit of 15 cells/mL is realized in the case of S/N = 3. In addition, the expression of CD44 in A549 and other five cancer cells is measured with this PEC method. Depending on our data, the expression of CD44 in different cancer cells is distinct, indicating great potential of this method in receptor protein-related studies.