Microfluidic paper-based fluorescence sensor for L-homocysteine using a molecularly imprinted polymer and in situ-formed fluorescent quantum dots.

Microfluidic paper-based analytical devices modified with molecularly imprinted polymers (µPADs@MIPs) were developed for fluorescent detection of targeted thiols via in situ UV-induced formation of quantum dots (µPADs@MIPs@QDs). The selectivity enhancement by the MIP layer formed on the filter paper surface was demonstrated for the isolation of L-homocysteine from wine. Followed by the addition of metal precursors solution (Zn/Cd/Cu) and UV irradiation, fluorescent quantum dots were formed thus enabling quantitative detection of the thiol (serving as a QD capping agent). The effect of different semiconductors was investigated to achieve a lower band gap and higher fluorescence intensity. Increasing fluorescence intensity in the presence of thiol groups was obtained for the following precursors mixture composition: ZnCdCu/S > ZnCd/S > ZnCu/S > ZnS. The proposed method has a good relationship between the fluorescence intensity of ZnCdCu/S QDs and L-homocysteine in a linear range from 0.74 to 7.40 µM with a limit of detection (LOD) and quantification (LOQ) of 0.51 and 1.71 µM respectively. This method was applied for the determination of L-homocysteine in white wine with RSD under 6.37%.

UV-induced Zn:Cd/S quantum dots in-situ formed in the presence of thiols for sensitive and selective fluorescence detection of thiols.

In this work, we explored a new approach to a simple and sensitive fluorescence detection of thiols. The approach takes advantage of an in-situ formation of UV light-induced fluorescent nanoparticles (ZnCd/S quantum dots), while utilizing the thiol group of the analyte as a capping agent. The selectivity is ensured by the selective isolation of the thiol analyte by a polydopamine molecularly imprinted polymeric (MIP) layer. Based on this approach, a method for determination of thiols was designed. Key experimental parameters were optimized, including those of molecular imprinting and of effective model thiol molecule (L-cysteine) isolation. The relationship between the fluorescence intensity of ZnCd/S quantum dots and the concentration of L-cysteine in the range of 12-150 µg/mL was linear with a detection limit of 3.6 µg/mL. The molecularly imprinted polymer showed high absorption mass capacity (1.73 mg/g) and an excellent selectivity factor for L-cysteine compared to N-acetyl-L-cysteine and L-homocysteine of 63.56 and 87.48, respectively. The proposed method was applied for L-cysteine determination in human urine with satisfactory results. Due to a high variability of molecular imprinting technology and versatility of in-situ probe formation, methods based on this approach can be easily adopted for analysis of any thiol of interest.

Integration of nickel doping with loading on graphene for enhanced adsorptive and catalytic properties of CdS nanoparticles towards visible light degradation of some antibiotics.

Water dispersible, highly efficient nickel doped CdS nanoparticles anchored on graphene nanosheets as a photocatalyst for cephalexin and sulfamethoxazole photodegradation have been prepared in a facile microwave-furnace assisted method. Each one of the two modifications has played a critical role in nanocomposite functioning. Defects originated by dopant boosted the lifetime of carriers and thereupon graphene matrix transferred them to contribute effectively the photocatalytic process. Characterization results revealed the formation of monocrystalline hexagonal phase of all products and that both doping and loading on graphene have red-shifted the absorption edge of CdS towards the visible light region. Furthermore, FTIR confirmed the successful reduction of graphene oxide by the subsequent preparation steps. Adsorption isotherms revealed the role of graphene in enhancing substrate adsorption. Nevertheless, dissimilar pathways of catalytic degradation were observed on the doped composite as cephalexin oxidation was principally mediated by the hole-hydroxyl radical mechanism, sulfamethoxazole oxidation favored the superoxide radical mechanism. This composite has shown, however, a high photostability and minimized ions release of the composite.