Determination of uric acid in human urine by capillary zone electrophoresis with indirect laser-induced fluorescence detection

Determination of strobilurin fungicide residues in fruits and vegetables by nonaqueous micellar electrokinetic capillary chromatography with indirect laser-induced fluorescence

A nonaqueous micellar electrokinetic capillary chromatography method with indirect LIF was developed for the determination of strobilurin fungicide residues in fruits and vegetables. Hydrophobic CdTe quantum dots (QDs) synthesized in aqueous phase were used as background fluorescent substance. The BGE solution, QD concentration, and separation voltage were optimized to obtain the best separation efficiency and the highest signal intensity. The optimal BGE solution consists of 40 mM phosphate, 120 mM sodium dodecyl sulfate, 15% v/v water and 15% v/v hydrophobic CdTe QDs in formamide, of which apparent pH is 9.5. The optimized separation voltage is controlled as 25 kV. The resultant detection limits of azoxystrobin, kresoxim-methyl, and pyraclostrobin are all 0.001 mg/kg, their linear dynamic ranges are 0.005-2.5 mg/kg, and the recoveries of the spiked samples are 81.7-96.1%, 86.5-95.7%, and 87.3-97.4%, respectively. This method has been proved to be sensitive enough to detect the aforementioned fungicides in fruits and vegetables at the maximum residue limits.

A Nicotinamide Adenine Dinucleotide Dispersed Multi-walled Carbon Nanotubes Electrode for Direct and Selective Electrochemical Detection of Uric Acid

A nanocomposite platform built with multi-walled carbon nanotubes (MWCNTs) and nicotinamide adenine dinucleotide (NAD(+)) via a noncovalent interaction between the large π systems in NAD(+) molecules and MWCNTs on a glassy carbon substrate was successfully developed for the sensitive and selective detection of uric acid (UA) in the presence of ascorbic acid (AA), dopamine (DA). NAD(+) has an adenine subunit and a nicotinamide subunit, which enabled interaction with the purine subunit of UA through a strong π-π interaction to enhance the specificity of UA. Compared with a bare glassy carbon electrode (GCE) and MWCNTs/GCE, the MWCNTs-NAD(+)/GCE showed a low background current and a remarkable enhancement of the oxidation peak current of UA. Using differential pulse voltammetry (DPV), a high sensitivity for the determination of UA was explored for the MWCNTs-NAD(+) modified electrode. A linear relationship between the DPV peak current of UA and its concentration could be obtained in the range of 0.05 - 10 μM with the detection limit as low as 10 nM (S/N = 3). This present strategy provides a novel and promising platform for the detection of UA in human urine and serum samples.

Sol-gel thin-film based mesoporous silica and carbon nanotubes for the determination of dopamine, uric acid and paracetamol in urine

This work describes the preparation, characterization and application of a hybrid material composed of disordered mesoporous silica (SiO2) modified with multiwalled carbon nanotubes (MWCNTs), obtained by the sol-gel process using HF as the catalyst. This hybrid material was characterized by N2 adsorption-desorption isotherms, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission microscopy (HR-TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). This new hybrid material was used for the construction of a thin film on a glassy carbon electrode. The modified electrode using this material was designated SiO2/MWCNT/GCE. The electrocatalytic properties of the electrode toward dopamine, uric acid and paracetamol oxidation were studied by differential pulse voltammetry. Well-defined and separated oxidation peaks were observed in phosphate buffer solution at pH 7.0, in contrast with the ill-defined peaks observed with unmodified glassy carbon electrodes. The electrode had high sensitivity for the determination of dopamine, uric acid and paracetamol, with the limits of detection obtained using statistical methods, at 0.014, 0.068 and 0.098 µmol L(-1), respectively. The electrode presented some important advantages, including enhanced physical rigidity, surface renewability by polishing and high sensitivity, allowing the simultaneous determination of these three analytes in a human urine sample.