Síntese de pontos quânticos de carbono fluorescentes assistida por micro-ondas para determinação de ácido ascórbico e riboflavina em suplementos vitamínicos

Abstract

In this study, the precursors ethylenediaminetetraacetic acid, sodium thiosulfate, and urea, which are commonly available in analytical laboratories, were used in a one-step synthesis of carbon quantum dots via a domestic 1,400 W microwave-assisted method. Characterization of the quantum dots included TEM imaging, FTIR spectroscopy, XPS analysis, XRD, Zeta potential measurement, UV-Vis absorption spectroscopy, and molecular fluorescence spectroscopy. Using two distinct instrumental, setups a diffraction grating with a photomultiplier detector (Agilent), and another with a CCD detector and 372 nm LED (Ocean Optics), the quantum dots exhibited stable optical properties for 240 days with blue fluorescence emission at 430 and 440 nm, respectively, water solubility, and a fluorescence quantum yield of 8.9%. They were applied in the determination of ascorbic acid (AA), and riboflavin (RB) content in vitamin supplements. The fluorescence mechanisms of the quantum dots included dynamic quenching for the PQC/Cr(VI) sensor based on the variation of Ksv with temperature change, and fluorescence recovery through suppression in the presence of AA, as well as static quenching based on the variation of Ksv with temperature change and internal filter effects in interaction with RB. A 24-factorial design was used to optimize the analytical parameters of PQC/Cr(VI)/AA, employing the variables PQC volume, buffer volume, 0.5 mol L-1 NaCl concentration, and Cr(VI) ion concentration, and a 23-factorial design for PQC/RB, using the variables PQC volume, buffer volume, and 0.5 mol L-1 NaCl concentration, calculated using Minitab free software. On the Agilent instrument, the PQC/Cr(VI) sensor used for AA determination through an on-off-on strategy resulted in a linear range of 0.5 to 50 μg mL-1 and an LOD of 0.15 μg mL-1, with sample recovery ranging from 80.72 to 107.72% and paired t-test results of 1.88 for liquid samples and 0.6 for solid samples; for RB determination, it yielded a linear range of 0.1 to 7 μg mL-1 and an LOD of 0.09 μg mL-1, with sample recovery from 85.07 to 112.23% and a paired t-test result of 0.74 for capsule samples. On the Ocean Optics instrument, the PQC/Cr(VI) sensor for AA determination with an on-off-on strategy produced a linear range of 0.25 to 50 μg mL-1 and an LOD of 0.1 μg mL-1, with sample recovery between 86.21 and 108.74% and paired t-test results of 3.10 for liquid samples and 0.14 for solid samples; for RB, it showed a linear range of 0.1 to 4.5 μg mL-1 and an LOD of 0.03 μg mL-1, with sample recovery from 88.97 to 112.96% and a paired t-test result of 3.05 for capsule samples. An analytical selectivity study, using various organic and inorganic compounds, was performed based on quenching effects, fluorescence recovery with different metal ions, and fluorescence recovery of the PQC/Cr(VI) sensor. Therefore, the synthesized quantum dots were used for the determination of AA and RB in vitamin supplements using two fluorescence instruments, and the application of this nanomaterial can be expanded to different analytes and matrices.