RESUMO
A new method for the determination of caffeine, a non-fluorescent analyte, based on the enhancement of the fluorescence of Rhodamine B dye on a membrane filter modified with multi-walled carbon nanotubes is proposed. The method comprises pre-concentration of caffeine on a solid support by chemofiltration in buffered solution onto multi-walled carbon nanotubes previously oxidised and dispersed in cationic surfactant admicelles. The effect of experimental parameters, including the nature of the buffer and pH, the nature of the solid support, filtration flow rate, dye and carbon nanotube concentration, and the nature of the surfactant and concentration were investigated by means univariation assays. Under optimum experimental conditions, the pre-concentration system gave detection and quantification limits of 0.3 and 1.1 µg l(-1), respectively. A wide linear range was achieved varying from concentrations of 1.1 to 9.7 × 103 µg l(-1) (r(2) = 0.999). Satisfactory recovery values were obtained using the method of standard addition, confirming the feasibility of this method for caffeine determination in energising dietary supplements and energy drinks.
Assuntos
Bebidas/análise , Cafeína/análise , Suplementos Nutricionais/análise , Nanotubos de Carbono , Rodaminas/química , Espectrometria de Fluorescência/métodos , Calibragem , Reprodutibilidade dos TestesRESUMO
BACKGROUND: In this work, a new methodology based upon enhancement of rhodamine B fluorescent signal is proposed for the quantification of caffeine traces. METHODS: Membrane filters treated with multiple wall carbon nanotubes were employed as solid support for determination step by solid surface fluorescence. RESULTS: Experimental variables that influence the preconcentration step and fluorimetric sensitivity have been optimized using uni-variation assays, presenting linearity from 1.1 to 9.7×10(3) µg/l, with a correlation coefficient of 0.99. At optimal conditions, a limit of detection of 0.3 µg/l and a limit of quantification of 1.1 µg/l were obtained. The method showed good sensitivity and adequate selectivity and was satisfactorily applied to the determination of trace amounts of caffeine in urine, plasma and serum belonging to subjects with different sex, ages and habit of caffeine intake. CONCLUSIONS: Chemofiltration step eliminated the highly fluorescent matrix, thus enabling and allowing CF quantification, in the presence of other methylxanthines. The proposed methodology represents an innovative application of the solid surface fluorescence using membrane filters modified with MWCNTs.