Europium-decorated graphene quantum dots as a fluorescent probe for label-free, rapid and sensitive detection of Cu2+ and L-cysteine

In this work, europium-decorated graphene quantum dots (Eu-GQDs) were prepared by treating three-dimensional Eu-decorated graphene (3D Eu-graphene) via a strong acid treatment. Various characterizations revealed that Eu atoms were successfully complexed with the oxygen functional groups on the surface of graphene quantum dots (GQDs) with the atomic ratio of 2.54%. Compared with Eu free GQDs, the introduction of Eu atoms enhanced the electron density and improved the surface chemical activities of Eu-GQDs. Therefore, the obtained Eu-GQDs were used as a novel "off-on" fluorescent probe for the label-free determination of Cu2+ and L-cysteine (L-Cys) with high sensitivity and selectivity. The fluorescence intensity of Eu-GQDs was quenched in the presence of Cu2+ owing to the coordination reaction between Cu2+ and carboxyl groups on the surface of the Eu-GQDs. The fluorescence intensity of Eu-GQDs recovered with the subsequent addition of L-Cys because of the strong affinity of Cu2+ to L-Cys via the Cu-S bond. The experimental results showed that the fluorescence variation of the proposed approach had a good linear relationship in the range of 0.1-10 mu M for Cu2+ and 0.5-50 mu M for L-Cys with corresponding detection limits of 0.056 mu M for Cu2+ and 0.31 mu M for L-Cys. The current approach also displayed a special response to Cu2+ and L-Cys over the other co-existing metal ions and amino acids, and the results obtained from buffer-diluted serum samples suggested its applicability in biological samples. (C) 2015 Elsevier B.V. All rights reserved.