Detection of lead (II) with a "turn-on" fluorescent biosensor based on energy transfer from CdSe/ZnS quantum dots to graphene oxide

Graphene oxide (GO) sheets are mixed with the aptamer-functionalized CdSe/ZnS quantum dots (QDs). Consequently, the aptamer-conjugated QDs bind to the GO sheets to form a GO/aptamer-QD ensemble, which enables the energy transfer from the QDs to the GO sheets, quenching the fluorescence of QDs. The GO/aptamer-QD ensemble assay acts as a "turn-on" fluorescent sensor for Pb2+ detection. When Pb2+ ions are present in the assay, the interaction of Pb2+ with the aptamer induces a conformational change in the aptamer, leading to the formation of a G-quadruplex/Pb2+ complex. As a result, the QDs that are linked to the G-quadruplex/Pb2+ complex are detached from the GO sheet, which "turns on" the fluorescence of the QDs. This sensor exhibits a limit of detection of 90 pM and excellent selectivity toward Pb2+ over a wide range of metal ions. The experiments have provided direct evidence that the fluorescence of QDs is quenched by GO via the nano-metal surface energy transfer (NSET) mechanism rather than the conventional Forster resonance energy transfer (FRET) process. (c) 2012 Elsevier B.V. All rights reserved.