Tuning quantum dots emission on DNA tetrahedron/silica nanosphere/ graphene oxide nanointerface for ratiometric fluorescence assay of Pb2+in multiplex samples

Background: Assembling framework nucleic acid (FNA) nanoarchitectures and tuning luminescent quantum dots (QDs) for fluorescence assays represent a versatile strategy in analytical territory. Rationally, FNA constructs could offer a preferential orientation to efficiently recognize the target and improve detection sensitivity, meanwhile, regulating size-dependent multicolor emissions of QDs in one analytical setting for ratiometric fluorescence assay would greatly simplify operation procedures. Nonetheless, such FNA/QDs-based ratiometric fluorescence nanoprobes remain rarely explored. Results: We designed a sensitive and signal amplification-free fluorescence aptasensor for lead ions (Pb2+) that potentially cause extensive contamination to environment, cosmetic, food and pharmaceuticals. Red and green emission CdTe quantum dots (rQDs and gQDs) were facilely prepared. Moreover, silica nanosphere encapsulating rQDs served as quantitative internal reference and scaffold to anchor a predesigned FNA and DNA sandwich containing Pb2+ binding aptamer and gQD modified DNA signal reporter. On binding of Pb2+, the gQD-DNA signal reporter was set free, resulting in fluorescence quenching at graphene oxide (GO) interface. Owing to the rigid structure of FNA, the fluorescence signal reporter orderly arranged at the silica nanosphere could sensitively respond to Pb2+ stimulation. The dose-dependent fluorescence signal-off mode enabled ratiometric analysis of Pb2+ without cumbersome signal amplification. Linear relationship was established between fluorescence intensity ratio (I555/I720) and Pb2+ concentration from 10 nM to 2 mu M, with detection limit of 1.7 nM (0.43 ppb), well addressing the need for Pb2+ routine monitoring. The designed nanoprobe was applied to detection of Pb2+ in soil, cosmetic, milk, drug, and serum samples, with the sensitivity comparable to conventional ICP-MS technique.