Metal-Enhanced Ratiometric Fluorescence/Naked Eye Bimodal Biosensor for Lead Ions Analysis with Bifunctional Nanocomposite Probes

A novel metal-enhanced ratiometric fluorescence/naked eye bimodal biosensor based on ZnFe2O4@Au-Ag bifunctional nano-composite and DNA/CeO2 complex for lead ions (Pb2+) has been successfully developed. The nanocomposite probe was composed of a magnetic ZnFe2O4 core and a Au-Ag hollow nanocube shell. Upon bioconjugation, bifunctional magnetic nanocomposites could not only make the probe possess excellent recyclability but also provide an enrichment of hot spots for surface enhanced fluorescence detection of Pb2+ by a metal-enhanced fluorescence effect. Typically, the bifunctional nanocomposites conjugated with double-stranded DNA (included Pb2+-specific DNAzyme strand and corresponding substrate strand) to form a Pb2+ biosensor Nanoceria as a fluorescence quencher strongly adsorbed DNA. Therefore, the formation of double-stranded DNA brought the labeled nitrogen Sulfur doped carbon dots (N,S-CDs) and CeO2 into close proximity, which significantly quenched the fluorescence of N,S-CDs. The presence of Pb2+ led to the breakage of the DNAzyme strand, resulting in the fluorescence signal of Cy3 decreasing, while the fluorescence intensity of N,S-CDs aggrandized. First, a preliminary test of Pb2+ was performed by the naked eye. The disengaged DNA/CeO2 complex could result in color change after adding H2O2 because of autocatalysis of , resulting in real time visual detection of Pb2+. If further accurate determination was required, the fluorescence intensity ratio of these two fluorescence indicators was measured at 562 and 424 nm (I-562 /I-424). A good linear correlation exists between the log(I-562/I-424) and the logarithm of Pb2+ concentrations ranging from 10(-12) to 3 X 10(-6) M. Remarkably, the detection limit of this ratiometric biosensor was 3 X 10(-13) M, which ascribed to its superior fluorescence enhancement. Interestingly, the developed bifunctional nanocomposite probe manifests good recyclability and selectivity. More importantly, the biosensor provided potential application of on-site and real-time unknown Pb2+ ions in real systems.