Plasmon-Enhanced Fluorescence in Coupled Nanostructures and Applications in DNA Detection

Plasmon coupling interactions between adjacent noble metal nanoparticles (NPs) can cause significantly enhanced local electric field in the gap region, which could be utilized to dramatically enhance the fluorescence intensity of chromophores. Here we performed a systematic study on the influence of different factors on plasmon coupling-enhanced fluorescence, including shape and size of metal NPs, dye distribution, and separation distance. Cyanine 5 (Cy5) acted as the fluorescence probe and DNA was employed to assemble nanostructures to immobilize Cy5 into the gap region of the coupled metal NPs. Fluorescence of Cy5 was prequenched by attaching DNA linked Cy5 to the surface of Au nanospheres (NSs). The quenched fluorescence of Cy5 was turned-on by forming nanoassembly through DNA hybridization with different enhancing substrates: Au NSs, Au nanorods (NRs) and Au@Ag NSs. Au@Ag NSs were found to give the largest fluorescence enhancement effect. Larger-sized Au@Ag NPs were found to display larger fluorescence enhancement effects compared to the smaller ones. Optimum fluorescence enhancement was observed at an intermediate interparticle separation distance of 8.2 nm, which was up to 100-fold enhancement compared to quenched Cy5-Au NSs, 5-fold enhancement compared to unquenched free Cy5 molecules. As prequenched fluorescence offers reduced background, this plasmon coupling-enhanced fluorescence phenomenon was further utilized to develop a simple fluorescence turn-on platform for highly sensitive and selective detection of DNA sequence. The limit of detection (LOD) of this method was estimated to 3.1 pM. The exceptional selectivity of this method allows us to distinguish single-base mismatch at room temperature. This plasmon coupling-enhanced fluorescence phenomenon could be further utilized to develop various platforms for highly sensitive sensing and imaging applications.