Ultrasensitive SERS detection of specific oligonucleotides based on Au@AgAg bimetallic nanorods

We synthesized a novel and sensitive Au/Ag bimetallic SERS-active nanotag, Au–Ag–Ag core–shell–shell nanorod (Au@AgAgNR). The Au@AgAgNR nanotag exhibited a strong SERS signal and was easily assembled from bilayer silver shells on an Au nanorod (AuNR) core with embedded Raman reporter molecules in the core–shell–shell gaps. The SERS activity of the nanotags was investigated with 2-mercaptopyridine (2-Mpy) as a Raman reporter, which could form pyridine/Ag+ coordination complexes to mediate the formation of silver shells. Specific enhancement of Raman signals was observed in the following order: AuNR < Au@AgNR < Au@AgAgNR. Then, Au@AgAgNR nanotags were coupled with magnetic beads (MBs) via specific DNA hybridization as a SERS sensor with a detection limit of 1 fM for a segment of the gene HPV-16. Factors affecting sensitivity and selectivity were investigated, including Raman dye concentration, silver nitrate dosage and the response to similar oligonucleotides. The proposed SERS sensor is expected to be a facile and sensitive method for specific gene detection.