Approaches for SERS-based DNA detection. (A) Label-free DNA detection using iodide-modified AgNPs , (B) sandwich assay for the detection of DNA using AuNPs probes labeled with oligonucleotides , (C) Raman dye labelled hairpin-DNA probes for the detection of DNA based on the on-off approach . (D) DNA detection based on the off-on approach. . Reproduced with permission from Refs. , , , .