*2.2. Nucleic Acid*

Detection of nucleic acids such as the DNA and RNA is significant for disease diagnosis and gene therapy [63–65]. SERS detection was widely used in nucleic acid analysis offering simple procedures, ultrasensitive and unique structural characterization. There are two major strategies for nucleic acid detection. Label-free detection strategy based on the distinctive SERS signals from four bases of DNA/RNA. The other strategy was modifying nucleic acid with extrinsic Raman labels or constructing SERS nanotags, which have more extensive applications than label-free strategy. A series of ingenious methods based on the properties of complementarity and amplification were developed for nucleic acid detection, such as the classical "sandwich" method, the hairpin DNA-assisted method and the signal amplification method.
