Surface-Enhanced Raman Scattering (SERS) sensors continue to attract attention as an analytical technique for chemical sensing and biomedical applications. SERS sensors offer multiple analysis advantages, including easy operation without complicated sample preparation, single-molecule sensitivity, high throughput, and point-of-care applications from commercially available portable Raman spectroscopes. Electric and magnetic field enhancements from localized surface plasmon (LSP) effects can reach several orders of magnitude higher than the incident field; this enhancement is preferred in most SERS sensors. The design and fabrication of plasmonic nanostructures is the key for high-performance SERS sensors, since the maximum field enhancement determines the sensors’ sensitivity, reproducibility, and applicability. 
One of the unique features of SERS sensors is that an analyte can be identified by its unique Raman spectrum, providing a route for label-free detection. Unfortunately, Raman scattering itself is inefficient because of the small scattering cross section. However, the analyte’s low scattering cross section can be overcome by designing plasmonic structures to either enhance the intrinsic SERS signal of the analyte, or use an extrinsic design where the SERS signal of a reporter molecule is only enhanced in the presence of the analyte. Figure 1 demonstrates the ability of this type of biosensor to measure the level of vascular endothelial growth factor (VEGF) in clinical blood plasma samples taken from breast cancer patients.