Gap-enhanced Raman tags: fabrication, optical properties, and theranostic applications

Gap-enhanced Raman tags (GERTs) are emerging probes of surface-enhanced Raman scattering (SERS) spectroscopy that have found promising analytical, bioimaging, and theranostic applications. Because of their internal location, Raman reporter molecules are protected from unwanted external environments and particle aggregation and demonstrate superior SERS responses owing to the strongly enhanced electromagnetic fields in the gaps between metal core-shell structures. In this review, we discuss recent progress in the synthesis, simulation, and experimental studies of the optical properties and biomedical applications of novel spherically symmetrical and anisotropic GERTs fabricated with common plasmonic metals-gold (Au) and silver (Ag). Our discussion is focused on the design and synthetic strategies that ensure the optimal parameters and highest enhancement factors of GERTs for sensing and theranostics. In particular, we consider various core-shell structures with build-in nanogaps to explain why they would benefit the plasmonic GERTs as a superior SERS tag and how this would help future research in clinical analytics and therapeutics.