Recent approaches toward creation of hot spots for SERS detection

• Top-down and bottom-up methods can be complementary for hot spot fabrication.
• Self-assembly allows tailoring nanoparticle organization for hot spot optimization.
• Reproducibility in SERS substrates is still a challenge.

Surface enhanced Raman scattering (SERS) spectroscopy is a powerful technique that provides molecular information through greatly enhanced Raman scattering from minute amounts of substance near nanostructured metallic surfaces. SERS is thus a promising technique for ultrasensitive sensing applications. Plasmonic nanostructures including metal nanoparticles and lithographically prepared nanostructures are ideal substrates to produce enhanced Raman signals. Numerous studies have been published on the production of SERS-active substrates for SERS measurements including solution phase methods and solid supports. In SERS applications, hot spots where the electromagnetic field is particularly intense, play a key role. In this review, we provide an overview of techniques designed for the creation of SERS hot spots both in solution and on solid supports. We first introduce the self-assembly of spherical and anisotropic nanoparticles in solution, to then focus on a wide variety of techniques to assemble nanoparticles onto solid supports. We also describe top-down approaches typically based on lithography techniques. Finally, we provide our own view on the current state of the field and the aspects where further development is expected.

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