Raman-Active Two-Tiered Ag Nanoparticles with a Concentric Cavity

Since the seminal discovery of surface-enhanced Raman scattering (SERS) in the 1970s[1–3] and the first demonstration of single-molecule detection by Nie et al. and Kneipp and co-workers in 1997,[4,5] SERS has been widely investigated for use in highly sensitive, real-time, nondestructive, and multiplexed molecular detection.[6–11] Reliable and straightforward formation of “Raman hot spots,” where a local electromagnetic field enhanced by surface plasmon resonance reaches its maximum value, is an essential prerequisite for practical applications of SERS. In many reports, Raman hot spots generated from aggregates or assemblies of nanoparticles have been used. However, a recent paper on the site distribution of SERS enhancement from Ag thin films on self-assembled nanoparticles shows that the hottest sites (enhancement factor > 109) account for only 0.006% of the total.[12] Even though high-end electron beam lithographic methods enable patterning to form more uniform and reproducible nanoscale hot spots,[13–15] these methods are not currently viable for large-scale fabrication of SERS substrates. Herein, we present an inexpensive and highly reliable SERS substrate where arrays of two-tiered Ag nanoparticles, each of which contains a cavity at the center, are generated by two simple steps of nano-imprinting and metal vacuum deposition. Because the individual Ag nano particles have their own hot spots, in the form of a nanoscale concentric cavity, it is possible to generate Raman hot spots reproducibly and without further reliance on nanoparticle aggregation or fortuitous interparticle distances. Using the fabricated two-tiered Ag nanoparticle array, highly sensitive detection of organic molecules with a wide dynamic sensing window was demonstrated.