Nanoparticle-decorated nanocanals for surface-enhanced Raman scattering.

The surface-enhanced Raman scattering (SERS) effect is considered important for fast detection of characteristic ‘‘fingerprint’’ signatures of analytes. In the SERS effect, a substantial Raman enhancement arises on localized spots (‘‘hot spots’’) in metallic nanostructures owing to strong local electromagnetic fields associated with the surface plasmon resonances of metal nanostructures. SERS on colloidal metal aggregates and nanowires and Raman resonances have been utilized for trace detection of explosives, chemical and biological warfare agents, internal mechanical stress, distribution and stress of carbon nanotubes, and toxic environmental pollutants. However, an outstanding challenge of SERS-based detection is the lack of robust and facile fabrication routines for SERS substrates with considerable enhancements. Traditionally, electrochemically roughened metal surfaces, colloids, island films, nanowires, periodic arrays, and self-assembled nanoparticles are employed as SERS substrates (for a recent review, see Reference [27]). However, the long-term stability of aggregated nanoparticles is the main obstacle for assembled structures, and the fabrication of complex surface structures (e.g. with microfabrication) is laborand cost-demanding and sometimes impossible to extend to large dimensions. Moreover, the sensitivity of simple 2D SERS substrates remains modest owing to a limited number of hot spots (usually below 10). To increase the sensitivity of SERS substrates, 3D porous structures have been suggested as active SERS substrates with the advantage of having a large surface area available for the formation of hot spots and the adsorption of target analytes. Consequently, 3D SERS substrates have been fabricated by depositing Au or Ag films on porous silicon, GaN, and filter paper. Alternatively, deposition of metal nanoparticles on the porous aluminum membranes, colloidal crystal templates, or microwires have been exploited. For instance, nanopores in gold films have been proven to show enhanced Raman scattering owing to an intense electromagnetic field generated by the surface plasmons. However, most of these studies have not fully utilized the advantages of 3D structures for SERS effects, mainly because of limited light propagation through porous materials owing to