Two-Step Photonic Reduction of Controlled Periodic Silver Nanostructures for Surface-Enhanced Raman Spectroscopy

Silver nanoparticle surface morphologies can be controlled using ultraviolet light or visible light-emitting diode (LED)-assisted photonic reduction of Ag ions in solution. Ultraviolet nanosecond laser interference lithography (LIL) is a cost-effective method for fabricating periodic structures in a polymer matrix, and in this work, we combine with photonic reduction to produce novel nanostructured substrates. Differing reduction times result in differing fringe widths and nanoparticle sizes, with nanoparticle shape gradually changing from spherical to plate-shaped with increasing (365 to 615 nm) excitation wavelengths. Such nanostructured substrates, when utilized for surface-enhanced Raman spectroscopy (SERS), realized analytical enhancement factors for naproxen of about 105 with plate-shaped nanoparticles, which was at least ten times greater than the enhancement afforded by spherical nanoparticles. A three-dimensional finite element method simulating localized surface plasmon resonance properties of these controlled silver structures demonstrated nanostructured plates effectively enhanced local electrical fields. Our results demonstrate that multi-wavelength photonic reduction integrated with LIL is an excellent means of providing sensitive substrates for SERS.