Developing highly reliable SERS substrates based on Ag grown on alumina nanomeshes anodized under 1 V for efficiently sensing Raman-active molecules

Abstract

We have developed silver-nanostructures grown on anodic alumina nanomesh (AAN) films to create new-type substrates for surface-enhanced Raman scattering (SERS). AAN with uniform thin sidewall of ∼ 5 nm was fabricated by anodizing Al sheets at 1 V in 6% H3PO4 solution. Subsequent AC electrochemical deposition of silver created an array of nanoparticles or nano-islands depending on growth time. The particle-island transition is non-monotonic evolution, since metal growth and dissolution compete in AC electrodeposition process. Systematic SERS study on various Ag-AAN films with trial probes of adenine solutions reveals collective contribution of electron-plasma oscillation and surface area of silver nanostructures in Raman enhancements. SERS signals are primarily contributed by surface area under excitation wavelength of 532 nm (away from plasmonic resonance). The average correlation coefficient between the SERS intensity and surface area was 0.85, indicating robust correlation. This value was reduced to 0.61 under excitation wavelength of 633 nm (closer to plasmonic resonance). Furthermore, increased Ag-deposition reduced the relative standard deviation of SERS intensities and thus improved both the uniformity and quality consistency of SERS substrates. Therefore, fabrication of SERS substrate with larger Ag surface area under similar SERS enhancement factors is suggested for high throughput in commercial sectors.