Synergistic Physical and Chemical Enhancement Effects Observed on Surface-Enhanced Raman Spectroscopy Substrates of Silver-Coated, Barrier-Type Anodic Alumina

Surface-enhanced Raman spectroscopy (SERS) represents an important spectroscopic technique with applications in many fields due to its ability to determine structural information of the analyte with high sensitivity, which requires the design of SERS substrates with high enhancement factors. Silver-coated porous anodic alumina (PAA) membranes arise as efficient and easy-to access SERS substrates with high enhancement factors. Related barrier-type anodic alumina (BAA) has not been investigated for SERS applications so far due to an absence of porosity and the anticipation of low SERS sensitivity. Now, we have found that non-porous BAA obtained through electrolysis using periodic and iodic acid followed by sputter deposition with silver exhibits higher SERS enhancement than that of just silver deposited on PAA where the enhancement factor of the best SERS substrate based on BAA from periodic acid shows a 2.6-fold increase compared to the prepared PAA-based SERS substrate. We have shown that physically derived factors as increased surface roughness, increased refractive index, or enhancement based on interference can be excluded upon comparison of different types of BAA. Chemical characterization revealed that iodine oxoacid-derived BAA, which is employed for the first time, contains iodine ionic species pointing toward a chemical enhancement mechanism aside from the physical enhancement effect.