Unique high-performance metal-nanoparticle-free SERS substrate with rapid-fabricated hybrid 3D-Nano-Micro-Cavities anodic alumina for label-free detection

Conventional surface-enhanced Raman scattering (SERS) substrates were usually nanofabricated by coating Ag or Au metal nano particles (MNPs) to modify the surface. However, the complex and time-consuming nanofabrication prevents those good-performance substrates from the practical applications together with the instability of MNPs as a challenge. Here, a unique Roughened-Surface-with-hybrid-3D-Nano-Micro-Cavities (RS3DNMC) SERS substrate free of MNPs is proposed for high-performance enhancement using facile and rapid electrochemically micromachining aluminum foil to form randomly-distributed 3D over-etched cavities of 1 ∼ 2 μm in diameter embedded with the nanopores of anodized aluminum oxide (AAO). The metal-film-coated AAO with RS3DNMC structure creates great SERS enhancement because of two mechanisms from the significantly reduced reflectance for increasing the photonic-scattering-and-multi-reflection and the more peripheral-and-gap hotspots of 3D-nano-micro-cavities compared to the 2D flat surface. Our SERS substrate merits a high analytical enhancement factor (AEF) of 1.15x106 and a low limit of detection (LOD) of 1x10-10 M for methylene blue (MB) with a good uniformity of 9.64% relative standard deviation (RSD) in a large area of 2.25 cm2. The melamine widely remained in human body and environment was detected at LOD of 0.05 ppm that is much lower than WHO-recommended criteria (1 ppm).