High-Throughput Fabrication of Bimetallic Au–Ag Annular Nanogap Arrays Featuring 5 nm Gaps for Surface-Enhanced Raman Spectroscopy

Abstract The confinement of light into nanometer-sized (typically <10 nm) metallic nanogaps can generate extremely high near-field enhancement, dramatically boosting absorption, emission, and Raman scattering of target molecules situated within the gaps. However, the scarcity of cost-effective, reliable, and high-throughput nanofabrication techniques capable of fabricating sub-10 nm gaps, particularly in binary nanostructures, has significantly hindered their practical applications. Here, a high-throughput nanofabrication strategy that combines anodic aluminum oxide (AAO) templates, molecular self-assembly, and glancing-angle ion-beam etching (IBE) is proposed to controllably produce large-scale bimetallic gold–silver (Au–Ag) arrays with approximately 5 nm gaps. These arrays serve as exceptional surface-enhanced Raman spectroscopy (SERS) substrates, allowing for the ultrasensitive detection of target molecules with limits of detection (LODs) as low as 10–11 M for rhodamine 6G (R6G) and 4-mercaptobenzoic acid (4-MBA) and 10–8 M for bilirubin. The bimetallic Au–Ag nanogap arrays leverage the synergistic effects of gold’s strong absorption and silver’s superior plasmonic properties, achieving significantly enhanced electromagnetic fields compared to monolithic Au–Au nanogap arrays. This cost-effective and efficient nanofabrication approach overcomes the limitations of conventional methods for fabricating binary nanostructures, providing a viable solution for developing high-sensitive and reliable SERS-based detection platforms toward chemical-/bio-sensing.