Intense terahertz field confined metallic nanoarchitectures for targeted actively capturing and specific biosensing via multi-step surface functionalization

Plasmonic resonators represent an emerging toolkit in biosensing by focusing electromagnetic waves within strongly enhanced electric field hotspots to facilitate light-matter interaction. However, the limited efficiency of field enhancement and the low spatial overlap between analytes and plasmonic hotspots impose fundamental limitations on achieving high-performance molecular assays. In this work, a terahertz (THz) biosensor for specific detection that allows molecules in solution to readily access predefined strong hotspot regions is presented. We construct heterogeneous nanogap structures by introducing a 20 nm-thick SiO 2 nanofilm to efficiently localize THz waves into the gaps providing over 300-fold field enhancement and optimal spatial matching with molecules. The sulfhydryl compounds are utilized to form Au-S bonds to cover large areas of the gold surface, preventing the aggregation of target molecules in the non-hotspot regions. Furthermore, the exposed silica surfaces within the nanogaps are functionalized with a biotin monolayer via silane coupling, enabling targeted active capture trace amounts of free streptavidin molecules in solution for specific detection at low concentrations down to 150 fM. This approach provides a valuable toolkit for ultra-sensitive biochemical detection and opens new avenues for the development of photonic devices in multifunctional, real-time, and on-chip sensing.