Accessible Sensing Sites in Metal–Insulator–Metal Plasmonic Nanostructure for Biosensing Applications

Localized surface plasmon resonance (LSPR) biosensing has emerged as a powerful technique for highly sensitive biomolecular detection. Metal–insulator–metal (MIM) nanostructures, featuring a plasmonic metal and a vertical gap, offer the potential for high sensitivity to background refractive index sensor measurements by generating strong near-field hotspots in the gaps. While they provide robust control of gap size, a challenge remains both to make the gaps accessible and to direct the binding of analyte to the gaps. We designed Au–SiO₂–Au MIM nanostructures with accessible gaps (aMIM) through FDTD simulation optimization and demonstrated fabrication and refractive index sensitivity experimentally. To direct detection events to the gap regions, we developed a 3-way copatterning of protein-rejecting brush polymers at and around the nanostructure, including functional groups for protein coupling only within the gaps. We demonstrated refractive index sensing directed within the gap using streptavidin as a model analyte, but the approach can be extended to a range of analytes. Our work highlights routes for site-specific binding at high-sensitivity sites around plasmonic sensors with the potential for use in biosensing.