Multi-resonant plasmonic nanodome arrays for label-free biosensing applications
Charles J. Choi, Stephen Semancik
The characteristics and utility of plasmonic nanodome arrays capable of supporting multiple resonance modes are described. A low-cost, large-area replica molding process is used to produce, on flexible plastic substrates, two-dimensional periodic arrays of cylinders that are subsequently coated with SiO2 and Ag thin films to form dome-shaped structures, with 14 nm spacing between the features, in a precise and reproducible fashion. Three distinct optical resonance modes, a grating diffraction mode and two localized surface plasmon resonance (LSPR) modes, are observed experimentally and confirmed by finite-difference-time-domain (FDTD) modeling which is used to calculate the electromagnetic field distribution of each resonance around the nanodome array structure. Each optical mode is characterized by measuring sensitivity to bulk refractive index changes and to surface effects, which are examined using stacked polyelectrolyte layers. The utility of the plasmonic nanodome array as a functional interface for biosensing applications is demonstrated by performing a bioassay to measure the binding affinity constant between protein A and human immunoglobulin G (IgG) as a model system. The nanoreplica molding process presented in this work allows for simple, inexpensive, high-throughput fabrication of nanoscale plasmonic structures over a large surface area (120 x 120 mm(2)) without the requirement for high resolution lithography or additional processes such as etching or liftoff. The availability of multiple resonant modes, each with different optical properties, allows the nanodome array surface to address a wide range of biosensing problems with various target analytes of different sizes and configurations.