Double-Langmuir model for optimized nanohole array-based plasmonic biosensors

The sensing mechanism of plasmonic nanohole arrays is investigated and a novel model is proposed to interpret their optical response over a wide dynamic range of concentrations (10^-13 - 10^-5 M), based on a double- Langmuir model. This model describes the signal response of the analyte binding as the sum of two independent contributions which are related to two different surface regions of the biosensor, namely the top gold surface of the nanohole array and the lateral gold area inside the nanoholes. Numerical simulations highlight the different near-field behaviour of these two regions and their very different refractive index sensitivities, which both support the double-Langmuir model. This is corroborated by experimental biosensing measurements with gold nanohole arrays with hexagonal symmetry, synthesized by nanosphere lithography. Their sensing performances are optimized by numerical simulations by changing their geometrical parameters (i.e., lattice constant, nanohole diameter and height) in order to achieve a maximum sensitivity. For the biosensing experiments, the biotin-streptavidin complex is used as a benchmark test for the optimized nanohole array and a robust calibration is provided by the double-Langmuir model obtaining a limit of detection of 0.3 ng/mL, which corresponds to an absolute analyte quantity of 0.02 fmol.