Photonic crystal and plasmonic nanohole based label-free biodetection.

Photonic crystals and plasmonic nanohole arrays are the conventional substrates for label-free biodetection applications. In this article, we readdressed these systems in terms of their sensing capability and provided a broad picture for a selection mechanism of optimum parameters providing strong sensing signals. We first investigated the physical origin of the transmission resonances supported by the two systems, which is the core of the label-free sensing mechanism, relying on strong light-matter interactions. We conducted an extensive theoretical study on optical and sensing properties of the two systems, e.g., linewidth of the optical modes, refractive index sensitivity and figure-of-merit capacities. Our theoretical analyses provided a rule-of-thumb method for the selection of geometrical device parameters of the two systems. In order to experimentally investigate the sensing properties, we fabricated the two systems via a lift-off free fabrication method based on electron beam lithography, where the plasmonic nanohole arrays are realized by covering the phonic crystal surface with a thin metal. As an example, we demonstrated the sensing strength of two systems with identical dimensions by monitoring the spectral variations within their optical responses. We also performed label-free sensing experiments through detection of protein mono- and bilayers, where the geometrical parameters favor the plasmonic sensor system. Integrating a high-resolution optical read-out scheme with a multi-spectral data tracking technique, we achieved an experimentally minimum detectable protein concentration as low as 200 pg/mL for the plasmonic nanohole array and 1 ng/mL for the photonic crystal-based sensing platform.