Crossed Surface Relief Gratings as Nanoplasmonic Biosensors

We present an original, low-cost nanoplasmonic (bio)sensor based on crossed surface relief gratings (CSRGs) generated from orthogonally superimposed surface relief gratings (SRGs) on gold-coated azo-glass substrate. This surface plasmon resonance (SPR)-based sensing approach is unique, since the light transmitted through a CSRG is zero except in the narrow bandwidth where the SPR conversion occurs, enabling quantitative monitoring of only the plasmonic signal from biomolecular interactions in real time. We validated the individual SRG plasmonic signature of CSRGs by observing their respective SPR excitation peaks, and tested them to detect both bulk and near-surface refractive index (RI) changes. Compared to simple SRGs, CSRGs portray a much-improved sensitivity of 647.8 nm/RIU, a resolution on the order of 10^-5 RIU, and a figure of merit (FOM) of 14 for bulk RI-change sensing. We also demonstrate their ability to perform as biosensors, through the detection and monitoring of near-surface biomolecular interactions in real time, a first for CSRGs. The minimum detectable concentration of biotin-streptavidin binding events was 8.3 nM. Due to their sensing abilities, low cost (<10 cents/unit), ease of fabrication, and inherent suitability for integration with microfluidics, we anticipate that CSRGs will stand as strong candidates in the portable diagnostics arena.