Nano-fabrication of plasmonic sensors

The fabrication process of our plasmonic sensors (depicted in Supplementary Figure 1) involved first generating a silicon ‘master,’ through a one-time photo-lithography process, which contained the desired nanostructures such as our hexagonal and square periodicity nano-hole arrays. Next, a monolayer of Tridecafluoro, 1, 1,2,2-Trtrahydrooctyl-1-trichlorosilane (Gelest, Inc.) was deposited via vapor deposition to form a ‘non-stick’ layer for the subsequent nano-imprint molding process. The masters were then used as molds by casting a drop of UV curable polyurethane acrylate (PUA-311RM, Minuta Technology, Inc.) onto the surface. A flexible cellulose-acetate film was used to disperse and flatten the liquid droplet over the silicon master, before being placed under a UV lamp (UV-A, 4W, 800 μW/cm², Thermo Fisher) for 2 hours to cure. After UV curing, the cellulose-acetate film was peeled from the master, completing the fabrication process of the ‘soft mold’. These soft molds therefore consist of the inverse geometries of that on the silicon master, and must be used for a secondary imprint molding process in order to recover the desired nano-hole array structure. The soft mold was then used to imprint its relief features onto liquid precursor of photo-curable polymer (NOA 81) deposited onto an oxygen plasma cleaned glass slide. After 25 minutes of curing time under the UV lamp, the soft mold was peeled away from the glass backed photo-polymer, completing the fabrication process of the desired quasi-3D nano-hole array. Finally, a bi-layer of 5 nm chromium and 50 nm gold was deposited onto the nanostructures by Electron Beam Evaporation (CHA Solution Electron Beam Evaporator) at a deposition rate of 2 Å/s and 3 Å/s, respectively, to complete the fabrication of the plasmonic sensor. Once the final plasmonic sensor was fabricated, a fluidic channel made of Polydimethysiloxane (PDMS) with a 300 μm height and 3 mm width was placed onto the surface with inlet and outlet holes to allow for unidirectional flow over the plasmonic sensor surface.