Arrays of Split-Ring Resonator embedded in a Free-Standing Gold Membrane for Surface-Enhanced Infrared Absorption Biosensing Platform

In this study, free standing gold (Au) membranes with embedded arrays of split-ring resonator (SRR) were fabricated using modified nanospherical-lens lithography. The diameter of the polystyrene nanosphere used is 2 mm. The SRR-shaped photoresist (PR) patterns were achieved by exposing ultraviolet light at a tilt angle while rotating the sample and subsequent PR development. A thin Au layer was evaporated onto the rotating sample at an oblique angle and the evaporated Au only covered the surface area, resulting the Au membranes with embedded SRR arrays. The Au membrane was released from the substrate by dipping into the solvent and was recovered onto a stainless-steel plate with micron-sized holes. The Au membrane suspended within the micron-sized hole is the free-standing gold Au membrane with embedded SRR arrays. Surface plasmon resonance (SPR) of the fabricated SRR is in the mid-infrared, which was confirmed by the theoretical simulation. The structural parameters of the SRR were tuned so the SPR is within the molecular infrared signature region between 400 to 4000 cm^-1. 4-aminothiophenol (4-ATP) molecules were applied to the Au membrane and their absorption signal around 1490 and 1600 cm^-1, which are correspondent to the vibrational modes of C=C bond and NH₂ scissoring, are greatly enhanced. Therefore, the proposed gold membrane with embedded SRR array can be ideal platform for surface enhanced infrared absorption spectroscopy.