Scalable Manufacturing, Transfer and Integration of Metasurfaces for Biosensing

Building upon a long-established history of plasmonic and nanophotonic biosensors, the recent advancements in metasurfaces have brought to light exciting new possibilities, such as tailor-designed high-Q resonances, spectrometer-less and imaging-based sensing. However, there is still a major challenge towards the widespread implementation of metasurface technology, for either sensing or wavefront engineering: optical metasurface manufacturing largely relies on electron beam lithography (EBL), which provide high resolution at a price of low throughput. It is difficult for EBL to offer either high-volume production or large area metasurfaces. Moreover, EBL-compatible substrates are planar and bulky, usually orders of magnitude thicker than the metasurface itself. Conventional substrates not only nullify the reduced footprint advantage of metasurfaces, but also limit their application scenarios.<br/>To address these challenges, we demonstrate scalable manufacturing of metasurfaces using the self-assembly approach known as nanosphere lithography, which is versatile in terms of material selection, nanopattern geometry and dimension tunablility. Through a process called “Marangoni convection”, polystyrene (PS) latex beads float at the air-water interface and eventually self-assemble into hexagonally close-packed pattern upon addition of surfactant. Nanosphere hexagonal arrays can be formed on a wider range of substrates, and then used as masks for metal deposition or etching to create complementary geometries of nanohole and nanodisc arrays, respectively. Oxygen plasma is used to etch the PS beads for fine tuning of the feature size in the nanoscale. After removing and cleaning of the PS beads layer, the metallic nanopatterns can be directly used as plasmonic metasurfaces for biosensing, or used as etching masks for pattern transfer into the underlying dielectric materials. Recently, dielectric metasurfaces are drawing increasing attention for sensing applications due to their advantages of negligible ohmic loss and high-Q resonances, compared to the plasmonic counterparts.<br/>Taking gold nanohole arrays as an example, the metasurface geometric parameters are tuned by the following: the period is determined by the original PS bead size, the nanohole diameter is determined by the Oxygen plasma etching, while the thickness is determined by electron beam deposition. We started with 600 nm PS beads assembly on glass coverslips, and etched the diameter to 360 nm for gold deposition of 50 nm. For the optofluidic integration, a polydimethylsiloxane (PDMS) chamber was formed in a mold, and then bonded on top of the metasurface after Oxygen plasma treatment. Microfluidic tubes were inserted to the PDMS chamber, forming inlet and outlet channels. In the medium of DI water, a transmission resonance dip was found near 700 nm wavelength using a spectroscopy setup. By changing media with different refractive indexes (RI), the bulk RI sensitivity was measured as 550 nm/RIU, with a limit of detection down to = 2×10^-4 RIU. The sensing performance matches well with finite-difference time-domain (FDTD) simulations, and approaches that of the state-of-the-art devices manufactured by EBL. The metasurface sensor was used to detect the specific binding between protein A/G and antibody, which can be further customized for various biosensing targets.<br/>In summary, we demonstrated the scalable manufacturing and optofluidic integration of metasurface biosensors. They provide high sensing performance comparable to EBL, while the high through and low cost make them more suitable for point-of-care testing. The scalable manufacturing and our recently developed metasurface transfer technique largely expand the selection of substrates, which will enable promising applications such as the integration of flexible/wearable devices and lab-on-fiber technology for in vivo sensing and diagnostics.