Metamaterials are arrays of meticulously engineered sub-wavelength structures that exhibit properties that cannot be found in bulk materials. We engineer 2D metamaterial surfaces – metasurfaces – as optical sensors with high sensitivity and Quality factor (Q-factor) for bio and gas sensing applications. These are made up of arrays of dielectric nanostructures by state-of-the-art nanofabrication techniques. Dielectrics — contrary to the commonly used plasmonic materials — are CMOS compatible and have no associated heat loss, making them advantageous for integration into portable, digital point-of-care devices for biosensing applications in the future. Biosensing relies on the enhancement of the electromagnetic field when light interacts with the sub-wavelength particles composing the metasurfaces. These fields are highly sensitive to the surface refractive index, allowing them to be used as label-free refractive index sensors. Generally, in response to the presence of molecules on the metasurface, a shift in resonant wavelength is observed. However, these measurements are sensitive to the intensity fluctuations of the illumination source and its wavelength calibration. To bypass this limitation, we explored a novel sensing concept based on diffraction-based metagrating, that combines the resonant behavior with the tuned diffraction from the metagrating. To assess our metagrating sensors' performance, we explored its behavior in a concentration range of glycerol with the refractive index ranging from 1.331 to 1.401. Additionally, the sensors were benchmarked using selective capturing of anti-Mouse IgG by surface-immobilized Mouse IgG (antibody) bioreceptors (a model antibody system) on the surface that demonstrated a promising limit of detection of ~770 fM. Additionally, these dielectric metasurfaces, unlike plasmonic metasurfaces, can be reused by removal of the covalently attached molecular layers quite easily. These metasurface sensors are explored to develop point-of-care diagnostic devices for the detection of diabetes or multiple sclerosis.

Dielectric metasurfaces as refractive index sensors: Towards high-sensitivity label-free optical sensing

ACS Fall 2021

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