Optical Biosensors Based on Nanostructured Silicon High-Contrast Gratings for Myoglobin Detection

Optical sensors are efficient, compact, and reliable tools that can be used to detect almost any kind of analyte. Among various sensing schemes, one of the most frequently used techniques relies on measuring the spectral shift of resonances due to a change in the refractive index caused by the presence of target molecules. High-contrast gratings (HCGs) are examples of nanoscale devices. Such a sensor's layout serves as a suitable platform for the detection of various kinds of biomarkers because it exhibits sharp optical resonances. Different types of HCGs include conventional, pedestal, half-buried, and floating HCGs. Modeling suggested that both pedestal and half-buried HCGs outperform conventional HCGs in terms of bulk or surface sensitivity. Here, we report on a comparison of the bulk and surface sensing performance of half-buried and pedestal HCGs. The pedestal HCGs provide a higher bulk refractive index sensitivity (536 nm/RIU as opposed to 409 nm/RIU) due to the extended surface area and better resonant conditions. On the contrary, the half-buried HCGs provide access to the regions with the highest electric fields of guided-mode resonances. As a consequence, their surface sensitivity surpasses that of the pedestal counterparts, as shown by dummy analyte layers of Al2O3 and TiO2 with different thicknesses. Both pedestal and half-buried HCGs were functionalized and employed for optical biosensing of cardiac biomarker myoglobin. The latter HCG structure showed a limit of detection (LOD) slightly lower than that of the former one: 35.7 versus 38.5 ng/mL. Both types of nanopatterned HCGs exhibited a LOD within the normal range of myoglobin levels in humans, demonstrating their potential as a myoglobin-sensing platform for clinical studies.