Protein Detection on an Optical Balance Diffraction Grating

We have developed a sensitive biosensor for protein detection called the diffraction land-contrast BioCD (DLC BioCD) in which a substrate is patterned into a diffraction grating that has vanishing first-order diffraction based on a sensitive balance of surface reflction. The graging is patterned using photolithography, and protein that is immobilized on the grating drives it off balance to generate a considerable diffraction signal. We fabricated a DLC surface based on a thermal oxide silicon wafer. Gratings consisting of grooves 65 nm deep with an 8 $\mu $m periodicity are etched into 200 nm SiO$_{2}$ on a silicon wafer. The first-order diffraction is proportional to $\left| {r_1 -r_2 } \right|^2$ where $r_1 $ and $r_2 $ are the reflection coefficients on 200 nm SiO$_{2}$/Si and 135 nm SiO$_{2}$/Si. $r_1 \approx r_2 $ for 488 nm wavelength light at normal incidence, and the grating generates nearly zero first-order diffraction. After applying a protein layer on the SiO$_{2}$, the complex values of $r_1 $ and $r_2 $ change with different signs on the complex plane. Therefore the change of $\left| {r_1 -r_2 } \right|^2$ caused by protein is maximized while the near-zero background significantly improves the sensitivity for protein detection. Experiments show that the signal-to-noise ratio of the protein signals is improved by a factor of 4 compared to a conventional BioCD, with further improvements possible.