Real-time multicolor antigen detection with chemoresponsive diffraction gratings of silicon oxide nanopillar arrays

In this study we fabricated nanopillar arrays (NPLAs) of silicon oxide, through a process involving very-large-scale integration and reactive ion etching, for use as two-dimensional periodic relief gratings on silicon surfaces. Antibodies were successively oriented on the protein G-modified pillar surface, allowing the system to be used as an optical detector specific for the targeted antigen. The surfaces of the antibody-modified NPLAs underwent insignificant structural changes, but upon immunocapture of antigens, the NPLAs underwent dramatic changes in terms of their pillar scale. Binding of the antibodies to the NPLA occurred in a way that allowed them to retain their function and selectively bind the antigen. We evaluated the performance of the sensor by capturing the target antigen on the NPLA and measuring the effective refractive index (neff). The binding of the antigen species to the NPLA resulted in a color change from pure green to pink, observable by the naked eye at an angle of 10–20°. Moreover, we used effective medium theory to calculate the filling factors inside the NPLA and, thereby, examine the values of neff during the structural changes of the NPLA. This technique eliminates many of the surface modifications and secondary immunochemical or enzyme-linking steps that are common required in immunoassays. Accordingly, these new films have potential applications as real-time optical biosensors.