Label-free detection of DNA hybridization using nanopillar arrays based optical biosensor

In this study we fabricated nanopillar arrays (NPLAs) of silicon, through a process involving very-large-scale integration and reactive ion etching, for use as two-dimensional periodic relief gratings on silicon surfaces. Oligonucleotides were successively immobilized on the pillar surface, allowing the system to be used as an optical detector specific for the targeted single-stranded DNAs (ssDNAs). The surfaces of the oligonucleotides-modified NPLAs underwent insignificant structural changes, but upon hybridizing with target ssDNA, the NPLAs underwent dramatic changes in terms of their pillar scale. Binding of the oligonucleotides to the NPLA occurred in a way that allowed them to retain their function and selectively bind the target ssDNA. We evaluated the performance of the sensor by capturing the target ssDNA on the NPLA and measuring the effective refractive index (neff). The binding of the target ssDNA species to the NPLA resulted in a color change from pure blue to red, observable by the naked eye at an angle of 15°. 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. Accordingly, these new films have potential applications as label-free optical biosensors.