Label-free Nanoscale Biosensing using a Porous Silicon Waveguide
The need to develop highly sensitive, selective, and cost-effective biosensors spans the areas of medicine, the environment, food safety, and homeland security. Accurate and reliable detection of trace quantities of small molecules is particularly challenging for existing sensor technology. Porous silicon is an excellent material for small molecule biosensing due to its large surface area and its capability of size selective sensing. In this work, a label-free porous silicon optical waveguide is demonstrated for the detection of short DNA oligonucleotides. The waveguide design enables the optical field to be localized exactly where biomolecules are attached to the sensor, leading to enhanced detection sensitivity. A prism is used to couple light into the waveguide at a particular angle where a sharp resonance dip is observed. The resonance angle is very sensitive to small changes in the porous silicon waveguide refractive index that occur, for example, when biomolecules bind inside the pores of the waveguide. Two different porous silicon waveguide structures are presented and compared: a p-type porous silicon waveguide with 20-30 nm pores in the Otto configuration and an n-type porous silicon waveguide with 100 nm pores in the Kretschmann configuration. Field profiles of the porous silicon waveguides are analyzed and their sensitivity enhancement over traditional sensor technology is shown for small molecule detection. In addition to experimental demonstration of quantitative and selective detection of DNA molecules using the porous silicon waveguide biosensor, the effects of biomolecular size on the sensitivity of porous silicon waveguide biosensors is discussed. With smaller pores, p-type porous silicon waveguides are more sensitive detecting shorter DNA molecules, while larger pore n-type porous silicon waveguides are capable of detecting longer DNA molecules that cannot easily infiltrate into p-type waveguides. Finally, further research is suggested to improve the sensor performance through modified design, alternate biomolecules to detect, and systems integration.