Sensing of biomolecules is gaining interest due to its applications in many research areas such as bacterial and virus detection, medical diagnostics, drug development, food and environmental control. Commercialized biosensors rely on detection of labeled molecules. The intermediate labeling step however complicates the detection process and decreases reliability, so there is a growing need for label-free detection methods for fast, sensitive and quantitative sensing. To this end we developed a highly miniaturized label-free biosensor based on optical microcavities in Silicon-on-Insulator. This material system offers a high refractive index contrast suitable for the fabrication of nanophotonic circuits including micron- and submicron sized optical cavities of very high quality. The shift of resonance wavelength that occurs when the dielectric surroundings of such a cavity is changed, can be used for sensing. We demonstrate an SOI optical microring resonator with radius 4 micron capable of detecting bulk refractive index changes of 10-5, well approaching the literature stated limit of detection of 10-6 RIU for biomolecular sensing. Modification of the silicon surface, allowing for immobilization of biomolecules, is characterized by X-ray photoelectron spectroscopy, ellipsometry and Atomic Force Microscopy (AFM). As an example of label-free affinity sensing, we use the avidin/biotin system to demonstrate good repeatability. The device is fabricated using Deep-UV lithography, which is the technology used for advanced complementary metal-oxide-semiconductor (CMOS) fabrication. This process allows for high levels of integration comparable to current electronic integrated circuits indicating the potential of multiparameter analysis and lab-on-chip applications. Moreover, it allows for high-throughput fabrication needed for commercial applications.
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