Silicon photonic resonator for label-free bio-sensing application

Abstract

In medical diagnostics there is an increasing demand for biosensors that can specifically detect biological analytes in a fluid, especially label-free sensing consistings of a transducer with biorecognition molecules immobilized on its surface without relying on fluorescent dye. In this research, we study the design and fabrication of a silicon nanowire photonic ring resonator and its feasibility as a biosensor. We have simulated and fabricated racetrack ring resonators which have a few tenths of micrometer gap, up to 0.5 µm between the input / output waveguides and the resonators. It is found that the devices can be designed with large Q factors and high sensitivity to biomaterial detection. The simulated results show that the ring resonator has a 15 nm resonance shift response per refractive index unit and presents the design and simulation of a tunable silicon on insulator ring resonator using tunable multimode interference (MMI) coupling structures. The new design is aimed to include the tuning capability for enhancing spectral response of the device in post-fabrication. Bioreceptor coating on the silicon surface is a crucial step in creating a functionalized device that can be used for bio-sensing. Conventional method which uses 3-Aminopropryltriethoxysilane (APTES) and glutaraldehyde (GA), or APTES-GA method, has the limitation on using GA crosslink, of which the two functional groups can bind to non-specific proteins, causing irregular binding. In this thesis, we propose a new coating technique to avoid such problem by applying APTES silanization with 1-Ethyl-3-(3-dimethyl aminopropyl)-carbodiimide (EDC) - N-Hydroxysuccinimide (NHS) protein crosslink, denoted by the APTES-(EDC/NHS) method. The EDC/NHS reaction has been shown to be able to immobilize protein in regular orientation due to consistent arrangement in C-terminal to N-terminal on surface. By applying APTES silanization, we circumvent the use of hazardous cleaning agent in conventional EDC/NHS technique. The experimental results in coating anti-Human TNF-alpha antibody have shown that the APTES-(EDC/NHS) technique has better repeatability in terms of lower standard deviation of thickness, measured by spectroscopic ellipsometry, as well as less roughness of the coated protein than the APTES-GA technique which are in agreement with the scanning electron microscope images, due to regular arrangement of coated antibody molecules. In addition, the coated antibodies have been confirmed by reaction experiments using Enzyme Linked Immunosorbent Assay (ELISA). The proposed APTES-(EDC/NHS) technique also takes shorter overall time since this technique can crosslink proteins to the silanized silicon surface in a single step.