Design, fabrication and evaluation of a label-free silicon-on-insulator “Lab on a Chip” optical biosensor

Abstract

This thesis presents the design, fabrication and testing on a Silicon-on-Insulator (SOI) “lab on a chip” immunosensors based on interferometer technology capable of label-free, real time, parallel detection and identification of multiple analytes with extremely high sensitivity. The basic principles for the biosensor device are evanescent wave sensing and interferometry. Light is guided through a high index contrast photonic wire by total internal reflection. The high index contrast core-cladding photonic wires supporting a vector TM-like mode can be designed to have extremely high sensing sensitivity for low analyte volumes. The optical field decays rapidly from the surface of the waveguide, penetrating into a test solution by ≈178nm. When a chemical, biochemical or biological reaction takes place in the sensing area, i.e. in the region of the decaying field near the surface, the light propagating through the sensing channel will experience a change in its effective refractive index causing a phase change per unit propagation length. With a reference channel built-in on chip, the resulting phase difference between the sensing and reference channels can be detected with a very high sensitivity of detection. The first sensor studied consists of Mach-Zehnder interferometers (MZIs) fabricated with silicon photonic wires. For a MZI with a sensing length of 1000μm the theoretical sensitivity is 3.1 × 10-7 Refractive Index Units (RIUs). Sensitivity is further increased by incorporating spiral waveguides to increase the length of the interferometer arms within a given wafer footprint. A spiral of four turns gives a sensing arm length of 3145μm, which takes up an area of 0.06mm2 and gives a theoretical MZI sensitivity of 9.9 × 10-8 RIUs. The sensitivity of detection of the biosensors developed is at least 10-100 times more sensitive than that of current commercial products. Parallel detection is achieved by exciting multiple sensors in parallel using a 1xN Multimode interferometer (MMI) where N≤20. The second sensor considered is a label-free self-aligned Plasmonic Interferometer (PI). The interfering plasmonic modes are excited on either side of a thin gold layer embedded into a silicon photonic wire. Only the mode on the upper surface interacts with the analyte. The resonant wavelength is thus sensitive to the analyte index. The PI designed for this project is 13μm in length and gives a predicted system wavelength shift responsivity of 500nm/RIU. To guide the way towards the successful design of these sensors commercial software is used to perform detailed simulation evaluations of straight and curved waveguides, Mach Zehnder Interferometers (MZIs), Plasmonic Interferometers (PIs), Multimode interferometers (MMIs), Directional Couplers (DCs), Y-junction splitters and Spot size converters. So that the proposed biosensors can operate as an immunosensor, two methods of selective surface functionalisation are developed to immobilise the antibodies on to the sensor waveguides. The first method attached a non-uniform layer of amine functional group to surface of silicon via a hydrosilylation process. The second method attached a uniform layer of amine functional group to the surface of silicon dioxide via a hydroxylation and silanisation process. Attaching a fluorescent tag to the amine functional groups allowed the imaging and assessment of the surface modification and selectivity. Four sets of biosensor devices were fabricated and tested showing promising results.

Date of Award	8 Nov 2017
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Yaping Zhang (Supervisor) & Trevor Benson (Supervisor)