| Author: | Wang, Han |
| Title: | Optofluidic chips with directly printed polymer optical waveguide sensors for label-free biodetection |
| Advisors: | Zhang, A. Ping (EEE) Wong, Siu Hong Dexter (BME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xvii, 109 pages : color illustrations |
| Language: | English |
| Abstract: | Optofluidic chips fusing photonic and microfluidic technologies have attracted remarkable attention due to their tremendous promise in the development of small-size high-performance biochips for biomedical detection and diagnostics. The progress of affordable diagnostic tools for everyday testing and extensive applications is impeded by the intricate fabrication procedures and restricted functionalization flexibility inherent in conventional silicon-based photonic chips. In this thesis, optofluidic chips based on directly printed polymer optical waveguide sensors, including Mach-Zehnder interferometer (MZI) waveguide sensors and micro-ring resonator (MRR) waveguide sensors, are developed, and their performances in label-free detection have been numerically and experimentally investigated. A new fabrication process for SU-8 optical waveguides was developed by using an in-house digital ultraviolet lithography (DUL) system. Compared to the photomask-based lithography technologies, DUL can save the cost for the fabrication of photomasks and shorten the lead time. On the other hand, it has the advantage of higher throughput over conventional electronic beam lithography (EBL) and direct laser writing (DLW) technologies. Some typical problems in digital lithography, such as proximity effect and stitching error, have been well compensated through the numerical pretreatment of pattern data and process optimization. Experimental results showed that at a wavelength of 1550 nm, the propagation loss of the manufactured straight waveguide is measured to be 0.238 dB/mm. The bending loss is below 0.1 dB/90°-arc if the bending radius is greater than 100 μm. Moreover, multi-mode interferometer (MMI) and Y-branch power splitters were experimentally fabricated to uniformly split the input power across a broad spectral range, spanning from 1500 nm to 1600 nm. With the own-established waveguide fabrication processes, an integrated optofluidic biochip with directly printed polymer optical waveguide MZI sensors has been developed. An asymmetric MZI sensor embedded with width-tailored waveguide was designed and fabricated for high-sensitivity label-free biodetection. In order to get a high-contrast interference at the output, a specifically constructed Y spitter has been designed to compensate for the light propagation loss in the sensor arm and make the light powers from the two arms suitably balanced. After coming with the transmission spectra of the MZI sensors before and after fabrication of Ormoclad cladding, it was verified that such a special designed MZI sensor can greatly enhance the extinction ratio (ER) of output spectrum to reach 13.48 dB when the sensor was finally packaged and immersed under water. After precise printing of an Ormoclad layer via an overlay exposure process and vertical integration with a microfluidic layer, an integrated optofluidic chip was fabricated for on-chip label-free biodetection applications. The on-chip integrated MZI sensor devices exhibited a remarkable high bulk refractive index sensitivity, i.e., 1695.95 nm/RIU. The demonstration of optofluidic chip's ability to detect disease biomarkers was conducted by quantifying the concentration of Human immunoglobulin G (HIgG). HIgG, a main antibody present in both blood and extracellular fluids, acts as a vital defense mechanism against bacterial and viral infections within the human body. After being functionalized with capture molecules, these MZI biosensors were capable of detecting HIgG analytes down to a low concentration of 1.78 pM, which is far beyond the performance of current commercial ELISA tests. Moreover, an optofluidic chip integrated with directly printed polymer optical waveguide MRR sensors has also been demonstrated. An all-pass MRR sensor that embedded with a width-tailored waveguide was designed and directly printed by using our own-built digital UV lithography processes. The Q factor of the fabricated MRR sensor is as high as 8604. An optofluidic chip has been fabricated, by vertically integrating the MRR sensors with a microfluidic layer, for on-chip label-free biodetection. Results showed that the bulk sensitivity of on-chip integrated MRR sensors is 22.55 nm/RIU. Considering that such MRR has a very narrow transmissive spectral dip (i.e., very high spectral resolution) because of high Q factor, it is believed that such an integrated optofluidic chips is also very promising for high-sensitivity biochemical applications, such as nucleic acid or virus testing. The incorporation of polymer optical waveguide biosensors in these optofluidic chips presents a range of advantages, such as miniaturization, cost-effectiveness, heightened sensitivity, and simplified functionalization. As a result, this breakthrough has the potential to revolutionize the development of compact devices and instrumentation for point-of-care testing and diagnostics. |
| Rights: | All rights reserved |
| Access: | open access |
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