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Towards optical biosensors based on whispering-gallery modes in microsphere resonators

Islam, Muhammad 2019. Towards optical biosensors based on whispering-gallery modes in microsphere resonators. PhD Thesis, Cardiff University.
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Whispering gallery mode (WGM) sensors have attracted a significant level of interest recently due to their high level of sensitivity in the field of life sciences as biosensors. However, integrated WGM sensor devices are still in their infancy. In this work, we introduced a sensor device structure which is a step closer to commercial exploitation and mass production of monolithic WGM sensor capable of multiplex detection. The project started with the aim to design and then fabricate a sensor device which can utilize the advantages of high Q factor of theWGM to detect biomolecules. The sensing device would be able to detect small concentrations of biomolecules and would support multiplex detection. In this project we have designed and fabricated the sensor device successfully. The design and fabrication of the sensor device consisted of fabricating an array of planar single mode SU-8 WGs on glass substrate, developing a method to spin coat and cure MY-133, which is a low RI material, matching the RI of water, and then immobilising the microspheres on top of the WGs. The first task in designing and fabricating the array of WGs was to select the materials and estimate the dimension of the WGs which would serve the purpose of the project. Therefore, we needed to find the appropriate materials, i.e., for the substrate, the core, and the cladding layer of the WG and the microspheres. The array of planar WGs were fabricated on a glass cover slip serving as cost-effective substrate, and allowing to observe and analyse the light coupling into the WGs and microspheres from the substrate side when the sample would be submerged inside the fluid chamber filled with water in our measurement setup. Since the sensor device will be submerged into the water filled fluid chamber, we researched materials of RI close to that of water at the operating wavelength of the DFB laser (784 nm) to be used as the cladding material of the planar WGs. We have researched materials which have a RI close to water at 784 nm. However many of these materials could only be cured by heating at high temperature. Such a temperature curing procedure was used previously in our group to cure the cladding layer on top the of SU-8WGs and then an attachment layer to glue the microspheres. This temperature curing procedure involved temperatures above 100�C, which is not suitable for microspheres functionalised with biomolecules such as antibodies. A few alternative materials were investigated to address this issue. After going through extensive safety protocols for processing in the clean room MY-133, a low RI material was chosen for processing. Apart from the safety issue, we choose MY-133 of RI 1.33 at 784 nm because it could be spin coated and then cured by UV light. The UV curing method provides is fast, and a low temperature method. Curing by UV light allows to work with biomolecules (antibodies) already functionalised on the microspheres. The principle of light curable materials was discussed in Sec. 2.6. We have developed a standard operating protocol to deposit MY-133 on SU-8 WGs given in Appendix B.2. We choose SU-8 as the core material because it is highly transparent for the laser wavelength, and it has a high RI of about 1.6, which is higher than both the glass substrate and the cladding layer, allowing for light to propagate by total internal reflection. After choosing the appropriate WG materials we estimated the WG dimension, discussed in Chap. 3. The thickness of the SU-8 layer for single mode propagation was estimated between 450 to 1000 nm for a 3 μm wide WG. We estimated that the thickness of the MY-133 on top of the WGs should be 550±50 nm for optimal coupling between the WGs and the microspheres. The next challenging part in this project that we faced after choosing the material and estimating the dimension of the WG was to choose the correct type of SU-8 to deposit the estimated target thickness of about 900 nm that can be reproducible. We tried first with SU-8 2000.5 as this type of SU-8 is usually used to deposit a typical layer of thickness less than 1 μm. However it was hard to reproduce the target thickness. We therefore used SU-8 2002, which is usually used to deposit thicker layer of SU-8. We experimented with SU-8 2002 and found out that it needed to be diluted. We diluted SU-8 2002 with SU-8 2000.5 and found that a ratio of SU-8 to 2000.5 (2:1) is suitable for a reproducible SU-8 WGs of 900 nm thickness. The second task in fabricating the planar WGs was to spin coat the diluted MY-133 and then cure the MY-133 layer. MY-133 can be purchased in the form of a gel and needs to be diluted by a fluorinated solvent in which MY-133 is completely soluble. We chose HFE-7500 for its high boiling point (128�C) compared to alternatives. The diluted MY-133 was spin coated to deposit an approximately 750 nm thick layer. We first tried to cure the deposited MY-133 layer by keeping the sample submerged under water while exposing to UV light. However, this process produced a tacky surface, so we had to develop a curing unit allowing to keep the sample under inert gas during exposure. The second generation of this curing unit can cure a MY-133 layer within one minute in inert gas such as nitrogen. Once the WGs were fabricated we characterised them. Usually DekTak can be used to measure the thickness of the WGs but in this project we have developed an alternative method to measure the height of the WGs optically by using DIC microscopy. It is a non-destructive method of characterising transparent material. We have verified the DIC technique using DekTak measurements on the same samples. We have also used DIC to determine the RI of the cured SU-8 layer. The third task in this project was to glue the PS microspheres on the WGs. For this we have spin-coated a layer of about 150 nm thickness of MY-133. Microspheres were then drop casted in water onto this layer, and they adhere to the layer, making contact with the cladding layer underneath. We characterised the PS microspheres by first estimating the RI of the PS microspheres using the DIC technique. Then we wanted to determine the footprint of the microspheres in the MY-133 gluing layer. In this project we have used PS microspheres of 30 μm diameter, which gives the submerged height of the microspheres in the MY-133 layer about 400 nm. However, the DIC microscopy technique resulted in a significantly lower height - a point which is open for future investigation. The next task was to cleave the sample so that it can be fitted within the fluid chamber. We have faced a set of issues, for example, when we were cleaving the samples to define facets of the WGs, the SU-8 layer was peeling off from the facet. In order to address this issue we have designed a cutting tool. The use of the cutting tool improved the peeling issue. Fig. B.1 shows sample where the SU-8 layer were peeled off when using a scriber to cleave the sample for opening the facet, and the improved result obtained using the cutting tool. We have developed an optical setup for biosensor experiments. The setup includes a distributed feedback (DFB) laser, a fluidic chamber, and a linescan camera to detect the WGMs of the MS among other optical components. The biosensor device was positioned inside the fluid chamber by resizing it using the cutting tool, retaining the majority of WGs intact to allow light travelling from WG input to output. We can align the laser light from the DFB laser to a single WG from the array of WG on the sensor device and observe the output light from the WG in a 2D camera. We can also determine the quality of the WGs by observing the light intensity at the end of the WG in that camera. We have further developed the experimental data acquisition software to generate and control trigger signals for the laser and camera in optical measurement setup. The software was also used for optical alignment and to save experimental data for future analysis. We have shown that the laser light couples to the WGs and analysed the blinking microspheres excited by the laser light. We have also analysed the scan recordings from the linescan camera. Finally, we presented the sensitivity result from our experiments in Chap. 5. This work is an important first step towards an integrated biosensor based on WGMs of microsphere resonators. We have successfully fabricated an on chip sensor device and coupled the device with the laser light. We have also successfully excited the microspheres. However, the Q-factor that we obtained from our experiments is not as high as we expected. We have made suggestions to improve the Q factor discussed in Chap. 5. In order to be able to couple an array of WGs to excite many microspheres for multiple detection one should use the originally planned cylindrical lens in the setup as shown in Fig. 4.1, combined with a micro-lens array. Such an array could be fabricated by 3D laser writing with a Nanoscribe tool now available in the department. We have fabricated a sensor device with non-functionalised microspheres. Thus this device can only be used for non specific sensing. Time did not allow us to fabricate a sensor device with functionalised microspheres but for specific sensing one needs functionalised microspheres glued on the sensor device. We have bought functionalised PS microspheres and discussed briefly about specific sensing in Chap. 5 as well. Importantly, the design and fabrication of the device was undertaken accommodating the use of batch-functionalised microspheres, so that once sensing with high-Q modes is achieved, introducing functionalised beads should not pose additional technical issues. Thus the next step would be to fabricate a sensor device with functionalised microspheres.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: Optical biosensor, Whispering Gallery Modes, Microsphere resonator, Waveguide, Differential Interference Contrast (DIC) microscopy, Microfabrication, SU-8 waveguide, Curing of low refractive index material, Biosensing setup,
Funders: President's Scholarship
Date of First Compliant Deposit: 8 July 2019
Last Modified: 10 Mar 2020 02:42

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