Doctor of Philosophy
Electrical and Computer Engineering
Dr. Jayshri Sabarinathan
Photonic crystals are the subject of intense study due to their capability to provide precise control of optical transmission characteristics through the choice of their periodic lattice parameters and the material with which they are fabricated. The sensitivity of photonic crystal band structure and defect transmission states to refractive index changes either at planar boundaries of a 2-D crystal or at individual lattice sites, make these structures potentially attractive for integrated point detection biosensor devices.
1-D Photonic crystal (PC) and 2-D PC based sensors were developed and studied in terms of sensitivity with different types materials functionalized on the surface. An extensive sensitivity analysis was done on both the devices to study the parameters that majorly effect device performance and the results from the proposed designs were found to be higher in comparison to published literature. Upon sucessfully designing of 1-D PC and 2-D PC sensors with high sensitivity these devices were fabricated to test them practically for various bio-materials. We demonstrated a 1-D PC sensor to detect pH change by coating thin film of hydrogel on PC surface which offers bio-compatibility for various bio-sensing applications. The 1-D PC sensor has optical sensitivities around 197nm/RIU in detecting refractive index change and 0.824nm per nanometer of thickness change of hydrogel on the surface of silicon gratings. We also developed a thin film coating procedure of hydrogel on 1-D PC sensor surface with high degree of uniformity. Thin film of hydrogels has many advantages over thick layer in terms of response time and enhancement in sensitivity. Silicon gratings were silanized so that the hydrogel is chemically bound to its surface and the micro-scale gratings devices were coated with a thin layer of 187.9nm of hydrogel using spin coating technique with high degree of uniformity. A fast response from thin layer of hydrogel integrating on optical sensor and high sensitivity makes these devices very attractive for bio-sensing applications.
Selectivity is another important aspect in the design of a biosensor. Thus, much effort has been exercised for developing a wide-range immobilization techniques that can covalently or ionically anchor to the surface of silicon based PCs devices. We demonstrate a 1-D PC sensor based biosensor to detect functionalized protein binding on its surface. The 1-D PC sensor has a sensitivity of 1.61nm per a nanometer of thickness change of bio-material on the surface of silicon gratings. Functionalizing proteins on gratings surface by eliminating unspecific bindings makes this device highly selective and efficient. Streptavidin of concentration 0.016µmol/ml was functionalized on silicon substrate and biotin of 12µmol/ml was used as a target molecule in our experiments for detection. Normal transmission measurements of 1-D PC sensors were made in air at different stages of immobilization, bare silicon grating, after attaching streptavidin and after trapping biotin. Besides protein detection, detection of DNA hybridization is also important for low volume, high selectivity biosensors. We also demonstrated the detection of functionalized DNA on 1-PC sensor surface as well as detection of the hybridization process with its complimentary DNA. The silicon 1-D PC designed have sensitivities upto 2.43nm per a nanometer of thickness change in bio-materials of refractive index 1.542 on its surface and 70nm/RIU in detecting refractive index change of 6nm thick bio-material coating on its surface. Immobilization of DNA on chip silicon device surface and there by hybridizing with its complimentary strand makes this device more selective and efficient in detection. The order of concentration of DNA solution used in this experiment is in nano-molar range and was functionalized on silicon substrate using layer by layer technique. Normal transmission measurements of 1-D PC sensors were made in air at different stages of immobilization, bare silicon grating, after functionalizing chemical linkers, after immobilizing ssDNA-A and after hybridizing with its complimentary ssDNA-B. Hybridization experiments were done with non-complimentary ssDNA-C as well and no shift in transmission spectra was found which shows very good selectivity.
Finally fabrication process for high sensitive 2-D PC coupled cavity device was also developed which needs to be tested in future.
Mudraboyina, Anil Kumar, "Planar Photonic Crystal for Biosensor Applications" (2011). Electronic Thesis and Dissertation Repository. 190.