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Title:A systematic study of ion-induced nanopatterning on photonic crystal-based label-free optical biosensor
Author(s):Cheng, Ming Kit
Advisor(s):Allain, Jean Paul
Contributor(s):Cunningham, Brian T.
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Ion irradiation
photonic crystal
optical biosensor
surface topography
surface chemistry
protein adhesion
titanium dioxide
thin film
diffraction gratings
Abstract:Photonic crystal (PC)-based biosensors are promising candidates for label-based fluorescence and label-free optical biosensors. This is due to the presence of PC, a periodically-structured dielectric material with diffraction gratings engineered on the surface that can control light propagation through its depth. PC enables resonance of a certain wavelength of light, leading to a significant increase in the intensity of light reflected off the surface, and hence the optical and fluorescent signal output from target analytes. In addition to the use of PC, sensor surfaces can be modified to become more biocompatible and sensitive to changes in target analyte concentrations. It is known that surface topography and chemistry are two main factors affecting protein affinity and conformation on biointerfaces, and the topography and chemistry can be tailored to improve their affinity and adhesion to the surface. It is also known that the presence of nanostructures can increase sensor sensitivity by providing a larger surface area for target adsorption. For example, a sensor surface coated with nanorods has been shown to increase the amount of proteins adsorbed by up to 4x. The tailoring of topography and chemistry can be achieved by the use of ion-beam nanopatterning, in which a beam of energetic ions is incident onto the surface with a tunable energy, incident angle and fluence, in order to disturb the equilibrium of the surface and induce redistribution of surface atoms, and hence changes in surface topography and chemistry. There has already been extensive research in ion-beam nanopatterning in numerous applications, implying its potential in surface modification. However, in these research either a bulk crystal or a thin film with a flat surface is used. When nanopatterning involves thin film with a non-flat surface due to the presence of larger pre-existing structures, such as the diffraction gratings on a PC surface, these pre-existing structures may have a shadowing effect, causing non-uniform sputtering of the surface, and resulting in variations of surface topography on different areas of the pre-existing structures, as well as affecting the PC’s ability to resonate and reflect light due to possible changes in the shape of these structures. Moreover, nanostructures on biosensors are often grown using conventional self-assembled deposition processes such as glancing angle deposition (GLAD), which has a low controllability on the size and shape of structures. To increase the variety of the size, shape and spacing of nanostructures, it is necessary to overcome the thermodynamic constraints by inducing a self-organization process on the surface, which can be initiated by ion-beam nanopatterning. Therefore, to address these two concerns, a systematic study on ion-beam nanopatterning on non-flat surfaces is needed. A systematic study of ion-beam irradiation on titanium dioxide (TiO2) thin film of ~ 100 nm thick coated on a non-flat polymer-based photonic crystal biosensor with one-dimensional rectangular diffraction gratings was carried out. Five parameters, i.e. ion species, ion energy, incident angle, beam orientation relative to the diffraction grating wave vector, and ion fluence, was varied in order to study their effect on the surface topography, chemistry and optical transmission property. It was found that ion-beam irradiation was able to induce topographical changes and growth of nanostructured ripples while keeping the surface chemistry unchanged. The topographical changes depended on the irradiation conditions as well as the position on the surface. Despite the topographical changes, either the surface chemistry remained unchanged, as in the case of O2+ irradiation, or the oxidation state of TiO2 was temporarily reduced, and then reverted back to the original state upon exposure to the atmosphere for a sufficient amount of time, as in the case of Ar+ irradiation. In addition, it was found that irradiation caused a blue-shift in optical resonant peak wavelength, probably due to a decrease in film thickness by sputtering. Nevertheless, the peaks remained sharp and intense enough to be distinguishable from the background signal in most cases, implying the sensor’s ability to detect peak wavelength changes, and thus act as a label-free biosensor to detect changes in target molecule adsorption based on the peak wavelength changes, is not affected. It was further found that an additional resonant peak was present when irradiation was carried out at an oblique angle of 30o relative to the edges of the diffraction gratings. This result is surprising, and the reason for this phenomenon was unclear. It might be related to the surface of the gratings being slanted towards one direction after irradiation. Further experiments in varying the beam orientation at a finer scale will be needed to examine the relationship between the topography and the appearance of extra resonant peaks. The goal of this study was to induce nanostructures of varying shapes and sizes, and hence increase the surface area which in turn improve the biosensor sensitivity up to the magnitude achieved by GLAD deposition of nanorods observed in a previous research conducted by Cunningham et al. (~4x increase in the amount of proteins adsorbed onto the sensor). However, it was found that the changes in topography due to irradiation were able to increase the surface area by only a few percent, much less than that achieved by GLAD (~4x). In addition, the study to relate the presence of nanostructures to sensitivity of the biosensor, i.e. to relate the increase in surface area due to nanostructures to the amount of proteins adsorbed onto the surface, was inconclusive. Proteins of different sizes and different adsorption mechanisms were used, but there was no clear trends of the amount of proteins adsorbed on surfaces as functions of separation of nanostructures and increase in surface area due to these structures. Moreover, the trends shown from XPS and optical transmission results, two techniques to determine the amount of target proteins present on the surface, did not match with each other. These experiments need to be repeated in the future to average out the data and hopefully observe more insightful trends. More samples with different ripple periodicity and surface area will also be needed.
Issue Date:2017-12-13
Rights Information:Copyright 2017 Ming Kit Cheng
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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