Optically Selective Nanostructures and the Optical Sensing Applications
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The ability to accurately detect biological or chemical agents, potentially at single molecule level, requires the development of techniques that are highly sensitive and specific. Sensitivity is the change of the measurement signal per concentration unit of analyte. The specificity is to demonstrate whether a sensor responds selectively to a specific group of analyte. The On potential methodology for realizing such sensors is the development of optical sensors. Specifically, optical-based sensors are relatively immune to electromagnetic interference, can be used to perform integrated multiplexed detection, and can be used perform remote sensing. In fact, the range of optical detection strategies and techniques are vast. Of specific interest to this thesis is the use of fluorescent molecules as labels since these methods have already demonstrated high sensitivity and have even demonstrated detection down to a single molecule level in specific applications . To improve the robustness of this technique, this thesis is focused on the development of structures that can enhance the collection efficiency of the fluorescence of sensor elements with integrated fluorescence molecules whose emission intensity is representative of the concentration of the analyte of interest. Therefore, this thesis is focused on the development of porous polymer photonic bandgap structures that can provide several optical methodologies for biochemical sensing – refractive index changes, transmission and reflection changes, and as light steering elements. The ability to "steer" the emission from sensor elements towards a photodetector is of particular interest to this thesis since it could provide a structure that could potentially be applied to many different sensor system. The increase in signal collection from "steering" will enhance the overall signal-to-noise in the detector systems. The designed photonic bandgap (PBG) structures (or photonic crystals (PC)) are good examples of the manipulation of nanoscale structure to affect the optical properties of a material system – in this case creating structures that forbid the transmission of certain wavelengths of light. These developed structures are one example of the recent global research effort in nanophotonics which include three main aspects: the nanoscale confinement of radiation, the nanoscale confinement of matter and nanoscale photoprocesses . In this thesis, I will focus on the development of nanostructured materials using a nanoscale photoprocess - holographic laser interference lithography. Specifically, this thesis focuses on the design, fabrication and characterization of holographic porous polymer photonic bandgap (P 3 BG) structures with specific application to the enhancement of fluorescence-based sensing. Functionalized P 3 BG structures such as flexible P 3 BG structures and concave shaped P 3 BG arrays are introduced. The potential of using these structures to enhance sensing will be discussed. In Chapter 1, I will introduce the basics of optical chemical and biomedical sensing. State-of-the-art and the future trends of modern optical sensors will be discussed. The fabrication of P 3 BG structures using the holographic lithography method is introduced in Chapter 2. The chemical components and photo physics of the photopolymer emulsion system are discussed in Chapter 2. In addition, the optimization of the resulting structures is presented. In Chapter 3, the application of the P 3 BG structures in optical sensing will be presented. First, the P 3 BG structures are demonstrated to be effective sensing elements for the detection of changes in refractive index. Second, the P 3 BG structures are combined with fluorescent elements and demonstrated as oxygen sensors. At the end of Chapter 3, I will show how the P 3 BG structures are integrated with xerogel based fluorescence sensors as a low-cost and effective optical filter. Functionalized flexible sensors are introduced in Chapter 4. These flexible sensors could be applicable to biomedical sensors that need to conform to curved surfaces. In this work, I demonstrated that it is possible to transfer the P 3 BG structures onto flexible substrate. Chapter 4 provides the characterization of the optical properties of the flexible P 3 BG structures. In addition to the flexible structures, nano-ribbon structures fabricated from the P 3 BG structures are discussed. In Chapter 5, P 3 BG micro-concentrator arrays, as another kind of functionalized P 3 BG structures are designed using reflection holography and fabricated using reflective microspheres. Microspheres ranging in size from several hundreds of microns to several microns are experimentally realized. The focusing properties of these structures are characterized. These structures can be applied not only to enhancement of fluorescence signals in optical sensing, but also to generating optical forces for laser tweezers. The final chapter, Chapter 6, provides a summary of the work accomplished in this thesis and provides some ideas for future studies of functional P 3 BG structures design and their possible sensing applications.