Development of Nanopore Based Label-Free Optical Sensors

Abstract

Optical sensors play an important role and are employed for more application in today’s lives than ever before. As an example, optical sensing systems have established strong footprints in quality assurance (i.e. ensuring safe levels of controlled substances in drinks and food products) and self-diagnostics (e.g. detection and quantification of glucose in blood or pregnancy assessment test). Conventional optical sensor read-out is based on colour change or signal variation (i.e. absorbance or fluorescence intensity) of the label/tag molecule (i.e. dyes) conjugated to the capture probes. However, requirement of expensive and sophisticated labels/tags and instruments, skilled personnel, and other inherent issues with the dye labels (i.e. short lift-time, concentration dependent quenching etc.) limit their broader application. Therefore, label-free sensors present a great advantage over their label based counterparts. Label-free optical sensors rely on changes in physical properties (e.g. refractive index: n) of the sensing substrate occurring during a binding event. Nanoporous substrates (i.e. porous silicon, nanoporous anodic alumina, and titania nanotubes arrays) prepared by simple and scalable electrochemical anodization process in combination with spectroscopy techniques that can be realized with miniature spectrometer (e.g. reflectometric interference spectroscopy, localized surface plasmon resonance spectroscopy etc.) can potentially overcome the limitations of label-based sensing systems. However, comprehensive and extensive fundamental research must be carried out in this field to make this technology feasible, efficient, reliable, sensitive, selective and inexpensive. In this scenario, this thesis puts forward a novel combination of nanoporous anodic alumina (NAA) and reflectometric interference spectroscopy (RIfS) for developing a highly sensitive detection system for environmental and biomedical sensing application. High surface area, modifiable surface chemistry, and optical activity make NAA a perfect substrate for highly sensitive label-free detection using RIfS platform. Moreover, the geometric features of NAA can be controlled during the fabrication process to generate more complex optical photonic structures. The simplicity and versatility of this combination (i.e. NAA and RIfS) also allows for real-time monitoring of the release of drug for the NAA pores. The most relevant features of this thesis are: 1. NAA Substrate and its Surface Chemistry: Optimization and fabrication of NAA substrate with straight pores using two step electrochemical anodization process. Optimization and modification of NAA surface chemistry with different silanes (e.g. amine terminated or thiol terminated) to impart it selectivity and specificity towards analyte molecules. 2. NAA Photonic Structures: Designing, fabrication, and optimization of NAA pore geometry (i.e. effective medium) to obtain photonic structures (i.e. Rugate filters) that display highly sensitive and selective detection capabilities in combination with RIfS. Comparison of sensing capabilities of NAA straight pores with NAA photonic structures. 3. Flow Cells for Sensing: Designing and fabrication of different types of flow cells including bulk and micro-fluidic flow cell that can accommodate NAA substrates. 4. Sensing of Heavy Metal Ions: Modification of NAA substrate with silane which specifically bind to heavy metal ions such as gold (III) and mercury (II) ions in model solvent (i.e. mili-Q water) and real-life samples (i.e. tap water and water from river Torrens in Adelaide, South Australia). 5. RIfS vs Photoluminescence using NAA Substrate: Sensing properties of NAA studied using RIfS and photoluminescence as the detection techniques, when analytes were introduced into NAA pores under non-specific and specific binding conditions. 6. Real-time Drug Release Monitoring from NAA Pores: NAA pores can act as nanocontainers which can hold substantial amounts of drug molecules that can be released over an extended period of time. NAA loaded with model drug acts as a way of measuring the drug release from its pores in real-time and under dynamic flow conditions using RIfS. The results presented in this thesis are expected to open doors for the development of more innovative and complex NAA photonic structures and surface chemistries aimed to produce highly sensitive and selective miniature, portable, and point-of-care analysis system for various industrial, environmental, and biomedical applications.