Development of Nanopore Based Label-Free Optical Sensors

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.Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 201

A nanoporous interferometric micro-sensor for biomedical detection of volatile sulphur compounds

This work presents the use of nanoporous anodic aluminium oxide [AAO] for reflective interferometric sensing of volatile sulphur compounds and hydrogen sulphide [H2S] gas. Detection is based on changes of the interference signal from AAO porous layer as a result of specific adsorption of gas molecules with sulphur functional groups on a gold-coated surface. A nanoporous AAO sensing platform with optimised pore diameters (30 nm) and length (4 µm) was fabricated using a two-step anodization process in 0.3 M oxalic, followed by coating with a thin gold film (8 nm). The AAO is assembled in a specially designed microfluidic chip supported with a miniature fibre optic system that is able to measure changes of reflective interference signal (Fabry-Perrot fringes). When the sensor is exposed to a small concentration of H2S gas, the interference signal showed a concentration-dependent wavelength shifting of the Fabry-Perot interference fringe spectrum, as a result of the adsorption of H2S molecules on the Au surface and changes in the refractive index of the AAO. A practical biomedical application of reflectometric interference spectroscopy [RIfS] Au-AAO sensor for malodour measurement was successfully shown. The RIfS method based on a nanoporous AAO platform is simple, easy to miniaturise, inexpensive and has great potential for development of gas sensing devices for a range of medical and environmental applications

One-pot synthesis of pH-responsive Eudragit-mesoporous silica nanocomposites enable colonic delivery of glucocorticoids for the treatment of inflammatory bowel disease

Oral glucocorticoids are backbones for the acute management of inflammatory bowel disease (IBD). However, the clinical effectiveness of conventional oral dosage forms of glucocorticoids is hindered by their low delivery efficiency and systemic side effects. To overcome this problem, a smart drug delivery system with high loading capacity and colonic release by coating functionalized mesoporous silica nanoparticles (MSNs) with a pH‐responsive polymer Eudragit S100 is proposed. In vitro dissolution tests show that Eudragit‐coated MSNs can limit the burst release of loaded prednisolone and budesonide in the gastric environment with more than 60% of the drugs released only at colonic pH (i.e., pH ≥ 7). In vivo therapeutic efficacy of budesonide‐loaded nanoparticles is tested in a murine model of dextran sodium sulfate‐induced colitis. An oral budesonide dose of 0.2 mg kg−1 nanoparticles with Eudragit coating improves the disease activity index compared to other groups. Interestingly, both coated and uncoated nanoparticles show pathological improvements demonstrated by similar levels of histological colitis score. However, coated nanoparticles significantly decrease mRNA expression of the cytokines (Il‐1β, Il‐17, and Il‐10) particularly in proximal colon, indicating colonic delivery. Overall, this study demonstrates the effectiveness of a simple method to fabricate targeted nanomedicine for the treatment of IBD.Peer reviewe

Controlling interferometric properties of nanoporous anodic aluminium oxide

A study of reflective interference spectroscopy [RIfS] properties of nanoporous anodic aluminium oxide [AAO] with the aim to develop a reliable substrate for label-free optical biosensing is presented. The influence of structural parameters of AAO including pore diameters, inter-pore distance, pore length, and surface modification by deposition of Au, Ag, Cr, Pt, Ni, and TiO2 on the RIfS signal (Fabry-Perot fringe) was explored. AAO with controlled pore dimensions was prepared by electrochemical anodization of aluminium using 0.3 M oxalic acid at different voltages (30 to 70 V) and anodization times (10 to 60 min). Results show the strong influence of pore structures and surface modifications on the interference signal and indicate the importance of optimisation of AAO pore structures for RIfS sensing. The pore length/pore diameter aspect ratio of AAO was identified as a suitable parameter to tune interferometric properties of AAO. Finally, the application of AAO with optimised pore structures for sensing of a surface binding reaction of alkanethiols (mercaptoundecanoic acid) on gold surface is demonstrated

Sensing and biosensing applications of nanoporous anodic alumina

Nanoporous anodic alumina (NAA) is fabricated by a simple yet cost-effective self-ordering anodization process of aluminium foils, which yields highly ordered and columnar nanopores. These ordered pores impart NAA with unique optical and electrochemical properties, which have been intensively researched to develop smart, efficient, cost-competitive and portable yet complex sensing and biosensing systems. This chapter provides detailed fundamentals of sensing techniques and recent advances in development of NAA based sensing and biosensing technologies

Electrochemical etching methods for producing porous silicon

Porous silicon produced by electrochemical etching of silicon has become one of the most popular materials used in many scientific disciplines as a result of its outstanding and unique set of chemical and physical properties and cost-competitive fabrication processes. To understand the electrochemical mechanisms taking place in the course of the etching of silicon is a key factor to control and modify the structure of this versatile porous material. This makes it possible to produce a broad range of structures, which can range from a porous matrix to arrays of nanowires. These structures are unique and bring new opportunities for multiple research fields and applications such as biotechnology, medicine, optoelectronics, chemistry and so forth. This chapter is aimed at compiling and summarising the fundamental aspects behind the production of porous silicon structures by electrochemical and metal-assisted etching of silicon wafers. Our objective is to provide a simple but detailed overview about the fabrication process of porous silicon, with special emphasis on the different fabrication conditions and geometric and morphological features of the resulting silicon nanostructures