Immobilization of Gold Nanoparticles for Colourimetric and Ratiometric Refractive Index Sensing

Abstract

The development of refractive index sensors is an expanding field of research, with applications in fields including medical diagnostics, food safety and public health. Nanotechnology has become widely implemented in various refractive index sensing techniques, resulting in substantial progress in detecting minute changes of refractive index. A literature review of the current refractive index sensing techniques which incorporate nanotechnology demonstrates that two main strategies for refractive index sensors have been developed, those that provide highly sensitive measurements and those that provide visual colourimetric measurements that can be detected by the naked eye. All colourimetric sensors based on gold nanoparticles offer a red to blue shift, however, this thesis outlines the development of a ratiometric colourimetric refractive index sensor that provides a unique blue to red shift. The development began with controlling the deposition of the various gold nanoparticle populations which are immobilized via electrostatic interactions between a weak polyelectrolyte and the gold nanoparticles. It was determined that by altering the pH of the polyelectrolyte as well as the size and concentration of the gold nanoparticles, modulation of the nanoparticle populations could be achieved. The nanoplasmonic surfaces were then shown to be effective sensors for the refractive index range of 1.00 to 1.47, providing both red to blue and blue to red colourimetric shifts, depending on the ratio of the different immobilized nanoparticle ensembles. The sensor surfaces were shown to be reusable, however the electrostatic interactions responsible for immobilizing the nanoparticles were weakened when exposed to various cleaning solutions and common solvents, resulting in nanoparticle loss from the sensor surface. Lastly, the optical response achieved by the refractive index sensors was simulated using COMSOL Multiphysics, which provided insight on the properties that resulted in the unique colourimetric shifts.