Multilayer thin film optical biosensors

This thesis describes a planar optically resonant multilayer thin film device and discusses its application as an evanescent field fluorescence immunosensor. Previous workers have used multiple total internal reflection (TIR) techniques and both monomode and multimode waveguides for planar evanescent field fluorescence immunosensing. TIR methods give poor sensitivities (insufficient for many analytes of clinical interest) due to low evanescent field strengths and high background levels whereas efficient and reproducible exciting light coupling into planar waveguides presents a major instrument design problem. Optically resonant multilayer devices offer the high evanescent field strengths and low penetration depths of waveguide devices whilst retaining the ease of input coupling of TIR devices. Multilayer devices also offer the prospect (as do waveguide devices) of surface patterning using techniques such as photolithography, to allow mutiple-analyte measurements on a single device. The theory of multilayer systems is briefly described followed by the fabrication of the multilyer devices using sol-gel silica and iron phosphate thin films - chosen because they allowed the inexpensive deposition of thin films from solution by dip and spin coating techniques. The multilayer devices were characterised by combining theory with observed resonance measurements, which allowed the refractive indices of the thin film materials to be estimated. The model assay was a fluorescence immunoassay for mouse-IgG in a buffer solution. The multilayer device gave a seven-fold sensitivity improvement over a TIR device. The assay was performed using a 633nm Helium-Neon laser and a 650nm semiconductor laser diode. The results showed that similar sensitivities could be obtained despite the five-fold lower optical power output of the laser diode. The theory of multilayer devices was extended to model the multilayer immunosensor, thus allowing the analysis of the sources of the fluorescence and background signals. This modelling revealed the immobilised capture antibody layer to be highly scattering and significant background signal contributions to be from scattered exciting light and from bulk solution fluorescence. The contribution to the background signal from scattering in the thin film layers was found to be insignificant.