Phase Modulator Ellipsometry Based Biosensor for Detection of E. coli K12

In recent years, rapid detection of pathogenic microorganisms in human food and water resources has been widely investigated to reduce the number of deaths as well as economic loss. Surface plasmon resonance (SPR) sensors as an optical method is used for detection of biological and chemical targets. These biosensors are extensively applied for monitoring of microorganisms such as bacteria. While most strains of <i>E. coli</i> are harmless, there are certain pathogenic strains that can cause illnesses in humans. Due to the size of <i>E.coli</i> bacteria (~ 1 micron), detection of these microorganisms brings challenges in experimental measurement. In this study, we aim to apply phase-modulator based ellipsometer to enhance the performance of conventional SPR sensors for E. coli detection.<br/>Surface plasmons as an surface charge density waves of free electrons at a metal surface propagate at metal-dielectric interface. In specific angle, the p-polarized incident light is strongly absorbed in consequence of coupling the evanescent wave generated at metal-dielectric interface and surface plasmon waves. Hence, the resonance can be observed as an sharp dip in reflectivity profile. Conventional SPR sensors monitor the changes in the intensity of reflected light according to the changes of refractive index of target sample. In resonance condition, not only the intensity of the incoming light changes, but also the phase change is strongly modulated. Due to the significant phase jump, this sharper shift can be studied to improve the performance of SPR sensors. Spectroscopic ellipsometry provides this opportunity to study both reflectance ratio and phase changes in reflected light from the sample of interest.<br/>Spectroscopic ellipsometry analyzes the changes in the polarization state of light after reflection or transmission from a surface and determines the optical properties and thickness of thin films. These changes in polarization states are studied by the amplitude ratio (Ψ) and the phase difference (Δ) between p- and s-polarized light, respectively. There are different measurement methods of ellipsometry based on the optical elements configuration and arrangement. Barium borate crystal based phase-modulator has been used in our optical setup to induce phase changes in the incoming light beam. Using a phase-modulator in optical design of ellipsometer makes it a faster measurement tool compared to the conventional ellipsometric measurement methods. Using this method each measurement takes around micro second. Therefore, this method can be used for detection of microorganisms with higher time resolution.<br/>This project has been focused on using conventional Kretschmann configuration including quartz prism and ~45 nm of homogenous gold film to support propagation of surface plasmon waves. Gold film is functionalized with porphyrin molecules as a linker with specific peptide group to bind with outer membrane of Gram-negative bacterium. This configuration has been used upon phase-modulator ellipsometer to monitor E. coli K12.