Plasmonic biosensors: dark-field optical microscopy as a tool to detect cytokines

Abstract

Plasmonic biosensors are amongst the promising types of biosensors for ultralow detection of biomolecules. One of the tools in plasmonic biosensors is dark-field optical microscopy. By exploiting an optical properties of noble metal nanoparticles, namely localized surface plasmon resonance (LSPR), dark-field microscopy is able to visualize small nanoparticles. LSPR response of metal nanoparticles changes as a result of the change in local environment of the nanoparticles. This phenomenon is used in plasmonic biosensor to detect biomolecules. In this project, performance of a colorimetric method on detection of Interleukin 6, a pro-inflammatory cytokine, was studied. Detection of ultra-small changes in concentration of IL6 has been attributed to early diagnosis of many inflammatory diseases and different types of cancers. Plasmonic biosensors are able to reach to single molecule detection level, yet normal employed procedures such as single particle spectroscopy, are time consuming. Consequently, it is not practical to analyse many single molecule events using these procedures. So, a fast colorimetric method, based on a code already developed in Smart Material and Surface (SMS) group, was employed for analysis of single plasmonic particles in a massive parallel way. First an ensemble method was proposed to check the viability of the analysis method for detection of IL6. Here 67 nm Au nanoparticles modified with Anti-IL6 antibodies interact with 20 nm Au nanoparticles modified by different Anti-IL6 antibodies which was used to capture IL6. As a result, a core-satellite assembly is formed which was immobilized on a surface and studies under darkfield microscopy. Since single 67 nm cores show different colour values than core-satellite assemblies, the colorimetric code was able to detect the antigen. In the next chapter a surface-based biosensor was proposed to improve sensitivity and change the concept from ensemble to single event measurement. Here darkfield images of immobilized 67 nm cores were analysed before and after addition of reporter nanoparticles. In the final chapter with the help of mapping between SEM and dark-field images the ability of colour analysis method to reach to single molecule counting was tested. Also, with the help of Comsol simulation best core-satellite design was proposed