Characterisation of Single Biomolecules With Optoplasmonic Resonators

Abstract

Biomolecules can be detected through induced changes in the optical whispering-gallery mode (WGM) resonance in a circularly symmetric dielectric. The spatial and temporal confinement of light in a WGM is further complemented by coupling to the localised surface plasmons (LSPs) of metallic nanoparticles attached to the WGM resonator. LSP-WGM hybridisation allows for the optical readout of single-molecule surface reactions on gold nanoantennae, the mechanisms for which are not yet fully understood from a theoretical perspective. The specificity of this modality is, moreover, a subject of intense research. In this thesis, we propose three strategies for characterising molecules with light. The first strategy is a prototype polarimeter that differentiates chirality based on a signal-reversible Faraday effect in a magneto-optical WGM microcavity. Thermal tuning integrated into the resonator minimises geometrical birefringence, in turn maximising Faraday rotation to optimise chiral sensitivity. There we endeavour to resolve single-molecule chirality. Without engineering reconsiderations, however, the polarimeter is found to be limited to bulk chiral analysis. The second strategy is an (optoplasmonic) LSP-WGM resonator with chiral gold nanoantennae. Signals from the molecules conjointly show a correlation with the molecular weight and diffusivity of detected DL-cysteine and poly-DL-lysine. Aside from these features, the sensing site heterogeneity on the chiral gold nanoparticles impedes chiral discrimination. The third strategy is a novel reaction scheme adapted to the optoplasmonic sensor. Aminothiol linkers functionalise the gold surface via amine-gold anchoring, setting up cyclical interactions with thiolated analytes by thiol/disulfide exchange. Unexpected perturbations in the LSP-WGM resonance are observed, such as linewidth oscillation without resonance shift attributed to optomechanical coupling between LSPs and the vibrational modes in a given analyte. This offers a new form of spectroscopy wherein single biomolecules could be characterised by their mass, size, and composition through monitoring secondary parameters of the optoplasmonic resonance.