High-throughput biosensing using chiral plasmonic nanostructures

The object of this thesis, is to demonstrate the potential capabilities of injection moulded chiral plasmonic nanostructures for enhanced sensing in biological systems. The key phenomenon employed throughout this thesis is the generation of electromagnetic fields, that produce a greater chiral asymmetry than that of circularly polarised light, termed ‘superchiral’ fields. These superchiral fields will be demonstrated as being an incisive probe into the structure, conformation, and orientation of proteins immobilised on the nanostructure surface of these injection moulded substrates. Initially, it will be shown how this phenomenon is sensitive to higher order changes in protein structure induced upon ligand binding, using an asymmetry parameter extracted from the optical rotatory dispersion (ORD) spectra. Where these changes would not be routinely detected by conventional chiroptical spectroscopy techniques, such as circular dichroism (CD). Further to this, as these nanostructures display the plasmonic analogue of the interference effect, electromagnetically induced transparency (EIT), a narrow transparency window is created within a broad reflectance spectrum. Where the spectra can be modelled using a simple coupled oscillator model, and the retardation phase effects extracted. This allows two new asymmetry parameters to be introduced for characterising any changes induced by the biological samples, the experimental separation parameter ∆∆S, and the modelled retardation phase asymmetries. These will be used to characterise the orientation of three structurally similar protein fragments, called Affimers, with the modelled phase asymmetries being shown as a particularly incisive probe into the surface immobilised orientation. Furthermore, conformational changes in the cancer relevant protein, HSP90, will be characterised upon the addition of increasing concentrations of the inhibitor molecule 17-AAG. With the orientation of the immobilised HSP90 protein being shown to influence the sensitivity observed for any protein-ligand interactions that occur. Finally, this phenomenon will be used to quantitatively detect elevated protein levels in a complex solution. Elevated levels of IgG will be measured in human blood serum solutions, utilising the isoelectric point of the proteins in solution to enhance the level of IgG adsorbed in the protein corona. This will demonstrate for the first time, the use of superchiral fields generated around injection moulded chiral nanostructures, to detect protein changes in complex real life solutions, such as human blood serum. Representing the first step in creating a high-throughput ultrasensitive system for a range of diagnostic applications