Nanoscale characterization of interactions between molecular specific plasmonic nanoparticles and living cells and its implications for optical imaging of protein-protein interactions
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Imaging of biomolecules on the nano-scale is a crucial developing technology with major implications for our understanding of biological systems and for detection and therapy of disease. Plasmonic nanoparticles are a key optical contrast agent whose signal is generated by the collective oscillation of electrons in the metal particle. The resonance behavior of the electrons depends strongly on the arrangement of neighboring nanoparticles in a structure. This property may be exploited in imaging applications to report information on nanoscale morphology of targeted biomolecules. While the effect of plasmon resonance coupling has been studied in dimers and linear arrays of nanoparticles, this phenomenon remains largely unexplored in the case of 2D and 3D assemblies which are important in molecular cell imaging. This dissertation demonstrates how the optical signal from assemblies of gold nanoparticles can be related to nanoscale morphology in cellular imaging systems. First, the scattering spectra from live cells labeled with gold nanoparticles were collected and compared to the nanoscale arrangement of the particles in the same cells as determined by electron micrograph. Then, trends in scattering spectra with respect to nanoparticle arrangement were analyzed using a model system that allowed precise control over arrangement of nanoparticles. Several approaches to creating these model systems are discussed including biochemical linking, capillary assembly of colloidal particles, and direct deposition of gold onto substrates patterned by electron beam lithography. Spectral properties of the assemblies including peak position, width, and intensity are gathered and related to model variables including interparticle gap and overall particle number. It is shown that the redshift in the scattering spectra from nanoparticle assemblies is derived from both the particle number and the gap and is due to near-field coupling of particles as well as phase retardation of the scattered wave. The redshift behavior saturates as the number of particles in the aggregate increases but the saturation point depends strongly on interparticle gap. The drastic dependence of the red-shift saturation on the gap between nanoparticles has not been previously described; this phenomenon can have significant impact on the development of nanoparticle contrast agents and plasmonic sensor arrays.