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POLARIZATION-RESOLVED OPTICAL MICROSCOPY FOR NANOPARTICLE IMAGING
Paranzino, Bianca
Paranzino, Bianca
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Thesis/Dissertation
Date
2025-08
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Chemistry
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https://doi.org/10.34944/p1z3-j351
Abstract
Noble metal nanoparticles have emerged in applications such as biosensing, photothermal therapy, and catalysis due to their unique optical properties and large surface area-to-volume ratio. Their unique optical properties arise due to their ability to support a plasmon, which is the collective oscillation of the surface conduction electrons, and is dependent on the metal type, particle morphology, and the refractive index of their surrounding environment. Due to the plasmon’s dependence on particle morphology, certain shapes outperform others in different applications, giving rise to specific structure-function relationships. Structural characterization techniques such as electron microscopy are often employed to determine particle morphology but suffer from low throughput, high cost, and are destructive in nature, making in situ measurements of particles during dynamic processes challenging. Optical microscopy offers a non-destructive alternative for nanoparticle characterization that is capable of in situ measurements but suffers from a fundamental resolution limit called the diffraction limit of light, which prevents structures smaller than roughly half the wavelength of light used to illuminate them from being resolved. In this dissertation, we demonstrate that multiple variants of polarization-resolved optical microscopy enrich the information gained from diffraction-limited images and provide insights into nanoparticle structure and properties that would be hidden in traditional optical imaging.In Chapters 1 and 2, we discuss the basic principles of polarization-resolved imaging and introduce two variants: calcite-assisted localization and kinetics (CLocK) microscopy and defocused imaging. In Chapter 3, we use CLocK imaging of gold nanotriangles and develop a shape assignment workflow to quickly classify nanoparticle shapes using the wavelength and degree of anisotropy of their scattered light. We demonstrate how different imaging modes are suited for specific challenges, from rapid sorting to more refined shape assignments, and correctly assign gold nanotriangles with 98% accuracy. In the next chapter, we use CLocK microscopy to monitor copper nanoparticle oxidation in situ based on changes in the wavelength and degree of anisotropy of their scattered light caused by oxidation-induced morphological changes. We found that not only does the imaging medium cause heterogeneity in oxidation kinetics, but particles on the same sample under the same imaging conditions can still experience differences in their oxidation rates. Finally in the third chapter we used a technique called defocused imaging to report on the polarization of the luminescence for a plasmonic nanoparticle and its effect on the observed emission polarization of surface-bound fluorescent dipoles. We observed that the luminescence emitted from a plasmonic nanoparticle does not retain the same polarization as the input excitation as previously predicted by theory but instead depolarizes the light in three dimensions, The nanoparticle’s depolarized electric field then depolarizes the light emitted by fluorescent molecules bound to its surface, causing the emission to no longer be representative of a single dipole
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