Sensing and Imaging Biomolecules with Plasmonic Nanoparticle Assemblies Coupled with Darkfield Microscopy

Noble metal nanoparticles exhibit unique optical properties arising from the resonant oscillations of their conduction electrons with light. This phenomenon is called localized surface plasmon resonance (LSPR). The LSPR frequency is extremely sensitive to the size, shape, refractive index at the metal-dielectric interface, and other nearby metal nanoparticles. In an assembly of proximal nanoparticles, the LSPR of individual particles can couple to yield enhanced light scattering and large spectral shifts, which are useful for many applications including diagnostics and sensing. This dissertation presents a complex nanostructure comprising a core gold nanoparticle surrounded by multiple satellite gold nanoparticles for biosensing application. Chapter 2 introduces the fabrication and characterization of the core-satellite assemblies via a layer-by-layer process. Using ATP-aptamer as the linker, we demonstrated the detection of ATP based on the disassembly of the nanostructure, which can be readily captured by darkfield microscopy. The detection limit, dynamic range, and sensitivity can be tuned by controlling the size of the assembly. We found that the aptamer-linked nanoparticle assemblies were selective to only ATP, and not other adenine-containing compounds. Additionally, sensing of ATP in buffer and in bulk cell lysates was demonstrated. Chapter 3 presents the methodology for detecting ATP directly from lysed cells, down to the single-cell level without the need for purification or extraction. The intracellular ATP levels of two ovarian cancer cell lines were quantified to elucidate the differences and cellular distribution, and the potential of the stick-and-peel platform for mapping the microenvironment of 2D heterogeneous surfaces was demonstrated. In chapter 4, the optical properties of nanoparticle assemblies were tuned by changing the morphology of the nano building block, where the incorporation of gold nanoshells as satellites led to an extended redshift of LSPR to a much longer wavelength compared with using solid gold nanoparticles as satellites. This tunability in the LSPR of the assemblies allows for color-based analysis and color-coding of the plasmonic sensors. Lastly, Chapter 5 outlines the development of a multiplexed assay using the nanoparticle assemblies. Two types of assemblies, targeting either ATP or a nucleic acid (DNA-210), were fabricated with different DNA linkers in the same sensing area. The multiplexing was demonstrated by the selective disassembly process. Moreover, the ability to tune the optical properties of nanostructures using different morphologies was integrated; two different morphology of nanostructures, i.e. solid-solid and solid-shell nanostructures, for two targets, ATP and DNA-210, respectively, were fabricated. Based on a difference in scattered color, two types of biosensors among thousands of nanoparticle assemblies can be easily identified. Finally, we demonstrated duplex detection based on the change in the scattering intensity and the color read-out. Reflecting on the contributions of our work, this dissertation advances the fundamental knowledge and practical design of chip-based sensing platforms comprising complex plasmonic nanostructures. The work contributes to the sensing field by addressing some of the challenges in point-of-care or point-of-need measurement applications and provides an alternative bioanalytical tool for single-cell based analysis.