Reversibly Reconfigurable Plasmonic Nanomaterials
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University of Washington Abstract Reversibly Reconfigurable Plasmonic Nanomaterials Soumyadyuti Samai Chair of the Supervisory Committee: Professor David S. Ginger Department of Chemistry Plasmonic nanoparticles have been extensively investigated in various fields, ranging from biosensing to nanophotonics, due to their characteristic optical features arising from localized surface plasmon resonance. A number of efforts have been made to tailor the optical properties of the nanoparticles by controlling their shape, size and chemical compositions that have advanced their applications in catalysis, molecular diagnostics, therapeutics, and designing electronic devices. Optical signatures of plasmonic nanoparticle assemblies depend on the near field coupling between the plasmon modes of the constituent particles that can be modulated by the distance and orientation between the particles. While the first-generation of plasmonic nanomaterials attempts to control the distance and directionality of the interparticle coupling by employing chemical reagents, recent developments to introduce stimulus-responsivity in the nanomaterials provide us opportunities to control the functional and optical properties of the such materials with external reagents such as light, heat, pH, electric field etc. These emerging plasmonic nanomaterials allow reversible reconfiguration of the structure that can be manipulated remotely, in a reagent-free manner, allowing reusability of the materials in all the applications. Such reconfigurable nanomaterials are obtained by combining the plasmonic nanoparticles with stimulus-sensitive materials. In this dissertation, I explore the use of photo-responsive DNA and thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM) hydrogels, to construct reconfigurable assembly of plasmonic nanoparticles and characterize the reversible change in their optical properties in response to external stimuli. DNA has been a powerful material in nanotechnology for engineering 3D plasmonic structures, plasmon rulers and chiral nanophotonic elements. Not only the length and structural conformations of the DNA allow a precise tuning of interparticle distance and geometry of the nanostructures, but also it matches with the decay length of the near-field plasmon coupling. Recent advents of the azobenzene-phosphoramidite chemistry have facilitated the design of photo-responsive nanomaterials assembly, where the structural reconfiguration and the optical properties are controlled by the reversible trans-to-cis azobenzene photoisomerization. Such nanomaterials have found potential applications in low-cost, remote plasmonic biosensing, and optically active nanodevices. The functionality of such optically reconfigurable nanomaterials is extremely sensitive to efficiency of azobenzene photoisomerization in the DNA sequences. So, in chapter 3, we study the trans-to-cis photoisomerization of azobenzene-modified DNAs by measuring the photoisomerization quantum yields in different DNA sequences at various temperatures. Notably we provide the first report that the quantum yields of photoisomerization of azobenzene incorporated to a DNA phosphate backbone is temperature-sensitive and the temperature-dependent behavior is related to the host DNA sequence and its melting temperature. This result is unique in the sense that this behavior is distinct from the photoisomerization of free azobenzene in solution, which is independent of temperature. We also examined the effect of DNA sequences on the cis-to-trans reverse thermal isomerization of azobenzene. Our results indicate that the reverse thermal isomerization process is not affected by the DNA sequences and follows first order kinetics with an Arrhenius activation energy similar to that of the free azobenzene in solution. These findings provide effective design principles for engineering more efficient photo-reconfigurable plasmonic nanomaterials using azobenzene-modified DNAs. Next, in chapter 4 we demonstrate the assembly of photoswitchable gold nanoparticle dimers using an azobenzene-modified hairpin DNA linker and optically characterize their optical reconfiguration upon reversible photoisomerization of azobenzene. The trans-to-cis azobenzene photoconversion upon UV light exposure leads to the unzipping of the hairpin DNA that increases the separation between the two nanoparticles. Blue light illumination reforms the hairpin structure and restores the closed form of the dimer. The light-induced reconfiguration of the interparticle distances is reflected in the reversible plasmonic shift of the dimer scattering spectra measured by single particle dark field spectroscopy over multiple cycles of UV and blue light exposure. Our results significantly contribute to the fundamental understanding of the dynamical optical and structural properties of the DNA-linked gold nanoparticle dimers and lay the ground work for using them as building blocks in the future plasmonic nanomaterials. Finally, in chapter 5, we assemble and characterize novel reconfigurable hybrid nanocomposite materials that combines the optical properties of plate-like anisotropic silver nanoprisms with thermally responsive PNIPAM microgels. We find that these composites exhibit large thermochromic shifts upon the reversible volume-phase transition of PNIPAM that results in an easily observed color switching of the solution with temperature cycling. We also show that both the nanoprism size and loading density on the microgel can be used to independently tailor the thermo-responsive optical properties of the composites in the visible and NIR region of the spectra. The hybrid microgels exhibit a strong, reversible change and high contrast in NIR scattering intensity, achieved by the thermally reversible modulation of the interparticle distance and near-field plasmonic coupling upon swelling-deswelling of the PNIPAM microgels. These results create opportunities to use the novel plasmonic materials in designing thermochromic sensors, NIR labels for imaging and smart materials for nanophotonics applications.
- Chemistry