Optical Characterization of Bio-Inspired Functional Materials, Metal Nanoparticles, and Their Composites

Abstract

The ability to characterize optically-active materials by their transmutable optical signatures can both yield noncontact information on the nanoscale behavior of the material and be used to predict its applicable functionality. The work presented herein covers the optical characterization of a variety of metal nanoparticles, stimulus-responsive bio-inspired polymers, and biotemplated metal nanoparticles. We first show the optical modeling of a variety of plasmonic nanoparticles by the finite-difference time-domain method, supporting experimental observation made by both traditional and novel spectroscopic methods. Using finite-difference time-domain modeling, we identify a large magnitude of interparticle coupling for reversible, reconfigurable nanoparticle composites with the thermoresponsive polymer microgel, poly(N-isopropylacrylamide) and compare the simulated results to experimental UV-Vis extinction spectra. We then confirm the experimental results of a new photoacoustic absorption spectroscopy technique by simulating the scattering, absorption, and extinction coefficients of different sized gold nanorods and compare with the experimental findings. Lastly, finite-difference time-domain modeling allows us to deconvolute complex scattering spectra and draw conclusions about the coupling of plasmonic-excitonic system which could not be otherwise inferred experimentally. Next, we characterize a self-folding polymer with pendant host-guest cyclodextrin-azobenzene derivatives by demonstrating reversibility of the azobenzene derivative isomerization and photoisomerization quantum yield differences providing evidence of self-folding via UV-Vis extinction spectroscopy. Finally, we employ de novo designed protein nanofibers to electrostatically template gold nanoparticles at the surface of a charged substrate and explore how substrate-nanoparticle repulsion via nanoparticle size and the pH of solution dictate the final assembly outcome using the well-studied the Derjaguin-Landau-Verwey-Overbeek theory to justify our results. We then employ hyperspectral darkfield microscopy to compare optical scattering observations for larger nanoparticles with the assembly conclusions made with small gold nanoparticles and motivate the use of optical scattering microscopy for future in situ characterization and observation of protein-templated nanoparticle assemblies.