Morphology and Assembly Control of Nanostructures for Plasmonic Optical Amplification

Abstract

Ability to control the morphology and assembly of metal nanostructure is highly desirable as the geometry of particles is directly related with the properties of the nanoparticles. Different shape and assemblies provide wide variety of unique optical properties to nanostructures. The surface plasmon of the metal nanoparticle is the origin of unique optical properties and tailoring the structure allows one to modulate the surface plasmon mode. The surface plasmon which interact with visible range of light provides brilliant color with excellent stability and numerous physical color through morphological change of nanostructures. The confinement of a surface plasmon to a small volume results in localized electromagnetic field concentrating incident radiation to a subwavelength physical region and leads to strongly enhanced surface-enhanced Raman scattering signal. Due to these compelling optical properties and applications, there have been many efforts to control morphology and assembly of nanostructures. However, the number of methodologies and morphologies that can be achieved is quite limited. Through this research, we intended to push the boundary in the field of the nanostructure fabrication. In this thesis, we investigated new methodologies to fabricate plasmonic nanostructures with novel design, created unprecedented plasmonic nanostructures, and evaluated the optical properties of newly produced nanostructures. By changing the ratio of cetyltrimethylammonium bromide (CTAB) and ascorbic acid (AA), we developed various shapes of gold nanoparticles. We chose the well-known ligand, CTAB and the reducing agent, AA and studied the role of them in shape control. The ratio of CTAB and AA is important to determine the relative growth of different crystallographic facet. Based upon the discoveries, we constructed the morphology diagram as a function of CTAB and AA concentration and successfully synthesized the rhombic dodecahedron and hexaoctahedron for the first time in CTAB and AA condition. This work can provide a useful guideline to control the morphology simply by controlling the relative ratio of CTAB and AA. We developed a new system that can direct the growth of nanoparticle by changing the functional group in the organothiol called organothiol assisted growth. Generally the organothiol molecule is used for functionalization of nanoparticle, but in here we revisited the role of organothiol in shape control and developed unprecedented nanoparticle morphologies having compelling properties. The thiol group in organothiol strongly links the molecule to the metal surface and at the same time, functional groups in the molecule make characteristic attachment onto the surface thus change the direction of crystal growth. In this system, we rationally designed organothiol molecules and utilized it to induce crystal morphology appropriate for each function. We identified benzenethiol system where the interaction energy between thiol and gold can be systematically varied and synthesized numerous high index nanoparticles. Especially, a new concave rhombic dodecahedron (RD) shape was fabricated using 4-aminothiophenol which has maximized binding energy due to electron donating amine. Sharp edges and multiple high index facets of concave RD generated strong electromagnetic field and exhibited superior catalytic performance. Furthermore, we discovered peptide organothiol molecule which is capable of developing chiral morphologies. In this organothiol additive growth, continuous attachment of organothiol on gold surfaces during the evolution of particle mediates the crystallographic growth direction. During the growing process, gold atoms and peptides continuously provided onto the cube seed and the peptides make peptide-inorganic stereoselective recognition on gold surface which leads to modified growth direction on the surface changing in shape consequentially. By incorporating peptide-gold enantioselective interaction into particle growth, here, we demonstrated various novel chiral structures controlled at nanometer resolution with strong optical activity and achieved new advances in chiral morphology fabrication. The synthesized nanoparticle showed strong optical activity at visible light and rotated the angle of linear polarized horizontal light at a 28 degree. This organothiol assisted growth system holds great potential in developing the fabrication of nanoparticle. As there are myriad combinations of functional group in organothiol molecule, the system enables one to make numerous shape and establish nanostructure library. In addition, this will provide important insight for the rational design of nanoparticle. Using genetically engineered M13 virus as assembling template, we fabricated closely aligned gold nanocube chain. Designing a surface-enhanced Raman scattering (SERS) nanoprobe with a controlled nanostructure and attachment of antibodies is an important issue in quantitative multiplexed detection. We have developed antibody containing SERS nanoprobe that is based on a genetically engineered, bifunctional M13 virus. At the end of the 1-µm-long filamentous virus, an antibody was expressed with high binding affinity against antigens. Along the length of the virus, gold nanocubes were closely aligned into chains through a gold-binding peptide sequence expressed on major coat proteins. These nanocube chains in a single virus amplified the Raman signals of various reporter molecules and thus served as a specific label to distinguish different antibodies. We successfully applied the virus-based platform to prostate specific antigen (PSA) detection and a quantitative immunoassay. Our results suggest a new possibility for the use of a multifunctional virus scaffold as a general SERS platform that can genetically carry various antibodies and templates for plasmonic nanostructures. We believe that our newly developed methodologies, various fascinating shape and assembly in this research will pave the way to develop novel plasmonic nanostructures and bring about new opportunities for light manipulation.