For nearly two decades, the unique size- and shape-dependent optical properties of gold nanorods (AuNRs) have inspired a great deal of scientific inquiry. Due to a phenomenon known as localized surface plasmon resonance (LSPR), AuNRs have been captivating researchers with their beautiful colors while their use in a broad range of applications—including sensing, imaging, cancer therapy, catalysis, and electronics, among other—have been investigated. Part of the interest in AuNRs is due to their versatility. Because the optical properties of AuNRs are shape dependent, AuNRs can be tailored so that their LSPRs meet the requirements of a broad variety of applications. Just as important have been advances for the surface modification of AuNRs. Future applications of AuNRs will depend on the ability to combine them with novel and functional materials and to control not just what kinds of chemical modifications are done to surfaces but also to be able to control where they occur. In Chapter 1, AuNRs and their properties are introduced. After a brief history on the seed mediated synthesis that is used to produce AuNRs, some of the most important methods for modifying their surfaces are described. These include the synthesis of silica shells on AuNRs, the exchange of ligands on their surface using thiols, and the use of positively and negatively charged polyelectrolytes to build up polymer coatings using layer-by-layer techniques. Finally, progress towards anisotropic surface modification is discussed.Chapter 2 focuses on nanorod matryoshkas—nanoparticles named for the Russian nested dolls—that consist of a AuNR core with a silica layer in the middle and an outer gold shell. Discrete dipole approximation (DDA) calculations on nanorod matryoshkas reveal that the magnitude of plasmonic enhancement near the gold nanorod surface and the location of enhancement (that is, at the sides or at the ends of the AuNRs) depend on the geometry of the nanorod matryoshka. Efforts toward the synthesis of gold nanorod matryoshkas are also described.One of the major goals in AuNR surface modification has been the achievement of anisotropic surface functionalization. Some of the most enticing results in the field involve blocking the ends of AuNRs with poly(ethylene glycol) methyl ether thiol (PEG-SH) so that the sides can be selectively coated in silica, but those reports have suffered from a lack of reproducibility. In Chapter 3, it is demonstrated that only PEG-disulfide—not PEG-thiol—can be used to block the ends. It is also shown that silica can be made to grow on only the ends of AuNRs using similar conditions to those previously reported for isotropic silica shells but by reducing the amount of silica precursor added. These results indicate that silica shell growth begins at the ends of AuNRs.Layer-by-layer techniques for coating AuNRs are extended to allow for the layer-by-layer synthesis of conformal metal-organic framework (MOF) shells in Chapter 4. MOFs are composed of metal-containing hubs connected by organic linkers and they are unique because of their highly porous structures. Hybrid materials of MOFs with plasmonic materials like AuNRs may be useful for applications such as sensing or catalysis. Shells of the MOF HKUST-1 weredeposited on AuNRs by alternating additions of copper (II) acetate and tetramethylammonium (1,3,5)-benzenetricarboxylate. It is shown that the initial surface chemistry of the AuNRs is crucial for the synthesis of conformal MOF shells. It is also demonstrated that using the tetramethylammonium salt of the organic linker can improve the colloidal stability of MOFs during layer-by-layer synthesis, minimizing aggregation of MOF coated AuNRs.In the final chapter, spray coating of ultrasonically nebulized dispersions of nanoparticles is introduced as an alternative method for transmission electron microscopy (TEM) sample preparation. TEM and related techniques are increasingly important tools for studying the structure, morphology, and chemistry of nanoparticles. Traditional methods of TEM sample preparation like drop-casting often introduce drying artifacts that can complicate the interpretation of observations made using TEM. For fragile support materials like graphene—which is very useful for quantitatively studying low atomic number materials like the ligands on nanoparticles—that are likely to rupture when samples are prepared with drop-casting, spray coating is a much gentler alternative. Additionally, because ultrasonic nebulization results in droplets of uniform size, the number of nanoparticles deposited together in clusters depends on their concentration, which may prove useful for estimating nanoparticle concentration using TEM.
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