|Abstract:||Gold nanorods (GNRs) show much promise for applications in biological, optoelectronic and energy applications. The resonant generation of a localized surface plasmon resonance (LSPR) at the GNR surface results in interesting optical properties and unique interactions with molecules. Combined with their biocompatibility, ease of synthesis and facile surface functionalization, these anisotropic metal particles are excellent scaffolds for the study of the interactions between nanoscale surfaces and their chemical/biological environments. Regardless of the application, however, GNR utility will not be fully realized until the chemical nature of the surface is understood and controlled.
GNRs can enhance various photophysical properties of molecules. In the case of two-photon absorption (TPA), cross-section enhancements have been shown to increase with strong distance-dependence. Here, a dual approach for the conjugation of a TPA chromophore to GNRs is presented, relying on layer-by- layer (LbL) polymer wrapping and direct thiol coating of the same parent chromophore structure. Together, these approaches allow for estimated chromophore-particle distances from <1nm to more than 15 nm. Composites were confirmed using conventional nanoparticle characterization methods. Imaging of GNR polymer shells indicated anisotropic composite structures, as confirmed by both conventional and cryo-TEM. Optical characterizations were performed using two-photon excited fluorescence and Z-scan techniques, to probe the TPA enhancement. The intrinsic nonlinear optical properties of GNRs is shown to contribute strongly to these measurements, suggesting the utility of these materials for bi-modal imaging platforms.
GNR properties, like their shape, are anisotropic. The LSPR-induced near- fields are heterogeneously distributed on the nanorod surface, with the tips being much “hotter” than the sides. To understand and utilize fully the spatially- dependent interactions of GNRs with their environment, the site-specific attachment of molecules is necessary. Few methods exist, however, to guide molecular localization. Here, the role of by-products in the synthesis of site- selective silica-coated GNRs is demonstrated, and the thickness tunability of the resulting core-shell materials is investigated. The redox state of methoxy- terminated poly(ethylene glycol) thiol attached to GNRs is shown to be relevant in guiding the deposition of silica, providing an important insight into the design of anisotropic composite nanomaterials.
Surface-initiated Atom transfer radical polymerization (SI-ATRP) is a popular method for grafting polymers from a surface. We demonstrate our ability to grow poly(N-isopropylacrylamide) (PNIPAM) shells on the GNR surface, toward a “smart” thermoresponsive polymer shell. The role of ligand choice, molar ratio of monomer to initiator and polymerization on presence and control of shell thickness are investigated. The introduction of a tetradentate (vs. the commonly-used tridentate) ATRP ligand was necessary for the growth of PNIPAM shells in our studies, and consideration of the molar ratio of monomer and initiator and reaction time allowed control of shell thickness and extent of aggregation.