Synthesis and Self-Assembly of Gold and Gold-Silver Nanorods
Summary
Gold nanoparticles are widely studied because of their interesting optical properties and chemical stability, with applications such as (photo)catalysis, surface-enhanced Raman spectroscopy, data storage and photothermal (cancer) treatment. In particular, gold nanorods (AuNRs) have been extensively studied due to their excellent plasmonic properties. The anisotropic shape of the nanorods results in both a strong and highly tunable plasmon resonance from 500 to 1200 nm, depending on the aspect ratio of the rods. In addition to this, the plasmonic properties of the AuNRs can be tuned and enhanced by self-assembling the rods into larger structures, whereby plasmonic hot spots are created between the particles. Enhanced plasmonic properties in the visible spectrum can be achieved by introducing a second metal such as silver, which has superior plasmonic properties compared to gold. In this project, we first synthesized Au-Ag nanorods with either a core-shell or a homogeneously alloyed structure. The latter could be achieved by heating the Au-Ag core-shell NRs under an inert or reducing atmosphere, without losing the anisotropy of the NRs. We studied the influence of composition and metal distribution on the optical properties and gained insight into the influence of the gas atmosphere on the alloying. Secondly, the plasmonic properties of AuNRs could be enhanced by self-assembly into novel plasmonic structures. The phase behaviour of silica-coated AuNRs was studied in 2D and in spherical confinement via solvent evaporation. Using this method it was possible to form spherical supra-particles consisting of ordered AuNRs, which may be interesting for applications such as SERS and plasmon-enhanced fluorescence due to plasmonic hot-spots between the tips of aligned AuNRs.