Controlling plasmonic 'hot spots' in nanoparticle assemblies using ligand place-exchange reactions

This thesis describes the 1) selective attachment of Au nanoparticles (NPs) to the edges of Au nanoplates via ligand place-exchange reactions, 2) preparation of coupled Au nanoplates via ligand place-exchange reactions, 3) attachment of Au NPs to Au nanorods via ligand place-exchange reactions, 4) study of the dynamics of ligand place-exchange reactions on Au nanoplates, 5) study of the changes in the optical properties of Au nanoplates upon attachment of Au NPs by observing the shift in the localized surface plasmon resonance (LSPR) band, and 6) study of the surface-enhanced Raman spectroscopy (SERS) enhancement of analyte molecules located in the "hot-spots" of coupled Au nanoplate-Au NP structures.;The optical properties of the Au nanoplate-Au NP assemblies were studied by monitoring the shift in the LSPR band of the Au nanoplates upon attachment of Au NPs and by measuring the SERS enhancement of the 4-ATP linker. The LSPR band red shifted about 1 nm upon attachment of each Au NP at the vertex and edge sites. The estimated shift is about 0.1 to 0.6 nm per Au NP for those attached to the terraces sites, where the shift is larger for Au NPs attached to outer terrace sites close to the edge and smaller for the inner terrace sites. Au nanoplate-Au NP assemblies with 100% of the NPs attached on the vertex sites of the nanoplates showed the largest enhancement relative to the Au nanoplates. The intensity was similar to the assemblies with many more NPs, but relatively fewer at the vertex/edge sites, indicating that the "hottest spot" for enhancement is located in the interparticle junction between the vertex of the nanoplate and the attached NP. The relative enhancement of the Au nanoplate-Au NP couple relative to the Au nanoplate alone was about 11 on a per Au NP basis. Fundamentally, the results suggest that a small number of "hottest spots" in the laser beam area dominate the Raman enhancement signal and practically, the nanoplate-NP coupling approach controllably at the vertex/edge sites can be very useful for amplifying the detection of analyte down to the single molecule level.;This research provides a simple approach for the preparation of various nanostructure assemblies with high accuracy and reproducibility. At the same time, it can potentially be used to control the location of analyte for LSPR and SERS sensing. Another important contribution of this study is the direct evidence for the mechanism and kinetics of the ligand place-exchange reaction on the nanoplate surfaces, which can be useful for controlling NP attachment to other structures as well.;We regio-selectively attached 20 nm Au NPs to Au nanoplates by first assembling hexanethiol (HT) onto Au nanoplates, then exchanging HT with 4-aminothiophenol (4-ATP) using different reaction times, and finally electrostatically attaching the negatively-charged Au NPs to the positively-charged 4-ATP ligands bound to the Au nanoplates. We prepared nanoplate-NP assemblies with 100% of the NPs attached to vertex sites or 100% attached to vertex and edge sites using 1 h and 2 h exchange times, respectively. The location and number of bound Au NPs to the nanoplates as a function of exchange time provided information about the mechanism of the ligand place-exchange reaction on the Au nanoplate surface. Ligand place-exchange starts from the vertex sites, then proceeds at the edge sites and finally occurs at smooth terrace sites for longer times. Direct exchange at terraces is possible but the data suggests it occurs mostly through lateral migration of the 4-ATP ligands from vertex/edge sites to the terrace. The data also suggests that phase segregation of 4-ATP ligands and HT ligands occurs on the terrace sites for longer times. The ligand place-exchange strategy also leads to interesting coupled Au nanoplate-Au nanoplate and Au nanorod-Au NP assemblies.