In Situ Monitoring of the 2D Aggregation Process of Thiol-Coated Gold Nanoparticles Using Interparticle Plasmon Coupling
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Near-field plasmon coupling between neighboring gold nanoparticles, measured by polarized optical waveguide lightmode spectroscopy, is employed to study the surface self-assembly of alcanethiol-capped gold nanoparticles during solvent evaporation. The waveguide used is a monomode optical fiber half-coupler. The sample is deposited on the surface of the waveguide and absorption spectra are continuously collected during the solvent evaporation process with a temporal resolution of 0.2 s. The absorption spectra show a progressive red shift of the plasmon peak caused by increasing interparticle near-field coupling. This shift can be used to determine the distance between particles by comparison to theoretical values calculated using the discrete dipoles approximation. The technique is demonstrated for the assembly of 10 nm gold particles capped with thiol ligands of two different lengths. Interestingly, in the case of dodecanethiol-capped particles, the extinction spectrum not only shifts to longer wavelengths, but also changes in shape during the drying process. About half a second before the solvent completely evaporates, the spectrum broadens as a second component appears. This feature is tentatively attributed to the formation of a significant population of particle clusters as a result of incomplete screening of van der Waals attractions by the shorter ligand.
KeywordsSelf-assembly Gold nanoparticles Plasmon coupling Optical waveguide spectroscopy
The authors acknowledge the Centre Québécois sur les Matériaux Fonctionnels (CQMF) and NanoQuébec, le Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) and the National Sciences and Engineering Research Council of Canada (NSERC) for financial support.
TE-polarized OWLS extinction spectra recorded as a function of time during solvent evaporation for citrate-reduced gold NPs capped with dodecanethiol (C12S–). Selected spectra are depicted in Fig. 7. The x-axis corresponds to a wavelength range of 400 to 950 nm. The y-axis corresponds to an absorbance scale of 0 to 0.7 (MPG 2,212 kb)
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