The effect of mixing bulky oils with alkanethiol-capped gold nanoparticles on nanoparticle self-assembly

Abstract

The mechanism by which alkanethiol-capped gold nanoparticles (AuNPs) interact with their surrounding environment and self-assemble is a subject of great interest because of the potential applications of AuNP films in electronics, optics, and as chemical sensors. Previous work on AuNP films at the air-water interface has shown that the addition of linear hydrocarbon oils to typically rigid and irreversible pure AuNP films has a modest effect in increasing NP-NP spacing to form more fluid and reversible films. These studies also suggested that the oils were able to effect these changes by interdigitating with the AuNP ligand shells. The present study aimed to improve upon these observed properties through the introduction of longer and bulkier oils to prevent AuNP aggregation using steric repulsion. AuNP-oil film compression isotherms and analysis by transmission electron microscopy, atomic force microscopy, and ultraviolet-visible (UV-vis) spectroscopy revealed that the addition of 1-phenyltridecane and hexadecane increased film fluidity and reduced AuNP aggregation during compression. Additionally, across all AuNP-oil combinations, no compression isotherm characteristic (i.e. earlier onset rise pressure or low hysteresis) was found to be a definite indication of AuNP-oil mixing and of being a good candidate for additional in situ analysis. The effect of increasing oil steric bulk was convoluted by concomitant changes in the stability and wettability of the oils, severely limiting the possible design parameters for oil structure. Finally, UV-vis spectroscopy measurements revealed large shifts in the λmax, which are promising indicators of AuNP-oil mixing. Therefore, in situ UV-vis spectroscopy is likely an ideal technique for assessing the mechanism by which oils mix with AuNP ligands. The shifts observed in the λmax themselves may be explained by the oil interdigitating with the AuNP ligand shell as well as changes in AuNP ligand tilt.