Self assembly of gold nanorods by electrostatic interaction

Abstract

Gold nanorods (GNRs) with aspect ratio ca. 4 for which the surface plasmon absorption maxima is ca. 800 nm had been synthesized using a seed-mediated growth method [14]. Their surface modification had been achieved by a versatile layer-by-layer (LbL) method using charged polyelectrolytes, poly(sodium-4 styrenesulfonate) (PSS, Mw 70 000 g/mol) and poly(diallyldimethylammonium chloride) (PDDA, Mw 40,000 g/mol). This method resulted in isolated negatively and positively charged gold nanorods and had been characterized by UV-Vis absorption, zeta potential, and transmission electron microscopy (TEM) measurements. Self assembly of these oppositely charged GNRs by electrostatic interaction had been investigated by titrating of negatively charged GNRs (PSS-coated-GNRs) with positively charged GNRs (PDDA-coated-GNRs) and monitoring the UV-Vis absorption spectra, hydrodynamic diameter, and zeta potential for each point of titration. The data indicated the presence of side-by-side and end-to-end self assembly. Electrostatic aggregation of oppositely charged GNRs leads to the formation of core-shell clusters in which the shell is formed by the nanoparticles, which are in excess. Gold nanospheres with diameter ca. 13 nm had been successfully synthesized using the method developed by Hiramatsu. et al. [16]. Functional groups, N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride (TMA) had been introduced to their surface leading to positively charged gold nanospheres. Their self assembly with negatively charged GNRs (PSS-coated-GNRs) had been investigated as well using similar methods of oppositely charge gold nanorods. The data indicated the presence of side-by-side and end-to-end self assembly and the core shell structure can explain this phenomena as well.