Effect of Surface Chemistry and Metallic Layer Thickness on the Clustering of Metallodielectric Janus Spheres

The noncovalent binding of the gold hemispheres of polystyrene/gold colloidal Janus spheres in aqueous solution was found to depend more significantly on the deposition thickness of the particle's gold layer than the chemistry of a covalently affixed self-assembled monolayer on the gold. By means of two-channel confocal laser scanning microscopy, salt-induced clustering was observed and quantified for Janus particles with gold hemispheres functionalized with a thiol self-assembled monolayer that varied in hydrophobicity and chain length. The thickness of the gold layer on the Janus particles was also varied from 10 to 40 nm. The measured cluster distributions were strongly salt dependent, with clustering absent at 1 mM salt but present at salt concentrations in the range of 2-3 mM. For Janus spheres with a 40 nm thick gold hemisphere, the effects of both thiol monolayer hydrophobicity and chain length were modest. Varying the gold layer thickness from 10 to 40 nm, however, had a significant effect on the cluster distribution; the most abundant cluster size shifted from one to seven particles as the gold layer thickness increased from 10 to 40 nm. Thus, the gold layer thickness had an effect stronger than that of either self-assembled monolayer hydrophobicity or chain length on the self-assembly of metallodielectric Janus particles into clusters. The dominant effect of the metallic layer thickness suggests that van der Waals forces between metallic surfaces are more important than hydrophobic interactions in determining the pair potential interactions of metallodielectric Janus particles.