Varying the Interparticle Distances of Gold Nanospheres and Nanorods via Polymer Ligand Chain Lengths

The surface modification and functionalization of nanoparticles with polymer ligands is the focus of current research in order to improve nanoparticle stability and to control nanoparticle assembly. The surface attachment of end-functionalized polymers is a versatile route to structurally well-defined stabilizing polymer shells. Here, we investigate for spherical and cylindrical gold nanoparticles the ligand exchange method to attach end-functionalized polystyrene chains with narrow molecular-weight distribution in order to control the interparticle distance, i.e., nanoparticle surface-to-surface distances, and to stabilize nonspherical particle shapes. For the ligand exchange, we use gold nanoparticles capped with citrate and cetyltrimethylammonium bromide as well as polystyrenes with thiol and multidentate amino end groups. We demonstrate that the interparticle distances for both spherical and cylindrical nanoparticles can be systematically varied via the molecular weight of the polymer ligands. We observe that in both cases, a plateau value of the interparticle distances is reached for large molecular weights. We find that for cylindrical nanoparticles, the polymers preferentially bind to the axial circumference, and to a lesser degree to the cylinder tips, in line with the observed shifts of the longitudinal and transverse plasmon resonances. We show that polymer ligands can be used for the synthesis of spherical seed particles without employing low-molecular-weight capping agents. The results provide further insights into the functionalization of spherical and cylindrical nanoparticles with polymer ligands to provide stability and distance control to tailor nanoparticle assembly.