Tunable Assembly of Gold Nanorods in Polymer Solutions to Generate Controlled Nanostructured Materials

TL;DR

This study demonstrates how polymer-induced depletion interactions can be used to assemble gold nanorods into tunable, nanostructured materials with potential applications in additive manufacturing and in situ material design.

Contribution

It introduces a method to control nanorod assembly using polymer concentration and size, revealing non-monotonic structural dependencies and a crossover in aggregation regimes.

Findings

Depletion interactions depend on correlation length, not polymer molecular weight.

Structural parameters peak at intermediate polymer concentrations.

Assembly control enables design of materials with specific nanoscale properties.

Abstract

Gold nanorods grafted with short chain polymers are assembled into controlled open structures using polymer-induced depletion interactions and structurally characterized using small angle x-ray scattering. When the nanorod diameter is smaller than the radius of gyration of the depletant polymer, the depletion interaction depends solely on the correlation length of the polymer solution and not directly on the polymer molecular weight. As the polymer concentration increases, the stronger depletion interactions increasingly compress the grafted chains and push the gold nanorods closer together. By contrast, other structural characteristics such as the number of nearest neighbors and fractal dimension exhibit a non-monotonic dependence on polymer concentration. These parameters are maximal at intermediate concentrations, which are attributed to a crossover from reaction-limited to diffusion-limited aggregation. The control over structural properties of anisotropic nanoscale building blocks demonstrated here will be beneficial to designing and producing materials \emph{in situ} with specific direction-dependent nanoscale properties and provides a crucial route for advances in additive manufacturing.