Linear assembly of gold nanoparticle clusters via centrifugation.

Centrifugation is widely used in the synthesis and handling of solution-phase nanoparticles to improve their purity and to change the composition of the solvent. Herein, we couple the optical properties of citrate-stabilized gold nanoparticles and silica encapsulation to investigate how centrifugation impacts the formation of stabilized, nanoparticle clusters in solution, without the use of linker molecules or asymmetric functionalization. Gold nanoparticles preconcentrated using a high (9,400) g force result in linear assemblies of gold cores that are spaced by ∼l-4 nm within Au n@SiO2 structures (n = number of gold, nanoparticle cores per silica shell) with ∼30% monomers, 30% dimers, 20% trimers, and 10% 4-7mers. In comparison, nanoparticles preconcentrated using (stirred) ultrafiltration and low (23) g force centrifugation have statistically identical cluster distributions (90% monomers, 9% dimers, and 1% trimers) whereas nanoparticles that are not preconcentrated always exhibit 100% monomers using the same silica coating procedure. We hypothesize that under high g force, the electrical double layer surrounding the gold nanoparticles is slightly polarized, thereby increasing the attraction between, nanoparticles and the formation of stable clusters. The conductivity of the solution plays an important role in this stabilization. This novel demonstration of linear cluster formation of gold, nanoparticles using centrifugation suggests that this commonly used preparative tool can both positively or negatively impact the fundamental properties of these materials and their use in various applications. © 2009 American Chemical Society.