Aggregation Behavior of Oppositely Charged Gold Nanorods in Aqueous Solution

The aggregation behavior of oppositely charged gold nanorods (GNRs) in aqueous solution has been investigated by zeta potential, UV-vis-NIR, DLS, and TEM measurements. The positively charged GNRs (p-GNRs) and negatively charged GNRs (n-GNRs) were prepared by layer-by-layer deposition of oppositely charged electrolytes on GNR surface. As p-GNRs are added into n-GNR solution (before reaching the isoelectric point), p-GNR/n-GNR aggregations of fairly constant size are formed with the minor component (p-GNRs) mostly at the center and the majority component (n-GNRs) at the outside. While the overall packing of the GNR aggregates is rather random, the local arrangements of GNRs show both side-by-side and end-to-end arrangements, resulting in elongated aggregates due to the anisotropic nature of GNRs. While no precipitation occurs before isoelectric point, the size of aggregates grows rapidly as the isotropic point is approached and rapid precipitation occurs at the isoelectric point, showing ionic-like behavior. Beyond the isoelectric point, partial dissolution of aggregates occurs.