Structural and equilibrium effects of the surface passivant on the stability of Au nanorods

Au nanomaterials are well-known for their optical properties, where Au nanorods have demonstrated unique capabilities because of their readily tunable size and shape. Unfortunately, functionalization of the material surface is challenging because of their lack of stability after only a few purification cycles. Here, we demonstrate that enhanced Au-nanorod stability can be achieved by purifying the materials using dilute cetyltrimethylammonium bromide (CTAB) wash solutions. To this end, purifying the materials in such a manner shifts the passivant on/off equilibrium to maintain surfactant adsorption to the metal surface, leading to enhanced stability. Interestingly, from this study, a bimodal distribution of Au nanorods was evident, where one species was prone to bulk aggregation, whereas the second population remained stable in solution. This likely arose from defects within the CTAB bilayer at the nanorod surface, resulting in selective material aggregation. For this, those structures with high numbers of defects aggregated, whereas nanorods with a more pristine bilayer remained stable. Coating of the Au nanorods using polyelectrolytes was also explored for enhanced stability, where the composition of the anionic polymer played an important role in controlling materials stability. Taken together, these results demonstrate that the stability of Au nanorods can be directly tuned by the solvent-exposed surface structure, which could be manipulated to allow for the extensive material functionalization that is required for the generation of nanoplatforms with multiple applications.