Stability and Electrostatic Assembly of Au Nanorods for Use in Biological Assays

The structure, stability, and aggregation potential of short Au nanorods under biological-based solution conditions have been studied. These attributes were studied using UV-vis spectroscopy, transmission electron microscopy, zeta-potential analysis, and dynamic light scattering. The stability and aggregation potential of the materials depended strongly upon both the purity and the solvent used to prepare Au nanorod solutions. When the Au nanorods were dissolved in Tris buffer at concentrations less than 10.0 mM, no aggregation was observed; however, when the solvent was comprised of Tris buffer with concentrations between 10.0 and 100 mM, significant aggregation of the materials occurred. This effect resulted in a dramatic broadening and shift in the absorbance maxima of the longitudinal surface plasmon resonance. At Tris buffer concentrations of greater than 100 mM, minimal to no aggregation of the materials in solution was observed. Such an ability is based upon electrostatic aggregation of the materials in solution mediated by the anions associated with the buffer system; at concentrations between 10.0 and 100 mM, the anions present electrostatically bind to the surfaces of the Au nanorods that are positively charged, resulting in cross-linking of the materials. At higher buffer concentrations, a sufficient number of anions are present in solution to template around the entire surface of each individual nanorod, in effect neutralizing the charge and producing an electronic double layer, which prevents aggregation. Such studies are timely as they represent an analysis of the stability and range of use of Au nanorods for biological-based applications where remarkable potential exists.