Bio-Inspired Assembly of Au Nanorods

Potential uses of Au nanorods in biological applications require control and stabilization of the composition and structure in desired media. Au nanorods exhibit unique optical properties that depend on both their shape and size. The aspect ratio of these nanorods can easily be determined by the position of the transverse and longitudinal surface plasmon bands in the visible region of the electromagnetic spectrum: the latter undergoes a proportional red shift with increasing aspect ratio. For studies in biological applications, Au nanorods are typically purified and placed in high ionic strength buffered solutions. After the synthesis, performed using the seed-mediated growth method, Au nanorods are purified by centrifugation to remove excess surfactant; however, this can lead to particle instability. Analysis of this purification process demonstrates that extensive washing can lead to distortions in the nanorod structure, eventually resulting in complete particle aggregation. Once effectively purified, the materials must be dispersed in appropriate media, which can cause electrostatic aggregation. A carefully designed analysis of the effects of Tris buffer concentrations below 400 mM indicates that electrostatic aggregation occurs with anions of the solvent between 10 and 100 mM only. This electrostatic interaction causes direct aggregation in intermediate buffer solutions characterized by UV-vis spectroscopy, TEM, ζ-potential, and DLS analyses. At higher concentrations, only independent species are observed. With an understanding of the appropriate biological conditions, systematic studies to determine the interactions of Au nanorods with the simple amino acid cysteine were conducted to determine the amino acid's mode of materials assembly. Understanding the mode of assembly is important for developing controlled structure and stability of Au nanorods in pH and ion strength dependent media.