Morphosynthesis of Gold Nanoplates in Polypeptide Multilayer Films

Abstract

Multilayer thin films formed from the layer-by-layer (LbL) assembly of cationic linear polyethyleneimide (LPEI) and anionic polyaspartic acid (pLAA) were used as reactive templates for in situ morphosynthesis of gold nanoplates. Size- and shape-controlled growth was made possible by the specific morphogenic action of the polypeptide pLAA and growth in a spatially confined environment. LPEI, in addition to serving as the counter polyelectrolyte for pLAA in the LbL assembly, also provided the facility for chloroaurate anion (AuCl4−) binding through its protonated amine groups (NH3+). The availability and accessibility of these binding sites were important to particle size control and could be varied by the pH of the pLAA solution used in the LbL assembly. Size-controlled synthesis of gold nanoplates was accomplished by controlling the secondary structure of pLAA in the multilayer film. In particular, an increase in the fraction of pLAA α-helices was found to increase the availability and accessibility of the reducing carboxyl side groups of pLAA, resulting in more facile reduction kinetics and smaller particle size in the product. The polypeptide-based reactive templates offered a finer level of kinetic control, enabling the formation of gold nanoplates in a size range (i.e., 50 nm) not possible in a solution phase synthesis (i.e., 500 nm), using reaction mixtures with the same overall composition (i.e., pLAA, LPEI, and HAuCl4) and under the same environmental conditions. The key parameters affecting the polypeptide film formation and their effects on the size-controlled synthesis of gold nanoplates were identified and discussed in detail in this report.