Controlling the morphology of poly(ethyleneimine)/gold nanoassemblies through the variation of pH and electrolyte additives
Krisztina Bali, Mónika Bak, Katarína Szarka, György Juhász, György Sáfrán, B. Pécz, Judith Mihály, Róbert Mészáros
Abstract
Recent investigations have revealed very promising analytical and medical applications of poly(ethyleneimine) (PEI) capped gold nanoparticles (Au NPs). One simple way for their synthesis utilizes the dual nature of PEI molecules, which can simultaneously act as reducing and stabilizing agents via their amine groups. However, the formation mechanism of these kinds of NPs as well as the dependence of their morphology and charge on the pH and electrolyte additives has not been explored yet. In the present paper, the role of these factors on the PEI assisted one-pot synthesis of gold nanoassemblies was studied systematically using IR and UV–Vis spectroscopy as well as DLS, electrophoretic mobility and TEM techniques. It was shown that these nanomaterials cannot be considered as simple PEI coated Au NPs. Instead, the gold particles are embedded in a rather specific polymer matrix, the structure of which ranges from a slightly cross-linked, positively charged thin polymer layer at low pH, to an extended, negatively charged and coherent amorphous polymer film in alkaline medium, provided that adequate type and concentration of supporting electrolyte is also present. These observations were rationalized through the unique pH and salt dependent mechanism of the initial gold(III)/amine complexation and that of the subsequent oxidation and cross-linking processes of the PEI molecules. This crucially determines the following nucleation and growth as well as the morphology, charge and size of the polymer entrapped gold NPs. The presented results may have important implications in novel applications of gold nanoassemblies.