Synthesis and Enhanced Colloidal Stability of Cationic Gold Nanoparticles using Polyethyleneimine and Carbon Dioxide

Abstract

Employing green methods in the design and synthesis of functionalized metallic nanoparticles poses significant challenges in terms of maintaining product integrity (size, shape, dispersity, and colloidal stability). In this study, the direct synthesis of cationic gold nanoparticles (GNPs) capped by low molecular weight (Mw ∼ 600) polyethylenimine was investigated. Specifically, three separate HAuCl4 reduction pathways were used to produce robust GNPs with sizes ranging from 4 to 20 nm in diameter with excellent size control. The inclusion of carbon dioxide as a nontoxic, nonflammable, and inexpensive component led to decreases in particle size and an increase in the colloidal stability of the GNPs. Furthermore, the thermally reversible reaction of CO2 with amines provides means to control the solvent pH through carbamate structures and, hence, the controllable formation of particle aggregates. The effects of pH, PEI concentration, and reduction method on the particle core size and stability were determined via transmission electron microscopy, UV–vis absorption spectra, and dynamic light scattering.