Solvent-Dependent Stabilization of Gold Nanoparticles: A Comparative Study on Polymers and the Influence of Their Molecular Weight in Water and Ethanol

Gold nanoparticles (AuNPs) are attracting more and more attention in life sciences, especially due to their versatile physicochemical properties. They are biocompatible, air and water stable and easy to synthesize. Yet, the colloidal stability of AuNPs not only in water but also in organic solvents remains a decisive factor for their practical usage. This study therefore investigated the influence of different polymer coating methods and polymer types using the postsynthetic addition reaction (PAR) and the one-pot synthesis with the polymers poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG), poly(vinylpyrrolidone) (PVP) and poly(acrylic acid) (PAA) each with different molar weight averages. Analysis of the AuNP@Polymer conjugates by transmission electron microscopy (TEM) finds essentially unchanged gold nanoparticle core sizes of 11-18 or 11-19 nm in water and ethanol, respectively. The hydrodynamic diameter from dynamic light scattering (DLS), which also includes the polymer shell, lies largely in the range from 20-70 nm and ultraviolet-visible spectroscopy (UV-Vis) showed gold plasmon resonance band maxima between 517-531 nm over both synthesis methods and solvents for most samples. The polymer PVA showed the best colloidal stability in both synthesis methods, both in water and after transfer to ethanol, and thus provides the best protection for the AuNPs against aggregation. An increased instability in ethanol could only be noted for the PEG coated samples apparently due to the less-coordinating ether groups of the polymer. For the polymers PVP and PAA the stability depended more specifically on the combination of synthesis method, polymer molecular weight and solvent.