Unique Complex Structure and Mechanism of Physisorption of Defect-Free Cyclic Poly(ethylene glycol) onto Gold Nanoparticles

Surface functionalization of gold nanoparticles (AuNPs) to impart colloidal stability is imperative for their applications, especially in biomedical fields. A drastic dispersion stabilizing effect of defect-free cyclic poly(ethylene glycol) (<i>c</i>-PEG) through their physisorption onto the surface of AuNPs was previously discovered, which was even superior to the <i>de facto</i> standard, thiolated PEG (HS-PEG-OMe). Here, we report a comprehensive characterization of the unique structure of the AuNPs/<i>c</i>-PEG complex using nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS). Furthermore, molecular dynamics (MD) simulations were employed to elucidate the distinct mechanism of physisorption. Quantification of the adsorbed <i>c</i>-PEG molecules by NMR revealed that the adsorption density (σ) is approximately 1 order of magnitude smaller than that of HS-PEG-OMe. Model-independent and -dependent analyses of the SANS data were used to further identify the relatively thin and sparse PEG layer as a characteristic of <i>c</i>-PEG adsorption. MD simulations of the physisorption events revealed the absolute value of free energy of adsorption for <i>c</i>-PEG to be greater than that for linear HO-PEG-OH. The favorable adsorption of <i>c</i>-PEG is suggested to arise from its less hydrated state and the larger number of water molecules that are released from the gold surface upon PEG adsorption.