Size-dependent wettability of carboxyl alkyl chain-modified gold nanoparticles

Abstract

The wettability of organic ligand-capped metal nanoparticles plays a crucial role in determining their behavior in diverse applications, including protein adsorption, protein corona formation, cellular uptake, toxicity, immune system recognition, drug release kinetics, and bioavailability. However, a comprehensive understanding of the size-dependent effects in metal nanoparticles with specific ligand modifications remains elusive. Here, we leverage molecular dynamics simulations to delineate the size-dependent wettability of Au nanocrystals modified with carboxyl-terminated alkyl chains at the same grafting density. Our results reveal a negative correlation between the water contact angle of gold nanoparticles and the radius of the Au-core for a given ligand length. This trend is determined by the spatial arrangement of carboxyl groups, where an increase in the size of Au-core leads to a reduction in the intermolecular spacing between carboxyl groups, promoting the formation of hydrogen bonds between the carboxyl groups and water molecules, thereby conferring a more hydrophilic character to the nanoparticles. Conversely, for a fixed Au-core radius, the water contact angle of nanoparticles increases as the length of alkyl chains increases. This distinct relationship arises from the extended separation between carboxyl groups in longer alkyl ligands, which reduces the hydrogen bond formation with water molecules and renders the nanoparticle more hydrophobic. These findings lay the groundwork for how nanoparticle wettability can be precisely modulated, a critical factor for optimizing their performance in various biomedical and industrial applications.