Effect of surface coverage and chemistry on self-assembly of monolayer protected gold nanoparticles: a molecular dynamics simulation study

Gold nanoparticles (AuNP) are being used in a variety of applications ranging from drug delivery systems to nano-coolants. The structure and surface properties of AuNP can be manipulated using coatings of thiols, carrying different charges. Different densities of surface coverage can result in the formation of different structures. A molecular basis to quantify the interactions between AuNP covered with different densities (20, 60 and 100%) of surface coverage and various thiol chains (neutral, cationic and anionic) is obtained using potential of mean force (PMF) calculations. Further self-assembly simulations were performed to study the formation of aggregates/dispersed solutions with different densities of surface coverage (20, 40, 60, 80 and 100%). It is found that neutral coatings of all surface coverage densities studied, and charged coatings (anionic and cationic) of less than 60% surface coverage density result in the formation of aggregates. The aggregation and dispersion of AuNPs is explained in terms of an interplay between van der Waals and electrostatic forces. The results obtained are in good agreement with the data available in the literature and suggest that aggregation behaviour can be controlled by modifying the surface coverage and chemistry.