Investigation on the Structural and Thermal Behaviors of Poly(amidoamine) Dendrimer-Encapsulated Au Nanoparticles of Different Sizes

The dynamical and thermal behaviors of Au nanoparticle (AuNP) and fourth generation poly(amidoamine) dendrimer (G4 PAMAM) of the dendrimer-encapsulated Au nanoparticle system under the dry environment were investigated by the molecular dynamics (MD) simulation. The consistent valence force field (CVFF) was used to describe the interaction of the G4 PAMAM dendrimer and the interaction between the dendrimer and the AuNP. The many-body tight-binding potential was adopted to describe the atomic interaction between the Au atoms of an AuNP. Three AuNPs, Au₁₁₆, Au₂₀₁, and Au₄₀₅, with the diameters of 1.45, 1.8, and 2.3 nm were concerned, respectively. Au₁₁₆ and Au₂₀₁ are nearly completely covered by G4 PAMAM dendrimer, but the branching chains of the dendrimer are no longer able to wrap around the Au₄₀₅.The conformation of G4 PAMAM dendrimer still possesses its unique spherical structure after encapsulating the AuNP, and the irregularity of the outer surface decreases as the size of the AuNP increases. For the thermal stability, it was found that the PAMAM dendrimer conducts thermal expansion when temperature increases from 298 K to about 600 K. At temperatures higher than 600 K, the dendrimer moves relative to the AuNP. It also can be seen that the melting points of PAMAM dendrimer-encapsulated AuNPs increase significantly when compared to bare AuNPs of the same size. This study would help us to clarify the characteristics and phenomenon of PAMAM-AuNP composites, and these theoretical insights thus provide valuable information for future strategic design of new high-performance catalysts and nanosensors.