FIGURE 1 MD simulations of Au-TMA nanoparticles and four molecular anions (P1-, P1-2, P3-3 and P2-4). Demonstrating the all-atomic (AA) model of the Au-TMA nanoparticle with 384 TMA ligands (left panel), the mapping of the TMA ligand to four CG beads (middle panel) and a snapshot of the Martini CG model of the Au-TMA nanoparticle (right panel) (A). In the AA model of the Au-TMA nanoparticle, gold atoms are shown as yellow spheres and the TMA ligands are shown as sticks. Carbon, nitrogen and sulfur atoms are shown in cyan, blue and brown, respectively. Hydrogen atoms are omitted for clarity. In the CG model of the Au-TMA nanoparticle, the TMA ligands are represented by one positively charged bead and three neutral beads (also see Figure S4A), and the CG TMA ligands are shown as sticks. The positively charged bead is shown in blue while the other three neutral beads are shown in cyan. Martini mapping of P1-, P1-2, P3-3, P2-4, P4-6and P6-6 anions is also presented (B). Whereas, the CG beads of P1-, P1-2, P3-3, P2-4, P4-6and P6-6 anions are shown as semi-transparent grey, pink, red, purple, magenta and orange spheres, respectively. Final configurations of the P1-, P1-2, P3-3 and P2-4 systems (C). The P1- and the P1-2 anions are shown in grey and pink spheres, respectively. The P3-3 and P2-4 anions are shown in red and purple sticks, respectively. All the anions within 1 nm distance from the Au-TMA nanoparticle surface is shown while sodium ions and water beads are not revealed for clarity. Time evolution of COM distances between Au-TMA nanoparticles for the four systems is also demonstrated (D) whereas the COM distance is measured by the COMs of the two gold cores. The time evolution of negative interface charge for the four systems (E). The interface region defined in the demonstrations is measured as space within 1.5 nm around the two Au-TMA nanoparticles.
Next, we studied the co-assembly of Au-TMA nanoparticles with molecular anions featured with six negative charges (i.e., P4-6and P6-6). The previous analyses of Au-TMA nanoparticle aggregation in the P1-, P1-2, P3-3 and P2-4 systems were also performed in the P4-6 and P6-6 systems. Interestingly, the COM distance of the P4-6 system converged rapidly to ~8.5 nm (Figure 2A) and was close to the results of the P2-4 and P3-3systems (Figure 1D). However, the COM distance of the P6-6 system reached ~9.5 nm at ~350 ns and remained steadfast indicating stable aggregations of Au-TMA nanoparticles. Moreover, the final structures at the end of MD simulations demonstrated a close association between Au-TMA nanoparticles in P4-6 and relatively loose contacts of Au-TMA nanoparticles in the P6-6 systems (inset of Figure 2A). We speculate that this is due to the fact that P4-6 is a linear-shaped anion consisting of four beads while P6-6 is a ring-shaped anion with six beads. Therefore, the charge density of the P4-6 anion is higher than that of the P6-6 anion and contains fewer beads. Despite both the P4-6 and P6-6 anions having the same number of negative charges they still maintain differences in molecular shapes and sizes. This may be the main responsible factor for obtaining the different COM distances between Au-TMA nanoparticles (more discussion below).