FIGURE 5 The anions distribution on the Au-TMA nanoparticles and transition probability matrices. Scheme for the definitions of variables DIST and Rmin (A). The two gold cores are denoted as I and II , respectively. Whereas, each anion on the Au-TMA nanoparticles calculated by its distance to the COMs of gold core I and II are presented (denoted ad Dist(I) andDist(II) , respectively). The variables DIST andRmin demonstrate the absolute difference and minimum of theseDist(I) and Dist(II) , respectively. The probabilities of anions distributions on the Au-TMA nanoparticles as a function ofDIST and Rmin are also shown (B). The schematic representation of the three anion classes (C). Whereas, anions in the solutions belonging to the first class are shown in grey. The anions on the Au-TMA nanoparticle surfaces with DIST greater than 2.0 (shown as pink) and DIST equal to or less than 2 nm (shown as green) are classified into interface anions (the second class) and outer anions (the third class), respectively. The transition matrix of anions in the P2-4, P3-3, P4-6 and P6-6 systems with demonstrated transitions between the three regions obtained from the last 1500 ns of MD simulations (D). The grey, pink and red squares denote anions in the solution, outer anions and interface anions, respectively. Whereas, the sum of each vertical column equals one.
2.4 | Dynamics of Au-TMA nanoparticles
In our last analysis, we measured the effects of molecular anions on the dynamics of Au-TMA nanoparticles in these aggregates. This was performed by estimations of the relative rotation of two Au-TMA nanoparticles. Herein, the relative rotation of Au-TMA nanoparticles was calculated by using one Au-TMA nanoparticle as the reference. The analysis was performed over the last 1500 ns of MD simulations with a time interval of 4 ns (Figure 6A). In comparison with the rotation angle of the P2-4 system, the rotation angle of the P3-3 system was significantly smaller which indicated a more dynamic rotation of Au-TMA nanoparticles in the P2-4 system (Figure 6B). This may be potentially contributed by the higher charge density and smaller anion size of the P2-4 anion. On the other hand, the dynamics of the P2-4 anions in the interface region were found higher than the interface of anions in the P3-3 system (see Figure 5D). This may be attributed to a lower rotational barrier of Au-TMA nanoparticles in the P2-4 systems than that of the P3-3 system. However, the rotation angle of the P3-3 system is close to that of the P4-6 system. Interestingly, close proximity transition probabilities involving anions in the interface region for both P3-3 and P4-6 systems were also observed (see Figure 5D). The data infers that the linear-shaped P4-6 anion with higher charge density matches the dynamics properties of the P3-3 anion because of its stronger interaction with Au-TMA nanoparticles. Moreover, the rotation angle in the P6-6 system was much smaller than that of the P4-6 system and consistent with the transition probability of these two systems. We speculate that the interfacial aggregates formed by P6-6 anions and sodium ions present a significant rotational barrier to the rotation of Au-TMA nanoparticles. We propose this effect to significantly reduce the dynamics of Au-TMA nanoparticles within the aggregate.