FIGURE 4 The charge distribution of aggregates formed by Au-TMA nanoparticles and anions (A). The grey dashed line marks the position of zero net charge density. Snapshots of the final structure of the P2-4, P3-3, P4-6and P6-6 systems (B). The simulation boxes are represented as black lines.
2.3 | Distributions and fluctuations of anions on the surface of Au-TMA nanoparticles
We analyzed the distribution of anions on the Au-TMA nanoparticles for the P2-4, P3-3, P4-6 and P6-6 systems. The symmetrical aggregates formed by the two Au-TMA nanoparticles were inspected by considering two variables (i.e., DIST andRmin ) as performed in a previous study[2c]. The analysis was aimed to characterize the positions of anions on the surface of Au-TMA nanoparticles. Herein, the two gold cores are denoted as I and II , respectively. The distribution of each anion on the Au-TMA nanoparticles was calculated by its distance to the COMs of the gold core I and II (denoted ad Dist(I) andDist(II) , respectively). The variables DIST andRmin are defined as the absolute difference and minimum ofDist(I) and Dist(II) , respectively (Figure 5A).The variable DIST characterizes the anion positions on the two nanoparticles, whereas Rmin indicates the radial distance to the closer Au-TMA nanoparticle.
We calculated the probabilities of anions distributions on the Au-TMA nanoparticles in the P2-4, P3-3, P4-6 and P6-6 systems as a function of DIST and Rmin over the last 1500 ns of MD simulations. The probability of anions distribution was firstly normalized by the number of anions in each system. This was necessary because of the higher number of anions in the P3-3 ad P2-4 systems than those in the P4-6and P6-6 systems (see Table S4). We found that the P3-3 anions were distributed mostly on the nanoparticle with DIST values greater than 2.5 nm while the P2-4 anions were located mainly on the nanoparticle with DIST values greater than 3.5 nm (Figure 5B). Meanwhile, as compared to the Rmin distributions of the P3-3system, the Rmin distribution of the P2-4system was found wider with an obvious difference of Rmin~4. This result implies TMA ligands specific higher penetration of P2-4 anions in contrast to P3-3 anions and may be contributed by the smaller size of P2-4 anions. On the other hand, the distributions of P4-6 and P6-6 on the Au-TMA nanoparticles were denser. This is consistent with the previous finding of strong electrostatic interactions between anions featured with six negative charges and Au-TMA nanoparticles. Meanwhile, the distributions of P4-6 and P6-6 on the Au-TMA nanoparticles were found quite similar except for barely seen distributions of P6-6 anions in proximity to the interface of the two nanoparticles (i.e., the DIST close to zero). This may be attributed to the larger COM distance of Au-TMA nanoparticles in the P6-6 system and aggregation of the interface region with sodium ions and P6-6 anions (see Figures 2B and 3B).
Furthermore, we characterized the anions into three classes based on its dynamics on the nanoparticles and in the solution. The anion in the solution was the first class subjected to this analysis (Figure 5C). Herein, we adopted a DIST criterion of 2 nm to further classify the two other classes of anions (interface and outer) on the nanoparticles based upon the anion distribution results (Figure 5B). Finally, the anions on the Au-TMA nanoparticle surfaces with DIST greater than 2.0 (pink) and DIST equal to or less than 2 nm (green) were classified as interface anions (the second class) while others as outer anions (the third class), respectively.
We further calculated the transition probabilities of these three regions during the last 1500 ns of MD simulations. We found that anions in the solutions and the outer regions were quite stable for all the four systems (Figure 5D). Notably, the anion transition between outer anions and interface anions in the P2-4 system was more dynamic than that of the P3-3 system. Similarly, the transition between outer anions and interface anions was more dynamic in the P4-6 system than that in the P6-6 system. Interestingly, close transition probabilities of outer anions and interface anions were observed for the P3-3 and P4-6 systems. These comparisons briefly summarize that the increase in anion charge mainly reduce the dynamics of anion transitions. In a similar way, the detailed analysis also employs that the ring structure and larger size of anions reduce the dynamics of anion transitions in contrast to the linear ones with smaller sizes.