FIGURE 2 MD simulations of Au-TMA nanoparticles and molecular anions featured six negative charges (P4-6 and P6-6). Time evolution of COM distance between Au-TMA nanoparticles in the P4-6 and P6-6systems. Final configurations of P4-6 and P6-6 systems after MD simulations are also demonstrated. The P4-6 and P6-6anions are shown in magenta and orange sticks, respectively. (A). Time evolution of negative interface charge for the P4-6and P6-6 systems (B). The different Stages includingI , II and III separated by orange dashed lines.
We further calculated the negative charges near the contact interface of the Au-TMA nanoparticles in P4-6 and P6-6 systems. Because of the slightly larger COM distance of Au-TMA nanoparticles in the P6-6 system than that of the P4-6 system (Figure 2A), we adopted a cutoff distance of 2 nm for the calculations of negative interface charges. The results show a rapid increase in the number of negative interface charges in the P4-6 system while remaining steady at ~150. This indicates that the equilibrium state is attained quickly after the aggregation of the two Au-TMA nanoparticles (Figure 2B). On the other hand, we observed that the number of negative interface charges as a function of time in the P6-6 system, could be divided into three stages. StageI corresponds to the sharp increase to ~150 charges during the first ~300 ns and remaining steady until 700 ns. This first stage indicated the encounter of Au-TMA nanoparticles and the formation of a metastable state. However, the number of negative interface charges increased gradually to ~200 until ~1300 ns defining the stageII (from ~700 ns to ~1300 ns). This second stage is suggesting aggregation of P6-6anions in the interfacial region. Finally, Stage III is considered as the equilibrium stage occurring after ~1300 ns where the number of negative interface charges remained steady. All the representative snapshots of these three stages are shown in Figure S8. Furthermore, higher negative interface charges in the P6-6 system (~200) were observed as compared to the P4-6 system (~150) during the final equilibrium stage. These results suggested the presence of stronger electrostatic attractions in the interfacial P6-6 than those of the interfacial P4-6 anions towards the Au-TMA nanoparticles.
These MD simulations predominantly revealed that the negatively charged phosphoric anions are attracted by the positively charged Au-TMA nanoparticles due to the presence of the electrostatic attractions. Meanwhile, anions diffuse more quickly than Au-TMA nanoparticles due to their significant smaller sizes. Moreover, the attained dynamic equilibrium of adsorption and desorption of anions on the Au-TMA nanoparticles is crucial. These anion densities near the Au-TMA nanoparticle surface play critical roles in mediating the aggregations of Au-TMA nanoparticles. The surface anion densities are influenced by the valency of molecular anion as it determines the electrostatic attraction strength with the positively charged Au-TMA nanoparticles according to Coulomb’s law (more discussion below).
2.2 | Structural analysis of aggregates formed by Au-TMA nanoparticles and ions
Our results showed that P2-4, P3-3P4-6 and P6-6 anions can mediate effective aggregations of Au-TMA nanoparticles. We further evaluated the potentially important role of sodium ions in these aggregations. This was performed by comparing the positive interface charge which consisted of sodium ions in our four systems (Figure 3A). Our results suggested a higher number of interface charges of the P3-3 system (~16) as compared to the positive interface charge of the P2-4 system (~8). Interestingly, this observation with higher charges of P2-4 than the P3-3 anion indicated that the number of positive interface charges may correlate with the anion size and shape. Furthermore, we also found more (~50) cations in the interfacial regions of the P6-6 system as compared to the P4-6 system (~24) during the equilibrium stage (i.e., after ~300 ns). This evidence further showed that the anion shape and size sustain influence on the interfacial ion distributions. Figure 3B shows the representative detailed snapshots of the interfacial regions of the P3-3, P2-4, P4-6and P6-6 systems.
Similarly, the positive interface charge results were also consistent with the calculated COM distances of Au-TMA nanoparticles. The COM distances of Au-TMA nanoparticles during the aggregations were found comparable (~8.5 nm, see Figures 1C and 2B). This is in agreement with the smaller number of sodium ions in the interfacial regions for P3-3, P2-4 and P4-6 systems (Figure 4). Meanwhile, the COM distance of Au-TMA nanoparticles was relatively large (~9.5nm, see Figure 2B) due to the significantly higher positive interface charge of the P6-6 system (Figure 3).