FIGURE 3 The sodium cations in the interface regions of Au-TMA nanoparticles. The time evolution of positive interface charge for the P2-4, P3-3 P4-6and P6-6 systems (A). The contact interface region was demonstrated as space within a cutoff distance of 1.5 nm around the two Au-TMA nanoparticles. Snapshots of the detailed interface regions of the P2-4, P3-3, P4-6and P6-6 systems (B). Whereas, the green spheres represent sodium ions.
In the next analysis, we systematically compared the electrostatic interactions in the aggregates by calculating the charge densities among the four tested systems. The charge distributions were calculated by summing the number of charges as a function of the distance from the surfaces of the two gold cores over the last 1500 ns of MD simulations. It is important to notice that the initial peaks of the charge density at ~1.5 nm for the four systems correspond to the positive charges of TMA ligands (Figure 4A). During this analysis, the peak heights of P2-4 and P3-3systems observed are lower than those of P4-6 and P6-6 systems which may be attributed to the smaller sizes of P2-4 and P3-3 anions. The smaller sizes of these anions also enabled them to partially penetrate the coating layer of TMA ligands and cause neutralization of the positive charges in these ligands.
As the positive charges of TMA ligands are further neutralized by the charges of anions, we also observed valleys for the charge density of these four systems. It can be described as charge reversal at the interface of Au-TMA nanoparticles. A detailed examination suggests that the valley positions are in the order of P2-4<P3-3<P4-6<P6-6, which can be attributed mainly to the increase in anion sizes. Meanwhile, the depths of the valleys are in the same order and correspond to an increase in negative charges near the surface of Au-TMA nanoparticles. It can be inferred that the electrostatic attractions between Au-TMA nanoparticles and surface anions are in the same order of P2-4<P3-3<P4-6<P6-6. Final snapshots of systems at the end of the MD simulation are demonstrated in Figure 4B. In comparison with the P2-4and P3-3 systems, the P4-6 and P6-6 systems showed much fewer anions in the solutions, especially for the P6-6 system. This also employs that the molecular anions absorbed onto the Au-TMA nanoparticles reverse the surface charge from positive to negative and allow sodium ions further attracted towards the surface of Au-TMA nanoparticles. It is then followed by the appearance of second peaks representing charge accumulations observed at ~2.2 nm. Interestingly, the heights of the second peaks were in the similar order of P2-4<P3-3<P4-6<P6-6. It also consistently indicates an order of electrostatic attractions between surface molecular anions and nearby cations.