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).