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.