FIGURE 6 Rotation angles of the two Au-TMA nanoparticles in the
aggregates. Scheme for the definition of the rotation angle (A) and the
results of rotation angles in the P2-4,
P3-3, P4-6 and
P6-6 systems calculated over the last 1500 ns of MD
simulations (B). The time interval of the trajectories is 4 ns. The
error bar denotes the standard deviations. The PMF profiles of
P2-4, P3-3, P4-6and P6-6 systems along the reaction coordinates were
calculated as the distance between the COMs of the two gold cores of the
Au-TMA nanoparticles (C).
Hence, the molecular anions play a significant role in mediating the
aggregations of Au-TMA nanoparticles and are referred as “ionic
glues”[14]. We further evaluated the interactions
between the two Au-TMA nanoparticles by calculating the potential of
mean force (PMF) profile. This was performed by analyzing the distance
between the COMs of the two gold cores in the Au-TMA nanoparticles as
the reaction coordinate. The results suggested a higher disassociation
free energy barrier in the P4-6 and
P6-6 systems as compared to the P3-3and P2-4 systems. Specifically, the disassociation
free energy barrier of P3-3 was found slightly lower
than that of P2-4 system despite a higher electric
charge (Figure 6C). In comparison, the PMF profile of the
P6-6 system showed a significantly larger
disassociation free energy barrier than the P4-6system. These observations further emphasized the critical roles of the
shape and size of these anions in mediating the aggregation of Au-TMA
nanoparticles.
Based on the overall results, we speculate two folds effect of anion
size and shape on these aggregations. Notably, the effects of anion ring
structures (P3-3 and P6-6) are
associated with their relative low configuration entropies. It may also
serve as a major contributor in altering electrostatic interaction with
Au-TMA nanoparticles and sodium ions. This effect resembles the
configurational entropy loss, where the ligand is rigidified in the
classic ligand-protein binding. This configuration entropy loss of
ligand is favorable for its binding to
protein[15]. On the other hand, the effects of
molecular size can be interpreted as the distributed charges being
superior to collected charges in adsorbing onto the Au-TMA
nanoparticles. Molecular anions with more distributed charges
(P3-3 and P6-6) can adsorb
efficiently onto Au-TMA nanoparticles. This infers stronger
electrostatic interactions with Au-TMA nanoparticles (see Figure 4A) and
further promotes the adhesions between Au-TMA nanoparticles.
3 | Conclusion
In this work, we mainly studied the effects of valency, shape and size
of molecular anions on their co-assembly with the positively charged
Au-TMA nanoparticles. The results of COM distance of gold cores showed
that anions with charges greater or equal to three serve as effective
“ionic glues”[14] to the Au-TMA nanoparticles in
consistence with the previous experimental
findings[2c]. Furthermore, the charge density
calculations suggested that ring-structured P3-3 anion
with a larger size can more efficiently reverse the surface charge
density from positive to negative and has a lower transition probability
in the interface region as compared to the linear P2-4anion. Similar observations were evident from comparison of the linear
P4-6 anion and the ring-structured
P6-6 anion with a larger size. These findings
suggested that ring structures and large sizes of the anions facilitates
the interactions with Au-TMA nanoparticles and reduces the electrostatic
repulsions between Au-TMA nanoparticles. Consistently, the PMF profiles
revealed that the disassociation free energy tends to increase as anion
valency increases. In our results, the P6-6 system
showed a significantly larger disassociation free energy barrier than
the P4-6 system. These findings suggested the critical
roles of the shape and size of an anion in mediating the aggregation of
Au-TMA nanoparticles. Furthermore, these Au-TMA nanoparticles in the
P6-6 system show relatively low rotational dynamics
but higher dissociation free energy. However, aggregates are formed by
sodium ions and P6-6 anions in the highly curved
interface regions of Au-TMA nanoparticles mediated by strong
electrostatic attractions between sodium ions and P6-6anions. These observations mainly attribute to the combination of the
high valency, ring structure and large size of P6-6anions. Our performed study emphasizes that the valency, shape and size
of molecular anions are important factors in mediating attractions
between the Au-TMA nanoparticles.
The potential future implication may be based on the
P6-6 system with augmented inter-particle distance due
to increased ion size. This property may aid as a potential method to
fine-tune the lattice parameters of electrostatic co-assembly formed by
charged nanoparticles and oppositely charge molecular
ions[16]. Furthermore, the phosphoric anions
related to the phosphate backbones of DNA may also be useful.
Previously, DNA and DNA origami have been widely adopted as templates in
nanoparticle assembly or co-assembly with
nanoparticle[17]. Therefore, designing DNA-based
nanostructures, such as DNA templates and DNA origami, for their
co-assembly with positively charged gold nanoparticles into highly
ordered superstructures may be inspired by the findings of the current
study.