KEYWORDS
Electrostatic interaction, nanoparticle, aggregation, co-assembly,
coarse-grained molecular dynamics simulation
1 | INTRODUCTION
The dynamic nature of the self-assembly of charged nanoparticles
mediated by crucial interactions with co-assembly partners affects a
wide range of physical, chemical and biological properties, which leads
to the formation of both ordered and disordered
superstructures.[1] For instance, spherical
nanoparticles functionalized with multiple charged ligands (also called
superions) and nanoscale analogues of simple ions exhibit similar
behaviours in several ways.[2] These oppositely
charged ‘superions’ can co-assemble into binary nanoparticle crystals
due to the electrostatic attractions and resemble the formation of salt
crystals by oppositely charged ions.[2a, 3]Despite the extensive studies of the self-assembly of
nanoparticles[4], there is still critical
information missing on the co-assembly of charged nanoparticles with
oppositely charged ions, such as the effects of valency, shape and size
of these superions. Very recently, Bian et al.[2c]reported that molecular ions with as few as three electric charges can
co-assemble with oppositely charged nanoparticles and effectively induce
attractions between charged nanoparticles in water. These interactions
between molecular ions and nanoparticles can remarkably modulate the
formation of colloidal crystals. The Classical
Derjaguin-Landau-Verwey-Overbeek (DLVO) theory[5]divides the interactions between colloidal particles in solution into
van der Waals (attractive) and the electrostatic (repulsive) as a
function of separation distance.[6] The theory has
been widely adopted for the explanations of colloidal stability and
rationalization of the forces acting on colloidal particles. However,
this theory is not readily applicable to charged nanoparticles due to
the non-additivity of nanoparticle interactions on the
nanoscale.[1a, 1b] In our study, a novel approach
in the fabrication of nanoparticle superstructures has been utilized.
The explored mechanism of interactions between molecular ions and
nanoparticles offers several applications in nanoscale synthesis and
engineering of nanoparticle superstructures.
Electrostatic interactions play a major role in the co-assembly of
molecular ions and nanoparticles.[7] However, the
detailed co-assembly mechanism of charged nanoparticles and oppositely
charged molecular ions remain to be elucidated. Herein, we focused on
probing the electrostatic co-assembly mechanisms of charged
nanoparticles and oppositely charged molecular ions. The study depicts
the effects of valency, shape and size of molecular ions on this
co-assembly. Previously, Bian et al.[2c] reported
the molecular dynamics simulations (MD) of trimethyl (mercaptoundecyl)
ammonium (TMA)-coated gold nanoparticles (denoted as Au-TMA
nanoparticles) and citrates using coarse-grained (CG) Martini 2 force
field[8]. However, the methodology deployed was an
implicit solvent method for the MD simulation. Such, implicit solvent
models may not accurately calculate solvation and sufficiently describe
the electrostatic interactions between Au-TMA nanoparticles and
citrates.[9] In contrast, we adopted the new
Martini 3 force field[10] and the corresponding
water model. The explicit water environment is considered more accurate
and realistic than the implicit solvent model for the depiction of
electrostatic interactions.[9b, 11] Moreover, the
Martini 3 force field shows significant improvements and offers broader
applications over the Martini 2 force field.[10,
12]
In our study we performed coarse-grained molecular dynamics (CGMD)
simulations of Au-TMA nanoparticles with a gold core diameter of 4.9 nm
and phosphoric anions. We used a series of phosphoric anions with
different valences for the depiction of co-assembly with Au-TMA
nanoparticles. The anions used in the study included dihydrogen
phosphate (P1-), hydrogen phosphate
(P1-2), trimetaphosphate (P3-3),
pyrophosphate (P2-4), tetraphosphate
(P4-6) and hexametaphosphate (P6-6).
These anions are denoted as Pnm, where n is the number
of phosphates and m is the net electrical charge. The previous study by
Bian et al.[2c] suggests P3-3,
P2-4 and P6-6 anions as effective
candidates in mediating attractions between Au-TMA nanoparticles. We
speculated that P3-3 and P2-4 anions
are comparable, with the P3-3 anion having a slightly
larger molecular size and lower charge density than the
P2-4 anion. Moreover, the P4-6 anion
was used as a reference to P6-6 due to its equal
charge despite having a smaller molecular size. Similarly,
P2-4 and P4-6 anions as linear
molecular anions while P3-3 and P6-6anions as ring-shaped. In our study, we mainly utilized
P1- and P1-2 anions due to their
relatively low valences. The selection of these molecular anions was
then followed by a systematic investigation of the effects of several
parameters including valency, shape and size of these molecular anions
on their electrostatic co-assembly with Au-TMA nanoparticles. Moreover,
the selected phosphoric anions are particularly interesting as they are
related to the phosphoric acid backbone of the DNAs. Therefore, the
co-assembly of phosphoric anions and gold nanoparticles in this study
may serve as a motivation for designing the DNA-based nanostructures,
such as DNA templates and DNA origami, for their co-assembly with the
positively charged gold nanoparticles into highly ordered
superstructures, as reported by previous experimental
studies.[13]
Our results mainly suggest that P1- and
P1-2 anions with electric charges less than three may
not mediate sufficient attractions between Au-TMA nanoparticles.
However, molecular anions with higher electric charges (i.e.,
P3-3, P2-4, P4-6and P6-6) successfully induce the aggregations of
Au-TMA nanoparticles. Furthermore, the size and shape of anions have
great influences on the distribution and dynamics of adsorbed anions and
the dynamics of nanoparticle aggregates. Our study emphasizes that the
valency, shape and size of molecular anions are important factors in
mediating attractions between the Au-TMA nanoparticles. These important
findings also pave the way to provide future guidance for the design of
DNA templates and DNA origami co-assembling with positively charged
nanoparticles.
2 | RESULTS AND DISCUSSION
2.1 | Interactions between positively charged Au-TMA
nanoparticles and molecular anions
Detailed models of Au-TMA nanoparticle and phosphoric anions are shown
in Figure 1A and Figure 1B, respectively. The model-building methodology
of Au-TMA nanoparticle and phosphoric anions are provided in the
Supporting Information. To evaluate the potential interactions, we
performed MD simulations of Au-TMA nanoparticles and a series of
phosphoric anions. Initially, two Au-TMA nanoparticles and phosphoric
anions were randomly distributed. Figure 1C shows the final structures
of the P1-, P1-2,
P3-3 and P2-4 systems after the MD
simulations. The results show that the two Au-TMA nanoparticles stay
separated from each other in the P1- and
P1-2 systems while clustered in the
P3-3 and P2-4 systems (Figure 1C).
To quantitively monitor the aggregations of Au-TMA nanoparticles, the
centre-of-mass (COM) distance between the two gold cores were calculated
for all these four systems. During the MD simulations, the distance
between the nanoparticle fluctuated for the P1- and
P1-2 systems, indicating the presence of dominant
electrostatic repulsions between the two Au-TMA nanoparticles (Figure
1D). In contrast, the two equally charged Au-TMA nanoparticles in the
P3-3 and P2-4 systems agglomerated
at ~850 ns and ~50 ns, respectively. The
aggregates formed in both systems remained stable during the rest of the
simulations maintaining a COM distance of ~8.5 nm. This
result showed that both P3-3 and
P4-6 anions can mediate efficient attractions between
Au-TMA nanoparticles and were consistent with previous experimental
findings[2c].
Since the attractions between Au-TMA nanoparticles are associated with
negative charges in the interfaces, it is essential to compute the
negative charge near the contact interface of the Au-TMA nanoparticles.
We adopted a cutoff distance of 1.5 nm and counted the number of
negative charges of the anions within a diameter of ~1.5
nm around the two Au-TMA nanoparticles. Consistent with the fluctuations
of inter-nanoparticle distance in the P1- and the
P1-2 systems (Figure 1D), the negative interface
charge of the two analyzed systems also fluctuated with the charge
number less than 40 (Figure 1E). Interestingly, the negative interface
charges reached ~85 for both the P3-3and P4-6 systems after the aggregation of the two
Au-TMA nanoparticles. This indicates that interfacial
P3-3 and P4-6 anions produce close
electrostatic attractions with the Au-TMA nanoparticles.