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.