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.