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Emergence of hierarchical structural complexities in nanoparticles and their assembly

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Science  23 Dec 2016:
Vol. 354, Issue 6319, pp. 1580-1584
DOI: 10.1126/science.aak9750

Probing packing rules

The crystals of a well-defined ligand-covered gold nanoparticle can reveal how packing into a lattice happens. Zeng et al. synthesized nanoparticles with a 246-atom gold core surrounded by 80 4-methylbenzenethiol ligands. These nearly spherical nanoparticles did not pack into a cubic arrangement but instead formed a lower-symmetry monoclinic structure. A hierarchy of interparticle ligand interactions controlled the packing, including sets of chiral packing arrangements that reversed between layers.

Science, this issue p. 1580

Abstract

We demonstrate that nanoparticle self-assembly can reach the same level of hierarchy, complexity, and accuracy as biomolecules. The precise assembly structures of gold nanoparticles (246 gold core atoms with 80 p-methylbenzenethiolate surface ligands) at the atomic, molecular, and nanoscale levels were determined from x-ray diffraction studies. We identified the driving forces and rules that guide the multiscale assembly behavior. The protecting ligands self-organize into rotational and parallel patterns on the nanoparticle surface via C-H⋅⋅⋅π interaction, and the symmetry and density of surface patterns dictate directional packing of nanoparticles into crystals with orientational, rotational, and translational orders. Through hierarchical interactions and symmetry matching, the simple building blocks evolve into complex structures, representing an emergent phenomenon in the nanoparticle system.

Hierarchical self-assembly of nanoparticles (NPs) into complex architectures across different length scales is an important capability in nanotechnology (14), especially for the bottom-up fabrication for electronics, sensors, energy conversion, and storage devices. Such self-assembly can be driven by entropy-dictated maximization of the packing density, as demonstrated in close packing of spheres, binary NPs, rods, and hard polyhedrons (59). Interparticle interactions, such as the electrostatic attraction (10), DNA binding (11, 12), and patchy NP surfaces (1315), have also been exploited to guide assembly into diverse lattice structures. Despite these advances, NP assembly has not achieved the same level of atomic accuracy as in biological systems.

We now demonstrate that NP-assembled structures can reach the same hierarchy and atomic accuracy as biomolecules at the interparticle and intraparticle levels. Through crystallization of uniform 2.2-nm gold NPs bearing p-methylbenzenethiolate (p-MBT) surface ligands [Au246(p-MBT)80], we fully resolve the entire self-assembled structures at atomic (packing of gold atoms), molecular (packing of surface ligands), and nanoscale (packing of NPs) levels by single-crystal x-ray diffraction (SC-XRD) (16). The precise structural information across scales allows an in-depth examination of the forces and the rules that govern the assembly behavior at each level. We reveal that the simple structure of protecting ligands can generate complex patterns on the NP surfaces, and the symmetry and density of the surface patterns further guide the packing of NPs into lattices with orientational, rotational, and translational order.

The Au246(p-MBT)80 NPs were synthesized by a two-step “size-focusing” method (16). Briefly, the ligand-coated NPs in a narrow size range from ~10 to ~70 kilodaltons were first made, and then the size focusing process gradually led to the stable Au246(p-MBT)80 NPs (figs. S1 and S2). The as-obtained product (~90% purity) was subject to further solvent fractionation to reach molecularly pure Au246 NPs. Optical absorbance spectra showed a prominent peak at 470 nm and several weak humps at 400 and 600 nm (fig. S3), indicating the nonplasmonic nature of the Au246(p-MBT)80 NPs. Single crystals were grown by diffusion of antisolvent (acetonitrile) into a toluene solution of the pure Au246 NPs. The structure was determined at the resolution of 0.96 Å by SC-XRD (tables S1 and S2). The individual NPs have a nearly spherical shape, with a metal core diameter of 2.2 nm and an overall diameter (including the ligand shell) of 3.3 nm (fig. S4).

In contrast to conventional spherical NPs, which are typically packed into superlattices with simple translational symmetry, such as face-centered cubic (fcc) or body-centered cubic (bcc) (11, 17), the Au246(p-MBT)80 NPs packed into a more complex monoclinic lattice (Fig. 1). In the (001) plane of the crystal lattice, the NPs organized into a square lattice (Fig. 1A), akin to the packing mode in the {100} plane of the fcc lattice. The interparticle distance of 3.1 nm (less than the 3.3 nm overall size of the NP) arose from ligand interlocking. These two-dimensional (2D) lattices stacked up obliquely, instead of perpendicularly along the z direction (Fig. 1B), so the overall 3D lattice deviated from the fcc lattice. The NP packing density of ~60% is less than the 74% of the fcc lattice. Such monoclinic packing should be driven by specific interparticle interactions, because entropy alone would favor close packing (59). Indeed, when zooming into the surfaces of the NPs, the monoclinic packing was correlated to the alignment of surface ligands among NPs (Fig. 1, D to F, green, blue, red).

Fig. 1 Packing structure of the derivatized Au NPs in single crystals.

(A to C) View from z direction (A), y direction (B), and x direction (C). Magenta, blue: Au NPs with different chirality; yellow: sulfur; gray: carbon. (D to F) Alignment of surface ligands among the NPs. Gray: ligands located at the waist of the NP; red, blue, green: ligands located at the poles.

The p-MBT ligands were highly ordered and self-organized into two different patterns on the gold sphere (Fig. 2A). At the pole site of the gold sphere, 25 of the p-MBTs were rotationally arranged into four pentagonal circles (Fig. 2B, highlighted in red, blue, blue, green). Each circle had the same “latitude” and rotational direction. Instead of “on-top” adsorption, the thiolates (characterized by S-C bond vectors) “tilted” away from the radial direction of the gold sphere because of the gold-thiolate binding geometry (18). This pattern we call α-rotation created a “singularity” (19) at the pole (Fig. 2B). At the waist site, six of the p-MBT ligands were aligned into three alternating parallel pairs to form a pattern we call β-parallel (Fig. 2C). Five of these β-parallel patches circled up and covered the waist of the NP. The packing densities of ligands are ~14 ligands nm–2 for α-rotation and ~6 ligands nm–2 for β-parallel as measured based on the surface area of the inner gold sphere (Fig. 2A, magenta polyhedron). The clockwise and counterclockwise rotational arrangement of p-MBT ligands induces chirality in the NP, and both chiral isomers (denoted R/L) participate in the crystal packing (Fig. 1, C and F). The NPs with the same chirality are packed in the same square layer, and the neighboring square layers are composed of NPs with opposite chirality (Fig. 1C).

Fig. 2 Self-assembled surface patterns of the ligands on the Au NPs.

(A) Overall structure of ligands on the surface of NPs. (B) Rotational packing of ligands at the pole site of the NP. (C) Parallel packing of ligands at the waist of the NP. (D) The C-H⋅⋅⋅π interactions for stabilizing the large-scale rotational patterns and parallel patterns.

Such rotational and parallel self-assembled surface patterns of ligands are reminiscent of the α helix and β sheet in proteins, which suggests that NPs could exhibit a level of structural complexity comparable to that of biomolecules. The secondary structures of proteins are mainly stabilized by the hydrogen bonds. Here, the surface patterns on the NPs are stabilized by intermolecular C-H⋅⋅⋅π interactions, in which the C-H bonds from the phenyl rings or the methyl groups interact with the π electrons (Fig. 2D). Such intermolecular interactions were observed in the packing structures of aromatic molecules and supramolecules, and the strength is about 1.5 to 2.5 kcal mol–1 (20, 21). Specifically, the C-H⋅⋅⋅π interactions in the α-rotation linked the 25 p-MBT ligands into five spirals, with the H⋅⋅⋅π distances ranging from 2.5 to 3.0 Å and C-H-π angle ranging from 112° to 147° (fig. S5). For the β-parallels, the alternating pattern among the three pairs was also stabilized by the C-H⋅⋅⋅π interactions (fig. S6). Within each parallel pair, the phenyl rings were offset to avoid the repulsion between π electrons. We reason that the surface patterns are intrinsic to NPs instead of being induced by crystallization, because the collective C-H⋅⋅⋅π interactions can generate an energy barrier and stabilize the pattern, similar to the case in which the hydrogen bonds can stabilize the DNA double helix in solution.

To study the interparticle interactions, we isolated the coordination environment of NPs (Fig. 3), which reflects the orientational, rotational, and translational symmetry of the crystal lattice. Each NP has six nearest neighbors. Four of them were within the same square layer and had the same chirality as the central one. (Fig. 3A, yellow arrows), The particle-to-particle distance was 31.0 Å, whereas the other two had opposite chirality (Fig. 3B, purple arrows) and the distance was 31.7 Å. These NPs interacted with the central one by aligning the α-rotation ligands at the pole sites, whereas there were fewer interactions of the β-parallel ligands at the waist site. The preferred alignment of α-rotation ligands was correlated to the higher packing density of ligands in α-rotation (~14 nm-2) compared to the density in β-parallel (~6 nm-2). In this way, the van der Waals interaction among the contacting ligands was maximized. This surface-density–dictated assembly strategy is in accordance with the assembly behavior observed in DNA-coated NPs (11, 12, 22), oleic acid–protected nanoplates (15), and nanocrystals (17).

Fig. 3 Interparticle self-assembly dictated by the ligand density and the symmetry of surface patterns.

(A and B) Coordination geometry of NPs in the crystal lattice: side view (A) and top view (B). (C) Contacting environment among the interparticle ligands. The outside pentagons are located at the bottom of the top three nanoparticles in (A), and the central pentagon is located at the top of the central nanoparticle. (D) Side-by-side stacking of the ligands in the NPs with the same chirality. (E) Point-to-point stacking of the ligands in the NPs with opposite chirality. (F) Scheme showing the directional packing of NPs achieved through matching the symmetry of surface patterns.

In addition to the maximization of ligand interactions, matching of surface symmetry was also important for ordered NP assembly. For a more detailed picture on interparticle interactions, we further isolated the contacting region among the top three and the central NPs (Fig. 3A, white frame), as reflected in the four pentagons shown in Fig. 3C. Notably, the central NP spontaneously matched with the same surface region of its neighbors when forming packing interaction (Fig. 3, C to E). Each contacting area was composed of about five pairs of symmetrically identical p-MBT ligands located at the corners of the contacting pentagons (Fig. 3, D and E). For the neighboring NPs with the same chirality as the central one, the pentagons were aligned side by side (Fig. 3D), whereas for the opposite chirality, the pentagons were aligned point to point (Fig. 3E). The spacing between interacting ligands was ~2.5 Å, and the shortest spacing could reach 1.9 Å (fig. S7). This short distance indicates tight packing among NPs. The symmetry-matching strategy is akin to the assembly of gears (Fig. 3F), with each contacting area resembling a tooth of the gear. Matching the symmetry facilitates the interlocking of surface ligands. Each pentagon has five potential contacting areas to connect with five other NPs, but only three are occupied because of the limited space around the NP.

The assembly structure provides a precise depiction of the long-standing issue of ligand effects on the packing structures of NPs (23, 24). It implies that the inhomogeneous but symmetric distribution of surface ligands can serve as “sticky bonds” for directional NP assembly. Such spontaneously organized surface patterns have an effect similar to that of artificially decorated surface patches in guiding assembly (3, 13, 14) . The perfect uniformity of the NPs was critical for maintaining the fidelity of surface patterns. The packing behavior also represents an emergent phenomenon (25, 26), in which small and simple entities (the surface ligands), through multiscale interactions, generate larger and more complex structures with new features that are not manifested in the simple entities. If the p-MBT ligand is viewed as the primary structure, then through the C-H⋅⋅⋅π interactions among the ligands, a more complex secondary structure of surface patterns is generated, and these surface patterns, via the density- and symmetry-dictated packing rules, further guide the packing of NPs into the tertiary structure of the crystal lattice. It is expected that by controlling the symmetries of surface ligand patterns, diverse packing structures of NPs could be achieved.

The assembly behavior within each NP also exhibits such order and hierarchy. The individual NP can be divided into four regions from the core to surface. The innermost part is a three-shell Au116 Ino decahedron (i-Dh) exposing {111} facets at the poles and {100} facets at the waist (Fig. 4A and fig. S8). The i-Dh is one of the energy minima for packing of metal atoms (2729). The second part is a transition layer containing 90 gold atoms (Fig. 4B, magenta), which “sphericizes” the i-Dh, lowers the surface energy, and provides “footholds” for anchoring surface protecting motifs. The two parts together give rise to an Au206 core. The average Au-Au bond length in the i-Dh was 2.87 ± 0.05 Å (fig. S9), whereas the bonds associated with the transition layer showed larger deviations (2.89 ± 1.12 Å) because of the binding effect of surface ligands. The third part is the Au-S interfacial layer, in which the surface dangling bonds of the Au206 core are anchored by the protecting motifs assembled from 40 gold and 80 sulfur atoms. The protecting motifs are highly diverse, in accordance with the rich surface features of the Au206 core (such as facets and grooves, Fig. 4B). At the poles the exposed, the exposed {111} facets are protected by –S–Au–S–Au–S– motifs and the {111}|{111} grooves are linked by simple bridging thiolates (Fig. 4C, bottom); at the waist, the {100}|{100} and {111}|{100} grooves are all covered by –S–Au–S– staple motifs (Fig. 4C, top). The fourth part is the surface carbon layer as discussed above (Fig. 4D). Although each part of the NP followed different assembling rules, the fivefold symmetry was always maintained (Fig. 4E). An intriguing question is whether the packing symmetry of the surface patterns emerged from the core or the core symmetry emerged from the surface. Although each part of NP contributes to the overall energy minimization, we deduce that the surface ligands play a pivotal role in guiding the intraparticle assembly, because the gold atoms are less selective when packing into specific structure types such as fcc, decahedron, or icosahedron (30). In addition, the rotational patterns of ligands induce the chirality in the interfacial layer and transitional layer (figs. S10 and S11), and packing of gold atoms alone cannot give rise to chirality. Also, the larger sphere of the ligands likely has a greater surface energy to minimize, and the structures of Au NPs were highly sensitive to the subtle changes of ligands (29).

Fig. 4 Intraparticle self-assembly in the Au NP.

(A) Au116 i-Dh kernel, top: side view; bottom: top view. (B) Transition layer structure as colored in magenta. (C) Gold-sulfur interfacial structure containing diverse surface protecting motifs. (D) Surface carbon layer and overall structure. (E) Intraparticle symmetry matching and emergent behavior.

Supplementary Materials

www.sciencemag.org/content/354/6319/1580/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S11

Tables S1 and S2

Reference and Notes

  1. Materials and methods are available as supplementary materials on Science Online.
Acknowledgments: R.J. thanks the Air Force Office of Scientific Research [award no. FA9550-15-1-9999 (FA9550-15-1-0154)] and the Camille Dreyfus Teacher-Scholar Awards Program for financial support. All data are available in the main text and in the supplementary materials. The x-ray crystallographic coordinates have been deposited in Cambridge Crystallographic Data Centre with CCDC number 1511348.
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