Scientists decipher the controllable crystallization of dynamic surface-induced gold nanoclusters

On November 11, 2022, Beijing time, Professor Xie Jianping of the National University of Singapore, Professor Han Yu of King Abdullah University of Science and Technology, and Professor Hannu Häkkinen of Jyväskylä University teamed up in the journal Nature Chemistry with the title “Supercrystal Engineering of Atomically Precise Gold Nanoparticles Promoted by.” Surface Dynamics”, published recent research progress.

Through the synergy between the electrostatic interaction and CH-π interaction between organic quaternary ammonium cations and gold nanoclusters, the research team realized the effective regulation of the dynamic adsorption/desorption of the surface structure of the cluster and the cross-linking between clusters, and then realized the rational control of the morphology, size and symmetry of the cluster supercrystals, which provided new ideas for the study of the self-assembly/crystallization law of inorganic nanoparticles and the chemistry of the interaction between particles.

The corresponding authors of the paper are Jianping Xie, Yu Han and Hannu Häkkinen; The first authors are Yao Qiaofeng, Liu Lingmei and Sami Malola.

Image credit: Sami Malola, University of Jyväskylä

Solid materials in nature are made up of basic structural primitives such as atoms, molecules, and ions. The properties of solid materials are closely related not only to the properties of the structural primitives themselves, but also to the packing or assembly mode between the structural primitives (e.g. graphite v.s. diamond). After nearly two centuries of vigorous development, inorganic nanoparticles have rapidly emerged into a new class of structural primitives with unique optical, electrical and magnetic properties, and are used to construct “metamaterials” that do not exist in nature. Compared with the structural characteristics of single inorganic nanoparticles (such as size, morphology, crystallinity, etc.), the rational regulation of the size, morphology and packing symmetry of the assembled body/supercrystal formed by nanoparticles is more difficult, which is a hot spot and difficulty in the research of nanoparticle self-assembly and metamaterials. In traditional self-assembly studies of inorganic nanoparticles, nanoparticles are often regarded as hard spheres with fixed morphology and surface properties, while ignoring the dynamic characteristics of nanoparticle structure and surface chemistry.

Thiol-protected gold nanoclusters are an emerging class of nanoparticles with ultra-small metal nuclei (<3 nm). They generally have the core-shell structure characteristics of metal nucleo-organometallic complexes, and their chemical composition and atomic packing structure can be accurately determined by high-resolution mass spectrometry and single crystal X-ray diffraction. Due to the significant quantum confinement effect and the diversity of cluster structures at this ultra-small scale, gold nanoclusters have a series of unique molecular properties, such as discrete electron energy levels, strong luminescence, intrinsic chirality, etc. More importantly, these molecular properties show atomic-level dependence on the size, structure and interaction of clusters, so they can be used to accurately track the self-assembly process of clusters. In addition, recent studies have shown that thiol-protected gold nanoclusters not only have a multi-level structure similar to proteins, but also exhibit significant dynamic characteristics (such as reversible isomerization, dynamic migration of surface organic protectant molecules, etc.). Therefore, thiol-protected gold nanoclusters are an ideal model for understanding the self-assembly/crystallization laws of inorganic nanoparticles, especially the correlation between the dynamic characteristics of inorganic nanoparticle structures and their self-assembly behavior.

Figure 1: Growth of Au25 clusters of rod-shaped supercrystals

a: Schematic diagram of crystallization process; b, e, g: SEM, TEM, XRD characterization of rod-shaped supercrystals; c, d, f: UV-vis absorption spectroscopy of rod-shaped supercrystals after redissolution in water, electrospray ionization mass spectrometry, 1H NMR characterization (Image credit: Springer Nature)

In view of this, the team of Professor Xie Jianping of the National University of Singapore, Professor Han Yu of King Abdullah University of Science and Technology, and Professor Hannu Häkkinen of Jyväskylä University worked together to make full use of the dynamic absorption/desorption characteristics of the surface structure of the gold nanocluster to realize the rational control of its supercrystal size, morphology and symmetry. The research team used the Au25(p-MBA)18 cluster (p-MBA as p-mercaptobenzoic acid) as the model system, and promoted the (p-MBA) on the surface of the Au25 cluster through the synergy of electrostatic interaction and CH-π interaction between tetraethylammonium cation and cluster surface ligands, and promoted the (p-MBA)-[Au(I)-(p-MBA)]2 Asymmetric desorption of nail-like complexes and their intercluster re-crosslinking. And then through the unique (p-MBA) -[Au(I)-(p-MBA)]The 4-cluster interlocking structure connects the Au25 clusters into one-dimensional nanowires, and achieves high homogeneity (~1 μm) growth of clustered rod-like supercrystals through the accumulation of one-dimensional nanowires (Figure 1). In contrast, after the tetraethylammonium cation is removed from the crystallization system, the Au25 (p-MBA) 18 nm cluster will be stacked into a regular octahedral supercrystal according to the hard sphere model.

Figure 2: Stacked structure of Au25 clusters in rod-shaped supercrystals

a, b: near-atomic resolution ultra-low irradiation TEM photographs; c: three-dimensional electron diffraction pattern; d: Simulation and experimental TEM photo comparison (i-iii), supercrystalline structure model (iv-vi) (Image credit: Springer Nature)

In order to further obtain the accumulation information of Au25 clusters in supercrystals, the research team first determined that the spatial group of supercrystals is R-3m by three-dimensional electron diffraction. More critically, using ultra-low irradiation transmission electron microscopy (TEM), the team also “visualized” clusters in supercrystals with near-atomic precision (Figure 2). In near-atomic resolution TEM photographs, (p-MBA)-[Au(I)-(p-MBA)]The 4-cluster inter-cluster link structure is clearly visible, which confirms the dynamic surface-assisted self-assembly mechanism proposed by the author. With the help of density functional theory (DFT) simulation and image similarity analysis algorithms, the authors also determined the orientation of clusters in the supercrystal lattice, and the resulting reconstructed cluster supercrystal model can perfectly reproduce the experimental TEM photos.

In view of the synergistic effect of electrostatic interaction and CH-π interaction in the formation of supercrystals, the size and morphology of supercrystals can also be rationally regulated by the types and concentrations of organic quaternary ammonium cations. For example, by regulating the concentration of tetraethylammonium cations, the morphology of Au25 cluster supercrystals can gradually evolve from regular octahedron to micron rods and nanorods (Figure 3). Spectroscopic characterization showed that rod-like supercrystal formation was induced by selective adsorption of 6 molecules of tetraethylammonium cations in clusters of Au25 (Figure 4).

Fig. 3: Effect of tetraethylammonium cation concentration on supercrystalline morphology of Au25 clusters

a-c, g-i: SEM photo; d-f, j-l: TEM photo (Image credit: Springer Nature)

Figure 4: Spectroscopic characterization of supercrystals formed at different tetraethylammonium cation concentrations

a: UV-vis absorption spectrum; b, c: electrospray ionization mass spectrometry; d-f: 1H NMR spectra (Image credit: Springer Nature)

Molecular dynamics simulations further show that tetraethylammonium cations can provide optimal CH-π interactions to stabilize intercluster (p-MBA)–[Au(I)-(p-MBA)]4 Link structure (Figure 5). Based on this finding, the team introduced the weaker tetramethylammonium cation into the crystallization system as a competitive ion, and achieved the growth of diamond-shaped sheet supercrystals through kinetic regulation (Figure 6).

Figure 5: Molecular dynamics simulation of CH-π interaction between tetraethylammonium cations and Au25 clusters (Image credit: Springer Nature)

Figure 6: Morphology characterization of Au25 cluster diamond-shaped sheet supercrystals

a: SEM photo; b-e: TEM photograph and its fast Fourier transform (Image credit: Springer Nature)

This work demonstrates the decisive influence of the dynamic characteristics of inorganic nanoparticle structures on their self-assembly behavior through the controlled crystallization of Au25(p-MBA) 18 nanometer clusters. At the same time, this work also proves that the synergy of electrostatic interaction and CH-π interaction can achieve fine regulation of cluster surface properties and self-assembly behavior. In addition to the above in-depth understanding of cluster self-assembly and interaction mechanism, this work also provides a supercrystalline structure elucidation method based on ultra-low irradiation TEM and theoretical simulation, which is a useful supplement to the X-ray diffraction structure analysis method of single crystals. (Source: Web of Science)

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