Spherical nanoparticles balance the high surface energy imposed by nanoscale dimensions against the surface energy minimization promoted by spherical geometry. Their surface properties are crucial to the effectiveness of their applications. However, limited by their tiny size and structural uncertainties, clear information on surface adsorption behavior is hard to be obtained through traditional analytical methods.

Spherical nanoclusters with atomically precise structures can serve as molecular models. Nevertheless, it is difficult to design and synthesize spherical clusters as well as to achieve a balance between stability and porosity in these discrete compounds in the solid state. Their host-guest chemistry studies are mostly conducted in solution, making direct characterization using single-crystal x-ray diffraction difficult.

In a study published in Nature Synthesis, a team led by Prof. ZHANG Jian and Prof. FANG Weihui from the Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences proposed a "co-encapsulation" strategy drawing inspiration from micellar self-assembly principles, and constructed the first spherical aluminum oxo cluster (SAlOC-1) through the synergistic guidance of flexible sterically hindered ligands and inorganic lone pair electrons.

The surface of SAlOC-1 contains a large number of monodentate ligands suspended on coordinatively unsaturated Al³⁺ ions, enabling it to simulate the complex surface environment of nanoparticles. SAlOC-1 retains its spherical morphology which maximizes exposure of surface supramolecular sites, while exhibiting unusual low symmetry, reducing disorder in crystallographic studies of guest molecules. It can bind up to 20 guests across a wide range at room temperature through a single-crystal-to-single-crystal transformation. 

Researchers showed that SAlOC-1 has unique advantages in guest recognition. It overcomes the limitations of host-guest chemistry of traditional discrete system in solution, and it is simple, rapid, and direct to operate. Furthermore, this surface host-guest chemistry is both universal and selective, and exhibits biomimetic characteristics of multi-component combination.

Theoretical calculations by Prof. LI Chunsen's group from FJIRSM showed that the mechanism by which SAlOC-1 captures guests is significantly different from classic framework materials, mainly relying on dynamically rotatable monodentate ligands acting as "molecular catchers" on flexible surfaces, thereby reducing dependence of the diffusion mechanism on high porosity.

This study provides new insights into the precise construction of spherical aluminum oxo clusters and the rational design of flexible surfaces. It also opens up new avenues for the solid-state host-guest chemistry of discrete compounds, the structural identification of organic molecules, and the surface modification of nanoparticles.