Polyethylene glycol is a linear chain structure composed of repeated ethylene oxide groups, with a hydroxyl group at each end, which can be formed by the gradual addition polymerization of ethylene oxide and water or ethylene glycol. Polyethylene glycol is a water-soluble polymer compound with different physical forms due to different molecular weights. It is a colorless, odorless, non-volatile viscous liquid with slight water absorption at room temperature when the molecular weight is 200-800; it is a waxy semi-solid when the molecular weight is 1000-2000; it gradually becomes a hard light white waxy solid or flake paraffin or fluid powder when the molecular weight is 3000-20000. With the increase of molecular weight, its water solubility, vapor pressure, toxicity, water absorption and solubility in organic solvents decrease accordingly, while the freezing point, relative density, flash point and viscosity increase accordingly. It is thermally stable, does not work with many chemicals, and does not hydrolyze.
Self-assembling colloidal nanocrystals into superstructures with specific arrangements and mesoscopic morphologies is crucial for the development of new materials. The self-assembly process of nanocrystals usually follows the principle of free energy minimization, thus tending to form two-dimensional/three-dimensional closely packed superstructures. This makes the realization of one-dimensional assembly of nanocrystals challenging. The main difficulty in constructing linear superstructures is the lack of directional interactions between particles. Although researchers have developed a series of strategies to promote the linear assembly of nanocrystals, most methods rely on the use of templates or external fields, which limits the subsequent application of materials and increases the complexity of the assembly process. In addition, linear assembly strategies based on macromolecules and DNA ligands still have shortcomings in terms of orientation control, preparation scale and cost.
A series of oleic acid-capped anisotropic rare earth nanocrystals were assembled into flexible chain superstructures with high crystal face selectivity. Different from the traditional linear assembly strategy of grafted macromolecules or DNA ligands, this method utilizes the specific adsorption of low molecular weight polyethylene glycol (PEG) on the (100) crystal face of oleic acid-grafted rare earth nanocrystals. This selectivity is achieved based on the difference in binding energy of oleic acid ligands on different crystal faces of rare earth nanocrystals. Furthermore, adjustment of the solvent polarity promotes the regioselective formation of PEG supramolecular bridges connected by intermolecular hydrogen bonds, which ultimately achieves linear assembly.
This facet-selective assembly approach is applicable to a variety of anisotropic rare earth nanocrystals, including nanorods, nanodisks, and nanodumbbells, as well as their pairwise combinations. Nanodisk chains often exhibit branched structures because of the larger hexagonal side lengths and lower steric hindrance of nanodisks. Nanodumbbells have a relatively limited effective interaction area, but they are still capable of facet-selective assembly, demonstrating the effectiveness of this assembly strategy. In addition, the concave features of the nanodumbbells give them two directional binding sites on each side. This induces the formation of more complex directional ordered superstructures, such as non-densely packed two-dimensional superstructures.
