Self-assembly of nanoparticles

by Ben Wang, Yabin Zhang, Zhiguang Guo, Li Zhang

Self-assembly refers to an autonomous process whereby disordered building blocks gradually form larger, well-organized patterns, driven by mutual interactions of the building blocks toward reducing the system free energy. The process obeys physical laws and can occur at all scales, in the form of static and dynamic self-assembly [1]. Both static and dynamic self-assemblies form stable or meta-stable patterns with highly ordered structures. Static self-assembly is commonly a relatively slow process and formed from the simple aging process, with the formed patterns integrally stable via a free energy minimization process. Dynamic self-assembly commonly forms stable patterns that are not in equilibrium. As a simple and low-cost approach, the bottom-up self-assembly strategy shows unique merits in large-scale batch fabrication of ordered patterns, compared with top-down nano-fabrication techniques, which usually require expensive cleanroom facilities. The self-assembly of micro-/nanoparticles, among various kinds of building blocks, has generated intense interest because the self-assembled patterns commonly possess unique physical properties and find various applications in the fields of nanophotonics, solar cells, catalysts, data storage, and so on [2].

For instance, statically self-assembled patterns with micro-/nano-nanoparticles show features with a comparable size to the wavelength of incident light, inducing selective Bragg’s diffraction of light and interference effects. While the light wavelength is within the range of visible light, the material will show a particular color. The color can even change with one’s viewing orientation due to the alternation of the constructive and destructive interference [3]. Distinguished from pigmentary color, this kind of color is generated from periodical micro-/nano-structures. Various brilliant colors in nature like the feathers of birds, shells of beetles, the skin of chameleons, and petals of flowers originate from the surface imprinted periodic micro-/nano-patterns, which offer interminable sources and examples for researchers to develop photonic materials with excellent optical properties mimicking nature, and even going beyond nature [4].

Unlike static self-assembly, the dynamic system is forcefully prevented from reaching a free energy minimum and requires a continuous supply of external energy to balance the intrinsic interactions between building blocks, sustaining stable dynamic patterns with on-demand controllable configurations. Short-range forces – either attractive or repulsive – such as electrostatic interaction, van der Waals attraction and steric repulsion, all work during the dynamic self-assembly process, making it more complicated than static cases. It is sensitive to tiny and local changes, disturbances among the building blocks, and the supplied external energy. Dynamic self-assembly is very common in nature and the biological world, such as starling flocks, schooling fish, and bacterial swarms; however, the development of artificial dynamic self-assembly lags behind the static systems. The dynamic assembled pattern shows its merit in the adaptability compared with the static ones. To date, researchers have developed some dynamic systems with various kinds of external energy input, such as magnetic field [5], electric field [6], ultrasound field [7], light field [8], [9], and hybrid energy source [10], which provide promising means to realize robot swarms at the small scales. Recently, with a combination of magnetic assembly and controlled locomotion, microrobotic swarms of magnetic colloid particles with reconfigurable patterns have been reported, resulting in various applications in cargo transportation and delivery [11], [12], magnetic hyperthermia [13], anti-diffusion [14], and heavy metal removal [15]. The dynamic patterns are generated and controlled on-demand, originating from dipole–dipole magnetic interactions between building blocks and fluidic drag effects. A magnetic field is used as the external energy source due to its merits in long-range and precise actuation. Moreover, a low-frequency magnetic field can penetrate deep tissues and is harmless to biological organisms, facilitating future in vivo applications of the dynamic swarming colloid particles, such as drug delivery and thrombolysis.

The image on the cover of this issue of Materials Today shows a large-scale and multi-layer photonic crystal with periodically distributed defects obtained from the static self-assembly process of a highly concentrated superhydrophilic mesoporous silica nanospheres on a silicon wafer via evaporation. The self-assembled vortex-like pattern is built from the uniform mesoporous silica particles with a size of approximate 100?nm. A lot of line defects were formed during the evaporation of the solvent. The line defects show well-organized arrangement on the silica pattern and reach up to about 2?mm in length originating from the center of the pattern. This kind of photonic crystal, offering plenty of defects with uniform and periodical arrangements, may provide potential applications in nanophotonics, environmental sensing, and anti-fouling materials [16].

Acknowledgment:

This work was partially supported by the General Research Fund (GRF) from the Research Grants Council (RGC) of Hong Kong with Project No. 439113, 14209514, 14203715, 14218516, HKSAR Innovation and Technology Commission (ITC) with Project No. ITS/440/17FP, and MRP/036/18X. CUHK Direct Grant for Research with Project No. 4055111, and the National Natural Science Foundation of China (NSFC) (No. 51735013).

Self-assembly of nanoparticles

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