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NANOPARTICLES
Self-assembly nds its own limits
Inorganic nanoparticles can self-assemble into uniformly sized supraparticles in a process governed by competition between electrostatic and van der Waals forces.
Paulette Clancy
The development of nanoparticle assemblies of less than 100nm was a watershed moment in the
convergence of experimental and molecular simulation studies: it marked the point at which simulation and experiment could nally operate on similar length scales. In combining these approaches, a powerful method for analysing nanoscale systems, which could uncover the molecular-scale mechanisms that underlie phenomena such as the self-assembly of nanoparticles into larger entities, was established. Writingin Nature Nanotechnology, ZhiyongTang, Sharon Glotzer, Nicholas Kotov and colleagues now report using such a combination to create a new class of self-limiting, self-assembled supraparticles1.
The assembly of nanoparticles into larger structures of specic size is achievable with physical processes such as patterning or spin coating, but the resulting assemblies can have limited stability and complexity. Chemical processing techniques, such as printing and lithography, can also be used, though these techniques can be complex and expensive. Alternatively, nature oers numerous examples of processes that produce intricate and functional structures through self-assembly, such as the folding of polypeptides into proteins and chemical crystallization. These examples have inspired researchers to develop synthetic self-assembled systems and a variety of new
functional materials, such as thin lms of covalent organic frameworks2. However, most of these systems involve self-assembly during a drying/evaporation process, andit is difficult to control the limits of the assembly process, especially when it takes place in solution.
Kotov and colleagues1 who arebased at the University of Michigan, Argonne National Laboratory and the National Center for Nanoscience and Technology, China have created a system in which uniformly sized supraparticles with diameters of up to 50nm can be synthesized from small CdSe nanoparticles with diameters of around 35nm. The size of the nal supraparticles is self-limited and related to size of the component nanoparticles.
The assembly process can be governed by thermodynamic and/or kinetic principles. Thermodynamics governs through a minimization of free energy (acombination of energetics and entropic eects) the preferred conguration of the system of particles. But this can be thwarted by kinetics essentially, the rates controlling the ordering process so that the nal conguration may...