Supraparticle physical chemistry

Chemistry has focused on molecules for centuries. Stitching elements picked up from the periodic table into molecules or supramolecules with the structural and functional complexity embodied in Nature is the logic and beauty of chemistry. Thanks to discrete energy band structures at different mesoscopic levels, colloidal particles nowadays add a third dimension to the periodic table. This is unfolding a fantasy of translating the language of molecular synthesis and self-assembly to colloidal particles, by which we can organize particles into structures that are usually thought to be thermodynamically and/or mechanically unstable; spatially and temporally manipulate the particle configurations to program material properties at the mesoscopic level; precisely model and study various fundamental association processes such as protein folding and crystallization occurring at the molecular level and below, and so on. In recent times we have witnessed an increasing number of efforts devoted to molecular mimetic self-assembly of colloidal particles. Various fancy jargons, such as colloidal atoms, superatoms, colloidal molecules, superparticles, supracrystals, and supraparticles, are increasingly emerging in the literature. This Themed Issue aims to provide a broad picture of this new and fast growing research trend with a strong focus on physical chemistry.

It has been known for a century and more that in an oil/water biphasic system, colloidal particles can function in a similar way to amphiphilic molecules; namely, they thermodynamically tend to adsorb on interfaces and stabilize emulsions, the so called Pickering emulsions. Binks et al. have demonstrated that particle-stabilized emulsions can exhibit a phase inversion behavior very similar to that of surfactant-stabilized ones (DOI: 10.1039/c0cp00558d and DOI: 10.1039/c0cp00581a). The interfacial activity of colloidal particles has inspired the use of a dazzling diversity of not only liquid/liquid but also solid/solid interfacial structures to direct self-assembly of colloidal particles. Ploshnik et al. have studied co-assembly of block copolymers and CdSe nanorods within ultrathin films and used the microphase separation of the former to direct the self-assembly of the latter (DOI: 10.1039/c0cp00277a). Harada et al. have studied co-assembly of chiral amphiphilic molecules and Fe₃O₄ nanoparticles in toluene and successfully constructed magnetic nanotubes as a result of cooperation of adsorption of the surfactants on nanoparticles, self-assembly of nanoparticles, and self-assembly of surfactants (DOI: 10.1039/c0cp00533a). Fakhrullin et al. have succeeded in using yeast cells to stabilize bubbles and adsorb to the surfaces of aragonite microneedles and calcite microcubes and create cell colloidosomes (DOI: 10.1039/c0cp00131g).

Distinct from surfactants that have spatially well-separated hydrophilic heads and hydrophobic tails, colloidal particles usually have isotropic surface chemistry. Although colloidal particles can function in a very similar way to surfactants at interfaces, they are only surface active and not amphiphilic, as evidenced by the fact that they cannot aggregate in liquid crystal structures as surfactants do in biphasic systems. Patching functional domains on the surfaces of colloidal particles is a prerequisite for the particles to mimic surfactant self-assembly behavior. Hwang et al. have demonstrated a electrohydrodynamic co-jetting strategy to create Janus, bi-compartmentalized, hybrid particles with one compartment loaded with magnetic nanoparticles for magnetic manipulation of particle self-assembly (DOI: 10.1039/c0cp00264j). Ye et al. have succeeded in directly synthesizing indium oxide nanoparticles with uniform but varied morphology and studied the shape effect on particle self-assembly (DOI: 10.1039/c0cp00138d). Hu et al. have reported a water-based strategy to etch silica to form exotic, hollow, silica particles with mobile Ag cores (DOI: 10.1039/c0cp00031k). Yabu et al. have demonstrated a very elegant way to directly produce polymer particles with sophisticated surface patterns and inner structures based on microphase separation of block copolymer blends (DOI: 10.1039/c0cp0011f).

Distinct from those between molecules, various weak interactions between colloidal particles directly and, most importantly, indirectly manifest themselves and result in undesirable self-assembly structures or smear out the effect of the patchy surfaces purposely created on colloidal particles. For instance, dipolar interactions can cause anisotropic self-assembly structures at different levels. The role of the thermodynamic balance of various inter-particle forces over the course of particle self-assembly is important to study. Xia et al. have carefully correlated the morphology of polyaniline particles with the thermodynamic balance between electrostatic repulsion and Van der Waals attractions between the primitive particles (DOI: 10.1039/c0cp00128g). Yang et al. have studied how electrostatic repulsion directs charged gold nanoparticles to self-assemble into chains in a controlled manner (DOI: 10.1039/c0cp00127a). Lilly et al. have created an electric double layer-like cloud of charged quantum dots to surround oppositely charged gold nanoparticles via electrostatic interactions and observed a strong photoluminescence enhancement of the quantum dots as a result of interaction with gold nanoparticles (DOI: 10.1039/c0cp00186d). Sun et al. have used cyclodextrins to direct hyperbranched polymers to soft spheres via strong host–guest inclusion complexation (DOI: 10.1039/c0cp002463e). Wang et al. have directed barium carbonate nanoparticles to self-assemble into mesocrystalline branched dumbbells using bioconjugates to alter the interaction balance between the particles (DOI: 10.1039/c0cp00819b). McDermott et al. have successfully manipulated the aggregation of oppositely charged, different, colloidal particles to form heterogeneous particle trimers (DOI: 10.1039/c0cp00254b).

Self-assembly of particles usually results in peculiar collective properties, which are neither seen in individual particles nor in bulk materials. Pileni’s tutorial perspective article has highlighted how the physical and chemical properties of 2D self-assemblies of inorganic nanoparticles are correlated with the packing parameters such as ordering degree and inter-particle spacing. Yang et al. have studied new thermoelectrical properties of Bi₂Te₃/CdTe nanosheets derived from self-assembly of CdTe nanoparticles (DOI: 10.1039/c0cp00079e). Tang et al. have produced fluorescent CdTe/polymer nanocomposites via controlled crystal growth of the nanoparticles in the polymer matrices. Lee et al. have employed photolithography to sculpt colloidal particle self-assemblies into spectrally-encoded photonic microcarriers for label-free, multiplex detection, biological assay (DOI: 10.1039/c0cp00134a). Rastogi et al. have studied sustained release kinetics of porous supraparticles with the help of microfluidic cells (DOI: 10.1039/c0cp00119h). Dechézelles et al. have purposely created planar defects in colloidal crystals to manipulate the photoluminescence of chromophores incorporated within the defect regions (DOI: 10.1039/c0cp00129e).

Unfortunately, we failed to include proper contributions focusing on characterization of self-assembly particles, which, however, is one of the most important issues in this field. Particle self-assembly in solution can be indirectly assessed via, for instance, light scattering. But direct visualization of particle self-assembly is not yet accessible, which therefore demands significant progress to be made in techniques such as electron microscopy and confocal microscopy. At the moment, the study of particle self-assembly must proceed with the great help of theoretical modeling. Sciortino et al. have contributed a numerical study of the phase diagram of self-assembly of Janus particles and indicated the possibility of directing such particles into micelles and vesicles (DOI: 10.1039/c0cp00504e).

Overall, due to the diversity of the research activities in the field of molecular mimetic self-assembly of colloidal particles and the scope of PCCP, the present Themed Issue focuses mainly on understanding of the mechanisms, especially inter-particle force balance mechanism, governing particle self-assembly. I hope that it will establish a solid physical chemistry ground for this new research to grow from. And last but not least, I wish thank all the authors for their contributions to this Themed Issue. A special acknowledgement goes to Yuandi Li, Philip Earis, and other editorial staff of PCCP for their excellent work on this Themed Issue.

 

Dayang Wang, University of South Australia, Australia.

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