Materials Science

Colloids get complex

A Correction to this article was published on 22 February 2006

Self-organization of soft-matter components can create complex and beautiful structures. But the intricate structures created by adding a second stage of organization could reveal more than just a pretty face.

The term ‘soft matter’ denotes materials that are easily deformed by external stresses, and encompasses liquid crystals, polymers, surfactants and colloids (particles dispersed within another medium). Their basic constituents have characteristic sizes of between several nanometres and several micrometres, and, crucially, have the potential to self-organize, forming beautiful, regular three-dimensional structures. A triplet of recent papers1,2,3 presents the latest such structures: complex colloids formed through self-organization on scales up to a micrometre.

Alternative terms that have been used to describe these colloid structures — ‘colloidal molecules’, or ‘patchy particles’ — hardly do justice to their intricacy. What is considered a complex colloid is, admittedly, somewhat arbitrary: the colloidal ‘ice-cream cones’ (Fig. 1a) produced some years ago4, which resulted from repeated polymerization and the subsequent phase separation of the polymers formed, would certainly have merited the term complex colloid. The innovation of recent efforts, however, is that structures are being designed with a second stage of self-organization in mind. Such an approach, in which colloidal particles are first formed at soft-matter scales, and then built up to far more intricate structures, should allow unprecedented control over the three-dimensional organization of materials, as well as the combination of different materials over several length scales.

Figure 1: A selection of complex colloids achieved by various means of self-organization.
figure1

a, ‘Ice-cream cones’ resulting from repeated polymerization and phase separation between polymers of different composition4. b, c, Through controlled drying of a binary dispersion in water-in-oil emulsion droplets1; b, both colloids same charge; c, colloids with opposite charge. d, Through silica deposition on liquid crystal phases formed by surfactants2. e, Through osmotic stress deformation of thin hybrid siloxane shells after growing them on monodisperse oil droplets3.(Courtesy of: a, John Wiley, Inc.; b, c, American Chemical Society; d, C. M. van Kats, D. C. 't Hart and J. D. Meeldijk; e, C. I. Zoldesi and A. Imhof. All scale bars are approximate.)

The results of Cho et al.1, published in the Journal of the American Chemical Society, exemplify the fruits of this technique. The authors created complex colloidal structures (Fig. 1b, c) by drying emulsion droplets containing “bidisperse” charged colloids, consisting of components of two quite different sizes — one on the nanometre and one on the micro-metre scale. Using the same or opposite charges on the two components, an amazing richness of structural motifs could be obtained. Equally impressive results have been published by Lin et al.2 (Fig. 1d) in Chemistry of Materials and by Zoldesi and Imhof3 (Fig. 1e) in Advanced Materials. Their structures were fabricated by depositing silica on liquid crystals formed by surfactants2, and through the regular deformation by osmotic stresses of thin siloxane shells grown around emulsion droplets that are monodisperse (all the same shape and size)3

It is important to mention at this point that complex shape is no prerequisite for complex interaction. The adsorption of charged molecules on colloids with sizes of the order of micrometres5 and several nanometres6 has been shown, for example, to result in charges small enough that ionic colloidal crystals — crystals comprising particles of opposite charge — can form. The added complexity of the interactions between oppositely charged spheres, compared with components with the same charge, has already resulted in the number of different types of binary colloidal crystal that have been fabricated doubling within a few months5,6.

Starting self-assembly with complex colloids, however, offers increased possibilities. This is nicely demonstrated by a modest goal that many groups are aiming for: the creation of colloidal crystals with diamond symmetry. Such structures could be used to create a photonic crystal with a robust ‘band gap’ that can inhibit the propagation of light and modify the spontaneous emission of photons at visible wavelengths. These crystals are potentially as useful for manipulating the flow of light waves as semiconductor crystals have become for manipulating the flow of electrons. The interest in these three-dimensional structures is so great that they have even been assembled do-it-yourself style by placing thousands of colloids into a diamond lattice one by one7. But, whereas just a few years ago most considered the creation of diamond lattices by self-assembly to be wishful thinking, several approaches now seem to make it an immediate prospect. Some proposed schemes, based on theory and computer simulations, make use of complex spherical colloids with either tetrahedrally arranged attractive patches8 or, remarkably, non-additive spherically symmetric potentials that might be produced using particles coated with complementary strands of DNA (that is, that can bind together to form double-stranded DNA)9.

For a second self-organization step to succeed in any system, all particles must be monodisperse and the yield of the first step must be high. The polydisperse emulsion droplets created by Cho et al.1 are therefore at present unsuited for self-organization into more complex three-dimensional structures, because no two colloid particles that they produce are exactly the same. But the path to monodispersity, clearly outlined by the authors, should not be too arduous.

Going even further than this, a crystalline arrangement of smaller monodisperse nanoparticles between larger spheres, or a mixture of oppositely charged and monodisperse nanocrystals of different composition, could be created. Transistors with a conventional two-dimensional layout can be made from self-organized colloidal crystals of nanoparticles10: by self-organizing such semiconducting colloidal crystals between larger colloids, such as those devised by Cho et al., individual transistors could be arranged into regular, three-dimensional structures. A three-dimensional wiring system could also conceivably be established by using spheres that comprise a conducting and an insulating part as the larger building blocks, allowing each transistor to be addressed individually as it sits in the lattice.

Even though the complex colloidal structures now being created are beginning to show a faint resemblance to the beautiful silica structures produced by diatoms, natural examples of self-organization on multiple length scales and from different materials are generally still far ahead of any human design. Such structures are always available in case we run out of inspiration11.

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van Blaaderen, A. Colloids get complex. Nature 439, 545–546 (2006). https://doi.org/10.1038/439545a

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