Materials science

Self-assembly gets new direction

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By controlling the placement of 'sticky' patches on particles, assemblies can be made that mimic atomic bonding in molecules. This greatly expands the range of structures that can be assembled from small components. See Article p.51

Imagine trying to assemble a bookcase from parts covered in glue that stick to each other equally well wherever they touch, regardless of their relative orientations. You would quickly find the task to be extremely challenging, because the components would keep joining together in haphazard configurations, rather than fitting neatly into their intended positions. Indeed, even relatively simple structures, let alone your bookcase, are impossible to create when the interactions between individual parts lack two key properties: specificity and directionality.

Scientists working with colloids — micro- and nanoscale particles suspended in a liquid — as components of self-assembling systems have found themselves in an analogous predicament. In general, the particles are spherical and uniformly sticky across their surfaces, and they interact through nonspecific forces. The lack of specificity has been addressed by attaching single-stranded DNA molecules to particles, so that they interact only with other particles bearing complementary DNA1. But imparting directional bonding interactions to colloidal particles has remained more of a challenge. On page 51 of this issue, Wang et al.2 take the concept of DNA-mediated interactions a step further with their report of micrometre-sized particles that have symmetrically arranged, 'sticky' patches of DNA on their surfaces. The patches force the particles to interact only along certain vectors, mimicking the connectivity of atoms in molecules. This work is a major advance on earlier attempts to generate directional interactions between particles3,4,5,6,7,8,9,10,11,12, and greatly increases the sophistication of structures that can be built 'bottom up' from smaller components.

Directional interactions between atoms — a concept called valency — are common and form the basis for the rich structural complexity of many naturally occurring materials, from organic molecules to atomic lattices. In atoms, coordination environments (the arrangements of atoms, molecules or ions bound to a central atom) typically adopt highly symmetrical geometries, such as linear, triangular, tetrahedral or octahedral. Electron orbitals are responsible for the directionality of this bonding, which chemists regularly study and manipulate. It has long been the goal of many colloid scientists to synthesize 'artificial atoms' that interact with these same symmetries, in principle enabling man-made components to assemble predictably and with the same diversity as atoms4,5.

Previous attempts at particle-based valency have struggled to localize sticky patches at symmetric sites and have been limited primarily to two-sided particles6,7,8,9. Others have used different particle shapes, such as rods, triangular prisms and octahedra, as a means to break the conventional spherical symmetry of the particle, inducing interactions that loosely mimic valency10,11,12. Wang et al. have vastly expanded the morphological diversity of such structures by creating particles with up to seven symmetrically positioned patches that precisely mimic atomic orbital arrangements.

To make these colloidal cousins to atoms, Wang et al. started with n polymer spheres packed into clusters with geometries that can be tailored to resemble various polyhedra13, such as triangles (n = 3), tetrahedra (n = 4), and octahedra (n = 6). By swelling the clusters in a controlled way from the centre outwards — by treating them with styrene — then polymerizing the styrene, the authors made particles that had 'islands' of the original spheres protruding from the newly formed surface (see Fig. 1b, c of the paper2). These small, exposed regions resemble patches, and maintain the geometry of the original cluster. The authors then attached single-stranded DNA molecules to the patches, which resulted in sticky regions that mediate inter-particle binding through hybridization with complementary DNA strands attached to patches on neighbouring particles. The locations of the patches provide directionality, whereas the sequence-dependent binding of DNA imparts specificity.

With their artificial 'atoms' in hand, Wang et al. went on to synthesize artificial 'molecules' by combining mixtures of particles that have matched valencies and complementary DNA strands (Fig. 1). For example, when they mixed small monovalent 'B' particles (which have one patch) with a larger, tetravalent 'A' particle (which has four patches), they obtained an AB4 cluster in which a central A particle was surrounded by four satellite B particles in a tetrahedral arrangement. Similarly, combinations of other appropriately matched particles yielded linear (AB2) or triangular (AB3) morphologies and even copolymer arrangements — long chains of alternating A and B particles. By increasing the patch size on divalent particles so that more than one monovalent particle can bind to a single patch, the authors produced even more-complex molecular motifs, such as systems that mimic the different arrangements of atoms around a double bond.

Figure 1: Self-assembly of patchy particles.
figure1

Wang et al.2 report the synthesis of micrometre-sized particles that have surface patches (red and green regions) positioned to mimic the arrangements of bonds around atoms. The authors attached single-stranded DNA (not shown) to these patches, so that the resulting particles bind only to neighbouring particles whose patches bear the complementary DNA sequence; in the examples shown, red patches bind to green patches. When matching particles are mixed together, they self-assemble into clusters that resemble the atomic arrangements of molecules, such as linear carbon dioxide (CO2), triangular boron trifluoride (BF3) and tetrahedral methane (CH4). In the molecular structure of methane, solid wedges indicate bonds projecting above the plane of the page, whereas broken wedges represent bonds projecting below that plane.

Despite their strong analogy to atoms, these patchy particles are quite large (500 to 900 nanometres in diameter), a feature that powerfully modifies the dynamics of their interactions compared with atoms. Wang and co-workers took advantage of this size difference to monitor the formation kinetics of their artificial molecules in real time using optical microscopy (for videos in the Supplementary Information to the paper, see go.nature.com/fagxzu). Such detailed kinetic studies based on direct observation are currently impossible with atoms, and so these particles might one day function as a model system to shed light on certain aspects of the dynamic and packing behaviour of matter at the smallest length scales.

In some respects, Wang and colleagues' particles are even more amenable to tailoring than naturally occurring atoms and molecules. For example, particles of different size can be combined in a multitude of configurations; the length and sequence of the DNA molecules on a given patch can modulate the spacing and connectivity between particles; and the symmetry and number of patches on a particle can be tuned to access geometries not found in any natural system. The authors' work therefore greatly expands the toolbox for assembling systems of colloidal particles. It will be surprising if some of the newly accessible configurations do not yield mechanistic insight into atomic systems, or give rise to materials that have previously unknown properties.

A major challenge for the future will be to expand Wang and co-workers' methods to generate even more sophisticated structures, for example by controlling the geometry of patches in a way that does not follow the highly symmetrical arrangements governed by simple polyhedra. Ring-like patches, linear patches and patches confined to an equatorial plane have all been shown theoretically to exhibit fascinatingly complex, self-assembled arrangements14.

In addition, the property of chirality — the characteristic of objects that have non-superimposable mirror-image forms — may be introduced into these structures if each patch on a single particle contains a different type of DNA strand that binds to a distinct kind of particle. A tetrahedral particle could therefore capture four separate, differently sized particles, creating a cluster with a chiral centre. Control over chirality at the micro- and nanoscale is rapidly becoming a goal of great scientific interest because exotic phenomena not found in nature are thought to arise from such structural motifs15. Finally, extension of the principles developed by Wang et al. to particles of alternative compositions (such as those made from noble metals, semiconductors or oxides) will allow optical, electronic and catalytic materials to be coupled in previously impossible architectures that have potentially new emergent properties.

Wang and colleagues' work demonstrates the power of valency when applied to man-made materials, and challenges the scientific community to find methods to generalize this principle to particles at smaller and smaller length scales. With such tools in hand, scientists and engineers may one day be able to construct materials from the bottom up with a precision that makes assembling a bookcase look like child's play.

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Correspondence to Chad A. Mirkin.

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Jones, M., Mirkin, C. Self-assembly gets new direction. Nature 491, 42–43 (2012) doi:10.1038/491042a

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