Colloid particles that form bonds to each other at specific orientations might self-assemble into all sorts of useful materials. The key — and the lock — to such binding has been discovered.
On page 575 of this issue, Sacanna et al.1 report a simple, scalable method for controlling the orientations of interactions between colloidal particles. Their technique can immediately be applied to existing processes for the self-assembly of colloidal particles. Moreover, because the resulting directional bonds are both switchable and mechanically flexible, previously inaccessible colloidal structures can now be imagined as targets for self-assembly, potentially allowing access to advanced, optically active materials.
Colloidal particles that are between roughly 100 nanometres and 1 micrometre in diameter make excellent building blocks for materials that interact strongly with light, because their size is about the same as the wavelengths of the visible spectrum. Everyone is familiar with the optical properties of colloids — the turbidity of milk and of silt-laden rivers is a consequence of the strong light scattering effected by dispersed colloid particles. If such particles self-assemble into colloidal crystals (three-dimensional arrays that have long-range order), then their turbidity is transformed into iridescence. Opals are naturally occurring examples. The optical properties of colloidal crystals can be tuned by changing their unit cells or inter-particle spacing, allowing useful materials to be made that have applications in processes such as chemical sensing2,3.
But progress towards building high-quality colloidal crystals has been slow. Although crystals in which particles are closely packed can be made, more complex arrangements, such as the tetrahedral lattice found in diamond, have proved elusive. Simulations of colloids that assume directional interactions between particles have identified pathways for assembling complex crystal structures4. However, these simulations are far ahead of reality because effective tools for controlling the direction of colloidal–particle interactions have been lacking. Currently, the best approaches are to use Janus spheres5 (microscopic particles that have two chemically or physically different hemispheres) or mixtures of oppositely charged colloids6.
Sacanna and colleagues' approach1 to directional bonds involves the use of 'lock' and 'key' particles. Their lock particles contain a dimple that can accept spherical key colloids of matching size (Fig. 1a). Generating the dimple on the lock colloid was no mean feat, and required the authors to develop some clever colloid chemistry. The yield and selectivity of the synthesis are particularly good, which is essential for future applications of the technique.
To bind the lock and key particles together, the authors exploit a force known as the depletion interaction that is unique to the colloidal scale. Depletion interactions arise when nanometre-sized polymers or particles (known as depletants) are added to colloidal solutions. Because colloidal particles are in constant random motion, they occasionally come into close proximity. When this happens, depletants are excluded from the gap between the larger colloid particles (Fig. 1b). The imbalance in depletant density inside and outside the gap sets up a difference in osmotic pressure that leads to a pairwise attraction between the colloid particles7,8.
The interaction can also be understood in terms of the volume of the colloidal system that is available to be occupied by the additives (the free volume). Depletants can't get any nearer to colloid particles than the distance of their own radius, thus creating an 'exclusion zone' around each colloid particle that depletants can't penetrate. When pairs of colloid particles come into contact, parts of their exclusion zones overlap, reducing the total volume of the colloid system that is inaccessible to depletants. Colloidal pairs are therefore attracted to each other because this increases the free volume of the depletants, a thermodynamically favourable effect.
Sacanna et al.1 recognized that the complementary binding of key particles into the dimples of locks maximizes the depletants' total free volume relative to nonspecific binding modes (in which keys bind to locks at positions other than the dimples). The attraction between the two types of colloid particle is therefore greatest along an axis that connects the centroids of the lock, the key and the dimple's radius of curvature (Fig. 1b), and forms the basis of a truly directional bond. The authors do not report the relative strength of the directional interaction compared with nonspecific binding modes, but the directional effect must be substantial, judging by the lack of nonspecific binding in their system at the lowest depletant concentrations in which lock-and-key binding is observed.
So how could such a bond be used? Sacanna et al. demonstrate that the interaction is selective — key colloid particles do not bind efficiently to locks if they are larger or smaller than the lock's pocket. This suggests that locks could pluck specifically sized colloid particles from a mixture. But this is probably not the most interesting use for this technology. Instead, the mechanical origin of the lock-and-key interaction indicates that the bond will form even when the two particles have different compositions or surface chemistry. This would therefore be a straightforward way to produce anisotropic particles, for example, that would respond to an external electric or magnetic field. Such field susceptibility would be useful for self-assembly processes, because it would allow particles to align collectively relative to the field's direction.
The fact that several locks may bind to a single key increases the possibilities of lock-and-key bonds still further. Multiple pockets on lock particles, if achievable, would introduce the colloidal equivalent of extended coordination complexes — two- and three-dimensional molecular arrays that self-assemble in fixed geometries from atoms and ligand molecules. The assembly of colloidal particles into similar arrays might allow access to desirable, but so far elusive, complex colloidal structures9.
It is possible, however, that lock and key particles will form glasses or gels, rather than regular lattices, through a process known as kinetic trapping. But the fact that each bond functions as a ball-and-socket joint — the key colloid is free to rotate in its binding pocket — should help to prevent this. Such flexibility will probably provide some essential 'wiggle room' in the later stages of particle assembly processes, in the same way that it is often helpful to be able to bend the final piece of a model aeroplane in order to manipulate it into place.
Colloid scientists are used to working in a sequential manner: first they make their building blocks, then they assemble them. In other words, once synthesized, the building blocks are fixed items that can't be changed. Happily, the building blocks of Sacanna and colleagues' system1 — the lock-and-key complexes — can be reconfigured at any point during a self-assembly process. To do this, the authors exploit the fact that the strengths of depletion interactions are linked to the size of the depletant particles. Using established methods to modulate the sizes of these particles in situ in their colloidal systems — for example, by making depletants from polymers that swell or shrink in response to temperature — the authors were able to form or break bonds on demand. But how much faster might colloidal crystals be assembled or disassembled if the colloidal particles themselves could be reconfigured on demand? This is just one issue, among many others raised by this work1, that colloid scientists will enjoy locking in on.
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Journal of Industrial and Engineering Chemistry (2018)
Scientific Reports (2018)
Journal of Molecular Liquids (2017)
Chemical Communications (2016)