The properties of all bulk materials depend on the properties of their building blocks, and the design of new materials relies on our understanding of this connection. Atoms and small molecules are the usual constituents that are considered, but the very specific structuring and bonding that are adopted in bulk materials makes a systematic study of the relationship between bulk materials and their building blocks challenging. Using more complex building blocks, for which it is possible to have complete control over shape and bonding, may make such a study achievable. For example, recent developments in DNA origami technology have enabled the assembly of DNA molecules into particles with almost any chosen shape. Writing in Nature Materials, Zvonimir Dogic, Hendrik Dietz and colleagues examine the link between the microscopic properties of DNA origami and the macroscopic properties of the resulting bulk materials — in this case colloidal liquid crystals.

Liquid crystals exhibit properties of both solids and liquids, offering huge technological potential, such as in liquid crystal displays. In cholesteric liquid crystals, the chirality of a constituent confers to the liquid crystal a helical structure and therefore an overall chirality. The helical structure arises from the arrangement of the constituent particles in layers within which they orient along a particular direction, which varies periodically between the layers. The period of this variation is termed pitch and corresponds to a specific wavelength of light that is reflected by the liquid crystal, which can be easily detected. “Despite the technological importance of these materials, there is at present neither a quantitative nor qualitative understanding of the relationship between the tendency of liquid crystalline materials to adopt macroscopic twist and the microscopic chirality of the constituent units,” explains Dogic. “One of the main obstacles to progress in this area has been the inability to continuously tune the chirality of the constituent building blocks. However, using the unique features of DNA origami we have been able to design a set of DNA filaments in which we can directly control the magnitude of the twist along the filament's long axis.”

Credit: Adapted with permission from Siavashpouri, M. et al. Nat. Mater. http://dx.doi.org/10.1038/NMAT4909 (2017), Macmillan Publishers Limited.

One of the main obstacles to progress in this area has been the inability to continuously tune the chirality of the constituent building blocks

Dogic, Dietz and co-workers produced and assembled DNA rods with twists of different degrees and directions, and demonstrated that the chirality of the rods influences the pitch of the cholesteric bulk material. For example, they noted that straight DNA rods produced a right-handed cholesteric phase, whereas those with a right-handed twist led to cholesteric phases with smaller pitch. In addition, they showed that by changing the composition of a mixture of DNA filaments with opposite twist they could tune the macroscopic chirality of the cholosteric phase.

Next, the researchers investigated whether the usual synthetic methods that are used for colloidal self-assembly could be applied to producenew materials that are based on assembled DNA origami. By changing the ionic strength of the solution and the strength of the attractive interactions, they were able to generate a series of colloidal structures: from 1D twisted ribbons, which can reach hundreds of micrometres in length, to 2D colloidal membranes. Furthermore, they were able to demonstrate that the structure, chirality and elastic properties of 1D twisted ribbons could be precisely engineered by tuning the geometry of the constituent filaments. “Twisted ribbons and amyloid fibres are ubiquitous structural motifs that are present in misfolded proteins, block copolymers and complex surfactants. Usually, their assembly in aqueous solvents is driven by the hydrophobic segments of the structural building blocks. However, we demonstrate a fundamentally different pathway for the assembly of 1D twisted ribbons, which does not rely on the chemical heterogeneity of the building blocks, but on the geometry of the elemental units,” explains Dogic. Initial experiments have demonstrated that two very different systems — DNA origami and filamentous bacteriophage — can follow the same assembly pathways, which indicates that the present findings could have a universal nature.