Several technological applications, spanning from energy storage to biomedicine, require the preparation of new materials with a finely organised 3D nanoscale structure. To achieve such delicate structures, they must be assembled from the bottom up, much like the iconic Lego bricks. However, unlike Lego, the building blocks in these scenarios are too small to be handled directly, so they must be encouraged to self-assemble.
Reliable self-assembly methods for the fabrication of nano-structured materials with prescribed properties would bring about a manufacturing revolution in many areas of advanced technology, including, but not limited to, (bio)sensing, photonics, light harvesting, energy storage, and molecular sieving.
In a recent contribution5, the team featuring Mr R. Brady, Prof P. Cicuta, Dr L. Di Michele (University of Cambridge), and Dr N. Brooks (Imperial College London), combined the best of both worlds, introducing a new type of amphiphilic DNA nanostructures, dubbed C-Stars5, capable of effectively forming macroscopic 3D crystalline aggregates. C-Stars are simple branched motifs self-assembled from a small number of synthetic single-stranded (ss)DNA molecules (Fig. 1a). Some of these strands are modified with a hydrophobic cholesterol molecule, ultimately positioned at the tip of each DNA arm. Since DNA itself is very hydrophilic, cholesterol modifications make C-Stars amphiphilic, and capable of self-assembling through the robust frustrated phase separation mechanism. At the same time, C-Stars retain all of the advantages of DNA-based materials, including the facile design and prototyping of different shapes, the easy functionalisation and the biocompatibility.
Figure 1 demonstrates the self-assembly principle of C-Stars. The ssDNA components are dispersed in buffered aqueous solution, and heated up to 90oC. At this high temperature, the DNA double helices (duplexes) cannot form, but the hydrophobic forces are still active, causing the cholesterol-bearing ssDNA strands to cluster into micelles (Fig. 1b). As the temperature is slowly decreased, the duplexes forming the C-Star junction start to self-assemble, linking together the micelles and forming a network whose structure is determined by the shape of the C-Stars, and thus encoded in the chosen sequence of DNA bases. The resulting aggregates grow over time and coalesce (Fig. 1b), eventually forming macroscopic networks exceeding 40 μm in size, equivalent to billions of individual C-Stars. When inspected with light microscopy, the aggregates display a polyhedral shape, strongly hinting at an underlying crystalline structure (Fig. 1c).
Brady RA, Brooks NJ, Cicuta P, Di Michele L. Crystallization of amphiphilic DNA nanostructures, Nano Letters 17 (5), 3276-3281, doi:10.1021/acs.nanolett.7b00980 (2017).
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