DNA Origami as Patchy Colloids—Diamond Cubic Lattice with a Photonic Band Gap in UV

Rational design of the structure of matter on the sub-micron scale can lead to interesting and fundamental material properties, for instance sub-wavelength periodicity of dielectric structures leads to photonic band gaps in photonic crystals(<i>1</i>). The diamond cubic lattice has long been known to possess one of the widest and experimentally most accessible photonic band gaps. Building such a lattice on the scale of visible wavelengths has proven difficult because either its small feature sizes were not accessible through lithographic methods, or its non-close packed and mechanically unstable structure was challenging to achieve with self-assembly. Tetravalent colloidal particles are the most obvious candidates for building blocks of such a structure, however they are difficult to manufacture and indiscriminately crystallise into either diamond cubic or hexagonal diamond structures(<i>2</i>). Use of torsional potential on the binding patches was proposed to bias their crystallisation exclusively to the diamond cubic lattice(<i>3</i>), but this makes the implementation of the tetravalent particles even more challenging.<br/><br/>We have solved both issues with the use of DNA origami(<i>4</i>). DNA origami uses the predictable binding of complementary DNA sequences of short oligonucleotides to fold a long (~8000nt) single-stranded DNA scaffold into almost arbitrary shapes on the scale of tens of nanometers(<i>5</i>, <i>6</i>). With this method we rationally designed four-legged tetrapods with a binding patch at the end of each of the legs. The binding was implemented by DNA overhangs that were positioned in a pattern with a three-fold symmetry to enforce a 60° rotation between neighbours as needed for the diamond cubic structure. With tuning of the binding sequences and crystallisation conditions we were able to grow crystals up to 50 micrometers in size with a periodicity of 170 nm. The pores in the crystals are about 100 nm in diameter, making them an interesting high surface-to-volume ratio material which could be used for catalysis or energy storage.<br/><br/>After crystallisation we mechanically stabilised the crystals by silicifying them with a sol-gel reaction and employed atomic layer deposition for conformal coatings of metal oxides. Deposition of thin layers of high refractive index materials like TiO₂ led to the appearance of a photonic band gap for wavelengths around 300 – 350 nm. The position of the photonic band gap redshifted with increasing thickness of TiO₂ coating as predicted by our numerical calculations.<br/><br/>1. C. A. Mirkin, S. H. Petrosko, Inspired Beyond Nature: Three Decades of Spherical Nucleic Acids and Colloidal Crystal Engineering with DNA. <i>ACS Nano</i> <b>17</b>, 16291–16307 (2023).<br/>2. F. Romano, E. Sanz, F. Sciortino, Crystallization of tetrahedral patchy particles in silico. <i>J. Chem. Phys.</i> <b>134</b>, 174502 (2011).<br/>3. Zhang, A. S. Keys, T. Chen, S. C. Glotzer, Self-Assembly of Patchy Particles into Diamond Structures through Molecular Mimicry. <i>Langmuir</i> <b>21</b>, 11547–11551 (2005).<br/>4. G. Posnjak, X. Yin, P. Butler, O. Bienek, M. Dass, S. Lee, I. D. Sharp, T. Liedl, Diamond-lattice photonic crystals assembled from DNA origami. <i>Science</i> <b>384</b>, 781–785 (2024).<br/>5. P. W. K. Rothemund, Folding DNA to create nanoscale shapes and patterns. <i>Nature</i> <b>440</b>, 297–302 (2006).<br/>6. S. M. Douglas, H. Dietz, T. Liedl, B. Högberg, F. Graf, W. M. Shih, Self-assembly of DNA into nanoscale three-dimensional shapes. <i>Nature</i> <b>459</b>, 414–418 (2009).