Casting inorganic structures with DNA molds
Authors: Sun Wei , Boulais Etienne , Hakobyan Yera , Wang Wei Li , Guan Amy , Bathe Mark , Yin Peng 
Published: October 2014

Introduction
The ability to manufacture inorganic nanoparticles (NPs) with arbitrarily prescribed three-dimensional (3D) shapes and positional surface modifications is essential to enabling diverse applications (e.g., in nano-optics and biosensing). However, it is challenging to achieve 3D arbitrary user-specified shapes with sub–5-nm resolution. Top-down lithography has limited resolution, particularly for 3D shapes; capping ligands can be used to tune the energy difference of selected crystallographic facets, but typically only for highly symmetric shapes with identical surface facets.

Rationale
We developed a framework to program arbitrary 3D inorganic NPs using DNA, which serves both as an informational “genome” to encode the 3D shape of a NP and as a physical “fabricator” to retrieve the information and execute the instruction to manufacture the NP. Specifically, our method uses a computationally designed, mechanically stiff synthetic DNA nanostructure with a user-specified cavity as a “mold” to cast the target inorganic NP. The mold encloses a small gold (Au) “seed.” Under mild conditions, the Au seed grows into a larger metal NP that fills the entire cavity, thereby replicating its prescribed 3D shape. The remaining DNA mold additionally acts as a spatially programmable functionalization surface.

Results
Using this DNA nanocasting method, we constructed three distinct sub–25-nm 3D cuboid silver (Ag) NPs with three independently tunable dimensions. The shape versatility of DNA-based nanocasting was further demonstrated via the synthesis of Ag NPs with equilateral triangular, right triangular, and circular cross sections. The material versatility was demonstrated via synthesis of a Au cuboid in addition to the Ag NPs. The DNA mold served as an addressable coating for the casted NP and thus enabled the construction of higher-order composite structures, including a Y-shaped Ag NP composite and a quantum dot (QD)–Ag-QD sandwiched structure through one-step casting growth.
We investigated the key design parameters for stiff DNA molds through mechanical simulations. Multilayered DNA molds provided higher mechanical stiffness for confining NP growth within the mold than single-layer DNA molds, as confirmed by experimental observation.
We additionally characterized plasmonic properties of the designer equilateral Ag triangle and Ag sphere through electron energy loss spectroscopy. Tuning of particle symmetry produced a shape-specific spectrum, which is consistent with the predictions of electromagnetism-based simulations.

Conclusion
DNA nanocasting represents a new framework for the programmable digital fabrication of 3D inorganic nanostructures with prescribed shapes, dimensions, and surface modifications at sub– 5-nm resolution. The key design strategy is to encode linear sequences of DNA with the sophisticated user-specified 3D spatial and surface information of an inorganic NP, as well as to retrieve and execute the information to physically produce this structure via geometric confinement. Such a method may lead to computationally designed functional materials for the digital manufacture of optical nanocircuits, electronic nanocomputers, and perhaps even sophisticated inorganic nanorobots, each with their blueprints (or “genomes’’) encoded in the DNA molecules that constitute their “nanofabricators.”