Nano-objects of desire: Assembling ordered nanostructures in 3D

"One of the major reasons why SA is not a technique of choice for practical applications is that the same SA process cannot be applied across a broad range of materials to create identical 3-D ordered arrays from different nanocomponents," explained corresponding author Oleg Gang, leader of the Soft and Bio Nanomaterials Group at the Center for Functional Nanomaterials (CFN) -- a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory -- and a professor of Chemical Engineering and of Applied Physics and Materials Science at Columbia Engineering. "Here, we decoupled the SA process from material properties by designing rigid polyhedral DNA frames that can encapsulate various inorganic or organic nano-objects, including metals, semiconductors, and even proteins and enzymes."

The scientists engineered synthetic DNA frames in the shape of a cube, octahedron, and tetrahedron. Inside the frames are DNA "arms" that only nano-objects with the complementary DNA sequence can bind to. These material voxels -- the integration of the DNA frame and nano-object -- are the building blocks from which macroscale 3-D structures can be made. The frames connect to each other regardless of what kind of nano-object is inside (or not) according to the complementary sequences they are encoded with at their vertices. Depending on their shape, frames have a different number of vertices and thus form entirely different structures. Any nano-objects hosted inside the frames take on that specific frame structure.

To demonstrate their assembly approach, the scientists selected metallic (gold) and semiconducting (cadmium selenide) nanoparticles and a bacterial protein (streptavidin) as the inorganic and organic nano-objects to be placed inside the DNA frames. First, they confirmed the integrity of the DNA frames and formation of material voxels by imaging with electron microscopes at the CFN Electron Microscopy Facility and the Van Andel Institute, which has a suite of instruments that operate at cryogenic temperatures for biological samples. They then probed the 3-D lattice structures at the Coherent Hard X-ray Scattering and Complex Materials Scattering beamlines of the National Synchrotron Light Source II (NSLS-II) -- another DOE Office of Science User Facility at Brookhaven Lab. Columbia Engineering Bykhovsky Professor of Chemical Engineering Sanat Kumar and his group performed computational modeling revealing that the experimentally observed lattice structures (based on the x-ray scattering patterns) were the most thermodynamically stable ones that the material voxels could form.

"These material voxels allow us to begin to use ideas derived from atoms (and molecules) and the crystals that they form, and port this vast knowledge and database to systems of interest at the nanoscale," explained Kumar.

Gang's students at Columbia then demonstrated how the assembly platform could be used to drive the organization of two different kinds of materials with chemical and optical functions. In one case, they co-assembled two enzymes, creating 3-D arrays with a high packing density. Though the enzymes remained chemically unchanged, they showed about a fourfold increase in enzymatic activity. These "nanoreactors" could be used to manipulate cascade reactions and enable the fabrication of chemically active materials. For the optical material demonstration, they mixed two different colors of quantum dots -- tiny nanocrystals that are being used to make television displays with high color saturation and brightness. Images captured with a fluorescence microscope showed that the formed lattice maintained color purity below the diffraction limit (wavelength) of light; this property could allow for significant resolution improvement in various display and optical communication technologies.

"We need to rethink how materials can be formed and how they function," said Gang. "Material redesign may not be necessary; simply packaging existing materials in new ways could enhance their properties. Potentially, our platform could be an enabling technology 'beyond 3-D printing manufacturing' to control materials at much smaller scales and with greater material variety and designed compositions. Using the same approach to form 3-D lattices from desired nano-objects of different material classes, integrating those that would otherwise be considered incompatible, could revolutionize nanomanufacturing."