Collaborators from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and Columbia Engineering have discovered that single-stranded DNA chain-coated nanocubes or cube-shaped nanoparticles arrange in a zigzag pattern. This occurrence in the nanoscale nor mcroscale has not been observed before.
Shapes are packed together to form various three-dimensional (3-D) structures such as ancient pyramids to modern buildings. The shape of an object is fixed at the macroscale. On the contrary, the shape of objects at the nanoscale can be modified when coated with organic molecules such as DNA, surfactants, and polymers. Organic molecules cover a soft shell around rigid nano-objects. This flexibility of the shells allows the reshaping of these molecules called as nanoscale sculpturing.
The findings of the study were published in the journal Sciences Advances.
"Nanoscale objects almost always have some kind of shell because we intentionally attach polymers to them during synthesis to prevent aggregation," explained co-author Oleg Gang, leader of the Soft and Bio Nanomaterials Group at the Center for Functional Nanomaterials (CFN)--a DOE Office of Science User Facility at Brookhaven Lab--and professor of chemical engineering and applied physics and materials science at Columbia University. "In this study, we explored how changing the softness and thickness of DNA shells (i.e., the length of the DNA chains) affects the packing of gold nanocubes."
The team of researchers discovered that the DNA-shell-coated nanocubes arrange in a similar manner as macroscopic objects like cubes in neat layers arranged on top of each other. The difference comes in when the shell's thickness increases as it changes to a unique kind of packing.
"Each nanocube has six faces where it can connect to other cubes," explained Gang. "Cubes that have complementary DNA are attracted to one another, but cubes that have the same DNA repel each another. When the DNA shell becomes sufficiently soft (thick), the cubes arrange into what looks like a zigzag pattern, which maximizes attraction and minimizes repulsion while remaining packed as tightly as possible.
"This kind of packing has never been seen before, and it breaks the orientational symmetry of cubes relative to the vectors (directions of the x, y, and z axes in the crystal) of the unit cell," said first author Fang Lu, a scientist in Gang's group. "Unlike all previously observed packings of cubes, the angle between cubes and these three axes is not the same: two angles are different from the other one."
A crystal lattice is composed of the smallest repeating part called a unit cell. Nanoparticles position themselves in this array of points in 3-D space, also known as the crystal lattice. There could be different orientation for shaped nanoparticles based on the corners, edges, or faces. The researchers observed that there is no fixed orientation because of this zigzag packing. The basis of their arrangement is on the cubes co-existing in an ordered lattice whether they the same or complementary DNA.
The body-centered cubic (BCC) and body-centered tetragonal (BCT) are the two kinds of lattice types. Both have the same particle placement in the center and corners of the cubes. They only differ in that BCC has equal length of unit cell sides while BCT does not, according to the research.
A dual technique of electron microscopy at the CFN and small-angle x-ray scattering (SAXS) were used to visualize the shape of the cubes and their packing behavior. The first technique requires that the materials are taken out of solution but the second permits in situ to give a detailed and precise structural information. The symmetries, distances between particles, and orientations of particles were possible through the scattering data. The zigzag arrangement is confirmed through the theoretical calculations performed by the Kumar Group at Columbia.
The next target of the team is to determine whether non-cube shaped soft-shelled nano-objects also behave themselves in the same manner.
"An understanding of the interplay between shaped nano-objects and soft shells will enable us to direct the organization of objects into particular structures with desired optical, mechanical, and other properties," said Kumar.
