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  • Mixing Nanoparticles to Make Multifunctional Materials
Oct 2013
UPTON, N.Y., Oct. 21, 2013 — A general approach for combining different types of nanoparticles opens opportunities for mixing and matching such particles with different optical, magnetic or chemical properties to form new, multifunctional materials.

The technique, developed at Brookhaven National Laboratory, involves pairing complementary strands of synthetic DNA. After coating the nanoparticles with a chemically standardized “construction platform” and adding extender molecules to which DNA can easily bind, the scientists attach the lab-designed DNA strands to the two different kinds of nanoparticles they want to link.

The natural pairing of the matching strands then self-assembles the particles into a 3-D array consisting of billions of particles. Varying the length of the DNA linkers, their surface density on particles, and other factors gives scientists the ability to control and optimize different types of newly formed materials and their properties.

DNA linkers allow different kinds of nanoparticles to self-assemble and form relatively large-scale nanocomposite arrays. This approach allows for mixing and matching components for the design of multifunctional materials. Courtesy of Brookhaven National Laboratory.

“Our study demonstrates that DNA-driven assembly methods enable the by-design creation of large-scale ‘superlattice’ nanocomposites from a broad range of nanocomponents now available — including magnetic, catalytic and fluorescent nanoparticles,” said Brookhaven physicist Oleg Gang, who led the research at the lab’s Center for Functional Nanomaterials (CFN). “This advance builds on our previous work with simpler systems, where we demonstrated that pairing nanoparticles with different functions can affect the individual particles’ performance, and it offers routes for the fabrication of new materials with combined, enhanced or even brand-new functions.”

Future applications could include quantum dots whose glowing fluorescence can be controlled by an external magnetic field for new kinds of switches or sensors; gold nanoparticles that synergistically enhance the brightness of quantum dots’ fluorescent glow; or catalytic nanomaterials that absorb the “poisons” that normally degrade their performance, Gang said.

“Modern nanosynthesis methods provide scientists with diverse types of nanoparticles from a wide range of atomic elements,” said Yugang Zhang, first author a paper on the work. “With our approach, scientists can explore pairings of these particles in a rational way.”

The team used a wide range of techniques, including x-ray scattering studies at Brookhaven’s National Synchrotron Light Source, and spectroscopy and electron microscopy at the CFN to understand the fundamental aspects of the various new materials they formed — particle shape, for example.

“In principle, differently shaped particles don’t want to coexist in one lattice,” Gang said. “They either tend to separate into different phases, like oil and water refusing to mix, or form disordered structures.”

The scientists discovered that DNA not only helps the particles mix, but it can also improve order for such systems when a thicker DNA shell around the particles is used.

They also investigated how the DNA-pairing mechanism and other intrinsic physical forces, such as magnetic attraction among particles, might compete during the assembly process, and explored how the particles were ordered in the superlattice arrays to answer the question of whether one type of particle always occupies the same position relative to the other type, or if are they interspersed more randomly.

“This is what we call a compositional order, which is important for example for quantum dots because their optical properties,” Gang said. “If you have compositional disorder, the optical properties would be different.” In the experiments, increasing the thickness of the soft DNA shells around the particles increased compositional disorder.

These fundamental principles give scientists a framework for designing new materials. The specific conditions required for a particular application will depend on the particles being used, Zhang said, but the general assembly approach would be the same.

“We can vary the lengths of the DNA strands to change the distance between particles from about 10 nanometers to under 100 nanometers — which is important for applications because many optical, magnetic and other properties of nanoparticles depend on the positioning at this scale,” Gang said. “We are excited by the avenues this research opens up in terms of future directions for engineering novel classes of materials that exploit collective effects and multifunctionality.”

The research, described in a paper published online Oct. 20 in Nature Nanotechnology, was funded by the US Department of Energy Office of Science.

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