Researchers have engineered tiny gold particles that can assemble into a variety of crystalline structures simply by adding a bit of DNA to the solution that surrounds them. Down the road, such reprogrammable particles could be used to make materials that reshape themselves in response to light, or to create novel catalysts that reshape themselves as reactions proceed.
“This paper is very exciting,” says Sharon Glotzer, a chemical engineer at the University of Michigan, Ann Arbor, who calls it “a step towards pluripotent matter.” David Ginger, a chemist at the University of Washington, Seattle, agrees: “This is a proof of concept of something that has been a nanoparticle dream.” Neither Glotzer nor Ginger has ties to the current research.
The dream started decades ago when chemists first discovered ways to synthesize nanoparticles, clumps of atoms below 100 nanometers in size. Researchers quickly began looking for ways to control how these particles assembled in order to build new materials from the bottom up.
Today, few materials are built from scratch. One exception is laser materials, which are used in everything from telecommunications gear to barcode scanners. But the materials, formed by adding atoms on a surface layer by layer, are very expensive to make, and severely limited in size. Assembling nanoparticles could offer a new way to grow larger—and more varied—materials cheaply. But in most cases, mastering the level of control needed to do this has turned out to be elusive.
One approach that has worked well was pioneered by chemist Chad Mirkin and colleagues at Northwestern University, Evanston, in Illinois. Mirkin’s team decorated the outer surface of gold nanoparticles with snippets of single-stranded DNA, and then used those strands like Velcro to link together neighboring particles. As the separate particles approached one another, the strands knitted themselves into the more common form of double-stranded DNA, holding the particles together. Over the years, Mirkin’s team showed that it could use this setup as a means to coax particles coated with different sequences to assemble into different types of crystals, creating powerful sensors for detecting specific DNA strands and proteins in the process. But despite the success of this approach, each time the team wanted to build a material with a new crystal orientation, it had to re-engineer its DNA linkers.
Not anymore. For their current work, Mirkin and his colleagues set out to create transmutable nanoparticles that, once formed, could be assembled into any one of a wide variety of building blocks that could then form crystalline materials. To do so, they still relied on coating gold nanoparticles with DNA. But instead of attaching single-stranded DNA to their particles, they linked “hairpins,” single strands of DNA in which the end sticking out from the nanoparticle loops back and binds to a portion of the DNA closer to the particle.
In this “closed” state, the hairpin DNA can’t bind to the DNA on other nanoparticles. But the researchers added another set of short DNA strands to their solution that were programmed to bind to the portion of the hairpins stuck to the nanoparticles. This released the end of the hairpin, creating a “sticky end” that was now free to bind to a complementary strand on another nanoparticle. The sequence of these sticky ends could be programmed to cause them to bind either to similar-sized particles or to larger ones.
The team also showed that it could combine multiple types of DNA hairpins. These could be released separately, on individual nanoparticles, thereby allowing the researchers to choose exactly which partners a nanoparticle will assemble with as a crystal grows.
The researchers also added more control over what crystals formed by changing the length of the DNA hairpins, the concentration at which they were assembled around the particles, and the concentration of different types of particles. They report in today’s issue of Science that they used these different knobs to create 10 different crystals. But according to Mirkin, his team already has the ability to cause particles to assemble into more than 500 different crystal forms. “This gives us the ability to make materials by design,” he says.
That ability should prove particularly useful in designing new optical materials, which depend on tight control over the spacing of nanoparticles in a crystal to determine what colors of light they transmit, reflect, and even emit. The new process allows for such control, Mirkin says. And much as in living organisms, it shows that a little bit of DNA can make a big difference.