PATEL: Mirkin and his colleagues are trying to dramatically speed up materials discovery. They have created a tool: a “megalibrary.” It’s an array of millions of little nanoparticles that fits on a microscope slide. Each nanoparticle has a different structure and composition.
CM: We’ve made libraries with as many as 5 billion particles and a million different variants, compositional and size variants.
PP: They make the libraries using a technique called polymer pen lithography. They use hundreds of thousands of tiny, sharp tips to deposit dots of polymer on the slide. Each dot is a little reactor, loaded with different metal salts. Heating the slide eliminates the polymer and reduces the metal ions.
CM: The atoms within one nanoreactor will coalesce, aggregate, and form a single nanoparticle. We can make structures with as many as eight different elements within one particle and every combination of those structures. It’s a very, very tiny reactor—a very, very tiny volume. That means you can confine within literally a countable number of atoms, which gives you the ability to precisely control the composition. And then if I control the size of the reactor I can also control particle size. So this is quite sophisticated, and opens the ability to look at the nanomaterial landscape like we’ve never had before. We’ve already used this approach to discover a new catalyst for single-walled carbon nanotubes. That was done with Wright Patterson Air Force Base. There was a sweet spot in terms of the compositional ratio that gave us greatest activity. All of that was refined and figured out by first making a megalibrary that had a systematic variation in particle size and also amounts of copper and gold.
PP: The researchers synthesized these catalysts in larger amounts to test them to make sure they worked. It took less than a week with this process to discover a completely new catalyst. Mirkin’s team has also used megalibraries to identify new catalysts for the hydrogen evolution reaction. These catalysts could help develop next-generation fuel cells that don’t have expensive platinum catalysts.
CM: In this case they were three-element nanoparticles that nobody, again, even contemplated before. So the question is what are you going to discover? “Almost anything” is the answer. Any structure that might lead to a property of interest. This is a way of discovering new nanomaterials with exciting physical properties and chemical properties. This is a completely new territory in terms of materials discovery.
The International Institute for Nanotechnology at Northwestern University is an umbrella organization that represents and unites more than $1 billion in nanotechnology research, education and supporting infrastructure.