An image of the combinatorial library of polyelemental nanoparticles created using dip-pen nanolithography. This novel nanoparticle library opens up a new field of nanocombinatorics for the rapid screening of nanomaterials for specific properties. Image: Peng-Cheng Chen/James Hedrick.A research team from Northwestern University has developed a tool with the potential for simultaneously testing millions and perhaps even billions or more different nanoparticles in order to identify the best particle for a specific task.
When materials are miniaturized, their properties – optical, structural, electrical, mechanical and chemical – change, offering new opportunities. But determining what kind of nanoparticle, in terms of size and composition, would perform best for a given application, such as catalysis, biodiagnostic labels, pharmaceuticals and electronic devices, is a daunting task.
"As scientists, we've only just begun to investigate what materials can be made on the nanoscale," said Chad Mirkin, professor of chemistry at Northwestern University and founding director of Northwestern's International Institute for Nanotechnology, who led the study. "Screening a million potentially useful nanoparticles, for example, could take several lifetimes. Once optimized, our tool will enable researchers to pick the winner much faster than conventional methods. We have the ultimate discovery tool."
Using a Northwestern technique for depositing materials on a surface, Mirkin and his team figured out how to make combinatorial libraries of nanoparticles in a very controlled way. (A combinatorial library is a collection of systematically-varied structures encoded at specific sites on a surface.) Their work is presented in a paper in Science.
The nanoparticle libraries are much like a gene chip, Mirkin says, where thousands of different spots of DNA are used to identify the presence of a disease or toxin. Gene chips allow thousands of reactions to be performed simultaneously, providing results in just a few hours. In a similar way, the libraries developed by Mirkin and his team will allow scientists to synthesize millions to billions of nanoparticles of different compositions and sizes, and then rapidly screen them for desirable physical and chemical properties.
"The ability to make libraries of nanoparticles will open a new field of nanocombinatorics, where size – on a scale that matters – and composition become tunable parameters," Mirkin said. "This is a powerful approach to discovery science."
"I liken our combinatorial nanopatterning approach to providing a broad palette of bold colors to an artist who previously had been working with a handful of dull and pale black, white and grey pastels," said co-author Vinayak Dravid, professor of materials science and engineering in Northwestern University’s McCormick School of Engineering.
Using five metallic elements – gold, silver, cobalt, copper and nickel – Mirkin and his team developed an array of unique structures by varying every elemental combination. In previous work, the researchers had shown that particle diameter can also be varied deliberately over a scale of 1–100nm.
Some of the resultant structures can be found in nature, but more than half of them have never existed before on Earth. And when pictured using high-powered imaging techniques, the nanoparticles appear like an array of colorful Easter eggs, each compositional element contributing to the palette.
To build their combinatorial libraries, Mirkin and his team used dip-pen nanolithography, a technique developed at Northwestern in 1999, to deposit individual polymer ‘dots’, each loaded with different combinations of metal salts, onto a surface. The researchers then heated the polymer dots, reducing the salts to metal atoms and forming a single nanoparticle. The size of the polymer dot can be varied to change the size of the final nanoparticle.
This control over both the size and composition of the nanoparticles is very important, Mirkin stressed. Having demonstrated control, the researchers used the tool to systematically generate a library of 31 nanostructures made from the five different metals.
To help analyze the complex elemental compositions and size/shape of the nanoparticles at the sub-nanometer scale, the team turned to Dravid, Mirkin's long-time friend and collaborator. Dravid, founding director of Northwestern's NUANCE Center, contributed his expertise and the advanced electron microscopes at NUANCE to spatially map the compositional trajectories of the combinatorial nanoparticles.
Now scientists can begin to study these nanoparticles as well as build other useful combinatorial libraries consisting of billions of structures that subtly differ in size and composition. These structures may become the next materials for powering fuel cells, efficiently harvesting solar energy and converting it into useful fuels, and catalyzing reactions that take low-value feedstocks from the petroleum industry and turn them into high-value products useful in the chemical and pharmaceutical industries.
This story is adapted from material from Northwestern University’s McCormick School of Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.