March 09, 2017, BY Matt Hughes

A scientific breakthrough spearheaded by Bucknell Professor Brian Smith, chemistry, could improve technologies as wide-ranging as more efficient solar cells, water purification and medicinal catalysts that mimic the body's own chemical components.

The research team investigated nanomaterials called covalent organic frameworks — porous, uniform grids of molecules that can act as a scaffold for storing energy, drugs or other cargo — in two-dimensional sheets. It's a process of engineering materials "from the bottom up," Smith explained, at the molecular scale.  

"We design molecules in such a way that when you mix them in a container, they react with each other and build a structure automatically," Smith said.

Professor Brian Smith, chemistry. Photo by Dave Ashby

In the 10-year history of the developing field, scientists have created these molecular frameworks, but in an unprocessable powdered form. Smith and coworkers, for the first time, managed to suspend the framework molecules as a milky liquid called a colloid, and then to evaporate away the liquid, leaving a thin, transparent sheet. It was a major step forward in providing practical applications for the technology.  

To explain why, Smith uses the different ways that sand and glass might be utilized as a metaphor. "Sand and glass are made of the same material, silica, but you can't use sand in the same way you could use a glass plane," Smith explained. "So the challenge is to transition from a powder to the same material in a useful form."

Smith, a first-year professor at Bucknell, developed the technique as a postdoctoral researcher at Cornell University with William Dichtel and multiple undergraduate students. Further study of the material involved collaboration with researchers at Northwestern University, Florida State University and the University of California, San Diego. They published the results of their study in the January edition of the open-access journal ACS Central Science. Smith was the paper's first author. 

The result of the team's work is a kind of molecular chicken wire or honeycomb grid with an incredible amount of surface area relative to its weight. A sample of the material that weighs as little as a dollar bill has enough surface area to cover two NBA basketball courts — "lots of nooks and crannies," Smith said. The size of pores in that honeycomb pattern can be tweaked by using different molecules to assemble them, allowing chemists to engineer finely tuned filters that trap larger molecules while letting smaller ones pass through.

Stacked in sheets, similar polymers could be used to create more efficient solar cells, or to drive reactions in industrial chemistry. Smith is particularly intrigued by their potential application in water filtration and desalination.

To create a covalent organic framework, monomers like those at left combine to form circular molecules, which can then bond together to form a two-dimensional honeycomb pattern.

"I'm interested in using frameworks in water desalination to strip chemicals from the water supply," Smith said. "Freshwater only accounts for at most 3 percent of water on Earth, and human use has increased tenfold in the past 100 years, so addressing the global water supply is an outstanding challenge. I'm interested in taking these and optimizing them to create an ideal filter membrane for desalination."

While still a long way off, one of the technology's most revolutionary applications could be in medicine. "Biological enzymes drive reactions in the body, and they have pockets where the molecules fit in just so, in order to start those reactions," Smith said. "The dream is, can we design those synthetically? Rather than relying on biology, we hope to design the structure to make a perfect pocket for the chemical to fit in and react — a completely synthetic enzyme."

Smith plans to continue his materials research in his lab at Bucknell, and to provide opportunities for Bucknell student researchers to partner with him in his work. The next step, he says, is to explore creating a three-dimensional framework.

"My main interest is in how we control the structure of materials to make them do something that they never could before," he said.

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