How Things Look When They’re Small Can Make a Big Difference

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If you’ve ever checked a weather report or turned a map upside down to orient it, then you know the value of a good model. So does George Schatz, the Morrison Professor of Chemistry and professor of chemical and biological engineering at Northwestern University. He is among the leading authorities on developing computer models and simulations used by researchers to help explain and predict the properties of things like DNA or nanoparticles. SiS asked him to tell us more about his work.

George SchatzGeorge SchatzYou’re a computational chemist. What does that mean?
It means I’m a chemist who mostly uses computers and math instead of beakers and Bunsen burners. Specifically, I use computational science, a field of study that analyzes and solves problems with mathematical models and computers. We use complex computer simulation programs to help researchers in a broad range of disciplines from science and engineering to medicine. Let me use a sports metaphor to help make it meaningful.

Take football. With the traditional concept of theoretical study we might develop a set of mathematical models and equations to shed light on what one or a few independent players do on the field. But to describe all the players on the field, and explain the interactive relationships among and between them, and the coaches, and the field conditions, as well as to predict all the potential scenarios that could result, traditional theoretical study is at a loss. Only with computational study could we provide a mathematical model that does all that.

Can you provide an example?
Let’s use one from nanoscience, a field that explores tiny particles that are less than 100 nanometers in size. A nanometer is one billionth of a meter, which is roughly the size of a single molecule.  Nanoparticles can have anywhere from a hundred to billions of atoms or more, with each atom needing at least six equations to describe its physical state and interaction with others.  So, for even the smallest nanoparticles, a researcher needs to solve hundreds to thousands of equations at the same time to understand their behavior. And the thing about nanoscience is that when things are that size, they show some unexpected properties.  

For example, nanoparticles can be different colors than their bulk materials. Chemists have known for centuries that the tiny particles of gold appear red, while silver is yellow. But in the 1990s, new tools and techniques became available that let scientists make use of these properties. Researchers realized that these differences in color and other optical and chemical properties might enable useful applications in medicine and other potential products.

But a problem remained. 

The researchers couldn’t predict how the nanoparticles behaved differently from their bulk compounds.  Here we were, with wonderful new tools and techniques, and the only way to apply the knowledge was the same way Thomas Edison is said to have invented the light bulb – by first finding 10,000 ways not to invent it. It was like trying to build a bridge without knowing the size of the river. 

So I worked with a team of chemists at Northwestern to design a computational program that would help predict and explain the optical properties of nanoparticles for different structures. We based that program on one we stumbled across that was developed by an astro-physicist at Princeton who was studying how light was scattered by dust particles in space. We said to ourselves ‘This is fundamentally similar to our problem of how light is scattered by nanoparticles. This could work for us!’

Gold nanoparticles linked by DNA (photo courtesy of George Schatz, cover art for: J. Phys. Chem. 113, (16) 2009.)Gold nanoparticles linked by DNA (photo courtesy of George Schatz, cover art for: J. Phys. Chem. 113, (16) 2009.)What other research areas interest you?
We’re still interested in the optical properties of nanoparticles and nanoparticle assemblies, and the potential applications these have for medical technology and other breakthroughs, but we are also exploring other things including DNA structure, optical properties and DNA-based nanomaterials. 

For example, when researchers at a lab here at NU discovered that the single strands of DNA they attached to gold nanoparticles melted at a precise temperature – something that does not happen to DNA in its natural state – they immediately saw potential practical implications. But why it melted this way remained a mystery. We worked with NU chemists and have calculated an answer.

What did your research show about the melting points of gold nanoparticles connected by DNA?
We found that when DNA links gold nanoparticles together, it makes a large, multiply connected network.  If a few DNAs in the network melt, this destabilizes the entire network.  We’ve learned from this how to make crystalline materials in which the gold particles are ordered in lattices by the DNA.  This results in porous structures that are of potential interest as sponge-like media for removing pollutants.

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