Understanding the Chemistry of Complex Molecules on Surfaces
Charles Sykes, PhD, joined the Department of Chemistry in 2005. His research group uses low-temperature scanning tunneling microscopy (STM) to visualize and control single atoms and molecules on conductive surfaces. “We can zoom in on a metal surface and see each individual atom,” says Sykes. “We can work on different molecular systems on top of those metals and build structures, and even get sub-molecular resolution and see where different parts of the molecule are and see how it moves.” Sykes is using STM on a variety of projects, including molecular motors, chiral drugs, and improved catalysis.
Sykes is the Usen Family Career Development Assistant Professor. He earned a combined BS/MS in chemistry from Oxford University and a PhD in chemistry from Cambridge University. He did postdoctoral research with Paul Weiss at Pennsylvania State University and with Michael Fiddy at the University of North Carolina – Charlotte.
“One of the projects we’re working on is looking at the rotation of individual molecules and how we could eventually build up to have a molecular rotor and motor,” says Sykes. “In nature, your body is full of molecular motors that move stuff around cells; they can even move whole cells. But apart from liquid crystals that are in LCD watches, mankind doesn’t have any type of molecule that functions like a useful motor.” Sykes and his research group are currently working on controling molecular rotation. They can start and stop the rotation of an individual molecule by altering its temperature, its supply of electrons, or its proximity to other molecules. For example, at near absolute zero (0 Kelvin, -273° Celsius) a thioether molecule anchored to a gold surface is stationary. At 80 K, the molecule spins at about 10 million times per second. The group can also make the molecule spin by giving it pulses of electricity with the microscope, and they can “brake” it by moving it closer to a chain of static molecules. Currently, the spinning switches randomly from clockwise to anticlockwise. The group is working on methods to get directed rotation. Sykes would be interested in discussing his molecular motor work with a physicist with expertise in the interactions of electrical and magnetic fields, and how these fields affect molecules.
Another project the Sykes group is working on is based on the discovery by a colleague that metal crystals can be cut to create surfaces that are chiral, that is, left- or right-handed. You can think of the surface as a layer of left- or right-handed gloves. “In biology, all your DNA and amino acids are one chirality and not the other,” says Sykes. “So if you take a drug, if it’s the wrong handedness, it can be dangerous or not do the right job.” He and his colleagues are looking at ways to use chiral surfaces to separate the two chiral forms of a drug or to specifically grow crystals of one chiral form or the other. The pharmaceutical industry currently has methods to do this, but any increase in the efficiency of the process could dramatically decrease the cost of drug production. “The most exciting thing about these surfaces is that in a square centimeter of surface, there are about 10 million million chiral sites, all of the same chirality,” says Sykes. “So if you put in molecules, you could have 10 million million molecules all interacting at chiral sites of the same symmetry. We’re now looking at how molecules interact [with these chiral surfaces]. Can you separate molecules that way or could you even put reactive gases on there and make chiral molecules? It is a very new field.”
A third Sykes project is related to increasing the efficiency of catalyzing certain chemical reactions. Palladium and platinum are good catalysts but are very expensive. “What we discovered is that individual palladium atoms will do the whole reaction; you don’t need a lot of palladium or platinum. You just need a good alloy surface because you can spread them out on the surface layer,” says Sykes. The group uses a copper surface and a tiny amount of palladium to catalyze reactions. They recently discovered that a single atom of palladium can grab a hydrogen molecule, split it apart, then pass the hydrogen atoms off onto the copper surface, where a second reaction could occur. “That’s catalytically interesting because you can put two catalysts together—the palladium does one reaction, the copper does another—and then you get a product,” explains Sykes. His research group is working to use this process to reduce the cost of producing hydrogen gas as an alternative energy source.
The Sykes lab has two low-temperature scanning tunneling microscopes. He can offer the unique capability of these microscopes (fewer than 100 exist in the world) to Tufts researchers who have a need for atomic resolution and an application that can function at extremely low temperatures. Test surfaces are usually metal but can also be oxides or semiconductive surfaces such as silicon. Very thin layers of an insulator such as sodium chloride can also be used. Sykes welcomes discussion of all of his projects with interested researchers.
For more information, please go to http://ase.tufts.edu/chemistry/sykes.