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Supramolecules are super for sensing

Postdoctoral associate Preston Snee, graduate student Aetna Wun and Professor Daniel G. Nocera (left to right) use a laser microsensor.
Postdoctoral associate Preston Snee, graduate student Aetna Wun and Professor Daniel G. Nocera (left to right) use a laser microsensor.
Photo / Donna Coveney

Large molecules composed of subunits designed to perform specific tasks can detect pollutants in water, help jets maneuver at high speeds and improve the efficiency of internal combustion engines, Professor of Chemistry Daniel Nocera said during an Earth System Initiative seminar on Friday.

Nocera's laboratory creates novel techniques and devices based on these supramolecules for chemical and physical sensing from the nanoscale to the megascale. "All sensing applications are based on our ability to control energy flow in newly created molecules and materials," he said. In the case of optical supramolecules, the molecules act as a complex system and perform their function when they interact with light.

The Earth System Initiative ( is a new research and education initiative that relates science and technology to the evolution, status and future of the Earth's systems. In the Sept. 26 seminar, Nocera described work that combines chemistry, optical physics and engineering.

By very precisely controlling energy flow in supramolecules, researchers can get the molecule to light up in the presence of pollutants. The Air Force, for instance, is interested in dealing with the problem of jet fuels contaminating groundwater. The goal is to create a 100-square-mile field full of tiny sensors that will shine bright green in the presence of the pollutant in groundwater plumes.

Nocera's research team created what he calls a "sugar-coated Dixie cup"--actually a tiny sensing supramolecule that captures the pollutant. In the cup, the pollutant absorbs light and passes the energy along to a light-emitting sensor strapped across the bottom of the cup.

When the optical supramolecules are reduced to the nanoscale, you just can't create enough glow to see the result with the naked eye. The next step the team tackled was to replace the emitting center with a laser at the molecular-length scale. The laser turns off and on when the pollutant enters the "Dixie cup."

Optical supramolecules also come in handy to measure the speed and direction of flow at a boundary interface such as where the ocean meets the air or where the air meets a plane wing. Nocera invented the molecular tagging velocimetry (MTV) technique by introducing optical supramolecules into flows.

By shining a laser on the flow, a glowing net can be created from light emanating from the supramolecules. Nocera can determine the vorticity by watching the net get stretched and pulled by the flow.

Using the MTV technique, Nocera has investigated how to manage the movement of a dense slurry of coal and water through pipelines for the U.S. Department of Energy. He also has implemented the MTV technique in automobile engines.

In internal combustion engines, fuel burns more cleanly if the air and fuel mix thoroughly. Again, vorticity maps from the MTV technique identified a place where flow got chaotic and inefficient. Placing a 400MHz speaker near the intake valve created enough vibration to change the flow and make the engine burn fuel more cleanly.

Likewise, jet wings develop air turbulence at their leading edges that affects their maneuverability. The MTV map revealed the entire turbulence flow field at the front edge of the wing boundary. The answer to eliminating the turbulence? Little rearward-facing steps on the wing that act like fish scales, which allow fish to reduce drag as they zip through the water. The same idea works on airplane wings.

A version of this article appeared in MIT Tech Talk on October 8, 2003.

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