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MIT researchers discover a new kind of chemical ‘glue’

30 May 2013

MIT researchers have developed a method for attaching molecules to metal surfaces that could find applications in medicine, electronics and other fields.

A gold surface (in yellow) with a polymer material (depicted as complex shapes of green, black and pink) attached to the gold surface by carbenes (depicted as blue spheres). Illustration: Christine Daniloff/MIT
A gold surface (in yellow) with a polymer material (depicted as complex shapes of green, black and pink) attached to the gold surface by carbenes (depicted as blue spheres). Illustration: Christine Daniloff/MIT

Over the past three decades, researchers have found various applications of a method for attaching molecules to gold; the approach uses chemicals called thiols to bind the materials together. But while this technique has led to useful devices for electronics, sensing and nanotechnology, it has limitations. Now, an MIT team has found a new material that could overcome many of these limitations.

The new approach uses a family of chemicals called 'carbenes' to attach other substances to gold — and potentially to other material surfaces as well. The work was led by MIT assistant professor of chemistry Jeremiah Johnson.

Thiols have two main limitations in binding other materials to gold, Johnson explains: The binding is relatively weak, so the attached molecules can come loose with heating, and the connection does not typically conduct electricity well, limiting use in electronic devices. 

The MIT team determined that certain carbenes can overcome both of these hurdles. In so doing, they reasoned that they might be able to open up a wide variety of applications, such as molecular electronics. 

“You could scale electronic components down to the molecular level by wiring a molecule between two electrodes,” says Johnson. “It would be the smallest possible component.” 

Others have tried to do this with thiol-based connections, but these junctions have a rather large resistance. By comparison, preliminary indications suggest that carbenes could provide highly conductive linkages.

These carbenes could function as 'surface anchors' to link many compounds to many different surface materials — a process known to chemists as 'functionalising' the surface. Johnson says you can count on one hand the number of methods you can use to functionalise surfaces, and they are different for different surfaces. "If we could find a general one, that could make a big difference,” he adds.

Carbenes — specifically, a type the MIT team calls addressable N-heterocyclic carbenes (ANHCs) — may provide such a generalised solution. While further experiments will be needed to confirm the material’s performance, the technology holds  much promise. 

It is already known that some carbenes can bond securely to a variety of metal surfaces, as well as many other materials. But there had been no investigation of their possible use as anchors, stably binding dissimilar materials.

Such combinations could be used as biosensors; for example, a molecule designed to bond with a specific biological marker could be attached to a gold wire, activating a circuit when that marker bonds with it. It could also be used to create protective surface coatings - antifouling surfaces to prevent buildup of biological deposits, or antibiotic coatings to prevent the spread of infections.

Another possible application might be to coat gold nanoparticles with a biomolecule that binds to tumours. The particles could then be heated using infrared light, killing the tumours with heat. ANHC coated surfaces could be beneficial in this regard, as they should be stable at higher temperatures, which would prevent particle degradation.  

Once specific applications are found, the material has great potential because “it’s cheap to make, and you can make it at large scale,” Johnson says.

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