How to attach molecules to gold nanoparticles

Method for attaching molecules to metal surfaces could find applications in medicine, electronics and other fields.
June 3, 2013

Diagram shows a gold surface (in yellow) with carbene anchors (green) attaching polymer molecules (purple ribbons) to the surface. MIT researchers found that such carbene anchors can be used to attach many different kinds of materials to a variety of surfaces. (Credit: Jeremiah Johnson et al./MIT)

An MIT team has found a new material that could overcome many of the limitations of current methods for attaching molecules to gold.

The new approach uses a family of chemicals called carbenes to attach other substances to gold — and potentially to other material surfaces as well.

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 envisioned that certain carbenes can overcome both hurdles. In so doing, they would enable a wide variety of applications, says assistant professor of chemistry Jeremiah Johnson, such as in the development of molecular electronics.

Molecular electronics

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

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

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

Biological markers

It was already known, Johnson says, 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.

Attacking tumors

Another possible application might be to coat gold nanoparticles with a biomolecule that binds to tumors. The particles could then be heated using infrared light, killing the tumors 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.

The work received funding from MIT’s Deshpande Center for Technological Innovation, the MIT Lincoln Laboratory, and the U.S. Department of Defense.