The hybrid technique developed by Hermann Gaub and colleagues combines the precision of an atomic force microscope tip with the "stickiness" of DNA molecules that bind only selectively to other molecules. "The AFM works like a 'nanocrane' that lifts up biomolecules, just like a normal, everyday crane lifts up life-sized objects," explained team member Mathias Strackharn. "The molecules can then be brought to a construction site, where they may be arranged at will."

Gaub and colleagues began by assembling aptamers (artificial nucleic-acid receptors that have a high affinity for certain ligands and antigens) for the small fluorescent molecule malachite green. The aptamer compounds were placed in a storage area from where they were then "picked up" and transferred to the construction site using the AFM tip. The AFM tip can be moved relative to the surface with nanometre-scale precision thanks to piezo stages. DNA molecules, brought onto the construction site in the same fashion, subsequently bound to the aptamers in a selective and complementary way to complete the assembly process – seen by the fact that the malachite green molecules emitted green fluorescent light.

Enzymes and other molecules

The team says that its single-molecule cut-and-paste method would easily work for a variety of other molecules, such as enzymes for example. "In many biomolecular processes, function is a result of the interplay between several components, and order and alignment of the involved molecules often play a role," Strackharn told A famous example is the cellulosome, a complex of enzymes needed to make the energy of plant sugars available to bacteria. "We could now investigate such complex systems on the single-molecule level, using our tools to find out how the molecules interact and how function depends on parameters like intermolecular distance."

The work was reported in Nano Letters.