“Our biological technique allows one to tune the electronic properties of nanotubes in a simple process,” researcher Hiroshi Matsui told nanotechweb.org. “Since nature always produces morphology-controlled nanocrystals in living systems with high accuracy and reproducibility, this biological approach also has an advantage over synthetic methods.”

Matsui and colleagues used histidine-rich peptides (HG12) as the template for depositing copper. They attached the HG12 peptides to the amide groups of nanotubes self-assembled from bolaamphiphile peptide monomers. Introducing a solution of copper chloride caused the HG12 peptides to coordinate copper ions, creating nucleation sites for the growth of copper nanocrystals. At this stage of the process, the scientists varied the pH between 4 and 10. Finally, the team reduced the copper ions with a solution of sodium boronhydride to create copper nanocrystals.

Copper nanocrystals grown at pH 6 had an average diameter of 10 nm. Crystals grown at a pH of 7 to 10, in contrast, had an average diameter of 30 nm. The scientists believe that, depending on the pH, the peptide sequence coordinates copper ions at different positions and folds into different conformations - this affects the size of the resulting copper nanocrystal. It’s also likely that attaching the HG12 peptides to the template nanotubes helps regulate the size of the nanocrystals.

According to Matsui, the biological nanotubes could have applications in electronics, sensors and optics. “This technique can be applied to any nanocrystal coatings whose peptide sequences are known to biomineralize certain ions,” he said. “For example, magnetic nanocrystals can be coated on peptide nanotubes that will be applied in magnetic recording media, spintronics, separation and catalysis.”

The researchers, who reported their work in the Proceedings of the National Academy of Sciences, are currently working on controlled coatings of semiconductor, magnetic and zeolite nanocrystals on peptide nanotubes.