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Thin films of silk produce & combine with metallic particles
Aug '09
Using thin films of silk as templates, researchers have incorporated inorganic nanoparticles that join with the silk to form strong and flexible composite structures that have unusual optical and mechanical properties. This bio-enabled, surface-mediated approach mimics the growth and assembly processes of natural materials, taking advantage of the ability of biomolecules to chemically reduce metal ions to produce nanoparticles—without harsh processing conditions.

Less than 100 nanometers thick, silk-silver nanoparticle composite films formed in this process can be used as flexible mirrors. The technique could also be used to create films that reflect light in specific wavelengths, anti-microbial coatings, thin film sensors, self-cleaning coatings, catalytic materials and potentially even flexible photovoltaic cells.

“We are taking advantage of biological molecules that have the ability to bind metallic ions of silver or gold from solution,” said Vladimir Tsukruk, a professor in the Georgia Tech School of Materials Science and Engineering. “These molecules can create mono-dispersed metallic nanoparticles of consistent sizes under ambient conditions—at room temperature and in a water-based environment without high vacuum or high temperatures.”

Sponsored by the Air Force Office of Scientific Research and the Air Force Research Laboratory, the research was described August 19 at the Fall 2009 National Meeting of the American Chemical Society.

The nanoparticles produced range in size from four to six nanometers in diameter, surrounded by a biological shell of between one and two nanometers. The silk template permits good control of the nanoparticle placement, creating a composite with equally dispersed particles that remain separate. The optical properties of the resulting film depend on the nanoparticle material and size.

“This system provides very precise control over nanoparticle sizes,” said Eugenia Kharlampieva, a postdoctoral researcher in Tsukruk's laboratory. “We produce well-defined materials without the problem of precipitation, aggregation or formation of large crystals. Since the silk fibroin is mono-dispersed, we can create uniform domains within the template.”

Atomic force microscope image shows a silk film on which gold nanoparticles were grown.

Fabrication of the nanocomposites begins by dissolving silk cocoons and making the resulting fibroin water soluble. The silk is then placed onto a silicon substrate using a spin-coating technique that produces multiple layers of thin film that is then patterned into a template using a nanolithography technique.

“Because silk is a protein, we can control the properties of the surface and design different kinds of surfaces,” explained Kharlampieva. “This surface-mediated approach is flexible at producing different shapes. We can apply the method to coat any surface we want, including objects of complex shapes.”

Next, the silk template is placed in a solution containing ions of gold, silver, or other metal. Over a period of time ranging from hours to days, nanoparticles form within the template. The relatively long growth process, which operates at room temperature and neutral pH in a water-based environment, allows precise control of the particle size and spacing, Tsukruk notes.

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