14 June 2012
First genetically evolved semiconductors created
by Will Parker
By directing the evolution of silicateins, the proteins responsible for the formation of silica skeletons in marine sponges, scientists at the University of California Santa Barbara have synthesized novel semiconducting materials.
"In the realm of human technologies it would be a new method, but it's an ancient approach in nature," said Lukmaan Bawazer, the first author of a paper detailing the work in the Proceedings of the National Academy of Sciences.
Bawazer said that silicateins, which are genetically encoded, serve as templates for the silica skeletons and control their mineralization, thus participating in similar types of processes by which animal and human bones are formed. Silica is the primary material in most commercially manufactured semiconductors.
In the study, polystyrene microbeads coated with specific silicateins were put through a mineralization reaction by incubating the beads in a water-in-oil emulsion that contained chemical precursors for mineralization: metals of either silicon or titanium dissolved in the oil or water phase of the emulsion. As the silicateins reacted with the dissolved metals, they precipitated them, integrating the metals into the resulting structure and forming nanoparticles of silicon dioxide or titanium dioxide.
With the creation of a silicatein gene pool, through what Bawazer only somewhat euphemistically calls "molecular sex" - the combination and recombination of various silicatein genetic materials- the scientists were able to create a multitude of silicateins, and then select for the ones with desired properties.
"This genetic population was exposed to two environmental pressures that shaped the selected minerals: the silicateins needed to make [mineralize] materials directly on the surface of the beads, and then the mineral structures needed to be amenable to physical disruption to expose the encoding genes," explained Bawazer. "The beads that exhibited mineralization were sorted from the ones that didn't, and then fractured to release the genetic information they contained, which could either be studied, or evolved further."
The process yielded forms of silicatein not available in nature, that behaved differently in the formation of mineral structures. The study notes that some silicateins self-assembled into sheets and made dispersed mineral nanoparticles, as opposed to more typical agglomerated particles formed by natural silicateins. In some cases, crystalline materials were also formed, demonstrating a crystal-forming ability that was acquired through the directed evolution process.
Importantly, silicateins are enzymes with relatively long amino acid chains that can fold into precise shapes, so there is the potential for more functionality than would be possible using shorter biopolymers or more traditional synthetic approaches.
Bawazer believes the process could potentially work with a variety of metals, evolving different types of materials. "By changing the laboratory-controlled environments in which directed evolution occurs, it might be possible to evolve materials with specific capacities, like high performance in an evolved solar cell, for example. I'd like to take it a step further and evolve material performance in a functional device," he said.
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