Semiconductors have become indispensable for modern technology. For example, oxide semiconductors such as zinc oxide (ZnO) are the materials of choice for transparent conducting layers for blue LEDs, liquid crystal displays, and solar cells. The large-scale production of such oxide semiconductors is an energy-intensive process. In order to decrease the energy expenditure, the search is on for new production processes that work under mild conditions. Researchers at the City University of New York's Hunter College have now developed a new approach that delivers zinc oxide at room temperature. Hiroshi Matsui and Roberto de la Rica report in the journal Angewandte Chemie that they use the enzyme urease as a "nanoreactor" for the production of crystalline zinc oxide.
Urease is found in plant seeds, bacteria, and various sea creatures. It splits urea into carbon dioxide and ammonia. Ammonia is an alkaline substance; when urease is active, the pH value on the surface of the enzyme rises.
Because of the negative charge on its surface, urease has a high affinity for positively charged metal ions. In a zinc nitrate solution, the zinc ions aggregate around the urease. The local pH value on the surface of the enzyme can be finely tuned to values ideal for the formation of zinc oxide. The increasing zinc ion concentration at a suitable pH value catalyzes the formation and growth of ZnO crystals. The enzyme cores thus become surrounded by nanoshells of zinc oxide. These have a diameter of about 18 nm. By using slightly modified urease molecules, the size of the zinc oxide shells can be varied.