The technological materials of the future will require methods of synthesis that are energetically efficient but also environmentally benign. In nature, numerous proteins are adept at controlling the crystallization of inorganic ions under atmospheric conditions. For this reason, research into how the mineralization properties of all sorts of biomolecules can be harnessed for the generation of materials for practical applications has intensified in recent years.

Dingguo Xia and colleagues from the Beijing University of Technology in China1 have now used enzymes to generate three different crystal structures of titania (TiO2) — a material used in a variety of applications from solar cells to catalysts and even paint pigments. Their new synthesis process requires only a titanium precursor, enzymes and time. After three weeks simply sitting on a shelf at room temperature, the solutions yielded nanoparticles of rutile, anatase and mixed-phase titania. The crystal form obtained depends on which precursor and enzyme are used.

“At first, we wanted to immoblize glucose oxidase and catalase onto a titania support to generate sensors. Lysozyme was used as a control,” says Xia. “But instead, we observed these interesting nanocrystal growth phenomena.”

Fig. 1: Transmission electron microscopy images of the glucose oxidase-mediated synthesis of porous titania (rutile) at different magnifications.

Spectroscopic techniques in combination with thermogravimetric analysis revealed that the anatase and the mixed-phase materials both remained intimately associated with the enzymes that had helped in their formation. Surprisingly, however, the rutile sample was almost free of the enzyme (Fig. 1). X-ray diffraction measurements made at intervals during crystallization revealed that the rutile sample initially contained anatase, whereas the other samples (anatase and mixed phase) remained in the same phase throughout crystallization. The researchers suggested that the formation of rutile is the result of weak interactions between the enzyme and precursor, allowing dissolution and recrystallization mechanisms to generate an enzyme-free crystal. They attribute the formation of anatase and the mixed phase, however, to strong interactions between the enzyme and the precursor, which prevent recrystallisation, resulting in a titania–enzyme composite.

Xia and his colleagues believe that their rutile nanocrystals, shown by electron microscopy to aggregate into a microporous material, might have an application as an anode in lithium-ion batteries. Preliminary experiments show promising electroactivity towards lithium insertion and good cycling rate capability.

“Our goal now is to be able to control the shape, structure and polymorphs of micro/nanoscale metal oxides using the unique properties of proteins,” explains Xia.