This false-color image (left) depicts the core lattice in blue, where drugs can be placed in compartment pores for targeting in the body. In the hexagon-shaped cylinder branches, other types of drugs may be place for delivery. Simultaneous delivery of pharmaceuticals can thus be optimized for each drug separately. The accompanying illustration (right) offers a clear vision of the left image. (Image credit: Cornell University)
Ulrich Wiesner, the Spencer T. Olin Professor of Materials Science and Engineering, and first authors, Cornell researcher Teeraporn Suteewong and graduate student Hiroaki Sai, have tweaked familiar “sol-gel” chemistry used to self-assemble porous silica particles, making the assembly shift gears partway through to create what amounts to two or more different nanoparticles joined together, while controlling how one particle grows out of another, a process referred to as epitaxial growth. “It’s the first time I’m aware of that the shapes of porous silica nanoparticles have been controlled via epitaxy,” Wiesner said. “The cubic lattice and the hexagonal lattice have a well-defined relationship.” The products so far are fairly simple particles with two or three compartments, but the methods might be extended to create much more complex structures, he said. The discovery was partly serendipitous. While making ordinary nanoparticles, the scientists saw a small fraction with hexagonally structured porous branches growing out of a cubic core particle. “We set out to understand what controls that,” Wiesner said. The starter for the process is a mixture of organosilanes, molecules built around carbon and silicon atoms, and surfactants. Surfactants, of which the prime example is soap, have one end that likes water and another “oily” end that tries to stay away from it. So in water surfactants form micelles, tiny spherical bundles with the water-loving end out and the oily part tucked away in the center. In the sol-gel process the micelles act as cages around which silica from the orgaosilanes forms, building particles about a hundred nanometers in diameter. When the micelles are washed away what remains is a porous silica structure with pores two to three nanometers in size.