Nano compartments may aid drug delivery, catalyst design
April 23, 2013
Cornell researchers have created spongelike nanoparticles with separate compartments that could deliver two or more different drugs to the same location, with precise control over the amounts, avoiding unpleasant side effects.
The technology might also be applied to catalysts used to enhance chemical reactions, which are sometimes formed into porous nanoparticles to expose more surface area. Compartmented particles could allow two or more catalysts to work in sequence.
The researchers modified familiar “sol-gel” chemistry used to self-assemble porous silica particles, creating two or more different nanoparticles joined together, while controlling how one particle grows out of another, a process referred to as epitaxial growth.
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. “The micelles are placeholders for the pores,” said Ulrich Wiesner, the Spencer T. Olin Professor of Materials Science and Engineering. (A nanometer is a billionth of a meter, about the length of three atoms in a row.)
The type of pore lattice depends, among other things, on the pH, or acidity, of the solution. The researchers added ethyl acetate, a chemical that breaks down in water and in the process makes the solution more acidic, to act as a timer to change the output of the reaction partway through.
At first a cubic lattice forms, building cubical particles. As acidity increases the reaction path changes to make a hexagonal lattice creating cylinders that begin to grow out of the faces of the cubes. The number of cylinders and their length can be controlled by the concentration of ethyl acetate, generating tripods or even tetrapods.
“Previous work in my group and that of others has focused on how to control the pore structure,“ said Wiesner. “Here we use the pore structure to control the shape of the nanoparticles.”
In a hint of the future, the researchers were able to connect two or three cubes with cylindrical bridges between them, perhaps the beginning of a network of cubes and tubes like a nanoscale hamster habitat. “We have learned to switch the growth conditions. If we can switch back we might be able to grow all sorts of funky architectures,” Wiesner said.
The National Science Foundation supported this research.