NEW ORLEANS -- In recent years chemists and materials scientists have enthusiastically searched for ways to make materials with nanoscale pores -- channels comparable in size to organic molecules -- that could be used, among other things, to separate proteins by size. Recently Cornell University researchers developed a method to "self-assemble" such structures by using organic polymers to guide the formation of ceramic structures.
Now they have advanced another step by incorporating tiny magnetic particles of iron oxide into the walls of porous ceramic structures in a simple "one-pot" self-assembly. Such materials could be used to separate proteins tagged with magnetic materials, or in catalytic processes.
"This enables access, for the first time, to protein-separation technology based on a combination of size exclusion with magnetically assisted separation," explains Ulrich Wiesner, professor of materials science at Cornell, in Ithaca, N.Y., lead investigator for the research. One application could be the separation of a single protein out of the thousands found in blood serum.
The new research will be described in a paper by Cornell graduate student Carlos Garcia and research associate Yuanming Zhang, Wiesner and Francis DiSalvo, Cornell professor of chemistry and director of the Cornell Center for Materials Research (CCMR), in a forthcoming issue of the authoritative German journal of chemistry, Angewandte Chemie. Wiesner will discuss this and other work on self-assembled polymer-ceramic hybrids at the 225th national meeting of the American Chemical Society in New Orleans at 1:30 p.m. CST Monday, March 24, as part of a symposium on hybrid materials.
Wiesner's team creates porous structures by mixing organic polymers -- in particular a class known as diblock copolymers -- with silica-type ceramics. Under the right conditions the materials self-assemble into polymer channels surrounded by a polymer-ceramic composite. This is "calcined," or exposed to extreme heat to vaporize organic components, leaving a ceramic honeycombed with tiny passages. By controlling the polymer molecular weight and the relative amounts of polymer and ceramic, they control the size of the passages. In the latest work, iron ethoxide powder is added to the polymer-ceramic mix. The iron is dispersed throughout the ceramic portion of the structure.
When the material is calcined in the presence of oxygen, the iron transforms into nanoparticles of crystalline iron oxide -- in a so-called "lamda" form that has magnetic properties -- embedded in the walls of the passages. The Cornell researchers note that apparently the surrounding silica-type matrix prevents the iron oxide from converting into a more stable, non-magnetic "alpha" form under calcination.
X-ray scattering and transmission electron microscopy (TEM) verified that the initial hexagonal cylinder composite structure is preserved under calcination. Measurements with a superconducting magnetometer verified that the nanometer-sized iron oxide particles within the pore walls are superparamagnetic -- that is, their magnetic properties can be switched on and off by the application of external magnetic fields. The TEM images show the iron oxide particles to be about 5 nanometers in size (a nanometer is one billionth of a meter), a figure that agrees with theoretical predictions based on magnetometer data.
One use for these novel materials, Wiesner suggests, would be to separate proteins or other biological molecules both by size exclusion and magnetic interactions. If a magnetic field is applied to the ceramic structure, molecules tagged with magnetic material would be held back. After other molecules have passed through, the field is turned off and the selected molecule is released.
The porous materials also could be used in catalytic conversion. Iron oxide, for example, is used as a catalyst in converting carbon monoxide to carbon dioxide. In theory, Wiesner says, these materials could be made with a wide variety of metals, making other catalytic processes possible. The material is stable at temperatures up to 800 degrees Centigrade (1,472 degrees Fahrenheit), making it usable in many high-temperature catalytic processes.
Other researchers have experimented with adding magnetic particles to a porous ceramic structure after it is formed, by depositing the particles on the inner surfaces of the pores. This risks clogging the pores, Wiesner says. In the latest experiments, the iron oxide particles are embedded within the ceramic walls. The form of iron oxide created in this process is known as lamda-Fe2O3 . Non-magnetic alpha-Fe2O3 , with a different arrangement of atoms in the molecule, is usually observed after exposure to the high temperatures of calcination. The research was supported by the National Science Foundation, Phillip Morris CCMR, which is a Materials Research Science and Engineering Center of the National Science Foundation.
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The Wiesner Research Group: http://www.ccmr.cornell.edu/~uli/
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