Imprinted Polymers Would Selectively Trap Metal Ions

January 21, 1997

By Paul Preuss, [email protected]

Chemist Richard Fish of the Lab's Environmental Energy Technologies Division has developed a new technique that holds promise for recovering precious metals such as gold and silver from aqueous solutions, or for cleansing polluted waters of mercury and other toxic metals.

The key to Fish's technique is the use of "imprinted polymers," molecular chains imprinted with empty binding sites that match the size and shape of specific kinds of metal ions. These polymers can be used to selectively trap and contain a desired species of ion for removal from solution.

To create his imprinted polymers, Fish first sandwiches a target metal ion such as zinc or mercury between a pair of special organic ligands known as TACNs (short for N-[4-vinylbenzyl]-1,4,7-triazacyclononanes). After these sandwich complexes are cross-linked into a polymer, the metal ions are washed away with a strong acid, leaving templates or empty sites of the right size to fit similar ions. The imprinted polymer is then ground to a fine powder. When the powder is confined so that an aqueous solution can be passed through it, as in a chromatography column, metal ions become trapped in the empty sites and thus are removed from the solution.

"Copper ions, for example, immediately turn the slightly off-white imprinted polymer to a very prominent green color," Fish says. "The copper can be recovered from the imprinted polymer by eluting the same chromatography column with a strong acid."

Metal ions have different ionic radii, the average distance at which the mutual repulsion of their electric charges makes them repel one another like hard spheres. In cases such as copper and zinc, where the ionic radii are similar, the imprinted polymer can bind more than one kind of metal ion.

Nevertheless, imprinted polymers can seek out specific ions. TACN polymers imprinted with zinc actually prefer copper ions, Fish explains. In a solution with equal concentrations of five different ion species including both copper and zinc, the polymer captured 157 copper ions for every ion of zinc.

"We think that thermodynamic effects are the overriding factor when two metal ions have similar radii," says Fish. "Despite the nice fit of the zinc ion in the polymer, the chemical combination of copper and TACN has better thermodynamic stability."

Yet the imprinted shape does play an important role. Once copper is removed from the solution, the same polymer much prefers the zinc ions over the other metal ions, grabbing zinc over nickel, the nearest competitor, in a ratio of nine to one.

In a separate test of the role of shape and ionic radius, divalent copper ions and trivalent ions of iron were dissolved in the presence of zinc-imprinted TACN polymer to see which would be selected. A free TACN ligand usually selects the highly reactive iron ions over copper, but in this experiment, the zinc-imprinted polymer favored copper ions by a ratio of 44 to one. The most likely answer lies with the size of the hole in the polymer. Zinc and copper ions have nearly similar radii, but the radius of the iron ions is significantly smaller.

Fish is also investigating the use of imprinted polymers as real-time probes for metal ions in the environment. He has designed TACN ligands with attached fluorescent groups which he then binds to mercury ions, for example. His final product will be an imprinted polymer that fluoresces when it encounters mercury ions.

The TACN monomer containing the fluorescent probe can be polymerized on the tip of an optic fiber. When the fiber is dipped in an aqueous solution, the intensity of the fluorescence can be measured immediately to reveal how much of the ion is present. Such fluorescent probes would be useful for determining the concentration of a wide variety of metal ions that pollute streams.

"Imprinted polymers can be highly selective, somewhat like antibodies -- but more easily prepared, with higher stability, and at a lower cost," Fish says, "which gives them great power to detect and remove specific substances from the environment."

Fish has published details about his metal-ion implanted polymers in a number of recent publications, including a chapter of the forthcoming American Chemical Society Symposium Series book, Recognition with Imprinted Polymers.

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