Imprinted Polymers Would Selectively Trap Metal Ions
January 21, 1997
By Paul Preuss, paul_preuss@lbl.gov
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.
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.
