Researchers discover simple coating technique using tannic acid and iron

July 12, 2013 by Bob Yirka, report
Microscopy images of FeIII -TA capsules: DIC image (B), AFM image (C), SEM image (D), and TEM image (E). Credit: Science/AAAS

( —A team of chemical researchers at the University of Melbourne in Australia has discovered a simple coating technique that uses nothing but tannic acid and iron ions. In their paper published in the journal Science, the team describes how they added tannic acid and iron ions to a water solution which resulted in the spontaneous creation of a thin film. The film, the group report, coats other materials that are put into the mix without the need for coaxing.

Tannic acid is one of a number of phenolics—a family of self-assembling . Others include lignin, found in wood and , found in skin. Tannic acid is of course, found in wine, and has historically been used to tan animal hides. In this new effort, the researchers looked to make use of the material's natural ability to coat surfaces.

When tannic acid is placed in water it tends to coat anything else that is placed in the water, metals, for example, plastics, or even viruses. Seeking to take advantage of that feature, the researchers added to the mix, and found that the ions stuck to the , resulting in the creation of a thin film. Next, the team added other materials to the mix to serve as various substrates and found that the acid-iron films would spontaneously coat each of them. For small particles, the result was the creation of capsules. Perhaps even more importantly, the team found that by altering the pH level of the solution in which the coating resides, the coating would disassociate itself. Other techniques have been developed to create coatings, of course, but none are as simple (just one step) or inexpensive.

Video demonstration of the assembly of FeIII-tannic acid films on particle templates. Credit: Hirotaka Ejima

This new way to create a coating is important as it contributes to ongoing research looking into ways to create and manufacture for use in medical and life science applications. It's not difficult to imagine capsules based on those developed by the team in Australia that hold medicine. Such capsules would release their payload only when they arrive at a part of the body that has the right pH level, allowing for targeted treatment.

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More information: One-Step Assembly of Coordination Complexes for Versatile Film and Particle Engineering, Science 12 July 2013: Vol. 341 no. 6142 pp. 154-157 DOI: 10.1126/science.1237265

The development of facile and versatile strategies for thin-film and particle engineering is of immense scientific interest. However, few methods can conformally coat substrates of different composition, size, shape, and structure. We report the one-step coating of various interfaces using coordination complexes of natural polyphenols and Fe(III) ions. Film formation is initiated by the adsorption of the polyphenol and directed by pH-dependent, multivalent coordination bonding. Aqueous deposition is performed on a range of planar as well as inorganic, organic, and biological particle templates, demonstrating an extremely rapid technique for producing structurally diverse, thin films and capsules that can disassemble. The ease, low cost, and scalability of the assembly process, combined with pH responsiveness and negligible cytotoxicity, makes these films potential candidates for biomedical and environmental applications.

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5 / 5 (1) Jul 12, 2013
So THAT is what's behind the slime in my coffee pot!
not rated yet Jul 12, 2013

I think you are on to something: I have found that spray bleach works very well in getting coffee residue out of my stainless steel coffee maker carafe. Perhaps the high ph is doing the trick!
1.3 / 5 (4) Jul 12, 2013
This should motivate weekend lab workers to just mix stuff from their old shelves and not throw away any fluffy junk that results. Obviously, few have such spontaneity, given how many decades this combination went undiscovered. The graphene Nobel prize is another example of how unplayful entire generations of graduate students can be.
1.3 / 5 (4) Jul 12, 2013
few have such spontaneity, given how many decades this combination went undiscovered
In this case you're rather demonstrating the lack of informations or the willingness to do a brief review at Google first. The tannic acid layers were tested long time ago as an anti-rust coatings just for protection of iron surfaces. These guys rather did a characterization of these films with modern methods. I wouldn't compare it with graphene finding.
not rated yet Jul 13, 2013
Sounds curiously like the iron gall ink that was the standard ink used in Europe from the 5th century to the 19th century.

It was made by mixing iron(II)sulfate with tannins from oak gall immediately before writing. When you write, the ink first appears light brown. After a while it oxidizes to brownish-black iron(III) tanate.

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