Tannic acid and iron combine to form a coating on polystyrene substrates as shown by atomic force microscopy.
In ancient Japanese culture, those who wished to prevent cavities coated their teeth with black solutions of iron mixed with vegetable tannins. This custom of ohaguro was abandoned by the Japanese in the 19th century, but chemists in Australia have resurrected a version of the technique with potential applications far beyond preventive dentistry.
A team led by Frank Caruso at the University of Melbourne has developed a simple one-pot recipe for a coating made of only Fe(III) and tannic acid, a polyphenol found in wood that is perhaps best known for improving the flavor of wood-casket-aged red wine.
The coating is unusually versatile. It can cover all manner of nano- and microscopic objects, including gold nanoparticles, calcium carbonate and silicon dioxide particles, and bacteria, regardless of whether the object to be coated is positively charged, negatively charged, or neutral (Science 2013, DOI: 10.1126/science.1237265). The discovery is patent-pending.
Since both Fe(III) and tannic acid are generally regarded as safe by regulators and are already used in food and biomedical applications, the coating could find uses right away in areas as diverse as drug delivery and corrosion protection, comments Christopher W. Bielawski, a chemist at the University of Texas, Austin. “This is going to make a big impact mainly because it is so simple,” he adds. “Many will wish that they had thought of it first.”
“The coating’s pH sensitivity is the really exciting aspect,” comments Phillip B. Messersmith, a biomedical engineer at Northwestern University. The coating forms above pH 6, but in more acidic environments it falls apart, revealing or releasing the contents of objects within it. One could envision using the coating as a way to deliver a drug to a cell’s acidic lysosome or endosome and then having the contents released in the organelle’s low-pH environment, he says.
Researchers use “tannic acid” to describe a family of molecules that contain a central glucose with one to five polygalloyl groups of varying lengths that emanate from the sugar base. Every Fe(III) atom can coordinate three pairs of hydroxyl groups found on tannic acid and thus can complex up to three different tannic acid molecules. Meanwhile, the abundance of hydroxyl groups in tannic acid means that each tannic acid molecule can complex up to a dozen or so iron atoms. The result is a cross-linked coating that is about 10 nm thick, Caruso says.
The technique seems like it could be easily expanded, Bielawski says. “There are a lot of other polyphenols out there besides tannic acid, so there’s considerable potential for changing the surface chemistries of the coating and creating new applications.”