Tannins belong to a class of chemical compounds called phenols, which include the lignin found in trees and the melanin found in our skin. Tannic acid is also found in wine. The researchers, led by Frank Caruso, made their coating from iron (III) ions and tannic acid, which crosslink together by coordination chemistry in a simple, one-step self-assembly process that takes just minutes.

The self-assembly can occur on any type of surface, including inorganic and organic molecules and living cells, and be used to coat objects of various sizes, shapes and structures, say the researchers. When deposited on particles and the template cores removed, capsules can be produced, explains Caruso. These capsules can be engineered with tuneable thicknesses and disassemble at different rates when the pH of the solution is changed (in this case, when it becomes more acidic) – something that could be exploited for the timed release of substances, such as therapeutic drugs, contained within the capsules.

Inexpensive and easy to make

The films also have the advantage of being inexpensive to produce. They are made from commercially available materials and are simple to fabricate, with no complex apparatus being required, and are Generally Recognised as Safe (GRAS) – that is they are biocompatible.

"Our coatings could potentially be used for releasing therapeutics, as mentioned, in biological imaging (for example, in PET, CT and MRI), and possibly environmental applications, such as metal ion separation," Caruso told nanotechweb.org.

Spurred on by these initial results, the team is now busy trying to make films and capsules that can carry and release a wide variety of therapeutics for combating different diseases.

The present work is reported in Science.