Silicic acid mediated formation of tannic acid nanocoatings

Tannic acid (TA) adheres to a broad variety of different materials and forms versatile surface coatings for technical and biological applications. In mild alkaline conditions, auto-oxidation processes occur and a firm monolayer is formed. Up to now, thicker coatings are only obtained in cross-linked multilayer fashion. This study presents an alternative method to form continuous TA coatings using ortho-silicic acid (Siaq). Adsorption kinetics and physical properties of TA coatings in the presence of Siaq were determined using a quartz crystal microbalance (QCM-D) and nanoplasmonic spectroscopy (NPS). Under supplement of 80 µM Siaq, an in situ TA layer thickness of 200 nm was obtained after 24 h. Dry state measurements indicated a highly hydrated layer in situ. Furthermore, chemical analysis by FTIR spectroscopy revealed possible complexation of TA by Siaq, whereas UV-vis spectroscopy did not indicate an interaction of Siaq in the auto-oxidation process of TA. Investigation of additional metalloid ions showed that germanic acid was also able to initiate a continuous coating formation of TA, whereas boric acid prevented its polymerization process. In comparison to TA, the coating formation of pyrogallol (PG) and gallic acid (GA) was not affected by Siaq. PG already formed continuous coatings without Siaq, while GA only formed a monolayer in its presence. However, Siaq induced a continuous layer formation of ellagic acid (EA). These results indicate the specific importance of ortho-silicic acid in the coating formation for polyphenolic molecules with multiple ortho-dihydroxy groups and open new possibilities to deposit TA on interfaces.