Polyphenols can form functional coatings on a variety of different materials through auto-oxidative surface polymerization in a manner similar to polydopamine coatings. However, the mechanisms behind the coating deposition are poorly understood. We report the coating deposition kinetics of the polyphenol tannic acid (TA) and the simple phenolic compound pyrogallol (PG) on titanium surfaces. The coating deposition was followed in real time over a period of 24 h using a quartz crystal microbalance with dissipation monitoring (QCM-D). TA coatings revealed a multiphasic layer formation: the deposition of an initial rigid layer was followed by the buildup of an increasingly dissipative layer, before mass adsorption stopped after approximately 5 h of coating time. The PG deposition was biphasic, starting with the adsorption of a nonrigid viscoelastic layer which was followed by layer stiffening upon further mass adsorption. Coating evaluation by ellipsometry and AFM confirmed the deposition kinetics determined by QCM-D and revealed maximum coating thicknesses of approximately 50 and 75 nm for TA and PG, respectively. Chemical characterization of the coatings and polymerized polyphenol particles indicated the involvement of both physical and chemical interactions in the auto-oxidation reactions.

Two blue multicopper oxidases (MCOs) (viz. Trametes hirsuta laccase (ThLc) and Myrothecium verrucaria bilirubin oxidase (MvBOx)) were immobilized on bare polycrystalline gold (Au) surfaces by direct adsorption from both dilute and concentrated enzyme solutions. The adsorption was studied in situ by means of null ellipsometry. Moreover, both enzyme-modified and bare Au electrodes were investigated in detail by atomic force microscopy (AFM) as well as electrochemically. When adsorbed from dilute solutions (0.125 and 0.25 mg mL −1 in the cases of ThLc and MvBOx, respectively), the amounts of enzyme per unit area were determined to be ca. 1.7 and 4.8 pmol cm −2 , whereas the protein film thicknesses were determined to be 29 and 30 Å for ThLc and MvBOx, respectively. A wellpronounced bioelectrocatalytic reduction of molecular oxygen (O 2 ) was observed on MvBOx/Au biocathodes, whereas this was not the case for ThLc-modified Au electrodes (i.e., adsorbed ThLc was catalytically inactive). The initially observed apparent k cat app values for adsorbed MvBOx and the enzyme in solution were found to be very close to each other (viz. 54 and 58 s −1

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Surface modification with polyphenolic molecules has been pursued in biomedical materials owing to their antioxidant, anti‐inflammatory, and antimicrobial characteristics. Recently, the use of silicic acid (Siaq) as a mediator for efficient surface deposition of tannic acid (TA) was reported, but the postulated Si‐TA polymeric networks were not characterized. Herein, we present unambiguous evidence for silicate‐TA networks that involve Si−O−C motifs by using solid‐state NMR spectroscopy, further supported by XPS and ToF‐SIMS. By using QCM‐D we demonstrate the advantages of Siaq, compared to using transition‐metal ions, to improve the coating efficiency under mildly acidic conditions. The presented homogenous coating buildup and validated applicability in inorganic buffers broadens the use of TA for surface modifications in technological and biomedical applications.

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). An in situ TA layer thickness of 200 nm was obtained after 24 h in solutions supplemented with 80 µM Siaq. 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 the polymerization process. In comparison to TA, the coating formation of pyrogallol (PG) and gallic acid (GA) was not affected by Siaq. PG formed continuous coatings also without Siaq, while GA only formed a monolayer in presence of Siaq. 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 of polyphenolic molecules with multiple ortho-dihydroxy groups and open new possibilities to deposit TA on interfaces.