Colorless Multifunctional Coatings Inspired by Polyphenols Found in Tea, Chocolate, and Wine**

Phenols and polyphenols are widely distributed in plant tissues, where they are linked to diverse biological functions such as chemical defense, pigmentation, structural support, and prevention of radiation damage. Members of this large family of compounds include epigallocatechin gallate (EGCG), epicatechin gallate (ECG), epigallocatechin (EGC), and tannic acid (TA), to name a few (Figure 1B). Plant polyphenols display a rich and complex spectrum of physical and chemical properties, giving rise to broad chemical versatility including absorption of UV radiation, radical scavenging, and metal ion complexation. In addition, significant attention has been given to the purported health benefits associated with consumption of foods and beverages rich in plant polyphenols. Of interest to us in this report is the strong solid–liquid interfacial activity exhibited by plant polyphenols, a property that is reflected in their historical use as binding agents (lignin) and in leather manufacturing (vegetable tannins). The high dihydroxyphenyl (catechol) and trihydroxyphenyl (gallic acid, GA) content of plant polyphenols further attracted our interest in the context of surface modification, as catechols are known to strongly bind to surfaces through covalent and noncovalent interactions and are prominent constituents of marine polyphenolic protein adhesives. Inspired by the high catecholamine content of mussel adhesive proteins and by the involvement of catecholamines in melanin biosynthesis, in situ oxidative polymerization of dopamine at alkaline pH was recently discovered as a universal route for deposition of multifunctional coatings onto surfaces. Although polydopamine (pDA) is simple to apply to substrates, deposits on a wide range of materials and offers many potential applied uses, the high costs of dopamine and the characteristically dark color of pDA coatings may be impediments for some practical applications. While a colorless pDA approach was recently reported, the method employed a 2:1 mixture of 2bromoisobutyryl-substituted dopamine to dopamine and was achieved at the expense of a roughly fourfold reduction in coating thickness. Herein, we describe the use of low-cost plant polyphenols, their building blocks and trihydroxyphenyl-containing molecules as precursors for the formation of multifunctional coatings. In contrast to previous studies where plant polyphenols have been investigated as monolayer adsorbates or as ingredients in multicomponent coatings, our strategy features either a plant polyphenol (TA) or a simple phenolic mimic (pyrogallol, PG) as the sole coating precursor. Plant polyphenol-inspired coatings retain many of the advantages of pDA and deposit under similar conditions, yet are colorless and derived in some cases from reagents hundredfold less costly than dopamine. The coating potential of plant polyphenols was first illustrated with a simple experiment involving unadulterated tea and wine, yielding the surprising finding that thin polyphenol coatings form spontaneously on surfaces exposed to these polyphenol-rich beverages (Figure 1). A freshly prepared green tea infusion left undisturbed in a covered porcelain cup for several hours and then rinsed with tap water, appeared to leave no residue or produce any observable color changes to the cup. However, the presence of a thin polyphenol coating was revealed by immersing the tea cup in a AgNO3 solution, resulting in the deposition of a dark metallic silver film on the surface, presumably through a redox reaction between Ag ions and the polyphenolic coating (Figure 1C). The tea bag used to prepare the infusion also appeared unchanged but darkened markedly upon treatment with AgNO3. The presence of surface-bound silver nanoparticles was evident upon inspection using scanning electron microscopy (SEM, Figure S2). Similarly, red wine also deposited an indiscernible polyphenol coating on glass that was visible only after treatment with AgNO3 (Figure 1D). Next we investigated crude extracts of red wine (RWE), cacao bean (CBE), and green tea (GTE) for their ability to form polyphenol films on polymeric, metallic, and native[*] T. S. Sileika, Dr. D. G. Barrett, Dr. K. H. A. Lau, Prof. P. B. Messersmith Biomedical Engineering, Northwestern University Evanston, IL 60208 (USA) E-mail: [email protected]