Grafting of gallic acid to metallic surfaces

The protection against bacteria, oxidation and corrosion is crucial for metals. Natural molecules can offer numerous of these properties; however they are poorly explored for the surface modification of metals. Gallic Acid (GA) was selected as a model molecule for the polyphenols to investigate the mechanisms involved in grafting to the metallic surfaces. Ti6Al4V, AISI-204-Cu and AISI-304 stainless steels were used as substrates. The GA solutions were prepared in different media (ultrapure water, TRIS/HCl buffer, TRIS/HCl buffer containing Ca2+ ions) to explore the role of the ionic composition and pH in the GA bonding ability to the metallic substrates. The solutions were analyzed by means of the UV and FTIR spectroscopies. The samples were characterized by means of the FTIR spectroscopy and Folin & Ciocalteu tests to evaluate GA presence and activity. Surface wettability was also evaluated. GA was effectively grafted to all the surfaces maintaining its redox activity. The FTIR and wettability tests confirmed its presence on the grafted surfaces. A crucial role of both pH and Ca2+ was demonstrated and correlated to the formation of coordination compounds in solution and at the metal surface. It was observed that the surface chemical composition and the presence of functional groups affect the molecular grafting ability.