Surface functionalization of graphene oxide with tannic acid: Covalent vsnon-covalent approaches

Four green, easy and inexpensive esterification approaches have been tested to prepare surface functionalized graphene oxide (GO) nanosheets with tannic acid (TA): direct, carbodiimide activated,oxalyl chloride acylation and via an acid-functionalized GO intermediate. The resulting samples have been extensively characterized to get knowledge on the GO-TA interactions and the degree of grafting, as well as their surface topography, level of hydrophilicity, solubility/dispersibility, thermal and antibacterial properties. The covalent grafting of TA renders the GO surface more hydrophobic, resulting in improved dispersion in organic solvents. Besides, TA acts as a crosslinker between the GO nanosheets, leading to higher thermal resistance. A synergistic effect of both GO and TA on inhibiting bacterial growth has also been found. The esterification via carbodiimide leads to the highest grafting degree, the best thermal stability and the most effective antibacterial activity. This work not only highlights the great potential of TA for both exfoliation and surface functionalization of GO, but also extends its applications in biomedicine and for the development of environmentally friendly nanocomposites to be used in several industries.