Self-Polymerized Plant Polyphenols as Antibacterial Agents

Polyphenols are a class of biologically active compounds that are derived from plant-based natural products such as fruits, vegetables, coffee, and tea. Polydopamine, dopamelanin, gallic acid, pyrogallol, and tannic acid contain the representative chemical features of polyphenols which can form coordination bonding with protons, ions, or electrons via redox reaction. Due to their physical, chemical, and electrochemical properties, polyphenols have attracted significant interest in various research areas, including charge storage, heavy metal ions removal, or drug delivery. Recent studies have shown that phenol-based polymeric particles can inhibit bacterial growth and prevent biofilm formation by releasing the reactive oxygen species in aqueous environment. However, the surface disruption of polymers in complex surroundings and the poor efficiency due to the low surface area to volume ratio of the coating remain the potential challenges that shorten the long-term stability.<br/>Herein we report the facile synthesis of a free-standing poly(pyrogallol) (1,2,3-trihydroxybenzene, PPG) polymer that can be used as an environmental friendly and efficient antibacterial agent. PPG consists of homogeneous nanofibrous microstructure containing the chemical moieties that allow the reversible redox reaction via catechol or gallol groups. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy corroborated the formation of the ether functional groups that are induced by the polymerization between the hydroxyl group and the carbons of the benzene. The antibacterial activity of PPG was examined using the gram-negative <i>Escherichia coli</i>. The antibacterial activity was quantitatively evaluated by a colony count method after incubating the <i>E. coli</i> with the concentration of 1.5 X 10⁵ CFU mL^-1. The survival rates were compared with the natural melanins extracted from <i>Sepia Officinalis</i> that contains the catechol groups. PPG achieved 63.1% and 99.9% <i>E. coli</i> inhibition within 30 min and 4 h of incubation, which is approximately 50% higher than the catechol-containing melanins. This is mainly due to the higher density of phenolic compounds available in PPG to produce ROS in the aqueous environment. The results herein suggest that PPG exhibits potential to be utilized as functional antibacterial agents for a variety of applications in biomedical and environmental engineering.