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The University of Auckland
From Tea Stains to Coatings: Polyphenol Chemistry towards Antimicrobial Surfaces
Name(s) of author(s):
Tea stains are ubiquitous and annoying due to their omnipotent fouling properties and are difficult to remove. However, such characteristics of tea stains may translate to ideal adhesive properties in terms of surface coatings. Inspired by the commonly seen tea and coffee stains, we report a surface-independent coating strategy that takes advantage of tea stains' strong adhesive property on diverse materials including metals, plastics and glass. A simple dip-coating procedure forms a thin layer of coating with a typical thickness ranging from 10-100nm. The as-formed coating is colourless and resistant to common solvents, including water. However, it can be easily removed under alkaline conditions. Such unique property prompted us to find out the science behind it.
Similar to the events happening in brewing a cup of tea, it is believed that the coating formation takes two steps. Firstly, initial surface adsorption is achieved through electrostatic interactions and dispersion forces between the surface and phenolic molecules, in which protonated amine group is believed to play a critical role. After the initial adsorption, polyphenol underwent oxidative polymerization, leading to highly cross-linked polymer chains and reduced solubility, which gradually deposited onto the initially adsorbed layer. In addition, the hydrated salts in the solution can be incorporated into the polymeric matrices, in which cation-Pi interaction is the dominant driving force.A significant advantage of polyphenol coating is that the various chemical bonds within the matrices make subsequent functionalization achievable. In such a way, we took advantage of the strong interaction between aromatic Pi electron with silver ions to the in situ synthesis of silver nanoparticles (AgNPs) for facile antimicrobial and antiviral surface strategy. An up to 7 log CFU/mL reduction of both Gram-positive and Gram-negative bacteria was achieved in 2-hour exposure of bacterial cells to the AgNP-modified polypropylene films. In the antiviral assay, where T7 bacteriophage was used as a model naked virus, an 8 log PFU/mL reduction was obtained within 2-hour contact.
Mr Boyang Xu acquired his Bachelor of Science degree at Shenyang Pharmaceutical University, China, during which he was educated and trained as a Medicinal Chemist.
After that, he received a Master of Science degree with a specialization in Food Science at The University of Auckland. He is now studying as a PhD student at the same university with a project focusing on surface coatings of food origins.