Colorless copper-containing coatings with high antimicrobial efficacy and formulation versatility†

Abstract

This work reports an antimicrobial (AM) copper(I)-containing additive for water-based formulations with high efficacy and minimal impact to the formulation's natural color. Determination of whether Cu¹⁺ ions can be maintained for long durations of time and induce high bioactivity when used in complex aqueous environments are known technical challenges towards using copper as an antimicrobial additive. Cu¹⁺ ions are the preferred oxidation state to achieve broad-spectrum AM efficacy. Also, Cu¹⁺ if stabilized in a formulation, can impart low color changes relative to the Cu²⁺ ions. To this end, we developed a UV-vis spectroscopy approach to track copper speciation. We used multinuclear NMR spectroscopy on copper-based additive mixtures to also demonstrate that Cu¹⁺ ions within the additive converts selected water-based formulations into antimicrobial white and clear coatings. The copper additive mixtures were made by extracting Cu¹⁺ ions from a previously reported copper-glass ceramic (CGC) powder that offered high antimicrobial efficacy, but CGC powders when used directly led to unacceptably high color in white paints and clear coatings. Ligands such as phosphites were shown to promote extraction and stabilization of Cu¹⁺ ions through coordination. The antimicrobial performance of the additives was tested in commercial formulations that included a white latex paint and clear coatings for wood and glass substrates. A reduction in Staphylococcus aureus (staph) bacteria of >99.9% (>log 3 kill), under test conditions that simulate realistic microbial contamination, was observed for these coatings with negligible change to the original color of the coating. Here, our findings demonstrate novel advancements in the field of inorganic antimicrobial clear coatings for a range of surfaces such as in daycare, hospitality, healthcare interior spaces, automotive interiors, and consumer electronics.