Hydrophilic ZnO Nanoparticle-Based Antimicrobial Coatings for Sandstone Heritage Conservation

Exploring efficient and sustainable antimicrobial materials is crucially important for built cultural heritage conservation. The synergic combination of hydrophilic ZnO nanoparticles (NPs), calcium phosphate oligomers (CPO), and oxidized sodium alginate (OSA) exhibits distinctive properties, such as the chemical stability of ZnO NPs being enhanced with strong intermolecular interactions. Herein, an OSA/CPO/ZnO NPs composite has been prepared by embedding hydrophilic ZnO NPs and CPO into OSA molecules. The metastable CPO units bond with OSA molecules via electrostatic attraction between the positively charged calcium ions and negatively charged carboxyl groups and then gradually transform into ultrafine hydroxyapatite nanolines. In addition, the OSA/CPO/ZnO NPs composite shows different particle sizes and zeta potentials depending on the concentration of ZnO NPs in the dispersions. In vitro antimicrobial tests show that these composites have significant antimicrobial activity, inhibiting the growth of bacteria Escherichia coli and Micrococcus yunnanensis and fungus Aspergillus sp. F6. The application of the OSA/CPO/ZnO NPs composite as protective coatings on sandstone presents a potent and long-lasting antimicrobial activity and also has no significant effect on the aesthetic appearance, indicating a potential candidate for cultural heritage conservation.