Antimicrobial Efficacy of Chitosan and Chitosan-Based Nanoparticles for the Conservation of Natural Pigments in Mural Paintings

Biodeterioration due to microbial colonization poses a severe threat to the integrity of ancient mural paintings. This study investigates the antimicrobial efficacy of utilizing chitosan and chitosan-based nanocomposites as a protective coating for the conservation of traditional natural pigments. These composites were added to the natural red, yellow, green, and blue pigments through two distinct application methodologies: bulk mixing into the pigment matrix and surface spray coating onto painted canvases. The antimicrobial profiles of pure chitosan, chitosan–titanium dioxide (TiO2) nanoparticles, and chitosan–zinc oxide (ZnO) nanoparticles were evaluated via the agar diffusion assay against key biodeteriorating agents, including the fungi Aspergillus niger and Candida albicans, and the bacteria Bacillus cereus and Staphylococcus aureus. The experimental results revealed that all tested configurations exhibited notable inhibitory effects. Crucially, the chitosan–ZnO nanocomposite demonstrated the highest antimicrobial efficacy across both application methods. Additionally, laterite-derived pigments (red and yellow) demonstrated better compatibility and increased antifungal performance. This study shows that chitosan–ZnO hybrids provide a pathway for the preservation of paintings using natural pigments under laboratory test conditions, leaving an opening for further long-term studies under real-world conditions.