Chitosan coatings reinforced with cellulose crystals and oregano essential oil as antimicrobial protection against the microbiological contamination of stone sculptures

The proliferation of microorganisms in outdoor stone sculptures and cultural objects can damage the structure and aesthetics of the materials through biodeterioration mechanisms. Biocides and synthetic products are often used to prevent this phenomenon, despite their negative impact on the environment and human health. Less toxic alternatives with reduced environmental impact can be an option for the preventive conservation of stone sculptures to reduce the environmental impact. In this work, chitosan formulations reinforced with two types of cellulose crystals (microcrystalline cellulose (MCC) or cellulose nanocrystals (CNCs)) and with or without citric acid and sodium tripolyphosphate were prepared. The films obtained with these formulations showed low solubility, and those only containing MCC or CNCs had the lowest wettability. The formulation containing 2% (w/v) MCC was selected for further analysis and supplemented with oregano essential oil (OEO) at 1% (v/v) and 2% (v/v), exhibiting low solubility, swelling and wettability when polymerised in film form. Inoculation of the films with Staphylococcus aureus, Bacillus cereus, Pseudomonas aeruginosa and Rhodotorula spp. resulted in total or partial inhibition of their growth, as well as a 60–100% reduction in Penicillium chrysogenum growth, depending on the concentration of OEO. The formulation with 2% (v/v) OEO was applied to samples of granite, marble and limestone, forming a protective, yet irregular coating on their surfaces. The wettability of the stones’ surfaces was reduced without becoming completely water-repellent and the coating did not cause visible colour changes.