Evaluation of the applicability of nano-biocide treatments on limestones used in cultural heritage

Abstract One of the main problems in the conservation of historical buildings and archaeological sites is the one caused by biodeterioration. Biopatina, biocrust or biofouling generate aesthetical changes and induce degradation processes within the stone matrix. In this work, three treatments based on silver nanoparticles and silver/titanium dioxide nanocomposites have been studied as potential biocides for limestones: citrate-stabilized silver nanoparticles, silver/TiO 2 nanocomposites and citrate-stabilized silver/TiO 2 nanocomposites. These treatments were synthesized following a bottom-up method, using or not a stabilizing agent (citrate), and have been characterized by UV-Visible spectrophotometry, Dynamic Light Scattering and Raman spectroscopy. These treatments were applied on limestones from three different Spanish quarries located in Utrera (Seville), El Puerto de Santa Maria (Cadiz) and Novelda (Alicante). The aesthetical modification of limestone surfaces was studied by colorimetric techniques and the effectiveness of protection against biofouling formation was tested using an accelerated biofouling growth assay. The best results were obtained for the treatments based on citrate-stabilized silver nanoparticles. The effectiveness of the treatments depends on their penetration depth in the stone matrix and in this study, we have used Laser Induced Breakdown Spectroscopy to determine the depth profiles of nanocomposite presence in the stone matrix. Our results demonstrate that nanocomposites based on citrate-stabilized silver nanoparticles can be useful for the treatment of historical buildings and archaeological sites made of limestone, without producing high colour increments. Additionally, we have demonstrated the suitability of Laser Induced Breakdown Spectroscopy for the detection of silver/TiO 2 nanocomposites and for the generation of depth profiles.