Hybrid beta-Cyclodextrin/silica systems for the preservation of biodeterioration of stone materials

Biodeterioration of stone monuments represents a critical issue for the preservation and conservation of Cultural Heritage, since different biological attacks may affect archeological areas, statues, buildings and hypogea contexts. Commercial products are commonly used as anti-biodeteriogeneous agents in traditional restoration treatments. Unfortunately, they are often toxic and environmentally-unfriendly materials, easily subjected to a leaching process that could induce dispersion of the chemicals into the surroundings and produce an unstable effect. In order to obtain an eco-friendly product and to increase the durability of the protective treatment, a hybrid organic-inorganic system was designed by using suitable nanocarriers for a controlled release of biocide agents. This system consists of a beta-Cyclodextrin (beta-CD) derivative supported by mesoporous silica, loaded with BIOTIN R®, a commercial product successfully used for the protection of stones surfaces from the proliferation of algae and lichens. By acting with different release kinetics, according to the homing site of the biocide agent, this hybrid system should extend the efficacy of treatment for a longer time, if compared to traditional treatments, stabilize the compound on the stone surfaces and reduce the environmental impact. Both beta-Cyclodextrin and mesoporous silica are very suitable as nanocarriers for this purpose. In addition, due to its inorganic nature, silica has a strong chemical and physical compatibility with stone materials and it can be further processed for the production of more advanced systems. In this work, carboxymethyl-beta-Cyclodextrin sodium salts (CM-beta-CD) and hexagonal mesoporous silica (HMS) were used. The organic-inorganic hybrid system was prepared by functionalizing HMS with aminopropyl groups (HMS-NH2), which act as linker for anchoring the beta-CD. The grafting of the CM-beta-CD onto HMS-NH2 produced the HMSNH-CM-beta-CD system. Subsequently, the entrapment of BIOTIN R® was performed on the HMS-NH-CM-beta--CD samples. The hybrid system and its single components were characterized by surface chemical analysis (X-ray Photoemission Spectroscopy, XPS) and structural analysis (X-Ray Diffraction, XRD). Thermal analysis (TGA, DTA) and porosimetry by gas adsorption (BET) were performed in order to study the loading process of the biocide into the nanocarriers. The kinetics of release of the biocide from all the loaded samples (single CM-beta-CD, single HMS and HMS-NH-CM-beta-CD) was investigated by UV-VIS spectroscopy using an acidic solution simulating a typical urban polluted environment (rain water).