Development of eco-friendly biocides based on Calamintha nepeta – loaded chitosan nanoparticles to be employed in the conservation of biocolonized cultural heritage materials.

Biodeterioration is a detrimental phenomenon having a high impact on the conservation of both organic and inorganic materials composing cultural heritage artefacts (i.e.: wood, stones, paper, parchment etc.). Researchers are focusing on the employment of natural and Phyto-derivative compounds as biocides for the microorganisms responsible of biodeterioration. In addition to their biocide properties, the employment of natural substances limits the ecological and healthy hazardous related to the employment of classical chemical biocides. Among the most promising candidates there are essential oils (EOs) that are volatile, heterogeneous mixtures of 20-60 organic compounds, with assessed biological properties. Researchers tested different EOs against microorganisms commonly found on cultural heritage materials, and some of them (the ones characterized by a majority presence of phenolic terpenes) gave good results in vitro [1]. The advancement of this kind of research foreseen the application of these substances directly on biocolonized materials. To achieve this aim, the EOs must be loaded in a matrix, that allows to 1) deliver the EOs on the substrata, 2) limit the volatility of the EOs, prolonging their persistence on surfaces. In two previous studies we tested three essential oils (from Thymus vulgaris, Origanum vulgare and Calamintha nepeta) dispersed in a PVA-based hydrogel matrix on a travertine biocolonized wall and on granite biocolonized samples, obtaining encouraging results, especially for C. nepeta oil [2,3]. In this research, we want to design a nano-particles system based on the presence of C. nepeta incapsulated in chitosan nanospheres, to be applied as biocide for organic and inorganic cultural heritage materials. Chitosan is a natural cationic polysaccharide derived from chitin, that is attracting attention because its non-toxicity, biocompatibility, biodegradability, and antibacterial properties, as well as the possibility of creating gels, films, and particles. The selected approach for the nano-encapsulation of C. nepeta oil consist in a two-step emulsification-ionic gelation procedure, which is particularly advantageous because it is a well-established, simple, and organic solvent-free approach for the formation of nanoparticles. Such method is based on the interactions between the negatively charged pentasodium tripolyphosphate (TPP) and positively charged molecules of chitosan, avoiding the employment of toxic crosslinking agents and other reagents to obtain the formation of inter- and intra-molecular linkages [4,5]. The characterization of C. nepeta – loaded nanoparticles includes dynamic light scattering, UV-Vis spectrophotometry, FTIR spectroscopy and SEM analyses to assess the encapsulation of the oil and the dimensions, the morphology and the shape of the nanoparticles. The achievement of this kind of formulations would lead to have 1) a completely green alternative to chemical biocides, but also 2) a very versatile product that exploits the inner properties of chitosan, that showed very encouraging results as a consolidants for wood artefacts [6] and textiles [7].