Plant essential oils suspended into hydrogel: Development of an easy-to-use protocol for the restoration of stone cultural heritage

The development of phototrophic biofilms deteriorating stone cultural heritage is a well-known phenomenon that causes not only discoloration but also micro-decohesion of substrates. Recently, essential oils (EOs) have resulted as successful natural biocides to be used for the restoration of cultural heritage as an alternative to chemicals. In a first experiment EOs of Lavandula angustifolia Mill. and Thymus vulgaris L. at 5% concentration were tested against cyanobacterial biofilms. Moreover, to develop a suitable protocol for in situ application, diverse concentrations (0.5% - 0.25% - 0.1%) of T. vulgaris EO were encapsulated within an alginate hydrogel and applied for different times on cyanobacterial biofilms. Chemical profiles of the two EOs were investigated via GC-MS, to identify the main plant compounds underlying the biocidal effects. Results showed that T. vulgaris EO, rich in thymol, had the greatest inhibitory effect of photosynthetic efficiency compared to essential oil of L. angustifolia, especially if suspended into hydrogel. The development of such innovative protocol allowed the concentration of the tested essential oil to be 0.1% (v/v), maintaining antimicrobial efficacy against cyanobacteria and reducing the volatility of the terpene components. This innovative method will facilitate the application also on vertical surfaces and the removal of the biofilm.