Development and application of cinnamaldehyde-loaded halloysite nanotubes for the conservation of stone cultural heritage

Abstract

Essential oils and their active antimicrobial components are promising in controlling the biodeterioration of outdoor stone cultural heritage, but their high volatility, hydrophobicity, and sensitivity to light, heat, and oxygen limit their practical application. To overcome the limitations of essential oils in the conservation of outdoor stone cultural heritage, we developed a sustained release system based on halloysite nanotubes. Firstly, we acid-etched the halloysite nanotubes by 4 M HCl to increase their specific surface area and pore volume, which thereby enhanced the loading capacity of halloysite nanotubes. Then, as a model active ingredient (AI), cinnamaldehyde was loaded into acid-etched halloysite nanotubes by vacuum method. A series of experiments revealed that the cinnamaldehyde loaded into acid-etched halloysite nanotubes not only improved their thermal stability but also could be slowly released into the deionized water, following Fickian diffusion. Finally, the cinnamaldehyde-loaded halloysite nanotubes were used in field experiments on Feilaifeng limestone. After a year of monitoring, the number of microorganisms on the limestone surface was greatly reduced after the treatment of cinnamaldehyde-loaded halloysite nanotubes. The cinnamaldehyde-loaded halloysite nanotubes showed great potential in controlling the biodeterioration of outdoor stone cultural heritage.