Subsurface treatment of TiO⁠2 nanoparticles for limestone:Prolonged surface photocatalytic biocidal activities

We report on a new subsurface deposition method for treating porous stone surfaces with titanium dioxide nanoparticles. Limestone samples were semi immersed in a TiO2 sol so that nanoparticles could be transported via capillary diffusion into pores and grains to accumulate in the near surface region where they aggregated and attached to the walls as water evaporated. The white glazing layer formed on the surface was wiped off to facilitate further diffusion. A few cycles of immersion, evaporation, and surface cleanup were repeated until the amount of TiO2 reached a saturation level. Lab tests showed that nanoparticles loaded with this method are more difficult to remove than in the case of direct coating of the surface. Even after accelerated washing and drying cycles by spraying water equivalent to 5000 mm rainfalls the sample surface still preserved good antifungal activity against P. chrysogenum and C. cladosporioides whereas such effects disappeared after the same washing process for specimens directly coated with similar amount of TiO2 nanoparticles.