Titanium dioxide based nanotreatments to inhibit microalgal fouling on building stone surfaces

Stones are among the most widespread traditional materials used in construction for both structural and ornamental purposes, especially for Cultural Heritage monuments. Stone materials both natural and man-made are quite prone to algal colonisation that may cause biodeterioration of building materials by both affecting the aesthetic of their surfaces and compromising their entirety and durability. Nano technology has been recently used in both Cultural Heritage and construction industry to improve the preservation of building surfaces and titanium dioxide (TiO2) in its nanometric form is among the most used and promising nanotechnological material. This study investigated the biocidal ability of photocatalytic TiO2-based nanocompounds (also in combination with Ag and Cu nanoparticles) applied on travertine surfaces by spray-coating in order to limit or inhibit algal fouling. The aesthetic compatibility with stone has been assessed using colorimetry. Algal fouling was simulated by means of an accelerated water run-off test under artificial solar light and weak UV irradiation. Antialgal capability of metallic nanotreatments was evaluated through the combination of different parameters monitored for 9 weeks: human perception of the colour change, reflectance reduction and measurement of area colonised by algae. Nanoproducts had a limited impact on surface colour of the substrate after application making them suitable for restoration. Even though photocatalysis prevented algal fouling only partially, some nanotreatments moderately reduced the bioreceptivity of coated stones mainly limiting the area colonised by microalgae. Further investigations are necessary, since the conditions used to accelerate algal growth may affect greatly the biocidal efficiency of nanotreatments. (C) 2016 Elsevier Ltd. All rights reserved.