## 3 Materials Used for Restoration and Conservation of Masonry Elements
### Biocidal Interventions
#### 3.2.2 Nanotechnological Approaches
Another important aspect related to the potential use of nanoparticles is related to their potential harmful effects. The toxicity of nanoparticles represents to this date a subject of research. However, considering the relatively low levels necessary for the achievement of biocidal effect, the nanomaterials can be considered relatively safe for use [78]. In our opinion, future studies presenting the application of nanomaterials in the cultural heritage area should be accompanied by thorough toxicological studies. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Nanomaterial** & **Nanomaterial Characteristics** & **Application** & **Treated Material** & **Ref.** \\ \hline Functionalized carbon nanofibers & 80–150 nm/1–2–14 nm diameter, & Surface treatment (removal of black and gray & Marble & [79] \\ and nanotubes & commercially available & patina), finishing cleaning method & & \\ \hline Ca(OH\({}_{2}\)) mixed with ZnO/TiO\({}_{2}\) & 500/10-30/-50 nm & Antimicrobial (against _Pacililian calcium_ and & Limestone & [80] \\ \hline Ag nanopowider/ silane/siloxane & \textless{}100 nm, PVP coated, commercially & Surface treatment emulsion for facades (against & \\ emulsions & available & algae and cyanobacteria biofouling & & \\ \hline \hline TO\({}_{2}\)/SiO\({}_{2}\) nanocomposites & Theoretical proposal, no studies & Preventing biodeteroation & Mortar & [56] \\ \hline Ag nanoparticles & Phytosynthesized, 39 to \textgreater{}100 nm & Surface treatment (against _Patchotetarium_ & Stucco (pozzolanic material), \\ \cline{3-3}  & & _cartoeven_ and _Alternaria alternate_) & calcite and basalt & [74] \\ \hline TiO\({}_{2}\) & 4 nm, commercially available & Surface treatment (antifouling, against _Chorella_ cf. & \\ \cline{3-3}  & & _milliols_ Andrews and _Cromoccacidensis fissurarum_) & Fired bricks & [82] \\ \hline TiO\({}_{2}\), TiO\({}_{2}\)/Ag, TiO\({}_{2}\)/Cu nanoparticles & TiO\({}_{2}\) – 40–50 nm & Surface treatment, spraying in three layers (against & \\ \cline{3-3}  & & & different algal species) & Traverine & [83] \\ \hline TiO\({}_{2}\), ZnO and Ag nanoparticles & Particles mean diameter 100 nm, & Antifouling agent for underwater stone materials & & \\ dispersed in melted siloxane wax & commercially available & (against epitidic and endificific micro-organisms) & & \\ \hline Pristine and functionalized silica (MCM41) & Commercially available & Controlled release of commercially available & Stone & [77] \\ \cline{3-3}  & & & biocides New Des 50 and biotin T. & \\ \cline{3-3}  & & & \\ \cline{3-3}  & & & \\ \cline{3-3}  & & & \\ \cline{3-3}  & & & \\ \cline{3-3}  & & & \\ \hline ZnO and Ag nanoparticles & 30/25 nm, commercially available & Dip-coated mortar disks (agains _B. cereus_ and _E. coli_) & Mortar & [71] \\ \hline Si nanocapsules & 148 nm & Controlled release of an eco-friendly biocidal agent for antifouling coatings & Proposed for cultural heritage applications & [84] \\ \cline{3-3} Si nanocapsules and nanoparticles & 128/39 nm, with entrapped active ingredient & Controlled release of a commercial biocidal agent & Proposed for cultural heritage applications & [85] \\ \hline Ag nanoparticles and Ag/TiO\({}_{2}\) nanocomposites & 36/72/94 nm hydrodynamic diameter, depending on the synthesis pathway & Surface treatment (against multiple Chlorophyta and cyanobacteria) & Limestone & [72] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Nanomaterials applied as biocidal agents in cultural heritage conservation (references presented in chronological order).