## 3 Materials Used for Restoration and Conservation of Masonry Elements
### Biocidal Interventions
#### 3.2.2 Nanotechnological Approaches  
Considering the shortcomings of the classical methods, a viable alternative is represented by the application of nanotechnology. Nowadays, nanotechnology emerged as a research field with successful application in the cultural heritage domain. As our group previously presented [67], the application of nanotechnology for the development of antimicrobial coatings represents an emerging field, which could offer valuable resources for specialists in restoration/conservation. In spite of the wide-spread use of nanomaterials, their application as biocidal agents in cultural heritage stone conservation represents the subject of a surprisingly low number of articles (details presented in Table 4). Usually, the use of nanomaterials in the field of antimicrobial protection of stone artifacts harvests the known potential of some metallic or metal oxide nanoparticles (such as silver, a wide known antimicrobial, ZnO, TiO\({}_{2}\), CuO, Cu, etc.) [68; 69; 70; 71; 72]. Among those examples, the hydrophobic film developed by [PERSON] et al. [68], incorporating a photocatalytic agent (TiO\({}_{2}\)) and an antimicrobial agent (Ag), showed very good antifouling properties for application on underwater marble artifacts. The procedure also proved to be reversible, after 20 months no trace of protective film being identified, suggesting a repetition of the treatment every 12-24 months. With a similar approach, [PERSON] et al. [73] used silver nanoparticles and Ag/TiO\({}_{2}\) nanocomposites as antimicrobial agents on limestone. By using two different synthesis procedures for silver nanoparticles--AgNPs (citrate and sodium borohydride reduction), the authors observed a direct correlation between the antimicrobial effect of the silver nanoparticles/silver nanocomposites and the hydrodynamic diameter. More than that, the AgNPs and Ag/TiO\({}_{2}\) nanocomposites obtained by the citrate method led to superior results regarding the biofuling reduction, even compared with a commercial biocide (>70%, compared with 53% for Biotin T) without significantly altering the aesthetic properties of the limestone (particularly AgNPs). The silver nanoparticles alone were also proven to be an efficient antimicrobial agent against bacteria and fungi developed on different materials (stucco, calcite and basalt), inhibiting the colonization of the stones (0.16%-3.6% colonization, compared with the untreated samples--8%-28% colonization, depending on the material) [74]. The authors observed a dependency on the AgNPs dose, and less on the NPs size (as is the case for the in vitro antimicrobial properties of the nanoparticles) [75; 76]. Another potential application of the nanotechnology is represented by the use of nanocapsules or nanoparticles for the controlled release of the biocidal agents. For example, [PERSON] et al. [77] proposed the use of pristine and functionalized silica (with amino or carboxylic groups) for the controlled release of the widely used biocides New Des 50 and Biotin T. The authors tested the efficiency of the proposed material both in vitro (against Gram-positive and Gram-negative bacteria--_Kokuriaizophrenia_, _Staphylococcus aureus_ and _Escherichia coli_). The proposed materials were tested by immersing in the solution a stone fragment (not defined) from a fountain located in Diamantina, Minas-Gerais, Brazil. The long-term effect was evaluated by determining the total viable bacterial counts after 1, 6 and 12 months (registering a 98.4% inhibition in bacterial counts after 12 months, compared with an untreated sample). This approach seems to be the most successful for future research works, as it harvests the nanotechnological advantages, together with the proven biocidal effect of commercial products. More than that, the method could be applied for the incorporation and controlled release of other types of biocides (such as, for example, the essential oils), which, without such delivery vehicles, hardly finds application in cultural heritage conservation.