Most historical buildings and monuments, that are a seamless part of our urban landscape, employ natural and artificial stone materials, which are subject to deterioration processes, due to the interaction with the environment (Jackson et al., 2005; Bourges et al., 2008; Torraca, 2009; Doehne and Price Clifford, 2010). In particular, urban environment accelerates the deterioration and may cause crumble of outdoor surfaces (such as architectural elements and sculptures), due to pollution and high concentration of fine particulate. In addition, biotic factors, especially microorganisms, are among the main agents of stone damage (McNamara and Mitchell, 2005; Caneva et al., 2008; Scheerer et al., 2009). Given the large number and the different nature of the stone materials and the various related degradation processes, finding potential supporting solutions is a challenging issue.

Among the consolidant products used for stone, TEOS-based materials have relevant applications and properties (Kim et al., 2009; Zárraga et al., 2010). When these products are applied on the decaying stone substrate, the tetraethoxysilane polymerizes into the porous structure, thus increasing the intragranular cohesion. Ideally, the TEOS matrix rebinds adjacent mineral grains (Zárraga et al., 2010), appreciably increasing the strength of the stone. However, although TEOS-based coatings are widely applied for consolidation interventions (Zárraga et al., 2010), it is well-known that during the drying phase these products undergo crack formation (Wen and Mark, 1995; Mosquera et al., 2003; Salazar-Hernández et al., 2009; Illescas and Mosquera, 2011; Xu et al., 2012; Pinho et al., 2013; Angulo-Olais et al., 2018).

Different authors have tested the effects of the addition of oxide nanoparticles. To quote a few of them, we mention that Milani et al. (2007) have investigated the realization of a new TEOS-based consolidant with colloidal silica particles. Kim et al. (2009) and the Mosquera's group (Mosquera et al., 2003) have also highlighted that the addition of nanoparticles into the TEOS matrix noticeably decreases the cracking during the drying phase.

Addition of TiO₂ nanoparticles, with antifouling and self-cleaning capabilities, seems a promising way in order to scale up the properties of TEOS based products; indeed titanium oxide properties have been widely tested in conservative treatments of stone surfaces (La Russa et al., 2012; Quagliarini et al., 2012, 2013; Kapridaki and Maravelaki-Kalaitzaki, 2013; Munafò et al., 2015; Gherardi et al., 2016; Xu et al., 2019). In the last years, self-cleaning and antifouling coatings have attracted the attention of many researchers in the field of stone conservation (Colangiuli et al., 2015; Lettieri et al., 2019). The self-cleaning properties of titanium oxide are due to its ability to photocatalyze the complete degradation of many organic contaminants and environmental toxins (Liu and Liu, 2016). Moreover, the photocatalytic activity of TiO₂ determines a collateral biocidal effect. The photocatalytic killing action by titania has been studied on a wide spectrum of organisms including bacteria, fungi, algae (Ruffolo et al., 2017). However, the structural properties of the target microorganism (e.g., the complexity and thickness of cell envelope) influence the microbiocidal efficacy of TiO₂: fungi, for instance, have a complex structure, which protects them from damage due to the radical species produced during the photocatalytic titania activities (Seven et al., 2004; Munafò et al., 2015).

In order to improve the antifouling effects, most coatings are prepared by dispersion of biocides molecules into the wet coating material (Omae, 2003; Almeida et al., 2007). This practice suffers from some drawbacks, such as poor control of the release rate of the antifoulants or degradation of the active substance. On the other hand, increasing the quantity of biocides into the coating, in order to extend its functionality over time is not economically, environmentally, and practically sustainable (Pinna et al., 2012; Fidanza and Caneva, 2019). Moreover, the high concentration of biocides into the coating may induce macroscopic damage of the film, detectable as a phase separation (Trojer et al., 2015).

Since several biocides are effective only for a short period of time after the application (between 6 months and 1 year), due to the cleaning of the surfaces after the treatments or due to the washing out of the compounds by rainfall, a frequent reapplication is needed (Jämsä et al., 2013). Thus, when indirect control methods are not possible (Almeida et al., 2007; Caneva et al., 2008; Pinna et al., 2012; Fidanza and Caneva, 2019), improving the performance of the antifouling coating is necessary from both economical and environmental points of view.

Several biocides have been used in antifouling treatments for buildings conservation (Nugari and Salvadori, 2003; Caneva and Tescari, 2017; Pinna, 2017). A lot of active principles widely used in the past, such as organ-stannic or organotin compounds, have been excluded for toxicological reasons. The quaternary ammonium salts (Quats, e.g., benzalkonium chloride) are among the most used materials, despite their lower efficacy with respect others (Nugari and Salvadori, 2003; Caneva and Tescari, 2017; Pinna, 2017). Considering its chemical properties and the documented efficacy, we selected, as an active principle, 2-mercaptobenziothiazole (MBT), which can be easily encapsulated into silica based nanocapsules (Maia et al., 2015). In commercial products (e.g., Vancide 51, Vanderbilt Company) MBT is included in combination with dimethyldithiocarbamate (Nugari and Salvadori, 2003). Nevertheless, in order to improve the stability of our silica nanocontainers, we have encapsulated and confined only the 2-mercaptobenziothiazole (Ruggiero et al., 2019). The potential oxidation of the biocide induced by the TiO₂ nanoparticles is expected to decrease thanks to the confinement into the silica containers that “protect” the biocide against oxidation. In this contest, we tested the addition of two different kinds of nanofillers: the commercial TiO₂ nanoparticles and, for the first time, the silica nanocontainers (Si-NC and Si-MNP) loaded with MBT synthesized in our laboratory. The addition of these nanofillers to the TEOS-based coating formulation will improve the physic-chemical properties of the proposed product while providing a multifunctional composite coating with antifouling and self-cleaning properties.

In particular, TiO₂ nanoparticles provide self-cleaning properties, while the two silica nanocontainers (hereafter labeled as Si-NC and Si-MNP) loaded with 2-mercaptobenzothiazole (MBT) will contribute to the antifouling action through a controlled release of the biocide. The first silica nanocontainer (Si-NC) shows a core-shell structure, and thus is called NanoCapsule. The second one (Si-MNP) consists instead of Mesostructured NanoParticles. Si-NC and Si-MNP nanocontainers have different size and different textural properties, which affect the loading capability and, consequently, the release rate of the MBT, as measured in water for 120 days (Ruggiero et al., 2019).

This study presents the design and the characterization of a composite coating, which may have application on outdoor stone monuments, while planning the necessary further tests on different lithotypes and biological communities. The incorporation of TiO₂ and SiO₂ nanocontainers into coating matrices is supposed to increase the quality of a multifunctional composite coating, as it combines the self-cleaning and antibacterial properties of TiO₂, the antifouling and mechanical strength of SiO₂ nanocontainers, and the controlled release of the encapsulated biocide. In order to select the best composition from a physic-chemical point of view, we compare a series of coatings by mixing in the starting sols different concentrations of nanoparticles.

In order to overcome the drawbacks of cracking of conventional TEOS-based materials, the proposed coatings was synthesized by combining three different strategies: (i) the addition of nanofillers, (ii) the addition of an non-ionic surfactant (n-octylamina) (González-Rivera et al., 2018), (iii) the modification of the elastic properties of the gel with Poly(dimethylsiloxane) hydroxy terminated (PDMS-OH) (Tamayo and Rubio, 2010; Zhang et al., 2010).

The coatings have been cast on plastic Petri dishes and examined after drying. First of all, their transparency has been investigated. Within the series shown in Table 2, dry coating (C) without nanoparticles is colorless and transparent but presents cracks. The addition of nanofillers into the sol changes the chromaticity of the final coatings: we notice a whitening of the charged coatings, with intensity depending on the type of nanoparticle and on their concentration. In particular, the addition of TiO₂ nanoparticles induces a whitening of the coatings with decreased transparency (first raw of Table 2). The coatings with Si-MNP maintains a good level of transparency with a few cracks. From the optical point of view, the reasonable amount of nanoparticles is 0.1% (w/v) and the best systems seem to be the coatings added with Si-MNP. The smaller size of Si-MNP in comparison with Si-NC allows the dispersion of these nanofillers into the coating matrix, reducing the optical effects correlated with the formation of aggregates.

In order to evaluate the effect of the nanofillers on the drying time of the gel, the weight loss of each sample has been measured. Figures 1A,B show the weight loss for the sols obtained by the addition of 0.1 and 0.2% w/v nanoparticles, respectively, over time (from 0 to 180 h), showing different drying rates.

The sample without nanoparticles shows the faster drying rate: the weight reaches a plateau after 145 h. Generally, the addition of the nanofillers slows down noticeably the drying rate, indeed the coatings with 0.2% w/v of fillers show a lower weight loss than the coatings with 0.1% w/v of fillers.

Comparing the green and magenta curves in Figures 1A,B, we notice that the coating charged with the Si-NC has a faster drying rate compared to that charged with the smaller sized Si-MNP. This difference persists, although reduced, when also TiO₂ particles are added; moreover, the presence of TiO₂ particles makes the drying process even slower. These observations, in the light of what reported in the previous paragraph, suggest that the nanoparticles are stratified in the coating, according to their average dimension, with the Si-NC occupying the bottom layers of it. As a matter of fact, it can be verified by naked eye that the Si-NC tend to deposit at the bottom of the coating and thus have a limited effect on the drying rate. On the contrary, the TiO₂ and/or Si-MNP, characterized by a smaller size than the Si-NC, seem to be more homogeneously dispersed in the coatings. This implies a slowing of the evaporation of the solvents and, consequently, a longer drying time of the coating. The presence of the nanofillers, distributed into the thickness of the coating, hinders this evaporation process by slowing the gelation and, consequently, the drying of the products. These results suggest that nanofillers interfere with the drying process. The coatings in this way penetrate into the porous substrate before the viscosity naturally increases upon the sol-gel transition (Milani et al., 2007). The most promising system seems to be once again the C_TiO₂/Si-MNP.

SEM micrographs, collected with InLens detector on the upper surface of the coating, are compared in Table 3. All the coatings show a homogeneous morphology, consisting of microstructured folds, formed by uniform nanospheres of size of the order of 13–15 nm. These spherical microstructures do not depend on the size distribution and the morphology of the nanofillers. The SEM micrographs are quite similar one to each other and suggest that, during the deposition of the coatings into the petri dish, the nanofillers are preferably distributed through the thickness, or in some cases precipitate at the bottom and for these reasons are not visible on the surface.

In order to investigate the textural properties of the hybrid coatings, we have measured the N₂ adsorption-desorption isotherms of the most promising samples as the as far as the optical properties and drying rates are concerned, i.e., those containing 0.1% w/v of nanoparticles. The corresponding textural data are reported in Table 4.

All the analyzed coatings show type IV adsorption isotherms, with a H1 hysteresis loop (see Figure 2A). This loop is more evident for the coatings loaded with TiO₂ nanoparticles. These isotherm profiles suggest that the proposed coatings are a network of silica nanoparticles and that the fillers (Si-NC/Si-MNP and TiO₂) are embedded and integrated into the porous structures (Milani et al., 2007).

The BET surface area of the empty coating is 440 m²g⁻¹, whereas for all sample with the addition of silica nanodevices and/or TiO₂ nanoparticles, the BET surface area is around 480 m²g⁻¹. In Figure 2B the peak of sample C is centered at 8.4 nm (BJH P.S.): the addition of Si-NC/Si-MNP and/or TiO₂ reduces the pore size to a value between 5.8 and 6.4 nm, without substantially changing the total pore volume (0.67 ÷ 0.76 cm³g⁻¹). These results suggest that the nanoparticles are integrated into the coating matrix, inducing an increase of BET surface area of about 10% and a decrease of the pore size of about 30% in comparison with the empty coating (C). The textural properties outline that the size of the two different silica fillers determines the properties of the coatings. When the TiO₂ nanoparticles are added into the coating formulations, the differences in surface area between the coating loaded with Si-NC and Si-MNP are reduced, as shown in Table 4. Moreover, TiO₂ nanoparticles induce an increase of the pore size in the coating loaded with Si-NC. This behavior suggests that the TiO₂ nanoparticles work in synergy with the Si-NC, positively shaping the network structure. The Si-NC, because of their diameter, tend to precipitate and agglomerate during the sol-gel process. Otherwise, the Si-MNP shape the network modifying the porosity of the coating in comparison with the empty coating. For this reason, when the TiO₂ nanoparticles are added to the coating with Si-MNP, the TiO₂ is embedded in the porosity created by the Si-MNP, decreasing the pore diameter of the C_ TiO₂/Si-MNP (Pinho and Mosquera, 2011).

Structural characterization of dried coatings has been performed by means of Fourier Transformed Infrared Spectroscopy. Figures 3–5 report the FTIR spectra of the coatings under study. The band corresponding to the Si–O–Si vibration (~1,050 cm⁻¹), being the most intense, has been used for the normalization of the infrared spectra. The spectra (Figure 3) show a few bands that are unequivocally assigned to vibrations of an organic fraction, namely PDMS-OH. In particular, the band at 2,907 cm⁻¹ corresponds to the symmetric stretching vibration of the C-H bonds, whereas the bands at 2,929 and 2,964 cm⁻¹ are assigned to the asymmetric stretching vibration of the C-H bonds in CH₂ and CH₃ groups, respectively (Tamayo and Rubio, 2010; González-Rivera et al., 2018).

The symmetric bending vibration of CH₃ groups in PDMS-OH contributes to a sharp band centered around 1,260 cm⁻¹ (Zhang et al., 2010; Goffredo et al., 2019) in Figure 4. The band at ~790 cm⁻¹ is related to the rocking modes of methyl groups and to the Si-C stretching in Si-(CH₃)_n, as reported in Figure 4 (Fidalgo et al., 2005; Johnson et al., 2013).

The copolymerization of Si-OH group of hydrolyzed TEOS and Si-OH of PDMS-OH is suggested by the band at 850 cm⁻¹ (Tellez et al., 2003), as reported in Figure 4. This implies that during the synthesis PDMS-OH and TEOS interact, producing a homogeneous hybrid coating.

Several vibration modes of siloxane bonds contribute to the most intense band of the infrared spectra, in the region between 1,000 and 1,250 cm⁻¹. In detail, the large band centered at 1,050 cm⁻¹ is related to the asymmetric ν(Si–O–Si) stretching of the Si-O-Si network and it can be deconvoluted in four components: two longitudinal (LO) and two transverse (TO) optic modes. These components correspond to the four-membered (SiO)₄ and the six-membered (SiO)₆ arrangement of siloxane rings (Fidalgo and Ilharco, 2004). The position and the relative intensities of the LO and TO modes, according to the literature, change when chemical groups or organic molecules modify the silica network (Fidalgo and Ilharco, 2004).

In this contest, a shift of the Si-O-Si asymmetric stretching bands toward higher wavelengths in comparison with the empty coating is observed (from 1,042 to ~1,050 cm⁻¹, Figure 5). These shifts are ascribed to the network deformation needed to accommodate the nanoparticles within the hybrid coating matrix, as summarized in Table 5. This phenomenology outlines that the distance between interacting species (e.g., the matrix and the nanoparticles) is reduced by the increase of the long-range Coulomb interactions. This finding is in agreement with the porosity decrease, suggested by N₂ adsorption-desorption measurements (Li et al., 2013; Xu and Li, 2013).

As reported in Table 5, the position of the band at ~555 cm⁻¹ (Figure 4), assigned to a coupled mode in four-member siloxane rings (SiO)₄, moves from a maximum of 555 cm⁻¹ to a minimum of 543 cm⁻¹, depending on the species and concentration of dispersed nanoparticles (Tamayo and Rubio, 2010).

In order to evaluate the chromatic stability of the coating over time, the total color variation (ΔE^*) of the samples has been measured at t = 0¹ and after 20, 40, and 60 days. All the coatings show a ΔE^* below the perceivable threshold. The chromatic feature of the coating does not change over time, regardless of the type and quantity of nanoparticles added as filler. Such absence of chromatic variation will be further tested in direct application of these coatings on outdoor building surfaces, following the standards in the field. Future plans also include investigating the photocatalytic activity of the commercial TiO₂ nanoparticles added to the coating, and the long-time effects on the stone properties after the treatment (e.g., color, porosity, and permeability changes).

In this work, we report the synthesis and the multi-analytical characterization of an innovative TEOS-based composite coating with the addition of two different silica nanofillers with antifouling properties. The best results in terms of optical properties (e.g., visual aspect and transparency), porosity and absence of cracking are obtained with the following nanofiller concentrations: 0.05% w/v of TiO₂ and 0.05% w/v of Si-MNP or Si-NC loaded with MBT. The coatings with Si-MNP as filler show better properties than the coatings with Si-NC: indeed, the size of the filler shapes the behavior of the coatings, modifying not only the visual aspect, but also the drying rate. Further tests are in progress to check the interaction of this new coating with the most common substrates of interest for conservation of buildings and Cultural Heritage, as for instance its photocatalytic and biocidal activities and long-lasting efficacy on different lithotypes.

The datasets generated for this study are available on request to the corresponding author.

LR made the synthesis, some characterizations, and drafted the manuscript. MI and LT made FT-IR analysis. MF made the colorimetric measurements. GC, MR, and AS coordinated the project and participated in the discussion of the results. All authors contributed to write the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.