Recent Advances in the Application of Metal Oxide Nanomaterials for the Conservation of Stone Artefacts, Ecotoxicological Impact and Preventive Measures

Abstract

Due to the ongoing threat of degradation of artefacts and monuments, the conservation of cultural heritage items has been gaining prominence on the global scale. Thus, finding suitable approaches that can preserve these materials while keeping their natural aspect of is crucial. In particular, preventive conservation is an approach that aims to control deterioration before it happens in order to decrease the need for the intervention. Several techniques have been developed in this context. Notably, the application of coatings made of metal oxide nanomaterials dispersed in polymer matrix can be effectively address stone heritage deterioration issues. In particular, metal oxide nanomaterials (TiO₂, ZnO, CuO, and MgO) with self-cleaning and antimicrobial activity have been considered as possible cultural heritage conservative materials. Metal oxide nanomaterials have been used to strengthen heritage items in several studies. This review seeks to update the knowledge of different kinds of metal oxide nanomaterials, especially nanoparticles and nanocomposites, that have been employed in the preservation and consolidation of heritage items over the last 10 years. Notably, the transport of nanomaterials in diverse environments is undoubtedly not well understood. Therefore, controlling their effects on various neighbouring non-target organisms and ecological processes is crucial.

1. Introduction

Building materials are subject to a complex environment in which biological, chemical, and physical variables often work in concert to induce degradation and significantly alter their appearance [1]. In general, cultural heritage items made using stone materials are composed of different pores, with pore sizes varying from one item to another [2]. This feature, associated with bad conservation status, enhances the capacity of stone to absorb water. Decay processes of stone materials are strongly related to their porosity properties due the fact that decay agents (including water) may easily penetrate under their surface [3]. Moreover, air pollution and climate changes participate in material deterioration as well [4]. Along with the presence of water and increase in surface roughness, these abiological factors enhance the chances of microorganism colonization and growth. To preserve tangible cultural heritage items worldwide, three main approaches are followed: restoration, remedial conservation, and most importantly, preventive conservation. In general terms, restoration is the repair of damage; such treatment seeks to restore the work of art as close as possible at the original status [5,6,7,8]. Remedial conservation is based on techniques to arrest current damage and reinforce the structure in order to preserve it [9]. Examples of remedial conservation include the stabilization of corroded metals, consolidation of mural paintings, dehydration of wet archaeological materials, etc. Instead, preventive conservation is generally carried out after conservation treatment in order to apply a “set of operations aiming to prolong the life of the material by avoiding/preventing future damage” [10]. In fact, preventive conservation is a methodology that intends to control the unavoidable deterioration of cultural goods by minimizing the most expensive interventions. Very often, these different approaches are consequential and are all applied together in a well-planned conservative intervention to preserve cultural heritage items.

At a high level, the preventive conservation of stone materials which have been used in cultural heritage items includes the application of protective coatings [11]. Protective materials that are frequently employed in the preservation of cultural heritage items include acrylic resins [12], silicone resins (polyalkyl alkoxysiloxanes or alkylalkoxysilanes) [13,14], acrylic resins combined with silicones [15], fluorinated polymers [16,17,18], and other synthetic organic polymers. Due to their water repellence features, these polymer-based coatings prevent water penetration inside pores and stop microorganisms from adhering to stone surfaces. Despite each of them exhibiting advantages, none of them can be considered the best solution for every cultural heritage item. Indeed, they also present severe limitations, such as migration inside the stone, porosity with time, consequent loss of protection effectiveness, low permeability to water vapour, poor durability, and insufficient compatibility of organic materials with substrates [3].

Alternatively, scientists have proposed several inorganic consolidants (water saturated solutions of Ca(OH)_2, ammonium oxalate, and diammonium hydrogenphosphate), which usually display better durability and higher physico-chemical compatibility with stone than organic ones [19]. Although they show good properties, there are other limitations (insufficient penetration depth, unsatisfactory strengthening effect, and need for many applications). In the last decade, coatings based on nanomaterials (Ca(OH)₂, Sr(OH)₂, Mg(OH)₂, and nano-apatite nanoparticles) have been used in the conservation of wall paintings, bas-reliefs, and ancient monuments due to their distinctive properties [19,20,21,22].

The optical, physical, and magnetic qualities provided by nanostructured materials make it possible to create transparent hydrophobic and/or antifouling protective layers on surfaces. Moreover, nanomaterials have larger surface areas compared to similar masses of larger-scale materials, thereby enhancing their chemical reactivity. They have high reactivity even when kept on the surface of restored materials. Furthermore, thanks to their tiny particle size these nanomaterials can penetrate deeply inside damaged stone materials. Recently, treatments of monuments materials have shifted into the application of nanomaterials with self-cleaning properties instead of coatings with combined biocidal and antipollution properties. In fact, the self-cleaning properties of these coatings could be more effective, having by design the aim of avoiding any attachment of inorganic and organic particles, including microorganisms. Many studies have been performed in this context [23,24,25,26,27]. In particular, heterogeneous photocatalysis using metal oxide nanomaterials such as TiO₂, ZnO, CuO, AgO, MgO, etc. has been considered as an interesting method and used extensively for cleaning applications [28,29]. The particular activity of these oxides, based on the generation of reactive oxygen species (ROS) when they are exposed to UV light, makes them very attractive compounds [30].

However, although on the one hand these nanomaterials possess discrete self-cleaning properties, they possess a certain toxicity as well. In fact, over the years an increasing number of studies have pointed out the risks of nanotoxicity, including mortality rates, new diseases linked to contact between nanomaterials and the human body, and commonly, impacts on other living beings [31].

The present review paper intends to first present the main factors that contribute to the deterioration of stone materials used in cultural heritage items. Next, we exhibit the main features of metal oxide nanomaterials and the recent developments in their use for heritage preventive conservation that have occurred in the last ten years. Finally, we end the review by discussing the impacts of nanomaterials on the environment, human beings, and other living beings along with the precautions that must be taken when working with them.

2. Decay Mechanisms of Stone Materials

Most of our cultural and historical heritage items are in danger due to different combined processes of physical, chemical, and biological decay. In outdoor settings, such deterioration often follows a process of loss, deterioration, corrosion, or fracturing, which is shown by a deep modification of the initial physical, chemical, and visive features. Deterioration can be caused by:

• Intrinsic factors: depending on the nature of the material, orientation, manufacturing technique, and procedures used to affect the work.
• Extrinsic factors: due to external sources such as environmental factors (temperature, air pollutants, relative humidity, and light), anthropogenic factors (handling, misuse, vandalism, tourism, etc.), catastrophic factors (earthquakes, fires, floods, etc.), and not less important, biological factors such as macro- and micro-organisms [1].

Among the main mechanisms of deterioration, three processes are the most known. The first mechanism consists of physical or mechanical processes in which material behaviour is altered because of various mechanical forces (tensile, compressive, etc.) without changing the chemical composition of the substance. The second process consists of chemical processes in which the matter is altered as a result of a chemical reaction. The third process involves biotic processes in which living organisms engage in mechanically and chemically attacking the material; this process is known as biodeterioration [10].

In addition to intrinsic factors, the following main causes of decay involved in the deterioration of cultural heritage items are explicated in the following parts: water, microorganism colonization, weather, and climate changes.

2.1. Intrinsic Properties of Stone Materials

Several kinds of stone are utilized for heritage buildings around the world, such as sandstone (e.g., Angkor monuments, Cambodia) [32], limestone (e.g., Megalithic Temples, Malta) [33], calcarenite (as in the Baroque town of Noto in Sicily, Italy) [34], Candoglia marble (e.g., Milan Cathedral, Italy) [35], granite (e.g., Évora Cathedral, Portugal) [36], and volcanic rock (e.g., the Churches of Lalibela, northern Ethiopia) [37]. This diversity may explain the variability in the intrinsic assessment of the features of stone materials, particularly their crystal structure, physical properties, and mineral constituents. Pires et al. (2022) have stated that the presence of clay minerals may affect the properties of building stones [38]. When exposed to water or moisture, common deterioration patterns can include color changes, increased fracture, and swelling those results in loss of material. On the other hand, porosity is a key factor in the lifetime of stone. In fact, when it comes to the processes of stone degradation, porosity is a crucial factor. In addition to measuring pore space volume, porosity provides information on pore connectivity [39]. Pore connectivity rises with porosity and is correlated with the pore space skeleton coordination number. Pore diameters in porous stone materials can range from millimeters to nanometers. Depending on how the pores are connected, the existence of very large pores may affect how quickly stone deteriorates [40]. Indeed, the degradation of stone is highly influenced by the existence of particularly large pores. Thus, pore size emerges as the key pore structure parameter in terms of heritage stone materials degradation. Different decay processes can take place in stone depending on pore size, including capillarity imbibition, water adsorption, capillarity condensation, and crystallization pressure due to salt and ice. Many physical and chemical degradation processes are caused by the penetration of water inside the pore network.

2.2. Water

Water is often the main factor behind the processes that alter and degrade cultural heritage items. Historical buildings are often made from highly porous materials with average porosity ranging from 30% to 40% [41]. Over the years, historical buildings constructed from porous materials experience increased porosity due to physical and chemical degradation processes, leading to additional water absorption and consequent weathering [42].

Stone in contaminated urban environments is subject to alteration processes due to significant temperature swings and the crystallization of water-soluble salts within the porous stone, which can impair its coherence and mechanical characteristics [43]. Salt crystallization is one of the most damaging abiotic mechanisms involved in the rapid disintegration of porous stone [1,19,44]. When stone is highly permeable to water, water is able to move through the layers of stone, with salt crystallizing where the water is evaporated. When water contained in stone evaporates, the salt concentration increases until precipitation occurs [45]. The types and sources of these salts vary and have changed over time, primarily as a result of human activity in urban areas [46]. Salt crystallization rate of decay is influenced by a variety of factors, including the kind of salt, the temperature and relative humidity the heritage materials’ location, the pore size and pore density of the stone used for construction, and the frequency of the wetting/drying cycles [47]. Eventually, these factors may result in moisture retention, slow carbonation beneath the surface, and mechanical weakness in porous stone. The majority of earlier studies on salt weathering have only looked at a single salt composition; however, there are always a variety of salts present in structures [48].

Pollution can affect marble and limestone buildings through the dissolution of calcite due to acid rain [1]. Acid rain refers to rain with a pH equal to 4 or lower. When the calcite in marble or limestone reacts with the sulfuric, sulfurous, and nitric acids from contaminated air and rain, the calcite dissolves [49]. Consequently, the removal of material and the loss of carved details can be observed in the exposed portions of buildings and statues [50]. In the past, volcanic activity was the main cause of acidification. Nowadays, air pollution due to human activity contributes to the dissolution of monuments and structures through acid rain.

In “cold regions” characterized by air temperatures below the freezing point, freeze–thaw cycles are another mechanism that contributes to the deterioration of stone [19]. Stone naturally contains pores that allow it to hold water both on the surface and inside pores for a long time [51]. The primary mechanism of rock deterioration results from the dual action of the water–ice phase transition. Alterations in the water state (liquid, solid, gas) in the pores due to variations in the ambient temperature induce deterioration of the rock structure due to the alternation between cold contraction and thermal expansion process [51]. The freezing–thawing action of the water inside the pores and fissures plays a central role in the alteration of rocks due to freezing when the temperature oscillates across 0 °C [52]. Water freezes at temperatures below 0 °C and converts into ice, which can cause internal stresses on the stone and lead to damage. In fact, crystallization pressure is produced when ice is created inside the pores, which consequently creates pressure against the material walls of the pores [1]. Previous studies have demonstrated that the characteristics of the stone (i.e., porosity, lithology, and water content) and the environment where heritage materials are located, including the frequency of the freeze–that cycle, amount of temperature variation, and resulting stress levels all have an impact on the freeze–thaw failure phenomenon [53].

On the other hand, atmospheric and soil moisture may increase the concentration of weathering agents, allowing water to enter the nucleus in the case of porous sandstone. The damaging effects of moisture on historical buildings can be observed in different ways, starting with salt erosion, chemical corrosion of the historical artefact, and the development of microorganisms such as fungi, algae, and moss, with cracks on the stone surfaces ultimately observed due to swelling and shrinkage [54]. High water availability may promote microbial growth by increasing intracellular water potential, consequently stimulating hydration and enzyme activity.

2.3. Microorganism Colonization

While stone artefacts decay as a result of physical and chemical processes, biological processes can cause aggressive damage as well [55]. In particular, microorganisms on stones can cause what is known as “biodeterioration”, which was initially established by Polynov in 1945 while researching soil formation [56]. In the following studies, “biodeterioration” has been broadly accepted as any biologically-caused unfavorable change in material appearance and properties [48,56]. Cultural heritage assets are a highly diverse environment and are inhabited by a variety of microorganisms. These have an undeniable impact on the degradation of works of art [57]. Biodeterioration is a global issue, as microorganisms can colonize cultural assets in any country regardless of the climatic conditions and the features of the item [58]. Different kinds of microorganism groups can adhere to and develop on stone materials depending on environmental factors and the chemical composition of stone artefacts. Bioreceptivity is affected by the intrinsic characteristics of stone, such as its surface roughness, open porosity, chemical composition, capillary water, and abrasion pH [59,60]. Moreover, depending on the in situ microclimate, ambient climatic conditions, and physiochemical and chemical item features, various microorganisms (i.e., bacteria, fungi archaea, algae, cyanobacteria, and lichens) can be deposited on stone heritage materials and develop into epilithic and/or endolithic biofilms [61].

Biological organisms are able to cause aggressive aesthetic and structural damage. Aesthetically, microorganisms result in the appearance of spots, discolouration, encrustation, streaks, and coloured patinas. Structurally, artefacts can be deteriorated through water retention, the disintegration, degradation, and breakage of materials, and alkaline dissolution [1]. To ensure efficient protection of monuments and temples, identification of the specific active microorganisms and their physiological roles is required for better knowledge of the degradation mechanisms and processes that cause deterioration. The mechanisms of degradation through biological processes are different, and depend on factors such as climatic conditions, stone features, and living microbial communities. As mentioned before, Pollutants are able to decay stone artwork through chemical weathering (salt crystallization/dissolution) and may in turn encourage microbial biodeterioration. On the other hand, it has been reported that fungi are some of the most active microorganisms that may utilize organic support as nutrition [62]. Due to their heterotrophic nature, fungi perform biodeterioration by transferring this support and creating specific chemicals, including inorganic and organic acids [62]. In particular, black meristematic fungi are an example of microorganisms well adapted to live on rock surfaces in harsh environmental condition, including stone monuments exposed outdoors [63]. Their resilience is related to polyextremotolerant characteristics such as the presence of melanin in the cell wall, meristematic development, oligotrophy, and slow growth. Moreover, the capacity of eukaryotic microalgae and prokaryotic cyanobacteria to chromatically adjust to various types of light enables them to grow on stone in archaeological sites with low light intensities, i.e., caves crypts, and catacombs [64,65,66]. Complex microbial communities need to be investigated and identified due to their high contribution to the decay process [65,67]. More details on the mechanisms of degradation of heritage stone materials through biodeterioration can be found in the reported literature [34,56,68,69].

2.4. Weather and Climate Changes

Cultural heritage places additionally suffer from the effects of weather and climatic changes. Weather conditions (e.g., temperature, humidity, etc.) play an important role in the degradation process. The negative effects of improper temperatures (too hot or too cold) lead to gradual degradation that may only become apparent over time, and the results could consequently be underestimated. Elevated temperatures can result in the desiccation of organic materials, causing them to become less flexible and break. Lamp et al. (2016) stated that thermal stress weathering contributes to flaking, cracking, and exfoliation of porous rocks [70]. The same authors declared that thermal stresses, hydration of clays, crystallization, subsequent hydration of salts, and freezing of pore water can all contribute to spalling in porous rocks found in temperate locations. In addition, stone may experience both macro- and micro-degradation processes under conditions of intense heat, which can result in structural instability (e.g., stone cracking, color change, textural alteration, mineralogical changes, etc.) [71]. Intense heatwaves often cause fires. Fire is a major threat to stone-built cultural heritage. It has been stated that a rise in cultural heritage exposure to fire in the Europe is highly predicted [72]. Moreover, alterations in temperature and rainfall affect the distribution and abundance of lichens and other organisms that can develop on stone substrates [71,73]. Relative humidity, or RH, is another factor that needs to be taken into consideration when studying decay factors. It is generally recommended that RH values at heritage sites be between 40% and 65%. Environment with inappropriate RH is likely to affect stone heritage materials and participate in their degradation. It has been revealed that biological deterioration of cultural materials is amplified by a rise in relative humidity in warmer climates [74]. On the other hand, for stone heritage materials, natural moisture condensation is considered to be a significant cause of degradation [75].

Climate change has a harmful impact on human beings, natural systems, and both natural and cultural world heritage items [76]. Experts from the Intergovernmental Panel on Climate Change (IPCC) claim that climate change has an impact on the frequency and severity of dangerous occurrences, including floods, landslides, and droughts [77]. Gradual variations in temperature, air moisture, wind speed, fire due to heatwaves, sea level rise, desertification, ocean alteration of properties, and other climate-related changes have a destructive impact on cultural heritage sites [71,76]. All of these threats and others have already been named by the United Nations Educational, Scientific, and Cultural Organization (UNESCO) [78]. For historical buildings, water is the primary cause of degradation. If precipitation increases due to climate change, soils may become saturated and downpipes and gutters may be overloaded, resulting in a higher danger of dampness penetrating into art or worked materials such as masonry walls [71]. Moreover, because soluble salts are present in stone materials, fluctuating variations in temperature and precipitation result in more salt crystallization cycles, and consequently more damage [71,79]. On the other hand, alteration of wind speed or direction during storms can destroy historic structures and archaeological sites [80]. Figure 1 summarizes the impact of the extrinsic degradation factors discussed in this section on cultural building items.

3. Metal Oxide Nanomaterials: Properties and Applications

Metal oxide nanomaterials with small crystallite sizes and high surface areas have attracted a great deal of interest due to their wide range of applications. Because of their distinctive features, synthetic metal oxide nanoparticles are among the most widely produced nanomaterials [81]. As the size of the nanoparticles decreases, more surface and interface atoms are produced. The specific size of the nanoparticle can determine its magnetic, conductive, chemical, and electrical capabilities [81]. Certain metal oxides possess photocatalytic features, making them able to absorb light, induce the charge separation process, generate electrons and holes, and oxidize organic pollutants. A photocatalyst is a substance that can absorb light and generate electron–hole pairs, which allow the participants to perform chemical transformations. The key characteristics of a photocatalytic system are an appropriate band gap, suitable shape, large surface area, stability, and reusability [82]. In this review, four kinds of different metal oxide NPs based on titanium, zinc, magnesium, and copper are discussed in detail in relation to their frequent use in treatment applications for stone heritage structures.

Titanium dioxide is a naturally occurring titanium compound. In the early 20th century, this material was used as a white pigment because of its high refractive index [83]. Titanium dioxide has been extensively used for many industrial applications, including as a white pigment included in paper, plastic, medicinal and cosmetic products, toothpastes, food coloring, and many others instances in which white coloration is desired [83]. The photocatalytic activity of TiO₂ was later discovered and published in Nature, and is called the “Honda Fujishima effect” [84]. Many studies have focused on its photocatalytic effectiveness under different conditions. These unique properties have widened the application of TiO₂ to environmental and ecological applications such as air purification, water treatment, self-cleaning coatings, and non-spotting glass, as well as bio-medicinal applications such as self-sterilizing coatings. The use of TiO₂ has received wide attention thanks to its biological and chemical stability, low cost, ease of production, and harmlessness to the environment [83]. Because of its distinctive properties, particularly its capacity for photocatalysis, TiO₂ is regarded as an interesting semiconductor [85]. When titanium dioxide absorbs enough energy, all photoinduced phenomena, including photocatalysis, superhydrophilicity, and photovoltaics, are initiated. The photocatalytic mechanism of TiO₂ is illustrated in Figure 2.

On the other hand, it has been claimed that titania has a substantially slower rate of charge carrier recombination compared to other semiconductors. This is advantageous because it has been proposed that chemical processes require photo-generated charge carriers (e⁻/h⁺) with a lifetime of at least 0.1 ns [30]. When a photon with an energy higher than the band gap energy is absorbed by a semiconductor, an electron passes from the valence to the conduction band, creating a valence band hole ($h_{Vb}^{+}$) and conduction band electron ($e_{cb}^{-}$) according to the following equation.

$${\mathrm{T}\mathrm{i}\mathrm{O}}_2+h\mathrm{ʋ}\to e_{cb }^{-}+h_{vb}^{+}$$

(1)

Hence, surface-based oxidation/reduction reactions can start as photogenerated electron–hole pairs, diffuse to the surface, and transfer to absorbed species (i.e., O₂, H₂O₂, OH⁻), resulting in the production of reactive oxygen species (ROS). These created species are able to degrade contaminants, resulting in their mineralization [86]. On the other hand, they have the ability to partially degrade bacterial outer membranes, penetrate cytoplasmic membranes, and induce peroxidation of membrane lipids, which results in cell death [87]. Jin et al. (2011) reported that TiO₂ NPs in the anatase phase contribute to ultrastructural damage to cells by the generation of ROS [88]. Indeed, the anatase phase causes both cytotoxicity and genetic toxicity, as the ROS are internalized in the cytoplasm, and some are lodged within the mitochondria and nucleus. More recently, Henningham et al. (2015) announced that ROS can effectively deteriorate bacterial nucleic acids, proteins, and cell membranes [89]. A report by Joost et al. (2015) considered that (HO^●) radicals play a significant role in seriously oxidatively altering the components of bacterial cellular membranes by starting the process that causes the peroxidation of fatty acids (radical chain reaction) [90]. Indeed, it was assumed that the formation of peroxides in oleic and linoleic acids was probably driven by (HO^●) radicals that attack a hydrogen atom in R-H and create a carbon radical R^●, then, molecular oxygen is added to R, creating a peroxyl radical ROO^●. Finally, the peroxyl radical abstracts a hydrogen from the R-H bond, creating a lipid hydroperoxide ROOH.

Zinc oxide (ZnO) NPs, which are insoluble in water and have the appearance of a white powder, have excellent chemical, electrical, and thermal stability due to their energy band of 3.37 eV and bonding energy of 60 meV [91]. They are safe, inexpensive, and simple to prepare [92]. Additionally, thanks to their optical, electrical, and photocatalytic features, ZnO NPs are used in several applications, such as chemical sensors, solar cells, and notably, photocatalysis [93]. It has been stated that ZnO reveals antimicrobial activity at the nanoscale [94,95,96,97,98]. Indeed, Mokammel et al. (2019) stated that the antibacterial effects of ZnO NPs can be ascribed to their capacity to break down bacterial cell walls and interfere with DNA replication [99]. Reactive oxygen species are produced when NPs release metal ions inside the cells. Several studies have demonstrated that ZnO NPs have a high capacity for producing ROS, which makes them very active in damaging cell walls and making the membrane more permeable. Brayner et al. (2006) suggested that ZnO NPs can occasionally become separated into Zn²⁺ ions; these ions are able to interact with intracellular components by diffusing through damaged cell membranes [100]. Additionally, it has been hypothesized that Zn²⁺ ions released during ZnO dissolution bind to the membranes of microorganisms, extending the lag phase of the microbial growth cycle [101]. Another explanation for the strong antibacterial activity of ZnO NPs is their high surface-to-volume ratio and surface abrasiveness. The low toxicity and great UV absorption of ZnO NPs, in addition to their self-cleaning and antibacterial activity, make them an excellent choice for use in many domains, in particular in the field of heritage building conservation.

Copper oxide (CuO) NPs have stable chemical and physical characteristics, are relatively inexpensive, and are photocatalytic. CuO NPs have the potential to be used as anti-infective agents thanks to their attractive crystal morphologies and incredibly large surface area. The antibacterial effect of CuO NPs may be attributed to different proposed mechanisms. Several research works suggests that CuO NPs may stick to bacterial cell walls due to their positive charge and interact with the carboxyl and amine groups that exist on the surfaces of microbial cells. Consequently, bacteria that possess a high density of these ionic groups, such as Bacillus subtilis, have greater affinity and are more prone to copper oxide NPs [102]. On the other hand, CuO NPs have less impact on Gram-negative bacteria such as Proteus spp. and P. aeruginosa [103]. Copper is an essential component of several enzymes found in living microorganisms. Consequently, Cu²⁺ must be present in relatively high concentrations in order to have toxic effects on microbial pathogens. Cu²⁺ ions have a role in the production of ROS, which interact with DNA and intercalate nucleic acid strands when present in large doses [103]. The production of Cu²⁺ may prevent many microorganisms from synthesizing amino acids. Additionally, the production of ROS may cause bacteria to experience membrane damage brought on by oxidative stress [104].

Thanks to special and distinctive features that make them useful in several fields (e.g., catalysis, sensing, medicinal and biological applications), magnesium oxide-based NPs (MgO NPs) have been the focus of interest over the past 20 years. MgO NPs are typically used in the medical field because of their resistance, biocompatibility, and high stability [105]. In addition to their biocompatibility, MgO NPs have been documented to display improved photocatalytic activity. Several studies have reported their efficiency in degrading organic dyes such as methylene blue, methyl orange, and Congo red stains [106,107,108]. On the other hand, MgO NPs have demonstrated anticancer, antioxidant, and antibacterial capabilities against both Gram-negative and Gram-positive bacteria in vitro, including Staphylococcus aureus and Escherichia coli [109,110]. Interestingly, MgO possesses antimicrobial activity without the need for activation by a light source (photo-activation). It has been reported that MgO NPs and bacterial cells interact to generate cell membrane permeability, oxidative stress, leakage of intracellular contents, and ultimately cell death [111,112]. In addition to their photocatalytic properties, metal oxide NPs, notably oxides of titanium, zinc, magnesium, and copper, possess many interesting features which are illustrated in Figure 3.

An important aspect to establish the effectiveness and the suitability of treatments based on nanoparticles is to consider the synthesis method used to obtain the nanomaterials. The experimental synthesis parameters can determine the morphology, particle sizes, agglomeration level, and crystalline structure of the resulting nanoparticles [27]. The synthesis process plays a critical role in controlling and obtaining ultra-fine nanopowders. On the other hand, the preparation procedure affects the physical and chemical features of metal oxides NPs [113]. Several methods have been developed to synthesize nano-size metal oxide particles, in particular TiO₂, ZnO, MgO, and CuO. The different processes are illustrated in Figure 4.

4. Application of Metal Oxide-Based Nanocomposite Coatings on Stone Building Materials

Metal oxide nanomaterials have been widely used in the field of conservation. Many studies performed in the field of conservation of heritage building materials deal with TiO₂ nanomaterials thanks to their superior photocatalytic activity. In this regard, D’Orazio and Grippo (2014) prepared a water-dispersed TiO₂/poly(carbonate urethane) nanocomposite through cold mixing of single components in a sonication process [114]. The prepared nanocomposite with the presence of 1% (w/w) of TiO₂ nanoparticles showed good self-cleaning properties by the degradation of methyl orange dye after 3 h of irradiation under UV light (Osram Ultra-Vitalux lamp, 300 W, 230 V) and was considered a promising coating for use on porous deteriorated stone such as Neapolitan yellow tuff, a common stone building material in the South of Italy since antiquity. It is worth noting that the type of photocatalyst is not the only factor that needs to be considered; a number of factors need to be taken into consideration in order to produce an acceptable protective coating, including the intrinsic characteristics of the stone (i.e., open porosity, composition, type of stone, etc.), the deterioration mechanisms, climatic factors, the compatibility between the developed product and the stone, the impact of the applied coating on the properties of the stone (i.e., breath function, water penetration, esthetic features, etc.), the coating procedure, and more. Among these factors, binders are crucial for assuring the effectiveness of the coating. A suitable binder must interact well with the substrate to prevent the polymer film from altering after prolonged exposure to sunlight, guarantee good dispersion of nanomaterials, adhere the photocatalyst properly to stone surfaces, have good water-repellent features, and be resistant against ageing. Different binders have been mixed with TiO₂ NPs and used to protect heritage materials, such as perfluoropolyether, acrylic polymer, and methyl acrylate copolymer [115]. Treatments such as Fosbuild, which is a commercial product composed of an aqueous emulsion of acrylic polymer (4 wt.%) mixed with TiO₂ (0.3 wt.% of anatase, 25 nm), show high water repellent features. However, there are no indications concerning the effects of the treatment on the breath function of the tested stone substrates. Recently, Polydimethylsiloxane (PDMS) has been employed due to its advantageous properties, such as high water-repellent property, resistance to fading from sunlight and UV rays, and resistance to deterioration from heat, water, or oxidizing chemicals, among others [116]. Kapridaki and Maravelaki (2013) worked on using PDMS as a binder to prepare nanocomposite containing TiO₂-SiO₂ NPs for stone protection [117]. They declared that PDMS offers hydrophobic coating, improves the durability and flexibility of the silica network, and prevents the gel from cracking during drying. Developed nanocomposites exhibited interesting properties (aesthetic modifications, water repellence behavior, acceptable water vapor permeability, good self-cleaning activity, and high antibiofilm performances). Tavares et al. (2014) elaborated a PDMS/TiO₂ nanocomposite by varying the amount of nanoparticles in the mixture at 0%, 0.5%, and 1% by weight [118]. In more detail, TiO₂ NPs were first synthesized by a microwave-assisted hydrothermal process; then, nanocomposite preparations were obtained by simple mixing of NPs with PDMS at different ratios and the coatings were processed by the spray method. The photocatalytic activity of different nanocomposite formulations was evaluated by the degradation of methylene blue dye at a suspension concentration of 1 × 10⁻³ mol/L under UVC lamps (254 nm, ≈4.9 eV) for three hours. Commercial TiO₂ (P25) was studied in research work for comparison purposes. The results demonstrated that TiO₂ concentration affects the photodegradation of methylene blue stain, as coating with 1% (by weight) TiO₂ showed a significantly faster degradation rate while pure PDMS films showed insignificant reduction of the dye. However, the P25 and 1% TiO₂ coatings exhibited similar behaviors. Moreover, the study lacked data on the impact of the coating on stone properties such as the breath function and on the durability of the coating. Kapridaki et al. (2018) [119] developed a hydrophobic and photoactive hybrid nanocoatings composed of three layers: (i) a tetraethoxysilane (TEOS)-nano-Calcium Oxalates consolidant; (ii) a hydrophobic layer composed of TEOS-PDMS; and (iii) TiO₂ nanoparticles as a self-cleaning layer. This method of conservation was proposed for lithotypes and mortars with various levels of porosity and petrographic features. The results showed that the treatment adhered well to stone surfaces, with only a small amount of material lost during the peeling test. All the substrates under study showed improved hydrophobic characteristics after treatment as demonstrated by the contact angle and capillary water absorption values. For the majority of the examined lithotypes, the permeability of water vapor was ensured to an acceptable level. The self-cleaning test was performed through discolouration of methylene blue dye on different lithotypes. The results demonstrated that the treatment displayed an enhanced self-cleaning activity, accomplishing the total degradation of MB on many of the tested specimens owing to the incorporation of nano-TiO₂ into the silica network, as shown by the creation of stable Si-O-Ti bonds detected by FTIR analysis. In addition to PDMS, other binders have been proposed; for example, Corcione et al. (2018) [120] elaborated a protective nanocomposite coating which was tested on Lecce stone. The coating was made of a hybrid methacrylic–siloxane resin modified with 1 wt.% oleic acid (OLEA) and 3-(trimethoxysilyl) propylmethacrylate (MEMO)-coated TiO₂ nanorods (NRs). OLEA-coated TiO₂ NRs were created in the anatase phase using a colloidal method, allowing for control of the size, shape, and crystalline quality of the NRs. TiO₂ NRs coated with two different amounts of OLEA/MEMO (1 and 3 wt.%, respectively) were dissolved in MEMO and stirred at room temperature. Brushing was used to apply the specified amount of the produced formulations, which was calculated by weighing the sample before and after the treatment. The discoloration capacity of the developed treatment was evaluated through Methyl Red (MR) as a model stain by dropping 200 μL of MR dye solution dispersed in isopropanol (3.5 × 10⁻³ M). The durability of the applied treatment was investigated by exposing stone substrates under outdoor conditions for one year. The results showed that treatment provided excellent surface hydrophobicity, as suggested by the considerable contact angle value (about 136°). According to the stone vapour water permeability test, the difference between treated and untreated stone was only 22%. This finding confirms that the treatment does not block the natural water vapour permeability of stone to the outside environment. In addition, the treatments induced chromatic variations of ΔE between 2.10 and 4.30. These results show that these coatings can be used without creating adverse aesthetic alterations of the stone substrate. The protective efficacy (PE) was evaluated by comparing the weight of water adsorbed by the treated stone specimens after 8 days compared to untreated specimens. The findings showed that the PE was almost 90% for the coating containing 1 wt.% of TiO₂ NRs (i.e., 10 mg cm⁻²), while it reached a value of only 20% for the treatment composed only by resin. The self-cleaning test showed that, due to the inclusion of the TiO₂ NRs, the nanocomposite coating exhibited a higher discolouration percentage (34%) than the resin-coated sample (20%) after 24 h of irradiation. In 2020, Azadi et al. developed multifunctional inorganic–organic hybrid coatings with self-cleaning, hydrophobic, thermal stability, and weathering resistance features for application to outdoor stone building artefacts [121]. Hybrid coatings were prepared from fluorinated acrylic copolymers containing a suitable silane functional group (as the organic substance) and TiO₂ nanoparticles (as the inorganic compound). The authors stated that TiO₂ NPs serve to enhance the thermal resistance and hardness of the coating in addition to inducing self-cleaning properties. The presence of organofluorine substances improves the water-repellent characteristics of the coatings and their resistance to weathering. Tetraethyl orthosilicate (TEOS) was included in the treatment to ameliorate the thermal resistance of the protective coatings. Nanocomposite thin films revealed acceptable colour variation (ΔE* ˂ 5), hydrophobic behavior (i.e., a contact angle around 131°), low water absorption percentage in the first three hours, better mechanical properties, and good photocatalytic activity through the degradation of methylene blue dye. The results showed that the presence of the organofluorine–titania hybrid led to better hydrophobicity by increasing the roughness of the surface. Interestingly, the coatings exhibited good resistance against aging.

As the main factor causing weathering is water, researchers have suggested using superhydrophobic materials with a static contact angle higher than 150° coupled with small contact angle hysteresis (typically, a roll-off angle below 10°). The capacity of superhydrophobic materials to inhibit water penetration allows them to significantly reduce the impact of water erosion. Moreover, the low roll-off angles of surfaces efficiently reduce the accumulation of pollutants and microorganisms, demonstrating promise for protecting stone artifacts [122]. Suitable roughness and low surface energy are required for the development of superhydrophobicity in substances. For this, researchers have suggested preparing nanocomposite coatings obtained from nanoparticles combined with organic coatings, with the nanoparticles offering a rough structure and the organic component providing small surface energy. In this context, Peng et al. (2022) [123] elaborated nanocomposite superhydrophobic coatings composed of silicon dioxide and titanium dioxide NPs, which were used to modify a waterproof coating obtained from dodecyltrimethoxysilane (DTMS). DTMS is utilized for many applications, such as fabric waterproofing, alloy anti-corrosion, and notably, sandstone protection [123]; however, reports have declared that DTMS has weak resistance to light and low durability. It is expected that the introduction of NPs into the structure of organic coatings will be able to enhance features such as substrate adherence, thermomechanical characteristics, chemical stability, resilience to wear, self-cleaning, and notably, resistance against UV light degradation, which would extend the durability and the hydrophobic character of DTMS. Peng and co-workers elaborated a superhydrophobic coating named DST composed of SiO₂ and TiO₂ with concentrations of 0.5% (w/w) and 0.01 (w/w), respectively. Developed nanocomposites were then tested on red sandstone collected from the Daming Place Building Material Market, Xi’an, China. The coatings showed overall chromatic variation of less than 2, which is acceptable for use in conservation. The results revealed that the NPs were well-dispersed in DTMS, with about 70% of the particle sizes ranging between 60 and 90 nm (Figure 5a). The nanoscale surface roughness (see SEM image in Figure 5b) induced superhydrophobic behavior, as suggested by very large static contact angle values (up to about 152°). The hydrophobic character of the coatings was further demonstrated by measuring their water repellence efficiency, which was more than 92% after 72 h of absorption compared to untreated samples (Figure 5c). Interestingly, their findings revealed that the simultaneous effects of TiO₂ and SiO₂ NPs results in amelioration of the thermal and chemical stability of DTMS in addition to improving its UVA resistance, consequently guaranteeing better durability (Figure 5d).

Further studies concerning the use of TiO₂ nanomaterials in coatings applied on different stone substrates have been developed over the last ten years. Applications in the field of heritage conservation are summarized in

Table 1. Application of metal oxide NPs as coatings for the treatment of heritage stone materials (references are presented in a chronological order).

Nanomaterials Composition	Material Size	Substrate	Experiment Conditions	Obtained Results	Ref.
TiO2	-	Travertine, a natural limestone	Aqueous TiO2 colloidal suspension (NPs amount: 1 wt.%) NPs prepared by sol–gel process Application procedure: spray method Applied amount: 0.20 g/m (SL: single layer) and 0.60 g/m (ML: Three layers) Self-cleaning test: Rhodamine B (RhB)	Acceptable colour variation (ΔE* = 2.15–2.5) Contact angle values (α) for SL ~70° and for ML ~85° Water absorption of treated surfaces decreased about 50% TiO2-based coatings-decolorized RhB up to 75%	[124]
SiO2-TiO2	40–100 nm	Greek marbles from Naxo	SiO2–TiO2 composites were prepared by sol–gel route Application procedure: brushing method Photocatalytic activity test: Methyl orange (MO)	Acceptable chromatic variation (ΔE* = 0.6–1.9) Contact angle (α): 93°–106° (Hydrophobic coatings) 1 WCA decreased by 88% Water vapor coefficients decreased by around 23% High self-cleaning activity: faster degradation of MO-ΔE*/ΔE*0 ~0.05 after exposition for 120 min	[125]
TiO2	Anatase: 3–6 nm Brookit: 5–10 nm	Pietra di Lecce	TiO2 NPs obtained by sol–gel Application procedure: brushing method Photocatalytic activity test: methyl orange (MO)	Acceptable colour variation (ΔE* = 2.7–4.9) Slight reduction in capillarity absorption Good MO stain degradation	[126]
TiO2	10–15 nm	-Apuan marble (AM) -Ajarte limestone (AL)	Two kinds of nanocomposite were prepared: ✓ WNC: alkyl alkoxy silane oligomers (15% w/w) with TiO2 NPs (0.96% w/w) in water ✓ ANC: alkyl alkoxy silane monomers (40% w/w) with TiO2 NPs (0.12% w/w) in 2-propanol Application procedure: capillary absorption method	Acceptable chromatic variation (ΔE* = 1–3.5). Contact angles (α): WNC = 114° ± 1° (AM); WNC = 122° ± 7° (AL); ANC = 130° ± 10° (AM), ANC = 141° ± 2° (AL) ANC provided the largest reduction in water uptake Higher photocatalytic activity: discoloration levels ~90% with both treatments against Rhodamine B.	[127]
SiO2-TiO2-PDMS	25 nm	Modica stone	The amount of polymer on stone surface ~50 g/m2 Optimization was performed based on TiO2 NPs amounts (2.5, 10, 20 and 40 g/m2) Application procedure: brushing Self-cleaning efficiency: (MB, 10% w/w in water solution)	Acceptable chromatic variation: amount of TiO2 < 7 g/m2 The stone surface saturation was obtained as 23.7 g/m2 and amount of TiO2 as >20 g/m2 Maximum discoloration rate: TiO2 ~20 g/m2; degradation of MB ~80%	[128]
TiO2	-	Trani stone: Low porosity (2%)	TiO2 NPs/fluoropolymer coatings Application procedure: brushing Photocatalytic activity test: stain (Rhodamine B)	Acceptable colour variation (ΔE* = 0.5–3.5) Contact angle measurements (α): 100°–130° before exposition to outdoor condition; α = 20°–60° after exposition Rhodamine B degradation ~95% (7.5 h)	[129]
SiO2-TiO2	20 nm	Portland cement (WC) paste	SiO2/TiO2 prepared by an improved sol–gel method Photocatalytic activity test: stain (Rhodamine B)	The photodegradation capacity of nanocomposite coatings was not considerably altered, while it dropped to around 9% in the case of P25	[130]
TiO2@SiO2 Core-Shell	Shell thicknesses: 1.95–6.13 nm	White Portland cement past	Commercial TiO2 (P25) was employed as the core structure, and the typical Stöber process was used to create the SiO2 shell TiO2@SiO2 suspensions were sprayed on specimen surfaces Self-cleaning test: Rhodamine B solution (20 mL, 10 mg/L)	The degradation rates of TiO2@SiO2 coatings were higher than that of pure P25 The maximum Rhodamine B degradation rates (68.1%) Shells thickness ≈ 3.92 nm; Surface areas ≈ 96.63 m2/g	[131]
MgO/TiO2	24 to 56 nm	-Red bricks -Gypsum mortars	Coatings were obtained through immersion for 15 min Hydrophobic composite coatings: oxide powders (0.5 wt.%) in modified sodium polyacrylate (NaPAC16) aqueous solution (0.1 wt.%) Antimicrobial activity test was performed against Staphylococcus aureus, Candida albicans, and Aspergillus niger Photo-catalytic test through MO photodegradation	Acceptable colour modifications (ΔE* < 5) MO degraded: over 80% by NaPAC16-TiO2 and 93.7% by NaPAC16-MgO NaPAC16-oxide NPs showed antimicrobial activity against all tested strains	[111]
TiO2 nanosheets-SiO2	Thickness ≈6.5 nm	Capri limestone (open porosity: 9%–12%)	Self-Cleaning test: methylene blue (MB) Soot photo-elimination test: soot water dispersion (80 mg/L)	Acceptable colour variations (ΔE* < 3) Better photocatalytic activity of Titania nanosheets Coating containing nanocomposite removed more than 50% of the soot stains after 600 h of irradiation	[132]
TiO2-SiO2	100 nm	Concrete	Soot was applied at a rate of 20 μL/cm2 Methylene blue (MB solution: 0.5 mM) Durability test: indoor conditions by MB dye degradation test	MB degradation (1 h): 95% with nanocomposite coating. The coated surfaces showed an almost unaltered structure after the outdoor durability tests; long-lasting effectiveness	[133]
TiO2-SiO2-PDMS	-	Cement mortar	Application procedure: spray method Self-cleaning test: MO stains	Hydrophobic coatings (static contact angle = 152.6°) The surface of the uncoated wall was shown to be easily polluted by MO and was difficult to clean with water, in contrast to treated specimens	[134]

¹ Water capillary absorption coefficients (WCA); Methylene blue (MB); Methyl orange (MO); Rhodamine B (RhB).

.

Biofouling plays a main role in the deterioration of submerged heritage stone. Indeed, underwater archaeological sites are deteriorating as a result of marine fouling [135]. After prolonged exposure times, a diverse community composed of plants, animals, and bacteria known as marine fouling takes place as a result of accumulation processes. For this reason, Roveri et al. (2018) prepared a nanocomposite coating composed principally from hydrophobized silica with AgO and ZnO nanoparticles (0.2% w/w) [136]. The tested stone substrates belonged to three different lithotypes: Apuan marble, Balegem limestone, and Schlaitdorf sandstone, with open porosities of 0.7 ± 0.1, 9.9 ± 0.8, and 16 ± 1 %vol, respectively. The nanocomposite coating showed acceptable chromatic variation (ΔE* ˂ 5), good effectiveness in decreasing water uptake, and an important decrease in surface wettability with a contact angle of about 140° but had high permeability reduction (about 70%–80%). On the treated Schlaitdorf and Balegem stone samples, it showed good ability to inhibit microorganism growth, while on the treated Apuan marble samples exhibited insignificant reduction of the CFU number for both Bacillus cereus and Pseudomonas putida. These results may be explained by the small amount of protective coating that the marble was able to absorb and the small quantity of bacteria that able to adhere to marble surfaces. To examine the durability of nanocomposites, three alternative artificial ageing protocols that account for the distinct impacts of heat, UV light irradiation, and meteoric run-off were carried out in laboratory settings. The findings showed good durability of the nanocomposite coating under different ageing conditions.

Factors such as weathering and mechanical damage can lead to partial or total collapse of heritage stone materials. Furthermore, microbial colonization, particularly by different strains of fungi, can affect the physical and mechanical properties of stone. In this context, Van der Werf et al. (2015) developed nanocomposites based on ZnO NPs embedded in water-repellent siloxanes-based ESTEL1100 and SILO111 matrices as antimicrobial coatings [137]. The ZnO NPs were elaborated through a reproducible electrochemical process, and different mixtures were applied on a real monument, specifically, the outside of the 12th century Church of San Leonardo di Siponto in Manfredonia, Italy. The results showed that the consolidant products ESTEL and SILO were able to successfully suppress the growth of Aspergillus niger (A. niger) fungus strains after treatment with 0.4% w/w ZnO NPs while barely altering the color of the stones. Marble is one of the most significant and fundamental architectural materials in heritage buildings. To completely protect exposed ancient marble architectural components (e.g., columns) from fungi, experts are looking to new technologies to achieve the ideal approach. In particular, ZnO photocatalytic inorganic nanoparticles have been used to produce protective surface coatings and prevent microbial and fungal growth on exposed marble columns. Aldosari et al. (2019) [138] developed ZnO nanoparticles that were mixed with a synthetic acrylic polymer prepared by emulsion polymerization to design a coating combining biocidal and consolidating properties for use on old marble columns. The prepared nanocomposite coating was tested on marble samples collected from several Egyptian archeological sites. The results showed that ZnO NPs were obtained with a spherical morphology and a diameter ranging from 15 to 50 nm. SEM micrographs showed that samples treated with polymer coatings containing the ZnO NPs exhibited homogeneous distribution of the particles that were uniformly dispersed on the surface without producing cracks or segregation, contrary to specimens that were coated with only polymer. Combined with good distribution of the NPs, the polymer provided a hydrophobic coating with a contact angle of 140° and acceptable chromatic variation. The findings showed that the fungal strains, A. niger and Penicillium sp. grew quickly and diffusely on the untreated marble specimens. The chemical composition, rougher surface, high initial porosity, and mineralogical features of marble made untreated surfaces more susceptible to microbial attack. Samples treated with only the polymer showed low capacity to inhibit fungal growth, which was attributed to the features of the polymer. Because the polymers are organic compounds, they can be decomposed by microbial action, and can theoretically even encourage the growth of certain microorganism species. Interestingly, the nanocoating exhibited high antifungal activity against the tested strains due to the high photo-killing performance of the ZnO particles. In fact, as discussed in the previous section, the ROS generated during the photocatalytic process can damage the membranes of microbial cells. It is worth noting that the effectiveness of the applied treatment was greatly influenced by the uniform distribution of the NPs across the entire treated surface. Moreover, the results revealed that incorporating ZnO NPs into polymer can improve the resistance of stone surfaces to changes in relative humidity and temperature in addition to improving their durability against UV aging.

The ability of microorganisms to produce biofilms is the main cause of monument biodeterioration. Organic and inorganic materials can be colonized by bacterial biofilms, which use them as chemical and energy sources, causing substantial and irreparable damage to artifacts regardless of their composition. Moreover, bacteria have the ability to adhere to surfaces and even to each other, forming colonies of cells that produce an extracellular matrix made of DNA, proteins, and polysaccharides [139]. In this regard, Schifano et al. (2020) [21] evaluated the efficiency ZnO nanorod-decorated graphene nanoplatelets (ZNGs) to inhibit biofilm development by bacterial species (Arthrobacter aurescens and Achromobacter spanius) isolated from deteriorated historical monuments, precisely, the Temple of Concordia in the Valley of the Temples in Agrigento, Sicily, Italy. Zinc oxide nanorods (ZnO NRs) were prepared by thermally decomposing zinc acetate dihydrate followed by sonication. Commercially available graphite intercalation compound was thermally expanded for 30 s at 1050 °C to create graphene nanoplatelets. The prepared nanocomposite showed high antibacterial activity against Streptococcus mutans in a previous work [140]. Next, ZnO NRs with rod-shaped particles, a diameter of around 36 nm, and a length ranging from 300 to 400 nm were prepared directly on the planar face of pristine graphene nanoplatelets to produce ZNGs. Specimens of Noto stone, Carrara marble, and yellow brick were spray-coated with 2 mL of ZNGs aqueous suspension using an airbrush. The substrate stones differed in terms of their qualities, including hardness, porosity, and alkalinity, which influence the resulting distribution. A reduction in bacterial viability of about 60, 70, and 90% was obtained for the treated Noto stone (porosity ≈ 38%), Carrara marble (porosity ≈ 0.4%), and yellow brick (porosity ≈ 28.5%), respectively. These results show a relationship between the antibiofilm performance of the ZNGs and the distribution of the nanostructure in relation to specimen porosity. For example, the antibiofilm impact was found to be slightly decreased by the high porosity of Noto stone. Additionally, the results showed that the structure of the produced nanocomposite favored the inhibition of biofilm growth as can be observed through high-resolution field emission scanning electron microscopy (FE-SEM) (Figure 6). The ZnO NRs acted as nanoneedles, breaching the bacterial wall, while the nanosheets of graphene nanoplatelets provided a wide surface area for the oriented development of the ZnO NRs over the graphene surface. More precisely, the shape of the ZnO NRs increased their ability to pass through cell membranes by enhancing the adherence of nanostructures to the cell wall and increasing their ability to damage bacterial surfaces.

Copper oxide NPs have been used in the field of heritage stone protection as well. CuO NPs are selected for this application due to their encouraging benefits, in particular their low cost compared to conventional biocidal agents. Furthermore, they are more stable than Ag⁰ and Cu⁰ NPs, which are susceptible to oxygen and sunlight, and most importantly, they do not require a source of light in order to be active. In this context, Zarzuela et al. (2016) [141] elaborated multifunctional nanocomposites composed of CuO and SiO₂ NPs through the sol–gel route as a protective coating for building stones. In their study, n-octylamine (n-8, 99%) was used as a surfactant to minimize surface tension and regulate pore size in order to produce xerogels free of cracks. The tested stone had a porosity of about 19% and was principally composed of calcite (45%), dolomite (>37%), and quartz (>7%). CuO nanoparticles with spherical shapes and sizes varying from 50 to 60 nm were integrated into silica matrix. The findings showed that nanocomposites containing an intermediate amount of CuO (i.e., 0.15% w/v) exhibited acceptable chromatic variation (ΔE ≈ 2.5), good biocidal activity against Escherichia coli and the yeast Saccharomyces cerevisiae, and good water repellence features. In fact, the static contact angle values were higher than 90° for the treated species, and the total water uptake (%TWU) dropped from 5.69 ± 0.05 for untreated stone to 0.14 ± 0.01 for treated stone. The mechanical properties of the coatings were tested through drilling resistance, peeling, and the Vickers hardness test. The results showed that the mechanical features of the stone were enhanced, proving the efficiency of the protective coating as a consolidant. This study showed promising results; however, the authors did not study the durability of the coating or its resistance in severe conditions. The same nanocomposite was used to create a novel kind of manufactured stone with thermal, chemical, and antibacterial resistance for possible use in cladding and tiles [142]. The authors proposed replacing the resin matrix with an amorphous silica matrix made via a sol–gel process utilizing quartz sand as the aggregate and adding copper oxide nanoparticles as a biocide agent. The authors suggested that this kind of stone could be used in the restoration of heritage buildings. CuO NPs were first introduced in concentrations ranging from 0.00% to 1.00% w/v relative to the silica oligomer. The prepared mixture was mixed with n-octylamine and de-ionized water, then sonicated using an ultrasonic probe (2 W/mL for 10 min). The quartz/SiO₂ sol paste was made using quartz particles of three distinct sizes (Figure 7). The paste was cast and cured into silicone molds under laboratory conditions (20 °C, 45% RH), then different tests were performed to investigate its performance. The results showed that the prepared materials had excellent surface hardness, heat resistance, and antifungal capabilities against yeast and Aspergillus carbonarius spores. Nevertheless, they showed reduced mechanical strength compared to a manufactured stone made of only resin matrix. The sol–gel kinetics and quartz sedimentation were affected by the concentration of CuO and the amount of water in the matrix, which influenced the structure and mechanical properties. In the same context, Kahrizsangi et al. (2016) [143] studied the impact of Cr₂O₃ nanoparticles on the mechanical and physical characteristics of an MgO-CaO refractory composition, with a focus on the enhancement of hydration resistance. Their results showed that the addition of 1.5 wt.% Cr₂O₃ NPs produced the best results, achieving the maximum improvement in mechanical and physical performance.

5. Metal Oxide NPs with Enhanced Photo-Response Activity

Despite the interesting properties of metal oxide NPs, because of their large bandgaps they can generally absorb only UV light, which makes up about 4% of the entire solar spectrum. Thus, developing NPs that can effectively remove contaminants and absorb visible light is highly desirable. Metal doping is one of several suggested methods for improving the ability of metal oxide NPs to absorb visible light [1].

Because nano-sized TiO₂ has been largely studied and tested for the conservation of cultural heritage materials, considerable efforts have been developed in order to shift the absorption region of titanium dioxide to the visible region by modifying its band gap energy. Many research groups have focused on developing synthesis routes to prepare visible light-responsive TiO₂ as well as to enhance its antibacterial activity by doping with noble metals. Localized surface plasmon resonance (LSPR) has attracted a great deal of attention in recent years in the field of photocatalysis. The excitation of plasmonic metals by LSPR permits the production of energetic electrons at the metal surface. Under visible light, they gain enough energy to facilitate the transition to a semiconductor’s conduction band and take part in the chemical reaction. In particular, doping TiO₂ NPs with silver has been shown to be a good method for improving the photo-response activity of TiO₂. In fact, silver ions have attracted considerable attention due to their amazing photocatalytic and antibacterial activity [144]. Silver ions represent a non-specific bactericide, contrary to antibiotics, as they can act against a large spectrum of bacterial and fungal species [145]. On the other hand, a novel approach has emerged as a feasible solution to produce effective materials by doping with rare earth ions (i.e., lanthanide ions). In this context, our research group developed TiO₂ NPs with enhanced photo-response properties elaborated through the sol–gel technique to produce a thin layer capable of protecting monumental stone [146,147]. Doping with lanthanide ions, more precisely Gadolinium ions (Gd³⁺), was chosen as a method to enhance the photoactivity of TiO₂ NPs by reducing the recombination of photo-generated charge carriers and employing the visible region of the solar spectrum. Doping with the more well-known silver ions was performed as well. Silver is a widespread choice due to its high reactivity and capacity to produce surface plasmons at desired wavelengths. Lecce stone, a fossiliferous biocalcarenite quarried near Lecce in the South of Italy, was used as a model of highly porous stone. Pure and Gd³⁺/Ag⁺ doped TiO₂ nanoparticles (NPs) were prepared by the hydrothermal assisted sol–gel route with different doping percentages (0, 0.1, 1, 3 and 5 mol%). Morphological observations showed that all prepared NPs had a spherical shape, with particles sizes varying from 10 to 30 nm. First, we started by performing preliminary analyses in order to optimize the powder/binder ratios. Nanocomposites containing pure TiO₂ NPs with different powder/binder ratios (0.1%, 0.2%, 0.5% and 1% w/v TiO₂ in polydimethylsiloxane, PDMS) were applied on Lecce stone (LS) specimens. Preliminary analyses were based on measuring overall chromatic variations and static contact angles after application of nanocomposite on the stone surface. Based on the obtained results, it was decided to perform the subsequent tests with a nanopowder/binder ratio equal to 1% (w/v), as this was considered the optimal ratio. However, during the tests it was noticed that the PDMS binder altered the original color of the treated stone surfaces. To resolve this problem, t-Butanol (TBA) was added to PDMS (1:1 PDMS/TBA equivalent ratio was used) to reduce the darkness caused by the polymer. Acceptable chromatic variations were observed after application of NPs/PDMS diluted with TBA. The preliminary tests indicated that the nanocomposite containing TiO₂ NPs doped with 3 and 5 mol% Ag induced excessive chromatic variations; thus, investigation of these concentrations was discontinued [146]. Moreover, it was decided to stop investigation of materials containing 3 and 5 mol% Gd-TiO₂ based on the results of the preliminary study concerning their photocatalytic and antibacterial activity. The next step was to apply different coatings composed of the synthesized nanoparticles and binder on samples of Lecce stone in order to verify their efficiency in protecting stone substrates from biodeterioration. Coatings with multifunctional properties composed of the synthesized nanoparticles (pure TiO₂, 0.1–1 mol% Gd-TiO₂, 0.1–1 mol% Ag) and binder were applied to Lecce stone (LS) specimens. In a comparative study, a series of samples was treated only with PDMS:TBA (1:1) and another series was kept untreated. The coatings were evaluated by performing contact angle, chromatic variations, SEM-EDS, optical observation, capillary absorption, water vapour permeability, self-cleaning, antimicrobial, and ageing tests (Figure 8). The results revealed that all the applied coatings showed acceptable chromatic variations with ∆E* < 5 and exhibited water repellent properties. Optical microscope analyses suggested that the nanoparticles and binder were homogeneously distributed on the surface of the LS. SEM-EDS analysis further confirmed the homogeneous distribution of the nanoparticles and polymer on the Lecce stone surface, with localization of some NPs in the pores, confirming that the protective coating was successfully deposited (Figure 8b). The kinetics of capillary suction were only affected by the polymer and NPs treatments during the first 30 min, as shown by the CA and Qf values, which indicate the amount of water absorbed in 96 h and the coefficient of average absorption in 30 min, respectively. In addition, it was found that the treatment decreased the vapour permeability of the LS; however, the breath function of the original material was not dramatically affected (permeability reduction lower than 40%). Additionally, the surface hardness was increased by adding NPs to the surface of the LS samples. An interesting finding was that the PDMS coating with 1 mol% Gd/Ag-doped TiO₂ NPs was much tougher than the coating containing undoped NPs. The self-cleaning test was carried out by applying methylene blue dye (0.1% w/v in ethanol solution) to LS specimens. Colour variation was measured before and after applying MB and again after 48 and 96 h of UV irradiation. The results exhibited that the samples treated with NPs displayed faster degradation of the organic dye pollutant, with higher activity of doped materials, except for 1 mol% Gd-TiO₂. Moreover, the coating composed of binder and 1% Ag-doped TiO₂ nanoparticles showed the best self-cleaning activity, with total discoloration after only 6 h (Figure 8i,l). The antimicrobial analysis showed that treatment inhibited the overall spreadability and colonization of different microorganisms on the LS surface. In particular, the nanocomposite coatings composed of PDMS and 1 mol% Ag-doped TiO₂ NPs displayed the highest activity (Figure 8r). Additionally, the durability of the nanocomposite coatings was evaluated following exposure to various ageing cycles (high humidity, solar light, and long-term microbial incubation) in order to evaluate their stability. The effect of solar irradiation on chromatic variations caused by the protective films was carried out by performing accelerated ageing tests. The results showed that the samples treated with only polymer and the specimens coated with undoped and 1% Ag-doped TiO₂ nanoparticles showed unacceptable chromatic variations after 1000 h of irradiation under solar light. Solar irradiation did not affect the water repellent properties of applied coatings, despite a slight decrease in contact angle measurements after 1000 h of irradiation under a visible lamp. Additional findings revealed the stability of the coatings, notably the 0.1 mol% Gd-TiO₂ NPs and 1 mol% Ag-TiO₂ NPs, in terms of self-cleaning and antimicrobial activity. According to Normal 20/85 (1996) [148], one of the requirements for accepting the application of such treatments is that a protective coating should not result in noticeable alteration of the heritage material and must be stable over time. In conclusion, the 0.1 mol% Gd-TiO₂ and 0.1 mol% Ag-TiO₂ materials can be considered good candidates to protect monument surfaces made from this kind of stone, while coatings containing nanoparticles doped with a concentration of 1 mol% Ag could be useful for other kinds of stone.

Next, our research group worked on preparing a nanocomposite coating for the preservation of Serena stone (SS) historic artifacts [149]. The study was carried out in order to determine the ideal application conditions of synthesized silver-doped TiO₂ NPs distributed in a PDMS binder as a protective coating for Serena sandstone materials. Ag-TiO₂ NPs were mixed with PDMS at a powder/solvent ratio equal to 1% w/v, while the binder contained PDMS (M.W. 4200) and Tert-Butyl alcohol (TBA) at a ratio equal to 1:10 (PDMS:TBA). The nanocomposite coating was then applied at an amount of 2 g/m² on SS specimens (Figure 9). After application of the nanocomposite protective material on historical stone surfaces by brushing, a coating with good water repellent features, self-cleaning activity, and antimicrobial performance was obtained.

Despite the enhancement of the photo-reactivity of TiO₂ NPs induced by doping, Ag can be easily oxidized upon coming into touch with TiO_2, because of its chemical reactivity [150]. To get around this issue, Ag NPs can be encapsulated in a material such as SiO₂ to balance the distance between TiO₂ and Silver. Indeed, incorporating the photocatalyst inside a mesoporous silica coating is considered an intriguing way to maintain strong surface adhesion and long-term wear resistance. In this context, Pinho et al. (2014) [151] elaborated mesoporous Ag-TiO₂-SiO₂ photocatalytic coatings for outdoor applications by integrating TiO₂ and Ag NPs into SiO₂ matrix in the presence of a surfactant (n-octylamine). The team of Cádiz investigated different sol formulations prepared at different TiO₂ and Ag concentrations. Prepared sols were applied onto limestone surfaces entirely made of calcite through a spraying approach until apparent refusal. All applied coatings showed acceptable colour variation except for a formulation composed by 4% (w/v) TiO₂ and 5% (w/w) Ag, which displayed undesired colour variation as a result of the high amount of Ag. A peeling test was performed to test the degree of adhesion of the applied nanocomposite coatings. The findings revealed that the specimens treated with sols and 1% (w/w) Ag in addition to samples containing 1% (w/v) TiO₂ exhibited only practically insignificant weight loss in contrast to untreated samples. These results demonstrate that TiO₂ and Ag were bonded to the SiO₂ matrix, which in turn was strongly attached to the tested stone. Total water uptake (TWU) values measured after 48 h proved that the applied coatings successfully inhibited water penetration inside the stone pores, as it was near zero for all of the treated samples and much lower than the corresponding untreated samples. The outcomes of the self-cleaning test revealed that the coating composed of silica, 1% (w/v) TiO₂, and 10% (w/w) Ag (named S1T10Ag) showed the highest activity, degrading most of the methylene blue dye, in contrast to coatings containing only SiO₂ and Ag NPs incorporated into silica matrix without the presence of TiO₂ NPs (Figure 10a). This confirms that the presence of titanium dioxide nanoparticles is indispensable for the self-cleaning activity. The authors deduced that the reduced average size and high dispersion of Ag NPs present in the S1T10Ag nanocomposite, along with the high absorption of visible light, are the key factors making these coatings highly effective in degrading the tested stain. TEM analysis revealed that the TiO₂ and Ag NPs included in the prepared nanocomposites were separated by a thin SiO₂ interlayer; thus, the thickness of the SiO₂ interlayer between the TiO₂ and Ag could probably be lowered by improving their dispersion, as the Ag nanoparticles would be located closer to TiO₂ NPs (Figure 10b). The authors stated that the surfactants help to reduce the amount of Ag species in the sols and increase the stability of the TiO₂ and Ag NP dispersion during the sol–gel transition. Although this study investigated the water repellence and self-cleaning performance of the coatings, more analysis is required to examine the biocidal activity and durability of the coatings in real conditions.

Cádiz’ team also used a sol–gel approach to develop a nanocomposite with self-cleaning and de-polluting activity based on a Gold (Au)-Titanium dioxide NP photocatalyst embedded in a silica matrix to protect heritage building materials from air contamination, [152]. Gold NPs were selected because of their excellent efficacy in improving the photo-response activity of TiO₂ NPs. In fact, the localized surface plasmon resonance (LSPR) of gold NPs varies from 500 to 600 nm, compared to silver NPs which have LSPR maximum absorption in the range of 400 nm (i.e., near to UV light); thus, they can absorb more solar light [153]. Despite studies concerning the use of gold nanoparticles for conservation of building materials being scarce because of its high price, the authors incorporated low amounts of gold (0.5% w/w Au/TiO₂) to overcome this issue. Gold NPs with two different average sizes (13 and 38 nm) were considered in the study. Three formulations with different Au/TiO₂ compositions (0.25%, 0.5% and 1% w/w) were dispersed in silica matrix in the presence of n-octylamine at a TiO₂/SiO₂ ratio of around 2.5% w/w. The prepared sols were first applied on Capri limestone, an oolitic limestone quarried from Cabra, Spain with open porosity of 12%. The findings revealed that coatings with different preparations preserved the appearance of the original material. The self-cleaning test showed that better MB stain removal efficacy was obtained after the addition of AuNPs, proving the improvement in TiO₂ photocatalytic capabilities brought on by the noble metal NPs. In particular, AuNPs with the lowest size (13 nm) and intermediate Au content (0.5% w/w Au/TiO₂) displayed the maximum photoactivity, probably due to better charge separation and lower photogenerated electron–hole pair recombination. In addition, it is well known that more small NPs have a higher surface area, leading to an increase in the available surface-active sites for TiO₂-Au interactions with pollutants. This optimum formulation was then selected to investigate its self-cleaning and depolluting features using real pollutants, particularly, soot and NO. The results showed that better soot removal was observed for specimens coated with Au-TiO₂/SiO₂ nanocomposite. NO conversion was almost double for specimens coated with nanocomposite compared to their counterparts coated with only TiO₂/SiO₂, demonstrating that AuNPs similarly accelerated the photo-oxidation of this pollutant. Despite the interesting outcomes found in the study, no data about the biocide properties of the coatings or their durability were presented. The Cadiz group also prepared of Ag/modified-TiO₂ NPs using -SH or -NH ended alkoxysilanes to functionalize TiO₂ NPs for possible use as coating to protect heritage stone materials with visible self-cleaning activity and biocidal characteristics [154]. In order to reduce the potential harmful effects of AgNPs, a low Ag/TiO₂ ratio (1%) was used. The authors stated that reducing the discharge of Ag⁺ into the environment would restrict the impact on non-target organisms such as aquatic fauna, soil bacteria, etc. In addition, excessive metal loading may increase the rate of electron–hole recombination, reduce the efficacy of the photocatalytic mechanism, and increase the creation of AgNP clusters during the synthesis process. The findings revealed that -NH functionalization produced the maximum stability, homogeneity, and visible range absorption. Moreover, the addition of -SH groups changed the size of the AgNPs and reduced their efficiency by producing Ag₂S and Ag/TiO₂ NPs with -SH functionalization, which exhibited no biocidal action. Next, in 2021, the Cádiz team in collaboration with Abdelmalek Essaadi University used Copper NPs to improve a TiO₂/SiO₂ nanocomposite [155]. In comparison to other noble metals, copper is more readily available on Earth and less expensive. Moreover, copper is accessible in different oxidation states (Cu⁰, Cu^I, Cu^II and Cu^III). The authors declared that a photocatalyst containing 5% copper displayed the maximum degradation of both MB (95%) and soot (50%) within 1 h and 168 h of irradiation in a solar degradation reactor composed of a 2500 W xenon arc lamp equipped with an outdoor UV filter. Nevertheless, a higher Cu amount resulted in a reduction of nanocomposite performance. This may be attributed to defects in the Cu NPs, which could have promoted photogenerated electron–hole recombination and consequently led to decreased TiO₂ photocatalytic activity. In contrast to the results obtained in the self-cleaning test, air de-pollution findings carried out by investigating nanocomposites’ performance through NOx reduction showed that their efficiency was proportional to the amount of copper. Indeed, samples coated with 15% Cu presented the highest NO oxidation activity, at 36.70%, while samples treated with pure TiO₂/SiO₂ showed only 25.56% oxidation activity. In order to investigate the durability of the coatings in real-life circumstances, samples were exposed in outdoor conditions for the first 12 months and following 17 months (February–February 2020; July–November 2020). Overall analysis of chromatic variations showed acceptable variation, with ΔE* = 3.93 and 3.28 for samples containing 5% and 15% Cu, respectively. In this study, chromatic variations were investigated only after the ageing test; more analyses are required to prove the durability of the coatings in real conditions. Other works have been performed to elaborate enhanced TiO₂ NPs as protective treatments in recent years, which are summarized in

Table 2. Applications of enhanced TiO₂ NPs as coatings for the treatment of heritage stone materials (references are presented in a chronological order).

Nanomaterials Composition	Particles Size	Substrate	Obtained Results	Ref.
Ag-TiO2	0.1–1 μm	Limestone slabs from quarry of Utrera (Seville, Spain)	Ag/TiO2: smaller particle sizes, more stable colloids, zeta potential < 20 mV, and antibacterial activity against E. coli Nanocomposite showed good inhibition activity against biopatina formation compared to independently generated Ag or TiO2 NPs	[156]
Au-SiO2-TiO2	10–30 nm	Limestone with an open porosity of around 12%	The thickness of coatings ranged from 3 to 12 µm ST12Au coating: highest self-cleaning efficiency by degrading methylene blue, acceptable chromatic variation (∆E* = 3.4 ± 0.5), adequate adhesion to the tested stone, and water repellence behaviour	[157]
Ag-TiO2	94–234 nm	Carbonate stone mainly composed from calcite (95%–98%) and quartz (2%–5%) with open porosity of 10%	Silver NPs were stabilized by citrate and prepared with TiO2 to obtain nanocomposites A noticeable chromatic variation was obtained; all coatings induced (∆E* > 5) Biopatina growth was significantly reduced by the developed nanocomposite	[158]
TiO2/Au-SiO2	20–25 nm	Fossiliferous limestone (calcite 98.5%, α-quartz 1.5%)	The resultant nanocomposites produced crack-free surface coatings on limestone, effective adhesion, increased stone mechanical properties, and imparted hydrophobic and self-cleaning capabilities	[159]
Fe-TiO2	13–21 nm	Limestone	The low iron doping titania (0.05% and 0.10% w/w) had a favorable impact on the photocatalytic destruction of methyl orange under visible radiation	[160]
Au/N-TiO2/SiO2	5.2 nm	Capri limestone composed from calcite (open porosity of 9%–12%) Granite Grey Pearl (with open porosity < 1%) HERPLAC® concrete (with an open porosity of 10%)	The introduction of AuNPs has the drawback of causing significant color variations Au/N-TiO2/SiO2 coatings can improve photocatalytic performance, but only when the conservation of the material aesthetic characteristics is not crucial	[161]
Ag-TiO2	12.5 ± 4.3 nm	Fossiliferous limestone, extracted from Cabra (Cordoba, Spain), composed of calcite (~100%) and with a 6% porosity	Superhydrophobic coatings were obtained with contact angle of 155° Coatings were able to degrade 75% of MB dye after 100 min and reduced microbial growth on tested stone with inhibition up to 20% (Saccharomyces cerevisiae) and 70% (Escherichia coli)	[162]
N-TiO2/SiO2	50–200 nm	Portland cement mortar	N-TiO2/SiO2 nanocomposites enhanced MB degradation compared to TiO2/SiO2 materials (MB removal of 85% and 78% after 1 h of irradiation, respectively).	[163]

.

ZnO NPs have been coupled with noble metals, particularly silver, to obtain enhanced nanocomposite coatings. It is well known that ZnO NPs have weak activity under visible light. Moreover, Ag nanoparticles are not stable when exposed to sunlight. These limitations can be eliminated when ZnO NPs are combined with Ag derivatives. Mu et al. (2021) [164] developed nanocomposite materials for use as antimicrobial coatings to protect heritage building materials, particularly limestone (natural rocks rich in CaCO₃). The antimicrobial activities of the prepared nanocomposites were tested against Gram-positive bacteria (Bacillus subtilis), Gram-negative bacteria (Escherichia coli), and fungi (Aspergillus niger). The results showed that the AgCl/ZnO combination exhibits less inhibitory activity against fungi than against bacteria. This might be explained by the sturdier cell walls fungi, which could serve as a defense against biocidal substances. However, additional research is required to assess the applicability of AgCl/ZnO to a larger variety of materials.

Fungi are among the most significant colonizers and biodegraders of stone substrates of all microbes. For this reason, there is an urgent need to develop antifungal materials [63]. In this regard, Fernández et al. (2017) [165] developed antifungal coatings with high potential for stone conservation. They prepared Zn-doped MgO (Mg_1−xZn_xO, x = 0.096) NPs by a sol–gel process for use as an antifungal coating. Their research intended to combine the potential of ZnO NPs as antimicrobial agents with the high compatibility of MgO NPs with stone materials. The photocatalytic and antifungal activities of Zn-doped MgO NPs were compared with single ZnO and MgO NPs. The results showed that the doped nanomaterials reveal improved self-cleaning capabilities. In fact, 87% of methylene blue dye was destroyed using Zn-doped MgO NPs after one hour of UV exposure, while ZnO and MgO NPs alone only degraded 58% and 38%, respectively. Antifungal action was tested against Aspergillus niger, Paraconiothyrium sp., Penicillium oxalicum, and Pestalotiopsis maculans on two calcareous substrates (Calcitic and dolomitic stones) that are extensively used in the cultural heritage of Mexico and Spain. Such strains have demonstrated potential activity in the solubilization of calcium carbonate plates and limestone through the creation of oxalic acid [166]. The results showed that treatment using doped nanoparticles stopped invasive microbial growth on both tested calcareous stone materials, particularly against Aspergillus niger and Penicillium oxalicum.

Weththimuni et al. developed a ZrO₂-doped ZnO-PDMS nanocomposite (doped NPs 0.5% (w/w) in PDMS) which is a multi-functional and durable coating for protecting different types of stone e.g., Lecce stone (LS), Brick (B), and Marble (M) [96]. Nanocomposite protective coatings provided homogeneous distribution on the considered stone surfaces, as confirmed by optical microscope (Figure 11a–f) and SEM (Figure 11g–l) analyses. The authors mainly focused their study on evaluating the durability properties and self-cleaning effect of the newly prepared coating with respect to the well-known PDMS coating. They were assessed after exposure to two different ageing cycles: solar ageing (300 W OSRAM Ultravitalux light with an UV-A component (315–400 nm, 13.6 W) and UV-B component (280–315 nm, 3.0 W) for 1000 h and humid chamber ageing (RH > 80%, T = 22 ± 3 °C, desiccator, 2 years). The preliminary results suggested that the PDMS (P) and ZrO₂-doped ZnO-PDMS (Zn-Zr-P) coatings did not significantly alter the original chromatic properties of any litho-types (∆E* < 5), while water repellent behavior and vapor permeability were preserved at acceptable levels compared to their unaged counterparts [95,167]. However, each stone (Lecce stone, Brick, and Marble) showed its own behavior towards water and water vapor due to their different porosity properties. After assessing the durability of the coatings using two different ageing cycles, their performance was evaluated by the self-cleaning test. The newly developed coating showed a higher photocatalytic effect (self-cleaning effect) than plain polymer coating (PDMS) on all of the tested stone types based on its ability to discolor methylene blue dye (e.g., Figure 11: Y image) when exposed to UV light (Figure 11m,n graphs). The highest efficiency was reported in the case of marble treated with nanocomposite coating (the discoloration factor D* was 72% in the case of ZrO₂-doped ZnO-PDMS-treated marble, while it was 56% in the case of plain PDMS). The doped NPs were able to increase the photocatalytic effect of the binder material (PDMS) to a good level and maintained their self-cleaning ability even after long-term ageing processes, indicating that the coating has good durability.

6. Risk of Toxicity and Preventive Measures for Use of Nanomaterials in Art Conservation

Handling products containing nanomaterials poses one of the biggest concerns when working on heritage conservation. However, the risk of nanomaterials does not exist only during the application of coatings on building surfaces. Actually, it starts when the synthesis procedure takes place, as operators are severely exposed when handling both the reagents and the resulting nanomaterials. Moreover, not only the operators but even people who are in the same work area are exposed to significant threats. Due to their small size, nanomaterials can pass into the human body, into animals, and even contaminate the environment.

The risks of nanomaterials in coatings during the application procedure depends on the application process. Nanomaterials dispersed in a solvent or mixture of solvents can be applied on building materials through three procedures: brushing, immersion, and spraying. In the case of application through immersion, the piece can be removed from the area and placed in a solution containing a mixture composed of nanomaterials for impregnation through capillary rise [31]. The application of coating through spraying processes can be considered the most hazardous procedure [168], as nanomaterials can enter into the body during the application process through the skin, mainly via the face, hands, and arms, in addition to possible access through the eyes, nose, and ears [31]. Simkó et al. (2010) [169] announced that there are various entrance points of nanomaterials into human body, with the most common being ingestion, aspiration by respiratory system, ear canals, tear ducts, and skin contact. Brushing processes can be considered less risky; however, nanomaterials can frequently persist on the brush, and contact between particles and the skin can take place accidentally. Additionally, the remaining amount can come into contact with laboratory workplace surfaces after the application process, where the risk of its spreading to other areas increases, and consequently could pose a threat to people, animals, and environment [31]. Therefore, it is crucial to implement the necessary safety precautions described later in this section.

Several factors are essential to take into consideration when evaluating nanomaterials toxicity, as presented in Figure 12, based on intensive research carried out on the ecotoxicity of nanomaterials conducted by international organizations as the European Commission [170]. It is known that the degree of cohesiveness between particles is fundamental to determining whether particles can be released into the environment. Agglomerates are defined as “weakly bound particles”, while aggregates are “strongly bound particles”. Thus, it is obvious that in the case of agglomerated nanomaterials the physical bond between particles can be easily broken and particles with tiny size can be simply released into the environment and human body. According to certain studies performed on the agglomeration of particles and their impact on health, agglomeration can more easily lead to diseases than aggregation [171]. On the other hand, particles exhibit numerous particular features depending on the synthesis technique, in particular morphology, which causes variations in their behavior and how they interact with their environment. Accordingly, toxicity may be higher in the ionic state, followed by spherical particles, and lower in the cases of cubic and prismatic particles [172]. However, other reports indicate that rod-like nanoparticles of iron oxide are more toxic than spherical ones [173]. Zhao et al. (2013) [174] stated that nano-hydroxyapatite with plate- and needle-shaped NPs produce a higher proportion of cell deaths compared to spherical and rod-shaped nanoparticles. Morphology has an impact on particle size, and consequently on surface area, which consequently affects the level of toxicity. It is well known that as the particle size decreases, the surface area increases, and consequently the level of interaction between NPs and their environment rises. The solubility of inorganic nanomaterials is another factor that needs to be considered. According to studies, it has been stated that soluble silver compounds are less hazardous than metallic silver and insoluble silver compounds [175]. The chemical composition of nanomaterials is one factor that strongly affects their toxicity. Compared to negative and neutral nanoparticles, positively charged NPs are more hazardous, as they are able to enter cells faster [176].

In general, basic nanomaterials act like gases, have quick diffusion capacities, and move over extended distances. When nanomaterials are released into the environment, they can come into contact with various organs of the human body and animals and react with different tissues or cells through a variety of access points. Studies on animals have revealed that certain NPs can pass through the blood–brain barrier, which generally defends the brain from toxins and contaminants in the bloodstream [177]. It has been reported that NPs inhaled by animals might result in inflammation of the lungs. In addition, NPs are able to move from the lungs to other organs and interfere with cell signaling [177]. Recently, Gomez-Villalba et al. (2023) [31] reported in detail on the side effects that may result from exposure to nanomaterials over time or continuously when performing ordinary conservation activities. They indicated that different organs could be affected due to nanomaterials accessing the body through different routes. For example, the nasal passage is distinguished from all pathways to the body. After entering the nasal channel, the particles move to the respiratory system and the lungs via the olfactory nerves (Figure 13a). Studies reported that diffusion, impaction, interception, and electrostatic attraction are the mechanisms causing deposition of particles in the pulmonary airways during the breath inspiratory phase [178]. In particular, it is important to know that how nanoparticle aerosols behave and how they affect human health depend on their electrical charge. The particle attraction on the surface of the respiratory tissue is caused by the presence of opposite charges between the particle and tissue, which can raise the amount of breathed-in particles that are deposited in the lungs. This process is influenced by the size of the particles. Indeed, the strength of this attraction is more efficient as particle size decreases. It has been stated that the Brownian deposition process predominates for atmospheric NPs when they are negatively charged and range in size from 6 nm to 30 nm [179]. Alveolar deposition increases dramatically in terms of surface area when the polarity of the nanoparticles changes, for instance, from 16 nm to 30 nm [179]. On the other hand, it is worth noting that nanoparticles that are collected at the nasal level are not only able to reach respiratory system and lungs, and could reach the brain through the olfactory nerves (Figure 13b). The eye is another organ by which nanoparticles can enter the brain, as shown in Figure 13c. Despite the existence of obstacles preventing materials from reaching the eyeball, particles with small size are able to create strong contact with the ocular surface [180]. Experts claim that the cornea is the primary location where nanomaterials could be deposited and persist for a long period, overcome the barriers of the ocular surface, and eventually reach the retina [181] and posterior segments of the eye.

Prow et al. (2010) [181] stated that migration over the epithelial barrier is able to generate cytotoxicity and inflammatory reactions. Moreover, NPs have the ability to cause cellular damage and a broad immunological reactions, which has an impact on the optic nerve, retina, lens, and macula [181]. The impact of NPs on the eyes depends not only on their particles size but on their nature. Zhu et al. (2019) reported that an increase in retinopathies might result from exposure to ZnO NPs [180], while, Wu and Tang (2018) stated that Ag and TiO₂ NPs can enter the central nervous system and cause neuroinflammation by eye-to-brain routes [182]. The brain is not the only organ that NPs can reach; the kidneys are among the most influenced organs due to their high susceptibility to toxic metals and their contribution to the elimination of NPs from the blood supply (Figure 13d). Metal and metal oxide NPs have the ability to promote the production of ROS, which can lead to oxidative damage, activation of antioxidant enzymes, and cell death through apoptosis [183]. The accumulation of nanomaterials is influenced by their size and morphology, while their toxicity relies on their concentration and solubility [183]. The liver can be affected by nanomaterials as well; NPs can access it through gastrointestinal route to interconnect with the liver cells, altering their structure and even their function (Figure 13e). Jayvadan and Champavat (2014) [184] reported a detailed investigation of the impacts of nanomaterials on the digestive system, the influence of the size, charge and solubility of particles, and the impact of the pH of the medium on their dispersion and cell-specific incorporation. The same authors stated that ingestion of TiO₂ NPs with size ranging from 25 to 80 nm can induce inflammation in stomach cells. Accumulation of ZnO NPs could result in intestinal and stomach inflammation and even intestinal blockages. In addition, depending on their size NPs can penetrate through the skin, hair follicles, pores, and wounds. Indeed, unlike particles with sizes larger than 30 nm, particles less than 10 nm can get inside human body and result in cell damage [185].

The negative effects of nanomaterials are not limited to the human body and animals; they can affect the environment as well. For example, silver NPs have been demonstrated to have harmful effects on plants, leading to chromosomal abnormalities [186]. Nanomaterials have been revealed to be effective against the natural enemies of mosquitos, posing a threat to public health by impairing the biological regulation of mosquito populations [187]. It has been reported that CuO NPs are harmful to the reproduction of Enchytraeus crypticus (i.e., the earthworm), which might modify soil processes, as earthworms play a crucial role in soil health [188]. Soil processes might be changed by impacts to the nitrogen cycle in plants due to TiO₂, Ag, and CuO NPs. Recently, a detailed study performed by Reyes-Estebanez et al. (2018) reported the ecotoxicological effect of engineered NPs on animals and plants [189].

In this review, we present the recent developments in the field of application of metal oxide nanomaterials for conservation of heritage building materials; thus, it is indispensable to present more details about the toxicity of these nanomaterials on the human body, animals, and the environment. Reports have investigated the impact of metal oxide nanomaterials, notably, TiO₂, ZnO, and CuO NPs [190,191,192]. As mentioned above, the main factors that affect the toxicity of metal oxide particles are their size, morphology, surface modification, solubility, and concentration. Oxidative stress and increased production of ROS are the main causes of metal oxide nanomaterial toxicity. Inflammation, cytotoxicity, DNA damage, chromosomal damage, and carcinogenic consequences as a result of oxidative stress are produced by the internalization of TiO₂ NPs into mammalian cells. Several studies have indicated that anatase phase NPs are more harmful than rutile, which reflects that the structure of TiO₂ polymorphs influences their toxicity. Numerous investigations have demonstrated that TiO₂ NPs were found in the gastrointestinal tract, lungs, spleen, liver, cardiac muscle, heart, and kidneys after oral exposure or inhalation [193]. Other studies have reported the risk of pulmonary harm when working with indoor paints containing TiO₂ nanopowders, which can induce inflammation and DNA damage [194]. Other studies have reported toxicity of TiO₂ NPs in plants [195], animals [196], and insects [197]. Several studies have demonstrated that ZnO NPs in a variety of forms, including rods and spheres, are likely to be harmful because of their capacity to enter the brain [198,199]. The harmful effects of ZnO nanoparticles due to ROS generation, oxidative stress, induction of apoptosis, inflammatory reactions, etc., have been demonstrated in numerous in vitro investigations [191]. Jia et al. (2017) [200] investigated the toxicity of ZnO NPs in the nervous system; their findings showed that both ZnO nanoparticles and Zn ions cause the production of ROS, which causes apoptosis and cytoskeleton disruption. Bioaccumulation ZnO NPs has been detected in the liver, gills, intestine, and brain of fish [201]. The researchers demonstrated that mature fish underwent neural and behavioral alterations as a result of exposure to ZnO nanoparticles [201]. Hepatocyte swelling has been detected in the livers of rats after various dosages of ZnO NPs [202]. Details of the impact of ZnO NPs on plants, soil, animals, and soil organisms can be found in [203]. Copper oxide NPs have the highest cytotoxicity and DNA damaging potential when compared to TiO₂ and ZnO NPs [204]. In vitro studies conducted on human cells exposed to CuO NPs have revealed their significant cytotoxicity, capacity to damage DNA, and ability to produce oxidative stress and cell death [205]. The capacity of particles to diffuse into tissues or cells, which is highly influenced by their surface charge, is necessary for the systemic dispersion of the particles [204]. After they enter cells, CuO NPs may interact with organelles to produce ROS, altering the normal cellular functions [206]. ROS stimulation of oxidative stress results in lipid peroxidation and damage to cellular defense mechanisms through the depletion of reduced glutathione [206]. CuO NPs are able to interact with the lung epithelium, resulting in inflammation [204]. The blood circulatory system carries NPs from the lungs to other parts of the body, where they accumulate in different bodily organs and have harmful effects at diverse locations [207]. According to the findings of in vivo research, oral administration of CuO NPs causes oxidative stress, increased ROS generation, inflammation, apoptosis, and histopathological abnormalities in a number of organs, including the liver, kidney, stomach, and bone marrow [208,209,210]. Other reports have presented the toxicity of CuO NPs on insects [211,212], animals [213], and plants [188,214]. Studies conducted on both in vivo and in vitro have revealed that NPs may be hazardous because they promote the formation of superoxide and other ROS, which results in an imbalance in the redox state and induces oxidative stress in cells [215]. It has been established that MgO NPs can penetrate through the skin, digestive system, and lungs, as well as accumulate in different tissues [216]. Other studies have demonstrated that MgO NPs are harmful to the human body, animals, and soil organisms when their concentration exceeds a certain level [216,217,218]. In this context, the toxicity of MgO NPs (10–15 nm) in Wistar rats was assessed in an in vivo setting [219]. The tested rats received varying dosages of MgO NPs through intraperitoneal injection to examine the effect of NP concentrations on their liver and kidney tissues. The findings demonstrated that when compared to the control group, high concentrations of MgO NPs (i.e., 250 and 500 g.mL⁻¹) dramatically increased white blood cells, red blood cells, hemoglobin, and hematocrit. Figure 14 reveals the different reported mechanisms of cell inactivation through metal oxide nanomaterials after the generation of reactive oxygen species.

According to the rules developed by various organizations, several precautions must be taken when working to conserve historical materials using nanomaterials. Nanomaterial application techniques must consider the maximum protection of workers against direct contact with NPs. In particular, the use of improper protective gloves when using brushing treatments increases the danger of skin contact with nanomaterials emitted into the environment. Thus, wearing gloves during brushing procedures is highly recommended. Moreover, in order to protect the skin gloves must be in accordance with the rules, extremely durable, not highly porous, and able to withstand contact with liquid solutions containing nanomaterials that may eventually react with the glove. A face shield, HEPA 14 filter mask, gloves, protective glasses, and suits are required for the operator when performing different application processes [31]. A glove box and pyramid portable glove bag are required to keep operators away from direct interaction with NPs. However, these are more useful when operators are working on specimens of small and medium size at the laboratory scale. Additionally, the laboratory needs to have a sufficient ventilation system and proper waste management containers. On the other hand, extra caution must be used when cleaning brushes, paintbrushes, and other tools, to prevent spilling their contents into the water pipes. As an alternative, they ought to be kept in cans that are clearly marked with information about the contents and risks [31]. Furthermore, selecting the most suitable application procedures depending on the nature and composition of coatings can decrease the risks of exposition to hazardous compounds. Figure 15 illustrates the main personal protection tools required for each application route and how the human body can be protected from nanomaterial penetration through the main access routes.

The access of nanomaterials in general and metal oxides in particular into the environment is feasible during synthesis procedures as well as during their application to heritage stone. The destiny of these nanomaterials after application has not yet been fully identified. Most studies have discussed the characteristics of coatings containing metal oxide NPs and their performance; however, almost no studies have dealt with the fate of these particles after their application, how they interact with environment, and how conservators might be able to remove coatings containing nanoparticles in order to perform renovations after several years. On the other hand, the control of treatments based on nanomaterials is almost always difficult for outdoor applications due to climate environments such as heavy rain, wind, flooding, etc. Alternatively, the use of low amounts of NPs dispersed and interconnected with a binder material, such as PDMS or any other binder, decreases the risk of toxicity. Indeed, metal oxide nanoparticles may be coated with polymeric suspension at low NP concentrations by forming chemical bonds between NPs and the binder, then links between the binder and the stone substrate, which decreases their release into environment or human body and moderate their harmfulness. Contrarily, preparing NP mixtures based on water or alcoholic suspensions increases the risk of NPs being dispersed outside, as water and alcoholic suspensions are easily evaporated over time and the NPs, without a bond or chemical link with the stone substrate, can easily become detached. Moreover, in general, combining NPs with organic fillers or surface functionalization can reduce their risk [96]. In fact, NPs used as aqueous dispersions or transformed to insoluble stable chemical compounds inside stone substrates (if possible) can decrease the risk of NPs being released into the environment [19]. This solution has several benefits due to its low cost and is safer for the environment as it is able to decrease the possibility of NPs accessing into the atmosphere. In addition, working with biocompatible nanomaterials, which have no negative impacts on the environment, has recently been considered as an alternative approach. The green route for synthesis of nanomaterials offers cost-effective, environmentally friendly, and non-toxic alternatives to traditional physical and chemical processes, and prevents operators from being exposed to hazardous chemical products.

7. Conclusions and Future Perspectives

Most of the cultural heritage materials of the globe are made of stone, which is deteriorating due to various causes. Several techniques have been developed for their conservation to preserve them for future generations. In particular, extensive research has been carried out on the development of coatings based on nanomaterial formulations with self-cleaning and/or antimicrobial activity for the protection of stone buildings. This review involves the recent advances in the development of metal oxide nanostructures as treatments for the cleaning, consolidation, and protection of natural stone in built heritage. Using metal oxide-based nanomaterial compounds has been considered an attractive approach by several research group working in this field due to the possible development of products with multifunctional features. In fact, coatings based on metal oxide nanomaterials can reduce water penetration thanks to their potential interaction with a wide range of hydrophobic binders, in addition to the so-called lotus effect. Moreover, their capacity to degrade pollutants such as stains [220], combat aerial pollutants such as nitrogen oxides gases (NO_x) [221], inhibit microbial growth, and promote resistance to ageing thanks to photocatalytic processes through the continuous generation of ROS when nanomaterials are exposed to a source of light has helped these kinds of nanomaterials to gain attention in this field. However, pure metal oxide NPs, particularly TiO₂ and ZnO nanomaterials, which are the most widely used metal oxides in this field, have two main limitations: weak solar energy absorption efficiency and fast recombination of photogenerated charge carriers. To overcome this issue, laboratory experiments have produced several promising outcomes, particularly for metal oxide nanomaterials coupled with metal ions/NPs. In the case of CuO and MgO, the use of hazardous materials, significant production expenses, and high energy consumption are their main drawbacks [222,223]. Green synthesis, which is based on using natural sources such as plants and microorganisms to create CuO or MgO nanomaterials, has developed as an option that is environmentally friendly with low costs and energy consumption.

Despite the extensive work which has been performed in the field of using metal oxide nanomaterials for the preservation of heritage stone artefacts, many existing works do not clearly mention the mechanical features of the coatings (e.g., peeling), their capacity to degrade a wide range of pollutants under normal solar light, their durability and resistance against ageing, or their microbial inhibitory capacity against a mixture of microorganisms. We expect that in the following years several research groups will work on developing multifunctional coatings with enhanced photocatalytic performance (i.e., active under natural solar light) with good resistance against ageing (at least 5 years) and which are able to support severe conditions (weather, climatic changes, pollution, etc.) and inhibit complex microbial mixture growth.

Because nanomaterials unfortunately have the potential to modify the environment and harm both human and animal health, it is necessary to work on new safe-by-design nanomaterials in order to reduce the environmental and toxic dangers, the consequences of release into the environment, and the required precautions. Furthermore, the potential for developing reversible treatments based on metal oxide nanomaterials is another issue that must be addressed in further research as a key request when discussing the preservation of stone artefacts.