## 1 Introduction  
Cultural heritage artworks have an inestimable value that must be protected and preserved to be passed on to future generations. They are affected by different alteration processes and degradation, which increase over time, and in outdoor conditions, they are exposed to a greater extent of atmospheric agents as well as to biological colonization [1, 2, 3].  
Thanks to technological advancements in recent times, applied techniques and the biocide products used to remove biofilms on monument surfaces have been improved [4, 5, 6, 7, 8]. Biocides are chemical substances which are efficient at killing the undesirable microorganisms that can occur on such surfaces, and they have been widely applied in the conservation of cultural heritage [9, 10, 11]. However, traditional commercial biocide can show some harmful effects on the environment and human health; furthermore, they can also show some interferences with the substrate, such as staining, changes in color, and chemical interactions [8, 10, 12]. Application methods and low doses can reduce the mentioned risks. Moreover, the direct application of biocide has relatively short-term effectiveness, and sometimes, the compounds can be removed and dispersed in the environment [2, 8, 10, 13].  
In the last decade, worldwide research has concerned the testing of innovative natural biocide and the development of organic and inorganic nanomaterials, associated with multifunctional coatings, for long-lasting reliability, and non-invasive and selective activity [7; 8; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. The use of natural active compounds coupled to this nanotechnology seems a good way to pursue an eco-friendly, sustainable, and safe approach for the conservation of cultural heritage, reducing the amount of the bioactive compound and obtaining a satisfactory, long-lasting, antifouling action [15; 16; 17; 25; 26; 27]. The literature reports the synthesis of different nanocontainers and loading techniques [21; 25]. However, nanosystems based on mesoporous silica materials, applied in a multifunctional coating, as a controlled release biocide system over time, are reported only in a few cases [19; 21; 28; 29; 30]. In previous works, two different silica-based nanocontainers (Ns), namely a core-shell nanocapsule (NC) and a mesoporous nanoparticle (MNP), have been synthesized and characterized [13; 14; 20; 21; 22]. Previous preliminary in vitro cultures exposed to the nanoparticles showed a high antifouling activity associated with a slight biocidal activity against photosynthesizing microorganisms [13]. However, the efficiency of these Ns in the release control of the engaged biocides and the improvement of biocide efficiency, allowing a reduction in the quantities of biocide required, have not been sufficiently addressed.  
In this work, our goal is to test different biocidal agents through in vitro tests against a significant component of the biofilm occurring on stone in conditions of high humidity and low solar radiation, and in parallel, evaluating different coatings' formulations. According to ecological trends and EU regulations [31], we tested natural zosteric sodium salt, a derivative of zosteric acid, as a natural antifoulant compound, in comparison with the commercial biocide 2-mercaptoethanolhoiazole widely used in the treatment of cultural heritage [9; 32; 33]. The choice of both biocides and, in particular, of zosteric sodium salt as a green biocide, despite the possibility of a negative effect being linked to sulfate residues [34], is related to the previous results for the nanocontainers' synthesis [13; 14; 20; 21; 22]. Such a first step will need further evaluation before a wider application.