## 1 Introduction  
In recent years, the application of green, renewable, and sustainable materials has become increasingly important for the realization of various added-value products with a low environmental impact. In this context, considering that artworks constitute a worldwide priorless good, conservation scientists, restoreers, curators, and researchers who deal with this field are trying to develop innovative solutions for the conservation and recovery of cultural heritage, taking into account environmental compatibility and sustainability [1]. Therefore, for this purpose, the research is focused on using nanotechnology to obtain more sustainable solutions; among these, nanocellulose, or cellulose in the form of nanostructures, has proved to be one of the most important biomaterials of modern times, and also easily usable in the field of cultural heritage given its compatibility with many types of artistic substrates, which mainly consist of cellulose [2; 3; 4]. This innovative bio-nanostructure can be obtained from cellulose extracted from different natural sources, such as plants, algae, small marine animals (tunicate), seeds (cotton), cane (bamboo, bagasse), straw (rice), and some bacteria, by several processes that are necessary for separating cellulose from the other components present in the sources, such as hemicellulose and lignin in the case of wood, or pectin, waves, and other hydrosoluble components in starting materials of different nature [5]. Due to all of the shapes that nanostructured cellulose can take, they have resulted in being an attractive alternative solution to non-renewable sources that are in continuous decline, to environmental pollution, to global warming, and to the energy crisis, as well as to the frequent use of invasive synthetic products for the preservation of artistic works [6]. The differences between several types of nanocellulose (NC), which are cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial nanocellulose (BNC),concern their dimensions and crystallinity; in fact, CNCs are nano-scale materials and correspond to the crystalline part of cellulose, therefore having a higher degree of crystallinity, whereas CNFs and BNC can be nano- or micro-scale materials [7]. The advanced characteristics of cellulosic nanomaterials make them materials that are widely usable for various applications, also including the restoration and recovery of cultural heritage, according to their eco-friendly properties and versatility [3]. For example, CNCs have a high Young's modulus, for which being relatedto the Kevlar material, and are thus used for the production of highly robust materials [8]. Moreover, their biodegradability and biocompatibility make crystalline nanocellulose non-toxic for humans and useful for the reconstruction of body tissues or for drug delivery as reported by [PERSON] and [PERSON] [9]. Nanocellulose has fire retardant properties and can be used as a UV shield [10]. It can also be inserted in a polymer matrix as an additive or applied on the surface of natural or synthetic textile fibers to improve their properties [11; 12]. According to [PERSON] et al., these nanomaterials are also renowned for the possibility of modifying their surfaces with various functional groups that enhance their compatibility with polymers to obtain multifunctional composite materials [13]. The enormous quantity of hydroxyl groups on the nanocellulose surface allows for its interaction with organic compounds and heavy metals present in the waste waters, acting as a filtering membrane [3; 14]. Additionally, thanks to its antimicrobial action, it can be used as wallpaper for hospitals or as food packaging materials and water filters [10]. In the field of cultural heritage, nanocellulose can be used in combination with some additives [15], such as silver nanoparticles (AgNPs) [16] or ZnO [17], thus providing an antibacterial and antifungal activity against common fungi usually found in archives or museums, and bacteria more present in common life.