## 3 Characterization Methods of Cellulose Nanomaterials

Table 3: _Cont._nanomaterials, used FTIR, TG-DTG, XRD, and SEM-EDS to demonstrate the successful surface modification of nanocellulose [12]. [PERSON] et al. [51] introduce, in their contribution, the contact angle measurement of starch films functionalized with nanocellulose in order to analyze the influence of chemical modifications on the surface polarity due to the nanocellulose. This is a significant aspect because, with this measurement, it is possible to evaluate the variation in the hydrophilic character of nanocellulose coatings, which affects the behavior of the treated artistic substrate [51; 52]. In addition to the above-mentioned techniques, [PERSON] et al. [49] used UV/visible spectroscopy to measure the optical properties of nanocellulose. In the conservation of cultural heritage, the appearance of works of art depends on the optical properties; unfortunately, UV light can also destroy covalent bonds in organic materials present on the artwork, causing many undesirable degradation effects, such as in polymers, woods, dyes, and pigments, leading to the photodegradation of the works themselves. For these reasons, it is important to evaluate the behavior of the consolidants against the UV radiation [53]. Moreover, in the same work of [PERSON] et al., the antibacterial analysis performed against Bacillus and E. coli bacteria using the disc diffusion method is reported [49]. In the case of artworks, it is very important to perform this analysis on the consolidants in order to ensure that irreparable damages due to bacteria do not occur. The antimicrobial activity of nanoparticles or other nanomaterials, such as nanocellulose, is usually carried out by optical and electronical microscopy [54]. Beyond that, the toxicity and cytotoxicity of nanocellulose are other important aspects to analyze, especially for applications in the medical and biotechnology fields, but also for the protection and health of operators working in the field of fine arts. To achieve this, four different approaches can be used: the first method consists of the assessment of cell damage in function of its morphology; the second one measures the cell damage; the third measures the cell growth; and, finally, the last method measures specific metabolic activities. [PERSON] [55] reported that most of the studies on the cytotoxicity of NCs do not show harmful effects on cells and there are not even adverse dermal or oral toxicity effects for the human health; however, the presence of surface functional groups and cytotoxic impurities due to the origin of the cellulose source, the preparation procedure, and the post-processing or sample preparation may be harmful; for these reasons, further studies and the development of new characterization methods of nanocellulose are required [9]. All of these investigation techniques allow us to obtain very interesting information about these innovative materials and to visualize the differences between each type of nanocellulose, starting from their shape and morphology up to their concentration of carbon and oxygen, the main elements present in nanocellulose. Therefore, the techniques previously mentioned are the most used for the first characterization of nanocellulose, but also for the analysis of nanocellulose-based consolidants applied for the recovery and conservation of cultural heritage [4; 16; 17; 24; 34; 36; 56; 57; 58; 59; 60; 61]. The description of all of these techniques and their principles of operation were recently reported by [PERSON] et al. [62]. Certainly, the development of new, faster, and reliable characterization techniques that are able to carry out less destructive and in situ analyses would allow for the spread of nanocellulose at an industrial level, and this is the challenge that we are trying to overcome, even if, for the application in the artistic patrimony conservation field, the nanocellulose mass available is currently sufficient for sustaining these uses [15; 42].