## 1 Introduction
Another major concern involves the interaction between artist-applied materials, particularly acrylic-based media, and environmental factors such as humidity, light exposure, and airborne pollutants [20; 21; 22]. Numerous studies have demonstrated that volatile organic compounds (VOCs), often released by conservation materials like adhesives, coatings, or display case polymers, can compromise the chemical and chromatic stability of acrylic films [23; 24]. The diffusion of VOCs and moisture into the polymer matrix can promote the formation of hydrogen bonds and initiate chemical reactions that alter the polymer structure, leading to internal stress, plasticisation, and progressive deterioration [25]. These effects are especially critical for water-based acrylic formulations, which are widely used in commercial markers such as UniPosca. Their relatively low glass transition temperature (Tg) makesthem sensitive to temperature fluctuations and promotes increased tackiness, dust accumulation, and the potential migration of additives under display or storage conditions. Additionally, photodegradation triggered by UV exposure may result in the breakdown of chromophores, binder oxidation, and discolouration of pigments, further compromising visual integrity [26; 27; 28]. While protective coatings are commonly employed in preventive conservation to enhance resistance to chemical and physical stressors, their application to contemporary art remains largely confined to urban artworks and outdoor murals [29; 30; 31; 32; 33; 34; 35; 36; 37]. In contrast, applying such treatments to works like Money Art, characterised by unconventional, fragile substrates such as banknotes, raises additional concerns. Other strategies include coatings based on pullulan, pectin, or starch derivatives, which have been evaluated for food packaging and are now being considered for conservation contexts due to their barrier properties and environmental compatibility [38]. Additionally, layered coatings incorporating CeO\({}_{2}\) nanoparticles and chitosan have been proposed as UV-protective, transparent barriers suitable for light-sensitive materials [39]. Among the most promising candidates is chitosan, a natural cationic polysaccharide that forms transparent, biocompatible films with intrinsic antimicrobial properties [40]. However, pure chitosan films often exhibit limited mechanical resistance and are susceptible to humidity-induced deformation. For this reason, recent studies have focused on hybrid chitosan-based systems, combining chitosan with cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs) to enhance dimensional stability and tensile strength. [PERSON] et al. (2022) [41] demonstrated the efficacy of such nanocomposites in preserving paper-based substrates under accelerated ageing, while maintaining transparency and reversibility. Recent studies have also demonstrated innovative uses of chitosan-based coatings for the conservation of modern materials. [PERSON] et al. (2024) [42] applied a chitosan coating to protect modified artworks against graffiti, highlighting its transparency, reversibility, and minimal aesthetic impact. Furthermore, chitosan's known antimicrobial and film-forming properties have made it a focus of recent research as a sustainable conservation material [43; 44; 45; 46]. Nonetheless, pure chitosan films tend to lack mechanical strength and dimensional stability [47; 48; 49; 50]. For this reason, blending chitosan with structurally supportive materials such as nanocellulose has become an area of active investigation. Nanocellulose contributes tensile reinforcement and enhances film integrity, forming hybrid coatings that combine the bioactivity of chitosan with superior mechanical properties [51; 52; 53; 54; 55; 56; 57]. While numerous studies have identified biological, mechanical, chemical, thermal, and environmental factors as primary contributors to banknote deterioration during circulation [16; 17], the present investigation focuses specifically on the stability of the applied pictorial layer.