## 1 Introduction  
The sudden outburst of the COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has contributed to the extensive production and consumption of facemasks worldwide, creating a new and serious environmental challenge. Facemasks are generally composed of nonbiodegradable polymers, such as polypropylene, polyethylene, polyurethane, polystyrene, polycarbonate and polyacrylamide, and the disposal of these facemasks generates a large amount of waste, causing severe ecological problems and becoming a new source of microplastics [1, 2]. Therefore, researchers have been looking for innovative strategies to develop face-masks with a similar or better performance to the existing nonbiodegradable ones, to minimize the negative environmental impact. One sustainable option is the exploitation of green chemistry as well as the replacement of conventional materials with eco-friendly ones, such as natural fibers and biodegradable polymers [2, 3, 4]. Recently, the electrospinning technique has acquired tremendous focus for the development of high-performance filters. This technique enables the production of fibers with diameters ranging from micrometers to nanometers, which show significant morphological advantages over current conventional filters. Moreover, the cost effectiveness, simplicity of use, versatility of using different biodegradable natural or synthetic materials and possibility of scalability make electrospinning more competitive and attractive compared to conventional nonwoven manufacturing methods [5; 6]. In fact, electrospun fibers exhibit outstanding properties, such as a high specific surface area, which results in the higher adsorption capacity of contaminants, controllable small diameters, highly porous structure with small pore sizes and internal interconnectivity, flexibility and possibility of functionalization with active agents. Due to their low weight and high permeability, these fibrous membranes provide high filtration efficiency, without adding extra weight and maintaining the wearer's comfort and breathability [7; 8; 9; 10]. To date, several polymers have been used to produce membranes by electrospinning for air filtration applications. As already mentioned, to face the accumulation of plastics in the environment, the use of biodegradable polymers is preferable [4]. Among them, synthetic PCL polymer has been demonstrated as a promising candidate for filtration applications, due to its low cost, biocompatibility, biodegradability, availability, excellent mechanical properties and ease of operation [11; 12; 13]. Besides acting as a physical barrier to prevent the penetration of small particles and microorganisms, electrospun membranes can be functionalized with several materials in order to achieve new functions, such as antimicrobial activity. Microorganisms can reach the surface of the mask, fail to be destroyed and then penetrate the filter layer, which compromises the protection of the wearer. Therefore, the development of facemasks able not only to act as a barrier for the particles and microorganisms but also to actively degrade them is crucial [14]. One possible approach to develop active filter layers is by the addition of antimicrobial agents, such as NPs. Their nanoscale size and large surface area-to-volume ratio give NPs unique characteristics, improving their catalytic, magnetic, mechanical and optical properties [15; 16]. Several metal and metal oxide NPs, such as Ag, TiO\({}_{2}\) and MgO, have been explored as effective antimicrobial agents against several microorganisms, such as bacteria, viruses and fungi [17; 18]. With the increasing concern about bacterial resistance, other alternatives to the use of antibiotics are required. In this way, NPs arise as suitable options since their mechanisms of action are different from traditional antibiotics [19; 20]. Furthermore, Ag [21], TiO\({}_{2}\)[22] and MgO [23] NPs are already shown to increase the filtration efficiency of electrospun membranes.