**1. Introduction**

Polyhydroxyalkanoates (PHAs) currently represent one of the most important alternative to petroleum-based materials in the frame of the Circular Economy [1]. PHAs, which are synthesized by a wide range of microorganisms as carbon storage material, are thermoplastic materials, biodegradable, and present similar physical properties to other plastics, e.g., polypropylene (PP) and polystyrene (PS), such as high mechanical strength and water resistance [2]. PHAs have been prompted as potential packaging applications due to their biocompatibility and physical properties [3]. However, the PHA production currently associates a high cost due to the carbon sources of the raw materials, i.e., low yield and productivity, and the down-stream process [4]. The synthesis of PHA through fermentation from industrial by-products and waste, particularly the use of mixed microbial cultures, is nowadays seen as an option to reduce the production costs [5].

Among PHAs, the most widely studied and easiest-to-produce member of this family is poly(3-hydroxybutyrate) (PHB). This isotactic homopolyester presents a relatively high melting temperature (T m) and good stiffness due to its high crystallinity (>50%). However, the use of PHB has been limited due to several drawbacks, particularly its poor impact-strength resistance and low thermal stability. To overcome these shortcomings, the use of its copolymers, such as those made with 3-hydroxyvalerate (3HV) or 4-hydroxybutyrate (4HB) to produce poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV) and poly(3-hydroxybutyrate-*co*-4-hydroxybutyrate) (P(3HB-*co*-4HB)), can improve these limitations and widen its processing window [6,7]. In particular, PHBV is a potential candidate to be applied for packaging of films, blow-molded bottles, paper coatings, etc. [8]. To this end, different studies have explored the use of PHBV due to its potential as a sustainable packaging material [9,10]. For instance, PHBVs have been applied in the form of films, fibers, and foams for everyday articles such as shampoo bottles and plastic beverage bottles due to its renewability, biodegradability, and high water vapor barrier [11]. In addition, the incorporation of antimicrobial and/or antioxidant substances into a PHA-based packaging material can result in high interest to improve both protection and shelf life of foodstuffs during the storage period [12–14].

Electrospinning is an innovative technology to generate ultrathin fibrous mats made of a wide range of polymer and biopolymer materials with fiber diameters ranging from several nanometers to a few microns [15]. Electrospun ultrathin fibers have prompted their use in a wide range of industrial sectors, including packaging applications [16,17]. This technique is highly suitable for the encapsulation and/or sustained delivery of active and bioactive substances at the nanoscale level due to both the high surface-to-volume ratios of the electrospun fibers and the high porosity of their mats [18,19]. In particular, electrospinning is interesting for the development of antimicrobial materials by either the use of inherently antimicrobial polymers or the nanoencapsulation of biocide substances [20]. As a result, within the frame of active packaging, different recent studies have reported the encapsulation of metal nanoparticles (MNPs) in electrospun matrices. For instance, poly(vinyl alcohol) (PVOH) and poly(N-isopropylacrylamide) (PNIPAAm) membranes containing silver nanoparticles (AgNPs) immobilized onto cellulose nanowhiskers (CNWs) presented antimicrobial activity against several Gram-negative (G-) and Gram-positive (G+) bacteria [21]. In another study, polyvinylpyrrolidone (PVP)/poly(ε-caprolactone) (PCL) nanofibers functionalized with zinc oxide nanoparticles (ZnONPs) and AgNPs, also prepared by electrospinning, showed a high antibacterial activity against *Staphylococcus aureus* (*S. aureus*) and *Escherichia coli* (*E. coli*) [22]. Similarly, electrospun chitosan/poly(ethylene oxide) (PEO) membranes containing AgNPs presented antimicrobial effect against *E. coli* [23]. Recently developed electrospun PHA materials containing AgNPs [24] and copper oxide nanoparticles (CuONPs) [25] have been also able to considerably reduce bacterial growth at very low contents. These novel NPs-containing electrospun materials offer significant potential as new antimicrobial coatings or interlayers, that is, internal layers in a multilayer system, for application in the design of active food packaging structures.

Natural antimicrobials, such as essential oils (EOs), are currently regarded as an alternative to synthetic preservatives of food because they are considered as Generally Recognized As Safe (GRAS) substances, being acceptable to consumers [26] and having the capacity to exert a multitude of biological effects [27]. For instance, eugenol, which has potential antimicrobial and antioxidant actions, has been effectively applied against foodborne pathogens [28,29]. However, EOs are frequently unstable and can be easily degraded in stressful situations such as in the presence of oxygen, temperature and light, so that they can lose their antimicrobial activity [30]. To avoid this issue, encapsulation is considered a good way to protect and preserve the effectiveness of active and bioactive substances [31]. In this sense, silica mesoporous supports (SMPSs) [32] show a grea<sup>t</sup> deal of potential for the storage and release of active substances [33,34]. In particular, the typical sizes of SMPSs range from microns to nanometers, presenting tailor-made pores of around 2–10 nm [35]. The particular morphology of SMPSs renders a very large specific surface area, up to 1200 m2/g and, then, an enhanced loading capacity for the encapsulation and release of natural antimicrobials [36]. Within SMPSs, Mobil Composition of Matter (MCM), including both MCM-41 and MCM-48, are among of the most popular mesoporous molecular sieves in which their pore diameter can be nicely controlled by adjusting their synthesis conditions and/or by employing surfactants with different chain lengths in their preparation [37]. Silica mesoporous materials are thus able to encapsulate organic molecules, forming host–guest complexes with volatile molecules (e.g., EOs) to efficiently control their volatility and reactivity. So far, many studies have employed MCM to encapsulate active substances with positive results in different applications, for instance, caprylic acid against foodborne pathogens [38], EOs as antifungal [36,39,40] and antimicrobial systems [41], and poplar-type propolis in drug delivery platforms [42]. In particular, the antimicrobial and antifungal effect of the EOs-functionalized supports improved compared to the free compounds due to the EOs encapsulated inside MCM released in a controlled manner [39–41]. These previous results sugges<sup>t</sup> that the immobilization of EOs onto silica supports can represent a novel strategy to develop a new generation of long-term antimicrobial systems that may not only enhance the antimicrobial activity of EOs, but also mask their characteristic odor/taste for food-related applications.

In this study, it is initially reported the preparation of nanometric MCM-41 particles loaded with eugenol, a phenylpropene and an allyl chain-substituted guaiacol that is primarily extracted from cinnamon, bay leaf, nutmeg, basil, and clove [43]. The resultant MCM-41 particles containing eugenol were thereafter incorporated, for the first time, into PHBV by electrospinning. The generated electrospun composite fibers were thermally post-treated to produce films that were characterized in terms of their morphology, thermal, mechanical, and barrier properties. Finally, the antimicrobial performance against foodborne bacteria was also determined. In a packaging context, the active tests were carried out as a function of time in open vs. close conditions in order to simulate potential real conditions.

#### **2. Materials and Methods**
