Development of biocidal polymeric films incorporating nanoemulsified essential oils

The proliferation of pathogenic microorganisms on various surfaces is a significant concern, prompting extensive research into the development of effective antimicrobial materials. One particularly important and effective approach that has emerged from this research is the use of biocidal polymer films and coatings to prevent microbial contamination. These systems can improve public health by reducing the spread of harmful pathogens on surfaces in various environments. These antimicrobial polymers are typically designed by incorporating biocidal agents into their matrices. In recent years, considerable attention has been given to the design and development of polymeric films and coatings with antimicrobial functionality due to the superior reinforcing capabilities and multifunctional properties conferred by these materials compared to conventional biocidal agents [1]. Natural compounds such as gellan gum, gelatin, chitosan and sodium alginate are widely used in the production of films with biological functions, such as antimicrobial properties [2], as well as for wound healing and drug delivery applications [3]. Essential oils (EOs) are natural products of plant origin that are hydrophobic and volatile. They are obtained through physical extraction methods from leaves, flowers, fruits, bark and other plant tissues. These complex mixtures are rich in terpenoids, terpenes and various aromatic compounds and have demonstrated potent antimicrobial activity [4]. In addition to their antimicrobial effects, EOs are gaining attention for their potential to combat biofilms—highly structured microbial communities that form on surfaces and pose significant risks to health safety [5]. Controlling biofilm-associated contamination is therefore critical in ensuring surface hygiene. Despite their advantages, EOs also present technological challenges such as volatility, low water solubility, and instability under environmental conditions. To overcome these limitations, encapsulation strategies such as nanoemulsions (NEs) have been developed. These systems improve the physical stability and enhance the antimicrobial efficacy of essential oils, mainly due to their nanometric droplet size (~200 nm), which increases the surface area and facilitates a more uniform interaction with microbial cells. Based on these considerations, lavender essential oil (LEO) and peppermint essential oil (MEO) were formulated into NEs. These nanoemulsified essential oils were subsequently incorporated into sodium alginate, chitosan, gelatin and gellan gum-based film-forming polymeric matrices, specifically designed for surface coating applications. The dispersion of essential oil-loaded NEs within the hydrophilic polymer matrices addresses the drawbacks typically associated with the direct addition of pure essential oils, such as phase separation and non-uniform distribution during film formation, while further improving the antimicrobial functionality of the resulting films. To optimize the formulation of the NEs, a ternary phase diagram was constructed by varying the ratios of essential oil, surfactants, and water. All prepared NEs were characterized for their droplet size, zeta potential, and polydispersity index (PDI) using dynamic light scattering (DLS), and their stability was systematically evaluated. To investigate potential changes in the chemical profile of the essential oils, the NEs were analysed using headspace gas chromatography–mass spectrometry (HS–GC/MS). The films were prepared using the solvent casting technique, which involves dissolving the polymer, plasticisers and additives in an appropriate solvent before casting the solution onto a substrate. The cast films were then dried at 40 °C for 15 hours to allow complete solvent evaporation. The chemical composition of the essential oils was characterised using gas chromatography–mass spectrometry (GC–MS). The in vitro biocidal activity of LOE and MOE was assessed prior to and following their incorporation into the biopolymeric films to evaluate the effect of the film matrix on the bioactivity of the encapsulated essential oils. In vitro antibacterial activity was evaluated by determining the minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) against one Gram-positive and one Gram-negative bacterium. Different concentrations of the optimal formulation were introduced into sodium alginate, gelatin or gellan gum films for testing. The optimised systems were characterised in terms of thickness and swelling capacity. Water vapour permeability and mechanical testing were also studied.