Biopolymer-Based Films Reinforced with FeₓOᵧ-Nanoparticles

Citation: Abdullah, J.A.A.; Jiménez-Rosado, M.; Benítez, J.J.; Guerrero, A.; Romero, A. Biopolymer-Based Films Reinforced with FexOy-Nanoparticles. Polymers 2022,14, 4487. https://doi.org/ 10.3390/polym14214487 Academic Editor: Alexey V. Lyulin Received: 26 September 2022 Accepted: 20 October 2022 Published: 23 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Biopolymer-Based Films Reinforced with FexOy-Nanoparticles Johar Amin Ahmed Abdullah 1,* , Mercedes Jiménez-Rosado 1, JoséJ. Benítez 2, Antonio Guerrero 1 and Alberto Romero 3,* 1Departamento de Ingeniería Química, Escuela Politécnica Superior, Universidad de Sevilla, 41011 Sevilla, Spain 2Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Calle Américo Vespucio 49, Isla de la Cartuja, 41092 Sevilla, Spain 3Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spain *Correspondence: [email protected] (J.A.A.A.); [email protected] (A.R.); Tel.: +34-954557179 Abstract: Nowadays, natural polymer-based films are considered potentially environmentally friendly alternatives to conventional plastic films, due to many advantageous properties, including their easy processability, high flexibility, non-toxicity, low cost, high availability, and environmental safety. However, they are limited in their application by a number of shortcomings, including their high water solubility and vapor permeability as well as their poor opacity and low mechanical resistance. Thus, nanoparticles, such as green Fe x O y -NPs, can be used to overcome the drawbacks associated with these materials. Therefore, the aim of this study was to develop three different polymer-based films (gelatin-based, cellulose acetate-based and chitosan-based films) containing green synthesized Fe x O y -NPs (1.0% w/w of the initial polymer weight) as an additive to improve film properties. This was accomplished by preparing the different films using the casting method and examining their physicochemical, mechanical, microstructural, and functional characteristics. The results show that the incorporation of Fe x O y -NPs into the different films significantly enhanced their physicochemical, mechanical, and morphological properties as well as their antioxidant characteristics. Consequently, it was possible to produce suitable natural polymer-based films with potential applications across a wide range of industries, including functional packaging for food, antioxidants, and antimicrobial additives for pharmaceutical and biomedical materials as well as pesticides for agriculture. Keywords: gelatin; cellulose acetate; chitosan; iron oxide nanoparticles; mechanical properties 1. Introduction In the past few years, nanocomposites derived from biopolymer-based films have been widely investigated due to their potential as a technology for innovative design uses in eco-friendly food packaging [ 1 ]. Nevertheless, they have some disadvantages, such as their inadequate barrier and mechanical properties for resisting water vapor and oxygen. Therefore, nanoscale reinforcements are studied. Films are thin (<1 mm), transparent, and stretchable plastics [ 2 ]. These materials provide excellent flexibility, are ideal for wrapping products of different sizes and shapes, and are typically produced from polyethylene (PE) or polypropylene (PP) [ 3 ]. These films possess several interesting properties, including their transparency (which allows visualizing the inside of the package), flexibility, adaptability, and impermeability (which provides a barrier that prevents air and moisture from flowing through, thereby protecting the content) [ 4 ]. All these properties are of interest for the food industry since films can prevent oxidation and reduction reactions and microbe interaction, thereby prolonging the shelf life of food products [5]. During the past few years, the demand for films has increased as a result of two critical factors: the importance of ensuring product safety during transport, thus increasing the Polymers 2022,14, 4487. https://doi.org/10.3390/polym14214487 https://www.mdpi.com/journal/polymers Polymers 2022,14, 4487 2 of 18 requirement for packaging, and that the trend today is toward more attractive products, which increasingly utilize films for both hygienic and aesthetic purposes [ 6 ]. In this sense, food packaging is designed to protect it from several physical, chemical, and biological factors. Furthermore, it provides information to consumers regarding the ingredients, nutritional value, and safety of food products. Most polymeric materials are used in the food industry as packaging materials for direct contact with food [ 7 ]. However, these packaging materials are poorly biodegradable, creating an environmental problem. In this way, biopolymers, including polyesters, proteins, polysaccharides, and lipids, are utilized in order to satisfy industrial needs and consumer demands as well as to minimize their impact on the environment [ 8 – 10 ]. This has resulted in the development of a variety of natural biopolymer-based films, including those based on gelatin, cellulose, chitosan, or derivatives thereof, which have good biodegradability without toxic effects [ 11 , 12 ]. The raw materials such as gelatin, cellulose acetate, and chitosan possess properties which are potentially widely applicable in a variety of areas. Gelatin is an edible biodegradable material derived from collagen, which is a proteinbased natural biopolymer that is combined with functional ingredients (it is largely composed of amino acids such as glycine, proline, and hydroxyproline) to form efficient packaging materials [ 4 ]. It presents several advantages over other materials, including its ability to easily form effective films, excellent flexibility, appropriate gas barrier properties, low cost, and high reliability and availability [ 13 , 14 ]. On the other hand, it also exhibits high water solubility, high vapor permeability, and poor mechanical and thermal resistance [ 15 , 16 ]. Cellulose acetate is an ester of cellulose derived from the esterification of several cellulosic raw materials found in cotton, rice straw, sugarcane, wood, recycled paper, and bagasse [ 1 , 17 ]. It has several advantages, including an outstanding optical clarity, film formation at low temperatures, and chemical and thermal resistance. However, it has some weaknesses, such as a low dimension stability at high temperatures, stiffness, and the need for plasticizers for its industrial processing [ 1 , 17 ]. Likewise, chitosan is composed of poly ( β -(1 → 4)-2-amino-2-deoxy-D-glucopyranose), which is a natural cationic polysaccharide derived from deacetylated chitin (n-acetylglucosamine and glucosamine units linked via β -(1 → 4)-glycosidic linkages). Chitin is very abundant, although less so than cellulose [ 18 , 19 ]. Nevertheless, it presents easy processability, non-toxicity, biocompatibility, biodegradability, genocompatibility, hemocompatibility, antibacterial activity, and environmental safety. In addition, it also has three functional groups (amine compounds (-NH 2 ) and primary and secondary hydroxyl (OH) groups), which make it highly reactive. However, further investigation is needed to retain and/or improve such properties in chitosan-based materials [18]. These raw materials are widely used for a variety of industrial purposes; among them is food protection against light, drying, oxidation, water vapor, and chemical contamination through the incorporation of antioxidants, antimicrobials, nutrients, antifungals and flavors [ 20 ]. For cosmetic and hygienic applications, they are used as interfaces in shampoos, hair gels, and hair care products as well as in other cosmetic products [ 21 – 24 ]. Furthermore, they are of interest to the makers of biomedical and pharmaceutical applications, such as in anticancer, antidiabetic, antimicrobial, antioxidant and antihypertensive agents, wound care, tissue engineering and gene therapy [ 25 , 26 ]; they are also used as gelling agents for plasma expanders and as soft and hard polymer capsule fillers. Additionally, they are able to microencapsulate oils and pharmaceuticals and stabilize emulsions [ 27 , 28 ]. They also serve as protective coatings that increase a photograph’s shelf life and protective films to polarize LCD panels [ 24 , 29 ]. They are also used in other applications, such as glass frames, paints, and membranes for the treatment of water and fertilizers [ 22 ]. Regarding packaging functions, containment, protection, convenience, and communication are the most important factors. Thus, food packaging prolongs the shelf-life of packaged foods and maintains their safety. There are significant economic losses associated with lipid food deterioration during storage. Aside from microorganisms, the principal causes of spoilage are oxygen and chemical reactions. Food products deteriorate due to oxidation, which Polymers 2022,14, 4487 3 of 18 significantly limits their shelf life. Food oxidation may result in the loss of natural value (including proteins, soluble vitamins, and fatty acids), a reduction in energy content, the production of undesirable odors and flavors, and the degradation of pigment along with changes in color, all of which make the food less attractive to the consumers. Rancidity and the changes mentioned are the results of auto-oxidation, which involves a free radical chain mechanism. Due to this, food packaging manufacturers are continuously seeking effective methods of reducing the oxidation of lipids in food [7]. There are still numerous open investigations aimed at highlighting the properties of biobased films, including the incorporation of nano-scale materials as reinforcement fillers, which is one of the most significant methods [ 30 ]. In this way, several metal oxide nanoparticles, nanocelluloses, and nanoclays have been incorporated into biopolymerbased films [ 1 , 31 – 37 ]. In addition, the inclusion of certain nano-sized composites can replace select chemicals, thereby reducing the toxicity and cost of materials [38]. Regarding nanoparticles, magnetic iron oxide nanoparticles (Fe x O y -NPs) are extensively employed in biomedical applications owing to their magnetic properties, bioavailability, and biocompatibility [ 39 ]. Fe x O y -NPs inhibit the growth of several types of foodborne pathogens, including Staphylococcus aureus (S. au), Escherichia coli (E. col), and Pseudomonas aeruginosa (p. aeruginosa) [ 40 ]. Fe x O y -NPs produce reactive oxygen species (ROS) such as • OH and • O 2− , causing damage to the DNA and proteins of bacteria and thus impairing mitochondrial function, without adversely affecting non-bacterial cells [ 2 , 18 , 20 ]. In addition, they are non-cytotoxic and nonhazardous at concentrations below 100 µ g/mL [ 41 ] and can be used as an oral treatment for anaemia or iron deficiency [ 42 ]. In this context, this type of Fe x O y -NPs has been proposed as an appropriate additive to incorporate into films and enhance their antimicrobial properties. Nevertheless, Fe x O y -NPs can be synthetized by various methods, including traditional and green methods. Normally, the traditional method uses chemical reducing agents (i.e., NaOH) that can produce impurities and toxic residues in the final nanoparticles. The green method replaces this chemical reducing agent with a green one based on a polyphenol-rich extract. Thus, purer nanoparticles can be obtained without toxic residues. In addition, the election of the synthesis method can also condition the final properties of the obtained nanoparticles. In this way, the chemical production of Fe x O y -NPs leads to hazardous and agglomerated nanoparticles with lower stability. On the other hand, the green method achieves smaller sized, more stable, less agglomerated, purer, and less toxic iron oxide nanoparticles [ 43 , 44 ]. Nevertheless, several factors affect the green synthesis of nanoparticles, both in the pretreatment (such as the extract preparation, metallic salt, pH, and time of the reaction) and in the final treatment (such as the calcination temperature and time). In this way, the active polyphenols should not be degraded during the synthesis to maintain the maximum functionality of the final product. Therefore, it is novel to report an effective green method to synthesize iron oxide nanoparticles and subsequently incorporate them as natural additives into natural polymer-based films to overcome their deficiencies. Thus, the main purpose of this study was to disperse these greenly synthetized nanoparticles at a concentration of 1.0% (w/wof the initial polymer weight) as an additive into three different films (gelatin-based, cellulose acetate-based, and chitosan-based films) to enhance their properties. To this end, films were processed by casting and were then characterized to compare their physicochemical, mechanical, microstructural, and functional properties. 2. Materials and Methods 2.1. Materials Food gelatin (Ge, type B 200/220 g blooms, <10 ppm of sulfur dioxide) was provided by Manuel Riesgo, S.A. (Madrid, Spain), and cellulose acetate (CA, 39.8 wt% of acetyl content, Mn ca. = 30,000 g/mol, DS = 2.45) and chitosan (degree of deacetylation 98%, Mv = 1.61 ×105 g·mol−1 ) were supplied by Sigma Aldrich (Darmstadt, Germany). Acetone (CH 3 ) 2 CO, Acetic acid CH 3 COOH 0.05 M, DPPH (2,2-diphenyl-1-picrylhydrazyl) and Polymers 2022,14, 4487 4 of 18 gallic acid (C 7 H 6 O 5 ) were supplied by Sigma Aldrich (Darmstadt, Germany). The reagents used were all of analytical quality. 2.2. Nanoparticles Preparation Green synthesized Fe x O y -NPs were obtained based on previous studies [ 45 – 47 ]. Briefly, they were synthesized by using colloidal precipitation method, mixing 20 mL of a polyphenol-rich solution (39 ± 2 mg GAE/g extract) extracted from Phoenix dactylifera L. with 20 mL of ferric trichloride hexahydrate FeCl 3· 6H 2 O (1M). In this case, NaOH 5 M was added dropwise to adjust the pH to 7.5, and the mixture was heated at 50 ◦ C for 2 h under constant stirring (600 rpm). Thereafter, the precipitate was filtered, washed, and dried in an oven at 100 ◦ C for 8 h. After that, it was subjected to a calcination treatment in a furnace at 200 ◦C for 2 h. 2.3. Film Processing Method According to the method described in a previous study, biofilms were processed by casting [ 46 ]. Thus, 2% w/vof biopolymers (gelatin, cellulose acetate, and chitosan) were dissolved in distilled water, acetone, and 0.05 M acetic acid, respectively. Then, each solution was stirred at 60 ◦ C for 2 h at 600 rpm. Subsequently, 1.0% w/wof Fe x O y -NPs were dispersed within the solutions using an ultrasound bath for 30 min (Ultrasounds, J.P Selecta, S.A., Barcelona, Spain) at 100 W sonication power and 50 Hz frequency. Finally, a constant volume of each solution (42.7 mL) was transferred into Teflon plates (7.6 cm in diameter), where they were dried at room conditions (22 ◦C and 35% relative humidity) for 72 h. For the characterization, the films were peeled off carefully and stored in a desiccator. The reference films were processed without the incorporation of FexOy-NPs. 3. Characterization Technique 3.1. Nanoparticles Characterization 3.1.1. X-ray Diffraction (XRD) The green synthesized Fe x O y -NPs were conducted with an XRD pattern obtained using a Brand diffractometer (Bruker model D8 advance A25 diffractometer with Cu anode) to confirm their crystalline phase. The diffractograms were recorded at 2 θ ( ◦ ) = [15–70 ◦ ]. According to previous studies, the Debye–Scherrer formula was used to calculate the size and crystallinity of the green synthesized FexOy-NPs [45,47]. 3.1.2. Transmission Electron Microscopy (TEM) A TEM characterization was performed to evaluate the crystal systems as well as the sizes of the green synthesized Fe x O y -NPs. TEM images were observed at 200 kV using a Talos microscope (Talos S200 microscope, Thermo Fisher Scientific, Waltham, MA, USA). Then, the images of Fe x O y -NPs were labelled using Image-J 1.53 q free software to evaluate the average diameter size [45]. 3.2. Physicochemical Properties of Films 3.2.1. Water Solubility The films were tested for water solubility according to previous studies with minor modifications [ 36 , 46 , 48 , 49 ]. Briefly, an initial weight (W i ) of the samples (1 cm × 1 cm) was obtained after keeping them in a laboratory oven at 105 ◦ C for 24 h. Afterwards, they were immersed in 25 mL of distilled water for 24 h, removed, and redried at 105 ◦ C for 24 h in order to obtain the final dry weight (W f ). Finally, the water solubility (WS%) was obtained using the following Equation (1): WS(%) = wi−wf wi ·100 (1) Polymers 2022,14, 4487 5 of 18 3.2.2. Contact Angle of Water (WCA) or Wettability of the Surface The static water contact angle (WCA) was implemented to evaluate the hydrophobicity of the film surfaces using an optical tensiometer (Attension TL100, KSV, Helsinki, Finland) in the sessile drop mode. A horizontally levelled film (1 cm × 1 cm) was mounted on a movable stage and a ≈ 2 µ L drop of DW (distilled water, Milli-Q grade) was placed onto the film surface using a µ -syringe. The drop contour was monitored for 20 s at 12 frames per second and the WCA was calculated on both sides. The frames that differed by more than 2 ◦ on the right and left sides were rejected. A minimum of five tests were conducted on each sample to ensure reproducibility. 3.2.3. Water vapor permeability (WVP) The measurement of the WVP of the different films was conducted based on the methodology defined in the ASTM E96 standard (ASTM, 2010) [ 50 ]. Thus, the films were preconditioned in a chamber at 25 ◦ C and 50 ± 2% relative humidity (RH) for 48 h. Each film was placed by sealing an aluminium Payne-type test cup filled up to 2/3 of its internal volume with distilled water. Cups were then placed inside a dry chamber containing silica gel and purged with a gentle dry N 2 stream that ensured virtually 0% RH. Dryness and temperature (22 ± 2 ◦ C) in the chamber were continuously monitored with a combined sensor. The weight loss (0.1 mg precision) of the cups was monitored at regular periods until reaching a constant trend. The water vapor transmission rate (WVTR, g/h · m 2· Pa) was determined as follows: WVTR =α A(2) where ( α ) is the linear regression slope (R 2 > 0.998) of weight loss versus time and (A) is the permeation area of the cup. Permeance =WVTR ∆p=WVTR S(R1−R2)(3) where ( ∆p) is the water vapor gradient between both sides of the film, which is calculated considering the water saturation vapor pressure ( S) at the experiment temperature ( 2646 Pa ) and the relative humidity at the water-exposed (R 1 ) and chamber-exposed (R 2 ) sides of the film, respectively, expressing it as fractions (R1−R2=1−0). WVP =Permeance ·f ilm thikness (4) 3.2.4. Optical Properties (Light Transmission) The transparency of each film was measured by UV-vis spectroscopy as described in a previous study [ 46 ]. Briefly, the transmittance of samples (1 × 2 cm 2 ) was noted at 600 nm with a UV-vis spectrophotometer (Model 8451A, Hewlett Packard Co., Palo Alto, CA, USA) by transforming the absorbance values according to Lambert–Beer law. The results are expressed as transmittance percentage T600 %, and then the transparency (T) of the films was obtained according to the method described by Peighambardoust et al. [51]: T=−LogT600 t(5) where T600 is expressed as a fraction and refers to the amount of light the system can transmit and t represents the thickness of the film (mm). A higher transparency value indicates reduced light transmission across the film or a higher opacity. 3.3. Mechanical Properties A static tensile test was conducted through a slight modification of the standard ISO 527–3:2019 [52] to assess the mechanical properties of the films. An axial force of increasing amplitude (at a rate of 10 mm/min) was applied to the samples until they broke, employing an MTS Insight 10 Universal Testing Machine (Darmstadt, Germany). Polymers 2022,14, 4487 6 of 18 In this test, the temperature and relative humidity (RH) were maintained at 22 ◦ C and 35%, respectively. This test allows for the obtainment of the maximum stress ( Polymers 2022, 14, x FOR PEER REVIEW 11 of 18 Figure 4. Photographs of the different films (gelatin-based, cellulose acetate-based, and chitosanbased composite films) with Fe x O y (1.0% w/w). Neat films (gelatin-based, cellulose acetate-based, and chitosan-based films) without Fe x O y nanoparticles incorporated were included as references. 4.3. Mechanical Properties The tensile profile of the different neat and composite films is shown in Figure 5. The mechanical parameters of the different films are also represented in order to facilitate the comparison of the systems (Table 2). As shown in Figure 5, all neat film systems were characterized by a short elastic zone and then a longer plastic region. This plastic zone was drastically reduced when the Fe x O y - NPs were incorporated into the film. The composites became more brittle by increasing the maximum stress (Ϭ max , MPa) and Young’s modulus ( Ε , MPa) and by decreasing the ε max . The Ϭ max and Ε of neat gelatin increased from 4.2 ± 0.1 to 11.1 ± 0.2 MPa and from 71.7 ± 17.8 to 555.6 ± 17.8 MPa, respectively, due to the incorporation of Fe x O y -NPs. Similarly, these parameters were found for neat cellulose acetate and neat chitosan (Table 2). On the other hand, the (Ϭ max , MPa) and ( Ε , MPa) of the neat cellulose acetate-based film were the lowest (Table 2), which may be due to the fact that cellulose acetate requires a plasticizer [1,17]. Similar findings were reported by Yadav (2018), who studied the incorporation of magnetite (Fe 3 O 4 -NPs) into cellulose-based films [85]. The addition of immiscible particles may lead to non-homogeneous networks in the films, thereby limiting their extensibility [86]. Through NP-protein hydrogen bonding, nanoparticles may enhance the mechanical resistance of polymer films [36]. The strain at break (ε max ) was reduced when Fe x O y -NPs were incorporated into the systems. This is because nanoparticle incorporation stiffens films due to the increase in solid material [36]. The incorporation of Fe x O y -NPs into polymer chains also reduces the cohesion forces between the chains, thereby reducing the strain at break [87]. Moreover, these results may also be attributed to the type and size of nanoparticles, as reported in a previous study [46]. Consequently, smaller nanoparticles could result in greater maximum stress and Young’s modulus, since better interconnections in the structure and a stronger network between nanoparticles and polymer chains were generated [88]. Furthermore, the presence of immiscible particles leads to less homogeneous structures, which in return reduces the connectivity of the polymer networks [89,90]. In addition, the calculated toughness was reduced when Fe x O y -NPs were incorporated into the cellulose and chitosan systems due to the lower deformation, but there was no significant difference observed with regard to the gelatin system (Table 2). Table 2. Results for thickness, mechanical parameters (Ϭ max , ε max , Ε , and toughness), and antioxidant activity values (DPPH IP (%)) of the different films embedded with Fe x O y nanoparticles (1.0%). Neat films (gelatin-based, cellulose acetate-based, and chitosan-based films) without Fe x O y -NPs blended were used as references. Different superscript letters (a–f) for each column ascertain heterogeneity of variances (p < 0.05). max , MPa), strain at break ( εmax , mm/mm), Young’s modulus or elasticity modulus (E, MPa), and toughness (kJ/m3). 3.4. Morphological Properties The morphological properties and microstructures of neat and composite film surfaces were determined using scanning electron microscopy (SEM). Furthermore, the thicknesses of the samples were measured with ImageJ free software (1.53q; NIH, Bethesda, MD, USA). Before the observation, thin gold coatings were applied to the samples to improve their conductivity and, therefore, the image resolution. They were photographed in a Zeiss EVO microscope (Pleasanton, CA, USA) accelerated at 10 kV and magnified by 3000 X [53]. 3.5. Functional Properties (Antioxidant Activity) The efficacy of the films as antioxidants was evaluated according to Mehmood et al.’s protocol [ 36 ] but with a few modifications [ 46 ]. In brief, 1 mL of film-forming solution was admixed with 1 mL of methanolic solution of DPPH (40 ppm), followed by incubation for 0.5 h at room temperature. Each solution’s absorbance was measured at 517 nm using a spectrophotometer. For positive control, gallic acid was employed. The DPPH inhibition (IP %) could be calculated according to Equation (6). IP (%) = A−B A×100 (6) where Ais the absorbance of the uninhibited DPPH solution (without film solution as antioxidant agent) and Bis the absorbance of the inhibited DPPH (with film-forming solution). 3.6. Statistical Analysis All measurements in this study were conducted at least three times for each sample. A mean value and standard deviation are presented as a summary of the results, which were estimated using IBM SPSS software. Furthermore, an analysis of variance (oneway ANOVA) with 95% statistical confidence interval was used to estimate significant differences (p< 0.05). 4. Results 4.1. Nanoparticles Characterization 4.1.1. XRD Figure 1shows the X-ray diffractogram of the green synthesized Fe x O y -NPs (the magnetite phase is indicated by red 2 θ◦ and planes, whereas the hematite phase indicated by black 2 θ◦ and planes). The crystallite size, crystallinity, and phase composition were further investigated. The diffraction peaks of Fe x O y -NPs were attributed to ≈ 98.3% of magnetite in polycrystalline structures (29.3% cubic structures, 32.6% trigonal with a hexagonal axis, and 36.2% monoclinic) and cubic hematite (1.7%) (JCPDS n ◦ . 00–210 − 1535, 00–152 − 8611, 00–153 − 2800, 00–900 − 7706, and 00–900 − 2673 standard iron oxide powder diffraction pattern) [ 54 – 58 ]. The mean size obtained through XRD was 10.2 ± 0.4 nm with 98.5% of crystallinity. These findings were further confirmed by TEM characterization. Polymers 2022,14, 4487 7 of 18 Polymers 2022, 14, x FOR PEER REVIEW 7 of 18 Figure 1. XRD spectra (JCPDS standard) of the green synthesized Fe x O y -NPs using aqueous extract of Phoenix Dactylifera L. 4.1.2. TEM The morphology and the size distribution histogram (fitted by Lorentz curve) of FexOy-NPs are presented in Figure 2, with an average diameter (D = 4.7 ± 2.5 nm). As can be seen, the synthesized Fe x O y -NPs showed cubic and hexagonal structures, with well dispersed, extremely small nanoparticles. However, slight aggregation can be observed, which was probably caused by the competitive interactions between iron ions on the magnetite surface (Fe 3 O 4 -NPs) and the functional groups (phenolic compounds -OH‾) of the extract [59,60]. The TEM results confirmed those obtained by XRD. Figure 2. Transmission electron microscopy (TEM) of the green synthesized Fe x O y -NPs using aqueous extract of Phoenix Dactylifera L. Figure 1. XRD spectra (JCPDS standard) of the green synthesized Fe x O y -NPs using aqueous extract of Phoenix Dactylifera L. 4.1.2. TEM The morphology and the size distribution histogram (fitted by Lorentz curve) of FexOy-NPs are presented in Figure 2, with an average diameter (D = 4.7 ± 2.5 nm). As can be seen, the synthesized Fe x O y -NPs showed cubic and hexagonal structures, with well dispersed, extremely small nanoparticles. However, slight aggregation can be observed, which was probably caused by the competitive interactions between iron ions on the magnetite surface (Fe 3 O 4 -NPs) and the functional groups (phenolic compounds -OH − ) of the extract [59,60]. The TEM results confirmed those obtained by XRD. Polymers 2022, 14, x FOR PEER REVIEW 7 of 18 Figure 1. XRD spectra (JCPDS standard) of the green synthesized Fe x O y -NPs using aqueous extract of Phoenix Dactylifera L. 4.1.2. TEM The morphology and the size distribution histogram (fitted by Lorentz curve) of FexOy-NPs are presented in Figure 2, with an average diameter (D = 4.7 ± 2.5 nm). As can be seen, the synthesized Fe x O y -NPs showed cubic and hexagonal structures, with well dispersed, extremely small nanoparticles. However, slight aggregation can be observed, which was probably caused by the competitive interactions between iron ions on the magnetite surface (Fe 3 O 4 -NPs) and the functional groups (phenolic compounds -OH‾) of the extract [59,60]. The TEM results confirmed those obtained by XRD. Figure 2. Transmission electron microscopy (TEM) of the green synthesized Fe x O y -NPs using aqueous extract of Phoenix Dactylifera L. Figure 2. Transmission electron microscopy (TEM) of the green synthesized Fe x O y -NPs using aqueous extract of Phoenix Dactylifera L. Polymers 2022,14, 4487 8 of 18 4.2. Physicochemical Properties 4.2.1. Water Solubility (WS) A key food packaging parameter is water solubility (WS). Thus, insoluble films are required for better moisture resistance and product safety [ 36 , 49 ]. The WS% values of the neat and composite films are illustrated in Table 1. Thus, the neat gelatin-based film was the most water-soluble (WS = 87.9%), which is probably due to gelatin hydrophilicity [ 61 ]. Nevertheless, the incorporation of Fe x O y -NPs significantly enhanced the water resistance of the films, causing a WS reduction ranging from 15% for the gelatin composite to 38% for the cellulose acetate composite. These results are consistent with that of Hosseini et al. (2015), who concluded that the incorporation of chitosan nanoparticles (6% w/w) into the gelatin matrix reduced the solubility from 71.8 to 62.6% [ 49 ]. However, the literature contains some contradictory results; for example, the incorporation of capsaicin- Fe+3 doped hollow metalorganic frameworks (cap- Fe+3 -HMOF-5) into gelatin/chitosan films increased the water solubility from 45.7 to 58.4%, according to Xiaojun et al. (2020) [ 62 ]. Neat acetate cellulosebased films exhibited much lower solubility (WS = 8.5%) due to their insolubility in water [ 63 ]. Furthermore, the incorporation of Fe x O y -NPs enhanced this insolubility (lowest WS = 5.3%, Table 1). A similar decrease in water solubility was also reported when cellulose nanofibers (C-NFs) were incorporated into cellulose acetate-based films [ 17 ]. Finally, the water solubility of the chitosan-based film was reduced from 18.1 ± 1.5 to 15.4 ±0.8% when the Fe x O y -NPs were incorporated. A similar decrease in solubility was reported with regard to the incorporation of SiO 2 -NPs-GA into chitosan films [ 64 ]. This could be due to the hydrophobic nature of chitosan. Generally, the water absorption reduction that was observed in all the films could be attributed to the formation of hydrogen bonds between the nanoparticles and polymer chains [ 61 ]. Thus, the incorporation of hydrophobic materials into polymer results in an increase in hydrophobic compounds, thereby reducing their solubility [ 65 ]. In addition, the high crystallinity of nanoparticles/nanofillers (in this work, 98.5%) was also reported as a contributing factor in improving water resistance [ 17 ]. Nevertheless, the water solubility of the films characterized in this work is higher than those obtained by a commercial material such as polyethylene. Regarding the different neat films, it is obvious that their water solubility was affected by their hydrophilicity/hydrophobicity characteristics, which was further confirmed by water contact angle measurements. Table 1. Physicochemical and optical values of the different films embedded with Fe x O y nanoparticles (1.0 %). Neat films (gelatin-based, cellulose acetate-based, and chitosan-based films) without Fe x O y nanoparticles incorporated were used as references. The parameters of a commercial material based on polyethylene were included as reference. Different superscript letters (a–f) for each column indicate heterogeneity of variances (p< 0.05). Sample WS (%) WCA (◦)WVTR (g/h·m2) Permeance (g/h·m2·Pa) ×10−2 WVP (g·m/h·m2·Pa) ×10−6 T600 (%) T Commercial reference <1% 96 4–23 1 0.1 8- - Gelatin FexOy-NPs 66.7 ±1.3 b94.6 ±1c62.2 ±2.3 e2.4 ±0.07 e2.5 ±0.08 f41.2 ±1.3 b3.65 ±0.01 e Neat gelatin 87.9 ±2.2 a82.2 ±0.2 f80.3 ±1.3 d3.0 ±0.04 d2.7 ±0.03 e61.0 ±1.1 a2.41 ±0.01 f Cellulose acetate FexOy-NPs 5.3 ±0.9 f105 ±0.8 a121.8 ±1.0 b4.6 ±0.07 b6.7 ±0.10 b10.35 ±0.8 f6.12 ±0.04 a Neat cellulose acetate 8.5 ±1.2 e92.7 ±0.4 d129.5 ±2.3 a4.9 ±0.03 a7.9 ±0.06 a14.89 ±1.3 e5.55 ±0.01 c Chitosan FexOy-NPs 15.4 ±0.8 d96.7 ±1.0 b88.1 ±1.3 c3.3 ±0.05 c3.0 ±0.04 d29.56 ±1.7 d5.79 ±0.01 b Neat chitosan 18.1 ±1.5 c89.3 ±1.8 e116.6 ±1.6 a4.1 ±0.04 a4.3 ±0.04 c34.4 ±1.2 c4.76 ±0.01 d 4.2.2. Water Contact Angle (WCA) In the design of food packaging films, the WCA plays an important role, indicating the hydrophilic/hydrophobic character or the surface’s wettability [ 51 ]. Hydrophilicity and hydrophobicity are related to WCA values, thus the smaller the angle (acute angle inside Polymers 2022,14, 4487 9 of 18 the drop), the greater the hydrophilicity, whereas the greater the angle (obtuse angles or over 90 ◦ ), the greater the hydrophobicity [ 51 , 66 ]. The water contact angles of neat films and composite films are illustrated in Figure 3, and their corresponding values are shown in Table 1. The WCA values varied with the polymer’s nature and were also affected by nanoparticle incorporation. For gelatin, a mild hydrophilic behavior was observed ( WCA = 82.2 ◦ ), despite the presence of hydrophilic chains in the polymer framework [ 67 ]. This behavior can be attributed to the orientation of the hydrophobic groups at the interface between gelatin and air during gelation or solvent evaporation. Therefore, hydrophilic groups (carboxyl and amino) tend to form internal hydrogen bonds, while hydrophobic groups (aryl and aliphatic) tend to form external hydrogen bonds. This indicates a relatively non-wettable surface of neat gelatin caused by the presence of nonpolar protein segments that are exposed at the exterior surface of the film upon the removal of the solvent [ 51 ]. The incorporation of Fe x O y -NPs increased the WCA to 94.6 ◦ , which may indicate the higher hydrophobicity of iron nanoparticles, this value is close to commercial material value. Iron ions form a crosslink complex that bonds to macromolecules through multiple means, such as hydrophobic interactions and hydrogen and ionic bonding [ 68 ]. These findings are consistent with those of previous studies [ 67 , 69 ]. Neat cellulose acetate-based films exhibited a moderate water contact angle value (WCA = 92.7 ◦ ) due to the hydrophobicity of cellulose acetate [ 70 , 71 ]. The incorporation of hydrophobic Fe x O y -NPs enhanced the hydrophobicity of the cellulose acetate-based nanocomposite films by increasing the WCA to 105 ◦ . This value is significantly higher than that of commercial material. It has been reported that hydrophobic nanoparticles increase water contact angles when exposed to membrane surfaces [ 66 ]. Similar findings were reported for the incorporation of Cu-NPs into cellulose acetate films [ 72 ] and silica particles into cellulose acetate/chitosan films (CA/CS-Si) [ 73 ]. Likewise, neat chitosan appeared to be relatively hydrophobic, with a WCA = 89.4 ◦ , most likely due to chitosan’s hydrophobic backbone [ 74 ]. The Fe x O y -NPs incorporated into the chitosan-based films increased the hydrophobicity of the film surface to 96.7 ◦ . Similarly, other nanoparticles were incorporated into chitosan-based films with similar results [ 75 ]. Moreover, the chemical effect of Fe x O y -NPs on the hydrophobicity of the composites can be complemented with the increase in surface roughness caused by nanoparticles (see Section 4.4.), and as previously reported for other blends [76]. Polymers 2022, 14, x FOR PEER REVIEW 9 of 18 4.2.2. Water Contact Angle (WCA) In the design of food packaging films, the WCA plays an important role, indicating the hydrophilic/hydrophobic character or the surface’s wettability [51]. Hydrophilicity and hydrophobicity are related to WCA values, thus the smaller the angle (acute angle inside the drop), the greater the hydrophilicity, whereas the greater the angle (obtuse angles or over 90 °), the greater the hydrophobicity [51,66]. The water contact angles of neat films and composite films are illustrated in Figure 3, and their corresponding values are shown in Table 1. The WCA values varied with the polymer’s nature and were also affected by nanoparticle incorporation. For gelatin, a mild hydrophilic behavior was observed (WCA = 82.2 °), despite the presence of hydrophilic chains in the polymer framework [67]. This behavior can be attributed to the orientation of the hydrophobic groups at the interface between gelatin and air during gelation or solvent evaporation. Therefore, hydrophilic groups (carboxyl and amino) tend to form internal hydrogen bonds, while hydrophobic groups (aryl and aliphatic) tend to form external hydrogen bonds. This indicates a relatively non-wettable surface of neat gelatin caused by the presence of nonpolar protein segments that are exposed at the exterior surface of the film upon the removal of the solvent [51]. The incorporation of Fe x O y -NPs increased the WCA to 94.6°, which may indicate the higher hydrophobicity of iron nanoparticles, this value is close to commercial material value. Iron ions form a crosslink complex that bonds to macromolecules through multiple means, such as hydrophobic interactions and hydrogen and ionic bonding [68]. These findings are consistent with those of previous studies [67,69]. Neat cellulose acetate-based films exhibited a moderate water contact angle value (WCA = 92.7 °) due to the hydrophobicity of cellulose acetate [70,71]. The incorporation of hydrophobic Fe x O y -NPs enhanced the hydrophobicity of the cellulose acetate-based nanocomposite films by increasing the WCA to 105 °. This value is significantly higher than that of commercial material. It has been reported that hydrophobic nanoparticles increase water contact angles when exposed to membrane surfaces [66]. Similar findings were reported for the incorporation of Cu-NPs into cellulose acetate films [72] and silica particles into cellulose acetate/chitosan films (CA/CS-Si) [73]. Likewise, neat chitosan appeared to be relatively hydrophobic, with a WCA = 89.4°, most likely due to chitosan’s hydrophobic backbone [74]. The Fe x O y -NPs incorporated into the chitosan-based films increased the hydrophobicity of the film surface to 96.7°. Similarly, other nanoparticles were incorporated into chitosan-based films with similar results [75]. Moreover, the chemical effect of Fe x O y -NPs on the hydrophobicity of the composites can be complemented with the increase in surface roughness caused by nanoparticles (see Section 4.4.), and as previously reported for other blends [76]. Figure 3. Photographs of the different films (gelatin-based, cellulose acetate-based, and chitosanbased composite films) with Fe x O y (1.0% w/w). Neat films (gelatin-based, cellulose acetate-based, and chitosan-based films) without Fe x O y nanoparticles incorporated were included as references. Figure 3. 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