Novel hybrid electrospun poly(ε-caprolactone) nanofibers containing green and chemical magnetic iron oxide nanoparticles

RESEARCH ARTICLE Novel hybrid electrospun poly(ε-caprolactone) nanofibers containing green and chemical magnetic iron oxide nanoparticles Johar Amin Ahmed Abdullah 1 | Víctor Perez-Puyana 2 | Antonio Guerrero 1 | Alberto Romero 2 1 Departamento de Ingeniería Química, Escuela Politécnica Superior, Universidad de Sevilla, Sevilla, Spain 2 Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, Sevilla, Spain Correspondence Johar Amin Ahmed Abdullah, Departamento de Ingeniería Química, Escuela Politécnica Superior, Universidad de Sevilla, 41011, Sevilla, Spain. Email: [email protected] Funding information MCIN/AEI/10.13039/501100011033/ FEDER, UE, Grant/Award Number: PID2021124294OBC21 Abstract This study investigates the effects of incorporating green and chemical magnetic iron oxide nanoparticles (GMIONPs and CMIONPs, respectively) into poly (ε-caprolactone) (PCL) nanofibrous membranes on their physicochemical, mechanical, morphological, and functional properties. The study evaluates the physicochemical and optical properties of the nanofibrous membranes using water contact angle (WCA), water vapor permeability (WVP), brightness, color determination, UV–visible and gap energy, and light transmission. The mechanical properties were evaluated using Young's modulus, maximum stress (Ϭ max , MPa), and the strain at break (ε max ), while the morphological properties were evaluated using confocal microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Functional properties were assessed in terms of antioxidant activity. The results show that incorporating MIONPs significantly affects the properties of the nanofibrous membranes. PCL/GMIONPs membranes exhibit better performance in terms of physicochemical, morphological, and functional properties than PCL/CMIONPs membranes. These findings suggest that PCL/MIONPs nanofibrous membranes could be a promising material for various biomedical applications. The incorporation of different types of magnetic iron oxide nanoparticles into PCL electrospun membranes is an innovative approach that opens up a wide range of potential applications. KEYWORDS antioxidant activity, electrospinning, hydrophobicity, magnetic iron oxide, nanoparticles, PCL, roughness 1|INTRODUCTION The electrospinning (ES) process is a fiber- or nanofibermaking technology that involves the acceleration of a jet of charged polymer solution under a controlled highvoltage electric field, leading to the creation of an interconnected film of nanofibers upon the surface of a drum collector. 1,2 A variety of polymers or polymer blends can be electrospun into nanofibers or nanofiber mats via ES 3,4 with additional materials in the form of Received: 12 April 2023 Revised: 8 June 2023 Accepted: 15 June 2023 DOI: 10.1002/app.54345 This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. © 2023 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC. 1of15 J Appl Polym Sci. 2023;140:e54345. wileyonlinelibrary.com/journal/app https://doi.org/10.1002/app.54345 nano-scale particles 5–7 or incorporating molecules into functional systems. 8–10 The ES can be carried out using a wide variety of material compositions and processing conditions to achieve a wide variety of applications, such as biomedical, biotechnological, and tissue engineering applications, 11–13 gas or liquid filtration, 14–16 and harvesting and storage of energy, 17–19 including hydrogen (H 2 )production, water purification, environmental protection, 20–23 and catalytic applications. 24–26 Poly(ε-caprolactone) (PCL) is a well-known aliphatic polymer that has been widely employed to create a broad range of synthetic nanofibers, due to its excellent features, for example, biodegradability, biocompatibility, bioresorbability, hydrophobicity, easy processability, and non-toxicity. 1,22 In addition to its intrinsic hydrophobic character, electrospun PCL nanofibers might also exhibit higher biocompatibility and hydrophilicity if they are modified with surface coatings, polydopamine treatments, plasma treatments, alkali treatments, polymer grafting, or using copolymer blends, making them an attractive material for a variety of biomedical applications. 27 For example, it has been observed that the combination with pluronic increases the surface roughness of electrospun PCL nanofibers, which improves the adsorption of proteins and cells. 28 Electrospun PCL-nanofiber scaffolds have also been modified with alkaline hydrolysis to induce hydrophilicity and wettability, resulting in effective cell growth. 29,30 In terms of packaging functions, the most relevant aspects are containment, protection, convenience, and communication. The deterioration of lipid-rich foods during storage, caused by microorganisms and oxygen spoilage, results in a substantial loss of economic value. The oxidation of food products significantly shortens their shelf life. During oxidation, food loses its natural value and energy, creating undesirable odors, flavors, and pigments in the food that make it less appealing. Since rancidity and the mentioned changes are a result of free radical chains, food packaging manufacturers are constantly seeking innovative methods to reduce lipid oxidation. 31 Nanoparticles' incorporation into polymer-based materials may solve this problem due to their excellent antioxidant and antimicrobial activity, surface-enhanced Raman scattering activity, and electrical conductivity. 4,32,33 In this sense, a synergistic combination of specific, optical, electrical, unique, catalytic, and functional properties of metal nanoparticles and polymer nanofiber materials (metalpolymer nanocomposites) has attracted a great deal of attention during the past few years. This is attributed to the excellent surface area of polymer nanofibers and the interaction between metal nanoparticles and polymer nanoparticles. A variety of metal oxide nanoparticles, including nanofillers, have been incorporated into polymer-based films/nanofibers mats in order to improve their properties. 34–41 As one of the most commonly used nanoparticles, magnetic iron oxide nanoparticles (MIONPs) have been employed in a wide range of applications due to their advantageous characteristics, including specificity, uniqueness, magneticity, and biocompatibility. 42 MIONPs exhibit a high inhibition ability against diverse foodborne pathogens' growth. 43 These nanoparticles can liberate the reactive oxygen species that damage bacteria DNA and proteins by impairing mitochondrial function and keeping non-bacterial cells unharmed. 41,42,44 MIONPs have been reported as danger-free, non-cytotoxic, and potential oral therapy to treat iron deficiency. 44,45 Generally, these nanoparticles may vary depending on the method used for their production, since they can be synthesized in a variety of ways, including traditional and environmentally friendly methods. The traditional method typically involves the use of chemical-reducing agents (such as NaOH), which can lead to the formation of impurities and potentially hazardous substances within the nanoparticles. As an alternative to these toxic reducing agents, the green method based on a green reducing agent (polyphenol-rich plant extracts) is used to produce smaller, purer, nontoxic, less aggregated, more stable nanoparticles with high functionality. 46,47 The incorporation of nanoparticles with different properties into polymer matrices is a promising strategy for creating materials with unique properties. However, the traditional method of mechanically mixing nanoparticles into polymer solutions often results in uneven particle distribution. To address this issue, this study utilized N,N-dimethylformamide (DFM) to reduce metal ions to zero-valent metal, resulting in the homogeneous dispersion of green and chemically synthesized magnetic iron oxide nanoparticles (GMIONPs and CMIONPs) in PCL electrospun membranes. Therefore, the main objective of this study was to analyze the effect of different concentrations of GMIONPs and CMIONPs on the physicochemical, mechanical, morphological, and biological properties of PCL nanofiber membranes. This approach offers a novel and promising approach for developing nanofibrous materials with enhanced properties for various biomedical applications. 2|MATERIALS AND METHODS 2.1 |Materials PCL (C 6 H 10 O 2 ) n (with average Mn 80,000; Sigma Aldrich, Saint Louis, Mi USA), chloroform (Friendemann Schmidt, Parkwood, Australia): DMF (CH 3 )2NC(O)H (Merck, ABDULLAH ET AL.2of15 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Darmstadt, Germany), gallic acid (C 7 H 6 O 5 ), and DPPH (2,2-diphenyl-1-picrylhydrazyl) were purchased from Sigma Aldrich. The other substances and reagents used in this study were all of analytical grade. GMIONPs (12 ± 1 nm, 84% cubic Fe 3 O 4 : 16% monoclinic Fe 2 O 3 and 95% crystallinity) and CMIONPs (24 ± 1 nm, 72% cubic Fe 3 O 4 : 28% trigonal with rhombohedral axis Fe 2 O 3 and 93% crystallinity) were synthesized based on previous studies. 48–50 Briefly, the process involves mixing 20 mL of total polyphenol content (39 ± 2 mg GAE/g extract) extracted from Phoenix dactylifera L. (as a reducing agent) with 20 mL of ferric trichloride hexahydrate FeCl 3 6H 2 O (as a precursor). The pH of the resulting mixture was adjusted to 7.5 by adding 5 M of NaOH. Then the mixture was heated at 50C for 2 h under continuous stirring. Following this, the precipitate obtained was filtered, washed with ethanol, and dried in an oven at 100C for 8 h. After that, it was calcined in a muffle at 500C for 5 h. 2.2 |ES process Solution ES was used to prepare PCL/GMIONPs and PCL/CMIONPs membranes in accordance with the methods described in a previous study with small modifications. 1 PCL was first dissolved in a co-solvent of chloroform: DMF (9:1 v/v) and stirred at room temperature to obtain the ES dope solution (10% w/v). Then, different concentrations (1.0%, 5.0%, and 10% w/w) of Fe x O y -NPs were dispersed in the dope solution using an ultrasound bath for 2 h (Ultrasounds, J.P Selecta, S.A., Barcelona, Spain at 100 W sonication power and 50 Hz frequency). Afterward, 10 mL dope solution was electrospun using a laboratory-scale ES machine (BioInicia, Fluidnatek LE-50 setup, Valencia, Spain). The voltage was adjusted to 12 kV, the needle was placed 13 cm from a rotating drum collector (which was rotating at 200 rpm) and the feed rate was kept constant at 0.9 mL/h. The electrospun PCL/Fe x O y nanofibers were then allowed to dry at room temperature, peeled off carefully, and kept in a desiccator for further use and characterization. 3|CHARACTERIZATION TECHNIQUES 3.1 |Physicochemical properties 3.1.1 | Water contact angle To determine the WCA for the different electrospun nanofibrous membranes (neat PCL, PCL/GMIONPs, and PCL/CMIONPs nanofiber membranes), an Optical Tensiometer (Attension TL100, KSV, Helsinki, Finland) was used. For the evaluation of the hydrophobicity of nanofiber surfaces, a static measurement of the WCA was conducted. Using a μ-syringe, distilled water drops (1 drop ≈2μL, Milli-Q grade water drop) were injected onto a horizontally leveled membrane (10 10 mm) for 20 s at a rate of 12 F/s (frames per second). The WCA was performed on both sides of the water drop to ensure symmetry and horizontal leveling. Frames with a difference of more than 2on either side of the center of the frame were rejected. A maximum of five tests were conducted on each sample to ensure reproducibility. 3.1.2 | Water vapor permeability The water vapor property of the different membranes was measured according to ASTM E96 (ASTM, 2010), 51 which has been described in a previous study. 52 Accordingly, water vapor permeability (WVP, g h 1 m 1 Pa 1 107) was calculated from Equation (1) as follows: WVP ¼Pt¼WVTR Δpt¼α AS RH1RH2 ðÞ t,ð1Þ where Pis the permeance (the degree to which the membrane admits a flow of water vapor, gh 1 m 2 Pa 1 ), WVTR α=AðÞis the water vapor transmission rate (the amount of water vapor transmitted through the membrane, gh 1 m 2 ), α, which can be obtained from the straight line slope, is the vapor flowrate (gh 1 ), A is the permeation area in m 2 ,Δp¼SRH1RH2 ðÞis the gradient of water vapor (expressed in Pa) between waterexposed surface (with relative humidity RH1¼1Þand chamber-exposed surface (with relative humidity RH2¼0), where S¼2646Pa and tis the membrane thickness expressed in m units. 3.1.3 | Optical properties Brightness and color determination Brightness and color measurements were performed with a portable spectrophotometer (SPECTROPHOTOMETER CM-700d, KONICA MINOLTA SENSING, INC., Tokyo, Japan), which is an instrument designed to measure the color of contact-type objects reflected at visible and nearvisible wavelengths (e.g., 360–740 nm). 53 Color transitions from dark to light are expressed by 3 coordinates (L*, ±a*, and ± b*) created by the CIE (Commission Internationale de l' Eclairage, 1976). 54 Thus, L* =0 indicates diffuse black, and L* =100 indicates higher specular diffuse white. The 3of15 ABDULLAH ET AL. 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License coordinate ±a* indicates the range diffused between red and green, where +a* indicates the red color and a* indicates green. The coordinate ±b* indicates the range diffused between yellow and blue, where +a* indicates the diffused yellow and b* indicates the diffused blue. Light transmission and transparency Transparency is one of the most attractive characteristics of the different nanofibrous membranes, which prevent light transmission while allowing visibility through the packaging material. An opaque membrane absorbs a significant amount of light. The optical properties of nanofiber membranes were assessed using a UV–vis spectrophotometer (Model 8451A, Hewlett Packard Co., USA) following the guidelines of a previous publication. 49 Briefly, the absorbance values of nanofiber membranes (1 2cm 2 ) were recorded at 600 nm and then converted to transmittance using the Lambert–Beer law. The results were presented as transmittance percentage (T600%), and then the transparency (T) of each nanofiber membrane was obtained from Equation (2) as follows 55 : T¼LogT600 t,ð2Þ where T600 is the transmittance fraction, which indicates how much light is transmitted through the system, and t is the thickness of the membrane (mm). Transparency increases as light transmittance across the membrane decreases, resulting in higher opaqueness. UV–visible and gap energy UV–visible absorption spectra were recorded on the nanofibrous membranes to confirm the incorporation of nanoparticles and calculate their gap energies using a UV-spectrophotometer (Varian Cary 6000i, USA). The UV–vis analysis provides an indirect measurement of absorbance and the determination of optical gaps or gap energy (E g , eV) of samples, as detailed in a previous study. 50 In brief, Egwas extracted using Tauc's relation illustrated in Equation (3): α¼BhvEg  n hv ,ð3Þ where αrepresents the dimensionless absorption coefficient, which was calculated from Equation (4): α¼1RðÞ 2 1R2,ð4Þ and Egis the optical gap energy (eV) with a transition nature depending on the n value: for direct and indirect allowed transition, n takes ½and 2 respectively; for direct and indirect forbidden transition, n takes 3/2 and 3 respectively. his Planck's constant, vis the frequency, B is a material-dependent constant whose dimensions depend on the value of n (B=1), and Ris the reflectance. 56 3.2 |Mechanical properties The static tensile test and mechanical parameters of the nanofibrous membranes were conducted according to a slight modification of the standard ISO 527-3:2019. 57 The different parameters, including ultimate tensile strength or maximum stress (UTS or Ϭmax, MPa), elongation or strain at break (εmax, mm/mm), and Young's modulus (MPa) were calculated during this test as described in a previous study. 52 3.3 |Morphological properties 3.3.1 | Scanning electron microscopy Scanning electron microscopy (SEM) imaging was used to assess the morphological and microstructural properties of nanofiber membranes. The specimens were observed using a Zeiss EVO microscope (Pleasanton, CA, USA) with an acceleration voltage of 10 kV and a magnification of 3000. 58 The samples were coated with a thin gold layer to improve their conductivity, as well as the micrograph quality. Thereafter, the membrane thickness was measured with ImageJ free software (1.53q; National Institutes of Health, Bethesda, Maryland, USA). 3.3.2 | Transmission electron microscopy The transmission electron microscopy (TEM) study was used to gather the elemental information and composition of nanofibers, as well as the nanoparticle distribution within nanofibers. It was performed at 200 kV with a Talos S200 microscope (Field Electron and Ion Company, FEI, USA). Further elemental information on the nanofibers was obtained through ImageJ free software labeling. 3.3.3 | Confocal microscopy The surface roughness profile was evaluated using a Confocal-Interferometric Optical Microscope (Sensofar S-NEOX, Sky Tech, Bukit Batok, Singapore) employing a Robust Gaussian filter at 100magnification and 8 μm ABDULLAH ET AL.4of15 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License amplitude (ISO 4287). 59 Generally, surface roughness is determined by either the centerline average of the surface irregularities or by measuring the distance between their lowest and highest points. In this way, two roughness parameters were calculated: roughness average (Ra), which is the arithmetic average of the absolute value of the profile height deviations over the mean length, and quadratic roughness (Rq or RMS), which is the root mean square average of the profile height deviation over the mean length. The images were further assessed with ImageJ free software. 3.4 |Functional properties (antioxidant activity) The antioxidant activity of the nanofiber membranes was tested as described in a previous study. 49 Briefly, 1 mL of each ES dope was mixed with 1 mL of DPPH solution (dissolved in methanol, 40 ppm) and then, the mixture was shaken and incubated for 30 min at 25C. The absorbance of the samples was recorded at 517 nm using a spectrophotometer. Gallic acid was used as a positive control and the DPPH inhibition (IP%) was calculated according to Equation (5). IP %ðÞ¼AB A  100, ð5Þ where Apresents the DPPH absorbance without ES dope and B presents the DPPH absorbance after mixing with ES dope. 3.5 |Statistical analysis A minimum of three measurements were conducted for each sample. The results were summarized as a mean value and standard deviation (M ± SD) using IBM SPSS software (IBM SPSS Statistics 26.0). Further significant differences (p< 0.05) were determined by using a oneway ANOVA at a 95% confidence level. 4|RESULTS 4.1 |Physicochemical properties 4.1.1 | Water contact angle The WCA is an integral aspect of many everyday applications, including food packaging, medicine, electronics, and painting. Thus, WCA provides good indications of the surface product wettability or hydrophobicity and of the efficiency of the process. 55 According to WCA values, hydrophilicity, and hydrophobicity are determined by the angle inside a drop in contact with a film surface (hydrophilic surface < 90< hydrophobic surface), and the increase in this angle indicates a higher degree of hydrophobicity. 55,60 Figure 1shows the WCA values of neat PCL and composite membranes (PCL/GMIONPs and PCL/CMIONPs, respectively), and Table 1provides the mean WCA values of each system. The measured WCA value for the neat PCL (118.9 ± 0.8) was consistent with its hydrophobic properties. The hydrophobic property of PCL is attributed to the presence of CH 2 groups in the main chain of the polymer. 61 This result resembles the previously reported WCA values of PCL electrospun fibers prepared in various solvent systems. For example, the WCA value of electrospun PCL (12% w/v in a mixture of acetic acid: formic acid 3:7) was 118. 62 Those systems of PCL with iron oxide nanoparticles exhibited an increase in WCA by increasing the nanoparticle concentration (1%, 5%, and 10%), leading to values of 123.3 ± 0.1, 127.5 ± 0.7, and 132.8 ± 1.5for PCL/GMIONPs and to 118.4 ± 0.5, 123.0 ± 0.1, and 124.2 ± 0for PCL/CMIONPs, respectively. The increased hydrophobicity of iron nanoparticles is evidenced by the formation of a crosslinked complex with macromolecules through different interactions, including hydrophobic interactions, hydrogen bonds, and ionic bonds. 63 The presence of hydrophobic nanoparticles on the surface of membranes has been reported to increase their WCAs. 60 A further contributing factor is an increase in surface roughness caused by nanoparticles, which enhances the chemical effect of MIONPs on the hydrophobicity of the composite membranes, in accordance with previous studies. 64 These findings are consistent with previous studies. 65–69 4.1.2 | Water vapor permeability It is of paramount importance to determine the WVP of many materials, such as plastic films and membranes, paper, and other sheet materials from which product packages are made. Maintaining the quality of a product in regions with diverse environmental conditions requires an understanding and control of the water vapor transmission rate (WVTR). Through WVTR measurements, the MIONPs were evaluated for their effect on the PCL membranes. Table 1also summarizes the results of WVTR, permeance, and WVP values. These parameters were reduced significantly when both GMIONPs and CMIONPs were included. The mean WVP of the neat PCL nanofibers was reduced by about 10%–38% with the incorporation of 5of15 ABDULLAH ET AL. 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License GMIONPs, and by 7%–26% with CMIONPs incorporation, compared to the WVP of the neat PCL nanofibers (Table 1). This could be due to the formation of wellconnected three-dimensional structures 70 and the generation of twisted pathways within the polymer matrix, which blocks and delays water molecules from crossing the matrix. 38 Furthermore, the nanoparticle dispersion restricts the movement of protein chains, thereby inhibiting water infiltration. 69,71 The filler's hydrophobic/ hydrophilic balance may be slightly altered by changing the cation. This is mainly due to nanoparticles exhibiting strong interactions with polymer chains, which may result in the consumption of hydrophilic groups, thereby reducing the transmission of water. 72 Several factors can influence the WVP of composite membranes, which comprise hydrophobicity/hydrophilicity, thickness, roughness, compaction, particle size, crystallinity, distribution, and orientation. 65,73,74 In this way, GMIONPs improved the WVP of the composite PCL membranes further compared to CMIONPs, due to their smaller sizes and higher crystallinity. Nanoparticles with higher crystallinity contribute to the enhancement of membrane crystallinity and hydrophobicity by reducing the free hydroxy groups in the membranes, thereby improving their resistance to moisture and reducing WVP. 34,75 In addition, the orientation of nanoparticles may take place, which may be affected by the homogeneity of the crystal system shown by the nanoparticles. The CMIONPs exhibited elongated agglomerates/aggregates, which can leave behind empty pores, thereby allowing moisture to penetrate. This may reflect the non-availability of the chemical stabilizing agents and the interaction between the different polycrystalline structures. 76 FIGURE 1 Water contact angle profile of PCL/GMIONPs and PCL/CMIONPs nanofibrous membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively, at different concentrations: 0, 1, 5, and 10% w/w). PCL nanofibrous membrane without any MIONPS (0%) was included as a reference system. [Color figure can be viewed at wileyonlinelibrary.com] TABLE 1 Physicochemical values of PCL/GMIONPs and PCL/CMIONPs nanofibrous membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively, at different concentrations: 0%, 1%, 5%, and 10% w/w). Sample C% (w/w) WCA () WVTR (gh 1 m 2 ) Permeance (gh 1 m 2 Pa) 102 WVP (gh 1 m 2 Pa 1 ) 107 Neat PCL 0% 118.9 ± 0.8 f 114.0 ± 0.3 a 4.31 ± 0.01 a 9.01 ± 0.04 a PCL/GMIONPs 1% 123.3 ± 0.1 d 103.9 ± 0.6 c 3.93 ± 0.01 c 8.29 ± 0.04 d 5% 127.5 ± 0.7 b 90.7 ± 0.4 e 3.43 ± 0.01 e 7.93 ± 0.03 e 10% 132.8 ± 1.5 a 82.9 ± 0.7 f 3.13 ± 0.01 f 7.82 ± 0.03 f PCL/CMIONPs 1% 118.4 ± 0.5 f 106.2 ± 0.7 b 4.01 ± 0.01 b 8.76 ± 0.04 b 5% 123.0 ± 0.1 e 101.0 ± 0.3 d 3.82 ± 0.01 d 8.72 ± 0.02 b 10% 124.2 ± 0 c 90.7 ± 0.5 e 3.43 ± 0.01 e 8.65 ± 0.03 c Note: Different superscript letters (a–f) of each column indicate heterogeneity of variances (p< 0.05). PCL nanofibrous membrane without any MIONPS (0%) was included as a reference system. ABDULLAH ET AL.6of15 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 4.1.3 | Optical properties Brightness and color determination Figure 2depicts photographs of the different samples studied. As can be seen, the PCL membranes with GMIONPs and CMIONPs became darker with increasing nanoparticle concentration. A longer processing time may also lead to darker images. For comparison purposes, it is important to maintain the same processing conditions and time for all electrospun membranes. Further analysis of the nanofibrous membranes was conducted using the three CEI coordinates (L*, ±a*, and ±b*) to identify the brightness and color space of the membranes, as summarized in Table 2. A decrease in L* values was observed when the PCL nanofibrous membranes were processed with MIONPs at 5% and 10% (w/w), although without significance at 1% (w/w). An increase in the diffuse red and yellow (+a* and +b*) or decrease in the diffuse green and blue (a* and b*) may be related to the nanoparticle type (e.g., Fe 2 O 3 -NPs, or Fe 3 O 4 -NPs). 77 Thus, when NPs were progressively incorporated into the PCL nanofibrous membranes, a* was found to vary from 0.06 to 5.36 with GMIONPs (84% Fe 3 O 4 and 16% Fe 2 O 3 ), and from 0.06 to 14.94 (Table 2) with CMIONPs (72% Fe 3 O 4 and 28% Fe 2 O 3 ). Similarly, coordinate b* increased with the increase in MIONPs concentration, being higher in PCL/CMIONPs than in PCL/GMIONPs, which may confirm that hematite has a higher proportion in PCL/CMIONPs. Another study reported a similar increase in the (a*, b*) coordinates to (2.95, 7.2) and (3.84, 1.29) when hematite (Fe 2 O 3 )constituted 14.9% and 15.3% of the total chemical composition, respectively. 77 Nevertheless, color change and other optical properties were further analyzed using UV–vis spectrophotometry. Light transmission and transparency Table 2summarizes the corresponding values for the membrane transmittance (T600%) and transparency (T). The incorporation of MIONPs into the PCL electrospun membranes resulted in a significant decrease in the T600%and an increase in the T index. Accordingly, the neat PCL exhibited white opaque membranes with the highest transmittance (0.94 ± 0.09%). When it was processed with GMIONPs (1%–10% w/w), the T600%was FIGURE 2 Photographs of PCL/GMIONPs and PCL/CMIONPs electrospun nanofibrous membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively, at different concentrations: 0%, 1%, 5%, and 10% w/w). PCL nanofibrous membrane without any MIONPS (0%) was included as a reference system. [Color figure can be viewed at wileyonlinelibrary.com] TABLE 2 Optical values (CEI coordinates [L*, a*, and b*], light transmission, and gap energy) of PCL/GMIONPs and PCL/CMIONPs nanofibrous membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively, at different concentrations: 1%, 5%, and 10% w/w). Sample C% (w/w) L* a* b* T 600 (%) TE g (eV) Neat PCL 0% 90.69 ± 0.25 a 0.06 ± 0.04 e 1.13 ± 0.03 g 0.94 ± 0.09 a 32.25 g 1.29 PCL/GMIONPs 1% 90.42 ± 0.85 a 0.03 ± 0.11 e 0.79 ± 0.28 f 0.64 ± 0.09 b 34.65 e - 5% 88.08 ± 0.03 b 3.35 ± 0.02 c 4.49 ± 0.04 d 0.19 ± 0.02 d 38.96 a - 10% 85.29 ± 0.94 c 5.36 ± 0.60 b 8.97 ± 0.06 b 0.13 ± 0.0.07 e 38.12 c 2.09 PCL/CMIONPs 1% 89.81 ± 0.94 a 0.93 ± 0.09 d 0.40 ± 0.19 e 0.65 ± 0.02 b 33.37f - 5% 84.02 ± 0.34 d 5.24 ± 0.24 b 6.33 ± 0.27 c 0.21 ± 0.06 c 38.91 b - 10% 75.75 ± 0.90 e 14.94 ± 0.48 a 23.11 ± 0.06 a 0.14 ± 0.08 e 37.34 d 1.47 Note: Different superscript letters (a–f) of each column indicate heterogeneity of variances (p< 0.05). PCL nanofibrous membrane without any MIONPS (0%) was included as a reference system. 7of15 ABDULLAH ET AL. 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License reduced by about 32%–87% by filling up the free space between the nanofibers, where the Tvalue was enhanced by about 7%–17% (Table 2,p<0.05). Lastly, when it was processed with CMIONPs (1%–10% w/w), the observed T600%was reduced by about 31%–85%, corresponding to 7%–17% of slightly different transparency to the GMIONPs (Table 1). This was attributed to the increase of the crystalline nanoparticles content in the polymer chain, which hinders their mobility. Nanofillers may fill up free spaces in polymer chains by being dispersed into the chains, thereby blocking the passage of light through the membrane. Similar results were found with other polymers blended with different nanoparticles. 36,41,55,78,79 UV–visible and gap energy: As shown in Figure 3, the comparison between the spectra of the pure PCL membrane with those of PCL and MIONPs based membranes clearly reflects the contribution of MIONPs, which lead to a remarkable enhancement of the absorption band in the UV–visible spectrum. In addition to the enhanced absorption band recorded at around 274 nm for both GMIONPs and CMIONPs, which increased by about 82% and 78%, respectively, they recorded broad absorption bands throughout the entire spectral range, particularly up to approximately 700 and 610 nm for the spectra on GMIONPs and CMIONPs, respectively. Furthermore, the corresponding direct band gap for each spectrum was determined to be increased by the presence of MIONPs; thus, the neat PCL showed a small gap energy (1.29 eV, Table 2), whereas the gap energies of PCL/GMIONPs and PCL/CMIONPs increased to 2.09 and 1.47 eV, in the order already mentioned. Incidentally, the direct gap energies of the isolated MIONPs were found to be 2.94 eV for the GMIONPs and 2.19 eV for the CMIONPs. This was due to the increase in Fe 3 O 4 proportion, as well as the smaller size and higher crystallinity, as reported elsewhere. 80 MIONPs represent the most significant absorption of visible light with direct and indirect band gaps of 1.6–3.01 eV, which cover most of the visible spectrum. 56,80,81 4.2 |Mechanical properties Figure 4a illustrates the tensile profile of PCL-based membranes, neat PCL, and PCL with 10% w/w of GMIONPs or CMIONPs. To facilitate better visualization, the tensile profile of PCL/GMIONPs and PCL/CMIONPs was included in Figure 4b,c. The mechanical properties FIGURE 3 UV–visible absorption spectra of PCL/GMIONPs and PCL/CMIONPs membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively, at (10% w/w). GMIONPs, CMIONPs, and PCL nanofibrous membranes without any MIONPS were included as reference systems. [Color figure can be viewed at wileyonlinelibrary.com] FIGURE 4 Tensile test profile of PCL/GMIONPs and PCL/CMIONPs membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively, at 10% w/w). PCL nanofibers without any MIONPS were included as a reference system. [Color figure can be viewed at wileyonlinelibrary.com] ABDULLAH ET AL.8of15 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License are summarized in Table 3. As expected, the neat PCL exhibited a shorter elastic area followed by a longer elongation area. In contrast, PCL/GMIONPS and PCL/CMIONPs nanofibrous membranes show higher values for Young's modulus (MPa) and the maximum stress (Ϭ max , MPa) with a shorter plastic region as compared to the neat PCL membrane. Young's modulus increased by about 99% and 97% and Ϭ max increased by about 97% and 94% over the value corresponding to the neat PCL membranes. Similar studies have found similar increases in these parameters. 82 The incorporation of immiscible nanoparticles may favor the formation of hydrogen bonding with NPs giving rise to nonhomogeneous networks within the fibers that may be responsible for the enhancement of the mechanical properties of the polymeric membranes. 41,83 However, higher concentrations of larger nanoparticles can lead to agglomeration of nanoparticles in/on the fiber by forming beads, which can restrict the mechanical properties. Furthermore, a remarkable decrease in the strain at break (ε max ) was observed when the PCL was reinforced with MOINPs, which indicates that the presence of solid material within PCL nanofibers may indurate the membrane by increasing the thickness and reducing the cohesion forces within the fibers. 41,84 The nanoparticles' type and size may contribute to this behavior. Therefore, the smaller crystalline sizes led to stronger networks between the nanoparticles and the polymer through the improvement of the interactions within the polymer structure, resulting in improved mechanical properties. Similar results with respect to the mechanical properties of polymers with nanoparticles have been previously reported, which may also be attributed to the type and size of nanoparticle interconnections. 49,85 Prior studies have reported similar results regarding the mechanical properties of polymers containing nanoparticles. 86–88 4.3 |Morphological properties 4.3.1 | Confocal microscopy: Roughness The roughness of the membrane surfaces is a crucial parameter for the growth and adherence of the lines of different cells for biomaterial purposes. 3D surface plots of the neat PCL, PCL/GMIONPs, and PCL/CMIONPs nanofibrous membranes are depicted in Figure 5. Roughness results (Ra and Rq) are summarized in Table 4. Accordingly, increases in Ra and Rq were observed when the PCL electrospun membrane was processed with both nanoparticle types. Nevertheless, Ra and Rq values (in nanometers) of PCL/CMIONPs were greater than TABLE 3 Results of the mechanical parameters of PCL/GMIONPs and PCL/CMIONPs nanofibrous membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively; concentration =10% w/w). Sample C% (w/w) Young's modulus (MPa) Ϭ max (MPa) ε max (mm/mm) Neat PCL 0% 0.3 ± 0.1 c 0.2 ± 0.1 c 3.9 ± 0.9 a PCL/GMIONPs 10% 87.6 ± 1.8 a 6.3 ± 0.1 a 0.4 ± 0.1 c PCL/CMIONPs 10% 15.8 ± 1.1 b 3.8 ± 0.2 b 1.6 ± 0.3 b Note: Different superscript letters (a-b) of each column indicate heterogeneity of variances (p< 0.05). PCL nanofibrous membrane without any MIONPS (0%) was included as a reference system. FIGURE 5 3D surface plot of PCL/GMIONPs and PCL/CMIONPs nanofibrous membranes (PCL blended with green and chemical iron oxide nanoparticles, respectively, at different concentrations: 0%, 1%, 5%, and 10% w/w). A 3D surface plot of PCL nanofibrous membrane without any MIONPS (0%) was included as a reference. [Color figure can be viewed at wileyonlinelibrary.com] 9of15 ABDULLAH ET AL. 10974628, 2023, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/app.54345 by Universidad De Sevilla, Wiley Online Library on [04/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License