Green Synthesis of FexOy Nanoparticles with Potential Antioxidant Properties

Citation: Abdullah, J.A.A.; Jiménez-Rosado, M.; Perez-Puyana, V.; Guerrero, A.; Romero, A. Green Synthesis of FexOyNanoparticles with Potential Antioxidant Properties. Nanomaterials 2022,12, 2449. https:// doi.org/10.3390/nano12142449 Academic Editor: Francisco Alonso Received: 20 June 2022 Accepted: 14 July 2022 Published: 17 July 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/). nanomaterials Article Green Synthesis of FexOyNanoparticles with Potential Antioxidant Properties Johar Amin Ahmed Abdullah 1,* , Mercedes Jiménez-Rosado 1, Víctor Perez-Puyana 2, Antonio Guerrero 1 and Alberto Romero 2,* 1Departamento de Ingeniería Química, Escuela Politécnica Superior, Universidad de Sevilla, 41011 Sevilla, Spain; [email protected] (M.J.-R.); aguerrer[email protected] (A.G.) 2Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spain; [email protected] *Correspondence: [email protected] (J.A.A.A.); [email protected] (A.R.); Tel.: +34-954557179 (J.A.A.A. & A.R.) Abstract: Iron oxide nanoparticles (Fe x O y -NPs) are currently being applied in numerous high-tech sectors, such as in chemical sectors for catalysis and in the medical sector for drug delivery systems and antimicrobial purposes, due to their specific, unique and magnetic properties. Nevertheless, their synthesis is under continuous investigation, as physicochemical methods are considered expensive and require toxic solvents. Thus, green nanotechnology has shown considerable promise in the eco-biogenesis of nanoparticles. In the current study, Fe x O y -NPs were synthesized by two different methods: via green synthesis through the use of polyphenols, which were extracted from Phoenix dactylifera L.; and via chemical synthesis, in which the reducing agent was a chemical (NaOH), and iron chloride was used as a precursor. Thus, polyphenol extraction and its ability to produce nanoparticles were evaluated based on the drying temperature used during the Phoenix dactylifera L. recollection, as well as the extraction solvent used. The results highlight the potential of polyphenols present in Phoenix dactylifera L. for the sustainable manufacture of Fe x O y -NPs. Finally, green and chemical syntheses were compared on the basis of physicochemical characteristics and functional properties. Keywords: iron oxide nanoparticles; green synthesis; Phoenix dactylifera L.; polyphenols; antioxidant activity 1. Introduction Nanomaterials (1–100 nm in size) are gaining considerable interest for their unique properties. Thus, they can be used in several sectors and applications, with continuous growth [ 1 – 3 ]. These nanomaterials can be obtained by various methods, which are classified into two types: natural nanomaterials, such as biogenic magnetite; and anthropogenic nanomaterials, which include all manufactured nanomaterials. These nanomaterials are generally labelled based on their shapes and structures (e.g., nanoparticles, nanotubes, nanofibers. etc.) [ 4 , 5 ]. Nevertheless, most of the physicochemical methods used to develop nanomaterials require the use of expensive and toxic solvents [ 6 – 8 ]. The use of environmentally friendly methods (green methods) to synthesize nanoparticles from noble metals has recently been investigated, with the aim of providing high-purity NPs through simple, economic and eco-friendly techniques [9–11]. For this purpose, plant and fruit extracts, as well as bio-organisms, have been used to achieve environmentally safe methods [ 12 – 16 ]. The importance of these green methods is not only based on environmental factors but also on other interesting features that they may provide. Thus, it has been shown that green methods are typically inexpensive, effective, rapid and non-toxic, generally producing nanoparticles with high crystallinity and various shapes and sizes [ 17 – 19 ]. These characteristics were reported to depend on two groups of parameters, including the composition of raw materials and additives used, as well Nanomaterials 2022,12, 2449. https://doi.org/10.3390/nano12142449 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022,12, 2449 2 of 19 as the processing conditions applied during the synthesis of NPs. Among compositional parameters, the most important are the nature and relative concentration of the plant extract (i.e., natural polyphenol content), the metal salts used as precursors, the extract/metallic salt ratio, the pH, and the addition of a reducing agent. The most important processing conditions are the drying temperature, grinding particle size and the extraction method employed, in addition to reaction temperature and time [ 20 , 21 ]. It has also been reported that the functionality of these extracts comes from their high polyphenol ratios, which reduce metal salts into high-purity nanoparticles due to their unique properties (reducing character, ability to form hydrogen bonds, nucleophilic activity, polarizability, acidity and chelation ability) [22,23]. As a suitable candidate for this purpose, Phoenix dactylifera L. is a rich source of polyphenol content. Its use was reported as an antioxidant additive in antimicrobial, antilipemic and antidiabetic activities [ 24 ]. Thus, its use in the synthesis of nanoparticles, particularly Fe x O y -NPs, could confer magnetic properties, high electrical conductivity, hightemperature properties and suitable biocompatibility, enabling them to be used in biomedical applications, such as drug delivery, antimicrobial treatments and diagnosis [25–30] ; in chemical applications, such as energy conversion, [ 31 ] catalysis [ 32 ] and environmental protection [ 33 , 34 ]; in electronic and optoelectronic applications [ 35 ]; and in agriculture and biotechnology applications in animals [36–40]. In order to maximize the polyphenol content in extracts, it is vital to understand the factors that determine polyphenol content in each processing stage, as well as the effect of phenolic content on the physicochemical characteristics of Fe x O y -NPs. It is known that the development of the phenolic content is modulated by pre-harvesting factors. These include intrinsic factors of the plant (e.g., Phoenix dactylifera L.), such as genetics and age, as well as environmental factors, such as exposure to solar radiation, rainfall and soil type [ 7 ]. In addition, the assessment of post-harvest parameters related to the processing and storage of the plant is also considered important (e.g., drying method, grinding, extraction method, solvent effect, extraction time and extraction temperature) [ 7 , 8 , 41 , 42 ]. According to some studies, drying temperature, grinding time and solvent type have a direct correlation with the phenolic compound extracted [ 8 , 43 ]. Various solvent systems were used to extract phenolic compounds from plants [ 44 ]. However, it was found that the type of extraction is the most important factor influencing the yield and antioxidant properties of polyphenols. This was associated with the abundance of antioxidant compounds with diverse polarity and different chemical properties, which make them solvent-soluble or not [ 45 ]. Phenolic content could have an important influence on some of the physical characteristics of nanoparticles, although information related to this influence is very limited [ 46 ]. A few authors have reported the possibility of establishing a direct relationship between phenolic compound concentration and Fe x O y -NPs size and type using plant extracts [ 46 , 47 ]. Specifically, the novelty of the present work lies in evaluating the influence of the polyphenol properties of Phoenix dactylifera L. on the synthesis of Fe x O y -NPs and their characteristics, such as their size, crystallinity, type and morphology. The previously mentioned studies did not provide information regarding the influence of phenolic compounds on the type and portions of Fe x O y -NPs, crystallinity degree or possible synthesis mechanisms. For example, in a 2019 study, Salgado et al. [ 46 ] reported a mixture phase of iron oxide nanoparticles (Fe x O y -NPs) from various extracts, modifying the reaction pH to 6 by adding 1 M of NaOH. There might be a further impact of the pH of the reaction on the phenolic compounds in the extracts, thereby affecting the Fe x O y -NPs. It should be noted that there was no modification to pH in this experiment, which allows for attribution of the actual impact of phenolic compounds present in the extract to those compounds. The main objective of this study was to synthesize Fe x O y -NPs through green synthesis using Phoenix dactylifera L. extracts. To this end, extracts were first collected using different drying temperatures (25, 50, 100 and 150 ◦ C) and extract solvents (water and ethanol). Nanoparticles were also synthesized by a chemical method to compare them with Nanomaterials 2022,12, 2449 3 of 19 green NPs. The comparison took into account the physicochemical characteristics of the nanoparticles and their antioxidant activity (TAC and DPPH). 2. Materials and Methods 2.1. Materials Folin–Ciocalteu reagent, sodium carbonate (Na 2 CO 3 ), methanol (CH 3 OH), ethanol 99.9%, iron (III) chloride hexahydrate 98% (FeCl 3· 6H 2 O), sodium hydroxide (NaOH), sulfuric acid (H 2 SO 4 ), sodium phosphate (Na 2 HPO 4 ), ammonium molybdate ((NH 4 ) 3 PMo 12 O 40 ), gallic acid (C 7 H 6 O 5 ), hydrochloric acid (HCl), 2,2-diphenyl-1-picrylhydrazyl (DPPH) and dimethyl sulfoxide anhydrous (DMSO) 99.9% were provided by Sigma Aldrich (Darmstadt, Germany). The rest of the reagents employed in this work were of analytical grade. Phoenix dactylifera L. leaves were obtained directly from trees from Seville (Seville, Spain), where the mean temperature is 19 ± 4 ◦ C (with minimum and maximum temperatures of 6 ◦ C and 36 ◦ C, respectively) and the average relative humidity is 53%, ranging between 33 and 73% (data from the National Oceanic and Atmospheric Administration, Washington, DC, USA). 2.2. Methods Figure 1shows a diagram of the process carried out to obtain NPs from plant sources. The polyphenols were first extracted from Phoenix dactylifera L. leaves, and later, nanoparticles were produced by green (GS) or chemical (CS) synthesis through colloidal precipitation. Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 21 ferent drying temperatures (25, 50, 100 and 150 °C) and extract solvents (water and ethanol). Nanoparticles were also synthesized by a chemical method to compare them with green NPs. The comparison took into account the physicochemical characteristics of the nanoparticles and their antioxidant activity (TAC and DPPH). 2. Materials and Methods 2.1. Materials Folin–Ciocalteu reagent, sodium carbonate (Na 2 CO 3 ), methanol (CH 3 OH), ethanol 99.9%, iron (III) chloride hexahydrate 98% (FeCl 3 ·6H 2 O), sodium hydroxide (NaOH), sulfuric acid (H 2 SO 4 ), sodium phosphate (Na 2 HPO 4 ), ammonium molybdate ((NH 4 ) 3 PMo 12 O 40 ), gallic acid (C 7 H 6 O 5 ), hydrochloric acid (HCl), 2,2-diphenyl-1-picrylhy- drazyl (DPPH) and dimethyl sulfoxide anhydrous (DMSO) 99.9% were provided by Sigma Aldrich (Darmstadt, Germany). The rest of the reagents employed in this work were of analytical grade. Phoenix dactylifera L. leaves were obtained directly from trees from Seville (Seville, Spain), where the mean temperature is 19 ± 4 °C (with minimum and maximum temperatures of 6 °C and 36 °C, respectively) and the average relative humidity is 53%, ranging between 33 and 73% (data from the National Oceanic and Atmospheric Administration, Washington, D.C, U.S). 2.2. Methods Figure 1 shows a diagram of the process carried out to obtain NPs from plant sources. The polyphenols were first extracted from Phoenix dactylifera L. leaves, and later, nanoparticles were produced by green (GS) or chemical (CS) synthesis through colloidal precipitation. Figure 1. Diagram of the synthesis of GS Fe x O y -NPs (green) and CS Fe x O y -NPs (red). 2.2.1. Polyphenol Extraction from Phoenix dactylifera L. Leaves First, Phoenix dactylifera L. leaves were thoroughly washed three times with distilled water and dried at different temperatures (25, 50, 100 and 150 °C) until the water loss percentage was constant. The drying kinetic parameters were determined by the transformation of moisture content into water loss percentage and applying a nonlinear curve (Gompertz fitting function), according to Equation (1): 𝑊𝐿(𝑡)=𝑊𝐿(∞)·𝑒 (·) (1) where 𝑊𝐿(𝑡) is the percentage of water loss over time (𝑡), 𝑊𝐿(∞) is the maximum percentage of water loss when 𝑡 → ∞ and 𝑘 is the drying rate constant (i.e., the rate constant of water loss percentage per time unit) [48]. Then, the samples were pulverized with an electric grinder (Cuisinart Grind Central DCG-12BC, East Windsor, NJ 08520 Printed in China) and sieved to obtain a fine powder. Figure 1. Diagram of the synthesis of GS FexOy-NPs (green) and CS FexOy-NPs (red). 2.2.1. Polyphenol Extraction from Phoenix dactylifera L. Leaves First, Phoenix dactylifera L. leaves were thoroughly washed three times with distilled water and dried at different temperatures (25, 50, 100 and 150 ◦ C) until the water loss percentage was constant. The drying kinetic parameters were determined by the transformation of moisture content into water loss percentage and applying a nonlinear curve (Gompertz fitting function), according to Equation (1): WL(t) = WL(∞)·e−e(1−k·t)(1) where WL(t) is the percentage of water loss over time ( t) , WL(∞) is the maximum percentage of water loss when t→∞ and k is the drying rate constant (i.e., the rate constant of water loss percentage per time unit) [48]. Then, the samples were pulverized with an electric grinder (Cuisinart Grind Central DCG-12BC, East Windsor, NJ 08520 Printed in China) and sieved to obtain a fine powder. Subsequently, the powders were characterized using a Mastersizer 2000 (Malvern Instruments Ltd., Malvern, UK) in order to determine the grain size distributions ( µ m) in the dried samples. Nanomaterials 2022,12, 2449 4 of 19 Next, 30 g of each powder was extracted using a Soxhlet extractor employing 300 mL of distilled water or ethanol 99.9% as solvents at 98 or 68 ◦ C, respectively, for 8 h. The solvent was recovered, and the yield of extraction was calculated using Equation (2) [49]: y=Mass o f crude extracted Inicial mass o f phoenix powder ×100 (2) The crude extracts were dissolved again in water or ethanol and centrifuged at 10,000 rpm for 15 min. Finally, the supernatant (extract) was collected and stoked at 4◦C for further use. 2.2.2. Total Polyphenolic Content (TPC) To determine the total polyphenol content (TPC) in Phoenix dactylifera L. extract, we used the method described by Ayala-Zavala et al. [ 50 ] and Zapata et al. [ 51 ]. According to this method, 50 µ L of the extract was mixed with 3 mL of distilled water and 250 µ L of the Folin-Ciocalteu 1N reagent until equilibrium was reached (8 min). After mixing, 750 µ L of 20%, Na 2 CO 3 and 950 µ L of distilled water were added. Finally, the absorbance was measured in a UV/VIS spectrophotometer (PG Instruments Ltd., model T70 + UV/VIS Spectrometer) at 765 nm after 30 min of incubation at room temperature. A calibration curve was also prepared for gallic acid. The results were expressed in mg gallic acid equivalents per gram of Phoenix dactylifera L. extract (mg GAE/g of extract). 2.2.3. Nanoparticle Preparation Green Approach In this approach, the GS FexOy-NPs were synthesized by mixing 20 mL of each extract (0.27 g/mL) with 20 mL of FeCl 3· 6H 2 O 1 M solution for 2 h at 50 ◦ C. Then, the mixture was filtered using filter paper (Whatman n ◦ 1) and washed at least three times with distilled water to ensure that any impurities and foreign particles suspended in the mixture were removed. The filtered solid was then kept at 100 ◦ C for 8 h and, subsequently, hot-calcined at 500 ◦C for 5 h to obtain the purest nanoparticles possible. Chemical Approach Nanoparticles were also synthesized chemically by mixing 20 mL of NaOH 1 M with 20 mL of FeCl 3 -6H 2 O 1M. The remaining steps were the same as those described for the green approach. 2.3. Characterization Techniques 2.3.1. X-ray Diffraction (XRD) Both green and chemically synthesized nanoparticles (GS Fe x O y -NPs and CS Fe x O y - NPs, respectively) were subjected to XRD characterization (Brand: Bruker Model D8 advance A25 with Cu anode) to determine the crystal phase composition of the samples. These tests allowed for calculation of the crystallinity index and the crystal size. The most intense peaks of each XRD pattern were used to determine the crystallite size, applying the formula of Debye-Scherrer [52] (Equation (3)): D=kλ β·cosθ(3) where D is the diffracting domain size, k is a correction factor (0.94), λ is the used wavelength (0.154178 nm), β = FWHM (full width at half maximum) and θ is the position of the main peak. Nanomaterials 2022,12, 2449 5 of 19 The crystallinity degree (%) of the GS and CS Fe x O y -NPs was calculated using Equation (4): [53] Cristanillity (%) = Area of cristalline peaks Cristalline peaks area +amorphous area ×100 (4) 2.3.2. Fourier Transform Infrared Spectroscopy (FTIR) FTIR was carried out to obtain information about the vibration modes of the bonds present in FexOy-NPs and their structure. The GS and CS Fe x O y -NPs and Phoenix dactylifera L. raw powder were subjected to FTIR spectroscopy (Nicolet iS50 FITR Spectrometer, ThermoFisher Scientific, Madison, WI, USA) from 4000 to 400 cm −1 with a resolution of 0.482 cm −1 to identify the bonds present in the nanoparticle structure. 2.3.3. Scanning Electron Microscopy (SEM) The morphology of both GS and CS Fe x O y -NPs, as well as their diameter distribution average, were measured by SEM with a Zeiss EVO SEM (Pleasanton, CA, USA) at 10 kV, recording images at different magnifications. The images were analyzed with ImageJ software (1.53q; National Institutes of Health, Bethesda, Maryland, USA) (free software) [54]. 2.3.4. Determination of Antioxidant Activity Normally, multiple methods are used to evaluate antioxidant activity due to its complexity and the diverse groups that can present antioxidant activity [ 55 ]. Therefore, two distinct methods were used for the extracts and GS and CS Fe x O y -NPs: total antioxidant activity phosphomolybdate test (TAC) and DPPH antioxidant activity. Total Antioxidant Activity (TAC) Determination. TAC analysis was performed following a previously described protocol [ 56 ] with a slight modification: 2 mL of sample was mixed with 2 mL of reagent (4 mM (NH 4 ) 3 PMo 12 O 40 , 0.6 M H 2 SO 4 and 28 mM Na 2 HPO 4 ). These mixtures were incubated for 90 min at 95 ◦ C in a water bath. Once tempered, the absorbance was determined by spectrophotometry at 695 nm, using gallic acid as a reference. Before the test, the NPs were dispersed in HCl. DPPH Antioxidant Activity Test This evaluation was performed using the protocol described in a previous study with a slight modification: a series of nanoparticle dispersion was prepared (4, 2, 1, 0.5, 0.25 and 0.125 mg/mL in DMSO), and subsequently, volumes of 2 mL of these dispersions were mixed with 2 mL DPPH. These mixtures were incubated for 30 min, and later, their absorbance was tested at 517 nm. The control solution was made up of 2 mL DPPH solution and 2 mL of DMSO. The DPPH activity of the extracts was evaluated out using the same protocol [56]. Finally,theinhibitionpercentageIC (%) ofeachconcentrationwasestablished( Equation (5) ) through the relationship between the absorbance values of the oxidized solutions in the absence of any antioxidant agent and those in the presence of antioxidant agents (A D and A Da , respectively). IC(%) = ((AD−ADa)/AD)·100 (5) In addition, the necessary antiradical to cause a 50% inhibition (IC 50 ) of each NP was determined using GraphPad Prism 9 software (GraphPad Prism version 9.0.0 for windows, San Diego, CA, USA, www.graphpad.com) [56]. 2.4. Statistical Analysis Statistical analyses were performed using IBM SPSS statistics 26 software and Graph- Pad Prism9 software. A one-way ANOVA was carried out to estimate the significant differences between observations. In at least three replicates, the data were expressed as mean ± SD. In addition, HSD Tukey tests were conducted to determine the significance level (p< 0.05). Nanomaterials 2022,12, 2449 6 of 19 3. Results and Discussion 3.1. Characterization of Raw Material and Extracts 3.1.1. Effect of Drying Temperature on Moisture Content in Phoenix dactylifera L. Figure 2shows the kinetics of moisture loss in Phoenix dactylifera L. The moisture content was converted to water loss percentage from the samples and fitted to a non-linear curve (Gompertz function) (R 2≥ 0.99). The maximum moisture loss percentage obtained was in the following order with significant differences (p< 0.05): T150 ◦C> T100 ◦C≈T50 ◦C > T 25 ◦C . The quickest sample was T 150◦C , reaching WL(∞) = 55.7 ± 1.7% (maximum moisture loss) in 8 h, whereas the slowest sample was T 25 ◦C , which required more time (1056 h) to reach equilibrium in WL(∞) = 51.1 ± 1.2%. Samples T 50 ◦C and T 100 ◦C did not show any significant difference with respect to WL(∞) , with values of 53.8 ± 2.6 and 54.2 ± 1.6% reached in 72 and 48 h, respectively, and drying rate values of k= 0.10 ± 0.02 and 0.22 ± 0.03 (H 2 O h −1 ). This indicates that accelerated dehydration occurred in the samples when the drying temperature was increased. In this way, T 150 ◦C reduced the time by 132, 9 and 6 times faster than T 25 ◦C, T 50 ◦C and T 100 ◦C , respectively, reaching a higher moisture loss. Similar results were reported by Larrauri et al. [ 57 ]. This disposal could be elucidated by the increased drying rate produced by higher temperatures. Thus, it was observed that when the drying temperature was increased, the drying rates (kinetic parameter k) increased significantly, as shown in Figure 2B. Similar results were reported by Patrón-Vázquez et al. [48] and Djaeni et al. [58]. Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 21 as mean ± SD. In addition, HSD Tukey tests were conducted to determine the significance level (p < 0.05). 3. Results and discussion 3.1. Characterization of Raw Material and Extracts 3.1.1. Effect of Drying Temperature on Moisture Content in Phoenix dactylifera L. Figure 2 shows the kinetics of moisture loss in Phoenix dactylifera L. The moisture content was converted to water loss percentage from the samples and fitted to a non-linear curve (Gompertz function) (R 2 ≥ 0.99). The maximum moisture loss percentage obtained was in the following order with significant differences (p < 0.05): T 150 °C > T 100 °C  T 50 °C > T 25 °C . The quickest sample was T 150°C , reaching 𝑊𝐿(∞) = 55.7 ± 1.7% (maximum moisture loss) in 8 h, whereas the slowest sample was T 25 °C , which required more time (1056 h) to reach equilibrium in 𝑊𝐿(∞) = 51.1 ± 1.2%. Samples T 50 °C and T 100 °C did not show any significant difference with respect to 𝑊𝐿(∞), with values of 53.8 ± 2.6 and 54.2 ± 1.6% reached in 72 and 48 h, respectively, and drying rate values of k = 0.10 ± 0.02 and 0.22 ± 0.03 (H 2 O h −1 ). This indicates that accelerated dehydration occurred in the samples when the drying temperature was increased. In this way, T 150 °C reduced the time by 132, 9 and 6 times faster than T 25 °C, T 50 °C and T 100 °C , respectively, reaching a higher moisture loss. Similar results were reported by Larrauri et al. [57]. This disposal could be elucidated by the increased drying rate produced by higher temperatures. Thus, it was observed that when the drying temperature was increased, the drying rates (kinetic parameter k) increased significantly, as shown in Figure 2B. Similar results were reported by Patrón- Vázquez et al. [48] and Djaeni et al. [58]. Figure 2. (A) Water loss as a function of drying time (t) at T 100 °C . (B) Maximum water loss percentage, 𝑊𝐿(∞), and kinetic parameter (k) as a function of drying temperature. 3.1.2. Drying Temperature, Grinding and Total Polyphenol Content Effect Table 1 shows the size average of powdered particles, extraction yield and total polyphenol content (mgGAE/g extract) as a function of drying temperatures (25, 50, 100 and 150 °C) and solvents (H 2 O or ethanol). The particle size was the smallest (36.98 ± 1.07 µm) at T 150 °C , increasing at lower drying temperatures. This effect may be a consequence of the fact that high temperatures favor mobility, facilitating the extraction of substances that may be present inside the leaf, giving rise to smaller particles after a grinding stage. The extraction yield increased with increased drying temperature. This increased extraction may also be attributed to alterations of internal components of cells induced at higher Figure 2. ( A ) Water loss as a function of drying time (t) at T 100 ◦C . ( B ) Maximum water loss percentage, WL(∞), and kinetic parameter (k) as a function of drying temperature. 3.1.2. Drying Temperature, Grinding and Total Polyphenol Content Effect Table 1shows the size average of powdered particles, extraction yield and total polyphenol content (mgGAE/g extract) as a function of drying temperatures (25, 50, 100 and 150 ◦ C) and solvents (H 2 O or ethanol). The particle size was the smallest ( 36.98 ±1.07 µm ) at T 150 ◦C , increasing at lower drying temperatures. This effect may be a consequence of the fact that high temperatures favor mobility, facilitating the extraction of substances that may be present inside the leaf, giving rise to smaller particles after a grinding stage. The extraction yield increased with increased drying temperature. This increased extraction may also be attributed to alterations of internal components of cells induced at higher temperatures, which may also cause these cells to lose a considerable amount of their active substance, regardless of the high yield achieved [ 8 , 59 , 60 ]. These effects are in line with the results of previous studies [ 60 ], alongside the effect of the solvents. Among the Nanomaterials 2022,12, 2449 7 of 19 tested solvents, ethanol achieved a higher extraction yield than water at all the studied temperatures, except at the highest temperatures, at which no significant differences were found. Nevertheless, all the samples achieved an extract yield of more than 50%, in agreement with the results of previous studies. These results were compared with those of other Soxhlet extracts [49,60]. The total polyphenolic compounds (TPC) and antioxidant activities were also evaluated (Table 1). The results indicate that drying temperature also affects the TPC. In this sense, the lower the drying temperature, the higher the TPC. Thus, the highest TPC was recorded at 25 ◦ C (42.28 ± 0.79 mg GAE/g extract and 38.64 ± 1.72 mg GAE/g extract for water and ethanol, respectively). However, this effect may be considered only a moderate trend up to a drying temperature of 100 ◦ C. At the highest drying temperature, at which the aforementioned loss of active substances from the cells should be relevant, the reduction in TPC was clear for the two solvents used. In addition, a higher TPC was observed in hot water as compared with the organic solvent, similar to the effect on TPC reported in other studies on plant extracts [ 41 , 59 , 61 , 62 ]. However, natural phenols have been reported to show a solubility preference for solvents with intermediate polarities, such as acetones and alcohols, rather than more or less polar solvents (e.g., water and ethyl acetate, respectively) [ 41 ]. According to Bhebhe et al. [ 41 ], an elevated water temperature may also have contributed to a higher TPC, applying the empirical rule previously mentioned, i.e., “like dissolves like”. This could indicate that Phoenix dactylifera L. contains more water-soluble polyphenolic compounds and phenolic acids, which have a solubility preference for water, unlike other solvents, due to their lower activity coefficient in water [63]. Table 1. Results of the average particle size of obtained powders, the extraction yield, total polyphenol content (TPC) of the extracts, IC50 DPPH free radical and total antioxidant activity (TAC) as a function of drying temperatures (25, 50, 100 and 150 ◦ C) and solvents (ethanol and H 2 O). Different superscript letters (a–d) within a column indicate significant differences among mean observations (p< 0.05). Sample Size (µm) Extraction Yield (%) TPC (mg GAE/g Extract) TAC (mg GAE/mg Extract) T25 ◦CEtanol Ext. 90.56 ±2.35 a53.77 ±0.77 c38.64 ±1.72 a,b 0.92 ±0.45 a,b T25 ◦CH2O Ext. 51.35 ±0.57 d42.28 ±0.79 a2.04 ±0.48 a T50 ◦CEtanol Ext. 47.75 ±1.48 b56.13 ±0.29 b35.13 ±3.17 b,c 0.87 ±0.21 a,b T50 ◦CH2O Ext. 54.45 ±0.77 b,c 38.47 ±2.01 a,b 1.95 ±0.75 a,b T100 ◦CEtanol Ext. 44.85 ±1.25 b58.53 ±0.99 a33.23 ±2.72 b,c,d 0.77 ±0.52 a,b T100 ◦CH2O Ext. 55.15 ±0.72 b,c 37.55 ±2.04 a,b 1.70 ±0.28 a,b T150 ◦CEtanol Ext. 36.98 ±1.07 a59.63 ±0.65 a29.02 ±1.50 d0.66 ±0.30 b T150 ◦CH2O Ext. 59.57 ±0.77 a29.59 ±0.85 d,c 1.49 ±0.63 a,b 3.2. Characterization of Nanoparticles 3.2.1. X-ray Diffraction (XRD) Figure 3shows the XRD diffractograms of GS-Fe x O y -NPs synthesized from the ethanolic and aqueous Phoenix extracts prepared from powders dried at different temperatures. The XRD diffractogram of CS-Fe x O y -NPs was also used for the sake of comparison. All the peaks observed in the GS Fe x O y -NPs diffractograms are indicative of a spinel structure. Thus, the peaks observed at 2 θ ( ◦ ) = 24.15, 33.15, 35.63, 40.86, 49.49, 54.07, 56.16, 57.45, 62.44 and 64.00 can be assigned to the crystallographic reflection planes (012), (104), (110), (113), (024), (116), (211), (122), (214) and (300), respectively, which correspond to the crystalline rhombohedral structure of hematite (Fe 2 O 3 ); space group: R-3c N ◦ : 167 with standard crystallographic parameter a = b = 5.035 Å, c =13.751 Å (JCPDS n ◦ . 01-080-2377 standard hematite powder diffraction pattern), as previously predicted by Ayachi et al. [ 64 ]. Moreover, the peaks observed at 2 θ ( ◦ ) = 30.07, 35.152, 43.04, 47.12, 53.391, 62.50, 65.71 and 66.50 may be assigned to the crystallographic reflection planes (220), (311), (400), (331), (422), (440), (531) and (442), respectively, corresponding to the crystalline cubic structure of magnetite (Fe 3 O 4 ); space group: Fd3m N ◦ : 227 with standard crystallographic parameter a, b and c = 8.3840 Å (JCPDS n ◦ . 01-075-0033 standard magnetite powder diffraction Nanomaterials 2022,12, 2449 8 of 19 pattern). These results are consistent with those obtained by Noukelag et al. [ 65 ], who synthesized hematite nanoparticles using Rosmarinus leaf extract, and Venkateswarlu et al. [ 66 ], who synthesized Fe 3 O 4 -NPs from Syzygium cumini seed extract. Other diffraction peaks of lower intensity were observed at 2 θ ( ◦ ) = 28.49 and 41.56, which may be attributed to crystallographic reflection planes (008) and (324), respectively, which correspond to the crystalline tetragonal structure of deviated magnetite (Fe 3-δ O 4 ); space group: P41212 N ◦ : 92 with standard crystallographic parameter a = b = 8.35 Å and c = 25.04 Å (JCPDS n ◦ . 01-080-2186 standard iron oxide powder diffraction pattern) [ 67 ]. The CS Fe x O y -NPs presented additional peaks at 2 θ ( ◦ ) = 27.43, 31.68, 45.43, 55.52 and 65.39, which may be attributed to crystallographic reflection planes (211), (220), (332), (440) and (622), corresponding to the crystalline cubic structure of iron oxide (Fe 2 O 3 ); space group: Ia3 N ◦ : 206 with standard crystallographic parameter a = 9.404 Å (JCPDS n ◦ . 00-039-0238 standard iron oxide powder diffraction pattern). Furthermore, peaks were observed at 2 θ ( ◦ ) = 31.68, 44.16, 56.52 and 66.29, which may be attributed to crystallographic reflection planes (314), (440), (643) and (531), corresponding to the crystalline orthorhombic structure of magnetite (Fe 3 O 4 ); space group: Pmc21 N ◦ : 26 with standard crystallographic parameter a = 11.868 Å, b = 11.851 Å and c = 16.752 Å (JCPDS n ◦ . 01-076-0957 standard iron oxide powder diffraction pattern) [68]. The parameters obtained from the XRD profiles are shown in Table 2. The proportions of Fe 2 O 3 and Fe 3 O 4 in the NPs were obtained by X-ray powder pattern analysis (PDF4) by means of X’Pert HighScore Plus software [ 69 ]. A high proportion of Fe 3 O 4 in the NPs was found in the presence of polyphenols. This may be related to the increased ability of phenolic compounds to reduce Fe+3 to Fe+2 (Fe3O4) according to Equation (6) [70]: 3Fe+3(aq) + 8 −OH−→Fe3O4↓+ 4H2O (6) However, the proportion of Fe 3 O 4 decreased when the drying temperature increased. A similar effect was related to the decrease in TPC with increasing temperature, as there were not enough polyphenols to reduce Fe 3+ to Fe 2+ [ 46 ], which may lead to magnetite oxidation according to Equation (7) [70]: 4Fe3O4+ O2→6Fe2O3(7) However, this effect cannot be explained solely by the reduction in TPC. Thus, the proportion of Fe 3 O 4 achieved at the highest drying temperature is much lower than that reported for the CS NPs, although the absence of some peaks in the XRD profile of CS FexOy-NPs indicates the deficiency of the reducing agent [4]. In contrast, the results suggest that a higher drying temperature inhibits the reduction of Fe 3+ to Fe 2+ . With respect to the crystal size, drying temperature seems to proportionally affect the nanoparticle size. Thus, the higher the drying temperature, the larger the nanoparticles obtained. The extracting solvent also affects nanoparticle size, although more gently. These results could confirm the relevance of the role of polyphenols, as increased TPC results in a smaller hydrodynamic system, affording smaller nanoparticles [ 71 , 72 ]. In this way, water gave rise to smaller nanoparticles than ethanol, whereas the largest sizes corresponded to the CS FexOy-NPs. With respect to crystallinity, the well-defined and sharper peaks in the XRD profiles support the good crystallinity degree obtained for the different GS Fe x O y -NPs. All the GS samples showed a crystalline degree more significant than that of CS Fe x O y -NPs, increasing with the drying temperature. The lower crystallinity obtained through chemical synthesis is probably due to the lower availability of stabilizing agents [4]. Nanomaterials 2022,12, 2449 9 of 19 Table 2. Results obtained for different parameters of GS Fe x O y -NPs as a function of drying temperature (25, 50, 100 and 150 ◦ C) and solvent (ethanol and H 2 O) and CS Fe x O y -NPs: composition (proportion of Fe 2 O 3 and Fe 3 O 4 ), crystallinity percentage, average diameter in nm (from DRX measurements and SEM images) and antioxidant activity (IC 50 DPPH free radical in mg/mL, the ratio between IC 50 of NPs and extracts, IC 50 NPs /IC 50 Ext and total antioxidant activity (TAC) in mg GAE/mg). Different superscript letters (a–d) within a column indicate significant differences among mean observations (p< 0.05). Sample Fe2O3(%) Fe3O4(%) Crystallinity (%) DDRX (nm) D SEM (nm IC50 (mg/mL) IC50 NPs/IC50 Ext TAC (mg GAE/mg) GS FexOy-NPs T25 ◦CEthanol Ext. 45.8 54.2 68.7 g36 ±10 37 ±1d0.29 ±0.11 b0.55 24.76 ±1.48 a T25 ◦CH2O Ext.46.3 53.7 66.5 h36 ±12 36 ±1d0.25 ±0.09 b0.54 18.34 ±1.09 a,b,c T50 ◦C Ethanol Ext.79.8 20.2 71.3 e38 ±10 38 ±1c,d 0.64 ±0.31 a,b 0.96 21.54 ±1.70 a,b T50 ◦CH2O Ext. 75.1 24.9 71.0 f38 ±737 ±1d1.39 ±0.67 a,b 2.14 15.82 ±0.70 b,c T100 ◦CEthanol Ext.87.6 12.4 76.0 c42 ±16 41 ±2b,c 1.14 ±0.90 a,b 2.58 21.41 ±0.29 a,b T100 ◦C H2O Ext. 88.9 11.1 73.8 d42 ±16 42 ±1b,c 0.68 ±0.41 a,b 1.66 12.43 ±2.20 c,d T150 ◦CEthanol Ext.90.2 9.8 85.9 b44 ±16 45 ±1b2.12 ±1.64 a4.07 15.31 ±0.78 b,c T150 ◦CH2O Ext. 92.8 7.2 86.6 a45 ±16 43 ±1b0.81 ±0.10 a,b 0.92 12.51 ±3.46 c,d CS FexOy-NPs 68.9 31.31 46.7 i59 ±24 59 ±20 a1.24 ±0.32 a,b -8.56 ±0.84 d Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 21 Figure 3. XRD spectra of the GS Fe x O y -NPs (JCPDS standard) using ethanolic (A) and aqueous (B) extracts obtained at different drying temperatures. The profile CS Fe x O y -NPs is included for comparison. 3.2.2. Fourier Transform Infrared Spectroscopy (FTIR) The FTIR profile of the different GS and CS Fe x O y -NPs were compared, as shown in Figure 4. In addition, they were related to those obtained for raw powder (Figure S1). Three distinct zones could be distinguished: the bonds of Fe-O in iron oxide (800–400 cm −1 ), the oleate’s COO - group (1800–900 cm −1 ) and the chains of the alkyl surface (3000– 2800 cm −1 ) [56]. The first observation was focused on the range of 800–400 cm −1 , where different Fe- O bands can be identified as both maghemite/hematite g-Fe 2 O 3 /α-Fe 2 O 3 and magnetite Fe 3 O 4 . 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