Green synthesis and characterization of zero-valent iron nanoparticles using stinging nettle (Urtica dioica) leaf extract

2.1 Materials

Dried leaves of U. dioica were purchased from a local shop (Shiraz, Fars, Iran). Ferric chloride (FeCl₃·6H₂O) was purchased from Merck Chemicals (Darmstadt, Hessen, Germany) and used as received. All glassware was acid washed and then rinsed with deionised water. Chemical reactions were conducted using Millipore water (Millipore Corp., Bedford, MA, USA, conductivity range=0.055−0.294 lS/cm).

2.2 Leaf extract preparation

Dried leaves were washed with deionized water to remove any mud and dust and were dried at room temperature. Using about a 5% (w/v) mixture of dried leaves in deionized water has been the most common ratio for leaf extract preparation in previous experiments [45], [46], [47], [48], [49], [50], [51], [52]. Therefore, in the current experiment, 5 g dried leaves were boiled in 100 ml deionized water for 15 min by using a heater mantel under reflux. The mixture was cooled to room temperature and filtered through a Whatman filter paper (Reeve angel, Grade 201). In order to remove leaf microparticles, the prepared extract was centrifuged at 2000 rpm for 5 min. The resulting clear solution was sealed in polypropylene tubes and refrigerated.

2.3 Synthesis of INPs

Synthesis of INPs was conducted using a 10 ml reaction liquid. In practice, 1 ml iron precursor (FeCl₃·6H₂O, 0.2 m) was added to 9 ml leaf extract while stirring vigorously at room temperature. After 12 h, the reaction mixture was centrifuged and the resulting black pellet was washed with deionized water and dried in an oven at 50°C.

2.4 Characterization of INPs

The visual appearance and morphology of prepared INPs were evaluated by transmission electron microscopy (TEM). A drop of nanoparticles dispersion in distilled water was dripped on to a carbon-coated copper grid and dried at room temperature. Micrographs were obtained using a Philips CM 10, TEM, operated at high voltage (HT) 100 kV [53], [54], [55]. Particle size analyses were conducted using ImageJ software version 1.47v, an image analysis software developed by the NIH (http://imagejnihgov/ij/). The hydrodynamic diameter and zeta potential of particles were measured using a Microtrac S3500 particle size analyser. The Fourier transform infrared (FTIR) spectroscopy analyses were done using KBr pellets. Prepared INPs were mixed and pressed with 150 mg KBr and the spectra were taken using a Bruker, Vertex 70, FTIR spectrometer. The IR absorption analyses were done from 4000 cm⁻¹ to 400 cm⁻¹ [20], [56]. The crystallinity and composition of the INPs were analysed via X-ray powder diffractometry (Siemens D5000). The X-ray diffraction pattern of the sample was collected over a 2-θ range from 20° to 90° at a scan rate of 2° min⁻¹ [57], [58]. Thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) analyses were done to determine the presence and quantification of organic compounds from U. dioica leaf extract in the final INPs product. Magnetic characteristics of nanoparticles were evaluated using a vibration sample magnetometer. The analysis was conducted at room temperature with increasing magnetic field up to 10 kOe and field sweeping from −10 to +10 kOe [17], [59].

3.1 Synthesis of INPs

By addition of iron precursor to the leaf extract a sudden change in the color of the reaction mixture was observed and a dark black suspension was formed. Instantaneous change in the reaction color and appearance of deep black suspension is a common phenomenon in green synthesis of INPs [48], [49], [60], [61], [62], [63] and considered as an indicator for reduction of iron ions and formation of INPs [46], [64], [65]. This phenomenon was also observed in semi-green synthesis [50], [66], [67], [68], [69] and chemical synthesis of INPs [18], [19]. Various parameters such as reaction time, metal precursor concentration, amount of leaf extract, and reaction temperature were reported to be the key parameters for the synthesis of nanoparticles in a green approach. The significance of these parameters does vary in different plant extracts [21], [38], [70], [71], [72]. While using a particular plant extract, various investigations should be done to evaluate the effects of each parameter on the characteristic features of the obtained particles. Finally, an optimization study would determine the optimal state of each parameter toward determining the best result.

3.2 Characterization of INPs

Visual appearance of the prepared INPs was evaluated by TEM analysis (Figure 1A). The obtained particles were dominantly spherical while some oval particles were also observed. Particle size distribution analyses were conducted and results are shown in Figure 1B. Diameters of INPs were measured in the range of 21 nm to 71 nm with a mean particle size of 46 nm. Green synthesized INPs with similar particle size distribution were also reported by using Adean blackberry, green tea, and eucalyptus leaf extract [46], [64], [67], [73]. Very small INPs (i.e. about 5 nm) were also synthesized by green methodology using mulberry, pomegranate, and Syzygium jambos leaf extract and soybean sprouts [61], [74], [75]. By contrast, Yuvakkumar and Hong [76] reported that using rambutan peel waste extract for the synthesis of INPs resulted in large spinel particles with 200 nm diameter. Based on the TEM micrograph, nanoparticles were surrounded by a biologic matrix from leaf extract. Particles were surrounded by a matrix and formed a complex. The sizes of INPs complexes with surrounding materials were measured in the range of 117–605 nm. Formation of nanoclusters with phytochemical capping was also reported for Andean blackberry leaf extract mediated synthesized INPs. The authors believed that various polyphenols in the leaf extract can be responsible for the formation of INP clusters [67].

Figure 1:

Transmission electron microscopy (TEM) micrographs of the green synthesized iron nanoparticles (INPs) (A) which indicate that nanoparticles are surrounded by a biologic matrix from leaf extract, and particle size distribution diagram of the INPs (B).

Precise particle size analyses were done to evaluate the hydrodynamic diameter of INP complexes. According to the results, the diameter of complexes was measured from 121.5 nm up to 2.75 μm. The particle size distribution curve (Figure 2) shows two distribution peaks at 243 nm (volume per cent: 13.4%) and 1.94 μm (volume per cent: 4.0%) which can be an indication for some agglomeration of INPs complexes. Muthukumar and Matheswaran have shown that biologic coating can reduce the rate of INPs agglomeration. They measured the size of biogenic synthesized INPs to be from 58 nm to 530 nm using a particle size analyser. However, chemically synthesized nanoparticles were agglomerated up to 2.3 μm [69]. Similar agglomeration was also reported by Niraimathee et al. [45] for the green synthesized INPs using Mimosa pudica root extract. They revealed particle size distribution of prepared particles ranging from 32 nm to 600 nm. The INPs complexes had a negative zeta potential of −82.6 mV. Biologic coatings from various plants can provide different zeta potentials ranging from −35 mV for green tea to about zero for Hordeum vulgare, and +71 mV for Amaranthus spinosus [60], [65], [69]. This variation is mainly due to different phytochemicals in various plants. Generally, the surface charge values above −30 mV are considered to be sufficient for suppressing particles agglomeration and providing a stable colloidal system [77].

Figure 2:

Particle size distribution curve of iron nanoparticles (INPs) complexes which were obtained by particle size analyser.

Figure 3 shows the FTIR absorption spectra of the synthesized INPs. The OH groups induce a broad indicative peak which appeared at 3467 cm⁻¹ [56]. The carbonyl group stretching vibration can be seen at 1636 cm⁻¹ and the peak at about 1070 cm⁻¹ produced by C–O bonds [56]. The peaks of aliphatic C–H appeared around 2800 cm⁻¹ [53], [54], [55]. Usually, presence of these peaks in the FTIR spectra of green synthesized nanoparticles is indicative of nanoparticles functionalization with organic compounds [20], [21], [22], [38]. It has been shown that exposure of INPs to aqueous leaf extract resulted in the coating and stabilization of INPs with herbal biochemical compounds [78]. Functionalization of nanostructures with hydrophilic groups such as hydroxyl, carbonyl, and C–O increases the interactions with water molecules and results in more stable colloids.

Figure 3:

Fourier transform infrared (FTIR) spectra of green synthesized iron nanoparticles (INPs); peaks at 1070 cm⁻¹, 1636 cm⁻¹, 2800 cm⁻¹, and 3467 cm⁻¹ were due to C–O, C=O, C–H, and OH bonds, respectively.

Iron oxide nanoparticles (IONs) exhibit two intense and sharp peaks at about 640 cm⁻¹ and 450 cm⁻¹ due to the presence of Fe–O bonds [17], [18], [19], [57], [58], [59]. These peaks were not observed in the FTIR spectrum of the prepared particles (Figure 3), which is indicative of zero-valent INPs. Similar spectra were also reported for chemically synthesized zero-valent INPs [79], [80]. Zero-valent INPs exhibit dual characteristics of iron oxide/hydroxide and of zero-valent iron [81]. There is a core-shell structure, zero-valent iron atoms forming the core which is surrounded by an iron oxide shell [80], [82], [83]. This feature is due to oxidation of exposed iron atom to iron oxide/hydroxide. The thin iron oxide shell resulted in the appearance of Fe–O absorption bonds, but in lower intensity than that usually observed for IONs [17], [59], [84]. Absence of these peaks in FTIR spectra of the prepared INPs indicated the absence of iron oxide shell. Organic coating from leaf extract seems to protect the surface of Fe⁰ atoms from oxidation.

The crystallinity of the prepared INPs was evaluated by X-ray powder diffraction analysis and results were evaluated by using PANalytical X’Pert HighScore software. As shown in Figure 4, there is no distinctive diffraction peak in the whole pattern, suggesting that the green synthesis approach resulted in the production of amorphous INPs. Fabrication of amorphous zero-valent INPs was also reported by using leaf extract of different plants and food industry wastes [46], [52], [75]. In addition, green synthesis of INPs by using extracts of green tea, eucalyptus, S. jambos (L.) Alston (Myrtaceae), and sorghum bran was reported to result in amorphous IONs [30], [46], [64], [74]. Some researchers also considered a small peak at 2θ=44.9° as an indicative peak for zero-valent INPs [48], [49], [51]. The broad shoulder peak from 10° to 20° of 2θ values was proposed to be due to the presence of organic materials from leaf extract which are responsible for capping and stabilizing nanoparticles [46].

Figure 4:

X-ray powder diffractometry (XRD) pattern of the green synthesized iron nanoparticles (INPs); no characteristic peak was observed which indicates the amorphous structure of nanoparticles.

TGA and DTG curves of green synthesized zero-valent INPs are provided in Figure 5. There is a significant weight loss in the TGA curve of the synthesized INPs. The initial weight loss below 200°C is usually attributed to evaporation of residual and adsorbed water [85]. Organic compounds are reported to exhibit a considerable thermal stability up to 200°C, whereas at higher temperatures, breakdown of the organic skeleton takes place [86], [87]. Decomposition of organic capping materials makes the main peak in the DTG curve as reported for dextran-coated magnetite nanoparticles [87]. About 30% weight loss was recorded due to decomposition of INPs biologic coating.

Figure 5:

Thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves of iron nanoparticles (INPs); the weight loss below 200°C was due to water evaporation and decomposition of biologic matrix resulted in 30% weight loss which makes the main peak in the DTG curve.

Magnetic characteristics of the prepared INPs were evaluated by a vibrating sample magnetometer at room temperature and the results are shown in Figure 6. Magnetization value of the synthesized INPs was measured to be 0.14 emu/g. Magnetization value about 130 emu/g was reported for chemically synthesized particles which contrasts with the current report [88]. High magnetization properties provide the possibility for magnetic handling of particles. However, high values of magnetization have undesirable impacts on INPs colloidal stability and formation of aggregated particles [88].

Figure 6:

Magnetization curve of iron nanoparticles (INPs); magnetization value of the synthesized INPs was measured to be 0.14 emu/g.