One-step green synthesis and characterization of iron oxide nanoparticles using aqueous leaf extract of Teucrium polium and their catalytic application in dye degradation

To prepare leaf extract, dried leaves of Teucrium polium were bought from a local mini market (Shiraz, Fars, Iran). Ferric chloride (FeCl3 · 6H2O) was procured from Merck, Hessen, Germany. All glassware was cleaned by using acid and DI water. Millipore water was used for chemical reactions. (Millipore Corp., Bedford, MA, USA, conductivity range = 0.055–0.294 lS cm⁻¹).

Surfaces of dried leaves were cleaned thoroughly with DI water to remove any debris and other contaminated organic contents, and were dried at ambient temperature. The most common ratio for leaf extract preparation was about a 5% (w/v) mixture of clean and dried leaves in deionized water. Consequently, 5 g of dried leaves was refluxed in 100 ml DI water for 15 min by using a heater mantel. The refluxed mixture was cooled at ambient temperature and filtered by using a Whatman filter paper (Reeve angel, Grade 201). A centrifuge was used to separate the leaf microparticles of prepared extract at rotating speed 2000 rpm for 5 min. The extracted clear solution was transferred to polypropylene tubes, sealed and refrigerated.

The IONPs were synthesized by using a 1 ml FeCl₃·6H₂O, 0.1 M to 9 ml leaf extract while stirring vigorously at ambient temperature for 24 h. The acquired reaction mixture was centrifuged. The resulting black pellet was collected and washed with DI water and dried in an oven at 50 °C.

The visual appearance and morphology of prepared IONPs were evaluated by transmission electron microscopy (TEM). In order to prepare sample, a carbon-coated copper grid was used and a drop of nanoparticles dispersed in distilled water was dripped on it to dry at ambient temperature. Micrographs were obtained using a Philips CM 10, TEM, operated at high voltage (HT) 100 kV. The hydrodynamic diameter of particles was measured using a Microtrac S3500 particle size analyser. X-ray powder diffractometry (Siemens D5000) was used for the crystallinity and composition evaluation of the IONPs at range of 20° to 90° at a scan rate of 2° × min ^{− 1}. The FTIR spectroscopy analyses were done using KBr pellets. Synthesized IONPs were mixed and pressed with 150 mg KBr and the spectra were taken from 4000 cm ^{− 1} to 400 cm ^{− 1} using a Bruker, Vertex 70, FTIR spectrometer. Magnetic properties of synthesized nanoparticles were assessed using a vibrating sample magnetometer at ambient temperature with increasing magnetic field up to 10 kOe and field sweeping from −10 to +10 kOe. Thermogravimetric analysis (TGA) was done to determine the presence and quantification of organic compounds from leaf extract in the final IONPs product.

The dye degradation experiments were conducted as described in previous literature [28]. For degradation of methyl orange (MO) dye, all processes were carried out under atmospheric pressure and in single-use 10 ml reaction volume, by stirring magnetically at 150 rpm. In the typical experiment, 10 mg of synthesized nanoparticles was added to solutions containing 8 ml methyl orange (25 mg l⁻¹) and 1 ml H₂O₂ (10%). The concentrations of MO were determined at different times (15 min, 30 min, 60 min, 120 min, 180 min, 240 min, 300 min, and 360 min) using a Hitachi U-0080D UV–vis spectrophotometer (Tokyo Japan) at ${\lambda }_{{\rm{\max }}}=465\,{\rm{nm}}$ and calibration curve obtained by using the standard MO solution. In addition, the decomposition ability of hydrogen peroxide and blank sample was tested without nanoparticles. All experiments were performed in triplicate also to evaluate.

In order for the green synthesis of IONPs as an eco-friendly, simple, reliable and cost effective method, which is significant in increasing their biomedical application, Teucrium polium was selected as a model plant. By addition of 0.1 M ferric (III) chloride to aqueous leaf extracts of Teucrium polium a rapid color conversion in the solution from yellow to dark black was created, which incidentally indicates a significant reduction potential of Teucrium polium leaf extract and the formation of iron-containing nanoparticles. The nanoparticles were characterized by PSA, TEM, XRD, FT-IR spectra, VSM, and TGA.

The morphology of the surface of synthesized nanoparticles was studied by TEM (figure 1(A)). This technique was applied to visualize the size and surface morphology of nanoparticles. The nanoparticles were spherical and have diameters in the range of 5.68 to 30.29 nm. The TEM image of IONPs is given in particles appears to have a characteristic of spherical like morphology with some aggregation. The small dimensions of nanoparticles prepared more specific surface to volume ratio and then more effective dye degradation.

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The average size measurement of the green synthesized nanoparticles was done to evaluate the hydrodynamic diameter of IONPs complexes by particle size analyzer (figure 1(B)). The particle size distribution curve revealed that the particle size distribution of nanoparticles was approximately between 87 to 1000 nm with mean particle size of 284.5 nm.

It is noticed that the particles size measured by PSA method is very different from particles size observed by TEM micrograph. In the PSA technique, the hydrodynamic diameter of the particles is measured, and as seen in the TEM image, the synthesized nanoparticles are surrounded by a biological coating created by the extract that is calculated in the estimation of the particle size by the PSA method, or the dark core related to IONPs, whereas the light shell corresponds to biological coating created by the extract that is calculated in the estimation of the particle size by the PSA.

The phase and crystalline nature of synthesized nanoparticles were also screened by x-ray diffraction (XRD) technique (figure 2). The pattern was insufficient in distinctive diffraction peaks and proposed that green synthesized nanoparticles were amorphous in nature, which was also in agreement with reported literature, indicating their amorphous structure. A broad peak presented at around 2θ of 10°−20° could correspond to coated organic materials from the reaction media which are responsible for stabilizing the synthesized nanoparticles [21, 30].

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The FTIR spectrum of green synthesized iron nanoparticles is shown in figure 3. After complete bio-reduction of iron ions, the treated Teucrium polium leaf extract was centrifuged to isolate the IONPs from the compounds present in the reaction media.

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FT-IR spectra showed absorption peak positioned at about 609.5 cm⁻¹ attributed to the vibration of the Fe–O stretching band. Besides, the obvious band located at 2067.5 cm⁻¹ position in the spectra corresponds to the –OH groups, demonstrating the existence of H bond or carboxylic acid. Additionally, the band absorption at 1635.5 cm⁻¹ can be attributed to C=C ring stretching that is the revealing of nanoparticle functionalization with organic compounds.

Due to their low surface charge, IONPs tend to accumulate in an aqueous medium. Consequently, citrate is used as a stabilizing agent in the chemical synthesis of nanoparticles [31]. Aqueous extract of plants coated/ stabilized IONPs by low molecular weight organic acids (such as citrate, malate, and oxalate) which prevent the interactions of IONPs together and enhance the interactions with water molecules resulting in more colloidal stability.

The magnetic property of the IONPs was measured with VSM at room temperature from −10000 to +10000 Oe (figure 4). The magnetization curve for IONPs was a linear graph with no hysteresis loop, which indicates that the produced nanoparticles have paramagnetic properties [28].

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The weight loss of the IONPs was screened in the range of 30 °C–600 °C using TGA (figure 5). The TGA plot showed a two-step thermal decomposition. In the first step at below 200 °C, the mass of IONPs fluctuated around 100% corresponding to the removal of physically adsorbed water, while the main weight loss in the second step up to 200 °C due to decomposition of biomolecule compound coated nanoparticles was accrued.

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Methyl orange (MO) removal ability of synthesized iron oxide nanoparticles was evaluated (figure 6). The hydrogen peroxide did not show any significant activity alone even after 6 h, while in the reaction accompanied by iron oxide nanoparticles, MO was degraded and the color disappeared with 73.6% efficiency after 6 h. The spectral band of MO is red shifting from 465 nm to 490 nm after 15 min contact in the solution containing iron oxide nanoparticles with H₂O₂. This variation in the location of absorption band could be due to the protonation of the MO molecules to form azonium ion on the molecular structure. The combination of iron oxide nanoparticles and H₂O₂ can cause the fabrication of free hydroxyl radical (OH·). These radicals can attack and cleave of the azo bond (–N=N–) in the MO leading to decolorization of the dye solution [26, 29, 32]. It is well known that plant-mediated synthesized INPs using different leaf extracts have different potentials for dye removal in liquid environments. For instance, our previous study showed that iron oxide nanoparticles biosynthesized by aqueous leaf extract of Daphne mezereum were able to remove about 81% of methyl orange dye after 6 h reaction. In another study, Muthukumar and Matheswaran investigated the MO decolorization kinetics by using both green and chemically synthesized IONPs. The results showed that the removal efficiency of IONPs biosynthesized by Amaranthus spinosus leaf extract was about 98.6%. Whereas, the decolorization capability of the chemically synthesized IONPs by NaBH₄ was even lower than these particles [33].

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