The Synthesis, Characterization, and Antimicrobial Activity of Magnetite (Fe₃O₄) Nanoparticles by the Sol–Gel Method ^†

Abstract

Transition metal oxide (TMO) nanoparticles have emerged as promising materials for various applications including color imaging, magnetic recording media, soft magnetic materials, heterogeneous catalysis, and different fields of biomedical science. Apart from the TMO, Fe₃O₄ nanoparticles hold great promise in a variety of biomedical uses such as drug delivery, cell separation, and MRI imaging. Magnetite (Fe₃O₄) nanoparticles exhibit their potential as antimicrobial agents due to their unique properties and interactions with microorganisms. This study focuses on the synthesis, characterization, and evaluation of the antimicrobial activity of magnetite (Fe₃O₄) nanoparticles that are prepared using the sol–gel method. The Fe₃O₄ nanoparticles were synthesized through a facile and cost-effective sol–gel route, involving the ferric nitrate and ethanol as precursors. Different characterization techniques, including Energy-Dispersive X-ray Spectroscopy (EDAX), X-ray diffraction (XRD), and UV-VIS NIR spectroscopy were employed to analyze the compositional analysis, crystalline structure, and optical properties of the nanoparticles. The EDAX and XRD analysis confirmed that the synthesized nanoparticles are near to stoichiometry and formation of single-phase magnetite nanoparticles. The obtained bandgap of synthesized nanoparticles is 5.03 eV. Additionally, the antibacterial activity of synthesized Fe₃O₄ nanoparticles was evaluated against a panel of bacteria, which included both Gram-positive (i.e., Staphylococcus aureus) and Gram-negative (i.e., Enterobacter aerogenes) bacteria. Investigations into the nanoparticles’ biocompatibility and long-term effects would be crucial for their safe and effective utilization in real-world applications.

1. Introduction

Transition metal oxide nanoparticles have turned out to be widely explored for many applications such as color imaging, magnetic recording, soft magnetic materials, sensors, supercapacitors, heterogeneous catalysis, and different fields of biomedical applications. Magnetite (Fe₃O₄₎ is a natural mineral of iron oxide. The multiple phases of iron oxides are important in academic and industrial research areas. In recent years, there has been a growing interest in the synthesis and characterization of magnetite nanoparticles due to their unique properties and wide range of applications [1]. Magnetite nanoparticles are composed of iron oxide and exhibit magnetic behavior, making them attractive for various fields such as drug delivery, cell separation, imaging (MRI), and in vivo therapy technology [2,3]. Magnetite possesses an FCC structure, where Fe has mixed valency of Fe²⁺ and Fe³⁺. The chemical formula of magnetite can be written as [Fe³⁺]_tetra [Fe²⁺Fe³⁺]_octaO_4, which falls into the Inverse spinel group [4]. Magnetite can be synthesized using several methods including coprecipitation [5], microemulsion [6], thermal decomposition [7], hydrothermal [8], ultrasonic [9], and sol–gel [10] methods. The sol–gel method is a chemical method for synthesizing various nanostructures, especially metal oxide nanoparticles. The sol–gel method is cost-effective and allows for good control over the chemical composition as well as the surface area of the nanoparticles. The purpose of this work is the preparation of magnetite nanoparticles via the sol–gel method. The elemental composition, structural analysis, optical properties, and antimicrobial activity of the synthesized magnetite nanoparticles were investigated.

2. Materials and Methods

2.1. Materials

From LOBA CHEMIE PVT.LTD., Mumbai, India, the ferric nitrate (Fe (NO₃)₃·9H₂O and ethanol (C₂H₅OH) of analytical grade were obtained. The materials were used without any further purification.

2.2. Synthesis of Magnetite Nanoparticles

In this process, first, 1.0 M ferric nitrate was dissolved in 20 mL ethanol and was vigorously stirred for 2 h at 50 °C. Then, the prepared sol was heated to 70 °C to obtain brown gel. The gel was aged at room temperature for about 1 h, and then the xerogel was annealed at 200 °C for 3 h in furnace. The calcined sample was crushed as fine powder by using a mortar and pestle. Finally, brown color magnetite nanoparticles were successfully synthesized.

2.3. Characterization

The EDAX characterization of synthesized magnetite nanoparticles was carried out for determination of elemental composition. The X-ray diffraction spectroscopy (XRD) (Bruker, Ettlingen, Germany, D8 Advance) was used for determination of the structural analysis of magnetite nanoparticles. The absorption spectra of the magnetite nanoparticles were determined by UV-VIS spectroscopy (Perkin Elmer, Waltham, MA, USA, LAMBDA 1050+). Antimicrobial activity of synthesized magnetite nanoparticles was evaluated by standardized test, aiming to establish the minimum inhibitory concentration.

3. Results and Discussion

In order to describe the elemental composition of magnetite (Fe₃O₄) nanoparticles, EDAX characterization was employed.

Table 1. Percentage of elements in magnetite (Fe₃O₄) nanoparticles from EDAX.

Element	Weight%
O K	24.7
Fe K	75.3

shows the EDAX analysis of magnetite nanoparticles, which shows that the atomic weight % of Fe is 53.38 and the atomic weight % of O is 46.62 in the Fe₃O₄. Atomic weight % of Fe and O is close to the Stoichiometric composition of Fe₃O₄. An EDAX image of magnetite nanoparticles is shown in Figure 1.

In order to describe the structural property of magnetite (Fe₃O₄) nanoparticles, X-ray diffraction (XRD) was employed. The X-ray diffraction (XRD) pattern of magnetite (Fe₃O₄) nanoparticles is shown in Figure 2. The diffraction peaks at 2θ = 9.77°, 17.99°, 21.64°, 24.36°, 31.40°, 35.75°, 40.20°, 45.93°, 52.77°, 62.47°, and 70.82° can be assigned to (1 0 0), (1 1 1), (2 0 0), (2 1 0), (2 2 1), (3 1 1), (3 2 1), (3 3 0), (4 2 2), (4 4 0), and (6 2 0), respectively. The crystal structure of the obtained magnetite (Fe₃O₄) nanoparticles is cubic, and the lattice parameter obtained is a = b = c = 8.409 A⁰ and the obtained data are well matched with JCPDS No: 019-0629.

In order to describe the optical property of magnetite (Fe₃O₄) nanoparticles, UV-visible spectroscopy was employed. The absorbance spectra, absorption coefficient, refractive index, extinction coefficient optical band gap, Urbach energy, and skin depth were among optical parameters described. The produced magnetite (Fe₃O₄) nanoparticles were found to have absorption peaks within the typical UV-vis absorption range, with a lower absorption wavelength of 203 nm being observed.

The relation between the absorption coefficient (α) and the incident photon energy (hν) can be determined by using Tauc’s relationship as follows [11]:

$$\mathit{\alpha }\mathit{h}\mathsf{\nu }={{\mathit{\alpha }}_{\mathbf{0}}(\mathbf{h}\mathsf{\nu }-{\mathit{E}}_{\mathit{g}})}^{\mathit{n}}$$

(1)

where α₀ is a constant and known as the band tailing parameter, Eg is the optical energy gap, and n is also a constant, which is known as the power factor of the transition mode. The direct optical bandgap is shown in Figure 3, which was found to be 5.03 eV. The indirect optical bandgap is shown in Figure 4 and was found to be 3.38 eV.

The optical density of the absorbance is proportional to both the thickness of the samples and the concentration of the absorbing material. The optical density of the magnetite nanoparticles can be estimated by using this simple equation [11]:

$${\mathbf{D}}_{\mathbf{o}\mathbf{p}\mathbf{t}}=\mathsf{\alpha }\mathbf{t}$$

(2)

where t is the thickness of the sample. The plotting of the optical density (D_opt) against the incident photon energy (hν), which is found to be 1.34 eV, as shown in Figure 5. The skin depth (δ) is related to the absorption coefficient (α) by the following simple relation [11]:

$$\mathsf{\delta }=\frac{1}{\mathsf{\alpha }}$$

(3)

The plot of the skin depth against the energy(hν), which is found to be 3.29 eV, as shown in Figure 6. The Urbach energy is calculated by ln α against the energy (hν) plot, which is shown in Figure 7. The Urbach energy of magnetite nanoparticles is given by the following relation [11]:

$$\mathit{\alpha }={\mathit{\alpha }}_0\mathbf{e}\mathbf{x}\mathbf{p}\left(\frac{\mathit{E}}{{\mathit{E}}_{\mathit{U}}}\right)$$

(4)

where E_U is the Urbach energy; E is photon energy, and α₀ is constant. The Urbach energy of magnetite (Fe₃O₄) nanoparticles is 1.36 eV. The plot of complex dielectric against wavelength clearly illustrates the variation of the real and imaginary dielectric constants with wavelength as shown in Figure 8.The corresponding wavelength to intersection of real and imaginary dielectric function is 197.35 nm.

• Antimicrobial activity

The antimicrobial activity of the magnetite (Fe₃O₄) nanoparticles was assessed by the agar well diffusion method. The bacterial cultures Staphylococcus aureus and Enterobacter aerogenes were poured over N-agar plates with 1% (W/V) top agar. The plates were allowed to be solidified at room temperature, and four wells were bored by a sterile cup borer. For the antimicrobial activity demonstration, the following concentrations of Fe₃O₄ nanoparticles were selected—200 ug mL⁻¹, 150 ug mL⁻¹, 100 ug mL⁻¹, and 50 ug mL⁻¹. The agar wells were filled up with magnetite (Fe₃O₄) nanoparticles solution. The antimicrobial activity was evaluated based on the zone of inhibition that appeared around the agar well, and the diameter was measured in mm. As shown in Figure 9a, the magnetite (Fe₃O₄) nanoparticles effectively inhibited the growth of Enterobacter aerogenes and Staphylococcus aureus at higher concentrations. The inhibitory effect is concentration-dependent, and the minimum threshold for the growth inhibition was found to be between 100 and 150 ug mL⁻¹. The result indicates that magnetite (Fe₃O₄) nanoparticles that are synthesized in-house have the potential to be used as a bacteriostatic as well as a bactericidal agent.

4. Conclusions

Magnetite (Fe₃O₄) nanoparticles were prepared by the sol–gel method at 200 °C. The sol–gel method offers several advantages for preparation of magnetite (Fe₃O₄) nanoparticles. The EDAX analysis of the magnetite nanoparticles showed that the atomic weight % of Fe is 53.38 and the atomic weight % of O is 46.62 in the Fe₃O₄. It is close to the Stoichiometric composition of Fe₃O₄. XRD shows that the crystal structure of the obtained Fe₃O₄ nanoparticles is cubic, the lattice parameter obtained is a = b = c = 8.409 A⁰, and the obtained data are well matched with JCPDS No: 019-0629. From UV-Visible analysis, the obtained direct optical bandgap is found to be 5.03 eV, and the indirect optical bandgap is found to be 3.38 eV. The Urbach energy of magnetite (Fe₃O₄) nanoparticles is 1.36 eV. The result indicates that magnetite (Fe₃O₄) nanoparticles that are synthesized in-house have the potential to be used as a bacteriostatic as well as a bactericidal agent.