Fabrication of New Fe₃O₄/PVA/(C₆H₇O₆Na)_n Nanohybrid Ferrogels for Antibacterial Applications

Abstract

New Fe₃O₄/polyvinyl alcohol (PVA)/sodium alginate (C₆H₇O₆Na)_n nanohybrid ferrogels for antibacterial applications were fabricated. The crystal and molecular structures along with optical and magnetic properties of the prepared samples were characterized. The antibacterial activity of the ferrogels against Bacillus subtilis and Escherichia coli was investigated using the agar dilution method. X-ray diffraction analysis showed that the Fe₃O₄/PVA comprised a PVA amorphous phase and a spinel-structured Fe₃O₄ crystalline phase. The Fe₃O₄/PVA crystallite size was 7.5–9.9 nm and the scanning electron micrographs showed that the Fe₃O₄/PVA agglomerated. The ferrogels were superparamagnetic with saturation magnetizations from 14.8 × 10⁻³ to 82.1 × 10⁻³ emu/g. The absorption of the ferrogels showed a bathochromic effect, accompanied by an increase in the bandgap from 2.09 to 2.18 eV with increasing Fe₃O₄ content in the ferrogels. The ferrogels demonstrated new potency as antibacterial agents against B. subtilis and E. coli, where their antibacterial performance increased with increasing Fe₃O₄ content.

Keywords:
Fe₃O₄/PVA/(C₆H₇O₆Na)_n; nanohybrid; ferrogels; superparamagnetic; antibacterial agent

1. Introduction

Over the past few decades, researchers have widely developed methods of preparing Fe₃O₄ nanoparticles (NPs) in various shapes and sizes and in the forms of powders, films, ferrofluids, and ferrogels. In particular, intensive research on Fe₃O₄ NPs in the form of ferrogels has been motivated by their advantageous properties compared with those of other forms, even ferrofluids. Theoretically, Fe₃O₄ ferrogels, which are gels with a cross-linked polymer network containing Fe₃O₄ NPs as a filler, offer specific advantages over conventional gels with respect to their high flexibility and high sensitivity to an external magnetic field, which implies that ferrogels can be easily positioned inside a living organism¹.

Fe₃O₄ NPs exhibit superior advantageous properties, including superparamagnetism, biocompatibility, and low toxicity. They are also inexpensive and easily synthesized², making such materials attractive in the biomedical field³, for example, as antibacterial agents for skin infections. In general, skin infections occur and often pose therapeutic challenges because of increasing concerns about multidrug-resistant bacteria⁴. Furthermore, gel-based skin antibacterial agents are particularly attractive because gels have advantages over powders and liquids. However, the Fe₃O₄-based antibacterial agents developed thus far have been based on powders. Therefore, the development of Fe₃O₄ ferrogels as antibacterial agents is strongly desired.

Fe₃O₄ ferrogels based on polyvinyl alcohol (PVA) have recently begun to attract researchers’ attention for possible use in various multipurpose applications^5–7. Recent research has shown that Fe₃O₄ ferrogels with a PVA matrix have excellent prospects for use in biomedical applications⁸. However, for skin antibacterial applications, PVA has a drawback because of its relatively high stiffness, which reduces antibacterial efficacy. Consequently, an appropriate biocompatible crosslinking agent that reduces the stiffness of Fe₃O₄/PVA ferrogels is urgently needed.

Sodium alginate, with chemical formula (C₆H₇O₆Na)_n, exhibits strong potential for use as a crosslinking agent that supports the antimicrobial performance of Fe₃O₄/PVA ferrogels. Theoretically, in addition to reducing the stiffness of PVA⁹, (C₆H₇O₆Na)_n offers other advantages of nontoxicity, biocompatibility¹⁰, bioresorption ability, biodegradability, good mechanical properties, and relatively low cost^11,12. Therefore, in the present work, we developed new Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels as antibacterial agents and investigated their crystal and molecular structures, optical properties, bandgaps, magnetic behaviors, and antibacterial efficacy.

2. Experimental Methods

2.1 Materials

The main precursors were FeCl₂·6H₂O, FeCl₃·6H₂O, HCl (12 M), NH₄OH (6.5 M), PVA (M_w ≈ 60,000), and (C₆H₇O₆Na)_n. All precursors were analytical grade to ensure high purity and quality of the samples. All precursors were purchased from Merck and were used without further purification.

2.2 Fabrication of Fe₃O₄/PVA Nanocomposites

The Fe₃O₄ NPs were prepared using a coprecipitation method following the procedure described in our previous work¹³. One g of PVA was dissolved in 20 mL of distilled water at 120 °C for 3 h using a magnetic stirrer with a speed of 720 rpm. Various masses of Fe₃O₄ (0.13, 0.26, 0.39, 0.52, and 0.65 g) were mixed with 2 mL of PVA solution at 120 °C for 30 min using a magnetic stirrer with a speed of 720 rpm to produce Fe₃O₄/PVA. The Fe₃O₄/PVA was collected after the mixture cooled to room temperature. The samples were labeled S1–S5 according to the respective masses of Fe₃O₄ (0.13, 0.26, 0.39, 0.52, and 0.65 g).

2.3 Fabrication of Fe₃O₄/PVA/(C₆H₇O₆Na)_n Ferrogels

The Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels were prepared through a sonochemical method. Three hundred mg of (C₆H₇O₆Na)_n was dissolved in 20 mL of distilled water at 120 °C under stirring using a magnetic stirrer with a speed of 720 rpm for 4 h, followed by a cooling process to room temperature. Next, 2 mL of (C₆H₇O₆Na)_n solution was reacted with the prepared Fe₃O₄/PVA (S1–S5 samples) at 120 °C for 30 min under stirring using a magnetic stirrer with a speed of 720 rpm. After the mixtures cooled to room temperature, the final products were obtained as solid-black gels in the form of Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels. The prepared Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels were labeled SF1–SF5 according to the respective S1–S5 samples. The schematic showing the construction of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels is shown in Figure 1.

Figure 1
Schematic showing the construction of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels.

2.4 Characterization

The structural, optical, and magnetic properties were characterized by X-ray diffraction (XRD) for S1–S5 samples, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) for S1–S5 samples, UV–Vis spectrophotometry for SF1–SF5 samples, Fourier transform infrared (FTIR) spectroscopy for SF1–SF5 samples, and vibrating sample magnetometry for SF1–SF5 samples. The antimicrobial performance of the SF1–SF5 samples against Bacillus subtilis (Gram-positive) and Escherichia coli (Gram-negative) was evaluated through the agar diffusion method. All characterizations were conducted at ambient temperature.

3. Results and Discussion

The XRD patterns of the Fe₃O₄/PVA used as the filler of the ferrogels (Figure 2) show that two phases were formed, i.e., amorphous and crystalline phases. The amorphous phase, with a broad diffraction peak detected at 2θ of approximately 19.5°, was attributed to PVA. This is consistent with the results of the previous work¹⁴. Meanwhile, the XRD patterns of the crystalline phase showed diffraction peaks at 2θ of 30.1°, 35.6°, 43.3°, 57.3°, and 63.0°, which represent the (200), (311), (400), (422), (511), and (440) planes of Fe₃O₄, respectively¹⁵. Furthermore, the intensities of the Fe₃O₄ peaks were observed to increase with increasing Fe₃O₄ content. Interestingly, the increase in Fe₃O₄ content led to a decrease in the intensity of the peak of the PVA amorphous phase because of the reduced PVA content. The crystallinity, i.e., the degree of regularity and structural order in the Fe₃O₄/PVA system, was found to increase from 22.8% to 61.0% with increasing Fe₃O₄ content.

Figure 2
XRD patterns of the Fe₃O₄/PVA.

The Fe₃O₄ crystal structure in this study exhibited a cubic spinel structure. The lattice parameters of the Fe₃O₄ in all samples were constant, with a value of a = b = c = 8.365 Å. In this crystal structure, the Fe²⁺ and Fe³⁺ ions are distributed in octahedral and tetrahedral sites. The distribution of these metallic ions determines the magnetic moments of Fe₃O₄, where the magnetic moments between ions in octahedral and tetrahedral sites are anti-parallel. Furthermore, the crystallite size (D) of the Fe₃O₄ of all samples was calculated using the Debye–Scherrer equation:D= 0.9λ/βcosθ, where λ, β, and θ are the wavelength of the X-rays, the full-width at half-maximum of the peak, and the diffraction angle, respectively¹⁶. The data analysis results in Table 1 show that D increased with increasing Fe₃O₄ content. The increasing Fe₃O₄ content tended to increase the extent of aggregation into clusters because of the concomitant increase in the van der Waals force. This result is consistent with the SEM images, as shown in Figure 3. The SEM images show that the Fe₃O₄ was covered with PVA, which led to aggregation¹⁷. As the Fe₃O₄ content was increased, the Fe₃O₄/PVA tended to form more clusters because of the reduced PVA content. Therefore, the Fe₃O₄ NPs, which have a high specific-surface-area-to-volume ratio, tended to agglomerate because of the interaction of magnetic particles or because of van der Waals force. In this regard, Roohani et al. showed that aggregation occurs as a result of hydrogen bonds between particles because PVA, which is rich in hydroxyl groups, cannot cover all of the hydrogen bonding sites of the Fe₃O₄ NPs¹⁸. The elemental composition of the samples is presented in Table 2. The elemental analysis results indicate that the Fe content increases and the C content decreases with increasing Fe₃O₄ used in the synthesis.

Table 1
Crystallite parameters of the Fe₃O₄/PVA

Sample	a = b = c (Å)	Crystallite size (nm)	Crystallinity (%)
S1	8.365	7.5	22.8
S2	8.365	8.0	30.6
S3	8.365	8.9	46.2
S4	8.365	9.4	53.4
S5	8.365	9.9	61.0

Figure 3
SEM images of the Fe₃O₄/PVA.

Table 2
Elemental composition of the Fe₃O₄/PVA

Sample	Wt. (%)
	Fe	O	C
S1	9.6	35.7	54.7
S2	12.9	37.1	50.0
S3	22.7	35.3	42.0
S4	29.9	30.5	39.6
S5	34.2	30.9	34.9

The transmittance spectra used to characterize the functional groups of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels are presented in Figure 4. The Fe–O stretching band originating from Fe₃O₄ is observed at 420–500 cm⁻¹ for the tetragonal sites and at 700 cm⁻¹ for the octahedral sites. Furthermore, stretching bands characteristic of PVA were detected at 857 cm⁻¹ (CC stretching)¹⁹, 1047–1147 cm⁻¹ (C–O)²⁰, 1350 cm⁻¹ (=C–O–C), and 2966 cm⁻¹ (CH₂asymmetric stretching)²¹. The stretching bands characteristic of (C₆H₇O₆Na)_n were detected in the range 1473–1560 cm⁻¹ (COO symmetric and asymmetric)²² and at 1708 cm⁻¹ (C=O)²³. Interestingly, the Fe₃O₄, PVA, and (C₆H₇O₆Na)_n, as the main components of the ferrogels, all contributed to the broad O–H band between 3305 and 3670 cm⁻¹.

Figure 4
FTIR spectra of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels.

The contribution of Fe₃O₄, PVA, and (C₆H₇O₆Na)_n to the functional bonding observed by the FTIR spectra initiates further discussion. The changes in intensities of chemical bands provide a clue on the mechanical reorientation of the molecules²⁴. The decreasing intensities of O-H band, CH₂ asymmetric stretching, =C–O–C, and C–O bonding is believed due to the decrease of PVA content. This trend is related to the increase of Fe₃O₄ and the presence of alginate. From the theoretical viewpoint, the presence of alginate reduces the stiffness of PVA-based system by virtue of its low elongation break²⁵. In other words, the long-chain structure of PVA in Fe₃O₄/PVA nanocomposites was destroyed by the addition of alginate. Zhang et al. claimed that one of the best ways to reduce the stiffness of fabric-reinforced PVA was by introducing Na-alginate²⁶. They discovered that mechanical strength and stiffness could be decreased due to the swelling behavior of composite hydrogels that diminish friction amongst interfibers²⁶. Furthermore, alginate is not only effective in reducing the stiffness of PVA-fibers, but also other polymers, such as poly-lactic-co-glycolic acid²⁷.

The absorbance spectra, as shown in Figure 5, were recorded to investigate the optical properties of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels. The peaks at 474 and 511 nm are consistent with the previous work²⁸. The absorbance intensity is associated with the number of particles present. Meanwhile, the wavelength of maximum absorption is associated with the particle size. Physically, these peaks represent the difference in energy levels between the σ and π* states²⁹. In addition, incorporating the Fe₃O₄ into ferrogels would increase the distribution of particles embedded in the (C₆H₇O₆Na)_n crosslinking agent. As a consequence, the conduction electrons on the surface of the Fe₃O₄ migrate from one atom to neighboring atoms so that the surface plasmon resonance shifts toward lower energies, which is associated with smaller wavelengths³⁰. In physics, this phenomenon is described as the bathochromic effect. The bandgap (E_g) of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n was calculated using the equation αhυ = A(hυ − E_g)^1/2, where α, h, υ, and A are the absorption coefficient, Planck’s constant, the frequency, and a constant, respectively³¹. The linear part of the curve was extrapolated to the intersection with the energy axis to determine the bandgap³². The data analysis is shown in Figure 6. The E_g of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels increased from 2.09 eV to 2.18 eV with increasing Fe₃O₄ content. This phenomenon is explained by Fe₃O₄ reduced dissociation as a result of its increased dispersion in the ferrogels. It implies that collisions among electrons in the magnetic particles were reduced, resulting in an increase of the energy on the surface. Other authors have reported an E_g of slightly greater than 2.2 eV for Fe₃O₄³³. This difference is attributed to the effect of the surface modification of Fe₃O₄ using PVA, which led to dispersion of the Fe₃O₄ in (C₆H₇O₆Na)_n to form nanohybrid ferrogels.

Figure 5
Absorbance spectra of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels.

Figure 6
Bandgaps of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels.

The magnetization curves of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels are depicted in Figure 7. The figure shows that all of the samples are likely to exhibit superparamagnetic character, as indicated by the characteristic S-like shape with the coercivity field close to zero³⁴. A quantitative analysis was carried out to fit the magnetization curves using the Langevin equation:

$$M=M_s\left(\coth \left(\frac{\mu H}{k_{B}T}\right)-\left(\frac{k_{B}T}{\mu H}\right)\right)$$ (1)

where M, M_s, μ, H, k_B, and T are the magnetization, saturation magnetization, magnetic moment, Boltzmann constant, and temperature, respectively. The magnetic parameters obtained from the data analysis are presented in Table 3. The magnetic moment of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels increased by the addition of Fe₃O₄ content. The magnetic moment plays an essential role in a material’s response to an external magnetic field. Interestingly, not only the increase in Fe₃O₄ content but also the increase in particle size contributed to the increase in M_s. Karaagac et al. have similarly reported that particle size strongly influences the M_s³⁵. Furthermore, the coercivity field also increased with increasing Fe₃O₄ content, which is attributable to the accompanying increase in particle size. Previous work has shown the same trend for Fe₃O₄/PVA with a ribbon-like structure³⁶.

Figure 7
Magnetization curves of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels.

Table 3
Magnetic parameters of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels

Sample	Ms (×10−3 emu/g)	Hc (T)	Magnetic moment (×10−19 J/T)
SF1	14.8	0.013	8.1
SF2	16.8	0.016	8.7
SF3	42.1	0.020	9.5
SF4	64.3	0.025	10.1
SF5	82.1	0.028	14.0

The antibacterial performance of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels against B. subtilis and E. coli is presented in Figure 8. This figure shows that the inhibition-zone diameter increases with increasing Fe₃O₄ content. The inhibition-zone is associated with the antibacterial performance of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels. Theoretically, the ability to destroy bacteria is strongly correlated with the ability of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n to generate reactive oxygen species (ROS)³⁷. ROS generate superoxide as a free radical that inhibits bacterial growth³⁸. Furthermore, in the case of Fe₃O₄ NPs incorporated into ferrogels, the inhibition-zone diameter was larger than that of pure Fe₃O₄. The enhanced bactericidal activity of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels is attributed to the better dispersion of the Fe₃O₄ particles in the ferrogels than in the pure Fe₃O₄, which tends to aggregate. In addition to ROS, particle size also plays an important role in bactericidal activity. Other authors have shown that Fe₃O₄ with small particle size and monodisperse shape exhibits better inhibitory activity against bacteria than Fe₃O₄ with large particle size and polydisperse shape³⁹. Theoretically, Fe₃O₄ NPs with an excellent particle size distribution are likely to penetrate more easily into the walls or pores of bacteria. As a comparison, Jalali et al. prepared Fe₃O₄ NPs by doping Co and Zn to strengthen their antibacterial potential; however, they observed an inhibition-zone diameter of only 6.4 mm for E. coli⁴⁰. In line with that, Salem and co-workers fabricated Fe₃O₄ NPs from marine algae; however, the inhibition zone diameter for E. coli was still approximately 6 mm and that for B. subtilis was approximately 5 mm. These values are still inferior compared with that of the ferrogels investigated in the present study.

Figure 8
Inhibition-zone diameter of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels.

The bactericidal mechanism of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels is presented in Figure 9. From a physics perspective, the negative surface charges on the Fe₃O₄/PVA nanocomposite particles repel the bacteria under electrostatic interaction mechanism. This repulsion makes the metal oxide NPs less effective in destroying bacteria. However, when Fe₃O₄/PVA are dispersed in (C₆H₇O₆Na)_n, the net charge at the surface of the particles becomes positive. This change in surface charge is beneficial in terms of catching the bacteria by Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels. Thus, the chance of ROS penetrating into the bacteria increases. This leads to increase in the diameter of the inhibition zone. Thus, the Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels exhibit superior antibacterial activity. Furthermore, because of the structural complexity of the Fe₃O₄ ferrogels, a theoretical explanation of the performance of the ferrogels related to their magnetic-sensitive behavior has not yet been established. Therefore, for the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels to be used on a large scale, further investigation is necessary, especially with respect to the optimum filler, matrix, cross-linker contents in the ferrogels.

Figure 9
Bactericidal mechanism of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n ferrogels.

4. Conclusion

We synthesized new Fe₃O₄/PVA/(C₆H₇O₆Na)_n nanohybrid ferrogels using a combination of coprecipitation and sonochemical methods. The XRD data indicated the presence of two phases: amorphous PVA and crystalline Fe₃O₄. The crystallite size and crystallinity increased with the addition of Fe₃O₄. The surface of the Fe₃O₄/PVA/(C₆H₇O₆Na)_n agglomerated because of the increase in Fe₃O₄ content, preventing PVA from covering it optimally. The nanohybrid ferrogels exhibited superparamagnetism, where the coercivity field and magnetization saturation increased with increasing Fe₃O₄ content. Remarkably, the antibacterial activity of the ferrogels was observed to increase with increasing Fe₃O₄ content, which was attributed to an increase in ROS penetrating the bacteria.

5. Acknowledgments

This work was financially supported by KEMENRISTEKDIKTI 2019 for AT.

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Publication Dates

• Publication in this collection
  05 Feb 2021
• Date of issue
  2021

History

• Received
  29 May 2020
• Reviewed
  17 Oct 2020
• Accepted
  27 Dec 2020