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Article

Synthesis and Antibacterial Evaluation of Silver-Coated Magnetic Iron Oxide/Activated Carbon Nanoparticles Derived from Hibiscus esculentus

by
Müslüm Güneş
1,
Erdal Ertaş
2,*,
Seyhmus Tumur
3,
Parvin Zulfugarova
4,
Fidan Nuriyeva
5,
Taras Kavetskyy
6,7,8,*,
Yuliia Kukhazh
6,
Pavlo Grozdov
6,
Ondrej Šauša
7,9,
Oleh Smutok
10,*,
Dashgin Ganbarov
11 and
Arnold Kiv
8,12
1
SBU Diyarbakir Gazi Yasargil Education and Research Hospital, Diyarbakır 21280, Turkey
2
Department of Food Technology, Vocational School of Technical Sciences, Batman University, Batman 72000, Turkey
3
Department of Civil Engineering, Dicle University, Diyarbakir 21280, Turkey
4
Department of Zoology and Physiology, Faculty of Biology, Baku State University, AZ1148 Baku, Azerbaijan
5
Department of Computer Science, Faculty of Science, Dokuz Eylul University, Izmiz 35390, Turkey
6
Department of Biology and Chemistry, Drohobych Ivan Franko State Pedagogical University, 82100 Drohobych, Ukraine
7
Institute of Physics, Slovak Academy of Sciences, 84511 Bratislava, Slovakia
8
South Ukrainian National Pedagogical University Named after K.D. Ushynsky, 65020 Odesa, Ukraine
9
Department of Nuclear Chemistry, Comenius University in Bratislava, 84215 Bratislava, Slovakia
10
Department of Chemistry and Biochemistry, Clarkson University, Potsdam NY 13699, USA
11
Nakhchivan State University, AZ7012 Nakhchivan, Azerbaijan
12
Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
 
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*
Authors to whom correspondence should be addressed.
Magnetochemistry 2025, 11(7), 53; https://doi.org/10.3390/magnetochemistry11070053 (registering DOI)
Submission received: 10 May 2025 / Revised: 16 June 2025 / Accepted: 19 June 2025 / Published: 21 June 2025
(This article belongs to the Special Issue Magnetic Nanoparticles and Nanocomposites for Bioanalytical and Biomedical Purposes)
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Abstract

:
The increasing prevalence of antimicrobial resistance alongside the pharmacological limitations and adverse effects associated with conventional antibiotics necessitates the development of novel and efficacious antimicrobial agents. In this study, magnetic iron oxide nanoparticles (MIONPs) were synthesized via a chemical co-precipitation method. Activated carbon (AC) derived from Hibiscus esculentus (HE) fruit was coated onto the nanoparticle surfaces to fabricate MIONPs/HEAC nanocomposites. To augment their antimicrobial properties, silver ions were chemically reduced and deposited onto the MIONPs/HEAC surface, yielding MIONPs/HEAC@Ag nanocomposites. Comprehensive characterization of the synthesized nanocomposites was performed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometry (VSM), dynamic light scattering (DLS), and zeta potential analysis. DLS measurements indicated average particle sizes of approximately 122 nm and 164 nm for MIONPs/HEAC and MIONPs/HEAC@Ag, respectively. Saturation magnetization values were determined to be 73.6 emu/g for MIONPs and 65.5 emu/g for MIONPs/HEAC. Antibacterial assays demonstrated that MIONPs/HEAC@Ag exhibited significant inhibitory effects against Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923, with inhibition zone diameters of 11.50 mm and 13.00 mm, respectively. In contrast, uncoated MIONPs/HEAC showed negligible antibacterial activity against both bacterial strains. These findings indicate that MIONPs/HEAC@Ag nanocomposites possess considerable potential as antimicrobial agents for biomedical applications, particularly in addressing infections caused by antibiotic-resistant bacteria.

1. Introduction

Nanoscience and nanotechnology are interdisciplinary fields dedicated to the design, synthesis, and application of materials with at least one dimension in the nanoscale range, typically between 1 and 100 nanometers. At this scale, materials exhibit unique physicochemical and biological properties, primarily due to their high surface area-to-volume ratios and the influence of quantum effects.
Nanoparticles (NPs) in particular display enhanced catalytic activity, improved mechanical and thermal stability, and distinctive optical, electrical, magnetic, and biological functionalities compared to their bulk counterparts [1,2,3]. These remarkable characteristics have established nanoparticles as promising candidates for a wide range of applications, especially in biomedicine, where they are intensively investigated for both diagnostic and therapeutic purposes. Recent research has increasingly focused on the precise engineering of nanoscale materials with well-defined compositions and morphologies, aiming to tailor their chemical and physical properties for advanced and targeted applications [4,5].
Among the various classes of nanomaterials, metallic nanoparticles (MNPs) have attracted considerable attention due to their broad functional versatility. Typically synthesized from metals with well-established therapeutic and catalytic properties, such as silver, gold, zinc, selenium, and palladium, MNPs have been utilized in applications ranging from drug delivery and catalysis to antimicrobial coatings and environmental remediation [6,7]. Notably, noble metal nanoparticles, particularly silver nanoparticles (AgNPs), have emerged as highly effective antimicrobial agents. They exhibit remarkable bactericidal, anti-inflammatory, antioxidant, and anticancer properties and have been shown to outperform conventional antibiotics in several comparative studies [8,9,10].
Activated carbon (AC) is extensively applied in environmental and biomedical domains owing to its high specific surface area, well-developed microporous structure, cost effectiveness, and environmental compatibility. The presence of surface functional groups, especially those containing oxygen and nitrogen, significantly enhances its chemical reactivity and adsorption capacity [11]. However, pristine AC may support microbial growth under certain conditions, which restricts its applicability in antimicrobial contexts. To mitigate this limitation, silver ions can be incorporated into the carbon matrix to produce silver-impregnated activated carbon nanocomposites, exhibiting enhanced antimicrobial efficacy. These hybrid materials have demonstrated promising potential in catalysis, filtration, and environmental remediation applications [12].
Additionally, magnetic iron oxide nanoparticles (MIONPs) have attracted significant attention owing to their superparamagnetic behavior and excellent biocompatibility, rendering them highly suitable for biomedical applications including targeted drug delivery, magnetic resonance imaging, and biosensing [13]. In accordance with green chemistry principles, sustainable synthesis methods employing plant-derived biomass have gained increasing interest as eco-friendly routes for the production of functional nanomaterials.
In this context, Hibiscus esculentus L. (commonly known as okra), a widely cultivated plant valued for its nutritional and medicinal properties, serves as a rich source of phytochemicals, including polyphenols, flavonoids, amino acids, fatty acids, and tannins. These bioactive compounds confer antioxidant, antimicrobial, and anti-inflammatory activities, making okra an excellent candidate for the eco-friendly synthesis of functionalized nanomaterials [14,15,16].
This study reports the green synthesis and comprehensive characterization of a multifunctional nanocomposite system comprising magnetic iron oxide nanoparticles supported on activated carbon derived from Hibiscus esculentus fruits (MIONPs/HEAC). The resulting material was further functionalized with silver ions to obtain MIONPs/HEAC@Ag, a nanocomposite engineered to integrate magnetic, adsorptive, and antimicrobial properties. The antimicrobial activity of the synthesized nanocomposites was systematically evaluated against Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922 using two complementary assay methods.
The novelty of this work resides in the integration of a naturally derived carbon support, magnetic nanoparticles, and silver ions into a single nanocomposite platform synthesized through an environmentally benign approach. This hybrid system provides a sustainable and efficient strategy for the development of advanced antimicrobial materials with promising applications in healthcare, environmental remediation, and biomedicine.

2. Materials and Methods

2.1. Chemicals

All reagents of analytical grade were obtained from Sigma-Aldrich (St. Louis, MO, USA), including ferrous chloride tetrahydrate (FeCl2·4H2O), ferric chloride hexahydrate (FeCl3·6H2O), silver nitrate (AgNO3), sodium borohydride (NaBH4), hydrochloric acid (HCl), sodium hydroxide (NaOH), ammonium hydroxide (NH4OH), and zinc chloride (ZnCl2). Throughout the study, deionized water was utilized for the synthesis of nanomaterials, preparation of solutions, and all related procedures.

2.2. Synthesis of HEAC

The Hibiscus esculentus fruits were initially rinsed with tap water, followed by 4–5 successive washes with deionized water to remove dust and contaminants. After cleaning, the fruits were air-dried in a fume hood (to maintain a sterile environment) at 25 °C for 96 h [17], then ground using an IKA M20 Universal grinder(Sigma-Aldrich, St. Louis, MO, USA), and stored for subsequent preparation of HEAC. Ten grams of Hibiscus esculentus fruit powder was mixed with 250 mL of 30% zinc chloride solution in a 500 mL beaker and mixed at 80 °C for one hour. The obtained product was washed several times to remove impurities after being cooled in the laboratory, and then freeze-dried. The dried powder was transferred to a porcelain crucible and carbonized in a muffle furnace at 500 °C for 2 h. After cooling to 25 °C in a desiccator, the resulting HEAC was washed with 0.1 N hydrochloric acid to remove residual remaining chlorine, zinc, and other ions. The obtained product was then rinsed with distilled water until neutral pH was achieved, freeze-dried, and stored in a dark place for further synthesis and characterization [18,19].

2.3. Synthesis of MIONPs/HEAC

The synthesis of MIONPs coated with AC derived from HE fruits was carried out with modifications to the protocol described in the literature [20]. Initially, 1.5 g of ferric chloride hexahydrate was added to 25 mL of ultrapure water in a 50 mL three-neck flask and stirred at 25 °C using a magnetic stirrer for 15 min. To prevent the hydrolysis of iron(III) ions, 2–3 drops of 37% hydrochloric acid solution were added dropwise during stirring. Then, 1.05 g of ferrous chloride tetrahydrate was added to the solution containing Fe3+ ions. The temperature was gradually increased to 90 °C and maintained under continuous magnetic stirring for 30 min. Subsequently, 10 mL of 26% NH4OH was added to the mixture, resulting in the formation of a black-colored product. The reaction solution was then mixed at the same temperature for an additional 30 min. Then, 100 mL of water containing 0.5 g of AC from HE fruits was added into the mixture. Stirring was maintained at 90 °C for an additional 60 min. After the reaction, the system was switched off and allowed to cool for 60 min. The formed MIONPs/HEAC nanoparticle solution was separated from the mixture using a neodymium magnet [13]. The product was washed 4–5 times with ultrapure water to remove any unreacted chemicals and then dried using a lyophilizer. The resulting MIONPs/HEAC nanoparticles were stored in a dark container for further characterization and experimental studies.

2.4. Synthesis of MIONPs/HEAC@Ag

To synthesize MIONPs/HEAC@Ag nanoparticles, 1 g of MIONPs/HEAC was ultrasonicated in 100 mL of ultrapure water. Then, 50 mL of 5 mM AgNO3 solution was added dropwise to the dispersion, which was maintained on a magnetic stirrer at 70 °C. Subsequently, this part of the mixture was stirred for an additional 60 min, and 0.6 g of sodium borohydride was rapidly introduced under continuous stirring to reduce Ag+ ions to Ag0. After the addition of sodium borohydride, the mixture was stirred at 70 °C for 3 h, leading to the formation of MIONPs/HEAC@Ag nanoparticles. The resulting nanoparticles were then separated from the solution using a neodymium magnet and cleaned 4–5 times with ultrapure water to eliminate any unreacted impurities [21,22]. The synthesis, characterization, and antimicrobial application mechanism of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles are illustrated in Figure 1.

2.5. Nanoparticle Characterization

Characterization of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles was carried out using various techniques. XRD analysis was performed with a Rigaku RadB-Dmax II, while FTIR spectra were obtained using an Agilent Cary 630 FTIR spectrometer (Agilent Technologies, Santa Clara, CA, USA) SEM imaging was performed with a LEO-EVO 40/Cambridge, and EDX analysis was conducted using a Bruker-125 eV/Berlin system. Particle size (DLS) and zeta potential were conducted using a ZetaSizer Nano ZS90.

2.6. Analysis of Antibacterial Effect of MIONPs/HEAC and MIONPs/HEAC@Ag

The antimicrobial activity of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles against E. coli and S. aureus was investigated using the disk diffusion and microdilution methods. Initially, bacterial cultures were grown in suitable media and incubated at 37 °C. Bacterial suspensions were adjusted to a concentration of 1.5 × 108 CFU/mL, in accordance with the McFarland 0.5 turbidity standard. For the disk diffusion assay, sterile paper discs were loaded with MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles and placed on Mueller-inton agar plates previously inoculated with the bacterial strains. Sterile ultrapure water was used as the negative control against the bacteria. The plates were incubated for 24 h at 37 °C to allow bacterial growth and interaction with the nanoparticles. Following incubation, the zones of inhibition were measured in millimeters using a caliper. In the microdilution assay, different concentrations of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles were prepared by serial dilution using a micropipette. Antibiotics diluted in the same manner served as a positive control. Bacterial suspensions, prepared according to the McFarland 0.5 standard, were added to the wells of a microplate containing the nanoparticle dilutions. After 24 h incubation at 37 °C, a growth control was used to evaluate bacterial growth in the absence of nanoparticles. The minimum inhibitory concentration (MIC) was determined by measuring the lowest concentration of nanoparticles that inhibited bacterial growth. This experimental approach provides a comprehensive analysis of the antibacterial properties of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles against S. aureus and E. coli by utilizing both disc diffusion and microdilution techniques [18,19].

3. Results and Discussion

3.1. Structural Characterization

3.1.1. FTIR Analysis

FTIR spectrometry was employed to investigate the synthesized HEAC, MIONPs/HEAC, and MIONPs/HEAC@Ag particles, with the corresponding spectra presented in Figure 2. In the FTIR spectra of the HEAC materials, a peak at 2113 cm−1 corresponds to the C=C stretching vibration, while a peak at 1990 cm−1 reveals the presence of the C=C=C allene structure, along with –OH and C groups at 1561 and 1401 cm−1. A peak at 1036 cm−1 is attributed to C–N stretching and N–H bending vibrations. For the MIONPs/HEAC nanoparticles, an absorbance peak at 3365 cm−1 is related to the –OH stretching vibrations, indicative of phenolic and alcoholic groups. When polysaccharides were incorporated into the MIONPs/HEAC@Ag nanoparticle, faint peaks at 2922 cm−1 appeared, indicating C–H stretching of aliphatic CH, CH2, and CH3 groups. Examining the spectra of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles, peaks at 2109 cm−1, 2113 cm−1, and 1990 cm−1 signify the presence of alkyne (≡C–H) and C–H functionalities. The carbonyl (C=O) peaks detected at 1636 cm−1 and 1654 cm−1 are linked to the presence of carboxylic acid and hydroxyl moieties, common in polyphenols, phenolic acids, and proteins. Furthermore, peaks at 1401 cm−1 reflect –OH and C–H bending vibrations, and those at 1039 cm−1 and 1077 cm−1 indicate the presence of C–O and C–N groups. A characteristic band at 536 cm−1 in both MIONPs/HEAC and MIONPs/HEAC@Ag spectra is indicative of the Fe–O bond, confirming the successful incorporation of iron oxide into the nanocomposites [13,18,19].

3.1.2. SEM Analysis

The surface morphology of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles was examined using scanning electron microscopy (SEM). Figure 3a,b present the SEM images of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles, respectively.
In Figure 3, the SEM micrographs of MIONPs/HEAC and MIONPs/HEAC@Ag reveal distinct surface morphologies and particle structures. The structure shown in Figure 3a displays a porous, low-density, relatively smooth, and layered surface morphology. Smaller granular particles are observed dispersed over and around these layered structures. The particle sizes in Figure 3a are more heterogeneous, presenting a complex architecture composed of both fine granular features and larger layered domains. Such morphologies are typical for materials exhibiting porous structures supported by carbon-based substrates. In contrast, Figure 3b shows a markedly increased agglomeration relative to Figure 3a, characterized by densely clustered spherical nanoparticles. This agglomeration is attributed to the deposition of silver (Ag) ions onto the MIONPs/HEAC surface, where the high surface energy of Ag nanoparticles facilitates aggregation. The relatively homogeneous size distribution observed in Figure 3b indicates a uniform dispersion of metallic silver (Ag0) across the surface. This uniformity likely results from interactions between Ag0 and the MIONPs/HEAC substrate, which mitigate the irregularities commonly present in porous and heterogeneous materials. The observed differences in porosity and surface morphology underscore the significant impact of Ag0 incorporation on the structural characteristics of the nanoparticles [23,24].

3.1.3. EDX Analysis

Energy-dispersive X-ray (EDX) spectroscopy was employed to determine the elemental composition of the surface of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles. The corresponding EDX spectra are presented in Figure 4, illustrating the presence and distribution of key elements in both nanoparticle samples.
The elemental composition of the synthesized MIONPs/HEAC nanoparticle was confirmed by EDX spectroscopy, revealing the presence of C (44.01%), O (20.27%), Fe (19.37%), Zn (14.99%), Ca (0.66%), Mg (0.35%), Si (0.33%), and Na (0.02%), which indicates the successful coating of HEAC activated carbon onto the magnetic MIONPs (Figure 4a). In contrast, the EDX analysis of the synthesized MIONPs/HEAC@Ag nanocomposite showed a composition of C (4.38%), O (32.04%), Fe (51.51%), Zn (5.73%), Na (0.07%), Cl (0.54%), and Ag (5.73%), confirming the successful deposition of silver onto the MIONPs/HEAC nanoparticles (Figure 4b) [25]. Elements other than C, O, Fe, and Ag are attributed to residual impurities remaining on the nanoparticle surfaces after washing.

3.1.4. DLS Analysis

Figure 5 presents the comparative particle size distributions of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles, as determined by Dynamic Light Scattering (DLS) after dispersing of 5 mg of each sample in 50 mL of ultrapure water using ultrasonication. The MIONPs/HEAC sample exhibited an average hydrodynamic diameter of approximately 122 nm, whereas the functionalization with silver ions led to a noticeable increase in particle size, with the MIONPs/HEAC@Ag sample reaching an average diameter of approximately 164 nm. The increase in particle size upon Ag functionalization can be interpreted through several structural and physicochemical mechanisms. DLS measurements inherently reflect account for the hydrodynamic diameter, which includes not only the solid core of the nanoparticle but also the surrounding surface layers, solvation shell, and any adsorbed molecules. In the case of MIONPs/HEAC@Ag, the surface deposition of Ag nanoparticles, either as a uniform shell or discrete clusters, contributes to the formation of a core-shell-type architecture, thereby increasing the overall hydrodynamic dimension. Moreover, silver functionalization may promote partial agglomeration or interparticle bridging, especially in the presence of bioactive molecules from the HEAC matrix that can interact with both the iron oxide core and silver nanoparticles. The DLS data strongly support the structural modification of MIONPs/HEAC following Ag functionalization, indicating the successful integration of silver and the transition toward a more complex, hybrid nanocomposite system [26].

3.1.5. Zeta Potential Analysis

Figure 6 presents the zeta potential measurements of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles. Zeta potential is a key parameter reflecting the surface charge of nanoparticles and plays a crucial role in determining the stability of colloidal suspensions. To ensure minimal agglomeration and clustering, it is necessary to achieve zeta potential values that are sufficiently positive or negative. The zeta potential values recorded for MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles were −16.8 mV and −16.3 mV, respectively, indicating moderate negative surface charges. These values suggest low tendencies toward agglomeration, contributing to the stability of the nanoparticle suspensions over prolonged periods. Accordingly, both MIONPs/HEAC and MIONPs/HEAC@Ag dispersions remained stable for several months, demonstrating the effectiveness of the synthesis and dispersion methods employed [18,19].

3.1.6. XRD Analysis

Figure 7 shows the XRD pattern employed to investigate and compare the crystalline phases of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles.
The X-ray diffraction (XRD) patterns of MIONPs/HEAC and MIONPs/HEAC@Ag magnetic nanocomposites are shown in Figure 7. Characteristic diffraction peaks corresponding to MIONPs/HEAC appear at (220), (311), (400), (511), and (440), while peaks observed at (111), (200), (220), and (311) are attributed to MIONPs/HEAC@Ag [27]. All diffraction peaks are in good agreement with standard reference data for MIONPs/HEAC (JCPDS 49-1287) and MIONPs/HEAC@Ag (JCPDS 65-3107). The average crystallite sizes of MIONPs/HEAC and MIONPs/HEAC@Ag were calculated using the Debye–Scherrer equation and found to be 21.85 nm and 22.40 nm, respectively. These values were estimated from the most intense peaks at 2θ = 35.44° and 35.50°, respectively. The slight increase in crystallite size after Ag incorporation indicates that silver does not significantly disrupt the crystalline structure of the MIONPs core, though it does contribute to a marginal size enhancement. Overall, the XRD results confirm the successful synthesis of both nanocomposites with well-defined crystalline structures, and verify the incorporation of silver into the MIONPs/HEAC matrix without inducing major phase alterations [13,18,19].

3.1.7. VSM Analysis

The magnetic properties of MIONPs and the MIONPs/HEAC@Ag nanocomposite were evaluated using vibrating sample magnetometry (VSM), and the corresponding magnetization curves are presented in Figure 8. The saturation magnetization of bare MIONPs was found to be 73.6 emu/g, whereas the MIONPs/HEAC nanocomposite exhibited a slightly reduced Ms value of 65.5 emu/g. This reduction in magnetic response can be attributed to several factors, including particle size variation, surface coating effects, and the presence of non-magnetic HEAC and adsorbed oxygen species. Despite the decrease in Ms, the MIONPs/HEAC nanocomposite retains sufficient magnetic responsiveness, enabling efficient separation from aqueous media using an external magnetic field. These findings confirm that the structural modification does not compromise the superparamagnetic nature of the MIONPs, thereby supporting their potential use in applications requiring magnetic recoverability [28,29].

3.2. Antibacterial Properties of MIONPs/HEAC and MIONPs/HEAC@Ag

The antimicrobial properties of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles were comprehensively evaluated using both the disk diffusion and microdilution methods. According to the results presented in Table 1, the MIONPs/HEAC@Ag compound in particular exhibited a markedly enhanced antibacterial effect against two model bacterial species, Staphylococcus aureus and Escherichia coli. Data obtained from microdilution tests revealed that these nanoparticles demonstrated greater inhibitory activity at lower concentrations, especially against S. aureus. These findings are consistent with the zones of inhibition observed in the disk diffusion method (Table 1). Upon examining Table 1, the observed differences in antibacterial activity can be attributed to intrinsic structural differences in the bacterial cell walls. Gram-positive bacteria such as S. aureus possess a thick peptidoglycan layer, forming a dense network reinforced by cross-linked peptide chains that provide mechanical strength to the cell wall. In contrast, Gram-negative bacteria such as E. coli possess a more complex cell wall architecture, comprising a thin peptidoglycan layer surrounded by an outer membrane enriched in lipopolysaccharides. This outer membrane acts as a selective permeability barrier to external agents, including nanoparticles, thereby limiting the entry of antimicrobial substances into the cell. Due to these structural differences, Gram-negative bacteria tend to exhibit greater resistance against nanoparticle-based antimicrobials. This explains the superior bactericidal performance of MIONPs/HEAC@Ag nanoparticles against S. aureus [30,31,32].
Silver-based metallic nanoparticles are extensively employed in antimicrobial research due to their remarkable bioactivity. Upon release or penetration into microbial environments, silver ions exhibit high reactivity and preferentially bind to microbial cell surfaces through electrostatic interactions. This initiates the production of reactive oxygen species (ROS), which induce oxidative stress by compromising membrane integrity and disrupting vital cellular processes. ROS also inflict damage on critical biomolecules, including RNA, DNA, and enzymatic systems, further impairing microbial viability. Notably, RNA is particularly susceptible to ROS-induced degradation, thereby amplifying the cytotoxic effects and ultimately leading to microbial cell death [33,34,35,36].
Figure 9 presents the antibacterial activity of MIONPs/HEAC@Ag against E. coli and S. aureus, which is attributed to the disruption of membrane integrity, oxidative stress induced by reactive oxygen species (ROS) generation, inhibition of protein biosynthesis, and DNA damage. These effects, primarily mediated by the silver component, ultimately lead to bacterial cell death.

4. Conclusions

The MIONPs/HEAC@Ag nanoparticles were synthesized by conjugating activated carbon (AC), derived from the fruits of Hibiscus esculentus, with magnetic iron oxide nanoparticles (MIONPs) prepared via the co-precipitation method. Subsequently, silver (Ag) ions were successfully incorporated onto the nanoparticle surface. Notably, no oxidation was observed during the synthesis of both MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles. The incorporation of AC and Ag into the nanoparticle structure was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy analyses. Moreover, the saturation magnetization of MIONPs decreased following AC and Ag incorporation, indicating the impact of these components on the magnetic properties of the nanocomposites. Both MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles exhibited antibacterial activity against Staphylococcus aureus and Escherichia coli, with the MIONPs/HEAC@Ag nanocomposite demonstrating enhanced antimicrobial efficacy. This improved activity is likely attributable to the synergistic effects of the magnetic properties and the bactericidal action of silver. In conclusion, MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles show significant potential as antibacterial agents for applications requiring effective bacterial inhibition.

Author Contributions

Conceptualization, M.G. and E.E.; methodology, E.E. and S.T.; software, P.Z.; validation, F.N., T.K., O.S.; formal analysis, Y.K. and P.G.; investigation, M.G., E.E., S.T., P.Z., F.N., T.K., Y.K., O.Š., O.S., P.G., D.G.; resources, M.G., E.E., T.K., O.S.; data curation, E.E., T.K., O.S.; writing—original draft preparation, M.G., E.E., S.T., P.Z., F.N., T.K., O.S., D.G.; writing—review and editing, T.K., O.Š., O.S., D.G., A.K.; visualization, T.K., O.S., A.K.; supervision, E.E., T.K., O.S.; project administration, E.E., T.K., O.S.; funding acquisition, E.E., T.K., O.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported in part by the Ministry of Education and Science of Ukraine (projects Nos. 0122U000850, 0122U000874, 0125U001054, 0125U002005, and 0125U002033), National Research Foundation of Ukraine (project No. 2020.02/0100), Slovak Grant Agency VEGA (projects Nos. 2/0166/22 and 2/0131/25), and Slovak Research and Development Agency (project No. APVV-21-0335). T.K. and Y.K. also acknowledge the SAIA (Slovak Academic Information Agency) for scholarships in the Institute of Physics of the Slovak Academy of Sciences in the framework of the National Scholarship Programme of the Slovak Republic. This work has also received funding through the MSCA4Ukraine project (grant No. 1128327), which is funded by the European Union. O.S. acknowledges US National Science Foundation (NSF) grant CBET-2235349, including IMPRESS-U supplement, and NSF-BSF grant CBET-2422672.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Magnetochemistry 11 00053 g001
Figure 1. Schematic illustration of the synthesis, characterization, and antimicrobial application mechanism of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles.
Figure 1. Schematic illustration of the synthesis, characterization, and antimicrobial application mechanism of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles.
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Figure 2. FTIR spectra of HEAC, MIONPs/HEAC, and MIONPs/HEAC@Ag.
Figure 2. FTIR spectra of HEAC, MIONPs/HEAC, and MIONPs/HEAC@Ag.
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Figure 3. SEM images of (a) MIONPs/HEAC and (b) MIONPs/HEAC@Ag nanoparticles, illustrating surface morphology and particle distribution.
Figure 3. SEM images of (a) MIONPs/HEAC and (b) MIONPs/HEAC@Ag nanoparticles, illustrating surface morphology and particle distribution.
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Figure 4. EDX spectra of (a) MIONPs/HEAC and (b) MIONPs/HEAC@Ag nanoparticles.
Figure 4. EDX spectra of (a) MIONPs/HEAC and (b) MIONPs/HEAC@Ag nanoparticles.
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Figure 5. Hydrodynamic size distribution of MIONPs/HEAC and MIONPs/HEAC@Ag nanocomposites determined by Dynamic Light Scattering (DLS) analysis.
Figure 5. Hydrodynamic size distribution of MIONPs/HEAC and MIONPs/HEAC@Ag nanocomposites determined by Dynamic Light Scattering (DLS) analysis.
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Figure 6. Zeta potential profiles of (a) MIONPs/HEAC and (b) MIONPs/HEAC@Ag nanoparticles.
Figure 6. Zeta potential profiles of (a) MIONPs/HEAC and (b) MIONPs/HEAC@Ag nanoparticles.
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Figure 7. XRD patterns of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles.
Figure 7. XRD patterns of MIONPs/HEAC and MIONPs/HEAC@Ag nanoparticles.
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Figure 8. VSM magnetization curves of MIONPs and MIONPs/HEAC nanoparticles measured at 300 K.
Figure 8. VSM magnetization curves of MIONPs and MIONPs/HEAC nanoparticles measured at 300 K.
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Figure 9. Representative scheme of antimicrobial mechanism of MIONPs/HEAC@Ag nanoparticles.
Figure 9. Representative scheme of antimicrobial mechanism of MIONPs/HEAC@Ag nanoparticles.
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Table 1. Antibacterial activity of MIONPs/HEAC and MIONPs/HEAC@Ag nanocomposites against Staphylococcus aureus and Escherichia coli. For comparative analysis, S. aureus strains were tested with colistin, and E. coli strains with vancomycin as reference antibiotics.
Table 1. Antibacterial activity of MIONPs/HEAC and MIONPs/HEAC@Ag nanocomposites against Staphylococcus aureus and Escherichia coli. For comparative analysis, S. aureus strains were tested with colistin, and E. coli strains with vancomycin as reference antibiotics.
MicroorganismDilution Method (µg/mL)Disc Method
Zone of Inhibition (mm)
AntibioticMIONPs/HEAC MIONPs/HEAC@AgControl Disc
Deionized waterMIONPs/HEACMIONPs/HEAC@Ag
E. coli2.0963.00.00.011.50
S. aureus1.0481.50.00.013.00
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Güneş, M.; Ertaş, E.; Tumur, S.; Zulfugarova, P.; Nuriyeva, F.; Kavetskyy, T.; Kukhazh, Y.; Grozdov, P.; Šauša, O.; Smutok, O.; et al. Synthesis and Antibacterial Evaluation of Silver-Coated Magnetic Iron Oxide/Activated Carbon Nanoparticles Derived from Hibiscus esculentus. Magnetochemistry 2025, 11, 53. https://doi.org/10.3390/magnetochemistry11070053

AMA Style

Güneş M, Ertaş E, Tumur S, Zulfugarova P, Nuriyeva F, Kavetskyy T, Kukhazh Y, Grozdov P, Šauša O, Smutok O, et al. Synthesis and Antibacterial Evaluation of Silver-Coated Magnetic Iron Oxide/Activated Carbon Nanoparticles Derived from Hibiscus esculentus. Magnetochemistry. 2025; 11(7):53. https://doi.org/10.3390/magnetochemistry11070053

Chicago/Turabian Style

Güneş, Müslüm, Erdal Ertaş, Seyhmus Tumur, Parvin Zulfugarova, Fidan Nuriyeva, Taras Kavetskyy, Yuliia Kukhazh, Pavlo Grozdov, Ondrej Šauša, Oleh Smutok, and et al. 2025. "Synthesis and Antibacterial Evaluation of Silver-Coated Magnetic Iron Oxide/Activated Carbon Nanoparticles Derived from Hibiscus esculentus" Magnetochemistry 11, no. 7: 53. https://doi.org/10.3390/magnetochemistry11070053

APA Style

Güneş, M., Ertaş, E., Tumur, S., Zulfugarova, P., Nuriyeva, F., Kavetskyy, T., Kukhazh, Y., Grozdov, P., Šauša, O., Smutok, O., Ganbarov, D., & Kiv, A. (2025). Synthesis and Antibacterial Evaluation of Silver-Coated Magnetic Iron Oxide/Activated Carbon Nanoparticles Derived from Hibiscus esculentus. Magnetochemistry, 11(7), 53. https://doi.org/10.3390/magnetochemistry11070053

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