Fe3O4 Coated SiO2 Magnetic Nanoparticles for Enhanced Antibacterial Activity and Electrochemical Sensing

Abstract

Multifunctional magnetic composite nanoparticles (NPs) with antibiotics have demonstrated symbiotic effects because of their promising antimicrobial properties. The antimicrobial agent reduces side effects and dosage, and increases drug delivery efficiency. In this study, SiO₂ coated over Fe₃O₄ magnetic nanoparticles (MNPs) were prepared by a solvothermal method. The MNPs were characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), and Fourier transform infrared spectroscopy (FTIR). The antimicrobial tests were carried out using the disk diffusion method. The electrochemical sensing was investigated by cyclic voltammetry with varying As(III) concentrations from 1–10 ppb. The microstructural results showed the formation of spherical-shaped Fe₃O₄@SiO₂ MNPs with 15–30 nm diameters. UV-vis results showed that Fe₃O₄ NPs promote visible light absorption of Fe₃O₄@SiO₂ MNPs because of well-structured and unvarying shell thickness which is beneficial for the absorption of organic dyes. With an increase in the concentration of As(III), there was a shift in potential and an increase in oxidation peak current, showing the electrocatalytic capacity of the modified electrode. The SiO₂ deposited on Fe₃O₄ displayed an admirable microbial operation. These Fe₃O₄@SiO₂ MNPs are easily absorbed by cells and have the potential to influence bacterial cells both within and outside of the cell membrane, making them an intriguing candidate for use in a variety of biological applications in the future.

1. Introduction

The increased risk of bacterial infections is a serious issue for public health, extensively, which decreases the regulation of standard antibiotics in the body and importantly increases clinical complications [1,2]. The death rate has increased to 25% in the world gradually. Therefore, antibiotics have been used rigorously to cure bacterial infections [3,4]. Some examples of antibiotics used are ampicillin, nystatin, and amphotericin. Contrary to how antibiotics like ampicillin impact the kidney, neurological system, and blood, nystatin is hydrophobic and prevents microbial infections. Therefore, to achieve the maximum benefit of antibiotics, increasing drug efficiency is vital for cellular growth. New antibacterial agents produced using nanomaterials have been developed in the past. Nanoparticles (NPs) have unique properties such as surface-to-volume ratios, and different physical–chemical and biological properties, and also provide wider usage in biomedical applications [5,6,7]. Popular iron oxide MNPs are highly susceptible to aggregation within the cellular environment. Thus, to protect from the aggregation of iron oxide NPs, various noble metal and oxide (Au, Ag, SiO₂, etc.) NPs have been coated over iron oxide NPs. The type of coating generally depends on the application. The right coating enables MNPs to target particular areas while remaining non-toxic. In the last decades, there have been many nanomaterials used as antibacterial agents such as Ag [7,8,9,10], Cu [11], Au [12,13], ZnO₂ [14], SiO₂ [15], and graphene oxide [16,17]. Among all of these, SiO₂ is the most exceptional antibacterial agent due to its high toxicity for microorganisms and low toxicity for human beings.

In addition, the most significant transition metal oxides with various technological implications are iron oxides. Various kinds of iron oxide polymorphs can be found in the natural world. The most prevalent polymorphs of iron oxides are hematite (α-Fe₂O₃), magnetite (Fe₃O₄), and maghemite (ϒ-Fe₂O₃) [18]. Magnetite Fe₃O₄ is the most fascinating of all iron oxides due to the presence of iron cations in two valence states, Fe³⁺ and Fe²⁺, in the inverse spinel structure. The cubic spinel Fe₃O₄ exhibits ferromagnetism below 585 °C [19]. However, two significant problems with magnetite NPs include fast agglomeration and oxidation by airborne oxygen. The most popular method of surface modification for conjugating organic or inorganic molecules to iron oxide nanoparticle (IONPs) surfaces is coating. This technique not only keeps IONPs from oxidizing and clumping together, but it also offers the chance for additional functionalization [20].

Around the world, heavy metals are a major cause of groundwater pollution where arsenic (As), one of these heavy metals, is a major factor in several illnesses. Skin lesions, skin cancer, Blackfoot disease, and ventricular fibrillation are a few examples of such skin disorders impacting many countries, e.g., Poland, Argentina, Chile, Vietnam, Cambodia, Malaysia, Taiwan, India, and Bangladesh [21]. Unfortunately, many of these places in Southeast Asia are underdeveloped and lack the resources to combat these issues. On these grounds, it becomes essential to choose a material with dual bactericidal mechanisms. In other words, the co-existence of Fe₃O₄ and SiO₂ NPs can activate antimicrobial as well as sensing mechanisms simultaneously which eventually lead to the development of a better antimicrobial behavior under visible light irradiation, seeming consistent with an earlier investigation on TiO₂-CuO NPs [22]. Both Fe₃O₄ and SiO₂ have been used in antibacterial and sensing mechanisms [23,24]. Due to this, it is necessary to evaluate the sensing performance of Fe₃O₄@SiO₂ using As(III) [25].

In this present work, SiO₂ is coated over Fe₃O₄ to protect from antibacterial growth. The Fe₃O₄@SiO₂ MNPs with a diameter of less than 20 nm have shown reduced antibacterial activity [21]. After coating with SiO₂, the properties of Fe₃O₄ change and it demonstrates chemical surface modifiability and low cytotoxicity [26]. Recent studies show the importance of active sites in the surface functionalization of MNPs. Qu et al. designed symmetric NiP₁N₃ atomic active sites for the electrochemical reduction of CO [27]. Recently, Huo et al. have synthesized Fe₃₆Co₄₄ bimetallic nanoclusters for catalytic hydrolysis of ammonia borane [28]. The surface of the SiO₂ shell at the Fe₃O₄ particle core provides the active sites for the attachment of bacteria. The catalytic active sites in the Fe₃O₄@SiO₂ MNPs are Fe⁺³. This Fe³⁺ ion possesses a Lewis acid character that further promotes surface modification with functional groups, bacteria, and other derivatives to demonstrate surface activity and antibacterial properties [29]. The Fe₃O₄@SiO₂ MNPs have a lower ability to eliminate Gram-positive bacteria than Gram-negative bacteria which is related to the strength of SiO₂ NPs opposing the microbial membranes. This stable nature of SiO₂ increases the difficulty in penetrating the Fe₃O₄ membrane for microbial activity [5,30,31]. Moreover, the main cause of human death is Gram-positive bacteria. The assemblage of SiO₂ coating becomes an impediment to antibacterial growth. Silica is the most auspicious and advantageous coating material, because it protects MNPs from agglomeration and oxidation at diverse pH values, improving chemical stability. Thus, SiO₂ has imperative properties, such as good compatibility, and hydrophilicity favors biomedical application [32]. SiO₂ has been frequently employed as a coating for magnetic nanoparticles because of its biocompatibility, nontoxicity, chemical inertness, extremely high specific surface area with many Si-OH bonds on the surface, non-magnetic characteristics, and biostability [33,34]. Since environmentally friendly technologies might lessen biological threats to living cells, they can be advantageous for the production of nano-antibacterial compounds [35]. Recently, several in-situ fabrication techniques were created in order to get rid of harmful reducing agents such as KBH₄ and hydrazine from the final system. Ag nanoparticles of various sizes and coverage were coated onto the surface of PS/PDA microspheres by leveraging the reducing property of polydopamine (PDA), and the resulting antibacterial agent displayed improved performance against Escherichia coli and Staphylococcus aureus [30]. The microemulsion method and Stober’s method, which involve alkaline hydrolysis of tetraethyl orthosilicate, are now the two main techniques for coating silica onto Fe₃O₄ NPs [18]. Micelles are used in the microemulsion technique to control and contain the coating. It creates core-shell NPs with a surfactant layer on the silica surface, partially negating the benefit of the silica surface’s simple bio-conjugations. Contrarily, core-shell Fe₃O₄@SiO₂ NPs without the need for a surfactant might be produced by the alkaline hydrolysis of tetraethyl orthosilicate (TEOS), which is stable and simple to disperse. The core-shell Fe₃O₄@SiO₂ NPs are unquestionably the best model NPs for the finding of bio-applications since they have a pristine silica surface. [14,36].

Recently, Fe₃O₄ MNPs have fascinated researchers because of their most promising magnetic properties, especially for medical applications such as targeted drug delivery, magnetic resonance imaging (MRI), hyperthermia therapeutic agents, and biomolecules separation because of low toxicity, biocompatible and surface-modifiable property [37,38]. Cu-based antimicrobial surfaces are enabling a revival in the fight against the epidemiology of healthcare-associated infections. Several strategies exist to create and change surfaces with superior antibacterial characteristics because of recent improvements in biotechnology and material science methodologies [39,40]. Though there are many other material or elements i.e., Au, Ni, Co, etc., we prefer using iron oxide because it has competitive advantages compared to others. Iron oxide is formed by a chemical combination of iron with oxygen, and it stands as the backbone of the current infrastructure. The synthesis of Fe₃O₄ is easily proceeded by the co-precipitation method using salts of ferric and ferrous under an inert atmosphere, using nitrogen gas keeping pH 12 for the solution. Iron oxide disperses well in a variety of liquid media with varying pH levels [41]. To improve the antimicrobial properties of Fe₃O₄, SiO₂ coatings have been investigated for enhancing their stability and biocompatibility [42]. The surface of the magnetic nanoparticles must be functionalized to increase their biocompatibility [33]. Silica has been utilized extensively as a coating for magnetic nanoparticles because of its biocompatibility, nontoxicity, chemical inertness, extremely high specific surface area with many Si-OH bonds on the surface, non-magnetic characteristics, and biostability [43,44]. One of the most used methods for silica coating is the Stöber synthesis method. The advantages of silica shell include its ease of further functionalization, improved water solubility, protection of the core from oxidation and degradation, increased resistance to highly acidic solutions, and ease of further conjugation with various functional groups. These features make it possible to couple and label bio targets with selectivity and specificity. The bio-toxicity analysis of the nanomagnetic silica-coated (Fe₃O₄@SiO₂) composite shows that it is biocompatible [45]. Fe₃O₄ NPs may therefore make good candidates for their prospective application in antimicrobial therapy. The objective of the current study is to create, characterize, and assess the antibacterial activity of microemulsion-produced Fe₃O₄ and silica-coated Fe₃O₄ MNPs [46]. We report on a modified Stöber approach to creating water-soluble core-shell Fe₃O₄@SiO₂ MNPs without the need for a surfactant in this study [36]. Environmentally friendly techniques are advantageous for creating nano-antibacterial agents since they can lessen the biological risk to cell life [35,47].

2. Experimental

2.1. Materials

Merck provided the tetraethyl orthosilicate (TEOS, 98%) and citric acid trisodium salt dehydrate (C₆H₅Na₃O₇.2H₂O, 99%), while Alfa Aesar provided the Ferric chloride hexahydrate (FeCl₃.6H₂O, 99%), ammonia hydroxide (25 wt%), ethylene glycol, and ethanol (99.9%). All the chemicals were of reagent grades. We used all of the chemicals as received without further purification.

2.1.1. Synthesis of Fe₃O₄ MNPs

Using salts of ferrous and ferric under an inert atmosphere created by nitrogen (N2) gas, we were able to create Fe₃O₄ MNPs. In 250 mL of ultra-pure de-oxygenated Millipore water (18.2 MΩ.cm), we varied FeCl₃ from 15 to 18 g and FeCl₂ from 5–7 g. The combined solution was stirred magnetically for two hours. After adding 2 M NaOH aqueous solution in the presence of N₂ gas, the solution precipitated at room temperature. The process was maintained at 700 °C for six hours with the solution’s pH remaining at 12.

According to Sophie et al., the precipitation of Fe₃O₄ was completed as anticipated at pH values ranging between 8 and 14. Following precipitation, the solution was cooled to room temperature, the precipitate was separated using a permanent magnet (2500 Gauss), and the precipitate was repeatedly washed with ultra-pure deoxygenated Millipore water until the pH of the solution was neutral. Finally, we used pure acetone to clean the Fe₃O₄ before drying it at 700 °C in a vacuum oven.

The overall precipitation reaction can be written as follows:

Fe²⁺ + 2Fe³⁺ + 8OH⁻ ↔ Fe (OH)₂ + 2Fe(OH)₃→Fe₃O₄↓ + 4H₂O

2.1.2. Synthesis of Fe₃O₄@SiO₂ MNPs

According to the Stöber method, core-shell Fe₃O₄@SiO₂ MNPs were created by hydrolyzing tetraethyl orthosilicate (TEOS) in the presence of Fe₃O₄ MNPs [48]. Using a solvothermal coating technique first, ethylene glycol (80 mL) and FeCl₃.6H₂O (3.2 g) were mixed by stirring for 30 min. Anhydrous sodium acetate (4.8 g) was then added to the mixture, and the mixture was stirred for the next 30 min. Yellow citrate (0.8 g) was also added to the solution mixture. After hydrothermal synthesis, the resulting black substance was cleaned and dried. Fe₃O₄ NPs (0.1 g) were dispersed in a water–ethanol mixture in 100:25 ratios to create the Fe₃O₄@SiO₂ structure. Next, NH₃.H₂O (1.5 mL) was added. The entire content was then submerged in an ultrasonic bath for 10 min. Following thorough mixing, 1 cc of TEOS was added, and the mixture was agitated for a further 6 h. The resultant product was then magnetized to separate it from the finished product, and it was then dried for 12 h at 80 °C in a vacuum oven.

3. Characterization

The characterization techniques were utilized to gain a better knowledge of surface chemistry and crystal structure. We employed X-Ray Diffraction (XRD) and UV-vis spectroscopy to study the structural and absorption characteristics of MNPs, respectively. Transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were also employed to evaluate the structural and absorption properties of MNPs at high resolution. The chemical bonds between the Fe₃O₄ core and the surface covering were revealed by FTIR findings. Finally, we performed antibacterial tests and electrochemical sensing studies. The samples for electron microscopy studies were prepared by ultrasonication of MNPs in ethyl alcohol and drying them on copper−carbon mesh grids.

3.1. Phase Evolution Studies

To study the phase evolution, we used XRD (Philips X’Pert PRO diffractometer) operated at 30 mA current and 40 kV. The XRD scans were performed in the range from 26–66° with the Cu Kα radiation (wavelength = 0.154 nm) and a scan rate of 0.05°/s. The phase analysis and evolution studies of various phases were done by matching the peaks in the XRD spectrum with the standard international center for diffraction Data- joint committee on powder diffraction standards (ICDD-JCPDS) database using X’pert High Score software provided in the XRD machine. The powder XRD diffraction technique was used to characterize magnetic nanoparticles and to gather basic data on the lattice parameter and size [49]. The well-known Scherer’s equation, $d=\frac{0.89\lambda }{\beta \cos \theta }$ was used to determine the particle size. Here, “d” stands for average crystal size, 0.89 is Scherer’s constant, λ is the X-ray wavelength, β is the full width at half-maximum of the diffraction peaks, and θ is the Bragg’s diffraction angle. The lattice parameter of Fe₃O₄ and Fe₃O₄@SiO₂ MNPs was calculated from Bragg’s law via the Nelson–Riley equation [49].

3.2. Surface Morphology Studies

Advanced transmission electron microscopy (TEM) was used to analyze the size and surface morphology of nanoparticles of each sample. To know surface morphology more deeply, transmission electron microscopy (JEOL-JEM 2100) operating at 20 kV was used. TEM images were acquired in bright field mode. The selected area diffraction patterns were recorded to examine the crystallinity of the produced nanoparticles. ImageJ version 4.0 was used to estimate the nanoparticle size and distribution.

3.3. UV-vis Spectroscopy

UV-vis spectroscopy (UV-vis) was carried out using a UV-vis spectrometer (Agilent Cary 60). The UV-vis diffuse reflectance spectra were recorded with solvent baseline correction in the range of 250 nm to 800 nm and a scanning rate of 24,000 nm min⁻¹.

3.4. FTIR Studies

FTIR studies were performed using a Perkin Elmer Spectrum One with a resolution of 4 cm⁻¹, using a scanning range of 500−4000 cm⁻¹ and a scanning rate of 0.20 cm/s.

3.5. Animicrobial Tests

Gram-positive (Bacillus pumilus), Gram-negative (Bacillus halodurons), and Candida albicans fungus were the targets of the antimicrobial testing. The various microbial strains were purchased from the biotechnology lab at SGT University. The microbial bacteria cultures were kept alive at a temperature of 35 ± 2 °C on nutrition Muller-Hinton agar. The bacteria cultures were kept in an appropriate media slant and kept at 51 °C until they were needed. In this study, dimethyl sulfoxide (DMSO) served as the negative control and the all-purpose antibiotic ampicillin served as the positive control. Additionally, Fe₃O₄@SiO₂ was tested for its antibacterial action using a variety of Fe₃O₄ concentrations, and the results were analyzed using a modified version of the agar well disc diffusion method from [30]. We made sterile nutritional plates and gave them a set amount of time to solidify (5 min). Using a chosen borer, 5 mm agar wells were punched in randomly chosen areas of various plates. A homogenous 1 mL inoculum suspension of Candida albicans, Bacillus pumilus, and Bacillus halodurons, as well as Gram-positive and Gram-negative bacteria, was applied to the agar plate surfaces. Furthermore, 15 mL and 20 mL of DMSO were used to solutionize 120 mg of pristine Fe₃O₄ and 150 mg each of Fe₃O₄ and Fe₃O₄@SiO₂ MNPs. As a result, solutions with concentrations of 15 mg/mL and 20 mg/mL were created. The agar well was then filled with 100 L of each prepared MNPs solution. The agar plates were set for overnight incubation at 35 ± 2 °C. Using a millimeter scale, the antimicrobial activity was assessed and recorded in mm with regard to the zone of inhibition. Around the agar well, crystal-clear inhibition zones were seen, which showed the development of the antimicrobial activity.

3.6. Cyclic Voltammetery Studies

Further, the electrochemical sensing of produced MNPs was carried out by using As(III) concentration from (1–10 ppb) by cyclic voltammetry studies. The measurements were conducted with a conventional three-electrode geometry system. The various electrodes used were a working electrode (unmodified or modified), a Ag/AgCl reference electrode saturated with 3 M KCl solution, and a platinum foil electrode as a counter, respectively. The different electrochemical measurements were carried out using Autolab potentiostat PGSTAT 302N (Metrohm Eco Chemie, Utrecht, The Netherlands). The Autolab machine was provided with the General Purpose Electrochemical Systems software module for data acquisition and analysis (software version 4.9). All the current-potential curves were referred to the AgAgCl reference electrode. The solution pH values were measured with a digital pH meter (model 827, Switzerland make).

4. Results and Discussion

4.1. UV-vis Spectroscopy

Figure 1 shows the UV-vis spectra of Fe₃O₄ and Fe₃O₄@SiO₂ MNPs. Due to a narrow band gap, Fe₃O₄ MNPs exhibit a broad band in the UV and visible spectrum [50]. Fe₃O₄ NPs were used to synthesize Fe₃O₄@SiO₂ MNPs, which resulted in a notable increase in visual absorption. This shows that Fe₃O₄ NPs efficiently increase Fe₃O₄@SiO₂ MNPs ability to absorb visible light due to their well-structured and uniform shell thickness, which may be important for the adsorption of organic dyes from the ambient. Previous studies have shown that the shell thickness of nanoparticles could effectively control the absorbance and antibacterial properties [51]. The visual absorbance dramatically enhanced following the fabrication of the Fe₃O₄@SiO₂ core-shell utilizing Fe₃O₄ NPs.

It was also seen that when the loading amount of Fe₃O₄ increased, the composites’ visual absorption gradually increased, and showing Fe₃O₄ could effectively promote visible light absorption. This shows that Fe₃O₄ NPs, which have a well-structured and homogeneous shell thickness, will actively participate in adsorption, as well as substantially increase the visible light absorption of Fe₃O₄@SiO₂. The formation of new peaks around 300 nm in the Fe₃O₄@SiO₂ core-shell are observed. A similar observation is reported by Nazarbady et al. [52], where a new absorbance peak was noticed in the case of 3-methacryloxypropyltrimethoxysilane around the wavelength of 300 nm. The authors attributed this observation to the extension of conjugation brought on by polymerization activity. In the present case, TEOS have been used for the fabrication of Fe₃O₄@SiO₂ MNPs and the peaks observed around 300 nm may be due to similar activity.

4.2. XRD Analysis

Figure 2 shows the XRD patterns of Fe₃O₄ and Fe₃O₄@SiO₂ MNPs. Fe₃O₄ XRD patterns showed sharp peaks, indicating that hydrothermal synthesis was successful in producing pure Fe₃O₄ MNPs. There was no other peak associated with the Fe₃O₄ NPs. It was discovered that the peaks of Fe₃O₄@SiO₂ MNPs were wider than those of Fe₃O₄ nanostructures. This indicated the refinement of the crystallite size of Fe₃O₄ after the introduction of SiO₂ NPs. The size of Fe₃O₄ NPs does not vary noticeably, but the SiO₂-shell coating increases because of the existing distance between NPs. According to the XRD patterns, magnetite’s typical crystal phase matches all of the highly crystalline peaks quite well. The presence of a bumpy peak between 20–30° is assigned to be amorphous SiO₂ [53].

The signals produced by Fe₃O₄ nanoparticles were thought to have been decreased by the SiO₂ layer. The fact that the XRD peaks for Fe₃O₄ and Fe₃O₄@SiO₂ MNPs were identical shows that the Fe₃O₄ structure was preserved in the core, where the SiO₂ cover did not affect the crystal structure of the Fe₃O₄ MNPs [54]. The Fe₃O₄ nanoparticle’s predicted mean crystallite size was discovered to be 25 ± 5 nm.

The crystal structure of the Fe₃O₄ NPs conforms to a cubic system with lattice parameters and d-spacing of 0.29 nm for the miller index of the major peak (311). For the lattice parameter and particle size of Fe₃O₄ NPs generated at varied temperatures, comparable findings were noticed. It was shown that when the temperature increased, both surfactants produced slightly larger NPs, which may be ascribed to the kinetics of agglomeration. The reaction temperature shift for such a method does not favor large particle formation. The mechanisms of growth and nucleation are significantly influenced by temperature. The particle size increases, and the particle size distribution becomes irregular as the temperature rises. Magnetite (Fe₃O₄) is a ferrimagnetic substance with a high Curie temperature of 592 °C among several nanoscale magnetic materials. In the past, navigators employed magnetite, one of the elements studied extensively, to locate the north pole of the earth. Fe₃O₄ loses its magnetic property above 700 °C, hence these materials can only be used below that temperature. For optimal use of ferrite NPs, one must have a thorough understanding of the stability, structure, thermodynamics, and reactivity of iron oxides at high temperatures [55].

As a result, the approach chosen makes it possible to synthesize the material at ambient temperature. It was discovered that both surfactants created slightly larger NPs as the temperature rose, which may be related to the kinetics of agglomeration. Large particle production is not encouraged by the reaction temperature change for such a technique. The temperature has a significant impact on the mechanisms of growth and nucleation [56]. When the temperature rises, the particle size increases, and the particle size distribution become erratic. The kinetic energy of collision increases as a result of an increase in collision frequencies, which gives nanoparticles a strong tendency to aggregate into larger particles and cross potential barriers between them. The apparent inverse spinel Fe₃O₄ MNPs has diffraction peaks at 30.21°, 35.63°, 43.34°, 53.71°, 57.40°, and 63.11°, which correspond to the orientations of face-centered cubic crystals (220), (311), (400), (422), (511), and (440) planes, respectively [57]. The rest of the peaks are identical to those seen in XRD patterns of Fe₃O₄, and the amorphous character of the peak correlates to SiO₂ materials [46].

4.3. FTIR Analysis

The FTIR analysis of Fe₃O₄ and Fe₃O₄@SiO₂ MNPs is demonstrated in Figure 3. The presence of Fe₃O₄ in the Fe₃O₄@SiO₂ NPs was confirmed by the IR band at 640 cm⁻¹ in the Fe₃O₄@SiO₂ spectra, which is caused by the stretching vibration of Fe-O-Fe. This was strengthened even more by the peak’s appearance at 1400 cm⁻¹, which was attributed to the stretching vibration of Fe-O-Si. The stretching and bending vibrations of O-Si-O were assigned to the bands at 1102 and 790 cm⁻¹ for silica-coated magnetic NPs, respectively [58].

The IR bands, which exhibit the vibrations M_t-O-M_o (600–550 cm⁻¹) and MO-O (440–470 cm⁻¹), respectively, where Mt and Mo represent the metal occupying tetrahedral and octahedral positions, again supported the existence of an inverse spinel type structure for Fe₃O₄ [33]. The presence of OH is shown in Figure 3 by the peaks at 3421 cm⁻¹, which are likely caused by air moisture. The bending vibration of (Si-OH) silanols is well suited to the infrared band at 960 cm⁻¹ [58]. Si-O-H stretching vibrations can be assigned the frequency at 1102 cm⁻¹ [59]. The signal at 1059 cm⁻¹ is attributed to Si-O-Si vibrations, according to the FTIR spectrum of Fe₃O₄@SiO₂ MNPs [60]. The result is consistent with those reported in the past where Alkoxysilanes’ 3-aminopropyl amino groups were attached to MNPs [61]. Two strong absorption bands at approximately 636 cm⁻¹ and 588 cm⁻¹ are indicative of the formation of magnetic nanoparticles. Additionally, the tetrahedral and octahedral sites of the spinel structure may be identified by the absorption bands at 545 cm⁻¹ and octahedral sites can be confirmed by the band at 588 cm⁻¹ [62].

4.4. Microstructural Analysis using TEM

The morphology and size of Fe₃O₄@SiO₂ MNPs were analyzed by TEM. The typical bright field image of NPs along with the particle size distribution is shown in Figure 4a,b. The bright field image of the Fe₃O₄@SiO₂ MNPs can be seen in Figure 4a. In Figure 4b, the histogram shows the particle size distribution, and the size of the particle varies from 15–30 nm with an average size of 21 nm.

The high-resolution TEM image shows the core-shell structure of Fe₃O₄@SiO₂ MNPs in Figure 4c. In Figure 4d, the selected area electron diffraction pattern (SAD) shows the different interplanar spacing of (220), (311), (400), (422), (511), and (440) with diffracting conditions and matched well with the XRD pattern, which also confirms that the prepared material is highly polycrystalline. The high-resolution TEM images shown in Figure 4e,f further depict the SiO₂ layer of the Fe₃O₄ core with a thickness of 2 nm. The measured interplanar spacing of the Fe₃O₄ is known to be 0.29 nm corresponding to plane (311) [43,63].

4.5. Antimicrobial Test

Antimicrobial activities of Fe₃O₄ and Fe₃O₄@SiO₂ MNPs examined by the disk diffusion method are illustrated in Figure 5. The test was performed in different cultures of Bacillus pumilus and Bacillus halodurons. The amount of sample had an inhibition zone Fe₃O₄ of 8.5 ±1.0 mm, 12 ± 1.5 mm, 13 ± 1.5 mm, 14 ± 1.0 mm with Fe₃O₄@SiO₂ MNPs 10 ± 1.2 mm, 12.5 ± 1.5 mm, 15 ± 1.0 mm, and 16 ± 1.8 mm with an increase in weight from 5 mg to 20 mg with the step of five in the Bacillus pumilus and Bacillus halodurans culture. Figure 5a,b shows the growth of Gram-positive and Gram-negative bacterial activity on Fe₃O₄@SiO₂ MNPs. The Bacillus pumilus grows in a nourishing environment depleted of SiO₂ in presence of some water activity; moreover, the Bacillus halodurans is a moderate halophilic bacteria. It has been observed that with the increase of SiO₂ concentration, the inhibition zone is also increased. The SiO₂ composited on Fe₃O₄ is a corporal contributory to the repression of the culture of Fe₃O₄ owning to magnetic dipole interaction. The bond of SiO₂ with the cell membrane in the inhibition zone can alter the structure of bacteria because of attraction in NPs in microbial activity; as a result, there is leakage of cellular content in microbial activity. The presence of SiO₂ on the Fe₃O₄ surface stops the antibacterial activity of MNPs; generally, SiO₂ acts as a molecular link between Fe₃O₄ and microorganisms.

Previous studies have shown that Gram-positive bacteria have thicker membranes as compared to Gram-negative bacteria, making the former more stable [64]. According to Chen et al., Escherichia coli was more susceptible to Fe₃O₄@SiO₂-Ag nanospheres than Staphylococcus aureus [64,65,66]. While Gram-negative bacteria are thin layered with a thickness of 8–10 nm and have a 20–30% lipid content, Gram-positive bacteria are thick layered or single layered with a thickness of 20–80 nm. It has been noted that the inhibitory zone grows along with an increase in SiO₂ concentration. Due to its biocompatibility, biodegradability, and surface-modifiability, Fe₃O₄ is the most promising magnetic material for medical applications [37,38]. In order to separate MNPs in water following antibacterial treatment, Fe₃O₄ superparamagnetic characteristics are mostly advantageous [65]. Additionally, according to Asab et al. [47], Gram-positive bacteria are typically more resistant to Fe₃O₄ NPs than Gram-negative bacteria concerning the composition of their cell walls, cellular physiology, metabolism, or level of interaction [66]. Moreover, Fe₃O₄ is loaded with the SiO₂ NPs in the form of Fe₃O₄@SiO₂ MNPs, which helps in removing the bacteria present over there. The combination of several antibacterial modalities helps in reducing the dosage, increasing efficiency, and reducing side effects [67].

4.6. Cyclic Voltammetry

The electrochemical sensing studies of the Fe₃O₄ and Fe₃O₄@SiO₂ were done by the cyclic voltammetry (CV) curves recorded using the As(III). Figure 6 shows the CV voltammogram response of Fe₃O₄@SiO₂ MNPs and Fe₃O₄@SiO₂ MNPs with varying As(III) concentrations from 1–10 ppb recorded at 10 mVs⁻¹ scan rate within the potential window of −0.2–0.8 V. It was seen that with different concentration of As in the solution, CV response curves change their shape and size appreciably. By increasing the concentration of arsenic, there was a shift in potential and an increase in the oxidation peak current, showing the electrocatalytic capacity of the modified electrode. As the pH 6.0 was close to the physiological solutions, the oxidation peak was lower in this study.

The curves of the electrodes trend towards cathodic and anodic peaks back to pseudocapacitive behavior as the redox reactions are established at the electrodes and the reversible reaction between the Fe²⁺/Fe³⁺ ions of the electrodes and the solution interface [68]. Fe₃O₄@SiO₂ MNPs displayed a well-defined oxidation wave. For Fe₃O₄@SiO₂ MNPs, the oxidation peak potential is noticed at +0.3 V. The anodic peak current response was higher when As(III) concentration was increased from 1 ppb to 10 ppb. The greater accessible surface area of the modified electrode was attributed to the electrocatalytic effect due to the nanoscale dimensions of the sample and the presence of SiO₂, which is essential for electrocatalytic oxidation [69,70].

Furthermore, we notice that the Fe₃O₄@SiO₂ provides a bigger CV response area than that of pristine Fe₃O. This indicates higher capacitive charge storage. The CV response peak of the hybrid Fe₃O₄@SiO₂ was larger than that of the pristine magnetite. Thus, it was inferred that SiO₂ coating enhances the capacitance of Fe₃O₄. At different gravimetric As(III) concentrations, the discharge curves of Fe₃O₄ and Fe₃O₄@SiO₂, and the potential shifted to the negative side gradually as depicted in Figure 6. According to a previous report, the specific capacitance of Fe₃O₄ and Fe₃O₄@SiO₂ samples can be calculated by using the following equation [68]:

$C_s=\frac{I\Delta t}{m\Delta E}$

Here, C_s, I, ∆𝑡, m, and ∆𝐸 are specific capacitance, discharge current, discharge time, the active mass of electrodes, and IR discharge drops. According to these findings, the specific capacitance of these prepared electrodes appears to fall as the current densities increase. This observation shows the pseudocapacitive behavior with favorable electrochemical sensing characteristics. A reduced specific capacitance can be attributed to the increase in current that reduces the discharge time of ions needed to enter through the electrodes. Additionally, these observations support the fact that the discharge time is reducing as the current density rises, signifying an increase in the drop voltage. The specific capacitance of Fe₃O₄ and Fe₃O₄@SiO₂ modified electrodes, calculated according to the above equation, were 166 and 201 A/g, respectively, at the charge current of 10 A/g. This shows that the SiO₂ coating on the Fe₃O₄ electrode is increasing the specific capacitance by enhancing the nucleation sites for energy storage. Therefore, the coating of SiO₂ to Fe₃O₄ offers significant electron transport on and within Fe₃O. All of these phenomena have led to the improvement of the electrodes for electrochemical sensing and reduced electrode resistance [71].

5. Conclusions

• The solvothermal approach has been used to successfully create Fe₃O₄ NPs coated with SiO₂ NPs. The XRD, UV-vis, and FTIR results demonstrate the creation of spinal Fe₃O₄@SiO₂ structure. A thorough examination of the TEM reveals the formation of Fe₃O₄ cell structure. Additionally, the size distribution of Fe₃O₄@SiO₂ MNP size distribution ranges from 10 to 30 nm.

• The antimicrobial tests showed a larger inhibition zone of Fe₃O₄ coated by SiO₂ as compared to the pure Fe₃O₄ NPs. Fe₃O₄@SiO₂ MNPs showed strong antibacterial characteristics by killing bacteria on the exterior, as well as inside, of their cell membranes.

• The electrochemical sensor based on Fe₃O₄ coated with SiO₂ NPs is capable of reducing and oxidizing arsenic with outstanding electrocatalytic activity. Fe₃O₄ covered with SiO₂ naturally enhances the sensitivity of the determination of arsenic (III) with a low detection limit because of its distinctive qualities, which include subtle electrical characteristics, good interaction, and strong adsorptive capacity.

• Varying the arsenic content resulted in a shift in potential and an increase in the oxidation peak current, which illustrates the improved electrode’s electrocatalytic capability.

• It was concluded that the combination of SiO₂ with the Fe₃O₄ NPs improves the antibacterial property of Fe₃O₄ and reduces the adverse effects. Additionally, the composite Fe₃O₄@SiO₂ can be used against bacteria as well as for the detection of arsenic pollutants as an electrochemical sensor.

Author Contributions

Conceptualization, M.K. and J.R.A.; methodology, M.; formal analysis, V.K. and S.N.; investigation, J.R.A., S.N., A.S.; writing—original draft preparation, M. and J.R.A.; writing—review and editing, M.K. and A.S.; supervision, M.K. and A.S.; project administration, A.S. and M.K.; funding acquisition, A.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not Applicable.

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

Acknowledgments

The authors are thankful to the Department of Material Science and Engineering, Indian Institute of Technology, Delhi for the help in XRD characterizations. The authors are also grateful to the SAIF, AIIMS, New Delhi, India for carrying out the TEM characterizations. MK and JRA are also thankful to Shree Guru Gobind Singh Tricentenary University Gurugram and LIET, Alwar for their support for this work.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Figure 1. UV-Vis spectra of Fe3O4 and Fe3O4@SiO2 MNPs samples.

Figure 2. XRD patterns of Fe3O4 and Fe3O4@SiO2 MNPs.

Figure 3. FTIR spectra of Fe3O4 NPs and Fe3O4@SiO2 MNPs.

Figure 4. TEM bright field image of (a) Fe3O4@SiO2 MNPs with (b) size distribution histogram of prepared NPs of 15–20 nm (c); (d) SAD pattern of Fe3O4@SiO2 MNPs; (e,f) High-resolution TEM imaging of (c).

Figure 5. (a) Antimicrobial activity on Fe3O4 in Bacillus halodurons culture; (b) Antimicrobial activities on Fe3O4@SiO2 MNPs in Bacillus pumilus culture.

Figure 6. Cyclic voltammetric response of bare Fe3O4@SiO2 MNPs, and Fe3O4@SiO2 MNPs with varying Arsenic concentrations from 1–10 ppb.

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