Functionalized Magnetic Nanomaterial Based on SiO₂/Ca(OH)₂-Coated Clusters Decorated with Silver Nanoparticles for Dental Applications

Abstract

In this study, an innovative dental functionalized magnetic nanomaterial was developed by incorporating hydrophilic magnetic clusters as an alternative to conventional isolated magnetic nanoparticles, introducing a novel structural and functional concept in dental applications. The ~100 nm magnetic clusters—composed of densely packed 7 nm Fe₃O₄ nanoparticles—were sequentially coated with a silica (SiO₂) layer (3–5 nm) to improve chemical and mechanical stability, followed by an outer calcium hydroxide [Ca(OH)₂] layer to enhance bioactivity and optical integration. This bilayer architecture enables magnetic field-assisted positioning and improved dispersion within dental resin matrices. Silver nanoparticles were incorporated to enhance antimicrobial activity and reduce biofilm formation. The synthesis process was environmentally friendly and scalable. Comprehensive physicochemical characterization confirmed the material’s functional performance. Saturation magnetization decreased progressively with surface functionalization, from 62 to 14 emu/g, while the zeta potential became increasingly negative (from −2.42 to −22.5 mV), supporting its ability to promote apatite nucleation. The thermal conductivity (0.527 W/m·K) closely matched that of human dentin (0.44 W/m·K), and the colorimetric analysis showed improved brightness (ΔL = 5.3) and good color compatibility (ΔE = 11.76). These results indicate that the functionalized magnetic nanomaterial meets essential criteria for restorative use and holds strong potential for future clinical applications.

1. Introduction

Over the past decades, dental restorative materials have significantly developed to respond to more and more challenging clinical expectations regarding esthetics, biocompatibility, and long-term durability. Composite resins, which are widely used in restorative procedures, typically consist of a polymer matrix reinforced with inorganic fillers. Although these materials have shown remarkable advantages in mechanical strength and color stability, conventional composites still have some limitations. Challenges such as microbial infiltration, pulpal sensitivity, and poor marginal sealing remain common, often resulting in secondary caries, micro-leakage, and pulpal inflammation [1,2,3,4].

To overcome these issues, recent advances in nanotechnology have facilitated the incorporation of nanoscale materials into dental composites to provide multifunctional capacities. Among these, magnetic particles—in particular, superparamagnetic iron oxide nanoparticles—have appeared particularly promising. Incorporating Fe₃O₄ magnetic nanoparticles into dental composites enables the design of intelligent materials that are sensitive to external magnetic fields [5,6,7]. Furthermore, studies have shown that magnetic nanoparticles can enhance the filler–matrix interaction, thus improving the overall compressive strength, flexural modulus, and wear resistance of the composite materials [8,9]. In addition, the magnetic behavior of these particles can be used to reduce gaps and ensure uniform layer distribution—key factors for improving the quality and longevity of restorations. Biologically, Fe₃O₄ magnetic nanoparticles have also demonstrated promising results in minimizing biofilm formation, especially when combined with antimicrobial agents or surface modifications. This opens a new avenue for the development of bioactive restorative materials with inherent antibacterial properties, aiming to avoid restoration failure caused by secondary caries and ensure long-term performance [10,11].

However, one of the major challenges when using magnetic nanoparticles in polymeric matrices is that, in their uncoated state, they exhibit chemical instability and a strong tendency to aggregate, which can limit their dispersion, alter their surface interactions, and ultimately reduce their functional efficiency. Surface modification techniques have been explored to specifically address this problem. Coating the magnetic cores with silica (SiO₂) has been effective in stabilizing the nanoparticles, improving the interfacial compatibility with the matrix, and enhancing the mechanical properties of the composite [10]. The silica shell acts as a physical barrier that prevents magnetic dipole–dipole interactions, thereby reducing the tendency of particles to aggregate [12,13]. In parallel, calcium hydroxide, Ca(OH)₂, is a well-known material in dentistry due to its antibacterial properties, its ability to promote dentin genesis, and its protective effect on the dental pulp [14,15]. Its use in composites also improves radiopacity and provides thermal insulation, both clinically desirable characteristics. Recent studies have shown that magnetic materials can be used successfully in dental applications, helping to improve the placement accuracy and stability of restorations. These findings support the potential clinical use of multifunctional magnetic composites like the one developed in this study [16,17].

The aim of this study is to develop and characterize a scalable, magnetically responsive, and antimicrobial nanomaterial for potential use in dental restorative applications. Improving the magnetic performance of restorative materials by incorporating compact clusters of hundreds of self-assembled Fe₃O₄ nanoparticles, so-called magnetic clusters, represents a promising strategy to overcome the limited responsiveness of conventional isolated nanoparticles. A key advantage of this material lies in its magnetic responsiveness, which enables easy and precise placement under the influence of an external magnetic field during dental procedures. This targeted application strategy offers significant improvements over conventional materials by enhancing interfacial adaptation and reducing the risk of microfractures and secondary caries. The silica (SiO₂) intermediate layer reinforces mechanical and chemical stability, the outer Ca(OH)₂ shell ensures bioactivity and optical compatibility, and the surface-functionalized silver nanoparticles provide broad-spectrum antimicrobial protection. Each component of this multifunctional architecture addresses critical limitations of existing dental materials, including poor control during placement, insufficient long-term stability, lack of bioactivity, and vulnerability to bacterial colonization.

2. Materials and Methods

2.1. Material Preparation

Magnetic fluid including OA-stabilized Fe₃O₄ nanoparticles in a toluene suspension was obtained from the Magnetic Fluids Laboratory, Timisoara [18]. Sodium lauryl sulfate (SLS) was used for magnetic cluster synthesis; tetraethyl orthosilicate (TEOS, 98%), ethanol, ammonia solution (25%) for silica coverage; calcium nitrate tetrahydrate, Ca(NO₃₎₂·4H₂O, sodium hydroxide NaOH were applied for calcium based coverage and silver nitrate; and AgNO₃ for the silver nanoparticles synthesis. Double-distilled water and ethanol were used as solvents. All reagents were obtained from Sigma-Aldrich and utilized without further purification. The synthesis procedure is presented in detail in the Results Section, alongside each corresponding functionalization and characterization step.

2.2. Material Characterization

The morphology of the functionalized magnetic nanomaterial was investigated using a Hitachi HD-2700 transmission electron microscope, operating at an accelerating voltage of 200 kV. Samples were prepared by drop-casting a diluted dispersion of the magnetic composite in ethanol onto carbon-coated copper grids, followed by drying at room temperature. The particle size was estimated from the TEM images using ImageJ software (version 1.53k), based on measurements of at least 100 individual particles. The results are expressed as mean particle size ± standard deviation. The thicknesses of the surface coatings, SiO₂ and Ca(OH)₂, were determined directly from high-magnification TEM images using the integrated measurement tool of the microscope’s software. The EDX spectra were collected at an accelerating voltage of 15 kV. The magnetic properties of the functionalized magnetic nanomaterial were investigated at room temperature using a vibrating sample magnetometer (VSM, Cryogenics Ltd., London, UK). The powdered samples were pressed into small pellets and mounted in non-magnetic sample holders. Magnetization curves were recorded under a variable magnetic field with the magnetic induction (B) ranging from −5 T to +5 T. Prior to measurement, all samples were dried to remove any residual moisture that could influence the results. XPS spectroscopy was performed using a SPECS spectrometer equipped with a dual-anode Al/Mg X-ray source and a PHOIBOS 150 hemispherical analyzer (SPECS, Berlin, Germany). Survey scans were acquired at a pass energy of 30 eV with an energy step of 0.5 eV, while high-resolution spectra of selected elements were obtained by averaging 10 scans using a pass energy of 30 eV and a resolution of 0.1 eV/step. Prior to analysis, the powdered samples were dried on indium foil. Spectral processing and peak fitting were carried out using CasaXPS software (version 2.3.19), applying a Gaussian–Lorentzian (GL) line shape and a non-linear Shirley background correction. The high-resolution spectra were deconvoluted to identify the contributions of distinct chemical bonding states. The color parameters of the functionalized magnetic nanomaterial were determined using a UV–Vis spectrophotometer (Jasco, Japan) equipped with a color analysis module included in the instrument’s UV-option software package. Measurements were performed in reflectance mode, and the samples were analyzed against a white standard background. The color coordinates (L*, a*, b*) were obtained according to the CIE Lab system. All measurements were carried out at room temperature, and each sample was analyzed in triplicate to ensure reproducibility. Dynamic light scattering (DLS) and zeta potential measurements were performed with a NanoZS instrument (Malvern Panalytical Ltd., Malvern, UK) equipped with a 633 nm He-Ne laser. The stock sample suspension was diluted with a NaCl aqueous electrolyte solution to obtain a final solid concentration of 0.1 g/L. The pH of the suspension was adjusted in the range of 3 to 10, and each value was measured immediately before transferring the sample into the measurement cell. All measurements were conducted at room temperature, and each sample was analyzed in triplicate to ensure reproducibility.

The thermal conductivity of the functionalized magnetic nanomaterial was evaluated with a Hot Disk TPS 2500S thermal analyzer (Hot Disk AB, Gothenburg, Sweden), employing the transient plane source (TPS) technique. The samples were pressed into disk-shaped pellets and placed in direct contact with the sensor. Measurements were performed at room temperature under ambient conditions. Each reported value represents the average of three independent measurements to ensure reproducibility. To evaluate the electrical conductivity of the functionalized magnetic nanomaterial, the four-point probe method was applied with the aid of a Fluke 8845A 6.5-digit precision multimeter (Fluke Corporation, Everett, WA, USA). The samples, pressed into uniform pellets, were tested at room temperature. Conductivity was calculated from the measured resistance and sample geometry, with all measurements performed in triplicate.

The functionalized magnetic nanomaterial was subjected to antimicrobial testing against E. coli (ATCC 25922), E. faecalis (ATCC 29212), P. aeruginosa (ATCC 27853), and S. aureus (ATCC 25923) via diffusion-based methods. An aqueous dispersion of the materials (30 mg/mL) was prepared and 20 µL of the suspension was applied to Whatman filter discs for subsequent testing. All bacterial strains were incubated under aerobic conditions at 37 °C for 24 h. Subsequently, the functionalized magnetic nanomaterials were evaluated for their ability to inhibit biofilm formation by E. coli and P. aeruginosa. For this, the materials were compressed into 6 mm diameter discs and placed in contact with bacterial suspensions (0.5 McFarland standard in 0.85% NaCl) for 24 h at 35 °C under aerobic conditions. After this period, the suspension was removed, and the discs were transferred onto fresh nutrient agar plates and incubated for an additional 24 h.

3. Results and Discussion

A novel functionalized magnetic nanomaterial was developed, consisting of hydrophilic magnetic clusters coated with a double layer of silica, SiO₂, and calcium hydroxide, Ca(OH)₂, and decorated with silver nanoparticles for enhanced antimicrobial properties. The synthesis protocol integrates several methods, previously applied individually, into a simplified and integrated process designed to ensure the required structural, chemical, and functional properties of the final material. The protocol is presented schematically in Figure 1 and includes the following four consecutive steps:

• Step 1: Formation of hydrophilic magnetic clusters via direct oil-in-water miniemulsion;
• Step 2: Coating the clusters with a silica layer using the Stöber sol–gel method;
• Step 3: Deposition of an outer calcium hydroxide shell by reducing a calcium precursor;
• Step 4: Decorated magnetic dental composite material with silver nanoparticles.

Each synthesis step was designed to be simple, reproducible, and environmentally sustainable, contributing to the development of a composite material with enhanced functionality suitable for dental restorative applications.

Figure 1. TEM and SEM images illustrating the morphology of the initial magnetic precursors: (A) Fe₃O₄-OA nanoparticles coated with oleic acid; (B) size distribution histogram of Fe₃O₄ nanoparticles; (C) image of the magnetic ferrofluid prepared from Fe₃O₄-OA dispersed in toluene; (D) SEM image showing a general view of the hydrophilic magnetic clusters; (E) TEM detail of a hydrophilic magnetic cluster.

Figure 1. TEM and SEM images illustrating the morphology of the initial magnetic precursors: (A) Fe₃O₄-OA nanoparticles coated with oleic acid; (B) size distribution histogram of Fe₃O₄ nanoparticles; (C) image of the magnetic ferrofluid prepared from Fe₃O₄-OA dispersed in toluene; (D) SEM image showing a general view of the hydrophilic magnetic clusters; (E) TEM detail of a hydrophilic magnetic cluster.

1.

Step 1—Synthesis of Hydrophilic Magnetic Clusters (MCs)

Among the recent strategies to improve the magnetic performance of magnetic nanoparticles (MNPs), the formation of magnetic clusters stands out due to its multiple advantages. This approach involves the self-assembly of hundreds of nanoparticles into compact, cohesive structures, known as magnetic clusters (MCs), which exhibit enhanced collective magnetic behavior.

The preparation of hydrophilic magnetic clusters (MCs) was carried out using a direct oil-in-water miniemulsion technique and is illustrated schematically in Scheme 1 (step 1 of the reaction), [19]. In the first step, two immiscible phases were mechanically mixed using a glass spatula: an aqueous phase (100 mL) containing the stabilizing surfactant sodium lauryl sulfate (SLS, 0.2 g) and an organic oily phase consisting of a ferrofluid (MF/toluene containing Fe₃O₄ nanoparticles stabilized with oleic acid, Ms = 445 G, ρ = 1.318 g/cm³), corresponding to 0.5 g of magnetite (Fe₃O₄) nanoparticles. Emulsification of the two phases was achieved using an ultrasonic sonotrode (UP200S Hielscher) for 2 min at 50% amplitude in continuous mode, which generated fine droplets of the organic solvent (toluene) containing dispersed magnetic nanoparticles within the aqueous medium. SLS surfactant promoted micelle formation during the process, where the surfactant molecules self-organized with their hydrophilic functional groups exposed to the aqueous medium and their hydrophobic alkyl chains embedded in the organic droplets. This resulted in a stable miniemulsion, in which the toluene droplets associated and were coated with the surfactant, encapsulating the magnetic nanoparticles. The resulting colloidal solution was then heated for 1 h at 80 °C to evaporate the toluene from the micellar structures. The hydrophilic magnetic clusters thus obtained were magnetically separated from the reaction medium, washed three times by applying a water–methanol mixture in order to remove residual impurities, and finally re-dispersed as a stable aqueous colloidal solution of known concentration.

2.

Step 2—Synthesis of SiO₂-covered functionalized magnetic nanomaterial (MCs-SiO₂)

Although magnetic clusters (MCs) represent advanced magnetic structures with excellent magnetic properties, their chemical—and especially mechanical—stability is not high enough for practical use in dental materials. To address this limitation, a silica (SiO₂) coating of controlled thickness was applied on their surface. This coating is chemically inert and resistant to the action of the various environments that may be experienced in the oral cavity while improving the mechanical robustness of the clusters and contributing to the overall stability of the resulting functionalized magnetic nanomaterial.

The silica shell was synthesized using the classical Stöber sol–gel method [20], which is via hydrolysis and the subsequent condensation of tetraethyl orthosilicate (TEOS) in an alcoholic medium, with ammonia serving as a catalyst, and is presented schematically in Scheme 1 (step 2 of the reaction). In our approach, the process was adapted to facilitate the deposition of silica directly onto the surface of the previously obtained magnetic clusters.

In our composite system, the SiO₂ layer serves primarily to enhance the mechanical and chemical stability of the magnetic clusters, facilitating their uniform dispersion and integration within the dental resin matrix. Unlike crystalline silica, which is typically used for structural or thermal functions in prosthodontic investment materials, the amorphous SiO₂ coating in this case plays a protective and stabilizing role, supporting the long-term performance of the composite under physiological conditions.

Specifically, 400 mL of an ethanol–water solution (ethanol–water volume ratio 4:1) was prepared in a Berzelius beaker. To this solution, 1.5 g of the silica precursor TEOS was added under continuous magnetic stirring at 500 rpm. Subsequently, 4.7 mL of the aqueous colloidal solution of magnetic clusters, corresponding to 1 g of wet sample, was introduced into the reaction mixture. While maintaining the same stirring conditions at room temperature, 8 mL of 25% aqueous ammonia solution was added dropwise to initiate hydrolysis and condensation. The reaction system was magnetically stirred at 500 rpm for 1 h under ambient conditions to ensure the uniform formation of the silica layer on the magnetic cluster surfaces. The silica-functionalized magnetic clusters were magnetically isolated from the reaction mixture at the end of the process. The resulting material was washed three times with 100 mL portions of deionized water to remove residual reactants, followed by re-dispersion in distilled water at specific concentrations for further use.

3.

Step 3—Synthesis of Ca(OH)₂-covered functionalized magnetic nanomaterial (MCs-SiO₂-Ca(OH)₂)

A major inconvenience in integrating magnetic materials into aesthetic and restorative dental materials is their naturally dark colors. Magnetic nanoparticles typically exhibit a black color, which may visually disturb the appearance of the final functionalized magnetic nanomaterial in an inappropriate way, compromising its aesthetic acceptability. To address this limitation, an external coating able to mask the dark color and enhance the visual properties is essential. In this work, calcium hydroxide (Ca(OH)₂) was selected as an outer coating layer due to its dual function: it provides a significant improvement in the overall aesthetics of the final functionalized magnetic nanomaterial and contributes to pulpal desensitization by promoting local calcification and stimulating the development of secondary dentin. Additionally, its radio-opacity acts as an effective barrier to thermal and electrical conduction. The Ca(OH)₂ layer was deposited onto the MCs-SiO₂ composite surface through the direct reduction of a calcium-based precursor in an alkaline (NaOH) medium, as illustrated in Scheme 1 (step 3 of the reaction) [21].

To synthesize MCs-SiO₂-Ca(OH)₂, 94.5 g of calcium nitrate (Ca(NO₃)₂·4H₂O) was initially solubilized in distilled water to obtain an 8.5 wt% solution. Subsequently, 1.2 mL of a colloidal suspension containing 1 g (wet mass) of MCs-SiO₂ magnetic composites was introduced into the solution. An alkaline medium prepared with sodium hydroxide, prepared at 1.6% concentration and containing 16 g NaOH, was added dropwise at 1 mL/min under intense magnetic agitation (1000 rpm) at ambient temperature. Roughly 30 min into the addition, turbulence was observed in the mixture, accompanied by the appearance of a white solid of Ca(OH)₂ began to form. After allowing the reaction to proceed for 3 h under ambient conditions, the resulting mixture was filtered, and the precipitate was rinsed three times with 100 mL portions of distilled water. The resulting solid was oven-dried at 60 °C for 1 h as the final step.

4.

Step 4—Decorated functionalized magnetic nanomaterial with silver nanoparticles

The integration of silver nanoparticles into the functionalized magnetic nanomaterial matrix plays a crucial role in enhancing its antimicrobial properties, effectively inhibiting biofilm formation and thereby extending the longevity of the dental restoration in the oral environment. Thus, silver nanoparticles (Ag⁰) were deposited on the surface of the magnetic clusters functionalized with a bilayer of SiO₂ and Ca(OH)₂ via a chemical precipitation method. Silver nitrate (AgNO₃) was used as a silver precursor. The reduction of Ag⁺ ions to metallic silver (Ag⁰) is facilitated by the combined action of the alkaline environment provided by ammonia and the presence of ethanol, which acts as a mild reducing agent. This method enables the controlled in situ formation and uniform deposition of silver nanoparticles on the composite surface without the need for strong chemical reducers and is presented schematically in Scheme 1 (step 4 of the reaction) [22].

To functionalize the composite material with silver nanoparticles, 0.5 wt% of the dental composite was dispersed in a 5% ammonia–alcohol solution and sonicated for 30 min to obtain a homogeneous colloidal suspension. Subsequently, an alcoholic solution containing 7.5 wt% silver nitrate (AgNO₃) was added dropwise under continuous magnetic stirring at 500 rpm. The reaction was allowed to proceed at room temperature for a duration of 3 h. At the end of the process, the silver-functionalized composite was separated from the reaction medium using a magnet, washed three times with a water–methanol mixture to remove residual reagents, followed by thermal drying at 60 °C in an oven.

In order to validate the potential of this composite material for dental restorative applications, a comprehensive point-by-point characterization was performed, aimed at evidencing each relevant property—morphological, structural, mechanical, magnetic, and biological—in direct correlation with the functional requirements of dental restoration.

Morphological characterization of the functionalized magnetic nanomaterial was performed using electron microscopy in both transmission and scanning transmission modes (TEM and STEM). The morphological evolution was monitored step by step throughout the synthesis process, enabling detailed observations at each stage in the construction of the final functionalized nanomaterial. In Figure 1A, magnetite (Fe₃O₄) nanoparticles characterized by a mean size of ~7 nm (Figure 1B) are shown, uniformly dispersed, and coated with a monolayer of oleic acid. This ferrofluid (Figure 1C) served as the precursor source of magnetite nanoparticles for the subsequent synthesis steps. Due to its excellent colloidal stability, ensured by the individual coating of each nanoparticle with a protective oleic acid layer, the natural tendency of magnetic aggregation and/or agglomeration was effectively suppressed.

Figure 1D,E illustrate the successful formation of hydrophilic magnetic clusters, composed of densely packed nanoparticles aggregated into well-defined structures with a controlled shape and size, averaging around 100 nm. Due to the presence of an individual oleic acid layer on each nanoparticle, these clusters exhibit enhanced magnetic receptivity through collective effects as well as improved colloidal stability in aqueous media, making them ideal building blocks for further functionalization in a dental composite system.

Figure 2 displays electron microscopy images obtained using both scanning (SEM), which provides detailed information about the surface topography of the materials, and transmission (TEM), which reveals internal morphology and fine structural features at the nanoscale of the magnetic clusters at each stage of the functionalization process, highlighting the structural changes induced by successive surface modifications. In Figure 2A, the hydrophilic magnetic clusters appear uniformly coated with a thin silica (SiO₂) shell, measured to be approximately 3–5 nm, exhibiting a smooth and continuous surface characteristic of amorphous silica structures. The deposition process does not induce any visible aggregation or agglomeration as each magnetic cluster remains individually encapsulated within a homogeneous silica layer.

This uniform coating significantly enhances the colloidal stability of the system and contributes to improved chemical and mechanical robustness of the resulting material, providing a solid foundation for further functionalization or dispersion in aqueous environments. In Figure 2B, the deposition of a secondary coating layer based on calcium hydroxide [Ca(OH)₂] results in an increased thickness of the structurally amorphous layer surrounding the magnetic core in the range of 4–6 nm, confirming the contribution of the calcium-based layer.

The SEM image of the cluster assemblies appears relatively aggregated; however, this effect is attributed to the drying process during sample preparation on the measurement grid, which causes shrinkage and compaction of the sample rather than indicating true aggregation in dispersion. In Figure 2C, particularly in the TEM image, the presence of colloidal silver (Ag) nanoparticles can be observed, with an average size of approximately 5–7 nm. These nanoparticles are relatively uniformly distributed within the outer layer, suggesting a controlled deposition that ensures their optimal spatial arrangement to achieve the desired antimicrobial effect.

Figure 3 presents the surface elemental distribution within the final dental composite material, as revealed by EDX elemental mapping. The purpose of this analysis was to generate detailed data on the presence and spatial distribution of key functional elements. The results demonstrated a uniform and continuous dispersion of silicon, calcium, and silver across the composite surface, confirming the successful and homogeneous integration of each functional layer into the final material.

To confirm the successful formation of the functionalized magnetic nanomaterial, their chemical composition was investigated using X-ray Photoelectron Spectroscopy (XPS). XPS measurements were carried out using a SPECS spectrometer equipped with a dual Al/Mg anode and a PHOIBOS 150 2D CCD hemispherical electron analyzer. The X-ray source employed was Al Kα radiation with an excitation energy of 1486.6 eV. The investigated samples, prepared as colloidal solutions, were dried on indium foil attached to a sample holder using carbon tape prior to introduction into the analysis chamber. Measurements were performed at room temperature under a vacuum of approximately 2 × 10⁻⁹ Torr. XPS data analysis was conducted using the CasaXPS software.

Figure 4 shows the high-resolution XPS spectra, representing the intensity as a function of binding energy (B.E.) for C1s, O1s, Si2p, S2p, and Fe2p, obtained from a representative sample of magnetic clusters stabilized with sodium lauryl sulfate (SLS) and coated with SiO₂.

The deconvolution of the XPS spectra reveals the chemical states of the detected elements, confirming the formation of clusters with SLS and SiO₂ coatings. The C1s spectrum contains two components attributed to C–C/CH and C–O groups, originating from the oleic acid bound to the surface of the magnetic nanoparticles within the clusters. The O1s spectrum includes three components assigned to Fe–O, C–O/OSO₃, and COO groups. The presence of the surfactant sodium lauryl sulfate used in the cluster formation is evidenced by the OSO₃-related contribution in the O1s spectrum and by the S2p spectrum.

The silica coating is confirmed by the SiO₂-related component in the O1s spectrum and the distinct signal observed in the Si2p spectrum. Due to the presence of the SiO₂ layer, the intensity of the Fe2p spectrum is significantly reduced, preventing the identification of the components corresponding to the iron oxidation states.

Figure 5 shows XPS analysis performed at high resolution for C1s, O1s, Ca2p, and Si2p obtained from a representative sample of magnetic clusters stabilized with sodium lauryl sulfate (SLS) and SiO₂/Ca-bilayer-coated clusters. The Ca2p spectrum, along with the Ca–O and OH components in the O1s spectrum, confirms the presence of the calcium hydroxide coating on the clusters. Deconvolution of the C1s spectrum reveals three components attributed to C–C/CH, C–O, and CO₃ groups. The appearance of the CO₃-related component at 290 eV indicates the formation of a surface layer of CaCO₃. This C1s component decreases in intensity after argon ion sputtering, as shown in Figure 6, supporting the conclusion that a thin layer of CaCO₃ forms only at the surface of the particles.

Magnetic characterization was performed using a vibrating sample magnetometer (VSM, Cryogenics Ltd.). Figure 7 presents a comparative analysis of the magnetization curves corresponding to materials obtained at different stages of the preparation of the magnetic dental composites. Compared to the uncoated magnetic cores, a gradual decrease in the saturation magnetization was recorded after surface modification, as follows: Ms = 62 emu/g for hydrophilic magnetic clusters; Ms = 32 emu/g for clusters coated with a SiO₂ layer; and Ms = 14 emu/g for clusters functionalized with a bilayer composed of SiO₂ and Ca(OH)₂. Since the inorganic SiO₂ and Ca(OH)₂ layers are non-magnetic, the absolute value of the saturation magnetization is not very relevant in this context. Considering the expected application of this material in dental restorations, where it will be located on the tooth surface under the influence of an external magnetic field, the relevant parameter is the magnetic moment of the composite system, determined by the magnetic core, i.e., the hydrophilic magnetic clusters.

These clusters exhibit relatively high magnetization values compared to previously reported models, which makes the novel functionalized magnetic nanomaterial promising for various magnetic field-assisted dental applications.

Evaluating chemical stability is a critical factor in the evaluation of the functionalized magnetic nanomaterial for dental use, given their long-term exposure to the complex biological environment of the oral cavity, particularly the continuous pH fluctuations. To evaluate the material’s behavior under such conditions, its chemical stability was analyzed under different pH environments. Hydrodynamic diameters were obtained via dynamic light scattering (DLS) characterization of the functionalized magnetic nanomaterial in different solution media, providing valuable information on their colloidal stability and aggregation tendencies in aqueous media. Figure 8 illustrates the evolution of hydrodynamic diameters for the magnetic composites throughout the synthesis process, as determined under varying pH conditions.

Figure 8 shows that the hydrodynamic diameters of hydrophilic magnetic clusters (MCs) and inorganic-coated composites (MCs-SiO₂ and MCs-SiO₂-Ca(OH)₂) are larger than their physical dimensions estimated from TEM measurements. This difference is normal because the hydrodynamic diameter also includes the effect of how the material surface interacts with water. In this case, the sulfonic groups on the surface of the hydrophilic clusters as well as both SiO₂ and Ca(OH)₂ layers exhibit hydrophilicity and interact with water through hydrogen bonding. These interactions explain the higher hydrodynamic diameter compared to the actual particle size determined by TEM.

For the investigated materials, a more pronounced increase in the hydrodynamic diameter was observed for magnetic clusters coated with a SiO₂ layer compared to simple magnetic clusters, functionalized with sulfonic groups, and those coated with Ca(OH)₂. This difference can be explained by the influence of the interfacial electric double layer generated at the boundary between the solid surface and the liquid medium. The SiO₂ surface is rich in silanol (-SiOH) groups, which can partially ionize in aqueous media, resulting in the formation of negatively charged surfaces. These charges attract counterions in solution, forming an extended electric double layer. This hydrated ionic layer increases the overall thickness around the particle, resulting in a larger measured hydrodynamic diameter [23]. In contrast, sulfonic groups on the surface of the hydrophilic clusters are more compact and produce a less-extended double layer, while the Ca(OH)₂ coating—although hydrophilic—has a lower degree of ionization and tends to form a weaker and less-stable electric double layer due to its lower acidity and moderate solubility [24]. The observed differences in hydrodynamic diameter are influenced not only by the surface chemistry of the particles but also by their electrostatic interactions in aqueous media, which can be explained by electric double-layer formation and classical colloidal stability theories [25,26].

However, when analyzing the final functionalized magnetic nanomaterial coated with both SiO₂ and Ca(OH)₂ layers, no significant changes in hydrodynamic diameter were observed across the tested pH values. This indicates that the composite maintains good stability regardless of the pH, which is particularly relevant given that dental materials operate in the oral environment, where pH can vary considerably, especially in saliva.

An essential property of materials used in dental restorative procedures is their ability to induce calcium and phosphate ion accumulation on the enamel surface. Zeta potential analysis offers an indirect method for evaluating the surface bioactivity of the material in aqueous solution. It is well-established that negatively charged surfaces favor the nucleation of apatite and facilitate local mineralization processes [27,28].

As shown in

Table 1. Zeta potential values MCs, MCs-SiO₂, MCs-SiO₂-Ca(OH)₂ in deionized suspension, at pH = 7.

Sample	MCs	MCs-SiO2	MCs-SiO2-Ca(OH)2
Zeta potential value (mV)	−2.42	−16.9	−22.5

, the magnetic clusters (MCs) exhibit a slightly negative zeta potential, which increases in absolute value as the surface is progressively coated with the two inorganic layers. This trend suggests an enhancement in the material’s bioactive potential. The consistently negative zeta potential values observed, especially at physiological pH (pH = 7), support the expectation that the functionalized magnetic nanomaterial may contribute to site-specific calcification and initiate the formation of reparative dentin.

Effective thermal insulation is essential for dental restorative materials to prevent thermal stress on surrounding tissues. Materials exhibiting elevated thermal conductivity can be problematic, particularly when placed close to the dental pulp as they can transmit temperature variations caused by hot or cold food, resulting in discomfort for the patient. Thermal conductivity is the measure of the ability of a material to conduct heat and is usually measured as the amount of heat transferred per unit time through a temperature gradient of 1 °C [29]. In the present research, we propose a novel functionalized magnetic nanomaterial incorporating an outer layer of calcium hydroxide (Ca(OH)₂). The material has been commonly utilized for decades as a lining and underlining agent in restorative dentistry because of its therapeutic potential in pulp exposure cases and its ability to support the generation of dentin bridges [30,31]. The Ca(OH)₂ layer contributes to pulpal desensitization and stimulates the natural deposition of minerals and initiates the process of secondary dentin development. In addition, Ca(OH)₂ is radiopaque and provides effective thermal and electrical insulation, improving the overall performance of the composite in clinical applications [32].

In order to evaluate the thermal insulation efficiency of the proposed functionalized magnetic nanomaterial, its thermal conductivity was determined and analyzed in relation to human dentin and several commonly used dental materials. The MCs-SiO₂–Ca(OH)₂ magnetic composites were characterized by a thermal conductivity of 0.527 W/m·K, nearly equivalent to that of dentinal tissue, 0.44 W/m·K [33]. In comparison with standard dental restorative substances, our functionalized nanomaterial presented lower thermal conductivity values, detailed as follows: 0.77 W/m·K for composite resin, 1.20 W/m·K for enamel, 1.13 W/m·K for zinc phosphate cement, and 38.5 W/m·K for Au–Ag–Pd alloys [34]. The data confirm that the developed magnetic nanomaterial has a thermal conductivity value similar to dentinal tissue and substantially below those recorded for conventional dental substances. This suggests that this material provides effective thermal insulation, making it a promising candidate for use in restorative dentistry.

Although electrical conductivity is not usually a primary consideration for dental materials, it can affect patient comfort in certain situations. For example, contact between a metal tool and a fresh metal filling can cause pain because saliva acts as an electrolyte, allowing electric current to flow between the metals, especially in teeth with deep restorations and limited dentin insulation. For this reason, in addition to thermal insulation, dental restorative materials must also have good electrical insulation. In this study, the MCs–SiO₂–Ca(OH)₂ functionalized magnetic nanomaterial exhibited an electrical resistivity of approximately 160 MΩ·cm, corresponding to a conductivity of 0.0065 S/cm. This places the material within the insulating range and supports its suitability for clinical use, offering both thermal and electrical insulation.

To highlight the potential of the magnetic dental composites for use in aesthetic restorative applications, colorimetric measurements were carried out on the functionalized magnetic nanomaterial obtained at various stages of the synthesis process. Color, defined as a perceptual quality determined by the spectral composition of incident light, was analyzed using the CIE Lab color space, a standardized model that quantifies color differences based on human visual perception [35]. The CIE Lab* color system was developed to mathematically represent color differences based on human visual perception, using three parameters: L (lightness), a* (green-to-red axis), and b* (blue-to-yellow axis). The perceived color difference between two materials can be expressed using the ΔE value, which quantitatively reflects the distance between two colors in the CIE Lab* space (Equation (1)).

$$∆\mathit{E}=\sqrt{{\left({∆\mathit{a}}^{\mathit{*}}\right)}^2+{\left({∆\mathit{b}}^{\mathit{*}}\right)}^2+{\left({∆\mathit{L}}^{\mathit{*}}\right)}^2}$$

(1)

The spectral analysis covered the visible range of 360–830 nm, and the data obtained offer a relevant basis for evaluating the visual compatibility of the developed composites with natural dental tissues.

Table 2. Lightness values (L), chromatic coordinates (a* and b*), and color difference parameters (ΔE, ΔL*, Δa*, and Δb*) relative to a standard dental reference sample [36].

Samples	Absolute CIE Values (L, a*, b*)			Degree of Color Difference (ΔE) (#)
	L*	a*	b*	ΔE	ΔL*	Δa*	Δb*
Standard Dental Sample	77.28	4.16	19.47	-	-	-	-
MCs	43.97	6.62	8.79	35.06	−33.31	2.46	−10.68
MCs-SiO2	54.16	14.43	18.37	25.32	−23.12	10.27	−1.1
MCs-SiO2Ca(OH)2	71.94	9.29	10.33	11.76	−5.34	5.13	−9.14

^#ΔL = L*_sample−L*_reference; Δa = a*_sample−a*_reference; Δb = b*_sample−b*_reference; ΔE = Equation (1).

provides an overview of the lightness parameter (L), the chromatic coordinates (a, b), and the resulting color deviation (ΔE), along with the individual components ΔL*, Δa*, and Δb*. These values were calculated by comparing each tested sample with a reference standard (commercial dental sample [36]). The materials were evaluated at different stages of the magnetic dental composite preparation process, with the reference sample included for comparison.

From a colorimetric perspective, the parameter ΔL* represents variations in the lightness between samples. A positive ΔL* value suggests that the sample appears lighter than the reference standard, whereas a negative value implies a darker appearance. The chromatic components Δa* and Δb* correspond to shifts along the red–green and yellow–blue axes, respectively, within the CIE Lab color space. A positive Δa* indicates a redder hue relative to the reference, while a negative Δa* denotes a greener tone. In the same way, a positive Δb* reflects a shift toward yellow, and a negative Δb* points to a shift toward blue when compared to the reference sample.

Following the evaluation of colorimetric parameters, several observations can be made regarding the tested samples. The absolute value of the lightness difference (ΔL*) gradually decreases as the magnetic clusters with successive surface modifications, indicating an increase in brightness compared to the initial, uncoated form. This trend results in a final ΔL* value of just 5.3 units relative to the reference standard. A consistent reduction in the total color difference (ΔE) is also noted, which reaches a value of 11.76 units, suggesting enhanced color compatibility. The application of a dual coating layer composed of SiO₂ and Ca(OH)₂ visibly improves the optical characteristics of the material. Notably, the final composite exhibits a color profile that aligns well with the standard range accepted for dental restorative materials. It should also be noted that, in real-world dental applications, the magnetic composite is intended to be integrated into formulations together with other structural components, which are likely to further balance and refine the final aesthetic outcome.

The antimicrobial potential of the as-prepared functionalized magnetic nanomaterial was evaluated through standardized diffusion assays [37] performed against a panel of clinically relevant bacterial strains—E. coli (ATCC 25922), E. faecalis (ATCC 29212), P. aeruginosa (ATCC 27853), and S. aureus (ATCC 25923)—through diffusion-based methods. These species were selected due to their known involvement in oral and systemic infections, as well as their differing cell wall structures (Gram-negative and Gram-positive), which allow for a broader assessment of antimicrobial effectiveness. The results of the inhibition zone measurements are presented in

Table 3. Results of the antibiogram assay for the functionalized magnetic nanomaterial tested against selected bacterial strains using the diffusion method. Values are expressed in millimeters as mean ± standard deviation from two independent experiments.

	E. coli	E. faecalis	P. aeruginosa	S. aureus
MCs-SiO2-Ca(OH)2-Ag	7.2 ± 1.1	5.3 ± 0.5	2.2 ± 0.6	6.1 ± 0.8

and provide a comparative overview of the bactericidal performance of the tested materials.

To investigate the antimicrobial properties, a biofilm formation assay was carried out against Escherichia coli and Pseudomonas aeruginosa of the functionalized magnetic nanomaterial containing MCs–SiO₂–Ca(OH)₂-Ag. The results indicated a mild bacteriostatic effect as uniform biofilm formation was observed following re-incubation, with visible bacterial growth present both on the material’s surface and in the surrounding medium (Figure 9). Although the biofilm was not fully inhibited, these findings suggest that the tested material may contribute to reducing bacterial adhesion and proliferation, supporting its potential use as an antimicrobial component in dental restorative applications when combined with other strategies. Although full biofilm inhibition was not achieved, the observed bacteriostatic effect may be attributed to the presence of Ag nanoparticles. Ag NPs are known to exert antimicrobial effects through ROS generation and disruption of bacterial membrane integrity, mechanisms which can impair both planktonic and biofilm-embedded cells [38,39]. While such pathways were not directly investigated here, they support the rationale behind Ag incorporation.

Although cytotoxicity testing was not within the scope of the present study, the previous literature reports have shown that similar composites containing calcium hydroxide and magnetite nanoparticles exhibit low cytotoxicity and good biocompatibility in vitro [40,41,42,43]. Future studies will include dedicated cytotoxicity assays in collaboration with dental research groups to confirm the safety profile of the proposed material.

Direct shear bond strength testing was not performed in this study; instead, a related material developed in our previous work—based on Fe₃O₄ nanoparticles coated with a simple SiO₂ layer—was evaluated using optical microscopy in collaboration with dental professionals [44]. The results demonstrated that the presence of iron-based MPs enabled magnetic field-assisted manipulation, leading to a significant reduction in the thickness of the adhesive layer. This thinning effect positively influenced the sealing capacity by lowering microleakage risk at the tooth–restoration interface. Building upon these encouraging results, we are confident that the current material, further enhanced by the use of clustered magnetic structures—a dual SiO₂–Ca(OH)₂ coating—and silver ion functionalization will exhibit at least comparable, if not improved, performance once optimized for clinical use. We look forward to validating these assumptions through dedicated bond strength analyses in collaboration with our dental partners.

Compared to conventional restorative materials such as glass ionomers and resin composites, the proposed magnetic composite introduces added value through magnetic responsiveness, antimicrobial effects via silver ions, and bioactivity through Ca²⁺ release. Its structure, based on Fe₃O₄ magnetic clusters, a SiO₂ layer for mechanical reinforcement, and a Ca(OH)₂ coating for remineralization and improved aesthetics offers a multifunctional platform. The ability to guide the material using an external magnetic field opens new application strategies, potentially reducing microfractures and secondary caries. While still in the prototype stage, this system provides a promising basis for the future development of advanced dental materials.

In the context of potential clinical translation, the safety of magnetic field-assisted application must be carefully considered. Based on our previous work [44], where magnetic materials were preliminarily evaluated using both conventional and magnet-assisted placement methods, we anticipate that localized static magnetic fields up to 0.5 T, generated by permanent neodymium magnets, are sufficient to guide the material into confined spaces during application. Neodymium magnets are already employed in various medical and dental applications due to their ability to generate localized static magnetic fields. They are incorporated into devices used in chronic pain therapy, arthritis treatment, wound healing, insomnia, and headache management [45], and have been reported to support tissue regeneration and bone formation through osteoblastic activation [46,47]. In the dental field, neodymium magnets have been used in orthodontic motion-generating systems, including molar distalization and palatal expansion appliances [48,49]. Furthermore, NASA has used neodymium magnets to preserve muscular tone in astronauts during spaceflight, highlighting their long-standing biocompatibility and functional integration in physiological regulation systems [50,51]. These examples support the rationale that magnetic guidance of restorative materials, when properly controlled, can be both effective and safe.

4. Conclusions

A novel multifunctional dental composite was developed by integrating Fe₃O₄-based magnetic clusters, a SiO₂ reinforcement layer, a Ca(OH)₂ bioactive coating, and silver nanoparticle functionalization. The material exhibits good colloidal stability, magnetic responsiveness, and uniform structure, enabling magnetic field-assisted placement with potential to reduce microfractures and secondary caries. Its physicochemical properties, including thermal insulation, electrical resistivity, and surface charge, align well with clinical requirements, offering potential benefits in patient comfort and restoration durability.

Its physicochemical and antimicrobial properties make it a strong candidate for improving restoration durability and limiting bacterial colonization. The scalable synthesis approach provides a versatile platform for the future development of advanced dental and biomedical nanocomposites.