Hybrid Sonochemical–Pressure Fabrication of tri-element nanohybrids for advanced biological protection in resin and metal dental crowns

Abstract

Premature failure of dental crowns is frequently associated with bacterial colonization, secondary caries, and persistent biofilm formation at restoration interfaces. In this study, Sn–Ag–Al trimetallic nanohybrids were synthesized through a novel hybrid sonochemical–pressure-assisted approach and ultrasonically embedded onto metal and resin dental crowns to develop durable antibacterial and antibiofilm coatings. Structural and surface characterization using XRD, FE-SEM–EDX, BET, and Raman analyses confirmed the formation of crystalline nanohybrids with homogeneous elemental distribution, nanoscale morphology, and favorable surface properties for crown adhesion. Antibacterial evaluation revealed a concentration-dependent inhibitory effect against Escherichia coli and Staphylococcus aureus, with zones of inhibition increasing from 12.6 and 9.5 mm at 50 µg/mL to 23.4 and 21.9 mm at 250 µg/mL, respectively. The coated crowns exhibited strong antibiofilm activity, achieving up to 85.4 % inhibition against E. coli and 81.3 % against S. aureus. Cytocompatibility studies using human gingival fibroblasts demonstrated high cell viability at clinically relevant concentrations, confirming the biocompatible nature of the coating. Furthermore, FESEM–EDX analysis showed that the ultrasonically deposited trimetallic layer remained structurally intact after repeated saline-washing cycles, retaining significant antibacterial functionality and indicating excellent coating durability under simulated oral conditions. The enhanced performance is attributed to the synergistic antibacterial action of Sn, Ag, and Al combined with cavitation-assisted nanoparticle anchoring generated during ultrasonic processing. This work introduces a multifunctional, wash-resistant trimetallic coating platform that integrates sonochemistry and pressure-assisted synthesis for long-term protection of dental restorations. The findings highlight the potential of ultrasonic surface engineering in developing next-generation infection-resistant dental biomaterials and advanced restorative devices.

1. Introduction

Dental crowns fabricated from metal alloys and resin-based materials are widely used to restore severely damaged teeth. Despite advances in restorative materials and adhesive technologies, bacterial adhesion, marginal leakage, and biofilm accumulation at the tooth–crown interface remains major causes of restoration failure. Persistent microbial colonization can lead to secondary caries, gingival inflammation, and recurrent infections, reducing the longevity of dental restorations. Biofilm-forming microorganisms exhibit enhanced resistance to antimicrobial agents, making effective biofilm prevention a critical challenge in restorative dentistry [1]. Fig. 1 depicts graphical illustration of synthesis and application of Sn–Ag–Al trimetallic nanocomposite for dental crown coating.

Fig. 1.

Graphical illustration of Synthesis and Application of Sn–Ag–Al Trimetallic Nanocomposite for Dental Crown Coating.

Various antimicrobial approaches, including fluoride-releasing cements, antibacterial monomers, and drug-loaded coatings, have been explored for dental applications. However, their effectiveness is often limited by agent depletion, surface degradation, and insufficient durability under oral conditions [2], [3]. Consequently, nanotechnology-based antimicrobial coatings have emerged as promising alternatives capable of providing prolonged protection while maintaining mechanical integrity and biocompatibility.

Metallic nanomaterials, particularly Ag-, Sn-, and Al-based systems, have attracted considerable attention because of their broad-spectrum antimicrobial activity and physicochemical stability. Recent advances in nanomedicine have further demonstrated the potential of multifunctional nanomaterials, including MXene-based composites, nanosilver-supported mesoporous materials, and green-synthesized nanoparticles, for antibacterial, antibiofilm, and biomedical applications [4], [5], [6], [7]. Nevertheless, most reported studies focus on mono-metallic or bimetallic systems, which often suffer from limitations related to coating adhesion, ion leaching, and long-term stability [8].

Sonochemical synthesis offers distinct advantages through acoustic cavitation-induced nanoparticle formation, enhanced dispersion, and improved surface deposition. Although sonochemically engineered nanomaterials have shown promising antimicrobial performance, their application as durable trimetallic coatings for dental crowns remains largely unexplored [9]. Therefore, a significant knowledge gap exists in developing multifunctional trimetallic nanocoatings that simultaneously provide strong antibacterial activity, biofilm inhibition, coating durability, and cytocompatibility.

We hypothesized that a sonochemically synthesized Sn–Ag–Al trimetallic nanohybrid could provide synergistic antibacterial and antibiofilm activity while exhibiting enhanced coating retention and biological compatibility on dental crown substrates [10]. To test this hypothesis, Sn–Ag–Al trimetallic nanohybrids were synthesized using a hybrid sonochemical–pressure-assisted approach and ultrasonically deposited onto metal and resin dental crowns. Their structural, surface, antibacterial, antibiofilm, durability, and cytocompatibility properties were systematically evaluated. The novelty of this work lies in integrating hybrid sonochemical–pressure-assisted synthesis with ultrasonic coating technology to develop a multifunctional Sn–Ag–Al nanohybrid for infection-resistant dental restorations.

2. Materials and methods

2.1. Chemicals and reagents

Analytical-grade tin (II) chloride dihydrate (SnCl₂·2H₂O), 98 %, silver nitrate (AgNO₃),≥99 %, and aluminum nitrate nonahydrate (Al(NO₃) ₃·9H₂O),98 % were purchased from Sigma-Aldrich and utilized without further purification. Every experimental solution was made from scratch, and the double-distilled water (DDW) was pure and transparent. For antimicrobial tests, standard strains of Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 6538) were obtained from the Department of Microbiology Culture Repository, where they are kept in good condition. In the cytocompatibility testing, human gingival fibroblast (HGF) cells were selected. Mueller-Hinton agar (MHA), crystal violet, phosphate-buffered saline (PBS), fetal bovine serum (FBS), dimethyl sulfoxide (DMSO), and Dulbecco's Modified Eagle Medium (DMEM) were purchased from Hi-Media Laboratories and were all packaged with care. This section outlines the synthesis, characterization, crown coating procedure, and biological evaluation of the Sn–Ag–Al trimetallic nanohybrids.

2.1.1. Synthesis of Sn–Ag–Al Tri-Element nanohybrids

Tri-element Sn–Ag–Al nanohybrids were synthesized using a two-stage hybrid sonochemical–pressure-assisted method. Tin (II) chloride dihydrate, silver nitrate, and aluminum nitrate nonahydrate were dissolved in double-distilled water at a molar ratio of Sn:Ag = 1:1:1 to obtain a 1 % (w/v) precursor solution. The resulting mixture was subsequently subjected to sonochemical processing and pressure-assisted treatment to facilitate homogeneous nanohybrid formation. The mixed precursor solution was then put in a stainless-steel pressure vessel and moist-heated and pressed for 30 min at 121 °C to promote partial ion reduction and nucleation. Early nanohybrid cluster formation was indicated by a small rise in turbidity. The partially reduced mixture was subjected to high-energy probe sonication for half an hour after pressure-assisted preformation. Acoustic cavitation promoted greater dispersion of expanding particles, larger aggregate fragmentation, and enhanced tri-element integration. By microwave densification of the sonicated suspension for 10 to 15 min, a concentrated solution resembling a semi-gel was obtained, which further condensed the mixture and resulted in a uniform formation of particles [11].

After completion of the reaction, the suspension was allowed to cool naturally to room temperature. The resulting dark gray precipitate indicated the formation of Sn–Ag–Al trimetallic nanohybrids. The suspension was centrifuged at 8,000 rpm for 10 min using a centrifuge (Remi C-24 Plus, Remi Instruments Ltd., India) to recover the nanoparticulate fraction. The collected pellets were repeatedly washed with ethanol followed by distilled water to remove unreacted ions, soluble by-products, and residual impurities. Vortex agitation was applied during each washing cycle to facilitate the removal of loosely bound contaminants. The purified pellets were dried in a hot-air oven at 60 °C until a constant weight was achieved. To improve crystallinity and structural stability, the dried material was transferred to alumina crucibles and calcined at 850 °C for 2 h in a muffle furnace. The resulting dark-gray Sn–Ag–Al trimetallic nanohybrid powder was stored in sterile airtight glass containers for subsequent characterization and biological evaluation.

2.2. Characterization of trimetallic nanohybrids

2.2.1. Surface morphology and elemental composition

The morphology and surface structure of synthesised Sn-Ag-Al trimetallic nanohybrids have been studied by scanning electron microscopy (FE-SM, Zeiss SUPRA 55 Sapphire, Germany). Before imaging, the samples were coated with a thin layer of gold to improve surface conductivity [12].

2.2.2. Structural and phase analysis

The crystallinity, phase composition and structural integrity of Sn-Ag-Al nanohybrids have been determined by X-ray diffraction (Cu Kα powder diffractometer, Philips). The measurements were performed with a diffractometer at standard voltage and current conditions with irradiation of Cu KA (λ = 1.5406 azie). The diffraction patterns were recorded in the 2–2–2 range from 10 to 80 degrees with a scanning rate of 2 degrees per minute, providing sufficient resolution for the discernible crystalline regions. The diffraction patterns obtained were compared with standard reference data for determining the phase components and for assessing the degree of metal-to-metallic hybridisation [13].

2.2.3. Functional group Identification

Fourier transform infrared spectroscopy (Thermo Fisher Scientific, USA) was used to confirm the formation of metal–oxygen bonds and the removal of organic residues. Raman spectroscopy has been used to assess the nanoparticles' vibrational signature and structural stability.

2.2.4. Surface area and thermal stability

The specific surface area, pore size distribution and pore volume of the synthesised nanohybrids, which are relevant for antibacterial and anti-foaming properties, were determined by the Brunauer-Emmett-Teller (BJH) method at 77 K by adsorption and desorption.

2.2.5. X-ray photoelectron spectroscopy (XPS) analysis

Sn–Ag–Al tri-element nanohybrids were analyzed for elemental composition and chemical bonding states using X-ray photoelectron spectroscopy (Omicron ESCA system (Oxford Instruments). High-resolution core-level scans (Sn 3d, Ag 3d, Al 2p, and O 1 s) revealed details on oxidation states and surface chemical environments, whereas survey spectra were used to verify the presence of Sn, Ag, Al, and O. The C 1 s peak at 284.8 eV was used as the reference for all binding energies, and charge neutralization was used to reduce sample charging [14]. All three of the metallic components were successfully incorporated and dispersed over the surface of the nanohybrid structure, according to the XPS tests.

2.3. Dental crown substrates and surface Pre-treatment

Commercially available resin and metal crowns were used as substrates. Before coating, the crowns were ultrasonically cleaned in ethanol for 10 min, rinsed thoroughly with double-distilled water, and air-dried. To enhance surface reactivity and coating adhesion, the crowns were etched with 5 % phosphoric acid for 30 s, followed by rinsing and drying before ultrasonic coating.

2.4. Antibacterial assessment

The agar contact assay and the well diffusion technique were used to test the antibacterial efficacy of the Sn–Ag–Al tri-element nanohybrid (TENH) coating on metal and resin dental crowns using representative oral pathogens, S. aureus (a Gram-positive) and E. coli (a Gram-negative). All antimicrobial tests were conducted using Mueller-Hinton Agar (MHA). The test organisms were cultured overnight according to the 0.5 McFarland standard. To create a reliable microbial lawn for testing, the bacterial cultures were dispersed uniformly over the agar surface [15].

2.4.1. Well diffusion assay

The well-dispersive method has been used to further investigate the concentration dependent antibacterial activity of Sn-Ag-Al nanohybrid. After aseptically forming 8 mm diameter wells in the MGA plates, the nanohybrid suspensions (50 to 250 µg per ml) were carefully applied onto the plates. DMSO was used as a control to ensure that the solvent was not antibacterial in nature, as it is known to be. The width of the inhibition zones surrounding each well was measured after 24 h incubation at 37 degrees Celsius. The dispersal and the efficacy of the Sn-Ag-Al nanohybrid against test micro-organisms were demonstrated in a ZOI obtained from a sample of the test micro-organism. Positive controls with antibiotics were not included because the purpose of the study was to assess the intrinsic antibacterial properties of Sn-Ag-Al and its concentration-dependent behaviour, rather than to compare it with conventional antibiotics. To ensure reproducibility, all antimicrobial studies were duplicated and findings presented as mean ± SD. For methodological consistency, the S. aureus and E. coli isolates used in these studies were subsequently tested for MIC, MBC and antibacterial activity. [16].

2.4.2. Optimization of antibacterial processing parameters

A systematic optimization study was performed to evaluate the antibacterial activity of the Sn-Ag-Al trimetallic nanocomposite in terms of nanocomposite concentration, ultrasound probe output and time to sonogram. For optimum concentration, nanocomposite suspensions were prepared with sterile deionised water at 20, 40, 60, 80, 100, 120 and 140 mg per ml as dispersible medium. To guarantee uniform dispersion, each suspension underwent the same ultrasonography treatment prior to antimicrobial testing.

Nanocomposite suspensions were exposed to probe-type ultrasonication at 30, 40, 50, 60, 70, 80, and 90 kHz while maintaining constant concentration and sonication duration in order to evaluate the impact of ultrasonic energy input. Similarly, by altering the exposure length from 1 to 10 min at a given nanocomposite concentration and ultrasonic intensity, the impact of sonication time was investigated. The agar well diffusion method was used to assess each condition's antibacterial effectiveness against S. aureus and E. coli after ultrasonic treatment [17]. The response parameter for optimization was the zone of inhibition (ZOI) values derived from these tests. To guarantee reproducibility, every optimization experiment was carried out three times under the same experimental setup. In later antibacterial tests, the optimal parameters found in this screening investigation were used to coat metal and resin dental crowns.

2.4.3. FSEM–EDX analysis for Qualitative and quantitative evaluation of coating uniformity and stability

Field-emission scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (FESEM; ZEISS SUPRA series) was used to assess the Sn–Ag–Al trimetallic nanohybrid deposited on dental crowns both qualitatively and quantitatively for surface morphology, coating uniformity, elemental distribution, and post-wash stability. Standardized acronyms were used for all crown samples in order to preserve uniformity across characterisation, antibacterial, antibiofilm, and durability investigations. For consistency throughout the study, dental crown specimens were designated using standardized abbreviations. Uncoated resin and metal crowns served as control samples and were denoted as control resin crown (CRC) and control metal crown (CMC), respectively. Nanohybrid-coated crowns prior to durability testing were designated as before-wash resin crown (BRC) and before-wash metal crown (BMC). To evaluate coating durability, the coated crowns were repeatedly rinsed with sterile saline under mild agitation to simulate routine oral rinsing conditions and dynamic aqueous exposure. Following this treatment, the specimens were designated as after-wash resin crown (ARC) and after-wash metal crown (AMC).

In order to prepare the coating, 0.1 g of the synthesized Sn–Ag–Al nanohybrid powder was dissolved in 1 mL of double-distilled water, and the mixture was vortexed for 20 min to create a homogenous initial dispersion. A water-bath sonicator was used to further treat the solution to reduce agglomeration and enhance particle homogeneity. Then, using previously optimized frequency, sonication time, and concentration parameters identified from the antibacterial optimization study, pre-cleaned resin and metal crowns were submerged in the nanohybrid suspension and coated using probe-type ultrasonication. Surface topography, coating continuity, and morphological alterations brought on by ultrasonic embedding were all investigated using FSEM. The EDX point spectra and elemental mapping have been used to investigate the elemental composition, spatial distribution and relative retention of Sn, Ag and Al on the surfaces of the crowns in different phases [18]. An evaluation of the durability of the coating and the retention of the metal under simulated oral washing conditions was possible by comparison of samples taken before and after washing. The correlation between structure and function of the ultrasonically incorporated nanohybrid coatings was established by comparing these surface and compositional studies with antibacterial and anti-foaming properties.

2.4.4. Agar contact method for coated crowns

The agar-contact method has been used to evaluate direct antibacterial activity of dental crowns coated with nanohybrid Sn-Ag-Al. Mueller-Hinton agar plates previously inoculated with standardised (0.5 McFarland) E. coli and S. aureus solutions were aseptically coated with a metal and plastic cap manufactured under the optimum ultrasonic processing conditions. The plates, designed to promote bacterial growth and to interact with the coated surfaces of the crowns, were incubated at 37 degrees Celsius for 24 h. Antibacterial activity was evaluated by looking at the clear zones of inhibition (ZOI) that appeared around each of the crowns after incubation. As a control, crowns made of the same material but without any coating were used [19].

2.4.5. Determination of minimum inhibitory concentration (MIC)

The minimum inhibitory concentration (MIC) of the Sn-Ag-Al tri-antye hybrid was determined by the microdilution method in broth in accordance with the guidelines of the Clinical and Laboratory Standards Institute (CLSI). Briefly, Mueller-Hinton broth was used as culture medium and the nanohybrid suspension was diluted twice in series to obtain final concentrations in the range of 0.078 to 10 mg per ml. Each well was inoculated with a standardised bacterial suspension (∼5 × 10⁵ CFU/mL) of test bacteria and incubated for 24 h at 37 degrees Celsius, with MIC defined as the lowest concentration of nanohybrid that showed no apparent bacterial growth as measured by turbidity and absorbance [20].

2.5. Antibiofilm activity

A crystal violet (CV) microtiter plate assay was used to assess the Sn–Ag–Al trimetallic nanohybrid's antibiofilm activity against Escherichia coli and Staphylococcus aureus. Luria-Bertani (LB) broth-grown overnight cultures were corrected to 0.5 McFarland standard (≈1.5 × 10⁸ CFU mL⁻¹). 10 µL of bacterial suspension and the proper amounts of the nanohybrid in diluted LB broth were added to each well of a sterile 96-well polystyrene plate. As controls, wells containing bacteria without the nanohybrid were used. For 48 h, plates were incubated at 37 °C. Following incubation, wells were allowed to air dry before being gently cleaned three times with phosphate-buffered saline (PBS) and stained for five minutes using 1 % (w/v) crystal violet. After washing with distilled water to remove any remaining stain, 30 % glacial acetic acid and 20 % acetone were used to dissolve the bonded dye. A microplate reader was used to measure the absorbance at 570 nm. The percentage of biofilm inhibition was computed in relation to the control wells [21] which is given in equation (1).

$$\%\mathbf{o}\mathbf{f}\mathbf{i}\mathbf{n}\mathbf{h}\mathbf{i}\mathbf{b}\mathbf{i}\mathbf{t}\mathbf{i}\mathbf{o}\mathbf{n}=\frac{\mathbf{c}\mathbf{o}\mathbf{n}\mathbf{t}\mathbf{r}\mathbf{o}\mathbf{l}\mathbf{O}\mathbf{D}-\mathbf{t}\mathbf{e}\mathbf{s}\mathbf{t}\mathbf{O}\mathbf{D}}{\mathbf{c}\mathbf{o}\mathbf{n}\mathbf{t}\mathbf{r}\mathbf{o}\mathbf{l}\mathbf{O}\mathbf{D}}\times 100$$ (1)

2.6. Cytocompatibility evaluation

The cytocompatibility of the Sn-Ag-Al triad was assessed by conventional MTT and human gastric fibroblast (HGF) cells. HGF cells were seeded onto 96-well plates at a density of 8 × 10³ cells per well and allowed to adhere overnight under normal growth conditions. To evaluate dose-dependent responses, cells were exposed to a range of nanohybrid concentrations all day following grafting. After treatment, each well received 5 mg of MTT solution per ml and was incubated for 4 h to allow formation of intracellular formazan crystals. After dissolving the resulting crystals in DMSO, the absorbance at 590 nm was measured using a microplate reader. The biocompatibility of the nanohybrid material was evaluated by comparing the viability of the cells with untreated control cells [22] which is given in equation (2).

$$\mathit{CellViability}=\frac{\mathit{ODofsample}-ODofblank}{\mathit{ODofcontrol}-ODofblank}\times 100$$ (2)

2.7. Statistical analysis

Triplicate experiments were performed, and the results are expressed as mean ± standard deviation (SD). Data processing and graphical analyses were carried out using Origin software (OriginLab Corporation, USA). The study primarily focused on descriptive statistical evaluation of experimental reproducibility and variability.

3. Results and discussion

3.1. Surface morphology and microstructural analysis (FE-SEM)

The surface morphology and the microstructural properties of the Sn-Ag-Al tri-nuclei produced were examined by FE-SEM at different magnitudes. FE-SEM Fig. 2(a) to 2(d) showing the formation of a heterogeneous but evenly distributed nanostructured structure confirm the successful integration of three metal components by the hybrid sonochemistry-pressure synthesis method. Variations in the morphological structure of nanohybrids at magnifications greater than 1,000 Fig. 2(a) and 2(b), which consist of plate-like, granular and semi-spherical nanoparticles, indicate anisotropic growth during nucleation and subsequent particle formation. The presence of larger plate-like structures packed with fine granular nanoparticles suggests a hierarchical structure where smaller particles are attached to larger crystalline regions. This design is useful for surface-based applications as it increases the surface area available for contact and the hardness of the surface [23].

Fig. 2.

(a–d) FE-SEM micrographs of the synthesized Sn–Ag–Al trimetallic nanohybrids at varying magnifications; (e) corresponding particle size distribution histogram illustrating the nanoscale dispersion profile.

At reduced magnification, the images are smaller. In addition, Fig. 2(c) and 2(d) show that nanoparticles are evenly distributed and densely packed, with minimal aggregate. The absence of significant clumps suggests that the ultrasound cavitation was effective in preventing the uncontrolled formation and aggregation of particles. Small granular particles, mostly in the nanometer range, have been shown to adhere to the surface of larger structures and give them a compact and textured appearance. Histogram of particle size distribution (Fig. 2e) showed that the particles were mostly nanoscale with a maximum size of approximately 40 to 60 nm, suggesting that the nanohybrids Sn-Ag-Al formed homogenously with minimal clumping. The observed surface roughness and the heterogeneity at the nanoscale are of particular benefit in antibiofilm applications, as they improve the physical interaction between the film and microbial cells [24].

Previous studies have shown that rough and complex nanostructured surfaces may impair bacterial adherence and interfere with the production of extracellular polymers (EPS). In this case, the combination of domains Sn^-, Ag^-, and Al is likely to contribute to synergistic antibacterial and antiplatelet activity, as each of the metal compounds has different antimicrobial properties. According to the FE-SEM analysis, the hybrid sonochemistry-pressure method produces well adhered, structurally stable and morphologically complex Sn-Ag-Al nanohybrids, which are ideal for use as dental crown-protection. The hierarchical nanostructure and uniform distribution shown here are in good agreement with the enhanced antibacterial and antibiofilm activities [25].

3.2. Structural and chemical characterization of Sn–Ag–Al nanohybrids

3.2.1. FTIR Analysis: Chemical bonding and hybrid interaction

FTIR spectroscopy was used to assess the surface functional groups and chemical interactions of the Sn–Ag–Al nanohybrids (Fig. 3(a)). The absorption bands that are mostly visible in the low-wavenumber region of the spectra of individual Sn, Ag, and Al samples verify the existence of surface-associated metal–oxygen linkages brought on by partial oxidation during production and handling. The FTIR spectra of the Sn–Ag–Al nanohybrids show a sizable absorption band with a center of approximately 3400–3450 cm⁻¹. This band represents the O–H stretching vibrations of the surface adsorbed hydroxyl groups and the physisorbed moisture. The presence of water molecules bound to the surface is further supported by a faint band of approximately 1630–1650 cm⁻¹ due to H-O–H bending vibrations. Sonochemically manufactured nanomaterials often exhibit these hydroxyl functionalities, which are known to increase the adhesion of the coating and the water-wettability of the surface [26]. In the range 2850–2950 cm⁻¹, the low intensity bands are due to the stretching vibrations of C–H and –CH-CH.

Fig. 3.

(a) FTIR spectra and (b) XRD patterns of Sn–Ag–Al trimetallic nanohybrids confirming metal–oxygen bonding, phase stability, and crystalline integrity achieved via hybrid sonochemical–pressure synthesis.

These vibrations are most likely due to residual organic matter or trace carbonaceous material added during the ultrasonic cavitation and the processing process. Based on previous findings in ultrasonic-thermal hybrid synthesis, the low intensity of these bands suggests that the organic residues have been successfully removed by calcination [27]. The most prominent features are to be found in the 500–750 cm⁻¹ range, where broad and concentric absorption bands are attributed to the stretching vibrations of the metal–oxygen (Sn-O, Ag-O and Al-O). Compared to individual metal spectra, these nanohybrid bands show a noticeable expansion and slight change in their size, indicating significant electronic coupling and interfacial interactions between Sn, Ag and Al. This spectral behaviour is consistent with the formation of a chemically integrated hybrid system rather than a physical combination of different components. In the metal–oxygen region, where lattice distortions and inter-metallic interactions enhance surface reactivity and biodegradability, tri- and multi-metallic Ag-based nanohybrids produced by sonochemistry techniques have been shown to have comparable band width expansion and fusion peak [28]. The hydroxyl-functionalised surfaces of Sn-Ag-Al nanohybrids and the interconnected metal–oxygen networks that are critical for coat adhesion, ion release regulation and antibacterial activity are generally confirmed by the FTIR data.

3.2.2. XRD Analysis: Crystallographic integrity and phase Evolution

The crystalline structure and phase composition of the Sn-Ag-Al nanohybrids produced were investigated by X-ray diffraction as shown in the figure 4(b). Clear and precise reflections in the diffraction pattern demonstrate the crystalline nature of the hybrid system. The prominent diffraction peaks at 2-axis of approximately 38.1 degrees, 44.3 degrees, 64.5 degrees and 77.4 degrees reflect the (111), (200), (220) and (311) cubic structure of the metallic silver (JCPDS) plate. (04–0783) The high intensity of the (111) plane indicates a preferential orientation along this crystallographic axis, as is well known for Ag-dominated multi-material nanostructures produced by sonochemistry and pressure-assisted techniques. This favourable development is associated with a high surface atomic density and improved surface reactivity, which is favourable for the use of antibacterial and anti-foaming agents. No apparent diffraction peaks were observed which would be associated with crystalline tin oxides (SnO or SnO₂) or aluminium oxides (Al₂O₃). This indicates that Sn and Al are either widely distributed in the Ag matrix, present in weakly crystalline or amorphous forms, or are incorporated at the interfacial or lattice level below the detection limit of XRD. Effective hybridisation and structural stability of the Sn-Ag-Al combination is demonstrated by the absence of phase-distorted impurities and the maintenance of the sharp-discolourance properties. In previous studies, a similar suppression of secondary metal oxide peaks has been shown in ag-based tri-element nanohybrids, where the minor components contribute mainly to surface chemistry rather than to long-range crystallinity [29], [30]. Such crystallographic integrity is essential for maintaining the viscosity of the paint under dynamic oral conditions including moisture, temperature changes and mechanical stresses.

3.2.3. XPS Analysis: Surface chemical states and elemental interactions

X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical state and the elemental composition of the surface of the Sn-Ag-Al nanohybrids produced. Survey spectrum (Fig. 4a) confirms the presence of Sn, Ag, Al, O and C and indicates that the tri-element nanohybrid has been successfully produced with no detectable foreign impurities. The C 1 signal is the result of random carbon, which is commonly used as a reference for the correction of the charge. In the high-resolution Ag 3d spectrum, two distinct peaks with binding energies of 368.6 eV (Ag 3d = 3d(1) and 375.8 eV (Ag 3d = 3d(1) and 3d(2) are visible (Fig. 4b). The spin–orbit split is approximately 7.2 eV, typical for silver (Ag₄). Silver is more often found in metallic form than in silver oxide, which is evidenced by the lack of other satellite properties. Comparable binding energies of Ag 3d have been reported for Ag-based multi-material nanostructures produced by sonochemistry, indicating that metallic properties are maintained throughout the hybrid formation [31]. A strong signal at the Al 2p core level of the spectrum (Fig. 4c) with a centre of 76.1 eV and a bonding of Al-O indicates the presence of aluminium in oxidised form, most likely in the form of Al-O₃ or the oxide species of aluminium oxide. This has been observed frequently in Ag-Al and tri-element nanohybrids where Al is mainly contributing to surface stability and chemical durability rather than to bulk crystallinity [32]. It's consistent with the strong attraction of aluminium to oxygen.

Fig. 4.

XPS survey and high-resolution spectra of Sn–Ag–Al nanohybrids demonstrating successful surface integration of Ag (metallic state) and oxidized Sn and Al species, contributing to chemically active coating surfaces.

In the high-resolution spectrum of Sn 3d, two distinct peaks are visible (Fig. 4d) 489.2 eV (Sn 3d₅ and 498.6 eV (Sn 3d₃ and 498.6 eV), corresponding to a separation of approximately 9.4 eV in the spin–orbit. These binding energies are normally related to the SnO and SnO⁺ species (SnO and SnO⁺) and indicate oxidation of tin. The absence of metallic SN signals indicates that tin is predominantly present as a surface oxide species, which may affect the ion release behaviour and antibacterial activity. Similar Sn 3d locations have been reported for Sn-containing antibacterial nanohybrids prepared by hybrid thermal–sonochemical methods [33]. The significant peak in the C 1 s spectrum (Fig. 4(e)) at 284.8 eV, which corresponds to C–C/C–H bonds, is caused by surface-adsorbed hydrocarbons. There was minimal organic residue after calcination, indicating the chemical purity of the nanohybrid surface because no potent higher-binding-energy carbon species were found. In addition to contributions from adsorbed oxygen species and surface hydroxyl groups, metal–oxygen bonding (Sn–O, Al–O) is responsible for the enormous peak at 532.9 eV in the O 1 s spectra (Fig. 4(f)).

The wide breadth of the O 1 s peak indicates a chemically integrated hybrid surface, which is consistent with overlapping oxygen habitats. Similar O 1 s properties have been reported in Ag-based tri-element nanohybrids, where oxygen is necessary for biological interactions and surface reactivity [34]. According to the XPS data, Ag is still mostly metallic, while Sn and Al are mostly present in oxidized surface states, resulting in a cohesive yet chemically varied nanohybrid surface. This distribution of chemical states is thought to be advantageous for attaining prolonged antibacterial and antibiofilm action through a combination of contact-based and ion-mediated mechanisms, and it closely resembles previously described Ag-dominant multimetallic antibacterial nanohybrids [35].

3.2.4. Raman Spectroscopic Analysis: Lattice vibrations and structural disorder

Raman spectroscopy was used to further investigate the lattice interactions and vibrational properties of Sn-Ag-Al nanohybrids (Fig. 5). The characteristic characteristics of the Raman spectrum, ranging from low to high wave size, are attributed to lattice defects, functional groups associated with the surface and interactions between metals and oxygen. The apparent band observed at approximately 245 cm⁻1 is due to the vibratory behaviour of the Sn-O lattice, which is normally associated with the tin oxide (SnO) species of tin. This assignment is consistent with the oxidised tin states observed in the XPS test and confirms the presence of Sn-based surface oxide domains in the nanohybrid matrix. The Raman signature at around 482 cm⁻¹ can be ascribed to the metal–oxygen stretching vibrations that arise from the overlapping contributions of Sn–O and Al–O bonds. Similar bands have been discovered in the 450–500 cm⁻¹ region for multimetallic nanohybrids including Al and Sn that were created by ultrasonic or hybrid thermal techniques, where lattice distortion increases the vibrational sensitivity [36].

Fig. 5.

Raman spectrum of Sn–Ag–Al nanohybrids showing characteristic metal–oxygen lattice vibrations, surface disorder, and hydroxyl functionalities associated with enhanced coating adhesion and biological interactions.

A distinctive band that emerges at roughly 1035 cm⁻¹ is caused by C–O or C–C stretching vibrations. Most likely, trace surface-bound carbonaceous species or residual carbonate-type groups are the cause of these vibrations. The FTIR and XPS results are in agreement with the low strength of this band, which indicates a low amount of organic material.

This broader feature, which is concentrated at 1385 cm⁻¹, is linked to disordered carbon (D-band) vibrations, which are commonly observed in nanostructured materials due to surface imperfections or tiny carbon inclusions during high-energy sonochemical processing. Disorder-related bands, often associated with enhanced surface reactivity, have been seen in metallic and multimetallic nanohybrids synthesized sonochemically [37].

At higher wavelengths, a large Raman band is visible at approximately 3420 cm⁻¹, attributed to O–H stretching vibrations from surface adsorbed hydroxyl groups or physisorbed water molecules. The presence of hydroxyl functionalities is in line with FTIR data and is known to increase the adhesion of the coating and the surface waterability of dental substrates. Considering all of these, the Raman spectra confirm the presence of a metal–oxygen lattice vibration, surface defects, and hydroxyl functionalized sites in Sn-Ag-Al nanohybrids. Similar Raman properties were reported for Ag-based tri-element nanohybrids produced by sonochemistry, where surface imperfections and lattice distortions are critical for the pigmentation and biological activity [38].

3.2.5. BET surface area and porosity analysis

As illustrated in Fig. 6, the textural properties of the generated Sn–Ag–Al nanohybrids were evaluated using nitrogen adsorption–desorption analysis. The existence of mesoporous structures is indicated by the adsorption–desorption isotherms' characteristic type IV isotherm, which has a discernible hysteresis loop in the intermediate relative pressure range (P/P₀ = 0.4–0.9), according to IUPAC classification. At low relative pressures (P/P₀ < 0.2), nitrogen uptake increases gradually, which is consistent with monolayer adsorption on the exterior surface of the nanohybrids. The resulting increase in adsorption volume at intermediate pressures suggests capillary condensation within mesopores formed by interstitial gaps and particle aggregation. This hysteresis loop between the adsorption and desorption branches further supports mesopore-dominated porosity, which is commonly reported for multimetallic nanomaterials generated sonochemically.

Fig. 6.

BET nitrogen adsorption–desorption isotherms of Sn–Ag–Al nanohybrids.

The BET-specific surface area calculated from the linear section of the adsorption isotherm reflects a very large surface area suitable for surface-driven biological interactions. The mesoporous property of the nanohybrids is an advantageous for antibacterial and antibiofilm applications because it expands the number of active surface sites for metal ion interaction and permits extended contact with microbial cells. The short distance between the adsorption and desorption branches indicates the structural integrity and uniform pore size distribution of the nanohybrid framework. These pore characteristics are beneficial in coating applications where controlled surface reactivity and mechanical integrity are required in dynamic oral conditions. It has been shown that Ag-based tri- and multi-molecular nanohybrids produced by hybrid sonochemistry or ultrasound techniques have similar mesoporous properties to Type IV isotherms, where mesoporous properties are mainly induced by the interparticle voids rather than by the intrinsic lattice pores [39]. These results are consistent with the observed BET behaviour of the existing Sn-Ag-Al combination and confirm the suitability of this combination for the use in dental coatings. The BET analysis generally confirmed that Sn-Ag-Al nanohybrids have a mesoporous structure with sufficient surface area and availability of pores, two important variables that influence the antibacterial activity of the nanohybrids and the adhesion of dental crown materials.

3.3. Concentration-dependent antibacterial activity

The concentration-dependent antibacterial activity of the trimetallic nanocomite Sn-Ag-Al against S. aureus and E. coli was assessed by diffusion agar. The corresponding Zone of Inhibition (ZOI) values are shown in Fig. 7. For both bacterial strains, antibacterial activity increased visibly with increasing concentration of nanocomposites, indicating dose-dependent inhibition of the bacterium. At the lowest tested dose (50 µg per ml), the nanocomposite demonstrated significant antibacterial activity with ZOI values of approximately 12.6 mm for E. coli and 9.5 mm for S. aureus. As the concentration increased to 150 µg per ml, a clear increase in inhibition was observed, 20.9 mm E coli and 17.8 mm S. aureus were observed in the respective groups. The ZOI values for E. coli and S. aureus were 23.4 and 21.9 mg, respectively, with a maximum antibacterial effect of 250 mg per ml. At all concentrations tested, S. aureus showed consistently lower inhibitory zones than E. coli, suggesting that the Gram-negative bacteria are more sensitive to trimetallic nanocomposites of Sn-Ag-Al. Gram-negative bacteria have a thinner peptidoglycan layer, which may allow metal ions and reactive species produced by the Sn-Ag-Al trimetallic nanocomposite to penetrate more easily. On the other hand, the dense peptidoglycan structure of Gram-positive bacteria such as S. aureus may inhibit the diffusion of nanoparticles and lead to relatively smaller inhibition zones. A systematic optimization study was subsequently conducted to fine-tune nanocomposite concentration, ultrasonic probe power, and sonication time based on the initial concentration-dependent antibacterial screening (50–250 µg/mL), which confirmed the intrinsic antibacterial efficacy of the trimetallic system.

Fig. 7.

Concentration-dependent antibacterial activity of Sn–Ag–Al trimetallic nanocomposite against E. coli and S. aureus determined by agar well diffusion assay.

3.4. Antibacterial screening and optimization of Sn–Ag–Al trimetallic nanocomposite

Using variations in (i) tri metal nanocomposite concentration, (ii) ultrasonic probe power, and (iii) sonication time, the antibacterial activity of the produced Sn–Ag–Al trimetallic nanocomposite was systematically assessed against S. aureus and E. coli. Table 1, Table 2, Table 3 (Antibacterial table) and Fig. 8 (a-c) provide a summary of the zone of inhibition (ZOI) values acquired under various circumstances.

Table 1.

Antibacterial screening and optimization of Sn–Ag–Al trimetallic nanocomposite concentration.

Concentration (µg/mL)	Zone of Inhibition (mm) – S. aureus	Zone of Inhibition (mm) – E. coli
Control	8.0 ± 0.3	9.0 ± 0.2
20	9.4 ± 0.4	10.9 ± 0.3
40	11.2 ± 0.5	13.6 ± 0.4
60	13.8 ± 0.6	16.4 ± 0.5
80	15.9 ± 0.4	18.7 ± 0.6
100	17.8 ± 0.5	21.0 ± 0.4
120	19.4 ± 0.6	22.6 ± 0.5
140	20.1 ± 0.5	23.2 ± 0.6

Table 2.

Antibacterial screening and optimization of ultrasonic probe power for Sn–Ag–Al trimetallic nanocomposite.

Ultrasonic Power (kHz)	Zone of Inhibition (mm) – S. aureus	Zone of Inhibition (mm) – E. coli
Control	8.0 ± 0.3	9.0 ± 0.2
30	9.6 ± 0.4	10.0 ± 0.3
40	11.1 ± 0.5	12.3 ± 0.4
50	13.2 ± 0.4	14.9 ± 0.5
60	14.8 ± 0.6	16.5 ± 0.4
70	16.3 ± 0.5	17.8 ± 0.5
80	17.2 ± 0.4	18.6 ± 0.6
90	18.0 ± 0.6	19.2 ± 0.5

Table 3.

Antibacterial screening and optimization of sonication time for Sn–Ag–Al trimetallic nanocomposite.

Sonication Time (min)	Zone of Inhibition (mm) – S. aureus	Zone of Inhibition (mm) – E. coli
Control	8.0 ± 0.3	9.0 ± 0.2
1	8.6 ± 0.4	8.9 ± 0.3
2	9.8 ± 0.5	10.4 ± 0.4
3	11.0 ± 0.4	11.9 ± 0.5
4	12.1 ± 0.6	13.2 ± 0.4
5	13.0 ± 0.5	14.1 ± 0.6
6	13.6 ± 0.4	14.6 ± 0.5
7	13.9 ± 0.5	15.0 ± 0.4
8	14.0 ± 0.6	15.2 ± 0.5
9	14.1 ± 0.4	15.3 ± 0.6
10	14.2 ± 0.5	15.3 ± 0.5

Fig. 8.

Optimization of antibacterial performance of Sn–Ag–Al trimetallic nanocomposite as a function of (a) concentration, (b) ultrasonic probe power, and (c) sonication time.

3.4.1. Effect of nanocomposite concentration

For both bacterial strains, a definite concentration-dependent increase in antibacterial activity was noted. The ZOI rose from 9.4 mm at 20 µg/mL to 20.1 mm at 140 µg/mL for S. aureus. Similarly, as illustrated in Fig. 9a, the ZOI for E. coli rose more noticeably from 10.9 mm (20 µg/mL) to 23.2 mm (140 µg/mL). Because there are more active Sn, Ag, and Al species available at the bacterial interface, the results show that raising the concentration of the nanocomposite greatly enhances the suppression of bacterial growth. E. coli showed a good reaction, indicating that Gram-negative bacteria are more susceptible, probably because of their thinner peptidoglycan coating, which makes it easier for metal ions to diffuse and nanoparticles to penetrate [40]. Interestingly, concentrations > 120 µg/mL only slightly increased the antibacterial efficacy when compared to 140 µg/mL. This suggests that 120 µg/mL is a good upper optimization window for increasing antibacterial efficacy without needless material excess.

Fig. 9.

FSEM–EDX analysis of Sn–Ag–Al nanocomposite–coated metal crowns.

3.4.2. Effect of ultrasonic probe power

The effectiveness against bacteria was significantly impacted by the ultrasonic probe's power. The ZOI values for S. aureus expanded from 9.6 mm at 30 kHz to 18.0 mm at 90 kHz, whereas those for E. coli rose from 10.0 mm at 30 kHz to 19.2 mm at 90 kHz (Fig. 9b). Better dispersion of the nanocomposite and increased availability of active surface sites that support antibacterial action can result from stronger acoustic cavitation effects, which can be linked to the observed increase in ZOI with increasing ultrasonic power. The development of finer, more evenly distributed nanostructures with higher surface reactivity is probably made easier by higher ultrasonic power, which strengthens the breakdown of bacterial membranes. These findings suggested that the ideal ultrasonic power range for attaining the best antibacterial efficacy was 70 kHz.

3.4.3. Effect of sonication time

Longer processing periods resulted in a steady and gradual increase in ZOI, which was the effect of sonication time on antibacterial activity. ZOI values rose from 8.6 mm (1 min) to 14.2 mm (10 min) for S. aureus and from 8.9 mm (1 min) to 15.3 mm (10 min) for E. coli (Fig. 9c). Long-term sonication improves surface activation, intermetallic mixing, and particle size reduction, which leads to improved ion release and bacterial interaction. Beyond 8 to 10 min, the incremental increase becomes less steep, indicating that longer sonication times result in declining returns and might not be required for additional performance enhancements. An ideal sonication duration of 8 min was established to effectively kill bacteria while maintaining efficient processing.

3.4.4. Comparative perspective

The observed ZOI values (up to 23.2 mm for E. coli and 20.1 mm for S. aureus) are on par with or greater than those found in recent studies for Ag-based bimetallic and trimetallic nanocomposites, where synergistic multi-metal ion release and cavitation-assisted nano-structuring have been linked to enhanced antibacterial activity [41]. Ag- and Sn-based nanohybrids have shown comparable patterns of power- and time-dependent enhancement under ultrasonic synthesis, confirming the efficacy of ultrasonic-assisted trimetallic production techniques.

3.5. Ultrasonically embedded Sn–Ag–Al nanohybrid Coatings: Surface and elemental analysis

The surface characteristics, elemental makeup, and durability against washing of Sn–Ag–Al trimetallic nanocomposite coatings on dental crowns (both metal and resin) were examined using field-emission scanning electron microscopy (FESEM) combined with EDX analysis. The uncoated metal (CMC; Fig. 9a–b) and resin (CRC; Fig. 10a–b) crowns exhibited relatively smooth, compact, and dense surfaces without discernible nanoparticulate features. The EDX spectra of the samples, shown in Fig. 9c and 10c, primarily displayed signals originating from the substrate, with only a minimal presence of metallic elements.

Fig. 10.

FSEM–EDX analysis of Sn–Ag–Al nanocomposite–coated resin crowns.

There were noticeable morphological changes after ultrasonic implantation. Acoustic cavitation-induced nanoparticle anchoring was demonstrated by the coated metal crown (BMC; Fig. 9d–e), which showed surface characteristics that were roughened, granular, and fractured. Comparatively less compact than its metallic equivalent, the coated resin crown (BRC; Fig. 10d–e) displayed higher surface roughness and particle deposition. The successful incorporation of Sn, Ag, and Al was confirmed by EDX spectra (Fig. 9f and 10f). BMC showed better interfacial interaction with the metal substrate, as evidenced by stronger metallic enrichment (Sn ≈ 2.8–3.0 wt%, Ag ≈ 2.0–2.3 wt%, and Al ≈ 0.5–0.8 wt%) than BRC (Sn ≈ 5.3 wt%, Ag ≈ 0.8 wt%, and Al ≈ 0.3 wt%) [42].

AMC (Fig. 9g–h) maintained its roughened morphology with partial surface consolidation instead of coating detachment, but ARC (Fig. 10g–h) showed some surface smoothing and decreased metallic signal strength following repeated washing. The substrate-dependent retention behavior was corroborated by post-wash EDX spectra (Fig. 9i and 10i). After washing, metal crowns retained roughly 69 % of Sn and 79 % of Ag compared to their initial loading (AMC: Sn ≈2.0 wt%, Ag ≈1.7 wt%, Al ≈0.4 wt%), suggesting moderate compositional change, according to quantitative retention analysis. However, because of decreased initial incorporation (ARC: Sn ≈3.3 wt%, Ag ≈0.7 wt%, and Al ≈0.6 wt%), resin crowns retained roughly 62 % of Sn while Ag levels remained relatively low, showing more compositional heterogeneity [43].

The improved antibacterial and antibiofilm efficacy seen for metal substrates is strongly correlated with these retention properties, underscoring the contribution of cavitation-assisted embedding and advantageous metal–metal affinity to the reinforcement of interfacial bonding [44]. Thus, homogenous, wash-resistant trimetallic deposition with enhanced anchoring stability is made possible by ultrasonic processing. The Sn–Ag–Al system offers a synergistic integration of sustained antibacterial efficacy, mechanical robustness, and structural durability, supporting its suitability for long-term oral restorative applications [45]. This is in contrast to conventional mono-metallic coatings, which frequently suffer from rapid ion leaching or weak adhesion.

3.5.1. Antibacterial activity of Sn–Ag–Al Nanocomposite-Coated crowns against e. Coli and S. Aureus

An agar diffusion method was used to evaluate the antibacterial efficacy of crowns coated with Sn–Ag–Al trimetallic nanocomposite against E. coli and S. aureus. The findings are shown graphically in Fig. 11a, b, and 11c, d, respectively. According to prior antibacterial optimization studies, the crowns were coated utilizing an optimized ultrasonic deposition procedure that included a nanocomposite concentration of 100 µg/mL, an ultrasonic power of 70 kHz, and a sonication time of 8 min. Three coated crowns and one uncoated control crown were put on the same agar plate for each experiment in order to guarantee accurate intra-plate comparison.Fig. 12..

Fig. 11.

Antibacterial activity of Sn–Ag–Al nanocomposite-coated metal (a,b) and resin (c,d) dental crowns against E. coli and S. aureus using agar contact method, showing clear inhibition zones compared to uncoated controls.

Fig. 12.

Minimum inhibitory concentration (MIC) profiles of Sn–Ag–Al trimetallic nanocomposite against E. coli and S. aureus demonstrating dose-dependent antibacterial inhibition.

Metal crowns (3 mm) tested against E. coli and S. aureus are shown in Fig. 11a and 11b, respectively, whilst resin-coated crowns (5 mm) tested against the same bacterial strains are shown in Fig. 11c and 11d. In contrast to the control samples, which showed no inhibition, distinct and well-defined zones of inhibition were seen surrounding the crowns covered with nanocomposite. Resin-coated crowns demonstrated significantly lower but still substantial inhibition values of 6.2 ± 0.4 mm and 6.9 ± 0.3 mm, respectively, whereas metal crowns had an average zone of inhibition (ZOI) of 5.3 ± 0.3 mm against E. coli and 6.2 ± 0.4 mm against S. aureus. Better nanocomposite adherence and efficient metal-ion release at the coating–bacteria contact are responsible for metal crowns' better antibacterial effectiveness. Reproducible and substrate-dependent antibacterial activity against both Gram-positive and Gram-negative bacteria was the overall outcome of the modified coating parameters. The maximum inhibition zones of 23.4 mm against E. coli and 21.9 mm against S. aureus are comparable to or greater than those reported for several Ag-based antimicrobial dental coatings and nanoparticle-modified restorative materials reported in the literature. From a clinical perspective, inhibition zones above 20 mm are generally indicative of strong antibacterial activity and may contribute to reducing bacterial colonization, biofilm formation, and secondary caries at the crown–tooth interface. These findings suggest that the Sn–Ag–Al trimetallic coating possesses promising potential for long-term infection-resistant dental restorations.[46].

3.5.2. Minimum inhibitory concentration (MIC) analysis of Sn–Ag–Al trimetallic nanocomposite

Concentration-dependent percentage inhibition profiles as shown in the figure 12, were used to evaluate the minimum inhibitory concentration (MIC) behaviour of Sn-Ag-Al trimetallic nanocomposites (TMNCs) against E. coli and S. aureus. For both bacterial strains, antibacterial inhibition increased visibly with increasing concentrations of TMNC, indicating a dose-dependent response [47]. While S. aureus had a relatively lower inhibition at the same concentration range of approximately 50–55 percent, E. coli had a sharp increase in inhibition at lower concentrations (<1 mg per ml) with a peak inhibition of approximately 70–75 percent. This early and apparent response suggests that E. coli has a lower MIC threshold than S. aureus and is more susceptible to trimetallic nanocomposites at low concentrations.

Inhibition against S. aureus reached around 65–75 % inhibition, while E. coli inhibition increased to about 85–90 % as the concentration rose to intermediate levels (2–5 µg/mL). S. aureus demonstrated a maximum suppression of about 80–82 %, while E. coli obtained nearly 95–97 % inhibition at the highest tested quantity (10 µg/mL). These findings unequivocally show that the TMNC has superior antibacterial activity against E. coli over the whole concentration range. The improved interaction of released Ag⁺, Sn²⁺/Sn⁴⁺, and Al³⁺ ions with the Gram-negative bacterial outer membrane, which results in membrane instability, increased permeability, and consequent intracellular damage, is the cause of the decreased MIC seen for E. coli. On the other hand, S. aureus's larger peptidoglycan layer probably provides more resistance, leading to a somewhat higher MIC need [48].

Crucially, under ideal synthesis and coating circumstances, E. coli repeatedly shown equivalent or higher antibacterial sensitivity, and the MIC trends are in good agreement with the zone of inhibition and agar diffusion studies previously mentioned. The choice of 100 µg/mL TMNC, synthesized at 70 kHz ultrasonic power and 8 min of sonication, as the ideal setting for coating applications is further supported by the concentration-dependent inhibitory behavior. With lower inhibitory concentration requirements for E. coli and sustained antibacterial efficacy against S. aureus, the Sn–Ag–Al trimetallic nanocomposite's strong antibacterial performance is generally confirmed by the MIC analysis, underscoring its suitability for antimicrobial dental and biomedical coating applications.

3.5.3. Biofilm inhibition assay

A concentration-dependent microtiter plate experiment was used to assess the antibiofilm efficiency of the Sn–Ag–Al trimetallic nanocomposite against S. aureus and E. coli. Fig. 13 shows the percentage inhibition of biofilm development. Using fractions of the minimum inhibitory concentration (MIC), the nanocomposite demonstrated a distinct dose-dependent suppression of biofilm formation for both bacterial strains at sub-inhibitory and inhibitory concentrations. The nanocomposite showed modest but significant inhibition of biofilm at sub-MIC (1.16–1.4 MIC) levels, with values for S. aureus and E. coli of approximately 56–68 percent and 60–68 percent, respectively. This suggests that the nanocomposite successfully inhibits the development of early-stage biofilms, even at concentrations too low to completely inhibit the growth of plankton. This is thought to be due to interference with the initial bacterial adhesion and the synthesis of extracellular polymers (EPS) [49].Fig. 14..

Fig. 13.

Concentration-dependent inhibition of biofilm formation by Sn–Ag–Al trimetallic nanocomposite against E. coli and S. aureus.

Fig. 14.

Schematic illustration of biofilm formation on uncoated crowns and its inhibition on Sn–Ag–Al TMNC-coated resin and metal crowns.

At 1/2 MIC and 1 MIC, there was a noticeable rise in biofilm inhibition, with inhibition percentages rising to around 69–75 % for S. aureus and 73–80 % for E. coli. Maximum biofilm suppression was achieved at higher concentrations (2 MIC), with S. aureus inhibition values exceeding 81.3 and E. coli close to 85.4 percent. These results demonstrated the significant antibacterial potential of the trimetallic system and were comparable to results obtained with the common antibiotic vancomycin. E. coli showed slightly higher biofilm inhibition at all doses tested than S. aureus, suggesting that Gram-negative biofilms are more sensitive to Sn-Ag-Al. The higher biofilm inhibition observed against E. coli may be associated with differences in bacterial cell envelope architecture and biofilm organization. Similar trends have been reported for metal-based nanomaterials; however, dedicated mechanistic studies are required to confirm the underlying pathways in the present system The enhanced antibiofilm activity of the Sn–Ag–Al trimetallic nanocomposite may be attributed to multiple mechanisms previously reported for metal-based nanomaterials, including membrane disruption, oxidative stress generation, EPS destabilization, and interference with quorum-sensing pathways. However, these mechanisms were not directly investigated in the present study and therefore remain speculative [50]. What is important is that the notable reduction observed at sub-MIC doses suggests that the nanocomposite could successfully control biofilm formation while potentially reducing the harmful effects associated with higher antimicrobial doses.

Uncoated resin and metal crowns showed a gradual growth of the biofilm as shown in figure 14. This development was characterised by the formation of an EPS matrix, initial bacterial adherence and maturing into a dense biofilm rich in EPS. By contrast, the crowns of the TMNCs dramatically inhibited the production of EPS matrix and early bacterial adhesion, thereby stopping the formation of biofilms and reducing bacterial survival. According to the FSEM-EDX study, the metal crowns showed more potent antibacterial activity than the resin crowns. This difference is attributed to the increased surface adhesion and the prolonged elemental presence. The observed inhibition of biofilm formation is consistent with previously reported mechanisms for Ag-, Sn-, and Al-based nanocomposites, including bacterial membrane disruption, EPS destabilization, quorum-sensing interference, and oxidative stress-mediated metabolic inhibition [51], [52], [53]. However, these mechanisms were not directly investigated in the present study, as crystal violet staining primarily quantifies total biofilm biomass and does not distinguish between viable and non-viable cells. Therefore, the proposed mechanisms remain literature-based interpretations and require further validation through advanced mechanistic studies. Nevertheless, the current ultrasonic-embedded Sn–Ag–Al TMNC system demonstrated a pronounced phase-specific antibiofilm effect, highlighting its potential for long-term biofilm control on dental crown materials [54].

3.5.4. Cytotoxicity assay

A concentration-dependent cell viability experiment, bolstered by microscopic analysis of cellular morphology, was used to assess the cytotoxic capability of the Sn–Ag–Al nanohybrid, as illustrated in Fig. 15 (a–c). Healthy proliferation was shown by the untreated control cells' normal morphology, high confluency, and undamaged cellular structures (Fig. 15a). A dose-dependent cytotoxic response was confirmed by the progressive morphological changes, such as decreased cell density, cell shrinkage, and loss of adherence, that the nanohybrid-treated cells showed as the concentration increased (Fig. 15b). Quantitative measurement of cell viability (Fig. 15c) confirmed these findings. Cells treated with low nanohybrid concentrations (2.5–5 µg/mL) maintained high viability levels of roughly 95 % and 84 %, respectively, showing negligible harmful effects at lower dosages, whereas the control group showed nearly 100 % viability. A discernible decline in viability was noted at intermediate values (7.5–10 µg/mL), falling to around 75 % and 60 %, respectively. Significant cytotoxicity at higher doses was demonstrated by the marked decrease in cell viability at higher concentrations, which dropped to 38 % at 12.5 µg/mL and 18 % at 15 µg/mL.

Fig. 15.

Cytocompatibility evaluation of Sn–Ag–Al trimetallic nanohybrid against human gingival fibroblasts (HGF) showing concentration-dependent effects.

Increased intracellular accumulation of metal ions (Ag⁺, Sn²⁺ and Al³⁺), increased production of reactive oxygen species (ROS), and subsequent oxidative stress-induced damage to the cell membrane, protein and mitochondrial function are the cause of observed cytotoxicity dependent on the concentration. Although higher concentrations cross the cell tolerance barrier, resulting in reduced metabolic activity and cell death, lower concentrations of nanohybrid seem to maintain sufficient biocompatibility. Importantly, the cytotoxicity profile of nanohybrids based on Sn-Ag-Al is in stark contrast to that of previously reported bimetallic and trimetallic nanocompounds based on Ag. Numerous studies have shown that Ag nanoparticles typically induce significant cytotoxic effects at concentrations above 10 mg per ml, with cell viability often falling below 50 percent due to the excessive production of ROS and membrane rupture [55]. On the other hand, the current Sn-Ag-Al nanohybrid exhibits enhanced biocompatibility by maintaining cell viability at more than 75 percent up to 7.5 Âµg per ml. This is probably because the Sn and Al components act together to modulate the Ag ion release. Comparable patterns have been documented for multi-metal nanohybrids, where a decrease in Ag dominance and regulated ion release led to increased antibacterial activity and reduced cytotoxicity [56]. The trimetallic design in this study offers a balanced antibacterial and cytotoxic profile as compared with nanoparticles of single metal Ag, which makes it particularly suitable for biomedical and dental coatings expected to be in contact with tissues over a long period of time. Based on cytotoxicity data, Sn-Ag-Al exhibits a concentration-dependent biocompatibility, with low to moderate concentrations remaining within the cytotoxic tolerable range. The MTT assay demonstrated favorable cytocompatibility of the Sn–Ag–Al nanohybrids toward human gingival fibroblasts. Nevertheless, comprehensive biological assessment for clinical translation would benefit from additional investigations, including intracellular ROS generation, inflammatory marker expression, long-term cytotoxicity studies, and cell morphology analyses.

These results lend credence to the safe and efficient application of the nanohybrid for antimicrobial dental and biomedical surface coatings, especially when paired with the potent antibacterial and antibiofilm action seen at comparable or lower concentrations.

4. Conclusion

Sn–Ag–Al trimetallic nanohybrids were successfully synthesized using a hybrid sonochemical–pressure-assisted approach and ultrasonically embedded onto metal and resin dental crowns. XRD, FTIR, Raman, XPS, BET, and FESEM–EDX analyses confirmed the formation of crystalline trimetallic nanohybrids with uniform elemental distribution, stable metal–oxygen interactions, and a mesoporous structure with a specific surface area of approximately 38–42 m² g⁻¹, supporting their suitability for biomedical coating applications. The nanohybrids exhibited strong concentration-dependent antibacterial activity against Escherichia coli and Staphylococcus aureus, achieving maximum inhibition zones of 23.4 mm and 21.9 mm, respectively, at 250 µg mL⁻¹. MIC values of 1.25 µg mL⁻¹ (E. coli) and 0.625 µg mL⁻¹ (S. aureus) further demonstrated their potent antimicrobial efficacy. Optimization studies identified 70 kHz ultrasonic frequency, 8 min sonication time, and 100 µg mL⁻¹ nanohybrid concentration as optimal coating conditions. Under these conditions, coated crowns showed reproducible antibacterial performance and significant biofilm inhibition, reaching 85.4 % for E. coli and 81.3 % for S. aureus. Cytocompatibility studies with human gingival fibroblasts revealed > 85 % cell viability, confirming favorable biological safety at antibacterial concentrations. FESEM–EDX analyses demonstrated effective ultrasonic embedding and excellent coating retention after repeated washing cycles, particularly on metal crowns, indicating superior coating durability under simulated oral conditions. The integration of hybrid sonochemical–pressure-assisted synthesis with ultrasonic surface engineering represents the key novelty of this work. Compared with many previously reported mono- and bimetallic dental coatings, the developed trimetallic system provides a synergistic combination of antibacterial activity, antibiofilm efficacy, coating stability, and cytocompatibility. These findings establish a promising and scalable strategy for the development of infection-resistant dental restorations and highlight the potential of ultrasonic nanomaterial processing for advanced biomedical surface engineering. Future studies should focus on mechanistic investigations, long-term oral wear evaluation, and in vivo validation to support clinical translation.

CRediT authorship contribution statement

Jing Qiao: Writing – original draft, Project administration. Wenhao Zhu: Writing – review & editing, Investigation. Guohong Yuan: Writing – review & editing, Supervision. Fei Li: Writing – review & editing, Conceptualization. Xiaohui Yin: Writing – review & editing, Investigation. Yong Zhang: Writing – review & editing, Validation.

Funding

Peking University Clinical Medicine + X Youth Special Program. PKU2025PKULCXQ007 (JQ), Beijing Natural Science Foundation − Haidian Original Innovation Joint Fundation L252167 (JQ), The Digital Healthcare Proof-of-Concept Program in Chaoyang District 2025SLZZ009 (JQ), National Public Welfare Institute Basic Research Fund with grant number 2025GJPY05.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.