Magnetoplasmonic core–shell structured Ag@Fe₃O₄ particles synthesized via polyol reduction process rendering dual-functionality for bacteria ablation and dyes degradation

2.2.1 Synthesis of Ag@Fe₃O₄

The core–shell structured Ag@Fe₃O₄ particles were synthesized via a one-step polyol reduction process (Dong et al., 2019; X.h. Meng, X. Shao, H.y. Li, F.z. Liu, X.p. Pu, W.z. Li, C.h. Su, One-step hydrothermal synthesis, characterization and visible-light catalytic property of Ag-reduced graphene oxide composite, Materials Research Bulletin 48(4), 2013; Hou et al., 2017; Jiao et al., 2018; Liang et al., 2023). Specifically, Fe(NO₃)₃·9H₂O (1.212 g), sodium acetate (2.153 g), and AgNO₃ (0.076 g) were sequentially added to ethylene glycol (30 ml) and stirred for 30 min. The resulting mixture was then transferred into a Teflon-lined stainless-steel autoclave and heated at 200 °C for 4 h. Finally, the product was washed three times with deionized water, subjected to centrifugation, and dried at 60 °C overnight to obtain the Ag@Fe₃O₄ particles.

2.2.2 Synthesis of solid Ag cores (c-Ag)

The Ag@Fe₃O₄ was immersed in a 1 mol L⁻¹ oxalic acid solution overnight, resulting in the formation of ferric oxalate and a yellow-green solution due to the reaction between Fe₃O₄ and oxalic acid, thus obtaining the c-Ag (Shrivastava et al., 2022; Yu et al., 2018; Zhang et al., 2014).

2.2.3 Synthesis of hollow Fe₃O₄ shells (s-Fe₃O₄)

The Ag@Fe₃O₄ was immersed in a 0.1 mol L⁻¹ Fe(NO₃)₃ solution and subjected to ultrasonication for 30 min. Afterwards, it was rinsed with deionized water three times and finally dried, resulting in the formation of s-Fe₃O₄ (Chen et al., 2014; Lu et al., 2007; Zhang et al., 2018).

2.4.1 Measurement for the diameter of inhibition zone (DIZ)

The activated E. coli was first diluted to a concentration of 5 × 10⁶ Colony-Forming Units (CFU) per milliliter using physiological saline. Subsequently, 100 μL of the diluted E. coli suspension was evenly spread onto the solid culture medium and placed within an Oxford cup. The c-Ag, s-Fe₃O₄, and Ag@Fe₃O₄ suspensions at various concentrations were placed in separate Oxford cups and incubated at 37 °C for 12 h prior to measuring the diameter of the inhibition zone (Aldosari et al., 2023; Xiong et al., 2013).

2.4.2 Measurement of the minimum inhibitory concentration (MIC)

The MIC refers to the minimum antibacterial concentration that can inhibit bacterial growth (Tang and Zheng, 2018; Mei et al., 2014). A negative control group (only broth) and a positive control group (mixture of bacteria and broth) were set up for comparison. The synthesized particles of various weight (1–10 mg), bacterial solution (100 μL with a concentration of 10⁶ colony forming units (CFU) per mL), and liquid broth (25 mL) were mixed thoroughly and cultured in a constant temperature incubator at 37 °C for 10 h. When the concentration of the composite material in the test tube surpasses the MIC, it effectively impedes bacterial growth and reproduction, resulting in a visibly clear solution within the test tube. Conversely, if the concentration of the composite material is below the MIC, it fails to hinder bacterial reproduction and growth, leading to a persistently turbid solution. Therefore, the MIC value of the composite material can be determined by visually observing the turbidity change in the test tube solution before and after incubation (Parvekar et al., 2020).

2.4.3 Cyclic antibacterial ability

The synthesized particles (10 mg) and a fresh bacterial solution (5 mL at a concentration of 10⁶ CFU per mL) were introduced into a sterile reagent bottle and thoroughly mixed, followed by incubation at room temperature for 150 min. The above suspension was subjected to magnetic separation, and the supernatant containing bacteria (100 μL) was extracted. Subsequently, it was diluted 1000-fold with normal saline before being spread onto solid medium and incubated at 37 °C for 24 h. The colony count was subsequently conducted, followed by the calculation of the bacterial inhibition rate. This was denoted as the first cycle of the antibacterial experiment. After the aforementioned magnetic separation process, the residual bacterial solution was discarded, and the unwashed separated particles were mixed with another fresh bacterial solution (5 mL at a concentration of 10⁶ CFU per mL) (Yong et al., 2018). The subsequent steps were then repeated for the next cycle.

The bacterial inhibition rate is calculated as follows (Wang et al., 2020; Shuai et al., 2020): $$R=\left(B-C\right)/B$$ where R (%) is the bacterial inhibition rate, B denotes the colony count in the control experiment without any treatment, and C represents the colony count after adding the Ag@Fe₃O₄.

3.1.1 SEM and TEM analysis

The scanning electron microscope (SEM) images reveal that the Ag@Fe₃O₄ particles exhibit a high degree of monodispersity, showcasing nearly spherical morphology with an average diameter of approximately 220 nm (Fig. 2a). Upon closer examination of the magnified SEM image, it becomes evident that these spheres possess a non-smooth surface with the presence of numerous small nanoparticles (Fig. 2b). The observed disparity between the inner core and outer shell in the transmission electron microscope (TEM) images of Ag@Fe₃O₄ implies the presence of core/shell structures (Fig. 2c, d). In these images, the Ag cores exhibit a dark appearance while the Fe₃O₄ shells appear lighter due to Ag's higher electron density, resulting in reduced electron transmission (M.E.n.F. Brollo, RománLópez-Ruiz, D. Muraca, S.J.A. Figueroa, K.R. Pirota, M. Knobel, , 2014). The silver cores demonstrate a nearly uniform size of approximately 70 nm, with multiple tiny Fe₃O₄ NPs comprising the shells that possess an average thickness of around 75 nm. The high-angle annular dark-field (HAADF) image (Fig. 2e) and energy dispersive X-ray spectrometer (EDX) mapping images (Fig. 2f) of Ag@Fe₃O₄ clearly demonstrate the distribution of Fe, O, and Ag within the Ag@Fe₃O₄ composite, confirming the successful integration of these elements in the Ag@Fe₃O₄ as well as the Ag-core Fe₃O₄-shell structure.

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Fig. 2

(a, b) SEM, (c, d) TEM, (e) HAADF image, and (f) EDX mapping images of the core–shell structured Ag@Fe₃O₄ nanoparticles; (g) SEM and (h) TEM images of the hollow s-Fe₃O₄ particles.

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The morphology and microstructure of the control samples, including Fe₃O₄ shells (s-Fe₃O₄) and Ag cores (c-Ag), are also examined. Results indicate that s-Fe₃O₄ exhibit a rough texture and discernible pores with a hollow structure (Fig. 2g, h), while c-Ag possess a spherical morphology (Fig. S1, Supporting Information), and their dimensions are consistent with the shell and core of Ag@Fe₃O₄, respectively. The weight percentages of Fe₃O₄ and Ag in the Ag@Fe₃O₄ are determined through the Atomic Absorption Spectroscopy analysis (Table S1, Supporting Information), which are 79.2 wt% and 20.8 wt%, respectively.

3.1.2 DLS analysis

The DLS technique is employed for the analysis of particle size distribution and polydispersity index (PDI) of the Ag@Fe₃O₄ particles. The DLS pattern (Fig. 3a) reveals that the average hydrodynamic diameter of Ag@Fe₃O₄ is 211.3 nm, which aligns with the TEM results. The detected polydispersity index is 0.213, confirming a relatively narrow size distribution of the Ag@Fe₃O₄ nanoparticles (Gong et al., 2007; Tayyaba et al., 2020).

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Fig. 3

(a) The size distribution of the Ag@Fe₃O₄ nanoparticles; (b) XRD patterns and (c) UV–vis spectra of the c-Ag, s-Fe₃O₄, and Ag@Fe₃O₄; (d) N₂ adsorption/desorption isotherm and pore size distribution of the Ag@Fe₃O₄; (e) magnetic hysteresis loops of s-Fe₃O₄ and Ag@Fe₃O₄, the insets are magnetic recovery images of Ag@Fe₃O₄ and magnified magnetic hysteresis loops of s-Fe₃O₄ and Ag@Fe₃O₄; (f) FTIR spectra of the s-Fe₃O₄ and Ag@Fe₃O₄; high-resolution XPS spectra of (g) Fe 2p and (h) Ag 3d in Ag@Fe₃O₄; (i) ESR spectra of Ag@Fe₃O₄ in aqueous solution at the time of 0 min and 2 min.

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3.1.3 XRD analysis

X-ray Diffraction (XRD) patterns provide valuable information on the crystalline structure and composition of materials. The prepared samples all exhibit distinct diffraction peaks, indicating their excellent crystallinity (Fig. 3b). The peaks of c-Ag at 2θ = 38°, 44.2°, 64.3°, and 77.4° correspond to the (111), (200), (220), and (311) crystal planes of Ag, respectively, as identified by the Joint Committee of Powder Diffraction Society PDF card No. 87–0717 (Akhavan, 2009; Jeon et al., 2003; Rajesh, 2012). The peaks of s-Fe₃O₄ at 2θ = 30.2°, 35.5°, 43.1°, 53.4°, 56.9°, 62.6° and 74.1° can be assigned to the (220), (311), (400), (422), (511), (440) and (533) crystal planes of Fe₃O₄, respectively, according to the PDF card No. 72–2303 (Wei et al., 2012; Vijayakumar et al., n.d.). The successful synthesis of Ag@Fe₃O₄ is confirmed by the presence of both Ag and Fe₃O₄ diffraction peaks in its pattern.

3.1.4 UV–vis analysis

The UV–vis spectroscopy offers valuable insights into the electronic structure of diverse materials, as the absorption of light at distinct wavelengths by different materials is attributed to their specific electronic transitions. Fe₃O₄ primarily absorbs light in the ultraviolet (UV) region due to electronic transitions involving its d orbitals. In the visible region, Fe₃O₄ does not have electronic transitions that coincide with the energy levels of visible light, resulting in no significant absorption peaks in this region, which is consistent with the spectrum of s-Fe₃O₄ (Fig. 3c) (Joshi et al., 2014; Sheng and Xue, 2012). The absorption peak of c-Ag is observed at 417 nm (Chen et al., 2010; Height et al., 2006), which is attributed to the phenomenon of surface plasmon resonance (SPR), where light energy is absorbed and leads to the collective oscillation of free electrons on the surface of silver nanoparticles. The peak for Ag@Fe₃O₄ is located at 486 nm, demonstrating a red shift in comparison to Ag. The observed outcome can be attributed to the fact that the refractive index of Fe₃O₄ (2.420) is higher than that of silver (0.399), resulting in a red shift of the SPR peak when silver core is coated with Fe₃O₄ shell (Anandan et al., 2008).

3.1.5 BET analysis

The specific surface area and pore characteristic (Fig. 3d) of Ag@Fe₃O₄ are determined by the BET analysis. According to the classification by the International Union of Pure and Applied Chemistry (IUPAC), the N₂ adsorption/desorption isotherm of Ag@Fe₃O₄ exhibits a Type IV curve with a hysteresis loop, indicating its mesoporous structure (Zhao et al., 2011). The data analysis confirms that the Ag@Fe₃O₄ nanoparticles exhibit a relatively high specific surface area of 13.8 m² g⁻¹ and possess abundant mesopores with an average size of 29.9 nm. The high specific surface area ensures thorough contact between the active sites on Ag@Fe₃O₄ and the target (bacteria or organic dyes), while the abundant mesopores facilitate efficient mass transfer of Ag⁺ or e⁻ from Ag@Fe₃O₄ to the target, both of which contribute to achieving exceptional antibacterial and dye degradation performance.

3.1.6 Magnetic hysteresis loops analysis

The Vibrating Sample Magnetometer (VSM) pattern typically exhibits a hysteresis loop, illustrating the correlation between magnetic field strength and material magnetization, thereby offering insights into the magnetic behavior of the material. The magnetization curve of the s-Fe₃O₄ and Ag@Fe₃O₄ particles at room temperature does not exhibit any noticeable remanence or coercivity, indicating their superparamagnetic behavior (Fig. 3e). The magnetic hysteresis loops indicate that the saturation magnetization of Ag@Fe₃O₄ is approximately 55.5 emu g⁻¹, which is lower than that of s-Fe₃O₄ (72.8 emu g⁻¹). The decrease can be attributed to the diamagnetic nature of Ag, which, when combined with the Fe₃O₄ shell, attenuates the magnetic properties of the s-Fe₃O₄. Nonetheless, the Ag@Fe₃O₄ still exhibits strong magnetic responsivity, as it has been reported that the application of a magnetic field can effectively facilitate the separation of materials from aqueous solutions when their saturation magnetization exceeds 16.3 emu·g⁻¹ (Liu et al., 2015). The strong magnetic responsivity of Ag@Fe₃O₄ is directly demonstrated in the inset in Fig. 3e, where the particles easily attract towards an external magnetic field when a cylindrical NdFeB permanent magnet is positioned adjacent to the vial containing the Ag@Fe₃O₄ suspension. During the process, the Ag@Fe₃O₄ particles rapidly adhere to the inner walls of the vials within 20 s, resulting in a transparent remaining liquid.

3.1.7 FTIR analysis

The distinct peaks in the FTIR spectra can provide valuable information regarding the chemical composition and structure characteristic of the material. The peaks observed at approximately 594 cm⁻¹ and 429 cm⁻¹ in the Ag@Fe₃O₄ spectrum (Fig. 3f) correspond to the stretching vibrations of the tetrahedral (Fe³⁺–O²⁻) and octahedral (Fe²⁺–O²⁻) groups, respectively, confirming the presence of Fe₃O₄ (Husain et al., 2019). The absorption bands at around 3456 cm⁻¹ and 1632 cm⁻¹ can be attributed to vibrational peaks originating from the hydroxyl group on the surface of Ag@Fe₃O₄. Apart from the difference in peak intensity, the peak position of s-Fe₃O₄ is approximately similar to that of Ag@Fe₃O₄ due to silver's lack of absorption in the infrared region.

3.1.8 XPS analysis

The XPS is utilized for the validation of the electronic state and chemical composition of the particles. The XPS full spectrum of Ag@Fe₃O₄ exhibits characteristic peaks corresponding to Fe 2p, O 1 s, and Ag 3d (Fig. S2a, Supporting Information). The peaks observed at 710.2 eV, 711.6 eV, 723.3 eV, and 725.0 eV in the Fe 2p spectrum correspond to Fe²⁺ 2p_3/2, Fe³⁺ 2p_3/2, Fe²⁺ 2p_1/2 and Fe³⁺ 2p_1/2, respectively, which is in consistent with the typical peaks in Fe₃O₄ (Fig. 3g) (Ma et al., 2015; Liang et al., 2018; Wilson and Langell, 2014; Yamashita and Hayes, 2008). The other four peaks are satellite peaks. The O 1 s spectrum can be deconvoluted into two distinct peaks, specifically at 529.8 eV (Fe-O) and 530.8 eV (H-O) (Fig. S2b, Supporting Information), which are associated with the Fe₃O₄ and surface hydroxyl, respectively (Mu et al., 2011). The Ag 3d spectrum is deconvoluted into two distinct peaks representing Ag 3d_5/2 (368.0 eV) and Ag 3d_3/2 (374.0 eV) (Fig. 3h), which align with the established binding energy of metallic silver (Lou et al., 2013; Mottaghi et al., 2014). The XPS results prove the successful synthesis of the Ag@Fe₃O₄ composite.

3.1.9 ESR analysis

The ESR technique is used to qualitatively and quantitatively detect the unpaired electrons present in atoms or molecules of materials, as well as explore the structural properties of their surroundings. The ESR patterns (Fig. 3i) demonstrate the absence of any signal peaks without light (0 min), while six characteristic peaks corresponding to superoxide radical (•O₂⁻) are observed under a mercury lamp for 2 min. The findings indicate that under light conditions, the Ag@Fe₃O₄ exhibits oxidative properties (Lee et al., 2022; Zhao et al., 2019).

3.2.1 Measurement for the diameter of inhibition zone (DIZ)

The antibacterial effectiveness of Ag@Fe₃O₄ against E. coli is evaluated through the assessment for diameter of inhibition zone (DIZ), minimum inhibitory concentration (MIC), and cycling antibacterial assay. The absence of an inhibition zone around s-Fe₃O₄ at both concentrations suggests that s-Fe₃O₄ does not possess antibacterial properties and is incapable of forming an inhibition zone (Fig. 4a). In contrast to s-Fe₃O₄, both c-Ag and Ag@Fe₃O₄ exhibit distinct inhibition zones, indicating their antibacterial properties. The DIZ of c-Ag and Ag@Fe₃O₄ is observed to increase with their concentration, indicating a positive correlation between antibacterial activity and particle concentration. The DIZ of Ag@Fe₃O₄ is evidently larger than that of c-Ag at equivalent concentrations, thereby indicating the superior antibacterial activity of Ag@Fe₃O₄. This is attributed to the tendency of c-Ag to aggregate, which results in a reduced surface area available for bacterial interaction and consequently hinders the release of silver ions responsible for antibacterial activity. In the case of Ag@Fe₃O₄, the porous Fe₃O₄ shells prevent the aggregation of Ag cores and facilitate the sustained release of silver ions, thereby contributing to its superior antibacterial performance.

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Fig. 4

(a) Inhibition zones of the as-synthesized c-Ag, s-Fe₃O₄, and Ag@Fe₃O₄ with different concentrations against E. coli, (b) measurement of the minimum inhibitory concentration of the Ag@Fe₃O₄, (c) cycling antibacterial assessment of the Ag@Fe₃O₄.

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3.2.2 Measurement of the minimum inhibitory concentration (MIC)

The broth microdilution method is employed to investigate the bactericidal effect of Ag@Fe₃O₄ at various concentrations, determining its MIC and quantitatively evaluating its efficacy against bacteria. A negative control group (only broth) and a positive control group (mixture of bacteria and broth) are set up for comparison. The bacteria continue to proliferate in the positive control group, resulting in the turbidity of the solution. The negative control group, however, exhibits a transparent solution due to the absence of bacteria (Fig. 4b). The solutions in the upper row tubes (0.04–0.20 mg mL⁻¹) demonstrate a turbid state similar to that of the positive control group, indicating the sustained proliferation and growth of bacteria at these concentrations of the composite material. Conversely, the solutions in the lower row tubes (0.24–0.40 mg mL⁻¹) exhibit clarity resembling that of the negative control group, thereby suggesting the inhibitory effect of the composite material on bacterial growth and reproduction at these concentrations. Consequently, the MIC for Ag@Fe₃O₄ is determined as 0.24 mg mL⁻¹ (equivalent to 0.042 mg mL⁻¹ in terms of Ag), representing the lowest concentration capable of impeding bacterial growth. The low MIC indicates the superior antibacterial performance of Ag@Fe₃O₄, which can be attributed to its intricate nanostructure. The porous Fe₃O₄ shell not only hinders the aggregation of Ag cores but also extends the release of silver ions, thereby contributing to an efficient and enduring antibacterial effect.

3.2.3 Cycling antibacterial assay

The reusability of Ag@Fe₃O₄ nanocomposite is evaluated by conducting a 10-cycle antibacterial experiment against E. coli (Fig. 4c). The results demonstrate that the antibacterial efficacy of Ag@Fe₃O₄ against E. coli remained stable during the first four cycles; however, it started to decline from the fifth cycle onwards, with an antibacterial efficiency of 74.6 % by the tenth cycle. This phenomenon can be attributed to the inevitable loss of Ag@Fe₃O₄ over multiple cycles, which leads to a diminished inhibitory effect on bacteria. Additionally, bacterial adhesion onto the surface of the material during cycling could hinder direct contact between Ag@Fe₃O₄ and bacteria, thereby reducing its effectiveness. In general, the Ag@Fe₃O₄ composite exhibits exceptional cyclic antibacterial performance due to its delicate structure, in which the porous Fe₃O₄ shell not only ensures sustained release of silver ions but also demonstrates excellent magnetic properties for efficient and high recovery rates under an external magnetic field.

The aforementioned advantages of Ag@Fe₃O₄ function synergistically and contribute to its exceptional antibacterial performance and reutilization, surpassing that of numerous previously reported Ag-based composites (Table S2, Supporting Information), thereby demonstrating its significant potential for medical applications and sewage treatment.

3.2.4 Possible antibacterial mechanism

The Ag@Fe₃O₄ composite demonstrates remarkable antibacterial efficacy, and multiple potential antibacterial mechanisms are implicated (Fig. 5).

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Fig. 5

The diagram illustrating the possible antibacterial mechanism of the Ag@Fe₃O₄.

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Firstly, the negatively charged bacterial cell wall strongly binds to Ag⁺ through Coulombic attraction. On one hand, these Ag⁺ impact the permeability of the bacterial membrane and disrupt both the microbial electron transport system and material transport system. On the other hand, the interaction between Ag⁺ and the cell wall triggers a peptidoglycan reaction, resulting in the degradation of crucial components and functional impairments within bacteria. Furthermore, penetration of Ag⁺ into the cell wall causes rupture and leakage of cellular fluid, ultimately resulting in bacterial death (Akhavan, 2009; Sotiriou and Pratsinis, 2010).

Secondly, upon penetration of the cell wall and entry into the bacterial cell, Ag⁺ selectively binds to specific groups (such as sulfhydryl group) within the bacterial cells, inducing protein coagulation. This process disrupts the enzymatic activity responsible for cellular synthesis, thereby impairing the division and reproduction ability of bacteria, ultimately leading to their demise (Yuranova et al., 2003; Ding et al., 2017).

Thirdly, the silver ions can induce DNA damage in bacteria. On one hand, silver ions can disrupt the structure and function of specific proteins, thereby reducing the efficiency of oxidative phosphorylation and impeding ATP synthesis, which results in impairment to DNA replication. On the other hand, the presence of silver ions induces the generation of reactive oxygen species (ROS, including the superoxide radical of •O₂⁻ confirmed by the ESR analysis), which can displace hydrogen atoms within DNA molecules. This displacement results in structural deformations in bacterial DNA, thereby impeding the synthesis of DNA and proteins and ultimately rendering the bacteria inactive (Li et al., 2005; Sondi and Salopek-Sondi, 2004; Li et al., 2021; Zhang et al., 2023).

The porous Fe₃O₄ shell in the Ag@Fe₃O₄ effectively impedes the aggregation of Ag, ensuring a sustained release of silver ions and exhibiting exceptional magnetic properties. All these factors of Ag@Fe₃O₄ function synergistically to efficiently eradicate bacteria and achieve a high recovery rate, offering great potential for the development of medical devices, antibacterial coatings, and sewage treatment applications.

3.3.1 Catalytic degradation of dyes

The as-synthesized Ag@Fe₃O₄ and its control samples of c-Ag and s-Fe₃O₄ are subjected to the degradation experiment of organic dyes (RB, R6G, and MB) in the presence of NaBH₄. The detailed experimental parameters are as follows: the catalytic temperature is room temperature (∼25 °C), the dye concentration is 5 mg L⁻¹, the solution pH is 7, and the dosage of Ag@Fe₃O₄ is 20 mg per each catalytic experiment. The results indicate very little degradation of RB after 15 min by either pure Ag@Fe₃O₄ (Fig. 6a) or pure NaBH₄ (Fig. 6b), while the presence of both Ag@Fe₃O₄ and NaBH₄ achieves near complete degradation of RB within only 5 min (Fig. 6c). The corresponding digital images visually demonstrate the degradation effect of both Ag@Fe₃O₄ and NaBH₄ towards RB since the solution color gradually faded with the reaction time (Fig. 6d), confirming their highly efficient degradation effect. The degradation effect is further quantitatively assessed by calculating their degradation efficiency (Fig. 6e). The degradation efficiency of both Ag@Fe₃O₄ and NaBH₄ towards RB can reach nearly 100 % within 5 min, whereas after 15 min, the pure Ag@Fe₃O₄ and pure NaBH₄ only achieve approximately 18.7 % and 36.8 %, respectively. The degradation efficiency for the control samples of c-Ag and s-Fe₃O₄ in the presence of NaBH₄ after 5 min are 48.2 % and 83.8 %, respectively, also exhibiting a limited degradation effect towards RB when compared to Ag@Fe₃O₄ (Figs. S3, S4, S5, Supporting Information).

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Fig. 6

The degradation of RB by pure Ag@Fe₃O₄, pure NaBH₄, and “Ag@Fe₃O₄ + NaBH₄”: (a-c) the UV–vis spectra at different time, (d) corresponding digital images, and (e) degradation efficiency over time.

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The superior degradation efficiency of Ag@Fe₃O₄ towards RB can be attributed to multiple factors. Firstly, the Fe₃O₄ shell effectively prevents the agglomeration of the Ag core, thereby preserving a high specific surface area and activity for the catalyst (M.a. Ghasemzadeh, J.s. Ghomi, S. Zahedi, Fe3O4 nanoparticles: A highly efficient and easily reusable catalyst for the one-pot synthesis of xanthene derivatives under solvent-free conditions, Journal of the Serbian Chemical Society 78(6), 2013; Wang et al., 2021). Secondly, the presence of mesopores and abundant active sites in the Fe₃O₄ shell enhances dye adsorption efficiency and facilitates increased contact between dye molecules and Ag core. Thirdly, the Ag with high electrical conductivity serves as an electron mediator, capturing electrons from the nucleophilic NaBH₄ and transferring them to the electrophilic RB during the reaction, ultimately resulting in efficient degradation of the RB (ZhongJie Jiang, 2005; Zheng and Wang, 2012). The degradation performance of RB by Ag@Fe₃O₄ surpasses that of pure c-Ag and s-Fe₃O₄, owing to the synergistic effect resulting from the aforementioned advantages of Fe₃O₄ shell and Ag core.

Similarly, the Ag@Fe₃O₄ demonstrates remarkable degradation efficiency towards various organic dyes, including R6G (Fig. 7) and MB (Fig. 8). The Ag@Fe₃O₄ can completely degrade R6G within 5 min in the presence of NaBH₄, outperforming the control samples of pure Ag@Fe₃O₄ and pure NaBH₄, as evidenced by the UV–vis spectra and corresponding digital images (Fig. 7).

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Fig. 7

The degradation of R6G by pure Ag@Fe₃O₄, pure NaBH₄, and “Ag@Fe₃O₄ + NaBH₄”: (a-c) the UV–vis spectra at different time, (d) corresponding digital images, and (e) degradation efficiency over time.

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Fig. 8

The degradation of MB by pure Ag@Fe₃O₄, pure NaBH₄, and “Ag@Fe₃O₄ + NaBH₄”: (a-c) the UV–vis spectra at different time, (d) corresponding digital images, and (e) degradation efficiency over time.

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The Ag@Fe₃O₄ also exhibit the superior degradation performance towards MB (Fig. 8). The remarkable degradation capability of Ag@Fe₃O₄ towards RB, R6G, and MB proves its relatively universal effect on degrading organic dyes, thereby indicating its potential for practical application in dye-containing wastewater treatment.

3.3.2 Cycling degradation property

To assess the practical applicability of Ag@Fe₃O₄ in wastewater treatment, the investigation into its cycling degradation property is conducted. During the experiment, Ag@Fe₃O₄ is easily reused by being collected with a magnet. The results demonstrate that even after 6 consecutive cycles of reutilization, the Ag@Fe₃O₄ still exhibits a remarkable degradation efficiency of 90.9 % towards RB, highlighting its exceptional potential for practical wastewater treatment (Fig. 9a). Similarly, the Ag@Fe₃O₄ exhibit excellent catalytic degradation efficiency towards Rhodamine 6G (Fig. 9b) and Methylene blue (Fig. 9c) after 6 cycles, both of which achieving 100 %.

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Fig. 9

Cycling degradation performance of the Ag@Fe₃O₄ towards (a) RB, (b) R6G, and (c) MB.

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The decrease in the degradation efficiency of Ag@Fe₃O₄ towards organic dyes with increasing cycle number may be attributed to several factors. Firstly, during the process of magnetic recovery, some loss of Ag@Fe₃O₄ nanoparticles is inevitable (such as incomplete separation of nanoparticles from the solution and adherence of particles to container surfaces), leading to a reduction in degradation efficiency during subsequent cycles due to the decrease in available catalyst. Secondly, during cycling, the surface of Ag@Fe₃O₄ nanoparticles can become fouled with reaction intermediates, by-products, or residual organic dyes, thereby obstructing active sites on the nanoparticles and diminishing their catalytic efficiency. Thirdly, the leaching and loss of the silver component in Ag@Fe₃O₄ during the degradation process can result in a reduction of catalytic activity, as silver plays a crucial role in the degradation mechanism (Wang et al., 2021).

Despite a decrease in degradation efficiency with an increasing cycle number, the overall degradation efficiency of RB, R6G, and MB can still exceed 90.9 % after 6 cycles, highlighting the superior degradation efficiency of Ag@Fe₃O₄ towards organic dyes. The catalytic degradation efficiency of the obtained Ag@Fe₃O₄ towards various organic dyes surpasses that of many previously reported Ag-based composites (Table S3, Supporting Information). Additionally, the Ag@Fe₃O₄ is cost-effective (around $0.17 per gram in terms of chemical reagents, the detailed calculation can be seen in Table S4, Supporting Information), underscoring its potential for practical applications in dye-containing wastewater treatment.

3.3.3 Possible degradation mechanism

The Ag@Fe₃O₄ exhibit exceptional catalytic degradation properties towards organic dyes, involving multiple potential mechanisms. The degradation of RB can be taken as an example (Figure S6, Supporting Information). The cationic dye RB is electrostatically attracted to the hydroxyl groups on the surface of the Fe₃O₄ shell or adsorbed onto the active sites on the Fe₃O₄ shell through van der Waals force. The Ag serves as an electron transfer mediator, facilitating the efficient electron transfer from the nucleophilic NaBH₄ to the electrophilic RB molecules. The electrons induce reduction of the chromophoric structure and disruption of the conjugated system in RB, leading to the decolorization of the dye. The final products may consist of Leuco-Rhodamine B, aminated and de-ethylated derivatives, as well as inorganic byproducts (potentially including borates) (Liu et al., 2023; Wang et al., 2016). The degradation process of R6G (Fig. S7, Supporting Information) and MB (Fig. S8, Supporting Information) is similar to that of RB, except for the potential final products differing from each other.