Introduction

Water resources, particularly groundwater, have become contaminated due to natural and artificial factors, as well as the release of various pollutants from different sources. Among the pollutants, dyes have emerged as a major concern, commonly carried by effluents from various industries, especially the textile industry (Kumar et al. 2022). Methylene blue (MB), crystal violet (CV), and malachite green (MG) are synthetic dyes widely employed in various industrial and laboratory applications (Oladoye et al. 2022). Despite their versatility and utility, these dyes have raised significant environmental concerns due to their potential to cause harm to ecosystems. MB and CV, when released into the environment through textile and industrial processes, can have adverse effects on water quality and aquatic life (Mani and Bharagava 2016). In particular, MG has been associated with environmental damage when used in aquaculture to treat fish diseases, as it can persist in aquatic environments and accumulate in fish tissues, posing risks to both aquatic organisms and human consumers (Kwan et al. 2020).

To minimize the risks of these diseases, it is essential to eliminate these pollutants from the water. Studies have revealed that untreated industrial effluents are one of the primary sources of these contaminants, which significantly affect the ecosystem and living organisms. Several technologies, time-consuming and non-destructive, are currently employed such as ion-exchange, adsorption, chemical precipitation, and membrane systems (Ahmed et al. 2022; Younas et al. 2021). Therefore, scientists are now developing environment-efficient technologies by using nanomaterials that demonstrate extensive potential for degrading hazardous organic dyes in water-based environments.

Nanotechnology as a highly concerned technology is increasingly used in environmental care (Shinde et al. 2023). In recent times, magnetic nanoparticles (MNPs) and bimetallic nanoparticles (BNPs) have gained significant attention due to their supermagnetic characteristics resulting from their small size (Idris and Roy 2023). These magnetic properties allow the potential use of these materials in eliminating and controlling environmental pollutants, as well as in the purification and prevention of their dispersion. Magnetite (Fe3O4) serves as an illustrative instance within this category and finds widespread use due to its robust magnetic attributes, expansive surface area, and limited toxicity, rendering it suitable for applications in both biomedicine and environmental remediation (Díez et al. 2022). These characteristics are contingent upon the methodology employed in the synthesis of MNPs, as it governs their dimensions, size distribution, shape, and consequently, their magnetic behavior. The characteristics of BNPs are influenced by various factors, including the type of reducing agents employed, the redox potentials of the metal ions, and van der Waals attractions among metal atoms (S. Ali et al. 2021). These factors determine whether BNPs will take on configurations such as core-shell, hetero-structure, multi-shell, cluster-in-cluster, or random alloy (Sharma et al. 2019). Additionally, the synthesis method used can impact the crystal structure and distribution of metal ions within the BNPs (F. Ali et al. 2023; N. Zhang et al. 2021). Consequently, the development and utilization of MNPs and BNPs, represent a promising avenue in the quest to address environmental challenges and advance various applications in environmental care (Cheng et al. 2023) and biomedicine (Allam et al. 2021).

Lately, there has been significant interest in the scientific community regarding green synthetic routes for producing nanoparticles (NPs). Many case studies focusing on green synthesis emphasize the use of methods and reagents that minimize the need for toxic reagents, solvents, or by-products during NPs preparation (Devi et al. 2023). The green approach, utilizing bio-active compounds and phytochemicals found in natural biomass, especially plant-based biomass like leaves, flowers, and flowers, has become the most commonly adopted method for preparing and stabilizing NPs (Laib et al. 2023; Rahman et al. 2022). This approach is straightforward, cost-effective, environmentally friendly, and safe for human use, making it highly advantageous for diverse NPs applications. Furthermore, being a one-step process, it also facilitates large-scale production.

Brown Egyptian propolis, derived from resinous substances collected by honeybees from various plant sources in Egypt, has garnered increasing attention for its diverse applications and numerous advantages. This type of propolis contains a complex mixture of bioactive compounds, including flavonoids, phenolic acids, and terpenes, which contribute to its potent antioxidant, antimicrobial, and anti-inflammatory properties (Matar and Andac 2023; Mohamed et al. 2022). These bioactive constituents have been harnessed for medicinal and therapeutic purposes, particularly in traditional and complementary medicine. However, the bio-fabrication of supermagnetic iron oxide nanoparticles (Fe3O4 NPs) and gold-iron oxide bimetallic nanoparticles (Au@Fe3O4 BNPs) using propolis extract has not been previously documented.

The current study was designed to address this gap by synthesizing Fe3O4 NPs and Au@Fe3O4 BNPs using Egyptian propolis extract. The synthesized Fe3O4 NPs and Au@Fe3O4 BNPs were then thoroughly characterized to confirm their particle size, morphology, crystallite size, particle composition, functional groups on the particle surface, and magnetic properties. Also, their photocatalytic potential for degrading basic dyes such as MB, CV, and MG dye as well as antiradical potential were tested. The successful utilization of propolis extract in synthesizing NPs could pave the way for further advancements and discoveries, making these NPs highly versatile for diverse applications in different scientific domains.

Materials and methods

Materials

The propolis samples were collected in the delta region, Egypt. Iron (III) chloride hexahydrate (FeCl3·6H2O), Iron (II) chloride tetrahydrate (FeCl2·4H2O), ammonium hydroxide solution (NH4OH, 28%), Tetrachloroauric (III) acid (HAuCl4·3H2O), and DPPH (1,1-Diphenyl-2-picryl-hydrazyl) were purchased from Sigma-Aldrich, USA. MB, CV, and MG were purchased from INTERLAB Laboratory Products, Turkey. Deionized water was used in all experiments.

Green synthesis of AuNPs, Fe3O4 NPs, and Au@Fe3O4 BNPs

Green synthesis of monometallic and BNPs using propolis extract involves the use of natural compounds present in propolis as reducing and stabilizing agents.

Fe3O4 NPs biosynthesis

Initially, a propolis extract was prepared using a 90% ethanol solvent at a 1:10 (w/v) ratio, according to our previous work (Matar and Andac 2023). Regarding the biosynthesis of supermagnetic Fe3O4 NPs, 5 mL of the extract was added to 50 mL of FeCl3·6H2O (0.2M) and FeCl2·4H2O (0.1M) solutions. The yellowish-colored solution turned black with the addition of extract. Subsequently, 5 mL of NH4OH (28%) was added dropwise, and the solution was vigorously stirred for 1 h. The precipitated Fe3O4 NPs were collected using a magnet, washed with deionized water several times, and then dried at 50 °C in an oven. Later, the powder Fe3O4 NPs was stored at room temperature for further use. The following experimental steps for the biosynthesis of Fe3O4 NPs using propolis extract are presented in Fig. S1 (see Supplementary Material).

AuNPs biosynthesis

For propolis extract-mediated biosynthesis of AuNPs, 1 mL of the extract was added to a solution of HAuCl4·xH2O (1 mM, 50 mL), and the mixture was stirred and heated at 45 °C. The reducing agents present in the extract convert the gold ions into AuNPs as evidenced by the change in color of the solution to color dark purple. The synthesized AuNPs were washed three times with deionized water and then centrifuged at 8000 rpm for 15 min to remove unreacted biomolecules. Finally, the optimum time for green synthesis of AuNPs was recorded at 90 min. After that, the AuNPs were dried and stored at room temperature for further use.

Synthesis of Au@Fe3O4 BNPs

Fe3O4 NPs synthesized as described in the previous step was used in the synthesis of Au@Fe3O4 BNPs. To achieve this, 0.1 g of the prepared Fe3O4 NPs was mixed with a 100 mL aqueous solution of 1 mM HAuCl4·3H2O under sonication for 30 min. The resulting brown-colored dispersion of Fe3O4 NPs in HAuCl4·3H2O solution was vigorously stirred at 45 °C for 10 min. 2 mL of propolis extract was then added drop by drop to the mixture, reducing the gold ions into AuNPs. The reaction was allowed to stir at 45 °C for 90 min. The brownish color solution changed to reddish-brown, indicating the formation and coating of AuNPs on the surface of the Fe3O4 NPs. After the synthesized Au@Fe3O4 BNPs were collected by a magnet, they were washed three times with deionized water and subsequently dried for further use. The following experimental steps for the biosynthesis of Au@Fe3O4 BNPs using propolis extract are shown in Fig. 1.

Fig. 1
figure 1

Schematic representation of the green synthesis of Au@Fe3O4 BNPs using propolis extract

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Characterizations

The synthesis of Fe3O4 NPs, AuNPs, and Au@Fe3O4 BNPs, as well as their photocatalytic activities, were monitored by UV-visible spectroscopy (Thermo Scientific™, USA) at a wavelength ranging from 200-800 nm. FTIR spectroscopy (Perkin-Elmer Spectrum Two FT-IR) was used to determine functional groups on the surface of the Fe3O4 NPs, AuNPs and Au@Fe3O4 BNPs at wavelengths of 4000 – 400 cm-1. FE-SEM images (JEOL-JSM-7001F, Japan) and HR-TEM images (FEI TALOS F200S TEM 200 kV, USA) were deployed to examine the morphology and size of NPs. The elemental composition of surveyed samples was measured with EDX spectra (Oxford X-Max 80 mm2, UK). The crystalline structure of the Fe3O4 NPs and Au@Fe3O4 BNPs powder sample was measured using XRD (Rigaku Smart Lab X-ray diffractometer with monochromatic Cu Kα radiation, Japan) at 2θ ranging from 10° to 80°. The zeta potential of the Fe3O4 NPs and Au@Fe3O4 NPs was carried out by the zeta sizer instrument (Malvern Zetasizer equipped with MPT-2 Titrator, Nano ZS). The Fe3O4 NPs and Au@Fe3O4 BNPs powder sample were analyzed using a vibrating sample magnetometer (VSM) with the model Dexing Magnet VSM 550. The magnetization curve was recorded at room temperature. The chemical state of Fe3O4 NPs and Au@Fe3O4 BNPs was investigated using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA).

Photocatalytic degradation performance of basic dyes

The photocatalytic activities of MB, CV, and MG dyes on Fe3O4 NPs and Au@Fe3O4 BNPs were studied under direct light exposure. Typically, 10 mg of Fe3O4 NPs and Au@Fe3O4 BNPs were added to each of the dye solutions (10 mL, 10 ppm) without the addition of any catalyst. Subsequently, the reaction system was irradiated under visible light (a FCL 22W Daylight fluorescent lamp). The mixture was sampled every 10 min and collected using magnets. A UV-Vis spectrophotometer was used to examine the absorbance of the supernatant obtained with the magnets. Meanwhile, we performed the sorption process of dyes by keeping the mixtures in the dark for the same duration as the removal efficiency time observed in photodegradation for each dye. The degradation efficiency (%) was determined based on the maximum degradation of MB, CV, and MG dye solutions at 665 nm, 580 nm, and 618 nm, respectively. The efficiency of dye degradation was evaluated by applying equation (1).

$$\textrm{Degradation}\ \textrm{efficiency}\ \left(\%\right)=\left(\frac{\textrm{Ao}-\textrm{A}}{\textrm{Ao}}\right)\times 100$$
(1)

Here, Ao and A are the absorbance of dye before and after photo-irradiation, respectively.

Antioxidant activity

The antioxidant activities of propolis extract, Fe3O4 NPs, and Au@Fe3O4 BNPs were evaluated using the DPPH free radical scavenging assay. In this assay, 0.5 mL of the propolis extract, Fe3O4 NPs, and Au@Fe3O4 BNPs at concentrations ranging from 20 to 100 μg/mL were mixed with 1 mL of a 0.1 mM DPPH solution. These mixtures were then incubated in darkness for 30 min. After incubation, the absorbance of these mixtures was measured at a wavelength of 517 nm. As a standard, ascorbic acid was used at the same concentration range. The percentage of free radical inhibition was determined using the following formula (2):

$$\textrm{DPPH}\ \textrm{rcavenging}\ \textrm{ability}\ \left(\%\right)=\left(\frac{\textrm{Ac}-\textrm{As}}{\textrm{Ac}}\right)\times 100$$
(2)

Here, Ac and As are the absorbance of control and after incubation of sample, respectively.

Results and Discussion

UV-visible spectrum analysis

Figure 2 displays the UV-visible absorption spectrum of the propolis extract, Fe3O4 NPs, AuNPs, and Au@Fe3O4 BNPs. The UV-visible spectrum of the propolis extract exhibited a characteristic absorption peak at 270 nm (Fig. 2a), indicating the presence of phenolic and flavonoid compounds in the extract (Matar and Andac 2023). In contrast, the spectrum of Fe3O4 NPs (Fig. 2b) displayed no discernible absorption peak within the measured wavelength range, consistent with previous reports (Desai et al. 2023). To investigate the influence of incubation time on AuNPs synthesis, we monitored the UV-visible spectra at various time intervals (Fig. 2c). Remarkably, we observed a clear trend of increasing surface plasmon resonance (SPR) peak intensity with prolonged incubation time. The highest intensity of the SPR peak, recorded at a wavelength of 547 nm, was achieved at 90 min of incubation, which is consistent with previous reports (Abdelsattar et al. 2022). This result indicates that the synthesis of AuNPs was completed within 90 min, along with the associated processes of reduction and stabilization. In the spectrum of Au@Fe3O4 BNPs, the SPR peak was notably shifted to a longer wavelength (564 nm), confirming the successful synthesis of these BNPs, with the absorption peak consistent with the presence of AuNPs (Fig. 2d). This shift in the absorption of Au@Fe3O4 BNPs can be attributed to the attachment of AuNPs to Fe3O4 NPs. Furthermore, when we applied a magnetic field, both Fe3O4 NPs and Au@Fe3O4 BNPs displayed attraction towards the magnet (as shown in Fig. 1). This magnetic behavior, along with the shift in the characteristic peak from 547 to 564 nm, collectively confirmed the synthesis of Au@Fe3O4 BNPs. These findings align with prior research in the field, where a redshift in the SPR peak of Au@Fe3O4 BNPs is attributed to the dielectric properties of the Fe3O4 NPs (Desai et al. 2023).

Fig. 2.
figure 2

UV-visible spectrum of (a) the propolis extract, (b) Fe3O4 NPs, (c) the effect of reaction time on the green synthesis of AuNPs, and (d) Au@Fe3O4 BNPs

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FTIR spectroscopy

The FTIR spectra of propolis extract, AuNPs, Fe3O4 NPs and Au@Fe3O4 BNPs are shown in Fig. 3a. The propolis extract exhibits a distinct peak at 3312 cm-1, attributed to the phenolic hydroxyl group. Meanwhile, other noticeable peaks at 2923, 2852, 1705, 1603, 1448, 1353, 1157, 1071 and 814, 767, 687 cm-1 (Fig. 3b) can be related to the C–H bond stretching, asymmetrical stretches of CH2, C=O stretching, C=C stretching, aromatic C–H, O–H bending, aromatic C–O bond stretching, stretching of C–O functional groups and C–H bending in the phenolic rings, respectively (Matar and Andac 2023). Along with the mentioned components, those can be derived from the secondary metabolites of the extract, in which polyphenols may facilitate the formation of NPs and prevent agglomeration. In the FTIR spectrum of Fe3O4 NPs, two distinct peaks at 3328 and 547 cm-1 are observed, associated with the characteristic vibrations of O–H and Fe–O, confirming the synthesis of Fe3O4 NPs (Desai et al. 2023). Additionally, the FTIR spectrum of Fe3O4 NPs displays narrow peaks at 1577, 1349, 1167, and 1097 cm-1, which can be attributed to the C=C stretching, O–H bending, aromatic C–O bond stretching, and stretching of C–O functional groups present in flavonoid compounds involved in biogenic synthesis. The FTIR spectrum of AuNPs exhibits characteristic peaks at 3215, 2922, 2851, 1706, 1608, 1447, and 1351 cm-1, corresponding to O–H stretching, C–H bond stretching, asymmetrical stretches of CH2, C=O stretching, C=C stretching, aromatic C–H, and O-H bending. Furthermore, the FTIR spectrum of AuNPs shows sharp peaks at 1156, 1030, and 821, 767, 696 cm-1, which can be attributed to the aromatic C–O bond stretching, stretching of C–O functional groups, and C–H bending in the phenolic rings present in flavonoid and phenolic compounds, which likely act as reducing or capping agents during the green synthesis. Similarly, in the FTIR spectrum of Au@Fe3O4 BNPs, the presence of a peak at 558 cm-1 indicates Fe–O bonding. The FTIR spectrum of Au@Fe3O4 BNPs exhibits peaks similar to the AuNPs spectra, with slight shifts in the distinct peaks to 3282, 2923, 2852, 1716, 1632, 1448, 1451, 1352, 1158, 1031, and 834, 768, 696 cm-1. These shifts are attributed to the biomolecules and functional groups responsible for attaching AuNPs to Fe3O4 NPs, thereby reducing and stabilizing them.

Fig. 3
figure 3

(a) FTIR spectra of purified Fe3O4 NPs, AuNPs and Au@Fe3O4 BNPs, (b) FTIR spectra of propolis extract, (c) XRD patterns of Fe3O4 NPs, AuNPs and Au@Fe3O4 BNPs, and (d) VSM analysis of Fe3O4 NPs and Au@Fe3O4 BNPs

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XRD studies

The XRD pattern was utilized to examine the crystalline nature and size of the AuNPs, Fe3O4 NPs and Au@Fe3O4 BNPs. In Fig. 3c, the XRD pattern reveals strong and intense peaks with no additional impurity crystalline phases, indicating the high crystallinity of these NPs. For AuNPs, Bragg's reflections were observed at 2θ angles of 38.14°, 44.41°, 64.67°, and 77.43°, corresponding to the (111), (200), (220), and (311) crystal planes, respectively. This pattern is consistent with PDF card no. 00-004-0784, confirming the presence of AuNPs. In the XRD pattern of Fe3O4 NPs, Bragg's reflections at 2θ angles of 30.41°, 35.49°, 43.41°, 54.01°, 57.18°, and 62.88° corresponded to the (220), (311), (400), (422), (511), and (440) crystal planes, respectively. This pattern is consistent with PDF card no. 01-080-6403, confirming the presence of Fe3O4 NPs. Upon analyzing the XRD pattern of Au@Fe3O4 BNPs, reflections were found at 2θ angles of 30.16°, 35.57°, 38.15°, 43.24°, 44.37°, 53.77°, 57.22°, 62.78°, 64.58°, and 77.42°, corresponding to Fe (220), Fe (220), Au (111), Fe (400), Au (200), Fe (422), Fe (511), Fe (440), Au (220), and Au (311) crystal planes, respectively. The XRD pattern of Au@Fe3O4 BNPs was similar to both patterns of AuNPs and Fe3O4 NPs, indicating the formation of Au@Fe3O4 BNPs.

Furthermore, the average crystallite size of the NPs was estimated using the Debye-Scherrer (D-S) equation, Williamson-Hall (W-H) method, and Halder-Wagner (H-W) method as described in Equations (3-5) (Nath et al. 2020; Radoń et al. 2023):

$$D=\frac{K\lambda}{\beta\ cos\theta}$$
(3)
$$\beta Cos\theta =\frac{K\lambda}{D_{XRD}}+4\varepsilon Sin\theta$$
(4)
$${\left(\frac{FWHM}{tan\theta}\right)}^2=\frac{K\lambda}{D_{XRD}}\ \frac{FWHM}{tan\theta sin\theta}+{\left(\frac{\varepsilon }{2}\right)}^2$$
(5)

where D is the crystallite size (nm), K is the Scherrer constant (0.94 for spherical crystallites), λ is the X-ray wavelength (0.154 nm), β (FWHM) is the full width of half-maximum signal, and θ is the Bragg’s angle. Accordingly, the average crystallite size of the AuNPs, Fe3O4 NPs, and Au@Fe3O4 BNPs calculated to be 8.59, 11.66, and 14.28 nm, respectively (Table S1).

Also, the average crystallite size of the NPs was determined using the W-H method by plotting βcosθ against 4sinθ and fitting a straight line to the data. In Fig. S2a, b, and e, the slope of the line represents the strain (ɛ), and the y-intercept provides the crystallite size. The average crystallite size of the AuNPs, Fe3O4 NPs, and Au@Fe3O4 BNPs, calculated from the W-H plot, was 6.8, 12.9, and 10.7 nm, respectively (Table S2).

Similarly, the H-W method was used to determine the average crystallite size of the NPs by plotting \({\left(\frac{\textrm{FWHM}}{\textrm{tan}\uptheta}\right)}^2\) against \(\frac{\textrm{FWHM}}{\textrm{tan}\uptheta \textrm{sin}\uptheta}\) (Fig. S2c, d, and f). The average crystallite size of the AuNPs, Fe3O4 NPs, and Au@Fe3O4 BNPs calculated from the H-W plot, was determined to be 6.7, 12.4, and 11.2 nm, respectively (Table S3). It’s noteworthy that the crystallite sizes obtained from both the W-H method and the H-W method are quite similar and also close to that of the S-D equation.

Magnetic properties

The magnetic characteristics of biosynthesized Fe3O4 NPs and Au@Fe3O4 BNPs were assessed using a vibrating sample magnetometer (VSM). In Fig. 3d, it is demonstrated that the Au@Fe3O4 BNPs prepared in this study possess magnetic properties primarily attributed to the presence of Fe3O4 NPs. To verify their magnetic characteristics, we conducted measurements of hysteresis loops at a temperature of 25 °C with an applied magnetic field ranging from -6000 to +6000 Oe. The saturation magnetization (Ms), coercivity (Hc), and saturation remanence (Mr) parameters of Fe3O4 NPs and Au@Fe3O4 BNPs are listed in Table S4. The saturation magnetization (Ms) values for Fe3O4 NPs and Au@Fe3O4 BNPs were found to be 57.22 and 30.99 emu/g, respectively, confirming the supermagnetic nature of both Fe3O4 NPs and Au@Fe3O4 BNPs. The slight decrease in the Ms of Au@Fe3O4 BNPs can be attributed to the presence of Au on the outer surface of the Fe3O4 NPs. This outcome highlights the outstanding magnetic properties of these NPs in comparison to other magnetic NPs (Desai et al. 2023). Due to their impressive magnetic response, these biosynthesized Fe3O4 NPs and Au@Fe3O4 BNPs hold promise for diverse biomedical and environmental applications. In terms of environmental applications, the magnetic properties of the NPs contribute to an increased surface area available for adsorption. This phenomenon is crucial for enhancing the adsorption capacity of the NPs, making them more effective than their non-magnetic counterparts in removing dye molecules from aqueous solutions. Additionally, the magnetic properties of the NPs play a crucial role in their performance in antioxidant applications. The ability to guide and target these NPs using a magnetic field allows for the precise delivery of antioxidants to specific sites within biological systems, thereby preserving their stability and biocompatibility. This targeted approach enhances the therapeutic potential of the NPs in mitigating oxidative stress and related disorders (Moskvin et al. 2021).

FE-SEM, HRTEM with SAED analysis and Zeta potential

The morphology and particle size of Fe3O4 NPs and Au@Fe3O4 BNPs were examined using FE-SEM and TEM (Fig. 4 and Fig. 5). Fig. 4a-c and Fig. 5a-c display FE-SEM and TEM images of Fe3O4 NPs and Au@Fe3O4 BNPs at different magnifications. These images show that the synthesized Fe3O4 NPs had uniform and monodispersive spherical shapes of varying sizes. In contrast, the Au@Fe3O4 BNPs appeared aggregated with spherical, rectangular shapes, and slightly larger sizes. This uniform dispersion of particles may be attributed to the presence of bio-analytes on the particle surface, which acted as stabilizing agents. The analysis of the TEM micrograph showed that the biosynthesized Fe3O4 NPs and Au@Fe3O4 BNPs had an average size of 5.45 ± 1.28 nm and 15.67 ± 4.02 nm, respectively, as shown in the size distribution histogram (Fig. 4d and Fig. 5d). In Fig. 4e and Fig. 5e, HRTEM images at 2 nm show fine fringes within the Fe3O4 NPs and Au@Fe3O4 BNPs. Fig. 5e also depicts the homogeneous presence of Fe3O4 NPs and AuNPs in the HRTEM image of Au@Fe3O4 BNPs, as confirmed through EDX analysis.

Fig. 4
figure 4

FESEM images at (a) 100 nm and at (b) 50 nm scale, TEM micrographs at (c) 100 nm scale, (d) particle size distribution, (e) HR-TEM image at 50 nm and showing fringe spacing at 2 nm scale, and (f) SAED pattern of biosynthesized Fe3O4 NPs

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Fig. 5
figure 5

FESEM images at (a) 100 nm and at (b) 50 nm scale, TEM micrographs at (c) 100 nm scale, (d) particle size distribution, (e) HR-TEM image at 50 nm and showing fringe spacing at 2 nm scale, and (f) SAED pattern of biosynthesized Au@Fe3O4 BNPs

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Furthermore, SAED patterns were utilized to examine the crystalline nature of the synthesized Fe3O4 NPs and Au@Fe3O4 BNPs (Fig. 4f) and (Fig. 5f). The d-spacing of Fe3O4 NPs and Au@Fe3O4 BNPs was calculated through SAED patterns using Image J software (Table S5). The SAED patterns of the Fe3O4 NPs and Au@Fe3O4 BNPs indicated the presence of intense bright spots on the diffraction rings, confirming the polycrystalline nature of these NPs. The diffraction rings of Fe3O4 BNPs were found at interplanar distances of 0.293, 0.252, 0.205, 0.169, 0.160, and 0.146 nm, corresponding to the (220), (311), (400), (422), (511), and (440) planes of Fe3O4. Whereas the SAED pattern of Au@Fe3O4 BNPs in Fig. 6f clearly depicts diffraction spots corresponding to the (220), (311), (400), (422), (511), and (440) planes of Fe3O4, and the planes (111), (200), (220), and (311) were observed for Au. Thus, the measurements obtained from the SAED pattern very closely align with the measurements of XRD analysis.

Fig. 6
figure 6

EDX spectrum and mapping images of (a) biosynthesized Fe3O4 NPs and (b) Au@Fe3O4 BNPs

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The zeta potential of the Fe3O4 NPs and Au@Fe3O4 NPs showed a negative charge, -26.3 mV and -34.7 mV, respectively (Fig. S3). The notable negative zeta potential observed in Fe3O4 NPs and Au@Fe3O4 NPs is associated with their prolonged stability and colloidal structure. This negative charge plays a crucial role in preventing the agglomeration of nanoparticles in the solution by fostering repulsion among them. Since the Fe3O4 NPs and Au@Fe3O4 NPs have a negative charge, they are likely to attract and interact with cationic dyes. In this case, cationic dyes such as MB, CV, and MG dye, which carry a positive charge, would be attracted to the negatively charged surface of these nanoparticles. This electrostatic interaction between the negatively charged nanoparticles and the positively charged cationic dyes can lead to adsorption of the dyes onto the nanoparticle surface.

EDX analysis

The elemental compositions of Fe3O4 NPs and Au@Fe3O4 BNPs were determined by EDX as shown in Fig. 6a, b. In Fig. 6a, the EDX spectrum of the Fe3O4 NPs indicates the predominant presence of carbon (C, 54.3 wt%), oxygen (O, 23.1 wt%), and iron (Fe, 22.6 wt%) within the NPs, confirming the successful synthesis of Fe3O4 NPs. In Fig. 6b, the EDX spectra reveal the presence of elemental carbon (C, 21.4 wt%), oxygen (O, 17.3 wt%), and iron (Fe, 53.9 wt%) along with gold (Au, 7.5 wt%) peaks, confirming the successful synthesis of Au@Fe3O4 BNPs.

Additionally, the EDX spectra of NPs indicate the presence of carbon and oxygen elements, attributed to extract role as the reducing agent in the biosynthesis process. In the mapping spectrum, the red, yellow, blue, and purple dots represent the distribution of C, O, Fe and Au elements within the NPs.

XPS analysis

XPS was used to investigate the oxidation state and chemical composition of the Fe3O4 NPs and Au@Fe3O4 BNPs. Table S6 presents comprehensive data regarding the elemental composition obtained through XPS analysis. Fig. S4 and Fig. 7 show the XPS data for Fe3O4 NPs and Au@Fe3O4 BNPs. Fig. S4a presents the survey scan of the Fe3O4 NPs, confirming the presence of C, O, and Fe in these NPs, which is consistent with the results of the EDX analysis. The Fig. S4c and Fig. 7c show the C 1s spectra of Fe3O4 NPs. As observed, C 1s spectra can be deconvoluted into three peaks at 284.6, 286.2, and 288.5 eV, corresponding to (C-C, C-H), C-O, and C=O, respectively (Ravelo-Nieto et al. 2023). In Fig. S4d and Fig. 7d, the O 1s spectra can be deconvoluted into two peaks located at 530.1 and 531.0 eV, corresponding to the oxygen species in the Fe-O component of magnetite and oxygen in the C-O or O-H components (López et al. 2023). The presence of carbon and oxygen in the Fe3O4 NPs was attributed to the capping of propolis extract biomolecules. Furthermore, Fig. S4b and Fig. 7b display the narrow scan of the Fe 2p spectrum, confirming the presence of characteristic peaks such as Fe2p3/2 (710.7 eV) and Fe2p1/2 (724.4 eV) in the NPs. These peaks can be further subdivided into six peaks at 710.1, 711.2, 713.7, 723.2, 724.6, and 727.2 eV. The peaks at 710.1, 711.2, 723.2, and 724.6 eV indicate the presence of Fe-O bonds in the Fe2+ ion, while the peaks at 713.7 and 727.2 eV were associated with Fe-O bonds in the Fe3+ ion within the Fe3O4 (Rajan et al. 2020). In Fig. 7a, the XPS survey spectrum of Au@Fe3O4 BNPs confirms the presence of C, O, Fe, and Au, which is consistent with the results of the EDX analysis. Notably, the intensity of the C 1s peak increased while the O 1s peak decreased noticeably. This change was due to the presence of Au on the outer surface of the Fe3O4 NPs (Fe3O4 NPs coated with Au). Fig. 7e shows the Au 4f spectra of Au@Fe3O4 BNPs. As observed, the Au 4f spectra can be deconvoluted into two doublet peaks at 84.1 and 87.8 eV, corresponding to Au 4f7/2, and Au 4f5/2, respectively (Naraginti and Li 2017). Moreover, there was almost no difference between the positions of the peaks in the C 1s and Fe 2p spectra for both Fe3O4 NPs and Au@Fe3O4 BNPs. Meanwhile, the peak intensity of the O 1s for Au@Fe3O4 BNPs decreased significantly, indicating that more AuNPs were coated on the surface of Fe3O4 NPs, as well as highlighting the strong interaction between AuNPs and Fe3O4. Taken together, these results confirm the successful fabrication of Au@Fe3O4 BNPs.

Fig. 7
figure 7

(a) XPS survey spectrum of Au@Fe3O4 BNPs, (b) Fe2p spectra, (c) C 1s spectra, (d) O 1s spectra, and Au 4f spectra

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Evaluation of photocatalytic performance

In this section, we assessed the photocatalytic performance of Fe3O4 NPs and Au@Fe3O4 BNPs as nanocatalysts by investigating their ability to degrade the dyes MB, CV, and MG, which are considered as toxic pollutants. After adding Fe3O4 NPs and Au@Fe3O4 BNPs to the dye solution and exposing it to fluorescent lamp light, we recorded the photocatalytic results of MB, CV, and MG dyes at regular time intervals using Fe3O4 NPs and Au@Fe3O4 BNPs. Over time, we observed changes in the absorption spectra of MB, CV, and MG, with characteristic absorption wavelengths (λmax) at 665 nm, 580 nm, and 618 nm, respectively. The absorption peaks of these dyes decreased with time, further supported by the fading of the dye color, as shown in the insets of Fig. S5 and Fig. S6.

Degradation of MB, CV, and MG dyes using Fe3O4 NPs

The photocatalytic performance of biosynthesized Fe3O4 NPs to degrade MB, CV and MG dyes was examined under fluorescent lamp light at room temperature. Fig. 8 represents UV-visible absorption plots of photodegradation of MB, CV, and MG dye treated with Fe3O4 NPs at different time intervals and their degradation percentages. Increasing the contact time resulted in a higher percentage of dye removal for all dyes examined. The ideal contact times for removing MB, CV, and MG were determined to be 70 min, 50 min, and 60 min, respectively, leading to the highest levels of dye removal (Fig. 8a, b, c). As displayed in Fig. 8d, e, f, Fe3O4 NPs degraded the MB, CV, and MG dye at a concentration of 10 ppm with 95.2%, 99.4%, and 96.2% degradation efficiency, respectively. Meanwhile, the removal of MB, CV, and MG dyes using Fe3O4 NPs was performed in the dark for 70 min, 50 min, and 60 min, respectively, as shown in Fig. S7. Still, the removal percentage efficiency was relatively lesser compared to the degradation efficiency of MB, CV, and MG dye in the presence of light (Table S7). The removal efficiency time for the dark reaction was 20 h for MB, CV, and MG dyes, whereas the degradation efficiency time for the light reaction was 70 min, 50 min, and 60 min for MB, CV, and MG dyes, respectively. It is well known that MB, CV, and MG act as cationic dyes in the aqueous solution. The presence of certain forces, such as hydrogen bonding, electrostatic interactions and hydrophobic-hydrophobic interactions, plays a crucial role in regulating the attachment of MB, CV, and MG dye (Cheng et al. 2023). Synthesized Fe3O4 NPs were produced using propolis extract containing bioactive compounds such as flavonoids and phenolic compounds. Analysis of FTIR and XPS spectra revealed the presence of functional groups such as hydroxyl groups (–OH) and carbonyl groups (C=O) on the surface of Fe3O4 NPs. These functional groups can potentially interact with the functional groups of dyes (MB, CV, MG) through hydrogen bonding (Fig. 10a). Additionally, hydrophobic-hydrophobic interactions may form due to the hydrophobic surface of the Fe3O4 NPs, which facilitates increased interactions between the Fe3O4 NPs and (MB, CV, MG) dyes in an aqueous medium (Fig. 10a). Furthermore, the high surface area and high negative surface charge of the Fe3O4 NPs facilitate the physisorption of these dyes onto the Fe3O4 NPs, as well as increased electrostatic interaction between the Fe3O4 NPs and the dye (Fig. 10a).

Fig. 8
figure 8

Absorption spectra of photocatalytic degradation of (a) MB, (b) CV, and (c) MG dye treated with Fe3O4 NPs at different time points and removal efficiency percentages of (d) MB, (e) CV, and (f) MG dye

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Degradation of MB, CV, and MG dyes using Au@Fe3O4 BNPs

High efficiency was observed in the removal of textile-toxic basic dyes (MB, CV, and MG dye) using biosynthesized Au@Fe3O4 BNPs as nanocatalyst at room temperature, without the addition of any catalyst. Figure 9 represents UV-visible absorption plots of photocatalytic degradation of MB, CV, and MG dye treated with Au@Fe3O4 BNPs at different time intervals and their degradation percentages. Increasing the contact time resulted in a higher percentage of dye removal for all dyes examined. The ideal contact times for removing MB, CV, and MG were determined to be 50 min, 30 min, and 50 min, respectively, leading to the highest levels of dye removal (Fig. 9a, b, c). As displayed in Fig. 9d, e, f, Au@Fe3O4 BNPs degraded the MB, CV and MG dye at a concentration of 10 ppm with 97.1%, 99.1%, and 98.1% degradation efficiency, respectively. Meanwhile, the removal of MB, CV, and MG dyes using Au@Fe3O4 NPs was performed in the dark for 50 min, 30 min, and 50 min, respectively, as shown in Fig. S8. Still, the removal percentage efficiency was relatively lesser compared to the degradation efficiency of MB, CV, and MG dye in the presence of light (Table S7). The removal efficiency time for the dark reaction was 20 h for MB, CV, and MG dyes, whereas the degradation efficiency time for the light reaction was 50 min, 30 min, and 50 min for MB, CV, and MG dyes, respectively. This improvement in dye removal percentage with extended contact time can be attributed to the stronger attraction between dye molecules and the active sites on the adsorbent molecules. The effectiveness of Au@Fe3O4 BNPs synthesized through the green method can be attributed to the presence of elemental gold and biomolecule compounds from propolis extract, forming a coating on the Fe3O4 NPs surface. Furthermore, the core-shell structure and the larger surface area of these NPs contribute significantly to their high adsorption capacity for cationic dyes (Das et al. 2023). It appears that the functional groups on the nanoparticle surface play a crucial role in the adsorption process, as indicated by FTIR and XPS analysis. These functional groups can potentially interact with the functional groups of dyes (MB, CV, MG) through hydrogen bonding. These functional groups can potentially interact with the functional groups of dyes (MB, CV, MG) through the presence of certain forces, such as hydrogen bonding, electrostatic interactions, and hydrophobic-hydrophobic interactions (Fig. 10b). There is no significant difference between the adsorption capacities of Fe3O4 and Au@Fe3O4 in the degradation of those dyes. In summary, the results demonstrate that biosynthesized Fe3O4 NPs and Au@Fe3O4 BNPs are effective in adsorbing basic dyes with cationic charged surfaces. Table S8 shows a comparative table of the photocatalytic activity of Fe3O4 NPs and Au@Fe3O4 BNPs with some reported materials.

Fig. 9
figure 9

Absorption spectra of photocatalytic degradation of (a) MB, (b) CV, and (c) MG dye treated with Au@Fe3O4 BNPs at different time points and degradation percentages of (d) MB, (e) CV, and (f) MG dye

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Fig. 10
figure 10

The adsorption mechanism of MB, CV, MG onto (a) Fe3O4 NPs and (b) Au@Fe3O4 NPs

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To check the reusability of Au@Fe3O4 NPs, FTIR analysis was conducted for both fresh and regenerated Au@Fe3O4 NPs. The desorption of the synthesized NPs involved washing them three times with ethanol and subsequently drying them in the oven. According to the FTIR results depicted in Fig. S9, there were no significant changes in the nature of the nanoparticles, confirming the stability of the regenerated Au@Fe3O4. Therefore, these green synthesized stable nanoparticles can be effectively utilized for the removal of toxic dyes from aqueous solutions.

FTIR analysis of MB, CV, and MG after adsorption

After the MB, CV, and MG dye adsorption processes using Fe3O4 NPs and Au@Fe3O4 BNPs, the intensity of the peaks in the adsorbents shifted without any change in their positions (Fig. 11). This shift can be attributed to the interaction and binding of the MB, CV, and MG dyes with the adsorbent surface. In the FTIR spectrum of Fe3O4 NPs-MB and Au@Fe3O4 BNPs-MB, the peaks corresponding to the -OH, C=O, C=C, and C-O-C functional groups shifted to lower wavelengths, indicating adsorptive interactions between these functional groups present on the surfaces of Fe3O4 NPs and Au@Fe3O4 BNPs with the MB dye (Matar and Andac 2023).

Fig. 11
figure 11

FTIR spectra of Fe3O4 NPs before adsorption of MB/CV/MG dyes, (a) after MB adsorption, (b) after CV adsorption, and (c) after MG adsorption; FTIR spectra of Au@Fe3O4 BNPs before adsorption of MB/CV/MG dyes, (d) after MB adsorption, (e) after CV adsorption, and (f) after MG adsorption

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In the FTIR spectra of Fe3O4 NPs-CV and Au@Fe3O4 BNPs-CV, important sharp peaks appeared at 1584 cm-1 and 1361 cm-1 for Fe3O4 NPs-CV, and at 1584 cm-1 and 1360 cm-1 for Au@Fe3O4 BNPs-CV. These peaks are attributed to the C=C aromatic stretching vibration and C–N aromatic tertiary amine stretching vibration, respectively (Foroutan et al. 2021; Shalaby et al. 2021). Distinctive changes were observed in the FTIR spectrum of Fe3O4 NPs-MG and Au@Fe3O4 BNPs-MG. A new sharp peak at 1735 cm-1 appeared in the FTIR spectrum of Fe3O4 NPs-MB, corresponding to the C=O stretching vibration. The other peaks at 1583 cm-1 and 1365 cm-1 are associated with C=C stretching in the benzene rings and C–N stretching vibration, respectively. Similarly, in the FTIR Spectrum of Au@Fe3O4 BNPs-MG, the peaks of the -OH, C=O, C=C, and C-O-C functional groups also shifted to lower wavelengths, along with the C–N stretching vibration (Zhang et al. 2022). These observations confirm the adsorption interaction of the functional groups on the surfaces of Fe3O4 NPs and Au@Fe3O4 BNPs with MB, CV, and MG.

Study of photodegradation kinetics

The Fe3O4 NPs and Au@Fe3O4 BNPs conducted kinetic investigations on the photodegradation of MB, CV, and MG dyes using the pseudo-first-order and pseudo-second-order models (Purkait et al. 2023). These models assist in determining the rate constants for the photodegradation of MB, CV, and MG dyes in the presence of Fe3O4 NPs and Au@Fe3O4 BNPs as photocatalysts, and the rate constants obtained were used to understand the mechanism of the reaction, as indicated by the following equations (6-7):

$$\ln {\textrm{A}}_{\textrm{t}}=-{\textrm{K}}_1\textrm{t}+\ln {\textrm{A}}_{{}^{\circ}}$$
(6)
$$\frac{1}{{\textrm{A}}_{\textrm{t}}}={\textrm{K}}_2\textrm{t}+\frac{1}{{\textrm{A}}_{{}^{\circ}}}$$
(7)

where K1 and K2 are the adsorption rate constants of the Langmuir-Hinshelwood model and the pseudo-second-order model. Once the suitable linear graphs for these models were plotted, we proceeded to compute the constant values, which are presented in Table 1.

Table 1 Kinetics parameters associated with the photocatalytic degradation of MB, CV, and MG dye using Fe3O4 NPs and Au@Fe3O4 BNPs
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Fig. 12 illustrates kinetic graphs representing both the pseudo-first-order model and pseudo-second-order kinetic models of MB-Fe3O4 NPs, CV-Fe3O4 NPs, and MG-Fe3O4 NPs. Table 1 presents a summary of the correlation coefficient (R2) values and various rate constants. The pseudo-first-order kinetics values of K1 and R2 for the MB-Fe3O4 NPs, CV-Fe3O4 NPs, and MG-Fe3O4 NPs are found to be (0.0430 min−1 and 0.9938), (0.0969 min−1 and 0.9730) and (0.0646 min−1 and 0.9466), respectively. In the case of pseudo-second-order kinetics values of K2 and R2 for the MB-Fe3O4 NPs, CV-Fe3O4 NPs, and MG-Fe3O4 NPs are found to be (0.1736 min−1 and 0.8454), (2.2191 min−1 and 0.5788) and (0.2263 min−1 and 0.9867), respectively. It's evident that, in terms of contact time, the pseudo-first-order kinetics proved to be the most favorable for degrading MB, CV, and MG using Fe3O4 NPs. Their respective R2 values were notably high at 0.9908, 0.9336, and 0.8834, respectively.

Fig. 12
figure 12

(a) Variation of absorbance (At) of MB, CV, and MG dye degradation with time; (b) pseudo-second-order kinetics of MB, CV, and MG dye degradation by Fe3O4 NPs; (c) Variation of absorbance (1/At) of MB, CV, and MG dye degradation with time; (d) pseudo-first-order kinetics of MB, CV, and MG dye degradation by Fe3O4 NPs

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Fig. 13 illustrates kinetic graphs representing both the pseudo-first-order model and pseudo-second-order kinetic models of MB-Au@Fe3O4 BNPs, CV-Au@Fe3O4 BNPs, and MG-Au@Fe3O4 BNPs. Table 1 presents a summary of the correlation coefficient (R2) values and various rate constants. The pseudo-first-order kinetics values of K1 and R2 for the MB-Au@Fe3O4 BNPs, CV-Au@Fe3O4 BNPs, and MG-Au@Fe3O4 BNPs are found to be (0.0652 min−1 and 0.9892), (0.1686 min−1 and 0.9886) and (0.0881 min−1 and 0.9717), respectively. In the case of pseudo-second-order kinetics values of K2 and R2 for the MB-Au@Fe3O4 BNPs, CV-Au@Fe3O4 BNPs, and MG-Au@Fe3O4 BNPs are found to be (0.3536 min−1 and 0.7059), (3.3387 min−1 and 0.8832) and (0.5187 min−1 and 0.9192), respectively. It's evident that, in terms of contact time, the pseudo-first-order kinetics proved to be the most favorable for degrading MB, CV, and MG using Au@Fe3O4 BNPs, with corresponding R2 values of 0.9751, 0.9768, and 0.9448, respectively.

Fig. 13
figure 13

(a) Variation of absorbance (At) of MB, CV, and MG dye degradation with time; (b) pseudo-second-order kinetics of MB, CV, and MG dye degradation by Au@Fe3O4 BNPs; (c) Variation of absorbance (1/At) of MB, CV, and MG dye degradation with time; (d) pseudo-first-order kinetics of MB, CV, and MG dye degradation by Au@Fe3O4 BNPs

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DPPH radical scavenging activity

The DPPH radical scavenging activity of propolis extract, Fe3O4 NPs, and Au@Fe3O4 BNPs was evaluated. In an ethanolic solution of DPPH, there exists a DPPH• free radical, which readily accepts either an electron or hydrogen to transform into a stable diamagnetic molecule known as DPPH-H (Hertadi et al. 2020). The introduction of propolis extract, Fe3O4 NPs and Au@Fe3O4 BNPs caused the donation of electrons to DPPH• free radicals, resulting in the conversion of the purple-colored free radicals into a pale yellowish DPPH-H form. Figure 14 represents UV-visible absorption plots of DPPH• scavenging using propolis extract, Fe3O4 NPs, and Au@Fe3O4 BNPs at various concentrations (20, 40, 60, 80, and 100 μg/mL), along with their respective percentages of inhibition of DPPH•. Consequently, there was a reduction in absorbance observed at 517 nm as the concentration of propolis extract, Fe3O4 NPs and Au@Fe3O4 BNPs increased. The antioxidant capacity of propolis extract, Fe3O4 NPs and Au@Fe3O4 BNPs exhibited an upward trend with increasing concentration, as depicted in Fig. 14d. Within the concentration range of 20–100 μg/mL, the inhibition of DPPH• free radicals increased from 15.8% to 85.2% for propolis extract, 11.2% to 84.2% for Fe3O4 NPs, and 22.7% to 92.6% for Au@Fe3O4 BNPs, compared to the standard ascorbic acid (which exhibited inhibition rates between 84.5% and 95.4%). The IC50 values for the propolis extract, Fe3O4 NPs and Au@Fe3O4 BNPs in the DPPH radical scavenging method were determined to be 55.23 ± 0.98 μg/ml, 61.09 ± 1.31 μg/ml, and 40.08 ± 1.94 μg/ml, respectively. Among them, Au@Fe3O4 BNPs exhibited the most potent antioxidant activity, as indicated by the lowest IC50 value in the DPPH assay.

Fig. 14
figure 14

UV-visible spectrum of DPPH• scavenging using (a) propolis extract, (b) Fe3O4 NPs and (c) Au@Fe3O4 BNPs; (d) Percentage inhibition of DPPH• using ascorbic acid (as standard), propolis extract, Fe3O4 NPs and Au@Fe3O4 BNPs. Statistical analysis: the values are presented as mean ± standard deviation

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Conclusion

A sustainable and cost-efficient approach has been utilized to produce environmentally friendly adsorbents as highly functional magnetic bimetallic NPs. Specifically, Fe3O4 NPs and Au@Fe3O4 BNPs were synthesized using propolis extract as both a reducing and capping agent. The UV-visible absorption spectra confirmed the presence of characteristic absorption peaks and the successful formation of these NPs. The FTIR analysis provided insights into the functional groups responsible for reducing and stabilizing the NPs. XRD analysis revealed the nature crystallinity of both Fe3O4 NPs and Au@Fe3O4 BNPs, with characteristic diffraction peaks matching their respective crystal structures. The VSM measurements demonstrated the supermagnetic nature of these NPs. Morphological analysis through FE-SEM and TEM showed that Fe3O4 NPs had uniform spherical shapes with an average size of 5.45 ± 1.28 nm, whereas Au@Fe3O4 BNPs exhibited various shapes and aggregated structures with an average size of 15.67 ± 4.02 nm. XPS analysis provided insights into the oxidation states and surface chemistry of Fe3O4 NPs and Au@Fe3O4 BNPs, further confirming the successful synthesis and the presence of functional groups on their surfaces. The photocatalytic performance of Fe3O4 NPs and Au@Fe3O4 BNPs in degrading cationic dyes (MB, CV, MG) was highly efficient, with Fe3O4 NPs exhibiting excellent dye removal percentages: 95.2% in 70 min for MB, 99.4% in 50 min for CV, and 96.2% in 60 min for MG. Au@Fe3O4 BNPs, with their unique surface properties, also demonstrated dye removal percentages: 97.1% in 50 min for MB, 99.1% in 50 min for CV, and 98.1% in 60 min for MG. The kinetic analysis revealed that the pseudo-first-order kinetics model was the most suitable for describing the dye degradation process. Moreover, the DPPH radical scavenging assay demonstrated the antioxidant potential of propolis extract, Fe3O4 NPs, and Au@Fe3O4 BNPs; among these, Au@Fe3O4 BNPs exhibited the highest antioxidant activity. These newly synthesized NPs hold promise for a wide range of applications, including wastewater treatment, catalysis, and biomedical fields, owing to their magnetic, catalytic, and antioxidant properties.