Synthesis and Characterization of Core–Shell Magnetic Nanoparticles NiFe2O4@Au

In this study, NiFe2O4@Au core–shell nanoparticles were prepared by the direct reduction of gold on the magnetic surface using amino acid methionine as a reducer and a stabilizing agent simultaneously. The obtained nanoparticles after three steps of gold deposition had an average size of about 120 nm. The analysis of particles was performed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-Vis spectroscopy techniques. The results indicate successful synthesis of core–shell particles with the magnetic core, which consists of a few agglomerated nickel ferrite crystals with an average size 25.2 ± 2.0 nm, and the thick gold shell consists of fused Au0 nanoparticles (NPs). Magnetic properties of the obtained nanoparticles were examined with magnetic circular dichroism. It was shown that the magnetic behavior of NiFe2O4@Au NPs is typical for superparamagnetic NPs and corresponds to that for NiFe2O4 NPs without a gold shell. The results indicate the successful synthesis of core–shell particles with the magnetic nickel ferrite core and thick gold shell, and open the potential for the application of the investigated hybrid nanoparticles in hyperthermia, targeted drug delivery, magnetic resonance imaging, or cell separation. The developed synthesis strategy can be extended to other metal ferrites and iron oxides.


Introduction
Reducing cancer mortality and increasing the success of cancer treatment begin with early diagnosis. Most early tumors do not produce any symptoms and are difficult to detect. Therefore, more sensitive materials that can be controlled from the outside and bound by cancer cells inside the body should be used for this purpose. Nanoparticles (NPs) with useful optical or magnetic properties are increasingly being used as a detective tool to spot tumors when they first develop [1,2]. It is possible to incorporate NPs into tumor tissue due to their small size. Then, an oscillating magnetic field can be applied so that magnetic nanoparticles interact with this field to generate heat and destroy cancer cells (the so-called hyperthermia). It is known that the heating of tissues up to 56 • C causes coagulation and carbonization of living cells. However, the temperature range from 40 • C to 46 • C can deactivate cancer cells without damaging healthy tissues. This process can be carried out under the influence of optical radiation (optical hyperthermia) or a high-frequency magnetic field (magnetic hyperthermia). A recent

Synthesis of Nickel Ferrite Nanoparticles
In this study, Ni ferrite nanoparticles were synthesized by the co-precipitation method. In a typical procedure, 10 mL Ni(NO 3 ) 2 (0.4 mol/L) was mixed with 10 mL Fe(NO 3 ) 3 (0.8 mol/L), 0.5 mL tartaric acid (0.1 mol/L), and 0.0728 g cetrimonium bromide (CTAB). The pH of the solution was dropped to 10 by adding 1 mol/L NaOH for 5 min. The mixture was stirred (1000 rpm) for 2 h at a temperature of 25 • C using a magnetic stirrer. The precipitate was centrifuged, washed with distilled water, dried in air at 80 • C for 2 h, and finally annealed in a muffle furnace for 3 h at 650 • C.

Gold Shell Formation
Hybrid NiFe 2 O 4 @Au nanoparticles were fabricated by methionine-directed gold deposition from HAuCl 4 solution according to the modified technique described in [15]. Shortly, 25 mg of nickel ferrite nanoparticles was dispersed in 20 mL of aqueous 0.03 mol/L l-methionine by ultrasonic agitation (ultrasonic bath "Sapphire") for 30 min. Then, 1 mL chlorauric acid (0.1 mol/L) was added to the mixture; the pH was adjusted to 10 by addition of 1 mol/L NaOH. The deposition process was conducted at 37 • C for 4 h under stirring (800 rpm). The products were centrifuged, washed with distilled water, and redispersed in ethanol for further investigations. The procedure of gold deposition was repeated 2 times under the same conditions.

Characterization/Analysis
UV-vis absorption spectra were collected in a glass cell with an optical path of 1 cm employing a Specol spectrometer (Carl Zeiss, Jena, Germany). X-ray photoelectron spectroscopy studies were performed using a hydrosol dried with highly oriented pyrolytic graphite (HOPG) and gently rinsed with water. The spectra were acquired using a Shimadzu XRD-600 diffractometer (Shimadzu Corporation, Kyoto, Japan) employing monochromatic Cu Kα radiation. The analyzer pass energy was 10 eV for high-resolution scans and 20 eV for survey spectra. An electron flood gun was applied to eliminate inhomogeneous electrostatic charging of the samples; the C 1s peak at 284.45 eV from HOPG was used as a reference. The high-resolution spectra were fitted after subtraction of Shirley-type background with Gaussian-Lorentzian peak profiles, using CasaXPS software (version 2.3.16, Casa Software, Teignmouth, UK). Transmission electron microscopy was carried out using an HT-7700 instrument (Hitachi Corporation, Hitachi, Japan) operating at an accelerating voltage of 100 kV. A nickel ferrite particle size distribution histogram has been obtained from more than three hundred particles. Magnetic properties of the obtained nanoparticles were examined with magnetic circular dichroism (MCD)-one of the most informative magneto-optical effects. By being linear in magnetization, the MCD allows one to trace the behavior of the magnetic moment of the sample in an external magnetic field and at various external influences. On the other hand, the study of the spectral dependences of MCD provides information on electronic excitations in a substance under the influence of optical radiation and allows one to obtain the structure of excited levels characteristic of each substance. To carry out the MCD measurements, transparent composite plates containing the NiFe 2 O 4 NPs were prepared according to the method described in [26]. Measurements were carried out in the spectral range of 1.3-3.9 eV in a magnetic field of 1.3 T at the temperature of 300 K also analogously to [26]. The measurement accuracy was about 10 −4 , and the spectral resolution was 20-50 cm −1 depending on the wavelength.

Fabrication and Characterization of Nickel Ferrite Nanoparticles
In this study, Ni ferrite nanoparticles were synthesized by tartaric acid and the cetrimonium bromide-assisted co-precipitation reaction of Ni(II) and Fe(III) salts (pH 10, NaOH) at 25 • C for 2 h followed by several rinsing and centrifugation procedures, as reported earlier [27,28]. The products annealed for 3 h at 650 • C were investigated by the methods of X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The crystalline nature of nickel ferrite NPs was verified by X-ray diffraction investigations. The representative XRD pattern of the NPs is shown in Figure 1a. The position and relative intensity of all diffraction peaks (18.44 (1)), and it was 24.8 nm.
where FWHM is the full-width at half-maximum intensity of the peak, 2θ is the scattering angle in radians, λ is the wavelength (1.5406 Å), b is a constant that is a function of the crystallite geometry, normally taking a value between 0.89 and 0.94, and D is the average size of the crystallites.

Fabrication and Characterization of Nickel Ferrite Nanoparticles
In this study, Ni ferrite nanoparticles were synthesized by tartaric acid and the cetrimonium bromide-assisted co-precipitation reaction of Ni(II) and Fe(III) salts (pH 10, NaOH) at 25 °C for 2 h followed by several rinsing and centrifugation procedures, as reported earlier [27,28]. The products annealed for 3 h at 650 °C were investigated by the methods of X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The crystalline nature of nickel ferrite NPs was verified by X-ray diffraction investigations. The representative XRD pattern of the NPs is shown in Figure 1a. The position and relative intensity of all diffraction peaks ( where FWHM is the full-width at half-maximum intensity of the peak, 2θ is the scattering angle in radians, λ is the wavelength (1.5406 Å), b is a constant that is a function of the crystallite geometry, normally taking a value between 0.89 and 0.94, and D is the average size of the crystallites.   Figure 1d). TEM analysis demonstrates that the particles are faceted in shape and their size is 20-30 nm. This was found to be in good agreement with the crystallite size (15 nm) obtained from the powder XRD data. As a result, we produce the material suitable for hybrid nanoparticles synthesis.   This was found to be in good agreement with the crystallite size (15 nm) obtained from the powder XRD data. As a result, we produce the material suitable for hybrid nanoparticles synthesis.

Fabrication and Characterization of NiFe 2 O 4 @Au
The literature demonstrates two main approaches to the synthesis of hybrid nanoparticles based on magnetite or ferrites and noble metals. The first is the adsorption of gold seeds on the surface of magnetic nanoparticles, the second is a direct gold reduction from HAuCl 4 solution on the NPs surface. We used the second approach adapting the technique from [15]. The deposition process was conducted using 2-amino-4-(methylthio) butanoic acid (methionine) as a reducer of gold species and a stabilizing agent at 37 • C for 4 h under vigorous stirring. The procedure of gold deposition (coating) was repeated two times under the same conditions. Among the 20 natural amino acids that are incorporated into proteins, only the tyrosine, tryptophan, histidine, cysteine, and methionine are oxidized at carbon electrodes and can be used for gold species reduction. However, methionine, unlike the other gold-reducing amino acids, does not form separate gold crystals in the bulk of the solution [15]. In addition, methionine plays the key role as the antioxidant, which defends the structure and stability of proteins [29][30][31][32][33]. Methionine is a sulfur-containing amino acid. The chemical linkage of the sulfur in methionine is a thiol ether. Combining several active centers in the methionine molecule (the sulfur atom, an amine group, and a carboxyl group) results in using it as an anchor between the NiFe 2 O 4 surface and gold. First, methionine is adsorbed on the nickel ferrite NPs surface via NH 2 -groups. Then, methionine reduces tetrachloroauric (III) acid to Au (I), which disproportions to form Au (III) species and metallic gold. Gold attaches to the NPs surface through a thiol ether atom of methionine.
In order to coat particles, nickel ferrite powder was mixed with aqueous solutions of methionine and gold (HAuCl 4 ) (1:6:1 molar ratio, respectively) and stirred for 4 h at 37 • C. The hybrid particles were separated by magnetic separation, washed, and dried.
The obtained particles redispersed in ethanol (sample 1) and the solution remaining after separation of the magnetic particles (sample 2) were studied by a spectrophotometer in the wavelength range of 400-1000 nm. Figure 2 shows that both samples have the absorption maximum of gold nanoparticles [30]: Sample 1 at 560 nm (Figure 2a), sample 2 at 580 nm. However, after the separation of nanoparticles, the solution contains a very small amount of gold. Its optical density is much lower than the optical density of the hydrosol of the obtained particles. This indicates that the reduction of gold occurs not in the volume of the solution but on the surface of the ferrite.

Fabrication and Characterization of NiFe2O4@Au
The literature demonstrates two main approaches to the synthesis of hybrid nanoparticles based on magnetite or ferrites and noble metals. The first is the adsorption of gold seeds on the surface of magnetic nanoparticles, the second is a direct gold reduction from HAuCl4 solution on the NPs surface. We used the second approach adapting the technique from [15]. The deposition process was conducted using 2-amino-4-(methylthio) butanoic acid (methionine) as a reducer of gold species and a stabilizing agent at 37 °C for 4 h under vigorous stirring. The procedure of gold deposition (coating) was repeated two times under the same conditions.
Among the 20 natural amino acids that are incorporated into proteins, only the tyrosine, tryptophan, histidine, cysteine, and methionine are oxidized at carbon electrodes [29,31] and can be used for gold species reduction. However, methionine, unlike the other gold-reducing amino acids, does not form separate gold crystals in the bulk of the solution [15]. In addition, methionine plays the key role as the antioxidant, which defends the structure and stability of proteins [33]. Methionine is a sulfur-containing amino acid. The chemical linkage of the sulfur in methionine is a thiol ether. Combining several active centers in the methionine molecule (the sulfur atom, an amine group, and a carboxyl group) results in using it as an anchor between the NiFe2O4 surface and gold. First, methionine is adsorbed on the nickel ferrite NPs surface via NH2-groups. Then, methionine reduces tetrachloroauric (III) acid to Au (I), which disproportions to form Au (III) species and metallic gold. Gold attaches to the NPs surface through a thiol ether atom of methionine.
In order to coat particles, nickel ferrite powder was mixed with aqueous solutions of methionine and gold (HAuCl4) (1:6:1 molar ratio, respectively) and stirred for 4 h at 37 °C. The hybrid particles were separated by magnetic separation, washed, and dried.
The obtained particles redispersed in ethanol (sample 1) and the solution remaining after separation of the magnetic particles (sample 2) were studied by a spectrophotometer in the wavelength range of 400-1000 nm. Figure 2 shows that both samples have the absorption maximum of gold nanoparticles [30]: Sample 1 at 560 nm (Figure 2a), sample 2 at 580 nm. However, after the separation of nanoparticles, the solution contains a very small amount of gold. Its optical density is much lower than the optical density of the hydrosol of the obtained particles. This indicates that the reduction of gold occurs not in the volume of the solution but on the surface of the ferrite. The obtained hybrid particles were examined by transmission electron microscopy. After a single coating (Figure 3a), the surface of magnetic particles is inlaid with gold seeds (2-3 nm), i.e., gold is not epitaxially deposited on the surface of the ferrite, and the coating is patchy [31]. The electron diffraction spectrum of NiFe2O4@Au NPs after a single coating (Figure 3b) contains diffraction rings corresponding to both nickel ferrite and gold. A thin layer around the particles in the TEM image in Figure 3a is noticeable. From our point of view, the layer is the product of The obtained hybrid particles were examined by transmission electron microscopy. After a single coating (Figure 3a), the surface of magnetic particles is inlaid with gold seeds (2-3 nm), i.e., gold is not epitaxially deposited on the surface of the ferrite, and the coating is patchy [31]. The electron diffraction spectrum of NiFe 2 O 4 @Au NPs after a single coating (Figure 3b) contains diffraction rings corresponding to both nickel ferrite and gold. A thin layer around the particles in the TEM image in Figure 3a is noticeable. From our point of view, the layer is the product of methionine oxidation and it allows for the attachment of the Au colloid to the nanocrystal surface. The adsorbed seeds are densely located, which allows us to expect their fusion during their growth. In order to grow the gold shell, the coating was repeated two more times, replacing the initial ferrite with inlaid magnetic cores.
Metals 2020, 10, x FOR PEER REVIEW 6 of 13 methionine oxidation and it allows for the attachment of the Au colloid to the nanocrystal surface. The adsorbed seeds are densely located, which allows us to expect their fusion during their growth. In order to grow the gold shell, the coating was repeated two more times, replacing the initial ferrite with inlaid magnetic cores. According to the TEM data (Figure 3c), an increase in the number of coating stages leads to the growth of seeds. We observe the formation of a continuous gold shell on the surface of the several agglomerated nickel ferrite crystals. The NiFe2O4@Au hybrid particles diameter increases from 20-30 to 120 nm, which indicates that the gold coating is most likely not a single layer. In addition, the gold coating has significant thickness; diffraction rings of gold are only observed in the electron microdiffraction pattern (Figure 3d). Gold is strongly fixed on the NPs and not detached from the magnetic core under the influence of a magnetic field and during post-synthetic processing (washing, redispersion, ultrasound treatment).  (Figure 4, curve 2). With an increase in the number of coating stages from two (curve 1, Figure 4) to three (curve 2, Figure 4), the intensity of the gold maximum increases and the relative Au 0 concentration increases from 15 to 36%. This indicates a denser coating of the magnetic nuclei. The difference between the TEM and XRD results is explained by the fact that these methods have different resolutions in depth.
We calculate the average nanoparticle crystallite size using the Scherrer equation from XRD patterns of NiFe2O4@Au NPs after two-stage and three-stage gold deposition (Figure 4). Moreover, the XRD peaks of the NiFe2O4 phase (30.33°; 35.73°; 57.45°) and XRD peak of the Au 0 phase (38.1°) According to the TEM data (Figure 3c), an increase in the number of coating stages leads to the growth of seeds. We observe the formation of a continuous gold shell on the surface of the several agglomerated nickel ferrite crystals. The NiFe 2 O 4 @Au hybrid particles diameter increases from 20-30 to 120 nm, which indicates that the gold coating is most likely not a single layer. In addition, the gold coating has significant thickness; diffraction rings of gold are only observed in the electron microdiffraction pattern (Figure 3d). Gold is strongly fixed on the NPs and not detached from the magnetic core under the influence of a magnetic field and during post-synthetic processing (washing, redispersion, ultrasound treatment).  (Figure 4, curve 2). With an increase in the number of coating stages from two (curve 1, Figure 4) to three (curve 2, Figure 4), the intensity of the gold maximum increases and the relative Au 0 concentration increases from 15 to 36%. This indicates a denser coating of the magnetic nuclei. The difference between the TEM and XRD results is explained by the fact that these methods have different resolutions in depth.
Metals 2020, 10, x FOR PEER REVIEW 7 of 13 were used for the calculation separately. The average nanoparticle crystalline size determined for the peaks of the NiFe2O4 phase for both samples (after two-stage and three-stage coating) was 25.2 ± 2.0 nm, which corresponds to the NiFe2O4 particle size before coating. At the same time, the NP sizes determined for the peaks of the gold phase was 23.5 nm (two-stage coating) and 25.5 nm (three-stage coating). These findings confirm that the gold shell covers several nickel ferrite crystals, whose size does not change in the gold deposition process. However, small gold seeds obtained in the first coating step significantly increase in size during the second step and form the continuous gold shell during the third stage. X-ray photoelectron spectroscopy was applied to study the surface composition and chemical state of elements in the hybrid nanoparticles NiFe2O4@Au ( Figure 5). The survey spectrum of gold-coated nickel ferrite nanoparticles (Figure 5a) contains peaks of Au 4f, S 2p, O 1s, and C 1s, while the lines of Fe 2p (Figure 5b) and Ni are almost absent, confirming the formation of a continuous gold shell ( Figure 3). The peak of N 1s was not observed either, while the Au/S atomic ratio was 3.3 (Table 1). This indicates a lack of methionine or methionine sulfoxide CH3-S(O)CH2CH2C(NH2)COOH that is believed to be the product of methionine oxidation by Au 3+ [32]. We calculate the average nanoparticle crystallite size using the Scherrer equation from XRD patterns of NiFe 2 O 4 @Au NPs after two-stage and three-stage gold deposition ( Figure 4). Moreover, the XRD peaks of the NiFe 2 O 4 phase (30.33 • ; 35.73 • ; 57.45 • ) and XRD peak of the Au 0 phase (38.1 • ) were used for the calculation separately. The average nanoparticle crystalline size determined for the peaks of the NiFe 2 O 4 phase for both samples (after two-stage and three-stage coating) was 25.2 ± 2.0 nm, which corresponds to the NiFe 2 O 4 particle size before coating. At the same time, the NP sizes determined for the peaks of the gold phase was 23.5 nm (two-stage coating) and 25.5 nm (three-stage coating). These findings confirm that the gold shell covers several nickel ferrite crystals, whose size does not change in the gold deposition process. However, small gold seeds obtained in the first coating step significantly increase in size during the second step and form the continuous gold shell during the third stage.
X-ray photoelectron spectroscopy was applied to study the surface composition and chemical state of elements in the hybrid nanoparticles NiFe 2 O 4 @Au ( Figure 5). The survey spectrum of gold-coated nickel ferrite nanoparticles (Figure 5a) contains peaks of Au 4f, S 2p, O 1s, and C 1s, while the lines of Fe 2p (Figure 5b) and Ni are almost absent, confirming the formation of a continuous gold shell ( Figure 3). The peak of N 1s was not observed either, while the Au/S atomic ratio was 3.3 (Table 1). This indicates a lack of methionine or methionine sulfoxide CH 3 -S(O)CH 2 CH 2 C(NH 2 )COOH that is believed to be the product of methionine oxidation by Au 3+ [32].  We compared photoelectron S2p spectra from the nanocomposite with those of methionine and the product of its interaction with aqueous AuCl4 − complex (Figure 5d-f). The spectrum of methionine ( Figure 5e) shows a predominant doublet with a binding energy S 2p3/2 of 162.6, which can be assigned to the thioether group C-S-C [34,35]. After the reaction with AuCl4 − at the molar ratio of methionine to Au of 10, the major doublet does not change their position, due to the large excess of methionine (Figure 5f), while an additional minor doublet with the binding energy S2p3/2 of 163.5 eV, which is attributable to a product of methionine oxidation, increases. At the same time, the atomic ratio of sulfur/nitrogen (Table 1) grows from about 1 to 1.4. This may be explained in terms of the dimerization of methionine with the formation of two-center three-electron (2c-3e) S-S bonds [36] or, most likely, the formation of gold sulfide. The lines with S2p3/2 binding energies of ~163.5, 162.5, and 161.6 eV, which are typically assigned to polysulfide, disulfide, and monosulfide species, respectively, were observed both for sulfur chemisorbed on gold and gold (I) sulfide that tends to decompose to Au 0 nanoparticles capped with sulfur [37,38]. This means that the interpretation of photoelectron spectra of gold sulfide still remains unclear.  Figure 2 (three-stage coating), S2p spectra from the methionine (e) and the product of its interaction with aqueous AuCl 4 − complex (f). We compared photoelectron S2p spectra from the nanocomposite with those of methionine and the product of its interaction with aqueous AuCl 4 − complex (Figure 5d-f). The spectrum of methionine ( Figure 5e) shows a predominant doublet with a binding energy S 2p 3/2 of 162.6, which can be assigned to the thioether group C-S-C [34,35]. After the reaction with AuCl 4 − at the molar ratio of methionine to Au of 10, the major doublet does not change their position, due to the large excess of methionine (Figure 5f), while an additional minor doublet with the binding energy S2p 3/2 of 163.5 eV, which is attributable to a product of methionine oxidation, increases. At the same time, the atomic ratio of sulfur/nitrogen (Table 1) grows from about 1 to 1.4. This may be explained in terms of the dimerization of methionine with the formation of two-center three-electron (2c-3e) S-S bonds [36] or, most likely, the formation of gold sulfide. The lines with S2p 3/2 binding energies of 163.5, 162.5, and 161.6 eV, which are typically assigned to polysulfide, disulfide, and monosulfide species, respectively, were observed both for sulfur chemisorbed on gold and gold (I) sulfide that tends to decompose to Au 0 nanoparticles capped with sulfur [37,38]. This means that the interpretation of photoelectron spectra of gold sulfide still remains unclear.
The S2p spectrum acquired from hybrid nanoparticles NiFe 2 O 4 @Au (Figure 5d) is better fitted using a high-intensity doublet with the S2p 3/2 peak at 163.3 eV and a smaller one at 161.8 eV. These signals should be attributed mainly to gold sulfide, and possibly minor chemisorbed sulfur Fe and Ni sulfides, as nitrogen, and so methionine is absent.
The Au 4f 7/2,5/2 spectrum of gold deposited on nickel ferrite (Figure 5c) differs from that of elemental gold and can be fitted by three components. The major doublet with the binding energy of the Au 4f 7/2 peak of 84.1 eV corresponds to metallic Au; the component at 85.0 eV is due to Au + species probably in gold sulfide, in agreement with the S 2p spectra and the relative element ratio (Table 1), and the third doublet has an Au 4f 7/2 binding energy as low as 83.1 eV. Such a significant negative shift is likely associated with electron transfer from the support to the Au particle [39,40].
It has been reported [41] that an excess of oxygen atoms is observed on the surface of oxides as a result of oxygen vacancies in the bulk of the oxide lattice. The gold atoms are adsorbed on these sites and the charge transfers from electron-rich O atoms to Au atoms. As shown in [42], the oxygen vacancies exist in most oxides and they are more typical for nickel ferrite.
Even though lots of research has been done, the chemical mechanism of methionine oxidation by [AuCl 4 ] − ions is still not understood. According to the literature data [32,33], the reaction mechanism takes place in a few steps. The first step is a very fast coordination of methionine (Met (2) Figure 6 shows the MCD spectra of samples containing NiFe 2 O 4 and NiFe 2 O 4 @Au NPs after three-stage gold deposition (empty and filled circles, correspondingly).
Metals 2020, 10, x FOR PEER REVIEW 9 of 13 The S2p spectrum acquired from hybrid nanoparticles NiFe2O4@Au (Figure 5d) is better fitted using a high-intensity doublet with the S2p3/2 peak at 163.3 eV and a smaller one at 161.8 eV. These signals should be attributed mainly to gold sulfide, and possibly minor chemisorbed sulfur Fe and Ni sulfides, as nitrogen, and so methionine is absent.
The Au 4f7/2,5/2 spectrum of gold deposited on nickel ferrite (Figure 5c) differs from that of elemental gold and can be fitted by three components. The major doublet with the binding energy of the Au 4f7/2 peak of 84.1 eV corresponds to metallic Au; the component at 85.0 eV is due to Au + species probably in gold sulfide, in agreement with the S 2p spectra and the relative element ratio (Table 1), and the third doublet has an Au 4f7/2 binding energy as low as 83.1 eV. Such a significant negative shift is likely associated with electron transfer from the support to the Au particle [39,40].
It has been reported [41] that an excess of oxygen atoms is observed on the surface of oxides as a result of oxygen vacancies in the bulk of the oxide lattice. The gold atoms are adsorbed on these sites and the charge transfers from electron-rich O atoms to Au atoms. As shown in [42], the oxygen vacancies exist in most oxides and they are more typical for nickel ferrite.
Even though lots of research has been done, the chemical mechanism of methionine oxidation by [AuCl4] − ions is still not understood. According to the literature data [32,33], the reaction mechanism takes place in a few steps. The first step is a very fast coordination of methionine (Met) to the Au(III) ion. Met acts as a bidentate S-and N-donor ligand and displaces the chloride anion in (2) Figure 6 shows the MCD spectra of samples containing NiFe2O4 and NiFe2O4@Au NPs after three-stage gold deposition (empty and filled circles, correspondingly). The spectra of both samples demonstrate the same shape: Strong positive maximum in the higher-energy part of the spectrum and two close negative maxima are the most prominent features of the spectra. Coincidence of the two curves evidences that the observed MCD is due to the magnetic NPs or magnetic core only. The gold shell can cause an increase in the MCD signal, but this question needs further investigation. As there are no data in the literature on the MCD spectra of stoichiometric NiFe2O4 (possibly because of the difficulties in the measurements in the transmitted light), we can compare them with the spectra of the off-diagonal components of the dielectric tensor, which were determined in several works based on the Kerr effect measurements in the reflected The spectra of both samples demonstrate the same shape: Strong positive maximum in the higher-energy part of the spectrum and two close negative maxima are the most prominent features of the spectra. Coincidence of the two curves evidences that the observed MCD is due to the magnetic NPs or magnetic core only. The gold shell can cause an increase in the MCD signal, but this question needs further investigation. As there are no data in the literature on the MCD spectra of stoichiometric NiFe 2 O 4 (possibly because of the difficulties in the measurements in the transmitted light), we can compare them with the spectra of the off-diagonal components of the dielectric tensor, which were determined in several works based on the Kerr effect measurements in the reflected light.
Such a comparison is fully justified by the fact that the MCD, like the Kerr effect, is determined by these components. In particular, the expression for MCD has the form [43]: where ε · xy and ε · xy are the real and imaginary parts of the off-diagonal component of the dielectric tensor ε, respectively; n and k are the refractive index and absorption coefficient, respectively; and λ is the light wavelength. In the considered spectral region, k is significantly lower compared to n, and θ depends mainly on the first term in brackets. The second term, as well the k and n dependences on the wavelength, can cause some deviations in the MCD spectrum from the spectrum of ε · xy . Comparison of the MCD spectra shown in Figure 6 with the ε · xy spectrum for the epitaxial NiFe 2 O 4 thin film presented in [44] demonstrates coincidence of all their main features, that is, the MCD spectrum observed in the fabricated NPs is typical for the nickel ferrite. At that, the negative MCD maximum centered near 2.55 eV and positive one centered near 3 eV can be associated, according to [44] where ε ⋅ xy and ε ⋅ ′ xy are the real and imaginary parts of the off-diagonal component of the dielectric tensor ε, respectively; n and k are the refractive index and absorption coefficient, respectively; and λ is the light wavelength. In the considered spectral region, k is significantly lower compared to n, and θ depends mainly on the first term in brackets. The second term, as well the k and n dependences on the wavelength, can cause some deviations in the MCD spectrum from the spectrum of ε ⋅ ′ xy .
Comparison of the MCD spectra shown in Figure 6 with the ε ⋅ ′ xy spectrum for the epitaxial NiFe2O4 thin film presented in [44] demonstrates coincidence of all their main features, that is, the MCD spectrum observed in the fabricated NPs is typical for the nickel ferrite. At that, the negative MCD maximum centered near 2.55 eV and positive one centered near 3 eV can be associated, according to [44] Figure 7 is typical for superparamagnetic NPs (NiFe2O4 NPs without gold shell show similar behavior). A similar dependence was observed recently in [45] for NiFe2O4 NPs of 9.2 nm in diameter synthesized by forced hydrolysis. Thus, the magnetic behavior of NiFe2O4@Au NPs corresponds to that presented in the literature for NiFe2O4 NPs.

Conclusions
This study describes a successful synthesis of core-shell nanoparticles with a magnetic nickel ferrite core and a gold shell using amino acid methionine as a reducer and a stabilizing agent simultaneously. The phase composition of the nanoparticles corresponds to the structure of NiFe2O4 and gold with characteristic reflexes: <30.33>; <35.73>; <43.43>, and <38.1>; <44.28>; <64.42>. The obtained nanoparticles consist of a magnetic core, which includes a few agglomerated nickel ferrite crystals with the average size of 25.2 ± 2.0 nm, and the thick gold shell consists of fused Au 0 NPs. The survey spectrum (XPS) of gold-coated nickel ferrite nanoparticles contains peaks of Au 4f, S 2p, O 1s, and C 1s, while the lines of Fe 2p and Ni are almost absent, confirming the formation of a continuous gold shell. The TEM and XRD data show an increase in the number of coating stages, which leads to the growth of seeds on a thick shell. The particle diameter increases from 20-30 nm (for NiFe2O4 NPs) to 120 nm (for hybrid particles). The formation of core-shell NiFe2O4@Au particles is proved by TEM, XRD, and XPS analysis. The magnetic properties of the obtained nanoparticles of magnetic

Conclusions
This study describes a successful synthesis of core-shell nanoparticles with a magnetic nickel ferrite core and a gold shell using amino acid methionine as a reducer and a stabilizing agent simultaneously. The phase composition of the nanoparticles corresponds to the structure of NiFe 2 O 4 and gold with characteristic reflexes: <30.33>; <35.73>; <43.43>, and <38.1>; <44.28>; <64.42>. The obtained nanoparticles consist of a magnetic core, which includes a few agglomerated nickel ferrite crystals with the average size of 25.2 ± 2.0 nm, and the thick gold shell consists of fused Au 0 NPs. The survey spectrum (XPS) of gold-coated nickel ferrite nanoparticles contains peaks of Au 4f, S 2p, O 1s, and C 1s, while the lines of Fe 2p and Ni are almost absent, confirming the formation of a continuous gold shell. The TEM and XRD data show an increase in the number of coating stages, which leads to the growth of seeds on a thick shell. The particle diameter increases from 20-30 nm (for NiFe 2 O 4 NPs) to 120 nm (for hybrid particles). The formation of core-shell NiFe 2 O 4 @Au particles is proved by TEM, XRD, and XPS analysis. The magnetic properties of the obtained nanoparticles of magnetic circular dichroism (MCD) are due to the magnetic core only. The magnetic behavior of NiFe 2 O 4 @Au is typical for superparamagnetic NPs and corresponds to that for NiFe 2 O 4 NPs without a gold shell. The continuous shell of the synthesized particles demonstrates properties that can be used for biomedical applications in cancer diagnosis and treatment. The developed strategy can be extended to other metal ferrites and iron oxides.