## 3 Results
### Physicochemical Analyses
Figure 7: The Au 4f photoelectron spectra of samples AuFe (**a**) and AuFe-MTX (**b**). The binding energies corresponding to the Au\({}^{0}\) and Au\({}^{+}\) states in the Au 4f spectrum of the bimetal sample with MTX are 0.1 eV higher than in the AuFe, whereas it is 0.5 eV higher for the Au\({}^{3+}\) state, which (by analogy with the work by [PERSON] et al. [35]) should be attributed to the interaction of gold atoms with the carboxyl groups of metatrexate. This leads both to an increase in the relative fraction of this state and to an increase in the binding energy of the corresponding doublet. The interaction of MTX with AuFe is accompanied by a threefold increase in the relative proportion of the Au\({}^{+}\) state. In other words, MTX acts as an oxidizer of gold. The Fe 2p spectra of the AuFe and AuFe-MTX samples (Figure 8) include Fe 2p\({}_{3/2}\)-Fe 2p\({}_{1/2}\) doublets with binding energies of 710.8-724.3 and 710.5-724.1 eV, respectively, and a satellite (sat) at ~718 eV. The latter is a fingerprint of the Fe\({}^{3+}\) state. The given energies also correspond to the Fe\({}^{3+}\) state. However, the difference in the binding energies of Fe 2p\({}_{3/2}\) and Fe 2p\({}_{3/2}\)-Fe 2p\({}_{1/2}\) spin-orbit splitting indicate a change in the electron density on the iron atoms after their interaction with MTX, which is also accompanied by a decrease in FWHM from 4.65 to 4.15 eV. The photoelectron spectra of Au 4f and Fe 2p indicate that the introduction of metatrexate into the AuFe changes the charge states of the Au atoms to a greater extent than those of the Fe atoms. In the Au 4f spectrum, a significant increase in the proportion of Au\({}^{+}\) and Au\({}^{3+}\) states is observed, while there is a slight decrease in the signal in the Fe 2p spectrum from the satellite peak and a slight shift of the Fe 2p peak to a low-energy region, indicating an increase in the proportion of the Fe\({}^{2+}\) state. This is manifested in the change in the Fe 2p/Au 4f signal intensity ratio, which indicates the segregation of gold atoms to the surface of the conjugate. Since the sampling depth of the Fe 2p photoelectrons is less than that of the Au 4f photoelectrons, this leads to a noticeable decrease in the Fe 2p's signal intensity. Nevertheless, the bulk content remains unchanged. Figure 9 shows the C 1s spectra of MTX, AuFe and AuFe-MTX samples. An important fact is that it reflects the practical disappearance of the signal at about 283.5 eV, which is characteristic of the metal-carbon bond or low-molar-mass C-C/C-H fragments [36], after the interaction of MTX with AuFe. Figure 10 shows the N 1s spectra approximated by three Gaussian peaks, the characteristics of which are provided in Table 5.  
The peak at 399.9 eV is a characteristic of MTX. The binding energy of 398.8 eV is close to that of pyridine and may also be attributed to the metal-nitrogen bond. However, due to the absence of such bonds in MTX, it can be attributed to intermolecular interactions. At the same time, when interacting with active iron black, the relative intensity of this peak increased approximately three times, and only 1.5 times with less active gold black [29]. In the AuFe-MTX sample, the relative intensity of this peak is the same as in the spectrum of the Fe-MTX sample [29]; therefore, it may be preferentially assigned to the Fe-N bond. Figure 8: The Fe 2p photoelectron spectra of samples AuFe (1) and AuFe-MTX (2). The peak at \(\sim\)401 eV occupies an intermediate position between the values of 400.6 and 401.46 eV, which is characteristic of the C(O)-N-C(O) and NH\({}^{3+}\) groups [37] and, apparently, can be attributed to the intermolecular interactions with the formation of hydrogen bonds between the oxygen atoms of the C(O)N groups and the hydrogen atoms of the neighboring molecule. The size distribution of the AuFe nanoparticles and their conjugates with MTX was analyzed using small-angle X-ray scattering.