## 3 Results and Discussion  
### Sample Characterization  
TEM images of uncoated Fe\({}_{3}\)O\({}_{4}\) and C@Fe\({}_{3}\)O\({}_{4}\) (Figure 1a,b) show individual particles to be spherical, although these undergo aggregation. Uncoated Fe\({}_{3}\)O\({}_{4}\) particles are larger (diameter 40-70 nm) than those of C@Fe\({}_{3}\)O\({}_{4}\). This finding is consistent with previous work reported by [PERSON] et al. [34] and [PERSON] et al. [47]. With the addition of D-glucose in the reaction media, a thin carbon shell coating clusters of the magnetite particles are formed (cluster size 50-80 nm, Figure 1b). HRTEM (Figure 1c) reveals nanostructural detail of these C@Fe\({}_{3}\)O\({}_{4}\) clusters, highlighting the amorphous carbon layer and crystalline nature of the magnetite core, as indicated by the lattice spacing (0.29 nm) being consistent with the (002) plane of magnetite [48]. The smaller size of C@Fe\({}_{3}\)O\({}_{4}\) particles relative to uncoated Fe\({}_{3}\)O\({}_{4}\) suggests that glucose acts as both a precursor of carbon shell formation and crystallization inhibitor, resulting in slow Fe\({}_{3}\)O\({}_{4}\) crystal growth. L-Cysteine loading appears to have no effect on the Cys-C@Fe\({}_{3}\)O\({}_{4}\) cluster size and the size of Fe\({}_{3}\)O\({}_{4}\) particles (Figure 2), reflecting the stability of carbon-coated magnetite towards surface functionalization. Figure 1: TEM images of (**a**) uncoated Fe\({}_{3}\)O\({}_{4}\), (**b**) C@Fe\({}_{3}\)O\({}_{4}\), and (**c**) C@Fe\({}_{3}\)O\({}_{4}\). Figure 3a shows PXRD patterns of uncoated Fe\({}_{3}\)O\({}_{4}\), C@Fe\({}_{3}\)O\({}_{4}\), and Cys-C@Fe\({}_{3}\)O4 particles and that of bulk L-cysteine for comparison. The characteristic peaks of magnetite (Fe\({}_{3}\)O\({}_{4}\), cubic phase, JCPDS file No. 75-1609) [36; 48] appear in the diffraction patterns of all particles. However, the intensity of these magnetite peaks in C@Fe\({}_{3}\)O\({}_{4}\) and Cys-C@Fe\({}_{3}\)O\({}_{4}\) samples is significantly lower than those in uncoated Fe\({}_{3}\)O\({}_{4}\), possibly related to shielding by the surface layers [49]. XRD peaks corresponding to graphitic carbon were not observed in C@Fe\({}_{3}\)O\({}_{4}\) and Cys-C@Fe\({}_{3}\)O\({}_{4}\), suggestive of amorphous carbon coatings in agreement with HRTEM observations. Additional XRD peaks corresponding to crystalline cysteine (2-theta: 18.3\({}^{\circ}\), 28.5\({}^{\circ}\), 31.4\({}^{\circ}\), and 34.2\({}^{\circ}\)) being observed in 1000 Cys-C@Fe\({}_{3}\)O\({}_{4}\) is probably the result from excess cysteine deposited on the carbon shell. The surface functionality of prepared sorbents was probed using FTIR. As shown in Figure 3b, the characteristic Fe-O stretching vibration appears at 570 cm\({}^{-1}\) in all Fe\({}_{3}\)O\({}_{4}\)-based samples [36]. Peaks corresponding to C-OH stretching and O-H bending (1069 cm\({}^{-1}\)) occur in C@Fe\({}_{3}\)O\({}_{4}\), with C=C vibrations at 1620 cm\({}^{-1}\) reflecting the successful carbonization of glucose to the coating layer [29]. Moreover, the relative intensity of the O-H vibration (3600-3100 cm\({}^{-1}\)) is significantly higher in C@Fe\({}_{3}\)O\({}_{4}\) samples relative to uncoated magnetite, indicating the greater abundance of these groups on the carbonized surface. The existence of these surface groups facilitates appendage of metal binding functionalities, in this case L-cysteine, to the particle periphery [34]. For Cys-C@Fe\({}_{3}\)O\({}_{4}\) samples, the presence of L-cysteine was confirmed by the N-H vibration overlapping with the O-H  
Figure 3: (**a**) Powder X-ray diffraction (PXRD) patterns, (**b**) FTIR spectra, (**c**) thermogravimetric (TG) curves, and (**d**) magnetic behavior of uncoated Fe\({}_{3}\)O\({}_{4}\), C@ Fe\({}_{3}\)O\({}_{4}\), and Cys-C@Fe\({}_{3}\)O\({}_{4}\) materials. The Y axis in (**a,b**) are given in arbitrary unit. stretching frequency at 3600-3100 cm\({}^{-1}\), and two bands at 1600 cm\({}^{-1}\) and 1240 cm\({}^{-1}\) attributed to C=O and C-O stretching of the amino acid, respectively [29].