## 3 Results and Discussion
### Sample Characterization
Moreover, further peaks at ~1500 cm\({}^{-1}\) (N-H bending) and 2590 cm\({}^{-1}\) (S-H stretching) are evident. These results confirm the grafting of L-cysteine to the particle surface, which most likely occurs via condensation between surface hydroxyl groups and the carboxylic acid moiety in L-cysteine [50]. CHNS analysis results were employed to confirm the presence of cysteine and to determine the cysteine content in Cys-C@Fe\({}_{3}\)O\({}_{4}\) samples. These results suggest that cysteine content increases with cysteine loading, as found to be 0.0563, 0.0963, and 0.1828 mmol per gram of solid samples of the 250 Cys-C@Fe\({}_{3}\)O\({}_{4}\), 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\), and 1000 Cys-C@Fe\({}_{3}\)O\({}_{4}\), respectively. Thermogravirtic (TG) analysis plots for all particle systems and that of bulk cysteine are illustrated in Figure 3c. The TGA profile of uncoated Fe\({}_{3}\)O\({}_{4}\) shows around 2% weight loss between 100 and 400 \({}^{\circ}\)C, possibly due to loss of residual water and surface hydroxyl groups. On the other hand, three distinct weight loss steps are evident for C@Fe\({}_{3}\)O\({}_{4}\): 40-200, 200, and \(>\)500 \({}^{\circ}\)C, corresponding to the loss of surface hydroxyl groups and water molecules and the combustion of amorphous carbon shells [34]. In addition, the TGA curve of bulk cysteine highlights decomposition occurring between 200 and 450 \({}^{\circ}\)C, while those of Cys-C@Fe\({}_{3}\)O\({}_{4}\) particles show three weight loss steps, below 230, 230-270, and \(>\)500 \({}^{\circ}\)C, which coincide with the loss of residual water and hydroxyl groups, cysteine decomposition, and the decomposition of the carbon shell, respectively [33; 50]. With no change in weight after 680 \({}^{\circ}\)C, the total weight loss results are indicative of 81% wt/wt. Fe\({}_{3}\)O\({}_{4}\) in C@Fe\({}_{3}\)O\({}_{4}\) material. On the other hand, the steep slopes at around 800 \({}^{\circ}\)C in in TG plots of 250 Cys-C@Fe\({}_{3}\)O\({}_{4}\), 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\), and 1000 Cys-C@Fe\({}_{3}\)O\({}_{4}\) systems indicate slower combustion (compared with C@Fe\({}_{3}\)O\({}_{4}\)), suggesting superior thermal stability arising from the presence of the cysteine layer. Furthermore, the residual mass of solid samples at 800 \({}^{\circ}\)C suggests the content of Fe\({}_{3}\)O\({}_{4}\) in 250 Cys-C@Fe\({}_{3}\)O\({}_{4}\), 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\), and 1000 Cys-C@Fe\({}_{3}\)O\({}_{4}\) to be not more than 50%, 49%, and 40% wt/wt., respectively, and reflect the increase in cysteine content with initial loading level, as indicated from elemental sulfur analysis. Magnetic properties of all Fe\({}_{3}\)O\({}_{4}\)-based samples were examined using VSM at room temperature. Results depicted in Figure 3d show that the saturation magnetization of uncoated Fe\({}_{3}\)O\({}_{4}\), C@Fe\({}_{3}\)O\({}_{4}\), 250 Cys-C@Fe\({}_{3}\)O\({}_{4}\), 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\), and 1000 Cys-C@Fe\({}_{3}\)O\({}_{4}\) samples were about 78, 51, 50, 49, and 45 emu.g\({}^{-1}\), respectively. Reductions in saturation magnetization after carbon coating and cysteine functionalization can be attributed to the decreasing fraction of magnetic material in these particles relative to diamagnetic coating materials (carbon and cysteine) [50; 51]. Figure S1 shows N\({}_{2}\) adsorption-desorption isotherms and pore size distributions of all samples, with porous structural data listed in Table 1. It can be concluded that isotherms of all materials are of type-IV with a type-H3 hysteresis loop, typical of mesoporous materials [51]. The greater specific area of C@Fe\({}_{3}\)O\({}_{4}\) (90 m\({}^{2}\cdot\)g\({}^{-1}\)) over that of uncoated Fe\({}_{3}\)O\({}_{4}\) (55 m\({}^{2}\cdot\)g\({}^{-1}\)) may be a consequence of the presence of pores in the carbon coating.