## 3 Results
### Surface-Casting  
The FA-PAA coating on the nanoparticles was confirmed from the FT-IR absorption spectrum of the FA-PAA-coated MnO nanoparticles (bottom spectrum in Figure 5a). The FT-IR absorption spectra of FA, PAA, and FA-PAA were also recorded for reference (top three spectra in Figure 5a). The C-H symmetric stretching peak at \(\sim\)2940 cm\({}^{-1}\) was observed in all FT-IR absorption spectra, supporting that the MnO nanoparticles were coated with FA-PAA. The C=C aromatic ring symmetric stretching peak at 1602 cm\({}^{-1}\) in the spectrum of FA also appeared in the spectrum of FA-PAA, confirming the successful conjugation of FA with PAA through an amide bond between the -NH\({}_{2}\) group of FA and the -COOH group of PAA. This peak also appeared in the spectrum of the FA-PAA-coated MnO  
Figure 4: XRD patterns before and after thermogravimetric analysis (TGA). The post-TGA peaks were assigned with (hkl) Miller indices. “L”: cell constant of MnO.  
Figure 3: Photographs of (**a**-**i**) an aqueous nanoparticle dispersion sample (since preparation, no nanoparticle precipitation was observed for >1 year, proving the good nanoparticle colloidal stability) and those of dilute aqueous nanoparticle dispersion samples in the presence of 1.0 mM (**a**-**ii**) Ca\({}^{2+}\) and (**a**-**iii**) Mg\({}^{2+}\) ions (no nanoparticle precipitation occurred for 5 days, indicating no effects of these ions on the nanoparticle colloidal stability). (**b**) Zeta potential curve. (**c**) Tyndall effect showing laser light scattering through a laser path (labeled with an arrow) for the aqueous nanoparticle dispersion sample (right vial), whereas no such effect was observed in the triple-distilled water (left vial), confirming the colloidal dispersion.  
nanoparticles, confirming that the MnO nanoparticles were coated with FA-PAA. The C=O symmetric stretching peak at 1688 cm\({}^{-1}\) in the FA-PAA spectrum was split into COO\({}^{-}\) antisymmetric stretching at 1540 cm\({}^{-1}\) and COO\({}^{-}\) symmetric stretching at 1396 cm\({}^{-1}\) in the spectrum of FA-PAA-coated MnO nanoparticles, which were red-shifted by 148 and 292 cm\({}^{-1}\), respectively. This is owing to the strong coordination bonding between the -COO\({}^{-}\) group of PAA of FA-PAA and the Mn\({}^{2+}\) species on the nanoparticle surface. Such splittings and red-shifts have been observed previously in various ligands with carboxylic groups bonded to metal oxides [31; 32], which support our results.  
The average surface-coating amount of FA-PAA was estimated from the TGA curve (Figure 5b). After heating the powder sample from room temperature to 700 \({}^{\circ}\)C, a decrease in mass was observed in the TGA curve. The initial decrease in mass (14.6 wt%) between the room temperature and \(\sim\)100 \({}^{\circ}\)C was owing to air and water desorption, whereas the following mass decrease (P) (38.1 wt%) was due to the FA-PAA removal from the nanoparticles by flowing hot nitrogen. The remaining mass (47.3 wt%) was attributed to the MnO nanoparticles, as confirmed from the XRD pattern obtained after TGA (Figure 4). Therefore, 44.6 wt% FA-PAA [obtained as 38.1/(1 \(-\) 0.146)] is estimated to be in the FA-PAA-coated MnO nanoparticles. The average number of FA-PAA molecules grafted per nanoparticle unit surface area, i.e., the grafting density (\(\sigma\)) [33], was estimated to be 0.6 nm\({}^{-1}\) using the above-mentioned P-value of FA-PAA, the d\({}_{\mathrm{avg}}\) estimated via HRTEM imaging, and the bulk density of MnO (5.45 g/cm\({}^{3}\)) [34]. The average number (N) of FA-PAA molecules coating a nanoparticle surface was determined as the product of \(\sigma\) by the nanoparticle surface area (\(\tau\mathrm{d}_{\mathrm{avg}}\)\({}^{2}\)), which was found to be \(\sim\)14. Table 1 summarized the surface-coating results.