## 3 Results
### Characterization of Bare and Grafted CeO\({}_{2}\) Particles
#### 3.1.2 Spectroscopic Characterization
Moreover, the UV technique was used to find the differences between the samples with polymer brushes and the initial CeO\({}_{2}\) in different pH 4, neutral at pH 7, and basic at pH 10). The different functional polymers grafted on the bare particles can lead to different responses in different pH levels, which can be seen in Figure 5.  
The CeO\({}_{2}\) showed a maximum absorption of around 302-303 nm [34; 35] or 350 nm [36]. The differences observed were because of the size of the particles obtained at different pH levels during synthesis. The environment can alter the absorbance maximum. It can lead to a bathochromic shift (red shift) when the maximum shifts to longer wavelengths or hyposochromic shift (blue shift) when the maximum shifts to shorter wavelengths. Many parameters can affect the shifting, like conjugation, steric hindrance, and changes in pH. In the case of acidic molecules, a change through acidic to basic conditions generally leads to red shifting, but the other parameters have to be taken into consideration. In addition, the red shift can be found in the presence of a high amount of hydrogen bonds [37]. From the UV absorbance results (Figure 5), it can be seen that the bare CeO\({}_{2}\) particles showed a blue shift in the absorbance compared with the composite particles for all three different pHs. This was something that was expected, since the chemical environment of the particles changed from the presence of the polymers. When studying the case of the bare CeO\({}_{2}\), the pH affected the nanoparticles, as a blue shift (from 340 nm to 306 nm)  
Figure 5: UV results of the CeO\({}_{2}\) and CeO\({}_{2}\)@polymers for three different pH synthesis conditions: acidic, neutral, and basic. was observed when the synthesis was performed at neutral conditions with respect to the acidic one. Then, for the basic synthesis conditions, a small red shift was observed (from 306 nm to 310 nm). These changes were likely induced by the size alterations of the CeO\({}_{2}\) particles because of the dominant presence of Ce\({}^{4+}\) oxygen charge transfer occurring on the low-coordination Ce\({}^{4+}\) ions [25; 38]. Ce\({}^{3+}\) tends to release into the solution if the pH is less than three, causing the particles to reductively dissolve and shrink but be retained in the nanoparticles if the solution is less acidic. For particles of any size, nearly every metal ion at the surface can be reduced [39]. In the case of CeO\({}_{2}\)@PAA, it can be seen that through acidic to neutral and basic pHs, the shift was also blue, meaning that the maximum decreased from 344 nm to 329 nm to 314 nm. This can be expected due to the lower amount of hydrogen bonding, which decreases for the PAA chains when the pH increases. At a low pH, the predominant forces have an associative behavior that changes to repulsive electrostatic at a high pH. The conformational changes of the PAA chains affect the size and aggregation of the particles with PAA, which has an impact on the maximum [40]. Similar behavior was observed for CeO\({}_{2}\)@PNVCL; the maximums were blue shifted from 345 nm to 330 nm to 314 nm, again arising from the hydrogen bonding and structural changes derived from the CeO\({}_{2}\). Moreover, for the case of CeO\({}_{2}\)@PMETAC, there was a blue shift again via acidic pH from 342 nm to 314 nm at a basic pH (the absorbance in the neutral pH was almost not detectable). The blue shift is connected with the interparticle distance that changes with the pH [41]. Finally, at all pH conditions, the maximum with polymer brushes was always at longer wavelengths, meaning a red shift occurred, which was related to the change of the chemical environment of the CeO\({}_{2}\) due to the presence of the polymer brushes.