## 3 Results and Discussions  
Figure 1a shows the fluorescence emissions of ceria NPs at around 530 nm, under a violet excitation of 430 nm, due to the formation of tri-valent trap states of cerium ions, which corresponded to 5d-4f transition [5]. In addition, the plasmonic resonance emissions spectra of the used Au NPs is shown in Figure 1b, and both CdS/ZnS, and CdSe/ZnS QDs are shown together in Figure 1c[16; 17]. The cross-section absorptions of plasmonic nanostructures show the wavelength resonance whether within the excitation wavelength of ceria at 430 nm or in the visible emissions at 530 nm. Figure 2 shows the fluorescence emissions of different samples of ceria NPs embedded with plasmonic nanostructures, under a violet excitation of 430 nm. All plasmonic-ceria NPs show an enhanced fluorescence visible emissions intensity compared to pure ceria NPs. That provides evidence of the optical coupling between the plasmonic resonance wavelength of the plasmonic nanostructures in general within either fluorescence excitation or the emissions spectrum region. However, the results show that Au NPs give a better emissions intensity when coupled with ceria fluorescence compared to quantum dots. Figure 1: (**a**) Fluorescence emissions intensity of ceria nanoparticles with no added plasmonic nanostructures under 430 nm excitation, and absorption spectra for (**b**) gold nanoparticles with a plasmonic resonance wavelength of around 530 nm, (**c**) CdS/ZnS with a plasmonic resonance wavelength of around 430 nm, and CdSe/ZnS quantum dots showing the plasmonic resonance wavelength at around 530 nm. Figure 2: Fluorescence visible emissions of different plasmonic-embedded-ceria NPs. Focusing on Au NPs as shown in Figure 2, the in-situ added Au NPs of 530 nm plasmonic resonance wavelength show an enhanced fluorescence emissions peak, which was found to be higher for added AuNPs as a post-synthesis of the same wavelength. Hence, the in-situ case may indicate that an optical coupling between the plasmonic resonance wavelength of Au NPs with the fluorescence emissions intensity spectrum of ceria NPs has a higher probability of occurring compared to a coupling with Au NPs during post-treatment. To study more features about the impact of added Au NPs on ceria NPs, Figure 3a shows the absorption spectroscopy analysis of ceria NPs, with and without added plasmonic nanostructures, along with the corresponding bandgap calculations in Figure 3b. Equation (1) illustrates the relationship between the allowed direct bandgap and linear region of the absorbance dispersion due to the Urbach tail phenomenon [18]  
\[\propto\text{E}=\text{A}\;(\text{E}-\text{Eg})^{1/2} \tag{1}\]  
where \(\alpha\) is the optical absorbance, A is a material-dependent constant related to effective masses of electrons and holes, E is the absorbed photon energy, and Eg is the allowed direct bandgap. It can be observed that the in-situ added gold-ceria NPs gives the biggest reduction of ceria bandgap closer to 3 eV, compared to whether there are other plasmonic-added cases or pure ceria Au NPs. This provides evidence that the in-situ Au NPs with coupling impact of fluorescence emissions causes more tri-valent ionization states of cerium ions, associated with the formation of oxygen vacancy. In turn, this shifts the bandgap of non-stoichiometric CeO\({}_{2-\text{x}}\) to the 3 eV bandgap range. Therefore, that contributes to the results of highest fluorescence emissions intensity found in Figure 3, due to the formation of more tri-valent ions associated with more O-vacancies. Therefore, the added in-situ Au NPs can help to provide a better formation of tri-valent cerium ions, which correspond to a better probability of 5d-4f transition and consequently leads to a higher fluorescence emissions intensity. Hence, the formation of more tri-valent cerium ions can be another possible factor for enhancing the fluorescence emissions, in addition to the possible coupling between ceria fluorescence emissions and plasmonic waves of Au NPs.