## 3 Results and Discussions
Both factors together contribute to a higher fluorescence intensity peak. Figure 4a shows a transmission electron microscope (TEM) image of in-situ gold-ceria NPs, as an example. The mean diameter of ceria NPs is around 10 nm, along with Au NPs with a 20 nm diameter. Figure 4b shows an XRD pattern of gold-ceria NPs and the peaks of cerium oxide, with no peaks indicating the formation of gold oxide. By using the first stable plane of ceria crystalline structure (111), the ceria NPs' mean diameter was verified using the TEM image with mean diameter of \(\sim\)10 nm. Figure 3: (**a**) Absorbance spectra, and (**b**) bandgap calculations of ceria NPs with and without added Au NPs. The added gold is in both cases of in-situ and post-synthesis. As an application, we have used our synthesized plasmonic-added-ceria NPs as an optical sensing material for the detection of metallic tiny particles in water, such as iron. The in-situ gold-added-ceria NPs was previously proved to have the highest fluorescence emissions intensity compared to post-synthesis gold-ceria, quantum dots-embedded-ceria, and pure ceria NPs. Therefore, we focus in this section on the application of in-situ gold-ceria NPs as the main sensitive host for the optical detection of tiny metallic iron particles. Figure 5 shows the fluorescence intensity quenching of in-situ gold-ceria NPs with an added increased concentration of iron tiny particles. It can be noticed that the fluorescence intensity was reduced, or quenched, to less than 40% of its initial emissions intensity with an added concentration of iron of up to 0.2 g/L. The fluorescence lifetime was also detected according to the variation of sensed tiny particle concentrations, as shown in Figure 6. The fitted data of the fluorescence lifetime of in-situ gold-ceria NPs was found to be 0.42 ms, 0.30 ms, and 0.23 ms for iron particles with concentrations of 0, 0.03, and 0.15 g/L, respectively. Therefore, the lifetime was decreased by up to approximately 55% of the initial lifetime at concentrations of iron quenchers of up to 0.15 g/L. Figure 4: (**a**) TEM image of in-situ added gold-ceria nanoparticles, and (**b**) XRD pattern of gold-ceria NPs. Figure 5: Fluorescence intensity quenching of in-situ gold-ceria NPs due to increasing iron particle concentrations “in g/L”. Based on the intensity change as shown in Figure 5, the relative change in the fluorescence emissions intensity at different iron concentrations is described in Figure 7, according to the Stern-Volmer relation [19]  
\[I_{o}/I=1+K_{SV}\left[Q\right] \tag{2}\]  
where both \(I_{o}\) and \(I\) are intensity peaks of fluorescence emissions in the absence and presence, respectively, of the iron tiny particles quenchers, \(K_{SV}\) is the Stern-Volmer quenching constant which indicates the sensitivity of gold-ceria NPs in detecting the iron using a fluorescence quenching mechanism [14], and [\(Q\)] is the iron quencher concentration. Figure 7 shows relatively linear behavior between the relative intensity and the quencher concentration at lower than 0.1 g/L, then the relation starts to be non-linear. In the linear region, the value of the calculated \(K_{SV}\) is 8.6 M\({}^{-1}\) for in-situ gold-ceria NPs due to the iron quencher, compared to 3.6 M\({}^{-1}\) for ceria NPs only. While the optical sensitivity of gold-ceria nanoparticles to metallic iron tiny particles is higher than for pure ceria, it is also less linear. Figure 6: Fluorescence intensity lifetime variation of in-situ gold-ceria NPs due to increasing iron particle concentrations. Figure 7: Relation between relative fluorescence intensity peak and variation of iron tiny particle concentrations for both gold-ceria nanoparticles and pure ceria nanoparticles “as a control sample”.