## 1 Introduction  
Cerium oxide (ceria) NPs can be considered to be one of the most promising nanostructures developed in the last two decades due to its oxide storage capacity based on reduction-oxidation (redox) characteristics [1]. The formed oxygen vacancies inside the crystalline structure of ceria are adsorbing centers for oxygen, radicals, and charged tiny metallic particles. Such vacancies are associated with the formation of tri-valent cerium ions trap energy levels, which are optically active through a reduced bandgap that is close to 3 eV. In addition, such tri-valent ions are responsible for visible fluorescence emissions when excited through violet or near ultra-violet (UV) exposure and 5d-4f energy levels transitions [2; 3; 4; 5]. Hence, both the oxygen vacancies and optical fluorescence characteristics of ceria lead to using the optical nanostructures in wide variety of applications such as optical sensors for water quality monitoring, biomarkers, and free radical scavengers in cancer treatments [6; 7; 8]. However, there is still a lack of study regarding how to improve the optical fluorescence emissions of ceria NPs. In this work, the fluorescence emissions of ceria nanoparticles can be enhanced by using different plasmonic nanostructures, due to their importance in different applications, including solid oxide fuel cells and electro-catalysis [9; 10; 11]. Our idea was to test the impact of optical coupling between plasmonic nanostructures and ceria NPs in both excitation and emissions spectra. Here, we studied the impact of different plasmonic nanostructures such as Au NPs, cadmium sulfide/zinc sulfide (CdS/ZnS), and cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots (QDs) on the fluorescence emissions intensity of ceria NPs. Au NPs, CdS/ZnS, and CdSe/ZnS QDs were selected with specific diameters to have resonance wavelengths matched with both the excitation and emissions wavelengths of ceria NPs, which were around 430 nm and 530 nm, respectively. Both plasmonic nanostructures have been added to ceria NPs whether in-situ, meaning within the initial precursors of the chemical precipitation synthesis process of ceria NPs, or post-treatment, which means to be added externally after the full preparation of ceria NPs.  
Then, the idea of our work was to check the best embedded plasmonic nanostructure along with its condition, in-situ or post-treatment, to determine the enhanced fluorescence intensity of ceria NPs. The impact of the best plasmonic nanostructure, from the fluorescence intensity perspective, on ceria NPs was studied through the absorbance dispersion and correlated bandgap calculations. Then, the fluorescence intensity was detected according to the added plasmonic nanostructures. Then, as an application, the best optical plasmonic-embedded-ceria NPs were selected as optical sensing materials for metallic tiny particles, such as iron, in aqueous media. This application is important for water pollution monitoring using an optical quenching technique rather than a traditional electrochemical one, which suffers from reference electrode fouling [12; 13]. Based on intensity quenching, the Stern-Volmer constant of the optimum selected plasmonic-ceria NPs, which indicates the sensitivity of quenching sensing, has been calculated to optically detect the added iron quenchers [14].