**1. Introduction**

Currently, resin composites and dental adhesive systems are used in restorative dentistry as primary direct restorative materials [1]. The failure of resin composites is mainly due to recurrent caries and fractures [2]. The diagnosis of recurrent caries underneath resin composite is a challenge for dentists [3]. Recurrent carious lesions may not always be seen during a clinical examination at the interface between the resin composite, and it requires radiographic evaluation for diagnosis [4,5].

Further, the application of adhesive systems can be seen radiographically as a radiolucent area, which mimics the radiographic appearance of carious lesions [6]. The radiolucency of dental adhesives can contribute to the misinterpretation of radiographic images [4,7,8]. Based on it, dentists may intervene surgically in existing composite restorations, replacing the resin [9].

A previous report has assessed and demonstrated the lack of radiopacity on many restorative dental materials [10]. In their results, all assessed materials were radiolucent and required alterations to their composition to facilitate their detection using radiographic images. The radiopacity of dental adhesives depends on their filler content, and it can be enhanced by incorporating elements with a high atomic weight as inorganic fillers [11].

Cerium dioxide (CeO2), a rare-earth oxide found in the lanthanide series of the periodic table, has been increasingly used as a nanotherapeutic material [12]. The numerous commercial applications for CeO2 also called ceria, include glass and glass polishing, phosphors, ceramics, catalysts, and metallurgy [13]. Cerium is found in various minerals, and its primary deposits are located in the United States (Florida and Idaho) and Brazil [14]. Interesting biological properties have been observed for both nanometric and micrometric CeO2 [15]. Thus, as further advancement in CeO2s applications, this compound has gained substantial interest in several innovative applications, mainly due to its redox property and catalytic activity [15,16]. In the dental field, CeO2 was primarily used for dental ceramics since this compound stimulates the natural fluorescence found in human dental enamel [11]. The high atomic number 58 of cerium suggests that it can promote considerable attenuation of a dental X-ray beam [17,18]. The incorporation of CeO2 in adhesives can be a valuable strategy to promote radiopacity and improve the detection of dental adhesives underneath resin composites. Therefore, the aims of this study were to formulate experimental dental adhesives containing CeO2 and investigate the effects of different loadings of CeO2 on their radiopacity and degree of conversion for the first time.
