**Cerium Dioxide Particles to Tune Radiopacity of Dental Adhesives: Microstructural and Physico-Chemical Evaluation**

**Isadora Martini Garcia 1, Vicente Castelo Branco Leitune 1, Antonio Shigueaki Takimi 2, Carlos Pérez Bergmann 3, Susana Maria Werner Samuel 1, Mary Anne Melo 4,5,\* and Fabrício Mezzomo Collares 1,\***


Received: 16 January 2020; Accepted: 6 February 2020; Published: 11 February 2020

**Abstract:** The insu fficient radiopacity of dental adhesives applied under composite restorations makes the radiographic diagnosis of recurrent caries challenging. Consequently, the misdiagnosis may lead to unnecessary replacement of restorations. The aims of this study were to formulate experimental dental adhesives containing cerium dioxide (CeO2) and investigate the e ffects of di fferent loadings of CeO2 on their radiopacity and degree of conversion for the first time. CeO2 was characterized by X-ray di ffraction analysis, Fourier transforms infrared spectroscopy, and laser di ffraction for particle size analysis. Experimental dental adhesives were formulated with CeO2 as the inorganic filler with loadings ranging from 0.36 to 5.76 vol.%. The unfilled adhesive was used as a control. The studied adhesives were evaluated for dispersion of CeO2 in the polymerized samples, degree of conversion, and radiopacity. CeO2 presented a monoclinic crystalline phase, peaks related to Ce-O bonding, and an average particle size of around 16 μm. CeO2 was dispersed in the adhesive, and the addition of these particles increased the adhesives' radiopacity (*p* < 0.05). There was a significant decrease in the degree of conversion with CeO2 loadings higher than 1.44 vol.%. However, all materials showed a similar degree of conversion in comparison to commercially available adhesives. CeO2 particles were investigated for the first time as a promising compound to improve the radiopacity of the dental adhesives.

**Keywords:** dental materials; dentistry; adhesives; light-curing of dental adhesives; composite resins; methylmethacrylate; oxides; cerium; polymers; dentine bonding agents
