Tehran University of Medical Sciences Journal of Craniomaxillofacial Research 2345-5489 12 4 2026 02 09 246 253 Maryam Samareh Khammar Department of Restorative and Cosmetic Dentistry, School of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. Ali Kazemian Department of Community Oral Health, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran. Niloofar Ebrahimzade Student Research Committee, School of Dentistry, Zahedan University of Medical Sciences, Zahedan, Iran. 2025 11 17 2026 01 06 Introduction: This study compared the effects of mouthwashes containing chitosan/zinc oxide nanoparticles, 0.2% chlorhexidine, 1.5% hydrogen peroxide (Colgate), and distilled water on the color and microhardness of bleached enamel.  Materials and Methods: Sixty bovine incisors were randomly divided into four groups (n = 15). After polishing and tea staining for six days, baseline color and microhardness were recorded using the CIELAB system and Vickers test. The enamel surfaces were bleached three times with 40% hydrogen peroxide. After ten days, measurements were repeated. The bleached samples were then immersed in 5 ml of each mouthwash twice daily for 14 days. The synthetic mouthwash contained 1% chitosan and 500 µg/ml zinc oxide nanoparticles. Color and microhardness were re-evaluated, and data were analyzed using the Kruskal–Wallis and post-hoc tests (α = 0.05).  Results: Significant color differences were observed between the mouthwash groups and water (p < 0.05), with no significant difference between chlorhexidine and the nanoparticle mouthwash. Both the nanoparticle and Colgate mouthwashes similarly increased enamel microhardness, while chlorhexidine caused a slight decrease. Conclusion: Mouthwash containing chitosan and zinc oxide nanoparticles improved bleached enamel microhardness without affecting color stability compared to 0.2% chlorhexidine.  Keywords: Chitosan; Mouthwashes; Nanoparticles; Tooth bleaching; Zinc oxide; Enamel; Remineralization. https://jcr.tums.ac.ir/index.php/jcr/article/view/581 https://jcr.tums.ac.ir/index.php/jcr/article/download/581/468