This randomized controlled clinical trial aimed to investigate the effectiveness of at-home , in-office and combined dental bleaching system.

The participants with at least one maxillary tooth demonstrating shade A3 or darker will be recruited and randomly allocated into 4 groups according to the different tooth bleaching protocol：at-home bleaching for group 1,in-office bleaching for group 2 ,combined bleaching for group 3 and group 4.Participates in group 1 will receive two weeks of at-home tooth bleaching （Lasts 14 days and at least 8 hours every day ）with 10% Carbamide peroxide(Opalescence PF 10%) , group 2 will receive two sessions （with a 1- week interval ）of in-office tooth bleaching with 40% hydrogen peroxide (Opalescence Boost PF 40%)，group 3 will receive one week of at-home tooth bleaching （Lasts 7 days and at least 8 hours every day ）and then the eighth day receive one session of in-office tooth bleaching , group 4 receive one session of in-office bleaching and lasts 7 days at-home bleaching start on the eighth day . Colour parameters (CIE L*, a*, b*) will be measured with a spectrophotometer (Vita Easyshade Advance 4.0) at baseline (T1), after the first bleaching session (T2), after the second bleaching session (T3), 1 month after the completion of bleaching (T4). CIEDE2000 colour difference (△E00), the Whiteness Index for Dentistry (WID) and Tooth sensitivity (TS) will also be calculated. The data will be statistically analysed through repeated ANOVA and Tukey's test (α = 0.05).

Inclusion Criteria:

• patients between 18 and 30 years of age, with fully erupted upper and lower incisors and canines without dental or periodontal disease or restorations, and with at least one maxillary tooth presenting color score A3 or darker, as measured with the Vita Classical guide (Vita Zahnfabrik, Bad Sa # ckingen, Germany) ordered by brightness

Exclusion Criteria:

• patients with systemic diseases or oral mucosal disorders, previous bleaching treatment, patients undergoing orthodontic treatment, pregnant or lactating women , people with known allergy to the product ingredients, smokers, and alcohol abusers, had severe internal tooth discoloration (e.g. tetracycline stains), had bruxism or any other pathology that could cause TS (such as gingival recession or dentin exposure).