Although several studies have assessed the efficacy of chlorhexidine in treating gingivitis and reducing biofilm formation, the side effects of a long-term use are unpleasant for patients. Melaleuca alternifolia oil has been considered as an alternative because of its anti-inflammatory properties. However, little is know about the efficacy of its nanoparticles. The purpose of this study is to compare the effect of 0.12% chlorhexidine (Periogard ®) and a nanoparticle solution of 0.3% M. alternifolia oil in reducing biofilm formation in plaque-free and plaque-covered teeth surfaces.

This will be a randomized crossover clinical trial, double blind, in which individuals will be randomly assigned into two different treatments: daily mouthrinse of chlorhexidine gluconate (0.12%) or daily mouthrinse of a nanoparticle solution of M. alternifolia (0.3%) during a period of oral hygiene withdrawal.

The test panel will consist of 59 students from the Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil. All subjects will receive written and oral explanations regarding the purpose and design of this study. Those who met the inclusion criteria will be selected for a dental screening appointment, being removed in the presence of any exclusion criteria. All volunteers will be asked to sign an informed consent before the experimental period.

The following clinical parameters will be assessed:

• Plaque index (PI) Quigley and Hein modified by Turesky.
• Gingival crevicular fluid (GCF) will be measured with an electronic gingival fluid measuring device (Periotron®).
• Gingival abrasion (GA) using 2-Tone disclosing solution.
• Perception of appreciation (Questionnaire-VAS scale). Descriptive analysis of PI data will be performed using averages, standard deviations and average percentage of sites with different PI scores. The normal distribution of the data will be evaluated by the Kolmogorov-Smirnov test. Analysis of variance (ANOVA) with repeated measures will be used to test for differences between groups. Paired student's t-test will be conducted to test whether there were significant differences between the baseline measurements of PI, GA, GCF and perception of appreciation after the use of mouthwash. The statistical tests will be performed using the program SPSS 20.0 (Statistical Package for the Social Sciences, Chicago, USA).

Inclusion Criteria:

• Individuals must be systemically healthy;
• Must have at least six teeth per quadrant;
• Must not be undergoing medical treatment;

Exclusion Criteria:

• Allergies or hypersensitivity to the components (Chlorhexidine Gluconate or M. alternifolia oil nanoparticles);
• Antibiotic therapy within 3 months prior to baseline examination;
• History of periodontal disease;
• Marginal bleeding index higher than 15%;
• Oral mucosal lesions;
• Pregnancy or breastfeeding;
• Presence of active infectious foci (endodontic abscesses);
• Presence of plaque retentive factors (caries, aesthetic or ill-fitting restorations, orthodontic appliances, fixed or removable prosthesis);
• Smokers;
• Systemic conditions (diabetes, immunosuppression);
• Use of any mouthwash within 21 days prior to baseline examination;