Abstract: BACKGROUND: Nano hydroxyapatite is an excellent bioactive material, which has a certain protective effect on demineralized enamel and dentin, but the effect and mechanism of its solution on remineralization of demineralized dentin are not clear. 
OBJECTIVE: To explore the feasibility of nano hydroxyapatite in the prevention and treatment of dentin sensitivity and dentin caries. 
METHODS: Abandoned human bicuspid specimens were collected, and 72 dentin slice samples were prepared from isolated bicuspids. The samples were randomly divided into 6 groups, with 12 samples per group: After demineralization, each group was immersed in 10% nano hydroxyapatite solution (group A), 6% bioactive glass solution (group B), 10% nano hydroxyapatite and 6% bioactive glass composite solution (group C), 0.05% sodium fluoride (group D), artificial saliva (group E), and deionized water (group F). After 14 days of treatment, working surfaces of samples were observed by scanning electron microscope. The surface composition was detected with energy spectrum analyzer. Dentin Rhodamine B staining was detected under a confocal laser microscope. 
RESULTS AND CONCLUSION: (1) Scanning electron microscopy: The dentinal tubules orifice in group A was completely sealed, and the surface of the dentin was uniformly covered by a layer of tightly connected granular material. The dentinal tubules in group C were partially sealed. A small amount of scattered particles were found on the dentin surface and in the dentinal tubules of group B, group D and group E, and the dentinal tubules were open. In group F, there was no particulate matter deposition on the dentin surface or in the dentinal tubules, and the dentinal tubules were open. (2) X-ray energy dispersive spectrocopy results: The Ca/P ratio of group A was 1.61, which was closest to the theoretical value of healthy dentin; the Ca/P ratio was higher in group A than that in the groups B, E, and F (P < 0.05). There was no significant difference in Ca/P ratio between group A and group C (P > 0.05). (3) Laser confocal microscopy test results showed that the fluorescence intensity and fluorescence area of group A were the smallest, and the mean fluorescence intensity was smaller in group A than that in other five groups (P < 0.05). The mean fluorescence intensity of group C was lower than that of groups B, D, E and F (P < 0.05). (4) It is indicated that nano hydroxyapatite has a good remineralization effect on demineralized dentin. The combination of nano hydroxyapatite and bioactive glass can also deposit on the dentin surface, but there is no synergistic effect. The sealing effect is not as good as that of nano hydroxyapatite alone. 

Key words: dentin, nano-hydroxyapatite, bioactive glass, demineralization, remineralization, dentinal tubule