Scanning electron microscopy evaluation
After the sealant was applied on the occlusal surface and the teeth were aged, two samples from each group were randomly selected to evaluate resin infiltration and the interface area. The teeth were sectioned transversally to the sealant–tooth interface and polished with 400, 600, 1000 and 2000 grit silicon carbide paper (SCi) with water cooling. Then the tooth surface was treated with 37% phosphoric acid for 10 s, rinsed for 30 s, and immersed in 5% NaOCl for 2 min. After rinsing, the teeth were dehydrated with ethanols. Next the samples were sputter-coated with gold in a vacuum evaporator, and the interfaces were examined with SEM (VEGA, Tescan, Brno, Czech Republic) at 1500× magnification.
Shear bond strength test
The root of 100 teeth was sectioned perpendicular to and 2 mm below the cementoenamel junction with a diamond saw. Then the enamel specimens were prepared in blocks so that the crown was embedded in acrylic resin while the buccal surface remained exposed. To obtain a flat surface the exposed enamel was polished with 600–1000 grit waterproof SCi. Each sample was demineralized, and the enamel was sonicated for 10 min to remove any debris due to surface polishing [24]. Then the teeth were randomly divided into 5 groups and pretreatment was applied to each sample as described above. Next a rubber cylindrical mold 3 mm in internal diameter and 3 mm in height was placed on the treated enamel and filled with fissure sealant material, which was then cured. Then the teeth were stored in humid conditions at 37 °C for 24 h, and SBS tests were done with a universal testing machine (Zwick-Roell, Zwic, Ulm, Germany). The load speed applied in a direction parallel to the bonded interface was 1 mm/min until failure occurred, and load failure was recorded in megapascals (MPa). Failure mode of the fractures was evaluated by two previously trained observers under blind conditions with a digital microscope (Dino Lite) at 25× magnification. The types of bond failure were recorded as follows: 1) adhesive fracture at the sealant–enamel interface, 2) cohesive fracture in the substrate (enamel or sealant), 3) mixed fracture when both adhesive and cohesive fractures occurred.
Energy dispersive X-ray spectroscopy and field emission scanning electron microscopy
A total of 32 teeth were selected and randomly divided into 4 groups of 8 specimens. Each tooth was sectioned transversally in the mesiodistal direction to obtain baseline sound enamel (the enamel surface of the lingually sectioned portion). Enamel blocks were prepared as described above. Then the buccal half of the sectioned teeth was cut in the buccolingual direction to obtain two quarters of the tooth. All parts of each tooth were sonicated for 10 min. The two buccal quarters were demineralized as described above. One quarter of each sample was pretreated (remineralized) as explained above, and the other quarter was used to study demineralization. Next the mineral content of each sample was measured by EDS at baseline (in sound enamel), after demineralization, and after immersion in nano-HA solution (remineralization). The sound sectioned part of each specimen was used as a reference for comparisons with the demineralized and remineralized enamel specimens. In other words all comparisons were done in all 32 tooth specimens.
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Before EDS analysis, the teeth were carbon coated. The scanning parameter settings and tooth position were held constant to standardize the analytical method and obtain comparable results. The spot size was kept at 100 nm for EDS and 2–3 nm for FESEM. The surface area assessed in each FESEM scan was 1.38 μm (at 75000×) at 15 kV. Scan duration was 45 s. In addition, EDS associated with high-resolution FESEM (MIRA3, Tescan, Brno, Czech Republic) was used for samples in groups 2 to 5 to observe nanoparticles in each sample (Fig. 1).
Statistical analysis
All data are presented as means ± standard deviations (SD), and were obtained with SPSS version 22.0 (IBM SPSS) software. Microleakage and SBS comparisons were done with the Kruskal–Wallis, one-way analysis of variance (ANOVA) and Duncan’s post hoc tests. Repeated measures ANOVA, the Sidak and Tukey post hoc tests were used to compare the effect of different nano-HA compositions and conditions. P values < 0.05 were considered statistically significant.
Results
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