Ad gel

The canals were rinsed with 3% EDTA solution (Smear Clean, Nipponshika Yakuhin, Yamaguchi, Japan) for 2 min and filled with 10% sodium hypochlorite gel (AD gel, Kuraray Noritake Dental, Tokyo, Japan) for 1 min [15] . The canal was finally irrigated with distilled water for 20 s and then dried with paper points. A dual-cure self-etch adhesive (Clearfil DC Bond, Kuraray Noritake Dental) was applied to the root canal dentin for 10 s. Excess adhesive at the apical side of the root canal was removed using paper points. The adhesive was then light-cured for 10 s with a cordless light-emitting-diode curing light (Mini LED3, SATELEC, Merignac, France), which had a maximal light density of 2,200 mW/cm 2 . Root canals were filled with a resin composite core material (Clearfil DC Core Automix, Kuraray Noritake Dental) and light-cured for 40 s. After light-curing, the specimens were stored in water for 24 h at 37 °C.

Three human non-caries molars were stored in HBSS and used within 6 months after extraction. Mid-coronal dentin surfaces were obtained by removing the occlusal third of the molar crowns using the low-speed diamond saw under water cooling. A standard smear layer was prepared using 600 grit SiC paper (Fig. 1a). The dentin surface was divided into four sections and each surface was treated as follows:
1. No treatment (Cont).
2. Etching: Etch the dentin surface for 15 s using 37% phosphoric acid (K-Etchant gel, Kuraray Noritake Dental), rinse, and air dry. 3. NC: Etch the dentin surface using phosphoric acid, rinse, air dry, and apply 10% NaClO gel (AD gel, Kuraray Noritake Dental) for 60 s. 4. NC+AC: Etch the dentin surface using phosphoric acid, apply NaClO gel and sulfinic acid sodium salt (Accel, Sun Medical) for 15 s, and air dry (Fig. 1g). Specimens were then observed by SEM (Fig. 1h).