Diamond paste

The selected bonded sticks were immersed in ammoniacal silver nitrate solution in darkness for 24 h, rinsed thoroughly in distilled water, and immersed in photo-developing solution for 8 h under a fluorescent light to reduce silver ions into metallic silver grains within voids along the bonded interface (Tay et al., 2002) . Sticks were polished with a wet 600-, 1000-, 1200-, 1500-, 2000-and 2500-grit SiC paper and 1 and 0.25 µm diamond paste (Buehler, Lake Bluff, USA) using a polishing cloth. The specimens were then ultrasonically cleaned, air dried, mounted on stubs, and coated with carbon (MED 010, Balzers Union, Balzers, Liechtenstein). The resin-dentin interfaces were analyzed using a field-emission scanning electron microscope operated in the backscattered mode (VEGA3 SB, TESCAN ORSAY HOLDING, Warrendale, PA, USA).
Three images were captured from each resin-dentin bonded stick. The relative percentage of NL within the adhesive and hybrid layers in each specimen was measured in all images using the public domain Image J software, a Java-based image processing software package developed at the National Institutes of Health (NIH) (Schneider et al. , 2012) by a blinded researcher (Reis et al., 2007) . The mean NL of all sticks from the same tooth was averaged for statistical purposes.

The resin-dentin interface was ultrastructurally observed by scanning electron microscopy (SEM). Bonded specimens from each group (n=5) were stored in distilled water maintained at 37ºC for 24 h, embedded in self-curing epoxy resin (Epon812, Nisshin EM, Tokyo, Japan), and stored at 37ºC for an additional 12 h. The embedded specimens were sectioned through the diameter of the composite resin post, and the surfaces of the cut halves were polished with an Ecomet 4/Automet 2 (Buehler) using SiC papers with a grit size of 600, 1,200, and 4,000 in succession. The surface was finally polished by a soft cloth using diamond paste (Buehler) with a grit size of 1.0 lm. All SEM specimens were dehydrated in ascending concentrations of tert-butanol (50% for 20 min, 75% for 20 min, 95% for 20 min, and 100% for 2 h) and transferred to a critical-point dryer for 30 min. These surfaces were subjected to Argon ion-beam etching (Type EIS-200ER, Elionix, Tokyo, Japan) for 30 s, with the ion beam (accelerating voltage, 1.0 kV, ion current density, 0.4 mA/cm 2 ) directed perpendicularly to the polished surface. Surfaces were coated in a vacuum evaporator (Quick Coater Type SC-701, Sanyu Denshi, Tokyo, Japan) with a thin film of gold and observed by SEM (ERA 8800FE, Elionix) at an accelerating voltage of 10 kV. All of the specimens were observed under SEM.