The effect of silver nanoparticles on composite shear bond strength to dentin with different adhesion protocols

Abstract In Dentistry, restorative materials and oral bacteria are believed to be responsible for restoration failure. To make long-lasting restorations, antibacterial agents should be made. Inorganic nanoparticles and their nano composites are applied as good antibacterial agents. Objective The purpose of this study was to investigate the effect of silver nanoparticles on composite shear bond strength using one etch and rinse and one self-etch adhesive systems. Material and Methods Silver nanoparticles were prepared. Transmission electron microscope and X-ray diffraction were used to characterize the structure of the particles. Nanoparticles were applied on exposed dentin and then different adhesives and composites were applied. All samples were tested by universal testing machine and shear bond strength was assesed. Results Particles with average diameter of about 20 nm and spherical shape were found. Moreover, it was shown that pretreatment by silver nanoparticles enhanced shear bond strength in both etch and rinse, and in self-etch adhesive systems (p≤0.05). Conclusions Considering the positive antibacterial effects of silver nanoparticles, using them is recommended in restorative dentistry. It seems that silver nanoparticles could have positive effects on bond strength of both etch-and-rinse and self-etch adhesive systems. The best results of silver nanoparticles have been achieved with Adper Single Bond and before acid etching.

Caries preventive measures with the purpose of reducing demineralization should be independent of the patient's compliance 5 . These preventive measures consist of antimicrobial bonding agents, antibacterial mouth rinses, and remineralizing agents adjacent to oral appliances. It has been demonstrated that the presence of silver in dental restorative materials is effective against caries producing bacteria, such as streptococci and lactobacillus 19 . It can also be effective as an antibacterial additive to dental restorations.
We do not know exactly whether the incorporation of silver NPs would interfere with bond strength of dental restorative materials 16,19 . Until now, only few studies have determined the effect of silver NPs on composite shear bond strength (SBS). Adhesion mechanism is based on the penetration of resin molecules into enamel and dentin 22 . Bond strength of dental composites to dentin is one of the main criteria in clinical durability of composite restoration.
Various bonding systems have been introduced in order to ful ll a reliable bond to tooth structure based on two main methods: the etch-and-rinse and the self-etching adhesive systems 7 . Smear layer removal and formation of collagen bril layer by means of acid conditioner to form hybrid layer are the main adhesion mechanisms in etchand-rinse systems 3 . The self-etching adhesive system is subdivided into two groups: two-step and one-step selfetching. Self-etching adhesive systems employ an acidic monomer as conditioner. In self-etch adhesives, acidic functional monomers react to the mineral content of tooth surface 24 . Self-etch adhesives are less time-consuming and technique-sensitive. Etch-and-rinse adhesive systems are still considered as a golden standard among bonding systems. However, dentists have a tendency to use adhesive systems with a simpli ed application procedure 12 . Therefore, in this study we examined whether an additional pretreatment with silver NPs provides any The products were washed with distilled water and collected by centrifugation at 15,000 rpm for 10 min (Hettich Universal 320, Tuttlingen, Germany). A transmission electron microscope (TEM) (JEM-1011, JEOL, Japan) was used to determine size, shape, and size distribution of silver NPs. Silver NPs were prepared by placing a drop of working solution on a TEM grid and dryed for TEM analysis. X-ray diffraction (XRD) was used as a secondary or complementary technique to determine particle size by analyzing diffraction peaks.

XRD pattern of silver NPs was obtained by a DX-1000
The effect of silver nanoparticles on composite shear bond strength to dentin with different adhesion protocols X-ray powder diffractometer (DX-1000X, Dandong Fangyuan, China) 17 .

Specimen preparation
Ninety extracted, noncarious human premolar teeth were cleaned and stored in 0.1% thymol solution for one week. The teeth were prepared using a diamond bur (4138 KG Sorensen, Barueri, Brazil) and a high-speed handpiece under water coolant up to the nearmost half of the DEJ to the pulp, which is the proposed location for measuring bond strength of composites, so that no pulp exposure occur in the preparation site. Then the exposed super cial dentin surface was polished using silicon carbide paper (600 grit) under water coolant to standardize the smear layer. The teeth were rinsed with distilled water to remove any debris and then mounted in acrylic resin (2×3×5 cm) and randomly divided into six groups (n=15).
The specimens' preparation was based on the type of employed adhesive system and tested silver NPs as follows: In all groups tested the application of silver NPs were followed by a 60-second rinse with water.
Adhesive systems were applied following the Tukey test with the level of signi cance at p=0.05.

TEM and XRD analysis
The size and morphology of particles were assessed using a TEM ( Figure 2). TEM image shows silver particles with average diameter of about 20 nm and with spherical shape.
A typical XRD pattern of the as-prepared silver NPs is shown in Figure 3. All peaks in the XRD pattern can be indexed as a face centered cubic (fcc) structure    In previous studies, primer and adhesives that contain silver NPs could kill the residual bacteria 8,18 . Bacteria at the interface of the tooth-restoration margins could harm the dental pulp and also affect bond strength.
The primer could be an important vehicle to deliver antimicrobial agents such as silver NPs to kill bacteria in the tooth cavity because it has direct contact with dentin 10,30 . Silver NPs has a strong antibacterial activity, low cytoxicity and acceptable biocompatibility with human cells, and a long-term antibacterial effect by means of sustained silver ion release 6  The present study demonstrated that bond strength  The present study measured dentin bond strength after one week of water-aging. More studies should be conducted to investigate the long-term bond durability of adhesive systems after long-term wateraging to investigate the effects of silver ion release and its effects on dental hard tissue remineralization, antibacterial characteristics, and the durability of the resin-dentin bond. In our in vitro study, we did not investigate the release of silver NPs into oral cavity and saliva, but these studies should be considered. It seems good to emphasize that more researches are