Ag NPs are frequently used in various fields and dentistry due to their antibacterial properties. The antimicrobial mechanism of Ag NPs is that Ag ions cause cell death by inactivating the enzymes of bacteria [35]. In studies, Ag NPs have been added to many dental materials such as dental composites, dental adhesives, glass ionomer cements, and their antimicrobial activity has been proven [11, 36–38]. Ag NPs impart antibacterial, antifungal and antiviral activity to dental resins [39]. Therefore, Ag NPs were preferred as antibacterial nanoparticle in our study.
A small amount of Ag NPs should be used so that it does not adversely affect the color, aesthetics and mechanical properties of dental materials [11]. The results of the studies showed that Ag NPs, used in appropriate proportions, imparts a strong antibacterial activity that greatly reduces biofilm growth and lactic acid production without adversely affecting other physical and mechanical properties of the resins [8–11]. In studies, the amount of Ag NPs in the dental adhesive is generally 0.05% and 0.1% by weight [33, 40]. It is aimed to obtain the highest efficiency at the lowest concentration from Ag NP used in materials expected to be developed with nanotechnological approaches. In our study, Ag NPs were tested in two ratios (0.05% and 0.1%), and sufficient antibacterial activity was achieved at the rate of 0.05%. For this reason, 0.05% Ag NPs were used in our study to eliminate the negative effects of high concentrations.
Today, various approaches are used in the synthesis of NPs. In current studies, NPs synthesized by green synthesis method attract more attention than chemically synthesized NPs [17, 18]. In chemical methods, toxic reducing and stabilizing agents are generally used for the synthesis of NPs. Toxicity is the main disadvantage in biological applications. Therefore, there is a need for non-toxic synthesis methods. In this regard, the green synthesis method provides a significant advantage [23, 25, 41]. Many different kinds of plant extracts have been used to obtain green synthesis NPs [26, 41]. Among them, chamomile plant was preferred as a reducing agent for Ag NPs synthesized by green synthesis method in our study because it does not cause color change and has antibacterial and antioxidant activity [41].
In current studies, graphene oxide is used as a platform to fix nanoparticles and prevent aggregation. Studies have shown that Ag NPs fixed on the nGO surface have greater antibacterial activity [27–29]. In current studies in dentistry, GO application has been used successfully in antimicrobial effect, regenerative dentistry, bone tissue engineering, drug delivery, increasing the physicomechanical properties of dental biomaterials, implant surface modification and oral cancer treatment. Due to the biocompatibility of graphene oxide and nanocomposites, it can be used successfully in bone regeneration, osseointegration and cell proliferation. In addition, due to its antibiofilm properties, the use of GO for biofilm and caries prevention is becoming widespread [42]. Therefore, in our study, Ag NPs synthesized by green and chemical method in order to increase the antibacterial activity of Ag NPs were fixed on nGO to obtain Ag@nGO NCs and added to the dental adhesive.
In our study, the lowest S.mutans survival rate, lactic acid production and colony count were observed in the group containing C-Ag@nGO NCs. Numerically, the lowest metabolic activity was seen in the group containing C-Ag@nGO NCs. However, this result did not create a statistical difference between the groups containing B-Ag NPs/Ag@nGO NCs and C-Ag NPs. In millimeters, the largest inhibition zone was seen in the group containing C-Ag@nGO NCs. According to the results of our study, our first and second hypotheses were rejected. The reason why the antibacterial activity of the groups containing C-Ag NPs/Ag@nGO NCs was higher than the group containing the B-Ag NPs/Ag@nGO NCs may be related to the synthesis method. The advantage of this method is that nanoparticles can be synthesized in the desired size and shape by chemical synthesis and the molecular stability of the obtained nanoparticles is good [43, 44]. However, the biggest disadvantage of the chemical synthesis method is the toxicity of the reducing agents used in the synthesis. Green synthesis has advantages such as being biocompatible and being able to be synthesized in large quantities at low cost. However, besides these advantages, the contents of the plants used in green synthesis reactions; This can slow down the reaction, decrease the stability of Ag NPs and cause aggregation [41, 27]. In addition, another disadvantage of the green synthesis method is that it is difficult to control the size and shape of nanoparticles [26].
According to our study results, although the adhesive groups containing B-Ag NPs/Ag@nGO NCshad less antibacterial effect than the adhesives containing C-Ag NPs/Ag@nGO NCs, they provided sufficient antibacterial activity when compared to the control group. Kulshrestha et al. [45] synthesized Ag@nGO NCs by green synthesis method using persian flower extract from Legistromia speciosa in a study they conducted and evaluated its antibacterial activity. According to the results of the study, Ag@nGO NCs reduced biofilm formation in both gram-negative (E. cloacae) and gram-positive (S.mutans) bacterial models. Similarly, in our study, the antibacterial activity of B-Ag@nGO NCswas found to be successful.
According to our study results, groups containing both B-Ag@nGO NCs and C-Ag@nGO NCs showed higher antibacterial activity than Ag NPs synthesized by the same method. The reason for these results is that GO, which is used as a platform to fix nanoparticles and prevent aggregation, provides more antibacterial activity by increasing the dispersion of Ag NPs. In a study, it was shown that the incorporation of Ag@GO NCs into conventional glass ionomer cements (CIS) significantly inhibited the growth of S. Mutans in vitro. Ag@GO NCs, added to the glass ionomer cement at a concentration of about 2% by weight, provided antibacterial properties without compromising the mechanical performance of the glass ionomer cements [27]. In the study of Wu et al. [29], the inhibitory effect of Ag@GO NCs on initial caries was investigated. According to the results of the study, Ag@GO NCs groups compared to the control groups; It caused a decrease in enamel surface roughness, shallow lesion depth and a decrease in mineral loss. In another study, Ag@GO NCswere used as an endodontic irrigation solution and was reported to have sufficient antimicrobial and antibiofilm capacity compared to the control group [46]. The results of our study are also compatible with these studies.
Bond strength tests are used to evaluate the bonding ability of restorative materials to enamel and dentine. Shear and tensile test methods are the most frequently preferred bond strength tests for this purpose. Studies have reported that the microtensile test method is one of the appropriate and safe test methods in testing the bond strength of adhesive materials to dentine [2, 31, 47]. Similar to previous studies, microtensile test was preferred to evaluate the bond strength of the experimental adhesives formed in our study to dentine [2, 31]. According to our study results, there was no statistical difference between the groups in the microtensile test results. Nanoparticles added to the adhesive resin did not reduce the adhesion force. “The B-Ag@nGO NCs of the dental adhesive contain nGO will not cause a decrease in the bond strength of the adhesive to dentine when compared to the control group.” Our null hypothesis of the form was accepted. In a similar study, Ag NPs were added to Scotchbond Multi-Purpose adhesive resin (SBMP) at different concentrations (50, 100, 150, 200 and 250 ppm). According to the results of the study, 200 and 250 ppm Ag NPs containing groups had similar microtensile bond strength to the control group [2]. In another study, 0.05% by mass of Ag NPs were added into Scotchbond Multi-Purpose adhesive resin (SBMP). Ag NPs added group showed similar microtensile bond strength when compared to the control group [31]. The results of our study on groups containing Ag NPs are compatible with these studies. However, there was no study in the literature in which Ag@nGO NCs were added to the dental adhesive. Therefore, comparison could not be made.
In our study, when the failure modes that occurred after the microtensile test were evaluated, the most common mix, followed by the cohesive, and the least adhesive type were observed. It has been reported that mixed failure mode is mostly seen at interfaces where the stress distribution is not homogeneous [48]. The low number of adhesive failure mode can be interpreted as strong adhesive bonding in the control and experimental groups. There are studies in the literature reporting that there is no correlation between bond strength and failure modes [49, 50]. In our study, when the bond strength values and failure modes of the groups were compared, it was seen that there was no correlation between these two parameters.