## 1 Introduction  
Adhesive dentistry has been greatly improved in the past few years as a result of several factors such as better knowledge of the morphology of the dental substrates that are adhered to dental composites, greater development of dental materials, and improvement of restorative techniques. However, the tooth-composite interface is still one of the most challenging and least predictable areas of restoration, mainly due to the great variability of the composition of the dental substrate; moreover, it is well-known that dentin exerts a pivotal role in this chain of events. In this context, the ability of restorative materials to adhere to the dentin surface is affected by several factors [1], which include the dentin heterogeneity regarding morphology, orientation, quantity, and size of dental tubules, all of them influencing the dentin degree of humidity [2; 3; 4; 5]. Since humidity plays a crucial role in the dentin bond phenomenon, all these factors may jeopardize the establishment of an effective and long-lasting bond strength between the tooth and the dental composite used for restoring the tooth. Besides,the bonded interface should be able to resist the stresses caused by the shrinkage produced during dental composite polymerization, which is related to gap formation at the tooth-composite interface [7; 8; 9]. In order to reduce the effect of the polymerization shrinkage stress, bulk-fill dental composites have been used. According to the manufacturers, these special types of restorative materials develop a lower polymerization shrinkage, theoretically inducing lower stress at the tooth-dental composite interface [10; 11]. From this perspective, bulk-fill composites might help with preserving the adhesive interface through less gap formation [12; 13]; however, there are controversial results regarding the shrinkage of these materials, showing that this low-shrinkage characteristic is highly dependent on the composition of each bulk-fill dental composite [14; 15]. Another factor that enormously affects the adhesive interface integrity is the bond strength provided by the adhesive system to the dentin, which should be able to counteract the shrinkage stress at the tooth-composite interface. Adhesive systems consist mainly of monomers, solvents, and photoinitiators [16]. It has been noticed that adhesives containing inorganic fillers lead to improved mechanical properties such as the ultimate tensile strength [17; 18], flexural strength [19; 20], and elastic modulus [18; 21], besides improving bond strength to dentin [18; 20; 22]. Particularly, the enhancement of the bond strength is attributed to the greater ultimate tensile strength of the adhesive and to the better stress distribution at the bond interface [20; 23]. On the other hand, it is questioned if the high modulus of elasticity--due to the higher content of inorganic fillers--could compromise the bonding interface since it could reduce its flow in the face of the contraction stresses generated [24]. In fact, studies that used flow composites without fillers [25] or a thick layer of unfilled adhesive as a liner in cavities restored with high modulus composites [26] produced better marginal sealing of restorations. According to the authors, they attribute this finding to the lower stresses generated by materials with a lower elastic modulus, generally due to the higher flow capacity in the early stages of polymerization. Therefore, aiming to verify whether charging dental adhesives with nanof fillers could influence the gap formation at the tooth-dental composite interface when bulk-fill or conventional composites are used in dental restorations with a high C-factor, the purpose of the present study was to formulate and characterize dental adhesives charged with different concentrations of nanofillers. Physicochemical properties, gap formation, and microtensile bond strength were evaluated.