Carbon dots as multifunctional primer to optimize dentin bonding durability

To explore the feasibility of carbon dots (CDs) serving as a simple but multifunctional primer for dentin bonding optimization, and investigate its contribution on bonding strength, anti-hydrolysis, anti-enzymolysis and antibacterial aspects.

CDs were characterized by HRTEM, XPS and spectrofluorimetry. Different concentration of 0, 1, 10 and 20 mg/mL CDs primers were applied on demineralized dentin surface, followed by adhesive coating and composite resin build-up. The samples were subjected to 10,000 times of thermocycling, one-month collagenase ageing, followed by microtensile bond strength (MTBS) and failure mode test. Interfacial nanoleakage and in situ zymography assay were used to evaluate the integrity and enzymolytic activity of hybrid layer. The antibacterial capability was assessed by live/dead bacterial staining, crystal violet staining and FESEM observation. Surface contact angle test and degree of conversion were used to estimate the effect of CDs on dentin surface and resin matrix. CCK-8, live/dead cell staining and hemolysis experiments were performed to prove the biocompatibility of CDs.

The CDs exhibited uniform small-sized shape and excellent fluorescence property with multiple functional groups terminated on the surface. The MTBS showed that the 20 mg/mL CDs pretreatment group demonstrated the most excellent bond strength regardless of ageing methods. Interfacial nanoleakage and in situ zymography assay proved that CDs could reduce the formation of water channels and activity of MMPs in hybrid layers. In addition, surface contact angle test and degree of conversion indicated that CDs pretreatment could increase the hydrophilia of dentin surface and polymerization of adhesive resin. The antibacterial and biocompatible assays confirmed that CDs could effectively inhibit the adhesion of S. mutans on the premise of good biosafety.

With the extremely small size and abundant functional groups, CDs can not only act as bridges to tightly connect demineralized collagen fibrils and adhesives, but also effectively penetrate into the gaps among collagen fibrils to reduce nanoleakage. In addition, they can inhibit the activity of MMPs and the adhesion of S. mutans without affecting biocompatibility. Therefore, these findings broaden the application area of CDs and lay a solid foundation for future scholars to apply carbon-based nanomaterials into adhesive dentistry.