Quantum Dots as Nonagglomerated Nanofillers for Adhesive Resins

Nanoparticles used in adhesive resins are prone to agglomeration, turning the material susceptible to physical failure. Quantum dots are nonagglomerated inorganic nanoparticles (1 to 10 nm) when in equilibrium. The aim of the present study was to synthesize and characterize zinc oxide quantum dots (ZnO_QDs) and to develop and evaluate an adhesive resin with the addition of ZnO_QDs. ZnO_QDs were formulated by self-organization in chemical reaction with isopropanol and added to 2-hydroxyethyl methacrylate (HEMA). HEMA containing ZnO_QDs was used for the experimental group and neat HEMA for the control group. Mean ZnO_QD diameter was evaluated in isopropanol and in HEMA by ultraviolet-visible spectroscopy. The adhesives were evaluated for degree of conversion ( n = 5), softening in solvent ( n = 5), ultimate tensile strength ( n = 5), microtensile bond strength ( n = 20) at 24 h and after 6 mo, SEM-EDS (scanning electron microscopy-energy-dispersive x-ray spectroscopy; n = 3), and superresolution confocal microscopy ( n = 3). Data of microtensile bond strength after 6 mo and Knoop hardness after solvent immersion were evaluated by paired t test with a 0.05 level of significance. The other data were evaluated by independent t test with a 0.05 level of significance. Ultraviolet-visible spectroscopy indicated that the mean ZnOQD diameter remained stable in isopropanol and in HEMA (1.19 to 1.24 nm). Fourier transform infrared spectroscopy analysis showed the peak corresponding to zinc and oxygen bond (440 cm^-1). The experimental group achieved a higher degree of conversion as compared with the control group and presented dentin/adhesive interface stability after 6 mo without altering other properties tested. SEM-EDS indicated 1.54 ± 0.46 wt% of zinc, and the superresolution confocal microscopy indicated nonagglomerated nanoparticles with fluorescence blinking in the polymerized adhesive. The findings of this study showed a possible and reliable method to formulate composites with nonagglomerated nanoscale fillers, shedding light on the nanoparticle agglomeration concern.