Optical and hardness performance of resin composites modified with Cu and ZnO nanoparticles for orthodontic applications

Incorporating metallic copper (Cu 0 ) and zinc oxide (ZnO) particles into orthodontic resin composites requires balancing functional modification, surface integrity, and optical acceptability. This study evaluated the effects of adding Cu 0 particles (0.05 and 0.075 wt%), ZnO nanoparticles (0.75 and 1 wt%), and their mixtures to a commercial methacrylate-based resin composite for clear aligner attachments. A multiscale characterization protocol included dynamic light scattering, zeta potential analysis, electron microscopy, ATR-FTIR, X-ray diffraction, X-ray fluorescence, thermogravimetric analysis, differential scanning calorimetry, Vickers microhardness, and CIELab-based optical analysis. Optical behavior was evaluated using color difference, translucency parameter, and contrast ratio at thicknesses of 1 and 2 mm. Statistical analysis was performed using ANOVA with Tukey’s post hoc test at a level α = 0.05. The Cu 0 particles exhibited a heterogeneous size distribution, a negative zeta potential (-29.9 mV), and cluster formation. The ZnO nanoparticles showed more homogeneous distribution and a positive zeta potential (+16.9 mV). ATR-FTIR indicated preservation of the methacrylate matrix, and XRD and XRF confirmed Cu- and Zn-containing phases. Thermal analysis showed similar glass-transition temperatures across formulations. Vickers microhardness decreased with formulation, from 43.4 ± 0.7 HV for the control group to 22.7 ± 1.67 HV for the highest-loaded hybrid formulation. Optically, ZnO-containing formulations showed the highest ΔEab values and greater opacity. Two-way ANOVA showed significant effects of formulation, thickness, and their interaction on ΔEab, TP, and CR. Among the modified composites, 0.05 wt.% Cu 0 + 0.75 wt.% ZnO provided the most balanced properties, combining intermediate color change with preserved surface hardness.