Naturally Colored ZrO₂-Based Hybrids Loaded with Olive Leaf Extract: Physicochemical, Thermal and Release Properties

Olive leaf extract (OLE) contains polyphenolic compounds with reported antioxidant and antimicrobial activities. In this study, OLE obtained by Naviglio extraction was incorporated into zirconia-based sol-gel hybrids at theoretical contents of 8, 25, 33, and 50 wt%. Colorimetric analysis showed that OLE acted as a natural coloring agent, producing composition-dependent changes from yellow to orange-brown tones. FT-IR spectra retained the characteristic bands of the zirconia-based matrix together with OLE-related contributions, while shifts in the O-H and Zr-O regions indicated changes in the local chemical environment after incorporation. Thermal analysis showed a progressive increase in the onset temperature of the main degradation stage, from 296.2 °C for bare ZrO₂ to 320.4 °C for ZrO₂/OLE50%, suggesting a stabilizing effect of OLE within the hybrid matrix. Release experiments in 95% (v/v) ethanol showed a clear dependence on OLE content. ZrO₂/OLE8% released almost completely within 5-6 h, whereas ZrO₂/OLE25% showed an initial burst followed by slower release, reaching 75-80% after 24 h. ZrO₂/OLE33% and ZrO₂/OLE50% showed slower release without a clear burst phase and released less than 30%. Agar-diffusion tests showed no enhancement of antibacterial activity against Escherichia coli or Enterococcus faecalis. These preliminary findings provide a basis for further investigation into the valorization of olive leaf extract as a natural component for the development of colored functional hybrid materials.