Orange Peel-Mediated Co-formation of a ZnO–Calcite Mixed-Phase Nanostructured Material: Physicochemical Characterization and Preliminary In vitro Antioxidant, α-Glucosidase Inhibitory, and Differential Cytotoxicity Assessment

Orange peel was used as a plant-derived medium to prepare a ZnO-calcite mixed-phase nanostructured material, which was physicochemically characterized and evaluated for DPPH radical-scavenging, α-glucosidase inhibitory, and in vitro cytotoxic activities. XRD confirmed the coexistence of hexagonal wurtzite ZnO and crystalline calcite, with ZnO apparent crystallite sizes ranging from 9.2 to 28.3 nm (mean 16.4 ± 7.3 nm). FTIR identified Zn-O vibrations, carbonate-related bands, surface hydroxyl groups, and residual organic functionalities, while FESEM revealed irregular agglomerates composed of nanoscale grains. EDX showed Zn, O, C, and Ca as the principal elements, and the material exhibited a zeta potential of -14.86 ± 0.52 mV in deionized water. The mixed-phase material showed concentration-dependent DPPH radical-scavenging activity, with an IC₅₀ of 24.64 µg/mL (95% CI: 22.61-26.79), compared with 28.91 µg/mL for ascorbic acid and 34.39 µg/mL for the orange peel extract. α-Glucosidase inhibition yielded an IC₅₀ of 0.2537 µg/mL (95% CI: 0.2251-0.2867), compared with 0.2067 µg/mL for acarbose. In MTT assays, the material showed IC₅₀ values of 100.0 µg/mL in HepG2 cells and 365.7 µg/mL in HDFn cells, corresponding to an SI of 3.66. These findings indicate preliminary differential cellular sensitivity together with measurable radical-scavenging and enzyme-inhibitory activities, while phase-specific mechanisms and biological safety require further investigation.