Mesembryanthemum crystallinum -guided green synthesis of ZnO nanoparticles with selective anticancer activity via ROS-mediated mitochondrial dysfunction.

Green synthesis of zinc oxide nanoparticles (ZnO-NPs) using Mesembryanthemum crystallinum L. (M. crystallinum), a halophytic plant adapted to saline environments of North Sinai, Egypt, offers a sustainable nanofabrication strategy with inherent therapeutic potential. High-resolution LC-ESI-QTOF-MS metabolomic profiling identified 13 major phytochemicals in the ethanolic leaf extract, dominated by citramalate (peak area 31,601), L-phenylalanine (30,377), and stress-responsive organic acids, reflecting the plant's halophytic adaptation. These redox-active compounds templated the biosynthesis of crystalline ZnO-NPs under ambient aqueous conditions, yielding spherical to quasi-spherical nanoparticles (4.4-12.2 nm primary size; 40-150 nm aggregates) with a hexagonal wurtzite structure, as confirmed by XRD and SAED. FTIR and EDX analyses verified surface functionalization by phytochemical capping agents (polyphenols, organic acids), while SEM revealed characteristic aggregation with rough surface morphology indicative of biomolecular adsorption. Biosynthesized ZnO-NPs exhibited selective cytotoxicity against human malignant melanoma A375 cells (IC₅₀ = 100.55 ± 8.6 µg/mL) with four-fold lower toxicity toward normal human skin fibroblasts (IC₅₀ = 406.01 ± 35.2 µg/mL; selectivity index ≈ 4.0). Mechanistic investigations demonstrated that nanoparticle internalization triggered robust ROS generation (~ 3.8 × 10⁴ fluorescence units), mitochondrial membrane depolarization, and mixed apoptotic/necrotic cell death (40% total death; 25% late apoptosis, 15% necrosis). Cell cycle analysis revealed cancer-selective G2/M arrest in A375 cells (20.87% vs. 8.59% control; p < 0.001) versus protective G1 arrest in fibroblasts (69.14% vs. 56.01% control; p < 0.001). Paradoxically, qPCR showed BAX downregulation in cancer cells but upregulation in normal cells, suggesting non-transcriptional execution of apoptosis in malignancies. These findings establish M. crystallinum as an ecologically sustainable nanofactory whose stress-adapted metabolome directs the formation of selectively cytotoxic ZnO-NPs acting through ROS-mediated mitochondrial dysfunction, a paradigm for designing plant-guided nanotherapeutics with defined molecular mechanisms.