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Wereport the synthesis and characterization of amorphous ironoxide nanoparticles from iron salts in aqueous extracts of monocotyledonous(Hordeum vulgare) and dicotyledonous(Rumex acetosa) plants. The nanoparticleswere characterized by TEM, absorbance spectroscopy, SAED, EELS, XPS,and DLS methods and were shown to contain mainly iron oxide and ironoxohydroxide. H. vulgareextracts producedamorphous iron oxide nanoparticles with diameters of up to 30 nm.These iron nanoparticles are intrinsically unstable and prone to aggregation;however, we rendered them stable in the long term by addition of 40mM citrate buffer pH 3.0. In contrast, amorphous iron oxide nanoparticles(diameters of 10–40 nm) produced using R. acetosaextracts are highly stable. The total protein content and antioxidantcapacity are similar for both extracts, but pH values differ (H. vulgarepH 5.8 vs R. acetosapH 3.7). We suggest that the presence of organic acids (such oxalicor citric acids) plays an important role in the stabilization of ironnanoparticles, and that plants containing such constituents may bemore efficacious for the green synthesis of iron nanoparticles. [ABSTRACT FROM AUTHOR]