Green synthesis of bimetallic Fe-Mn oxide nanoparticles and evaluation of their antioxidant, antibacterial, and T1-T2 contrast-enhancing properties

Bimetallic (Fe-Mn) and monometallic (Fe and Mn) oxide nanoparticles were successfully synthesized via a green approach using akapulko (Cassia alata) leaf extracts, followed by surface modification with polyacrylic acid (PAA) to enhance their potential for antioxidant, antibacterial, and biomedical imaging applications. Comprehensive characterization confirmed their spherical morphology. Transmission electron microscopy (TEM) revealed nanoscale sizes of 16.82 ± 4.06 nm for Fe-O, 179 ± 37.0 nm for Mn-O, and 21.12 ± 7.98 nm for Fe-Mn oxide nanoparticles. Notably, Fe-Mn oxide nanoparticles demonstrated strong antioxidant activity, with an IC₅₀ value of 3.16 ± 0.002 ppm. They showed high relaxivity values (r₁ = 21.59 ± 6.01 mM⁻¹s⁻¹ and r₂ = 50.84 ± 1.70 mM⁻¹s⁻¹), yielding an r₂/r₁ ratio of 2.47±0.60, confirming their dual-mode (T₁/T₂) MRI contrast agent capability. Overall, these green-synthesized nanoparticles hold considerable promise as eco-friendly MRI contrast agents with potent antioxidant properties. Although the antibacterial assay revealed no antibacterial activity against Gram-positive Staphylococcus aureus UPCC 1143 (ATCC 6538) and Gram-negative and Escherichia coli UPCC 1195 (ATCC 25922), this result allows valuable insight, and provides for a precise focus on the nanoparticles' most effective applications. This study provides a novel green-chemistry platform for producing multifunctional Fe-Mn oxide bimetallic nanoparticles with encouraging in vitro performance. The demonstrated potent antioxidant activity, combined with their dual-mode (T₁ and T₂) MRI contrast capabilities, highlights their promise as a sustainable and advanced alternative for biomedical applications. Keywords: Fe-Mn Oxide nanoparticles, green synthesis, akapulko (C. alata), dual- functional, MRI contrast agents