Study of the effect of ultrasonication on MnO2: Morphological evolution from nanosheets to nanofibers and its implications for biomedical redox modulation

Triclinic birnessite (TMA-birnessite) was synthesized and systematically transformed through controlled ultrasonication (10–30 min) to optimize its morphology and redox reactivity for biomedical use. Transmission electron microscopy revealed a progressive morphological evolution: from stacked nanosheets to individual monolayers, and ultimately to elongated nanofibers, driven by anisotropic fragmentation along Mn(III)O 6 chains. X-ray diffraction analysis (Williamson-Hall model) indicated a 70% reduction in crystallite size (from 9.6 to 3.1 nm) and a sixfold increase in microstrains (from 0.2 to 1.3%), reflecting extensive defect generation. Ultrasonication enhanced colloidal stability in aqueous media, as demonstrated by improved zeta potential (from –10 mV to –35 mV) and reduced hydrodynamic diameter (from 544 nm to 173 nm), though stability remained limited under physiological ionic strength. Kinetic studies using a pseudo-first-order model revealed morphology-dependent redox reactivity toward glutathione (GSH). Nanofibers exhibited a 1.5-fold faster GSH-mediated reduction rate ( k ≈ 0.097 vs. 0.061 min -1 for nanosheets at 4.0 mM GSH), indicating superior antioxidant depletion—critical for targeting tumor microenvironments. Conversely, methylene blue assays indicated a higher oxidative activity for nanosheets, likely related to their higher surface Mn(III) content (41 vs. 33% in nanofibers), but this test alone does not allow unambiguous identification of the reactive species involved. This interpretation was therefore validated using the terephthalic acid fluorescence probe, which specifically confirmed hydroxyl radical generation in the MnO 2 /H 2 O 2 system and revealed a stronger OH • production for nanosheets than for nanofibers, particularly in the presence of GSH. These results establish a clear morphology-activity relationship: nanofibers primarily act as GSH scavengers, while nanosheets serve as ROS generators. This tunability positions ultrasonicated birnessite as a promising platform for biomedical applications requiring balanced antioxidant depletion and ROS generation. • Ultrasonication transforms birnessite from stacked nanosheets into nanofibers • Crystallite size drops to ~3.1 nm and microstrains rate rises, indicating more defects • Nanofibers deplete glutathione faster, enhancing redox modulation • Nanosheets generate slightly more OH • from H₂O₂ due to higher surface Mn³⁺ content