Green Synthesis of α-Bi2O3 Nanosheets Using Azadirachta indica Leaf Extract: Toward Antimicrobial and Hemostatic Biofilm Applications

The green synthesis of nanomaterials offers an eco-friendly pathway to engineer complex inorganic morphologies via biomolecule-mediated processes. Here, we report the synthesis of two-dimensional α-Bi₂O₃ nanosheets through a sustainable route employing an aqueous extract of Azadirachta indica (neem) leaves. Rich in flavonoids, terpenoids, saponins, and other phytochemicals, the extract is proposed to modulate nucleation and growth by altering the local chemical and physical environment. Scanning electron microscopy revealed predominantly lamellar nanosheets and plate-like structures, while X-ray diffraction confirmed the formation of well-crystallized α-Bi₂O₃ with an average crystallite size of ~45 nm (Scherrer equation). Control experiments performed under identical conditions without the extract yielded amorphous nanoparticles, highlighting the morphology-directing role of the phytochemical medium. Secondary XRD reflections suggest the transient formation of a lamellar BiONO₃ intermediate, which likely undergoes a topotactic transformation into α-Bi₂O₃ during calcination while preserving its morphology. This green synthetic strategy reduces environmental impact and enables morphological control, opening pathways for tailored functional properties. The resulting α-Bi₂O₃ nanosheets are currently being integrated into pectin-based biofilms to explore their antimicrobial potential and enhance hemostatic performance. This work forms part of an ongoing research project supported by the São Paulo Research Foundation (FAPESP, Grant No. 2024/21092-5), aimed at developing multifunctional bioactive materials for biomedical applications.