| Summary: | Less toxic, environmentally safe green-mediated iron (III) oxide nanoparticles (Fe<sub>3</sub>O<sub>4</sub>-NP) synthesized using <i>Callistemon viminalis</i> (<i>C. viminalis</i>) leaf (Fe<sub>3</sub>O<sub>4</sub>-NPL) and flower (Fe<sub>3</sub>O<sub>4</sub>-NPF) extracts is reported in this work for the first time. Total flavonoids and phenols present in the plant extracts were determined. Characterization of the nanoparticles was carried out using Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible spectroscopy (UV–VIS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Malvern zeta sizer. Other properties of the nanoparticles were investigated using the thermogravimetric analyser and cyclic voltammetry. The average particle sizes obtained for Fe<sub>3</sub>O<sub>4</sub>-NPL and Fe<sub>3</sub>O<sub>4</sub>-NPF were 17.91 nm and 27.93 nm, respectively. Fe<sub>3</sub>O<sub>4</sub>-NPL exhibited an excellent electrochemical activity when compared with Fe<sub>3</sub>O<sub>4</sub>-NPF based on a stability study using cyclic voltammetry and regression value. Additionally, Fe<sub>3</sub>O<sub>4</sub>-NPF displayed excellent antimicrobial activity against <i>Bacillus cereus</i>, <i>Salmonella enteritidis</i>, and <i>Vibrio cholerae</i> with zones of inhibition of 13, 15, and 25 mm, respectively. Simple, cheap, and less toxic green-mediated iron (III) oxide nanoparticles synthesized from <i>C. viminalis</i> leaf (Fe<sub>3</sub>O<sub>4</sub>-NPL) and flower (Fe<sub>3</sub>O<sub>4</sub>-NPF) extracts hold the potential of being used to control the activity of pathogenic bacteria of health importance and as an electrochemical sensor for both biological and environmental analytes.
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