<i>Tagetes erecta</i>-Mediated Biosynthesis of Mn₃O₄ Nanoparticles: Structural, Electrochemical, and Biological Investigations

Mn₃O₄ nanoparticles (NPs) find diverse applications in the fields of medicine, biomedicine, biosensors, water treatment and purification, electronics, electrochemistry, and photoelectronics. The production of Mn₃O₄ NPs was reported earlier through various physical, chemical, and green routes, but no studies have still been performed on their biosynthesis from <i>Tagetes erecta</i>. We synthesized manganese oxide NPs, i.e., (Mn₃O₄)_L and (Mn₃O₄)_P NPs, by utilizing leaves and petals, respectively, of <i>T. erecta</i> as reducing and stabilizing agents. The investigated green path is eco-friendly and does not involve any hazardous raw materials. The structural properties of NPs were determined by X-ray diffraction (XRD) analysis, spectroscopies (Fourier transform infrared (FTIR), Raman, and UV-visible), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The NPs were also evaluated for their electrochemical properties by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). XRD analysis was performed to verify their tetragonal geometry, and the crystallite size (19.24 nm) of (Mn₃O₄)_P was smaller than that (20.84 nm) of (Mn₃O₄)_L NPs. SEM images displayed a porous and spherical morphology with a diameter of 14-35 nm. FTIR spectra of (Mn₃O₄)_L and (Mn₃O₄)_P displayed Mn-O vibrations at 605.69 and 616.87 cm^-1, respectively, and the hydrous nature of the material. Raman spectroscopy revealed the existence of tetrahedral and octahedral units along with A_1g, T_2g, and E_g active modes of Mn₃O₄ and 2TO mode. UV-visible analyses of (Mn₃O₄)_L and (Mn₃O₄)_P NPs showed absorption peaks at 272.3 and 268.8 nm, along with band gaps of 4.83 and 5.49 eV, respectively. TGA curves displayed good thermal stabilities up to 600 °C and a loss of moisture content. DSC curves exhibited exothermic/endothermic peaks with glass transition temperatures of 258.9 and 308.7 °C for (Mn₃O₄)_P and (Mn₃O₄)_L, respectively. The CV curves showed redox peaks and confirmed that the electrochemical reaction takes place in the Mn₃O₄ material. GCD scans revealed the capacitive behavior of NPs and their suitability as electrodes in energy storage devices. However, (Mn₃O₄)_L will act as a good material for energy storage applications as compared to (Mn₃O₄)_P NPs. The synthesized NPs were also tested for their antibacterial efficacy by biofilm inhibition and agar well diffusion methods. The NPs showed higher activities against <i>Staphylococcus aureus</i> (Gram-positive) than against <i>Escherichia coli</i> (Gram-negative), and (Mn₃O₄)_P was more bioactive than (Mn₃O₄)_L.