Tagetes erecta-Mediated Biosynthesis of Mn₃O₄ Nanoparticles: Structural, Electrochemical, and Biological...

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 Tagetes erecta. We synthesized manganese oxide NPs, i.e., (Mn₃O₄)_L and (Mn₃O₄)_P NPs, by utilizing leaves and petals, respectively, of T. erecta 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 Staphylococcus aureus (Gram-positive) than against Escherichia coli (Gram-negative), and (Mn₃O₄)_P was more bioactive than (Mn₃O₄)_L.