This short review article presents a summary of ecofriendly ways to synthesize cobalt oxide (Co3O4) nanoparticles using biomaterials like plant extract, bacterium and fungus. Currently, researchers have focused their concentration on the biosynthesis of nanoparticles using biomaterials containing phenols, carbonyls, amines, carbohydrates, proteins and amino acids which act as bio-templates, reducing agents, stabilizing agents, capping agents and chelating agents for nanoparticles. This article also discussed the reports of characterization of cobalt oxide nanoparticles by UV-Vis spectroscopy, diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), selected area electron diffraction (SAED), scanning electron microscopy (SEM), high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and Brunauer-Emmett-Teller (BET) analyzer which were carried out by various researchers. The applications of these biosynthesized cobalt oxide nanoparticles in a wide range of potential zones are listed including antibacterial activity, photo catalyst, sensor and supercapacitor.

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In this present study, an easy, eco-friendly, and efficient method for the biogenesis of manganese dioxide nanoparticles (MnO2 NPs) using Momordica charantia leaf extract is discussed. The MnO2 NPs were synthesised by reduction of potassium permanganate using Momordica charantia leaf extract as a reducing agent. Fourier-transform infrared spectra exposed the contribution of the biomolecules in the Momordica charantia leaf extract for the formation of MnO2 NPs. The UV–visible spectrum of the biosynthesized MnO2 NPs displayed absorption peaks at 371 nm, which was the absorption maximum of MnO2 NPs. Crystal phase and crystalline size of the biosynthesized MnO2 NPs was characterized by X-ray diffraction analysis. The X-ray diffraction pattern indicated that the average size of MnO2 NPs was about 36.01 nm. The field emission scanning electron microscopy analysis revealed that the biosynthesized MnO2 NPs have irregularly spherical shape with 16 – 63 nm in size. EDAX confirmed the presence of Mn and O in the MnO2 NPs. The antibacterial activities of MnO2 NPs were evaluated against Bacillus amyloliquefaciens, Bacillus subtilis, and Bacillus cereus. The total antioxidant capacity was evaluated by phosphomolybdenum method. The biosynthesized MnO2 NPs have significant antibacterial activity and antioxidant activity.

Co3O4 - MnO2 - ZnO NPs were synthesized by making use of the equimolar solutions of cobalt chloride, manganese(II) sulfate and zinc sulfate in aqueous sodium hydroxide and refluxed at elevated temperature. The synthesized mixed nano oxides were characterized by FT-IR, XRD, UV - Vis DRS, TEM, SAED, SEM, EDAX, AFM, TG/DTG and DSC. The FTIR spectra confirm the presence of M-O bonds (M = Co, Mn, Zn). The size of the Co3O4 - MnO2 - ZnO NPs are found to be 10.13 - 25.65nm through XRD studies. The XRD patterns also expose that the particle size is significantly increased with increasing concentration of the precursors. UV-Vis diffuse reflectance spectra (DRS) show that the ternary nanoparticles can be activated by UV and visible light illumination. The TEM, SEM and AFM micrographs of 0.1M Co3O4 - MnO2 - ZnO NPs show roughly spherical shape with size ranging from 10 - 30nm. EDAX analysis confirms the presence of Co, Mn, Zn and O. From TG-DTG and DSC studies, it is found that Co3O4 - MnO2 - ZnO NPs are thermally stable. The photocatalytic activity of Co3O4 - MnO2 - ZnO NPs was determined for degradation of methylene blue (MB) under sunlight. Among the samples, 0.1M Co3O4 - MnO2 - ZnO NPs exhibits the best performance (91.98%) for the degradation of MB in 90 min of irradiation time under sunlight. The factors influencing the photocatalytic activity are pH of dye solution, photocatalyst particle size, photocatalyst dosage and dye concentration.

Aims: To determine the antimicrobial activity of zirconium oxide nanoparticles (ZrO2 NPs) synthesized by Justicia Adhatoda leaf extract. Study Design: Synthesis, characterization and antibacterial activity determination of ZrO2 NPs. Place and Duration of Study: PG and Research Department of Chemistry, V.O.Chidambaram College, Tuticorin, Tamilnadu, India, between April 2020 and April 2021. Methodology: Justicia adhatoda leaf extract was used to synthesize ZrO2 NPs. UV-Visible spectroscopy was used to characterize ZrO2 NPs. Using Fourier transform infrared spectroscopy, the function of biomolecules in plant extract in the synthesis of ZrO2 NPs was identified. XRD was used to determine the particle size of nanparticles. ZrO2 NPs were evaluated for antimicrobial activity. Results: The synthesis of ZrO2 NPs was clearly visible in an absorbance band at 321 nm in the UV–visible spectrum. The absorption peak of ZrO2 NPs in the FTIR was 880 cm-1, confirming the Zr–O vibrational mode in ZrO2 NPs. As evidenced by XRD measurements, the average crystallite size of ZrO2 NPs was found to be 40 nm. The biosynthesized ZrO2 NPs were found to have potent antibacterial action against Escherichia coli bacteria and Staphylococcus aureus bacteria. Conclusion: ZrO2 NPs mediated by Justicia adhatoda leaf extract have demonstrated substantial antibacterial activity