Green fabrication of manganese oxide nanoparticles using Psidium guajava extracts for antioxidant, antimicrobial and photocatalytic degradation applications

Manganese oxide (MNO) nanoparticles (NPs) were synthesized via economical, and eco-friendly green method using Psidium guajava fruit pulp, peel, and leaf extracts. Three different samples of MNO-NPs, labelled as GPP, GPL, and GLS, were synthesized by using guava pulp, guava peel, and guava leaf respectively. These samples were characterized by using FTIR, XRD, and SEM methodology. The XRD pattern compared with the characteristic JCPDS Cards of Mn₃O₄-NPs clearly exhibited that the utilized green methodology was successful in fabricating the Mn₃O₄-NPs. The crystallite sizes of the NPs calculated using the Debye–Scherrer formula were found to be 17.35, 16.99, and 16.97 nm for GPP, GPL, and GLS-NPs indicating that the guava leaves extract exhibited the best potential for the fabrication of the smaller sized NPs. The SEM and FTIR further supplemented this observation as the GLS-NPs were found to be uniform and monodispersed NPs with very little agglomeration. The NPs were further evaluated for their antibacterial, antifungal and antioxidant potential. The comparative analysis revealed that the smaller size of GLS-NPs provided edge to these NPs in terms of the biological applications as the best results in terms of antibacterial potentials (against S. aureus, M. luteus, B. halodurans, and E. coli strains), antifungal potential (against A. niger and A. flavus), and antioxidant potential (with 93.4% inhibition for scavenging radicals) were documented via use of GLS-NPs. The photocatalytic degradation (PD) efficacy of the MNO-NPs was also investigated by using the methyl orange (MO) dye. An advanced statistical tool of response surface methodology (RSM) was implemented for carrying out the optimization of the understudy reaction. Degradation values of 93.62%, 94.26%, and 96.84% were documented for the respective GPL-NPs, GPP-NPs, and GLS-NPs which highlighted that the GLS-NPs was found to be more suitable for carrying out the PD of MO. The optimized reactions condition specified by RSM approach improved the degradation percentage to 97.32% for the GLS-NPs indicating that the interaction terms as suggested by the quadratic model were effective in improving the degradation efficiency of MO. Finally, our results demonstrated that MNO-NPs fabricated by using Psidium guajava extract exhibited promising antimicrobial, antioxidant and photocatalytic degradation properties.