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This study is aimed to synthesize zinc oxide nanoparticles (ZnO NPs) from Aloe barbadensis Miller leaf extract using the green synthesis method at room temperature. The biosynthesized NPs are characterized by X‐ray diffraction (XRD), UV–vis spectroscopy (UV–vis), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscope (SEM). The XRD pattern has revealed the hexagonal wurtzite structure with a crystallite size of 16 nm for synthesized ZnO NPs. The FTIR results display the formation of ZnO NPs and the presence of other functional groups. The UV–vis absorption spectrum shows an absorption band at 347 nm and the value of the bandgap is found to be 3.75 eV. The surface morphology of synthesized NPs is visualized by SEM images, it is observed that most of the NPs are needle in shape. Further, these NPs will find applications in biosensing and photocatalytic activity. [ABSTRACT FROM AUTHOR]

• Fabrication of ZnO and Cr doped ZnO NPs by adopting green and chemical methods. • Structural & optical contrasts in green vs. chemically synthesized ZnO & Cr-ZnO NPs. • Crucial insights into Cr-doped ZnO NPs behaviour. • Green Cr doped ZnO NPs: future photodegradation catalyst. • Potent anticancer activity by synthesized NPs. Herein, we focused on the combined investigation of physicochemical traits and multifunctional bioactivity of ZnO and Cr-doped ZnO NPs synthesized by chemical and green synthesis methods. The green synthesis was accomplished by utilizing Azadirachta Indica leaf extracts which include the phytochemicals azadirachtin and nimbolide function as prominent therapeutic agents in diagnosis. The XRD and TEM analysis evidences all the prepared NPs exhibit hexagonal wurtzite structure, and the change in crystallinity of ZnO NPs with Cr substitution has been examined. The optical properties show that Cr inclusion effectively changes the optical band gap of pure ZnO NPs, confirming the quantum confinement effect. The synthesized NPs were employed to investigate photocatalytic degradation of Rhodamine B (Rh B) dye in an aqueous environment under UV irradiation. The Rh-B dye effectively degraded within 180 min in the presence of synthesized nano catalyst at room temperature. Further, the synthesized NPs were subjected to investigate their bio-potency for microbicidal and cytotoxicity properties. The biosynthesized Cr-ZnO NPs exhibit significant cytotoxicity against A-549 lung cancer cells with the lowest IC 50 value (21.79 µg/ml) and also demonstrate good antioxidant activity with the highest percentage of inhibition 81.23 % as measured by the DPPH scavenging assay in contrast to other NPs. In addition, the chemically synthesized Cr-ZnO NPs found the highest inhibition activity (90.78 %) for antidiabetic efficiency of Alpha-amylase enzyme assay and also shows significant antibacterial activities with a higher zone of inhibition (17 mm) against gram-positive Bacillus cereus over other samples. Thus, this study highlights ZnO NPs versatile attributes, paving the way for promising biomedical and environmental uses. [Display omitted] [ABSTRACT FROM AUTHOR]