The ability of organisms and organic compounds to reduce metal ions and stabilize them into nanoparticles (NPs) forms the basis of green synthesis. To date, synthesis of NPs from various metal ions using a diverse array of plant extracts has been reported. However, a clear understanding of the mechanism of green synthesis of NPs is lacking. Although most studies have neglected to analyze the green-synthesized NPs (GNPs) for the presence of compounds derived from the extract, several studies have demonstrated the conjugation of sugars, secondary metabolites, and proteins in these biogenic NPs. Despite several reports on the bioactivities (antimicrobial, antioxidant, cytotoxic, catalytic, etc.) of GNPs, only a handful of studies have compared these activities with their chemically synthesized counterparts. These comparisons have demonstrated that GNPs possess better bioactivities than NPs synthesized by other methods, which might be attributed to the presence of plant-derived compounds in these NPs. The ability of NPs to bind with organic compounds to form a stable complex has huge potential in the harvesting of precious molecules and for drug discovery, if harnessed meticulously. A thorough understanding of the mechanisms of green synthesis and high-throughput screening of stabilizing/capping agents on the physico-chemical properties of GNPs is warranted to realize the full potential of green nanotechnology.

This article reports the green fabrication of cerium oxide nanoparticles (CeO₂ NPs) using Olea europaea leaf extract and their applications as effective antimicrobial agents. O. europaea leaf extract functions as a chelating agent for reduction of cerium nitrate. The resulting CeO₂ NPs exhibit pure single-face cubic structure, which is examined by X-ray diffraction, with a uniform spherical shape and a mean size 24 nm observed through scanning electron microscopy and transmission electron microscopy. Ultraviolet-visible spectroscopy confirms the characteristic absorption peak of CeO₂ NPs at 315 nm. Fourier transform infrared spectroscopy reflects stretching frequencies at 459 cm^-1, showing utilization of natural components for the production of NPs. Thermal gravimetric analysis predicts the successful capping of CeO₂ NPs by bioactive molecules present in the plant extract. The antimicrobial studies show significant zone of inhibition against bacterial and fungal strains. The higher activities shown by the green synthesized NPs than the plant extract lead to the conclusion that they can be effectively used in biomedical application. Furthermore, reduction of cerium salt by plant extract will reduce environmental impact over chemical synthesis.

In this study, we have investigated the effect of copper oxide nanoparticles (CuO-NPs) on callogenesis and regeneration of Oryza sativa L (Super Basmati, Basmati 2000, Basmati 370 and Basmati 385). In this regard, CuO-NPs have been bio-synthesized via Azadirachta indica leaf extract. Scanning electron microscope (SEM) analysis depicts average particle size of 40±5 nm with highly homogenous and spherical morphology. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) have been employed to confirm the phase purity of the synthesized NPs. It is found that CuO-NPs exhibit very promising results against callus induction. It is attributed to the fact that green synthesized CuO-NPs at optimum dosage possess very supportive effects on plant growth parameters. In contrast to callogenesis, differential regeneration pattern has been observed against all of the examined O. sativa L. indigenous verities. Overall observation concludes that CuO, being one of the essential plant nutrients, has greatly tailored the nutritive properties at nano-scale.

The use of nanostructures (NS) in plant tissue culture can be beneficial only if we have their complete bio-safety and biocompatibility profile. In the recent past, increasing rate of NS-cytotoxicity has alarmed the plant biotechnologist to think critically. Keeping this in view, our current study represents a comparative cytotoxicity examination of green fabricated highly potent CeO₂-NS and Ag-NS. Phyto-reductants from O. europaea extract simultaneously acted as reducing as well as capping agent in tuning the shape of CeO₂-NS and Ag-NS to as small as 18 nm and 26 nm respectively. High-performance liquid chromatography (HPLC) analysis of O. europaea extract supplemented by thermo-gravimetric analysis (TGA) findings further validates the capping action of bioactive agents from the plant extract. X-ray diffraction (XRD) testing confirms the phase purity and crystalline nature of prepared CeO₂-NS and Ag-NS. Fourier transform infrared (FTIR) spectroscopy shows typical Ce–O bond vibrations at 457 cm⁻¹ and Ag around 563 cm⁻¹ to 641 cm⁻¹. Moreover, the optimized NS shows differential cytotoxicity when tested on callogenesis and organogenesis of Lycopersicon esculentum. Findings from DPPH assay shows that CeO₂-NS are biocompatible and have tremendously enhanced the antioxidant potential of L. esculentum with greater biomass, contrastingly Ag-NS are found phyto-toxic with lower antioxidant potential and have a negative impact when added during plant tissue culture.