Phytochemical evaluation and biosynthesis of Ocimum tenuiflorum based iron nanoclusters: An integrated experimental and DFT approach

Tulsi ( Ocimum tenuiflorum ), revered as ‘Holy Basil’ in Indian culture, is a medicinal plant traditionally used to treat various ailments, including respiratory issues and insect bites. This study investigates the phytochemical properties of aqueous and ethanolic extracts from Tulsi leaves and their application in synthesizing iron nanoclusters. The leaf extracts were prepared using distilled water and 20 % ethanol, with qualitative tests revealing the presence of carbohydrates, proteins, steroids, flavonoids, and phenols. Quantitative analysis indicated that the ethanolic extract contained a higher concentration of phytochemicals than the aqueous extract; further active phytochemicals were confirmed by gas chromatography-mass spectrometry (GC-MS). Green synthesis of black colored iron nanoparticles (FeNPs) using O. tenuiflorum extract was done and confirmed formation of spherical shaped nanoparticles of 20–30 nm size and crystalline nature using TEM and XRD analysis respectively. Furthermore, density functional theory (DFT) calculations examined the interactions between active phytochemicals from Tulsi extracts and Fe 2+ ions. Notably, 1,16-cyclocoryan-17-oic acid-19,20-didehydro-methyl ester formed a stable nanocluster with Fe 2+ (Complex I), exhibiting an adsorption energy of −27.60 eV, while methyl eugenol (Complex II) showed an adsorption energy of −26.40 eV. Quantum Theory of Atoms in Molecules (QTAIM) analysis revealed closed-shell interactions in both complexes. The 3D Interaction Region Isosurface (IRI) confirmed chemical bond formation in Complex I, whereas Complex II displayed a mix of covalent interactions and van der Waals forces. Natural Bond Orbital (NBO) analysis indicated that Complex I is more stable than Complex II due to favorable interaction energies. These findings suggest that Complex I could have significant applications in various fields.