Swift Synthesis of Nickel Nanoparticles and their Antibacterial Studies through Green and Sustainable Method Using Terminalia Mantaly Leaf Extract

Abstract

In this paper, we report a swift and simple green synthesis method to produce nickel oxide nanoparticles (NiO NPs) utilizing, for the first time, Terminalia mantaly leaf extract to improve their properties. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), and photoluminescence studies were carried out on the synthesised samples. Antibacterial studies were performed, and results are presented in this paper. The results confirm that Terminalia mantaly biomolecules induce the reduction of nickel ions to NiO NPs and function as a capping and stabilising agent, enhancing biological performance.

Introduction

Nanoparticles have attracted considerable attention because of their unique physicochemical properties, including high surface area, tunable morphology, optical and magnetic characteristics, and excellent chemical reactivity. Although conventional synthesis methods such as sol–gel, hydrothermal, laser ablation, and co-precipitation are widely used, they often involve hazardous chemicals, expensive equipment, and high energy consumption. Green synthesis using plant extracts has emerged as an environmentally friendly and cost-effective alternative, where plant phytochemicals serve as natural reducing and stabilizing agents to produce stable nanoparticles with controlled size and morphology.

Nickel oxide (NiO) is an important transition metal oxide with a wide band gap (3.6–4.0 eV) and excellent electrochemical, magnetic, optical, and mechanical properties. NiO nanoparticles have found applications in catalysis, supercapacitors, solar cells, environmental remediation, drug delivery, and biomedical fields because of their antioxidant, antibacterial, antifungal, anti-inflammatory, anticancer, and enzyme inhibitory activities. However, conventional methods for synthesizing NiO nanoparticles often require complex procedures and high energy input. Green synthesis provides a sustainable alternative with lower environmental impact and simplified processing.

In this study, Terminalia mantaly leaf extract was used as a natural reducing and stabilizing agent to synthesize NiO nanoparticles. The leaves were washed, dried, powdered, and extracted with hot distilled water. Nickel nitrate hexahydrate was mixed with the leaf extract, heated to form a paste, dried, and calcined at 400°C to obtain greyish-black NiO nanoparticle powder.

The synthesized nanoparticles were characterized using XRD, FTIR, FESEM, EDX, and UV–Visible spectroscopy.

• XRD analysis confirmed the successful formation of highly crystalline face-centered cubic NiO nanoparticles. Diffraction peaks matched JCPDS card No. 78-0643, and the average crystallite size calculated using the Scherrer equation was 25.27 nm. The lattice parameter, interplanar spacing, microstrain, and dislocation density were also determined, indicating good crystallinity and structural quality.
• FTIR spectroscopy revealed the characteristic Ni–O stretching vibration at 454 cm?¹, confirming NiO formation. Additional absorption bands corresponding to hydroxyl, carbonyl, C–H, and C–O groups indicated the presence of phytochemicals from the leaf extract, which acted as reducing and capping agents during nanoparticle synthesis.
• FESEM images showed quasi-spherical to polyhedral nanoparticles with particle sizes ranging from 20–60 nm, forming porous agglomerated clusters. The morphology suggested effective nanoparticle formation with high surface area. EDX analysis confirmed the elemental composition of nickel and oxygen without detectable metallic impurities, demonstrating the high purity of the synthesized NiO nanoparticles.
• UV–Visible spectroscopy exhibited a strong absorption edge near 328 nm, characteristic of NiO nanoparticles. The optical band gap determined from the Tauc plot was 3.24 eV, consistent with reported values for nanocrystalline NiO. The slight increase in band gap compared with bulk NiO was attributed to nanoscale particle size, crystallinity, lattice strain, oxygen vacancies, and phytochemical capping effects. These optical properties indicate the suitability of the synthesized NiO nanoparticles for photocatalytic, optoelectronic, and antibacterial applications.

Conclusion

Comparisons of the two bacterial strains show that the samples exhibited antibacterial activity against both Staphylococcus aureus and Enterococcus faecalis. However, the extent of inhibition varied with the concentration of the sample loaded. There was no difference between the maximum inhibition zones observed for E. faecalis (10.2 mm) and S. aureus (9.6 mm), indicating that E. faecalis was slightly more susceptible under the experimental conditions. Lower inhibition values (2.0-4.4 mm) indicate relatively weak antibacterial activity, while higher inhibition values (more than 6 mm) indicate relatively moderate antibacterial activity. Overall, the results support the conclusion that the synthesised nanoparticles exhibit concentration-dependent antibacterial activity against all Gram (+) bacterial strains tested and have the potential to be used as environmentally friendly antimicrobial agents.

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Copyright © 2026 Bandari Lavanya, Y. Aparna. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.