Green synthesis, characterization, antibacterial and photocatalytic activity of black cupric oxide nanoparticles
Authors: Siddiqi Khwaja Salahuddin , Rashid M. , Rahman A. , Tajuddin  , Husen Azamal , Rehman Sumbul 
Published: December 2020

Abstract
Background
Biogenic fabrication of nanoparticles from naturally occurring biomaterials involves plants, herbs, bacteria and fungi using water as neutral solvent, while chemical synthesis involves hazardous chemicals and leaves unwanted byproduct which unnecessarily pollute the environment. In order to prevent atmospheric pollution a safe, clean and green strategy for the synthesis of cupric oxide nanoparticles from aqueous leaf extract of Diospyros montana has been employed. D. montana of Ebenaceae family is a poisonous tropical plant which grows wild in Asia. Its extract is commonly known as fish poison. The rate of formation of NPs from plant extract is thought to be facile and rapid relative to those formed by fungi and bacteria, but it depends on the concentration of reducing chemicals available in the extract. We report, in this communication, a benign method of biogenic synthesis of cupric oxide nanoparticles (CuO-NPs) from leaf extract of D. montana and their characterization by UV–visible, FTIR, SEM, TEM, DLS, SAED and EDX analyses. Their antimicrobial activity against seven Gram-positive and four Gram-negative bacteria has been screened. Photocatalytic degradation of methylene blue by ascorbic acid as reducing agent and cupric oxide nanoparticles as catalyst has been done under sunlight.

Results
Cupric oxide nanoparticles of varying size starting from 5.9 to 21.8 nm have been fabricated from aqueous leaf extract of D. montana at room temperature. The pure extract absorbs at 273 nm while CuO-NPs exhibit a broad peak at 320 nm. FTIR spectrum of the leaf extract shows the presence of a double quinonoid molecule. There are three types of CuO-NPs with different hydrodynamic radii. Their average hydrodynamic radii fall between 495 ± 346 nm. SEM and TEM images show spherical shaped CuO-NPs of different size. SAED suggests crystalline nature of CuO-NPs. They are highly polydispersed in solution. EDX analysis reveals the presence of Ca, C, O, Na and Si besides copper. Oxygen content is over 50% by mass. Reduction of methylene blue dye (MB) by ascorbic acid as reducing agent, in presence of CuO-NPs as catalyst, has been achieved in 90 s at room temperature while their reduction by ascorbic acid alone takes more than 10 min. Antibacterial activity of CuO-NPs against seven Gram-positive (Staphylococcus aureus, Streptococcus mutans, Streptococcus pyogenes, Streptococcus viridans, Staphylococcus epidermidis, Corynebacterium xerosis and Bacillus cereus) and four Gram-negative bacterial strains (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Proteus vulgaris) has been investigated. The results indicated that NPs are highly effective against growth inhibition of Gram-positive bacteria than Gram-negative bacteria. Copper oxide nanoparticles are even more toxic than the standard antibiotic, norfloxacin.

Conclusion
In this project cupric oxide NPs of 5.9–21.8 nm have been fabricated from aqueous leaf extract of D. montana. It is most inexpensive and easy process to fabricate NPs from plant material because no toxic chemicals are used. Since CuO-NPs are toxic to several Gram-positive and Gram-negative bacterial strains, attempt may be made to use them as antibacterial agent to protect food, vegetable and crops. Also, the reduction of methylene blue dye by ascorbic acid as reducing agent in presence of CuO NPs as catalyst has been done very efficiently at a rapid rate which prompts us to use them as catalyst in the reduction of dyes, other toxic materials and industrial effluents. Further investigation of other beneficial properties of CuO-NPs can also be explored.