Keywords

1 Introduction

As per the estimations of National Science Foundation (NSF), Alexandria, USA, the global market for the nanotechnology-based products would be reaching three trillion USD by the year 2020 (Roco 2011). More than a thousand commercial products containing nanoparticles (NPs), which range from 1 to 100 nm, are currently available in the market (Vance et al. 2015) with wide-ranging applications in the fields of biomedicine, wastewater treatment, environmental remediation, food processing and packaging, agriculture, horticulture, and crop protection (Husen 2017; Siddiqi and Husen 2016, 2017a, b; Siddiqi et al. 2018a, b, c).

In comparison to their bulk counterparts, NPs have a greater chemical reactivity, strength, and some novel properties due to their increased surface-to-volume ratio and quantum size effect. The surface plasmon resonance (SPR) exhibited by metal NPs is one of their most important characteristics. NPs can be produced by the breakdown (top-down) or the buildup (bottom-up) methods (Husen and Siddiqi 2014), involving various physicochemical techniques. However, these production methods are usually expensive, labor-intensive, and potentially hazardous to the environment and living organisms. The most acceptable and effective approach for NP preparation is the bottom-up approach. Biological methods for NP synthesis utilize a bottom-up approach with the help of reducing and stabilizing agents.

Copper oxide nanoparticles (CuO NPs) are especially known for their catalytic, electric, optical, photonic, superconducting, and biological properties (Padil and Cernik 2013). However, their large-scale production has introduced risk to the environment and human health. Considering the wide-ranging applications and increasing demand of metal NPs, an alternative, cost-effective, safe, and green technology for large-scale production of CuO NPs is required. This chapter discusses the pros and cons of plant-mediated synthesis of CuO NPs, with the main focus on reduction, capping, and stabilization of NPs, and casts a cursory glance on their prospective applications.

2 Plant-Mediated Synthesis of Copper Oxide NPs

Extracts of various parts of different plant species have been used for synthesis of CuO NPs, as shown in Table 8.1, proving the ability of plant extracts to reduce copper ions to copper metals, yielding Cu NPs. Plant-mediated synthesis of MNPs is energetically efficient and can even be carried out at room temperature and is generally completed within few minutes.

Table 8.1 Plants and their parts used for biosynthesis of CuO NPs, together with their size, shape, and significance
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Plant parts (leaf, root, bark, etc.) are thoroughly washed with tap water in order to remove dust particles, sun-dried for 1–2 h to remove the residual moisture, cut into small pieces, and extracted. The extract is purified by filtration and centrifugation. Different concentrations of plant extract and metal salts (e.g., cupric sulfate, cupric chloride, copper nitrate, cupric acetate) are incubated in a shaker for different time intervals, at different pH and temperature, for NP synthesis. Formation of NPs is monitored by change in color of the reaction mixture. At the end, the reaction mixture is centrifuged at low speed to remove any medium components or large particles. Finally, the NPs can be centrifuged at a high speed or with a density gradient, washed thoroughly in water/solvent (ethanol/methanol) and collected in the form of a bottom pellet.

In an experiment, CuO NPs were synthesized from black bean extract and characterized by XRD, FTIR, XPS, Raman spectroscopy, DLS, TEM, SAED, SEM, and EDX (Nagajyothi et al. 2017). XRD studies have shown that the particles were ∼26.6 nm in size and spherical in shape. Rehana et al. (2017) used leaf extract of various plants (Azadirachta indica, Hibiscus rosa-sinensis, Murraya koenigii, Moringa oleifera, and Tamarindus indica) for synthesizing CuO NPs. UV-vis spectroscopy revealed the band centered between 220 and 235 nm, typical for CuO NPs. The SEM and TEM studies confirmed the spherical shape, with an average size of 12 nm, while SAED revealed the crystalline nature of the NPs. FTIR spectroscopy displayed bands at ∼490 and ∼530 cm−1 corresponding to metal–oxygen (Cu–O) vibration that supports the availability of monoclinic phase of CuO NPs. These NPs also displayed band in the region of 3442–3474 cm−1 due to O–H stretching vibration, whereas the band at 2370 and 2385 cm−1 was ascribed to the primary amines. The band at 1663 and 1674 cm−1 was reported due to amide I; however the band at 1530 and 1535 cm−1 was ascribed to the amide II region, which was characteristic of proteins and/or enzymes. Further, bands at 1030–1110 cm−1 corresponded to C–O stretching vibrations. Rehana et al. (2017) have suggested that the phenolic compounds and flavonoids acted as capping agents, thus preventing agglomeration, and stabilized the NP formation. Vishveshvar et al. (2018) synthesized CuO NPs using an aqueous solution of copper (II) sulfate and Ixora coccinea leaf extract. These particles were characterized using UV-vis spectroscopy, SEM, TEM, and FTIR spectroscopy. UV-vis spectroscopy showed a wavelength region from 200 to 300 nm. SEM images exhibited the formation of NP clusters of an average size of 300 nm. Further, TEM images of NPs, separated after ultrasonication of the dispersion, showed an average particle size of 80–110 nm. The FTIR spectroscopy peaks revealed the bonding vibrations such as Cu–O and O–H. In another recent study, Mehr et al. (2018) fabricated CuO and ZnO NPs by using the Ferulago angulata extract as a mild and nontoxic reducing agent and an efficient stabilizer without adding any surfactants and characterized them with the help of XRD, FTIR, and FESEM. The NPs produced were crystalline in nature, with high purity and an average size of ~ 44 nm. The FTIR spectrum of NPs showed two peaks at 912 and 620 cm−1. Dobrucka (2018) synthesized CuO NPs (10 ± 5 nm in size) using the extract of Galeopsidis herba and characterized them by UV-vis, SEM, TEM, and FTIR spectroscopy and EDS profile. SEM images confirmed the spherical shape of the NPs. FTIR spectrum showed bands at 417, 408, and 398 cm−1 indicating the formation of metal–oxygen stretching of CuO nanostructure. These NPs showed antioxidant as well as the catalytic activity in the reduction of malachite green. Octahedral and spherical CuO NPs were prepared using copper sulfate and Aloe vera aqueous extract (Kerour et al. 2018). The SEM images revealed octahedral and spherical agglomeration of NPs. XRD confirmed the cubic structure of NPs, which depends upon the crystallite size concentration of Aloe vera aqueous extract. The FTIR vibration measurements validated the presence of pure Cu2O in the samples. The UV-vis spectra indicated that the prepared Cu2O had a gap energy estimated from 2.5 to 2.62 eV. The photocatalytic activities enabled an improved and fast degradation of methylene blue in aqueous solution at room temperature under solar simulator irradiation (Kerour et al. 2018). Further details are given in Table 8.1.

2.1 Mechanism of CuO NP Synthesis

Plant extracts contain a wide range of metabolites that can act both as reducing and stabilizing agents in the metal NP synthesis. Bioreduction is a relatively complex process. As a reducing agent, biomolecules in the extract provide electrons to metal ions, thus reducing it into the elemental metal. After reduction, the atom so formed acts as the nucleation center, which is immediately followed by a period of growth when the smaller neighboring particles amalgamate to form a larger NP. In the final stage of synthesis, the plant extracts’ ability to stabilize the NP ultimately determines its energetically favorable and stable morphology. In order to avoid further growth and maintain the particle in the nano-range, a substance called capping agent is added. Biomolecules present in the plant extract may act as the reducing agent, or the same molecules may function as both the reducing agent and the capping agent. Since different plant extracts contain different phytoconstituents, the NP formation mechanisms vary among different plant species, and their details are yet to be fully elucidated.

Secondary metabolites, such as terpenoids, polyphenols, flavonoids, alkaloids, phenolic acids, etc., are responsible for the reduction of metal ions to zerovalent metals or the stabilization of MNPs. Flavonoids constitute a large group of polyphenolic compounds that comprise of several classes, viz., anthocyanins, isoflavonoids, flavonols, chalcones, flavones, and flavanones. The transition of flavonoids from the enol to the keto may lead to reduction of the metal. The ability of flavonoids to chelate metal ions is well documented. Some flavonoids, such as quercetin and santin, are known to possess strong chelating activity due to the presence of hydroxyls and carbonyl functional groups (Anjum et al. 2015). Apart from the crude extract, individual pure secondary metabolites have the ability to synthesize metal NPs (Sahu et al. 2016; Kasthuri et al. 2009). It has been reported that amino acids, sugars, and fatty acids available in gum karaya could act as a reducing and capping agent for the formation of metal oxide NPs (Silva et al. 2003).

3 Controlling the Shape and Size of CuO NPs

Despite a significant progress in the biosynthesis of NPs, little could be achieved in controlling the shape of the metal NPs by biological routes. Polydispersity of nanoparticles still remains a challenge. During the course of biological synthesis of metal NPs, a number of physical and biological parameters, including pH, reactants’ concentration, reaction time, and temperature, govern nucleation and the subsequent formation of the stabilized NP. The oxidation/reduction state of proteins and enzymes present in the cell-free extract is highly dependent upon the pH, making it a substantial factor to determine the shape and size of NPs. Further, metal ion concentration and pH, reaction time, and temperature can also affect the rate of nucleation and growth of NPs. As the reaction temperature increases, the reaction rate increases consuming metal ions to form the nuclei, thereby enhancing the biosynthesis process. Alteration of the reaction time can lead to variation in growth rate of seed particle, generating multi-shaped NPs. It is very clear that a single parameter cannot decide the fate of nuclei; rather it is the balance between all the parameters that can generate different shapes and sizes of NPs produced.

4 Characterization of CuO NPs

For characterization of synthesized NPs, several techniques, including ultraviolet-visible (UV-vis) spectroscopy, transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), Fourier transform infrared (FTIR) spectroscopy, X-ray fluorescence (XRF) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), particle size analysis (PSA), Malvern Zetasizer (MZS), energy-dispersive X-ray spectroscopy (EDX/EDS), nanoparticle tracking analysis (NTA), X-ray reflectometry (XRR), Brunauer–Emmett–Teller (BET) analysis, selected area electron diffraction (SAED), and atomic force microscopy (AFM), are used (Table 8.2).

Table 8.2 Characterization techniques, their use, and limitations
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5 Applications of Copper Oxide NPs

5.1 Biological Application

Various studies have established that CuO NPs possess potent antimicrobial, antioxidant, and anticancer activities (Table 8.3).

Table 8.3 Antimicrobial , antioxidant, anticancer, and catalytic activities of CuO NPs
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5.1.1 Antimicrobial Activity

The last decade introduced opportunities to investigate the bactericidal effect of metal NPs. Their small size and high surface-to-volume ratio allow them to interrelate strongly with microbial membranes which are not merely due to the release of metal ions in solution (Subhankari and Nayak 2013). Niraimathi et al. (2016) and Kala et al. (2016) have reported the antimicrobial activity of copper bionanoparticles in the recent past. Acharyulu et al. (2014) studied the antimicrobial activity of biosynthesized CuO NPs from Phyllanthus amarus leaf extract against multidrug-resistant Gram-positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria. Abboud et al. (2013) investigated that CuO NPs produced by using brown algae extract (Bifurcaria bifurcata) show high antibacterial activity against two different strains of Enterobacter aerogenes (Gram-negative) and Staphylococcus aureus (Gram-positive). Suleiman et al. (2013) claimed that biologically synthesized copper NPs can be used for treating several diseases; however, it requires clinical studies to ascertain their antimicrobial potential and efficacy. Padil and Cernik (2013) suggested the antimicrobial activity of CuO NPs against common pathogens E. coli and S. aureus. Das et al. (2013) and Heinlaan et al. (2008) demonstrated the antioxidant and antibacterial behavior of these NPs, whereas Hariprasad et al. (2016) observed their good antibacterial activity against E. coli, S. aureus, Bacillus cereus, and Pseudomonas aeruginosa. According to Naikaa et al. (2015), the synthesized CuO NPs were effective against the pathogenic bacteria S. aureus and Klebsiella aerogenes. Devasenan et al. (2016) also confirmed the ability of CuO NPs to inhibit the growth of various pathogens.

5.1.2 Antioxidant Activity

Antioxidant activity of nanomaterials is well established. Photooxidation of polymer creates aldehydes, ketones, and carboxylic acids at the end of the polymer chain. Antioxidants terminate these chain reactions by removing free radical intermediates and inhibit other oxidation reactions. The antioxidant and DNA-cleavage properties of CuO NPs biosynthesized with the help of Chammomile flower extract were reported by Duman et al. (2016). They suggested that CuO NPs can act as a chemical nuclease, can generate DNA cleavage, and may be useful for preventing cell proliferation. The improved antioxidant efficacy in Andean blackberry fruit than Andean blackberry leaf extract may be due to the presence of more bioactive molecules in the former than in the latter, which have a role as an encapsulating agent in CuO NPs. The highest antioxidant efficacy of CuO NPs against DPPH is probably derived through the electrostatic attraction between negatively charged bioactive compounds (COO−, O−) and neutral or positively charged NPs. CuO NPs bind to phytochemicals, and their bioactivity increases synergistically. Antioxidant activity of CuO NPs was measured by Das et al. (2013) using (2,2 diphenyl-1-picrylhydrazyl) DPPH method where DPPH was used as a radial source. Guin et al. (2015) found the biologically synthesized CuO NPs to be significantly effective against oxidative stress and less toxic than the precursor material.

5.1.3 Anticancer Activity

Copper oxide NPs also exhibit anticancer activity, as reported recently by Nagajyothi et al. (2017). Clonogenic assays have confirmed that the NPs-incubated cancer cells are not able to proliferate well. The CuO NPs can induce apoptosis (cell death) and suppress the proliferation of HeLa cells. These NPs have a high anticancer cytotoxicity on human colon cancer lines (HCT-116) with IC50 value of 40 μg mL−1. According to Rehana et al. (2017), CuO NPs are cytotoxic against four cancer cell lines, viz., human breast (MCF-7), cervical (HeLa), epithelioma (Hep-2), and lung (A549), and one normal human dermal fibroblast (NHDF) cell line. The anticancer activity of brown algae-mediated CuO NPs was determined by MTT assay against the cell line (MCF-7) (Suleiman et al. 2013). CuO NPs synthesized by using stem latex of Euphorbia nivulia (common milk hedge) could be encrusted and stabilized by peptides and terpenoids present in the latex. These NPs have shown toxic effect against human adenocarcinomic alveolar basal epithelial cells (A549 cells) (Valodkar et al. 2011a, b).

5.1.4 Antirheumatic Activity

In a recent investigation, it was found that CuO NPs prepared using Cassia auriculata extract can be used as a vehicle in drug delivery as antirheumatic agent for rheumatoid arthritis treatment (Shi et al. 2017).

5.1.5 Catalytic Effect

According to Devi and Singh (2014), CuO NPs prepared from the leaf extract of Centella asiatica at room temperature show a catalytic effect. NPs have many active sites as compared to the bulk material because of their small size and large surface-to-volume ratio. These NPs could be used for the photocatalytic degradation of methyl orange. In the absence of reducing agents in aqueous medium, these NPs reduce methyl orange to its leuco form.

Nasrollahzadeh et al. (2015a) investigated the effectiveness of CuO NPs for the synthesis of propargylamines by elaborating the reaction conditions in A3 coupling reaction between piperidine (1.2 mmol) and phenylacetylene (1.5 mmol) with benzaldehyde (1.0 mmol) as a model reaction. An assorted range of propargylamines was obtained in a superior yield. In addition, the reclaim and separation of CuO NPs were very easy, effectual, and economically viable. In another study, CuO NPs were found to be exceptionally heterogeneous catalyst for ligand-free N-arylation of indoles and amines. An excellent yield of N-arylated products was obtained, and the catalyst could be recovered and recycled for auxiliary catalytic reactions with approximately no loss in activity (Nasrollahzadeh et al. 2016).

6 Conclusion

Due to rich plant diversity, phytosynthesis of CuO NPs is proficient in producing superficial, ecologically safe, and economically viable NPs, in comparison to physical and chemical methods. During the bioreduction process, the biomolecules present in plant systems play a significant role. Different techniques used for the characterization of biosynthesized NPs include UV-vis spectroscopy, FTIR spectroscopy, XRD, SEM, EDX, Raman spectroscopy, DLS, TEM, SAED, etc. Applications of CuO NPs are significant especially in biomedicine and catalysis. The recent use of engineered CuO NPs in drug and gene delivery, in addition to their well-known catalytic effect and the antimicrobial, antioxidant, and anticancer activities, has attached a special prestige to them.