Abstract
In the last decades, the idea of green nanotechnology has been expanding, and researchers are developing greener and more sustainable techniques for synthesizing nanoparticles (NPs). The major objectives are to fabricate NPs using simple, sustainable, and cost-effective procedures while avoiding the use of hazardous materials that are usually utilized as reducing or capping agents. Many biosources, including plants, bacteria, fungus, yeasts, and algae, have been used to fabricate NPs of various shapes and sizes. The authors of this study emphasized the most current studies for fabricating NPs from biosources and their applications in a wide range of fields. This review addressed studies that cover green techniques for synthesizing nanoparticles of Ag, Au, ZnO, CuO, Co3O4, Fe3O4, TiO2, NiO, Al2O3, Cr2O3, Sm2O3, CeO2, La2O3, and Y2O3. Also, their applications were taken under consideration and discussed.
1. Introduction
Green nanotechnology is the biosynthesis of nanomaterials from natural bioactive agents such as plant materials, microbes, and various biowastes such as agricultural residues, eggshells, vegetable waste, fruit peels, and others, accompanied by the use of nanoproducts to achieve sustainability [1, 2]. It is a low-cost, simple, safe, low-risk, nontoxic, and environmentally friendly approach [3, 4]. Green nanotechnology is a vital feature of clean technologies aimed at environmental remediation and the conversion of extra-bioactive products into more lucrative and environmentally friendly green nanomaterials. Human biowaste is increasing in tandem with population growth, posing a significant danger to environmental purity. As a result, finding a safe and economic solution to this problem, which has a high environmental cost, has become critical [5–7]. Many researchers have lately considered strategies to benefit from these bioresources by recycling them into acceptable and environmentally beneficial products from this standpoint [8]. One of these methods is green nanotechnology, which uses recyclable bioresources in the production of nanomaterials [9–11]. High pressure, energy, temperature, or toxic substances are not required. It is a biosafe and low-cost method for fabricating nanomaterials for applications in catalysis, solar cells, medicinal medicines, industrial industries, sensors, water desalination, water purification, and air purification [12, 13] (see Figure 1).
The prevalence of green route techniques is based on the fact that they allow the production of nanomaterials in a regulated and clean environment hence environmentally friendly [14]. There are various available methods or approaches to the green route that include utilizing plant extracts and synthesizing nanoparticles using fungi, bacteria, viruses, and algae [15]. Each of these green methods portrays specific advantages over environmental conservation and disadvantages.
The first green route technique is plant extracts, and it is considered the most reliable approach when it comes to eco-friendly, biocompatible, and safe nanoparticles. The approach has various advantages that include the free availability of plants that are harmless, easy to handle, and cheap material to allow various nanoparticle syntheses [16]. The approach also entails using distinct parts of the plant such as fruits, flowers, leaves, and roots with various biomolecules such as amino acids and carbohydrates. The approach also depicts some disadvantages that include the potential loss of original color, shape, and size of the plant’s part used in the synthesis process [16]. This is because, during plant incubation, the precursor salts change the solution color to reflect the iron salts used in intensifying the nanoparticle production [17]. Another approach involves green route synthesis utilizing bacteria, which also involves various advantages. The advantage is that bacteria can reduce the metal ions, mainly when manipulating bacteria [18]. The easier-to-manipulate bacteria have seen the enhanced adoption of bacterial synthesis of nanoparticles [19]. However, this approach is limited or disadvantaged since it needs the control of multiple factors such as cell wall functional groups and temperature control, making the entire process complex [1]. The third green route approach includes fungi-mediated biosynthesis and is considered an efficient process. The advantage of this method is the high presence of intracellular enzymes in various shapes and the ability of competent fungi to synthesize a larger number of nanoparticles [20]. The demerit of this approach is based on high variability when it comes to starting biosynthetic biomaterial where some involve fungal biomass, while others use fungal cell filtrate [21]. The last method of nanoparticle synthesis includes the use of algae that do not regard leaf or root structure. The merit associated with this method is the ability to synthesize different metallic types using microalgae [22]. The disadvantage is based on the procedure of using algae that involves washing seaweeds and drying them over the sunlight, which may lead to the loss of many materials. Table 1 summarizes the major merits and demerits of the main green routes.
Although green nanotechnology is an interesting and diverse subject of study, it is still considered a new technology. Most of the recent articles that deal with the synthesis of nanometal and metal oxides via green methods have been presented in this review.
2. Recent Studies of Nanoparticles Synthesized via the Green Methods
2.1. Silver Nanoparticles (Ag NPs)
Sekatawa et al. used Camellia sinensis and Prunus africana leaves to evaluate green-synthesized nanomaterials such as silver nanoparticles (Ag NPs) as antibiotic alternatives. Biosynthesized AgNPs had maximal growth inhibitory zones of 21 mm against carbapenem-resistant bacteria, with minimum inhibitory concentration and minimum bactericidal concentration of 125 and 250 g/ml, respectively [24].
Dutta et al. have developed a one-pot green synthesis technique for silver nanoparticles (Ag NPs) utilizing Citrus limetta peel extract. Micrococcus luteus, Streptococcus mutans, Staphylococcus epidermidis, Staphylococcus aureus, and Escherichia coli were all found affected by synthesized Ag NPs. Ag NPs also showed antifungal action against Candida species, as well as anti-biofilm and cell membrane permeabilization properties [25].
The bioreduction of silver nitrate using Parkia speciosa leaf aqueous extract resulted in the green production of silver nanoparticles (Ag NPs). Water treatment through photocatalytic methodology (methylene blue under solar irradiation) (Figure 2), antioxidant (DPPH radical scavenging method), and antibacterial capabilities (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Bacillus subtilis) were all demonstrated for the biosynthesized Ag NPs [26].
Silver nanoparticles were additionally prepared with silver nitrate as a precursor and an aqueous extract of fresh Gomphrena globosa (Globe amaranth) leaves as a reducing and stabilizing agent. Silver ions (Ag+) are rapidly reduced to metallic silver nanoparticles (Ag0) by active phytochemicals in the leaves. Three Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis, and Micrococcus luteus) and three Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae) demonstrate good antibacterial activity against the biosynthesized Ag NPs [27].
Alginate, fucoidan, and laminaran are polysaccharides extracted from marine algae. The activity of Saccharina cichorioides and Fucus evanescens as reducing and stabilizing agents in the biogenic production of silver nanoparticles was investigated. The antibacterial and cytotoxic characteristics of the nanoparticles were also tested. Furthermore, silver nanoparticles had significant antibacterial activities, which were more prominent in fucoidan-derived samples [28].
For the assessment of effective antibacterial, anticancer, and photocatalyst properties, metallic silver nanoparticles (Ag NPs) were biosynthesized utilizing Sambucus ebulus extract [29]. Synthesized Ag NPs@SEE showed improved performance on cancer cell lines and good antibacterial capabilities against Gram-positive microorganisms (MIC value of 1.5 g/ml for S. aureus). Silver nanoparticles generated by Sambucus ebulus have the potential to be used as low-cost and efficient nanoparticles for environmental and biological applications, according to these studies [29].
Yugay et al. described a green chemistry technique for the biological production of Ag NPs utilizing extracts from Panax ginseng non-transformed callus, rolC transgenic callus, and hairy roots, as well as an assessment of their effectiveness against crop-damaging fungal pathogens [30]. Fusarium graminearum, Fusarium avenaceum, Fusarium poae, and Fusarium sporotrichioides, which are involved with Fusarium head blight disease in cereals, were extremely affected by the biosynthesized nanoparticles. Furthermore, nanosilver’s antifungal activity was effectively applied to the surface sterilization of infected wheat kernels without affecting seed germination [30].
As an antibacterial, flexible strain sensor, silver nanoparticles were prepared by solid-state reduction of hydroxyethyl cellulose and compounded into a chemically cross-linked hydrogel. The composite hydrogels have been used to develop an antibacterial strain sensor with a gauge factor (GF) of 4.07 [31].
Using a bacterial T10 strain isolated from an atypical environment, functionalized silver nanoparticles were synthesized [32]. Within 1 hour of incubation, stable and well-disseminated Ag NPs were produced extracellularly utilizing the bacterial extract. Antimicrobial activity against clinical infections was observed, as well as a synergistic impact with antibiotics. By coating Ag NPs on bandages, the antibacterial activity of the resulting Ag NPs was evaluated against clinical pathogens. These findings show that the bacterial extracts are a good source for green Ag NP production and might be utilized to combat infections [32].
Hamida et al. isolated, purified, cultivated, and molecularly characterized a new cyanobacteria species (Desertifilum sp. IPPAS B-1220) and then utilized its aqueous extract to produce silver nanoparticles [33]. These studies demonstrated for the first time the ability of a new cyanobacteria strain, Desertifilum IPPAS B-1220, to produce tiny NPs that were effective anticancer and antibacterial materials against cancer cell lines and harmful bacterial strains [33].
From fungal metabolites of Penicillium oxalicum, biogenic production of silver nanoparticles (Ag-NPs) as potential antibiotics has been carried out [34]. The antibacterial activity of biosynthesized silver nanoparticles was tested using the well diffusion technique and a UV-visible spectrophotometer against Staphylococcus aureus, S. dysenteriae, and Salmonella typhi. For both S. aureus and Shigella dysenteriae, the maximum zone of inhibition was 17.5 ± 0.5 mm (mm), whereas for Salmonella typhi it was 18.3 ± 0.60 mm. P. oxalicum that biosynthesized silver nanoparticles have remarkable antibacterial activity. These findings suggest that biosynthesized silver nanoparticles have substantial promise for a variety of biological applications, including bactericidal agents against resistant bacteria, infection prevention, wound healing, and anti-inflammation [34].
A new fungus, Piriformospora indica, has been identified as a good source for creating simple and consistent Ag NPs. These biogenic nanoparticles are cytotoxic and were found to have inherent properties that make them appropriate for anticancer action. Human breast adenocarcinoma (MCF-7), human cervical carcinoma (HeLa), human liver hepatocellular carcinoma (HepG2) cell lines, and embryonic kidney cell line (HEK-293) were used to test the in vitro cytotoxicity of biologically generated AgNPs (BSNPs) and chemically synthesized AgNPs (SNPs). In comparison with SNPs, the antiproliferative results demonstrated that BSNPs had high cytotoxic action against MCF-7, HeLa, and HepG2 cell lines [35].
2.2. Gold Nanoparticles (Au NPs)
In comparison with gold bulk structure, gold nanoparticles have unique and unexpected capabilities that have been proposed for a variety of applications including therapeutic agent delivery, antifungal, antibacterial, anticancer, colorimetric sensor, photodynamic treatment, and electrochemical sensor [36, 37].
Mentha aquatic extract was used to produce Au nanoparticles utilizing a biosynthetic method [38]. The Au NPs were produced and employed as a conductive mediator to modify the tramadol electrochemical sensor. For the measurement of tramadol in aqueous solution, the modified paste electrode containing Au NPs and 1-butyl-3-methylimidazolium tetrachloroborate demonstrated good catalytic activity.
Fanoro et al. [39] used Combretum erythrophyllum plant leaves to describe a simple, green, and cost-effective plant-mediated production of gold nanoparticles (Au NPs). CE leaf extract was used as a reducing and capping agent in the synthesis, which was carried out at room temperature. With a minimum inhibitory concentration of 62.5 g/mL, the Au NPs showed broad-spectrum antibacterial activity against pathogenic Gram-positive (Staphylococcus epidermidis (ATCC 14990), Staphylococcus aureus (ATCC 25923), and Mycobacterium smegmatis (MC 215)) and Gram-negative bacteria (Proteus mirabilis (ATCC 7002) and Escherichia coli (ATCC 25)). Additionally, the as-prepared Au NPs were extremely stable, with good cell viability in both normal (BHK-21) and malignant cancer cell lines (cervical and lung cancer).
The reduction of hydrogen tetrachloroaurate (III) (HAuCl4·3H2O) solution by the aqueous leaf extract of Ananas comosus [40] was used to produce a bio-directed synthesis of gold nanoparticles (Au NPs). The solid-phase breakdown of low-density polyethylene (LDPE) film was used to investigate the photocatalytic capability of Au NPs. Nanocomposite film photoinduced degradation was greater than pure LDPE film. After 240 hours of sun irradiation, an LDPE film containing 1.0% Au NPs had a degradation efficiency of 90.8%. The nanoparticles’ durability was demonstrated by their capacity to be reused in the photocatalytic degradation reaction for up to five cycles without significant loss of catalytic efficacy [40].
The green synthesis of Au NPs and co-functionalization with Curcuma pseudomontana extracted curcumin (CUR) is studied [41]. The human red blood cell technique revealed a maximal inhibitory efficacy of 94% for CUR-Au NPs. CUR-Au NPs also showed strong antioxidant and radical scavenging properties. To successfully utilize the biological use of the particles, this work establishes a platform for unique synthetic Au NPs created by employing reducing agents in the form of isolated curcumin [41].
The effective characteristics of Au NPs synthesized from an aqueous extract of Pimenta dioica leaves were studied by Fadaka et al. [42]. The catalytic activity of produced nanoparticles was tested by degrading a methylene blue dye in the presence of NaBH4 as a reducing agent, which took just two minutes. Au NPs have a strong antioxidant capacity. Furthermore, the produced Au NPs had a substantial inhibitory impact against both Gram-positive and Gram-negative bacteria, with zones of inhibition of 4 and 9 mm for S. aureus and E. coli, respectively [42].
Perveen et al. [43] synthesized gold nanoparticles from Trachyspermum ammi seed extract (TA-Au NPs), tested their efficiency against drug-resistant Listeria monocytogenes and Serratia marcescens biofilms, and assessed their anticancer potential against HepG2 cancer cell lines. The biofilm inhibitory activity of synthesized TA-Au NPs against L. monocytogenes (73%) and S. marcescens (81%) was demonstrated. Important factors in biofilm formation and maintenance, including exopolysaccharide (EPS), motility, and CSH, were strongly reduced at the studied subminimum inhibitory doses (sub-MICs). Furthermore, TA-Au NPs successfully annihilated mature biofilms of S. marcescens and L. monocytogenes by 64% and 58%, respectively. Reduced biofilm formation in test pathogens might be explained by the induction of intracellular ROS generation in TA-Au NPs treated bacterial cells. The administration of TA-Au NPs caused cellular growth to be stopped in a concentration-dependent manner. In HepG2 cancer cell lines, TA-Au NPs reduce intracellular GSH, making the cells more susceptible to ROS production and inducing death [43].
Fresh peel aqueous extract of Benincasa hispida was used as a reducing and stabilizing agent, resulting in a rapid and environmentally friendly approach for the synthesis of gold nanoparticles (GNPs). The produced GNPs exhibited significant antibacterial activity against a variety of Gram-positive and Gram-negative microorganisms. In vitro cytotoxicity of the biosynthesized GNPs against a human cervical cancer cell line was also impressive. Peel extracts of B. hispida may be utilized to easily synthesize GNPs, which can be exploited as a natural source of antibacterial and anticancer agents [44].
The capability of the marine bacteria Paracoccus haeundaensis to synthesize gold nanoparticles (Au NPs) outside of the cell has been established [45]. On HaCaT and HEK293 normal cells, the Au NPs do not limit growth, but they do inhibit growth in A549 and AGS cancer cells in a concentration-dependent manner. As a result, Au NP synthesis using P. haeundaensis is straightforward and nontoxic to human cells, implying that they could be useful in biomedical applications [45].
Syed et al. [46] focused their study on gold sulfide nanoparticles (Au2S NPs) and employed the fungus Humicola sp. to biosynthesize these NPs. Normal and cancer (Daudi, ZR-75-1) cell lines showed decreased cytotoxicity when exposed to Au2S NPs. This innovative fungal-based approach offers an inexpensive and environmentally friendly biosynthesis of Au2S nanoparticles that might be useful in bioimaging and labeling applications [46].
Acay [47] investigated the bioactive characteristics of gold nanoparticles (Au NPs) produced from edible Morchella esculenta (ME) using a hot water extract method in room settings (ME-Au NPs). In the A549 and HepG2 cell lines, the ME-Au NPs demonstrated excellent antibacterial and cytotoxic action against a variety of pathogenic microorganisms. This work showed that an inexpensive and harmless fungal extract may be employed as a reducing and stabilizing agent in the synthesis of size-controlled, large-scale, and biocompatible Au NPs for future diagnostic and therapeutic applications.
Gold nanoparticles with anticancer properties were created using green synthesis and an endophytic strain, Fusarium solani ATLOY-8, isolated from the plant Chonemorpha fragrans. These NPs were found to be cytotoxic to cervical cancer cells (He La) and human breast cancer cells (MCF-7) in a dose-dependent manner [48].
2.3. Zinc Oxide Nanoparticles (ZnO NPs)
Metal or metal oxide NPs have emerged as attractive materials in the evolution of nanoscience [49]. Zinc oxide nanoparticles (ZnO NPs) in particular have amazing uses in a wide range of domains, including cosmetics, optical and electrical sectors, biomedicine, and catalysis [49]. Several research studies have shown cost-effective solutions involving diverse chemicals, plants, and microorganisms mediated ZnO NPs, including photocatalytic degradation [41], sensing studies [50], solar cell [51], photocatalytic activity, photocorrosion resistance, and recyclability [52]. Additionally, these nanoparticles are exploited as medication carriers, as well as in the cosmetics and pharmaceutical sectors [53]. ZnO nanoparticles have the potential to be employed as an antibacterial agent in biological applications to kill pathogenic germs [54].
The synthesis of nanoparticles via biological pathways, particularly employing fungal extracts, is gaining popularity in the field of nanotechnology due to its low cost and environmental friendliness, as well as its broad antibacterial performance. ZnO nanoparticles were produced from the white-rot fungus Phanerochaete chrysosporium using 0.01 M ZnSO47H2O and 0.1 N NaOH as precursors. P. chrysosporium is a wood-degrading fungus with two types of extracellular peroxidases: lignin peroxidase (Lip) and manganese peroxidase (Man) (MnP). Under the experimental conditions, ZnO nanoparticles also exhibited efficient antibacterial action against Staphylococcus aureus and Escherichia coli. As a result, Sharma et al.’s work investigated fungal-mediated production and antibacterial applications of ZnO nanoparticles [55].
Zaka et al. developed a technique for green synthesis of ZnO and Ag NPs via the callus extract (CE) of the medicinally significant Cannabis sativa. Four bacterial strains were employed to investigate the applicability of these biosynthesized NPs on biological entities, including Bacillus subtilis, Klebsiella pneumonia, Staphylococcus aureus, and Pseudomonas aeruginosa. For the antifungal test, five fungal strains were used: Mucor, Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, and Fusarium solani. The HepG2 cell line was also used in the cytotoxicity experiment. The study revealed significant antibacterial and antifungal activity. It also outperformed the control in terms of cytotoxicity [56].
Antifungal properties of ZnO-based nanomaterials, which include reduction in growth and reproduction of pathogenic fungus such as Fusarium sp., Rhizoctonia solani, and Macrophomina phaseolina, were one property of green ZnO NPs. Zaki et al. demonstrated the extracellular production of ZnO NPs with the assistance of a possible fungal antagonist (Trichoderma harzianum). The study’s findings demonstrate a unique fungicidal action in an in vitro assay for the total suppression of fungal growth of investigated plant pathogenic fungi, as well as a significant reduction in cotton seedling disease signs under greenhouse conditions. The formulation of a trichogenic ZnO NPs form considerably increased its antifungal activity. Finally, the use of biocontrol agents such as T. harzianum might be a safe technique for the medium-scale production of ZnO NPs and their application for fungal disease control for cotton [57].
Ameen et al. reported the synthesis of ZnO NPs with diameters ranging from 13 to 15 nm utilizing Acremonium potronii, a novel fungus species found in fruits, soil, and marine environment. ZnO NPs can destroy around 93% of the dye. Their study indicates the potential of fabricated ZnO NPs as dye removal catalysts and provides a platform for A. potronii use [49].
Zinc nanoparticles were synthesized utilizing a new simple green chemistry approach that included Ulva lactuca seaweed extract as a reducing and capping agent. Overall, they concluded that the synthesis of multifunctional Ul-ZnO NPs using widely accessible seaweed products may be advocated as a viable eco-friendly alternative to chemical techniques currently employed for nanosynthesis of antimicrobials and insecticides based on their findings [58].
The synthesis of zinc oxide nanoparticles utilizing Aspergillus niger (A/ZnO NPs) is another green approach of preparation. As an alternative to chemical and/or physical approaches, biosynthesizing zinc oxide NPs using fungal extracts may be a more efficient and environmentally friendly solution. Furthermore, for process optimization, the results of the biomedical available test may be employed in the synthesis of ZnO NPs, which are used for large-scale fabrication in a variety of medical applications [59].
Zinc oxide nanoparticles (ZnO NPs) were also prepared utilizing the marine sponge extract Spongia officinalis, as well as analysis of their antibacterial and possible insecticidal effects. Overall, their work suggests that S. officinalis-mediated ZnO NPs might be used for mosquito control and medication development [60].
Photocatalytic degradation activity is a promising practical way for removing dye toxicity. ZnO NPs were generated by biogenic synthesis. In terms of structural, optical, thermal, and photocatalytic characteristics, ZnO NPs have the necessary physicochemical qualities. The leaf extract of Syzygium cumini was utilized to synthesize nanoparticles for the elimination of MB dye toxicity with many qualities such as eco-friendliness, low cost, nontoxicity, and low time consumption. Synthesized nanoparticles were successful in degrading the MB dye by about 91.4% at pH 7 and 180 minutes of exposure time in sunshine (Figure 3) [61].
Umavathi et al.’s group have worked to create zinc oxide (ZnO) nanoparticles using a leaf extract of Parthenium hysterophorus. ZnO nanoparticles were shown to have significant antibacterial action against bacterial and fungal species. Sesamum indicum seed germination and vegetative development have been substantially enhanced [62].
Green synthesis of ZnO NPs was also performed utilizing aqueous leaf extract of Becium grandiflorum (AM: “Yedegamentisie”). During the synthesis of ZnO NPs, plant extract biomolecules (such as phenols, flavonoids, saponins, glycosides, steroids, tannins, and alkaloids) were utilized as capping and reducing agents. The synthesized ZnO NPs have been used to eliminate MB dye from an aqueous solution by acting as a photocatalyst and adsorbent. It also demonstrated antimicrobial activity against two Gram-positive (Staphylococcus epidermidis and Staphylococcus aureus) and three Gram-negative bacteria (Escherichia coli, Klebsiella pneumonia, and Pseudomonas aeruginosa) [63].
The antibacterial properties of zinc oxide nanoparticles (ZnO NPs) against Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, and Pseudomonas aeruginosa, as well as bacteria commonly found in human mouths and associated with dental conditions, including Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia, Streptococcus mutans, and Streptococcus sanguinis, have been evaluated. Green synthesis was used to grow ZnO NPs using the Mexican plant Dysphania ambrosioides, also known as “epazote,” which has been used as a dewormer by native Mexican populations and is now widely used in traditional Mexican cuisine. It is rich in organic compounds such as flavonoids and terpenes, which may aid in the synthesis of nanoparticles (NPs). Most of the bacterial strains employed in this investigation were susceptible to synthetic and commercial NPs, with Prevotella intermedia being the most sensitive to ZnO NPs, according to the antibacterial test [64].
Additionally, zinc oxide nanoparticles (ZnO NPs) were prepared by combustion technique using ecologically benign green extract (Lantana camara flowers) as a fuel. Surendra group examined at a great photodegradation (98%) performance of methylene blue (MB) dye under UV light, as well as an electrochemical assessment that showed an efficient oxidation-reduction process carried out in 0.1 M KCl with graphite electrode paste. Furthermore, the anti-inflammatory activity of produced ZnO NPs was investigated, and it was discovered that ZnO-2 is a highly effective PLA2 inhibitor with a MIC of 41 g/mL [65].
Developing a sustainable and effective method for dealing with organic contaminants in industrial waste is a never-ending challenge. The extract of Passiflora foetida fruit peels, which acts as a reducing (i.e., metal salt reduction) and stabilizing agent, was used to produce ZnO nanoparticles (ZnO NPs) using a controlled ultrasonic cavitation approach. Given their excellent reusability and photocatalytic efficacy, as well as their flexibility to green synthesis, green-manufactured ZnO NPs are a promising candidate for wastewater treatment [66].
Serrà et al. have demonstrated that biomimetic ZnO-modified micro/nanofern fractal architectures with highly enhanced sunlight photocatalytic efficiency and photocorrosion resistance can be synthesized using a novel, simple, inexpensive, and green electrochemical deposition approach in high stirring conditions. As a result, the ZnO@ZnS bioinspired micro/nanoferns are very promising photocatalysts for water remediation due to their ease of fabrication and ability to dramatically increase sunlight photocatalytic efficiency, as well as their practically negligible photocorrosion and simple recyclability in terms of non-catalyst poisoning [52].
Soto-Robles et al. described an accessible and simple green strategy for the production of ZnO nanoparticles utilizing Justicia spicigera extract. In terms of photocatalytic activity, the catalysts decomposed about 90% of the dye in just 120 minutes [67].
Zinc oxide nanoparticles (ZnO NPs) were also synthesized using Nilgirianthus ciliatus leaf extract. The findings of the experiments demonstrate that the ZnO NP produced in this work has outstanding antibacterial and anticancer action. Furthermore, normal L929 fibroblast cell lines are nontoxic. As a result, the research on green-produced ZnO NP will be useful in advancing its future research in biological areas [68].
Additionally, diverse vegetable extracts (onion, cabbage, carrot, and tomato) make zinc oxide nanoparticles (ZnO NPs), which succeeded over other techniques of synthesis in terms of simplicity, environmental friendliness, and the removal of hazardous substances. The dye-sensitive solar cell based on ZnO NPs has been fabricated, and the device’s efficiency has been determined by monitoring current density-voltage behavior under artificial sunlight. The improved dye molecule adsorption onto the surface of ZnO NPs accounts for the higher efficacy of the constructed dye-sensitive solar cell. As a result, using green-produced ZnO NPs to create dye-sensitive solar cells is a simple and practical solution for future well-being [51].
The biosynthesis of zinc oxide nanoparticles with Cocos nucifera leaf (CNL) extract has been described, as well as the evaluation of their antibacterial activity before and after calcination using various microorganisms. These ZnO nanoparticles can be used as both an antibacterial agent and a biocompatible carrier molecule in drug delivery [69].
Green procedures for the fabrication of ZnO nanoparticles with Lactobacillus spp. extract are cost-effective and eco-friendly, according to studies. Green ZnO nanoparticles have improved antibacterial and biocompatibility testing. MTT test results on cell viability tests revealed high biocompatibility efficacy against the HT29 cancer cell line in this investigation. The remarkable inhibition zones indicated the antibacterial effectiveness of ZnO nanoparticles against several Gram-positive and Gram-negative bacterial and fungal diseases (mm). They have found that bio-mediated ZnO nanostructures are an effective novel antibacterial and anticancer material in this study [70].
Zinc oxide nanocomposites were synthesized using intracellular fabrication by lactic acid bacteria. The antibacterial activity of ZnO NPs was further examined via fluorescence microscopy against two clinically relevant drug-resistant pathogens, Gram (+) and Gram (−). Furthermore, the acquired results allow us to characterize the intracellular ZnO NP production pathway for the first time. The carboxyl functional group was shown to be the most important in the production of ZnO NPs [71].
Lactobacilli strains were used to biosynthesize zinc oxide nanoparticles (ZnO NPs). The findings revealed that the minimal amount of biosynthesized ZnO NPs attributed to the formation of integrated yogurt properties. Furthermore, the biosynthesized ZnO NPs added to yogurt might be an excellent supply of zinc for those who are zinc-deficient, such as the elderly or vegetarians who do not consume meat yet are at risk of zinc insufficiency [72].
Hairy-shaped ZnO nanostructures were fabricated using ureolytic bacteria and applied as a photocatalyst for the degradation of dyes such as methylene blue (MB), methyl orange (MO), rhodamine B (Rho-B), and fresh textile effluents. The hairy-shaped ZnO nanostructure has good photocatalytic activity of textile dye in sunlight, which is connected with light intensity, according to this study [73].
The biological uses of zinc oxide nanoparticles (ZnO NPs) are numerous. Bacillus megaterium (NCIM 2326) cell-free extract was used as a bioreductant to produce anisotropic ZnO NPs with a predetermined form and size. ZnO NPs were tested in normal human mesenchymal stem cells for their multifaceted effect on Helicobacter pylori strains (hMSc). Furthermore, the ZnO NPs were shown to be more biocompatible with human mesenchymal stem cells (hMSCs) and to be possibly safe in mammalian cells. They found that the anti-H. pylori dose of ZnO NPs was safe for hMSC and could be used effectively as a nanoantibiotic, which supports this study [74].
2.4. Copper Oxide Nanoparticles (CuO NPs)
Many researchers were interested in copper oxide nanoparticles (CuO NPs) because of its electric, catalytic, optical, textile, photonic, monofluid, and pharmacological properties, which are dependent on the shape and size of the nanoparticles [75].
Aerva javanica plant leaf extract was utilized to greenly synthetize CuO NPs. The antibacterial potential of the CuO NPs was investigated against a variety of bacterial and fungal diseases. CuO NPs have the highest antibacterial activity against all of the tested bacterial and fungal pathogens, according to the findings. Copper oxide nanoparticles were compared to the medicines norfloxacin and amphotericin B for antimicrobial activity. Copper oxide nanoparticles had minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of 128 g/mL against all tested bacterial pathogens. CuO NPs had a MIC of 160 g/mL and a minimum fungicidal concentration (MFC) of 160 g/mL. CuO NPs can therefore be used as broad-spectrum antibacterial. At doses below 60 g/mL, the cytotoxic activity of the produced CuO NPs revealed that toxicity was insignificant [75].
Calotropis procera-mediated production of copper oxide nanoparticles (CuO NPs) utilizing aqueous plant extract and their anti-pathogenic activity against phytopathogens was also applied. The findings demonstrated that greenly generated CuO NPs had high biological potential and powerful antioxidant activity, indicating that they might be employed in phytopathology to attack plant infections [76].
Rubia cordifolia is an important botanical resource in traditional Chinese medicine and Ayurveda in India. The goal is to make biocompatible copper oxide nanoparticles (CuO NPs) from R. cordifolia bark extracts, define the chemical transitions involved, and investigate their biomedical and environmental implications. As a result, this is the first study to account for the many uses of phyto-CuO NPs. Furthermore, the green synthesis of CuO NPS has potential uses in therapeutic development for life-threatening illnesses and environmental concerns [77].
Thespesia populnea is a Malvaceae tree that grows in tropical climates, particularly in mangrove forests. Thespesia populnea aqueous bark extract is combined with copper metals efficiently against skin infection-producing microorganisms, according to this study. CuO NPs have higher antimicrobial activity against skin infection-causing bacteria including Staphylococcus aureus (MTCC 102), Streptococcus pyogenes (MTCC 102), Pseudomonas aeruginosa (MTCC 358), and fungal strains such as Trichophyton rubrum (MTCC 296) and Candida albicans (MTCC 183). When compared to other microbial strains, Trichophyton rubrum (MTCC 296) displayed a large zone of inhibition [78].
Allium sativum extract was used in the green production of CuO nanoparticles. The antimicrobial activity revealed that bacteria and fungi were effectively infected. The antioxidant activity indicated free radical scavenging activity’s capability. Anti-inflammatory action against egg albumin has been discovered. The anti-larvicidal action on Anopheles subpictus mosquito larvae revealed the significant potential nature of affection. CuO nanoparticles made from environmentally friendly synthesis have unique uses in pharmaceuticals, optics, batteries, gas sensors, catalysts, cosmetics, and semiconductors in the twenty-first century. CuO nanoparticles may also be used in the formulation of drugs for human life-threatening disorders and different harmful cancer therapies in the future [79].
Ghareib et al. have used prefabricated Aspergillus fumigatus mycelium to biosynthesize copper oxide nanoparticles (CuO NPs). These NPs have been shown to have antibacterial action against two significant human infections, Staphylococcus aureus and Klebsiella pneumonia. They also managed to degrade 97% of the methylene blue (MB) dye in direct sunshine after 200 minutes [80].
CuO nanoparticles were effectively biosynthesized for the creation of antibacterial active textiles using active enzymes/proteins released by fungus while taking into account their cytotoxicity. The fungus, Aspergillus terreus strain AF-1, was permitted to exude active components such as enzymes and the protein required to cap the CuO NPs after they had been generated. Cotton textiles were treated with CuO NPs at a safe dosage, and their antibacterial activity was assessed by comparing the fabrics before and after treatment. Fabrics were treated with CuO NPs at a tolerable dosage (100 g/mL), resulting in probable antibacterial activity against pathogenic bacteria, according to the cytotoxicity data [81].
El-Batal et al. have studied whether a certain fungus can biosynthesize copper oxide nanoparticles (CuO NPs) with the help of gamma rays and how effective it is as an antibacterial agent in agricultural areas. Penicillium chrysogenum filtrate was used to make CuO NPs using copper sulfate at varied gamma radiation dosages. CuO NPs may therefore be used as a substantial antibacterial agent in the agricultural domain to inhibit the multiplication of plant harmful fungus and bacteria due to their exceptional properties [82].
Oc-5 and Acv-11, two endophytic actinomycetes isolates, were isolated from healthy leaves of the medicinal plant Oxalis corniculata L. in this investigation. Using the 16S rRNA gene sequence, these isolates were identified as Streptomyces zaomyceticus Oc-5 and Streptomyces pseudogriseolus Acv-11. These strains’ biomass extracts were utilized to make copper oxide nanoparticles in a more environmentally friendly way (CuO NPs). The findings of this study clearly demonstrated that biosynthesized CuO NPs have good bioactivity and thus provide a foundation for the development of various biotechnological applications in the near future [83].
Copper oxide nanoparticles (TA-CuO NPs) were produced utilizing a cell-free extract of Trichoderma asperellum, and their photothermal induced anticancer activity was evaluated in this study. The research supported the development of anticancer nanotherapeutics using biogenic TA-CuO NPs for promising in vitro photothermolysis of cancer cells [84].
The ureolytic fungus Neurospora crassa, Pestalotiopsis sp., and Myrothecium gramineum were studied for their ability to produce nanoscale copper carbonate and the function of fungal extracellular protein. These findings reveal that fungal extracellular protein plays a key role in the production and size of certain nanometal carbonates and give direct experimental evidence for the synthesis of copper carbonate nanoparticles using fungal ureolytic activity. This method may be used to make particular and/or unique metal carbonate nanoparticles with practical applications, as well as precursors to other biomineral products including oxides [85].
Copper oxide nanoparticles (CuO NPs) are used in a variety of industries, including biomedicine. CuO NPs were produced from the filtrate of Aspergillus terreus. Biotechnology and induced radiation were used to create a new composite of P (AA-AN) NPs CuO. The batch approach was used to estimate the distribution coefficient value (Kd) of 47Sc(III) and 47Ca(II) ions for the synthetic novel composite. A chromatographic column filled with the novel composite material was used to investigate the radiochemical separation of 47Sc(III) from an irradiated calcium target. Using 1 M HCl, a recovery yield of 78.2% for 47Sc(III) was achieved. The eluted 47Sc passed quality control tests (chemical, radionuclide, and radiochemical purities) and was found to be suitable for nuclear medicine applications [86].
Metal nanoparticles (NPs) made through mycofabrication serve an important role in cancer therapies and provide a medical strategy. The study employed an endophytic fungus isolated from the Aegle marmelos medicinal tree in the Western Ghats of India to make copper oxide nanoparticles (CuO NPs). Among the 16 pigmented endophytic fungal strains recovered from the collected materials, the endophytic fungus FCBY1 had the strongest antagonistic and antioxidant capabilities [87].
The CuO NPs had the best antibacterial and antifungal activity against human clinical infections, and the particles also explained free radical/ROS scavenging at low concentrations. The anticancer actions of CuO NPs were explained in a concentration-dependent way, and the findings of this study demonstrate the importance of CuO NPs in cancer therapies [87].
Copper oxide nanoparticles (CuO NPs) were generated by a biological approach employing marine endophytic actinomycetes as a reducing and stabilizing agent. As a result, they concluded that actinomycetes-mediated CuO NPs have great medicinal applications against biofilm-producing bacteria and cancer cells and that they would be employed in future biomedical investigations [88].
Biosynthesis of non-precious metal nanoparticles is of great interest because metallic nanoparticle-based disinfection is a potential strategy for microbial pollution management in drinking water. Lv et al.’s group have presented a unique and effective technique for directly microbiological production of copper nanoparticles (Cu NPs) by Shewanella loihica PV-4, as well as their antibacterial activity. Cu NPs with strong antibacterial activity are particularly appealing for drinking water disinfection because of their cost-effective and environmentally sustainable production. Microorganisms are effective mechanisms for making biocompatible metal nanoparticles. Copper, a necessary component of life, offers promising medicinal properties. Copper, on the other hand, lacks a suitable form for efficient in vivo delivery, limiting its use. They used a copper-resistant bacterial strain from a copper mine to make biosynthesized copper nanoparticles (BCuNPs). BCuNPs had a greater antibacterial impact and was proven to be safer than copper sulfate against normal cell lines such HaCat, Vero, and hFOB [89].
Microorganism-based biosynthesis has developed as an environmentally acceptable, clean, and viable alternative to chemical and physical processes. The manufacture of copper oxide nanoparticles (CuO NPs) utilizes cell-free culture supernatant of marine Streptomyces sp. The effect of enhancing CuO NPs against paracetamol-induced liver damage was confirmed by histological inspection of the CuO NP groups [10].
Kouhkan et al.’s group have indicated the lethal effects of copper oxide nanoparticles (CuO NPs) produced by probiotic bacteria (Lactobacillus casei subsp. casei) against Gram-negative and Gram-positive bacteria, as well as cancer cell lines. CuO NPs may have antibacterial and cytotoxic effects on cancer cells by limiting their development, raising oxidative stress, and causing apoptosis, according to these findings [90].
Nabila and Kannabiran’s group have evaluated the antibacterial activity of copper oxide nanoparticles (CuO NPs) produced by actinomycetes against a variety of human and fish infections [91]. The antibacterial activity of actinomycetes-mediated biosynthesized CuO NPs shows that it can fight bacterial infections in both humans and fish. This is the first study of CuO NP production mediated by actinomycetes [91].
To create well-dispersed Cu nanoparticles (Cu NPs) encapsulated in carbonized bacterial cellulose, a simple one-step approach is presented (CBC). The findings reveal that the composites produced have a flawless core-shell structure and exhibit outstanding oxidation resistance and stability [92].
Figure 4 shows TEM images of Ag NPs, Au NPs, ZnO NPs, and CuO NPs synthesized via different green methods.
2.5. Cobalt Oxide Nanoparticles (Co3O4 NPs)
Nanotechnology is the fabrication, characterization, and application of nanoscale materials. Nanomaterials have drawn researchers from various sectors in recent decades due to their high surface-to-volume ratio and other unique and outstanding features. Cobalt and cobalt oxide nanoparticles (NPs) have a wide range of medicinal applications. Because of its antioxidant, antimicrobial, antifungal, anticancer, larvicidal, antileishmanial, anticholinergic, wound healing, and antidiabetic capabilities, cobalt and cobalt oxide NPs have been widely used in lithium-ion batteries, pigments and dyes, electronic thin film, capacitors, gas sensors, heterogeneous catalysis, and environmental remediation. Cobalt and cobalt oxide NPs have been synthesized using a variety of chemical and physical methods [93].
Green synthesis was used to prepare Co3O4 NPs from Euphorbia heterophylla L. leaf extract (ELEs). In the preparation of Co3O4 nanoparticles, ELE uses secondary metabolite molecules such as alkaloid as a weak base source and saponin as a capping agent. Co3O4 NPs were characterized via FTIR, UV-Vis, and XRD TEM images. Photocatalytic activity of Co3O4 NPs in the removal of methylene blue was about 63.105% for 3 hours [94].
To produce cobalt oxide as a reducing agent, Mohammadi et al. used walnut green skin extract, which contains a high amount of phenolic component. Green synthesis was used to make the nanoparticles, which were then identified and examined using SEM, XRD, FTIR, and VSM. The magnetic characteristics of the cobalt oxide nanoparticles generated by the VSM were measured, and the results showed that the nanoparticles were super-magnetic [95].
GCoO NPs were produced by co-precipitation with the grape Jumbo Muscadine (Vitis rotundifolia). UV-Vis, FTIR, XRD, and SEM were used to evaluate the produced GCoO NPs. The degradation of Acid Blue 74 (AB-74) dye was used to determine the photocatalytic activity of the GCoO NPs, and a full degradation of 98% was achieved during a reaction period of 150 minutes at pH 10 and a concentration of 60 mg/100 mL. The results of this investigation showed that GCoO NPs have good performance compared with some of the prior findings, making them a promising candidate for use as a catalyst alternative to current wastewater treatment methods [96].
Matinise et al. [97] used Moringa oleifera extract to establish a better, less expensive, more dependable, simple, and precise process for fabricating cobalt oxide (Co3O4) nanoparticles. Various characterization approaches were used to investigate the electrochemical activity, crystalline structure, shape, isothermal behavior, and optical features of Co3O4 nanoparticles. The production of Co3O4 nanoparticles was validated using XRD and EDS. In 3 M KOH solution, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were used to examine the pseudo-capacitor behavior of spinel Co3O4 nanoparticles on nickel foam electrode. The CV curve displayed two redox peaks, showing the Ni/Co3O4 electrode’s pseudo-capacitive properties. The electrochemical process on the surface electrode is kinetically and diffusion controlled, as evidenced by the EIS data, which indicated a tiny semicircle and Warburg impedance. At a discharge current density of 2 A/g, the charge-discharge findings indicate that the specific capacitance of the Ni/Co3O4 electrode is approximately 1060 F/g.
Co3O4 NPs were synthesized utilizing Populus ciliata (safeda) leaf extract and cobalt nitrate hexahydrate as a cobalt source. Different techniques such as FTIR, XRD, TEM, and SEM were used to analyze the synthesized NPs. The antibacterial activity of the produced Co3O4 NPs was tested against Gram-negative and Gram-positive bacteria, and it was discovered to be active against Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumonia), Bacillus subtilis (B. subtilis), and Bacillus licheniformis (B. licheniformis). One-way ANOVA with “Dunnett’s multiple comparison test” was used to examine the activity findings statistically. B. subtilis had the highest mean activity (21.8 ± 0.7), and E. coli had the lowest mean activity (14.0 ± 0.6) [98].
The cobalt oxide nanoparticles (Co3O4 NPs) were synthesized using Piper nigrum (P. nigrum) leaf extract as a green technique and calcined at different temperatures. XRD was used to investigate the crystalline nature. The amorphous phase changed to the crystalline phase as calcination increased. The UV-visible light absorbance phenomena were observed using diffuse reflectance spectroscopy (DRS), and the band gap energy was calculated using Tauc’s plot, which was found to decrease with calcination. The surface morphology was studied using SEM, and the surface functional groups were classified using FTIR spectroscopy. The antibacterial efficiency of Co3O4 NPs was tested against Gram-positive and Gram-negative bacteria and their antioxidant capability using ABTS assay [99].
Korkmaz [100] investigated Co3O4 NPs produced using a green synthesis method to avoid the toxicity of chemical production. The reducing agent was Allium tuncelianum, an endemic species in Tunceli, and the cobalt source was Co(NO3)2·6H2O. FTIR, XRD, SEM, and HR-TEM methods were used to characterize the synthesized Co3O4 NPs. The XRD pattern revealed that the Co NPs were face-centered cubic (FCC) in nature, with an average crystallite size of about 23 nm and a COD (R2): 0.982 value. In SEM images collected at 200 nm, the Co3O4 NPs showed a typically spherical form. In comparison with RPE normal cells, Co3O4 NPs had a low IC50 value against HeLa cells.
2.6. Magnetic Iron Oxide Nanoparticles (Fe3O4 NPs)
For the manufacture of metal oxide nanoparticles, a green technique has gained a lot of interest. Iron oxide nanoparticles are an example of metal oxide nanoparticles (IONPs). Because of their magnetic character, which allows them to be easily retrieved from the reaction mixture by applying an external magnetic field, IONPs have attracted a lot of attention in recent years. Although there are a range of chemical and physical synthesis processes available, green synthesis is safer, more sustainable, and physiologically acceptable. The main biological resources used for green synthesis are plants and microorganisms [101].
Sathishkumar et al. [102] developed a sustainable green chemical approach to produce magnetic Fe3O4 nanoparticles using the aqueous fruit extract of edible C. guianensis. Different high-throughput characterization techniques, including UV-visible, FTIR, XPS, DLS, and zeta potential analysis, were used to confirm the synthesized NPs. XRD, AFM, HRTEM, and SQUID VSM also show that crystalline CG Fe3O4 NPs with a mean diameter of 17 ± 10 nm are produced. CG Fe3O4 NPs have a remarkable bactericidal activity against a variety of human diseases, demonstrating their antibacterial potential. CGFe3O4NPs had a substantial dose-dependent cytotoxic effect on treated human hepatocellular carcinoma cells (HepG2).
An affordable co-precipitation approach was used to make Fe3O4 NPs with four different weight percentages of Punica granatum fruit peel extract as a green stabilizer. The NPs had spherical forms with an average size of 14.38 nm, as seen in transmission electron microscopy images. UV-Vis spectroscopy and band gap results suggested that the Fe3O4 NPs stabilized with the extract were successfully prepared. The particle zeta potential was improved from 29.24 to 35.62 mV after adding the stabilizer concentration. Under hyperthermia, the thermoresponsive performance of Fe3O4 nanofluids with green extract may provide a surprising heating capability. The samples had adequate magnetic resonance imaging (MRI) signals, according to the results. 5-fluorouracil, an anticancer medication, was successfully loaded onto Fe3O4 NPs with 2 weight percent of the extract, resulting in a maximal release of 79% in a pH 7.4 environment. In cytotoxicity tests, drug-loaded Fe3O4 NPs at concentrations of 15.62 and 31.25 μg·ml−1 killed 29% and 35% of HCT116 colorectal cancer cells, respectively. Fe3O4 NPs produced in green-synthesized could be a viable choice for magnetic hyperthermia therapy, MRI nanoagents, and medication administration in colorectal cancer [103].
Aqueous extract of Graptophyllum pictum leaf (GPLE) was used to successfully produce nanoscale magnetite particles (MNPs) in an environmentally acceptable manner. GPLE served as a base source and a capping agent in the production of MNPs. Through calcination, alkaloids in GPLE were hydrolyzed in water and hydroxylated Fe2+ to generate Fe3O4 nanoparticle powder. MNP production was observed by a color change from pale yellow to dark brown after the addition of leaf extract. UV-Vis, XRD, and FTIR were used to characterize the produced nanoparticles. MNP production suggested the surface plasmon resonance at a maximum wavelength of 291 nm, according to the findings. The average size of a crystallite is 23.17 nm. The MNPs created using a green synthesis process show promise in a variety of medical applications, including medication delivery [104].
The ability of Lagenaria siceraria leaf extract to green synthesize (Fe3O4 NPs) was discovered, and their properties were investigated using UV-Vis, SEM, EDX, XRD, zetasizer, and FTIR. Fe3O4 NPs synthesized in this way were naturally stabilized, cubic in shape, and ranged in size from 30 mm to 100 nm. The leaf phytochemicals have an important role as a reducing agent, assisting in the environmentally benign synthesis of Fe3O4 NPs with increased antioxidant properties. The antibacterial properties of synthesized Fe3O4 NPs were tested against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. The zone of inhibition for Escherichia coli was found to be 10 mm, and for Staphylococcus aureus, it was found to be 8 mm. Natural Fe3O4 NPs with herbal properties can thus be employed in a variety of biological applications [105].
A green approach for producing magnetic iron oxide nanoparticles (Fe3O4) was developed, with an aqueous extract of spent tea waste as the reducing agent, which was then used to make the magnetic and biodegradable Fe3O4/cellulose nanocomposite. Advanced techniques such as UV-Vis, FTIR, XRD, SEM, TGA, and VSM were used to compare the nanostructures. The produced nanocomposite exhibited a spherical form with an average particle size of 15.5 nm, which was less than the mean particle size of pure Fe3O4 nanoparticles (28 nm). The produced nanocomposite also had a greater thermal resistance (450–800°C) than pure cellulose, according to these findings. The magnetic property of the nanocomposite (25 emu/g), which was lower than that of pure Fe3O4 nanoparticles (45 emu/g), was another notable aspect. Furthermore, the swelling capacity of the nanocomposite was investigated as one of its functional capabilities, and it was found to be 139.3 g/g, which was higher than the swell capacity of pure cellulose (66.8 g/g). According to the findings, the produced Fe3O4/cellulose nanocomposite is recommended for use in metronidazole drug delivery systems due to its adequate and acceptable qualities, which include high absorption capacity, regulated magnetic transferability, biodegradability, and nontoxicity [106].
Using aqueous extracts of Pandanus odoratissimus leaves, one-step green chemistry was used for the preparation of Fe3O4 NPs at room temperature. With an average diameter of ∼5.0 nm, Fe3O4 NPs have a homogeneous particle size distribution. The nanoparticles’ BET surface area and average pore diameter were found to be ∼150 m2/g and ∼3.0 nm, respectively. The phase purity of the produced materials was additionally confirmed by FTIR, Raman, EDAX, and XPS. Fe3O4 NPs were used as electrocatalysts in 0.1 M KOH electrolyte solution for electrochemical water splitting processes. The dual nature of Fe3O4 electrocatalysts in water electrolysis for oxygen reduction reaction and oxygen evolution reaction is confirmed by polarization investigations [107].
Viju Kumar and Prem [108] fabricated iron oxide nanoparticles from Phyllanthus niruri leaf extract using a green technique. Additionally, the antibacterial property of the green-produced iron oxide nanoparticles was evaluated. For iron nanoparticles, a comparison with the chemical technique of creation is also made. The IR, UV-Vis, surface morphology, and size determination using TEM, SEM, and XRD are all used to characterize nanoparticles. Analytical tests revealed that the iron oxide nanoparticles generated using these two approaches are almost comparable in size and shape. The antibacterial activity of the produced iron oxide nanoparticles against E. coli and P. aeruginosa was substantial. The studies indicated that employing plant extracts to synthesize iron oxide nanoparticles is more helpful because it is a cost-effective, energy-efficient method.
2.7. Titanium Dioxide Nanoparticles (TiO2 NPs)
The green sustainable production of titanium dioxide nanoparticles (TiO2 NPs) sparked a lot of interest in the preceding quarter. The bioreduction and capping processes are aided by bioactive components found in organisms such as plants and bacteria [109].
Photocatalysis using semiconductor nanoparticles is considered being the most effective method for reducing toxic dyes. Singh et al. [110] have used green-manufactured titanium dioxide nanoparticles (TiO2NPs) made from Phyllanthus emblica (amla) leaf extract to remove Coralline Red dye. Chemical analysis and microscopy methods were used to characterize the synthesized NPs, which indicated the synthesis of crystalline, spherical NPs with a size range of 20–30 nm and tiny aggregates. Furthermore, under solar illumination, synthesized TiO2 showed significant photocatalytic degradation capacity for Coralline Red dye, with apparent rate constants (KAPP) and degradation efficiencies of 0.005 min1 and 93%, respectively. As a result, this study adds to the growing body of knowledge in green chemistry research for developing a water purification platform that is both successful and cost-effective.
A green and simple synthesis of TiO2 NPs was achieved utilizing Carica papaya leaf extract, which might be used as a photocatalyst. The tetragonal crystal structure of TiO2 NPs is shown by XRD analysis, which also demonstrates their crystalline character. TEM and FE-SEM images reveal the cage-like morphology and offer diametric measurements of the spherical TiO2 NPs, which are well accounted to be less than 20 nm. The average particle size is 15.6 nm, according to the calculations. The mesoporous nature of TiO2 particles is revealed by BET measurements, which show surface area and mean pore size of 81.653 m2g−1 and 8.0615 nm, respectively. The UV-visible spectrum reveals a significant absorption peak at 303 nm, and the matching band gap energy value of 3.85 eV is determined. Within 180 min of incubation under UV exposure at the optimum dosage of 25 mg, TiO2 NPs show remarkable photocatalytic effectiveness (91.19%) toward photodegradation of RO-4 dye [111].
Maurya et al. [112] used Bixa orellana seed extract to prepare mesoporousTiO2 nanoparticles from titanium (IV) butoxide solution, which they then used to make a DSSC photoanode. The technology provides DSSC that is environmentally friendly, less expensive, and more efficient. XRD, SEM, TEM, and N2 sorption measurements were used to describe the structure, morphology, size, and porosity of TiO2 nanoparticles. The desorption technique was used to determine the amount of dye loading by TiO2, which demonstrated substantially higher dye loading for plant seed-generated nanoparticles (G-TNP). The photovoltaic conversion efficiencies of TNP and G-TNP were 1.03% and 2.97%, respectively. As a result, the produced mesoporous TiO2 nanoparticles can be used as potential photoanode materials in DSSC applications.
Sethy et al. [113] used an aqueous solution of Syzygium cumini leaf extract as a capping agent to fabricate TiO2 NPs. The photocatalytic removal of lead ions from industrial effluent was tested using these green-generated TiO2 NPs. HRTEM, EDS, FTIR, XRD, DLS, and BET were used to characterize the nanoparticles obtained. The results showed that produced TiO2 NPs have a spherical morphology with a large surface area of 105 m2/g. In a self-designed reactor, photocatalytic investigations of TiO2 NPs for lead removal from explosive wastewater were conducted. The lead content was determined using inductively coupled plasma spectroscopy (ICP). Chemical oxygen demand (COD) was reduced by 75.5%, and lead (Pb2+) was reduced by 82.53%, according to the findings. This is the first time that green TiO2 NPs have been used in this way.
For the first time, TiO2 nanoparticles were synthesized from lemon peel extract using a green synthesis approach. Hesperidin flavanol, found in a hydrolyzed extract of lemon peel, releases aglycone, which acts as a capping and reducing agent. XRD, EDX, TEM, SEM, and UV-Vis techniques are used to characterize the synthesized materials. According to the SEM results, the produced specimen is entirely made up of agglomerated structures. A thorough examination using TEM reveals that the particles are spherical in shape, with particle sizes ranging from 80 to 140 nm. The band gap of 3.08 eV is visible in the Tauc plot created from the UV-Vis spectra. The produced TiO2 photocatalytic activity to the breakdown of rhodamine B (RhB) is also investigated. The results showed that synthesized particles have a photocatalytic activity of more than 70%, which is significantly higher than commercial TiO2 particles [114].
Green synthesis of titanium dioxide nanoparticles (TiO2 NPs) and generation of a nanocomposite were achieved using pristine pomegranate peel extract (PPP) (PPP-TiO2). SEM, dynamic light scattering (DLS), X-ray powder diffraction (XRD), and zeta potential (ζ-potential) were used to analyze the produced nanocomposite. The DLS data clearly reveal that TiO2 NPs have a peak intensity of 100%, a Z-average value of 620 nm, and a PDI of 0.178, indicating that they are monodisperse and homogeneous. Bimodal distribution peaks in the PPP-TiO2 DLS result indicate the existence of both TiO2 and PPP in the composite. The mean hydrodynamic diameter (Z-average) of PPP-TiO2 was found to be 1230 nm, with 87.5% peak intensity. The ζ-potential measurements indicate that PPP-TiO2 NPs (−11.4 mV) are more stable than TiO2 NPs (−6.96 mV). The biological activity was tested against Gram-positive and Gram-negative microorganisms using the well diffusion method, the microbial inhibition concentration (MIC), the minimum bactericidal concentration (MBC), and the live/dead cell assay. For the same harmful bacteria, the antibacterial activity of PPP-TiO2 was 1.5 times higher than that of PPP plus TiO2 NPs. Furthermore, the results revealed that the diameter inhibition zone (DIZ) inhibited S. aureus better than E. coli and P. aeruginosa. Furthermore, the microbial populations and organic matter in real water samples were determined using biological oxygen demand (BOD5). The results showed that samples containing PPP-TiO2 had lower BOD5 values than TiO2 NPs [115].
Thakur et al. [116] used Azadirachta indica leaf extract to synthesize titanium dioxide nanoparticles. The presence of terpenoids, flavonoids, and proteins, which are thought to be responsible for the production and stabilization of titanium dioxide nanoparticles, is indicated by FTIR research, while the crystalline character of titanium dioxide nanoparticles is revealed by XRD studies. The spherical form and size ranged from 15 to 50 nm, according to transmission electron microscopy photographs. TiO2 nanoparticles were spherical in shape and ranged in size from 25 to 87 nm, according to SEM analysis. Escherichia coli, Bacillus subtilis, Salmonella typhi, and Klebsiella pneumoniae were used to test the antibacterial efficacy of produced TiO2 nanoparticles and TiO2 compound. The results showed that TiO2 nanoparticles inhibited all of the bacteria from growing. When comparing TiO2 nanoparticles to TiO2 compound, the antibacterial activity is more prominent. Salmonella typhi and Escherichia coli had the lowest MIC (minimum inhibitory concentration) values of 10.42 g/mL of nanoparticles, but Klebsiella pneumoniae had the lowest MBC value of 83.3 g/mL.
2.8. Nickel and Nickel Oxide Nanoparticles (Ni and NiO NPs)
Ni NPs and NiO NPs are attracting increasing interest in the research field. These nanoparticles have applications in many fields including energy, electronics, catalysis, environment, biological sensing, biomedicine, and corrosion inhibition [117].
Plant-based synthesis of Ni and NiO is used widely with a production rate that is very high compared with using microorganism. Infrared spectroscopy showed that the reduction in metal salts into their corresponding NPs was caused by secondary metabolites such as terpenoids, flavones, pyrones, aldehydes, amides, and carboxylic acids that were made from plant extracts [117].
Nickel (Ni) nanoparticles (NPs) were effectively prepared using metal salt and an extract of the leaves of the wild plant Calotropis gigantea, which acts as a reducing and stabilizing agent. After analyzing catalytic efficiency, the maximum methylene blue dye degradation efficiency was determined to be 98.8%. Synthesized NPs had shelf life of 3 months. Biological tests revealed that the prepared Ni NPs exhibited antibacterial properties against E. coli and Bacillus subtilis [118].
Additionally, Nio/NiO nanoparticles were prepared using the extract of the seeds of Lactuca serriola. The prepared Ni NPs were tested for their antibacterial activity and photocatalytic behavior. The prepared composite demonstrated antibacterial activity and promising photocatalytic feature [119].
There is increasing interest in preparing NiO NPs using different parts of plants including leaves, seeds, bark, and gum. The prepared green particles were applied in many fields due to their low toxicity. One of the major fields in application is using these nanoparticles as catalysis. NiO NPs were found to be photocatalysis for many pollutants [120–123]. They were also found to be electrocatalysis for oxygen evolution reactions [124].
Olajire and Mohammed prepared NiO NPs from leaf extract of Ananas comosus and utilized the synthesized NPs to promote polymer photodegradation. Using the solid-phase degradation of low-density polyethylene (LDPE) film, NiO NPs showed promising photocatalytic behavior. They accordingly suggested to include NiO NPs into the polymer matrix to promote its photodegradation [120].
The seeds extract of Hordeum vulgare plant, which is high in phenolic content and antioxidants, was used to reduce the precursor metal salt into nickel (Ni) and nickel oxide (NiO) nanoparticles (NPs). Ni and NiO were used as photocatalysts for the degradation of methylene blue dye [121].
Another utilization of seed extract was using the extract of Salvia hispanica L. (chia) seeds as the capping agent to prepare Ni NPs. The prepared nanoparticles displayed superparamagnetic behavior identified by VSM analysis and photocatalytic capacity and low cytotoxicity that is concentration-dependent. These interesting properties made greenly synthesized NiO NPs a strong candidate for a variety of applications, including disease prevention and the elimination of residual toxins [122].
Olive tree leaves were also used as a reducing agent for the production of NiO NPs using Ni(NO3)2 as a precursor and D-sorbitol as the capping agent. The NPs were used for adsorption application where they were tested against two dyes, methylene blue and methyl orange. The results showed adsorption capabilities of 96% for methylene blue and 88% for methyl orange [123].
Green synthesis of nickel oxide nanoparticles using phytochemicals from peels was also investigated. Selvanathan et al. used three different sources to prepare NiO NPs. Using nickel (II) acetate tetrahydrate as a precursor, three different green methods were used. Two of the used green methods are peels, which are the peel extract of papaya and dragon fruit, in addition to the leaf extract of aloe vera. The resulting NPs were then employed as electrocatalysts in the oxygen evolution reaction with aloe vera extract-mediated NiOx NPs showing the highest electrocatalytic activity [124].
C. gigantea leaf extract was also used to prepare NiO NPs using a solution combustion technique. The nanoparticles were used as a sensor for nitrite pollutant in water and showed excellent photocatalysis behavior toward methylene blue with antibacterial activity [125].
NiO NPs have many applications in the energy field. The green-synthesized NPs were employed in energy storage devices, photodiodes, and supercapacitor.
NiO NPs were also synthesized using maize (Zea mays L.) dry silk extract. NiO NPs were used as the negative electrode in an asymmetric supercabattery device, and the activated carbon was the positive electrode. Due to the excellent electrochemical energy storage properties, this device can be considered strong candidate for future energy storage applications [126].
Cactus plant extract was used for the synthesis of NiO NPs. The synthesized NPs exhibited capabilities to be used for pseudo-capacitance [127]. Allium cepa bulb extract was also used to produce nickel oxide nanoparticles with greater dielectric constants [128]. Guava leaves aqueous extract was also utilized as a green source to prepare NiO NPs that has potential application as electrodes in supercapacitor devices [129].
The biological application of NiO NPs synthesized using plant includes numerous researches. Applications include antibacterial and anticancer activity, acaricidal efficacy, biosensing, cellular metabolism, and pest management.
Nickel oxide nanoparticles (NiO NPs) were created using E. heterophylla (L.) leaf extract as a reducing/capping agent. On human erythrocytes, NiO NPs reveal important nontoxic qualities and interference in the activity of the coagulation cascade on PRP and PPP in human blood. NiO NPs have been shown to have strong bactericidal action against pathogenic bacterial strains. The nanoparticles also exhibit considerable cytotoxicity against human lung cancer (A549) and human hepatocarcinoma (HepG2) cell lines [130].
Suresh and Balamurugan prepared NiO NPs utilizing Moringa leaf extract using a sonication-assisted green synthesis approach. The prepared NPs showed antibacterial activity against Staphylococcus aureus and Escherichia coli [131].
Additionally, Eichhornia crassipes (Ec) extract was used to make nickel oxide nanoparticles (NiO NP), and the NP’s influence on fermentative hydrogen generation was studied. Supplementing with ecosynthesized Eco-NO2 NP was found to increase fermentation hydrogen generation and controls changes in critical node metabolites and gene expression that are necessary for cellular function and efficiency [132].
A new pesticide for arthropod pests based on greenly synthesized NiO NPs was studied by Abdel-Ghany et al. NiO was prepared using the aqueous extract of Melia azedarach ripened fruits. The synthesized NiO NPs were evaluated for their in vitro acaricidal efficacy against the camel tick Hyalomma dromedarii. NiO NPs showed a significant effect against various developmental stages of the tick [133].
The aqueous leaf extract of Rauwolfia serpentina was used to prepare NiO NPs. The prepared NPs were investigated against pulse beetle, Callosobruchus maculatus. Insects treated with NiO nanoparticles exhibited dose-dependent reductions in fecundity and lengthened developmental periods. In addition, no significant influence on seed germination was seen across treatments except at 40 ppm. So, making NiO NPs in an environmentally friendly way could be a big part of getting rid of bruchids in black gram and other pulses [134].
Sarkar et al. used an extract of the plant Coriandrum sativum to make nickel oxide nanoparticles (NiO NPs). UV-visible spectroscopy, UV-thermal melting, circular dichroism, and fluorescence spectroscopy were used to investigate the interaction of produced NiO NPs with calf thymus DNA (CT DNA). CT DNA was used in place of nucleic acid biosensors. The NiO NPs bonded electrostatically with CT DNA in all experimental investigations. It was concluded that NiO NP-nucleic acid coupled materials can be utilized as nanobiosensors for a variety of applications in pharmacy, environment, and detection [135]. Additionally, NiO NPs were used to enhance glucose sensing. The nanoparticles were manufactured using Nigella sativa extract were incorporated on glassy carbon electrode and used as sensor for glucose determination. The proposed sensor had a quick response time and high sensitivity to glucose with the detection limit of 3 μM [136].
Another important application of greenly synthesized NiO NPs was in the field of enhancing mechanical and chemical properties. The prepared nanoparticles when incorporated or coated on different matrices exhibited properties such as wettability, biocompatibility, improved thermal behavior, and anticorrosion behavior leading to different applications.
Ramalingam et al. used Gymnema sylvestre extracts to prepare nickel oxide nanoparticles. Electrospun polycaprolactone/gelatin hybrid nanofibrous mats were infiltrated with the produced nanoparticles creating nanofibrous mats with superior wettability, improved mechanical characteristics, biocompatibility, and robust antibacterial activity. These properties make these mats an efficient anti-infective wound dressing candidate [137].
Using Delonix elata leaf extract as a reducing and capping agent, nickel oxide nanoparticles (NiO NPs) were created utilizing an ultrasonic wave-assisted green synthesis method. The electrochemical corrosion behavior of NiO NPs was studied in the presence of different aqueous electrolyte media with different pH values. The synthesized NiO NPs were applied as coating on Zn and Mg leading to effectively reduced corrosion of the plates in all the media with a stronger effect in the acidic media (1 M H2SO4) [138]. In addition, Suresh and Balamurugan prepared NiO NPs utilizing Moringa leaf extract using a sonication-assisted green synthesis approach. The prepared NPs increased the corrosion resistance of Zn, [131].
In contrast to plants, there is very little literature on the synthesis of NiO nanoparticles utilizing fungus. Nonetheless, nickel oxide nanoparticles were produced by some types of fungi and algae. Fungi, both living and dead, were employed to create NiO NPs. Fungi may generate nanoparticles within their cells in a process known as intracellular synthesis or outside their cells in a process known as extracellular synthesis. Because of the restricted space within the cells, nanoparticles in the intercellular process are often smaller than those in the extracellular process [139].
Marine macroalgae extract was also employed as a reducing and coating agent to produce nickel oxide nanoparticles. The nanoparticles had a crystalline form and a spherical shape with a mean particle size of 32.64 nm. TGA measurements revealed the presence of organic elements originating from the sea on the surface of NiO NPs. The prepared nanoparticles were found to be very effective catalyst for the production of pyridopyrimidine derivatives in aqueous conditions. The green catalysis led to faster reactions, higher yields (up to 96%), recyclability (7 runs), and environment-friendly process [140].
Figure 5 exhibits TEM images of NiO NPs, Co3O4 NPs, Fe2O3 NPs, and TiO2 NPs synthesized via different green techniques.
2.9. Aluminum Oxide Nanoparticles (Al2O3 NPs)
Biological methods are employed to create Al2O3 NPs in diverse morphologies and sizes from plant components such as leaves, seed fruits, and flowers. The hydrophilic heterocyclic components are primarily responsible for nanoparticle production and stability [141]. Al2O3 NPs were also synthesized using food such as tea and caffeine [142] and using algae [143] and fungi [144].
Sumesh et al. used the aqueous leaf extract of “Muntingia Calabura” to prepare aluminum oxide nanopowder. The prepared particles were employed as a filler powder for the sisal/coir, sisal/banana, and banana/coir-based hybrid composites. Adding only 3% of weight of Al2O3 NPs had a significant effect on their mechanical properties and degradation temperature [145, 146]. The nanopowder made the flexural properties of hybrid fibers better by reducing the empty space and making sure that the fibers and matrix were well attached [147].
Al2O3 NPs prepared using the extract of neem leaves were also applied to enhance the mechanical properties of the cutting tools made of cement carbide. The nanoparticles were self-adherent to the cutting texture gaps. The presence of the nanoparticles was found to improve the properties and hence the performance of the cutting tools [148].
Additionally, the extract of Aerva lanata leaves and terminalia chebula seeds was used to produce Al2O3 NPs. The size and shape of the nanoparticles formed were different, having a shape of spherical agglomeration and a size in the range of 50–70 nm in the case of Aerva lanata leaves while having a sphere and a size range of 50–100 nm in the case of Terminalia chebula seeds [149].
Five different plants were also used to prepare Al2O3 NPs. Syzygium aromaticum, Origanum vulgare, Origanum majorana, Theobroma cacao, and Cichorium intybus extracted were utilized in the microwave-assisted synthesis using aluminum nitrate as a precursor. The average size of clusters of nanoparticles varied with different routes from 60 nm to 300 nm [150].
Using Lyngbya Majuscula algae extract, high-efficiency, cost-effective, and green aluminum nanoparticles (Al2O3 NPs) were synthesized. Al2O3 NPs showed antibacterial efficacy toward several bacterial strains, Streptococcus aureus, Bacillus subtilis, Klebsiella pneumoniae, Salmonella Paratyphi, Candida albicans, and Aspergillus flavus [143].
Using an extract from the algae Sargassum ilicifolium as bioreducing and a stabilizing agent, ceramic alpha aluminum oxide nanoparticles were prepared. The synthesized Al2O3 nanoparticles exhibited high purity, were a rhombohedral shape, and were alpha crystalline in nature with an average size of 20 ± 2.1 nm [151].
Al2O3 NPs were prepared using the fungus Colletotrichum sp. The in vitro studies of the nanoparticles’ antibacterial properties against several foodborne pathogens showed that Al2O3 NPs were especially effective against F. oxysporum and can be considered as an effective antimicrobial agent to prevent food deterioration caused by food pathogens [144].
2.10. Chromium and Chromium Oxide (Cr and Cr2O3)
Cr2O3 stands out among metal oxides due to its unique thermodynamic stability, hardness, chemical resistance, and antiferromagnetic characteristics. Cr2O3 also exhibits n-type and p-type semiconductor properties. For making Cr2O3 NPs, researchers have looked into reducing and stabilizing agents made from different biological sources [152].
Satgurunathan et al. prepared chromium nanoparticles (Cr NPs) using the Allium sativum aqueous extract and potassium dichromate as a precursor. The synthesized nanoparticles were found to be effective in boosting the growth of prawn Macrobrachium rosenbergii (Cr NP) by using them as dietary supplements [153].
Chromium oxide (Cr2O3) nanoparticles were also synthesized using Abutilon indicum (L.) sweet leaf extract as a reducing and capping agent. The prepared nanoparticles had antioxidant and anticancer efficacy toward cancerous cells MCF-7 compared with the chemically prepared nanoparticles. Additionally, the biosynthesized nanoparticles were found to be more biocompatible with Vero cell lines than nanoparticles prepared by chemical methods (Figure 6). This increased efficiency for the greenly synthesized nanoparticles could be attributed to the synergetic effect of the plant phytochemicals and the nanoparticles [154].
Ahmed Mohamed et al. used Hyphaene thebaica as a bioreductant to produce chromium oxide nanoparticles (Cr2O3 NPs) for nanomedicinal uses. The nanoparticles had a size of around 25–38 nm. In vitro experiments were used to assess the biological characteristics of Cr2O3 NPs that showed biocompatibility and antibacterial and antioxidant activity. Additionally, the greenly synthesized nanoparticles showed inhibition of poliovirus at moderate levels [155].
Hassan et al. used Callistemon viminalis (bottle brush) flower extracts as a reducing and capping agent to make Cr2O3 nanoparticles (NPs). The prepared nanoparticles had high crystallinity and different sizes based on the annealing temperature with diameters of 15 and 17 nm by annealing them at 400 and 500°C, respectively. The prepared nanoparticles showed strong antibacterial activity. An MTT cytotoxic test was performed on Leishmania tropica amastigotes and promastigotes to test anticancer activity, yielding IC50 values of 44 g/ml and 10.56 g/ml, respectively. The anticancer activity test was also performed against HepG2 cancer cells, showing IC50 of 46.32 g/ml. Furthermore, Cr2O3 NPs showed strong DPPH radical scavenging, moderate reducing power, and overall antioxidant potential [156].
A simple, straightforward green synthesis technique was used by Sackey et al. to biosynthesize selective single-phase black-Cr2O3 nanoparticles. The procedure includes utilizing the extract of sweet potato skins and chromic nitrate salt as a precursor. A linear increase in the magnetism of Cr2O3 nanoparticles with the field strength was found, which was suggested to be due to the presence of uncompensated spins at the nanoparticles’ surface, leading to a nonmagnetic or antiferromagnetic state. The structural, electronic, and magnetic characteristics of Cr2O3 were further confirmed using density functional theory (DFT) [157].
Sharma and Sharma prepared Cr2O3 NPs from Cannabis sativa leaf extract. The nanoparticles demonstrated anticancer activity against HepG2 cell line. They also showed anticorrosion behavior showing a maximal corrosion inhibition efficacy at 303 K [158].
Pomegranate extract was also used to produce chromium oxide nanoparticles from K2Cr2O7 salt solutions. The prepared nanoparticles were utilized in a nanocomposite with polyamide. The produced nanocomposite was applied to adsorb U(VI) from polluted water [159].
Chromium oxide nanoparticles were produced by reducing potassium dichromate solution with natural honey as a reducing agent. Natural honey contains carbohydrates that are constituted a major component in the synthesis process. The prepared nanoparticles expressed both antioxidant and antibacterial activities [160].
Bacteria strains were also used as a reducing and capping agent to prepare chromium oxide nanoparticles. Erwinia amylovora bacteria were used to biosynthesize Cr2O3 NPs. The size of NPs was observed to be 32.35 nm nanometers. The prepared nanoparticles were tested for antibacterial activity, and they showed inhibition for bacterial growth [161].
Using effluent from electroplating bacteria strain Bacillus subtilis, Kanakalakshmi et al. prepared Cr(III) nanoparticles. The nanoparticles ranged in size from 4 to 50 nm. Several bacterial strains were inhibited by the prepared nanoparticles suggesting their antibacterial efficacy [162]. Fungi were also employed to make Cr2O3 nanoparticles. Using a biological method, Chromium oxide nanoparticles were created using a fungal extract of Aspergillus Niger. According to the XRD analysis and EDX, the size of biologically generated Cr2O3 NPs is 36 nm with a hexagonal shape. The EDX demonstrated further that the produced nanoparticles were pure with small amount of impurities in the sample [163].
Figure 7 depicts some of TEM images of Al2O3 NPs and Cr2O3 NPs synthesized via different green methods.
2.11. Rare Earth Metal and Their Oxides Nanoparticles (RE NPs)
Rare earth oxides are attracting increasing interest due to various properties including luminescence efficiency, catalysis, biological activity, photocatalysis, electronics, and sensors. Green synthesis techniques involving diverse plants such as algae, bacteria, and fungus are replacing the usage of physical and chemical resources in the creation of nanostructures.
2.11.1. Samarium and Samarium Oxide Nanoparticles (Sm and Sm2O3 NPs)
The inherent characteristics of samarium oxide (Sm2O3) piqued the interest of researchers. Sm2O3 is used for IR radiation absorption in glass, as catalytic oxides for organic reactions, and as neutron absorption control rods in nuclear power reactors. Sm2O3 is a high k-dielectric substance that has been intensively researched for its prospective use in several electronic devices.
Samarium nanoparticles were produced from ginger extract utilizing a green chemistry synthesis technique. The synthesized metal nanoparticles showed anticancer efficacy against human colorectal cancer cells. The cytotoxicity results showed that IC50 values for samarium nanoparticles on the HCT116 cell line had a value of 90 (equivalent to 23.1 mg/ml) and 81 M (equivalent to 20.7 mg/ml) after 24 and 48 hours of incubation durations, respectively [164].
Muthulakshmi et al. prepared samarium oxide nanoparticles using the hydrothermal method and utilized the leaf extract of Andrographis paniculata. The prepared nanoparticles had body-centered cubic structure and average size of 30–50 nm. The metal oxide nanoparticles showed antibacterial, anti-inflammatory, and antioxidant efficacy. The antibacterial behavior was observed against E. coli and S. aureus, and the antioxidant activity was found to be stronger than vitamin C [165].
Samarium oxide nanoparticles were also prepared using the flower extract of Hibiscus syriacus Ardens. The greenly synthesized nanometal oxide was used as a catalyst for benzimidazole derivative synthesis. Many advantages were attributed to the use of Sm2O3 NPs including high yield, faster reactions, simpler synthesis process, and greener approach. The catalysis was recovered and reused 4 times while maintaining its activity [166].
Sm2O3 nanoparticles (NPs) were synthesized using Caesalpinia pulcherrima leaf extract (CPE), as a reducing and capping agent. Sm2O3 NPs had a cubic shape and an average size of 73.27 nm. The photocatalytic activity of Sm2O3 NPs is operated at an of 4.76 eV, which is in the UV region. Sm2O3 NPs were found to obtain photocatalysis behavior toward malachite green under UV lighting. 80.14% of malachite green degraded in two hours when utilizing Sm2O3 nanoparticles as a photocatalyst [167].
Microwave-assisted synthesis of samarium oxide (Sm2O3) nanoparticles with a narrow size distribution without the need of a surfactant or template was effective, and the approach was dubbed “green synthesis” since strong bases and acids were replaced with environmentally benign urea. The particle size changed with changing the concentration of urea. The lowest size of nano-Sm2O3 nanoparticles (13–16 nm) was discovered at a urea concentration of 1.2 mol/L by adjusting the urea content. The prepared nanoparticles showed outstanding UV-responsive characteristics and distinguished deep UV and NIR regions, which proposed their possible application in photoelectric devices [168].
Another method employed the bacterium Anabaena cylindrica, which bioaccumulates Sm3+ and then generates nanosized intracellular Sm particles exclusively in its less numerous heterocysts (HCs). A. cylindrica was discovered to be an appropriate representative microorganism for green Sm recovery and the production of easily identifiable Sm NPs. A. cylindrica was only effective in its HCs and not in its vegetative cells (VCs), which had an average size of 9–12 nm [169, 170].
2.11.2. Cerium Oxide Nanoparticles (CeO2 NPs)
Due to its various uses in catalysis, cancer treatment, photocatalytic degradation of pollutants, sensors, and polishing agents, cerium oxide nanoparticles (CeO2 NPs) are gaining increasing demands in a variety of sectors. Various green approaches have been used to prepare cerium oxide NPs; however, plant extract is most commonly used [171–173]. Food-based items such as starch, sugar, a mixture of seashell, Aloe vera (A. vera), grapes, pomegranates, and diluted egg whites have also been utilized as extracts to generate CeO2 NPs [171]. Natural macromolecule polymers may also be employed as templates for bio-directed synthesis of CeO2 NPs [174].
Cerium oxide nanoparticles were prepared using the leaf extract of Brahma Kamal in aqueous solution. The resulting nano-oxide was employed as an efficient catalysis in the reaction of amino acyl chloride derivatives with NaHSe followed by adding α-bromoesters to synthesize selenoesters of amino acids [175].
Ditlopo et al. used Hoodia gordonii natural extract as a chelating agent in the biosynthesis of CeO2 nanocrystals. In the solar spectrum, the diffuse reflectivity profile of such CeO2 revealed a unique UV selectivity. While UV reflectance is modest, it reaches 63% in the visible and near infrared. Their relative generation of reactive oxygen species (ROS) was found to be less than 1 throughout a broad concentration range (0.5–1000 g/ml). This high photostability combined with good UV selectivity suggested employing green CeO2 nanocrystals in nanocosmetics [176].
Cerium oxide (CeO2) nanoparticles (NPs) were produced using Rubia cordifolia L. leaf extract as a reducing agent. CeO2 NPs underwent partial agglomeration that took a net structure. The greenly synthesized nanoparticles showed anticancer activity using MTT and reactive oxygen species (ROS) quantification tests against MG-63 human osteosarcoma cells. The treated cells were found to have loss of cell membrane integrity, oxidative stress, and apoptosis as a result of treating them with cerium oxide (CeO2) nanoparticles [177].
CeO2 nanoparticles were also synthesized using Azadirachta indica plant extract. Adding biosynthesis-derived cerium oxide nano-additions to the LGO25 fuel mix was found to be an excellent way to improve diesel engine performance while lowering emissions [178].
Cerium oxide nanoparticles (CeO2 NPs) were synthesized in an ecologically friendly and cost-effective manner using bioactive natural ingredients such as flower honey, pine honey, chestnut honey, and turmeric extracts. The synthesized nanoparticles’ sizes, surface charges, morphologies, band gap energies, antioxidants, and photocatalytic activity were compared. The synthesized CeO2 NPs were of spherical forms. Honey-derived NPs had particle sizes of 1.23, 2.61, and 3.02 nm for blossom, chestnut, and pine honey, respectively. Turmeric extract yielded NPs with a particle size of 6.08 nm. According to the DPPH technique, all green-synthesized nanoparticles had strong antioxidant activity, with IC50 values ranging from 32.4 to 64.5 μg/ml. Under UV light irradiation, high degradation efficiencies (between 83.9 and 93.4%) for methylene blue dye were achieved due to the CeO2 NPs’ strong redox property [179].
Interestingly, an innovative method using marine oyster extract has also been employed to prepare CeO2 NPs. Marine oyster extract is an effective source of bioreducing and capping/stabilizing agents that was used in a one-pot method to prepare CeO2 NPs. The nanoparticles had face-centered cubic shape with an average size of 15 nm according to SEM and TEM. The prepared nanoparticles exhibited biocompatibility. Furthermore, the nano-oxide showed photocatalytic activity toward methylene blue with degradation of 99% [180].
Khaligh and Asoodeh prepared cerium oxide nanoparticles (CeO2 NPs) utilizing Spirulina platensis microalgae. After preparation was done, CeO2 NPs were nanoencapsulated using an ultrasonic emulsification process to produce create cerium oxide nanoemulsion (CeO2-NE). CeO2-NE had a stronger impact on cancerous colon cell HT29, bacterial growth, antioxidant capability, and ferric ion reduction [181].
Using the sol-gel process, carrageenan hydrogel was used to produce cerium oxide nanoparticles (CeO2 NPs) using cerium nitrate as a precursor. CeO2 NPs had an estimated band gap of 2.69 eV and a diameter varied between 18 and 60 nm, with an average size of 34 nm. In vitro cytotoxicity testing on the WEHI 164 cell line revealed low toxicity, which proposed CeO2 NPs as strong candidate for medicinal applications [182].
2.11.3. Lanthanum and Lanthanum Oxide Nanoparticles (La and La2O3 NPs)
Because of its many uses in electronics, sensors, insulators, antimicrobial agents, biomedicines, and biocatalysts, lanthanum oxide nanoparticles (La2O3 NPs) have been gaining increasing interest [183]. Several plant extracts have been utilized for the green synthesis of La2O3 [184].
Biogenic lanthanum (La) nanoparticles were manufactured using the leaf extract of Muntingia calabura. SEM revealed that the size of La nanoparticles was less than 100 nm. The prepared metal nanoparticles possessed antioxidant activity that was studied using DPPH test and revealed an inhibition of 70.06%. The prepared nanometal also showed antibacterial efficacy toward several bacterial strains. A blood compatibility investigation revealed that La nanoparticles have a role in blood coagulation and thrombolytic action. The prepared La NPs were also tested for their photocatalytic behavior showing an efficiency in degrading Coomassie brilliant blue dye in visible light [185].
Lanthanum oxide nanoparticles were prepared using Datura metel leaf extract. With the increase in time and incubation, the precursor of La(OH)3 changed to LaOOH and finally to pure hexagonal La2O3 nanocrystals [186].
Utilizing the leaf powders of Centella asiatica and Tridax plants, lanthanum oxide (La2O3) nanoparticles (NPs) were produced using a simple green combustion process. The efficiency of both oxides NPs was investigated using electrochemical and photocatalytic methods. La2O3 NPs prepared using Tridax plants showed better photocatalytic behavior with a degradation of 60%. Both La2O3 NPs showed promising antibacterial properties [187].
2.11.4. Yttrium Oxide Nanoparticles (Y2O3 NPs)
Yttrium oxide (Y2O3) nanoparticle is well-known oxide due to its technological applications. It has also been employed in imaging, polarizer, cancer treatments, and antibacterial and antioxidant applications. Using green methods, it has been prepared using plant leaf and fruit extracts. The greenly synthesized particles were specifically tested as catalysts, antibacterial agents, and anticancer agents [188].
Liriope platyphylla rhizome extract was used to make green Y2O3 NPs using yttrium nitrate hexahydrate as a precursor. The leaf extract works as a reducing agent. The greenly synthesized oxide nanoparticles were used as a heterogeneous green catalyst for the synthesis of 1,3-thiazolidin-4-ones, which has special importance in medicine. Unlike many used catalysts, the prepared green Y2O3 NPs had a good-to-excellent yield [189].
Nagajyothi et al. prepared yttrium oxide nanoparticles (Y2O3 NPs) utilizing the aqueous fruit extract of Forsythiae Fructus and an average size of 11 nm. The green Y2O3 NPs showed substantial anticancer action against renal carcinoma cells but has no toxicity on the normal cells [190].
Figure 8 illustrates TEM images of rare earth metal NPs synthesized via different green methods.
The major criteria of metallic nanoparticles, which are recently synthesized via plants, microorganisms, and other green sources and covered in this review, are summarized in Tables 2–4, respectively.
3. Conclusion
Finally, the goal of this review was to present the most recent research on nanoparticle synthesis using green technologies, their characterization, and their use as materials in large-scale fields. The various biosources, such as aqueous plant extracts, bacteria, and fungus, were covered in this study. All of the elements are present: Ag, Au, ZnO, CuO, Co3O4, Fe3O4, TiO2, NiO, Al2O3, Cr2O3, Sm2O3, CeO2, La2O3, and Y2O3. In addition, the size of green nanoparticles was reviewed in each of the research papers.
Conflicts of Interest
The authors declare that there are no conflicts of interest.