Green Synthesis of Metal and Metal Oxide Nanoparticles Using Different Plants’ Parts for Antimicrobial Activity and Anticancer Activity: A Review Article

Abstract

Nanotechnology emerged as a scientific innovation in the 21st century. Metallic nanoparticles (metal or metal oxide nanoparticles) have attained remarkable popularity due to their interesting biological, physical, chemical, magnetic, and optical properties. Metal-based nanoparticles can be prepared by utilizing different biological, physical, and chemical methods. The biological method is preferred as it provides a green, simple, facile, ecofriendly, rapid, and cost-effective route for the green synthesis of nanoparticles. Plants have complex phytochemical constituents such as carbohydrates, amino acids, phenolics, flavonoids, terpenoids, and proteins, which can behave as reducing and stabilizing agents. However, the mechanism of green synthesis by using plants is still highly debatable. In this report, we summarized basic principles or mechanisms of green synthesis especially for metal or metal oxide (i.e., ZnO, Au, Ag, and TiO₂, Fe, Fe₂O₃, Cu, CuO, Co) nanoparticles. Finally, we explored the medical applications of plant-based nanoparticles in terms of antibacterial, antifungal, and anticancer activity.

1. Introduction

Technology and science are moving at the highest rate for developing green technology. Nanotechnology is one of the most interesting topics utilized to produce and employ materials having interatomic structural characteristics. Nanotechnology emerged as scientific innovation in the 21st century. Nanoparticles can be defined as particles having the size in the range of 1–100 nm and exhibited dimensions on a scale of one-billionth of a meter [1,2,3]. Nanoparticles are advanced materials in technology and science and have various applications in agriculture [4,5], medical [6], electronic [7], chemical [8], and pharmaceutical [9] fields. The biosynthesis of nanoparticles with desired morphology (shape, size, and crystalline nature) has been one of the basic aims in chemistry that can be utilized for various applications, e.g., catalysis, biomedical, lower-cost electrode, and biosensor [10,11,12]. Except for their unique chemical and physical properties, nanoparticles behaved as a bridge between molecular or atomic structure and bulk materials. Thus, they are the best candidate for many important applications such as biotechnology, trace substance identifications, medical, and electrochemistry [13,14,15,16]. Different synthetic approaches have been used for the fabrication of nanoparticles with desired the morphology and size. Although these approaches have resulted in superior nanoparticles, still a basic understanding of the improved fabricating process is required that could be utilized at the commercial and industrial levels. To achieve nanoparticles of desired morphology, two different basic approaches of synthesis (such as bottom-up and top-down methods) have been studied in the existing literature, shown in Figure 1. Conventionally, nanoparticles are synthesized through a diverse range of preparation methods such as ball milling, sputtering, lithographic techniques, and etching [17]. The utilization of the bottom-up approach (in which nanoparticles are prepared from simpler substances) also involves various protocols such as sol–gel process, molecular/atomic condensation, chemicals’ vapor deposition, laser pyrolysis, and spray pyrolysis, shown in Figure 1 [18]. New fields, i.e., green synthetic methods, are attaining remarkable attention in current development and research on materials science. Mainly, green synthesis of nanoparticles, prepared through regulation, clean-up, control, and remediation processes will uplift their ecofriendliness. Some fundamental principles of bio-synthesis can therefore be described by various components such as reduction of pollution, utilization of non-toxic solvent, prevention of waste, and renewable feed-stock [19]. Biosynthesis is essential to avoid the formation of harmful by-products through an environmentally friendly and sustainable approach. Biosynthesis of metal and metal oxide nanoparticles has been adopted to accommodate several biological entities such as plant extracts, bacteria, and algae. Among the existing green approaches of preparation for metal and metal oxide nanoparticles, using the plant is a rapid, easy, and simple process to synthesize nanoparticles at a large level as compared to algae-, fungi-, and bacteria-based prepared nanoparticles The prepared green nanomaterials have a great application in the pharmaceutical industry such as novel pharmaceuticals preparation, drug delivery personification procedures, and synthesis of functional nanodevices [20].

Here, we summarized the current research on the biosynthesis of metallics and their oxide nanoparticles with their advantages as compared to physical and chemical synthetic approaches. Additionally, we also described the essential role of various biological components (amino acid, carbohydrate, flavonoid, terpenoid, protein, and polyphenol) and solvent systems in the synthesis of metal and metal oxide nanoparticles. The objective of this review was to promote green synthesis, which is simple, cost-effective, and ecofriendly, so the novelty of this review article lies in explaining the recently reported (2019–2021) green synthetic methods of metal and metal oxide nanoparticles from plants and their capacity as antimicrobial and antibacterial agents.

2. Green Chemistry and Sustainable Principle

Green chemistry for sustainable development has been reported for less than 15 years [21]. Sustainable development can be termed as the development that accounts for and includes the needs of presently fulfilling and the capability of incoming generations [22]. Sustainable development has unique importance for industrial chemistry because it is concerned with pollution and the use of natural sources [23]. Chemistry has been considered a toxic branch of science, and, normally, the word chemical is associated with toxicity and hazards [24]. Generally, there are many methods to reduce risk by using protection, which is known as protective gear. When these methods fail, the risk of toxicity and hazards is increased. Due to high toxicity and hazards, the outcome can be more harmful, like injuries and deaths [25,26]. Therefore, safe, sustainable methods and procedures help to decrease toxicity and hazard to reduce the danger of accidents and damages [27,28].

3. Synthesis of Metal and Metal Oxide Nanoparticles Using Plants

In biological synthesis (using different organisms such as plants, bacteria, fungi, algae, and actinomycetes) of metal or metal oxide nanoparticles, ecofriendly accepted “green chemistry” ideas have been employed [29]. Biological synthesis of nanoparticles via biological organisms is summarized as a green substitute for the synthesis of nanoparticles having desired properties. In biological synthesis, both types of organisms (i.e., unicellular and multicellular) are permitted to react [30]. Plants are well-known chemical factories of nature that are inexpensive and ecofriendly. Plants have shown remarkable potential in heavy metals’ detoxification and collection, by which environmental contamination and pollutants’ problems can be resolved because the traces of these heavy metals are also hazardous. There are many benefits for nanoparticles’ synthesis via plant extract as compared to other biosyntheses such as by bacteria, fungi, actinomycetes, and algae [31]. One advantage of plant-mediated NPs is that the kinetics for this method are sufficiently higher than other biological methods. Different parts of plants, i.e., leaf, stem, seed, fruit, and roots, have been extensively used for the biosynthesis of nanoparticles because of the presence of remarkable phytochemicals [32]. For the synthesis of nanoparticles, specific parts of the plant are washed with tap or distilled water, after squeezing, filtering, and adding respective salt solutions, whose nanoparticles we wish to synthesize. The color of the solution begins to change, thus revealing the synthesis of nanoparticles, which we can separate easily.

4. Role of Capping Agents in the Synthesis of Metal and Metal Oxide Nanoparticles

Capping agents play an essential role in the synthesis of nanoparticles’ formation. The main role of the capping agent is to stabilize and functionalize the nanoparticles. By using a capping agent, we can impart the useful or desired properties to nanoparticles by controlling size and protecting the surface area and morphology. Various surfactants have been used as a capping agent for changing the desired morphology of nanoparticles, but these surfactants are very tough to remove. Moreover, these surfactants are toxic to our ecosystems [33]. Due to these limitations, there is a need to use ecofriendly capping agents and develop a green route at a commercial and non-commercial level for nanoparticles’ formation.

5. Role of Phytochemicals in the Synthesis of Metal and Metal Oxide Nanoparticles

The biosynthesis of nanoparticles compromises three main ingredients, e.g., solvent medium, reducing agents, and stabilizing agents [34,35,36]. To prepare plant-mediated nanoparticles, the photo component of plant extract serves as a reducing and stabilizing agent. Now, researchers have focused on plant-mediated nanoparticles’ biosynthesis due to more advantages over conventional physical and chemical synthetic procedures [37,38,39,40,41,42,43,44].

5.1. Role of Amino Acid in Green Synthesis of Nanoparticles

Synthesis of nanoparticles using bio-molecules has recently attained much interest because of their non-hazardous nature and because they do not involve harsh methods. Amino acid serves as an excellent capping and reducing agent to prepare nanoparticles having a specific structure. Maruyama and coworkers prepared gold nanoparticles with a size range of 4–7 nm by amino acid as a capping agent. There are 20 different types of amino acids. Among these different types, they used L-histidine, which reduced tetraauric acid to gold nanoparticles. The concentration of amino acid (L-histidine) affects the size of nanoparticles. The size of nanoparticles is decreased with an increase in the concentration of amino acids [45]. Qing-Hua Xu reported a single-step formation of gold nanoparticles by using two amino acids (glutamic and histidine) [46]. Meghana Ramani synthesized ZnO nanoparticles of different shapes and sizes by using three types of amino acids such as l-glutamine, l-alanine, and l-threonine. These amino acids played an important role as a capping agent. The surface modification of ZnO nanoparticles due to a capping agent was confirmed by FTIR spectroscopy [47].

5.2. Role of Protein in Green Synthesis of Nanoparticles

In the biosynthesis of nanoparticles, the vital role of proteins cannot be ignored. Proteins can offer a vital role of reduction, by which they donate e (electron) to the Ag+ ion that leads to the synthesis of silver nanoparticles. Recent studies report the potential role of proteins in the formation of silver nanoparticles by using Capsicum annum [48]. The absorption spectra of UV-Vis (ultraviolet-visible spectroscopy) showed a strong absorption peak at 210 nm, which the authors attributed to the existence of a peptide bond. While the peak was around 280 nm, the UV-Vis absorption spectrum showed the existence of amino acids, e.g., phenylalanine and tyrosine, that tend to react with silver ion. In another study, casein was utilized as a stabilizing and reducing agent in the synthesis of silver nanoparticles [49]. For the confirmation of the role of protein in the process of formation of nanoparticles, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of leaf extracts of Olax scandens and concentrated supernatants of O. scandens-based silver nanoparticles was performed. The results illustrated that some low-molecular-weight peptide bonds present in the extract of leaf were absent in the obtained nanoparticles. Such proteins of low-molecular-weight have been utilized in biosynthesis phenomena. NPs unite with proteins and form a dynamic nanoparticle-protein corona [50]. Similar results were studied in the soya been-mediated synthesis of gold nanoparticles [51].

5.3. Role of Carbohydrates or Saccharides in Green Synthesis of Nanoparticles

The recent study also showed the role of carbohydrates of saccharides in the synthesis of nanoparticles. Raveendran and coworkers utilized sugar and starch as nanoparticles-reducing or -stabilizing agents, respectively. In another study, glucose and hemicellulose were utilized for the synthesis of nanoparticles [52,53]. Polysaccharides are the major class of carbohydrates’ molecules with repeating units of mono- and disaccharides that linked each other by a glycosidic bond. Polysaccharides served as a capping agent in the synthesis of nanoparticles. These are the advantages of polysaccharides as a reducing agent:

• Low cost
• Stable
• Safe
• Nontoxic
• Hydrophilic

The green synthesis is performed in the presence of water as a solvent, therefore removing the use of a toxic solvent [54,55]. One of the unique characteristics of polysaccharides is that they sharply accelerate the kinetics of sol-gel methods because of their catalytic effect [56]. Shu-Juan Bao reported an eco-friendly, bio-mimetic method for the synthesis of TiO₂ nanomaterials. Polysaccharides not only have been found to modify the size, shape, and structure of TiO₂ but have induced various phases; rutile phase has been achieved in the presence of chitosan and the anatase phase has been achieved in the presence of starch.

Dextran is a branched polysaccharide that consists of many glucose molecules with a chain of different lengths. Gold nanoparticles were synthesized using natural honey that served as a reducing agent. Fructose in the honey was considered to serve as a reducing agent, while proteins were responsible for the stabilization of nanoparticles [57]. Gold nanoparticles were synthesized using amino cellulose that acted as capping as well as reducing agents.

5.4. Role of Phenolics Acid in Green Synthesis of Nanoparticles

Phenolic acid is a very essential phytochemical that belongs to the polyphenols’ family. It is composed of two important functional groups, i.e., carboxylic acid and phenolic ring. Various types of phenolic acid, i.e., ellagic acid, caffeic acid, protocatechuic acid, and gallic acid, are reported as reducing agents for the preparation of metal nanoparticles [58]. It is reported that silver nanoparticles can be prepared upon the formation of a transitional complex of Ag⁺ with gallic acid. Consequently, through oxidation phenomena, it converts to quinine that forms silver nanoparticles [59,60].

5.5. Role of Flavonoid in Green Synthesis of Nanoparticles

Flavonoid is the component of plants’ pigment that compromises a class of secondary metabolites because of their diversity and biological synthesis in plants [61]. Up until now, 7000 different flavonoid molecules have been reported. Flavonoids can be present in various forms, e.g., flavonol, isoflavones, anthocyanidins, flavones, flavanones, and flavan3-ol. Flavonoids are considered a basic bio-reducing agent of plant extract and their reducing ability is due to their ability to donate hydrogen ions or electrons [62]. Various articles have been published on the electron or hydrogen ion-releasing capability of flavonoids utilized for the preparation of nanoparticles [63,64,65]. Thus, flavonoids in an extract of the plant are currently utilized as a necessary tool for the primary assessment of untapped plants for the preparation of nanoparticles [66]. In another report, it was reported the production of free hydrogen ions during keto-enol conversion of flavonoids, e.g., rosmarinic acid and luteolin can be intended to reduce silver ions for the synthesis of silver nanoparticles [67]. In another report, it was reported hydrogen ions of flavonoids during the reduction of metal salt can oxidize to the carbonyl group [68]. In an extract of Ocimum basilicum, the transformation of enol- to keto- is a key factor in the biosynthesis of silver nanoparticles [69].

5.6. Role of Terpenoids in Green Synthesis of Nanoparticles

Terpenoids or isoprenoids are very important phytochemical that belongs to naturally synthesized terpenes. They are derivatives of essential oil, which are a mixture of secondary metabolites induced by plants. Previous studies have described the importance of these metabolites in the preparation of silver nanoparticles [70]. It was reported that two important terpenoids, e.g., sesquiterpenoids and monoterpenoids, are basic constituents for the silver nanoparticles’ synthesis [71]. Various reports showed the importance of essential oil of different plants’ species in the synthesis of silver nanoparticles such as Cocos nucifera [72], rosemary [73], Ricinus communis [74], and Anacardium occidentale [75].

6. Synthesis of Metal or Metal Oxides’ Nanoparticles by Using Plants

In this review article, we summarized the basic principles or mechanisms of the green synthesis method, especially for metal or metal oxide (i.e., ZnO, Au, Ag, TiO₂, Fe, Fe₂O₃, Cu, CuO, Co) nanoparticles.

6.1. Zinc Oxide Nanoparticles

Recently, Zinc oxide nanoparticles have emerged as one of the most significant metal oxide nanoparticles due to carrying specific differences in morphology (size, shape, and crystalline nature), applications, low toxicity, economic benefits, and bio-compatibility [76,77,78]. Zinc oxide nanoparticles can be prepared from various parts of a plant such as a leaf, Stem, root, flower, seed, and fruit.

6.1.1. Synthesis of ZnO Nanoparticles Using Leaf: (2019–2021)

To date, numerous leaves’ extracts have been used for the synthesis of ZnO nanoparticles, as shown in

Table 1. Synthesis of ZnO nanoparticles from leaf extract.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Walnut leaf	Leaf	15–40 nm	Triangular	E. coli (ZOI = 7–9 mm) and S. aureus	2021	[79]
2	Lippia adoensis	Leaf	22.6–26.8 nm	Predominantly spherical	S. aureus (ZOI = 6–14 mm), E. faecalis (ZOI = 6–10 mm), E. coli (ZOI = 6–12 mm) and K. pneumonia (ZOI = 6–12 mm)	2021	[80]
3	Cayratia pedata	Leaf	52.24 nm.	-	Utilized in the immobilization of the enzyme (Glucose oxidase)	2021	[81]
4	Piper betle	Leaf	112 nm	Hexagonal shape and spherical	S. aureus (ZOI = 2–3 mm) and E. coli (ZOI = 1–4 mm)	2021	[82]
5	Becium grandiflorum	Leaf	20 nm	-	S. aureus (ZOI = 7 mm) E. coli, (ZOI = 6 mm), K. pneumonia (ZOI = 8 mm), and P. aeruginosa (ZOI = 11 mm) Degradation of methylene blue (69% degraded after 200 min)	2021	[83]
6	Achyranthes aspera	Leaf	28.63–61.42 nm	Hexagonal	S. gallinarum (MIC ≥ 0.195 mg ± 0.00) and S. enteritidis (MIC ≥ 0.390mg ± 0.00)	2021	[84]
7	Arthrospira platensis	Leaf	30–55 nm	Spherical	subtilis (ZOI = 24.1 ± 0.3 mm), S. aureus (ZOI = 21.1 ± 0.06 mm), P. aeruginosa (ZOI = 19.1 ± 0.3 mm), E. coli (ZOI = 19.9 ± 0.1 mm), and C. albicans (ZOI = 21.6 ± 0.6 mm) Showed significantly high cytotoxic efficacy against cancerous cell	2021	[85]
8	Sambucus ebulus	Leaf	17 nm	Hexagonal	cereus, S. aureus, and E. coli Photo-catalytic degradation of Methylene blue ((80% degraded after 200 min)	2020	[86]
9	Anacardium occidentale	Leaf	107.03 ± 1.54 nm and 206.58 ± 1.86 nm	Spherical	S. aureus (ZOI = 1.06 ± 0.14 mm), E. aquaticum (ZOI = 1.99 ± 0.11 mm), K. pneumoniae (ZOI = 2.08 ± 0.03 mm), E. coli (ZOI = 1.49 ± 0.09 mm), and A. baumanii (ZOI = 2.99 ± 0.01 mm)	2021	[87]
10	Eucalyptus globulus Labill.	Leaf	27–35 nm	-	-	2020	[88]
11	Cassia fistula and Melia azadarach	Leaf	3–68 nm	-	Cassia fistula E. coli (ZOI = 21 ± 0.68 mm at 10 µL) and (ZOI = 44 ± 3.00 mm at 200 µL) and S. aureus (ZOI = 14 ± 0.54 mm at 10 µL) and (ZOI = 32 ± 2.30 mm at 200 µL) Melia azadarach E. coli (ZOI = 20 ± 0.56 mm at 10 µL) and (ZOI = 40 ± 0.48 mm at 200 µL) and S. aureus (ZOI = 21 ± 0.68 mm at 10 µL and (ZOI = 38 ± 0.55 mm at 200 µL)	2020	[89]
12	Euphorbia hirta	Leaf	5–20 nm in diameter	-	-	2020	[90]
13	saffron leaf	Leaf	Less than 50 nm in diameter	Spherical	At 25 (μg/disc) Concen of ZnOPs S. Typhimurium (ZOI = 12 ± 0.27 mm), L. monocytogenes (ZOI = -), and E. faecalis (ZOI = 11 ± 0.39 mm). At 50 (μg/disc) Concen of ZnOPs S. Typhimurium (ZOI = 23 ± 0.29 mm), L. monocytogenes (ZOI = -), and E. faecalis (ZOI = 14 ± 0.30 mm). At 50 (μg/disc) Concen of ZnOPs S. Typhimurium (ZOI = 26 ± 0.27 mm), L. monocytogenes (ZOI = -), and E. faecalis (ZOI = 18 ± 0.39 mm). Free radical scavenging activity was reported in DPPH and FRAP (64%). Degradation of methylene blue (69% degraded after 200 min).	2020	[91]
14	Azadirachta Indica	Leaf	25.97 nm	Hexagonal	E. coli (ZOI = 9.3 mm), Degradation of methylene blue (Degraded 35.5%. 45.7%, 63.9%, 72.1%, and 80.2% at 40, 80, 120, 160, and 200 min, respectively).	2020	[92]
15	Aquilegia pubiflora	Leaf	34.23 nm	Spherical or elliptical	aeruginosa (ZOI = 10.3 ± 0.19 mm) and F. solani (ZOI = 13 ± 14 mm) Antiparasitic potential.	2020	[93]
16	Broccoli extract	Leaf	4–17 nm	Hexagonal	Catalytic activity against methylene blue (74%) and phenol red (71%).	2019	[94]
17	Costus igneus	Leaf	26.55 nm	Hexagonal	At 40 (μg/mL) Concen of ZnOPs S. mutans (ZOI = 2.83 ± 0.15 mm), L. fusiformis (ZOI = 4.73 ± 0.25 mm), P. vulgaris (ZOI = 4.13 ± 0.14 mm), and V. parahaemolyticus (ZOI = 4.2 ± 0.1 mm). At 50 (μg/mL) Concen of ZnOPs S. mutans (ZOI = 4.83 ± 0.15 mm), L. fusiformis (ZOI = 6.6 ± 0.1 mm), P. vulgaris (ZOI = 5.3 ± 0.2 mm), and V. parahaemolyticus (ZOI = 5.13 ± 0.17 mm). At 70 (μg/mL) Concen of ZnOPs S. mutans (ZOI = 5.86 ± 0.18 mm), L. fusiformis (ZOI = 8.53 ± 0.20 mm), P. vulgaris (ZOI = 6.33 ± 0.15 mm), and V. parahaemolyticus (ZOI = 6.56 ± 0.11 mm). Antidiabetic activity. Free radical scavenging activity was reported in DPPH (75%)	2019	[95]
18	Pandanus odorifer	Leaf	90 nm	Spherical	B. subtilis (ZOI = 26 mm) and Gram-negative E. coli (ZOI = 24 mm).	2019	[96]
19	Solanum torvum	Leaf	34–40 nm	Spherical	Decreased serum uric acid level. Could affect hepatic and renal performance in rats.	2019	[97]

.

Walnut aqueous leaf extract was utilized for the bio-synthesis of ZnO nanoparticles with a size range of 15 to 40 nm and evaluated against E. coli (ZOI = 7 to 9 mm) and S. aureus (Gram-positive bacterial stain) [79]. Demissie, Meron Girma, et al. reported Lippia adoensis aqueous leaf extract inspired preparation of ZnO nanoparticles and investigated against Staphylococcus aureus (ZOI = 6–14 mm), Enterococcus faecalis (ZOI = 6–10 mm), Escherichia coli (ZOI = 6–12 mm), and Klebsiella pneumoniae (ZOI = 6–12 mm) [80]. Cayratia pedata-based ZnO nanoparticles were also synthesized by Jayachandran et al. The mechanism of Cayratia pedata-based nanoparticles’ synthesis is shown in Figure 2 [81].

Piper betle aqueous leaf extract was applied for the synthesis of ZnO nanoparticles with an average size of 112 nm. S. aureus (ZOI = 2–3 mm) and E. coli (ZOI = 1–4 mm) are main causes of surgical site infection (SSI). Globally, SSI accounts for 2.5% to 41.9%, and an even higher rate in developing countries. Surgical site infection affects not only the health of patients but also the development of the country. The anti-bacterial agents are a significantly effective solution to lower this rate and Piper betle-mediated ZnO nanoparticles were proven to show excellent antibacterial activity against S. aureus and E. coli [82]. Becium grandiflorum was reported for the biosynthesis of ZnO nanoparticles and evaluated antimicrobial activity against S. aureus (ZOI = 7 mm), E. coli (ZOI = 6 mm), K. pneumonia (ZOI = 8 mm), and P. aeruginosa (ZOI = 11mm) bacteria, shown in Figure 3. Methyl blue dye from an aqueous solution was effectively removed by synthesized ZnO nanoparticles [83].

In another study, ZnO nanoparticles were prepared from aqueous leaf extract of Achyranthes aspera and evaluated for antibacterial activity against S. gallinarum and S. enteritidis using the agar wall diffusion method. The author also observed that Achyranthes aspera-mediated ZnO nanoparticles showed zone of inhibition (ZOI) of 31 mm against S. enteritidis and S. gallinarum showed 30 mm [84]. Spherical-shaped ZnO nanoparticles with a size range of 30–55 nm were fabricated using an aqueous leaf extract of Arthrospira platensis. The results showed that antimicrobial activities of Arthrospira platensis-mediated nanoparticles were dose-dependent. Their application as an anti-microbial agent was studied and formed clear zones of 24.1 ± 0.3, 21.1 ± 0.06, 19.1 ± 0.3, 19.9 ± 0.1, and 21.6 ± 0.6 mm, at 200 ppm against B. subtilis, S. aureus, P. aeruginosa, E. coli, and C. albicans, respectively. These antibacterial activities were reduced as synthesized ZnO concentration decreased. ZnO nanoparticles showed significantly higher cytotoxic efficacy against cancerous cells than normal cell lines [85]. Hexagonal-shaped ZnO nanoparticles with a crystallite size of 17 nm were produced from ethanol leaf extract of Sambucus ebulus. The synthesized ZnO nanoparticles showed acceptable photo-catalytic degradation of Methylene blue dye. Sambucus ebulus-mediated ZnO nanoparticles explain efficient antioxidant and antibacterial activity [86]. Droepenu, Eric Kwabena, et al. reported the biosynthesis of ZnO nanoparticles using Anacardium occidentale and tested against S. aureus (ZOI = 1.06 ± 0.14 mm), E. aquaticum (ZOI = 1.99 ± 0.11 mm), K. pneumoniae (ZOI = 2.08 ± 0.03 mm), E. coli (ZOI = 1.49 ± 0.09 mm), and A. baumanii (ZOI = 2.99 ± 0.01 mm) [87].

From 2020 to 2019, several publications reported ZnO nanoparticles synthesized using leaf extract of various plants, e.g., Eucalyptus globulus Labill [88], Cassia fistula and Melia azadarach [89], Euphorbia hirta [90], saffron leaf [91], Azadirachta Indica [92], Aquilegia pubiflora [93], Broccoli extract [94], Costus igneus [95], Pandanus odorifer [96], and Solanum torvum [97].

6.1.2. Synthesis of ZnO Nanoparticles Using Roots and Root Hairs (2020–2021)

Roots and roots’ extracts are also well established for the synthesis of ZnO nanoparticles. To date, different roots’ extracts have been used for the synthesis of ZnO nanoparticles, as shown in

Table 2. Different roots and root hairs’ extracts have been used for the synthesis of ZnO nanoparticles.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Rubus Fairholmianus	Root	11.44 nm	Spherical	S. aureus (MIC = 157.22 μg/mL)	2021	[98]
2	Phoenix dactylifera	Root hair	30.87–47.89 nm	-	K. pneumoniae (ZOI = 2.4 cm), S. aureus (ZOI = 3.0 cm), Salmonella typhi (ZOI = 2.8 cm), E. coli (ZOI = 2.7 cm), and P. aeruginosa (ZOI = 1.6 cm) Anticancer cytotoxicity	2021	[99]
3	Raphanus sativus	Root	15 and 25 nm	Hexagonal	S. aureus (ZOI = 21.23 ± 1.16 mm) and E. faecalis (ZOI = 11.23 ± 0.58 mm)	2020	[100]
4	Sphagneticola trilobata L	Root	-	Irregular	-	2020	[101]
5	Moringa oleifera	Root	~25 nm	Hexagonal	B. Subtilis (ZOI = 12.5 mm) and E. coli (ZOI = 11.6 cm)	2020	[102]

. In 2021, the synthesis of spherical ZnO nanoparticles with an average size of 11.34 nm using Rubus Fairholmianus root (Dimethyl sulfoxide) extract was reported and tested against S. aureus (MIC = 157.22 μg/mL) [98]. In another study, aqueous root hair extract of Phoenix dactylifera was utilized for the synthesis of ZnO nanoparticles with a size range of 30.87 to 47.89 nm. ZnO nanoparticles were found to be 45% more cytotoxic than well-known chemotherapeutic drugs (doxorubicin). Especially Triple-negative breast cancer cells were found to be weaker to Phoenix dactylifera-mediated nanoparticles than doxorubicin. Phoenix dactylifera-mediated were observed to be 82.26% cytotoxic to lungs cancer cells. Phoenix dactylifera-mediated ZnO nanoparticles exhibited promising antibacterial action against K. pneumoniae (ZOI = 2.4 cm), S. aureus (ZOI = 3.0 cm), Salmonella typhi (ZOI = 2.8 cm), and E. coli (ZOI = 2.7 cm) [99]. Liu, Di, et al. reported Raphanus sativus-mediated ZnO nanoparticles exhibited antibacterial activity against S. aureus (ZOI = 21.23 ± 1.16 mm) and E. Faecalis (ZOI = 11.23 ± 0.58 mm) [100]. Recently, Sphagneticola trilobata L was also reported to synthesize ZnO nanoparticles, which were mainly irregular in shape [101]. Moringa oleifera was applied for the formation of hexagonal-shaped ZnO nanoparticles with a size of ~25 nm. The prepared ZnO nanoparticles were tested for their antibacterial action against B. Subtilis (ZOI = 12.5 mm) and E. coli (ZOI = 11.6 cm) [102].

6.1.3. Synthesis of ZnO Nanoparticles Using Stem and Stem Bark: (2019–2021)

Biosynthesis of ZnO nanoparticles using stem or stem bark has gained immense attention recently. Synthesis of ZnO nanoparticles using stem and stem bark is shown in

Table 3. Synthesis of ZnO nanoparticles using stem and stem bark.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Amygdalus scoparia	Stem	-	-	At 100 (μg/mL) Concen of ZnOPs B. Subtilis (ZOI = 25 mm), S. aureus (ZOI = 28 mm), S. typhimurium (ZOI = 21 mm), E. coli (ZOI = 28 mm), E. aerogenes (ZOI = 22 mm), K. aerogenes (ZOI = 21 mm), P. oryzae (ZOI = 18 mm), C. glabrata (ZOI = 16 mm), F. thapsinum (ZOI = 16 mm), C. albicans (ZOI = 16 mm), F. semitectum (ZOI = 18 mm), and C. neoformans (ZOI = 18 mm) Exhibited excellent photocatalytic activity Exhibited excellent inhibitory effect on cancer line	2021	[103]
2	Cinnamomum verum	Stem bark	-	Hexagonal	aureus (MIC = 125 μg/mL) and E. coli (MIC = 62.5 μg/mL)	2020	[104]
3	Mussaenda frondose	Stem bark	5–20 nm	Hexagonal	S. aureus (ZOI = 21.51 mm), B. subtilis (ZOI = 19.13 mm), and P. aeruginosa (ZOI = 20.31 mm). Photocatalytic activity and biological applications such as antidiabetic, anticancerous, anti-inflammatory, and antimicrobial activity	2020	[105]
4	Albizia lebbeck	Stem bark	-	-	Tested against S. aureus (ZOI = 4.50 ± 0.30 mm), B. cereus (ZOI = 8.83 ± 0.42 mm), S. typhi (ZOI = 91.3 ± 0.41mm), K. pneumonia (ZOI = 7.30 ± 0.29 mm), and E. coli (ZOI = 10.57 ± 0.320). Free radical scavenging activity was reported in H2O2 (IC50 0f 48.5, 48.7, and 60.2 μg/mL for 0.1 M, 0.05 M, and 0.05 M, respectively) and cytotoxicity against breast MD-MB and MCF-7 cancer cell lines	2019	[106]

. Amygdalus scoparia-mediated ZnO nanoparticles were synthesized by Jobie, Fatemeh Norouzi, et al., who investigated their antibacterial action against B. subtilis (ZOI = 25 mm), S. aureus (ZOI = 28 mm), S. typhimurium (ZOI = 21 mm), E. coli (ZOI = 28 mm), E. aerogenes (ZOI = 22 mm), K. aerogenes (ZOI = 21 mm), P. oryzae (ZOI = 18 mm), C. glabrata (ZOI = 16 mm), F. thapsinum (ZOI = 16 mm), C. albicans (ZOI = 16 mm), F. semitectum (ZOI = 18 mm), and C. neoformans (ZOI = 18 mm). Synthesized ZnO nanoparticles exhibited excellent photocatalytic activity, shown in Figure 4.

Prepared ZnO nanoparticles exhibited an excellent inhibitory effect on cancer line cells, whereas they had no hazardous effect on normal line cells. Amygdalus scoparia-mediated ZnO-cured diabetic rats illustrated an excellently higher level of insulin and lower alanine transaminase (ALT), aspartate aminotransferase (AST), and blood glucose as compared to a Streptozotocin (STZ)-induced diabetic group and other cured groups [103]. Cinnamomum verum bark was reported for the formation of ZnO nanoparticles and tested against S. aureus (MIC = 125 μg/mL) and E. coli (MIC = 62.5 μg/mL) [104]. Hexagonally shaped ZnO nanoparticles were synthesized by using aqueous root extract of Mussaenda frondosa with a size range of 5–20 nm, and its antimicrobial efficiency was evaluated against S. aureus (ZOI = 21.51 mm), B. subtilis (ZOI = 19.13 mm), and P. aeruginosa (ZOI = 20.31 mm). This study reported photocatalytic activity and biological applications such as antidiabetic, anticancerous, antioxidant, anti-inflammatory, and antimicrobial activity [105]. Albizia lebbeck aqueous stem bark extracts were utilized for the formation of ZnO nanoparticles and tested against S. aureus (ZOI = 4.50 ± 0.30 mm), B. cereus (ZOI = 8.83 ± 0.42 mm), S. typhi (ZOI = 91.3 ± 0.41mm), K. pneumoniae (ZOI = 7.30 ± 0.29 mm), and E. coli (ZOI = 10.57 ± 0.320 mm). The Albizia lebbeck stem bark extracts-mediated ZnO nanoparticles exhibited strong antioxidant and cytotoxicity against breast cancer cell lines [106].

6.1.4. Synthesis of ZnO Nanoparticles Using Flower Extract: (2019–2021)

Flowers were also used in the biosynthesis of ZnO nanoparticles, as shown in

Table 4. Synthesis of ZnO nanoparticles using flowers.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Cassia auriculata	Flower	-	Flake structured	S. pneumonia, S. aureus, E. coli, and K. pneumonia (the size of zone observed ranged from 18 mm to 25 mm against the abovementioned pathogens) Anticancer agent against MG-63 cells (15 and 20 μg)	2021	[107]
2	Punica granatum	Flower	-	Irregular shaped	At 100 (μg/mL) Concen of ZnOPs S. diarizonae (ZOI = 10.00 mm), B. cereus (ZOI = 12.33 ± 0.58 mm), S. aureus (ZOI = 10.50 ± 0.87 mm), P. aeruginosa (ZOI = 10.00 ± 1.00 mm), S. pneumonia (ZOI = 14.00 ± 1.00 mm), K. pneumonia (ZOI = 11.00 ± 1.00 mm), E. faecalis (ZOI = 9.67 ± 0.58 mm), S. typhi (ZOI = 8.67 ± 1.15 mm), E. coli (ZOI = 10.00 ± 1.00 mm), L. monocytogenes (ZOI = 14.33 ± 0.58 mm), E. faecium (ZOI = 15.83 ± 0.76 mm), A. hydrophila (ZOI = 13.83 ± 0.29 mm), and M. catarrhalis (ZOI = 12.00 ± 1.00 mm)	2020	[108]
3	Moringa Oleifera	-	13.2 nm	-	-	2020	[109]
4	Matricaria chamomillaL	Flower	62.4 nm	-	Pv. Oryzae (ZOI = 2.2 cm)	2019	[110]
5	Syzygium aromaticum	Flower	30–40 nm	Triangular and hexagonal	Potential application in agriculture and food industries	2019	[111]

. Biological synthesis of flake-structured ZnO nanoparticles was achieved by using Cassia auriculata. The prepared ZnO nanoparticles were used against various bacterial strains to evaluate their antimicrobial efficiency against S. pneumonia, S. aureus, E. coli, and K. pneumonia (the size of the zone observed ranged from 18 mm to 25 mm against the abovementioned pathogens) and anticancer agent against MG-63 cell. The cell adhesion assay was carried out to investigate the anticancer efficiency of Cassia auriculata-mediated ZnO nanoparticles against MG-63 cells [107]. In another study, the antimicrobial efficacy of Punica granatum flower-mediated ZnO nanoparticles was assessed against S. diarizonae (ZOI = 10.00 mm), B. cereus (ZOI = 12.33 ± 0.58 mm), S. aureus (ZOI = 10.50 ± 0.87 mm), P. aeruginosa (ZOI = 10.00 ± 1.00 mm), S. pneumonia (ZOI = 14.00 ± 1.00 mm), K. pneumonia (ZOI = 11.00 ± 1.00 mm), E. faecalis (ZOI = 9.67 ± 0.58 mm), S. typhi (ZOI = 8.67 ± 1.15 mm), E. coli (ZOI = 10.00 ± 1.00 mm), L. monocytogenes (ZOI = 14.33 ± 0.58 mm), E. faecium (ZOI = 15.83 ± 0.76 mm), A. hydrophila (ZOI = 13.83 ± 0.29 mm), and M. catarrhalis (ZOI = 12.00 ± 1.00 mm) [108]. Moringa Oleifera aqueous flower extract was used for the production of ZnO nanoparticles with a size of 13.2 nm [109]. Matricaria chamomilla L promoted the preparation of ZnO nanoparticles with an average size of 62 nm, reported by Ogunyemi, Solabomi Olaitan, et al. [110]. Hexagonally and Triangularly shaped ZnO nanoparticles with size range 30 to 40 nm synthesized by using the flower of Syzygium aromaticum were reported by Lakshmeesha, Thimappa Ramachandrappa, et al. [111].

6.1.5. Synthesis of ZnO Nanoparticles Using Seed (2020–2019)

The aqueous seed extract of the different plants has been widely used as a reducing and capping agent for the biosynthesis of ZnO nanoparticles, as shown in

Table 5. Synthesis of ZnO nanoparticles using Seed.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Parthenium hysterophorus	Seed	10 nm	Hexagonal	-	2020	[112]
2	Lettuce	Seed	50 nm	-	Effect on the process of seed germination	2020	[113]
3	Eriobotrya japonica	Seed	50 nm	-	-	2020	[114]
4	Longan seed	Seed	10–100 nm	Hexagonal	Evaluated through de-colorization of Orange II (70%), methylene blue (MB 90%), and methyl orange (80%)	2019	[115]
5	Trigonella foenum-graecum	Seed	70–90 nm	Irregular spherical and flake	Potential application in agriculture and food industries	2019	[116]

. Ware, Umavathi, Saraswathi, et al. reported the ecofriendly synthesis of ZnO nanoparticles using seed extract of Parthenium hysterophorus with a size of 10 nm [112]. In another study, Lettuce aqueous seed extract was utilized as a reducing agent for the formation of ZnO nanoparticles with an average size of 50 nm, and its effect on the process of seed germination was investigated [113]. Ngom, I. et al. reported the formation of ZnO nanoparticles using seed extract of Moringa Oleifera [114]. Longan aqueous seed extract was employed for the production of pure hexagonal-phase ZnO nanoparticles with a size range of 10–100 nm. The photocatalytic activity of Longan seed-mediated ZnO nanoparticles was evaluated through de-colorization of Orange II, methylene blue (MB), and methyl orange [115]. Trigonella foenum-graecum aqueous seed extract was also successfully applied for the biosynthesis of irregular spherical and flake-shaped ZnO nanoparticles with a size range of 70 nm to 90 nm. The photocatalytic activity of trigonal Trigonella foenum-graecum-mediated ZnO nanoparticles was evaluated through de-colorization of methylene blue [116].

6.1.6. Synthesis of ZnO Nanoparticles Using Fruit and Fruit Peel: (2019–2021)

ZnO nanoparticles synthesized using fruit and fruit extract are shown in

Table 6. Synthesis of ZnO nanoparticles using Fruit and Fruit peel.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Passiflora foetida	Fruit peel	58 nm	Hexagonal	Showed remarkable efficiency toward Rhodamine B (91.06%) and MB dye (93.25%)	2021	[117]
2	Myristica fragrans	Fruit	41.23 nm	Spherical	E. coli (ZOI = 15 ± 1.54 mm), K. pneumoniae (ZOI = 27 ± 1.73 mm), P. aeruginosa (ZOI = 17 ± 1.66 mm), and S. aureus (ZOI = 21 ± mm) Exhibited excellent larvicidal activity against Aedes aegypti Leishmanicidal activity was also examined against amastigote and promastigote parasite	2021	[118]
3	Citrus sinensis	Fruit peel	33.1 ± 11.7	Hexagonal	E. coli and S. aureus. Toxicity toward human umbilical vein endothelial	2020	[119]
4	Orange	Fruit	10–20 nm	Spherical	-	2020	[120]
5	Ailanthus altissima	Fruit	5–18 nm	-	E. coli and S. aureus	2019	[121]

. Khan, Mujahid, et al. reported the synthesis of hexagonal ZnO nanoparticles with an average size of 58 nm using an aqueous extract of Passiflora foetida fruit peels. Passiflora foetida-based ZnO nanoparticles showed remarkable efficiency toward Rhodamine B and MB dye (91.06%) and MB dye (93.25%), respectively [117]. Aqueous fruit extracts of Myristica fragrans were used to prepare elliptical- and spherical-shaped ZnO nanoparticles with a mean size of 41.23 nm and tested against E. coli (ZOI = 15 ± 1.54 mm), K. pneumoniae (ZOI = 27 ± 1.73 mm), P. aeruginosa (ZOI = 17 ± 1.66 mm), and S. aureus (ZOI = 21 ± mm). Prepared nanoparticles exhibited excellent larvicidal activity against Aedes aegypti. Similarly, significant leishmanicidal activity was also examined against amastigote and promastigote parasites. The biologically prepared ZnO nanoparticles exhibited excellent antioxidant and biocompatible nanoparticles. Photocatalytic activities of prepared ZnO nanoparticles were evaluated through decolorizations of methylene blue [118]. Hexagonal ZnO nanoparticles with an average size of 33.1 ± 11.7 nm were derived from aqueous peel extract of citrus sinensis and tested against E. coli and S. aureus. The toxicity of citrus sinensis-based ZnO nanoparticles toward human umbilical vein endothelial cells was dose-dependent. The feasibility of human umbilical vein endothelial cells (HUVECs) increased when reacted with 6.25 mg/L of prepared ZnO nanoparticles. However, feasibility lowered sharply, to around 20%, when the concentration of prepared ZnO nanoparticles increased to 25 mg/L or higher. It showed that prepared ZnO nanoparticles enhance the growth of HUVECs at low concentrations [119]. Aqueous fruit extract of orange was utilized for the formation of spherical ZnO nanoparticles with a size range of 10–20 nm and was evaluated against E. coli and S. aureus [120]. In another study, ZnO nanoparticles were prepared using the fruit of Ailanthus altissima with a size range of 5–18 nm and tested against E. coli and S. aureus [121].

6.2. Gold Nanoparticles

Among the various metal and metal oxide nanoparticles, gold nanoparticles have specific morphology (size, shape, and crystalline nature), controlled geometry, and stable nature [122]. Gold nanoparticles are utilized in light-harvesting assemblies, electronics, molecular switches, and sensing [123,124,125,126]. Gold nanoparticles are also utilized in the diagnosis, detection, and cure of various diseases [127,128]. Gold nanoparticles, with their multiple properties, gained attention and their features can be modified by altering the shape, size, and aspect ratio. Gold nanoparticles are proven to be exclusive in biomedical applications. They are used as a tool for early cancer diagnosis, heart diseases, and the presence of infectious agents. The non-toxic and biocompatible nature of gold nanoparticles makes them a good candidate for drug and gene delivery. They can modify their surface with antibodies and other drug molecules. They carry drugs that are released at the target site selectively [129,130]. Gold nanoparticles are also explored for gene delivery due to their optimal properties. It is reported that, for the enhancement of the genetic material of any plant, the DNA coated with gold nanoparticles is injected into the plant cell and results in transformation [131]. Gold nanoparticles, due to their biochemical inertness and their unique optical-electronical properties, are also broadly applied in analytical sciences. Sensory probes, conductors, electronic chips, photovoltaics, and fuel cells are advanced technical applications of gold nanoparticles. Gold nanoparticles are widely used in fluorescence, surface plasmon resonance, lateral flow immunochromatographic assay (LFICA), enzyme-linked immunosorbent assay (ELISA), and SERS immunoassays of biomolecules (Elahi, N.). The current advancement in imaging techniques such as Computed tomography, X-ray, and SERS is also based on the high-density resolution of gold nanoparticles. [132]. They are also found in many chemical reactions as a catalyst.

6.2.1. Synthesis of Gold Nanoparticles from Plant

Gold nanoparticles can be prepared from various parts of the plant such as leaf, Stem, root, flower, seed, and fruit. Synthesis and the mechanism of formation of gold nanoparticles using plant are illustrated in Figure 5 and Figure 6, respectively.

6.2.2. Synthesis of Gold Nanoparticles Using Leaves: (2019–2021)

Leaves’ extracts are used to synthesize gold nanoparticles, as shown in

Table 7. Synthesis of Gold nanoparticles using leaf extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Lantana camara, Populus alba, and Hibiscus arboreus	Leaf	~16.3 ± 0.7 nm	-	E. coli and S. aureus (MIC value of ~100 µg/mL.) For the degradation of MB and CR dye	2021	[134]
2	Limnophila rugosa	Leaf	122 nm	Spherical	Tremendous catalytic activity in the reduction of different nitrophenols	2021	[135]
3	Ananas comosus	Leaf	18.85 ± 6.74 nm	Spherical	Exhibited a degradation efficacy of 90% after 240 hrs	2021	[136]
4	Phragmites australis	Leaf	18 nm	Spherical	Showed tremendous anticancer efficiency. Exhibited good quenching for 2,2-diphenyl-1-picrylhydrazyl free radical with scavenging % equal to 10.26. Excellent photocatalytic activity, as they completely degraded the MB in just 60 sec.	2021	[137]
5	Mentha longifolia	Leaf	36.4 nm	Spherical	Tremendous anti-breast cancer efficiency against HS319.T (IC50 = 224 ± 0 µg/mL), MCF7 (IC50 = 264 ± 0 µg/mL) and UACC-3133 (IC50 = 201 ± 0 µg/mL) cell lines	2021	[138]
6	Sageretia theazans	Leaf	36 and 13 nm.	-	S. aureus (ZOI = 10 ± 0.54 mm), K. pneumonia (ZOI = 12 ± 0.2 mm), and B. subtilis (ZOI = 6 ± 0.4 mm). The antioxidant efficiency was investigated with DPPH scavenging activity, the maximum scavenging efficiency was observed at 100 µg/mL	2021	[139]
7	Coriandrum sativum	Leaf	32.96 ± 5.25 nm	Spherical	-	2021	[139]
8	Persicaria salicifolia	Leaf	5 and 23 nm	Spherica	Inhibiting the proliferation and growth of breast cancer cells (MCF7 cell line). Showed antioxidant activity	2021	[140]
9	Curcumae kwangsiensis	Leaf	~8–25 nm	Spherical	Showed tremendous antioxidant property toward common free radical, e.g., BHT (IC50 = 153 µg/mL). Excellent anti-ovarian cancer activity against Sw-626 (IC50 = 166 µg/mL), SK-0V (IC50 = 204 µg/mL), and PA1 cell lines (IC50 = 153 µg/mL)	2021	[141]
10	Centaurea behen	Leaf	50 nm	Spherical	Revealed cytotoxicity against THP-1 cell line. The IC50 for prepared nanoparticles was measured at about 25 µg/mL. Revealed maximum DPPH scavenging efficiency of 14%	2021	[142]
11	Ziziphus nummularia	Leaf	11–12 nm	Spherical	Excellent antioxidant activity toward ABTS (IC50 = 690 μg/mL), DPPH (IC50 = 520 μg/mL), and SO (IC50 = 330 μg/mL).	2021	[143]
12	Jasminum auriculatum	Leaf	8–37 nm	Spherical	E. coli, K. pneumonia, S. pyogenes, S. aureus, and Candida (fungus). Showed excellent Antimicrobial commotion toward Inhibitory effect in the proliferation of the human cervical cancer cell line (IC50 = 104 μg/mL).	2020	[144]
13	Vitex negundo	Leaf	Below 100 nm	Spherical rod-shaped	B. Subtilis (ZOI = 14 ± 0.7 mm), P. aeruginosa (ZOI = 13 ± 0.6 mm), S. aureus (ZOI = 12 ± 0.7 mm), and E. coli (ZOI = 23 ± 0.6 mm). Exhibited tremendous antioxidant activities against H2O2 (78%) scavenging, Nitric oxide scavenging (83%), and DPPH (79%). Exhibited tremendous anti-inflammatory activity.	2020	[145]
14	Pongamia pinnata	Leaf	10–25 nm	-	Tested against oomycetes SR1(MIC80 = 1.6) and BP1120 (MIC80 = 0.8)	2020	[146]
15	Lactuca indica	Leaf	13.5 nm	Spherical	Exhibited remarkable degradation of methyl orange (2.05 × 10−3) and 4-nitrophenol (1.3 × 10−3).	2019	[147]
16	Croton Caudatus	Leaf	20 and 50 nm	Spherical	-	2019	[148]
17	Sansevieria roxburghiana	Leaf	-	Spherical, hexagonal, rod, and decahedral	Degradation of MB (49.62%), bromothymol blue 88.16%), acridine orange (40.44), phenol red (85.88), and Congo red (93.09).	2019	[149]
18	Simarouba glauca	Leaf	-	Prism and spherical	At 2.7 mL Gold solution S. aureus (ZOI = 2.0 mm), B. subtilis (ZOI = 1.0 mm), E. coli (ZOI = 0.6 mm), P. vulgaris (ZOI = 0.2 mm), K. pneumonia (ZOI = No), and S. mutans (ZOI = 1.6 mm).	2019	[150]
19	Alcea rosea	Leaf	4–95 nm	Triangular, spherical, hexagonal, and pentagonal	Exhibited anti-oxidant commotion against ABTS (47.16 to 64.82%) and DPPH (15.95 to 51.53%)	2019	[151]
20	Bauhinia pupurea	Leaf		Hexagonal, nanorod, and triangular	B. Subtilis, P. aeruginosa, S. aureus, Anticancer effects toward Lung carcinoma cell A549 (IC50 = 36.39 μg/mL). Exhibited high antioxidant efficiency against DPPH (IC50 = 27.21 μg/mL).	2019	[152]
21	Coleus aromaticus	Leaf	-	-	epidermis (ZOI = 27 mm) and E. coli (ZOI = 22 mm) Cytotoxicity toward liver cell (HepG2) cell line	2019	[153]
22	Annona muricata	Leaf	25.5 nm	Spherical mono-dispersed	S. aureus (40%), E. faecalis (46%), K. pneumonia (52%), and C. sporogeneses (54%), flaws (31%), C. albicans (42%), F. oxysporum (50%), and P. camemberti (66%).	2019	[154]

. In 2021, gold nanoparticles were derived from aqueous leaf extract of Lantana camara, Populus alba, and Hibiscus arboreus with a size of ~16.3 ± 0.7 nm. The antibacterial activity of prepared gold nanoparticles was tested against E. coli and S. aureus (MIC value = 100 µg/mL). The bio-synthesized gold nanoparticles were also utilized for the degradation of MB and CR dye [134]. In another report, Limnophila rugosa was used for the production of spherical-shaped gold nanoparticles with a mean particles’ size distribution of 122 nm. Limnophila rugosa-capped gold nanoparticles showed tremendous catalytic activity in the reduction of different nitrophenols, e.g., 4-nitrophenol, 3-nitrophenol, and 1,4-nitrophenol [135]. Olajire, A.A. et al., reported the biological synthesis of spherical gold nanoparticles with a mean size of 18.85 ± 6.74 nm by utilizing aqueous leaf extract of Ananas comosus. Low-density polyethylene with 1% gold nanoparticles exhibited a degradation efficacy of 90% after 240 h [136]. Recently, a single-step and eco-friendly biosynthesis of gold nanoparticles was reported by El-Borady et al., utilizing leaf of Phragmites australis. The results exhibited the production of spherical-shaped gold nanoparticles with about 18 nm diameter. Phragmites australis-based gold nanoparticles showed tremendous anticancer efficiency with an IC_50. Prepared gold nanoparticles also exhibited good quenching for 2,2-diphenyl-1-picrylhydrazyl free radical with scavenging % equal to 10.26. Phragmites australis-based gold nanoparticles also showed excellent photocatalytic activity, as they completely degraded the MB in just 60 s. Mentha Longifolia-mediated nanoparticles had tremendous anti-breast cancer efficiency against HS319.T, MCF7, and UACC-3133 cell lines [137]. An environmentally friendly method for the biological formation of gold nanoparticles was developed by Shah, Sumaira, et al., using ethanol leaf extract of Sageretia theazans. The biological activity of Sageretia theazans-based gold nanoparticles was evaluated against S. aureus, K. pneumonia, and B. subtilis. The antioxidant efficiency was investigated with DPPH scavenging activity; the maximum scavenging efficiency was observed at 100 µg/mL [138]. In another report, gold nanoparticles were bio-synthesized by reducing gold metal ions upon interlinking with aqueous leaf extract of Coriandrum sativum. TEM was utilized to calculate the size range of spherical gold nanoparticles, which was in the range of being 32.96 ± 5.25 nm [139]. In another study, an instantaneous, single-step, inexpensive, ecofriendly production of gold nanoparticles via aqueous leaf extract Persicaria salicifolia was reported by Hosny, Mohamed, et al., resulting in the production of violet-colored, spherical-shaped gold nanoparticles with diameters between 5 and 23 nm. The cytotoxicity study of Persicaria salicifolia-mediated gold nanoparticles using sulforhodamine-B assay showed tremendous cell capability in inhibiting the proliferation and growth of breast cancer cells (MCF7 cell line). Additionally, prepared gold nanoparticles showed antioxidant activity [140]. In 2021, gold nanoparticles were biosynthesized through the mixing of aqueous leaf extract of Curcumae Kwangsiensis with a size of ~8–25 nm. Gold nanoparticles showed tremendous antioxidant properties toward common free radicals, e.g., DPPH. Prepared gold nanoparticles had excellent anti-ovarian cancer activity against Sw-626, SK-0V, and P cell lines [141]. In another study, the aqueous extract Centaurea behen was utilized for the simple and environmental production of gold nanoparticles. For testing of the cytotoxicity effect of C. behen extract and gold nanoparticles, an MTT test was performed. C. behen-mediated gold nanoparticles revealed the cytotoxicity against THP-1 cell line. The IC₅₀ for prepared nanoparticles was measured for about 25 µg/mL, whereas C. behen extract could not achieve the IC₅₀. Similarly, for testing of antioxidant property of gold nanoparticles, a DPPH test was performed. Gold nanoparticles revealed maximum DPPH scavenging efficiency of 14% [142]. Padalia, Hemali, et al., reported the biosynthesis of gold nanoparticles utilizing Ziziphus nummularia and their anticancer and antioxidant activities. TEM exhibited the biosynthesized gold nanoparticles to be 11–12 nm in size and spherical. The biosynthesized particles exhibited dose-dependent cytotoxicity toward the human breast cell line, fibroblast normal cell line, and breast cancer cell line. The biologically prepared gold nanoparticles showed excellent antioxidant activity toward ABTS (IC₅₀ = 690 μg/mL), DPPH (IC₅₀ = 520 μg/mL), and (IC₅₀ = 330 μg/mL) [143].

In 2019–2020, several publications reported gold nanoparticles synthesized using leaf extract of various plants, e.g., Jasminum auriculatum [144], Vitex negundo [145], Pongamia pinnata [146], Lactuca indica [147], Croton Caudatus [148], Sansevieria roxburghiana [149], Simarouba glauca [150], Alcea rosea [151], Bauhinia pupurea [152], Coleus aromaticus [153], and Annona muricata [154].

6.2.3. Synthesis of Gold Nanoparticles Using Root Extracts: (2021–2019)

The seed extract of the different plants has been widely used as a reducing and capping agent for the biosynthesis of gold nanoparticles, as shown in

Table 8. Synthesis of Gold nanoparticles using root extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Licorice	Root	2.647–16.25 nm	Circular	P. aeruginosa (ZOI = 25 ± 0.17), E. coli (ZOI = 29 ± 0.35), S. aureus (ZOI = 26 ± 0.29), S. typhi (ZOI = 26 ± 0.15), B. subtilis (ZOI = 25 ± 0. 15), P. citrinum (ZOI = 19 ± 0.21), A. niger (ZOI = 17 ± 0.29), Candida albicans (ZOI = 14 ± 0.21), F. oxysporum (ZOI = 18 ± 0.33), and A. flavus (ZOI = 16 ± 0.15). Antioxidant activity toward DPPH and ABTS.	2021	[155]
2	Phragmites australis	Root	-	-	Cytotoxicity toward human lung cancer cells (A549 cell line). Antioxidant efficiency was less than 10%	2021	[156]
3	Codonopsis pilosula	Root	20 ± 3.2 nm	Spherical	E. coli (ZOI = 7.0 ± 0.42 mm), B. subtilis (ZOI = 12.0 ± 0.85 mm), and S. aureus (ZOI = 17.0 ± 1.2 mm).	2020	[157]
4	Euphorbia fischeriana	Root	20–60	-	-	2019	[158]
5	Paeonia moutan	Root	25.08 ± 3.73 nm	-	Hindered the inflammation in murine microglial (BV2)	2019	[159]

. Licorice aqueous root extract was utilized for the formation of circular gold nanoparticles with a size range of 2.647 nm to 16.25 nm and tested toward P. aeruginosa (ZOI = 25 ± 0.17), E. coli (ZOI = 29 ± 0.35), S. aureus (ZOI = 26 ± 0.29), S. typhi (ZOI = 26 ± 0.15), B. subtilis (ZOI = 25 ± 0. 15), P. citrinum (ZOI = 19 ± 0.21), A. niger (ZOI = 17 ± 0.29), Candida albicans (ZOI = 14 ± 0.21), F. oxysporum (ZOI = 18 ± 0.33), and A. flavus (ZOI = 16 ± 0.15). Licorice-based gold nanoparticles exhibited antioxidant activity toward DPPH and ABTS. The cytotoxicity of prepared particles was examined by utilizing the MTT approach against liver (HePG-2) and breast cancer (MCF-7) cell lines [155]. In another research, the author adopted an environmentally friendly and sustainable method to synthesize gold nanoparticles by utilizing Phragmites australis aqueous root extract. The cytotoxicity of prepared particles was examined by utilizing an MTT approach against human lung cancer cells (A549 cell line). Antioxidant efficiency was less than 10%. The prepared gold nanoparticles showed excellent efficiency in removing methyl orange and methyl blue [156]. In 2020, spherical gold nanoparticles were derived by a green method utilizing Codonopsis pilosula with the size of 20 ± 3.2 nm and tested against E. coli (ZOI = 7.0 ± 0.42 mm), B. subtilis (ZOI = 12.0 ± 0.85 mm), and S. aureus (ZOI = 17.0 ± 1.2 mm) [157]. Zhang, Tipeng, et al. prepared the gold nanoparticles using Euphorbia fischeriana aqueous root extract with the size of 20–60 nm [158]. In 2019, Paeonia moutan methanol root extract was used to synthesize gold nanoparticles. The cytotoxicity of prepared particles was examined by utilizing an MTT approach against the murine microglial (BV2) cells. Paeonia moutan-mediated gold nanoparticles hindered the inflammation in murine microglial (BV2) [159].

6.2.4. Synthesis of Gold Nanoparticles Using Stem Extracts: (2021–2019)

Recently, Brassica oleracea var. Acephala cv galega was utilized to biosynthesize spherical gold nanoparticles with an average diameter of 25.08 ± 3.73 nm, as shown in

Table 9. Synthesis of Gold nanoparticles using stem extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Brassica oleracea var. Acephala cv galega	Stem	25.08 ± 3.73 nm	Spherical	The antioxidant assay was carried out in the root extract after the formation of gold nanoparticles	2021	[160]
2	Apium graveolens	Stem	-	-	Utilized as a catalyst for reduction of 4-nitophenol	2020	[161]
3	Angelica aiges	Stem	-	-	Degraded the Malachite (67%) and eosin dye (64%)	2019	[162]

. Additionally, the antioxidant assay was carried out in the root extract after the formation of gold nanoparticles [160]. Khoshnamvand, M. et al., reported the synthesis of gold nanoparticles by utilizing Apium graveolens aqueous stem extract. The prepared particles could be utilized as a catalyst for the reduction of 4-nitophenol [161]. Gold nanoparticles were biosynthesized utilizing the stem of Angelica aiges by a green approach. Prepared particles degraded the Malachite and eosin dye [162].

6.2.5. Synthesis of Gold Nanoparticles Using Flower Extracts: (2021–2019)

In 2021, saffron stigma-mediated gold nanoparticles were produced by Alhumaydhi, Fahad A., et al. and tested against E. coli, as shown in

Table 10. Synthesis of Gold nanoparticles using flower and seed extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Saffron	Flower	-	-	-	2021	[163]
2	ClitoriaTernatea	Flower	18.6 nm	Spherical	-	2020	[164]
3	Musa acuminata	Flower	12.6–15.7 nm	-	K. pneumonia (ZOI = 12 mm), P. aeruginosa (ZOI = 9 mm), E. faecalis (ZOI = 10 mm), S. typhi (ZOI = NO), E. coli (ZOI = 7), S. aureus (ZOI = 11 mm), and P. mirabilis (ZOI = 12 mm). Exhibited antioxidant activity toward DPPH (IC50 = 390 μg for ethanol and 460 μg aqueous)	2019	[165]
4	Elettaria cardamomum	Seed	16.6 nm	-		-	[166]

[163]. In 2020, spherical gold nanoparticles were derived from Clitoria ternatea, having particles size of 18.16 nm [164]. Musa acuminata ethanol and aqueous extract were utilized for the biosynthesis of gold nanoparticles, having a size range of 12.6–15.7 nm and evaluated against K. pneumoniae (ZOI = 12 mm), P. aeruginosa (ZOI = 9 mm), E. faecalis (ZOI = 10 mm), S. typhi (ZOI = NO), E. coli (ZOI = 7), S. aureus (ZOI = 11 mm), and P. mirabilis (ZOI = 12 mm). Prepared gold nanoparticles exhibited antioxidant activity toward DPPH [165]. Perveen, Kahkashan, et al. reported the biosynthesis of gold nanoparticles utilizing Elettaria cardamomum and their anticancer and antioxidant activities are shown in

Table 11. Synthesis of Silver nanoparticles using leaf extracts (2021).

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Mentha longifolia	Leaf	10.23 ± 2 nm	Round oval	At 2.0 (μg/mL) Concen of AgNPs S. aureus (ZOI = 12 ± 0.03 mm), B. subtilis (ZOI = 10 ± 0.01 mm), and K. pneumonia (ZOI = 0).	2021	[192]
2	Ocimum americanum	Leaf	48.25 nm	-	At 100 (μg/mL) Concen of AgNPs S. aureus (ZOI = 18.33 ± 0.33 mm), P. aeruginosa (ZOI = 17.66 ± 0.66 mm), V. cholera (ZOI = 15.66 ± 0.88 mm), Aeromonas sp (ZOI = 13. 33 ± 0.33 mm), bacillus sp (ZOI = 16.33 ± 0.33 mm), and E. coli (ZOI = 7.66 ± 0.33 mm) Anti-oxidant activity was examined by H2O2 (58.71%) and DPPH (75%). Photocatalytic degradation of Eosin dye (91.17%)	2021	[193]
3	Clerodendrum inerme	Leaf	-	-	A. niger (ZOI = 17 mm), A. flavus (ZOI = 22 mm), and antibacterial i.e., B. subtilis (ZOI = 15 mm) and S. aureus (ZOI = 14 mm) The anti-oxidant and cytotoxic activities were also examined by utilizing DPPH free radical scavenging and MTT process (78.8 ± 0.19%)	2020	[194]
4	Salvia officinalis	Leaf	41 nm	Spherical	Exhibited less cytotoxicity toward HeLa cells’ line and exhibited excellent anti-plasmodial efficiency (IC50 = 3.6 lg/mL)	2021	[195]
5	Alstonia venenata	Leaf	27–36 nm	Spherical	The larvicidal efficiency on early-third-instar larvae was sufficiently higher for silver nanoparticles as compared to extract. The larvicidal activity was tested toward Culex quinquefasciatus with IC50 equivalent to 14.50 lg/mL, Anopheles stephensi with IC50 equivalent to12.28 lg/mL, and Aedes aegypti with equivalent to LC5013.49 lg/mL	2021	[196]
6	Sida retusa	Leaf	20–40 nm.	Spherical	S. aureus (ZOI = 17 mm), B. subtilis (ZOI = 14 mm), E. coli (ZOI = 15 mm), and S. typhi (ZOI = 15 mm)	2021	[197]
7	Carica papaya	Leaf	-	-	Anticancer activity toward various human cancer cells. The cytotoxic commotion was performed toward various human cells and non-tumorigenic keratinocytes’ cells. Cure of DU145 cell with papaya-mediated silver nanoparticles (0.5–5.0 μg/mL) for 1 or 2 days decreased the total cell number by 21–36%	2021	[198]
8	Carissa carandas	Leaf	35 ± 2 nm at 25 °C and 30 ± 3 nm at 60	-	E. faecalis (ZOI = 7.0 ± 0.0 mm), S. flexneri (ZOI = 8.0 ± 1.0 mm), S. typhimurium (ZOI = 8.0 ± 1.0 mm), and gonococci spp (ZOI = 6.0 ± 0.0 mm) Exhibited excellent antioxidant activity through DPPH assay (IC50 = 68.12 ± 1.27).	2021	[199]
9	Malva parviflora	Leaf	50.6 nm	Spherical	Inhibited the growth of F. oxysporum (81%), A. alternate (82%), H. rostratum (89%), and F. solani (81%).	2021	[200]
10	Ziziphus nummularia	Leaf	25.6 nm	Oval and Spherical	Exhibited good DPPH activity (IC50 = 520 mg/mL) and ABTS activity (IC50 = 55 mg/mL)	2021	[201]
11	Otostegia persica	Leaf	36.5 ± 2.0 nm	Spherical	S. pyogenes (ZOI = 14 ± 0.4 mm), S. aureus (ZOI = 16 ± 0.1 mm), B. subtilis (ZOI = 15 ± 0.3 mm), P. aeruginosa (ZOI = 21 ± 0.5 mm), S. typhi (ZOI = 19 ± 0.4 mm), and E. coli (ZOI = 17 ± 0.1 mm) Exhibited excellent anti-oxidant activity (84%) compared to Otostegia persica leaf extract (64%).	2021	[202]
12	Lotus lalambensis	Leaf	-	-	C. albicans (MIC = 125 μg/mL)	2021	[203]
13	Symplocos racemosa	Leaf	-	-	P. aeruginosa (ZOI = 22 mm)	2021	[204]
14	Aloe vera	Leaf	-	-	P. aeruginosa (ZOI = 14.00 ± 1.00 mm), S. aureus (ZOI = 21.00 ± 1.00 mm), E. coli (ZOI = 20.00 ± 2.00 mm), and Enterobacter sp (ZOI = 32.00 ± 2.00 mm)	2021	[205]
15	Ruellia tuberosa.	Leaf	55.65 nm	Spherical	Cytotoxic potency against A549 lung cancer line with IC50 = 68 μg/mL. Degraded the Coomassie brilliant blue and crystal violet absorbance (peaks of the degraded CV and CBB were recorded at 586 and 590 nm)	2021	[206]
16	Psidium guajava	Leaf	-	-	Anti-chikungunya potency	2021	[207]
17	Euphorbia sanguinea	Leaf	-	-	Photocatalytic degradation of CR (90% within 1 h)	2021	[208]

. TEM exhibited the biosynthesized gold nanoparticles to be 16.63 nm in size and spherical [166].

6.3. Silver Nanoparticles

The synthesis of silver nanoparticles has gained remarkable attention due to their application in climate change, contamination [167], anti-microbial activities [168,169], information storage [170], bio-medical applications [171], energy generation [172], clean water technology [173], catalysis [174], biological sensors [175,176], optoelectronics [177], Lithium-ion batteries [178], and DNA sequencing [179]. Silver nanoparticles have vast applications in biomedicine due to their unique biological properties depending on their structure and size. Silver nanoparticles possess a very wide-spectrum, high antimicrobial activity [180]. They effectively kill microbes at a very low concentration [181]. Silver nanoparticles from free radicals alter the properties of microbial membranes and ultimately cause damage. Silver nanoparticles interact with microbial DNA and inhibit microbial activities [182]. Medical applications of silver nanoparticles are not only limited to antimicrobial treatments but they are also extended to bone healing [183], wound healing, vaccine development, the anti-diabetic effect [184], etc. Recent studies also proved silver nanoparticles as efficient candidates against various cancers. The surface-to-volume ratio of silver nanoparticles affects anticancer activity. Strong anticancer activities of silver particles are reported when size is reduced to even Angstrom [185]. It is also used in the treatment to control multi-drug-resistant microorganisms [186]. Silver nanoparticles are also used as a tool in dentistry [187,188]. Apart from biomedical applications, silver nanoparticles are widely used in various analytical techniques because of their unique physicochemical properties. They play an important role in biosensor and imaging technologies [189]. Many analytical techniques are also using silver nanoparticles in instrumentation [190]. They are used as fillers in biomaterials. Silver nanoparticles’ films have been recently used as an alternative food packaging material [191]. There are various methods, such as Supercritical Fluid Synthesis, Laser Ablation, Laser Pyrolysis, Ball Milling, Ultrasonic Synthesis, etc., that have been used for the synthesis of silver nanoparticles. Recently, the biological synthesis of silver nanoparticles by using biological organisms such as plants, fungi, algae, and bacteria as capping and reducing agents and their anti-microbial activity has been studied. The different biological molecules such as tannins, ketones, flavonoids, protein, and aldehydes are responsible for the synthesis of silver nanoparticles by oxidation of Ag⁺ to Ag⁰.

6.3.1. Synthesis of Silver Nanoparticles

Silver nanoparticles can be prepared from various parts of the plant such as leaf, stem, root, flower, seed, and fruit.

6.3.2. Synthesis of Silver Nanoparticles Using Leaf Extracts: (2019–2020)

To date, numerous leaves’ extracts have been used for the synthesis of silver nanoparticles, as shown in Table 11 and

Table 12. Synthesis of Silver nanoparticles using leaf extracts (2019–2020).

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Borago officinalis	Leaf	40 nm	Irregular	The bio-synthesized silver nanoparticles were hazardous to Spodoptera littoralis	2020	[209]
2	Tragopogon collinus	Leaf	7 nm	-	At 6000 (μg/mL) Concen of AgNPs S. aureus (ZOI = 2 mm) and E. coli (ZOI = 4 mm) At 7000 (μg/mL) Concen of AgNPs S. aureus (ZOI = 5 mm) and E. coli (ZOI = 7 mm) At 8000 (μg/mL) Concen of AgNPs S. aureus (ZOI = 10 m) and E. coli (ZOI = 8 mm)	2020	[210]
3	Melia azedarach	Leaf	18–30 nm	Spherical	Verticillium dahlia	2020	[211]
4	Mentha aquatica	Leaf	41 nm	Spherical	P. aeruginosa (MIC = 2.2μg/mL), E. coli (MIC = 58μg/mL), B. cereus (MIC = 20), and S. aureus (MIC = 198μg/mL)	2020	[212]
5	Ziziphus joazeiro.	Leaf	-	-	E. coli ATCC 25922 and S. aureus ATCC 25923	2020	[213]
6	Elytraria acaulis	Leaf	5–100 nm	Cuboid	S. typhi (ZOI = 11.5 ± 2.5 mm), S. Epidermis (ZOI = 14.3 ± 1.7 mm), E. coli (ZOI = 11.2 ± 1.6 mm), and B. subtilis (ZOI = 12.0 ± 2.4 mm). Anti-oxidant activity toward DPPH (84.47%) and ABTS (85.25%) Cytotoxic commotion was performed against A549 cell line (IC50 = 79.6 µg/mL)	2020	[214]
7	Hyptis suaveolens	Leaf	29.19–52.27 nm	-	Scavenged H2O2 (54.21–70.11%) and DPPH (77.75–83.19) Stopped coagulation of blood	2020	[215]
8	Caesalpinia pulcherrima	Leaf	9 nm	Spherical	Cytotoxicity toward HeLa cell line (IC50 = 4.44 mg/mL)	2020	[216]
9	Gomphrena globosa	Leaf	-	Spherical	subtilis (ZOI = 40 mm), P. aeruginosa (ZOI = 38 mm), M. luteus (ZOI = 48 mm), E. coli (ZOI = 53 mm), and K. pneumonia (ZOI = 39 mm)	2020	[217]
10	Plumbago auriculata	Leaf	20 to 500 nm	-	At 5 (μg/mL) Concen of AgNPs S. aureus (ZOI = 10 ± 1.5 mm), B. subtilis (ZOI = 8 ± 0.5 mm), K. pneumonia (ZOI = 11 ± 0.5 mm), and E. coli (ZOI = 10 ± 0.8 mm), At 10 (μg/mL) Concen of AgNPs S. aureus (ZOI = 10 ± 0.8 mm), B. subtilis (ZOI = 10 ± 1.7 mm), K. pneumonia (ZOI = 11 ± 0.8 mm), and E. coli (ZOI = 10 ± 0.6 mm), At 15 (μg/mL) Concen of AgNPs S. aureus (ZOI = 8 ± 0.7 mm), B. subtilis (ZOI = 8 ± 0.9 mm), K. pneumonia (ZOI = 12 ± 1.0 mm), and E. coli (ZOI = 12 ± 1.0 mm), At 20 (μg/mL) Concen of AgNPs S. aureus (ZOI = 10 ± 1.5 mm), B. subtilis (ZOI = 8 ± 1.0 mm), K. pneumonia (ZOI = 14 ± 1.7 mm), and E. coli (ZOI = 12 ± 2.5 mm), Inhibited the growth of Culex quinquefasciatus (45.1 µg/mL) and Aedes aegypti	2020	[218]
11	Cucumis prophetarum	Leaf	30–50 nm	-	At 20 (μg/mL) Concen of AgNPs S. typhi (ZOI = 15 ± 0.2 mm) and S. aureus (ZOI = 11 ± 0.4 mm) At 50 (μg/mL) Concen of AgNPs S. typhi (ZOI = 17 ± 0.5 mm) and S. aureus (ZOI = 14 ± 0.3 mm) At 75 (μg/mL) Concen of AgNPs S. typhi (ZOI = 20 ± 0.6 mm) and S. aureus (ZOI = 18 ± 0.4 mm) Anti-oxidant activity toward DPPH (IC50 = 29.2 µg/mL) and ABTS (IC50 = 34.5µg/mL)	2020	[219]
12	Polygonatum graminifolium	Leaf	3–15 nm	Spherical	E. coli (ZOI = 27 mm) and S. aureus (ZOI = 16 mm)	2020	[220]
13	Cocos nucifera	Leaf	14.2 nm	Cubic	E. coli (ZOI = 16.0 ± 0.11 mm), B. subtilis (ZOI = 10.0 ± 0.05 mm), S. aureus (ZOI = 12.0 ± 0.06 mm), and S. typhimurium (ZOI = 13.0 ± 0.12 mm),	2020	[221]
14	Mimosa albida	Leaf	6.5 nm ± 3.1 nm	-	Exhibited anti-oxidant activity (IC50 = 7563 ± 967)	2020	[222]
15	Capparis zeylanica	Leaf	-	Spherical	S. Paratyphi (ZOI = 18 mm), S. dysenteriae (ZOI = 19 mm), S. epidermidis (ZOI = 22 mm), E. faecalis (ZOI = 20 mm), A. niger (ZOI = 21 mm), and C. albicans (ZOI = 20 mm)	2020	[223]
16	Holoptelea integrifolia	Leaf	32–38 nm.	Spherical	Showed antioxidant activities toward DPPH (74.59 ± 3.08%) Showed antiinflammatory (binding constant 2.60 ± 0.05 × 10−4) and antidiabetic (86.66 ± 5.03%) activities.	2019	[224]
17	Annona reticulatal	Leaf	-	Cubic	P. Aeruginosa (MIC = 62.5 μg/mL), E. coli (MIC = 62.6 μg/mL), S. aureus (MIC = 31.5 μg/mL), B. cereus (MIC = 125 μg/mL), and C. albicans (MIC = 62.5 μg/mL)	2019	[225]
18	Combretum erythrophyllum	Leaf	13.62 nm	Spherical	S. Epidermidis (ZOI = 12 mm), P. vulgaris (ZOI = 11 mm), S. aureus (ZOI = 15 mm), and E. coli (ZOI = 12 mm)	2019	[226]
19	Berberis vulgaris	Leaf	30–70 nm	Spherical	S. aureus and E. coli	2019	[227]
20	Catharanthus roseus	Leaf	-	-	P. Aeruginosa (ZOI = 6 mm), S. dysenteriae (ZOI = 8 mm), S. aureus (ZOI = 8 mm), and B. anthracis (ZOI = 12 mm),	2019	[228]
21	Ganonerion polymorphum	Leaf	20–60 nm	Hexagonal and Spherical	B. Cereus (99.75%) and E. coli (99.94%)	2019	[229]
22	Premna integrifoliaL	Leaf	9–35 nm	Spherical	E. faecalis (MIC = 60 μg/mL), V. parahaemolyticus (MIC = 10 μg/mL), S. dysenteriae (MIC = 20 μg/mL), S. aureus (MIC = 30 μg/mL), and S. flexneri (MIC = 70 μg/mL) Showed anti-oxidant activity (IC50 = 524.19 ± 2.63 µg/mL) and cytotoxic to cancer cell line (SiHa).	2019	[230]
23	Piper betle	Leaf	6–14 nm	Spherical	At 1000 (μg/mL) Concen of AgNPsF. Solani (ZOI = 3.13 ± 0.25 mm) and A. brassicae (ZOI = 67. 21 ± 3.15 mm)	2019	[231]

. Rauf, Abdur, et al. reported the formation of AgNPs using Mentha longifolia aqueous leaves’ extracts. The round oval morphology of silver nanoparticles with a mean size of 10.23 ± 2 nm was revealed by TEM. Mentha longifolia-based silver nanoparticles showed tremendous antibacterial effect toward S. aureus (ZOI = 12 ± 0.03 mm), B. subtilis (ZOI = 10 ± 0.01 mm), and K. pneumonia (ZOI = 0) and antioxidant activities [192]. In another research, the biological fabrication of silver nanoparticles was explained by Ocimum Americanum, with a particle size of 48.25 nm. The biologically prepared silver nanoparticles showed anti-bacterial activity against S. aureus (ZOI = 18.33 ± 0.33 mm), P. aeruginosa (ZOI = 17.66 ± 0.66 mm), V. cholera (ZOI = 15.66 ± 0.88 mm), Aeromonas sp (ZOI = 13. 33 ± 0.33 mm), Bacillus sp (ZOI = 16.33 ± 0.33 mm), and E. coli (ZOI = 7.66 ± 0.33 mm). The anti-oxidant activity was examined by H₂O₂ and DPPH. Silver nanoparticles showed excellent photocatalytic degradation of Eosin dye [193]. By utilizing Clerodendrum inerme as both a capping and reducing agent, Khan, Shakeel Ahmad, et al., synthesized silver nanoparticles and evaluated them for various biological activities, e.g., anti-mycotic, i.e., A. niger (ZOI = 17 mm) and A. flavus (ZOI = 22 mm), and antibacterial, i.e., B. subtilis (ZOI = 15 mm) and S. aureus (ZOI = 14 mm), activities. The antioxidant and cytotoxic activities of prepared gold nanoparticles were also examined by utilizing DPPH free radical scavenging (78.8 ± 0.19%) and the MTT process [194]. In another study, Salvia officinalis hexane, ethyl acetate, and ethanol leaf extract were utilized in the formation of AgNPs. The biologically produced silver nanoparticles exhibited less cytotoxicity toward the HeLa cells’ line and exhibited excellent anti-plasmodial efficiency (IC50 = 3.6 lg/mL) [195]. A rapid and eco-friendly approach for preparing spherical silver nanoparticles with size of 27–36 nm by utilizing Alstonia venenata was performed. The larvicidal efficiency on early-third-instar larvae was sufficiently higher for silver nanoparticles as compared to extract. The larvicidal activity was tested toward Culex quinquefasciatus with IC₅₀ equivalent to 14.50 lg/mL, Anopheles stephensi with IC₅₀ equivalent to 12.28 lg/mL, and Aedes aegypti with IC₅₀ equivalent to 13.49 lg/mL [196]. Previous work reported the bio-fabrication of spherical silver nanoparticles with a diameter of 20–40 nm, utilizing Sida retusa and tested toward S. aureus (ZOI = 17 mm), B. subtilis (ZOI = 14 mm), E. coli (ZOI = 15 mm), and S. typhi (ZOI = 15 mm). [197]. In 2021, Singh, Surya P., et al. reported the formation of AgNPs from Carica papaya aqueous leaf extract and its anticancer activity toward various human cancer cells. The cytotoxic commotion was performed toward various human cell and non-tumorigenic keratinocytes’ cells. Cure of DU145 cell with C. papaya-mediated silver nanoparticles (0.5–5.0 μg/mL) for 1 or 2 days decreased the total cell number by 21–36% [198]. In another research, the author achieved silver nanoparticles with particles’ sizes of 35 ± 2 nm and 30 ± 3 nm using Carissa carandas aqueous leaf extract. Biologically synthesized silver nanoparticles exhibited excellent anti-oxidant activity through DPPH assay. Prepared silver nanoparticles also showed remarkable ant-bacterial activity toward human pathogenic bacteria, e.g., E. faecalis (ZOI = 7.0 ± 0.0 mm), S. flexneri (ZOI = 8.0 ± 1.0 mm), S. typhimurium (ZOI = 8.0 ± 1.0 mm), and gonococci spp (ZOI = 6.0 ± 0.0 mm) [199]. Malva parviflora ethanol and waterleaf extract were utilized to synthesize spherical silver nanoparticles. The biologically synthesized silver nanoparticles inhibited the growth of F. oxysporum (81%), A. alternate (82%), H. rostratum (89%), and F. solani (81%) [200]. Ziziphus nummularia aqueous leaf extract was utilized to synthesize silver nanoparticles. These silver nanoparticles exhibited efficient anti-microbial commotion against S. aureus, C. rubrum, S. typhimurium, P. aeruginosa, C. neoformans, C. albicans, and C. glabrata. Silver nanoparticles also exhibited good DPPH activity (IC₅₀ = 520 mg/mL) and ABTS activity (IC₅₀ = 55 mg/mL) [201]. A previous study confirmed for first time the capability of Otostegia persica for the bio-synthesis of silver nanoparticles. These particles exhibited excellent anti-oxidant activity compared to the Otostegia persica leaf extract. These particles also showed potential anti-bacterial activity toward S. pyogenes (ZOI = 14 ± 0.4 mm), S. aureus (ZOI = 16 ± 0.1 mm), B. subtilis (ZOI = 15 ± 0.3 mm), P. aeruginosa (ZOI = 21 ± 0.5 mm), S. typhi (ZOI = 19 ± 0.4 mm), and E. coli (ZOI = 17 ± 0.1 mm) [202]. In another study, Abdallah, Basem M., et al. aimed to produce silver nanoparticles from Lotus lalambensis aqueous leaf extract and their anticandidal activity toward C. albicans (MIC = 125 μg/mL) [203]. Spherical silver nanoparticles were produced utilizing Symplocos racemosa. Anti-microbial activity of biologically prepared silver nanoparticles was studied on P. aeruginosa (ZOI = 22 mm) [204]. Silver nanoparticles were biosynthesized by an environmentally friendly hydrothermal approach using Aloe vera aqueous leaf extract, used to evaluate antibacterial potency against P. aeruginosa (ZOI = 14.00 ± 1.00 mm), S. aureus (ZOI = 21.00 ± 1.00 mm), E. coli (ZOI = 20.00 ± 2.00 mm), and Enterobacter sp (ZOI = 32.00 ± 2.00 mm [205]. In another report, Seerangaraj, Vasantharaj, et al. aimed to biosynthesize spherical silver nanoparticles with particles’ size of 55.65 nm by utilizing Ruellia tuberosa. Biologically prepared silver nanoparticles exhibited cytotoxic potency against A549 lunger cancer line with IC₅₀ = 68 μg/mL. These silver nanoparticles were also degraded the Coomassie brilliant blue and crystal violet [206]. Sharma, Yashika, et al. evaluated the anti-chikungunya potency of Psidium guajava aqueous leaf extract and the biologically prepared silver nanoparticles [207]. Ekennia, Anthony C., et al. reported the formation of spherical silver nanoparticles using Euphorbia sanguinea and its photocatalytic degradation of CR (90% within 1 h) [208].

In the period of 2019–2020, several publications reported Silver nanoparticles synthesized using leaf extract of various plants, e.g., Borago officinalis [209], Tragopogon collinus [210], Melia azedarach [211], Mentha aquatica [212], Ziziphus joazeiro [213], Elytraria acaulis [214], Hyptis suaveolens [215], Caesalpinia pulcherrima [216], Gomphrena globosa [217], Plumbago auriculata [218], Cucumis prophetarum [219], Polygonatum graminifolium [220], Cocos nucifera [221], Mimosa albida [222], Capparis zeylanica [223], Holoptelea integrifolia [224], Annona Reticulatal [225], Combretum erythrophyllum [226], Berberis vulgaris [227], Catharanthus roseus [228], Ganonerion polymorphum [229], Premna integrifolia L [230], and Piper betle [231].

6.3.3. Synthesis of Silver Nanoparticles Using Root Extracts: (2021–2020)

Roots are also well established for the synthesis of silver nanoparticles, as shown in

Table 13. Synthesis of Silver nanoparticles using root extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Ricinus communis	Root	29 nm	Spherical	E. coli (73%), K. pneumonia (60%), S. aureus (56%), S. pneumonia (60%), A. niger (77%), and A. alternate (75%) Exhibited remarkable efficiency toward Urease (IC50 = 36.81 ± 0.05 μg/mL) and Xanthine (IC50 = 3.60 ± 0.04 μg/mL)	2021	[232]
2	Duchesnea indica	Root	20.49 nm	Spherical	E. coli (MIC = 0.53 mg/mL), S. typhi (MIC = 0.01 mg/mL), A. alternate (MIC = 0.51 mg/mL), and M. canis (MIC = 0.53 mg/mL)	2021	[233]
3	Salvadora persica	Root	37.5 nm	Rod and Spherical	S. epidermidis ATCC12228 (MIC = 0.39 µg/mL) and E. coli (MIC = 0.19 µg/mL)	2021	[234]
4	Asparagus officinalis	Root	-	-	Cytotoxic toward cervical cancer cell line (SiHa) (IC50 = 44 lg mL−1)	2021	[235]
5	Astragalus tribuloides Delile	Root	34.2 ± 8.0 nm	Spherical	S. aureus (ZOI = 18 mm), S. flexneri (ZOI = 27 mm), E. coli (ZOI = 24 mm), and B. cereus (ZOI = 16 mm) Excellent anti-oxidant (64% property higher than extract (47%)	2020	[236]
6	Berberis asiatica	Root	14 nm	Spherical	S. typhimurium (ZOI = 7 mm), E. coli (ZOI = 11 mm), S. aureus (ZOI = 12 mm), and K. pneumonia (ZOI = 6 mm)	2020	[237]

. In 2021, Gul, Anadil, et al. bio-synthesized spherical silver nanoparticles utilizing Ricinus communis methanolic root extract with an average size of 29 nm and used them to evaluate against E. coli (73%), K. pneumonia (60%), S. aureus (56%), S. pneumonia (60%), A. niger (77%), and A. alternate (75%). The results illustrated that the prepared nanoparticles exhibited remarkable efficiency toward Urease (IC₅₀ = 36.81 ± 0.05 μg/mL) and Xanthine (IC₅₀ = 3.60 ± 0.04 μg/mL) [232]. In another report, spherical silver nanoparticles with average size of 20.49 nm were prepared by utilizing Duchesnea indica and tested toward E. coli (MIC = 0.53 mg/mL), S. typhi (MIC = 0.01 mg/mL), A. alternata (MIC = 0.51 mg/mL), and M. canis (MIC = 0.53 mg/mL) [233]. Arshad, Hammad, et al. reported a simpler, quicker, and ecofriendly approach to prepare silver nanoparticles by utilizing Salvadora persica aqueous root extract and tested them toward S. epidermidis ATCC12228 (MIC = 0.39 µg/mL) and E. coli (MIC = 0.19 µg/mL) [234]. In another report, Tripathi, Deepika, et al. examined the cytotoxic efficiency of Asparagus officinalis-mediated silver nanoparticles toward a cervical cancer cell line (SiHa) [235]. In 2020, silver nanoparticles were prepared through an inexpensive and ecofriendly approach by utilizing Astragalus tribuloides Delile. The resultant silver nanoparticles showed excellent anti-oxidant properties compared to the extract. The resultant silver nanoparticles were also used to evaluate bacterial activity toward S. aureus, S. flexneri, E. coli, and B. cereus [236]. In another research, an environmental process was used for the preparation of silver nanoparticles by Berberis asiatica aqueous root extract and tested against S. typhimurium (ZOI = 7 mm), E. coli (ZOI = 11 mm), S. aureus (ZOI = 12 mm), and K. pneumoniae (ZOI = 6 mm) [237].

6.3.4. Synthesis of Silver Nanoparticles Using Stem and Stem Bark Extracts: (2019–2021)

Biosynthesis of ZnO nanoparticles using stem or stem bark has gained immense attention nowadays, as shown in

Table 14. Synthesis of Silver nanoparticles using stem and stem bark extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Grewia lasiocarpa	Stem bark	diameter between 38.3 and 46.7 nm	Spherical	S. aureus (MIC = 15.67 ± 2.08 µg/mL). Exhibited cytotoxicity toward HeLa (IC50 = > 1 μg/mL).	2021	[238]
2	Euphorbia nivulia	Stem bark	20–90 nm	Spherical	K. pneumoniae (MIC = 23.5 ± o.5 µg/mL), B. cereus (MIC = 27 ± 1 µg/mL), S. aureus (MIC = 24.5 ± 1.5 µg/mL), P. aeruginosa (MIC = 30.5 ± 0.5 µg/mL), B. subtilis (MIC = 29 ± 1 µg/mL), and C. albicans (MIC = 26 ± 1 µg/mL)	2021	[239]
3	Boswellia dalzielii	Stem	2 nm to 101 nm	-	Anti-oxidant activity of prepared silver nanoparticles was tested using DPPH (TEAC = 300.91)	2020	[240]
4	Piper chaba	Stem	19 nm	Spherical	Degradation of MB and reduction of 4-nitrppheno	2020	[241]
5	Garcinia kola	Stem	-	-	E. faecalis (ZOI = 2 mm), B. cereus (ZOI = 4 mm), C. sporogenes (ZOI = 6 mm), and E. coli (ZOI = 10 mm)	2020	[242]
6	Vigna unguiculata	Stem	~25 nm	-	-	2019	[243]

. In 2021, the author reported an environmentally friendly preparation of silver nanoparticles using Grewia lasiocarpa aqueous stem extract. The spherical shape of bio-synthesized nanoparticles was shown by SEM and HR-TEM. The prepared silver nanoparticles exhibited cytotoxicity toward HeLa (IC₅₀ > 1 μg/mL). The prepared silver nanoparticles were also used to evaluate bacterial activity against S. aureus (MIC = 15.67 ± 2.08 µg/mL) [238]. Euphorbia nivulia was utilized to prepare spherical silver nanoparticles with a size of 20–90 nm and tested against K. pneumoniae (MIC = 23.5 ± 0.5 µg/mL), B. cereus (MIC = 27 ± 1 µg/mL), S. aureus (MIC = 24.5 ± 1.5µg/mL), P. aeruginosa (MIC = 30.5 ± 0.5 µg/mL), B. subtilis (MIC = 29 ± 1 µg/mL), and C. albicans (MIC = 26 ± 1 µg/mL) [239]. In another study, silver nanoparticles were derived from Boswellia dalzielii aqueous stem extract. The anti-oxidant activity of prepared silver nanoparticles was tested using DPPH (TEAC = 300.91) [240]. A biosynthesis of spherical silver nanoparticles with an average size of 19 nm was performed utilizing Piper chaba aqueous stem extract. The prepared silver nanoparticles efficiently catalyzed the degradation of MB and reduction of 4-nitrpphenol [241]. In 2020, Akintelu, Sunday Adewale, et al. tested the anti-microbial commotion of Garcinia kola-based silver nanoparticles against E. faecalis (ZOI = 2 mm), B. cereus (ZOI = 4 mm), C. sporogenes (ZOI = 6 mm), and E. coli (ZOI = 10 mm) [242]. In another report, Dawodu, Folasegun A., et al. explained a quicker, inexpensive process for the preparation of silver nanoparticles with a mean size of ~25 nm by utilizing Vigna unguiculata aqueous stem extract [243].

6.3.5. Synthesis of Silver Nanoparticles Using Seed Extracts: (2019–2021)

The seed extract of the different plants has been widely used as a reducing and capping agent for the biosynthesis of silver nanoparticles, as shown in

Table 15. Synthesis of Silver nanoparticles using seed extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Trigonella foenum-graecum	Seed	-	-	B. cereus (ZOI = 10 mm), E. coli (ZOI = 14 mm), and S. aureus (ZOI = 5.0 mm)	2021	[244]
2	Morinda citrifolia	Seed	3 nm	Spherical	S. aureus (ZOI = 9.81 mm) and E. coli (ZOI = 10.63 mm)	2021	[245]
3	Mangifera indica	Seed	-	-	B. cereus (ATCC11778) (K. pneumonia (NMCIM2719), S. aureus (ATCC29737), P. aeruginosa (ATCC9027), C. rubrum (ATCC14898), E. coli (NCIM2931), S. typhimurium (ATCC23564), C. neoformans (ATCC34664), C. albicans (ATCC2091), and C. glabrata (NCIM3438)	2021	[246]
4	Annona squamosa L.	Seed	22 nm	Spherical	Showed excellent catalytic activity against degradation of Coomassie brilliant blue dye	2021	[247]
5	Rosa canina	Seed	150 nm	Rod and Spherical	-	2021	[248]
6	Nigella sativa	Seed	-	-	Showed good photocatalytic activity on degradation of Congo red (Degraded 96%, 97%, and 98.5%, at 0.2, 0.15, and 1.3 min, respectively).	2021	[249]
7	Moringa oleifera	Seed	-	Spherical	Enhanced wound contraction and tissue growth wall	2021	[250]
8	Bunium persicum	Seed	35 to 70 nm	-	Inhibited Urease and tyrosinase	2021	[251]
9	Syzygium cumini	Seed	-	-	F. nucleatum (MIC = NO), A. naeslundii (MIC = 125 µg/mL), S. aureus (MIC = 125 µg/mL), S. mutans (MIC = 250 µg/mL), S. epidermidis (MIC = 31.2 µg/mL), V. dispar (MIC = 62.5 µg/mL), and S. oralis (MIC = 31.2 µg/mL)	2021	[252]
10	Vitis vinifera	Seed	10–50 nm	-	-	2021	[253]
11	Gingerand Nigella sativa	Seed	~12–8 nm	-	P. Aeruginosa and E. coli	2020	[254]
12	Cuminum cyminum L.	Seed	~100 nm	Spherical	Effective against human breast cancer cells (IC50 = 1.25 µg/mL)	2020	[255]
13	Punica granatum	Seed	10 to 35 nm	Spherical	-	2020	[256]
14	Salvia hispanicaL.	Seed	7 nm	Spherical	S. aureus (ZOI = 14.9 mm) and E. coli (ZOI = 18.5 mm)	2019	[257]
15	Avicennia marina	Seed	5–10 nm	-	pneumoniae ATCC 700,603 (ZOI = 12.5 ± 0.01 mm), E. faecalis ATCC 5129 (ZOI = No), S. aureus ATCC 43,300 (ZOI = 3.25 ± 0.02 mm), P. aeruginosa ATCC 27,853 (ZOI = 12.5 ± 0.05 mm), and E. coli ATCC 35,218 (ZOI = 6.25 ± 0.05 mm)	2019	[258]
16	Tectona grandis	Seed	10–30 nm	-	B. cereus (ZOI = 12 mm), E. coli (ZOI = 17 mm), and S. aureus (ZOI = 16 mm)	2019	[259]

. In 2021, Awad, Manal A., et al. explained a green approach of biosynthesis of silver nanoparticles utilizing Trigonella foenum-graecum and tested it against B. cereus (ZOI = 10,0.9 mm), E. coli (ZOI = 14 ± 2.0 mm), and S. aureus (ZOI = 5.0 ± 2.0 mm) [244]. Morinda citrifolia was utilized to prepare spherical silver nanoparticles with an average size of 3 nm and used to evaluate bacterial activity toward S. aureus (ZOI = 9.81 mm) and E. coli (ZOI = 10.63 mm) [245]. In another work, round silver nanoparticles were derived from Mangifera indica aqueous seed extract and tested against B. cereus (ATCC11778), K. pneumonia (NMCIM2719), S. aureus (ATCC29737), P. aeruginosa (ATCC9027), C. rubrum (ATCC14898), E. coli (NCIM2931), S. typhimurium (ATCC23564), C. neoformans (ATCC34664), C. albicans (ATCC2091), and C. glabrata (NCIM3438) [246]. The formation of spherical nanoparticles with an average of 22 nm utilizing Annona squamosa L. was reported by Jose, Vimala, et al. The bio-synthesized silver nanoparticles showed excellent catalytic activity against degradation of Coomassie brilliant blue dye [247]. Saygi, Kadriye Ozlem, et al. reported Rosa canina aqueous seed extract-inspired biosynthesis of spherical and rod shape silver nanoparticles with a mean size of 150 nm [248]. In another study, an advanced approach for the synthesis of silver nanoparticles utilizing Nigella sativa aqueous seed extract was reported by Chand, Kishore, et al. The prepared silver nanoparticles showed good photocatalytic activity on the degradation of Congo red [249]. Perveen, Rehana, et al., described a facile and green process for the preparation of silver nanoparticles by utilizing Moringa oleifera seed polysaccharide. The conclusion drawn from the above study was that prepared silver nanoparticles were spherically shaped. Moringa oleifera-mediated silver nanoparticles can enhance wound contraction and tissue growth wall [250]. Khan, Ibrahim, et al., reported the eco-friendly, facile, and rapid biosynthesis of silver nanoparticles utilizing Bunium persicum alcohol/methanol seed extract with a mean size range of 35 to 70 nm. Bunium persicum-mediated silver nanoparticles inhibited Urease and tyrosinase [251]. de Carvalho Bernardo, Wagner Luís, et al. reported a facile and rapid preparation of silver nanoparticles utilizing Syzygium cumini ethanol seed extract and tested against F. nucleatum (MIC = NO), A. naeslundii (MIC = 125 µg/mL), S. aureus (MIC = 125 µg/mL), S. mutans (MIC = 250 µg/mL), S. epidermidis (MIC = 31.2 µg/mL), V. dispar (MIC = 62.5 µg/mL), and S. oralis (MIC = 31.2 µg/mL) [252]. Encapsulated silver nanoparticles were bio-synthesized by biogenic synthesis utilizing Vitis vinifera with a size range of 10–50 nm [253].

From 2020 to 2019, several publications were reported in which silver nanoparticles were synthesized using seed extract of various plants such as Ginger and Nigella sativa [254], Cuminum cyminum L. [255], Punica granatum [256], Salvia hispanica L [257], Avicennia marina [258], and Tectona grandis [259].

6.3.6. Synthesis of Silver Nanoparticles Using Flower Extracts:(2021)

Flowers were also used in the biosynthesis of silver nanoparticles, as shown in

Table 16. Synthesis of Silver nanoparticles using flower extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Avera lanata	Flower	7.6 nm	-	DPPH radical scavenging analysis showed antioxidant activity of prepared silver nanoparticles (IC50 = 50.08 ± 3.34)	2021	[260]
2	Fraxinus excelsior	Flower	-	-	Used as environmentally friendly material for the coloration of woven glass fabrics	2021	[261]
3	Jasmine	Flower	40 nm	-	Prepared silver nanoparticles degraded the MB (78% after 120 min).	2021	[262]

. In 2021, biosynthesis of silver nanoparticles with a mean size of 7.6 nm was conducted utilizing Avera lanata. The DPPH radical scavenging analysis showed the antioxidant activity of prepared silver nanoparticles [260]. A rapid, facile, sustainable, and controlled process was reported for the synthesis of silver nanoparticles by utilizing Fraxinus excelsior aqueous and ethanolic flower extract. The prepared silver nanoparticles can be used as an environmentally friendly material for the coloration of woven glass fabrics [261]. Aravind, M., et al. derived silver nanoparticles with an average size of 40 nm utilizing jasmine aqueous extract (flower) and tested them against S. aureus and E. coli. The abovementioned prepared silver nanoparticles degraded the MB [262].

6.4. Titanium Oxide Nanoparticles

Titania (existing as TiO₂ nanoparticles) constitutes specific thermal, magnetic, optical, and electric properties. Normally, Titanium oxide existed in three forms e.g., brookite crystalline polymorphs’ form, anatase form, and rutile form. The most important applications of TiO₂ are photocatalytic degradation and splitting [263], electronic and electrochromic [264], sensing instruments [265], and photovoltaic cells [266]. Among all other metal nanoparticles’ oxide, titanium oxide nanoparticles showed distinctive morphologies (size, shape, and texture) and surface chemistry. It is utilized in the preparation of papers, foodstuff, tints, cosmetics, and medicine [267]. Colloidal titanium oxide nanoparticles are utilized in the degradation of hazardous chemicals in water [268,269]. Conventionally, titanium oxide nanoparticles are prepared using chemical and physical techniques, e.g., chemical precipitation, chemical vapor deposition, sol-gel, and hydrothermal [270]. All these conventional approaches require high pressure, temperature, and toxic chemicals [271]. However, environmentally friendly, rapid, and inexpensive methods are required to prepare nanoparticles on a larger scale with lesser toxicity [272]. This could be only possible by utilizing biological extract (plants, bacteria, algae, and fungi) through green chemistry.

Synthesis of Titanium Oxide Nanoparticles from Leaves, Roots, Flowers, Seeds, and Fruit Peel Extracts: (2019–2021)

Among the biological extracts, plants are considered as one of the most favorable agents for the preparation of titanium oxide nanoparticles, as shown in

Table 17. Synthesis of Titanium Oxide nanoparticles from leaves, roots, flowers, seeds, and fruit peel extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Shape	Biological Activities	Year of Publication	Ref.
1	Mentha arvensis	Leaf	20–70 nm	Spherical	At 10 (μg/mL) Concen of AgNPs P. Vulgaris (ZOI = 25 mm), E. coli (ZOI = 20 mm), S. aureus (ZOI = 21 mm), A. niger (ZOI = No), A. fumigates (ZOI = 6 mm), and A. cuboid (ZOI = No)	2021	[275]
2	Pouteria campechiana	Leaf	-	Spherical	Exhibited larvicidal activity toward Aedes aegypti	2021	[276]
3	Coleus aromaticus	Leaf	12–33 nm	Hexagonal	S. boydii (ZOI = 30 mm) and E. faecalis (ZOI = 33 mm) Larvicidal activity toward fourth stages of instars’ larvae of Aedes aegypti. Cytotoxic activity toward HeLa cell line	2021	[277]
4	Ochradenusarabicusis	Leaf	20–40 nm	-	S. aureus (MIC = 31.25 µg/mL) and P. aeruginosa (MIC = 128 µg/mL)	2021	[278]
5	Aegle marmelos	Leaf	150 nm	Spherical	Removed ornidazole from wastewater.	2020	[279]
6	Azadirachta indica	Leaf	25–87 nm	Spherical	B. subtilis (MIC = 25 μg/mL), E. coli (MIC = 10.42 μg/mL), K. pneumoniae (MIC = 16.66 μg/mL), and S. typhi (MIC = 10.42 μg/mL).	2019	[280]
7	Carica papaya	Leaf	20 nm	Spherical	Photocatalytic activity (91.19%) against degradation of RO-4 dye	2019	[281]
8	Aloe barbadensis	Leaf	~20 nm	Spherical	Anti-biofilm activity toward P. aeruginosa (ZOI = 30.69 ± 3.78 mm)	2019	[282]
9	Glycyrrhiza glabra	Root	69 nm	Spherical	Cytotoxicity toward HEP2 and vero cell line	2019	[283]
10	Jasmine	Flower	31–42 nm	Spherical	Exhibited excellent degradation toward methylene blue dye (92% after 120 min).	2021	[284]
11	Myristica fragrans	Seed	-	-	Showed degradation against Congo red (99% after 45 min) and methylene blue (97% after 60 min).	2021	[285]
12	Cuminum cyminum	Seed	15.17 nm	-	-	2021	[286]
13	Trachyspermum ammi	Seed	16.63 nm	Spherical and spheroidal	-	2021	[287]
14	Bixa orellana	Seed	13 ± 2 nm	Spherical	-	2019	[288]
15	Nephelium lappaceumL.	Fruit peel	70–90 nm	-	Cytotoxicity was tested against MDA-MB-231 (death rate of cell = 73.65 µg/mL)	2019	[288]

. Various types of phytochemicals (phenol, amino acid, carbohydrate, and flavonoid) in plants regulate the biosynthesis of titanium oxide nanoparticles through stabilization and reduction processes [273]. The reaction starts strenuously when a titanium salt (precursor) is mixed with plant extract and color change (light-green to dark) shows the first sign of biosynthesis of titanium oxide, as shown in Figure 7 [274]. Synthesis of titanium oxide nanoparticles using various parts of plant is shown in

Table 18. Synthesis of copper and copper oxide nanoparticles using leaves, seeds, flowers, and fruit peel extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Cu/CuO NPs	Shape	Biological Activities	Year of Publication	Ref.
1	Terminalia chebula	Leaf	100 nm	CuO	Rod-like shape	Applications on diesel engine.	2021	[301]
2	Cedrus deodara	Leaf	100 nm	CuO	Spherical	S. aureus (MIC = 25 lg∕mL) and E. coli (MIC = 150 lg∕mL)	2021	[302]
3	Psidium guajava	Leaf	40–150 nm	CuO	Oval	epidermis (ZOI = 1.8 mm), E. coli (ZOI = 2 mm), S. pneumoniae (ZOI = 1.4 mm), and P. aeruginosa (ZOI = 3 mm)	2021	[303]
4	Sesbania aculeata	Leaf	-	Cu	-	C. lunata (ZOI = 22 mm) and Phoma destructiva (ZOI = 23 mm)		[304]
5	Celastrus paniculatus	Leaf	2–10 nm	CuO	Spherical	F. Oxysporum (maximum mycelial inhibition = 76.29 mm)	2020	[305]
6	Catha edulis	Leaf	-	CuO	Spherical	K. Pneumonia (ZOI = 29 ± 0.03 mm), E. coli (ZOI = 32 ± 0.02 mm), S. aureus (ZOI = 22 ± 0.01 mm), and S. pyogenes (ZOI = 24 ± 0.02 mm)	2020	[306]
7	Ageratum houstonianumMill	Leaf	~80 nm	Cu	Cubic, rectangular, hexagonal	E. coli (ZOI = 12.43 ± 0.233 mm). Photocatalytic property of prepared particles was tested toward an azo dye Congo red (40%).	2020	[307]
8	Jatropha curcas	Leaf	10 ± 1 and 12 ± 1 nm	Cu	-	Photocatalytic activity toward methylene blue (70%)	2020	[308]
9	Citrofortunella microcarpa	Leaf	-	CuO	-	Photocatalytic activity against Rhodamin B (98%)	2020	[309]
10	Enicostemma axillare	Leaf	330 nm	CuO	-	-	2019	[310]
11	Camelia sinensis	Leaf	60 ± 6 nm	CU	Spherical	Photocatalytic degradation (83.7%) of prepared copper nanoparticles was tested by utilizing bromophenol blue	2019	[311]
12	Annona squamosa	Seed	-	CuO	Spherical	Microbacterium testaceum (ZOI = 17 mm) and E. coli (ZOI = 21 mm)	2021	[312]
13	Azadirachta indica	Seed	41 ± 21 nm	CuO	-	Positive effect on nutrition, growth, and enhanced seed germination	2020	[313]
14	Elettaria cardamom	Seed	1–100 nm	CuO	-	-	2020	[314]
15	Wheat	Seed	22 ± 1.5 nm	CuO	Spherical	Described catalytic activity toward 4-nitrophenol removal (97.6% after 5 days)	2019	[315]
16	Ocimum tenuiflorum	Flower	5–10 nm	Cu	Spherical	Amino acid detection	2019	[316]
17	Stachys Lavandulifolia	Flower	20–25 nm	CuO	Spherical	-	2021	[317]
18	Punica granatum	Fruit peel	38.50 nm	CuO	-	-	2020	[318]

.

6.5. Copper and Copper Oxide Nanoparticles

Among all metal or metal oxide nanoparticles, copper oxide nanoparticles get more interest due to their multiple applications [289]. Copper oxide is a p-type semiconductor having a narrow bandgap of 1.7 eV. [290]. Biomedical applications of copper oxide nanoparticles involved antifouling, antioxidant, anti-microbial targeted drug delivery, and antibiotics. Copper oxide nanoparticles also have applications in other fields of science such as gas sensors, environmental remediation, nanocomposites’ synthesis, magneto-resistant material, textiles, high-temperature superconductor, and conducting material [291,292,293,294]. Various physiochemical methods have been extensively used to prepare copper oxide nanoparticles [295]. However, these methods have some flaws, e.g., releasing different hazardous chemicals, time consuming, and high cost. Thus, there is a need for a simple, quicker, eco-friendly, and inexpensive method to prepare nanoparticles with phase selectivity, purity, and homogeneity in morphology [296]. Biological synthesized copper oxide nanoparticles exhibited excellent anti-microbial activity [297]. Green approaches have led to developing a simple, cost-effective, and environmentally friendly process for the biosynthesis of nanoparticles [298].

Synthesis of Copper and Copper Oxide Nanoparticles Using Leaves, Seeds, Flower, and Fruit Peel Extracts: (2019–2021)

Plants create various secondary metabolites and consist of phytochemicals, which are excellent bioresources for the fabrication of copper and copper oxide nanoparticles (Table 18). The most favorable phytochemicals in plants are flavonoids and phenols, present in various parts of the plant, i.e., stems, leaves, fruits, seeds, and flowers. These phenolic phytochemicals have ketone and hydroxyl groups, taking part in the iron chelation and subsequently describing an excellent antioxidant activity [299]. Nanoparticles synthesized through this green approach increase instability, fend off the deformation and agglomeration of nanoparticles, and increase the phenomena of adsorption of phytochemicals on the nanoparticles’ surface, which increase the reaction rate of nanoparticles [300]. One of the common approaches in preparing copper and copper oxide nanoparticles is mixing a stoichiometric concentration of plant extract to a stoichiometric concentration of copper salt, heating the nano solution to a suitable temperature, with contentious stirring (shown in Figure 8). The mechanism of formation of copper oxide nanoparticles is shown in Figure 9.

6.6. Iron or Iron Oxide Nanoparticles

The structure of nanoparticles contains the magnetic core and their combinations, which have magnetic features in the presence of a textan erior magnetic field. Various types of iron oxide nanoparticles, each with its peculiar properties, magnetic behavior, formulas, and applications. The magnetic behavior is because of the motion of electrons [319]. Depending on the response to an external magnetic field, there are six types of material: super magnetic, ferromagnetic, diamagnetic, paramagnetic, antiferromagnetic, and ferrimagnetic. Due to the presence of one electron in the third sub-shell of intermediate metals, e.g., cobalt, iron, and nickel, which is in the absence of an external magnetic field, ferromagnetism behavior is produced [320]. Magnetic property is also exhibited by Ferromagnetic material. Paramagnetic and magnetic phenomena are also reported among magnetic materials. Because of the superparamagnetic feature of the magnetic nanocatalyst, these nanoparticles have been utilized in various fields. These nanoparticles consist of gadolinium, nickel, cobalt, iron metal, and metal oxide, e.g., Fe₂O₃ [321]. Among different types of metal and metal oxide nanoparticles, iron and iron oxide nanoparticles have exhibited high efficiency in various biomedical and industrial applications. There are eight types of iron oxide nanoparticles, among which magnetite, hematite, and maghemite have very useful applicants. Each of these three oxides has specific catalytic, magnetic, and biochemical properties. Hematite is extensively utilized in pigments, catalysts, and catalysis. It is also a reagent for the preparation of magnetite and maghemite, which have been kept in sight for various applications.

Synthesis of Iron and Iron Oxide Nanoparticles from Leaf, Flower, Seed, and Fruit Extracts: (2019–2021)

Different and cost-efficient synthesis processes have been used by utilizing plants, as shown in

Table 19. Synthesis of Iron and Iron oxide nanoparticles from Leaf, flower, seed, and fruit extracts.

Sr. No	Reducing Agent	Part of Plant	Size	Iron/Iron Oxide NPs	Shape	Biological Activities	Year of Publication	Ref.
1	Romalina sinensis	Leaf	20–70 nm	Iron Oxide	Spherical	-	2021	[322]
2	Chlorophytum comosum	Leaf	100 nm	Iron	-	P. aeruginosa, E. faecalis, E. coli, and S. aureus. The prepared iron nanoparticles showed Methyl orange degradation (77% after 7 h)	2021	[323]
3	Laurus nobilis	Leaf	8.03 ± 8.99 nm	Iron Oxide	Spherical and hexagonal	E. coli (ZOI = No), L. monocytogenes (ZOI = 12 mm), S. aureus (ZOI = No), P. spinulosum (ZOI = 14 mm), and A. aspergillus (ZOI = 13 mm)	2020	[324]
4	Carica papaya	Leaf	-	Iron Oxide	-	S. aureus (ZOI = 14 mm), Klebsiella spp (ZOI = 9 mm), and E. coli (ZOI = 9 mm). Exhibited against BHK-21 and Hela cell lines	2020	[325]
5	Eucalyptus robusta	Leaf	-	Iron	-	S. aureus (ZOI = 1.15 ± 0.05 mm), B. subtilis (ZOI = 3.60 ± 0.40 mm), P. aeruginosa (ZOI = 29 ± 0.03 mm), and E. coli (ZOI = 1.10 ± 0.10 mm)	2020	[326]
6	Ruellia tuberosa	Leaf	52.78 nm	Iron Oxide	-	K. pneumonia (ZOI = 12 mm) and E. coli (ZOI = 17 mm)	2019	[327]
7	Avicennia marine	Flower	30–100 nm	Iron oxide	Honeycomb	-	2021	[328]
8	Punica granatum	Seed	25–55nm	Iron oxide	Semi spherical	Exhibited efficient degradation toward reactive blue (95.08% after 56 min)	2019	[329]
9	Borassusflabellifer	Seed	10–40 nm	Iron oxide	Hexagonal	At 50 (μg/mL) Concen of Fe2O3NPs B. subtilis (ZOI = 18 mm), E. coli (ZOI = 14 mm), S. aureus (ZOI = 11 mm), C. albicans (ZOI = 9 mm), and A. niger (ZOI = 9 mm) At 100 (μg/mL) Concen of Fe2O3NPs C. subtilis (ZOI = 24 mm), E. coli (ZOI = 14 mm), S. aureus (ZOI = 18 mm), C. albicans (ZOI = 10 mm), and A. niger (ZOI = 11 mm) At 500 (μg/mL) Concen of Fe2O3NPs B. subtilis (ZOI = 26 mm), E. coli (ZOI = 23 mm), S. aureus (ZOI = 20 mm), C. albicans (ZOI = 13 mm), and A. niger (ZOI = 15 mm)	2020	[330]
10	Iraqi grapes	Fruit	29–37 nm	Iron oxide	-	E. coli (ZOI = 19 mm) and S. aureus (ZOI = 18 mm)	2020	[331]
11	Cornelian cherry	Fruit	20–40 nm	Iron oxide	Spherical	-	2020	[333]

. Synthesis of iron or iron oxides nanoparticles using plants are showed in Figure 10. Arjaghi, Shayan Khalili, et al. synthesized the spherical iron oxide nanoparticles with a size range of 20–70 nm by utilizing Ramalina sinensis [322]. In another report, Chlorophytum comosum aqueous leaf extract was utilized for the biosynthesis of iron nanoparticles with a size of 100 nm and tested against P. aeruginosa, E. faecalis, E. coli, and S. aureus. The prepared iron nanoparticles showed Methyl orange degradation (77% after 7 h) [323]. Jamzad, Mina, et al. carried out an experiment to derive spherical and hexagonal iron oxide nanoparticles utilizing Laurus nobilis and tested them against E. coli (ZOI = No), L. monocytogenes (ZOI = 12 mm), S. aureus (ZOI = No), P. spinulosum (ZOI = 14 mm), and A. aspergillus (ZOI = 13 mm) [324]. In 2020, Bhuiyan, Md Shakhawat Hossen, et al. reported the biosynthesis of iron oxide nanoparticles by utilizing Carica papaya aqueous leaf extract and tested them against S. aureus (ZOI = 14 mm), Klebsiella spp (ZOI = 9 mm), and E. coli (ZOI = 9 mm). The prepared iron oxide nanoparticles were tested against BHK-21 and Hela cell lines [325]. Vitta, Yosmery, et al. achieved iron nanoparticles from Eucalyptus robusta aqueous leaf extract and tested their antimicrobial commotion against S. aureus (ZOI = 1.15 ± 0.05 mm), B. subtilis (ZOI = 3.60 ± 0.40 mm), P. aeruginosa (ZOI = 29 ± 0.03 mm), and E. coli (ZOI = 1.10 ± 0.10 mm) [326]. In 2019, iron oxide nanoparticles with an average size of 52.78 nm were synthesized utilizing Ruellia tuberosa aqueous leaf extract and tested against K. pneumoniae (ZOI = 12 mm) and E. coli (ZOI = 17 mm) [327]. In 2021, Avicennia marine aqueous flower extract was utilized to synthesize iron oxide nanoparticles with an average size of 30–100 nm [328]. Semi-spherical Iron oxide nanoparticles with a size range of 25 to 55 nm were prepared through a green process utilizing Punica granatum seed. The prepared iron oxide nanoparticles exhibited efficient degradation toward reactive blue (95.08% after 56 min) [329]. An eco-friendly biosynthesis of iron oxide nanoparticles utilizing Borassus flabellifer ethanol seed coat extract was reported by Sandhya et al. and was tested against B. subtilis, E. coli, S. aureus, C. albicans, and A. niger [330]. Aziz, Wisam J., et al. reported the biosynthesis of iron oxide nanoparticles by utilizing Iraqi grapes’ aqueous extract and tested against E. coli (ZOI = 19 mm) and S. aureus (ZOI = 18 mm) [331]. Rostamizadeh, Elham, et al. fabricated the iron oxide nanoparticles by utilizing Cornelian cherry aqueous extract [332].

6.7. Cobalt and Cobalt Oxide Nanoparticles

Cobalt is a transition (d-block) metal that has useful effects on human health [333,334]. It is an essential part of Cobalamin (Vitamin B12), which is helpful in the cure of anemia as it excites the production of red blood cells [334]. Cobalt has unique catalytic, electrical, and optical properties that make it favorable for a vast range of applications involving catalysts, nano-electronic devices, and nano-sensors [335]. Cobalt can show variable oxidation states, e.g., CO⁴⁺, CO³⁺, and CO²⁺, that make it favorable to be utilized in various fields [336]. Now, cobalt nanoparticles have attracted remarkable interest because they are cheaper than other metal or metal oxide nanoparticles and exhibit various properties, e.g., magnetic and electrical, because of their huge surface area [337,338].

Synthesis of Cobalt or Cobalt Oxide Nanoparticles Using Plants

Synthesis of cobalt and cobalt oxide nanoparticles by a green approach utilizing plants, in general, includes washing or drying the plants’ part (leaves, root, stem, flower, seed, and fruit), as shown in

Table 20. Synthesis of cobalt or cobalt oxide nanoparticles using plants.

Sr. No	Reducing Agent	Part of Plant	Size	Cobalt/Cobalt Oxide NPs	Shape	Biological Activities	Year of Publication	Ref.
1	Hibiscus rosa sinensis	Leaf	-	Co3O4	-	P. aeruginosa (ZOI = 20 ± 1.47 mm), E. coli (ZOI = 16 ± 1.61 mm), and Proteus vulgaris (ZOI = 21 ± 1.32 mm)	2021	[340]
2	Conocarpus erectus L	Leaf	4.9 nm	Co	Spherical	-	2021	[341]
3	Citrus medica	Leaf	100 nm	Co3O4	-	Degradation of methyl orange (90% after 1 h)	2021	[342]
4	Foenum-graceum L.	Leaf	13.2 nm	Co3O4	Quasi-spherical	-	2020	[343]
5	Populus ciliata	Leaf	-	Co3O4	-	At 2(mg/mL) Concen of CoNPs B. Lichenifermis (ZOI = 14.1 ± 0.4 mm), E. coli (ZOI = 1.10 ± 0.5 mm), B. subtilis (ZOI = 19.7 ± 0.4 mm), and K. pneumonia (ZOI = 12.8 ± 0.2 mm) At 4(mg/mL) Concen of CoNPs B. Lichenifermis (ZOI = 19.2 ± 1.2 mm), E. coli (ZOI = 15.1 ± 0.6 mm), B. subtilis (ZOI = 21.2 ± 0.5 mm), and K. pneumonia (ZOI = 17.8 ± 0.9 mm) At 2(mg/mL) Concen of CoNPs B. Lichenifermis (ZOI = 22.5 ± 0.9 mm), E. coli (ZOI = 16.0 ± 0.8 mm), B. subtilis ZOI = 24.5 ± 1.3 mm), and K. pneumonia (ZOI = 20.4 ± 0.7 mm)	2020	[344]
6	Selinum wallichianum	Leaf	-	Co	-	-	2019	[345]

. The plant materials (fresh or powder) are boiled with water and the resultant extract is filtered. Phytochemical properties are because of the presence of biomolecules in plant extracts, e.g., vitamin, phenol, protein, carbohydrate, flavonoid, and more. Adding cobalt salt to plant extract reduces and stabilizes the cobalt ion to prepare cobalt and cobalt oxide nanoparticles [339].

7. Antibacterial Activities of Metal and Metal Oxide Nanoparticles

Research is going on and a vast amount of literature exists on the antimicrobial activity of metal and metal oxide nanoparticles. The silver nanoparticles excellently discompose the polymer sub-units of the cell membrane in micro-organisms. The plant-mediated silver nanoparticles consequently rupture the cell membrane, destroying the protein synthesis mechanism in the bacteria [346]. The higher concentration of silver nanoparticles has rapid membrane permeability, as compared to a lower concentration, and subsequently breaks the cell wall of bacteria, as shown in Figure 11 [347]. The highest conductivity was seen in Rhizophora apiculate-reduced silver nanoparticles, which exhibited a lower number of a bacterial colony in the experimental plate than the silver nitrate-treated cells, which may be because of a larger surface area and smaller size of nanoparticles. These two factors increase the permeability across the cell membrane and cell destruction [348]. The green-synthesized silver nanoparticles were prepared using Citrus sinensis peel extract and evaluated for antibacterial activity toward S. aureus, P. aeruginosa, and E. coli [349].

8. Antifungal Activity

The fungicidal mechanism of plant-mediated metal and metal oxide nanoparticles has greater potential as compared to commercial antibiotics, e.g., amphotericin. The plant-mediated silver nanoparticles have clearly exhibited the membrane breakage in Candida sp. and damage in fungal components (intercellular) and, consequently, cell function was destroyed [350]. Most commercial drugs have limited clinical applications and have more adverse effects. Consequently, the commercial antifungal agents induce side effects, e.g., liver damage and renal failure and nausea, diarrhea, and body temperature increased after utilizing the drugs. The cell wall of fungi is made up of protein and fatty acid. The plant-mediated silver nanoparticles have promising activity toward spore-producing fungus and efficiently damage the fungal growth. In the fungal cell membrane composition and structure, significant changes were seen by interacting it with metal and metal oxide nanoparticles [351].

9. Anticancer Activity

Cancer is an uncontrollable cell proliferation, having extensive changes of enzymatic parameters and bio-chemicals, which is the universal behavior of cancer cells. The overexposure of cellular growth will be triggered, and the cell cycle mechanism in cancerous cells will be arrested by utilizing plant-based nanoparticles [352]. The plant-based metal or metal oxide nanoparticles have excellent effects on different cancer cell lines, e.g., Hela, Hep 2, and HCT 116 cell lines. To date, various works reported that plant-mediated nanoparticles have the ability to control cancer cell growth. The bettered cytotoxic effect is because of secondary metabolites and some other non-metal composition in the prepared medium [353,354]. The bio-synthesized silver nanoparticles triggered the cell cycle and enzymes in the bloodstream [355]. Furthermore, the plant-derived nanoparticles control the formation of free radicals from the cell. Free radicals normally are the cause of cell proliferation and harm normal cell function. The moderate quantity of gold nanoparticles is the cause of the apoptosis mechanism in tumor cells (malignant cells) [356]. The metal and metal oxide nanoparticles have proven their application in medical science to diagnose and cure different types of cancer cells. The plant-mediated nanoparticles are advanced and revolutionized to cure the malignant deposits without disturbing the normal cell line.

10. Challenge and Future Perspectives of Plant-Mediated Metal and Metal Oxide Nanoparticles

To date, various plants’ extracts have been investigated for the preparation of metal or metal oxide nanoparticles and have been excellently used in a wide range of applications due to their huge abundance in nature. Recently, plant extracts have been investigated for their effectiveness in the formation of nanoparticles, which are based on the compositions of diverse phytochemicals and plant sources. However, the specific phytochemicals causing the reduction, capping, and stabilization of nanoparticles in the green synthesis mechanism are still not completely understood. Thus, further studies are required to understand these details. A suggested approach that might be able to describe the responsible phytochemicals serving as stabilizing and reducing candidates involves isolation protocols of the pure compound to identify specific phytochemicals. Additionally, the composition of phytochemicals in plant extract can be determined through various analytical techniques, e.g., ICP-AES (Inductively coupled plasma—atomic emission spectroscopy), HPLC (High Pressure Liquid Chromatography), NMR (Nuclear magnetic resonance), and GC-MS (gas chromatography-Mass spectroscopy) and various quantitative and qualitative chemical processes can be utilized to know the variety of phytochemicals through Coomassie blue assays, phenol-sulfuric acid assays, and colorimetric assays. The main challenge may be in knowing the basic profile of bio-molecules needed for serving as reducing agents of metal ions. Despite the various benefits of plant extracts, there are many other hindrances that should be accounted for before they can be applied practically, e.g., structure, control of shape, size, monodispersity, and crystallinity of plant-mediated nanoparticles. This template morphology is also connected to the phytochemicals that exist in the plant extract. Additionally, some other factors affect the morphology of nanoparticles such as metal ion concentration, reaction temperature, plant extract concentration, and pH. Furthermore, as described, the capability to attain a high yield of nanoparticles is also influenced, as is the reduction power of the plant.

The stability of plant-mediated nanoparticles is another important parameter to consider. It is very necessary to ensure that plant-mediated nanoparticles can remain stable for a long time without any changes in morphology. Another condition that should be discussed is an estimation of toxicity and biocompatibility of plant-mediated nanoparticles to human health and the ecosystem, which are still not described efficiently and are frequently reported.

More integrated, detailed, and systematic research work is still needed to fully define the human and ecological toxicity profile of plant-mediated nanoparticles to develop a stable system for the preparation of nanoparticles with well-defined size, morphology, and efficient homogeneity.

11. Conclusions

Biosynthesis of metal and metal oxide nanoparticles has been advanced and is a highly attractive research field of science over the last decades. Thus, knowledge of green chemistry and the use of green routes for the synthesis of nanoparticles is increasing day by day in order to get an environmentally friendly process. Various types of natural extracts (such as plants, fungi, algae, and bacteria) have been utilized for the preparation of nanoparticles. Among all the above mentioned sources, plants have been considered to possess remarkable efficiency as capping, reducing, and stabilizing agents for the preparation of nanoparticles with desired morphology due to the presence of Phytomolecules. This review delivers an excellent platform to researchers or the scientific community to gain diverse information related to detailing green synthesis of metal or metal oxide nanoparticles using multiple plant parts. Fundamentally, the greener production of metal or metal oxide nanoparticles using plant extracts has different biological applications such as anticancer, anti-microbial, and antifungal activity.