3.1  Introduction

Nanomaterials are going to be ubiquitous in almost all sciences, technologies, and commercial products. Metallic nanoparticles are one of the most applied nanomaterials with diverse properties and functions. Unique physicochem-ical properties of these materials make them the arrowhead of applied nanotechnology. These particles are now being used in various fields such as biotechnology, chemical engineering (Ebrahiminezhad et al. 2017a, 2016i, Raee et al. 2018, Ranmadugala et al. 2017a, b, c, d, e, 2018, Viola et al. 2018), civil engineering (Seifan et al. 2017, 2018a, b, c) and medical sciences (Ebrahiminezhad et al. 2016a, e, b, 2012a, 2014b, 2015a). Also these compounds make a promising future in environmental sciences (Ebrahiminezhad et al. 2017e). Expansion in the applications of metallic nanoparticles resulted in the increasing demand for the production of these materials. Since now, physical and chemical processes, both in top-down and bottom-up approaches (Figure 3.1), have been well developed for the preparation of nanomaterials (Ebrahiminezhad et al. 2012b, 2013, 2014a). Bottom-up is the more accepted approach due to better control on the synthesis process and properties of the prepared nanomaterials. This approach is performed in a chemical process by using ionic precursors of intended metal (Ebrahiminezhad et al. 2017a, Taghizadeh et al. 2017). However, the process is usually multistep and requires harsh conditions with high energy consumption. Chemical approaches also employ organic solvents and toxic chemicals which raise environmental concerns (Ebrahimi et al. 2016a, b, Iida et al. 2007, Li et al. 2009, Mandal et al. 2005, Park et al. 2004, Si et al. 2004, Sun et al. 2007, Wang et al. 2001, Yu et al. 2006). In addition, remnants of chemicals and organic solvents as contaminants can limit biological and biomedical applications of the chemically synthesized nanostructures. Production of stable and biocompatible nanoparticles is another concern in chemical synthesis. From chemical and colloidal point of view, naked nanoparticles are not stable and need to be coated or capped with biocompatible and hydrophilic compounds.

Green chemistry has emerged as a new concept in chemistry and synthesis technologies to eliminate hazardous chemicals and organic solvents especially from industrial process. This technique is based on swapping chemical compounds and organic solvents with biochemical species and aqueous matrix, respectively (Ebrahiminezhad et al. 2016a, b, c). Plants and microorganisms are the most studied sources for biologic compounds in regard to green synthesis. These organisms can produce biochemical compounds which are capable to synthesize and stabilize metallic nanoparticles. Since now, various microorganisms including bacteria, fungi, cyanobacteria, microalgae, and yeasts have been used for the synthesis of metallic nanoparticles (Ali et al. 2011, Chokshi et al. 2016, Ebrahiminezhad et al. 2016a, 2017h, Jena et al. 2015, Kannan et al. 2013, Kathiraven et al. 2015, Kianpour et al. 2016, Mohseniazar et al. 2011, Prasad et al. 2013, Sinha et al. 2015, Xie et al. 2007a, b). Both intracellular and secretory compounds were shown to be effective in this regard (Ebrahiminezhad et al. 2016a, Xie et al. 2007a, b). Using microorganisms for the production of nanoparticles has some major drawbacks. Most of the employed organisms are opportunistic or pathogenic, and nanoparticles that derived from such sources cannot be approved for pharmaceutical and biomedical applications. On the other hand, microbial cells usually grown in the complex and expensive media through labour-intensive and time-consuming process (Ebrahiminezhad et al. 2017h, Kianpour et al. 2016, Raĭkher et al. 2010). Disposal of culture wastes such as produced biomass and waste media is the other problem.

Figure 3.1   Top-down and bottom-up approaches in the fabrication of nanoparticles.

To eliminate technical problems in microbial synthesis, plant-mediated synthesis is a promising substitution toward green and economic synthesis of nanoparticles. Now various plants, such as trees, flowers, and herbs, are being used for this purpose (Cruz et al. 2010, Ebrahiminezhad et al. 2016b, c, Mo et al. 2015). But herbs are among the widely used ones due to being cheap, abundant, ubiquitous, safe, and rich in plenty of organic and reducing compounds which are effective for nanoparticles synthesis. Most importantly, using herbs in the production process resulted in the fabrication of particles with unique properties (Ebrahiminezhad et al. 2016c, 2017e). All parts of plants can be applied in the green synthesis of nanoparticles, considering leaf extract being the mostly used one. For instance, leaf extract of eucalyptus, black and green tea, Lippia citriodora (Lemon Verbena), maple (Acer sp.), Lantana camara, Artemisia annua, and Ephedra intermedia, and Mediterranean cypress (Cupressus sempervirens) were used for the synthesis of different metal nanoparticles (Ajitha et al. 2015, Basavegowda et al. 2014, Cruz et al. 2010, Ebrahiminezhad et al. 2016f, 2017e, Mo et al. 2015, Wang et al. 2014b, Xiao et al. 2015). Other parts of plants such as flower extract of Matricaria chamomilla, stem extract of Desmodium gangeticum, coffee powder extract, Piper longum and Crataegus douglasii, fruit extract, orange peel extract, Cinnamon zeylanicum bark extract, seed exudate from Sterculia foetida and Medicago sativa, and oil of Plukenetia volubilis L. were also used (Azad and Banerjee 2014, Ebrahiminezhad et al. 2016c, Ghaffari-Moghaddam and Hadi-Dabanlou 2014, Kahrilas et al. 2014, Kumar et al. 2014c, d, Lukman et al. 2011, Nadagouda and Varma 2008, Rajasekharreddy and Rani 2014, Reddy et al. 2014, Sathishkumar et al. 2009).

In this chapter, we discuss the plant-mediated green synthesis of nanoparticles in detail with focus on the molecular mechanism, effective parameters, and how to boost the synthesis reaction. Finally, there is a review on the plant-mediated synthesized nanoparticles and their characteristic features.

3.2  Mechanism of the Green Synthesis

Plant extracts are commonly rich in phytochemicals which are able to act as reducing agent and convert metal ions to metal nanoparticles. There is no exact knowledge about the effective molecules in the reduction of metal ions. But, some investigations have shown that oxygen-bearing functional groups such as hydroxyl, phenol, carboxyl, and carbonyl are the sites for entrapment and reduction of metal ions (Kozma et al. 2015). It is evident that plants with high contents in phenolic compounds are the best choice for green synthesis purposes. Phenolic compounds from Salvia officinalis, Zataria multiflora, Melaleuca nesophila, Eucalyptus tereticornis, Rosmarinus officinalis, Mansoa alliacea, and green tea were found to be the effective molecules in the formation of nanoparticles (Markova et al. 2014, Prasad 2016, Soliemanzadeh et al. 2016, Wang et al. 2015, 2014c). But some of these studies, especially those that examined the green tea extract for the synthesis of iron-based nanoparticles, have claimed that polyphenol compounds chelate iron ions to form nanostructures of iron complexes (Markova et al. 2014, Wang et al. 2015, 2014c).

Naked nanoparticles are chemically and colloidally unstable and should be coated or capped with a protective and hydrophilic compound (Ebrahiminezhad et al. 2012b). Also, the coating should be biocompatible if it is planned to apply nanoparticles in biological and biomedical fields. In the common chemical synthesis procedure, naked nanoparticles are coated by using biocompatible coatings such as polymers, carbohydrates, amino acids, lipoamino acids, and silica (Ebrahiminezhad et al. 2012b, 2013, 2014b, 2015a, Gholami et al. 2016, 2015, Gupta and Wells 2004, Muddineti et al. 2016, Ragaseema et al. 2012). In addition to fabricate nanoparticles, plant extracts provide stabilizing or capping agents which can engulf synthesized nanoparticles and make them colloidally and chemically stable (Ebrahiminezhad et al. 2016f, 2017c, f). Also, microparticles in the plant extract can act as a platform for the formation and stabilization of nanoparticles (Ebrahiminezhad et al. 2016g). Some evidences have shown that carbohydrates are the molecules that act in the way to engulf and stabilize nanoparticles (Ebrahiminezhad et al. 2016c, f, 2017g).

3.3  Effective Parameters in the Green Synthesis

3.3.2  pH of the Plant Extract

The pH of reaction solution is always critical factor in the chemical and biochemical reactions. For instance, in the chemical synthesis of magnetite nanoparticles, it is critical to raise the reaction pH near 9–11 to start the formation of magnetite nucleus and growth of magnetite nanoparticles. This extreme pH is usually obtained by using ammonium hydroxide and some other basic compounds such as KOH. Green synthesis is not exceptional, and the pH of the synthesis reaction has an immense effect on the prepared nanoparticles. It has been shown that plant extracts with different pH values provide particles with different characteristics. Acidic extracts provide smaller and more stable nanoparticles in contrast to less acidic or neutral extracts (Makarov et al. 2014b, Njagi et al. 2011). It is believed that capping of the prepared nanoparticles with organic acids such as oxalic acid or citric acid from plant extract resulted in the increase in the zeta potential of prepared particles. It is worth to mention that the zeta potentials more than 25 mV (negative or positive) enhance the colloidal stability of nanostructures (Ebrahiminezhad et al. 2016a).

Figure 3.2   Schematic illustrations for the green synthesis of metallic nanoparticles.

3.4  Assisted Green Synthesis of Nanoparticles

Green synthesis of nanoparticles can be assisted chemically or physically. In some cases, natural compounds from plant extract have no sufficient power to fabricate nanoparticles, and hence, you need an external power to complete the job. Also, it is possible that you want to fabricate nanoparticles with special characteristics that cannot be achieved simply by using a plant extract. In some cases, plant extract is used just as a source of natural coating materials and capping agents, and hence, the formation of nanoparticles should be driven by another source. This external source can be a chemical agent such as a powerful reducing agent or agents that are used for the chemical synthesis of nanoparticles. Also, fabrication of nanoparticles can be enhanced by using physical treatments, mostly irradiation. In the following, we explain these two techniques for the assisted green synthesis of nanoparticles.

3.5  Additives in Green Synthesis

The major disadvantage in the plant-mediated synthesis is anisotropic and non-uniform growth of the particles. Hence, one of the recent innovations to improve quality of the green synthesized nanoparticles is to employ some additional molecules (additives) such as surfactants (Khan et al. 2012a, b). The presence of various additive compounds in the reaction mixture has different effects on the synthesized nanoparticles such as creating desirable shape, capping the particles, increasing stability, and size control (Cao et al. 2016, Pandian and Palanivel 2016).

β-Cyclodextrin (βCD) is one of the well-employed molecules in this order. Zhuang et al. (2015) investigated the effect of βCD on morphology and reactivity of synthesized nanoparticles. They indicated that the addition of βCD to the reaction mixture resulted in a decrease in the particle size from 60 to 20 nm (Zhuang et al. 2015). In another study, it has been shown that Ag nanoparticles with desirable morphology can be obtained by adding suitable amount of cetyltrimethylammonium bromide (CTAB) to the synthesis reaction. In addition to shape directing role, CTAB is also able to increase the reactivity of phytochemicals to reduce Ag⁺ ions (Khan et al. 2012b). All surfactants cannot play controlling role in the green synthesis reaction. For instance, sodium dodecyl sulphate (SDS) has no significant effect on the green synthesis of silver nanoparticles by using Paan (Piper betle) leaf petiole extract (Khan et al. 2012a).

3.6  Iron Nanoparticles

Iron-based nanoparticles are one of the most studied and employed nanostructures in various fields of science and technology. Naturally, iron has different states of oxidation with different physicochemical properties. So, nanostructures of iron in various oxidation states pose unique properties which make them suitable for specific applications. Since now, a lot has been done to fabricate iron-based nanoparticles using green synthesis approach as tabulated in Table 3.1. Attempts for the green synthesis of various iron-based nanoparticles are explained in the following sections.

3.7  Silver Nanoparticles

Silver (Ag) nanoparticles are one of the most studied and applied nanoparticles in science and technology. Silver has gained historical applications in the human communities since ancient time (Ebrahiminezhad et al. 2016e). As a potent antimicrobial agent, the silver cations were used in burns, wounds, and ulcer treatments. As early as 4,000 B.C.E, silver was known to the Chaldeans. Silver colloids have gained medical applications even before discovering microbial life as the causative agent for infectious disease. Romans and Egyptians were found silver compounds as functional food preservative. Also, Hippocrates and Macedonians employed silver to prevent and treat wound infections. They found silver-based compounds as potent compound not just against infections but also as a wound-healing agent (Ebrahiminezhad et al. 2016e). Discovering and application of antibiotics as a potent antimicrobial agent, in the early 1940s, with very light side effects drove ancient silver away from the spotlight. Nowadays by emerging resistance strains, silver is back in a novel structure known as “nanosilver”. Vast investigations about the biological and antimicrobial properties of nanosilver, the mechanism behind it, and the fabrication of silver nanoparticles have been done (Ebrahiminezhad et al. 2016a, e). Like other nanostructures, the physical and chemical approaches were the first methods for silver nanoparticles fabrication (Ebrahimi et al. 2016a, b). But, by increase in the commercial and scientific applications of silver nanoparticles, the demand for this nanostructure was increased significantly. Hence, the production techniques were shifted to the sustainable process and so many investigations were done to fabricate silver nanoparticles in green manner using microorganisms and plants (Ebrahiminezhad et al. 2016a, b).

Since now, a lot has been done to facilitate the production of silver nanoparticles by plant-mediated synthesis (Table 3.2), and diverse parts of plants have been used for this purpose. Leaf extract is the most employed part, and the potential of various plant leaf extracts such as Artemisia annua, Lippia citriodora (Lemon Verbena), Lantana camara, Zataria multiflora, Cupressus semper-virens (Mediterranean Cypress), maple (Acer sp.), green and black tea, and eucalyptus has been evaluated (Ajitha et al. 2015, Basavegowda et al. 2014, Cataldo 2014, Cruz et al. 2010, Ebrahiminezhad et al. 2016g, Manjamadha and Muthukumar 2016, Mo et al. 2015, Nakhjavani et al. 2017, Vivekanandhan et al. 2014). In addition to leaves, the extract of plant other parts was also employed such as Alcea rosea and Matricaria chamomilla flower extract (Ebrahiminezhad et al. 2016b, c), Ephedra intermedia stem extract (Ebrahiminezhad et al. 2016f), Cinnamon zeylan-icum bark extract (Sathishkumar et al. 2009), and Curcuma longa tuber extract (Sathishkumar et al. 2010).

Very little work has been performed to understand the exact mechanism behind the plant-mediated synthesis of silver nanoparticles. Various biomolecules including enzymes, proteins, carbohydrates, vitamins, alkaloids, phenolic acids, terpenoids, and polyphenols are the main components in plant extracts that play an important role in the bioreduction of silver ions to silver nanoparticles (Ebrahimi et al. 2016a, b, c, f, g, 2017c). Silver nanoparticles have a high surface energy state that makes them less stable and prone to aggregation in colloidal systems. Functional groups of the plant metabolites such as carbonyl groups have the potential to bind to the silver nanoparticles and prevent agglomeration. Thus, it is suggested that the biological components of plant extracts could be involved in reduction, formation, and stabilization of silver nanoparticles (Ahmed et al. 2016, Makarov et al. 2014a).

It has been shown that some factors including plant source, concentration of initial silver ions, and reaction conditions such as temperature, pH, and time are the key factors in the biosynthesis of silver nanoparticles using plant extracts. These factors have significant impacts on physicochemical characteristics of the produced particles (Chung et al. 2016).

3.8  Gold Nanoparticles

The simplicity of using plant extract for reducing a metal salt has also led to a massive investigation for the fabrication of gold nanoparticles. Apart from the applications in DNA labelling, drug delivery, and biosensors, gold nanoparticles have been known for the antimicrobial activity against human and animal pathogens (Mittal et al. 2013). As shown in Figure 3.3, gold nanoparticles can also serve as catalyst for the reduction of nitro compounds such as 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) (Ghosh et al. 2012). A single-step procedure for plant-mediated gold nanoparticles preparation has attracted much attention as it is a rapid, cost-efficient, environment-friendly, and safe for clinical practices. In this approach, the plant extract is responsible for the bioreduction of Au ions, their growth, and stabilization (Mittal et al. 2013).

Many phytochemical compounds have been reported to be present in different parts of plants which can act as a reducing agent for the reduction of Au³⁺ ions to gold nanoparticles. Gold nanoparticles can be synthesized into various shapes such as nano-sphere, nano-rod, nano-star, nano-triangular, nano-cage, nao-prism, nano-plate, and nano-belt. Variation in the shape and size of the prepared particles results in the production of nanoparticles with different physicochemical properties (Ganeshkumar et al. 2012). Shape and size are among the most important features for gold nanoparticles to modulate the optical properties in electronic and sensor applications and to minimize any potential toxic side effects in biomedical and biological purposes (Shankar et al. 2005). Huang et al. (2007) reported that Au ions can be reduced to triangular or spherical shaped nanoparticles using leaf extract of Cinnamomum camphora at ambient temperature. It was noted that the formation of nanoparticles by C. camphora strongly relies on the dosage of the plant extract dried biomass. Biochemical compounds such as terpenoids, flavones, and polysaccha-rides in the C. camphora biomass can act as capping agent for the fabricated nanoparticles. Once the biomass concentration increased from 0.5 to 1 g, the interaction between biomolecules present in plant extract and the surface of nanoparticles led to a size reduction. Since now, various parts of different plants such as root, pod, peel, leaf, tuber, flower, fruit, and seed have been used for the green synthesis of gold nanoparticles. These data are provided in detail in Table 3.3 along with the resulted particles size and shape.

Figure 3.3   Catalytic activity of gold nanoparticles for reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP).

3.9  Zinc Oxide Nanoparticles

Among metallic nanoparticles, zinc oxide (ZnO) nanoparticles have attracted much attention because they are non-toxic and hygroscopic. These properties make them a potent material for applications as a catalyst in many organic transformations and in optics and sensors (Madhumitha et al. 2016, Roopan and Khan 2010). In this section, the plant-mediated synthesis of ZnO nanoparticles is discussed. Zinc salts such as zinc nitrate, zinc acetate, and zinc sulphate are the most applied zinc precursor. Biochemical reduction of zinc ions starts when the plant extract is mixed with zinc salt aqueous solution and a change in the colour of the reaction mixture is an indication of nanoparticles formation. Once the nanoparticles formed, they are harvested by using centrifugation and kept in an oven for drying.

The size and the morphology of fabricated nanoparticles are among the most important features that can influence their effectiveness in different applications. Table 3.4 illustrates the characteristics of ZnO nanoparticles fabricated using different plant parts. Similar to the chemical approaches for the nanoparticles synthesis, there are different factors that can affect the nanoparticles size. The morphological structure of nanoparticles has a significant effect in controlling the physical, chemical, electrical, and optical characteristics of them (Edison and Sethuraman 2012). It has been reported that the concentration of plant extract as well as the reaction temperature, pH, and time plays a key role in changing of the nanoparticles size, morphology, and quality. In this context, the scientists have attempted to modify the biosynthesis process and optimize the nanoparticle synthesis process by determining the optimal levels of effective variables. For instance, it was shown that the increase in the concentration of plant extract results in a reduction of nanoparticle size (Fu and Fu 2015). Anbuvannan et al. (2015a) found that the increase in the concentration of Anisochilus carnosus leaf extract from 30 to 50 mL leads to a 31% decrease in particle size while having no significant effect on the morphology of fabricated particles. The same observation was noticed by Elumalai et al. (2015) when the authors used various concentrations of leaf extract from Vitex trifolia. Due to the polarity and electrostatic attraction, the nanoparticles tend to agglomerate when a lower concentration of plant extract as a capping agent is used. However, the binding of smaller size nanoparticles which causes the formation of secondary bulk nanoparticles is responsible for agglomeration at the higher volume of leaf extract. Likewise, studies have shown that the changes in the pH of reaction solution result in the fabrication of nanoparticles with different shapes and sizes. It has been reported that a lower pH contributes to the commemoration of large particles (Dubey et al. 2010c). A higher pH has reported as a favourable condition for the formation of smaller metallic nanoparticles (Armendariz et al. 2004). The variation in particle size is due to the accessibility of functional groups at different pH levels during particles nucleation (Shah et al. 2015). The reaction time is another factor that influences both the particle size and the reaction yield. It has been noted that the increase in the reaction time results in a higher production of nanoparticles and this can increase the likelihood of agglomeration (Veerasamy et al. 2011). On the other hand, no significant reduction of precursor solution occurs in the early reaction time (Ahmad et al. 2016), while maintaining the reaction contributes to the further growth of particles and subsequently, a larger particle size is produced. The reaction temperature has also a significant effect on the nanoparticles size. It was found that the nanoparticles synthesis rate increases at elevated temperatures (Song et al. 2009). The authors noted that 100% of precursor converted to nanoparticles at 95° C in 5 min which this value was significantly higher than synthesis at room temperature at a given time.

3.10  Copper Nanoparticles

Copper nanoparticles show catalytic, antioxidant, antibacterial, and antifungal activities (Din et al. 2017). Leaves, barks, seeds, peels, fruits, roots, coir, and gum have been used as different parts of plants for the green synthesis of copper nanoparticles. Based on the literature, different aqueous solutions of precursors such as copper chloride, copper nitrate, copper sulphate, and copper acetate have been used for the preparation of copper nanoparticles. The reaction between plant extract and copper salt under a strong stirring condition at a controlled pH and temperature results in the formation of copper nanoparticles.

Table 3.5 shows the various sources of plant extract and reducing agents for the biosynthesis of copper nanoparticles. It has been reported that Calotropis procera L. latex is rich in protein including antioxidant enzymes (AOEs), cysteine protease with free thiol (–SH) group, and tryp-tophan (Freitas et al. 2007). Therefore, its extract can be used for the green fabrication of nanoparticles. The Calotropis procera L. latex-mediated synthesis of Cu NP was performed in the presence of copper acetate at room temperature and the particles subjected to characterization study (Harne et al. 2012). The characterization results showed monodisperse nanoparticles with an average diameter of 15 ± 1.7 nm. It was also found that the capping with latex proteins brings about a long-term stability of nanoparticles in an aqueous medium. In another investigation, the extract of Henna (Lawsonia inermis) leaves as a source of lawsone (2-hydroxy-1,4-naphthoquinone, C10H6O3), gallic acid, glucose, mannitol, fats, resin, mucilage, and alkaloids was used for copper nanoparticles synthesis (Cheirmadurai et al. 2014). Similarly, the extract obtained from dried leaves of Ginkgo biloba L. was identified as a source of polyphenolics and shown potential for the biosynthesis of copper nanoparticles (Nasrollahzadeh and Mohammad Sajadi 2015). The particles were synthesized upon the addition of Ginkgo biloba L. extract to copper chloride solution at a temperature of 80°C and pH of 9 for 30 min under vigorous shaking condition. Authors noted that the fabricated nanoparticles were stable for one month and had a narrow particle size distribution ranging from 15 to 20 nm. Copper nanoparticles were also synthesized using the aromatic dried flower buds of Myrtaceae tree (Syzygium aromaticum) (Subhankari and Nayak 2013). The copper precursor (5M copper sulphate) and plant extract were incubated for 1 h to allow the formation of nanoparticles nuclei. The characterization study was then performed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and a particle size of 14–50 nm was noticed. Depending on the types of employed copper salts, different particle size and shape have been reported. For instance, Shanker and Rhim (Shankar and Rhim 2014) observed various particle shapes when they added different precursor salts into a solution containing ascorbic acid as a reducing agent. The presence of copper acetate resulted in the formation of rod shape nanoparticles, while the addition of copper chloride and copper sulphate led to triangular and spherical shape, respectively. In another study, it was shown that the dropwise addition of olive tree (Olea europaea) leaf extract into an aqueous solution of copper sulphate at 100°C for 24 h results in the production of copper oxide (CuO) nanoparticles (Sulaiman et al. 2018a). The synthesized CuO nanoparticles showed a peak absorbance at 298 nm in UV-vis spectroscopy analysis, and the particles had a crystalline nature with a diameter of 20 nm.

3.11  Aluminium Oxide Nanoparticles

Aluminium oxide (Al₂O₃) nanoparticles are the other materials that have been used for diverse applications such as biomedical, antigen and drug delivery, biofiltration, and biosensors. These particles are biocompatible, and chemically inert and stable, and can be readily functionalized in the surface (Jalal et al. 2016, Saber 2012). The main attributes of plant-mediated synthesized Al₂O₃ nanoparticles are the exploitation of cost-and energy-efficient approach without using expensive equipment and toxic chemicals that are being used in chemical and physical methods.

Various sources of stabilizing agents and precursors for the green production of Al₂O₃ nanoparticles are given in Table 3.6. The reactions for the plant-mediated synthesis of Al₂O₃ nanoparticles with the help of heating are presented in the Eqs. 3.6 (Jalal et al. 2016). According to Eqs. 3.4, aluminium oxide-hydroxide (boehmite, AlOOH) can be synthesized by heating aluminium nitrate at different temperatures, and the generated AlOOH is used for the fabrication of stable Al₂O₃ nanoparticles. The lemon-grass extract is a source of carboxylic acids (HO₂CR), and its reaction with AlOOH results in the production of carboxylate-functionalized Al₂O₃ nanoparticles (carboxylate-alumoxane, Eq. 3.5). As shown in Eq. 3.6, a stable form of Al₂O₃ (α-phase) is then produced upon the calcination of carboxylate-alumoxane at a temperature of 1100°C for 3 h.

3.12  Titanium Dioxide Nanoparticles

Due to their physicochemical and biological properties, titanium dioxide (TiO₂) nanoparticles have a wide range of applications. For instance, their potential oxidation strength, high photo stability, non-toxicity, and antibacterial activities make them useful in air and water purification as well as dye-sensitized solar cells (Mishra et al. 2014). In continuation of efforts for the synthesis of TiO₂ nanoparticles using chemical and physical methods, a greener approach using the plant extract has been tested. Table 3.7 shows the characteristics of TiO₂ nanoparticles fabricated using different plant parts. Sundrarajan and Gowri (2011) used Nyctanthes leaf extract due to its functional antioxidant, antifungal, and antimicrobial activities for the fabrication of TiO₂ nanoparticles. The leaf powder was obtained by grinding of shade-dried leaves. The plant extract was then achieved by mixing the leaf powder and ethanol under reflux condition at a temperature of 50° C for 5 h. The TiO₂ NPs were obtained upon the reaction of ethanolic leaf extract and titanium tetraisopropoxide at 50°C. After 4 h of vigorous stirring, the nanoparticles were formed and the separated particles were subjected to a calcination process at 500° C for 3 h. Their characterization data show that the resulted nanoparticles were crystalline with a spherical shape ranging from 100 to 150 nm. In another investigation, Calotropis gigantea flower extract and TiO(OH)₂ were employed as a stabilizing and precursor agent, respectively (Marimuthu et al. 2013). The mixture of plant extract and TiO(OH)₂ was kept stirring for 6 h and then subjected to ultrasonication for 30 min prior to nanoparticle separation. The results from the analyses show the fabricated nanoparticles were spherical with an average size of 160-220 nm. Rajakumar et al. (2012) proposed Eclipta prostrata for the biosynthesis of TiO₂ nanoparticles. The nanoparticles were synthesized upon the reaction of E. prostrata and TiO(OH)₂ under ambient temperature for 24 h. The SEM micrographs showed poorly dispersed spherical clusters with agglomeration size up to 95 nm. Few years later, the authors used the same protocol and precursor with a different plant extract “Mangifera indica L.” for the biosynthesis of TiO₂ nanoparticles (Rajakumar et al 2015) Their results show that the utilization of Mangifera indica L. significantly contributed to decreasing the particle size. The characterization data show that the prepared nanoparticles had a spherical shape with an average size of 30 ± 5 nm.

3.13  Conclusion and Perspective

Plant-mediated synthesis is now a growing field in the synthesis of nanostructures. Since now, there is no detailed knowledge about the exact mechanism in the green synthesis reactions. In the case of some nanoparticles such as silver and gold nanoparticles, it is revealed that the reaction is a reduction reaction. This reaction is mediated by oxygen-bearing functional groups in the phytochemicals such as phenolic compounds. Iron complex nanoparticles are formed by chelating ferrous ions in the polyphenol compounds from plant extract. But, still there are many undiscovered areas in this field that needs to be explored. Questions such as the effective molecules in the green synthesis reactions, shape directing functional groups and biomolecules, interactions of phytochemicals with plant-mediated synthesized nanostructures, and effects of plant extract preparation on the synthesis reaction need to be answered. Nevertheless, green synthesis is now one of the most economic, environment-friendly, and facile techniques in the fabrication of nanostructures. Soon, it is expected to be one of the main approaches for the large-scale production of nanostructures worldwide.