**2. Biosynthesis of metallic nanoparticles**

Synthesis of nanoparticles generally involves two approaches: the top-down and bottom-up approaches. The top-down approach requires physical methods to disintegrate bulk materials into smaller units in the nano range. Typical methods such as electron beam lithography, laser ablation, spray pyrolysis, arc discharge, milling and so forth are physical methods used in a top-down approach. On the other hand, the bottom-up is simply a build-up of nanomaterials from smaller atoms/molecules through chemical or biological methods. In the chemical method, chemical substances

#### *Phyto-Metallic Nanoparticles: Biosynthesis, Mechanism,Therapeutics, and Cytotoxicity DOI: http://dx.doi.org/10.5772/intechopen.112382*

with reducing properties are used to convert the metallic salt solution to a nanoscale [14]. Commonly used chemical reducing agents are sodium citrate, Cetyltrimethylammonium bromide (CTAB), sodium borohydride (NaBH4), sodium tetrachloroaurate dihydrate salt (NaAuCl4.2H2O) etc. The physical and chemical methods are highly expensive and non-green methods resulting in increased environmental pollution and the release of toxic chemicals. Over the past few decades, there has been a surge in green chemistry through plant-based metallic nanoparticles research-based. The concept of green chemistry seeks to ensure the sustainable safety of humans, and the environment and process efficiency with the use of biodegradable and eco-friendly materials [15]. Thus, the biosynthetic route to PM-NPs helps to achieve this purpose with wide application in nanomedicine.

The biosynthesis of metal nanoparticles is a paradigm shift from the conventional physical and chemical methods due to their drawbacks. It is a biological method that applies green technology in the production of chemical substances using natural products of plants, microorganisms, and biomolecules from animals. The method has attracted huge attention as it displays efficient and effective utilization of the principle of green chemistry. Synthesis using plant extract and microorganisms is well documented with the advantage of being eco-friendly and does not involve the use of hazardous chemicals [2, 16]. However, the higher experimental cost and slow synthesis time due to microbial culture and growth, and the risk of infection with microorganisms are major concerns with the use of microorganisms. Biosynthesis through plant phytochemicals is relatively fast, safe, and suitable for large-scale synthesis [17]. Another biological method reported involves the use of natural products of animal origin. Although rarely used, this method involves biomolecules such as peptides or blood serum extracted from animals in nanoparticle synthesis [18, 19]. Animal blood serum consists of fibrinogen, globulin, albumin blood proteins and polypeptides that are biocompatible for use in the synthesis of NPs. These biomolecules are potentially reducing agents for metal ion reduction due to their involvement in oxidation/reduction reactions in animals. The redox properties, availability from slaughterhouses and their biocompatibility are reasons researchers used the blood serum as an alternative to the conventional synthesis of metallic nanoparticles [17].

In PM-NPs, extracts, and isolated bioactive compounds from plants serve as both reducing and stabilizing agents in place of chemical-reducing agents during biosynthesis. The effectiveness of biosynthesis with phytochemicals over non-biogenic synthesis was evidenced by Buono et al. [20] in the biogenic zinc oxide nanoparticles (ZnO-NPs) synthesis from the extract of *Lemna minor* (duckweed). The bioactive compounds of plants are mainly secondary metabolites (alkaloids, terpenoids, flavonoids, phenolics, steroids) that are not involved in the vegetative growth of plants but have an important function in the survival of plants as defense agents against herbivores, metal transporting agents, antibiotic agents, enzymes inhibitors etc. [3]. Activities of these secondary metabolites in green nanotechnology are reported to elicit bioreduction of metal ions to stable oxidation states through electron donations to metal ions to form stable atoms [14]. Because of this role of phytochemicals in green nanotechnology, much attention has been given to their extraction and isolation of the pure compounds.

#### **2.1 Extraction and isolation of phytochemicals**

Plant phytochemicals used for the synthesis of PM-NPs are mostly obtained from the leaves, stem, or tuber of the plant through decoction, infusion in water or aqueous ethanol [21]. The plant materials collected are cleaned of dirt and dried under shade for a few days until completely dried before use. The shade-dried material is ground to particulate matter to achieve complete extraction and a high percentage yield. The extract obtained is then sieved and filtered using Whatman No.1 filter paper. Further purification by centrifugation (600–800 rpm for 20 minutes), washing and filtration with an appropriate syringe filter give a pure extract of the plant. The freshly prepared aqueous extract can be made to powder by heating at 70°C or through lyophilization [5]. The above method is considered simple and conventional but with the limitation of involving the use of excess solvent and longer extraction time. The nonconventional methods for extraction of plant phytochemicals involve the application of improved technology like microwave, ultrasound, pressurized liquid, and enzymes assisted extraction [21, 22]. Both methods yield crude extract which serves as a bioreducing/capping agent for the synthesis of PM-NPs. To identify specific phytocompounds present in the crude extract and their roles in biosynthesis, the crude extract is subjected to further extractions/isolation and characterization procedures such as HPLC, LC-MS, Automated Flash chromatography, NMR etc. [3, 23]. The identification of the roles of pure compounds from a crude extract in nano synthesis is interesting to phytochemistry and plant enthusiasts. However, for cost consideration. a literature search of the chemical profile of the plant species and purchase of chemically synthesized compounds for use in synthesis is mostly employed.

#### **2.2 Phytochemicals in metallic nanoparticles synthesis**

The exploration of alternatives route to the synthesis of metallic nanoparticles used for nanomedicines lead to the discovery of plant species and phytochemicals having reducing potentials for metal ions. The phytochemicals for metallic nano synthesis could be derived from the whole plant material or specific parts such as the leaves, stem, flowers, seed, or root. Generally, plant species and their phytochemicals could be nano-active or inactive by showing potential for the bio-reduction of metal ions. As discussed in the introduction section, the ability of phytochemicals to reduce metal ions is usually first determined by the SPR of the bio-reduced metal measured by recording the UV-Vis scan ranging from 300 to 800 nm [15]. The frequency of SPR absorption depends on the metal ion involved, nanoparticle size, shapes, aggregation, and crystallinity of the nanoparticles. Generally, AuNPs exhibit SPR at 500–600 nm, AgNPs show absorption at 400–500 nm, PdNPs and PtNPs at 300–400 nm, and CuNPs (280–330 nm) [23, 24]. **Table 1** shows the SPR range, color changes and other properties of some reported PM-NPs. Measurement of SPR of the metal ion is followed by confirmatory analysis through physicochemical characterization by one or more of; energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transformed infra-red spectroscopy (FTIR), high-resolution transmission electron microscope (HRTEM), dynamic light scattering (DLS), thermogravimetry analysis etc. Several scientific reports have shown that nano-active plants consist of secondary metabolites rich in flavonoids and polyphenols [32, 33]. PM-NPs synthesis begins with the selection of potential plant species. This is usually through phytochemical screening of the plant species to identify the presence of secondary metabolites known with capping/reducing potentials of metal ions. The screening may be done through spectrophotometric assay for polyphenol and flavonoid content of the total extract or by reacting the total extract with specific reagents that give a characteristic color change to secondary metabolites [20, 34]. Del Buono et al. [20] showed

