Biological Synthesis of Metallic Nanoparticles from Different Plant Species

Green chemistry for the synthesis of different nanoparticles (NPs) from metal has become a new and promising field of research in nanotechnology in recent years. The inspire applications of metal oxide NPs have attracted the interest of researchers around the world. Various physical, chemical and biological methods in material science are being adapted to synthesize different types of NPs. Green synthesis has gained widespread attention as a sustainable, reliable, and eco-friendly protocol for biologically synthesizing a wide range of metallic NPs. Green synthesis has been proposed to reduce the use of hazardous compounds and as a state of a harsh reaction in the production of metallic NPs. Plants extract used for biosynthesis of NPs such as silver (Ag), cerium dioxide (C 2 O 2 ), copper oxide (CuO), Gold (Au), titanium dioxide (TiO 2 ), and zinc oxide (ZnO). This review article gives an overview of the plant-mediated biosynthesis of NPs that are eco-friendly and have less hazardous chemical effects.


Introduction
In the last decade, novel synthesis methods for nanomaterials such as quantum dots (QDs), carbon nanotubes (CNTs), graphene, and their composites have been an interesting area in nanotechnology [1]. Despite the progress of the use of small metallic materials listed as nano, the debate still continues in many aspects associated with this new technological revolution. The conceptual beginnings of green chemistry and nanotechnology are among the great scientific developments that have influenced the design of experiments with the goal of environmental protection. Now the reduction in size of green chemistry and nanotechnology is one of the great scientific developments which are to preserve the environment with experiments [2]. Nanoparticles are the most fundamental component in the making of nanostructures. They are much smaller than the everyday objects around us governed by Newton's laws of motion but larger than an atom or a simple molecule that is the subject of quantum mechanics [3]. Nanoparticles exhibit specific properties that depend on their shape, size, and morphology and enable them to interact with plants, microbes, animals [4]. Nanoparticles are a subclass of ultrafine particles with a length of more than 1 nm and less than 100 nm in two or three dimensions and which cannot exhibit size-related depth properties

Plant-based biosynthesis of metallic NPs
Many researchers have discovered many plant species and components that contain antioxidant compounds such as nitrogenous base amino acids, polyphenols, and sugars [15]. These compounds perform the function of a capping agent for the synthesis of nanoparticles [16]. Generally metal nanoparticles are manufactured in two ways such as bottom-up (fabrication of material from atom to bottom atom) and top to bottom (cutting of bulk material to obtain nano-sized particles). Metal and metal oxide NPs made from plant extract are generally stable even after 1 month and do not undergo any change [17]. Green synthesis of various metallic nanoparticles has reevaluated plants for their natural ability to reduce toxic and hazardous chemicals (Figure 1). Nanoparticles of plant-related parts such as leaves, stems, flowers, bark, roots, seeds, and their metabolites have been used for biosynthesis [19]. Generally, there are three aspects that need to be considered for green biosynthesis solvent medium, non-toxic ion reducing agents, and environmentally safe NPs stabilizers [20].

Biosynthesis of silver (Ag) nanoparticles
Plants have the ability to store heavy metals in their various parts. As a result, biosynthesis techniques employing plant extracts have produced a simple means to stabilize efficient growth, simple, cost-effective synthesis process, along with traditional preparation methods for the production of NPs [21]. Silver NPs have been synthesized and studied extensively due to their unique chemical, physical and

Biosynthesis of cerium dioxide (CeO 2 ) nanoparticles
Plants produce biologically active compounds to protect themselves. In plants, it is well organized and unique with various valuable metabolites that use phytochemical biologically active substances. These biocompounds are responsible for the reduction of metal ions during the synthesis of NPs [55]. Cerium dioxide has high catalytic properties due to their broad band gap energy and binding energy, indicating many applications [56]. Depending on the chemistry, the reaction kinetics are affected by the reaction temperature, and this determines the time that most of the temperature difference occurs as a result of changes in the synthesis of NPs [57]. Various compounds such as cerium nitrate, cerium acetate, and cerium chloride have been used as synthesis precursors, and extracts of several plant parts such as

Biosynthesis of copper oxide (CuO) nanoparticles
The parameters of plant extracts such as phytochemicals, metal salt concentration, temperature and pH control the rate of formation of nanoparticles as well as their stability and yield [59]. Extracts of plant leaves are considered a good source for metal and metal oxide NPs synthesis. Additionally, plant leaf extracts play a greater role as reducing and stabilizing agents of the biosynthesis process, the phytochemical composition of plant leaf extract is also an important factor in the synthesis of CuO NPs [60]. Green synthesis of Cu NPs can reduce the use of many hazardous chemical substances; many methods use L-ascorbic acid as a reducing agent in synthesis, and the synthesized Cu NPs were highly stable (

Biosynthesis of gold (Au) nanoparticles
Water is the most accessible and cheapest solvent on earth. Since the advent of nanotechnology, water has been used as a solvent for the synthesis of various NPs [62]. Most research on the biosynthesis of NPs was done from angiosperms using plant extracts, mainly in gold NPs synthesis. Mechanisms of metal depletion and stabilization have also been explored by phytochemicals of plants. They can be helpful in forming NPs [63]. Plant extracts showed the green synthesized NPs to be effective in ions reduction. High variability suggests that this may be one of the reasons that produced large amounts of Au NPs using plant extract (

Biosynthesis of titanium dioxide (TiO 2 ) nanoparticles
The high prevalence of TiO 2 NPs is due to their many versatile applications resulting from the stability of chemical structure, optical, electrical, and physical properties. These characteristics offer a wide range of TiO 2 is present in three different mineral forms such as anatase, rutile, and brocite. It is generally preferred due to its photocatalytic activity [9]. The synthesis of TiO 2 NPs by plant leaf extracts is mixed with metal precursor solutions in various reactions and temperature conditions [65]. The high production and widespread use of TiO 2 NPs have led to little effort for biogenic production. Several plant species have been investigated for the production of TiO 2 NPs. Several processes have been developed in the last two decades to synthesize biogenic NPs. These methods effectively control the properties of NPs in which most of the potential toxicity materials are used ( Table 1) [66].

Biosynthesis of zinc oxide (ZnO) nanoparticles
The biosynthesis of ZnO NPs an alternative to physical and chemical methods for the formation of NPs, it is representing an area of significant discovery for a wide range of applications (Figure 2) [73]. In recent times, the green method for the synthesis of NPs has become an area of great interest in this direction because the use of conventional chemical methods is very expensive, and as a reducing agent of chemical compounds or organic solvents use is required which produces toxicity [74]. Various studies where the plant-based synthesis of ZnO NPs from various zinc compounds was made using extracts from different parts of the plants are represented in Table 1 [75].
XPS spectra of AgNPs were measured using a hemispherical analyzer (Physical Electronics 1257 system). For the XPS, a twin anode (Mg and Al) with X-ray source was operated at 400 W of constant power and using Al Kα radiation (1486.6 eV). The samples were placed in a sample stage with an emission angle of 45°. The measurements were carried out by suspending AgNPs on a gold film while gold was served as metallic reference. Au 4f binding energy was 84 eV for samples without any charging effect (Figure 3).
Raman spectra were measured using a Bruker Raman spectrometer; model Senterra with laser excitation at 633 nm, and laser power at 10 mW. Spectral data were collected using a 50× microscope objective (NA = 0.51) with 30 s integration time. The surface-enhanced Raman scattering (SERS) samples were prepared by mixing 360 μL of colloidal solution with 40 μL of aqueous solutions of the probe molecule, resulting in a final R6G concentration of 1.0 × 10 −5 mol L −1 (Figure 4).

Conclusion
Today, nanotechnology is considered to be very essential to help promote ultra-modern farming systems with very little environmental damage. Since the beginning, methods of synthesis of plant-based NPs using bioprocess have been considered eco-friendly and cost-effective for the last decades. Many types of natural extracts such as plant leaves have been employed as efficient resources for the synthesis of NPs. A large number of NPs are being explored in many areas of agriculture and biotechnology.

Conflict of interest
Authors declare that there is no conflict of interest for publication of this article.