Sec-Met: An Effective Source for Green Nanostructural Synthesis

300,000,000 biodegradable plant species are presently available in our Mother Earth, with some at the threshold of extermination to be safeguarded for the future engendered (generation) [37]. The biodegradable plant kingdom is acknowledged well as a suitable substitute for physical/chemical techniques in the green nanostructural formulations [38]. The plant kingdom, a royal source of rich secondaiy metabolites, that serves for defence mechanism can be tapped from its botanical extracts, for an effective utilization in green and clean methods of formulation [39]. GNT effectively employs this potential from plant to engineer NPs [40].

Different plant segments rich in biomolecules (BMs) like (seed, shoot, root, stem, leaf, bud, flower, bark, latex, rhizomes, and fruit) effectively used in the formation of organic/inorganic/metallic NPs by green nanoroutes have been reported by authors [41,42]. Plant’s botanical, extremely rich in BMs like flavonoids, polyphenols, alkaloids, proteins, amino acids, carbohydrates, terpenoids, etc., serve as powerful bioreducer and biostabilizer in green nanostructural synthesis [43]. Plant-mediated, Sec-Met/primary-metabolites, used for biosynthesis of nanostructures are represented in a systematic way in Fig. 4.2.

Many factors like climatic environment, minerals present, nature of the soil, pollution, etc., regulate the concentration and the composition of Sec-Met of the plant [44, 45]. An important keynote linked with variations in morphological size and geometric shape of

Figure 4.2 Plant Sec-Met/primary-metabolites, used for biosynthesis of nanostructures.

the formulated NPs by green-routes depends upon the structural conformation and molecular concentration of these highly energetic BMs in the plant extract [46].

Utilization of plant Sec-Met in technological advancement is visualized for the green nanostructural fabrication of NPs. These as-synthesized green nanostructures from Sec-Met philosophies find their utility in a wide arena like vegetative food and seafood [47], agricultural [48], pharmaceutical [49], biomedical usages [50], ecological health [51], textile [52], optics [53], and energy [54]. Green nanostructures deliver reliability and sustainability and provide a significant substituted solution for ecological challenges in different venues like waste-water treatment [55], solar-energy garnering [56], H₂ production [57], drug-delivery [58], catalysis [59], and pollution control [60].

Innovative Focuses on Green Nanostructural Formulation

Scientific ignorance is a great question now. Research innovations are proceeding to face this challenging situation. Nanotechnology is in the first place, duly supported by green-nano routes amidst huge scientific conception with deep-rooted contemplation [61] to solve this difficulty. The three 'E's i.e. 'Energy'/'Environmentar/'Economy' at the marginal level of crunches at present, due to extreme catastrophe are facing challenges to meet sustainability. GNT provides the best solution for these unanswered questions.

The activity of NPs rests upon several aspects like constituent’s morphological factors, size dissemination, shape formation, constituent's composition, surface-chemistry, reductant, stabilizer/ capping promoters, agglomeration control, dissolution-rate, reactivity, stability, and solubility [62]. These aforesaid features are governed by the prerequisites (T C, pH, time, methods used, and other reaction conditions) for the NPs formation [63].

GNT utilizes these 3 core constituents, like precursors (organic/ inorganic-metallic/non-metallic), reducer, capper/stabilizer for its process of fabrication, with 3 core steps as (1) reduction to

Figure 4.3 Formation of green nanostructured particles from plant's botanicals.

undergo nucleation, (2) growth of nucleation to form particles, (3) stabilization/capping to give stability to the particles formed [64]. The 3 core aspects involved in the biosynthesis of green nanostructures are (1) use of non-toxic biodegradable component, (2) use of no solvent or water solvent system, (3) economical, biological reducer and stabilizer [65]. Figure 4.3, depicts the formation of nanoparticles by green nanosynthesis using plant extracts.

Bio-synthetic methods deliver controlled desired sizes and shapes according to the needs of the application [66]. Plant's bioor- ganic extracts as reducer and stabilizer deliver the requirement in a distinguished way [67]. The greatest benefit of utilizing plants phyto-molecules is the potential accessibility from a wide range of biological reserves. As a time-saving process, a higher degree of stability and quick solubility in water for the formulated NPs, are the added benefits of this green method [68].

Reducing and stabilizing capacity of the plants depends on the phyto-molecules that participate from various cell tissues within it [69, 70]. Mechanistic exploration being comparatively limited, is yet to be delivered in-depth. Ample literature is available for green synthesis, using various parts of plant species. Authors have exploited inorganic salt precursors for nanoparticle formulation using various approaches of green synthesis. Extraction of plant biomolecules with a large volume of potentials in it, like electrongiving tendency, an important keynote, can be a better eco-friendly, safe option in GNT synthesis for reduction of inorganic ions when compared with other reducing agents.

Many frontline floral species have been exploited in the fields of medicinal, phytochemical, pesticidal and clinical trials. Seeds of Tectona grandis i.e. Teak [71], Peel of—Rambutan Nephelium lappaceum [72-74], Citrus reticulata (Mandarin) [75], banana [76], the shell of Plukenetia volubilis [77], Anacardium occidentale [78], the bark of Cinnamon zeylanicum [79], leaf of Ziziphora tenuior [80], Azadirachta indica [81], Plantago asiatica [82], flowers of Jacaranda mimosifolia [83], Quisqualis indica [84], Mari Gold [85], plant of Garcinia xanthochymus [86], resinous gum of gum karaya [87], gum Arabic [88], Desmodium gangeticum [89], and wheat husk-ash [90] have been generously used in the formulation of Ag, ZnO, Cu, CuO, Au, Ni, and other NPs. Plant’s vital Sec-Met present in the above-said species like sugars, enzymes, polyphenols, flavonoids, terpenoids, fats, alkaloids and others have an essential part in reduction and capping of the salt-precursors into NPs.

Less energy, with a single step approach, is the desired platform in GNT synthesis. Elevated temperatures are an unnecessary component in cost-effective GNT synthesis. Changes in pH, time, temperature, the concentration of extract and salt precursors, modify the structural parameters of the formed NPs. The optimization of various reaction factors leads to the desired outcomes. Table 4.1 gives input on some contribution in green nanosynthesis.

Scientific communities are continuously developing green nanoroutes for the formation of nanoparticles with safe deliveries that limit the negative consequences to humans and the environment. It is important to authenticate the keynotes pertaining to the validation of the nanoproducts formed, its reliability, stability, formation and application operations to ensure a safe endpoint. The pros and cons of nanoparticles have been reported in terms of agriculture and medicine. Nanotoxicity has been commented on but requires clarity in depth.

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