Nano-Technological Intervention in Agricultural Productivity. Javid A. Parray

Metal NPs comprise metal‐made precursors. Because of the well‐localized surface plasma resonance (LSPR) characteristics, such NPs have distinct optoelectronic properties. In the visible region of the solar electromagnetic spectrum, the NPs of alkali and noble metals such as Cu, Ag, and Au have broadband absorption. In today's cutting‐edge materials, the facet, size, and shape‐controlled synthesis of metal NPs are critical [4]. Metal NPs are finding applications in many research fields because of their advanced optical properties. The coating of gold NPs is widely used to enhance electronic streaming for scanning electron microscopy (SEM) sampling, thus helping to accomplish good SEM images.

Figure 1.1 Description of fullerenes or buck balls (a) C₆₀ and (b) C₇₀.

      Source: From Khan et al. [2]. © 2017, Elsevier.

1.2.3 Ceramic NPs

      NPs from ceramics are inorganic non‐metallic solids synthesized by heat and cooling. They are used in amorphous, polycrystalline, solid, porous, or hollow [18] forms. Because of their use in catalysis, photocatalysis, colour photodegradation, and imaging, these NPs offer substantial interest from researchers [19].

1.2.4 Semiconductor NPs

      Semiconductor materials have properties between metals and non‐metals, and because of this property, they find different applications in the literature [20]. Semiconductor NPs have large band gaps, demonstrating significant modifications to their properties with band gap tuning. Items of great significance also include photocatalysis, photo‐optics, and electronic devices [21]. Because of their appropriate band gap and band edge positions, the range of semiconductor NPs is considered exceptionally efficient in water splitting applications [22].

1.2.5 Polymeric NPs

      These substances are used for a wide range of commercial applications, such as fillers [15], effective adsorbents of environmental remedial gases [16], and as a support medium for various inorganic and organic calculators because they give specific physical, chemical, and mechanical characteristics [23].

1.2.6 NPs Based on Lipids

In many biomedical applications, they are used, and they contain lipid molecules. Usually, the lipid NP is spherical, 10–1000 nm in diameter. Similar to polymeric NPs, lipid NPs have a solid lipid core and lipophilic molecules within the matrix. The external centre of these NPs has been stabilized by surfactants or emulsifiers [24]. Lipid nanotechnology [25] is a specific field that focuses on the design and synthesis of lipid NPs for various applications, such as drug carriers and delivery [26] and the release of RNA cancer therapy [27].

1.3 Synthesis of Nanoparticles

Various techniques can be used for the synthesis of NPs. Nevertheless, these approaches are generally divided into two main categories, i.e. (i) top‐down approach and (ii) bottom‐up approach [28, 29] (Figure 1.2). These methods are further divided into different subclasses based on process, reaction state, and adopted protocols.

1.3.1 Top‐Down Synthesis

      The larger molecules are disintegrated into smaller units by a destructive process, and these units are transformed into useful NPs, for example, grinding/milling, CVDs, physical vapour deposition, etc. [29]. This approach is generally used to synthesize NPs from coconut shells (CSs). The milling method is used, whereby raw CS powders were finely milled using ceramic balls and a well‐known planetary mill at different time intervals. Via other characterization techniques, the influence of the milling period on the overall size of the NPs is shown.

Furthermore, as time increases, the size of the NP crystallite (Scherrer equation) decreases. In this process, it was also found that the brownish colour faded away with the increment of each hour because of the reduced size of the NPs [30]. Various characterization techniques demonstrated the effect of milling time on the overall size of the NPs. The synthesis of spherical magnetite NPs from natural iron oxide (Fe₂O₃) ore was shown in the presence of organic oleic acid by a destructive top‐down method with a particle size ranging from 20 to 50 nm [31]. To synthesize spherical particles of colloidal carbon using a control scale, a primary top‐down route was used. The synthesis technique was based on the continuous chemical adsorption of polyoxometalates on the carbon interfacial surface. Adsorption has transformed black carbon aggregates into relatively smaller spherical particles with a high dispersion capacity and a narrow distribution of size [32]. Microstructures have found that the quantity of carbon particles is lower during the sonication period. Combined grinding and top‐down sonication techniques synthesized a sequence of transition metal dichalcogenide nanodots (TMD‐NDs) from their crystallites. Every TMD‐ND with a size of less than 10 nm shows excellent dispersion and is demonstrated by the narrow distribution of the measure [33]. Highly photoactive Co₃O₄ NPs have recently been produced by top‐down laser fragmentation, i.e. a top‐down process with an average size of 5.8 ± 1.1 nm. Powerful laser irradiations produce well‐uniformed NPs with adequate oxygen vacancy [34].

Figure 1.2 The synthetic models of NPs: top‐down and bottom‐up approach.

      Source: Modified from Iravani [29].

1.3.2 Bottom‐Up Synthesis

This reverse approach is used to synthesize NPs from relatively more straightforward substances and is also called an approach to building up. Examples include sedimentation and reduction techniques. It encompasses sun‐freezing, green synthesis, spinning, and biochemical synthesis [29]. Mogilevsky et al. [35] used this technique to synthesize TiO₂ anatase NPs to the graphene domains. Alizarin and titanium isopropoxide precursors have been used to synthesize the photoactive composite for methylene blue photocatalytic degradation. The X‐ray diffraction (XRD) framework has verified the anatase form [35]. Well‐uniform spherical shaped Au nanospheres have been synthesized with monocrystalline using top‐down laser irradiation technique [36, 37]. Recently, the solvent exchange method has been used to deliver medical cancer drugs to achieve limited‐size low‐density lipoprotein (LDL) NPs. The standard approach followed by growth, i.e. up process, is nucleation in this technique. The LDL NPs were obtained without phospholipid use and had high hydrophobicity, which is a prerequisite for drug delivery implementation. [38]. The monodispersed spherical bismuth (Bi) NPs, with top‐down and bottom‐up approaches, are synthesized with excellent colloidal properties [39]. In bottom‐up ethylene glycol, bismuth acetate was melted, although bismuth was converted into a molten state in the top‐down process. In boiled diethylene glycol, the molten drop then has been emulsified for NPs. Both methods generated different NPs in size from 100 to 500 nm [39] (Figure 1.3a,b). Green and biogenic bottom‐up processing is cost‐effective and environmentally sustainable, where biological processes, such as using plant extracts, achieve the synthesis of NPs. For the synthesis of NPs, bacteria, yeast, fungi, Aloe vera, tamarind, and even human cells are used. Au‐NPs were synthesized from wheat biomass and oat [40] and used as a reduction agent by microorganisms and plant extracts [41, 42]. Figure 1.3 demonstrated