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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 the bottom‐up (Figure 1.3a) method: decomposing a molecular precursor into simple metal atoms, which transform into colloids and the top‐down (Figure 1.3b) method.

Figure 1.3 Illustration of synthesis of nanoparticles: (a) top‐down method and (b) bottom‐up method.

Source: From Wang and Xia [39]. © 2004, American Chemical Society.