**2. Classification of Nanoparticles**

Classification of NPs varies according to their origins, structures, shapes, dimensions, chemical and phase compositions, physical and chemical properties, and crystallinity [29,38,39]. For example, NPs can be obtained naturally from dust, volcanic eruptions, and living organisms such as bacteria, fungi, algae, plants, etc., and artificially by utilizing chemical, physical, and biological synthesis routes such as colloidal, chemical precipitation, laser ablation, sputtering, and micro- and macro-organism-mediated synthesis approaches [30,31,40]. Furthermore, NPs can be classified into organic, inorganic, and semi-organic types according to their chemical nature [29,41]. Similarly, NPs can be categorized according to their shape, including rod, spherical, cubic, triangular, octahedral, pentahedral, flower, star, etc. [4,39], and on the basis of the number of their dimension in nanoscale, including zero dimension (0D) such as quantum dots, one dimension (1D) such as nanowires and nanorods, two dimensions (2D) such as nanolayers and nanoplates, and three dimensions (3D) such as nanocoils and nanoflowers [42]. In terms of magnetic properties, NPs can belong to either the paramagnetic category, which includes iron oxide and zinc sulfide NPs, or the diamagnetic category comprising titanium oxide and magnesium ferrite NPs [43] (Figure 1).

**Figure 1.** Classifications of nanoparticles (NPs).

## **3. Synthesis Routes of Nanoparticles**

Nanofabrication routes can generally be classified into two main groups: top-down methods, such as physical synthesis approaches, and bottom-up synthesis methods, such as chemical and biological synthesis processes (Figure 2) [44].

**Figure 2.** Synthesis routes of nanoparticles.
