**8. 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

#### **Figure 2.**

*Scanning electron microscopic images of biosynthesized NPs: (A) Silver NPs [67], (B) Cerium dioxide NPs [68], (C) Copper oxide NPs [69], (D) Gold NPs [70], (E) Titanium dioxide NPs [71], (F) Zinc oxide NPs [72].*

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**).

*Biological Synthesis of Metallic Nanoparticles from Different Plant Species DOI: http://dx.doi.org/10.5772/intechopen.101355*

**Figure 3.** *X-ray photoelectron Spectroscopy of AgNPs [76].*

**Figure 4.** *Surface-enhanced Raman scattering spectrum of AgNPs [77].*
