**10. Laser ablation of copper nanoparticles**

Copper nanoparticles (Cu-NPs) have more application conductive coatings, lubricants, sintering additives [87], and biosensors [88]. Copper nanoparticles are anti-inflammatory [89], reduce gastrointestinal mucosa [90], antioxidative [91], anti-ulcer [92], and are useful in preventing skin photosensitivity [93]. The copper nanoparticles strongly absorb the light beams about 600 nm, arising from the localized surface plasmon resonance (LSPR).

Recently, the application of vegetable oils such as palm oil [22], coconut oil [71], walnut oil [23], and castor oil [31] for dispersing the nanoparticles was considered for the preparation of nanometals [94]. These natural compounds contain triglycerides and non-polar long carbon chains that prevent nanoparticles agglomeration through steric repulsion [71].

Many methods based on the reaction of metal ions were presented to prepare copper nanoparticles. For example, solution phase [95], photochemical [96], sonochemical [97], and electrochemical synthesis methods [14] are the famous methods that are utilized for preparation of Cu-NPs in an aqueous solution. Laser ablation [98] is a green technique for the synthesis of copper nanoparticles. In the literature, the preparation of copper nanoparticles in distilled water, acetone, and ethanol [99] was reported using laser ablation. Malyavantham et al. [100] utilized the laser ablation technique to fabricate the Au-Cu NPs alloy. Copper nanoparticles were formed in the spherical shape (**Figure 4a**) in an aqueous solution. The UV-visible peak arose the localized surface plasmon resonance about 630 nm (**Figure 4b**). The influence of the colloidal absorption on the formation efficiency of copper nanoparticles was also the significant parameter to prepare copper nanoparticles. The formation efficiencies of Cu-NPs using 532 and 1064 nm laser beam were much closer than those of silver nanoparticles because the absorption at 532 nm in copper colloids was lower than that in silver colloids.

**Figure 4.** (a) TEM image of copper nanoparticle in oil and (b) UV-visible spectrum of copper nanoparticles produced using laser ablation.
