Table 1.

Different methods for M-NPs synthesis.

	Method	Graphical Representation	Short Description	Reference
Top-Down synthesis	Mechanical Milling		This approach includes the disintegration of particle aggregates, particle shape, and particle surface.	[20,21,22,23]
	Chemical Etching		Uses a strong acid or a corrosive liquid to cut a metal surface and create a design in the metal.	[24,25]
	Sputtering		There is a particular sputtering technique called magnetron sputtering, which consists of the delivery of a high voltage across a low-pressure gas (normally argon) to create a plasma of high energy composed of electrons and gas ions, which will strike a target containing the desired coating material.	[26,27]
	Laser Ablation		Is based on the production of micropatterns through the ablation (removal) of fractions of a substrate through the action of a focused pulsed laser beam.	[20,28]
	Electro Explosion		Single-step process in which a delicate wire of a conductive metal is exploded by an electric discharge that is caused by a high-power DC source. This electronic discharge creates a massive temperature that vaporizes the thin wire, turning it into gas atoms, which in their turn are chilled and, finally, the NPs are synthesized.	[29,30]
Bottom-up synthesis	Chemical Deposition		In particular, electrochemical deposition or electrochemical precipitation involves the passage of an electric current between an anode (sacrificial) and a cathode localized in an electrolyte. The anode is oxidized into metal ions and these are then reduced to metal by the cathode with the help of stabilizers.	[31,32]
	Spinning		This method involves the use of a spinning disc reactor. The disc spins at different speeds and the spinning causes the fusion and precipitation of atoms, which are then collected.	[5,33]
	Sol–Gel		A wet-chemical process, with sol being a colloidal solution of solids suspended in a liquid phase that serves as a metal precursor and is then dispersed into the gel, a host liquid leading to the formation of a solid macromolecule submerged in the solvent. After, there is a separate phase where the gel is dried and dehydrated to recover the NPs.	[20,24,33,34,35]
	Biosynthesis		Metalsynthesis is bio-mediated by microbes or through biosynthesis. Biological synthesis of nanomaterials is the best alternative being cost-effective, environmentally friendly, advantageous and does not comprise any input of toxic chemicals. Between the candidates to biosynthesize M-NPs are bacteria, fungi and yeast; algae; plants, flowers and fruits; and viruses.	[18,19,23,36,37,38]