**4. Approaches for obtaining NPs**

Many convincing procedures have been awarded for processing Ag NPs in the past years [30]. NP production is briefed under two elemental categories (**Figure 4**):

*Top-down Approach*: - Solid bulk material is applied with external force, forcing its break down into nanostructures.

*Bottom-up Approach*: - Its initiates from atomic scale & generates NPs, until appropriate size and shape is gathered [32].

Physical, Chemical, & Biological Methods to process the Ag NPs are explained below:

#### **4.1 Physical methods**

These could be defined as the green mechanism because of non-synthetic liquids in the tidy, thin films. Moreover, high consistency of NP distribution, is one of the pros of physically processed NPs as identify with any other methods [22]. This method consists of mechanisms like [30]:


#### **Figure 4.**

*Representation of two elemental categories, 'top-down' & 'bottom-up' approaches for the nanoscale material processing [31].*

**113**

*Silver Nanoparticles - Preparation Methods and Anti-Bacterial/Viral Remedy Impacts…*

A chemistry professor from Princeton - John Turkevich's work in the 1950s, gave a highlight to chemical methods. The chemical processing by natural and artificial reducing surfactants is the preferred method for constructing Ag NPs. Reducing agents like sodium citrate, ascorbate, elemental hydrogen, are generally employed

ions reduction in aqueous or non-aqueous solution. These agents shrink

• *Photochemical* – In an electron-contributing reagent, it requires UV irradiation of the solid forerunner solution. This process produces NPs subjected solid-

• *Microemulsion* – It is thermodynamically balanced colloidal diffusion of oil in water or vice-versa with a stabilizer. Ultralow interfacial tension, large interfa-

• *Hydrothermal* - It is a solution reaction-originated approach which produce NPs

• *Chemical Reduction* - A usual process utilizing natural & artificial reducing agents like sodium citrate, sodium borohydride (NaBH4), elemental hydrogen, N-dimethylformamide (DMF), & poly (ethylene glycol)-block copolymers for

• *Electrochemical Mechanism* - A common process, containing a two-electrode mechanism for the electrochemical fusion. It has utmost preference over other

These are categorized under green synthesis and comprises polysaccharides, organic & irradiation techniques, preferred over other traditional practices requiring synthetic agents with hefty harms [43]. Because of benefits like organic surfactants (bacteria, fungi, yeast, and plant), cheap and eco-friendly techniques, a vast interest is gained by these methods. Even with natural surfactants, it has a usual

• *Bacteria* – It is capable to weaken heavy metal ions & has its preferences for generating NPs. E.g.: *Pseudomonas stutzeri* and *Pseudomonas aeruginosa* [44].

• *Plants* - Metabolites in plant crude extract are capable of metal ions reduction

• *Fungi* - They offer high tolerance NPs and are easy to handle. Its extracellular

• *Algae* - These are single/multi-cellular organisms endured in surroundings. Processing at low temperature with high energy efficiency, less noxious to the environment are its pros. E.g.: *Chlorella Vulgaris*, Spirulina platensis [47].

protein extract can stabilize NPs. E.g.: Fusarium oxysporum [46].

bottom-up approaches because of pure quality of NPs received [42].

cial area & monodispersed NPs, are pros of this technique [40].

at room-temperature to extremely high-temperature [41].

of NPs during their preparation and to restrict them to stay onto material surfaces [37]. To stabilize particle hike and to sustain them from sedimentation, accumulation, or losing surface properties, comprising functionalities to unite with molecule skin, the presence of surface-active agents like acids, amines, or alcohol are must

[36]. Protective agents are obligatory to stabilize breaking-up

*DOI: http://dx.doi.org/10.5772/intechopen.99368*

[22]. This method comprises of [30, 38]:

ion break-down [22].

reduction process [30]. Green synthesis pursues with:

into metallic NPs. E.g.: *A. indica*, *A. sessilis* [45].

state status & at low temperatures [39].

**4.2 Chemical methods**

ions forming Ag0

for Ag+

Ag+

**4.3 Biological methods**

Ag+

*Silver Nanoparticles - Preparation Methods and Anti-Bacterial/Viral Remedy Impacts… DOI: http://dx.doi.org/10.5772/intechopen.99368*
