**2.6 Extracellular synthesis of silver nanoparticles**


### **2.7 The optimum condition for the synthesis**

One of the most critical factors in bacteria-mediated Ag NP synthesis is a high pH. In the presence of silver ions, high pH catalyzes the opening of monosaccharide rings to open chain aldehyde forms, which then undergo oxidation to the appropriate carboxylic acid while simultaneously reducing silver ions to Ag NPs. Reductases of oxidoreductase enzymes are also activated by high pH [16]. Some bacterial proteins are involved in the synthesis of silver nanoparticles. It binds the thiol region at alkaline conditions (there is no need for agitation). In addition, alkaline ions are very much required for the reduction of metal ions. Under the alkaline state, it enhances the enzyme activity to do a reduction

*Bacterial Silver Nanoparticles: Method, Mechanism of Synthesis and Application in Mosquito… DOI: http://dx.doi.org/10.5772/intechopen.104144*

#### **Figure 3.**

*Mechanism of extracellular synthesis of silver nanoparticles - the bacteria can release the metabolite or type of reductase enzyme to the environment. It oxidizes the metal to an inactive form.*

mechanism. In acidic conditions, it will take up to 4 days for silver nanoparticle synthesis. In alkaline conditions, the nanoparticle will be synthesized within 4 hours [12].

The high temperature will increase the dynamics of ions and the formation of more nucleation regions due to the obtainability of OH ions and the conversion of silver metal to the silver nanoparticle. At 60°C, nanoparticles are redacted up to 2–15 nm; in acidic conditions (50 nm), size is not reduced [12].

#### **3. Characterization of silver nanoparticle**

Once the synthesis procedure is completed, it is necessary to characterize the nanoparticles to know their structure, size, purity, and efficacy by studying their physiochemical properties, size, shape, surface area, and homogeneity. UV- visible spectroscopy, FTIR, TEM, SEM-EDAX, X-ray diffraction (XRD), and atomic force microscopy (AFM) are the tools used to characterize the synthesized nanoparticles [17].

#### **3.1 UV: visible spectroscopy**

UV-visible spectroscopy is a primary tool to analyze the availability of nanoparticles in the reaction mixture at 200–500 nm. It is based on the transition of electrons from

one molecular orbital to another due to the absorption of electromagnetic radiation of UV and visible regions. It is the type of absorption spectroscopy when electromagnetic radiation interacts with matter, and the incident light can be reflected off, absorbed by, or transmitted through a sample. Electromagnetic radiation is absorbed by atoms or molecules, transitioning from lower energy to an excited state. Then the energy matched the difference in energy between two energy samples [18].

### **3.2 Fourier transform infrared spectroscopy**

FTIR is used to analyze the surface chemistry of silver nanoparticles. The range that covers the electromagnetic spectrum is 1 micrometer to 100 micrometers. This spectroscopy is a type of vibrational spectroscopy. At the temperature above absolute zero, the bonds within molecules will vibrate. There are two main types of bond vibrations - stretching and bending. A stretching vibration occurs along the line of the chemical bond, whereas a bending vibration is any vibration that does not occur along the line of the chemical bond. It provides that all the functional groups are present in silver nanoparticles [18].

### **3.3 Transmission electron microscope**

In the TEM, a condenser lens focuses the electron beam onto the specimen, transmitting electrons through the specimen. The portion of the beam absorbed by the specimen is minimal; to be absorbed, an electron must lose all its energy to the specimen. Some electrons scattered through the specimen focus on forming an image, like how an image in a light microscope is formed. A phosphorescent screen, a photographic plate, or a high-resolution camera can be used to view the image [18].

## **3.4 Scanning electron microscope**

The scanning electron microscope views the surface nature of specimens. The sample is fixed, dried, and coated with a thin layer of heavy metal, such as gold or silver, scanned with a very narrow beam of electrons. Molecules in the specimen are excited, and they release secondary electrons. Molecules in the specimen are excited, and they release secondary electrons captured by a detector, generating an image of the specimen's surface. The resolving power of scanning electron microscopes, limited by the thickness of the metal coating, is only about 10 nm; they produce three-dimensional in SEM [18].
