*3.2.4 Fourier transform infrared spectroscopy (FTIR) analysis*

FTIR absorption spectra of AgNPs are shown in (**Figure 5**). The different possible functional groups at various positions will be determined by using FTIR analysis. The band at 1559 cm−1 indicates the presence of amide group [32] arises due to carbonyl stretch in proteins. It can be stated from FTIR analysis. The band at 1610 cm−1 is attributed to the stretching vibration of (NH) C=O group. That amide groups present in carbohydrates, proteins are dominant reducing agents and play an important role in the bio-reduction of Ag<sup>+</sup> ions to Ag0 leads to nanoparticles synthesis.

#### **3.3 Catalytic reduction of 4-nitrophenol**

In order to study the efficiency of bio-synthesized AgNPs, the catalytic reduction of 4-nitrophenol was carried out in an aqueous medium by using NaBH4 as a reductant at room temperature [33–35]. The 4-nitrophenol (0.1 mM) shows an absorption peak at 400 nm in the visible region with NaBH4 (**Figure 6**). In a control experiment, it can be concluded that the reduction does not occur in the absence of AgNPs, even after the addition of excess NaBH4. After the addition of AgNPs, the gradual decrease in intensity of the absorption peak at 320 nm was observed due to the formation of 4-aminophenol. The complete reduction of p-nitrophenol was also supported by a change in color from yellow to colorless.

### **3.4 Antibacterial activity**

Antibacterial activity of biosynthesized AgNPs was investigated against human pathogens. The biosynthesized AgNPs showed a high inhibitory effect on bacteria, and it may serve as an option for decreasing bacterial infections [36]. The zone of inhibition was found to be as per **Table 1**.

**Figure 5.** *FTIR image of AgNPs.*

#### **Figure 6.**

*The UV–vis spectrum of 4-nitrophenol reduction catalyzed by AgNPs using NaBH4.*


#### **Table 1.**

*Antibacterial activity of* Syzygium cumini *AgNPs measured as the zone of inhibition in (mm).*
