## 3 Results and Discussion
### Preparation and Characterization of Silver Nanoparticles  
AgNPs were prepared by chemical synthetic procedure under different conditions. We used: silver salt (AgNO\({}_{3}\)) as a metal precursor, sodium borohydride (NaBH\({}_{4}\)) as a reductant and gemini surfactants (16-6-16 in experiments 1-3 and 7-8, 16-O-16 in experiments 4-6) as stabilizing agents (Figure 4). Experiments 1-6 were performed according to preparation described by [PERSON] [87]. Different molar ratios of AgNO\({}_{3}\) and gemini surfactants were applied to study the formation of AgNPs (Table 2). In these cases, nAgNO\({}_{3}\) was taken in excess of 6-fold to nNaBH\({}_{4}\). Experiments 7 and 8 were carried out by another procedure, with 16-6-16 as stabilizing agent in silver-gemini surfactants molar ratio 5 (Table 2). In these cases, nNaBH\({}_{4}\) was taken in excess of 6-fold to nAgNO\({}_{3}\). \begin{table}
\begin{tabular}{c c c c c} \hline
**Surfactant** & **CMC (mM)** & \(\alpha\) & \(\beta\) & \(\Delta\text{G}^{\text{o}}_{\text{mic}}\) (kJ/mol) \\ \hline
16-6-16 & 0.034 & 0.55 & 0.45 & \(-\)50.16 \\
16-O-16 & 0.031 & 0.40 & 0.60 & \(-\)58.32 \\ \hline \end{tabular}
\end{table} 
Table 1: Critical micelle concentration, degree of ionization, counterion binding parameter, and Gibbs free energy of micellization in 25 \({}^{\circ}\)C.  
Figure 4: Schematic representation of the formation of layer of gemini surfactants, surrounding the Ag nanoparticles. The color of AgNPs solution depends on the degree of dilution of the colloids, starting from yellow and going to intense orange with the increase of colloids concentration. Gemini surfactant allows for keeping the nanodispersion homogeneous, with a color depending on the nanodispersion dilution. In our work, in all experiments, the intense orange color was formed, showing that the reaction was followed by the formation of nanoparticles [87; 88; 89; 90]. The color of AgNPs solution is suggested to appear due to localized surface plasmon resonance (LSPRs). LSPR is a collective excitation of the free electrons in the conduction band around the surface of the nanoparticles. The electrons are specified to particular vibration modes, which depend on particle size and shape. Therefore, AgNPs could be detected and characterized by UV-Vis. With increasing of particle size, the absorption wavelength shifts to longer wavelengths [91]. UV-Vis spectroscopic analysis is an essential technique for observation of AgNPs formation and stability in solution. The absorbance peak of silver nanoparticles usually occurs at wavelength range of 350-450 nm, and moves to longer wavelengths with progressive particle size [91]. In Figure 5a, the UV-Vis spectra of AgNPs, which were stabilized with 16-6-16 is presented. The spectra were measured for three various Ag-to-gemini surfactant molar ratios: nAg/n16-6-16 = 2.5; 5; 10. We can see that from the highest gemini concentration (experiment 1) to the lowest (experiment 3), the peak of absorbance increases gradually: 0.286, 0.332 and 0.359. As it is known, the absorbance is directly connected with the concentration, thus, since the difference is not significant, it can be concluded that 16-6-16 has a strong stabilizing effect on AgNPs in the extensive range of nAg/n16-6-16 molar ratio values. However, the nAg/n16-6-16 molar ratio at 2.5 indicates a slightly weak stabilizing effect since the concentration of AgNPs is not high. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Experiment** & **Gemini Surfactant** & **Molar Ratio nAg/nGemini** & **Concentration of AgNO3 [mM]** \\ \hline
1 & 16-6-16 & 2.5 & 0.75 \\
2 & 16-6-16 & 5 & 0.75 \\
3 & 16-6-16 & 10 & 0.75 \\
4 & 16-O-16 & 2.5 & 0.75 \\
5 & 16-O-16 & 5 & 0.75 \\
6 & 16-O-16 & 10 & 0.75 \\
7 & 16-6-16 & 5 & 2 \\
8 & 16-6-16 & 5 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental data of different procedures for AgNPs obtaining. Figure 5: UV-Vis spectra of silver nanoparticles stabilized with gemini surfactants (**a**) 16-6-16; (**b**) 16-O-16 at different nAg/nGemini values.