FT-IR analysis

In green synthesis the plant root extract plays dual role in synthesis of AgNPs both as capping agent and as bioreductant in the reaction. After the synthesis of AgNPs they are purified by centrifuging at 9000 rpm for 20 min and the pellet of AgNPs was collected. The centrifugation step was repeated thrice by washing the pellet with sterile distilled water to remove the any unbound residues of the plant root extract of Glycyrrhiza glabra. The purified AgNPs are now used for IR analysis. The IR results reveals that the spectra of root extract and the AgNPs have more similarities, confirming that the plant root extract components like flavonoids and poly phenols are involved in the bioreduction and formation of AgNPs. The IR spectrum reveals that there are some marginal shifts in the peak positions of AgNPs, which is due bioreduction. Further the FT-IR data was analyzed to distinguish the possible bio-reducing biomolecules present in the plant root extract. The FT-IR spectra of root extract Fig. 2a reveals strong spectral peaks at 3744, 3425, 2923, 2854 and 2311 cm⁻¹, 1634, 1550 and 1457 cm⁻¹, 1383, 1161 and 1046 cm⁻¹ of different functional peaks. Whereas the AgNPs, Fig. 2b showed strong characteristic bands of different functional groups at 3413, 2922, 1624, 1344 and 1043 cm⁻¹.

a IR spectrum of the root extract of G. glabra, b IR spectrum of the synthesized AgNps

The results are compared among the strong and broad peaks of root extract and biosynthesized AgNPs are as follows i.e. 3425 and 3413 cm⁻¹ which corresponds to –NH stretching in amide II and also the hydrogen-bonded hydroxyl (OH), were observed in both the extract and AgNPs. The peaks at 2923 and 2922 cm⁻¹ corresponds to asymmetric stretching of C–H group and the other peaks corresponding to 1634 and 1624 cm⁻¹ were identified as the characteristic peaks for the C–H, C–C, C–O stretching and the amide I group due to the carbonyl vibrational stretches in amide group linkages in proteins. The peaks at 1383 and 1344 cm⁻¹ bands corresponds to C–N stretching of aromatic amines and finally the intense peaks at 1046 and 1043 cm⁻¹ are the bands corresponding to characteristic peaks of C–OH stretching of secondary alcohols, carboxylic acids, ester and ether groups.(Shameli et al. 2012; Kora et al. 2012). It is already known that the noble metal nanoparticles have useful functional groups on the surface to form a protective layer and gives stability to the nanoparticles. The functional groups such as amino (–NH), carboxylic acid (COOH), mercapto (SH) and cyano (CN) and other groups have been proved to have high affinity towards metal nanoparticles (Teranishi et al. 1998; Corbierre et al. 2001; Mandal et al. 2002; Shan et al. 2003). In the present study we identified that different functional groups such as –NH, OH, C–H, C–C, C–O have been actively participated in the synthesis of silver nanoparticles using the root extract of Glycyrrhiza glabra. Glycyrrhiza glabra has Glycyrrhizin as the major water-soluble constituent. Glycyrrhizin is a triterpenoid saponin that is present within range of 2–14 %. So the water soluble Glycyrrhizin and other bioactive components may have actively participated in synthesis of AgNPs.