Addition of sodium citrate into the beakers containing aqueous solution of AgNO3 led to the change in the colour of the solution from colorless to brownish yellow within reaction duration due to excitation of surface plasmon resonance (SPR) vibrations in Ag NPs. The colour of the solution is brownish yellow indicating formation of Ag NPs (Figure 1 inset). Ag NPs synthesized using sodium citrate were analyzed by UV spectra of Plasmon resonance band observed at 410 nm (Figure 1) (Zia et al. 2016). SEM micrograph shows the morphology of the Ag NPs which are spherical and well dispersed (Figure 2a). TEM results showed well dispersed spherical particles with a size of ca. 23.4 nm in diameter (Figure 2b&c).
The spectroscopic detection of metal ions Cd2+, Cu2+, Fe2+, Hg2+, Mn2+, Ni2+, Pb2+ and Zn2+ at the fixed concentrations of 500 µL of a 1 × 10−5 M were added to Ag NPs solution and the corresponding changes of the absorption coefficient were observed from brownish yellow to light blue color (Figure 3a-h) (Kamel et al. 2019; Boruah et al. 2019). In presence of heavy metal ions with Ag NPs, a new peak at around 925, 898, 643, 665, 688, and 838 nm of Cd2+, Cu2+, Fe2+, Hg2+, Mn2+ and Zn2+ appeared in addition to the peak found at 410 nm of Ag NPs (Figure 4a-e, h). Further, the addition of Ni2+ and Pb2+ metal ion solution with Ag NPs increased the SPR band from 410 nm to 436 and 462 nm respectively (Figure 4f, 4g). Even though Pb interacts with Ag NPs surface plasmon resonance (SPR) peak at 436 near to Ag NPs similar reported (Anambiga et al. 2013). Ag NPs undergo agglomeration upon exposure to increasing concentrations of heavy metal ions. Color change occurred due to the aggregation between Ag NPs with metal ions (Xiong and Li 2008). The ability of silver nanoparticles to form agglomeration caused the SPR to broaden and shift to longer wavelengths (Sulistiawaty et al. 2015). In particular Hg2+ ions interact with Ag NPs precipitation and the SPR band absorbance at 410 nm. Besides, the colour of the Ag NPs became transparent immediately when added to the Hg2+ ions solution. In contrast, increasing the concentration of Ag NPs to Hg2+ solution changed colour into yellow precipitate (Uddin et al. 2017). Similarly, the Ag NPs were interacting with Hg2+ and Mn2+ ions, a blue shift in the SPR band was observed by UV-vis spectroscopy detection. Hg2+ ions were added to the Ag NPs solution and the colour of the solution changed from yellow to colorless. This interaction is due to oxidation of Ag0 to Ag+ during the process of reduction of Hg2+ ions (Annadhasan et al. 2014).
FT-IR measurements were carried out to identify the interaction of Ag NPs and metal ions. FT-IR spectra of Ag NPs functional groups 2924, 2856, 2338, 1714, 1638 cm−1 respectively, CdSO4 and CdSO4 + Ag NPs show several significant absorption peaks such as 2956, 2128, 1638 and 2327, 1639 cm−1 disappears in CdSO4 + Ag NPs. The peaks 2338 shifted into 2339 (Figure. 5a-c). FT-IR spectra of Ag NPs, CuSO4, CuSO4 + Ag NPs, absorption peaks at 2924, 2855 and 2448, 2092 cm−1 were disappeared in CuSO4+AgNPs and new peaks at 2956 shifted into 2969, 1737 shifted into 1738 (Figure 6). Similar study of FeO nanoparticles observation confirms a successful modification of the surface of the nanoparticles with Cd and Cu ions (Klekotka et al. 2018). There were significant changes on the FT-IR spectrum of Ag NPs after interaction with FeSO4 ions. The Ag NPs and FeSO4 absorption peaks at 2956, 2855, 2338, 2128, 1714, 2326, 1642 cm−1 were appearing but disappeared in the FeSO4 + Ag NPs whereas new peaks appeared at 2926, besides the peak at 2924 shifted into 2916 (Figure 7). The FT-IR spectrum of Ag NPs, HgCl2 and HgCl2 + Ag NPs was represented in Figure 8. The presence of a sharp absorption band at 2956, 2125, 2188 and 2097 cm−1 in Ag NPs and HgCl2. HgCl2 + Ag NPs new peak 2855, 1737 shifted into 2853, 1738. FT-IR spectral data revealed absorption peaks at 2956, 2338, 2128, 1737, 1714 and 2199 cm−1 were present in Ag NPs and MnCl2. Whereas the peak at 2855, 1638 shifted into 2853, 1640 present in MnCl2 + Ag NPs (Figure. 9). FT-IR spectra of Ag NPs, NiSO4 and NiSO4 + Ag NPs based on the functional groups 2924, 2856, 2338, 1714, 1638 cm−1 and 3293, 2332, 1638 cm−1 peaks were present and 2332, 2128, 1714, 1638 cm−1 peaks were disappeared in Ag NPs and NiSO4. While 3459, 3016, 2132 and 1434 cm−1 peaks were present in NiSO4 + Ag NPs. 2128 cm−1 line is observed in p-polarization and is absent in s-polarization shifted into 2132 –C≡C– stretch and the presence of a broad absorption band at 3392 and 3459cm−1 can be attributed the –OH stretching presented due to the adsorption of water in air when FT-IR sample disks were prepared in an open air (Figure 10). FT-IR analysis on Ag NPs, Pb(NO)3 and Pb(NO)3 + Ag NPs reveals that the peaks at 2956, 2924, 2855, 2338, 2398 and 1768 cm−1 were present [Ag NPs, Pb(NO)3] but disappear in the Pb(NO)3 + Ag NPs. The broad absorption peak 2128, 1638 shifted into 2125 cm−1, 1654 cm−1 confirms the formation of Pb-Ag NPs complex (Figure 11) (Kamel et al. 2019; Anambiga et al. 2013). FT-IR absorption peaks at 2128, 1714 and 1616 cm−1 were present in Ag NPs and ZnSO4 whereas disappear in ZnSO4 + Ag NPs. A ZnSO4 + Ag NPs new peak appears at 2871 and 1980. The peaks were 2338, 2077 and 1737 cm−1 shifted into 2340, 2079 and 1741 cm−1 (Figure 12). Based on the present study and previous literature report, the conceivable predicted mechanisms of Ag NPs interaction with metal ions which is shown in Figure 13.