The UV-visible absorption spectra of samples at different pHs are given in Fig. 2. The prepared samples displayed characteristic SPR absorption peaks of Ag-NPs between 412 nm and 439 nm. The intensity of the SPR band is directly related to the amount of Ag-NPs [33] and the broadening and shifting in the absorption peak are associated with the size and shape of the particles [18]. The position of kmax and the full width half maximum (FWHM) cal- culated from the absorption peaks in the UV-visible spectra are summarized in Table 1.