UV-vis spectroscopy is a very useful and reliable technique for the primary characterization of
synthesized nanoparticles which is also used to monitor the synthesis and stability of AgNPs .
AgNPs have unique optical properties which make them strongly interact with specific wavelengths of light . In addition, UV-vis spectroscopy is fast, easy, simple, sensitive, selective for different types of NPs, needs only a short period time for measurement, and finally a calibration is not required for particle characterization of colloidal suspensions [76–78]. In AgNPs, the conduction band and valence band lie very close to each other in which electrons move freely. These free electrons give rise to a surface plasmon resonance (SPR) absorption band, occurring due to the collective oscillation of electrons of silver nano particles in resonance with the light wave [79–84]. The absorption of AgNPs depends on the particle size, dielectric medium, and chemical surroundings [81–85]. Observation of this peak—assigned to a surface plasmon—is well documented for various metal nanoparticles with sizes ranging from 2 to 100 nm [74,86,87]. The stability of AgNPs prepared from biological methods was observed for more than 12 months, and an SPR peak at the same wavelength using UV-vis spectroscopy was observed.