Ag nano particles are not active in UV. the molecules that have double bond are active under UV radiation. so this peak absolutely is lemon.

good luck

If you got colored silver nanoparticles (AgNPs) they will show a UV peak in visible region depending on their size/color(Also the Dispersion media, shape, pH etc) in the range of 400-750 nm.

This peak seems to come from organic ligand, you may give a cross check, just take UV of lemon extract without silver.

First take the absorption spectrum of Ag and Lemon separately and then compare the spectra of mixture of both. The shape of silver plasmonic peak will depend on the shape and size of the silver nanoparticles. However, the position and shape of peak of composite (Ag + Lemon) will certainly be different than the location of peak obtained separately and that will be dependent on the individual peaks of Ag and Lemon.

Take 1mM of Silver Nitrate salt (AgNO3) and add lemon extract drop wise at temperature 80 degree at the same time observe the pale yellow color while adding extract (reducing agent) you will gate silver nanoparticles (UV-Visible peak 410-440 nm)

One of the reason for getting silver nanoparticles peak at 323 nm instead of around 420 is the presence of either elliptical or nanobar type shape of silver nanoparticles than the spherical shape. Elongated or nanobar type samples will produce two resonance peak due to longitudinal and transverse resonance. In such case one peak will be at lower wavelength and other one at higher wavelength than the peak due to spherical particles (425 nm). Similar results can be obtained from

Ind. J. phys. 90 107-116 (2016)