*2.3. AgNPs Sensing Tests*

A fixed volume of AgNPs in water (typical concentration 1.6 mg/mL) was mixed with a fixed volume of water solution containing the heavy metal ions at specific concentration. After five minutes of interaction of the nanoparticles with the metal ions, the optical absorption spectra were collected in order to verify possible changes (shape, energy and intensity) of the typical localized surface plasmon resonance (LSPR). The response to several metal ions at di fferent concentrations from 25 ppm down to 1 ppm was tested by UV-Vis spectroscopy [27].
