Gold nanoparticles (Au NPs) were entrapped within, and in between, a cross-linked thin chitosan film, casted onto a glass substrate. The glass/chitosan/Au NPs interface was formed by casting Au NPs-chitosan solutions onto flat glass supports, followed by exposure to Au NPs in solution and repeating the casting procedure for the desired number of layers. The optical properties of the resulting interfaces and their ability to sense various heavy metal ions, such as Fe3+ and Cu2+, were investigated utilizing the phenomenon of localized surface plasmon resonance (LSPR) visualized by UV/V is absorption spectroscopy. The interfaces investigated showed different optical behavior to the presence of the different metal ions. An increase of the LSRP absorption maximum at lamdamax = 517 nm was observed upon Fe3+ binding with a detection limit of 0.5 microM and a linear range up to approximately 2 microM. Due to a lower binding capability of Cu2+, the detection limit is 0.5 mM with a linear range up to approximately 5 mM.