Laser-induced localized formation of silver nanoparticles on chitosan films: study on particles size and density variation

We present the in situ localized formation of silver nanoparticles in chitosan films by pulsed UV laser irradiation, as a result of the photoreduction of the silver nitrate precursor loaded throughout the volume of the polymer. The UV pulsed irradiation is also found to be responsible for the photofragmentation of the previously formed nanoparticles, leading to their average size reduction as the number of pulses increases. In fact, their diameter changes from similar to 150 to similar to 30 nm for irradiation with 5 to 200 pulses, respectively. After irradiation the formation of nanoparticles continues for several days, since the already formed nanoparticles act as seeds for the reduction of the unreacted precursor. Indeed, few weeks after irradiation, the chitosan films present a metallic mirror-like appearance on the previously irradiated areas, as they are fully covered by silver. Taking advantage of all these simultaneous mechanisms, and controlling the number of pulses and elapsed time after irradiation, Ag nanoparticles of specific size can be formed in situ on desired areas of the film. Through this process is envisioned the fabrication of nanocomposites with functional properties.