In situ generation of fluorescent silver nanoclusters in layer-by-layer assembled films

Abstract

Fluorescent Ag nanoclusters have been easily generated in layer-by-layer (LBL) films of poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA) by photo-reduction of Ag⁺ ions, which were previously loaded via an ion exchange mechanism. Under UV irradiation, Ag⁺ ions were reduced to Ag atoms by the photo-generated radicals and then aggregate to form fluorescent nanoclusters. The Ag⁺ concentration plays a key role in the determination of the growth rate of Ag particles. Careful tuning of the Ag loading in the film prevented the growth of the fluorescent nanoclusters into larger non-fluorescent nanoparticles. Fluorescent Ag nanoclusters can be generated in both wet and dry films. However, the fluorescence intensity achieved in dry films is lower than that in wet films because the tight confinement of the polymer network retards the diffusion and aggregation of the photo-generated Ag atoms. Interestingly the fluorescence intensity can be enhanced significantly by a post-treatment in water, as it allows the Ag atoms to diffuse and aggregate to form more fluorescent nanoclusters. This work shows that the PVPON/PAA LBL films can serve as nanoreactors that allow the in situ generation of fluorescent Ag nanoclusters, yielding fluorescent Ag nanoclusters in a technically important form, in addition to providing a simple way to control the growth of the nanoparticles.