Electrostatically assembled fluorescent thin films of rare-earth-doped lanthanum phosphate nanoparticles

We report on the formation of nanostructured, ultrathin films of a new class of fluorescent nanoparticles (NPs) within the same size range as quantum dots, but with a different kind of fluorescence, by using the layer-by-layer (LbL) technique. The fluorescence of these rare-earth-doped lanthanum phosphate (LaPO4) NPs is due to the bulk properties of the material and is therefore independent of their size. Additionally, different colors are available due to variation of the dopants used in their synthesis (e.g., Ce, Tb, Eu, and Dy). These NPs were electrostatically assembled to form thin films on planar quartz supports as well as on polystyrene (PS) microspheres with the aid of oppositely charged polyelectrolyte interlayers. Regular growth of the multilayers was observed on both quartz and PS substrates, providing control over the composition and, in turn, the fluorescence intensity of the planar films and composite spheres. The resulting films exhibit the same UV and fluorescence characteristics as the corresponding aqueous NP dispersions. NPs with different fluorescence characteristics (i.e., colors) retain their individual optical properties when premixed and assembled into thin films and can be detected individually using only a single excitation wavelength. Biofunctionalization of LaPO4 NP-tagged PS spheres and of the individual LaPO4 NPs is expected to yield versatile labels that could be exploited in various biological applications.