Adsorption and desorption behavior of an anionic pyrene chromophore in sequentially deposited polyelectrolyte-dye thin films

Alternating thin films of the anionic dye pyrenetetrasulfonic acid (4-PSA) and poly(allylamine hydrochloride) (PAH) were formed by the sequential deposition of 4-PSA and PAH from aqueous solution onto solid substrates. Film growth was followed by absorption spectroscopy, and the resulting polyelectrolyte dye multilayer assemblies were characterized by using fluorescence spectroscopy, X-ray reflectivity, and atomic force microscopy (AFM) measurements. 4-PSA was successfully deposited in alternation with PAH when the ionic strength of the BAH solution was greater than 0.1 M. At each PAH adsorption step a given amount of previously adsorbed 4-PSA was extracted. The amount of dye released depended on the film thickness and the salt concentration in the PAH solution from which PAH was subsequently adsorbed. However, the total amount of dye remaining in the film after PAH deposition was essentially independent of the salt content in the PAH solution. Both X-ray reflectivity and AFM measurements revealed a high roughness of the 4-PSA/PAH multilayer films (ca. 20 Angstrom). To gain a better understanding of the parameters that control dye adsorption and extraction, dye adsorption was also investigated by immersion of preformed PAH and poly(styrene sulfonate) (PSS) multilayer films of various thickness into 4-PSA solutions. The amount of 4-PSA absorbed by the films increased with film thickness, and saturated at a thickness above about 20 nm. For these films, subsequent adsorption of a PAH layer extracted most of the dye, except for a small amount approximately equal to that present in one 4-PSA/PAH bilayer prepared by the alternate adsorption process. These observations indicate that the dye molecules are adsorbed to a finite depth of about 20 nm, and after deposition of the oppositely charged PAH, a small amount that is independent of film thickness remains. The presence of this remaining 4-PSA is necessary to complex and bind the next PAH layer, hence allowing continued multilayer film growth. It was also found that the 4-PSA adsorption/desorption process depended on the drying process that was employed during the formation of the PSS/PAH multilayer films. The amount of dye as well as the penetration depth was reduced when the film was dried between deposition of each layer, and minor changes in the film structure were observed by X-ray reflectivity measurements.