Sensing Cu2+ by controlling the aggregation properties of the fluorescent dye rhodamine 6G with the aid of polyelectrolytes bearing different linear aromatic density

The importance of the linear aromatic density of polyelectrolytes on the ability to bind and influence the state of aggregation of dyes such as rhodamine 6G is highlighted. The corresponding complexes present different interaction patterns with metal ions such as Cu²⁺, undergoing different spectroscopic changes. The chemical bases of these changes are discussed. The different polyelectrolytes studied, poly(sodium 4-styrenesulfonate), poly(sodium 4-styrenesulfonate-co-sodium maleate) at two different comonomer compositions (3:1 and 1:1), and poly(sodium acrylate-co-sodium maleate), bear different linear aromatic density and induce different R6G binding patterns, as seen by diafiltration and UV–vis spectroscopy of absorbance and fluorescence. As the linear aromatic density increases, smaller dye aggregates are induced in the systems. Thus, in the presence of a large excess of the polyelectrolyte showing the highest linear aromatic density, the dyes disperse on the polymer domain and no aggregation is detected. The interaction is less sensitive to the cleaving effect produced by the addition of NaCl 0.1M for the complexes that include the polyelectrolytes with the highest linear aromatic density. In the presence of Cu²⁺, the complexes formed with the polyelectrolytes showing the lowest linear aromatic density tend to cleave, producing the release of the dye from the polymer domain. On the contrary, the complexes formed with the polyelectrolytes showing the highest linear aromatic density effectively retain the dye in the presence of the divalent metal ion. Based on fluorescence changes by the addition of different amounts of Cu²⁺ to the solution, the potential of the polyelectrolyte/R6G complexes in applications as sensing materials is discussed.