Enhanced fluorescence effect of acridine orange sorbed on 2:1 layered clay minerals

Many dye molecules form different aggregation states in aqueous solution under different concentrations. Acridine orange (AO) is a cationic fluorescence dye and is commonly used in DNA analyses. Most measurements for light absorption and fluorescence emission of dyes sorbed on solid matrix reported in literature were performed in clay-dye suspension. Rarely, it was evaluated in solid state. In this study, the fluorescence effect of AO, after being sorbed on 2:1 layered clay minerals montmorillonite (MT), rectorite (RT), and illite (IT), was evaluated in terms of contributions by different forms of AO (monomer, dimer, H- and J-aggregates). As the amounts of AO sorbed increased, the proportion of absorption spectra made by monomer and dimer decreased, while that made by of H- and J-aggregates increased. In sorbed state, the absorbance of AO monomer and dimer was obvious while that attributed to H- and J-aggregates was higher in comparison to that in AO solution. At AO loading of 30 mu mol/g on MT and 24 mu mol/g on RT and IT, fluorescence emission reached maximum intensity. While the majority of the fluorescence was attributed to monomer and dimer, an unknown peak at 580-590 nm was resolved in the fluorescence spectra. Due to its red shift, it was attributed to J-aggregates on the surface of the minerals. The contribution by H-aggregates was observed in absorption spectra, but not in fluorescence spectra, which was speculated that the blue shift caused by H-aggregates overlapped with the excitation wavelength of 490 nm. The 2:1 clays could serve as a substrate for optimal dye sorption to achieve maximum fluorescence emission effect and practical applications of such dye-clay composites in powder form could be further explored.