AN/SIO showed an absorption peak at 520 nm, as the AN solution. Thus, the electrostatic interaction was rather weak.
In addition, the red shift may be caused by the coplanarization in the structure of anthocyanin. Since the flavylium skeleton of the anthocyanin is thought to be strongly adsorbed parallel to the clay mineral layer, it is expected that the orientation of the flavylium skeleton is forced to be more planar than in a solution. Therefore, the coplanarization of the benzopyrylium ring and phenyl group of the anthocyanin can also result in the red shift of the 523-nm peak. The observed spectral change might reflect the sum of these effects.