Absorption and luminescence spectra of cochineal

Carminic acid which can be obtained as extract (cochineal) from certain scale insects is an important red dye with many applications. It shows a red fluorescence under ambient conditions. The lowest-wavelength absorption and the corresponding emission undergo a red shift when carminic acid interacts with metal cations such as Zn2+, Al3+, Gd3+ and Th4+ in aqueous solution. The Al3+ complex of carminic acid which is isolated as Ca2+ salt (carmine) displays this fluorescence also in the solid state while the Ni2+ complex is not luminescent under any conditions owing to the presence of low-energy ligand field states.