||Researches on the optical biosensors for cysteine have been developed in supramolecular and biological chemistry. Because cysteine is an amino acid that contains sulfur, it is allowed to bind in a special way and maintain the structure of proteins in the body. Mercury ion also has received increasing attention because it has specific bindings with some materials such as sulfur-mercury and thymine-mercury-thymine interaction. Using these unique mercury interactions, we have prepared conjugated polyelectrolyte-mercury-thymine complex. Aggregation of the conjugated polymer chains resulted in enhanced exciton migration, leading to long wavelength fluorescence. If cysteine was added in complex solution, fluorescence of polymer exhibited original blue emission. In this way, the fluorescence color change of complex can be used to detect cysteine. Furthermore, using the feature of enhanced exciton migration present in the solid state, we have performed the in vivo monitoring of cysteine.