Sulfo-Cy3-PEG2-TCO disodium is a derivative of the Cy3 (Cyanine3) dye, incorporating sulfonate and TCO groups. The TCO group in Sulfo-Cy3-PEG2-TCO disodium undergoes a highly specific click chemistry reaction with tetrazine functional groups, resulting in covalent bonding. This allows Sulfo-Cy3-PEG2-TCO disodium to be covalently attached to certain biomolecules (particularly antibodies, proteins, etc.), enabling the tracking of their localization and dynamic changes within biological samples.