Sulfo-Cy3-PEG2-TCO is a fluorophore featuring a sulfonate group and a TCO moiety. TCO is a click chemistry handle which readily reacts with tetrazine groups on a target molecule. Cy3 is a cyanine dye with excitation and emission maxima at 555 nm and 570 nm respectively. The sulfonate group on the Cy3 dye increases this compound’s solubility in water.