Fluorescent dye that is water-soluble and forms stable Schiff Base with carbonyl group. Fluorescent Dye 488-I Hydrazide conjugates are significantly brighter than fluorescein conjugates and much more photostable. Additionally, the fluorescence of Fluorescent Dye 488-I Hydrazide is not affected by pH between 4-10. Compared to Alexa Fluor 488, Fluorescent Dye 488-I Hydrazide of single isomer has much hihger purity, bringing better performance consistancy from batch to batch.