Hello Nicole Abou Hamad

FITC is unstable in water. I would suggest that if you do not prefer DMSO, there are other organic solvents which could try such as DMF or ethanol to make a stock solution. You could first make a FITC stock solution in organic solvents like DMF or ethanol and then further dilution of the FITC stock solution could be made in an aqueous buffer (like HBSS) instead of water.

Best.