You will just have to be careful with background issues, even the serum-free media might have higher background than just Ringer's. How will you buffer pH?
If you need to get concentration values in nM you will have to apply a ratiometric method, and perform a calibration protocol. The serum free media might affect the background signal, but in my experiences, this is ok with Fluo-8, which gives a good Signal/noise ratio.