what shoud i say in a follow-up email to prospective PhD supervisors after I submitted my applications
For those with whom you communicated previously, a follow up is appropriate. But it can be very short and simple. Just say that you have applied and thank them for their help. Add that you remain interested in their research and in working with them in the future. 

You don't need to remind them of anything. The university procedures will take care of that. The "thank you" part is the most important.