[Neuroscience] which dye do I choose? It shouldnot cross the synapse , and keep the fluorescence for a long time (a few months)?

[kongzd from gmail.com]

Hi
my experiment is transplanted the neurons to the nerve tissue of host.
Now I want to observe the synapse between the transplanted neurons and
host neurons. So before transplantation, i go to find a dye to
staining the  transplanted neurons.
this dye shouldnot cross the synapse,so I can obsevre the synapse in
electron scope.
and the experimentsin vivo lasts a long time ,I want to it keep the
fluorescence for about 4 month.

is there a dye recommended? does DiI or Dio meet the requirement? I
really donnot konw if DiI can cross the synapse.
thank you.