Why is Fura-2 unable to cross cell membranes?

Fura-2 is a pentacarboxylate ratiometric indicator dye that binds to free intracellular calcium. It is widely used to measure intracellular calcium. The one limitation of Fura-2 is that it lacks the acetoxymethyl groups that are required to make the molecule neutral. As a charged molecule it is unable to cross cell membranes by itself. Loading them inside the cell requires the use of mechanical or chemical assistance. Fura-2 AM, cell-permeable derivative that is more lipophilic, is often used as a popular alternative to Fura-2.