DAPI is not very soluble in PBS, even after the addition of heat and sonication. It dissolves better in water than in any buffer. It will not dissolve in PBS because it has a ph of 7.4, while water’s ph is 7.  It’s important to add that because DAPI is hydrophobic, it still has a hard time dissolving in water. You would have to  sonicate the DAPI for it to dissolve.