SW116 free base is a selective fluorescent ligand for sigma-2 receptor with a K i of 14 nM. SW116 free base exhibits in methanol a maximal excitation wavelength of 333 nM, and a maximal emission wavelength of 506 nM. SW116 free base can be internalized into MDA-MB-435 cells, and reach 50% maximum fluorescent intensity in 24 minutes. SW116 free base can be as a fluorescent probe used in cancer research.