SBFI AM is a membrane-permeant, fluorescent Na+ indicator dye. It is selective for Na+ over K+ with Kd values of 20 and 120 mM for these ions, respectively. [1]Increasing concentration of Na+ increases the ratio of excitation efficiency at 330-345 nm to that at 370-390 nm with emission collected at 450-550 nm. [1] Therefore, ratio fluorometry and imaging can be performed with the same wavelengths. SBF AM is suitable for single intracellular dye injection and is typically used for high-resolution measurements of Na+ concentration in intact tissue through two-photon confocal imaging.[2]