Evans Blue, also known as azo blue, molecular formula C₃₄H₂₄N₆Na₄O₁₄S₄, molecular weight 960.80, CAS No. 314-13-6, belongs to a commonly used azo dye preparation, because of its molecular weight is similar to plasma albumin, and in the blood and plasma albumin has a high affinity, so in neuroscience research is often used for tracing to observe the integrity of the blood brain barrier (BBB), also used in cell staining to distinguish between living and dead cells, and also to determine blood volume. Because of its molecular weight similar to plasma albumin and its high affinity for plasma albumin in the blood, it is often used in neuroscience research for tracing and observing the integrity of the blood-brain barrier (BBB), as well as for cellular staining to distinguish between live and dead cells, and for measuring blood volume. Evans Blue is used as a clinical drug for the determination of plasma and blood volume, as well as for the localization of arterial cannulas. Under normal condition, plasma albumin can not pass through the blood-brain barrier, so after staining, if the blood-brain barrier of the nervous system is intact, Evans Blue bound to plasma albumin can not color it, on the contrary, if the blood-brain barrier of the nervous system is destroyed, Evans Blue can enter the nervous system and color it, there are strong peaks at fluorescence wavelengths of 470 nm, 540 nm, and there are weak peaks at 680 nm. It can be detected using chemical dialysis and colorimetric methods.

Evans blue and Tepan blue are both cell-active dyes, commonly used to detect the integrity of the cell membrane and whether the cells are alive; live cells will not be stained blue, while dead cells will be stained light blue, Evans blue staining by counting directly under the microscope or counting after taking pictures under the microscope, can be a more accurate quantification of cell survival, of which 0.5% is the most commonly used concentration; live cells can not be stained by Evans blue because of its exocytosis function. Live cells are unable to be stained by Evans Blue, so dead cells can be distinguished from live cells under the microscope by this method, but dead and necrotic cells cannot be distinguished.