'Blue Informant' in the Dark: Understand Luminol Blood Reaction in One Minute

The Luminol Reaction is a famous chemiluminescent reaction widely used in criminal forensics and legal medicine to detect trace amounts of blood. Even if bloodstains at a crime scene have been wiped away or are old, spraying luminol reagent will reveal a bright blue-green fluorescence emitted by excited-state products in a dark environment. Its core principle lies in redox reactions and catalysis. Luminol reagent is typically a mixture of luminol, hydrogen peroxide, and an alkaline substance. When the reagent comes into contact with blood, the iron ions (Fe²⁺ or Fe³⁺) in hemoglobin act as highly efficient catalysts, accelerating the decomposition of hydrogen peroxide and releasing oxygen. The oxygen then oxidizes the luminol dianion in the alkaline environment, forming an unstable peroxide. As its structure returns to the ground state, it releases energy in the form of light, producing a blue glow for about 30 seconds. While the luminol reaction is highly sensitive (up to one in a million) and does not destroy DNA for extraction, it has the drawback of false positives. Household chlorine bleach, bleaching powder, transition metals like copper and alloys, and even animal blood and urine can trigger the same catalytic luminescence reaction. Therefore, investigators typically need to conduct further tests for confirmation.