Methylene blue is one of the oldest organic dyes. It was first prepared in 1876 by H. Caro—BASF’s research director at the time—for use as a cotton dye. In 1880, microbiologist R. Koch established it as a medical stain. It is also used as an indicator for redox reactions, a photosensitizer for generating singlet oxygen, and a drug to treat methemoglobinemia, among other conditions. Until recently, dyestuff-grade methylene blue was used in medicine, but a pharmaceutical grade is now under development.
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