Methylene Blue
A troublemaker in disguise.
Methylthioninium chloride, better known as methylene blue (MB), has been around since the 1870s, first synthesised by German chemist Heinrich Caro. Since then, it has earned a place on the World Health Organization’s list of essential medicines, found widespread use as a dye, and—where Electrochemical Insights readers are most likely to know it—as an electrochemical redox probe.
MB appears throughout the scientific literature, often tethered to aptamers in biosensing platforms or used in solution-phase studies to investigate fundamental electrochemical processes.
However, much of the core literature on methylene blue is old, and many researchers overlook the nuances of how this molecule actually behaves in their systems.
This is especially relevant in the context of aptamer-based biosensors, where MB remains the most common redox label used by manufacturers. Its popularity stems from a combination of low cost, high coupling efficiency, and good stability. But does its ubiquity mean it’s always the right choice?
Chemical Structure of Methylene Blue
MB is a water-soluble, heterocyclic aromatic compound consisting of a central benzene ring structure with two functional side chains. It is photosensitive and typically exists in equilibrium between its monomeric, dimeric, and aggregate forms. In aqueous solutions at low to moderate concentrations, the monomeric form dominates.
MB also exhibits several behaviours that warrant close scrutiny.
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