formazans

Formazans are a family of organic dyes formed by the reduction of tetrazolium salts to formazan compounds. The resulting dyes are typically intensely colored and can vary in hue depending on the substituents on the starting tetrazolium salt. Formazans are commonly poorly soluble in water, a property that influences how they are used in assays; many protocols require dissolving the precipitated dye in a solvent or using a water-soluble formazan derivative.

In laboratory settings, formazans are used as redox indicators. The reduction reaction is facilitated by cellular dehydrogenases or chemical reducing agents, making the amount of formazan a proxy for metabolic activity, viability, or cytotoxic effects. This principle underpins widely used colorimetric assays in cell biology, most notably the MTT, XTT, MTS, and WST families. XTT, MTS, and WST variants are designed to yield water-soluble formazans, simplifying measurement.

Limitations include potential non-specific reduction, dependence on the cellular redox state, and interference by compounds that affect dye solubility or light absorption. The choice of tetrazolium reagent and solubilization method affects assay sensitivity and linearity.

Formazans have also been used in materials science and diagnostics as redox indicators, though their primary prominence is in bioassays. Safety considerations involve handling chemical dyes and solvents according to standard lab protocols.