Cadmium (Cd) contamination in water and soil is considered an environmental pollutant. Food crops can absorb and accumulate bioavailable Cd. Continuous monitoring of Cd levels in the environment can minimize exposure and harm to humans. Visual pigments have been demonstrated to have great potential in the development of minimal-equipment biosensors. In the present study, a metabolically engineered bacterium was employed to produce blue-purple pigment violacein responsive to toxic Cd(II). The high stability of the bisindole pigment contributed to determining the violacein at wavelengths of 578 nm. Visual and quantifiable signals could be captured after a 1.5-h Cd(II) exposure. This novel biosensor showed significantly stronger responses to Cd(II) than to other heavy metals including Pb(II), Zn(II), and Hg(II). A significant increase in pigment signal was found to respond to as low as 0.049 μM Cd(II). The naked eye can detect the color change when violacein-based biosensor is exposed to 25 μM Cd(II). A high-throughput method for rapid determination of soluble Cd(II) in environmental water was developed using a colorimetric microplate.
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