Highly selective colorimetric sensor for cysteine and homocysteine based on azo derivatives

A simple colorimetric method for the determination of cysteine and homocysteine has been developed. The reaction of the azo dyes containing an aldehyde group with cysteine or homocysteine afforded very stable derivatives thiazolidines or thiazinanes under neutral pH conditions. The method is selective and sensitive for cysteine and homocysteine detection without the interference of other amino acids. Importantly, the recognition of Cys and Hcy could be observed by naked eyes.