Determination of L-cysteine base on the reversion of fluorescence quenching of calcein by copper(II) ion

We report on a simple and sensitive method for the determination of L-cysteine (Cys). It is based on a redox reaction between the non-fluorescent Cu(II)-calcein complex and Cys which results in fluorescence recovery of calcein. When Cys is added to a solution of the Cu(II)-calcein complex, Cu(II) is reduced to Cu(I), and calcein is released to form a strongly fluorescent complex with Zn(II). The effect was used to develop a fluorescence enhancement method for the determination of Cys. Under the optimum conditions, the increase in signal intensity is linear in the range from 3.0 x 10(-7) to 1.2 x 10(-5) mol L-1, with a correlation coefficient (R) of 0.9978. The limit of detection (3 sigma) is 4.0 x 10(-8) mol L-1. The relative standard deviation (RSD) in the determination of 11 samples containing 5.0 x 10(-6) mol L-1 of Cys was 3.5%. There is little interference by common ions and other amino acids. The method, which is simple, rapid, and sensitive, was successfully applied to the determination of Cys in human serum samples.