A selective naked-eye detection and determination of cysteine using an indicator-displacement assay in urine sample

A simple and selective colorimetric method for the naked-eye detection and determination of cysteine in biological fluids using Zincon complexed with Zn2+ ions as an indicator displacement assay (IDA) is described. Adding cysteine to a solution of the Zincon-Zn2+ complex resulted in a ligand exchange reaction between Zincon and cysteine, with about 133 nm blue shift in the absorbance maximum (from 617 nm to 484 nm), accompanied by an obvious color change of the solution from blue to orange-yellow. The association constant for the binding of Zincon to Zn2+ was determined with a colorimetric titration of (K = 1.01 × 107) and that for the binding of cysteine to Zn2+ was found to be (K = 4.10 × 1011) by the non-linear regression analysis, the Newton-Gauss-Levenberg-Marquartd (NGLM) algorithm. A good linear response of absorption intensity as a function of cysteine concentration was obtained ranging from 4.0 to 94.0 μM with the detection limit of 1.0 μM. The relative standard deviation for 10 replicate detections of 100 μM cysteine was 0.24%. The developed bioassay was successfully applied to selective detection and determination of cysteine in human urine samples with recoveries between 91.0% and 113.0%.