Development of a Reversible Indicator Displacement Assay Based on the 1-(2-Pyridylazo)-2-naphthol for Colorimetric...

A new colorimetric chemosensor for naked-eye detection and determination of cysteine (Cys) based on indicator displacement assay (IDA) was designed using 1-(2-pyridylazo)-2-naphthol (PAN). The indicator exchange occurred between PAN and Cys by the addition of Cys to the Cu(PAN)₂ complex, which is accomplished by an immediate visible color change from magenta to yellow, in the solution phase and paper-based test strips. The proposed method exhibits 0.35 μmol L^-1 detection limit and good linearity in the range of 2.25-42.91 μmol L^-1. Paper test strips presented a detection limit of 38.0 μmol L^-1, fabricating an easy to use test kit for compatible "in-the-field" detection of Cys. The computer image analysis of the paper test strips, obtained from the CMYK color analysis system, showed a linear increase in Y (yellow) intensity with enhancement in the Cys concentration of 50.0-550.0 μmol L^-1. Additionally, the absorption and color change obtained in this chemosensor operate as an "INHIBIT" logic gate considering Cu²⁺ and Cys as inputs. Eventually, based on such a fast, reversible, and reproducible signal, a molecular-scale sequential memory unit was designed displaying "Writing-Reading-Erasing-Reading" and "Multi-Write" behavior. The developed chemosensor presented a satisfactory repeatability, intermediate precision, and successful application for the selective determination of Cys in human biological fluids. Graphical abstract .