A selective colorimetric sensing strategy for cysteine based on an indicator-displacement mechanism

Abstract

Rapid determination of cysteine in aqueous solution is important for the diagnosis and treatment of some diseases. We herein present a simple yet facile colorimetric indicator-displacement assay for cysteine in aqueous solution with a sensitive optical readout based on the competitive binding affinity between the 1-(2-pyridylazo)-2-naphthol (PAN) indicator and cysteine target towards the host Zn²⁺. In the absence of cysteine, Zn²⁺ can induce an appreciable yellow-to-pink color change for the PAN solution through the formation of a PAN–Zn²⁺ complex. However, adding cysteine to the above PAN–Zn²⁺ complex solution resulted in a ligand exchange reaction between PAN and cysteine with about 57 nm blue-shift in the absorbance maximum (from 527 nm to 470 nm), accompanied by a visible color change of the solution from pink to yellow. The obtained indicator-displacement assay provides a good detecting performance for cysteine with a detection limit of 7.32 μM (S/N = 3) using a UV/vis spectrometer as well as with a good linear range from 25 to 1000 μM. In particular, the proposed sensing system could be successfully used for the fabrication of a colorimetric INHIBIT logic gate and converted into the test paper and image capture analytical devices for the onsite visual determination of cysteine in practical applications.