A spiropyran-based ensemble for visual recognition and quantification of cysteine and homocysteine at physiological levels

(Chemical Equation Presented) Isomerization of the ligand, followed by cooperative metal-ligand interactions leads to the ternary ensemble shown on the right, which has potential for the recognition and quantification of cysteine and homocysteine. The free ligand (left) is red-violet in solution, but when metal ions and cysteine or homocysteine are simultaneously introduced, the solution turns yellow.