Rationally designed chromogenic chemosensor that detects cysteine in aqueous solution with remarkable selectivity

A sensing ensemble for cysteine was assembled conveniently by simply mixing N,N,N′,N′-tetra-(2-pyridylmethyl)-p-xylylenediamine (TPXD), cadmium perchlorate, and pyrocatechol violet in an 1:2:1 molar ratio in water of neutral pH. In the ensemble, [Cd2(TPXD)]4+ formed from TPXD and cadmium perchlorate serves as the receptor, and pyrocatechol violet functions as the indicator in sensing the analyte. The detection can be made either spectroscopically from the decrease of the UV–visible absorbance at 665 nm or visually from the color change to yellow upon addition of an aqueous solution of the analyte to the solution of the ensemble. The association constant (Kass) for the binding of the indicator to the receptor was determined with an isothermal titration calorimeter to be (2.77±0.98)×105 M−1 and that for the binding of cysteine to the receptor was obtained to have (1.62±0.97)×107 M−1 by the non-linear regression analysis of the titration curve obtained by titration of the solution of the ensemble with cysteine solution. The chemosensor showed excellent selectivity for cysteine over other amino acids including homocysteine.