A reversible competition colorimetric assay for the detection of biothiols based on ruthenium-containing complex

A novel reversible colorimetric sensor, which based on a competitive ligation of Hg2+ by thiols, cysteine (Cys) or glutathione (GSH), and thiocyanate (SCN) on the N3 dye (bis(4,4′-dicarboxy-2,2′-bipyridine) dithiocyanato ruthenium (II)), was developed for the detection of biothiols. First, Hg2+ ions coordinate to the sulfur atom of the dyesʹ SCN groups, and this interaction induces a change in color from red to yellow, owing to the formation of a complex of Hg2+–N3. Then, in the presence of biothiols, the red color of N3 is recovered concomitantly with the dissociation of the Hg2+–N3 complex, due to the extraction of Hg2+ by biothiols. Thus the corresponding color variation in the process of the dissociation of the Hg2+–N3 complex can be employed for the quantitative detection of thiols using UV–vis spectroscopy. In particular, the transformation can be readily viewed with the naked eye. A good linear relationship between the change in absorbance (ΔAbs) of Hg2+–N3 at 461 nm and the thiol concentration was obtained in the range of 0.5–25 μM, and the detection limits are then calculated to be 57 and 52 nM for Cys and GSH, respectively. The proposed colorimetric assay displays a high selectivity for Cys over various other amino acids and GSSG (oxidized glutathione).