Pattern-Based Recognition of Thiols and Metals Using a Single Squaraine Indicator

The design of a sensor array that uses a single entity as both the host and the indicator (squaraine dye, SO) to differentiate a series of metal ions and a series of thiols is reported. The metal ions and thiols act as both analytes and modulators of the squaraine response allowing pattern-based discrimination. Mercury(II), palladium(II), copper(II), iron(II), and nickel(II) can be discriminated when combining SO with five thiols: propane thiol (PT), 3-mercaptopropionic acid (MPA), naphthalene-2-thiol (NT), 2,3-dimercaptopropanol (DMP), and 2-acetylamino-3-mercaptopropionic acid methyl ester (ACM). Likewise, the five thiols can be discriminated using SO and the five metals. For example, SO in combination with 2-acetylamino-3-mercaptopropionic acid methyl ester (ACM) afforded very good differentiation of all five metal ions. However, propanethiol, 3-mercaptopropionic acid, and naphthalene-2-thiol produced very similar differentiation of the considered metal ions. On the other hand, all metal ions considered in this study are able to discriminate 2,3-dimercaptopropanol (DMP) and 2-acetylamino-3-mercaptopropionic acid methyl ester (ACM) clearly and completely, both from one another and from the other three thiols (PT, NT, MPA). Importantly, mercury(II) is the only metal ion able to effect the discrimination of naphthalenethiol (NT) from PT and MPA, thus giving the best discrimination overall. The study shows that complex discrimination of widely diverse classes, metal ions and thiols, can be achieved via a single receptor/indicator.