We investigated a colorimetric chemosensor for Hg2+ based on a mixture of xylidyl blue I as an anionic organic dye and poly(diallyldimethylammonium chloride) as a cationic polyelectrolyte in an aqueous solution at pH 7.5. The addition of Hg2+ to the mixture induced a bathochromic shift in the absorption spectra with a distinct color change from red to green which was readily identifiable by the naked eye, whereas the other metal ions gave rise to insignificant color changes. By contrast, upon adding Hg2+ to xylidyl blue I alone, the solution underwent no significant change in color. Moreover, a stoichiometric ratio for the complex between xylidyl blue I and Hg2+ in the presence of poly(diallyldimethylammonium chloride) was determined to be 1:1 by the absorption titration curve and Job's plot. Thus, the mixture can be used as a selective naked‐eye colorimetric chemosensor for Hg2+ over other common metal ions. This study raises the possibility that the combination of an organic dye and an oppositely charged polyelectrolyte is a potential candidate for the easy construction of a new chemosensor system. © 2018 Society of Chemical Industry

A new naked‐eye colorimetric sensor for detecting Co2+ in a pure aqueous solution at pH 7.0 was developed by mixing an anionic organic dye, 2‐(5‐bromo‐2‐pyridylazo)‐5‐(N‐propyl‐N‐sulfopropylamino)phenol (5‐Br‐PAPS) with a cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (PDADMAC). Upon addition of Co2+, the absorption maximum exhibited a remarkable bathochromic shift from 426 to 635 nm (Δ = 209 nm) with a detectable color change from yellow, green to blue, while other metal ions did not induce such a change. The results showed that the 5‐Br‐PAPS and PDADMAC mixture can be used as a selective and sensitive naked‐eye colorimetric chemosensor for Co2+. This study afforded a simple strategy to construct a novel chemosensor by a simple mixture of an organic dye and an oppositely charged polyelectrolyte. © 2018 Society of Chemical Industry

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The pyrogallol red (PR)-based Cu complex was proven to be an effective and selective colorimetric chemosensing ensemble for recognition of oxalate over other anions in a perfect aqueous solution. The addition of oxalate to the PR-Cu complex resulted in a colour change from purple to orange colour due to the regeneration of PR by the chelation of oxalate with Cu , while other anions did not induce any significant colour change. Moreover, it was revealed that no obvious interference was observed during the titrations with oxalate into each other anion. Therefore, the PR-Cu complex can be used as a simple and practical colorimetric chemosensor for detecting oxalate.

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