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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

The commercially available natural organic dye, carminic acid (CA), an anthraquinone derivative bearing hydroxyl and carboxyl groups as recognition sites was found to be a colorimetric probe for Pb in perfect aqueous solution under neutral conditions with specific selectivity and high sensitivity. Upon addition of Pb, the absorption maximum of CA showed a large red shift, and the resulted color change from red to purple could be easily identified even by the naked eye. The chemical stoichiometric ratio between CA and Pb was determined to be 1:2 through Job plot, Pb titration, and kinetic experiments. Moreover, other environmental relevant metal ions induced no or minimal spectral and color changes. The reversibility of Pb to CA with EDTA even through several cycles was established for practical applications. The results indicated that CA can be a good candidate for simple, convenient and reversible colorimetric detection of Pb in aqueous solution even though it was hard to be applied to determine Pb on the water testing by US EPA.

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Novel fluorescent chemosensors (2,3-NE, 2,3-NE-Ac) containing naphthalene as fluorescent signaling moiety and urea groups as anion binding sites were designed and synthesized, and their spectral behaviors toward various anions were investigated. The chemosensor 2,3-NE was prepared by reacting 2,3-diaminonaphthalene with ethyl isocyanatoacetate and 2,3-NE-Ac was obtained by saponifying 2,3-NE with Na 2 CO 3 . The chemosensor 2,3-NE was shown to signal selectively the detection of fluoride ion in the fluorescence spectra in CH 3 CN/DMSO (9:1, v/v) even extremely low concentration of fluoride anion. The significant spectral changes indicate that 2,3-NE can be a highly sensitive fluorescent chemosensor for fluoride anion by highly efficient quenching between the excited state of the fluorophore and fluoride ion. Job plot studies revealed that the presence of fluoride ion induces the formation of a 2:1 complex between 2,3-NE and fluoride ion. In addition, the binding mode with fluoride ion was investigated by 1 H NMR experiments. The chemosensor 2,3-NE-Ac also recognized fluoride anion, but is approximately 10-fold lower in sensitivity than 2,3-NE by the influence of the carboxylic acid groups. 2,3-NE can become a promising chemosensor for fluoride ion with high sensitivity and selectivity.