A highly selective and dual responsive test paper sensor of Hg2+/Cr3+ for naked eye detection in neutral water

Abstract

A highly selective and sensitive colorimetric and fluorogenic sensor (L₁) for Hg²⁺/Cr³⁺ is reported. This reagent (L₁) was synthesized by reacting 4-((4 (dimethylamino)phenyl)diazenyl)benzene-1-sulfonyl chloride, which has a dimethylaminophenyldiazenyl fragment as a photoactive signalling unit, with 2,2′-(3,3′-azanediylbis(propane-3,1-diyl))diisoindoline-1,3-dione as the receptor fragment. The reagent was characterized by standard analytical and spectroscopic techniques. Electronic spectral studies revealed that the reagent was selective for Hg²⁺ and Cr³⁺ in the presence of all other metal ions of Group 1A, IIA and all other common transition metal ions. On binding of L₁ to the Hg²⁺ or Cr³⁺ centres, a new intense absorption band with a λ_max of 509 nm appeared with associated changes in the visually detectable solution colour from yellow to red. Fluorescence spectral studies revealed a significant enhancement in the emission intensities upon coordination to Hg²⁺ or Cr³⁺ without any change in the emission wavelength. This could be explained by the efficient interruption of the photo induced electron transfer signalling mechanism involving an unshared pair of electrons from the central tertiary amine centre. An easy to prepare paper test kit, which was obtained by soaking the filter paper in a dichloromethane solution of L₁, presents an approach that could be successfully used in the detection of Hg²⁺ or Cr³⁺ ions present in neutral aqueous media. This indicates the potential application of this dip strip type sensor for the detection of Hg²⁺ and Cr³⁺ in neutral aqueous environments without any spectroscopic instrumentation. Importantly, this reagent binds specifically to Cr³⁺ in the presence of an excess of iodide ions, which act as a masking agent for Hg²⁺. To the best of our knowledge, there are very few examples of detection limits lower than the present test strip for Hg²⁺ in the literature, while, for Cr³⁺, no such report is available.