Abstract

Aromatic isocyanides including isocyanonaphthalene derivatives have been proven to be very effective fluorescent sensors for the quantification of Hg(II) ions in water. Thus, the reaction of 1,5-isocyanoaminonaphthalene (1,5-ICAN), which is one of the most important members of this family, with water and HgCl2 as the oxidation agents, was studied by fluorescence spectroscopy and mass spectrometry in order to get deeper insight into the kinetics and mechanistic details of this complex reaction. The reactions of 1,5-ICAN with water and HgCl2 were performed in various water/co-solvent mixtures of different compositions. The co-solvents used in this study were both aprotic solvents including tetrahydrofuran, acetonitrile and N,N-dimethylformamide and protic solvents, such as ethanol and 2-propanol. It was found that in aprotic solvents the conversion of the isocyano group to amino moiety takes place, while in protic solvents the corresponding carbamate (urethane) group is formed in addition to the amino moiety. The variation of the resulting fluorescence intensities versus time curves were described using an irreversible, consecutive reaction model, in which the formation of isocyanate and carbamic acid intermediates, as well as diamino and carbamate (in the case of protic solvents) products were assumed. The formation of these intermediates and products was unambiguously confirmed by mass spectrometric measurements. Furthermore, by fitting the model to the experimental fluorescence versus time curves, the corresponding rate coefficients were determined. It was observed that the overall rate of transformation of the isocyano group to amino moiety increased with the water concentration and the polarity of the co-solvent. It was also supported that formation of diamino and carbamate derivatives in protic solvents takes place simultaneously and that the ratio of the amino to the carbamate function increased with the increasing water concentration. In addition, with an extension, the model presented herein proved to be capable of describing the kinetics of the transformation of 1,5-diisocyanonaphthalene (1,5-DIN) into 1,5-diaminonaphthalene (1,5-DAN) in the mixtures of water/aprotic solvents.

Highlights

  • Mercury, either in metallic, covalent or ionic form is one of the most toxic heavy metals found in our environment [1]

  • We have shown that 1,5-isocyanoaminonaphthalene (1,5-ICAN) can react selectively with Hg(II) ions in the presence of water to convert the isocyano to amine group [14]

  • The conversion of the isocyano moiety to amino group in the presence of Hg(II) ions is accompanied by a considerable blueshift in the observable fluorescence spectrum, enabling a very sensitive and selective ratiometric determination of Hg(II) ions in aqueous solution with a limit of detection (LOD) as low as 6 nmol/L

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Summary

Introduction

Either in metallic, covalent or ionic form is one of the most toxic heavy metals found in our environment [1]. The conversion of the isocyano moiety to amino group in the presence of Hg(II) ions is accompanied by a considerable blueshift in the observable fluorescence spectrum, enabling a very sensitive and selective ratiometric determination of Hg(II) ions in aqueous solution with a limit of detection (LOD) as low as 6 nmol/L. Another efficient ratiometric fluorescent probe based on 1,8-naphthalimide with an isocyano group for the quantification of Hg(II) ions in aqueous medium has been designed and investigated [15]. We report a detailed kinetic study on the conversion of isocyano to the amino group of 1,5-ICAN in the presence of Hg(II) ions in various water/co-solvent solutions and water-content including co-solvents of both protic and aprotic ones

Experimental Materials
Monitoring the Reaction Using Steady State Fluorescence Spectroscopy
Evaluation of the Kinetic Data
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