Pyridine Derivatives as Fluorescent Sensors for Cations ^†

Abstract

A fluorescence grating sensor based on a pyridine derivative was synthesized. The results showed that it can rapidly identify and detect toxic heavy metal ions, such as Cr²⁺, Hg²⁺, Ni²⁺ and Co²⁺. When these toxic heavy metal ions were added to the fluorescence array sensor, different fluorescent responses were produced due to the different binding capacities of the metal ion complex. These toxic heavy metal ions present in environmental water were identified using the fluorescence grating sensor. This sensor provides a fast and convenient method for monitoring toxic heavy metal ions in wastewater.

1. Introduction

Environmental pollution is one of the main concerns of contemporary research.

The content of all metal ions becomes toxic when it exceeds the acceptable threshold.

The recognition of toxic metal ions is receiving particular attention, as they cause serious damage to human health and the environment [1,2].

Over the last few decades, several small-molecule-based fluorescence chemosensors have been developed [3].

There is a wide range of highly electronegative fluorescent heteroaromatics (nitrogen, oxygen, etc.) [4]. In addition, pyridine ring molecules are used as nonlinear optical materials, electrical materials and chelating agents in metal ligand chemistry [5].

From the literature, it is clear that among the fluorescent sensors developed for cation detection, those sensitive to mercury, lead, chromium, nickel, cobalt and thallium are of crucial interest due to their high toxicity [1,3,6,7,8].

In the present work, we report on the synthesis of chemosensors containing the pyrinidic moiety as the fluorophore unit, so we have studied their influence on the detection of toxic cations, including chromium, iron, cobalt, nickel and copper, in acetonitrile/water (1:1) mixtures given the importance of developing water-soluble probes for metal detection in biological and environmental systems.

2. Materials and Methods

These fluorometric chemosensors were analyzed at room temperature. The fluorescence excitation wavelength was set at 432 nm. All ligands were prepared in acetonitrile, maintaining a concentration of 10⁻⁶ M. Metal salt solutions were prepared by diluting the solutions with distilled water. The samples to be analyzed were prepared from a mixture of acetonitrile/water (1:1) at a concentration of 1 × 10⁻⁶ M for compound 2 and at a concentration of 1 × 10⁻⁴ M for the ions studied.

Salts used were as follows:

• CrCl₂ 6H₂O, CoCl₂ 6H₂O, CuCl₂, AlCl₃ 6H₂O, NiCl₂, FeCl₂, FeCl₂ 4H₂O, CaCl₂, ZnCl₂, FeCl₃, HgCl₂, BaCl₂ 2H₂O.
• Fe₂(SO₄)₃ 9H₂O, CuSO₄, CuSO₄ 5H₂O, MnSO₄, BaSO₄, ZnSO₄, NiSO₄, MgSO₄ H₂O, HgSO₄.

3. Results and Discussion

3.1. Synthesis of 2-Amino-3-cyanopyridine Derivatives from Alkenes

The first route involves the preparation of alkene 1 from Knoevenagel condensation between aromatic aldehyde and malononitrile in water at room temperature. Next, 2-amino-3-cyanopyridine derivative 2 was obtained by reacting alkene, acetophenone derivative and ammonium acetate. The reaction took place by conventional solvent-free heating (Scheme 1). The reaction mixtures were subsequently washed with diethyl ether and a small amount of ethanol. The crude product was purified by recrystallization in ethanol to give the product in good yield.

Compound 2 was evaluated as a fluorometric chemosensor, and we evaluated its sensitivity and selectivity in the presence of several metals: Hg²⁺, Mg²⁺, Ba²⁺, Ca²⁺, Mn²⁺, Cu²⁺, Cr²⁺, Co²⁺, Ni²⁺, Zn²⁺, Fe²⁺, Fe³⁺ and Al³⁺.

3.2. Fluorometry Analysis

The results obtained imply that compound 2 does indeed act as a chemoselective agent for Cr²⁺ ions in the presence of a chloride ion (Figure 1a). As expected, our chemosensors had different fluorometric responses towards the different cations studied: compound 2 interacted with Cr²⁺, Co²⁺ and Cu²⁺, revealing remarkable fluorescence intensity at 5796.4, 4626.2 and 4324.4 a.u., respectively, while compound 2 interacting with Ba²⁺ revealed fluorescence quenching (Figure 1a). In addition, we studied the fluorescence behavior associated with the detection of iron with different degrees of oxidation (Fe(II) and Fe(III)). Similarly, compound 2 revealed sensitivity and selectivity for these two ions, with a fluorescence intensity of 1578.7 a.u. for Fe²⁺ and 862.5 a.u. for Fe³⁺ in the presence of a chloride ion (Figure 1a). In contrast, for the same compound, the addition of Fe³⁺ resulted in a significant deviation, reaching 18.208 a.u. in the presence of sulfate as a counter ion (Figure 1b). Compound 2 in the presence of cations such as Cu²⁺, Mn²⁺, Ba²⁺, Zn²⁺, Ni²⁺ and Mg²⁺ showed an increase in fluorescence intensity compared to the ligand. No decrease in fluorescence intensity was observed in the presence of sulfate ions (Figure 1b).

4. Conclusions

Compound 2 revealed good selectivity and sensitivity for the detection of Cr²⁺, Co²⁺ and Cu²⁺ compared with the other metals studied; this ligand has chelating power with Fe³⁺, and it gave rise to a significant deviation in fluorescence intensity with sulfate as a counter ion. From this study, we conclude that product 2 can be considered an original fluoro-ionophore due to its good selectivity and sensitivity to the cations studied as well as to their appropriate counter ions.

Relevant fluorometric responses of compound 2 were observed for the detection of transition metals in acetonitrile/water (1:1) mixtures. However, given the importance of water-soluble probes, the development of this selective sensor will enable many real challenges in the recognition of toxic trace metals in different biological and environmental samples to be solved quickly and easily.

Binding to metal ions is probably attributed to the nitrogen atoms of pyridine moieties as donors. The size and charge of metal ions also affect fluorescence absorption.

Enhancing or reducing fluorescence absorption makes it easier to detect a particular metal ion than many others.