Pb 2+ Sensors

1.3.1.1:

Phenanthroline-based colorimetric sensors 1 and 2 have been designed, synthesized, and compared with phenanthrene-based receptor 3 for sensing of Pb²⁺ by color change.

Receptor 1 imparts color change (from yellow to red) selectively with Pb²⁺ in acetonitrile/water (9:1) as well as in methanol/water (9:1) when in the presence of other metal ions studied (Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Mn²⁺ as their perchlorate salts). Receptor 1 also shows fluorescence enhancement upon addition of lead perchlorate in acetonitrile/water (9:1) solvent possibly due to the chelation enhanced fluorescence (CHEF) effect (See Figure 1.13).

However, the binding behavior of 2 with Pb²⁺ is found to be less effective compared to that of receptor 1.³²

Figure 1.13. UV/Vis spectra of receptor 1 with the addition of different metal ions (left);

Fluorescence spectra of receptor 1 the addition of increasing amounts of Pb²⁺ [0, 0.5, 1, 1.5, 2, 3, 5, and 7 equiv.] (right).

17 1.3.1.2:

A novel organic–inorganic fluorescent material as chemosensor containing 2-substitute imidazole-[4,5-f]-1,10-phenanthroline derivative was prepared by sol–gel reaction, and the binding ability of hybrid material with metal ions was evaluated, and the results indicated that the hybrid material can selectively recognize Pb²⁺ (Figure 1.14). By examining the ability of hybrid material to adsorb Pb²⁺ in solid liquid phase, 98.3% of Pb²⁺ was adsorbed onto the surface of hybrid material, and the hybrid material can be repeatedly utilized with suitable treatment, where the “off-on-off” process was due to the pH modulation of the phenanthroline ligand. The combination of well-defined inorganic matrix and functionalized organic receptor can play a pivotal role in the development of a novel generation of functionalized composites.³³

Figure 1.14. Organic–inorganic hybrid material containing 1,10-phenanthroline (left);

Adsorption capacity (%) of hybrid material upon the addition of metal ions (right).

1.3.1.3:

The ferrocene-imidazopyrene dyad, bearing the imidazole ring as the only receptor site, acts as a redox and optical molecular sensor for ion pairs, exhibiting an easily detectable signal change in the redox potential of the ferrocene/ferrocinium redox couple and in the emission spectrum. Perturbation of the emission spectrum follows the order Pb²⁺ > Hg²⁺ > Zn²⁺ for

18 cations and H2PO4

> AcO^- for anions (Figure 1.15). ie. The ferrocene-imidazopyrene dyad 3 behaves as a host-separated ion pair receptor. A salient feature of this simple receptor is the presence of only one receptor site, the imidazole ring, which is able simultaneously to recognize an anion and a cation through variation of the oxidation potential of the ferrocene/ferrocinium redox couple and a remarkable perturbation of the emission spectrum.³⁴

Figure 1.15. (Left): (a) Changes in the absorption spectra of 3 (black) (5 x 10^-5 M) in CH3CN upon addition of increasing amounts of Pb(ClO₄)₂ , until 1 equiv (purple). (b) Visual features observed by passing from 3 to the complex 3.Pb²⁺. (Right): Changes in the fluorescence emission spectra of 3 (black) (c = 1 x 10^-5 M in CH3CN) upon addition of increasing amounts of (a) Pb(ClO₄)₂ until 0.5 equiv (purple) and (b) [(n- Bu)₄ N]H₂PO₄ until 2 equiv (blue).

19 1.3.1.4:

Among Ferrocene-imidazoquinoxaline dyads 6a and 6b, showed selective sensitivity towards Pb²⁺ and Hg²⁺ respectively (Figure 1.16) Dyad 6a behaves as a highly selective redox, chromogenic and fluorescent chemosensor molecule for Pb²⁺ cations in CH3CN solutions; the oxidation redox peak is anodically shifted (ΔE1/2 = 110 mV); in the absorption spectrum a new low-energy band appeared at λ= 463 nm, and the emission band is red-shifted (Δλ = 31 nm) along with an important chelation-enhanced fluorescence factor (CHEF = 276), upon complexation with this metal cation. The dyad 6b, bearing two additional pyridine rings as substituents, has shown its ability for sensing Hg²⁺ cations through three different channels:

the oxidation peak is anodically higher shifted (ΔE1/2 = 300 mV), a new low-energy band appears in the absorption spectrum at λ = 483 nm, and the emission band was also red-shifted (Δλ = 28 nm) and underwent an important chelation-enhanced fluorescent factor (CHEF = 227). The changes in their absorption spectra are accompanied by color changes from yellow to orange which allow their potential use for the “naked eye” detection of these metal cations.

Linear sweep voltammetry revealed that Cu²⁺ cations induced oxidation of the ferrocene unit in both dyads, which is accompanied by an important increase of the emission band.³⁵

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Figure 1.16 (Left): (a) Changes in the fluorescence spectra of 6a (1 x 10^-5 M) in CH3CN upon addition of Pb²⁺ (dotted line) and Cu²⁺(dashed line) metal cations (λexc = 330 nm). (b) Fluorescence emission intensity of 6a upon addition of 0.5 equivalents of Pb²⁺ in the presence of 0.5 equivalents of interference metal ions in CH₃CN. (Right): Changes in the fluorescence spectra of 6b (1 x 10^-5 M) in CH3CN upon addition of Hg²⁺ (dotted line) and Cu²⁺ (dashed line) metal cations (λexc = 310 nm). (b) Fluorescence emission intensity of 6b upon addition of 0.5 equivalents of Hg²⁺ in the presence of 0.5 equivalents of interference metal ions in CH3CN.

1.3.1.5:

Ferrocene–imidazophenazine dyads 4 and 7 showed selective sensing towards Pb²⁺ and Hg²⁺

respectivily (Figure 1.17). Dyad 4 behaves as a highly selective chemosensor molecule for Pb²⁺ cations in CH₃-CN/H₂O (9:1). The emission spectrum ( λexc = 317 nm) undergoes an important chelation-enhanced fluorescence effect (CHEF = 47) in the presence of Pb²⁺ cations. A new low-energy band appeared at 502 nm, in its UV/vis spectrun, and the oxidation redox peak is anodically shifted (ΔE1/2 = 230 mV). The presence of Hg²⁺ cations also induced a perturbation of the redox potencial although in less extension than those found with Pb²⁺ cations. Dyad 7, bearing two fused pyridine rings, has shown its ability for sensing Hg²⁺ cations selectively through three channels: electrochemical, optical, and fluorescent; the

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oxidation redox peak is anodically shifted (ΔE1/2 = 200 mV), a new low-energy band of the absorption spectrum appeared at 485 nm, and the emission spectrum (λexc = 340 nm) is red-shifted by 32 nm accompanied by a remarkable chelation-enhanced fluorescent effect (CHEF

= 165). Linear sweep voltammetry revealed that Cu²⁺ cations induced oxidation of the ferrocene unit in both dyads. ¹H NMR studies have been carried out to obtain information about the molecular sites which are involved in the binding process.³⁶

Figure 1.17. (Left): Changes in the absorption spectra of 4 upon addition of increasing amounts of Pb(ClO4)2. (Middle): Changes in the fluorescence emission spectrum of 4 upon addition of Pb(ClO₄)₂. (Right): Changes in the fluorescence emission spectrum of 7 upon titration with Hg(OTf)2.

22 1.3.1.6:

A new pyrene-based sensor that functions as a fluorescent probe for Pb²⁺ sensing with high selectivity (Figure 1.18) . LFS-1 coordinates Pb²⁺ with 1:1 complex stoichiometries. LFS-1 displayed significant pyrene excimer emission as well as the quenching of monomer in the presence of Pb²⁺. In contrast to LFS-1, LFS-2 showed fluorescence quenching upon addition to Pb²⁺ but without emission of the pyrene excimer, indicating distinct mechanisms underlying fluorescence quenching and the formation of the pyrene dimer necessary for excimer formation. These measurements emphasize a requirement for sufficient flexibility in the probe scaffold in the rational design of fluorescent sensors requiring pyrene–pyrene interactions.³⁷

Figure 1.18. (Left and Middle): Selective association between LFS-1 and Pb²⁺. (Left):

Fluorescence emission spectra of LFS-1 (1.0 μM) upon addition of different metal ions (100 equiv) in 10 mM HEPES buffer (containing 10% DMSO at pH 7.4) (λex = 355 nm). (Middle):

The gray bars represent the ratio of excimer emission fluorescence at 469 nm to monomer emission at 395 nm (I469 /I395 ) in presence of indicated cations. (Right): Selective fluorescence quenching of LFS-2 upon metal binding with no excimer formation.

Fluorescence emission spectra of LFS-2 (1.0 μM) upon addition of different metal ions (100 equiv) in 1.0 mM HEPES buffer (containing 10% DMSO at pH 7.4) (λex = 355 nm).

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