Chromogenic ‘naked eye’ and fluorogenic ‘turn on’ sensor for mercury metal ion using thiophene-based Schiff base

Abstract

2-((3-Methylthiophen-2-yl)methyleneamino)benzenethiol (Probe 1) has been synthesized and successfully applied for the selective recognition of mercury metal ions and utilized as a fluorescence turn-on sensor for Hg²⁺ ion via chelation enhanced fluorescence (CHEF). The mechanism of the interaction of Probe 1 with metal ions has been deduced by the absorption, emission, ¹H-NMR, and MALDI-TOF Mass analysis that intimate the favourable coordination of Hg²⁺ metal ion by the mercapto unit. The 2 : 1 stoichiometry of the sensor complex 1 + Hg²⁺ was calculated from the Job’s plot based on UV-Vis absorption spectra. The binding constant (log β₂) of the 1 + Hg²⁺ complex was found to be 13.36 by Hill plot. The limit of detection (LOD) was also calculated from the fluorescence emission titration and found to be 20 μM. Moreover the density functional theory (DFT) studies were investigated for the 1 + Hg²⁺ binding mechanism. Cyclic voltammograms also confirmed the recognition of Hg²⁺ metal ion to the Probe 1.

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1. Introduction

Many human disorders are caused by the exposure of the toxic metal ion: like mercury, lead, cadmium, silver. Many efforts have been devoted to the development of fluorogenic and colorimetric sensors for the highly toxic mercury ion. The mercury metal ion is highly toxic, non-biodegradable,¹ hazardous in nature,^2–4 and toxic for humans, including brain, kidney and lung damage. The results of mercury poisoning in several diseases, including acrodynia, and Minamata disease. The toxicity of Hg²⁺ in humans is caused by the easy coordination with biological ligands such as proteins, DNA and enzymes due to its affinity towards thiol groups. It's present significant hazards to public health because of its presence in drinking water. Therefore, the development of rapid, cost-effective and enzyme-free colorimetric sensors for the easy and fast detection of toxic metal ions by the naked eye, without resorting to any expensive instruments, is still an active ongoing research area.

The sources of Hg²⁺ contamination in water include gold mining, rubber processing, fertilizer industries, oil refining, and wood pulping. All the different forms of mercury ion, such as zero oxidation, mercuric ion in Hg²⁺, and mercurous in Hg₂²⁺, have toxic effects on the environment. Due to the bioaccumulation and magnification of Hg²⁺ in the aquatic food chain, Hg²⁺ has serious environmental toxicity. So construction of a chemosensor for Hg²⁺ ion detection is in demand. Due to the low level of metals in the samples and complexity of the matrices, the analysis of metal ions in the environment, clinical, industrial and biological samples is still a challenging task. Many sensors have been reported based on a fluorescence ‘turn-off’ response due to the quenching nature of heavy metal Hg²⁺ ions having large spin–orbit coupling constants.^5–9 And recently, several ‘turn-on’ fluorescence sensors for Hg²⁺ ion have been reported.^10–15 However, chemosensors to detect Hg²⁺ ion via enhanced fluorescence are very rare. We herein report a new sensor motif to detect Hg²⁺ through a fluorescence “turn-on” response in a partially aqueous medium.

The World Health Organization (WHO) and Environmental Protection Agency (EPA) have defined the limited concentration of these metal ions in drinking water. There are many important techniques which facilitate the quantification of these metal ions like: Atomic Absorption Spectroscopy (AAS) and ICP-MS.^16–19 Due to their high cost and high maintenance, colorimetric sensors have been developed. For the recognition of soft, heavy metal Hg²⁺ ions, nitrogen and sulphur binding sites might be a choice as they are present in the thiophene-based Schiff base.

2. Experimental section

2.1. Reagents and instrumentation

Chloride and nitrate salts of metal ions were all of analytical reagent grade and purchased from Merck. These reagents were used without further purification. 3-Methylthiophene-2-carbaldehyde, 2-aminothiophenol, 3-aminophenol were purchased from Sigma-Aldrich. The UV-Vis analysis of all the solutions was recorded on a Shimadzu, UV-3600 double beam spectrophotometer using a 10 mm path length silica cell. IR spectra were recorded with a Perkin-Elmer FT-IR 1000 spectrophotometer as films between KBr. CHNS analysis was recorded on an Elementar model Vario EL-III. NMR spectra were recorded on a Bruker AVANCE 500 MHz spectrometer. MALDI-TOF mass spectra were recorded on a Bruker Ultra-fleXtreme-TN-MALDI-TOF spectrometer using HABA (2-(4-hydroxyphenylazo)benzoic acid) as a matrix. Fluorescence emission spectra were recorded using RF-5301PC with a standard quartz cell of 3 cm path length. Cyclic voltammetric studies were carried out at room temperature on a CHI760E electroanalyser. The potential range was +1.5000 V to −1.5000 V at a scan rate of 0.1 V s⁻¹ using a glassy carbon electrode as the working electrode, a Ag/AgCl electrode as the reference electrode and Pt wire as the auxiliary electrode, and 0.1 M tetrabutylammonium perchlorate (TBAP) was used as the supporting electrolyte. All solutions were purged with nitrogen before the experiment. DFT computation studies were organized in the Gaussian 09 W programme in gas phase using a B3LYP function with 6-31G (d, p) for the metal-free ligand and LANL2DZ for the metal–ligand complex.

2.2. Synthesis of Probe 1 and 2

2-((3-Methylthiophen-2-yl)methyleneamino)benzenethiol (Probe 1). 3-methylthiophene-2-carbaldehyde (2 mmol) in methanol solution was added to a round bottom flask and stirred until the aldehyde was completely dissolved. After that, 2-amino thiophenol (2 mmol) was added and then refluxed for 16 h until a yellowish precipitate of Probe 1 formed and was recrystallised using ethanol.

Yield: 77%. Anal. calc. for C₁₂H₁₁NS₂: C, 61.76; H, 4.75; N, 6.00, S, 27.48 found: C, 62.01; H, 4.50; N, 6.71; S, 26.99. IR data (KBr, ν_max cm⁻¹): Ar–H: 3117, S–H: 2359, C–N: 1568, C–C: 1414, C–O: 1247. UV-visible (MeOH, λ_max nm⁻¹): 287, 355. ¹H NMR (DMSO, 500 MHz, δ/ppm): 8.78 (s, 1H), 7.36 (d, 1H), 7.21 (d, 1H), 7.10 (t, 1H), 6.92 (dd, 1H), 6.87 (d, 1H), 6.83 (t, 1H), 4.66 (s, 1H), 2.44 (s, 3H), ¹³C NMR (DMSO, 125 MHz, δ/ppm) 151, 148, 142, 136, 135, 131, 130, 128, 120, 115, 114, 83.

3-((3-Methylthiophen-2-yl)methyleneamino)phenol (Probe 2). 3-Methylthiophene-2-carbaldehyde (5 mmol) was dissolved in methanol and a methanolic solution of 3-amino phenol (5 mmol) was added dropwise. Next this solution was refluxed for 20 h. A brownish precipitate formed and was recrystallised using ethanol.

Yield: 62%. Anal. calc. for C₁₂H₁₁NOS: C, 66.33; H, 5.10; N, 6.45; O, 7.36; S, 14.76 found: C, 66.10; H, 4.83; N, 6.79; O, 8.72; S, 13.56 IR data (KBr, ν_max cm⁻¹): O–H: 3433, C–N: 1607, C–C: 1422, UV-visible (MeOH, λ_max nm⁻¹): 383. ¹H NMR (DMSO, 500 MHz, δ/ppm): 8.65 (s, 1H), 7.38 (d, 1H), 7.24 (d, 1H), 6.95 (m, 4H), 3.19 (broad s, 1H), 2.48 (s, 3H), ¹³C NMR (DMSO, 125 MHz, δ/ppm) 158, 150, 144, 141, 138, 136, 134, 131, 129, 122, 114, 55.

The structures of both the Schiff bases are shown in Scheme 1. The characterization data of Probe 1 and 2 are shown in the ESI (Fig. S1 to S6†).

Scheme 1 Synthesis of thiophene-based Schiff bases (Probe 1 and 2).

3. Results and discussion

Mercury ions have a binding affinity with the sulphur atom because of the soft ligand. For this purpose, we proposed the synthesis of a ligand with a mercapto unit and explored the selective recognition of mercury ions. Mercury binds with the ligand moiety in a linear pattern.

3.1. Naked eye detection of metal ion

To investigate the metal ion recognition with the synthesized Schiff bases (Probe 1 and 2), colorimetric studies were carried out with various metal ions. The solution of all the metal ions and Probes were prepared (50 mM) in methanol/H₂O (8/2: v/v solution). 3 equivalents of metal solution were added to the Probe 1 solution, and the sudden color change of Probe 1 from light yellow to yellowish orange was observed with the Hg²⁺ ion (Fig. 1). Furthermore, the selectivity of Probe 1 towards mercury ions was confirmed by UV-Vis and fluorescence studies. Probe 2 did not show any change in color with different metal ions.

Fig. 1 Images of the colorimetric changes of Probe 1 with Hg²⁺ ions in a methanol/H₂O (8/2; v/v) solution.

3.2. UV-Vis studies of Probe 1 and 2 with metal ions

To investigate the selective recognition of Hg²⁺ with Probe 1, UV-Vis studies were carried out in methanol/H₂O (8/2: v/v solution). Probe 1 showed absorption peaks at 287 nm due to a π–π* transition and at 355 nm due to an n–π* transition. Upon the addition of mercury solution to the Probe 1 solution (50 mm), a new absorption band occurred at 430 nm in the UV-Vis spectra due to the metal recognition (Fig. 2a), and the other metal ions did not show any significant changes in the UV-Vis spectra with Probe 1. Probe 2 did not show any absorption changes upon addition of the metal ions (Fig. 2b).

Fig. 2 UV-Vis spectra of Probe 1 (a) and Probe 2 (b) with different metal ions.

To know the binding stoichiometry of the 1 + Hg²⁺ complex, equimolar solutions of Probe 1 and Hg²⁺ ions were prepared (50 μM in methanol/H₂O (8/2: v/v solution)) and the absorption spectra were taken by a continuous variation of 1 and Hg²⁺ solution.

Fig. 3a shows the changes in the absorption spectra and Job’s plot,²⁰ which show the 2 : 1 stoichiometry. UV-Vis spectra were shown to change with the addition of Hg²⁺ ion in Probe 1 (100 μM), a concomitant increase in the absorbance at 430 nm and decreased in the absorbance at 355 nm and 287 nm were observed. The formation of the isosbestic point confirmed (Fig. 3b) the change from uncomplexed species (Probe 1) to complex species (1 + Hg²⁺). The red shift in the UV-Vis spectra (90 nm) was responsible for the sudden color change of the solution of Probe 1, i.e., light yellow to yellowish orange upon the addition of Hg²⁺ ions. The new absorption band occurred due to the intramolecular charge transfer between Probe 1 and mercury metal ions.

Fig. 3 (a) UV-Vis spectra with continuous variations in mole fractions of Probe 1 and metal ions. The inset shows a Jobs plot with an equimolar concentration (50 μM). (b) UV-Vis spectra shows the isosbestic point (100 μM).

3.3. Fluorescence emission spectra with metal ions

All the experiments were carried out in methanol/H₂O. Initially, Probe 1 (20 μM) was in methanol/H₂O; a 8/2 v/v solution was tested with 60 μM (3 equiv.) of metal ions (Na⁺, K⁺, Fe²⁺, Cr³⁺, Ni²⁺, Co²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Cu²⁺, Mn²⁺, Hg²⁺, and Ag⁺).

Upon treating Probe 1 with 3 equiv. of metal ions, a turn-on emission occurred at 503 nm with Hg²⁺ ions (λ_ex = 365 nm) (Fig. 4). The ‘turn on’ fluorescence selectivity of Hg²⁺ ions was caused by chelation enhanced fluorescence (CHEF). The covalent bond formation between the –SH group and Hg²⁺ ions exhibited the enhanced fluorescence response.

Fig. 4 Fluorescence spectra of Probe 1 (5 μM) towards different metal ions in methanol/water (8/2 v/v solution) (λ_ex = 365 nm).

From the UV-Vis studies and fluorescence studies, it was found that the Probe 1 is highly selective for Hg²⁺ ions (Table 1). To confirm this selectivity, a single and dual metal study has done with Probe 1. In the course of the dual-metal studies, two equal amounts of both metal ion Hg²⁺ and other metal ions (60 μM + 60 μM) were used. The interference effect of the secondary metal ion for the selectivity of Hg²⁺ ions was also carried out and is shown in Fig. 5. The blue bar indicates the single metal ions (Na⁺, K⁺, Fe²⁺, Cr³⁺, Ni²⁺, Fe²⁺, Co²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Mg²⁺, Cu²⁺, Mn²⁺, Hg²⁺, and Ag⁺) with Probe 1 and the red bar indicates the 1 + Hg²⁺ with interfering ions. Fig. 5 shows the enhanced fluorescence intensity with Hg²⁺ ions. No other metal ions showed an enhancement in fluorescence intensity. No metal ions interfered with the selectivity of Hg²⁺ (red bar). So with the help of the interference study, it was confirmed that Probe 1 was highly selective and sensitive for Hg²⁺ metal ions.

Table 1 UV-Vis and fluorescence wavelengths of Probe 1 and 1 + Hg²⁺ and the LOD calculated by fluorescence titration

Sample	UV (nm) study	Fluorescence study	LOD by fluorescence titration
Probe 1	287, 355 nm	—	—
1 + Hg^2+	287, 355, 430 nm	503 nm enhanced fluorescence	20 μM

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Fig. 5 Interference study with different metal ions at 503 nm in methanol/H₂O solution, Probe 1 + metal ions (blue bar) and 1 + metal ions + Hg²⁺ (red bar).

3.4. Fluorescence titration^21,22 of Hg²⁺ ions with Probe 1

For the fluorescence titration, 10 μL aliquots of Hg²⁺ ions of fixed concentrations were added to a 3 mL solution of Probe 1 (Fig. 6). The binding constant was calculated by fluorescence titration using the Hill method.²³ Hill plot was constructed by plotting graph between log[(I − I₀)/(I_max − I)] against log(Hg²⁺), where I₀, I and I_max are the emission intensity values of Probe 1, 1 + Hg²⁺ complex and after the complete binding of 1 + Hg²⁺ complex respectively.

Fig. 6 Fluorescence emission titration of Probe 1 with various concentrations of Hg²⁺ metal ions. Inset shows Hill plot.

The log β₂ value for the Hg²⁺ ions with Probe 1 was found to be 13.36 with a 20 μM limit of detection (LOD)²⁴ (Fig. 7), which is lowered than previously reported in the literature.

Fig. 7 Limit of detection (LOD) calculated by fluorescence emission spectra.

3.5. pH studies

The optical behaviour of Probe 1 with Hg²⁺ ion was evaluated at varying pH by absorption and emission spectroscopy. All the above analysis was carried out at a neutral pH. In the absorption analysis, the spectra remained the same at pH 6–10, but in a more alkaline medium (pH 11–14), disappearance of the absorption band at 421 nm was observed, which was due to the intramolecular charge transfer between Probe 1 and the Hg²⁺ ions. At low pH 1–5 a total blue shift occurred (Fig. 8). In the emission spectra, Probe 1 showed enhancement in the intensity with Hg²⁺ ions, accordingly at pH 7–11. Probe 1 with Hg²⁺ at pH 1–6 illustrated a marginal enhancement in the emission intensity at 503 nm and a decrease in the emission intensity as we increased the pH to 12 to 14 (Fig. 9). The whole study suggested that the chemosensor was properly used for the recognition of Hg²⁺ ions in pH 7–10.

Fig. 8 Variations in the absorption spectra of Probe 1 with Hg²⁺ ions at different pH in MeOH/H₂O solution.

Fig. 9 Variation in the emission spectra of Probe 1 with Hg²⁺ ions at different pH in MeOH/H₂O solution.

3.6. Stoichiometry of binding²⁵

Stoichiometry is an important tool for sensing mechanisms. To find out the binding site of Probe 1, the 2 : 1 stoichiometry of the 1 + Hg²⁺ complex was calculated using a Job’s plot. By the Job’s plot it was confirmed that the absorption maxima occurred at 0.66 mole fractions, which facilitated the 2 : 1 stoichiometry complex.

3.7. Nature of binding interactions

To sustain this hypothesis of the interaction of Probe 1 towards Hg²⁺ ions, ¹H-NMR studies were carried out. The ¹H-NMR spectra of Probe 1 in CDCl₃ showed a sharp singlet at δ 8.78 ppm of –CH protons (Ha) and doublets and triplets were assigned to aromatic protons. The resonance signal appeared at δ 4.66 and 2.44 ppm and were assigned to the –SH proton and –CH₃ proton, respectively. After the addition of Hg²⁺ ions, the downfield shift of the aromatic protons (Δδ = 0.02) occurred due to the interaction of a π–el⁻ cloud of aromatic rings to the Hg²⁺ metal ions, and a significant downfield shift of the resonance signal of the –SH proton occurred. With an increase in the concentration of Hg²⁺ ions the –SH proton signal completely disappeared. The ¹H NMR studies, as discussed thus, clearly suggested that the complexation occurred after complete deprotonation of the mercapto proton.

Moreover, to confirm the stoichiometry of the 1 + Hg²⁺ complex, MALDI-TOF spectra were recorded (Fig. S7 in the ESI†), which showed a molecular ion peak, m/z at 667.356 (calcd 667.022). ¹H NMR spectral data analysis of a complex, 1 + Hg²⁺ and MALDI-TOF spectra clearly suggested the interaction of Probe 1 with Hg²⁺ through the S atoms in the mercapto unit in 2 : 1 stoichiometry (Fig. 10).

Fig. 10 ¹H-NMR spectra of Probe 1 and 1 + Hg²⁺ in CDCl₃ and a possible mode of binding interaction.

3.8. Computational studies

Theoretical calculations²⁶ were conducted using a density functional theory (DFT) method. Geometry was optimized in gas phase using a Gaussian 09 W computational program with a B3LYP function by the basis set 6-31G (d, p) for the metal-free ligand and LANL2DZ for the metal bound ligand. Computational studies of Probe 1 and 1 + Hg²⁺ also confirmed the ligand to metal charge transfer. Fig. 11 shows the optimized structure of Probe 1 and 1 + Hg²⁺ and the HOMO–LUMO band gap, which confirmed the ligand to metal charge transfer. To find out the mechanism of binding interaction of Probe 1 with Hg²⁺, the optimized structure of 1 + Hg²⁺ indicated that the S–Hg–S linkage formation had a bond angle of 179.9°. Mercury metal forms a complex with the Schiff base in a linear fashion. The HOMO–LUMO band gap of the 1 + Hg²⁺ complex was decreased because of the ligand to metal charge transfer occurring between Probe 1 and the Hg²⁺ ions. The optimized structure and HOMO–LUMO diagram of Probe 2 is shown in Fig. S8.†

Fig. 11 DFT optimized structure and the HOMO–LUMO band gap of Probe 1 and the 1 + Hg²⁺ complex.

3.9. Cyclic voltammetry

Further, the electrochemical behavior of Probe 1, Probe 2 and Probe 1 with the Hg²⁺ ion was tested in methanol solution. The cyclic voltammograms of Probe 1 show two irreversible reduction peaks and three oxidation peaks. The reduction peaks occurred at −1.22 and −0.635 V and the oxidation peaks at 0.202, 0.516 and 1.07 V. After the addition of a methanolic solution of Hg²⁺ ions, the first reduction potential of Probe 1 was showed a 0.131 V cathodic shift. The first oxidation peak showed a 0.373 V anodic shift. Probe 1 showed a higher oxidation potential after binding with Hg²⁺ ions due to the charge transfer from the ligand to metal ions (Fig. 12). These results are also supported by theoretical calculations. All the CV data are summarised in Table 2. The cyclic voltammograms of Probe 2 had one irreversible reduction peak and two irreversible oxidation peaks. The reduction peak occurred at −0.60 V and the oxidation peaks occurred at 0.88 and 1.30 V. The cyclic voltammograms of Probe 2 are mentioned in the ESI (Fig. S9†).

Fig. 12 Cyclic voltammograms of Probe 1 (top) and the 1 + Hg²⁺ (bottom) complex.

Table 2 The redox potentials of Probe 1 and 1 + Hg²⁺ in methanol at 298 K

Sample	Oxidation peak (V)			Reduction peak (V)
Probe 1	1.07	0.516	0.202	−0.635	−1.22
1 + Hg^2+	1.13	0.852	0.575	−0.766	−1.19
Probe 2	1.30	0.88	—	−0.60	—

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3.10. Reversibility of the proposed sensor²⁷

The reversible behaviour of the proposed sensor was checked with EDTA disodium salt. Probe 1 showed the ‘turn-on’ fluorescence emission at 503 nm with Hg²⁺ metal ions in methanol/H₂O (8/2: v/v solution). After the addition of 10 mM of EDTA (in water) into the 1 + Hg²⁺ solution (yellowish orange color), the fluorescence enhanced emission was quenched and the color disappeared. Furthermore, the solution was reused for the chemosensing of Hg²⁺ metal ions. The fluorescence intensity and reversible colorimetric images are shown in Fig. 13.

Fig. 13 Sensor reversibility and no. of cycles of 1 + Hg²⁺ with EDTA.

The proposed chemosensor for Hg²⁺ metal ions was compared with those previously reported in the literature. From Table 3, the proposed sensor results seem good with respect to the limit of detection (LOD).

Table 3 Comparison of proposed chemosensor with previously reported literatures

Previous literature	Solvent	Limit of detection (LOD)
Dalton Trans., 2013, 42, 4456 (ref. 28)	CH3CN	50 mM
Org. Biomol. Chem., 2012, 10, 5410 (ref. 29)	H2O–CH3CN (10 : 90, v/v)	0.226 mM
Tetrahedron Lett., 2010, 51, 3286 (ref. 30)	CH3CN–DDW	30 mM
Spectrochim. Acta, Part A, 2012, 93, 245 (ref. 31)	DMSO	5.0 mM
Org. Lett., 2010, 12, 476 (ref. 32)	C2H5OH/H2O (1 : 1; v/v)	80 μM
This work	CH3OH/H2O (8/2; v/v)	20 μM

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4. Conclusion

Thiophene-based Probes (1 and 2) have been synthesised and characterised by various spectroscopic techniques such as UV-Vis, FT-IR, and NMR. Furthermore, Probe 1 was used for fluorometric and colorimetric sensing of Hg²⁺ metal ions without any interference from other metal ions. The binding affinity of Hg²⁺ ions with Probe 1 was also confirmed by the optical studies, NMR, mass studies, DFT optimization and electrochemical behaviour. The designed chemosensor was accomplished for the detection of 20 μM Hg²⁺ ions in a partially aqueous medium.

Acknowledgements

Ms Divya Singhal is highly grateful to CSIR (Council Scientific of Industrial Research) for providing funding for this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11043b

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