Preparation of the chemosensor (FSU)
The chemosensor [N-(Phenylcarbamoselenoyl) furan-2-carboxamide] was synthesized and characterized by referring to a procedure reported previously and then purified by column chromatography. The compound (FSU) is a brown-colored solid, stable in air, non-hygroscopic, and shows better solubility in dimethyl sulphoxide (DMSO). The general protocol for the synthesis of FSU is summarized in scheme 1.
The derivative of selenourea (FSU) was synthesized with a good yield. Elemental analysis and other spectroscopic tools were used to characterize the compound. Electronic spectra of the FSU described an absorption peak around 305 nm (π → π*).The bands that appeared approximately at a range of 3263 cm−1 for amide N−H were highlighted in the FT-IR spectra of FSU. The Selenourea N−H was observed at 3118 cm−1 and this value is shifted to a lower region of wavenumber, it can be explained based on the hydrogen bonding between N–H of Selenourea and the carbonyl oxygen. The C=O frequency for FSU was observed at 1665 cm−1 region. The characteristic absorption band of C=Se vibrations appeared at the range of 1274 cm−1 which agreed with the previously reported selenourea compounds [26]. In the 1H NMR spectra of FSU, the N−H proton which is present in between carbonyl and selenocarbonyl groups appeared as singlets around 10.89 ppm and the peak appeared at the range of 10.56 ppm was assigned to N−H proton which is attached between selenocarbonyl and phenyl ring. All other peaks corresponding to the aromatic protons were observed at the expected region (6.75-8.50 ppm). The entire characterization data mentioned above is represented in Fig.S1 to S4.
Fluorescence studies
The efficiency of a chemosensor is usually discussed based on its selectivity. The present work reveals the ability of the chemosensor FSU to detect Hg2+ ion in presence of different metal cations such as Co2+, Cr3+, Ni2+, Zn2+ Mg2+, Cu2+, Mn2+, Ca2+, Cd2+, Ga3+, Pb2+, Fe2+, Na+, and K+ of 10-fold excess in concentration in the reaction medium. As described in Fig. 1a, only Hg2+ ion can induce a significant fluorescence quenching at 440 nm in the presence of various cations in the FSU solution. Other cations are not capable enough to produce a significant variation in the fluorescence intensity of chemosensor FSU. Apart from this, metal ions’ interference studies were also performed. As described in Fig. 2a, even after the addition of different cations to the complex FSU-Hg2+ system, the emission intensity almost remains constant. This clearly indicates that the recognition process of Hg2+ is not influenced by other cations. As mentioned previously, the chemosensor FSU was specific to the detection of Hg2+ ions. Furthermore, the time of response study for the chemosensor FSU towards Hg2+ was studied (Fig. S6). The emission intensity of the complex FSU-Hg2+ remains almost the same at different time intervals (0-180s), indicating that the complex formed is stable.
The effect of pH in the sensing of Hg2+ ions was also investigated by conducting pH study within the range of 1 to 14 (Fig. S5). Hydrochloric acid and sodium hydroxide solutions were used to adjust the pH of the sensing medium. The fluorescent intensity of the chemosensor FSU was almost constant in the preferred pH range describing its consistency. The fluorescence emission intensity of the FSU-Hg2+ complex gradually increases and reaches maximum at a pH of 4. When the pH was shifted from acidic to neutral in the range of 6 to 9, fluorescence intensity decreased and that may due to the maximum interactions occurred between selenium and mercury in neutral medium. Also, in basic medium the fluorescence intensity of the system increased slightly that reveals the less stability FSU-Hg2+ complex in basic pH. So the results of pH study reveals that the FSU-Hg2+ system is stable in neutral pH rather than acid and basic pH. So all the fluorescence studies were conducted in aqueous pH.
Fluorescence titration experiments were performed (Fig. 2b) to get a better insight into the mechanism of sensing and detection limit. Fig. 2b demonstrated that the fluorescence emission of the chemosensor FSU at 440 nm was decreasing upon the successive addition of (8x10-5 M) Hg2+ solution in water (0-1.6 equivalents). The fluorescence intensity of the chemosensor FSU remains unchanged even after 1.6 equivalents of Hg2+ ions were added and after that quenching was not observed even the amount of the Hg2+ ion raised in the medium. The chemosensor fsu can be recommended as a good candidate for mercury (II) ion sensing due to its ability to quench the fluorescence intensity of FSU.
A calibration plot was constructed between Hg2+ ion concentration and the fluorescence emission values (y=0.9830) (Fig. 3). The equation 3σ/K was used to determine the detection limit of the chemosensor FSU towards Hg2+ ion and it was found to be 7.35×10-7 M which is lower than the detection limits of many reported sensors for Hg2+ ion (Table S1) [S.I 27-32]. When this value is compared with the value reported by WHO, it is below the acceptable limit of Hg2+ in drinking water [33]. Also, the correlation constant value was found to be 1.413×103 M-1 using the B-H equation developed from the titration values (Fig. 4). Thus, the results mentioned above described that the chemosensor FSU is a better chemical tool for the sensing of Hg2+ ions and a standard plot was developed with better linearity (R2=0.9871) for the quantitative analysis of mercury.
Further confirmation of the chemosensor metal complex was done by means of a mass analyzer. The HR-MS spectra confirmed the formation of ligand (FSU) and the complex FSU-Hg2+. The peak observed at M/Z = 922.009 corresponds to [M+ACN+Na+] where M= FSU-Hg2+ (Fig. 5) and which further reveals the 2:1 ratio of chemosensor and metal ions. Fluorescent titration values were used to build the Job’s diagram (Fig. 6) and it was further used to confirm the 2:1 ratio of chemosensor and Hg2+ ions. As mentioned in Fig. 6, two straight lines meet at a point where the mole fraction is 0.4, which confirms the 2:1 (ligand to mercury) stoichiometry of complex formation between the chemosensor (FSU) and Hg2+ [34].
The plausible binding mechanism of the FSU-Hg2+ complex (Fig.8) was proposed based on, the FT-IR, NMR titration and DFT calculation studies. The fluorescence property of the chemosensor FSU can be explained based on the ICT process taking place within the compound [35-36]. While upon the addition of Hg2+ to the chemosensor, the ICT process was interrupted and that resulted in fluorescence quenching. Since, there was no significant change in the chemical shift values of the –NH protons, the 1H NMR titration study clearly suggested that the –NH moieties were not involved in coordination with the mercury ions (Fig. 7). It was further confirmed by the FT-IR analysis. The FT-IR spectra of both FSU and the complex FSU-Hg2+ were recorded (Fig. S3). From the spectra, it is clear that except for –C=Se moiety, the stretching frequencies of all other functional groups remain almost the same. After the addition of Hg2+ -C=Se stretching frequency was lowered from 1274 cm-1 to 1250 cm-1 thereby confirming the direct interaction of selenium and mercury.
We also carried out the reversibility study of the chemosensor FSU, because it is also important when one considers its metal ion sensing application. The FSU-Hg2+ complex (8 × 10−5 M−1 - 1× 10−3 M) in DMSO/ water (95/5, v/v) system was reversed by EDTA addition (2.0 × 10−3 M). The Hg2+ complex formation with the chemosensor FSU disappeared after the addition of EDTA (Fig. S7), showing the reversible sensing action.
DFT calculations
In order to verify the results obtained experimentally, density functional theory was executed for both chemosensor FSU and FSU-Hg2+ complex using the Gaussian 09 software with B3LYP/ 6-31G (d, p)/LanL2DZ level of theory. The optimized structures of the chemosensor FSU and the complex FSU-Hg2+ are depicted in Fig. 9. The density functional theory calculations clearly described that the electron density of FSU is mainly distributed in the region of –C=Se and –C=O at HOMO and LUMO energy levels. After the complexation of FSU with Hg2+, the electron density around –C=O and –C=Se decreased significantly and shifted towards Hg2+. These results demonstrated that electron transfer occurred within the chemosensor and it was blocked by the addition of Hg2+ which resulted in weak fluorescence intensity. As depicted in the Figure (Fig. 9), the HOMO-LUMO for chemosensor FSU and the complex FSU-Hg2+ were found to be 0.1134 and 0.1001 eV respectively. Critical chemical reactivity parameters like softness, hardness (η), electronegativity (χ), chemical potential (μ), electron-affinity (A), and ionization energy (I) were calculated based on the HOMO-LUMO energy gap. These properties have been defined as follows [37].
η = (I-A)/2 μ= - (I-A)/2 χ = (I+A)/2
Where I and A were obtained from HOMO and LUMO energies as I = -EHOMO and A = -ELUMO as per Janak theorem and Perdew et al. [38].
Table 1. Calculated energy values for FSU using B3LYP/6-31G (d, p) basis set
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Parameters
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Energy (au)
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-1537.22
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Dipole moment (Debye)
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6.1802
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EHOMO (eV)
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-0.2095
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ELUMO (eV)
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-0.1094
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EHOMO-LUMO (eV)
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0.1134
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EHOMO-1 (eV)
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-0.2177
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ELUMO+1 (eV)
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-0.1044
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E(HOMO-1)-(LUMO+1) (eV)
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0.1133
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Hardness (η)
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0.0567
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Chemical Potential (μ)
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-0.1595
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Electronegativity (χ)
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|
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0.1595
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Electrophilicity index (ω)
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|
|
0.4487
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Hardness (η) which is directly related to stability was found to be 1.7448 eV for FSU. The tendency of electrons to escape from an equilibrium system was termed chemical potential and that was found to be -3.7045 for FSU. The global electrophilicity index (ω), a global reactivity index that is related to η and μ, first introduced by Parr et al. [39]. It is the measure of the stabilization attained by the system in terms of energy when it acquired extra electronic charge from the surroundings and is given by ω = μ2/η. The corresponding value for fsu is 7.8652 e.V. Receptor FSU provides a proper molecular structure to coordinate with the Hg2+ ion. The obtained results also revealed that the HOMO-LUMO energy gap corresponding to the complex FSU-Hg2+ was lower when compared to the chemosensor FSU thereby form a stable complex. A strong interaction between the chemosensor FSU and Hg2+ ions imparted a distortion of electronic structure of chemosensor FSU that led to the quenching of the fluorescent emission.
Molecular electrostatic potential (MEP) surface analysis
The MEP analysis is an effective theoretical tool for predicting reaction behaviour of molecules. MEP analysis allow us to envisage the distribution of electro cloud of compounds and provide other charge related properties. ESP study is one of the important theoretical methods by which organic chemists are analyzing the interactions among drug-receptor and enzyme-substrate along with hydrogen bonding interactions.
A comparative view of molecular electrostatic maps (MEP) is shown in Fig. 10. MEP of FSU was calculated using BL3YP/6-31G (d, p) basis set. The graphic representation with the rainbow colour scheme of ESP lies in the range of -6.018e-2 to +6.108e-2 for FSU. The high electron density (negative potential) indicates regions that are red colour. The low electron density (positive potential) indicates regions that are blue colour. As can be seen in the MEP for FSU, the three regions around carbonyl oxygen and carbonyl selenium of FSU are red and hence possess high electron density or in other words a site more exposed to electrophilic attack [40].