Retracted Article: Hydrogen sulfate ion sensing in aqueous media based on a fused pyrimido benzothiazole derivative

Abstract

A sensitive and selective receptor 3-cyano-4-imino-2-methylthio-4H-pyrimido[2,1-b][1,3]benzothiazole (SVK-1) bearing a fused pyrimido benzothiazole structure was developed for the recognition of HSO₄⁻ anions. UV-vis and fluorescence emission spectroscopy were employed for the recognition for HSO₄⁻ over other anions such as Cl⁻, Br⁻, I⁻, F⁻, AcO⁻, H₂PO₄⁻, ClO₄⁻ and NO₃⁻ in an aqueous medium.

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In recent times selective sensing and recognition of anions has received considerable attention due to its significant role in agricultural, biological, industrial and environmental sciences.^1–3 Therefore, development of chemosensors for the selective recognition and sensing of anions has been a significant goal for researchers worldwide. In many agricultural fertilizers and industrial raw materials HSO₄⁻ can be found.⁴ However, HSO₄⁻ can enter the environment as a pollutant from these means and others.⁴ In alkaline conditions HSO₄⁻ dissociates to form toxic sulfate ion (SO₄²⁻), which is responsible for irritation of eyes and skin. In an aqueous media selective recognition of HSO₄⁻ is a challenging task since it has a large standard Gibbs energy of hydration (−1080 kJ mol⁻¹).⁵ In aqueous media this strong preference toward hydration hinders receptors from recognising HSO₄⁻. To construct a receptor that shows high hydrogen sulfate detection selectivity in anion recognition chemistry is a great challenge. A review of the literature revealed that few fluorogenic chemosensor for selective recognition of HSO₄⁻ anions have been reported on the aqueous medium.^6–14 It is well known that in nature HSO₄⁻ is recognised and transported by proteins via hydrogen bonding.¹⁵ Several receptors that have been reported are based on hydrogen bonding complex formation with HSO₄⁻.^16–21 However, they suffer from either no specific selectivity or sensitivity.^21–24 Thus there is a need to develop simple molecule for selective detection of HSO₄⁻. Recently, the 3-cyano-4-imino-2-methylthio-4H-pyrimido[2,1-b][1,3]benzothiazole (SVK-1) and its derivatives have been used as a potentially bioactiveheterocycles.²⁵ The SVK-1 exhibiting several binding sites would prove effective sensor for anion recognition. Thus, in this paper, we demonstrate that the SVK-1 probe can serve as a selective and sensitive sensor towards HSO₄⁻ in aqueous medium.

Following the protocol by Kuberkar et al., we prepared SVK-1 by reacting bis(methylthio)methylene malononitrile 2 with 2-amino benzothiazole 1 in the presence of a catalytic amount of anhydrous potassium carbonate in dry DMF at reflux.²⁵ The crude product was purified by recrystallization from DMF : EtOH (1 : 1, v : v) solvent mixture to give analytically pure SVK-1 (Scheme 1).

Scheme 1 Synthetic pathway of receptor SVK-1.

SVK-1 in an ACN : H₂O (1 : 2 v/v) solvent mixture shows a strong blue fluorescence under irradiation at 365 nm. Given this, naked eye detection of different anions was trialed. To individual solutions of SVK-1, 100 equivalents of the F⁻, Br⁻, Cl⁻, I⁻, HSO₄⁻, H₂PO₄⁻, NO₃⁻, ClO₄⁻, and AcO⁻ (as its TBA salt) were added. The fluorescence at 365 nm changed from a strong blue to an off-blue, with a decrease in intensity upon the addition of HSO₄⁻ only as shown in Fig. 1.

Fig. 1 Color changes of receptor SVK-1 (3 × 10⁻⁵ M) in ACN : H₂O (1 : 2, v : v) under 365 nm upon addition of 100 equiv. of F⁻, Br⁻, Cl⁻, I⁻, HSO₄⁻, HPO₄⁻, NO₃⁻, ClO₄⁻, and AcO⁻ (as its TBA salts).

Furthermore, the UV-vis absorption spectroscopy was used to investing selective sensing ability of receptor SVK-1 upon addition of various anions as shown in Fig. 2A. The absorption spectra of SVK-1 in ACN : H₂O (2.5 : 7.5, v : v), shows three characteristic peaks at 261 nm, 313 nm and 378 nm along with shoulder peak at 275 nm (Fig. 2A). Upon addition of various anions Br⁻, Cl⁻, F⁻, I⁻, AcO⁻, H₂PO₄⁻, ClO₄⁻ and NO₃⁻ (as its TBA salt) to the a solution of SVK-1 no change in peak intensity and position was observed (Fig. 2A). However, as was seen with the naked eye detection, the addition of HSO₄⁻ ion to the receptor SVK-1 showed dramatic shift in absorption as well as a change in intensity (Fig. 2A).

Fig. 2 UV-vis absorption of SVK-1 (1 × 10⁻⁵ M) (A) upon addition of various anions in ACN : H₂O (2.5 : 7.5, v : v) and (B) with an incremental amount of HSO₄⁻ (1 × 10⁻³ M) ion in ACN : H₂O (2.5 : 7.5, v : v) up to 30 equivalent.

This clearly showed that SVK-1 acts as a selective receptor for HSO₄⁻. To understand this process in depth, systematic concentration dependent titration of HSO₄⁻ with SVK-1 was performed as shown in Fig. 2B. It can be clearly seen that upon incremental addition of HSO₄⁻ (as its TBA salt) to the receptor SVK-1 results in a gradual decrease in peak intensity at 261 nm along with a decrease in intensity and a blue-shift of the peak at 378 nm peak (which is shifted to 317 nm) (Fig. S5†). Along with above changes in peak position and intensity, a new peak at 356 nm (Fig. 2B and S6†) also appeared. We presume that this is due to the formation of complex of the =N–H and CN of receptor with the HSO₄⁻ anion. The formation of four well defined isosbestic points at 262 nm, 299 nm, 317 nm and 364 nm were observed upon UV-vis titration of receptor SVK-1 with HSO₄⁻ anion, indicating complex formation in the titration solution (Fig. 2B). The Benesi–Hildebrand equation was employed to investigate binding constant (K_a) of HSO₄⁻ to the receptor SVK-1 and was determined to be 1.6421 × 10³ M⁻¹ (see ESI Fig. S7†).

The UV-vis spectroscopic techniques we investigated the effect of competing anions on the sensing properties of SVK-1 towards HSO₄⁻. This was achieved by treating SVK-1 with interfering anions such as Br⁻, Cl⁻, F⁻, I⁻, AcO⁻, H₂PO₄⁻, ClO₄⁻ and NO₃⁻ and to that a solution HSO₄⁻ was added. In the presence of interfering anions the UV-vis absorption spectrum change observed is similar to that of the absorption spectrum obtained by the addition of only HSO₄⁻ to the receptor SVK-1 (Fig. 3 and ESI Fig. S8†). The obtained results about competitive selectivity of receptor SVK-1 towards HSO₄⁻ indicated that the receptor SVK-1 is a selective sensor for HSO₄⁻ in the presence of other competing anions.

Fig. 3 Competitive selectivity of receptor SVK-1 (1 × 10⁻⁵ M) towards HSO₄⁻ in the presence of other anions (30 equiv.) in ACN : H₂O (2.5 : 7.5, v : v).

Furthermore, fluorescence emission spectroscopy was employed to investigate the sensing ability of SVK-1 towards various anions. The emission peak of receptor SVK-1 appeared at λ_em = 457 nm (λ_ex = 350 nm), upon addition of Br⁻, Cl⁻, F⁻, I⁻, AcO⁻, H₂PO₄⁻, ClO₄⁻, HSO₄⁻ and NO₃⁻ (as its TBA salt) to the receptor SVK-1 in ACN : H₂O (2.5 : 7.5, v : v), no significant changes in the emission spectrum of SVK-1 were observed for Br⁻, Cl⁻, F⁻, I⁻, AcO⁻, H₂PO₄⁻, ClO₄⁻ and NO₃⁻, however, quenching of emission was observed for HSO₄⁻ (Fig. S9†). The titration of HSO₄⁻ in ACN : H₂O (2.5 : 7.5, v : v) with receptor SVK-1 resulted in a gradual decrease in fluorescence intensity with increasing HSO₄⁻ (Fig. 4). Here we presume that decrease in fluorescence intensity observed is due to the formation of a complex of HSO₄⁻ with SVK-1, supporting the initial naked eye detection method (Fig. 1) used with SVK-1. These observations indicated that HSO₄⁻ interacts with the SVK-1 receptor more strongly over the other anions.

Fig. 4 Changes in fluorescence emission spectra (λ_ex = 350 nm) of receptor SVK-1 (1 × 10⁻⁵ M) with an increasing amount of HSO₄⁻ (1 × 10⁻³ M) ion in ACN : H₂O (2.5 : 7.5, v : v) up to 30 equivalent.

The above study indicated that the receptor SVK-1 showed selective sensing for HSO₄⁻ ions over other anions (Br⁻, Cl⁻, F⁻, I⁻, AcO⁻, H₂PO₄⁻, ClO₄⁻ and NO₃⁻) this is likely due to the additional hydrogen bonding with nitrile subunit, which was also supported by DFT calculations. It is notable that SVK-1 containing –CN and –NH subunits was suitable for synergistic binding with HSO₄⁻ ion. Furthermore, it is interesting to record that only HSO₄⁻ anion (pKa 1.99 in aqueous solution) causes remarkable changes both in UV-vis absorption and fluorescence emission spectra. The plausible complex formation (YMN-1) between receptor SVK-1 and HSO₄⁻ is as shown in Fig. 5A. FT-IR spectroscopy was employed to investigate binding interactions of receptor SVK-1 with HSO₄⁻ ions (Fig. 5B). The broad band at 3431 cm⁻¹ was assigned to be N–H stretching vibrations. The intense band at 2201 cm⁻¹ is attributed to –CN functional group. The addition of HSO₄⁻ to receptor SVK-1 caused a change in intensity and peak position of –CN and –NH from 2201 cm⁻¹ to 2368 cm⁻¹ and 3431 to 3406 cm⁻¹ respectively indicating strong complexation between the receptor SVK-1 and HSO₄⁻ ion via intermolecular hydrogen-bonding formation (Fig. 5A and B). On the basis of obtained FT-IR and emission spectra, we can presume that the fluorescence emission peak was quenched with the addition of HSO₄⁻ anion and resulted into formation of complex YMN-1. The complex between HSO₄⁻ and the receptor e.g. YMN-1 upon excitation (λ_ex = 350 nm) results in quenching of fluorescence (Fig. 4).

Fig. 5 (A) The plausible structure of the complex formation mechanism resulted between receptor SVK-1 and HSO₄⁻ anion; (B) the FT-IR spectra of the receptor SVK-1 (i) and its complex with HSO₄⁻ ion YMN-1 (ii).

The Gaussian 09 ab initio/DFT quantum chemical simulation package was employed to get results of the calculations in the present work.²⁶ The geometries of molecules HSO₄⁻, SVK-1 and complex YMN-1 were optimized at B3LYP/6-311++G (d,p) level of theory. The polarisable continuum model (PCM) is used to investigate the effect of solvent (water) on geometries. To ensure structures to be real, frequency calculations were carried out. In Fig. S10† and 6 optimized structures of HSO₄⁻, SVK-1 and complex YMN-1 and few geometrical parameters (Fig. S11†) of the molecules are given to notify interactions. Hydrogen bonding interactions are seen in YMN-1 complex (Fig. 5 and S11†). On comparison of bond lengths of OH, NH and CN bond before and after interaction. It can be seen that OH, NH bond lengths elongates and CN bond length shortens after complexation between HSO₄⁻ and SVK-1. Relatively stronger hydrogen bonding interaction is seen between OH part of HSO₄⁻ ion and CN of SVK-1 (hydrogen bond length 1.871 Å) when compared with that of between O-part of HSO₄⁻ ion and NH of SVK-1 (hydrogen bond length 2.165 Å). This can be attributed to less hydration around OH part of HSO₄⁻ ion when compared with that of at O-part of HSO₄⁻ ion. Calculated IR spectra were compared to support the experimental observations resulted from analysis of FTIR spectra. It is seen that calculated vibrational wavenumbers for OH and NH stretching before complexation are 3777 and 3510 cm⁻¹, whereas upon complex formation those are shifted to 3467 and 3451 cm⁻¹ respectively. This lowering in stretching frequencies indicate strong complexation between HSO₄⁻ ion and SVK-1 via intermolecular hydrogen-bonding formation.

Fig. 6 Frontier molecular orbitals of SVK-1 and its complex with HSO₄⁻ (YMN-1).

Further the geometries of SVK-1 and YMN-1 obtained at B3LYP/6-311++G (d,p) level were subjected to time-dependent density functional theory (TD-DFT) calculations for the study of absorption properties of SVK-1 molecule and YMN-1 complex. Calculated TD-DFT excitation properties of SVK-1 and YMN-1 are given in Table S1.† The TD-DFT results for SVK-1 shows absorption at 232 nm, 241 nm, 257 nm, 315 nm, 360 nm, and YMN-1 shows at 239 nm, 257 nm, 316 nm, 364 nm, which are in considerable agreement with the experimental absorption recorded for SVK-1 molecule and YMN-1 complex (Fig. 2A) respectively. All transitions and pictures of the orbitals involved in excitations are shown in Table S1, Fig. S12a and S13b.†^27,28 Calculated absorption of SVK-1 at 360 nm is with oscillator strength 0.2473, where as calculated absorption of YMN-1 at 364 nm is with oscillator strength 0.2607, which involves transition from HOMO to LUMO. This increase in the oscillator strength upon complexation can be attributed to enhanced donor acceptor character in YMN-1 leading to fluorescence quenching via photoinduced electron transfer (PET). Upon complexation between receptor SVK-1 and HSO₄⁻ ion the YMN-1 complex possess small energy differences (4.001 eV) (Fig. 6), the electronic transition from the nitrogen lone pair to the corresponding π-bonding orbital lead to fluorescence quenching (Fig. 4).

The detection limit of HSO₄⁻ was evaluated to determine the practical applicability of the receptor SVK-1. The equation used for calculation of limit of detection is 3S/ρ, where the standard deviation of three blank measurements is S and was calculated by using . Where N = number of samples, x̄ = mean of sample value and x = individual sample value and the slope between absorbance intensity versus sample concentration is ρ. The limit of detection of HSO₄⁻ was calculated by plotting calibration curve between the changes observed in absorbance intensity at 356 nm of SVK-1 and the concentration of HSO₄⁻. It was observed that a plot of hydrogen sulfate ions showed linear relationship with an r² value of 0.9932. The SVK-1 has limit of detection to be 1.6067 × 10⁻⁷ M for HSO₄⁻ (see ESI Fig. S13†). The proposed receptor SVK-1 was also compared with the reported receptor for HSO₄⁻ ion detection (Table 1). The present anion detection method is the most selective and sensitive protocol that offers a considerable enhancement in the limits of detection compared with anion detection methods reported in the literature. The novelty of this work is the ease at which SVK-1, being done in one step and is shown to be only selective for sensing of the HSO₄⁻ ion, even in the presence of other “competing” ions.

Table 1 Comparison of the receptor SVK-1 with the reported receptors in the literature

Sr. No.	Authors	Solvent system	Limit of detection (HSO4^−)	Ref.
1	Li et al.	Water : DMSO (3.8 : 6.2, v : v)	2.0 × 10^−6 M	29
2	Kim et al.	CH3CN : H2O (1 : 1, v : v)	3.75 × 10^−6 M	30
3	Singh et al.	Water	5.67 × 10^−9 M	31
4	Jiang et al.	Water	5 × 10^−6 M	32
5	Wang et al.	Water	5 M	33
6	Kuwar et al.	Water	0.25 × 10^−3 M	34
7	Kaur et al.	Water	37 × 10^−3 M	35
8	Kaur et al.	Water	1.12 × 10^−3 M	36
9	Bhosale et al.	CH3CN : H2O (2.5 : 7.5, v : v)	1.6067 × 10^−7 M	Present work

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In conclusion, we have demonstrated the synthesis and sensor properties of the fused pyrimidobenzothaizole receptor SVK-1 in water. The receptor exhibited excellent sensor properties towards HSO₄⁻ ion proved by using fluorometric and optical detection. Upon addition of HSO₄⁻ blue fluorescence changes to off blue under long range UV light. UV-vis results showed good selectivity for HSO₄⁻ ion over other anions such as Br⁻, Cl⁻, F⁻, I⁻, AcO⁻, H₂PO₄⁻, ClO₄⁻ and NO₃⁻. Furthermore, the addition of HSO₄⁻ ion to the receptor SVK-1 resulted into quenching of the emission peak due to ICT off phenomenon. The complexation between SVK-1 and HSO₄⁻ anion through hydrogen-bonding was demonstrated by FT-IR spectra and DFT calculations. The present method of HSO₄⁻ anion detection provides new platform for developing novel receptors for anion recognition.

Acknowledgements

S. V. B. (IICT) and S. D. P. are grateful for financial support from the DAE-BRNS (Project Code: 37(2)/14/08/2014-BRNS), Mumbai, and Intelcoat project CSC0114, CSIR, New Delhi, India. R. S. B. acknowledges financial support from CSIR, New Delhi under the SRA scheme [(13(8772)-A)/2015-Pool]. S. V. B. (RMIT) acknowledges financial support from the Australian Research Council under a Future Fellowship Scheme (FT110100152). ALP acknowledges use of the High Performance Computing Facility (Linux Cluster) and Gaussian 09 procured under the DST-FIST Scheme (Sanction No. FS/FST/PSI-018/2009).

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Footnote

† Electronic supplementary information (ESI) available: Additional figures, detail experimental protocol, full characterisation and spectroscopic data of all new compounds. See DOI: 10.1039/c6ra01980c

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