Benzothiazole-based heterodipodal chemosensor for Cu2+ and CN– ions in aqueous media

Tetrahedron Letters 60 (2019) 151075

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Benzothiazole-based heterodipodal chemosensor for Cu2+ and CN– ions in aqueous media Won Sik Na a, Pushap Raj b, Narinder Singh b,⇑, Doo Ok Jang a,⇑ a b

Department of Chemistry, Yonsei University, Wonju 26493, Republic of Korea Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India

a r t i c l e

i n f o

Article history: Received 21 June 2019 Revised 16 August 2019 Accepted 21 August 2019 Available online 22 August 2019 Keywords: Chemosensor Copper Cyanide Aqueous

a b s t r a c t A simple and selective chemosensor, A, was developed for recognizing Cu2+. The emission spectra of receptor A showed a fluorescence quenching response upon addition of Cu2+ with a low detection limit of 4.51 nM, significantly less than the WHO recommended guideline for drinking water. In addition, the formed A∙Cu2+ complex was examined for secondary sensing of anions. The A∙Cu2+ complex showed selectivity for CN– via a recovering emission profile of A. Ó 2019 Elsevier Ltd. All rights reserved.

Introduction Recently, the development of active receptors for sensing metal ions and anions has received considerable attention due to their importance in biological and environmental processes [1–4]. Among metal ions, copper is the third most abundant and is a vital trace element found in natural systems, playing an essential role in many physiological processes [5,6]. Most copper-containing proteins and enzymes are redox active, while copper acts as a cofactor in electron and dioxygen transfer reactions and is a catalyst for many redox reactions [7,8]. However, increased concentrations of copper in biological systems can lead to the generation of reactive oxygen species (ROS) that destroy biomolecules such as proteins, enzymes, lipids, and nucleic acids [9,10]. Several diseases such as Alzheimer’, Parkinson’s, Menkes, and Wilson diseases are associated with copper dysregulation [11]. The excessive utilization of copper in daily life is a primary reason for copper pollution in the environment. According to the guideline provided by the world health organization (WHO), the permissible level of copper in drinking water is 2 ppm, and in blood, it should not exceed 100– 150 lg/dL [7]. Therefore, it is of great importance to develop an efficient strategy to detect copper in environmental and biological samples. Recently, several methods including liquid–liquid extraction (LLE), flame atomic absorption spectrometry (FAAS), and inductively coupled plasma mass spectroscopy (ICP-MS) have been ⇑ Corresponding authors. E-mail addresses: [email protected] (N. Singh), [email protected] (D.O. Jang). https://doi.org/10.1016/j.tetlet.2019.151075 0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.

developed for detection of copper in various sample [12–14]. However, the major concern associated with the above techniques is their cost, time-consumption, and complex procedures required for sample handling [15]. Therefore, the development of fluorescent chemosensors for copper ions have attracted significant interest due to their high sensitivity, easy operation, and onsite detection capabilities [16,17]. Numerous organic receptors have been fabricated with two or three pods to which copper-specific binding site are attached [18–22]. The selectivity of these receptors depends on the pod rigidity, cavity size, and orientation of the binding sites [23]. Cyanide (CN ) is highly toxic to mammals due to its strong binding to Fe3+ in heme, which leads to the inhibition of the electron transport chain in the respiratory system [24–26]. Despite its toxicity, it is a commonly used reagent in many industrial processes, including mining, fabric synthesis, and electroplating [27]. The industrial use of cyanide and its basic transportation is the primary route of human exposure [28]. According to the WHO guidelines, the permissible level of cyanide in drinking water is <0.2 ppm [29]. In the literature, a number of organic- fluorescence-probebased approaches have been developed for the detection of cyanide ions, such as nucleophilic addition reactions [30–32], single-electron transfer reactions [33], H-bonding [34], and displacement approaches [35–39]. However, transition-metal-complex-based sensing probes show multiple advantages for chemosensing application over organic fluorescence probes owing to their unique characteristics such as high sensitivity, large stokes shift, high quantum yield, and long phosphorescence lifetime

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[40–42]. Generally, transition-metal-complex-based fluorescence probes utilize either ‘‘turn-on” or ‘‘turn-off” signaling modes after binding with the target analyte [43,44]. Herein, we focused on the metal displacement approach as it exhibits many advantages over covalent-based detection methods, including reversibility, short response time, reusability, and reduced interference from competing anions and solvents [45]. A benzothiazole-based heterodipodal receptor was designed for the selective detection of copper ions. The molecular design of the receptor was engineered with the benzothiazole moiety as a signaling unit. A sulfonyl group was introduced to manipulate the binding site between the two pods, thereby allowing sulfur and oxygen to bind to the copper ion. The free hydroxyl group has an acidic hydrogen, which makes the sensor sensitive to pH. The binding of copper with this cavity leads to a change in the electronic structure of the receptor. Therefore, tunability in terms of the emission signal was achieved. This tunability of the emission signal was used for analyte estimation. In addition, the resulting complex A∙Cu2+ was used for sensing an array of anions, selectively detecting cyanide ions at a low detection limit.

453 nm when excited at 390 nm. Upon addition of Cu2+, significant fluorescence quenching (quenching efficiency: (I-I0)/ I0  100 = 97%) was observed due to the paramagnetic nature of the copper ions (Figs. 1 and S2). The other tested metal ions did not produce significant changes in the emission spectrum of receptor A, indicating that it selectively bound Cu2+. To confirm the interactions of receptor A with Cu2+, a fluorescence titration experiment was performed. Successive addition of Cu2+ was performed (0–12 equivalents) and a linear decrease in the 453 nm emission intensity was observed (Figure 2). The Benesi-Hildebrand plot

Results and discussion Synthesis of receptor A Compound 1 was synthesized via nucleophilic substitution of 4(diethylamino)-2-hydroxybenzaldehyde with 4-methylbenzenesulfonyl chloride in the presence of K2CO3 dissolved in acetonitrile (ACN). Receptor A was synthesized by condensation of 2aminobenzenethiol with compound 1, as shown in Scheme 1 and the product was characterized by NMR, mass spectrometry, IR spectroscopy, and elemental analysis.

Cl O S O

rt, 3 h, 87% O

O

0 equiv

700 600 500 400

Cu2+

300 200

12 equiv

100 0 400 425 450 475 500 525 550 575 600 625 650 675 700 Wavelength (nm) Figure 2. Changes in the fluorescence intensity of A (300 nM) upon addition of Cu2+ (0–12 equiv.) in ACN/H2O (1:99, v/v, HEPES 10 mM, pH 7.4) excited at 390 nm.

N K2CO3, CH3CN

OH

900

Fluorescence intensity

The UV–vis absorption spectra of receptor A in ACN:H2O (1:99, v/v, HEPES 10 mM, pH 7.4) exhibited absorption bands at 390 and 270 nm (e = 91200), arising from the p-p* and n-p* transitions of receptor A, respectively. For cation binding studies, receptor A was treated with a variety of metal ions, including Zn2+, Pb2+, Na+, Mg2+, K+, Cd2+, Ca2+, Ba2+, Al3+, Co2+, Ni2+, Fe2+, Fe3+, and Cu2+, and UV–vis absorption spectra were recorded (Figure S1). None of the tested analytes caused significant changes in UV–vis absorption spectrum of receptor A except for Cu2+ which showed a hypochromic shift at 390 nm and hyperchromic change at 270 nm. The emission spectrum of receptor A was monitored at

+

1000

800

Cations binding studies

N

Figure 1. Fluorescence quenching of A (300 nM) upon addition of various metal salts (10 lM) in ACN/H2O (1:99, v/v, HEPES 10 mM, pH 7.4) at 453 nm (excited at 390 nm).

O O S O 1

N SH 1

+ NH2

DMSO 60 oC, 24 h, 51% N

S

O O S O

A Scheme 1. Synthesis of heterodipodal receptor A.

Figure 3. Benesi-Hildebrand plot for the determination of the stability constant of A toward Cu2+ at 453 nm.

W. Sik Na et al. / Tetrahedron Letters 60 (2019) 151075

was used to determine the binding constant of receptor A for Cu2+, which was 2.4 ± 0.3  106 M 1 as shown in Figure 3 [46]. This high binding constant verified complex formation. A calibration plot of the fluorescence intensity as a function of Cu2+ concentration

3

was generated, exhibiting a good linear correlation with R2 = 0.99, indicating that receptor A is suitable for sensing Cu2+ (Figure 4). The detection limit of receptor A for Cu2+ was 4.51 nM (Figure S3) [47], which is considerably less than the WHO suggested limit for drinking water (31.5 mM) [48]. To determine the stoichiometry of copper binding, Job’s plot was obtained, revealing

Figure 4. Calibration curve of A (300 nM) binding with Cu2+ (0–700 nM) in ACN/ H2O (1:99, v/v, HEPES 10 mM, pH 7.4) at 453 nm (excited at 390 nm). Figure 7. Changes in the fluorescence intensity of the A∙Cu2+ complex (300 nM) upon addition of CN– (0–200 equiv.) in ACN/H2O (1:99, v/v, HEPES 10 mM, pH 7.4) excited at 390 nm.

Figure 5. Fluorescence intensity of A (300 nM) when exposed to various metal ions (3 lM) at 453 nm.

Figure 8. Calibration curve of the A∙Cu2+ complex (300 nM) toward CN– (0–42 mM) in ACN/H2O (1:99, v/v, HEPES 10 mM, pH 7.4) at 453 nm (excited at 390 nm).

Figure 6. Changes in the A∙Cu2+ complex (300 nM) fluorescence intensity upon addition of a TBA anion salts (30 mM) in ACN/H2O (1:99, v/v, HEPES 10 mM, pH 7.4) excited at 390 nm.

Figure 9. Fluorescence intensity of the A∙Cu2+ complex (300 nM) against potentially interfering TBA anions (6 lM) at 453 nm.

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a 1:1 stoichiometry between receptor A and Cu2+ (Figure S4) [49]. This binding stoichiometry was further supported by the obtained ESI-MS data which showed a peak at m/z 515.05 and was assigned to A∙Cu2+ (calculated: 515.05; Figure S5). A competitive binding experiment was performed to assess any possible interferences from competitive metal ions (Figure 5). None of the competitive analytes tested significantly interfered with the detection of Cu2+. Response time is another essential factor for analyte analysis. The response time for receptor A when exposed to Cu2+ was studied by recording fluorescence spectra at regular time intervals (Figure S6), showing that 60 s was sufficient to detect Cu2+. Anion binding studies

Figure 10. Reversible switching of the fluorescence intensity of A (300 nM) by alternate addition of Cu2+ and CN in ACN/H2O (1:99, v/v, HEPES 10 mM, pH 7.4) at 453 nm (excited at 390 nm).

Figure 11. Photographs of A containing Cu2+ and CN– under UV irradiation (365 nm).

To investigate the formed A∙Cu2+ complex as a secondary sensor for anions, 300 nM solution of the A∙Cu2+ complex was screened with 30 mM of the tetrabutylammonium (TBA) salts of Br–, Cl–, ClO–4, CN–, F–, H3PO–4, HSO–4, N–3, NO–3, CH3COO–, S2–, and SCN–. None of the tested anions induced significant changes in the emission profile of the A∙Cu2+ complex, as shown in Figure 6. However, upon addition of CN–, the emission intensity of the A∙Cu2+ complex showed significant enhancement. Fluorescence titration was performed, showing a linear increase in the emission intensity with increasing concentration of CN– (0–200 equiv.; Figure 7). The obtained emission pattern was similar to the emission pattern of the free receptor A. Furthermore, the calibration plot was generation of the fluorescence intensity vs. concentration, showing a good correlation coefficient value (R2) = 0.99 (Figure 8). Competitive binding studies were performed to investigate the effect of competitive anions on CN– detection. The competitive analytes were added to the A∙Cu2+ complex solution along with CN–, and the fluorescence emission was measured at 453 nm, showing that no competitive analyte interfered with the CN– detection (Figure 9). In order to determine the stoichiometry of CN– binding, Job’s plot was obtained, revealing a 1:1 stoichiometry between the A∙Cu2+

Figure 12. Structure optimization and energy correlation diagram showing the HOMO-LUMO energy of A and the A∙Cu2+ complex.

W. Sik Na et al. / Tetrahedron Letters 60 (2019) 151075

complex and CN– (Figure S7). These studies indicated that the A∙Cu2+ complex could be used for the selective detection of CN– in a complex solution. The limit of detection (LOD) was 7.46 lM, which is close to the WHO recommended value for drinking water (Figure S8) [47]. The reversibility of the A∙Cu2+ complex sensor toward CN– was tested via alternate addition of Cu2+ and CN– (Figure 10). The fluorescence intensity showed alternate fluorescence quenching and enhancement at 453 nm. This process was repeated for five cycles and only a minor loss in fluorescence efficiency was observed.

Possible sensing mechanism The color of A upon addition of Cu2+ and CN– was measured under UV light irradiation, as shown Figure 11. The addition of Cu2+ to a solution of A was accompanied by a color change from blue to green under UV light irradiation. The green solution changed to blue upon addition of CN–, indicating that ligand A was regenerated. Comparative FTIR spectra of A and the A∙Cu2+ complex are presented in Figure S9, showing that variation in the frequency of sulfonyl (S@O) group occurred upon binding with Cu2+. The antisymmetric and symmetric vibration modes of S@O in compound A were observed at approximately 1378 and 1276 cm 1, which merged to form a band at 1385 cm 1. This spectral feature is associated with binding of the sulfonyl group with Cu2+. The fluorescence of A was quenched by addition of Cu2+ by chelation enhanced fluorescence quenching (CHEQ) owing to the paramagnetic effect from spin-orbit coupling of Cu2+ [50]. DFT calculations were performed to support the experimental evidence of A and Cu2+ binding. The HOMO of A showed that the electron cloud was situated on the benzothiazole ring. However, the electron density in the LUMO was delocalized over the sulfonyl group and benzothiazole ring. The HOMO and LUMO energies of A were 5.11 and 3.05 eV, respectively. Upon coordination of Cu2+, the HOMO and LUMO energies of the A∙Cu2+ complex were 4.92 and 4.71 eV, respectively (Figure 12). The HOMO-LUMO gaps of A and A∙Cu2+ complex were calculated to be 2.06 and 0.21 eV, respectively, showing that the coordination of Cu2+ by A led to a decreased HOMO-LUMO gap and stabilized copper complex formation. A plausible binding mode of A with Cu2+ and CN– is presented in Scheme 2. The reversible switching fluorescence intensity cycles of A by alternate addition of Cu2+ and CN showed that the process was reversible. The fluorescence profile of A was quenched by Cu2+

O N

S

O S O

2+ S Cu

A.Cu2+

A

[Cu(CN)x]n-

A benzothiazole-based heterodipodal receptor was synthesized and used for selective binding of Cu2+. The emission spectra of receptor A exhibited remarkable fluorescence quenching upon addition of Cu2+, confirming its high selectivity in the presence of other metals. The binding stoichiometry between receptor A and Cu2+ was studied using a Jobs plot and ESI-MS, which proved that a 1:1 complex formed between A and Cu2+. The A∙Cu2+ complex was further examined for its selective binding with anions, and the emission studies showed that the A∙Cu2+ complex demonstrated selectivity toward CN– in a reversible sensing process. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2018R1D1A1B07041198). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.tetlet.2019.151075. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

[21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

CNFluorescence "ON"

Conclusions

[17] [18] [19] [20]

N O S O O

N

CHEQ upon adding Cu2+ and was recovered by addition of CN– to the A∙Cu2+ complex, indicating that Cu2+ was released from the A∙Cu2+ complex, forming [Cu(CN)x]n– [38].

[16]

N

Fluorescence "OFF"

Scheme 2. Proposed binding modes.

5

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