Robust, sensitive and facile method for detection of F−, CN− and Ac− anions

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 186 (2017) 8–16

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Robust, sensitive and facile method for detection of F−, CN− and Ac− anions P. Madhusudhana Reddy a,1, Shih-Rong Hsieh b,1, Jem-Kun Chen c, Chi-Jung Chang a,⁎, Jing-Yuan Kang a, Chih-Hsien Chen a a b c

Department of Chemical Engineering, Feng Chia University, 100, Wenhwa Road, Seatwen, Taichung 40724, Taiwan, ROC Department of Surgery, Taichung Veterans General Hospital, 1650 Taiwan Boulevard Section 4, Taichung 40705, Taiwan, ROC Department of Materials Science and Engineering, National Taiwan University of Science and Technology, 43, Sec.4, Keelung Rd, Taipei 106, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 17 March 2017 Received in revised form 23 May 2017 Accepted 5 June 2017 Available online 6 June 2017 Keywords: Naked eye Colorimetric Sensor Anions Test stripes

a b s t r a c t Sensing of F−, CN− and Ac− is important from the viewpoint of both medically and environmentally. Particularly, sensing of the anions in 100% water by a colorimetric chemical sensor is a highly difficult task as water molecules interfere the sensing mechanism. In this regard, sensor R1, having azo and nitrophenyl groups as signaling units and thiourea as a binding site was prepared. This sensor exclusively detected CN− ion over other testing anions in 30% aq. DMSO solution by exhibiting distinct spectral and visual color changes. However, in 15% aq. DMSO solution, R1 exhibited obvious spectral and color changes in response to F−, CN− and Ac−. On the other hand, we have also designed sensor, R2, having same signaling units of R1, but a different binding site of urea group. Surprisingly, in contrast to R1, R2 exhibited obvious spectral and color changes in 5% aq. DMSO solution only. Further, economically viable “test stripes” were prepared in a facile mode to detect the CN− in 100% aqueous solution. Such stripes can serve as a practical colorimetric probe for “in the field” detection of the ions and thus avoid additional expensive equipment. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Over the past few years, development of chemical sensors for anion sensing presents both the promise and challenge of advancing science in a variety of key areas, including medicine, analytical and environmental [1,2]. Moreover, these sensors have several advantages such as high selectivity, sensitivity, and real-time monitoring of anions over other methods [3,4]. These sensors recognize anions via hydrogenbonding interactions or deprotonation [5]. Among various anions, F−, CN− and Ac− have been intensively studied owing to their important roles in our life [5–18]. For instance, F− ions play vital role in human's daily life such as water supply treatment, osteoporosis treatment, dental care and even in chemical warfare agents [15–18]. CN− ions also play significant role in numerous chemical processes, such as electroplating, plastics manufacturing, gold and silver extraction, tanning, and metallurgy [5,6]. In living organisms such as acetyl coenzyme A, Ac− ions play an important role. Ac− ions in the form of sodium acetate involved in biological process like enzyme metabolism and used as main ingredient in some medicines. In addition, the main component present in vinegar is acetic acid [14]. Despite the extensive advantages [5–18], excess intake of F− can lead to fatal problems including skeletal fluorosis, bone diseases, mottling of tooth enamel, and lesions of the thyroid, liver and ⁎ Corresponding author. E-mail address: [email protected] (C.-J. Chang). 1 These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.saa.2017.06.004 1386-1425/© 2017 Elsevier B.V. All rights reserved.

other organs [19,20]. Thus, the diversity of its functions, both beneficial and otherwise, makes the detection of F− ions important. Similarly, CN− is also known to be toxic anion in biology and environment [21–23]. The interest in monitoring of CN− ions stems from the fact that the estimated production is nearly 140,000 tons per year worldwide [24–26]. On the other hand, Ac− also received an increased intensity in the study as it plays a prominent role as acetyl coenzyme A in living organisms [14]. Azo dyes have found potential applications in the textile industry and the holographic data storage application due to their vivid color and photorefractive properties [27–30]. To date, several sensors have been developed to detect the F−, CN− and Ac− [13,14]. Particularly, azo dye based chemical sensors have attracted much attention among the scientific community [31–33]. However, many sensors were reported to work either in an organic solvent or organic solvent-water mixture with a very little portion of water. For instance, Dang et al. [31] have studied disperse orange 3 (DO3) based sensors with different substituents of the electron withdrawing (EWG) and electron donating groups (EDG). According to their results, the sensor (1) with EWG substituent has shown the association constant of 3.3 × 104 ± 2.6 × 102 for F− ion, while the sensor (2) with EDG substituent exhibits the association constant of 8.6 × 104 ± 3.4 × 103 for the same ion in DMSO solution. In another instance, Cheng et al. [32] described the azo-based sensor for the detection of CN− ion with the detection limit of 1.1 μM in CH3CN solution. Niu et al. [33], prepared DO 3 based chemical sensor, which can detect the CN− ion with the binding constant (K− CN) and

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detection limit of 2.8 × 103 M−1 and ~11 μM, respectively, in 10% (v/v) aq. acetonitrile (CH3CN) solution. Moreover, it was reported that the detection limit values were increased when the ratio of water was increased in the organic solvent-water mixture [34]. This increasing behavior of detection limit can be attributed to the strong competition between water and donor site to anion [35]. Moreover, the applications of most of the existing sensors further limited due to their performance in solution only, but not as solid stripes [25]. Therefore, it is very important to develop potential colorimetric sensors for identification of hazardous anions, at their minimum concentration levels, in the organic, organo-aqueous mixture and water for an environment or biological applications. In order to find out more efficient naked eye anion sensor and to explore the optimized sensor design, we have designed thiourea/urea based sensors, R1 and R2. In these systems, an acceptor nitro-phenyl moiety and a donor, thiourea/urea, are connected through a conjugated azo system. Further, the benzoyl group was introduced at adjacent position to the binding groups of R1 and R2 to amplify the acidic strength of \\NH, and thus make\\NH protons to be available easily to the anions. Therefore, the sensing performance of R1 and R2 was improved. The change in the nature of the donor can be produced through hydrogen bonding and thus can result in a change of UV–visible spectra, and consequently, change in color of the sensor solution. In the presence of relatively basic anions such as F−, CN− and Ac−, sensors R1 and R2 display noticeable color changes from pale yellow to orange and pale yellow to yellow due to the formation of hydrogen bonds or deprotonation in DMSO and DMSO-water mixture. Anion complexation-induced changes in UV–vis absorption and 1H NMR spectra and solution color can be evidenced for the intermolecular interactions between sensors and anions. After establishing the anion sensing properties of sensors in solution, we finally developed the “test stripes” to detect the anions. The chromogenic response of these sensors can be detected by the naked eye. As a result, these sensors might allow us to screen the anions in 100% aqueous medium.

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124.3, 125.0, 127.4, 128.1, 132, 133.2, 141.9, 149.2, 150.1, 168.1, 178.8; HRMS (m/z): found 405. 2.2.2. Chemical Sensor R2 Sensor R2 was also prepared in a single step as illustrated schematically (Scheme 1b in Supporting Information). Sensor R2 was synthesized by reacting the benzoyl isocyanate (3) with DO 3 (1) at mole ratios of 1:1, and a reaction was proceeded at 60 °C, for 6 h. The chemical structure of R2 was characterized by the 1H NMR (Fig. S4), 13C NMR (Fig. S5) and HRMS (Fig. S6) and the corresponding spectra were provided in Supplementary Information. 1H NMR (400 MHz, DMSO-d6) δH: 7.55 (t, 2H), 7.66 (t, 1H), 7.87 (d, 2H), 8.01 (d, 2H), 8.05 (d, 2H), 8.06 (d, 2H), 8.42 (d, 2H), 11.04 (s, 1H), 11.13 (s, 1H);); 13C NMR (DMSO-d6): δ = 122.1, 122.5, 124.2, 125, 128, 128.9, 132.2, 134, 142, 148, 149.5, 150, 168; HRMS (m/z): found 389.1. 2.3. Instrumentation HR4000 high-resolution UV–Vis spectrometer (Labguide Co.) was used for absorption measurement. Proton nuclear magnetic resonance (1H NMR) spectra were measured on Agilent Technologies DD2 spectrometer (600 MHz) with chemical shifts reported in d6-DMSO. Highresolution mass spectra (HRMS) were performed on a Finnigan-MAT-95XL mass spectrometer (Thermo Quest Finnigan, Bremen, Germany). 2.4. Preparation of “Test Stripes” Initially, DMSO solution of R1 was prepared at a concentration of 5 mM. Filter papers (2 cm × 1 cm) were immersed into the DMSO solution of R1 and subsequently dried in an oven at 50 °C for 30 min. Finally, the drop of interested contaminated water (50 mM of TBAF, TBACN and TBAAc in water medium) samples was dropped on to these filter papers for the identification of anions.

2. Experimental Section

3. Results and Discussion

2.1. Materials

3.1. Sensing Properties of Sensor R1

4-(4-Nitrophenylazo) aniline (disperse orange 3, DO 3), tetrabutylammonium cyanide (TBACN), tetrabutylammonium thiocyanate (TBASCN), tetrabutylammonium acetate (TBAAc) and tetrabutylammonium fluoride (TBAF) were purchased from Aldrich. Tetrabutylammonium hydrogensulfate (TBAHSO4) and dimethyl sulfoxide (DMSO) were received from Sigma-Aldrich. Tetrabutylammonium bromide (TBABr) was provided by the Alfa Aesar. Tetrahydrofuran (THF) was supplied by Macron fine chemicals. Benzoyl isothiocyanate and benzoyl isocyanate were received from Acros organics. All the chemicals were received at their highest purity and used without further purification.

Anion sensing properties of R1 were studied in DMSO and DMSOwater mixture (15% and 30% aq. DMSO). The anion sensing ability of R1 was evaluated by using the series of anions (F−, Br−, CN−, HSO− 4 , SCN− and Ac−) as their tetra-n-butylammonium (TBA) salts. The sensing ability of R1 was scrutinized by its colorimetric and UV–vis spectral changes upon the addition of different anions. As shown in Fig. 1(a-i), in the absence of anions, the absorption spectrum of R1 was characterized by the peak located at ~ 390 nm. However, this peak was shifted to ~470 nm (Bathochromic shift or red shift) with the addition of relatively basic anions such as F−, CN− and Ac−. The bathochromic shift in the characteristic peak of R1 can be regarded to the change in electronic transitions induced by the interaction between R1 and anions (F−, CN− and Ac−). On the other hand, negligible changes were observed in the UV–vis spectrum of R1 with other anions in DMSO solution (Fig. 1(a-i)). Here it is worth to mention that the changes in the UV–vis spectra of R1 upon addition of F−, CN− and Ac− ions were associated with the colorimetric changes that were easy to recognize by the naked eye (Fig. 1(a-ii)). Such colorimetric changes are really valuable in anion sensing application as this process can avoid expensive instrumentation. The anion induced color changes in R1 solution can be attributed to the complex formation between an anion and sensor or anion-induced deprotonation [36–39]. Further, such complex formation or deprotonation may lead to the enhancement in electron density in the sensor. Because of this enhancement in electron density in the sensor, the charge-transfer interactions between the electron rich and electron deficient moieties are improved and thus color change can be observed by naked

2.2. Synthesis of Chemical Sensors R1 and R2 2.2.1. Chemical Sensor R1 Sensor R1 with thiourea binding site was prepared in a single step as illustrated schematically (Scheme 1a in Supporting Information). To a solution of DO 3 (1) in anhydrous THF, benzoyl isothiocyanate (2) was added at mole ratios of 1:1, and a reaction was proceeded at 60 °C, for 6 h. Once the reaction was finished, the precipitate was collected and washed with dry THF. After washing, the product was subjected to vacuum drying process to obtain the solid flakes. The chemical structure of R1 was characterized by the 1H NMR (Fig. S1), 13C NMR (Fig. S2) and HRMS (Fig. S3) and corresponding spectra were provided in Supplementary Information. 1H NMR (400 MHz, DMSO-d6) δ: 7.56 (t, 2H), 7.68 (t, 1H), 8.01 (d, 2H), 8.04 (d, 2H), 8.09 (d, 2H), 8.10 (d, 2H), 8.45 (d, 2H), 11.71 (s, 1H), 12.83 (s, 1H); 13C NMR (DMSO-d6): δ = 122.1,

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Fig. 1. UV–vis absorbance spectral changes and corresponding photographs for colorimetric changes of sensor R1 (30 μM) in DMSO (a-i, a-ii), 15% aq. DMSO (b-i, b-ii) and 30% aq. DMSO (ci, c-ii) upon addition of different anions (30 equivalents) of tetrabutylammonium.

eye [40,41]. These results demonstrated that the simple azo dye based R1 is quite suitable as a colorimetric chemosensor capable of detecting anion species in an organic medium. However, in order to be an efficient anion sensor, a sensor must work in aqueous based systems. Therefore, in the present study, 15% and 30% aq. DMSO solutions were selected as solvent media to check the sensing performance of R1 in water based systems. UV–vis and colorimetric results of R1 that were obtained in 15% and 30% aq. DMSO solutions presented in Fig. 1(b-i, b-ii) and (c-i, c-ii), respectively. As shown in Fig. 1(b-i, b-ii), the UV–vis spectral changes and corresponding color changes of sensor R1 upon addition of selected anions in 15% aq. DMSO solution were similar with those obtained in DMSO solution (Fig. 1(a-i, a-ii). The results confirm that the addition of water to DMSO up to 15% did not interfere in the anion sensing performance of R1. The sensing performance of R1 in 15% aq. DMSO solution could be rationalized as follows. The compound R1 bearing thiourea group which has been proven to be a good hydrogen bond donor and thus make strong hydrogen bonds with the anions, which water molecules could not break [42]. Therefore, the possibility of the interaction of the anions with the sensor was improved greatly [43]. It is worth to mention that the color difference among F−, CN− and Ac− was not observed either in pure DMSO or 15% aq. DMSO (Fig. 1(a-ii and b-ii). The similar − kind of results have been observed for F−, H2PO− 4 and Ac upon interaction with azophenol-thiourea based anion sensor [41]. However, it is surprising that the R1 detected exclusively CN− in 30% aq. DMSO solution as shown in Fig. 1(c-i, c-ii). The exclusive detection of CN− in 30%

aq. DMSO solution by R1 can be attributed to its comparatively lower hydration energy (ΔHhyd = − 67 kJ/mol) than that of the F− (ΔHhyd = −505 kJ/mol) and Ac− (ΔHhyd = −375 kJ/mol) [44,45]. To check the possible interference of other anions on the detection of F−, CN− and Ac− ions by R1, the UV–vis spectral changes and colorimetric changes of R1 in the presence of F−, CN− and Ac− mixed with various anions. For instance, UV–vis spectral changes and colorimetric changes of R1 in the presence of F− mixed with SCN−, Br− and HSO− 4 in 15% aq. DMSO solution (Fig. 2). To our surprise, the spectral and colorimetric changes of R1 were similar to that in the presence of F− ion alone (Fig. 1). The similar results were obtained for R1 with CN− and Ac− (Fig. 2). These results suggest that R1 displays a good selectivity for F−, CN− and Ac− over other competing anions. Therefore, these results clearly indicated that R1 can act as a specific chromogenic chemosensor for F−, CN− and Ac− in an organic-water mixture, which is reminiscent of industrial waste water. 3.2. Sensing Properties of Sensor R2 Having established the anion sensing ability of R1, we next have our attention to sensor R2. R2 also showed marked UV–vis spectral changes towards F−, CN− and Ac− ions in DMSO solution as shown in Fig. 3(a-i). The corresponding samples have also shown the color changes from pale yellow to yellow as shown in Fig. 3(a-ii). As shown in Fig. 3(a-i), R2 in DMSO has a maximum absorption peak centered at the wavelength of ~ 384 nm in the absence of anions. However, this maximum

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Fig. 2. UV–vis absorbance spectral changes of sensor R1 (30 μM) upon addition of F−, CN− and Ac− (30 equivalents) of tetrabutylammonium in the presence of 30 equivalents of various anions (HSO4−, SCN− and Br−) in 15% aq. DMSO (a) and corresponding photographs for colorimetric changes (b).

absorption peak was shifted towards longer wavelengths of ~438, ~435 and ~430 nm upon addition of F−, CN− and Ac−, respectively. In contrast, R2 resulted in no change either in color or UV–vis spectra upon ad− dition of the remaining anions (Br−, HSO− 4 and SCN ). It is worth to mention that the solution of sensor R1 showed more intense color change and longer wavelength shift than the sensor R2 for the same anions. These results suggest that the sensor R1 with thiourea binding site can form the stronger hydrogen bonds with the anions. The ability of formation of strong hydrogen bonds by thiourea stems from its more acidic nature than the urea (pKa (thiourea) = 21.1, pKa (urea) = 26.9 in DMSO) [46]. In contrast to sensor R1, R2 was not able to detect the anions in 15% aq. DMSO. Therefore, we have selected the 5% aq. DMSO as a solvent medium to study the sensing abilities of R2. The difference in the sensing abilities between R1 and R2 can be attributed to the difference in the acidic strengths of -NH of urea and thiourea. As shown in Fig. 3(b-i) and 3(b-ii), the color and absorption spectra of R2 in 5% aq. DMSO solution were not different from those of R2 in pure DMSO. R2 was pale yellow in color showing maximum absorption wavelength at about ~385 nm

in 5% aq. DMSO solution in the absence of anions. The addition of F−, CN− and Ac− to the R2 solution resulted in changes both in color and maximum absorption wavelength. Therefore, these results dictated that upon the addition of anions, the R2 formed the complex with the anions through hydrogen bonds as expected, and thus lead to apparent color change and shift in maximum absorption wavelength. In contrast, no other anion produced such changes in colorimetric and UV–vis spectra of R2. Here it is worth to mention that, the anion induced bathochromic shift was higher in pure DMSO when compare to that in 5% aq. DMSO solution. The maximum absorption wavelength of R2 was found to be ~ 426, 425 and 389 nm, in the presence of F−, CN− and Ac−, respectively. These results suggest that there is a competition between water and a binding site for the anions. Further, in order to check the sensing ability of anions in the presence of other competitive anions, the UV–vis and colorimetric changes of R2 were monitored in the presence of interested anion mixed with various anions. For instance, as shown in Fig. 4, the spectral and colorimetric changes of R2 in the presence of F− ion mixed with various anions were similar to those of R2 in the presence of F− ion alone. Similar results were

Fig. 3. UV–vis absorbance spectral changes of sensor R2 (30 μM) upon addition of different anions (30 equivalents) of tetrabutylammonium in DMSO (a-i)) and corresponding photographs (a-ii)). UV–vis absorbance spectral changes of sensor R2 (30 μM) upon addition of F−, CN− and Ac− (30 equivalents) of tetrabutylammonium in 5% aq. DMSO (b-i)) and corresponding photographs for colorimetric changes (b-ii)).

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Fig. 4. UV–vis absorbance spectral changes of sensor R2 (30 μM) upon addition of F−, CN− and Ac− (30 equivalents) of tetrabutylammonium in the presence of 30 equivalents of various − − anions (HSO− 4 , SCN and Br ) in 5% aq. DMSO (a) and corresponding photographs for colorimetric changes (b).

obtained for R2 with CN− and Ac−. These results suggested that R2 display a good selectivity for F−, CN− and Ac− over other competing anions. 3.3. Stoichiometric Characteristics of Sensor R1 CN− anion titrations of sensor R1 were undertaken in 15% aq. DMSO solution. In the UV–vis titration with CN− ion, the sensor R1 showed characteristic absorption peak at ~390 nm. Upon the slow addition of an incremental amount of CN− ions, the absorption peak at ~ 390 nm slowly moved to a longer wavelength of ~450 nm with one isosbestic point at ~ 410 nm (Fig. 5a). This gradual incremental amount of CN−

ions to the R1 solution induces color changes from pale yellow to orange. Further addition of more equivalents of CN− ions to the R1 solution did not bring any noticeable changes both in UV–vis spectra and color. To know the binding mechanism, the Job's plot was constructed as shown in Fig. 5b. From the Job's plot, a maximum absorption was observed when the molar fraction of CN− reached 0.5, which is indicative of a 1:1 stoichiometry binding between R1 and CN− [47]. The Benesi– Hildebrand method [48] was used to determine the binding constant (K) of complexation between R1 and CN− as shown in Fig. 5c. The bind4 −1 ing constant for CN− (K− . FurtherR1-CN) was found to be 3.7 × 10 M more, the detection limit of sensor R1 for the analysis of CN− was determined through standard deviations and linear fittings [49]. By

Fig. 5. UV–vis absorbance spectral changes of sensor R1 (30 μM) upon addition of different amounts of CN− (a); Job's plot for the binding of R1 with CN−, absorbance was plotted as a function of the molar ratio of [R1]/([R1] + [CN−]) and the total concentration of CN− with sensor R1 was 50 μM (b); Benesi–Hildebrand plot of the sensor R1 binding with the CN− ion associated with the absorbance change at 443 nm (c); Detection limit of R1 for CN− ion. All the measurements made in 15% aq. DMSO solution.

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plotting the absorbance changes as a function of the concentration of CN− as given in Fig. 5d, the detection limit of R1 for the CN− was found to be 13 μM, based on the 3σ/slope. On the other hand, the addition of F− and Ac− whose binding constants for the 1:1 stoichiometry host–guest complex were calculated 4 −1 as 6. 37 × 104 M− 1 (K− (K− R1-F) and 1.9 × 10 M R1-Ac), respectively, (Fig. S7 and S8) led to the similar changes of the absorption spectrum with that of CN−. From these results it was found that the binding affinities of anions for R1 (in 15% aq. DMSO solution) were determined to be F− N CN− N Ac−. As already established elsewhere, the anion binding affinity of the urea or thiourea sensors generally correlates to the basicity of the target anions [50,51]. Similar to such sensors, the examined R1 containing thiourea donor showed binding constants and colorimetric change in accordance with the basicity indices of the introduced anion species. Being most basic, F− forms the strongest hydrogen bonds, CN− as the second most basic anion forms the relatively weaker hydrogen bonds with R1. R1 has shown the highest binding constant value with F− following CN− and Ac−. Therefore, a sufficient basicity of the anions is necessary to be recognized. Fortunately, the anions of interest for the scientific community are basic enough to be recognized by the chemical sensors. Further, in addition to the intrinsic basicity of anions, the anion detection properties of the colorimetric sensors also influenced by the presence of strong electron withdrawing group (EWG). The presence of EWG, decreases the electron density at the receptor nitrogen atom and hence increase the strength of the receptor-anion interaction, which results in enhanced. In contrast, the electron donating groups (EDGs) increase the electron density on the receptor nitrogen atom, therefore reducing the ability of receptor-anion interactions by the detector molecules. For F− and Ac− R1 has exhibited a detection limit of 15 μM and 26 μM, respectively (Fig. S7 and S8).

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3.4. Stoichiometric Characteristics of Sensor R2 The UV–vis spectral changes of R2 upon the gradual addition of an incremental amount of CN− ion were displayed in Fig. 6. As shown in Fig. 6a, with increasing concentration of CN−, the maximum absorption of the peak centered at 385 nm shifted to the longer wavelength of 426 nm with an isosbestic point located at the wavelength of 405 nm. This single isosbestic point indicated the presence of two species, such as R2 and complex of R2 with an anion, in the solution. Job's plot was constructed using the UV–vis spectra of R2 in the presence of CN− ions as shown in Fig. 6b. Binding mechanism and complex formation of R2 with the CN− ions were obtained in 5% aq. DMSO from the Job's plot. As shown in Fig. 6b, the absorbance exhibited a maximum at a molar fraction of 0.5, indicating that the most likely binding mode for CN− and R2 was a 1:1 stoichiometry. Therefore, R2 attract the most basic CN− ion through the two hydrogen atoms of urea moiety to form the R2-CN− complex. The Benesi–Hildebrand method [48] was used to determine the binding constant of complexation between R2 and CN− as shown in Fig. 6c. The binding constant for CN− was found to be 2.4 × 104 M−1 (K− R2-CN). Furthermore, the detection limit of sensor R2 for the analysis of CN− was determined through standard deviations and linear fittings [49]. By plotting the absorbance changes as a function of the concentration of CN− as given in Fig. 6d, the detection limit of R2 for the CN− was found to be 14 μM, based on the method used for R1. In order to elucidate the binding mechanism of F− and Ac− with R2, UV–vis titration experiments were further extended. R2 has shown the UV–vis spectral changes upon the gradual addition of F− and Ac−. It is worth to mention that, the titration profile of R2 with F− (Fig. S9) and Ac− (Fig. S10) was almost similar to that with CN−. As shown in Fig. S9a, gradual increment of F− ions leads to the shift of absorption peak

Fig. 6. UV–vis absorbance spectral changes of sensor R2 (30 μM) upon addition of different amounts of CN− (a); Job's plot for the binding of R2 with CN−, absorbance was plotted as a function of the molar ratio of [R2]/([R2] + [CN−]) and the total concentration of CN− with sensor R2 was 50 μM (b); Benesi–Hildebrand plot of the sensor R2 binding with the CN− ion associated with the absorbance change at 465 nm (c); Detection limit of R2 for CN− ion. All the measurements made in 5% aq. DMSO solution.

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from 385 nm to 425 nm with an isosbestic point at 405 nm. Analogously, the gradual increment of Ac− ions leads to the shift of absorption of the band from 385 nm to 450 nm with an isosbestic point at the wavelength of 408 nm. Further, the job's plot analysis confirmed the 1:1 stoichiometry for the R1-F− (Fig. S9b) and R1-CN− (Fig. S10b). The binding constants for R2 with F− (Fig. S9c) and Ac− (Fig. S10c) were estimated to 4 −1 be 1.9 × 104 (K− (K− R2-F) and 2.4 × 10 M R2-Ac), respectively, by using the Benesi-Hildebrand equation. The detection limits of R2 for F− (Fig. S9d) and Ac− (Fig. S10d) were found to be 7 and 5.8 μM, respectively. 3.5. 1H NMR Titration Studies of Sensors R1 and R2 It is known that the molecular level mechanism for the interactions between sensor-anion can be understood with the help of 1H NMR titration. Therefore, in the present study the 1H NMR titration experiments were carried out by the incremental addition of anions (F−, CN− and Ac−) to solutions of the sensors R1 and R2 in DMSO-d6. We traced the aromatic protons and NH protons to extract the binding mechanism between anions and sensor molecules. With increasing addition of CN−,

an upfield shift was observed in the signals of aromatic protons of both sensors R1 and R2 as shown in Fig. 7. These upfield shifts can be attributed to the complex formation and deprotonation of NH protons of R2 and R1, respectively, by CN− ion. Because of this complexation and deprotonation, the electron density on the phenyl ring passed through-bond propagation, and thus the increased electron density generates a shielding effect. This shielding effect produced an upfield shift with C\\H protons [52]. On the other hand, it was found that the amide NH proton signals of sensor R2 were exhibited downfield shift, with increasing CN− concentration from 0 eq. to 0.5 eq. indicating the formation of a host–guest hydrogen-bonding complex and an overall change of the electron distribution [53]. The formation of R2-CN− complex due to H-bonding make NH protons to be deshielded and thus downfield shifts were observed with NH protons. The responses of the R1 and R2 towards F− and Ac− were also investigated by means of 1H NMR titration experiments and the corresponding spectra were presented (Fig. S11 and S12). As shown in Fig. S11a, the peaks correspond to \\NH protons were disappeared, while aromatic protons have shown the upfield shift upon addition of even 0.5 eq. of

Fig. 7. 1H NMR spectra of sensors R1 (a) and R2 (b) (5 mM) in DMSO-d6 upon successive addition of CN− ions.

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Fig. 8. Photographs for the detection of anions in 100% water on the filter papers of sensor R1.

F− to the R1 solution. However, for R2, the NH peaks were shifted to downfield, while aromatic protons shifted to upfield. Moreover, F−, being small in size and high in charge density, initially forms a hydrogen-bonded complex and then, in the presence of excess F−, causes deprotonation. However, the amide NH proton signals of sensor R1 were disappeared upon addition of F−, suggesting the deprotonation. Therefore, such strong interaction of the amide NH in the sensors R1 and R2 with F− might cause internal charge transfer, which accounted for the increase in the intensity of the absorbance band and the generation of orange color. Studies of sensor R1 with Ac− yielded a downfield shift for NH protons at 0.5 eq., while these peaks were disappeared with increasing the concentration (Fig. S12a). These results indicated that Ac− form the strong hydrogen bonded complex with R1 at lower concentrations, while deprotonation of R1 occurred at higher concentration. On the other hand, sensor R2 has shown weak interaction with Ac−, discernible through the remarkable displacement of the urea NH protons towards lower chemical shift and aromatic protons towards the upfield shift. (Fig. S12b). These results were consistent with the stoichiometric parameters as determined by UV–vis titration studies.

3.6. Solid Test Kit of Sensor R1 for “In-The-Field” Detection of Anions in 100% Water Solution The results of the R1 system in solutions of DMSO and 15% aq. DMSO solutions motivated us to prepare the test stripes for on-site detection of anions. Such test stripes avoid the additional equipment for on-site detection and thus make the detection process hassle free and economic viable. The anions were recognized by dropping the aqueous solution of anions (50 mM). As shown in Fig. 8a, the color change was not clearly visible in wet condition. However, the color change becomes clearly visible to the naked eye after drying the test stripes, using hair dryer, for 1– 2 min (Fig. 8b). This difference can be attributed to the presence of water molecules in the wet condition. Here, it is worth to mention that the color change was obvious only for CN− even after drying when compared to that of F− and Ac−. This could be attributed to very high solvation energy for F− (ΔHhyd = − 505 kJ/mol) in water and thus R1 has shown very weak/negligible binding. This weak binding subsequently leads to the poor color change as shown in the Fig. 8. A similar explanation can also be regarded to the poor detection of Ac− ions as this ion shows high solvation energy (ΔHhyd = − 375 kJ/mol) in water. Owing to such high solvation energy values in water, F− and Ac− lost their basicity and consequently could not be recognized. On the other hand, the comparatively lower hydration energy for the CN− (ΔHhyd = −67 kJ/mol) than that of the F− and Ac− explains the selectivity of cyanide ion [44,45]. Therefore, the present test stripes can use exclusively to detect the CN− ions in aqueous systems. The major advantage of the present stripes is that they can be utilized in 100% aqueous medium. As a chemosensor with excellent performance, the R1 can find practical applications of great importance.

4. Conclusions We synthesized selective and sensitive chemical sensors with thiourea (R1) and urea (R2) binding sites to detect the environmentally and biologically significant anions. Sensor, R1 exclusively detected CN− ion over other testing anions in 30% aq. DMSO solution by exhibiting distinct spectral and visual color changes. However, R1 can detect the F− and Ac− anions in 15% aq. DMSO solution. The intermolecular interactions between anion and sensor were attributed to the observed spectral and color changes. These intermolecular interactions can be accounted to the hydrogen bonding and followed by deprotonation event in thiourea binding site of R1, which were confirmed by a 1 H NMR titration experiment. On the other hand, sensor R2, detected the ions only in 5% aq. DMSO solution. In this context, a detailed comparison of sensing performance of R1 and R2 was provided by taking the nature of binding sites and analyte into account. Sensor R1 with thiourea binding group exhibited lower sensing limits of 15 and 13 μM for F− and CN− ions, respectively, in 15% aq. DMSO solution. While, R2 with urea binding group exhibited sensing limits of 7 and 14 μM for F− and CN− ions, respectively, in 5% aq. DMSO solution. Further, the test stripes of R1 exclusively detected the CN− ions in 100% aqueous medium. Therefore, we believe that this investigation provided useful insights into the molecular design for the cost-effective practical sensors that can detect biologically significant anions in industrial waste water. Acknowledgments The authors are grateful for the financial support from Feng Chia University and the Taichung Veterans General Hospital under contract number TCVGH-FCU1048201. The authors appreciate the Precision Instrument Support Center of Feng Chia University for providing the measurement facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.06.004. References [1] Q. Wang, Y. Xie, Y. Ding, X. Li, W. Zhu, Colorimetric fluoride sensors based on deprotonation of pyrrole-hemiquinone compounds, Chem. Commun. 46 (2010) 3669–3671. [2] P.A. Gale, Anion receptor chemistry: highlight from 2008 and 2009, Chem. Soc. Rev. 39 (2010) 3746–3771. [3] E. Saikia, M.P. Borpuzari, B. Chetia, R. Kar, Experimental and theoretical study of urea and thiourea based new colorimetric chemosensor for fluoride and acetate ions, Spectrochim. Acta A Mol. Biomol. Spectrosc. 152 (2016) 101–108. [4] D. Singhal, N. Gupta, A.K. Singh, The anion recognition properties of a novel hydrazone based on colorimetric and potentiometric studies, Mater. Sci. Eng. CMater. Biol. Appl. 58 (2016) 548–557.

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