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A novel colorimetric HSO4− sensor in aqueous media
Spectrochimica Acta Part A 90 (2012) 152–157
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel colorimetric HSO4 − sensor in aqueous media Ping Li a,b,c , You-Ming Zhang a,b,c , Qi Lin a,b,c , Jun-Qiang Li a,b,c , Tai-Bao Wei a,b,c,∗ a
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Lanzhou, Gansu 730070, PR China Key Laboratory of Polymer Materials of Gansu Province, Lanzhou, Gansu 730070, PR China c College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China b
a r t i c l e
i n f o
Article history: Received 6 October 2011 Received in revised form 1 January 2012 Accepted 16 January 2012 Keywords: Colorimetric sensors Hydrogensulfate recognition Aqueous solution Colorimetric test kit
a b s t r a c t A novel and sensitive anion receptor 3, bearing Schiff base structure, nitrophenyl azobenzol and carboxyl groups, was developed and characterized as a single chemosensor for the recognition of HSO4 − anion. The different responses of UV–vis spectra and color changes of 3 could be applied to the recognition for HSO4 − over other anions such as F− , CI− , Br− , I− , AcO− , H2 PO4 − and CIO4 − by the naked eye. Furthermore, the anion binding interaction of receptor-anion was also studied using UV–vis and 1 H NMR titration which revealed that 3 displayed a remarkable binding ability for the HSO4 − with an association constant Ka = 6.59 × 104 M−1 . And the detection limitation of HSO4 − with the receptor 3 was 2.0 × 10−6 mol L−1 in aqueous solution. Most importantly, the qualitative detection of HSO4 − using receptor 3 was attempted with test kit which was made from receptor 3. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction The sensing and recognition of anions has emerged recently as a key research field within the generalized area of supramolecular chemistry for the important role placed by anions in a wide range of industrial, agricultural, biological systems and environmental problems [1–3]. Therefore, the recognition and sensing of anionic analytes has attracted considerable attention recently and as a significant goal of research programs [4–10]. In addition, hydrogensulfate anion can be found in many agricultural fertilizer, industrial raw materials and their deleterious effect as pollutants [11]. For example, hydrogensulfate anion is present in nuclear fuel waste along with other oxoanions, which eventually get into the environment. Given that the hydrogensulfate anion has a large standard Gibbs energy of hydration (−1080 kJ mol−1 ), the recognition and separation of the hydrogensulfate anion from an aqueous media is a challenging task [12]. However, only few examples of hydrogensulfate anion recognition have been reported in recent years. In nature, the hydrogensulfate anion is recognized and transported by the hydrogensulfate-binding protein through hydrogen bonds [13]. Accordingly, the hydrogen bond complexes were widely used in design of anion sensor because a stable object hydrogen bonding complexes could be formed to use the amides [14–16], thioureas [17–19], ureas [20–23] and imidazolium [24] which regarded as the
∗ Corresponding author at: College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China. Tel.: +86 931 7973191; fax: +86 931 7971216. E-mail address: [email protected] (T.-B. Wei).
hydrogen donor to form R H· · ·X− anion hydrogen bonds and the tautomeric azophenol-containing receptors. However, the detection of anions based on tautomeric azophenol-containing receptors is rarely reported in the literature, although such cation receptors which undergo azophenol to quinone-hydrazone tautomerization upon presence of cations, have been successfully shown [25–27]. Hence, there is a need for us to develop colorimetric anion receptor with anion-induced azophenol to quinone-hydrazone tautomerization as signaling mechanism. On the other hand, in biological and environmental systems, anion-receptor interactions commonly occur in aqueous media. Therefore, much attention has been paid to develop anion sensors that work in the aqueous phase [28–32]. The challenge is that strong hydration in the aqueous phase stops the sensors from recognizing the anions. So far, only a few receptors have been synthesized that are able to recognize anions in the aqueous phase. Herein, as one part of our research interesting in anion recognitions [26,31,33–36], we attempted to design an easy to synthesis, highly selective and sensitive sensor for HSO4 − . Thus, the design of receptor as the chemosensor was mainly based on the fact that: (i) the receptor contains both carboxyl group and Schiff base structure as the binding sites that could recognize HSO4 − anion selectively; (ii) in order to achieve “naked-eye” recognition, the nitrophenyl azobenzol group as the chromophore was designed. We further anticipated that compound 3 could display azophenol to quinone-hydrazone tautomerization, stimulated by anionic species in solution. Just as expected, the remarkable color change from orange to light yellow was seen during the spectral titration process.
1386-1425/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2012.01.026
P. Li et al. / Spectrochimica Acta Part A 90 (2012) 152–157
2. Experimental 2.1. Apparatus Melting points were measured on X-4 digital melting-point apparatus and were uncorrected. The infrared spectra were performed on a Digilab FTS-3000 FT-IR spectrophotometer. UV–visible spectra were recorded on a Shimadzu UV-2550 spectrometer. 1 H NMR spectra were recorded on a Varian Mercury plus-400 MHz spectrometer with DMSO as solvent and TMS as an internal reference of analytical grade. Electrospray ionization mass spectra (ESI-MS) were measured on an Agilent 1100 LC-MSD-Trap-VL system. Elemental analyses were performed by Thermo Scientific Flash 2000 organic elemental analyzer. 2.2. Chemicals All reagents obtained commercially for synthesis were used without further purification. In the titration experiments, all the anions were added in the form of tetrabutylammonium (TBA) salts, which were purchased from Alfa-Aesar Chemical, stored in a vacuum desiccator containing self-indicating silica and dried fully before using. 2.3. General method 2.3.1. General procedure for UV–vis All the UV–vis experiments were carried out in DMSO or DMSO/H2 O binary solution on a Shimadzu UV-2550 spectrometer at 298.5 K, unless otherwise mentioned. Any changes in the UV–vis spectra of the synthesized compound were recorded on addition of tetrabutylammonium salt while keeping the ligand concentration constant in all experiments. Tetrabutylammonium salt of anions (F− , CI− , Br− , I− , AcO− , H2 PO4 − , HSO4 − and CIO4 − ), were used for the UV–vis experiments. Affinity constants of receptor 3 for anionic species were determined by non-linear fitting analyses program ORIGIN according to the equation reported by Valeur, 1:1 host–guest complexation [37]. 2.3.2. General procedure for 1 H NMR For 1 H NMR titrations, two stock solutions were prepared in DMSO-d6 solution (TMS is used as an internal standard), one of them containing host (0.01 M) only and the second one containing an appropriate concentration (0.5 M) of guest. Aliquots of the two solutions were mixed directly in NMR tubes and 1 H NMR of the host–guest system was detected. 2.4. Synthesis and characterization of receptor 3 The synthesis route of receptor molecule 3 is demonstrated in Scheme 1. Intermediate 5-(p-nitro-phenylazo)-salicylaldehyde (1) and 5-amino-1,3,4-thiadiazol-2-carboxylic acid (2) were prepared according to the literature reported [38,39]. Intermediate 5-(p-nitro-phenylazo)-salicylaldehyde (0.542 g, 0.002 mol) and 5amino-1,3,4-thiadiazole-2-methane acid (0.290 g, 0.002 mol) were mixed in absolute ethanol solutions (20 mL) with acetic acid as a catalyst. Then, the resulting solution was stirred under refluxed conditions for 5–6 h at 79 ◦ C, then cooled to room temperature and the solvent was removed by evaporation. Finally, the brown crude solid was purified by recrystallistaion from hot solution of DMF/H2 O and the desired pure receptor 3 was obtained in 87% yield. Mp: Intermediate 1: 186–188 ◦ C (lit. 187–189 ◦ C); Intermediate 2: 184–186 ◦ C (lit. 185–187 ◦ C); receptor 3: 212–213 ◦ C. 1 H NMR (DMSO-d6 , 400 MHz) ı 11.79 (s, 1H, COOH), 10.38 (s, 1H, OH), 8.44 (d, J = 8.8, 2H, ArH), 8.26 (s, 1H, ArH), 8.18 (m, 3H, ArH), 8.16 (m, J = 2.8, 1H, ArH) 7.25(d, 1H, CH); 13 C NMR (DMSO-d6 , 400 MHz)
153
ı 206.42, 190.20, 186.64, 183.25, 167.41, 165.94, 164.40, 155.13, 148.10, 144.76, 130.00, 125.00, 124.74, 123.22, 122.80, 118.62. ESIMS, m/z: 398.1. Anal. Calcd. for C16 H10 N6 O5 S: C, 48.24; H, 2.51; N, 21.11; S, 8.04; Found: C, 48.22; H, 2.48; N, 21.14; S, 8.06. 3. Results and discussion 3.1. UV–vis spectral recognitions and colorimetric signaling The anion binding affinity of receptor 3 was primly investigated by UV–vis spectra in the absence and presence of adding various anions such as F− , CI− , Br− , I− , AcO− , H2 PO4 − , HSO4 − and CIO4 − using tetrabutylammonium (TBA) as a counter cation. The experiment was performed by preparing 2.0 × 10−5 mol L−1 solution of receptor 3 in the mixture of dimethylsulphoxide (DMSO) and thirdly distilled water with the volume ratio H2 O/DMSO (3.8:6.2, v/v). There were two characteristic absorption peaks for UV–vis spectra of receptor 3 at 488 nm and 376 nm in the absence of anions. The receptor 3 responded with dramatic color changes when particular TBA anionic salts were added to solution above respectively. Upon the addition of HSO4 − anion, there was a prominent change that an absorption peak at 488 nm disappeared while a new and strong absorption peak at 300–450 nm appeared (Fig. 1(a)) in the corresponding UV–vis absorption spectra due to complexation between receptor 3–HSO4 − molecules. In addition, there appeared a dramatic color change from orange to faint yellow (Fig. 1(c)) for the solution of receptor 3 along with the addition of HSO4 − (TBA). On the contrary, addition of other anionic species to the solution of receptor 3 (TBA) no significant modulation of color was observed in the UV–vis absorption spectra. The result shows the specificity of the chemosensor 3 for binging HSO4 − anion was realized successfully (For interpretation of the references to color in the text, the reader is referred to the web version of the article.). 3.2. UV–vis spectral titrations In order to estimate the specific properties for selective recognition of HSO4 − and colorimetric changes associated with the receptor 3, the receptor 3 toward HSO4 − anion was studied by UV–vis absorption spectra titration experiments. The experiments were conducted using a 2.0 × 10−4 M solution of receptor 3 in aqueous solutions (H2 O/DMSO, 3.8:6.2, v/v) (Fig. 2(a)). Upon the addition of HSO4 − (0.1 M) anion to the aqueous solution, a significant decreasing of the UV–vis absorbance at 480 nm and a new band centered at 370 nm were observed. There was an isosbestic point at 398 nm, which indicates that receptor 3 reacts with HSO4 − anion to form a stable complex. By nonlinear least-squares fitting of the spectroscopic titration curves at max = 480 nm for the receptor 3, the association constant Ka of the receptor 3 toward HSO4 − was calculated as 6.59 × 104 M−1 (R = 0.998). Furthermore, the lowest detection limitation of receptor 3 toward HSO4 − was obtained according to UV–vis titration profile and was at least down to 2.0 × 10−6 mol L−1 in aqueous solution [33]. To know the stoichiometry between the receptor and HSO4 − in the aqueous solution, the Job’ plot (Fig. 2(b)) from which 1:1 stoichiometry was found has been drawn. Taken together, these results illustrated that receptor 3 is binding with HSO4 − as specific chemosensor. Thus, the receptor 3 could potentially be used as an anion probe for monitoring HSO4 − in physiological and environmental systems. 3.3.
1H
NMR titrations
To further elucidate the binding mode of the receptor 3 with HSO4 − , 1 H NMR-titration spectra were undertaken, which illustrated the characteristic structural changes that occurred upon interaction with HSO4 − (5.0 × 10−1 mol L−1 ) as their TBA salts in
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P. Li et al. / Spectrochimica Acta Part A 90 (2012) 152–157
Scheme 1. Synthesis of receptor 3.
Fig. 1. (a) UV–vis absorption spectra of receptor 3 (c = 2.0 × 10−5 M) in the presence of 50 equiv. of various anions in H2 O/DMSO (3.8:6.2, v/v) binary solution at room temperature. (b) Optical density of 3 at 480 nm upon the addition of different anions. (c) Color changes observed upon the addition of various tetrabutylammonium salt anions (50 equiv.) to solutions of receptor 3 (2.0 × 10−5 M) solutions.
P. Li et al. / Spectrochimica Acta Part A 90 (2012) 152–157
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Fig. 2. (a) Titration curves of receptor 3 in H2 O/DMSO (3.8:6.2, v/v) (c = 2.0 × 10−5 M) upon addition of HSO4 − . Inset: the nonlinear fitting curve of change in absorbance of receptor 3 at 480 nm with respect to amounts of HSO4 − in H2 O/DMSO (3.8:6.2, v/v) solutions. (b) The Job’s plot of 3–HSO4 − complex indicates a 1:1 stoichiometry.
DMSO-d6 solution (1.0 × 10−2 mol L−1 ). As shown in Scheme 2, there was one intramolecular hydrogen bond in the molecular structure of 3: O Ha · · ·N C. The formation of this hydrogen bond led to the 1 H NMR chemical shifts of COOH and phenol OHa appearing at high-field. Owing to the fact that O Ha · · ·N C is a very strong intramolecular hydrogen bond, as shown in Fig. 3, the 1 H NMR chemical shifts of COOH and phenol OHa appeared at the high-field of the molecular 3 at ı 11.79 and 10.38 ppm, respectively. After the addition of 0.5 equiv. of HSO4 − , an intergradation was formed between receptor 3 and HSO4 − (Scheme 2) and induced
the breaking of O Ha · · ·N C, which caused the 1 H NMR chemical shift of the COOH downfield and broadened to 11.86 ppm. Simultaneously, there was a new signal peak appeared at ı 9.75 ppm which attributed to the NHa proton. Additionally, with the continuous addition of HSO4 − anion at 1.0 and 2.0 equiv., the resonance at ı 11.86 ppm for COOH as before and the signal of NHa proton shifted towards upfield at 9.65 ppm completely. It is clearly that a signal at 9.65 ppm appeared completely and showed upfield with the addition of HSO4 − more than 0.5 equiv., which ascribed to the proton transference from OHa to N atom. As a result, there
Scheme 2. The proposed reaction mechanism of the receptor 3 with HSO4 − .
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P. Li et al. / Spectrochimica Acta Part A 90 (2012) 152–157
Fig. 3.
1
H NMR spectra of receptor 3 (0.01 M) upon addition of 0.5, 1 and 2 equiv. of HSO4 − in DMSO-d6 .
formed four hydrogen bonds for HSO4 − with the S atom, COOH, C N and phenol O groups, respectively. Namely, before all the 0.5 equiv. of HSO4 − was added, the receptor 3 and the stable 3–HSO4 − complex coexisted in the solution. After 1 and 2 equiv. of HSO4 − had been added, the intergradations completely changed into stable 3–HSO4 − complex. Thus, the results of 1 H NMR titration and spectrum titration implicated that the tautomeric equilibrium occurred during the anion recognition process. It was interrupted the conjugation of the azo chromophore because of these changes. As mentioned above, the proposed anion recognition process in solution is shown in Scheme 2. Correspondingly, the color of solution containing the receptor 3 changes from orange to faint yellow. 4. Application To investigate the practical application of receptor 3, colorimetric paper-made test strips were prepared by immersing filter papers into a H2 O/DMSO (3.8:6.2, v/v) solution of receptor 3 (0.01 M) and then drying it in vacuum surrounding (Fig. 4(a)). The test strips
containing 3 were utilized to sense different anions. After several drops of different anions (1.0 × 10−2 M) were added to test strips, the obvious color change from orange to faint yellow was observed only for HSO4 − anion solution (Fig. 4(b)) and no significant color change for other anions tested. Therefore the colorimetric test strip would have potential application to detect HSO4 − anion easily and rapidly. 5. Conclusions In conclusion, we have designed and synthesized a novel colorimetric anion receptor 3 which bears carboxyl and azobenzol groups and shows colorimetric selectivity for HSO4 − in solutions (H2 O/DMSO, 3.8:6.2, v/v). The highly single selectivity and strong affinity for HSO4 − over other anions in the aqueous solution were demonstrated via its optical response, which ascribed to the a tautomeric equilibrium occurred during the anion recognition process and hydrogen bonding mentioned-above. The different electronic properties of the tautomer are responsible for the observed color and spectral changes. On the other hand, the detection limitation of the receptor 3 toward HSO4 − is 2.0 × 10−6 mol L−1 which indicates that this receptor could potentially be useful as a probe for monitoring HSO4 − levels in physiological and environmental systems. In addition, based on the colorimetric response of 3 to HSO4 − , test strip containing 3 was fabricated, which also exhibits a high sensitivity to HSO4 − in aqueous solution. Acknowledgements This work was supported by the NSFC (No. 21064006) and the Natural Science Foundation of Gansu (1010RJZA018). References
Fig. 4. Photographs of the colorimetric test kit with 3 for detecting HSO4 − anion in aqueous solution. (a) Before the addition of HSO4 − anion and (b) after the addition of HSO4 − anion.
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Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel colorimetric HSO4 − sensor in aqueous media Ping Li a,b,c , You-Ming Zhang a,b,c , Qi Lin a,b,c , Jun-Qiang Li a,b,c , Tai-Bao Wei a,b,c,∗ a
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Lanzhou, Gansu 730070, PR China Key Laboratory of Polymer Materials of Gansu Province, Lanzhou, Gansu 730070, PR China c College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China b
a r t i c l e
i n f o
Article history: Received 6 October 2011 Received in revised form 1 January 2012 Accepted 16 January 2012 Keywords: Colorimetric sensors Hydrogensulfate recognition Aqueous solution Colorimetric test kit
a b s t r a c t A novel and sensitive anion receptor 3, bearing Schiff base structure, nitrophenyl azobenzol and carboxyl groups, was developed and characterized as a single chemosensor for the recognition of HSO4 − anion. The different responses of UV–vis spectra and color changes of 3 could be applied to the recognition for HSO4 − over other anions such as F− , CI− , Br− , I− , AcO− , H2 PO4 − and CIO4 − by the naked eye. Furthermore, the anion binding interaction of receptor-anion was also studied using UV–vis and 1 H NMR titration which revealed that 3 displayed a remarkable binding ability for the HSO4 − with an association constant Ka = 6.59 × 104 M−1 . And the detection limitation of HSO4 − with the receptor 3 was 2.0 × 10−6 mol L−1 in aqueous solution. Most importantly, the qualitative detection of HSO4 − using receptor 3 was attempted with test kit which was made from receptor 3. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction The sensing and recognition of anions has emerged recently as a key research field within the generalized area of supramolecular chemistry for the important role placed by anions in a wide range of industrial, agricultural, biological systems and environmental problems [1–3]. Therefore, the recognition and sensing of anionic analytes has attracted considerable attention recently and as a significant goal of research programs [4–10]. In addition, hydrogensulfate anion can be found in many agricultural fertilizer, industrial raw materials and their deleterious effect as pollutants [11]. For example, hydrogensulfate anion is present in nuclear fuel waste along with other oxoanions, which eventually get into the environment. Given that the hydrogensulfate anion has a large standard Gibbs energy of hydration (−1080 kJ mol−1 ), the recognition and separation of the hydrogensulfate anion from an aqueous media is a challenging task [12]. However, only few examples of hydrogensulfate anion recognition have been reported in recent years. In nature, the hydrogensulfate anion is recognized and transported by the hydrogensulfate-binding protein through hydrogen bonds [13]. Accordingly, the hydrogen bond complexes were widely used in design of anion sensor because a stable object hydrogen bonding complexes could be formed to use the amides [14–16], thioureas [17–19], ureas [20–23] and imidazolium [24] which regarded as the
∗ Corresponding author at: College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China. Tel.: +86 931 7973191; fax: +86 931 7971216. E-mail address: [email protected] (T.-B. Wei).
hydrogen donor to form R H· · ·X− anion hydrogen bonds and the tautomeric azophenol-containing receptors. However, the detection of anions based on tautomeric azophenol-containing receptors is rarely reported in the literature, although such cation receptors which undergo azophenol to quinone-hydrazone tautomerization upon presence of cations, have been successfully shown [25–27]. Hence, there is a need for us to develop colorimetric anion receptor with anion-induced azophenol to quinone-hydrazone tautomerization as signaling mechanism. On the other hand, in biological and environmental systems, anion-receptor interactions commonly occur in aqueous media. Therefore, much attention has been paid to develop anion sensors that work in the aqueous phase [28–32]. The challenge is that strong hydration in the aqueous phase stops the sensors from recognizing the anions. So far, only a few receptors have been synthesized that are able to recognize anions in the aqueous phase. Herein, as one part of our research interesting in anion recognitions [26,31,33–36], we attempted to design an easy to synthesis, highly selective and sensitive sensor for HSO4 − . Thus, the design of receptor as the chemosensor was mainly based on the fact that: (i) the receptor contains both carboxyl group and Schiff base structure as the binding sites that could recognize HSO4 − anion selectively; (ii) in order to achieve “naked-eye” recognition, the nitrophenyl azobenzol group as the chromophore was designed. We further anticipated that compound 3 could display azophenol to quinone-hydrazone tautomerization, stimulated by anionic species in solution. Just as expected, the remarkable color change from orange to light yellow was seen during the spectral titration process.
1386-1425/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2012.01.026
P. Li et al. / Spectrochimica Acta Part A 90 (2012) 152–157
2. Experimental 2.1. Apparatus Melting points were measured on X-4 digital melting-point apparatus and were uncorrected. The infrared spectra were performed on a Digilab FTS-3000 FT-IR spectrophotometer. UV–visible spectra were recorded on a Shimadzu UV-2550 spectrometer. 1 H NMR spectra were recorded on a Varian Mercury plus-400 MHz spectrometer with DMSO as solvent and TMS as an internal reference of analytical grade. Electrospray ionization mass spectra (ESI-MS) were measured on an Agilent 1100 LC-MSD-Trap-VL system. Elemental analyses were performed by Thermo Scientific Flash 2000 organic elemental analyzer. 2.2. Chemicals All reagents obtained commercially for synthesis were used without further purification. In the titration experiments, all the anions were added in the form of tetrabutylammonium (TBA) salts, which were purchased from Alfa-Aesar Chemical, stored in a vacuum desiccator containing self-indicating silica and dried fully before using. 2.3. General method 2.3.1. General procedure for UV–vis All the UV–vis experiments were carried out in DMSO or DMSO/H2 O binary solution on a Shimadzu UV-2550 spectrometer at 298.5 K, unless otherwise mentioned. Any changes in the UV–vis spectra of the synthesized compound were recorded on addition of tetrabutylammonium salt while keeping the ligand concentration constant in all experiments. Tetrabutylammonium salt of anions (F− , CI− , Br− , I− , AcO− , H2 PO4 − , HSO4 − and CIO4 − ), were used for the UV–vis experiments. Affinity constants of receptor 3 for anionic species were determined by non-linear fitting analyses program ORIGIN according to the equation reported by Valeur, 1:1 host–guest complexation [37]. 2.3.2. General procedure for 1 H NMR For 1 H NMR titrations, two stock solutions were prepared in DMSO-d6 solution (TMS is used as an internal standard), one of them containing host (0.01 M) only and the second one containing an appropriate concentration (0.5 M) of guest. Aliquots of the two solutions were mixed directly in NMR tubes and 1 H NMR of the host–guest system was detected. 2.4. Synthesis and characterization of receptor 3 The synthesis route of receptor molecule 3 is demonstrated in Scheme 1. Intermediate 5-(p-nitro-phenylazo)-salicylaldehyde (1) and 5-amino-1,3,4-thiadiazol-2-carboxylic acid (2) were prepared according to the literature reported [38,39]. Intermediate 5-(p-nitro-phenylazo)-salicylaldehyde (0.542 g, 0.002 mol) and 5amino-1,3,4-thiadiazole-2-methane acid (0.290 g, 0.002 mol) were mixed in absolute ethanol solutions (20 mL) with acetic acid as a catalyst. Then, the resulting solution was stirred under refluxed conditions for 5–6 h at 79 ◦ C, then cooled to room temperature and the solvent was removed by evaporation. Finally, the brown crude solid was purified by recrystallistaion from hot solution of DMF/H2 O and the desired pure receptor 3 was obtained in 87% yield. Mp: Intermediate 1: 186–188 ◦ C (lit. 187–189 ◦ C); Intermediate 2: 184–186 ◦ C (lit. 185–187 ◦ C); receptor 3: 212–213 ◦ C. 1 H NMR (DMSO-d6 , 400 MHz) ı 11.79 (s, 1H, COOH), 10.38 (s, 1H, OH), 8.44 (d, J = 8.8, 2H, ArH), 8.26 (s, 1H, ArH), 8.18 (m, 3H, ArH), 8.16 (m, J = 2.8, 1H, ArH) 7.25(d, 1H, CH); 13 C NMR (DMSO-d6 , 400 MHz)
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ı 206.42, 190.20, 186.64, 183.25, 167.41, 165.94, 164.40, 155.13, 148.10, 144.76, 130.00, 125.00, 124.74, 123.22, 122.80, 118.62. ESIMS, m/z: 398.1. Anal. Calcd. for C16 H10 N6 O5 S: C, 48.24; H, 2.51; N, 21.11; S, 8.04; Found: C, 48.22; H, 2.48; N, 21.14; S, 8.06. 3. Results and discussion 3.1. UV–vis spectral recognitions and colorimetric signaling The anion binding affinity of receptor 3 was primly investigated by UV–vis spectra in the absence and presence of adding various anions such as F− , CI− , Br− , I− , AcO− , H2 PO4 − , HSO4 − and CIO4 − using tetrabutylammonium (TBA) as a counter cation. The experiment was performed by preparing 2.0 × 10−5 mol L−1 solution of receptor 3 in the mixture of dimethylsulphoxide (DMSO) and thirdly distilled water with the volume ratio H2 O/DMSO (3.8:6.2, v/v). There were two characteristic absorption peaks for UV–vis spectra of receptor 3 at 488 nm and 376 nm in the absence of anions. The receptor 3 responded with dramatic color changes when particular TBA anionic salts were added to solution above respectively. Upon the addition of HSO4 − anion, there was a prominent change that an absorption peak at 488 nm disappeared while a new and strong absorption peak at 300–450 nm appeared (Fig. 1(a)) in the corresponding UV–vis absorption spectra due to complexation between receptor 3–HSO4 − molecules. In addition, there appeared a dramatic color change from orange to faint yellow (Fig. 1(c)) for the solution of receptor 3 along with the addition of HSO4 − (TBA). On the contrary, addition of other anionic species to the solution of receptor 3 (TBA) no significant modulation of color was observed in the UV–vis absorption spectra. The result shows the specificity of the chemosensor 3 for binging HSO4 − anion was realized successfully (For interpretation of the references to color in the text, the reader is referred to the web version of the article.). 3.2. UV–vis spectral titrations In order to estimate the specific properties for selective recognition of HSO4 − and colorimetric changes associated with the receptor 3, the receptor 3 toward HSO4 − anion was studied by UV–vis absorption spectra titration experiments. The experiments were conducted using a 2.0 × 10−4 M solution of receptor 3 in aqueous solutions (H2 O/DMSO, 3.8:6.2, v/v) (Fig. 2(a)). Upon the addition of HSO4 − (0.1 M) anion to the aqueous solution, a significant decreasing of the UV–vis absorbance at 480 nm and a new band centered at 370 nm were observed. There was an isosbestic point at 398 nm, which indicates that receptor 3 reacts with HSO4 − anion to form a stable complex. By nonlinear least-squares fitting of the spectroscopic titration curves at max = 480 nm for the receptor 3, the association constant Ka of the receptor 3 toward HSO4 − was calculated as 6.59 × 104 M−1 (R = 0.998). Furthermore, the lowest detection limitation of receptor 3 toward HSO4 − was obtained according to UV–vis titration profile and was at least down to 2.0 × 10−6 mol L−1 in aqueous solution [33]. To know the stoichiometry between the receptor and HSO4 − in the aqueous solution, the Job’ plot (Fig. 2(b)) from which 1:1 stoichiometry was found has been drawn. Taken together, these results illustrated that receptor 3 is binding with HSO4 − as specific chemosensor. Thus, the receptor 3 could potentially be used as an anion probe for monitoring HSO4 − in physiological and environmental systems. 3.3.
1H
NMR titrations
To further elucidate the binding mode of the receptor 3 with HSO4 − , 1 H NMR-titration spectra were undertaken, which illustrated the characteristic structural changes that occurred upon interaction with HSO4 − (5.0 × 10−1 mol L−1 ) as their TBA salts in
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Scheme 1. Synthesis of receptor 3.
Fig. 1. (a) UV–vis absorption spectra of receptor 3 (c = 2.0 × 10−5 M) in the presence of 50 equiv. of various anions in H2 O/DMSO (3.8:6.2, v/v) binary solution at room temperature. (b) Optical density of 3 at 480 nm upon the addition of different anions. (c) Color changes observed upon the addition of various tetrabutylammonium salt anions (50 equiv.) to solutions of receptor 3 (2.0 × 10−5 M) solutions.
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Fig. 2. (a) Titration curves of receptor 3 in H2 O/DMSO (3.8:6.2, v/v) (c = 2.0 × 10−5 M) upon addition of HSO4 − . Inset: the nonlinear fitting curve of change in absorbance of receptor 3 at 480 nm with respect to amounts of HSO4 − in H2 O/DMSO (3.8:6.2, v/v) solutions. (b) The Job’s plot of 3–HSO4 − complex indicates a 1:1 stoichiometry.
DMSO-d6 solution (1.0 × 10−2 mol L−1 ). As shown in Scheme 2, there was one intramolecular hydrogen bond in the molecular structure of 3: O Ha · · ·N C. The formation of this hydrogen bond led to the 1 H NMR chemical shifts of COOH and phenol OHa appearing at high-field. Owing to the fact that O Ha · · ·N C is a very strong intramolecular hydrogen bond, as shown in Fig. 3, the 1 H NMR chemical shifts of COOH and phenol OHa appeared at the high-field of the molecular 3 at ı 11.79 and 10.38 ppm, respectively. After the addition of 0.5 equiv. of HSO4 − , an intergradation was formed between receptor 3 and HSO4 − (Scheme 2) and induced
the breaking of O Ha · · ·N C, which caused the 1 H NMR chemical shift of the COOH downfield and broadened to 11.86 ppm. Simultaneously, there was a new signal peak appeared at ı 9.75 ppm which attributed to the NHa proton. Additionally, with the continuous addition of HSO4 − anion at 1.0 and 2.0 equiv., the resonance at ı 11.86 ppm for COOH as before and the signal of NHa proton shifted towards upfield at 9.65 ppm completely. It is clearly that a signal at 9.65 ppm appeared completely and showed upfield with the addition of HSO4 − more than 0.5 equiv., which ascribed to the proton transference from OHa to N atom. As a result, there
Scheme 2. The proposed reaction mechanism of the receptor 3 with HSO4 − .
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Fig. 3.
1
H NMR spectra of receptor 3 (0.01 M) upon addition of 0.5, 1 and 2 equiv. of HSO4 − in DMSO-d6 .
formed four hydrogen bonds for HSO4 − with the S atom, COOH, C N and phenol O groups, respectively. Namely, before all the 0.5 equiv. of HSO4 − was added, the receptor 3 and the stable 3–HSO4 − complex coexisted in the solution. After 1 and 2 equiv. of HSO4 − had been added, the intergradations completely changed into stable 3–HSO4 − complex. Thus, the results of 1 H NMR titration and spectrum titration implicated that the tautomeric equilibrium occurred during the anion recognition process. It was interrupted the conjugation of the azo chromophore because of these changes. As mentioned above, the proposed anion recognition process in solution is shown in Scheme 2. Correspondingly, the color of solution containing the receptor 3 changes from orange to faint yellow. 4. Application To investigate the practical application of receptor 3, colorimetric paper-made test strips were prepared by immersing filter papers into a H2 O/DMSO (3.8:6.2, v/v) solution of receptor 3 (0.01 M) and then drying it in vacuum surrounding (Fig. 4(a)). The test strips
containing 3 were utilized to sense different anions. After several drops of different anions (1.0 × 10−2 M) were added to test strips, the obvious color change from orange to faint yellow was observed only for HSO4 − anion solution (Fig. 4(b)) and no significant color change for other anions tested. Therefore the colorimetric test strip would have potential application to detect HSO4 − anion easily and rapidly. 5. Conclusions In conclusion, we have designed and synthesized a novel colorimetric anion receptor 3 which bears carboxyl and azobenzol groups and shows colorimetric selectivity for HSO4 − in solutions (H2 O/DMSO, 3.8:6.2, v/v). The highly single selectivity and strong affinity for HSO4 − over other anions in the aqueous solution were demonstrated via its optical response, which ascribed to the a tautomeric equilibrium occurred during the anion recognition process and hydrogen bonding mentioned-above. The different electronic properties of the tautomer are responsible for the observed color and spectral changes. On the other hand, the detection limitation of the receptor 3 toward HSO4 − is 2.0 × 10−6 mol L−1 which indicates that this receptor could potentially be useful as a probe for monitoring HSO4 − levels in physiological and environmental systems. In addition, based on the colorimetric response of 3 to HSO4 − , test strip containing 3 was fabricated, which also exhibits a high sensitivity to HSO4 − in aqueous solution. Acknowledgements This work was supported by the NSFC (No. 21064006) and the Natural Science Foundation of Gansu (1010RJZA018). References
Fig. 4. Photographs of the colorimetric test kit with 3 for detecting HSO4 − anion in aqueous solution. (a) Before the addition of HSO4 − anion and (b) after the addition of HSO4 − anion.
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