Cyanide detection using a benzimidazole derivative in aqueous media

Accepted Manuscript Cyanide Detection Using a Benzimidazole Derivative in Aqueous Media Jing-Han Hu, Jian-Bin Li, Juan-Juan Chen, Jing Qi PII: DOI: Reference:

S1386-1425(14)00948-2 http://dx.doi.org/10.1016/j.saa.2014.06.060 SAA 12319

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

12 February 2014 4 June 2014 8 June 2014

Please cite this article as: J-H. Hu, J-B. Li, J-J. Chen, J. Qi, Cyanide Detection Using a Benzimidazole Derivative in Aqueous Media, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http:// dx.doi.org/10.1016/j.saa.2014.06.060

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Cyanide Detection Using a Benzimidazole Derivative in Aqueous Media Jing-Han Hu*, Jian-Bin Li, Juan-Juan Chen, Jing Qi

------------------------------------------E-mail: [email protected] ------------------------------------------College of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu, 730070，P. R. China

Abstract: A novel cyanide selective fluorescent chemosensor S1 based on benzimidazole group and naphthalene group as the fluorescence signal group had been designed and synthesized. The receptor could instantly detect CN− anion over other anions such as F−, Cl−, Br−, I−, AcO−, H2PO4−, HSO4−, SCN− and ClO4 − by fluorescence spectroscopy changes in aqueous solution (H2O/DMSO, 8:2, v/v) with specific selectivity and high sensitivity. The fluorescence color of the solution containing sensor S1 induced a remarkable color change from pale blue to mazarine only after the addition of CN− in aqueous solution while other anions did not cause obvious color change. Moreover, further study demonstrates the detection limit on fluorescence response of the sensor to CN− is down to 8.8×10−8 M, which is far lower than the WHO guideline of 1.9×10−6 M. Test strips based on S1 were fabricated, which could act as a convenient and efficient CN− test kit to detect CN− in pure water for “in-the-field” measurement. Thus, the probe should be potential applications in an aqueous environment for the monitoring of cyanide.

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Keywords: Benzimidazole; Fluorescence; Sensors; Cyanide

1. Introduction The cyanide anion (CN−) is known to be an extremely toxic anion and can directly lead to the death of human beings in several minutes because it strongly binds cytochrome-c, thereby disrupting the mitochondrial electron-transport chain and causing a decreased oxidative metabolism and oxygen utilization[1-2]. The cyanide ion also detrimentally affects vascular, visual, central nervous, cardiac, endocrine, and metabolic functions. However, cyanide toxicity, large quantities of cyanide salts are still widely used in industrial production such as metallurgy (1.5 million tons per year), electroplating, and the synthesis of fine chemicals. As a result, the purposeful design and synthesis of efficient chemsensors to selectively detect CN− ions at the environmental and biological fields have been in recent years attracted much attention. Although a wide variety of chemical and physical sensors for the detection of CN− have been reported[3-4], most of physical methods require expensive and sophisticated equipment or involve time-consuming and laborious procedures that can be carried out only by well-trained professionals, which seriously stumbling block to the practical application of these CN− sensors[5]. For purposes of simplicity, convenience, easily prepared and high sensitivity[6] CN− fluorescent chemosensors have become particularly attractive. Up to now, some organic molecules and transition metal complexes able to signal the presence of cyanide based on supramolecular approaches according to the

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hydrogen-bonding motifs with dramaticlly changes in their absorption and emission properties have been already identified [7-11]. Some of these chemosensors can even detect micromolar amounts of cyanide[12-14]. However, most of them suffer the severely interference from coexisting anions such as F−, AcO−, and H2PO4−. In addition, many of them are reported to work only in organic media[15-17], therefore, it is still worthwhile to exploit new fluorescence sensors for selective recognition of toxic anions (CN−) in the organo-aqueous media and then in pure water solution for environment system or biological applications. Our research group has a longstanding interest in molecular recognition[18]. Herein, we have designed 2-(2-hydroxy-1-naphthalenyl)-1H-benzimidazole (S1), S1 possesses a hydroxyl and an imine group of benzimidazole and it is well-established that the NH and OH group due to strong acid is readily deprotonated when basic ions appear. The naphthalene group is introduced as fluorophore to achieve fluorescent recognition. The response of this new probe S1 to CN− in aqueous solution was investigated as was the detection limit to CN−. The mechanism of this process has been investigated by 1H NMR and UV spectrum and ESI-mass spectrometry. 2. Experimental section 2.1 Materials and physical methods All reagents and solvents were commercially available at analytical grade and were used without further purification. 1H NMR and

13

C NMR spectra were recorded

on a Mercury-400BB spectrometer at 400 MHz. Chemical shifts are reported in ppm downfield from tetramethylsilane (TMS, δ scale with solvent resonances as internal

3

standards) UV–vis spectra were recorded on a Shimadzu UV-2550 spectrometer. Photoluminescence spectra were performed on a Shimadzu RF-5301 fluorescence spectrophotometer. Melting points were measured on an X-4 digital melting-point apparatus (uncorrected). Infrared spectra were performed on a Digilab FTS-3000 FT-IR spectrophotometer. 2.2 General procedure for UV-vis experiments All UV-vis spectroscopy was carried out just after the addition of tetrabutylammonium anion salt in DMSO/H2O (2:8, v/v) solution, while keeping the ligand concentration constant (2.0×10-5 M) on a Shimadzu UV-2550 spectrometer. The solution of CN− was prepared from the NaCN. 2.3 General procedure for fluorescence spectra experiments All fluorescence spectroscopy was carried out just after the addition of tetrabutylammonium anion salt in DMSO/H2O (2:8, v/v) solution, while keeping the ligand concentration constant (2.0×10 -5 M) on a Shimadzu RF-5301spectrometer. The solution of anions were prepared from the tetrabutylammonium salts of F−, Cl−, Br−, I−, AcO−, H2PO4 −, HSO4−, ClO4 −, SCN−, but CN− was prepared in NaCN. The excitation wavelength was 360 nm. 2.4 General procedure for 1H NMR experiments For 1H NMR titrations, the sensor of stock solutions was prepared in DMSO-d6, the cyanide anion was prepared in distilled water. Aliquots of the two solutions were mixed directly in NMR tubes. 2.5 General procedure for test strips experiments

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Test strips were prepared by immersing filter papers into a DMSO/H2O binary solution of S1 (0.01 M) following by exposing it to air to dry it. The test strips containing S1 were utilized to detect CN−. When CN− solution was added on the test kits, the fluorescence turn on response can be obvious observed under UV irradiation. 2.6 Synthesis of S1 The synthesis route of receptor molecule S1 is demonstrated in Scheme 1. To an ethanol solution (25 mL) of 2-Hydroxy-1-naphthaldehyde (0.86g, 5 mmol) and NaHSO3 (0.624g, 6 mmol) as a catalyst was stirred 4h at the room temperature, and added a DMF (15 mL) of O-phenylenediamine (0.54g, 5 mmol) to the maxed solution. Then, the reaction of mixture solution was stirred at 353 K for 2 h. After cooling to room temperature, and dropwise added the pale yellow reaction solution to the 450 mL of distilled water, produced a large number of yellow precipitation, quietly placed, filtered, and washed with distilled water three times, then recrystallized with absolute ethanol to get yellow crystal of S1 in 70% yield (m.p. 252-255 ), 1H NMR (DMSO-d6, 400MHz) δ: 12.19 (s, 2H), 8.19 (d, 1H), 7.95 (d, 1H), 7.89 (d, 1H), 7.70~7.58 (m, 2H), 7.50 (t, 1H), 7.36 (t, 1H), 7.32 (d, 1H), 7.27~7.21 (m, 2H); 13

C-NMR (DMSO-d6, 100MHz) δ: 155.76, 149.47, 132.06, 131.84, 128.45, 127.99,

127.42, 124.13, 123.27, 122.12, 118.64, 108.45. IR (KBr) v: 1622.13cm-1 (CH＝N), 3370.90cm-1 (NH), 3468.73cm-1 (OH). ESI-MS calcd for C17H12N2O+H 261.0, found 261.2. HO NH2 + NH2

O

Ethanol DMF

HO

NaHSO3 80oC

N N H S1

5

Scheme 1 Synthetic procedures for receptor S1 3. Results and discussion Receptor was found to have limited solubility in water, and this compelled us to use these sensor in mixed solvent, such as H2O/DMSO (8:2, v/v), for recognition studies of S1. Fluorescence spectral response of chemosensor S1 were tested with aqueous solutions of the tetrabutylammonium salts of all common anionic analytes such as (F−, CI−, Br−, I−, AcO−, H2PO4−, HSO4−, CIO4− and SCN−) as well as CN−. In aqueous solution H2O/DMSO (8:2, v/v), chemosensor S1 produced a band at λ

max

=

388 nm in the absorption spectrum recorded at a 2×10-5 M concentration of the sensors in a H2O system, the emission spectrum of S1, which is excited at 360nm, exhibits an emission maximum at 450 nm with a low quantum yield (ΦR = 0.34). Changes in spectral pattern were observed only in the presence of added 20 equivalent of CN− and showed a strong fluorescence response with a increasement quantum yield (ΦR = 0.50), and responded with a dramatic color change, from pale blue to mazarine. No change in spectral pattern for receptor S1 in the presence of other anions suggests either a very weak or no interaction between these anions and the compound (Fig. 1, Fig. 2).

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Fig. 1. Fluorescence spectra of S1 and in the presence of 20 equiv. of various anions in H2O/DMSO (8:2 v/v) binary solution at room temperature. The inset shows the fluorescence intensity of S1 at λmax = 450 nm with respect to the various of anions.

Fig. 2. Color changes observed upon the addition of various anions (20 equiv.) to solutions of sensor S1 (2×10 -5 M) in DMSO/H2O (2:8, v/v) solutions. Fluorescent titration was performed to gain insight into the recognition properties of receptor S1 as a CN− probe (Fig. 3). The emission band at 450 nm of chemosenor S1 remarkably increased as the CN− concentration increased from 0 to 26 µL. In the meantime, the detection limit of the fluorescence spectra measurements, as calculated on the basis of 3SB/S[19] (where SB is the standard deviation of the blank solution and S is the slope of the calibration curve; Fig. 4), showed a detection limit of

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approximately 8.8×10 -8 M for CN−, which is far lower than the WHO guideline of 1.9 µM cyanide.

Fig. 3. Fluorescence titration spectra of S1 (20 mM) in (H2O/DMSO, 8:2,v/v) solution upon adding of an increasing concentration of CN− (λex = 360 nm). The inset shows fluorescence change of S1 observed upon excitation at 360 nm after the addition of CN− anions (0-26µL).

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Fig. 4. Fluorescence detection limit spectra of S1 (20 mM) in (H2O/DMSO, 8:2,v/v) solution upon adding of an concentration of CN− (0.24 µL ). The proposed mechanism potentially the recognition behavior of S1 is shown in Scheme 2. These results reveal that the recognition mode includes two stages: (i) deprotonation process (for adding small quantities of cyanide is) and (ii) the receptor S1 recover a process of intranuclear hydrogen bonding interactions (for adding high quantities of cyanide is). In other words, the sensor of S1 response to CN− could undergo deprotonation, though intramolecular hydrogen bonding and cyclization. It leads to increase the sensor of conjugate rigid plane structure and induce its intramolecular charge transfer (ICT). Consequently, the observed fluorescence enhancement is most likely caused by the ICT. Therefore, It can be clearly seen the chemosensors S1 selectively response of cyanide over other anions such as F−, Cl−, Br−, I−, AcO−, H2PO4−, HSO4−, ClO4− and SCN− in aqueous solutions (H2O/DMSO, 8:2, v/v).

a c HN H

N

b H O H

a c HN H

N

b H O

d H

+CNDeprotonation

c

N

N

c

H

N

N

H

H-bond interaction

O

O

d

d

d H

H S12

S1

Scheme 2 Possible sensing mechanism To justify the mode, 1H NMR titration experiments were conducted to further investigate the interaction of S1 with CN− in DMSO-d6, as shown in Fig. 5. With 1.0 equiv. NaCN was added, signal of –OH and –NH at 12.19 (s, 2H) ppm had completely disappeared, which stated clearly the –OH and –NH had deprotonation

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and the other hydrogen proton chemical shifted of the aromatic ring gradually to upfield for the shielding effect. The signal of naphthalene at 8.19 (d, Hc) were attractive to N atom of the benzimidazole involved in intramolecular hydrogen bonding and a new six-membered ring was formed by the hydrogen bond connection, so the signal of Hc apparently moved to downfield. Therefore, 1H NMR titration experiments suggested that the validity of the mechanism submitted and the cause of the fluorescence enhancement presented.

Fig. 5. Partial 1H NMR spectra of S1 (0.01 M) and in the presence of varying amounts of CN−.
On the other hand, the UV-vis spectrum of the resulting sample was also recorded and demonstrated to the recognition mechanism of the sensor S1 with CN− (Fig. 6). The appearance of absorption peak at 305 and 348 nm exhibited for compound S1. The former absorption peak at 305 nm could be assigned primarily to

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the π-π* transition of the naphthalene-based, whereas the longer wavelength absorption band for receptor S1 at 348 nm was attributed to the intra-ligand π-π* transition of the –C=N bonds from benzimidazole moiety. Compared with the absorption peak of the ligand S1, on responding to the CN−, the corresponding new band

at

328

and

388

nm

could

be

observed

and

attributed

to

a

[π]benzimidazole→[π*]naphthalene based intramolecular charge transfer band, which were red shifted distinctly. The UV-vis data enough testified the probe of S1 underwent a series of reaction and acquired the more larger conjugated molecules S12. Moreover, the mass spectrum obtained and confirmed the ion peaks were detected at m/z 261.2, ＋

which are corresponding to [S1＋H]

(Fig. S2). The probe with NaCN confirmed the

revival of m/z 259.9 also demonstrated the presence of compound [S12+2H] (Fig. S3). In conclusion, though the 1 H NMR titration experiments, the UV-vis spectrum and mass spectral analyses implicated that cyanide interacts with the free ligand S1 promoting a deprotonation process and H-bond reaction mechanism.

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Fig. 6. UV-vis spectra changes of S1 (20 mM) in DMSO/H2O (2:8, v/v) upon addition of 50 equiv. of CN−. 4. Application The realization of quick response to cyanide is quite meaningful for the sensor in its practical application in portable sensing devices. To facilitate the use of S1 for the detection of cyanide, test strips were prepared by immersing filter papers into a DMSO/H2O binary solution of S1 (0.01 M) following by exposing it to air to dry it. Intriguingly, the fluorescence color can be changed immediately from blue to mazarine once the test paper was immersed into an aqueous solution (5 µM) of cyanide under UV irradiation (Fig. 7). Thereby chemosensor S1 exhibits excellent fluorescence sensing performance, which will be very useful for the fabrication of sensing devices with fast and convenient detection for cyanide and other anions.

Fig. 7. Fluorescence changes of test strips for detecting CN− in aqueous solution using UV lamp at r t. 5. Conclusion In conclusion, we have presented a facile, rapid and efficient chemosensor S1, which showed specially selectivity and highly sensitivity fluorescence recognition for CN− in DMSO/H2O (2:8, v/v) solutions. Moreover, the sensor demonstrates the detection limit on fluorescence response of the sensor to CN− is down to 8.8×10−8 M,

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which is far lower than the WHO guideline of 1.9×10−6 M. In addition, test strips based on S1 were fabricated, which could serve as a practical fluorescence sensor to detect CN− in field measurements or in test kits. We believe that these characteristics of S1 make it attractive for further molecular modifications and underlying applications as fluorescence sensor for CN−.

Acknowledgment We gratefully acknowledge the support of the Colleges and Universities graduate advisor research project in Gansu Province (No. 0804-11).

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1. Receptor S1 shows highly sensitive fluorescence turn on response for CN−. 2. Sensor S1 demonstrates specifically selective fluorescence recognition to CN−. 3. Test strips of S1 were prepared and convenient and rapidly detected CN−. 4. The probe S1 responded with CN− in aqueous solution system. 5. Chemosensor S1 was designed with ease of synthesis.

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H O

O

H

N H

H

N

N

N H

NaCN

H

Graphical abstract: A novel cyanide specifically selective and highly sensitive fluorescent enhanced chemosensor S1 based on benzimidazole group and naphthalene group was designed and synthesized.

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