Chinese Chemical Letters  2014, Vol.25 Issue (01):35-38   PDF    
引用本文
Jun Liu, Qi Lin, Hong Yao, Miao Wang, You-Ming Zhang, Tai-Bao Wei.Turn-on fluorescence sensing of cyanide ions in aqueous solution[J]. Chin. Chem. Lett., 2014,25(01): 35-38   
Turn-on fluorescence sensing of cyanide ions in aqueous solution
Jun Liu, Qi Lin, Hong Yao, Miao Wang, You-Ming Zhang, Tai-Bao Wei     
* Corresponding authors at:Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
Received:2013-7-14, Revised:2013-9-18, online:2013-11-12.
E-mail addresses:weitaibao@126.com
Abstract: A sensitive fluorescent probe, 2,2'-bisbenzimidazole (L), for CN- has been developed. This structurally simple receptor displays great selectivity for the cyanide anion over other common inorganic anions in an aqueous environment. In addition, further study demonstrates the lower detection of the fluorescence response of the sensor to CN- is in 10-9 mol/L range. Thus, the present probe should be applicable as a practical system for the monitoring of cyanide concentrations in aqueous samples.
Key words: Fluorescent probe     2,2'-Bisbenzimidazole     Cyanide     Aqueous solution    
1. Introduction

Anion recognition is an area of growing interest in supramolecular chemistry due to its important role in a wide range of environmental, clinical, chemical, and biological applications [1, 2, 3, 4]. Cyanide detection is particularly important due to its extreme toxicity in physiological systems and the increasing environmental concerns caused by its widespread industrial uses in petrochemical, gold mining, photographic, and steel manufacturing [5, 6, 7, 8, 9]. To detect cyanide anions, fluorescence sensing is one of the most powerful methods owing to its simplicity and high sensitivity [10, 11, 12, 13].

In recent years, some organic molecules and transition metal complexes capable of signaling the presence of cyanide by pronounced changes in their absorption and emission properties have been already identified [14, 15, 16]. Some of these chemosensors can even detect micromolar amounts of cyanide [17, 18, 19]. However, most of them suffer the deleterious interference of other anions and in addition, many of them are reported to work only in organic media [20, 21, 22]. Consequently, the search for effective sensing systems in aqueous environment is still a great challenge.

With the above-mentioned criteria in mind, herein, we have designed 2,2'-bisbenzimidazole (L), which behaves as a highly sensitive fluorescence turn-on sensor for the cyanide anion in aqueous solutions. In this article, we illustrate the logic behind our molecular design and we report the synthesis and characterization of L with its fluorescent response to cyanide.

2. Experimental

2.1. Apparatus and instruments

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. Mass spectra were measured with a Bruker Daltonics Esquire 6000. Fluorescence spectra were recorded on a Shimadzu RF-5301 fluorescence spectrometer. 1H NMR spectra were recorded on a Varian Mercury plus-400 MHz spectrometer with DMSO as solvent and analytical grade TMS as an internal reference.

All reagents were obtained commercially for synthesis and used without further purification. In the titration experiments, stock solutions of the tetrabutyl ammonium salts of F-, Cl-, Br-, I-, AcO-, H2PO4 -, HSO4 -, ClO4 -, but CN- was prepared in NaCN. All were purchased from Alfa-Aesar Chemical, stored in a vacuum desiccator containing self-indicating silica and dried fully before using.

The synthesis route of receptor molecule L is demonstrated in Scheme 1. Components O-phenylenediamine (5 mmol) and oxalic acid (2 mmol) were mixed in glycol (40 mL), with PPA (polyphosphate phosphoric acid) as a catalyst. Then, the resulting solution was stirred under refluxed conditions for 2 h at 160 ℃. After cooling to room temperature, the bright green precipitate was filtrated, washed with distilled water three times, then recrystallized with glacial acetic acid to get green crystals of L in 96% yield (mp > 300 ℃), 1H NMR (400 MHz, DMSO-d6): δ 11.91 (s, 2H), 7.77 (s, 1H), 7.57 (s, 1H), 7.30 (s, 2H), 7.07-7.14 (m, 4H); 13C NMR (100 MHz, DMSO-d6): δ 115.263, 123.127, 143.923, 155.354, 172.210. IR (KBr, cm-1): v 1620.13 (CH55N), 3250.78 (NH). MS/ (EI): m/z 235.2 (L+H)+; Anal. Calcd. for C14H10N4: C 71.78, H 4.30, N 23.92; found: C 71.69, H 4.10, N 24.21.

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Scheme 1.Synthetic procedures for receptor L.
Fluorescence spectroscopy was carried out after the addition of perchlorate metal salts and the tetrabutyl ammonium salts in DMSO, while keeping the ligand concentration constant (2 × 10-5 mol/L) on a Shimadzu RF-5301 fluorescence spectrometer. The excitation wavelength was 352 nm.

3. Results and discussion

To evaluate the selectivity of L, we measured the fluorescence intensity of L in the presence of various anions (F-, CI-, Br-, I-, AcO-, H2PO4 -, HSO4 - and CIO4 -) as well as CN-. Only when 20 equiv. of CN- were added to the H2O/DMSO (1/1, v/v) solutions of sensor L, the chemosensor responded with a dramatic color change, from colorless to bright blue (Fig. 1). Receptor L produced a band at λmax = 365 nm in the absorption spectrum recorded at a 2 × 10-5 mol/L concentration of the receptor in a H2O system. Using these anions (20 equiv.), compound L showed a large fluorescent enhancement only with CN-. However, under identical conditions, no obvious changes were observed for other tested metal ions (Fig. 2).

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Fig. 1.Visual fluorescence emissions of probe L upon the addition of various anions (20 equiv.) in DMSO/H2O (1/1, v/v) solutions on excitation at 365 nm using UV lamp at r.t.

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Fig. 2.Fluorescence spectra of L and in the presence of 20 equiv. of various anions in H2O/DMSO (1/1, v/v) binary solution at room temperature.
Fluorescent titration was performed to gain insight into the recognition properties of receptor L as a CN- sensor (Fig. 3). Upon addition of CN- to receptor L, the emission band at 365 nm gradually increased. Furthermore, the detection limit on fluorescence response of the sensor to CN- is 2.1 × 10-9 mol/L.

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Fig. 3.Fluorescence spectral titration of sensor L with CN- in (H2O/DMSO, 1/1, v/v) solution.
To confirm the binding stoichiometry between L and CN- in the aqueous solution, a job plot was performed. The results illustrate that the receptor-guest complex concentration approaches a maximum when the mole fraction of the host is about 0.5, which indicates that the anion forms a 1:1 complex with the receptor (Fig. 4).

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Fig. 4.Job’s plot of L and CN-, which indicated the stoichiometry of L–CN- complex is 1:1.
Fig. 5 illustrated the fluorescence response of L to CN- in the presence of other anions. From the bar diagram, one could easily understand that the effects on emission intensity of L upon the addition of higher concentrations of various anions were almost negligible except for CN-. Therefore, it was clear that other ions’ interference was negligibly small during the detection of CN-.

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Fig. 5.Competitive experiments toward the fluorescence enhancement effect of CN- in presence of a large number of other anions (20 equiv.).
Upon mass spectral analysis, an ion peaks was detected at m/z 235.2, which corresponds to [L+H]+. The peak at m/z 257.2 also demonstrated the presence of [L+Na-H]+ (L+Na+ complex). Thus, on the basis of above observations, in accordance with the 1:1 stoichiometry, sensors are most likely to chelate with metal ion via its imidazole N atoms (Scheme 2). Particularly, the benzimidazole system is non-planar. Whereas, the binding mode of L-Na+ can enhance the co-planarity of the conjugated system, largely promote the strong bright blue fluorescence emission. Besides the color change, infrared (IR) spectra of the diferric complex are compared to the metal-free ligand spectra in Fig. 6. In addition to the dominant C=N bending terms at 1618 cm-1 for L-Na+, there are also imidazole-group-derived signals (black line in Fig. 6). The shift to low wavenumbers of these absorption bands suggests the coordination of sodium ions with the C=N groups of L induced the formation of the complex.

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Scheme 2.Possible sensing mechanism.

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Fig. 6.Infrared spectra of L (red line) and its complex (black line).
1H NMR titration experiments were conducted to further investigate the interaction of L with CN- in DMSO-d6 (Fig. 7). When one equiv. of NaCN was added, the signal of -NH at 11.91 and 13.54 ppm exhibited an upfield shift, which suggested that the - NH group perhaps underwent a process of deprotonation. The deprotonation of -NH group led to an increasing in electronic density of the imidazole group. In addition, the color change for the sensor L from colorless to bright blue is due to the deprotonation of the -NH the group. As a result, they formed the stable L-Na+ complex. Thus, the results of 1H NMR titration, spectrum titration and mass spectral analyses implicated that this anion interacts with the free ligand promoting a deprotonation process on the imidazole nitrogen atoms.

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Fig. 7.1H NMR spectra of receptor L (0.01 mol/L) upon addition of 1 equiv. of CN- in distilled water.
To facilitate the use of L for the detection of cyanide, we prepared test papers of it by dipping a filter paper into the solution of L in H2O/DMSO following by exposing it to air to dry. Fig. 8 demonstrates the process of the change fromcolorless to bright blue by adjusting the concentrationof the added cyanide from10-9 mol/L to 10-5 mol/L. This color change was very apparent and could be easily seen by the "naked eye". In brief, the detection limit on fluorescence response of the sensor to CN- is in the 10-9 mol/L range.

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Fig. 8.Detection limit in fluorescent mode.
The initial colorless test strips turn to bright blue when the concentration of CN- was 1.0 × 10-5 mol/L; however, the immersion of these test strips in the solution mixture of other anions (1.0 × 10-5 mol/L), did not cause any color change, and the colorless strips remained unaffected. Thus, these strips could be easily carried by people for the detection of CN- at any moment (Fig. 9).

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Fig. 9.Fluorescence changes of test strips for detecting NaCN in aqueous solution using UV lamp at r.t.
4. Conclusion

A new simple and easily prepared 2,2'-bisbenzimidazole (L) described here displays fluorescence response driven by CN-. It can be synthesized easily and rapidly with good selectivity and sensitivity for CN-. As a result of the markable increases of the fluorescence intensity in the presence of CN- in H2O systems, it is possible for this ligand to directly sense and detect CN- by fluorescence intensity. These characteristics of L make it attractive for further molecular modifications and potential applications as fluorescence sensor for CN-.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21064006, 21262032 and 21161018), the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (No. IRT1177), the Natural Science Foundation of Gansu Province (No. 1010RJZA018), the Youth Foundation of Gansu Province (No. 2011GS04735) and NWNU-LKQN-11-32.

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