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A Highly Sensitive and Selective Supramolecular Fluorescent Chemosensor for Dichromate Ion Detection and Application to Real Samples.

Byline: Farid Ahmed, Sadia Khalid, Kiramat Shah and Muhammad Raza Shah

Abstract: We are reporting here the synthesis, characterization and photophysical properties of a new chemosensor containing triazole heterocycle as binding site for the selective detection of dichromate ion. The fluorescent chemosensor 6 was synthesized using click reaction. The structure of chemosensor 6 was characterized using UV-visible, NMR and mass spectroscopic techniques. Compound 6 displayed extremely high sensitivity to Cr2O72- by fluorescence quenching mechanism. Competitive experiments in presence of other anions, such as F-, Cl-, Br-, I-, N3-, SO42-, CH3COO-, ClO4-, NO3-, CO32- and the effect of pH on the effectiveness of the chemosensor 6 have also been explored. Chemosensor 6 displayed good selectivity to Cr2O72- ions over other anion tested as no significant interference was observed.

Under optimized conditions, the response of chemosensor 6 was linearly proportional to the concentration of Cr2O72- in a range from (10-100 uM) with determination coefficient of 0.9911. This method work effectively for the direct detection of Cr2O72- ion in real samples.

Keyword: Triazole, Supramolecular Chemosensor, Fluorescent, Optimal conditions, Determination Coefficient

Introduction

Chemosensor for the recognition of particular analytes is an important area under investigation now a day in the field of supramolecular chemistry. The detection of anion is much important due to their role in the chemical, biological and environmental chemistry [1-3]. The field of anion recognition is more demanding as compared to cation sensing due to many reason including size, shape and their solvent effect [4]. The anions are larger in size as compared to cation thus the charge to radius ratio of anion is smaller than cation which decrease the electronic interactions [5]. In anion recognition solvent also play important role because they can easily form hydrogen bonding with hydroxylic solvents which reduce the attraction of particular analyte with chemosensor. Chromium belongs to transition series of the periodic table. It is very common and hazardous water contaminant that has gained great attention [6-8].

Naturally chromium occurs in plants, rocks, soils, animals, and volcanic emissions. Main sources of Chromium (VI) in the waste product are various industrial processes such as chrome pigment, chrome-plating, corrosion control, wood industry, leather preserving, and use of chromium as catalysts for various reactions. Chromium found in several oxidation states in the aqueous system in which the most existing one is (VI) oxidation state. Chromium compounds are highly toxic due to the oxidizing power of chromium [9]. Chromium compounds are extensively used in chrome plating to protect metals form corrosion and for the improvement of paint adhesion.

Various analytical methods are reported in the literature used for the determination of chromium includes spectrophotometery [10, 11], spectrofluorimetry [12], ion chromatography with atomic absorption spectrometry [13, 14], differential pulse polarography [15], inductively coupled plasma mass spectrometry [16], differential pulse voltammetry and gas chromatography [17]. The above mentioned methods have some disadvantage, most of them require expansive instruments and time consuming sample preparation steps. On the other hand method based on fluorescent chemosensor is more attractive because it is less laborious, highly selective and extremely sensitive. Chemosensor based on click generated triazole for selective recognition of analyte gaining more attraction recently due to ease of their synthesis. The unique ability of 1, 2, 3-triazole ring due to its involvement in hydrogen bond formation and dipole- dipole interaction have made click chemistry more prominent [18].

We report here a novel chemosensor for the selective detection of dichromate ion based on bis-triazole. The chemosensor contain nitrogen and oxygen electron rich environment for the coordination with chromate ion. So this chemosensor exhibited good selectivity working concentration of dichromate ion in DMF/water.

Experimental

Materials and Instruments.

All the chemicals, salts and solvents were purchased from commercial supplier and were used without any pretreatment. 1H NMR spectra were recorded on a Bruker NMR spectrometer 400 MHz (Switzerland). Chemical shifts are reported in ppm down field from TMS (internal standard) .The fluorescence spectra were recorded with a Shimadzu RF-5301 spectrofluorometer. Melting points were determined on a Stuart SMP10 (uncorrected) and Mass spectra were recorded on EI and ESI-MS.

(Equations)

Experimental

Synthesis and characterization of supramolecule 6

Chemosensor 6 was synthesized by the reaction of the azide 5 with terminal alkynes 3, using click chemistry as shown in scheme 1.

The alkynes 3 were obtained by the reactions of 1 with propargyl bromide in alkaline medium. All the synthesized compounds were characterized through spectroscopic techniques i.e. 1H NMR, EI, ESI-MS and UV visible spectroscopy.

Synthesis and characterization of 2, 2'-bis (prop-2-ynyloxy)-1, 1'-binaphthyl (3)

(Equations)

To a solution of compound 1 500 mg (1.38 mmol) in ethanol (30 mL) K2CO3 485 mg (3.50 mmol) was added and mixture was refluxed for half hour. Then propargyl bromide 500 mg (4.37 mmol) was added and reaction mixture was stirred at 60 C till the completion of reaction. The progress of the reaction was monitored by using thin layer chromatography (TLC DCM: hexane 3:7). The crude product was then mixed with water and extracted thrice with dichloromethane (3x40 mL). The organic layer was dried using anhydrous MgSO4 and concentrated under vacuum.

The mixture was then purified through column chromatography (dichloromethane: hexane 2:8) which gives desired compound 3 as white solid in 78% (500 mg) yield. m.p.124-126 C, EI-MS m/z 362.0, 1H NMR (CDCl3, 300MHz) d: 7.98 (d, 2H, ArH, J= 9.3 Hz), 7.88 (d, 2H, ArH, J= 8.4 Hz), 7.59 (d, 2H, ArH, J= 9.0 Hz), 7.34-7.40 (dt, 2H, ArH, J12= 1.2 Hz, J13=6.6 Hz), 7.20-7.27 (dt 2H, ArH, J12= 0.9 Hz, J13=6.6 Hz), 7.14 (d, 2H, ArH, J=8.4 Hz), 4.59 (t 4H, PhOCH2, J=2.1 Hz), 2.37 (t, 2H, CH, J=2.4 Hz).

Synthesis of 1-(azido methyl) naphthalene (5)

(Equations)

In a 50 mL round bottom flask compound 4 500 mg (2.84 mmol) and sodium azide 190 mg (2.90 mmol) were added in 20 mL ethanol and refluxed for 6 hours. After the complete consumption of starting material shown by TLC (DCM: Hexane 1:9) extra solvent were removed using a rotary evaporator. The mixture obtained was diluted with water and extracted thrice using ethyl acetate (3x30 mL).

The combined organic portion was concentrated under vacuum to obtain desired product 5 as a yellow liquid. Yield 95% (750 mg), EI-MS give molecular ion peak at m/z 183.0, 1H NMR (300 MHz, CDCl3) d: 4.77 (s, 2H, NCH2), 7.45 (t, 2H, ArH, J= 3.6 Hz), 7.52 (m, 2H, ArH), 7.84 (d, 1H, ArH, J= 2.7 Hz), 7.88 (d, 1H, ArH, J= 9.0 Hz), 8.00 (d, 1H, ArH, J= 8.1 Hz).

Synthesis of 2, 2-bis (prop-2-ynyloxy)-1, 1-binaphthyl (6)

In a 100 ml round bottom flask compound 3 300 mg (0.82 mmol) and compound 5 300 mg (0.86 mmol) were dissolved in DMF (20 mL). Then 5 mol% aqueous solution of CuSO4 (6.5 mg) and 20 mol% sodium ascorbate (33 mg) was added to the reaction mixture. The reaction mixture was then heated to 70 C until the complete consumption of the reactants. After the completion, reaction was quenched by the addition of ice cooled water.

The precipitates formed was filtered, washed with water to remove inorganic salts. The residue obtained was then dried under reduced pressure. The mixture was then separated using silica gel chromatography to give compound 6 as white powder in 70% yield. m.p 178-180 C, ESI-MS [M+H] gives molecular ion peak at m/z 729.29, 1HNMR (DMSO, 400 MHz) d: 7.91 (m, 8H, ArH), 7.85 (d, 2H, ArH, J=8.0 Hz), 7.48 (m, 8H, ArH), 7.33 (s, 2H, CH), 7.27 (t, 4H, ArH, J=7.2 Hz), 7.10 (t 2H, ArH, J=7.6 Hz), 6.78 (d 2H, ArH J= 8.4 Hz), 5.90 (s, 4H, N-CH2), 4.88 (d, 4H, PhOCH2, J=12.4 Hz).

Results and Discussion

The spectral analysis was performed in DMF/H2O (1:1, v/v) on a Shimadzu RF-5301 spectrometer. Triazole 6 gives excitation at 270 nm while emission wavelength was selected at 375 nm. The changes in the excitation and emission intensity of chemosensor 6 were recorded upon addition of different anion (100 uM) while keeping the triazole concentration constant (150 nM) in all experiments. Deionized water was used for the preparation of anions (F-, Cl-, Br-, I-, Cr2O7, N3, SO4, CH3COO, ClO4, NO3, CO3) solutions.

Spectral analysis

For the determination of recognition properties of chemosensor 6 towards different anions ( F-, Cl-, Br-, I-, Cr2O7, N3, SO4, CH3COO, ClO4, NO3, CO3) fluorescence spectroscopy was used.

Excitation and emission spectra of compound 6 with various anions were recorded in DMF/Water (1:1 v/v) as depicted in Fig. 1. Reduction in the fluorescence intensity of chemosensor 6 at 270 and 375 nm was observed upon mixing with one equivalent of dichromate ion. While addition of other tested anions does not produce any notable change in excitation and emission spectrums of chemosensor 6. These results showed that chemosensor 6 is selectively interact with dichromate ion.

To further evaluate the binding ability of chemosensor 6 for dichromate ion, concentrations experiments were carried out. Keeping the concentration of chemosensor 6 (150 nM) constant, the concentration of dichromate was varied from 10 uM to 100 uM. Figure 2a and b clearly showed that the quenching effect is directly proportional to the concentration of dichromate ion. Figure 2c showed the plot of the change in intensity of chemosensor 6 at 375 nm against the concentration of dichromate ion and a linear relationship was obtained over a wide range of dichromate concentration (10 uM -100 uM) with R 0.9911. The limit of detection for-2-dichromate ion was 5 uM.

Selective response for a particular analyte is the main characteristic of a chemosensor. Selectivity is the comparative response of the chemosensor to a particular analyte, in comparison to the other interfering ions present in the same solution. To determine the selectivity of the chemosensor 6 for dichromate ion, we examined the effect of other competing anions on the detection dichromate ion. The effect of interfering anions such as F-, Cl-, Br-, I-, N3 , SO4 , CH3COO , ClO4 , NO3 , and CO3 ions on the fluorescence response of chemosensor 6 and dichromate is shown in figure 3. Figure 3 showed that no significant change in the fluorescence intensity of chemosensor 6 was observed by the addition of other competing anions as compared with dichromate alone. The results of this study showed that the chemosensor 6 is highly selective and these anions seem not to interfere with the determination of Cr2O7.

pH of the medium also effect the stabilization of the complex formed between host and guest molecule. The effect of pH on the recognition of dichromate was carried out under optimal experimental conditions. Dilute hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions were used for the maintenance of the pH. Figure 4 showed that fluorescence quenching of triazole 6 remain also same in the pH range (6-12). Where as in the pH range (2-5) quenching effect was decreased due to protonation effect.

The binding stoichiometry between triazole 6 and dichromate ion was determined using the job's plot method. The total concentration of the complex was kept constant during this study while the mole fraction of dichromate ions was changed from 0.1-1.0. The plot of dichromate mole fraction against fluorescence intensity (Fig. 5) showed extreme quenching effect when the mole fraction of dichromate ion is 0.5. This showed 1:1 binding ratio bet host and guest.

We also check the detection of dichromate ion in real sample. Tap water was collected from university of Karachi and spiked with dichromate ion (100 uM). Fluorescence spectrum of compound 6 (150 nM) with dichromate ion (added in tap water) was recorded as shown in figure 6. Results of this study showed that the electrolytes of tap water donot interfer in the detection of dichromate ion.

Conclusions

A novel BINOL based triazole chemosensor has been synthesized and characterized using click chemistry. The newly synthesized triazole based chemosensor was used successfully for the detection of dichromate anion. Compound 6 shows high selectivity and sensitivity for Cr2O7 over other anion tested. The fluorescence excitation and emission intensity of triazole 6 is directly proportional to the dichromate ion concentration. As compared to previously known methods the chemosensor 6 is more selective, sensitive and was synthesized simply by using easily available starting materials under click condition. Moreover the proposed method could be used successfully for accurate determination of dichromate in real samples.

Conclusions

A novel BINOL based triazole chemosensor has been synthesized and characterized using click chemistry. The newly synthesized triazole based chemosensor was used successfully for the detection of dichromate anion. Compound 6 shows high selectivity and sensitivity for Cr2O7 over other anion tested.

The fluorescence excitation and emission intensity of triazole 6 is directly proportional to the dichromate ion concentration. In comparison with other methods, the purposed sensor is more sensitive and it was synthesized easily by using easily available materials under click condition. Moreover the proposed method could be used successfully for accurate determination of dichromate in real samples.

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Author:Ahmed, Farid; Khalid, Sadia; Shah, Kiramat; Shah, Muhammad Raza
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Feb 29, 2016
Words:2791
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