Novel Carbonothioate-Based Colorimetric and Fluorescent Probe for

Jul 27, 2016 - The development of probes for selective detection of mercury ions (Hg2+) is an important mission to accomplish because of the toxicity ...
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A novel carbonothioate-based colorimetric and fluorescent probe for selective detection of mercury ions Wei Shu, Yawei Wang, Liu Wu, Zuokai Wang, Qingxia Duan, Yibo Gao, Caiyun Liu, Baocun Zhu, and Liangguo Yan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02158 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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A novel carbonothioate-based colorimetric and fluorescent probe for selective detection of mercury ions Wei Shu, Yawei Wang, Liu Wu, Zuokai Wang, Qingxia Duan, Yibo Gao, Caiyun Liu, Baocun Zhu,* and Liangguo Yan * School of Resources and Environment, University of Jinan, Shandong Provincial Engineering Technology Research Center for Ecological Carbon Sink and Capture Utilization, Jinan 250022, China *

Corresponding author. Fax: +86-531-82767617; Tel.: +86-531-82767617

E-mail address: [email protected] (B. Zhu) and [email protected] (L. Yan)

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Abstract The development of probes for selective detection of mercury ions (Hg2+) is an important mission to accomplish, due to the toxicity and universality of Hg2+. Herein, we

designed

and

synthesized

a

novel

fluorescent

probe

O-(N-butyl-1,8-naphthalimide)-4-yl-O-phenyl carbonothioate (CBONT) for selective and sensitive detection of Hg2+ by turn-on fluorescence spectroscopy. Probe CBONT exhibited a fast response for Hg2+ with excellent sensitivity (LOD = 1.9 nM, 3σ/slope), and it might be attributed to the adoption of new recognition receptor of carbonothioate moiety. Additionally, probe CBONT could serve as a “naked-eye” indicator for Hg2+. Finally, probe CBONT could be successfully applied to detect the concentrations of Hg2+ in real water samples. Our proposed recognition receptor would open up new exciting opportunities for designing highly selective and ultrasensitive fluorescent probes for the determination of Hg2+ in real water samples.

Keywords : Fluorescent probe; Hg2+; naked-eye detection; carbonothioate; 4-hydroxynaphthalimide

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1. Introduction As one of the most typical and ubiquitous heavy metal, mercury (Hg) is the most toxic nonradioactive element,1-2 has caused serious damage to the environment and human body due to its durability, easily absorbed by aquatic organisms and high biological accumulation.3-5 It is well-known that the Environmental Protection Agency (EPA) has set a 2 ppb (0.01 µM) maximum tolerable level of mercury contamination in food and drinking water,6-7 because a very small amount of mercury ions could pose a serious threat to the central nervous and endocrine systems.8-9 Therefore, developing simple methods for the highly sensitive and selective detection of Hg2+ is very important. To date, many methods have been applied to detect Hg2+, such as inductively coupled plasma mass spectrometry, atomic absorption spectroscopy and electrochemical10-12 . Although above methods are sensitive and accurate, they often require involve time-consuming sample preparation steps, sophisticated instruments and a large amount of sample. In contrast, colorimetric probe could detect the target analytes by the naked-eye, without the aid of any advanced instruments.13 On the other hand, fluorescent probes might be the most attractive method for the detection of Hg2+ because of its high sensitivity and operational simplicity.14-18 A number of colorimetric and/or fluorescent probes for Hg2+ have been reported,19-22 however, most of them still suffered from longer response time, poor selectivity, low sensitivity, color/fluorescence quenching, bad water solubility, a smaller Stokes shift, and synthetic difficulties.23-25 Therefore, constructing a simple water-soluble turn on fluorescent probe for visualization of Hg2+ with high selectivity and sensitivity became our task. To address the above-mentioned challenges, the adoption of new specific reaction between recognition receptor and Hg2+ is a crucial. Very recently, we have reported a

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carbamothioate-based

fluorescent

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probe

O-(N-butyl-1,8-naphthalimide)-4-yl-N,N-dimethylthiocarbamate (CBAMT) for Hg2+ with good features of high selectivity and sensitivity in aqueous solution and water samples.26 The reaction between probe CBAMT and Hg2+ was very slowly, so H2 O2 (100 mM) was added to promote the cleavage of carbamothioate group, and restore the green fluorescence quickly. Obviously, the addition of H2 O 2 (100 mM) could result in a harshly testing conditions. Then, we expected the carbonothioate moiety would respond Hg2+ rapidly without the help of H2 O2 , thus leading to a superior reaction time and sensing performance in comparison with CBAMT.26 To verify our hypothesis and acquire a new Hg2+ probe with superior property, we designed a new fluorescent probe CBONT based on a recognition receptor of carbonothioate (Scheme 1). In this new probe CBONT, we still chose N-butyl-4- hydroxy-1,8-naphthalimide as the fluorophore for its outstanding ICT structure and desirable photophysical properties,27 and carbonothioate moiety as the recognition receptor for the first time. As expected, probe CBONT shows meaningfully properties, shown as the following: (1) quicker response to Hg2+ than probe CBAMT, and without the assistance of any reagent; (2) excellent water solubility and selectivity; (3) high sensitivity (LOD = 1.9 nM, 3σ/slope); (4) a large Stokes shift (102 nm); (5) practical application in real water samples.

2. Experimental 2.1 Materials and general methods Unless otherwise stated all the chemicals used in this work were gained from commercial suppliers and used without further purification. Ultrapure water was prepared through Sartorious Arium 611DI system and used throughout the experiments. Silica gel (200-300 mesh, Qingdao Haiyang Chemical Co.) was used for column chromatography.

1

H NMR and

13

C NMR were recorded on a Bruker AV-400

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spectrometer with chemical shifts reported as ppm (in DMSO-d6 , TMS as internal standard). Electrospray ionization (ESI) mass spectra were measured with an LC-MS 2010A (Shimadzu) instrument. Fluorescence emission spectra were carried out on a Horiba FluoroMax-4 spectrophotometer with excitation wavelength of 450 nm. All the fluorescence spectra were uncorrected. 2.2. Synthesis of Probe CBONT According to our previous report, N-butyl-4-hydroxy-1,8-naphthalimide was readily synthesized by the reaction between N-butyl-4-chloro-1,8-naphthalimide and N-hydroxyphthalimide.26 N-ethyldiisopropylamine

(350

N-butyl-4- hydroxy-1,8-naphthalimide

μL) (538

was mg,

added 2

to mmol)

a

solution and

of

phenyl

chlorothionocarbonate (516 mg, 3mmol) in 15 mL dry dichloromethane. Then the resulting mixture was stirred at room temperature for 10 h. After evaporation of the solvent, the product was purified by silica column chromatography CH2 Cl2 to obtain probe CBONT (736 mg, 91%). FT-IR (KBr) ν: 2959.85, 2934.10, 2874.55, 2857.92, 1698.06, 1658.31, 1626.29, 1589.53, 1490.55, 1386.82, 1358.06 1284.17, 1265.04, 1233.79, 1200.10, 1148.40, 1085.67, 1003.06, 935.59, 876.29, 863.82, 784.04, 776.33, 716.62. 1 H-NMR (400 MHz, CDCl3 ) δ (*10-6 ): 0.94(t, J = 6 Hz, 3H), 1.32-1.41(m, 2H), 1.59-1.67(m, 2H), 4.05(t, J = 8 Hz, 2H), 7.41(t, J = 8 Hz, 1H), 7.49(d, J = 8 Hz, 2H), 7.54-7.59(m, 2H), 7.91(d, J = 8 Hz, 1H), 7.98(t, J = 8 Hz, 1H), 8.50(d, J = 8 Hz, 1H), 8.57(d, J = 8 Hz, 1H), 8.59(d, J = 8 Hz, 1H).

13

C-NMR (100 MHz, CDCl3 ) δ (*10-6 ): 14.16,

20.27, 30.05, 39.25, 121.20, 121.63, 122.20, 123.15, 124.84, 127.70, 128.15, 128.79, 129.17, 130.45, 131.80, 131.86, 153.45, 153.72, 163.05, 163.61, 193.89. ESI-MS calcd for C23 H20 NO4 S [M+H]+ 406.1113, found 406.1098.

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3. Results and discussion 3.1. Characteristic spectra The characteristic spectra of free probe CBONT toward mercury species was measured under aqueous solution containing (5 mM HEPES, pH=7.4). As shown in Fig. 1a, the free probe CBONT had weak fluorescence. While Hg2+ was added to the solution of probe CBONT, the fluorescence intensity at 552 showed a large enhancement. In the absence of mercury species, the probe solution exhibits one major absorption peak at 350 nm. Then the addition of Hg2+ to the probe solution resulted in a new absorption band at 450 nm with the color changes from colorless to yellow (Fig. 1b and Fig. S1). The enhancement of fluorescence intensity and red-shifted absorption spectra

induced

by

Hg2+

might

be

attributed

to

the

formation

of

N-butyl-4- hydroxy-1,8-naphthalimide with a much stronger “push-pull” component. The results implied that mercury species could prompt the split of a carbonothioate group (Scheme 2). In other words, our proposed probe had the capability to detect mercury species by the naked-eye (Fig. S2), absorption spectra and fluorescence spectra. 3.2. Time-dependence of detecting Hg2+ The reaction time of probe CBONT with Hg2+ was evaluated under the above- mentioned analytical conditions and the results are shown in Fig. 2. The solution of free probe CBONT was very stable, and had nearly no fluorescence. The fluorescence intensity at 552 nm increases gradually with increasing reaction time after Hg2+ was added. In addition, the fluorescence intensity remained when the reaction time exceed 10 min. Therefore, our proposed probe CBONT could provide a rapid analytical method for the detection of Hg2+.28-36 3.3. Quantification of Hg2+

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In the fluorescent spectrum, as expected, the continuous addition of Hg2+ to the solution of probe CBONT resulted in a gradually increase of the fluorescence intensity at 552 nm (Fig. 3). Additionally, the fluorescence intensity remained plateau when the concentrations of Hg2+ exceed 50 µM (Fig. S3). As shown in inset of Fig. 3, there was a good linearity between the fluorescence intensity and the concentrations of Hg2+ in the range of 0.1 to 10 µM. The limit of detection (LOD) of Hg2+ was calculated to be 1.9 nM (3σ/slope). The LOD could meet the criteria of 2 ppb maximum tolerable level of mercury contamination in drinking water set by the Environmental Protection Agency (EPA). That is to say, probe CBONT is sufficiently sensitive for detecting mercury ion by a turn-on fluorescence method. 3.4. Selectivity to Hg2+ Then we evaluated the specificity of probe CBONT towards Hg2+. The probe CBONT was treated with other potentially competing analytes to examine the selectivity. As shown in Fig. 4, only Hg2+ resulted in a fluorescence enhancement, and nearly no fluorescence intensity changes were observed towards other competitive analytes, such as Ag+, K+, Na+, Ca2+, Mg2+, Fe2+, Fe3+, Al3+, Cu2+, Cd2+, F-, Cl-, NO 3 -, NO2 -, CO 32-, HCO 3-. To further demonstrate the ability to detect Hg2+ in the presence of other competitive analytes, the anti- interference of probe CBONT was studied (Fig. 5). The results implied that probe CBONT had excellent selectivity toward mercury species in the presence of other competitive ions. This might be ascribed to the specific reaction of sulfur atom with strong electron-donating ability with Hg2+.26 Additionally, the pH dependency of CBONT and CBONT towards Hg2+ were researched. The results showed our probe could be used in a wide range of pH conditions. (Fig. S4).

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3.5. Analytical application Encouraged by the above excellent properties, we attempted to investigate the practical application of probe CBONT for the selective detection of Hg2+ in three real water samples, and the results were summarized in Table 2. Hg2+ had been not found in the three water samples through our proposed method and cold vapor atomic absorption spectrometry. Then 1 µM, 4 µM and 10 µM Hg2+ was added to above- mentioned samples respectively. The good recoveries of these samples proved the accuracy of the proposed method for the determination of Hg2+. The results further demonstrated the utility of our proposed method for the effective and fast Hg2+ detection. 3.6. Mechanism of probe CBONT in sensing mercury species To fully explore the sensing mechanism of probe CBONT for Hg2+ and determine the produced species, the reaction products of probe CBONT and Hg2+ were subjected to electrospray ionization mass spectral analysis. With the presence of Hg2+, the peak of probe CBONT (m/z 406.1098 [M+H]+ ) disappeared, while a new peaks emerged at m/z 268.0981

[M-H]- ,

which

N-butyl-4- hydroxy-1,8-naphthalimide.

represented Additionally,

the

formation

we synthesized

of

CBONT-Hg

through the reaction of CBONT and Hg2+. The CBONT-Hg was characterized to be N-butyl-4- hydroxy-1,8-naphthalimide by FT-IR, 1 H-NMR and

13

C-NMR. Therefore, we

proposed that the fluorescence turn-on reaction process most likely undergoes the proposed mechanism as shown in Scheme 2. 4. Conclusions In summary, we have designed and successfully synthesized a novel fluorescent probe CBONT for selective and sensitive detection of Hg2+ by turn-on fluorescence

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spectroscopy. The synthesis of probe CBONT was given in Scheme 1. The structure of the target probe CBONT was confirmed by 1 H NMR,

13

C NMR, and HRMS. This new

probe displayed quick response (Table 1), a large Stokes shift (102 nm), excellent sensitivity and selectivity for Hg2+ in aqueous solution and real water samples. Importantly, our proposed recognition mechanism would open up new exciting opportunities for designing highly selective and ultrasensitive fluorescent probes for the determination of Hg2+ in real water samples. Acknowledge ments We gratefully acknowledge financial support from the National Nature Science Foundation of China (No. 21107029), Outstanding Young Scientists Award Fund of Shandong Province (BS2013HZ007), Postdoctoral Science Foundation of China (2013M541953), Graduate Innovation Foundation of University of Jinan (YCXS15016) for financial support.

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Figure captions Fig. 1 (a) The fluorescence spectra of probe CBONT (5 µM) in the presence and absence of Hg2+ (10 µM) in HEPES (5 mM, pH 7.4) aqueous solution. (b) Absorption spectra of probe CBONT (20 µM) toward Hg2+ (40 µM) in HEPES (5 mM, pH 7.4) aqueous solution. Each spectrum was acquired 10 min after Hg2+ addition. Excitation wavelength = 450 nm. Fig. 2 Time-course for probe CBONT (5 µM) in the presence of Hg2+ (20 µM). Fig. 3 Fluorescence spectra of probe CBONT (5 µM) in the presence of increasing concentrations of Hg2+ (final concentration: 0, 0.1, 0.5, 1, 2, 4, 6, 8, 10 µM). Inset: The linearity between fluorescence intensities at 552 nm and increasing concentrations of Hg2+. Each spectrum was acquired 10 min after Hg2+ addition. Error bar = RSD (n = 5). Fig. 4 Fluorescence spectra of probe CBONT (5 µM) in the presence of Hg2+, Ag+ (10 µM) and other analytes (50 µM). Fig. 5 Fluorescence responses of probe CBONT (5 µM) toward various analytes (50 µM except for specific label) in the presence of Hg2+ (10 µM). a: only Hg2+, b: K+, c: Na+, d: Ca2+, e: Mg2+, f: Fe2+, g: Fe3+, h: Al3+, i: Ag+ (10 µM), j: Cu2+, k: Cd2+, l: F-, m: Cl-, n: NO3 -, o: NO2 -, p: CO32-, q: HCO 3 -. Bars represent the fluorescence intensities at 552 nm. Error bar = RSD (n = 5)

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Fig. 1a

Fig. 1b

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Fig. 2

Fig. 3

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Fig. 4

Fig. 5

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Schemes and scheme captions Scheme 1 Synthesis of probe CBONT. Scheme 2 Reaction mechanism of CBONT for mercury species.

Scheme 1

Scheme 2

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Tables Table 1 Comparison of fluorescent probes for Hg2+. Probe

λex/λe m (nm)

Reaction Time (min)

LOD (nM)

Solution (v/v)

References

365/500

30

100

CH3 CN/H2 O (1:99)

[36]

380/458



80

THF/H2 O (9:1)

[24]

520/586

60

60.78

CH3 CN/H2 O (1:99)

[25]

560/613

40

10

480/535



7.8

CH 3CH 2OH

[30]

450/552

>25

2.4

Aqueous solution

[26]

450/552

10

1.9

Aqueous solution

This work

O N O

S HN

S S N

N

N

N S HN

N H

O N

N

H N

NH S

N

O

N

NC CN

NC O

CH 3CH2OH/H 2O

(1:1)

O

O N

N

S

N

N N B F F

O

O

[35]

N

N

O

O

S N

O

N

O

O O

S

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Table 2 Analysis results of Hg2+ in three water samples. Water samples

Sample A

Sample B

Sample C

LOQ (µM)

0.3

0.5

0.5

Found Hg2+

No

No

No

Added (µM)

Found (µM) (n = 3)

Recovery (%)

1.00

0.83±0.11

83.54

4.00

3.87±0.15

96.75

10.00

10.08±0.17

100.84

1.00

0.94±0.08

94.62

4.00

4.18±0.12

104.50

10.00

10.89±0.21

108.98

1.00

1.03±0.09

103.48

4.00

4.31±0.13

107.75

10.00

11.23±0.15

112.38

Notes: Sample A: tap water from University of Jinan; Sample B: lake water from Jia Zi Lake, University of Jinan; Sample C: from the Yellow river at Jinan, China.

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