A fluorescent chemosensor for Hg2+ based on naphthalimide derivative by fluorescence enhancement in aqueous solution

Analytica Chimica Acta 717 (2012) 122–126

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A fluorescent chemosensor for Hg2+ based on naphthalimide derivative by fluorescence enhancement in aqueous solution Chun-Yan Li a,c,∗ , Fen Xu a , Yong-Fei Li b,∗∗ , Kai Zhou b , Yu Zhou a a b c

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, PR China College of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry & Chemical Engineering, Hunan University, Changsha 410082, PR China

a r t i c l e

i n f o

Article history: Received 14 October 2011 Received in revised form 8 December 2011 Accepted 10 December 2011 Available online 27 December 2011 Keywords: Fluorescence Chemosensor Hg2+ Naphthalimide

a b s t r a c t Naphthalimide derivative (compound 1) containing hydrophilic hexanoic acid group was synthesized and used to recognize Hg2+ in aqueous solution. The fluorescence enhancement of 1 is attributed to the formation of a complex between 1 and Hg2+ by 1:1 complex ratio (K = 2.08 × 105 ), which has been utilized as the basis of fabrication of the Hg2+ -sensitive fluorescent chemosensor. The comparison of this method with some other fluorescence methods for the determination of Hg2+ indicated that the method can be applied in aqueous solution rather than organic solution. The analytical performance characteristics of the proposed Hg2+ -sensitive chemosensor were investigated. The chemosensor can be applied to the quantification of Hg2+ with a linear range covering from 2.57 × 10−7 to 9.27 × 10−5 M and a detection limit of 4.93 × 10−8 M. The experiment results show that the response behavior of 1 toward Hg2+ is pH independent in medium condition (pH 4.0–8.0). Most importantly, the fluorescence changes of the chemosensor are remarkably specific for Hg2+ in the presence of other metal ions, which meet the selective requirements for practical application. Moreover, the response of the chemosensor toward Hg2+ is fast (response time less than 1 min). In addition, the chemosensor has been used for determination of Hg2+ in hair samples with satisfactory results, which further demonstrates its value of practical applications. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In detrimental transition and post transition metal ions, mercury is considered a highly dangerous element because both elemental and ionic mercury can be converted by bacteria in the environment to methyl mercury, which subsequently bioaccumulates through the food chain [1]. It is estimated by the United Nations Environment Programme (UNEP) that ca. 7500 tons of mercury is released into the environment annually [2]. Accordingly, the development of methods to detect Hg2+ is of considerable significance and has become the important subject of current chemical research. Fluorescent devices for the sensing and reporting of chemical species are currently of significant importance for chemistry, biology, and environmental science [3–6]. The design and synthesis of sensitive and selective fluorescent chemosensor are

∗ Corresponding author at: Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, PR China. Tel.: +86 731 58292205; fax: +86 731 58292477. ∗∗ Corresponding author. Tel.: +86 731 58293549; fax: +86 731 58293284. E-mail addresses: [email protected] (C.-Y. Li), [email protected] (Y.-F. Li). 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.12.018

fundamental targets for organic and analytical chemists. So far, there have been some successful achievements in the development of fluorescent chemosensors for Hg2+ [7–12]. The chemosensors possess the advantages of simple preparation, reasonable selectivity and improved sensitivity. However, most of these chemosensors possess either one, two or all of the following disadvantages. Firstly, mercury ions are known as fluorescence quenchers via enhanced spin–orbit coupling, or energy or electron transfer [13–18]. And the quenching not only is disadvantageous for a high signal output upon complexation but also hampers temporal separation of spectrally similar complexes with time-resolved fluorometry. Secondly, mercury ions are relatively easy to be detected in organic solvent, but they are rather difficult to recognize directly in aqueous environment [11,19]. These disadvantages need to be addressed when designing Hg2+ chemosensor for environmental and biological applications. Therefore, novel selective chemosensors for Hg2+ with fluorescence enhancement in aqueous solution become our target. Naphthalimide derivatives are chosen because they are excellent fluorophores with high stability and quantum yield. And some of them have been used as fluorescent chemosensor for pH [20,21], metal cations [22–24] and anions [25] in recent years. These make naphthalimide derivatives potential carriers for preparation of new optical chemosensors. However, the chemosensors based on

C.-Y. Li et al. / Analytica Chimica Acta 717 (2012) 122–126

O

O Br

NH

N

EtOH

CH3OCH2CH2OH

O

O

HOOC

2 O

O N N O

HN

Br

HOOCC5H10NH2

O

HOOC

123

N

NH Cl

N

N O

3

N

N CH3CN

HOOC

1

Fig. 1. Chemical structure and synthetic route of compound 1.

naphthalimide derivatives are commonly applied in organic solutions because lipophilic alkyl or phenyl groups were connected to imide moieties of naphthalimide derivatives [26]. And the hydrophilic group such as hexanoic acid group can make naphthalimide derivatives at imide moieties operate in aqueous solution [27]. In this paper, naphthalimide derivative (compound 1) containing hydrophilic hexanoic acid group was selected as the fluorescence carrier for obtaining a new chemosensor for Hg2+ in aqueous solution. Compound 1 was synthesized and showed remarkable fluorescence enhancement because of the complexation reactions between compound 1 and Hg2+ .

2. Experimental 2.1. Reagents Twice-distilled water was used throughout all experiments. 4-Bromo-1,8-naphthalic anhydride, 6-aminohexanoic acid, piperazine and 2-(chloromethyl)pyridine were purchased from Sigma–Aldrich. All other chemicals were of analytical reagent grade, purchased from Shanghai Chemical Reagent Corporation (Shanghai, China) and used without further purification. Thin layer chromatography (TLC) was carried out using silica gel 60 F254, and column chromatography was conducted over silica gel (200–300 mesh), both of which were obtained from the Qingdao Ocean Chemicals (Qingdao, China).

2.2. Syntheses The synthetic scheme for compound 1 from commercially available compounds is shown in Fig. 1. Compounds 2 and 3 were synthesized according to the procedure reported by Li et al. [28]. Compound 1: 2-(chloromethyl)pyridine (0.60 g, 3.66 mmol) was dissolved in acetonitrile (25 mL), and compound 3 (0.26 g, 0.65 mmol) was added. The reaction mixture was stirred and refluxed for 6 h. After the solvent was evaporated under reduced pressure, the crude product was purified by silica gel column chromatography using CH2 Cl2 /EtOH (10:1, v/v) as eluent to afford a yellow solid: yield 0.123 g (60%). 1 H NMR (400 MHz, DMSO) ı 8.67 (d, J = 7.2 Hz, 1H), 8.59 (d, J = 8.0 Hz, 2H), 8.43 (d, J = 8.0 Hz, 1H), 8.06 (d, J = 7.6 Hz, 1H), 7.87 (t, J = 7.8 Hz, 1H), 7.72 (t, J = 7.4 Hz, 1H), 7.36 (d, J = 8.0 Hz, 1H), 7.24 (t, J = 6.0 Hz, 1H), 4.20 (q, J = 6.7 Hz, 2H), 3.83 (s, 2H), 3.31 (s, 4H), 2.88 (m, 4H), 2.47 (t, J = 7.6 Hz 2H), 1.75–1.82 (m, 4H), 1.47–1.54 (m, 2H). HRMS m/z calcd for C28 H30 N4 O4 (M+H) 487.2345, found 487.2337. Anal. calcd. for C28 H30 N4 O4 : C, 69.12; H, 6.21; N, 11.51; O, 13.15. Found: C, 69.04; H, 6.22; N, 11.53; O, 13.16.

2.3. Apparatus UV–vis absorption spectra were recorded with a Perkin Elmer Lambda 25 spectrophotometer. All fluorescence measurements were carried out on a Perkin Elmer LS 55 fluorescence spectrometer with excitation slit set at 13.0 nm and emission at 5.0 nm. The pH measurements were carried out on a Mettler-Toledo Delta 320 pH meter. Data processing was performed on a Pentium IV computer with software of SigmaPlot. 2.4. Measurement procedure A 1 × 10−4 mol L−1 stock solution of compound 1 was prepared by dissolving compound 1 in absolute ethanol. A stock standard solution of Hg2+ (0.01 mol L−1 ) was prepared by dissolving an appropriate amount of mercury nitrate in water and adjusting the volume to 500 mL in a volumetric flask. This was further diluted to 1 × 10−3 –1 × 10−8 mol L−1 stepwise. The solution of 1-Hg2+ was prepared by adding 1.0 mL of the stock solution of compound 1 and 1.0 mL of the stock solution of Hg2+ in a 10 mL volumetric flask. Then the mixture was diluted to 10 mL with Tris–HNO3 buffer solution. In the solution thus obtained, the concentrations were 1 × 10−5 mol L−1 of compound 1 and 1 × 10−3 –1 × 10−8 mol L−1 of Hg2+ . The solution was protected from light and kept at 4 ◦ C for further use. Blank solution of compound 1 was prepared under the same conditions without Hg2+ . For pH buffer solutions from 2.0 to 6.0, appropriate amounts of sodium acetate and acetic acid were used; for buffers from 7.0 to 8.0, tris(hydroxymethyl)aminomethane and nitric acid mixtures were used at the concentration of 0.1 M. The fluorescence measurements were carried out at the maximum excitation wavelength of 401.0 nm and the maximum emission wavelength of 529.0 nm. Before each measurement, the solutions were allowed to stand for a few minutes to complete formation of metal–ligand complex. 3. Results and discussion 3.1. Spectral characteristics Fig. 2 shows the fluorescence spectra of compound 1 exposed to solutions containing different concentrations of Hg2+ recorded at excitation wavelength of 401.0 nm and emission wavelength of 450.0–660.0 nm. The spectrum of 1 without Hg2+ exhibits one fluorescence emission peaks at 529 nm, which is typical fluorescence emission peak of naphthalimide. As can be seen from Fig. 2, compound 1 itself exhibited weak fluorescence emission in Tris–HNO3 buffer solution. Addition of Hg2+ to the solution of compound 1 results in a remarkable enhancement of fluorescence signal. The

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1.0

500

100 μM

0

300

.6

α

Fluorescence intensity

.8

400

200

.4

100 0

.2

450

500

550

600

650 1

Wavelength(nm)

2

3

4

0.0

Fig. 2. The fluorescence emission spectra of compound 1 (10 ␮M) at pH 7.00 in the presence of different concentration of Hg2+ (0, 0.10, 0.50, 1.0, 2.0, 5.0, 10, 20, 50, 100 ␮M). Excitation wavelength was 401.0 nm.

fluorescence intensity of compound 1 gradually increases with increasing Hg2+ concentration. These results provide a proof for the reaction of compound 1 with Hg2+ , which constitutes the basis for the determination of concentrations of Hg2+ with compound 1. In order to understand better the variation of fluorescence response of compound 1 toward Hg2+ , the UV–visible spectra of compound 1 in the absence of Hg2+ and in the presence of Hg2+ were recorded in Fig. 3. The absorption spectrum of compound 1 indicates an absorption band at 396 nm. The addition of Hg2+ leads to decreasing absorption. The experimental results show very large changes in absorption intensity and small spectral shifts. This phenomenon is probably due to the complexation of aliphatic amine in compound 1 and Hg2+ . Similar results were reported by Qian et al. [29]. So the changes of fluorescence spectra of compound 1 seem more likely to be caused by the change of fluorescence intensity rather than spectral shifts. These spectroscopic features are diagnosis of the reaction of Hg2+ and compound 1. 3.2. Principle of operation The complexation equilibrium of compound 1 (L) and Hg2+ (M) with an association constant K can be expressed by the following equation, K

mM + nL←→Mm Ln

(1)

[Mm Ln ]

(2)

K=

[M]m [L]n

compound 1 compound 1+Hg

Absorbance

0.6

-8

-7

-6

-5

-4

-3

2+

log[Hg ] Fig. 4. Relative fluorescence intensity ˛ as a function of log[Hg2+ ]. The curves fitting the experimental data were calculated from Eq. (4): (1) m:n = 3:1, K = 1.03 × 1016 ; (2) m:n = 2:1, K = 4.34 × 1010 ; (3) m:n = 1:1, K = 2.08 × 105 ; (4) m:n = 1:2, K = 2.08 × 1010 . (䊉) Data points experimentally obtained.

where Hg2+ and compound 1 are established with formation of a complex with a complexing ratio of m:n. The relative fluorescence intensity ˛ is defined as the ratio of free L, [L]f , to the total amount of L, [L]t , in the solution. It can be experimentally determined by measuring the fluorescence intensity of compound 1 in the solution. ˛=

[L]f [L]t

=

F − F0 Fb − F0

(3)

Here Fb is the fluorescence intensity of compound 1 in the blank buffer solution and F0 represents the fluorescence intensity of compound 1 in the solution when compound 1 is completely complexed with Hg2+ . F is the fluorescence intensity of compound 1 actually measured when in contact with Hg2+ solutions of a given concentration. The relationship between the ˛ and Hg2+ concentration [M] can be represented as ˛n 1 = n−1 1−˛ nK[L]t [M]m

(4)

The response of compound 1 for different concentrations of Hg2+ is shown in Fig. 4. Four curves are calculated using Eq. (4) with different K and ratios of Hg2+ and compound 1. It can be seen that the curve with 1:1 complex ratio and an appropriate K of 2.08 × 105 fits best to the experimental data. The curve can serve as the calibration curve for the detection of Hg2+ concentration. Moreover, the Job’s plot (Fig. S2 in Supporting Information) clearly confirms the 1:1 stoichiometry for 1-Hg2+ complex. A practically usable range for quantitative determination covers a range from 2.57 × 10−7 to 9.27 × 10−5 M (0.05 ≤ ˛ ≤ 0.95) [30]. The detection limit is 4.93 × 10−8 M (defined as three times standard deviation of blank solution).

0.4

3.3. Effect of pH 0.2

0.0 300

350

400

450

500

Wavelength(nm)

Fig. 3. Absorption spectra of compound 1 (5 ␮M) in the absence of Hg2+ (solid line) and in the presence of Hg2+ (dotted line).

The pH value of the environment around the fluorescent chemosensor usually shows somewhat of an effect on its performance toward target metal ion due to the protonation or deprotonation reaction for the fluorophore and the hydrolysis reaction for the metal ions in the basic condition. The effects of pH on the fluorescence response of compound 1 to Hg2+ were therefore investigated. A solution of the high concentration of Hg2+ might cause precipitation of HgO in the alkaline condition, so these experiments were carried out at a pH range from 2.0 to 8.0, with the

C.-Y. Li et al. / Analytica Chimica Acta 717 (2012) 122–126 5

5

4

4

125

50 μM 3

F/F0

F/F0

3

5 μM

2

2

1

1

0.5 μΜ

0

0 2

4

6

8

pH

Fig. 5. Effect of pH on the fluorescence intensity of 1 (10 ␮M) in the presence of Hg2+ (the concentration was fixed at 1 × 10−5 mol L−1 ).

concentration of compound 1 fixed at 10 ␮M and of Hg2+ at 10 ␮M, respectively (Fig. 5). It can be seen that the fluorescence intensity of the chemosensor decreases with decreasing pH value in the section of lower pH value, which might be caused by the protonation of 1 without binding with the metal ion. In a wide range of pH from 4.0 to 8.0, acidity does not affect the determination of Hg2+ with 1. In other words, the response behavior of 1 is pH independent in medium condition, which is convenient for practical applications of the proposed chemosensor in determination of Hg2+ in actual samples. 3.4. Selectivity The effect of interferents on the fluorescence determination of Hg2+ by the chemosensor based on compound 1 was investigated. The experiments were carried out by recording the change of the fluorescence intensity before and after adding the interferent (the concentration was fixed at 1 × 10−3 mol L−1 ) into the Tris–HNO3 buffer solution of pH 7.0. The results for common inorganic ions are presented in Fig. 6. Experimental results show that the ions have no obvious interference for Hg2+ detection. Therefore, the chemosensor has a better selectivity for Hg2+ over other metal ions tested under the same conditions. 3.5. Response time and reversibility Besides high sensitivity and selectivity, a short response time and reversible response are other two necessities for a fluorescent chemosensor to dynamically monitor Hg2+ in environmental sam-

Fig. 6. Effect of different interferents (1 × 10−3 mol L−1 ) on the fluorescence intensity of the proposed chemosensor. F and F0 are the fluorescence intensities of compound 1 with and without adding the interferent.

0

100

200

300

400

Time (s)

Fig. 7. Kinetics of the fluorescence enhancement of 1 (10 ␮M) in the presence of different concentrations of Hg2+ . Fluorescence intensity was recorded at 529.0 nm. Excitation wavelength was fixed at 401.0 nm.

ples in real time. To study the response time of the chemosensor based on compound 1 to Hg2+ , the kinetics of fluorescence enhancement at 529 nm upon analyzing different concentrations of Hg2+ by the new developed chemosensor were recorded, and results were shown in Fig. 7. The response time of the chemosensor to Hg2+ is concentration-dependent, as the time required to reach equilibrium increases with the increase of Hg2+ concentrations. However, in all cases, the stable reading could be obtained in less than 1 min. Therefore, this chemosensor could be used for real-time tracking of pH. From Fig. 7 one can also discover that once a plateau is reached, the fluorescence intensity at 529 nm stays almost unchanged the rest of the time, indicating that the chemosensor is photostable under irradiation with visible light. The chemical reversibility behavior of the binding of 1 with Hg2+ was then studied. Because of the high stability of the EDTAHg2+ complex (stability constant log KEDTA-Hg = 21.7 [31]), it could be expected that the addition of EDTA will liberate Hg2+ from the metal–ligand complex, releasing free compound 1. Therefore, 1 equiv of EDTA (20 ␮M) was added to the Hg2+ (20 ␮M) complex of 1 (10 ␮M) in buffered solution, which shows a remarkable decrease of fluorescence signal at 529 nm. The regenerated free 1 can then participate in another Hg2+ binding process. These results demonstrate that the Hg2+ binding of 1 in buffered solution is chemically reversible, which benefits from the dynamic monitoring of the concentration change of Hg2+ in various samples. Therefore, this chemosensor could be used for real-time tracking of Hg2+ . 3.6. Investigation of sensing mechanism When a fluorescence group links with an electrondonating group (it generally includes one or more amido nitrogen atoms), the photo-induced electron transfer (PET) will occur between the fluorescence group and the electron-donation group [32,33]. In compound 1, the aliphatic secondary amine is connected with naphthalimide fluorophore. Aliphatic amine shows the property of a strong electron donor in an alkaline medium and the PET occurs from the aliphatic amine to the naphthalimide fluorophore. Under these conditions the fluorescence of compound 1 is in “offstate” because the fluorescence of naphthalimide fluorophore is quenched by the PET process. The PET process can be stopped by the complexation of aliphatic amine and Hg2+ and the fluorescence of the fluorophore restored. Consequently, the fluorescence of compound 1 in the presence of Hg2+ would be “switched on”. The proposed fluorescence enhancement mechanism is shown in Fig. 8. More direct evidence was obtained by comparing the ESI mass spectra of compound 1 and 1-Hg2+ (Figs. S3 and S4 in the

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Table 1 Determination of Hg2+ concentration in hair samples with the proposed chemosensor and CV-AFS. Hair sample

Proposed chemosensor meana ± SDb (␮g g−1 )

1 2 3 4

0.383 0.422 0.276 0.328

a b

± ± ± ±

CV-AFS meana ± SDb (␮g g−1 )

0.04 0.06 0.05 0.06

0.388 0.425 0.267 0.322

± ± ± ±

Relative error (%)

0.02 0.05 0.03 0.07

1.3 0.7 −3.4 −1.9

Mean of three determinations. Standard deviation.

PE T

PET O

O N N

Hg2+

N

N

N

N

N

Hg2+

O

O

N

HOOC

HOOC Off

On

Fig. 8. Proposed scheme of fluorescence enhancement mechanism of compound 1 and Hg2+ .

Supporting Information). In Fig. S3, the unique peak at m/z = 487.2 corresponding to [1 + H]+ . While in Fig. S4, with excess Hg2+ added to 1, the peak at m/z = 487.2 disappeared, and the main peak at m/z = 689.2 corresponding to [compound 1 + Hg2+ − H]+ was clearly observed, which indicated the formation of a complex of compound 1 with Hg2+ .

3.7. Preliminary analytical application The practical application of the designed chemosensor was evaluated by determination of Hg2+ in hair samples. The hair samples were obtained from four volunteers. All the samples collected were possessed according to the procedure reported by Aksuner et al. [34]. Table 1 lists Hg2+ concentration in human hair measured by the proposed chemosensor as well as cold vapor atomic fluorescence spectroscopy (CV-AFS). As is shown in Table 1, the concentration of Hg2+ , determined by the proposed chemosensor, was in good agreement with that obtained by CV-AFS. The 1-based chemosensor seems to be useful for the determination of Hg2+ in actual samples.

4. Conclusion We have developed a fluorescent method for Hg2+ in aqueous solution by using functionalized naphthalimide (compound 1). The fluorescence intensities of the compound 1 increase with increasing Hg2+ concentration, which is attributed to the complexation reaction between compound 1 and Hg2+ . The comparison of this method with some other fluorescence method for the measurement of Hg2+ based on naphthalimide indicated that the methods can be applied in aqueous solution rather than organic solution. And the proposed method was used for the determination of Hg2+ in real samples with satisfactory results.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21005068), Hunan Provincial Natural Science Foundation of China (11JJ3023), and State Key Laboratory of Chemo/Biosensing and Chemometrics Foundation (200906).

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