A colorimetric and highly sensitive and selective chemodosimeter for Cu2+ and its application in live cell imaging

Tetrahedron Letters 55 (2014) 6269–6273

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A colorimetric and highly sensitive and selective chemodosimeter for Cu2+ and its application in live cell imaging Jia-Hai Ye a,⇑, Jing Xu a, Huachao Chen b, Yang Bai b, Wenchao Zhang a, Weijiang He b,⇑ a b

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China

a r t i c l e

i n f o

Article history: Received 3 June 2014 Revised 11 September 2014 Accepted 19 September 2014 Available online 28 September 2014 Keywords: Fluorescence BODIPY Chemodosimeter Cu2+ Colorimetric Live cell imaging

a b s t r a c t A new BODIPY (4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)-based colorimetric and fluorescent chemodosimeter 1 has been designed and synthesized. Specific turn-on fluorescence response of 1 for Cu2+ was investigated in CH3CN–H2O (1:1, v/v) solution, which can be deduced to a reaction of Cu2+ promoted selectively hydrolysis of C@N bond to generate formyl-BODIPY. The chemodosimeter 1 can selectively recognize Cu2+ rather than other metal ions in aqueous solution. Confocal fluorescence imaging of cells for detecting Cu2+ in vivo was carried out successfully. The favorable features of chemodosimeter 1 toward Cu2+ include significant fluorescence enhancement (1340-fold), high selectivity, and low detection limit (0.2 lM). Ó 2014 Elsevier Ltd. All rights reserved.

Introduction Copper is the third-most-abundant (after iron and zinc) among the essential transition metal ions in the human body and plays a critical role in various physiological processes.1,2 Despite its important role in organisms, however, copper becomes a toxic and hazardous element in hampering the self-purification capability of the sea or rivers and destroys the biological reprocessing systems in water at high concentrations.3 Moreover, alterations in the cellular homeostasis of copper ions can cause neurodegenerative disorders, such as Menkes and Wilson’s diseases,4 familial amyotrophic lateral sclerosis, prion disease, Parkinson’s and Alzheimer’s diseases,5 probably by its involvement in the production of reactive oxygen species. Although many fluorescent probes for Cu2+ based different sensing mechanisms have been reported,6,7 the development of effective receptors for recognizing Cu2+ is still highly desirable. Recently, an alternative approach is reactivity-based detection, which utilizes irreversible chemical reactions to transform nonemissive precursors to fluorescent products and tracks timedependent cumulative effects of metal exposure.3,8 So far, some reaction-based chemodosimeters have been developed to achieve emission enhancement by reacting with Cu2+ to yield fluorescent

⇑ Corresponding authors. Tel.: +86 25 8430 3116; fax: +86 25 8431 5857. E-mail address: [email protected] (J.-H. Ye). http://dx.doi.org/10.1016/j.tetlet.2014.09.083 0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

products of little affinity to Cu2+, such as Cu2+-promoted hydrolysis of amides,9a esters,7l,9b–d hydrazone,9f lactams,9e,f and lactone,7a oxidative C-O bond cleavage,7j oxidative cyclization of azoaromatics,7k oxidation of dihydrorosamine,7g phenothiazine,10a catechol,10b phenol10c and DNA.10d However, chemists still need to design novel ones, which can overcome the limitations, including ultraviolet excitation, operation at nonphysiological pH for fluorescence imaging, being used only in pure organic solvents, requiring specific reaction conditions, showing low Cu2+ selectivity in the presence of other metal cations.7a,b,9a,d–f,10–12 The 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-sindacene (BODIPY), represents a family of extremely versatile fluorophores owing to its outstanding chemical, thermal, and photophysical properties such as photostability, large extinction coefficients, high luminescence efficiency, long wavelength emission, reasonably long excited singlet state lifetimes, relative insensitivity to environment perturbations, high fluorescence quantum yields, and fine tunable optical properties through chemical modifications.13 However, there have been relatively few fluorescent chemodosimeters developed for Cu2+ detection.7b,e,l Subsequently, developing new fluorescent chemodosimeters that could recognize Cu2+ ion in aqueous media with high selectivity and sensitivity is still challenging and very attractive. Herein, we want to report compound 1 (Scheme 1) as a new highly sensitive and selective turn-on fluorescent chemodosimeter for the detection of Cu2+ in aqueous solution and in living cells.

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OMe

N B F

CHO

N F

N

2 N

NNH2

B

CHO

F OH

HO

F

1

OH

H 2NNH 2

3

N

N

2 4 % yield

5 5 % yield

Scheme 1. Synthesis of chemodosimeter 1.

Results and discussion

(a) 1.2

1.0

0.8

Absorbance

1.2

Absorbance

The synthetic pathway of compound 1 is outlined in Scheme 1. The target compound 1 was synthesized from the readily available substrates 2 and 3 through condensation reaction efficiently (Scheme 1).14–16 The chemical structure of compound 1 was characterized by spectroscopic and elemental analysis data. Initially, the UV–visible absorption and fluorescence titrations of the compound 1 were carried out using a solution of 10 lM of compound 1 in CH3CN/H2O (1:1, v/v) solution upon the addition of Cu2+ ion. A solution of 1 (10 lM) in CH3CN/H2O (1:1, v/v) exhibited an absorption band at 519 nm and was very weakly fluorescent. However, the addition of Cu2+ induced significant changes in both the absorption and emission spectra (Figs. 1, S2). Then, Cu2+-titration and spectral responses were investigated in detail. The absorption spectral changes of 1 upon addition of Cu2+ and other cations were measured to evaluate their sensing abilities. As shown in Figure 1, probe 1 (10 lM) in CH3CN/H2O (1:1, v/v) exhibited a clear absorbance at 519 nm. With the addition of Cu2+, a new absorption peak centered at 490 nm appeared at the expense of the original absorption peak at 519 nm, and the color of the solution changed from redpurple to orange. In contrast, no obvious responses could be observed upon the addition of Li+, Na+, K+, Ba2+, Pb2+, Cu+, Co2+, Al3+, Ag+, Hg2+, Mg2+, Ni2+, Cd2+, Ca2+, Fe3+, Mn2+, Cr3+, and Zn2+, indicating the special selectivity toward Cu2+ (Fig. S3). In addition, three isosbestic points at 498, 416 and 270 nm were observed, which indicated that Cu2+ can be easily detected from other metal ions by the naked eye due to the formation of a new compound from compound 1 upon the addition of Cu2+. Next, the fluorescence titration experiments for 1 (10 lM) with different concentrations of Cu2+ were carried out (Fig. 2). Significant enhancement in the featured emission of formyl-BODIPY 2 at 503 nm was observed upon gradual addition of Cu2+. The emission intensity reached its maximum when 100 equiv of Cu2+ was added, with an enhancement by nearly 1340-fold. While other metal ions show no fluorescence enhancement at 503 nm except Fe3+ shows a slight enhancement. Therefore, compound 1 could detect Cu2+ with significant sensitivity (Fig. S3b). Moreover, there were three linear dependences of the fluorescence intensity between the Cu2+ concentrations from 0.1 lM to 0.1 mM, from 0.1 mM to 0.4 mM, and from 0.4 mM to 1.0 mM, respectively, (Fig. S4b). The detection limit for Cu2+ was determined as 0.2 lM under the experimental conditions, which is sufficiently low to allow the fluorescent detection of micromolar concentrations of Cu2+ in drinking water and living systems. Selectivity based competition of the analyte over other metal ions was a very important parameter to evaluate the performance

0eq Cu2+

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(b) Figure 1. (a) Photographs of compound 1 only and upon addition of 4 equiv of different metal ions in CH3CN/H2O (1:1, v/v) after 3 h. (b) Absorbance spectra of compound 1 (10 lM) in the presence of different concentrations of Cu2+ ions (0– 85 equiv) in CH3CN/H2O (1:1, v/v). The inset shows the absorbance of 1 at 519 nm and 490 nm as the functions of Cu2+ concentration, respectively.

of a fluorescence probe. The interference of other metal ions to the detection of Cu2+ was further investigated. Cu2+ (4 equiv) were added into the solution of CH3CN/H2O (1:1, v/v) solution containing 1 (10 lM) in the presence of other metal ions (4 equiv). As shown in Figure 3, the selectivity profile diagram reveals that

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10µM 1+4eq other cations 10µM 1+4eq Cu2++4eq other cations

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100eq Cu2+

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0eq Cu2+

20

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2+ Li+ Na+ K+ g2+ a2+ a2+ Al3+ b2+ Cr3+ n2+ e3+ o2+ Ni2+ Cu+ Ag+ n2+ d2+ Z C P Cu M F C M C B

1000

Figure 3. Fluorescence responses of 1 (10 lM) to various cations in CH3CN/H2O (1:1, v/v) media. The black bars represent the emission intensities of 1 in the presence of other cations of interest. The red bars represent the change of the emission that occurs upon the subsequent addition of 40 lM of Cu2+ to the above solution. The intensities were recorded at 503 nm, excitation at 450 nm.

800 600 400 200 0 0

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2+

Mole equiv of Cu

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(b) Figure 2. (a) Photographs of compound 1 only and upon addition of 4 equiv of different metal ions in CH3CN/H2O (1:1, v/v) after 3 h under UV light (365 nm). (b) Fluorescence spectra of compound 1 (10 lM) in the presence of difference concentrations of Cu2+ ions (0–100 equiv) in CH3CN/H2O (1:1, v/v) (kex = 450 nm). The inset shows fluorescence intensity change as a function of Cu2+ concentration.

Cu2+-induced fluorescence enhancement remains unperturbed and does not get any interference by the coexisting metal ions. All of these results indicate that the selectivity of probe 1 for the Cu2+ ion over other competitive cations in aqueous media is remarkably high. In addition to metal ion selectivity, for some biological applications, it is very important that the fluorescent probe can be suitable for measuring specific cations in the physiological pH range. To study the practical applicability, the effect of pH on the fluorescence response of probe 1 to Cu2+ was investigated. As shown in Figure 4, Probe 1 was stable and weakly fluorescent over a wide pH range (pH = 5.0–10.0). On the other hand, the fluorescence response of the probe 1 toward the addition of Cu2+ was indeed pH dependent. Probe 1 displayed an efficient fluorescence response to Cu2+ in the pH range of 5.0–8.0. Notably, the fluorescence enhancement was significantly greater at physiological pH (pH = 7.2 nearby), which suggests that probe 1 could be applied for detecting Cu2+ in biological environment. The absorption and emission spectra of the 1-Cu2+ system are characteristic of formyl-BODIPY.14 In addition, the cleavage of C@N bond can be catalyzed by Cu2+ to produce the carbonyl fluorescent compound.7m Therefore, the sensing mechanism of probe 1 can be proposed as the Cu2+-induced selective hydrolysis of 1 to release formyl-BODIPY. Initially, Cu2+ ion coordinated to probe

1 due to its high affinity to various amines and hydroxy ligands. Subsequently, the hydrolysis of the C@N bond of dihydrazone unit near BODIPY occurred to produce formyl-BODIPY which allows the strong green fluorescence emission (Fig. 5). In an effort to gain more detailed information on better understanding the sensing mechanism, 1H NMR spectroscopy was employed. The reaction of probe 1 with Cu2+ ion was carried out in CD3CN for 10 min, then a 1H NMR spectrum of the resulting solution was recorded (Fig. 5). The 1H NMR spectrum shows the disappearance of two singlet peaks at 8.74 ppm (Ha) and 8.65 ppm (Hb) corresponding to the protons on the CH@N bonds in dihydrazone unit of probe 1, respectively, (Fig. 5a). Meanwhile, the singlet peaks at 9.96 ppm (Hc) and 10.02 ppm (Hd) correspond to the two protons of the CHO moieties in the hydrolysis products formyl-BODIPY 2 (Fig. S15) and salicylaldehyde, respectively, (Fig. 5b). This result clearly confirms that probe 1 was hydrolyzed effectively in the presence of Cu2+ to generate highly fluorescent product, formyl-BODIPY. Due to the favorable properties of probe 1 in vitro, the potential utility in living cells was studied. Hela cells were incubated with 1 (20 lM) for 30 min at assessed 37 °C show almost no intracellular fluorescence. However, when Cu2+ (20 lM) was introduced into 65

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OMe

OMe Hb

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O C Hc

Cu2+

HO

C

N Ha

N B

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weak fluorescence

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strong fluorescence

+

Hd OH

(a)

(b) Figure 5. (a) Proposed sensing mechanism and (b) partial 1H NMR spectra of 1 (1 mM) in CD3CN upon addition of (a) 0, (b) 1 equiv of Cu2+.

a

b

c

(a) a

(b) b

(c) c

Figure 6. Confocal fluorescence images of living Hela cells: (1a) Cells loaded with probe 1 (10 lM) at 25 °C for 30 min. (kex = 488 nm; band path = 490–700 nm); (1b) Bright field image of 1a. (1c) Overlaid images of panels 1a and 1b. (2a) Probe 1 loaded Cells with Cu2+ (20 lM) at 25 °C for 3 h (kex = 488 nm; band path = 490–700 nm); (2b) Bright field image of 2a. (2c) Overlaid images of panels 2a and 2b.

the cells via incubation under the same condition after 1–3 h, the significant increase of the fluorescence from the intracellular area was observed (Fig. 6). Obvious fluorescence changes indicated that probe 1 permeated the cell membrane and was capable of imaging Cu2+ in the living cells.

Acknowledgments This project was funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Supplementary data

Conclusion In conclusion, we developed a new colorimetric and turn-on fluorescent probe 1 based on BODIPY to monitor Cu2+ in aqueous media at a low detection limit (0.2 lM). This novel probe operated through an irreversible hydrolysis of C@N bonds and thus can be classified as a chemodosimeter. Probe 1 displayed a significant sensitivity (1340-fold enhancement) and high selectivity toward Cu2+ even coexistence with other metal ions. Furthermore, we have demonstrated the application of this probe in imaging intracellular Cu2+ in Hela cells.

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