Fluorescent sensor for sequentially monitoring zinc(II) and cyanide

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Fluorescent sensor for sequentially monitoring zinc(II) and cyanide anion in near-perfect aqueous media Hyo Jung Jang, Ji Hye Kang, Misun Lee, Mi Hee Lim, and Cheal Kim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03826 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Industrial & Engineering Chemistry Research

Fluorescent sensor for sequentially monitoring zinc(II) and cyanide anion in near-perfect aqueous media

Hyo Jung Jang,a Ji Hye Kang,a Misun Lee,b Mi Hee Lim,b* Cheal Kima*

a

Department of Fine Chemistry and Department of Interdisciplinary Bio IT Materials, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea. Fax: +82-2973-9149; Tel: +82-2-970-6693; E-mail: [email protected] b

Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. Fax: +82-52-217-5409; Tel: +82-52-217-5422; E-mail: [email protected]

Abstract We report a new sensor, 1, for sequentially detecting Zn2+ and CN- based on fluorescence. Sensor 1 was prepared through the reaction of 3-aminobenzofuran-2-carboxamide with 4diethylaminosalicylaldehyde. Sensor 1 showed a selective ‘off-on’ fluorescent response toward Zn2+, distinguishing Zn2+ from Cd2+. The limit of 1 (0.35 µM) for monitoring Zn2+ is lower than the World Health Organization (WHO) guideline (76 µM) for water available for drinking. Importantly, sensor 1 could sense Zn2+ in living cells and aqueous media. In addition, the resulting 1-Zn2+ complex functioned as an efficient ‘on-off’ sensor for CN-. The mechanisms of 1 for detecting Zn2+ and CN- were explained by spectroscopic, spectrometric, and theoretical studies.

Keywords: fluorescent chemosensor, sequential detection, zinc, cyanide, cell imaging

1

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1. Introduction Zinc ion is an essential cofactor in a variety of biological pathways involved in pathology, immune function, brain function, mammalian reproduction and neuronal signal transmission.1–3 A lack of Zn2+, however, leads to the unbalanced metabolisms, such as retarded growth in children, high blood cholesterol, and brain disorders, and neurodegenerative diseases.4–6 To gain a better understanding of Zn2+ in human body, the development of probes for sensitively and selectively detecting Zn2+ would be valuable. Nonetheless, the previously reported chemosensors have difficulty in differentiating Zn2+ over Cd2+ due to their similar physical properties. Therefore, it has been challenging to devise chemosensors selective for Zn2+ over Cd2+.7–20 The toxicity of cyanide anion, CN-, one of the harmful anions, is associated with binding to the iron in cytochrome c oxidase, which interrupts electron transport and causes hypoxia.21,22 In addition, through lungs, gastrointestinal tract and skin, cyanide could be absorbed, which results in convulsion and unconsciousness leading to death.23 Even with its toxicity, cyanide anion is extensively utilized in chemical operations, including manufacture of plastics, extraction of gold and silver, and metallurgy.24–26 Thus, it is very critical to have tactics to selectively and sensitively monitor CN-.27–36 There are traditional techniques, such as atomic absorption spectroscopy and electrochemical systems, have been employed for detection of Zn2+ and CN- in environmental and biological samples.37,38 Such means, however, requires expensive instrument and complex settings for regular analysis. Different from such methods, fluorescence-based detection for analytes has been known to be simple, efficient, and sensitive.39,40 Therefore, the invention of chemosensors able to achieve fluorescent responses to Zn2+ and CN- has received attention. Development of chemosensors for sequentially indicating diverse cations and anions, such as Cu2+/CN-,22,33,41,42 Cu2+/HS-,43 Hg2+/I-,44 Zn2+/HS-,45,46 Al3+/F-,47 Cr3+/F-,48 Ni2+/CN-,49 and Zn2+/H2PO4-/CN-,50 has been recently reported. Especially, due to cost and efficiency, some efforts on the sequential detection employing a single probe has been made. An examples of the sequential sensing of both Zn2+ and CN- via a single chemosensor in pure water, however, has not been reported to date, while only two chemosensors are known to detect sequentially Zn2+ 2


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and CN- in a mixture of water and organic media.51,52 Herein, we report on a new chemosensor, 1, capable of sequentially detecting Zn2+ and CNbased on fluorescence in a near-perfect aqueous media. Our sensor, 1, indicated an enhancement in fluorescence upon incubation with Zn2+ showing the specificity for Zn2+ over Cd2+. In addition, 1 is observed as a chemosensor to monitor Zn2+ in living cells and aqueous systems. Moreover, the resultant 1-Zn2+ complex is shown to recognize CN- with ‘on-off’ fluorescence signals. 2. Experimental Section 2.1. Materials and methods All solvents and reagents used for our studies were obtained from Sigma-Aldrich. 1H and

13

C

NMR spectra were aquired on a Varian 400 MHz and 100 MHz spectrometer [chemical shifts (δ) were recorded in ppm]. Fluorescence responses of 1 to analytes were monitored by a Perkin Elmer model LS45 fluorescence spectrophotometer. UV/Vis and mass spectrometric spectra were collected by a Perkin Elmer model Lambda 25 UV/Vis spectrometer and a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument, respectively. 2.2. Synthesis of 1 3-Aminobenzofuran-2-carboxamide

(180

mg,

1

mmol)

was

introduced

into

4-

diethylaminosalicyl aldehyde (23 mg, 1.2 mmol) in EtOH (2 mL). The resulting mixture was stirred at room temperature for 24 h. Orange precipitates generated were filtered and washed multiple times with MeOH and Et2O, followed by being dried under vacuum to obtain the final product [pure orange solid; yield 0.29 g (82.62 %]]. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 12.77 (s, 1H of hydroxyl group), 9.08 (s, 1H of imine group), 8.01 (s, 1H of aromatic ring), 7.88 (s, 1H of amide group) 7.63 (s, 1H of amide group), 7.47 (m, 4H of aromatic ring), 6.34 (d, J = 8 Hz, 1H of aromatic ring), 6.09 (s, 1H of aromatic ring), 3.39 (s, 4H of alkyl group), 1.11 (s, 6H of alkyl group); 13C NMR (100 MHz, DMSO-d6): δ (ppm) 164.96, 160.44, 153.048, 139.84, 134.08, 132.63, 132.36, 128.21, 124.72, 122.29, 120.54, 119.54, 117.34, 112.89, 31.13. ESI-MS (m/z) for [1 + H+ + 2DMSO + MeOH]+: Calcd 540.22; found, 540.40. 3



2.3. Fluorescence titrations The stock solution of 1 (0.35 mg, 0.001 mmol) was prepared in dimethyl sulfoxide (DMSO) (1 mL). To generate the final concentration (20 µM) of 1, the stock solution was diluted by 10 mM bis-tris, pH 7.0 . For Zn2+ detection, Zn(NO3)2 (20 mM in the bis-tris buffer) was titrated to the solution of 1 (20 µM). After incubation of the mixture for a few seconds, fluorescence measurements at room temperature were conducted. For sensing CN-, Zn(NO3)2 (57 µL of 20 mM) was treated with 1 (final concentration, 20 µM). Then, the solution (0-96 µL) of tetraethylammonium cyanide (TEACN, 0.1 mmol), dissolved in bis-tris buffer (1 mL), was added to the resulting solution of 1 and Zn2+. Upon mixing them for a few seconds, fluorescence spectra at room temperature were recorded.

2.4. UV-vis titration measurements UV-vis spectra of 1 (20 µM, 3 mL) upon titration with Zn(NO3)2 (20 mM, 0-270 µL) in the bistris buffer solution (10 mM, pH 7.0) were obtained at room temperature. In addition, in the case of CN-, changes of the optical spectra were measured at room temperature when TEACN (100 mM, 0-66 µL; bis-tris buffer) was introduced into the solution containing 1 (20 µM) and Zn(NO3)2 (20 mM, 270 µL; bis-tris buffer).

2.5. Job plot measurements Sensor 1 (0.35 mg, 0.001 mmol) was dissolved in DMSO (1 mL) and 600 µL of the sensor 1 (1 mM) was diluted to 29.4 mL solution to make the final concentration of 20 µM. The optical spectra of 1 (20 µM; 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL) in bis-tirs buffer were obtained. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the diluted Zn2+ solution ([Zn(NO3)2] = 20 µM) were mixed with each solution of sensor 1. The total volume of each cell is 3 mL. UV-vis spectra of the mixed solutions were measured at room temperature. To generate the 1-Zn2+ complex, Zn(NO3)2 (30 µL, 20 mM) was transferred to the solution of 1 (20 µM). The solutions (2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL) of the 1-Zn2+ complex were prepared. The solution of the cyanide solution (6 µL, 100 mM) was prepared by dilution with 30 mL of bis-tris buffer. The diluted cyanide solutions (0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 4


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and 2.7 mL) were mixed with each solution of the 1-Zn2+ complex followed by incubation for a few seconds. UV-vis spectra of the resultant solutions were collected at room temperature. The total volume of all solutions was 3 mL. 2.6. Competition experiments Studies of 1’s selectivity for Zn2+ over other metal ions. MNO3 (M = Na and K; 0.02 mmol), M(NO3)2 (M = Mn, Co, Ni, Cu, Zn, Cd, Mg, Ca, and Pb; 0.02 mmol), M(NO3)3 (M = Al, Ga, In, Fe, and Cr; 0.02 mmol) or M(ClO4)2 (M = Fe; 0.02 mmol) was dissolved in bis-tris buffer (1 mL). Each metal solution (57 µL, 20 mM) was introduced to the solution of 1 (3 mL, 20 µM). After mixing them for a few seconds, fluorescence spectra were taken at room temperature. Studies of the selectivity of the Zn2+-1 complex for CN- over other anions. Tetraethylammonium (TEA)X (X = F, Cl, CN, Br, and I; 0.1 mmol), (TBA)X (X = OAc-,BzO-, SCN-, N3-, and H2PO4; 0.1 mmol), NaX (X = NO2-, S2-, SO42-, and HSO3-; 0.1 mmol), and PPi were separately dissolved in bis-tris buffer (1 mL). Each anion solution (90 µL, 100 mM) was added to the solution of the 1-Zn2+ complex (20 µM, 3 mL) followed by treatment of the CN- solution (90 µL, 100 mM). Fluorescence spectra of the resulting solutions after incubation for a few seconds were obtained at room temperature. 2.7 pH effect A series of solutions having different pHs was prepared. Zn(NO3)2 (57 µL, 20 mM in bis-tris buffer) was transferred to each solution of 1 (20 µM) with the desired pHs. After mixing the resulting solution for a few seconds, fluorescence spectra were obtained at room temperature. For the detection of CN-, 60 µL of 1 (20 µM) and 57 µL of Zn(NO3)2 (20 mM) were introduced in bis-tris buffer (2.94 mL). Then, 90 µL of CN- (100 mM) was added to the solution of the 1Zn2+ complex. Fluorescence spectra were recorded right after mixing the resulting solutions at room temperature. 2.8. Cell imaging experiments HeLa cells were purchased from ATCC (Manassas, USA) and maintained in media [Dulbecco Modified Eagle Medium (DMEM); 10% fetal bovine serum (FBS, GIBCO, Grand Island, NY, 5



USA); 100 U/mL penicillin (GIBCO); 100 mg/mL streptomycin (GIBCO)]. The cells grew at 37 °C in a humidified atmosphere containing 5 % CO2. Cells were seeded onto a six well plate at a density of 20,000 cells per 100 µL and incubated for 16 h at 37 °C. Cells were washed with the phosphate-buffered saline (PBS; 2 mL) before addition of 1 and Zn(NO3)2. Cells were treated with 1 (10 µM; 1% v/v final DMSO concentration;) and Zn(NO3)2 (dissolved in water; 1 mM) followed by 5 min incubation. Cell imaging was carried out by an EVOS FL fluorescence microscope (Life technologies) with a GFP light cube [excitation: 470 ± 11 nm; emission: 510 ± 21 nm]. 2.9. Cytotoxicity studies The MTT assay was employed to measure the viability of cells incubated with 1 (MTT = 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide). Cells were plated in a 96 well plate (8,000 cells per 100 µL for 5 min incubation; 1,500 µL cells per 100 µL for 24 h incubation in 100 µL per well). Cells were added with 1 at different concentrations (5, 10, 20, and 50 µM). In addition, 1 (10 µM; 1% v/v DMSO) and Zn(NO3)2 (250, 500, and 1000 µM) were introduced to cells. After 5 min or 24 h incubation, MTT (25 µL of 5 mg/mL in PBS) was introduced into each well, and the plate was incubated at 37 °C for 4 h. Formazan produced by cells was solubilized overnight at room temperature in the dark upon treatment with an acidic solution containing dimethylformamide (DMF; pH 4.5, 50% v/v, aq) and sodium dodecyl sulfate (20% w/v). The absorbance was measured at 600 nm using the microplate reader. Cell survival was calculated compared to cells treated with an equivalent amount of DMSO. 2.10. Reversibility of fluorescent responses Zn(NO3)2 (57 µL, 20 mM) was introduced into the solution of 1 (3 mL, 20 µM). The fluorescence spectrum of the resulting solution was measured at room temperature before and after addition of the solution of ethylenediaminetetraacetic acid (EDTA) disodium salt dehydrate (57 µL, 20 mM). To test the reversibility, an additional solution of Zn2+ (57 µL, 20 mM) was introduced for a minute. The fluorescence spectrum was also taken at room temperature. The same experimental procedure was repeated multiple times. 2.11. Water samples 6


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Fluorescence responses of 1 to Zn2+ in water samples (drinking and tap water) were monitored. The stock solution of 1 (60 µL, 1 mM) and bis-tris buffer (0.3 mL, 100 mM) were mixed with the solutions of samples (2.64 mL). The resulting solutions were allowed to stand for 2 min at 25 ◦C for fluorescence measurements. 2.12. NMR titrations For the titrations with Zn2+, four different equivalents (0, 0.5, 1 and 2) of Zn2+ dissolved in DMSO-d6 (0.3 mL) were separately added into the NMR tubes containing 1 (0.35 mg, 1 µmol) dissolved in DMSO-d6 (1 mL). After incubating the solutions for a few seconds, 1H NMR spectra were measured. In the case of the titrations with CN-, three NMR tubes of 1 [0.35 mg, 0.001 mmol, DMSO-d6 (1 mL] with 1.0 equiv of Zn2+ were prepared and incubated for a minute at room temperature. Three different equivalents (0, 2, and 4) of CN- dissolved in DMSO-d6 (0.3 mL) were separately treated with the solutions of the 1-Zn2+complex for a few seconds. 1H NMR spectra of the resulting solutions were recorded. 2.13. Theoretical studies Theoretical studies on the basis of the hybrid exchange-correlation functional B3LYP 53,54 were performed employing the density functional theory (DFT) and time-dependent DFT (TD-DFT) with the Gaussian 03 program.55 The main atoms were used by the 6-31G (d, p) basis set

56,57

and the zinc element was applied by the LANL2DZ effective core potential (ECP).58–60 All geometries were optimized in the ground states (S0). For our calculations, the solvent effect of water was handled by the Cossi and Barone’s conductor-like polarizable continuum model (CPCM).61,62 To examine the transition energies for the optimized structures of 1 and the 1-Zn2+ complex, the lowest 20 singlet-singlet transitions by TD-DFT calculations at S0 were calculated. In electronic transitions, the contribution of molecular orbitals (MO) was analyzed by GaussSum 2.1.63 3. Results and discussion

7



As shown in Scheme 1, 1 was synthesized through the reaction of 3-aminobenzofuran-2carboxamide with 4-diethylaminosalicyl aldehyde in EtOH in a high yield (83 %). The product 1 was fully characterized by 1H and 13C NMR spectroscopy and mass spectrometry (MS). 3.1. Response of 1 to Zn2+ The fluorescent responses of sensor 1 toward various metal ions, such as Al3+, Ga3+, In3+, Cd2+, Cu2+. Fe2+, Fe3+, Mg2+, Cr3+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+, Pb2+ and Zn2+ were detected in bis-tris buffer (10 mM, pH 7.0) (Figure 1). Upon excitation at 438 nm, sensor 1 exhibited a weak intensity of fluorescence emission (Ф = 0.0071 at 523 nm). When 19 equiv of metal ions were added to the solution of 1, only Zn2+ presented a noticeable increase in fluorescence (Ф = 0.1219). The other metal ions, however, indicated no obvious alteration of fluorescence. Together, our results suggest that sensor 1 is observed to be a “turn-on” chemosensor specific for Zn2+ over the other metal ions, including Cd2+, tested for our studies. To further investigate the chemosensing property of 1, the titration of 1 with Zn2+ was conducted and the change in fluorescence was monitored (Figure 2). With treatment of Zn2+, the fluorescence intensity of 1 at 523 nm was enhanced and saturated with 19 equiv of Zn2+. Through the UV-vis titration, the photophysical property of 1 was also investigated (Figure S1). When Zn2+ was added to the solution of 1, the intensities of the optical peaks at 288 nm and 437 nm were reduced and the absorption bands at 318 nm and 473 nm appeared. The clear isosbestic points were indicated at 297 nm, 340 nm, and 457 nm, implying that one product was formed upon binding of 1 to Zn2+. The Job plot64 indicated a complexation between 1 and Zn2+ with 1:1 stoichiometry (Figure S2), further verified by electrospray ionization mass spectrometry (ESI-MS) (Figure S3). The MS spectrum of 1 upon addition of 1 equiv of Zn2+ displayed the generation of a complex ([1 + Zn2+ + NO3- + NaNO3 + HNO3], m/z = 625.10, calcd, 625.05). The binding constant (3.0 x 104 M-1) of 1 for Zn2+ was determined by the fluorescence titration based on the Benesi-Hildebrand equation (Figure S4).65 This binding value is in the range of those of the previously reported chemosensors for Zn2+(1.0-1.0 x 1012).11,66–68 The detection limit of 1 for Zn2+ was found to be 0.35 µM according to 3σ/slope (Figure S5).69 This detection limit is much lower than the WHO guideline for drinking water (76 µM).70,71 8


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To verify the application of 1 as a chemosensor for Zn2+, the experiments were performed with Zn2+ in the presence of various metal ions, including Al3+, Ga3+, In3+, Cd2+, Cu2+. Fe2+, Fe3+, Mg2+, Cr3+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+, and Pb2+ (19 equiv). As depicted in Figure S6, other metal ions, except for Cu2+, Fe2+, Fe3+, Cr2+ and Co2+, were shown to have small or no obvious interference with the detection of Zn2+. Their interference might be due to the intrinsic quenching properties to fluorescence.72–75 Importantly, 1 could detect Zn2+ over Cd2+. Therefore, 1 could be utilized as a fluorescence sensor selective for Zn2+ even with being competing metal ions present. To validate the binding mode of 1 with Zn2+, the titration experiments were carried out by 1H NMR (Figure 3). With treatment of 1.0 equiv of Zn2+, the protons of the amide (H1) and the phenol (H10) were shifted to downfield (0.33 ppm for H1 and 0.18 ppm for H10), and the proton of imine (H6) was shifted to upfield (0.1 ppm). The observation of these chemical shifts suggested that the oxygen (the phenol group) and nitrogen (the imine and amide) donor atoms might be bound to Zn2+. There was no further chemical shift in the protons with excess amounts of Zn2+ (>1.0 equiv), indicative of the generation of the 1-Zn2+ complex with 1:1 stoichiometry. Based on the results obtained by Job plot, ESI-MS, and 1H NMR, the structure of the 1-Zn2+ complex might be proposed as described in Scheme 2. For applications of 1, the fluorescence of 1 toward Zn2+ was examined at various pHs. The fluorescence intensity of 1-Zn2+ was exhibited to be dependent on pH (Figure S7). The intense and stable fluorescence of 1-Zn2+, observed at pH 7, suggests its potential utilization under physiological conditions.76 To evaluate potential biological applications of 1 to monitor Zn2+, we performed the imaging experiments in living cells. HeLa cells were added with 1 for 5 min before treatment with Zn2+ (Figure 4). Note that no toxicity of sensor 1 (up to 20 µM; particularly at the concentration (i.e., 10 µM) used for imaging) was confirmed upon incubation with cells for 5 min and 24 h (Figure S8). The fluorescence intensity of 1 was observed to be noticeably increased in the presence of Zn2+, compared to that in the absence of Zn2+. These results suggest that sensor 1 could be used for detecting Zn2+ in cellular environments.

9



For

the

reversibility

of

1

for

Zn2+,

we

conducted

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the

experiments

using

ethylenediaminetetraacetic acid (EDTA, 19 equiv) against the 1-Zn2+ complex (Figure 5). When EDTA was introduced to the 1-Zn2+ complex, the fluorescence intensity at 523 nm was diminished. Upon additional treatment of Zn2+ with the resulting solution with EDTA, the original fluorescence intensity of the 1-Zn2+ complex was recovered. Even after multiple cycles (i.e., alternative treatment of EDTA and Zn2+), the reversible fluorescence changes were indicated. Thus, our data imply that 1 is able to reversibly monitor Zn2+. Moreover, to further examine the application of 1 toward real samples, the calibration curve of 1 for the quantitation of indicating Zn2+ was obtained (Figure S9). The fluorescence response of 1 is shown to be proportional to the concentration of Zn2+ [correlation coefficient of R2 = 0.9974 (n = 3)]. To demonstrate the practical application of 1, the testing systems included drinking and tap water. As summarized in Table 1, the satisfactory recoveries and R.S.D. values of the water samples were confirmed suggesting that sensor 1 could be applied to detection of Zn2+ in environmental settings. 3.2. Theoretical calculations To elucidate the mechanism of 1 for sensing Zn2+, theoretical calculations were carried out with the 1:1 stoichiometry of 1 and Zn2+, confirmed through the analyses of Job plot and ESI-MS. The optimized structures of 1 and the 1-Zn2+ complex were first obtained by the B3LYP/6-31G (d, p) method basis set and B3LYP/LANL2DZ/ECP basis set using the Gaussian 03 program (Figure 6). The structure of 1, energetically optimized, displayed a planar structure with the dihedral angle (-1.157o) of 1O, 2C, 3C, and 4C (Figure 6a). The 1-Zn2+ complex indicated a distorted structure showing the dihedral angle (-31.580o) of 1O, 2C, 3C, and 4C (Figure 6b). The singlet excited states were calculated by the TD-DFT methods to determine the electronic properties of 1 and the 1-Zn2+ complex. The contribution of the molecular orbitals (MOs) at the first lowest excited state of 1 and the 1-Zn2+ complex was identified to be the transition of HOMO to LUMO (408.11 nm and 429.72 nm, respectively; Figures S10 and S11). As shown in Figure S12, no distinct variations of the electronic transitions between 1 and the 1-Zn2+ complex were exhibited, whereas only a bathochromic shift (408.11 nm to 429.72 nm) was shown upon chelation of 1 to Zn2+. Therefore, the fluorescent enhancement of 1 for Zn2+ is shown to be 10


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mainly originated from the chelation-enhanced fluorescence (CHEF) effect. 3.3. Fluorescence and optical responses of the 1-Zn2+ complex for CNSome metal complexes reported the selectivity for specific anions (for example, Co-CN, Cu-CN and Al-F).22,47,77 To verify the selectivity of the 1-Zn2+ complex for anions, when 150 equiv of various anions, such as CN-, OAc-, F-, Cl-, Br-, I-, H2PO4-, HP2O73-, BzO-, N3-, SCN-, NO2- and S2-, were added to the 1-Zn2+ complex, CN- showed remarkable ‘on-off’ fluorescent quenching in a near-perfect aqueous solution (Figure 7). In addition, H2PO4- presented to some extents the decreased fluorescence intensity of the 1-Zn2+ complex, but the fluorescence intensity was still discernible. Other anions exhibited either no or little quenching in emission, relative to that of sensor 1 only. Thus, our results indicated that the 1-Zn2+ complex could serve as a sensor for CN- based on fluorescence. To the best of our knowledge, the 1-Zn2+ complex is the first zinc complex to detect cyanide in near-perfect aqueous media (Table S1). In order to investigate the ability of the 1-Zn2+ complex to recognize CN-, fluorescence titrations were performed (Figure 8). Upon treatment of CN- to the solution of the 1-Zn2+ complex, the fluorescence intensity at 523 nm was diminished and saturated at 150 equiv of CN-. The absorbance change of the 1-Zn2+ complex with CN- was monitored by UV-vis titration experiments (Figure S13). As depicted in Figure S13, the two-step alteration of the absorption bands was indicated. With up to 20 equiv of CN-, the absorbance at 437 nm was decreased, while a new absorption band at 490 nm was slightly enhanced showing an isosbestic point at 485 nm (Figure S13b). Upon introduction of up to 100 equiv of CN- the absorbance at 437 nm was continuously decreased, and the absorption peak at 252 nm was increased indicating an isosbetic point at 255 nm (Figure S13c). According to the two-step UV-vis process, the first step might be proposed to be associated with ‘anion-exchange approach’ between NO3- and CNfollowed by ‘displacement approach’ as the subsequent second step for the release of Zn2+ from the 1-Zn2+ complex.78 As shown in Scheme 3, when CN- (< 20 equiv) was incubated with the 1Zn2+ complex, the nitrate anion of the 1-Zn2+ complex was replaced with CN-. Further addition of CN- (> 20 equiv) was observed to generate the stable Zn(CN)x complex by the displacement method with the consequent release of the anionic form (1-). To determine the binding mode of the 1-Zn2+ complex with CN-, Job plot analysis was 11



conducted (Figure S14). The Job plot confirmed a 1:1 stoichiometry ratio of the 1-Zn2+ complex to CN-, further supported by the analysis of ESI-MS (Figure S15). The spectrum of the 1-Zn2+ complex upon addition of 1 equiv of CN-, obtained by ESI(+)MS, showed the generation of the species corresponding to [1 + H+ + 2·DMSO + MeOH]+ (m/z = 540.40; calcd, 540.22), proposing the release of 1 from the 1-Zn2+ complex. From the data of fluorescence titrations, the binging constant between the 1-Zn2+ complex and CN- was calculated as 2.3 x 103 M-1 by Benesi-Hildebrand equation (Figure S16). The detection limit of the 1-Zn2+ toward CN- was found to be ca. 39 µM on the basis of 3σ/slope (Figure S17). To identify the interaction between the 1-Zn2+ complex and CN-, 1H NMR titrations of the 1Zn2+ complex were carried out (Figure S18). Upon addition of 4 equiv of CN-, the 1H NMR spectrum of the 1-Zn2+ complex was changed to that of 1. These results suggest that CN- might detach Zn2+ from the 1-Zn2+ complex through the displacement approach (Scheme 3). To determine the ability of the 1-Zn2+ complex to be a fluorescence sensor for CN-, the competition experiments were conducted in the presence of CN- and other anions, such as OAc-, F-, Cl-, Br-, I-, H2PO4-, BzO-, N3-, SCN-. NO2-, SO42-, HSO3-, PPi, and S2- (Figure 9). Other background anions indicated no obvious interference with the detection of CN-. The data demonstrate that the 1-Zn2+ complex could be a sensor selective for CN- based on fluorescence with the competing anions being present. For practical applications, the pH dependence of the 1-Zn2+ complex was examined at various pHs (Figure S19). The 1-Zn2+ complex exhibited significant fluorescence responses at an environmentally relevant pH (i.e., pH 7).79 These results indicate that the 1-Zn2+ complex could sense CN-. 4. Conclusion A simple and easy-to-make benzofuran-based fluorescent chemosensor, 1, was constructed to sequentially detect Zn2+ and CN- in the near-perfect aqueous media. Sensor 1 is demonstrated to selectively and reversibly detect Zn2+ through turn-on fluorescence with difference between Zn2+ and Cd2+. The detection limit of Zn2+ is below the WHO level for detecting Zn2+ (76 µM) in drinking water. Sensor 1 is verified to be potentially useful for monitoring Zn2+ in real water 12


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samples and living cells. Moreover, the resulting 1-Zn2+ complex can also be used as a fluorescent turn-off sensor for CN- in aqueous media, which is, to the best of our knowledge, the first example that a zinc complex could visualize cyanide based on fluorescence in a nearperfect aqueous solution. The mechanism by the 1-Zn2+ complex for sensing CN- was proposed to be the two-step process, i.e., ‘anion-exchange approach’ and ‘displacement approach’. Therefore, our overall results show the feasibility of developing a new type of chemosensors for the sequential recognition of Zn2+ and CN- showing on-off fluorescence signals in aqueous environments.

Acknowledgments This work is supported by the Korea Environment Industry & Technology Institute (KEITI) through "The Chemical Accident Prevention Technology Development Project" funded by Korea Ministry of Environment (MOE) (No. 2016001970001 to C.K.); the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2017R1A2B3002585 to M.H.L.).

Supplementary Information Supplementary data (additional experimental data) associated with this article can be found. This material is available free of charge via the Internet at http:// ~.

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Scheme 1. Synthetic route to 1.

21



Scheme 2. Proposed binding mode of 1 with Zn2+.

22


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Scheme 3. Proposed mechanism of the 1-Zn2+ complex for sensing CN-.

23



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Table 1. Determination of Zn2+ in water samples.a Sample Drinking water

Tap water a

Zn(II)

added

Zn(II)

found

Recovery

R.S.D. (n = 3)

(µmol/L)

(µmol/L)

(%)

(%)

0.00

0.00

-

-

8.00

8.08

101.0

0.94

0.00

0.00

-

-

8.00

8.25

103.1

3.18

Conditions: [1] = 20 µmol/L in bis-tris buffer (10 mM, pH 7.0).

24


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Figure Captions Figure 1. Fluorescence spectral changes of 1 (20 µM) in the presence of different metal ions (19 equiv) such as Al3+, Ga3+, In3+, Cd2+, Cu2+. Fe2+, Fe3+, Mg2+, Cr3+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+, Pb2+ and Zn2+ with an excitation of 438 nm in bis-tris buffer (10 mM, pH 7). Figure 2. Fluorescence spectral changes of 1 (20 µM) in the presence of different concentrations of Zn2+ ions in bis-tris buffer (10 mM, pH 7). Inset: Fluorescence intensity at 523 nm versus the number of 19 equiv of Zn2+ added. Figure 3. 1H NMR titration of 1 with Zn2+. Figure 4. Fluorescent imaging of HeLa cells incubated with 1 and Zn(II). Cells were incubated with 1 and Zn(II) for 5 min. Conditions: [1] = 10 µM; [Zn(II)] = 1000 µM; 37 °C; 5% CO2. The scale bar is 50 µm. (a) Bright field image of the cells treated with 1. (b) Fluorescence image of treated with 1. (c) Bright field image of the cells treated with both 1 and Zn2+. (d) Fluorescence image of the cells treated with both 1 and Zn2+. Figure 5. Fluorescence intensity changes of 1 (20 µM) after the sequential addition of Zn2+ and EDTA. Figure 6. Energy-minimized structures of (a) 1 and (b) the 1-Zn2+ complex. Figure 7. Fluorescence spectral changes of the 1-Zn2+ complex upon addition of 150 equiv of various anions. Figure 8. Fluorescence spectral changes of the 1-Zn2+ complex in the presence of different concentrations of CN- in bis-tris buffer solution (10 mM, pH 7). Inset: Fluorescence intensity at 523 nm versus the number of 150 equiv of CN- added. Figure 9. Competitive selectivity of the 1-Zn2+ complex toward CN- (150 equiv) in the presence of other anions (150 equiv) in bis-tris buffer (10 mM, pH 7).

25



Figure 1

26


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Figure 2

27



Figure 3

28


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Figure 4

29



Figure 5

30


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(a)

(b)

Figure 6

31



Figure 7

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Figure 8

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Figure 9

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Graphical abstract

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Fluorescent sensor for sequentially monitoring zinc(II) and cyanide

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