Efficient Ensemble System Based on the Copper Binding Motif for

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Efficient Ensemble System Based on the Copper Binding Motif for Highly Sensitive and Selective Detection of Cyanide Ions in 100% Aqueous Solutions by Fluorescent and Colorimetric Changes Kwan Ho Jung and Keun-Hyeung Lee* Bioorganic Chemistry Laboratory, Center for Design and Applications of Molecular Catalysts, Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, South Korea

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 5, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.analchem.5b01982

S Supporting Information *

ABSTRACT: A peptide-based ensemble for the detection of cyanide ions in 100% aqueous solutions was designed on the basis of the copper binding motif. 7-Nitro-2,1,3-benzoxadiazole-labeled tripeptide (NBD-SSH, NBD-SerSerHis) formed the ensemble with Cu2+, leading to a change in the color of the solution from yellow to orange and a complete decrease of fluorescence emission. The ensemble (NBD-SSH−Cu2+) sensitively and selectively detected a low concentration of cyanide ions in 100% aqueous solutions by a colorimetric change as well as a fluorescent change. The addition of cyanide ions instantly removed Cu2+ from the ensemble (NBD-SSH−Cu2+) in 100% aqueous solutions, resulting in a color change of the solution from orange to yellow and a “turn-on” fluorescent response. The detection limits for cyanide ions were lower than the maximum allowable level of cyanide ions in drinking water set by the World Health Organization. The peptide-based ensemble system is expected to be a potential and practical way for the detection of submicromolar concentrations of cyanide ions in 100% aqueous solutions.

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high percentage of organic solvent in sample media for their proper operation for the detection of cyanide ions in aqueous media.26−40 Although some of them could work in 100% aqueous media, they did not show enough sensitivity for the maximum allowed cyanide ions (1.9 μM) in drinking water set by the WHO.41−46 Only a few chemosensing ensembles based on CdTe quantum dots, graphite carbon(IV) nitride, and a fluorescent solid probe were reported to satisfy the submicromolar detection limit for cyanide ions in 100% aqueous media.47−49 As they are all of the solid probe type, there are some limitations of handling and quantifying the probe itself. Thus, it is highly challenging to develop new chemosensing ensembles for the sensitive and selective detection of submicromolar concentrations of cyanide ions in 100% aqueous solutions. In the present work, we have first used a peptide as a component of the Cu2+-based ensemble for the detection of cyanide ions for the following reasons. Generally, peptides are highly soluble in aqueous solutions and have potent binding affinity for specific analytes in aqueous solutions.50−61 In

yanide is considered to be one of the most toxic anions even in very small concentrations because it binds to metallic cofactors in metalloenzymes, impairing important cell functions.1 Cyanide exerts severe toxicity to mammals, leading to loss of consciousness and eventually death through inhibition of cytochrome c oxidase in the mitochondrial respiratory chain.1 As cyanide was used in various important industrial processes and was naturally generated, it accidently contaminated aquatic environments.2 Thus, the World Health Organization (WHO) strictly regulated the maximum allowable level (1.9 μM) of cyanide ions in drinking water.3 In recent years, optical sensing methods for cyanide ions using chemodosimers (reactive probes) and chemosensors have received great attention due to their simplicity, high sensitivity, and inexpensive instrumentation.4−25 Among them, displacement methods using the Cu2+-based chemosensing ensembles have been extensively studied for highly selective detection of cyanide ions in aqueous solutions.26−49 In these displacement methods, the chemosensor formed a stable ensemble with Cu2+, which showed little fluorescence by the quenching effect of Cu2+.33 The addition of cyanide ions removed Cu2+ from the ensemble due to the formation of very stable [Cu(CN)x]n−, resulting in an increase in fluorescence. On the other hand, almost all reported displacement methods using the Cu2+-based ensembles showed some limitations. Most of them required a © XXXX American Chemical Society

Received: May 28, 2015 Accepted: August 31, 2015

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DOI: 10.1021/acs.analchem.5b01982 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

(0.1 mmol) was added 4-chloro-7-nitro-2,1,3-benzoxadiazole (100 mg, 0.5 mmol, 5 equiv) in DMF (3 mL) containing DIPEA (104.5 μL, 0.6 mmol, 6 equiv), and the resulting solution was kept for 6 h at room temperature. After the coupling reaction was complete, the deprotection and the cleavage from the dried resin were accomplished by treatment with a mixture of TFA/TIS/H2O (95:2.5:2.5, v/v/v) at room temperature for 4 h. After removal of the excess TFA by N2 blow-off, the crude product was precipitated by the addition of cold ether. The solid precipitate was centrifuged, washed with ether, and lyophilized under vacuum. The crude product was further purified with semipreparative HPLC using a water (0.1% TFA)/acetonitrile (0.1% TFA) gradient. The peptide mass was characterized with an ESI-TOF HRMS spectrometer (Compact, Bruker). The homogeneity (>98%) of the peptide compound was confirmed by analytical HPLC on a C18 column (Shimadzu). Characterization data for NBD-SSH: orange solid; mp 133 °C; 1H NMR (400 MHz, D2O, 25 °C) δ 8.66 (s, 1H), 8.55 (d, J = 8.8 Hz, 1H), 7.35 (s, 1H), 6.41 (d, J = 8.4 Hz, 1H), 4.75−4.71 (m, 2H), 4.55−4.53 (m, 1H), 4.16− 4.15 (m, 2H), 3.92−3.89 (m, 2H), 3.35−3.30 (m, 1H), 3.21− 3.13(m, 1H); 13C NMR (100 MHz, D2O, 25 °C) δ 174.0, 171.6, 171.2, 163.0, 162.6, 144.2, 143.7, 138.1, 133.6, 128.6, 122.4, 117.4, 61.6, 61.0, 59.4, 55.9, 52.4, 26.4; HRMS (ESITOF) calcd 492.1586 (m/z) [M + H+]+, obsd 492.1586 (m/z) [M + H+]+. General UV/Vis and Fluorescent Measurement. A stock solution of NBD-SSH at a concentration of 2 mM was prepared in one-third deionized water and stored in a cold and dark place. The stock solution was used for absorption and fluorescence experiments after appropriate dilution. The concentration of NBD-SSH was confirmed by absorbance at 478 nm for the NBD group. UV/vis absorption spectra (300− 650 nm) of the samples in a 10 mm path length cuvette were measured using a PerkinElmer UV/vis spectrophotometer (model Lambda 45). The fluorescence emission spectra (480− 700 nm) of the samples in a 10 mm path length cuvette were measured using a PerkinElmer luminescence spectrophotometer (model LS 55) with excitation at 469 nm. The slit widths for excitation and emission are mentioned in the captions of the figures. All absorbance and fluorescence measurements were carried out in aqueous buffered solutions (10 mM HEPES, pH 7.4) except pH titration experiments. The pH titration experiments were carried out using the proper aqueous buffer systems. The response to Fe3+ was measured in the presence of citrate, which acts as a stabilizing reagent for Fe3+ at pH 7.4. The absorption and fluorescence emission spectra of the ensemble (NBD-SSH−Cu2+) in the presence of anions were measured with continuous stirring to reach an equilibrium of the system. Determination of the Dissociation Constant (Kd). The dissociation constant was calculated on the basis of the fluorescent titration curve of NBD-SSH with metal ions. The fluorescence signal, F, is related to the equilibrium concentration of the complex (HL) between the host (H) and metal ion (L) by the following expression:


addition, the binding affinity of the peptides for a specific analyte could be tuned by changing the amino acid sequences. We prepared a chemosensing ensemble using the aminoterminal copper and nickel (ATCUN) binding peptide motif and Cu2+ and investigated the detection ability for cyanide ions in aqueous solutions. The new ensemble based on the ATCUN binding peptide motif showed several promising sensing properties: (1) highly selective and sensitive detection of cyanide ions among various anions in 100% aqueous solutions, (2) colorimetric detection as well as fluorescent “turn-on” type detection of cyanide ions, and (3) a submicromolar detection limit for cyanide ions in 100% aqueous solutions. To the best of our knowledge, this is the first example of a peptide-based ensemble system for the sensitive detection of cyanide ions in 100% aqueous solutions. As shown in Figure 1a, GlyGlyHis was a well-known peptide sequence for the ATCUN binding motif.62,63 X-ray crystallo-

Figure 1. Structures of (a) the complex between Cu2+ and the ATCUN motif (GGH) and (b) NBD-SSH.

graphic studies revealed that an N-terminal amino group, two amide groups, and an imidazole group were involved in the formation of a square planar Cu2+ complex,62 and further study confirmed that the ATCUN binding motif corresponded to the NH2-X-X-His sequence (where X is any amino acid).63 A tripeptide (SerSerHis) as a binding motif for Cu2+ was synthesized instead of GlyGlyHis because SerSerHis was expected to have better solubility in aqueous solutions. As shown in Figure 1b, 7-nitro-2,1,3-benzoxadiazole (NBD) was conjugated at the N-terminal of the peptide. As the NBD fluorophore has a high quantum yield and absorbs visible light,64−68 we expected that the NBD-labeled peptide ensemble could detect cyanide ions by a fluorescent change as well as a colorimetric change.

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EXPERIMENTAL SECTION Reagents. Fmoc-Ser(tBu)-OH, Fmoc-His(Trt)-OH, 1hydroxybenzotriazole (HOBt), and Rink amide MBHA resin (100−200 mesh, 0.45 mmol/g) were purchased from Bead Tech. N,N′-Diisopropylcarbodiimide was purchased from TCI. 4-Chloro-7-nitro-2,1,3-benzoxadiazole and N,N-dimethylformamide (DMF) were purchased from Acros Organics. The other reagents for solid-phase synthesis, including trifluoroacetic acid (TFA), triisopropylsilane (TIS), diethyl ether, N,N-diisopropylethylamine (DIPEA), and piperidine, were purchased from Sigma-Aldrich. All perchlorate salts of metal ions, tetrabutylammonium salts and potassium salts of cyanide, thiocyanate, fluoride, chloride, bromide, and iodide, and sodium salts of nitrate, acetate, bicarbonate, sulfate, perchlorate, phosphate, and arsenate were purchased from Sigma-Aldrich. Solid-Phase Synthesis and Characterization. NBD-SSH was synthesized by solid-phase peptide synthesis using Fmoc chemistry according to the reported procedure.69 The coupling of 4-chloro-7-nitro-2,1,3-benzoxadiazole was performed by applying the following procedure: To the resin-bound peptide

F = Fo + ΔF[HL]

[HL] = 0.5[KD + L T + HT − {( −KD − L T − HT)2 − 4L THT}1/2 ] B


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Analytical Chemistry where Fo is the emission intensity of the free sensor and ΔF is the change of emission intensity due to the formation of HL, LT and HT are the total concentrations of metal ions (L) and chemosensor (H). Determination of the Detection Limit. To determine the signal-to-noise ratio (S/N), the emission intensities of NBDSSH in the presence of 1 equiv of Cu2+ were measured 10 times and the standard deviation of the blank measurements was determined. Three separate measurements of the emission intensity were performed in the presence of cyanide ions, and the mean intensity was plotted as a concentration of cyanide ions to determine the slope. The detection limit was calculated using the following equation:70


detection limit = 3σ /m

where σ is the standard deviation of the emission intensity of NBD-SSH in the presence of 1 equiv of Cu2+ and m is the slope between the emission intensity and concentration.

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RESULTS AND DISCUSSION Colorimetric and Fluorescent Responses of NBD-SSH to Cu2+ in Aqueous Buffered Solutions. NBD-SSH was easily synthesized in high yield (72%) using solid-phase peptide synthesis (Scheme S1).69 NBD was directly conjugated at the N-terminal of the peptide (SerSerHis). As the free N-terminal amine in the ATCUN binding motif plays an important role in the tight binding with Cu2+,62,63 we expected that the binding event of Cu2+ to NBD-SSH and the displacement event by cyanide ions would induce the change of the absorbance and/or the fluorescence of the NBD. NBD-SSH was characterized by ESI high-resolution MS (ESI-HRMS), 1H NMR, and 13C NMR (Figures S1−S3). The photophysical property of NBD-SSH was investigated by absorption and fluorescence spectroscopy. As a millimolar concentration of NBD-SSH was fully soluble in water, the spectroscopic experiments were carried out in 100% aqueous buffer solution without an organic cosolvent. The absorption spectra and the fluorescence emission spectra of NBD-SSH in the presence of an increasing concentration of Cu2+ were measured. As shown in Figure 2, free NBD-SSH showed a strong absorption band at 460 nm and a strong emission at 525 nm. Upon the addition of Cu2+, a gradual red shift (12 nm) of the maximum absorption and an increase of absorbance were observed, resulting in a color change from yellow to orange. The fluorescence emission at 525 nm decreased as the concentration of Cu2+ increased. About 1.0 equiv of Cu2+ was required for the complete colorimetric and fluorescent changes to occur. The binding stoichiometry of the complex between NBD-SSH and Cu2+ was investigated by Job’s plot analysis (Figure S4). When the mole fraction of Cu2+ was increased to 0.5, the intensity variation at 525 nm reached the maximum. This result indicated that NBD-SSH formed a 1:1 complex with Cu2+. Assuming the formation of a 1:1 complex, the dissociation constant of NBD-SSH for Cu2+ was calculated to be 327 nM (R2 = 0.975) by nonlinear least-squares fitting of the emission intensity at 525 nm (Figure S5). This result indicated that NBD-SSH had a potent binding affinity for Cu2+ ions. The colorimetric and fluorescent responses of NBD-SSH to various metal ions (Ag+, Al3+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, and Zn2+) were investigated in aqueous solutions (Figure S6). Among the tested metal ions, only Cu2+ sensitively induced the red shift of

Figure 2. (a) Absorption spectra and (b) fluorescence spectra of NBD-SSH (30 μM) with increasing concentration of Cu2+ in aqueous buffered solutions (10 mM HEPES, pH 7.4) (λex = 469 nm, slit 10 nm/5.5 nm).

the absorption bands and the complete decrease of fluorescence emission. Even though the peptide showed some responses to Ni2+ and Hg2+, the spectrum changes induced by these metal ions were much smaller than that induced by Cu2+. In addition, the colorimetric and emission responses to Cu2+ were not considerably interfered with by other metal ions, including Ni2+ and Hg2+ (Figure S7). Overall the results indicate that NBDSSH has a more potent binding affinity for Cu2+ than any other metal ions and NBD-SSH can be utilized as the component for the ensemble with Cu2+ for the detection of cyanide ions. Colorimetric and Fluorescent Responses of the Ensemble to Cyanide Ions in Aqueous Buffered Solutions. We prepared the ensemble (NBD-SSH−Cu2+) by addition of 1 equiv of Cu2+ to the aqueous solutions containing NBD-SSH and investigated the responses to cyanide ions in 100% aqueous solutions at pH 7.4. As shown in Figure 3a, a gradual blue shift of the maximum absorption band and a decrease of absorbance were observed by increasing the concentration of cyanide ions. About 20 equiv of cyanide ions was required for the complete change of the colorimetric response. Meanwhile, the fluorescence emission at 525 nm increased gradually up to 325-fold upon the addition of cyanide ions and reached the saturation point at about 20 equiv of cyanide ions, as shown in Figure 3b. The absorption and fluorescent emission spectrum of the ensemble in the presence of cyanide returned to the original spectrum of NBD-SSH. The fluorescent intensity change induced by cyanide ions as a function of time was measured. The response time of the peptide-based ensemble to cyanide ions was less than a few minutes in 100% aqueous solutions (Figure S8). This result indicated that the ensemble showed a better response than the C


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Figure 4. (a) Absorption spectra, (b) fluorescence spectra (λex = 469 nm, slit 10 nm/5.5 nm), (c) visible color change under ambient light, and (d) emission color change under UV light (λex = 365 nm) of a UV lamp of NBD-SSH−Cu2+(30 μM) in the presence of various anions (22 equiv) in aqueous buffered solutions (10 mM HEPES, pH 7.4).

Figure 3. (a) Absorption spectra and (b) fluorescence spectra of NBD-SSH−Cu2+ (30 μM) with increasing concentration of cyanide ions in aqueous buffered solutions (10 mM HEPES, pH 7.4) (λex = 469 nm, slit 10 nm/5.5 nm).

previously reported ensemble system to cyanide ions because the previously reported ensemble required a high number of equivalents of cyanide ions (∼100 equiv) or long incubation times (>60 min) to achieve a complete response.35,40,42−44 To evaluate the selective detection of cyanide ions in 100% aqueous solutions, the absorption spectra and the fluorescence emission spectra of the ensemble (NBD-SSH−Cu2+) were measured in the presence of various anions (F−, Cl−, Br−, I−, NO3−, CN−, SCN−, AcO−, HCO3−, SO42−, ClO4−, HPO42−, HAsO42−). As shown in Figure 4, only cyanide ions induced the blue shift of the absorption band and the turn-on response of the fluorescence emission among various competing anions, whereas the other anions did not induce any significant change in the absorption and emission spectra. Figure 4c presents a visible color change of the ensemble under ambient light, and Figure 4d presents a visible emission change of the ensemble under UV light. The ensemble solution containing cyanide ions displayed a yellow color and emitted bright fluorescence. In contrast, the ensemble solution in the absence or in the presence of other competing anions displayed an orange color and emitted little fluorescence. We investigated whether the peptide-based ensemble selectively detected cyanide ions in the presence of the other competing anions (Figure 5). The colorimetric responses to cyanide ions were not changed considerably in the presence of other anions. In addition, the fluorescent turn-on response to cyanide ions was not affected by the presence of other anions. The results indicated that the ensemble (NBD-SSH−Cu2+) selectively detected cyanide ions in the presence of various

Figure 5. (a) Absorbance and (b) emission intensity of NBD-SSH− Cu2+ (30 μM) in the presence of 22 equiv of other anions (white bar) and in the presence of cyanide ions and other anions (light gray bar) in aqueous buffered solutions (10 mM HEPES, pH 7.4) (λex = 469 nm, slit 10 nm/5.5 nm). The data were obtained from the mean of three independent measurements, and the error bars represent the standard deviations.

competitive anions in 100% aqueous solutions by colorimetric as well as fluorescent responses. The detection limits of the ensemble (NBD-SSH−Cu2+) for cyanide ions in 100% aqueous solutions were investigated using the S/N ratio and the signal-to-background ratio (S/B). The detection limit for cyanide ions using the S/N ratio was calculated to be 24.9 nM (R2 = 0.992) in 100% aqueous solutions on the basis of 3σ/m, where σ is the standard deviation of the blank measurements and m is the slope of the intensity at 530 nm as a function of the concentration of cyanide ions (Figure S9).70 Since the detection limit using the S/B ratio is regarded as the most reliable and accurate,25,46 the detection limit for cyanide ions using the S/B ratio was also determined on the basis of a plot of the ratio of the fluorescence intensity (I/I0) at D


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ensemble, resulting in the return to the Cu2+-free absorbance and emission intensity of the peptide. ESI high-resolution mass spectrometric experiments were carried out to investigate the formation of the ensemble (NBDSSH−Cu2+) and the displacement of Cu2+ by cyanide ions. As shown in Figure 8 and Figure S11, the mass spectrum of NBD-


525 nm versus the concentration of cyanide (Figure 6). The detection limit for cyanide ions was calculated to be 724 nM

Figure 6. Ratio of the emission intensity of NBD-SSH−Cu2+ (0.5 μM) at 525 nm as a function of the cyanide ion concentration in aqueous buffered solutions (10 mM HEPES, pH 7.4) (λex = 469 nm, slit 15 nm/15 nm).

Figure 8. ESI-HRMS spectra of NBD-SSH (10 μM) with Cu2+ (1 equiv).

SSH in the presence of Cu2+ showed a peak at 551.0567 (m/z) corresponding to [M + Cu2+ − 3H+]− in the negative ion mode. The isotopic patterns of [M + Cu2+ − 3H+]− were in agreement with the calculated values. This result confirmed that NBD-SSH formed a 1:1 complex with Cu2+. After addition of cyanide ions into the solution of the ensemble (NBD-SSH− Cu2+), a new peak at 492.1587 (m/z) corresponding to [M + H+]+ was observed in the positive ion mode (Figure S12). This result indicated that cyanide ions successfully removed Cu2+ from the ensemble. The binding mode and working mechanism of the ensemble (NBD-SSH−Cu2+) for the detection of cyanide ions were proposed by spectroscopic data and the ESI-HRMS spectrum, as shown in Scheme 1. The pH titration result showed that the

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(R = 0.998) on the basis of the intensity ratio measured in 100% aqueous solutions. The detection limit for cyanide ions was lower than the maximum allowable level of cyanide ions (1.9 μM) in drinking water set by the WHO.3 This result indicated that the ensemble has hypersensitivity for cyanide ions in 100% aqueous solutions. Binding Mode and Working Mechanism of the Ensemble for the Detection of Cyanide Ions. The fluorescent and colorimetric responses to Cu2+ and cyanide ions were measured at various pH values to investigate the working pH range and the binding mode (Figure 7 and Figure

Scheme 1. Proposed Binding and Detection Mode of Cyanide Ions Using the Peptide-Based Ensemble

Figure 7. Emission intensity of NBD-SSH−Cu2+ (30 μM) in the presence and absence of cyanide ions (22 equiv) at different pH values (λex = 469 nm, slit 10 nm/5.5 nm).

imidazole group of the peptide played a critical role in the binding with Cu2+. Considering the binding mode of the ATCUN motif (GGH) with Cu2+,62,63 NBD-SSH might form a similar square planar complex with Cu2+ using two amide groups, an imidazole group, and an amino group of the NBD fluorophore. Cu2+-induced deprotonation of the NH of NBD might induce an increase in the conjugation of the benzoxadiazole part, resulting in a red shift of the absorption band.65 Simultaneously, the bound Cu2+ in the peptide quenched the emission of the NBD. After the addition of cyanide ions, the absorption and emission intensity returned to the metal-free spectrum. The mass spectrum including the peak corresponding to [M + H+]+ confirmed the removal of Cu2+ from the ensemble by cyanide ions.

S10). At acidic pH, NBD-SSH did not display colorimetric and fluorescent responses to Cu2+. Considering the pKa value of the imidazole group (pKa ≅ 6) of His, the imidazole group of the peptide (NBD-SSH) might play a critical role in the binding to Cu2+. The protonated imidazole group could not bind with Cu2+ ions. At neutral and basic pH values, NBD-SSH showed significant colorimetric and fluorescent responses to Cu2+. The addition of cyanide ions to the ensemble induced the color change of the solution from orange to yellow and complete enhancement of fluorescence emission. This result indicated that cyanide ion successfully removed Cu2+ ions from the E


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Analytical Chemistry To the best of our knowledge, this is the first example in which a peptide has been used as a Cu2+-based ensemble for the detection of cyanide ions. The peptide-based ensemble sensitively and selectively detected submicromolar concentrations of cyanide ions in 100% aqueous solutions by a colorimetric change as well as a fluorescent change. Given the growing interest in detecting cyanide ions in water, the peptidebased ensemble is expected to be a potential tool for the detection of cyanide ions. Further work is currently under way to investigate the correlation between peptide sequences and the binding affinity for Cu2+ for the sensitive detection of cyanide ions.

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CONCLUSION In summary, we successfully developed a novel peptide-based ensemble for the detection of cyanide ions in 100% aqueous solutions. The NBD-labeled ATCUN binding motif (NBDSSH) formed an ensemble with Cu2+, leading to a change in the color of the solution from yellow to orange and a complete decrease of the fluorescence emission. The ensemble (NBDSSH−Cu2+) sensitively and selectively detected submicromolar concentrations of cyanide ions in 100% aqueous solutions. The addition of cyanide ions successfully removed Cu2+ from the ensemble, leading to a change in the color of the solution from orange to yellow and a turn-on fluorescent response. The detection limits for cyanide ions were lower than the maximum allowable level of cyanide ions in drinking water set by the WHO.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b01982. Experimental procedure, 1H NMR, 13C NMR, and HRMS spectra, and additional spectroscopic data of NBD-SSH (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82-32-867-5604. Phone: +8232-860-7674. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant (2014R1A2A1A11051727) from the National Research Foundation and a grant (2015000540007) from the Korea Environmental Industry and Technology Institute.

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REFERENCES

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