Highly selective colorimetric and fluorescent detection for Hg²⁺ in aqueous solutions using a dipeptide-based chemosensor

Abstract

A peptidyl chemosensor (1) showed sensitive colorimetric and fluorescent responses to Hg²⁺ and Ag⁺ among 16 metal ions in aqueous solutions. 1 detected sensitively Hg²⁺ and Ag⁺ by color change with a red shift of the maximum absorbance band at 452 nm; a red color for Hg²⁺ and an orange color for Ag⁺. Furthermore, 1 showed a highly selective colorimetric response to Hg²⁺ among 16 metal ions in aqueous solutions containing NaCl. The selective colorimetric detection of Hg²⁺ was not interfered by other metal ions. About 2 equiv. of Hg²⁺ was enough for the saturation of color and fluorescent change of 1 in aqueous solutions. The dissociation constant for Hg²⁺ was 2.4 × 10⁻¹⁰ M² (R² = 0.993) and the detection limit for Hg²⁺ was 25.6 nM (R² = 0.994). The binding mode of 1 with Hg²⁺ ions was proposed on the basis of the results of pH titration experiment, NMR titration experiment, and mass spectrometry.

---

Introduction

There are high demands for new detection methods of heavy and transition metal ions (HTM) because these metal ions show severe toxicities to living organisms. In particular, Hg²⁺ ion is regarded as one of the most toxic metal ions.^1,2 Current detection methods for Hg²⁺ ions use atomic absorption spectrometry and inductively coupled plasma-mass spectrometry and share some drawbacks of needing expensive instruments and time-consuming procedures.^3,4

In recent years, colorimetric chemosensors for Hg²⁺ ion have attracted considerable attention due to their simple detection with a naked eye, inexpensive instrumentation, and its potential environmental applications.^5–20 Therefore, a range of colorimetric chemosensors for Hg²⁺ ions have been reported.^5–20 As the bioaccumulation of mercury in living organisms including humans occurs mostly through water contamination with Hg²⁺ ions, ideal colorimetric chemosensors for Hg²⁺ ions should dissolve well in aqueous solutions and sensitively detect low levels of Hg²⁺ ions in aqueous solutions.^1,2 Furthermore, most colorimetric chemosensors required high percentages of organic solvents for their operations in aqueous solutions due to poor solubility in water and suffered from interference of other metal ions because of a low binding affinity for Hg²⁺ ions in aqueous solutions.^5–20 In addition, successful colorimetric detection for Hg²⁺ ion in aqueous solution was mostly achieved by chemodosimeters (reactive probes), however they irreversibly detected Hg²⁺ ions.^7–12,19 Therefore, it is highly challenging to synthesize new reversible colorimetric chemosensors for Hg²⁺ ions with high water solubility, high sensitivity, and high selectivity.

In recent years, fluorescent chemosensors for HTM based on amino acids or peptides have received much attention because they dissolved well in aqueous solutions and exhibited selective and sensitive fluorescent responses to specific metal ions.^21–32 Besides, it is very rare to develop colorimetric chemosensors based on peptides for monitoring HTM. In the present study, we synthesized a new colorimetric chemosensor based on a dipeptide containing a disulfide bond for the following reasons. Disulfide bonds of some proteins including human serum albumin participated in the binding site for Hg²⁺ ions with a specific geometry conformation.^33,34 In addition, Hg²⁺ ions interacted directly with a disulfide bond of the protein and sometimes formed a bis(mercapto) bond.³⁴ Thus, we synthesized a 7-nitro-2,1,3-benzoxadiazole (NBD) labelled dipeptide (1) containing a disulfide bond and investigated the detection ability for HTM ions in aqueous solutions. NBD has been used as a chromophore for in vivo and in vitro analysis of amino acid neurotransmitters and biological active peptides because NBD has a visible light absorption, high extinction coefficients, high quantum yield, and high chemical stability.^35–45

Interestingly, the NBD labelled dipeptide (1) selectively detected Hg²⁺ and Ag⁺ among 16 metal ions in aqueous solutions by a colorimetric and fluorescent change. In aqueous buffered solution containing NaCl (1 mM), 1 showed an exclusive selectivity for Hg²⁺ and the detection of Hg²⁺ was not interfered by other heavy metal ion. 1 as a colorimetric chemosensor showed promising properties for detecting Hg²⁺ ions such as fast, high water solubility, reversible detection, high selectivity, colorimetric and fluorescent response to Hg²⁺ ions. Further study indicates that the disulphide bond of 1 played a critical role for the selective recognition of Hg²⁺ ions among various heavy metal ions.

Materials and methods

Materials

1-Hydroxybenzotriazole (HOBt) and Rink Amide MBHA resin (100–200 mesh, 0.5 mmol g⁻¹) were purchased from Bead Tech. Fmoc-Cys(Trt)-OH was purchased from NovaBioChem. N,N′-Diisopropylcarbodiimide (DIC), trifluoroacetic acid (TFA), tri-isopropylsilane (TIS), triethylamine (TEA), diethyl ether, acetonitrile (CH₃CN), piperidine (pip) were purchased from Sigma Aldrich. 4-Chloro-7-nitro-2,1,3-benzoxadiazole (4-ClNBD) and N,N-dimethylformamide (DMF) were purchased from Acros Organics. All perchlorate and chloride salts of metal ions were purchased from Sigma Aldrich and metal ion stock solution was prepared in high purity de-ionized water.

Solid phase synthesis of 1 and 2

1 was synthesized in solid-phase synthesis using Fmoc chemistry (Scheme 1). Activated Fmoc-Cys(Trt)-OH (176 mg, 0.3 mmol) with DIC (47 μl, 0.3 mmol) and HOBt (40 mg, 0.3 mmol) was loaded to Rink amide MBHA resin (200 mg, 0.1 mmol) according to the reported procedure.⁴⁶ After deprotecting the Fmoc group with 25% piperidine in DMF followed by washing, NBD chloride (4-ClNBD, 60 mg, 0.3 mmol) was coupled with the amino group of the amino acid on resin in the presence of triethylamine (83 μl, 0.6 mmol). After completion of the coupling reaction, the cleavage of 2 from the resin was accomplished with TFA/TIS/H₂O (95 : 2.5 : 2.5, v/v/v) at room temperature for 4 h. After removing the excess TFA by N₂, the crude product was precipitated by addition of cold diethyl ether at −20 °C into the cleavage solution. The product was collected by centrifugation. The product was washed with diethyl ether (−20 °C) and then was dried under vacuum. 1 was synthesized from 2 by air oxidation instantly in 50% CN₃CN/H₂O. 1 and 2 were purified with semi-preparative HPLC using water (0.1% TFA)/acetonitrile (0.1% TFA).

Scheme 1 Solid phase synthesis scheme of 1 and 2.

1. Yield 60%, orange solid; m.p. 135 °C; ¹H NMR (400 MHz, 50% CD₃CN/D₂O, 25 °C) δ 9.23 (d, J = 8.8 Hz, 2H), 7.16 (d, J = 8.8 Hz, 2H), 5.56 (s, 2H), 4.56 (t, J = 12 Hz, 2H), 4.36–3.97 (m, 4H); ¹³C NMR (400 MHz, 50% CD₃CN/D₂O, 25 °C) δ 172.49, 144.64, 137.58, 128.78, 124.28, 114.99, 101.16, 56.71, 40.12; ESI-HRMS calcd: m/z 565.06 [M + H⁺]⁺, C₁₈H₁₇N₁₀O₈S₂. Found: m/z 585.07 [M + H⁺]⁺, C₁₈H₁₇N₁₀O₈S₂ (Fig. S1–4†).

2. Yield 80%, yellow orange solid; m.p. 142 °C; ¹H NMR (400 MHz, 50% CD₃CN/D₂O, 25 °C) δ 8.48 (d, J = 8.0 Hz, 1H), 7.54 (d, J = 8.0 Hz, 1H), 3.8 (t, J = 3.6 Hz, 2H), 3.09–3.05 (m, 1H); ¹³C NMR (400 MHz, 50% CD₃CN/D₂O, 25 °C) δ 179.88, 143.68, 132.10, 124.86, 99.77, 81.33, 51.07, 31.21; ESI-HRMS calcd: m/z 284.04 [M + H⁺]⁺, C₉H₁₀N₅O₄S. Found: m/z 284.05 [M + H⁺]⁺, C₉H₁₀N₅O₄S (Fig. S5–8†).

Absorbance and fluorescence measurements

A stock solution of 1 with the concentration of 1 mM was prepared in CH₃CN and stored in a cold and dark place. The concentration of 1 was confirmed by the absorbance at 478 nm for NBD group (λ_max = 478 nm, ε = 18 492 cm⁻¹ M⁻¹). This stock solution was used for absorbance and fluorescence experiments after appropriate dilution. The Absorbance and fluorescence titration was carried out in aqueous buffered solution (10 mM HEPES, at pH 7.4). UV-Vis absorbance spectrum (300–650 nm) of a sample in a 10 mm path length quartz cuvette was measured using a Perkin-Elmer UV-Vis spectrophotometer (model Lambda 45). The emission spectrum (490–650 nm) of the sample was also measured using a Perkin-Elmer Fluorescence spectrometer (model LS 55). The absorbance and emission spectra of 1 were measured in the presence and absence of metal ions (Na⁺, K⁺ and Al³⁺ as chloride anion and Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ as perchlorate anion). Emission spectra were measured by excitation with 469 nm. The slit widths for excitation and emission were 5 nm and 5 nm, respectively.

Ellman’s test

Ellman’s reagent (5,5′-dithiobis-(2-nitrobenzoic acid), DTNB) was used to quantify the thiol groups in the compound.⁴⁷ DTNB reacted with the thiol group in the compounds and formed mixed disulfides, resulting in the release of thionitrobenzoic acid (14 150 M⁻¹ cm⁻¹ at 412 nm), the color of which was measured at 412 nm with a spectrophotometer to quantify the thiol group. A stock solution of the compound was diluted in 10 mM Tris buffer solution at pH 8. After mixing with DTNB solution, an absorbance at 412 nm was measured for the quantification of the thiol group.

Determination of dissociation constant (K_d)

The dissociation constant (K_d) was calculated based on the titration curve of 1 with metal ions. The dissociation constant was determined by a nonlinear least squares fitting of the data with the following equation, as referenced elsewhere.⁴⁸
where A is absorbance signal, A₀ and A_∞ are the final absorbance signal, [G] is total concentration of metal ion (G).

Determination of detection limit

To determine the S/N ratio, the absorbance of free 1 was measured ten times and the standard deviation of blank measurements was determined. Three independent measurements of the absorbance were performed in the presence of metal ions and each average value of the absorbance was plotted as a function of the concentration of metal ions for determining the slope. The detection limit was calculated using the following equation: detection limit = 3σ/m, where σ is the standard deviation of the intensity of free 1, m is the slope between the absorbance at 464 nm vs. concentration.⁴⁹

Results

As shown in Scheme 1, a NBD labelled peptide (1) was synthesized easily using solid phase synthesis with high yield (80%).⁴⁶ 1 and 2 were characterized by ¹H NMR, ¹³C NMR, and ESI-HRMS data, respectively (Fig. S1–8†) and the high purity (>96%) was confirmed by analytical HPLC. As Ellman’s test has been used to quantify the thiol groups of various compounds including peptides, Ellman’s test was used to confirm the complete disulfide bond formation of 1.⁴⁷ As shown in Fig. S9,† an absorbance of 1 at 452 nm was not changed in the presence of DTNB, which indicated that 1 has a disulfide bond by complete oxidation of the thiol group. The disulfide bond of 1 was not reduced by 1 mM DTT as a reducing agent at room temperature for 1 h 1 (10–100 μM) was dissolved well in aqueous solutions containing a small percentage of organic solvent.

UV-Vis spectra of 1 were measured in the absence and presence of various metal ions (Ag⁺, Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, and Zn²⁺) in aqueous buffered solution (10 mM HEPES, pH 7.4) containing 3% (v/v) CH₃CN. 1 showed selective colorimetric responses to Hg²⁺ and Ag⁺ among 16 metal ions; pink color for Hg²⁺ and orange color for Ag⁺ (Fig. 2). Emission spectra of 1 in the presence of various metal ions revealed that 1 showed selective fluorescent responses to Hg²⁺ and Ag⁺ among 16 metal ions in aqueous solutions.

Fig. 1 Proposed binding mode and color change of 1 with Hg²⁺ ions.

Fig. 2 (a) UV-Vis absorbance, (b) fluorescence emission spectra and visible color change of 1 (15 μM) in aqueous buffered solution (10 mM HEPES, pH 7.4) containing 3% (v/v) CH₃CN in the presence of various metal ions (60 μM) (λ_ex = 469 nm, slit 5/10 nm).

To improve selective detection of Hg²⁺, a colorimetric response of 1 to HTM was measured in aqueous buffered solution containing NaCl (1 mM). Interestingly, 1 showed an exclusively selective color change to Hg²⁺ ions, as shown in Fig. 3. In this condition, 1 showed pink color for Hg²⁺ ions and the absorbance induced by Hg²⁺ ions was almost the same as that measured in aqueous solution without NaCl. Emission spectra of 1 in the presence of various metal ions indicated that 1 showed a turn-off response to Hg²⁺ ions in aqueous buffered solution containing NaCl (1 mM). To investigate the role of the disulfide bond of 1 for the selective detection of Hg²⁺ ions, we synthesized NBD labelled cysteine (2) and measured UV-Vis absorbance and emission spectra in the presence of metal ions. The presence of a thiol group of 2 was confirmed by Ellman’s test (Fig. S9†). The colorimetric and fluorescent responses to metal ions were investigated in aqueous buffered solution (10 mM HEPES, pH 7.4). 2 showed sensitive colorimetric and fluorescent responses to several HTM such as Ag⁺, Cd²⁺, Co²⁺, Hg²⁺, Pb²⁺, and Zn²⁺ (Fig. S10†). This indicates that the thiol group of 2 acted as a ligand for various metal ions, whereas the disulfide bond of 1 acted as a selective ligand for Hg²⁺ ions. Cysteine, containing a thiol group, is regarded as an important ligand for several heavy metal ions in various metalloproteines that play an important role in the detoxification and transport of HTM.^50–52

Fig. 3 (a) UV-Vis absorbance, (b) fluorescence emission spectra and visible color change of 1 (15 μM) in aqueous buffered solution (10 mM HEPES, pH 7.4, 1 mM NaCl) containing 3% (v/v) CH₃CN in the presence of various metal ions (60 μM) (λ_ex = 469 nm, slit 5/10 nm).

UV-Vis titration experiments were carried out for measuring the binding affinity of 1 for Hg²⁺ (Fig. 4). Upon addition of Hg²⁺, a gradual red shift (30 nm) of the maximum absorbance at 452 nm with decreasing absorbance intensity was observed and the change of absorbance was complete by 2 equiv. of Hg²⁺. The clear isosbestic point at 480 nm in the titration of 1 with Hg²⁺ suggests that 1 may form one complex with Hg²⁺. The Job’s analysis of 1 with Hg²⁺ showed that the maximum absorbance was observed at 0.67 mole fraction (Fig. S11†). This indicated that 1 might form a 1 : 2 complex with Hg²⁺.

Fig. 4 (a) UV-Vis absorbance and (b) fluorescence emission spectra of 1 (15 μM) in the presence of Hg²⁺ (0, 5, …, 60 μM) in aqueous buffered solution (10 mM HEPES, pH 7.4) containing 3% CH₃CN (λ_ex = 469 nm, slit 5/10 nm).

We also investigated the formation of the complex between 1 and Hg²⁺ by ESI-mass spectrometry (Fig. S12†). After adding Hg²⁺ ions into the solution of 1, a new peak [M + Hg²⁺ − 3H⁺]⁻ was observed in the mass spectrum with negative ion mode. This confirmed that 1 interacted strongly with Hg²⁺ and formed a 1 : 1 complex with Hg²⁺. In general, it was difficult to observe a peak corresponding to the 1 : 2 complex with Hg²⁺ in mass spectrum due to an ionization process in the gas phase. To confirm the 1 : 2 complex between the dipeptidyl chemosensor and Hg²⁺ in solution, we investigated the binding stoichiometry of the monomer form 2 of the dipeptidyl chemosensor with Hg²⁺. As shown in Fig. S13,† upon addition of Hg²⁺, about 1 equiv. of Hg²⁺ required for the complete change of absorbance. This result suggested that the monomer form of the dipeptide might form a 1 : 1 complex with Hg²⁺. Furthermore, the data of the absorbance titration fitted the equation for the 1 : 2 complex model rather than the 1 : 1 complex model. The dissociation constant (K_d) of 1 for Hg²⁺ was calculated to be 2.4 × 10⁻¹⁰ M² (R² = 0.993) (Fig. S14†). Fluorescence titration experiments were carried out. The emission intensity change at 528 nm as the function of Hg²⁺ ions fitted a non-linear equation for the 1 : 2 complex model to calculate the dissociation constant. The dissociation constant was calculated to be 1.9 × 10⁻¹⁰ M² (R² = 0.993) for Hg²⁺ (Fig. S14†). This K_d value was consistent with that measured in UV-Vis titration experiment.

The detecting ability of 1 for Hg²⁺ was investigated in the presence and absence of other metal ions (Fig. 5). The Hg²⁺-dependent color change of 1 was not affected considerably by the presence of other metal ions. 1 showed a pink color to Hg²⁺ even in the presence of Ag⁺, suggesting that 1 could allow naked eye detection of Hg²⁺ even in the presence of Ag⁺ because of the potent binding affinity for Hg²⁺.

Fig. 5 UV-Vis absorbance of 1 (15 μM) in the presence of Hg²⁺ and other metal ions (60 μM) in aqueous buffered solution (10 mM HEPES, pH 7.4) containing 3% CH₃CN.

1 exhibited a sensitive colorimetric response to low levels of Hg²⁺ in an aqueous solution. The detection limit of 1 for Hg²⁺ was determined to be 25.6 nM (R² = 0.994) in aqueous solutions (Fig. S15†). We also investigated the anion effect on the detection for Hg²⁺ in aqueous solutions. The colorimetric response of 1 to Hg(ClO₄)₂, HgCl₂, Hg(OAc)₂ and Hg(NO₃)₂ were measured (Fig. S16†). 1 showed a similar colorimetric response to Hg²⁺ regardless of the nature of the counter anion. This indicates that 1 sensitively detected Hg²⁺ independent of the counter anion.

To investigate the potential practical application of 1, we tested whether 1 could detect Hg²⁺ in real groundwater from local mountain (Odae mountain) in Korea. As the analysis of mercury ions in this groundwater by cold vapor atomic absorption indicated that there was no Hg²⁺ in this groundwater, sample solutions containing Hg²⁺ were spiked into the groundwater at concentrations ranging from 200 nM to 60 μM. As shown in UV-Vis titration experiments (Fig. S17†), the addition of increasing concentration of Hg²⁺ induced a red shift (30 nm) of the maximum absorbance at 452 nm with decrease of the absorbance intensity in the real sample solution. The dissociation constant (K_d) of 1 for Hg²⁺ in a real sample solution was calculated to be 3.9 × 10⁻¹⁰ M² (R² = 0.997) based on the UV-Vis titration result (Fig. S17†). The K_d value for Hg²⁺ measured in groundwater was consistent with that measured in aqueous buffer solution prepared by distilled water. We also measured the detection limit of 1 for Hg²⁺ in this groundwater. The detection limit of 1 for Hg²⁺ was determined to be 34.3 nM (R² = 0.987) in groundwater (Fig. S18†). These analytical data indicated that the dipeptidyl chemosensor 1 showed a sensitive response to Hg²⁺ ions in real groundwater and provided a simple method for the detection of Hg²⁺ in practical water samples.

The reversible sensing ability of 1 for Hg²⁺ ions was tested in aqueous solutions (Fig. S17†). After addition of Hg²⁺ to the solution of 1, a red shift in the absorption was observed, indicating the formation of the complex between 1 and Hg²⁺. And then the addition of EDTA as a chelating agent for Hg²⁺ ions resulted in the return of the metal free spectrum, which confirmed the reversible sensing ability of 1 for Hg²⁺ ions.

We proposed the binding mode of 1 with Hg²⁺ based on the results of the pH titration experiment, ESI mass spectrometry, and UV titration experiment (Fig. 1). As shown in Fig. 6, 1 did not show a colorimetric response to Hg²⁺ in acidic conditions (<pH 4) but showed significant colorimetric responses to Hg²⁺ in neutral and basic conditions. The no response to Hg²⁺ in acidic conditions was due to the amine group (pK_a ≅ 4) of NBD fluorophore of 1 which is protonated in acidic conditions. This result confirmed that the amine group of 1 played a critical role in the binding with Hg²⁺. The red shift in the absorption of 1 in the presence of Hg²⁺ revealed that the chelation of Hg²⁺ with 1 resulted in the increase of the conjugation of the NBD moiety. Thus, the chelation of Hg²⁺ must induce the deprotonation of the NH group of 1. The chelation of the amino group with Hg²⁺ and deprotonation were also confirmed by ESI mass spectrum (Fig. S12†). The peak corresponding to [M + Hg²⁺ − 3H⁺]⁻ in the negative ion mode also suggests that Hg²⁺ chelated the amine group of benzoxadiazole moiety, resulting in the induction of the deprotonation process. The binding mode of 1 with Hg²⁺ was further investigated by NMR spectroscopy (Fig. S20†). ¹H NMR titration experiments were carried out in CD₃OD and DMSO-d6 because this solvent system provided enough solubility for the complex between 1 and Hg²⁺ and 1 showed a colorimetric response to Hg²⁺ ions in this solvent system (Fig. S21†). When Hg²⁺ was added to the solution containing 1, downfield shifts in H(6) and H(5) corresponding to the protons of the benzoxadiazole moiety were observed. The downfield shifts of H(6) (Δ = 0.18 ppm) and H(5) (Δ = 0.16 ppm) may be attributed to the chelation of Hg²⁺ with the amine group of benzoxadiazole moiety, resulting in the induction of the deprotonation process. The downfield shifts in H(3) and H(2), corresponding to the protons of α amino carbon and the methylene of –CH₂–S–S–, suggested chelation of Hg²⁺ with the amino and disulphide groups of 1. Overall, the results support the proposed binding mode of 1 with Hg²⁺, as shown in Fig. 1.

Fig. 6 UV-Vis absorbance spectra of 1 (15 μM) in different pH of 10 mM buffer solution in absence of these metal ions (■) and in presence of Hg²⁺ ().

Conclusions

1 showed sensitively colorimetric and fluorescent responses to Hg²⁺ and Ag⁺ among 16 metal ions in aqueous solutions. In aqueous solution containing NaCl, 1 showed exclusively selective colorimetric and fluorescent responses to Hg²⁺ among 16 metal ions. The colorimetric change of 1 was saturated by about 2 equiv. of Hg²⁺ and the sensitive detection of 1 for Hg²⁺ was not considerably affected by other heavy metal ions. 1 showed a nanomolar detection limit for Hg²⁺ (25.6 nM, R² = 0.994) in aqueous solutions. The binding mode study showed that the dipeptidyl chemosensor formed a 1 : 2 complex with Hg²⁺ and the disulphide bond of the sensor played a critical role in the selective binding of Hg²⁺ ions.

Acknowledgements

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.

Notes and references

1. EPA, Mercury Update: Impact on Fish Advisories (EPA Fact Sheet EPA-823-F-01-011), in Office of Water, EPA, Washington DC, 2001.
2. T. W. Clarkson, Environ. Health Perspect., 2002, 110, 11.
3. J. M. Lo, J. C. Yu, F. I. Hutchison and C. M. Wai, Anal. chem., 1982, 54, 2536.
4. K. A. Anderson, Mercury analysis in environmental samples by cold vapor techniques, in Encyclopedia of analytical chemistry, ed. R. A. Meyes, Wiley, New York, 2006.
5. H. N. Kim, W. X. Ren, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2012, 41, 3210.
6. D. Sareen, P. Kaur and K. Singh, Coord. Chem. Rev., 2014, 265, 125.
7. E. M. Nolan and S. Lippard, J. Chem. Rev., 2008, 108, 3443.
8. M. E. Jun, B. Roy and K. H. Ahn, Chem. Commun., 2011, 47, 7583.
9. K. Kaur, R. Saini, A. Kumar, V. Luxami, N. Kaur, P. Singh and S. Kumar, Coord. Chem. Rev., 2012, 17, 1992.
10. L. Prodi, F. Bolletta, M. Montalti and N. Zaccheroni, Coord. Chem. Rev., 2000, 205, 59.
11. X. Chen, T. Pradhan, F. Wang, J. S. Kim and J. Yoon, Chem. Rev., 2012, 112, 1910.
12. D. Cho and J. L. Sessler, Chem. Soc. Rev., 2009, 38, 1647.
13. J. Jiang, W. Liu, J. Cheng, L. Yang, H. Jiang, D. Bai and W. Liu, Chem. Commun., 2012, 67, 8371.
14. E. Coronado, J. R. Galan-Mascaros, C. Marti-Gastaldo, E. Palomares, J. R. Durrant, R. Vilar, M. Gratzel and M. K. Nazeeruddin, J. Am. Chem. Soc., 2005, 127, 12351.
15. M. Zhu, M. Yuan, X. Liu, J. Xu, J. Lv, C. Huang, H. Liu, Y. Li, S. Wang and D. Zhu, Org. Lett., 2008, 10, 148.
16. S. Y. Moon, N. R. Cha, Y. H. Kim and S. J. Chang, J. Org. Chem., 2004, 69, 181.
17. H. Yang, Z. Zhou, F. Li, T. Yi and C. Huang, Inorg. Chem. Commun., 2007, 10, 1136.
18. J. Y. Park, B. G. In, L. N. Neupane and K. Lee, Analyst, 2015, 140, 744.
19. Y. Yang, K. Yook and J. Tae, J. Am. Chem. Soc., 2005, 127, 16760.
20. H. Li, Y. Li, Y. Dang, L. Ma, Y. Wu, G. Hou and L. Wu, Chem. Commun., 2009, 29, 4453.
21. M. D. Shults, D. A. Pearce and B. Imperiali, J. Am. Chem. Soc., 2003, 35, 10591.
22. B. R. White, H. M. Liljestrand and J. A. Holcombe, Analyst, 2008, 1, 65.
23. S. Jang, P. Thirupathi, L. N. Neupane, J. Seong, H. Lee, W. I. Lee and K. Lee, Org. Lett., 2012, 18, 4746.
24. L. N. Neupane, P. Thirupathi, S. Jang, M. J. Jang, J. H. Kim and K. Lee, Talanta, 2011, 3, 1566.
25. L. N. Neupane, J. Park, J. H. Park and K. Lee, Org. Lett., 2013, 2, 254.
26. D. Kim, J. Seong, H. Lee and K. Lee, Sens. Actuators, B, 2014, 196, 421.
27. J. Hatai, S. Pal, G. P. Jose and S. Bandyopadhyay, Inorg. Chem., 2012, 19, 10129.
28. J. Kim, C. R. Lohani, L. N. Neupane, Y. Choi and K. Lee, Chem. Commun., 2012, 48, 3012.
29. B. P. Joshi, C. R. Lohani and K. Lee, Org. Biomol. Chem., 2010, 8, 3220.
30. M. Yang, P. Thirupathi and K. Lee, Org. Lett., 2011, 19, 5028.
31. C. R. Lohani, J. M. Kim and K. Lee, Tetrahedron, 2011, 67, 4130.
32. Y. Li, X. Zhang, B. Zhu, J. Xue and J. Yan, J. Fluoresc., 2011, 21, 1343.
33. Y. Li, X. Yan, C. Chen, Y. Xia and Y. Jiang, J. Proteome Res., 2007, 6, 2277.
34. R. I. Sperling and Z. Steinberg, Biochemistry, 1974, 13, 2007.
35. J. H. Kim, J. Y. Noh, I. H. Hwang, J. J. Lee and C. Kim, Tetrahedron Lett., 2013, 30, 4001.
36. J. Tan and X. Yan, Talanta, 2008, 1, 9.
37. S. Liu and S. Wu, J. Fluoresc., 2011, 4, 1599.
38. F. Qian, C. Zhang, Y. Zhang, W. He, X. Gao, P. Hu and Z. Guo, J. Am. Chem. Soc., 2009, 4, 1460.
39. Z. Xie, K. Wang, C. Zhang, Z. Yang, Y. Chen, Z. Guo, G. Lu and W. He, New J. Chem., 2011, 3, 607.
40. Z. Xu, G. H. Kim, S. J. Han, M. J. Jou, C. Lee, I. Shin and J. Yoon, Tetrahedron, 2009, 65, 2307.
41. Y. B. Ruan, S. Maisonneuve and J. Xie, Dyes Pigm., 2011, 90, 239.
42. K. Liu, Y. Zhou and C. Yao, Inorg. Chem. Commun., 2011, 14, 1798.
43. N. Wanichacheva, M. Siriprumpoonthum, A. Kamkaew and K. Grudpan, Tetrahedron Lett., 2009, 50, 1783.
44. Z. Wang, M. A. Palacios, G. Zyryanov and P. Anzebacher, Chem.–Eur. J., 2008, 14, 8540.
45. Y. Fu, X. Zeng, L. Mu, X. K. Jiang, M. Deng, J. X. Zhang and T. Yamato, Sens. Actuators, B, 2012, 164, 69.
46. G. B. Fields and R. L. Noble, Int. J. Pept. Protein Res., 1990, 35, 161.
47. G. L. Ellman, Arch. Biochem. Biophys., 1959, 82, 70.
48. A. M. D. L. Pena, T. N. J. B. Zung and I. M. J. Warner, Phys. Chem., 1991, 95, 3330.
49. G. L. Long and J. D. Winefordner, Anal. Chem., 1983, 55, 712A.
50. L. E. Kerper, N. Ballatori and T. W. Clarkson, Am. J. Physiol., 1992, 262, 761.
51. P. Rousselot-Pailley, O. Seneque, C. Lebrun, S. Crouzy, D. Boturyn, P. Dumy, M. Ferrand and P. Delangle, Inorg. Chem., 2006, 45, 5510.
52. L. Rulisek and J. Vondrasek, J. Inorg. Biochem., 1998, 71, 115.

---

Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR, ESI-MS data, and additional experiment result. See DOI: 10.1039/c5ra05842b

---