Colorimetric detection of Fe3+ and Fe2+ and

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Dyes and Pigments 139 (2017) 136e147

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Colorimetric detection of Fe3þ and Fe2þ and sequential fluorescent detection of Al3þ and pyrophosphate by an imidazole-based chemosensor in a near-perfect aqueous solution Tae Geun Jo a, Kwon Hee Bok a, Jiyeon Han b, Mi Hee Lim b, Cheal Kim a, * 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 Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, Republic of Korea

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 October 2016 Received in revised form 26 November 2016 Accepted 26 November 2016 Available online 28 November 2016

A novel chemosensor was designed and synthesized for various analytes: Fe3þ, Fe2þ, Al3þ and pyrophosphate. The sensor showed a selective color change from yellow to orange toward both Fe3þ and Fe2þ in a near-perfect aqueous solution, which could be reusable simply through treatment with ethylenediaminetetraacetic acid. The detection limits (0.27 mM and 0.32 mM) for Fe3þ and Fe2þ were much lower than the environmental protection agency guideline (5.37 mM) in drinking water. The sensor could be used to quantify Fe3þ in real water samples. Moreover, this sensor acted as a ‘turn-on’ and ‘turn-off’ type fluorescent sensor toward Al3þ and pyrophosphate. The sensing mechanism of the sensor for Al3þ could be explained by chelation-enhanced fluorescence effect, which was supported by theoretical calculations. Through a metal-complex displacement method, the sensor-Al3þ complex selectively responded to pyrophosphate over various anions especially including phosphate-based anions. Interestingly, the sensor could be used to sequentially detect both Al3þ and pyrophosphate in the living cells. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Multiple analytes Colorimetric Fluorescent Theoretical calculations Cell imaging

1. Introduction The development of chemical sensors for biologically and environmentally important metal ions and anions has received considerable attention because of their significant roles in industry, medicine, human health and the environment [1e6]. The chemical sensors have analytical merits, such as high selectivity, eidetic recognition, rapid response and real time monitoring [7e11]. Among heavy metals, iron is one of the indispensable metal ions and plays an important function in a wide range of organic and biological processes such as oxygen-carrying, cellular metabolism, enzymatic reaction and various bio-syntheses [12,13]. However, the deficiency or overload of iron in humans cause various diseases such as anemia, liver damages and hemochromatosis [14e16]. For these reasons, detecting iron ions has steadily attracted a great deal of attention in various areas [17,18]. Aluminium, the third most prevalent metallic element in the earth, is extensively used in various fields, including food

* Corresponding author. E-mail address: [email protected] (C. Kim). http://dx.doi.org/10.1016/j.dyepig.2016.11.052 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

packaging, pharmaceuticals, water purification, and the manufacturing industry [19,20]. Because of its wide use, aluminium ion can be easily accumulated in human body. The accumulation of the ion can lead to many hazardous diseases such as Parkinson's disease and Alzheimer's disease [21e23]. Hence, the development of sensors for aluminium is highly desirable in environmental and biological systems [24]. Pyrophosphate (P2O4 7 , PPi), the product of adenosine triphosphate (ATP) hydrolysis under cellular conditions, has been received attention due to its important roles in many crucial reactions, such as energy transduction, metabolic processes and DNA/RNA polymerization [25,26]. Also, the detection and quantification of PPi are of importance in cancer and various disease research areas. Therefore, there have been tremendous efforts to develop the sensors for PPi [27,28]. Recently, a metal-complex displacement method was recognized as one of the successful strategies in the design of detector for PPi, with a specific interaction between metal ions and PPi [29]. Up to now, most of the metal-complex displacement-type probes for PPi used Zn2þ, Fe3þ and Cu2þ as a metal source [25e27,29e32], while only few examples were reported for Al3þ used as a metal source [33e37]. Imidazole derivatives have been utilized as a good sensor to

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detect the metal ions because they have excellent fluorogenic and chromogenic properties [38e41]. Also, the julolidine moiety is well known as a good fluorophore and chromophore [42e44]. In this regard, we designed and synthesized a new chemosensor 1 based on the imidazole and julolidine moieties, which was expected to detect various analytes through the change of unique photophysical properties. Herein, we report on the development of a multiple-target colorimetric and fluorescent chemosensor 1, which could detect Fe2þ and Fe3þ by color change from yellow to orange and Al3þ by fluorescence enhancement in a near-perfect aqueous environment. Moreover, the resulting 1-Al3þ complex could be used for detection of PPi by the displacement reaction. Based on Job plots, UV-vis titrations, ESI-mass spectrometry analyses, 1H NMR titrations and theoretical calculations, their binding structures and sensing mechanisms were proposed and explained. 2. Experimental section 2.1. General information All solvents and reagents (analytical grade and spectroscopic grade) were purchased from Sigma-Aldrich and used without further purification. Both 1H NMR and 13C NMR were recorded on a Varian 400 MHz and 100 MHz spectrometer, respectively. The chemical shifts (d) were recorded in ppm. Absorption spectra were recorded at room temperature using a Perkin Elmer model Lambda 2S UV/Vis spectrophotometer. Electrospray ionization mass spectra (ESI-mass) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQ™ Advantage MAX quadrupole ion trap instrument. Fluorescence measurements were performed on a Perkin Elmer model LS45 fluorescence spectrophotometer. Elemental analysis for carbon, nitrogen, and hydrogen was carried out by using a Flash EA 1112 elemental analyzer (thermo) in Organic Chemistry Research Center of Sogang University, Korea. 2.2. Synthesis of 1 A synthesis of sensor 1 was performed by stirring a mixture of 5amino-1H-imidazole-4-carboxamide (0.13 g, 1 mmol) and 8hydroxyjulolidine-9-carboxaldehyde (0.22 g, 1 mmol) in ethanol (15 mL) for 5 h at room temperature until a red precipitate appeared. The resulting precipitate was filtered and washed with methanol and diethyl ether. The yield: 0.19 g (60.0%) and the melting point: 230e235  C. IR (KBr): n (cm1) ¼ 3136 (m), 2944 (m), 2853 (m), 2360 (s), 1613(s), 1535(s), 1439(s), 1302 (s), 1183(s). 1 H NMR (DMSO-d6, 400 MHz): d 13.66 (s, 1H), 12.61 (s, 1H), 8.69 (s, 1H), 7.93 (m, 3H), 7.25 (s, 1H), 3.42 (m, 4H), 2.63 (m, 4H), 1.86 (m, 4H). 13C NMR (DMSO-d6, 100 MHz): d 161.8, 158.4, 152.0, 151.9, 142.3, 136.1, 134.89, 117.1, 111.6, 106.8, 105.8, 50.9, 50.0, 26.8, 21.0, 20.6, 20.0 ppm. ESI-mass: m/z calcd for C17H19N5O2þHþ ([M þ Hþ]), 326.16; found, 326.10. Anal. Calc. for C17H19N5O2 (325.37): C 62.75; H, 5.89; N, 21.52%; found: C, 62.42; H, 5.96; N, 21.88%. 2.3. Colorimetric sensing for iron 2.3.1. UV-vis titration measurements A stock solution (3 mM) of sensor 1 in DMSO was diluted in bistris buffer solution (10 mM, pH 7.0) to make the final concentration of 20 mM (3 mL). Fe(ClO4)2 (or Fe(NO3)3) (0.01 mmol) was dissolved in bis-tris buffer solution (500 mL). Then, 0e4.8 mL of the Fe2þ (or 0e3.6 mL of the Fe3þ) solution (20 mM) were transferred to the solution prepared above. After stirring the solutions for a few seconds, UV-vis spectra were recorded at room temperature.

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2.3.2. Job plot measurements The stock solutions of sensor 1 (3 mM) in DMSO and Fe(ClO4)2 (or Fe(NO3)3) (20 mM) in bis-tris buffer solution were prepared, respectively. The sensor 1 solution (400 mL) was diluted to 29.6 mL of bis-tris buffer solution (10 mM, pH 7.0) to make the final concentration of 40 mM 60 mL of Fe(ClO4)2 (or Fe(NO3)3) solution (20 mM) was diluted to 29.94 mL of bis-tris buffer solution (10 mM, pH 7.0). 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the sensor 1 solution were taken and transferred to vials. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the Fe2þ (or Fe3þ) solution were added to each sensor 1 solution to make a total volume of 3 mL, separately. After stirring the solutions for a few seconds, UV-vis spectra were recorded at room temperature. 2.3.3. Competition tests MNO3 (M ¼ Na, K, 0.01 mmol) or M(NO3)2 (M ¼ Zn, Cd, Cu, Mg, Co, Ni, Ca, Mn, Pb, 0.01 mmol) or M(NO3)3 (M ¼ Al, Ga, In, Fe, Cr, 0.01 mmol) or M(ClO4)2 (M ¼ Fe, 0.01 mmol) was separately dissolved in bis-tris buffer solution (500 mL). 4.2 mL of each metal solution (20 mM) was diluted to bis-tris buffer solution. 4.2 mL of the Fe2þ (or 3.0 mL of the Fe3þ) solution (20 mM) was added to the solutions prepared above. Then, 20 mL of the sensor 1 solution (3 mM) was added to the mixed solutions to make a total volume of 3 mL. After stirring the solutions for a few seconds, UV-vis spectra were recorded at room temperature. 2.3.4. pH experiments A series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bis-tris buffer. After the solution with a desired pH was achieved, a stock solution of 1 (3 mM) was diluted with buffers to make the final concentration of 20 mM (3 mL). Then, 4.2 mL of the Fe2þ (or 3.0 mL of the Fe3þ) solution (20 mM) was transferred to each sensor 1 solution prepared above. After stirring the solutions for a few seconds, UV-vis spectra were recorded at room temperature. 2.3.5. Determination of Fe3þ in water samples UV-vis spectral measurements of water samples (drinking, tap and artificial polluted water) containing Fe3þ were performed by adding 20 mL of 3 mM stock solution of 1 and 0.60 mL of 50 mM bistris buffer stock solution to 2.38 mL sample solutions. After stirring the solutions for a few seconds, UV-vis spectra were recorded at room temperature. 2.4. Fluorescent sensing for Al3þ and PPi 2.4.1. Fluorescence titration measurements For Al3þ, a stock solution (3 mM) of sensor 1 in DMSO was diluted in bis-tris buffer solution (10 mM, pH 7.0) to make the final concentration of 20 mM (3 mL). Al(NO3)3 (0.1 mmol) was dissolved in bis-tris buffer solution (1 mL). Then, 0e144 mL of the Al3þ solution (100 mM) were transferred to the solution of 1 (20 mM, 3 mL) prepared above. After stirring the solutions for a few seconds, fluorescence spectra were recorded at room temperature. For PPi, a stock solution (100 mM, 104 mL) of Al3þ in bis-tris buffer solution was diluted in the solution of 1 (20 mM, 3 mL). Na4P2O7 (PPi, 0.1 mmol) was dissolved in bis-tris buffer solution (1 mL). Then, 0e12.6 mL of the PPi stock solution (100 mM) were transferred to the solution of 1-Al3þ complex prepared above. After stirring the solutions for a few seconds, fluorescence spectra were recorded at room temperature. 2.4.2. UV-vis titration measurements For Al3þ, a stock solution (3 mM) of sensor 1 in DMSO was

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diluted in bis-tris buffer solution (10 mM, pH 7.0) to make the final concentration of 20 mM (3 mL). Al(NO3)3 (0.1 mmol) was dissolved in bis-tris buffer solution (1 mL). Then, 0e144 mL of the Al3þ solution (100 mM) were transferred to the 1 solution (20 mM, 3 mL) prepared above. After stirring the solutions for a few seconds, UVvis spectra were recorded at room temperature. For PPi, a stock solution (100 mM, 104 mL) of Al3þ in bis-tris buffer solution was diluted in the solution of 1 (20 mM, 3 mL). PPi (0.1 mmol) was dissolved in bis-tris buffer solution (1 mL). Then, 0e25.2 mL of the PPi solution (100 mM) were transferred to the solution of 1-Al3þ complex prepared above. After stirring the solutions for a few seconds, UV-vis spectra were recorded at room temperature.

to make the final concentration of 20 mM (3 mL). Then, 104 mL of the Al3þ stock solution (100 mM) was transferred to each sensor 1 solution prepared above. After stirring the solutions for a few seconds, fluorescence spectra were recorded at room temperature. For PPi, a series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bis-tris buffer. After the solution with a desired pH was achieved, the stock solutions of Al3þ (100 mM, 104 mL) and 1 (3 mM, 20 mL) were diluted and mixed in the buffer solution. Then, 21.6 mL of the PPi stock solution (100 mM) was transferred to each 1-Al3þ solution prepared above. After stirring the solutions for a few seconds, fluorescence spectra were recorded at room temperature.

2.4.3. Job plot measurements For Al3þ, 400 mL of the sensor 1 solution (3 mM) was diluted to 29.6 mL of bis-tris buffer solution to make the concentration of 40 mM 12 mL of Al(NO3)3 solution (100 mM) was diluted to 29.988 mL of bis-tris buffer solution. 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the sensor 1 solution were taken and transferred to vials. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the Al3þ solution were added to each sensor 1 solution separately. Each vial had a total volume of 3 mL. After stirring the solutions for a few seconds, fluorescence spectra were recorded at room temperature. For PPi, 1 (0.02 mmol) in DMSO (1 mL) and Al(NO3)3 (0.02 mmol) in bist-ris buffer were prepared, respectively. The two solutions were mixed to make 1-Al3þ complex. 27, 24, 21, 18, 15, 12, 9, 6 and 3 mL of the 1-Al3þ complex solution (3 mM) were taken and transferred to vials. Each vial was diluted with bis-tris buffer solution to make a total volume of 2.985 mL. PPi (0.01 mmol) was dissolved in bis-tris buffer (1 mL). 3, 6, 9, 12, 15, 18, 21, 24 and 27 mL of the PPi solution were added to each diluted 1-Al3þ solution. Each vial had a total volume of 3 mL. After stirring the solutions for a few, UV-vis spectra were taken at room temperature.

2.4.6. Imaging experiments in living cells HeLa cells (ATCC, Manassas, USA) were maintained in media containing Dulbecco's Modified Eagle Medium (DMEM), 10% fetal bovine serum (FBS, GIBCO, Grand Island, NY, USA), 100 U/mL penicillin (GIBCO), and 100 mg/mL streptomycin (GIBCO). The cells grew in a humidified atmosphere with 5% CO2 at 37  C. Cells were seeded onto 6 well plate (SPL Life Sciences Co., Ltd., South Korea) at a density of 150,000 cells per 1 mL and then incubated at 37  C for 12 h. For fluorescence imaging experiments, cells were first treated with 1 (dissolved in DMSO; 1% v/v final DMSO concentration; 20 mM; at room temperature) for 10 min. After incubation with various concentrations of Al(NO3)3 (dissolved in water; 1% v/v) for 10 min, cells were washed twice with 2 mL of 10 mM bis-tris buffer (pH 7.4, 150 mM NaCl). In case of fluorescence quenching experiments, cells were first treated with 1 (dissolved in DMSO; 1% v/v final DMSO concentration; 20 mM; at room temperature). After 5 min, Al(NO3)3 (dissolved in water; 200 mM; 1% v/v) was incubated with cells for 10 min. Various concentrations of PPi (dissolved in bis-tris buffer; 1% v/v) were introduced to cells for 5 min and the cells were washed with 3 mL of bis-tris buffer three times. Imaging was performed with an EVOS FL fluorescence microscope (Life technologies) using a GFP light cube [excitation 470 (±11) nm; emission 510 (±21) nm].

2.4.4. Competition tests For Al3þ, MNO3 (M ¼ Na, K, 0.1 mmol) or M(NO3)2 (M ¼ Zn, Cd, Cu, Mg, Co, Ni, Ca, Mn, Pb, 0.1 mmol) or M(NO3)3 (M ¼ Al, Ga, In, Fe, Cr, 0.05 mmol) or M(ClO4)2 (M ¼ Fe, 0.1 mmol) was separately dissolved in bis-tris buffer solution (1 mL). 104 mL of each metal solution (100 mM) was diluted to 2.772 mL of bis-tris buffer solution, separately. 104 mL of the Al3þ solution (100 mM) was taken and added to the solutions prepared above. Then, 20 mL (3 mM) of the sensor 1 was taken and added to the mixed solutions. After stirring the solutions for a few seconds, fluorescence spectra were recorded at room temperature. For PPi, a stock solution (100 mM, 104 mL) of Al3þ in bis-tris buffer solution was diluted in the solution of 1 (20 mM, 3 mL). The tetraethylammonium salts (0.1 mmol) of F, CN, Cl, Br and I, the tetrabutylammonium salts (0.1 mmol) of AcO, H2PO 4,  BzO, N and the sodium salts (0.1 mmol) of SH, 3 and SCN adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and PPi were separately dissolved in bis-tris buffer (1 mL). 21.6 mL of each anion solution (100 mM) was dilluted to 2.978 mL of 1-Al3þ complex solution, separately. Then, 21.6 mL of the PPi solution (100 mM) was taken and added to the solutions prepared above. After stirring the solutions for a few seconds, fluorescence spectra were recorded at room temperature. 2.4.5. pH effect For Al3þ, a series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bis-tris buffer. After the solution with a desired pH was achieved, the stock solution (3 mM) of 1 was diluted in buffers

2.4.7. 1H NMR titrations For 1H NMR titrations of sensor 1 with Al3þ, three NMR tubes of sensor 1 (3.3 mg, 0.01 mmol) dissolved in DMSO-d6 were prepared and then three different concentrations (0, 0.005 and 0.01 mmol) of Al(NO3)3 dissolved in DMSO-d6 were added to each sensor 1 solution. After stirring the solutions for a few seconds, 1H NMR spectra were recorded at room temperature. 2.4.8. Theoretical calculations All DFT/TDDFT calculations based on the hybrid exchange correlation functional B3LYP [45,46] with 6-31G** basis set [47,48] were carried out using Gaussian 03 program [49]. In vibrational frequency calculations, there was no imaginary frequency for the optimized geometries of 1 and 1-Al3þ, suggesting that these geometries represented local minima. For all calculations, the solvent effect of water was considered by using the Cossi and Barone's CPCM (conductor-like polarizable continuum model) [50,51]. To investigate the electronic properties of singlet excited states, timedependent DFT (TDDFT) was performed in the ground state geometries of 1 and 1-Al3þ. The twenty lowest singlet states were calculated and analyzed. The GaussSum 2.1 [52] was used to calculate the contributions of molecular orbitals in electronic transitions. 3. Results and discussion The chemosensor 1 was synthesized by the condensation

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reaction of 8-hydroxyjulolidine-9-carboxaldehyde with 5-amino1H-imidazole-4-carboxamide in ethanol at room temperature (Scheme 1), and characterized by 1H and 13C NMR, ESI-mass spectrometry and elemental analysis. 3.1. Colorimetric sensing for Fe2þ and Fe3þ The UV-vis spectral changes of 1 were investigated with the addition of various metal ions such as Al3þ, Ga3þ, In3þ, Zn2þ, Cd2þ, Cu2þ, Fe2þ, Fe3þ, Mg2þ, Cr3þ, Co2þ, Ni2þ, Naþ, Kþ, Ca2þ, Mn2þ and Pb2þ in bis-tris buffer solution (10 mM, pH 7.0). As shown in Fig. 1, there were no significant spectral and color changes in the presence of most metal ions, whereas Fe2þ and Fe3þ ions caused both distinct spectral changes and pronounced color changes from yellow to orange. These results suggested that 1 could be used as a colorimetric chemosensor for Fe2þ and Fe3þ ions via direct visualization in a near-perfect aqueous media. First of all, the UV-vis titration experiments were conducted to investigate the binding property of 1 with Fe3þ ions (Fig. 2). Upon the addition of Fe3þ, the absorbance band at 433 nm decreased gradually, while the absorbance at 325 nm and 520 nm increased with two clear isosbestic points. The points indicated that the binding between 1 and Fe3þ ions afforded only one species. The UV-vis variation of 1-Fe2þ complex was also nearly identical to that of 1-Fe3þ (Fig. S1). The peaks at 520 nm with high molar extinction coefficients, 3.3  103 M1cm1 (ε520nm) for Fe3þand 2.5  103 M1cm1 (ε520nm) for Fe2þ, are too large to be Febased DMSO-d6 transitions. Thus, these new peaks might be attributed to a metal-to-ligand charge-transfer (MLCT) [53e57], resulting in the color change of the solutions. The binding stoichiometries between 1 and Fe2þ/3þ were determined by the Job plot analyses [58], which showed 2:1 complexations of 1 and Fe2þ/3þ (Fig. S2). The stoichiometries of 1 and Fe2þ/3þ were further confirmed by ESI-mass data analyses (Fig. 3 and Fig. S3). The positive mass spectrum of 1 for Fe3þ indicated that the first major peak at m/z ¼ 704.20 was assignable to 2∙12HþþFe3þ [calcd, m/z: 704.23] (Fig. 3). The positive-ion mass spectrum of 1 with Fe2þ was also nearly identical to that of Fe3þ (Fig. S3). These results led us to propose that Fe2þ of the complex formed from the reaction of Fe2þ with 1 might be rapidly oxidized to Fe3þ by air [54,55]. To verify our proposal, we examined the spectral changes of the solution of 1 with Fe2þ under the degassed conditions (Fig. S4). When Fe2þ reacted with 1 under the anaerobic condition, there was no spectral and color change. Under the aerobic condition, however, we observed the significant spectral and color changes of the solution, which were identical to those of the Fe3þ-2∙1 complex. These results indicated that the Fe2þ-2∙1 complex formed under the degassed conditions was oxidized to the Fe3þ-2∙1 complex in air. In addition, the time-dependent changes for the reaction of sensor 1 with Fe2þ/3þ was evaluated (Fig. S5). The formation time

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(120 s) of Fe3þ-2∙1 obtained from the reaction of 1 with Fe2þ was 3 times slower than that (40 s) obtained from the reaction of 1 with Fe3þ. This observation further proved that Fe2þ of the Fe2þ-2∙1 complex formed from the reaction of Fe2þ with 1 might be rapidly oxidized to Fe3þ by air. Based on the Job plot, ESI-Mass spectrometry analysis, and the degassing experiment, we proposed the structure of Fe3þ-2∙1 complex as shown in Scheme 2. On the basis of the Li's equations [59], the association constants (K) of 1 with Fe2þ and Fe3þ were calculated as 1.4  104 and 2.8  104, respectively (Fig. S6). The detection limits of sensor 1 for Fe2þ and Fe3þ were calculated to be 0.32 mM and 0.27 mM on the basis of 3s/K (Fig. S7) [60]. These values are much lower than the guideline (5.37 mM) set by the Environmental Protection Agency (EPA) for iron in drinking water [61]. In order to check the selectivity of 1 towards Fe3þ ions over the various metal ions, competitive studies were carried out (Fig. 4). In presence of the competing metal ions, there was no significant interference in the detection of Fe3þ. Only Cu2þ influenced somewhat the interaction of 1 with Fe3þ. Fe2þ also showed a similar tendency to Fe3þ (Fig. S8). These results indicated that sensor 1 could be efficiently used for the selective detection of Fe2þ/3þ. For practical applications, the effect of pH on the color and absorbance changes of 1 to Fe2þ and Fe3þ was studied in various pH range (2e12) (Fig. S9). The Fe2þ/3þ-2∙1 complexes exhibited the stable absorption and color changes between pH 3 and pH 11, which warrant that 1 could detect Fe2þ and Fe3þ via naked eye in a wide range of pH (3e11). For the practical application of the sensor 1 toward Fe3þ, the real sample analysis was performed for quantitative measurement of Fe3þ. As shown in Fig. S10, a good calibration curve was obtained for the determination of Fe3þ. Then, 1 was applied for the determination of Fe3þ in both tap and drinking water samples. As shown in Table 1, a suitable recovery and Relative Standard Deviation (R.S.D.) values were obtained. These results indicated both the suitability and applicability of the sensor for the detection of Fe3þ in the real samples. The reusability of sensor is an imperative ability to develop the practical chemosensor. Hence, the reversible ability of 1 was examined by adding ethylenediaminetetraacetic acid (EDTA) to the solution of Fe3þ-2∙1 complex (Fig. S11). The complex solution color was recovered from orange to yellow (the original color of 1). Upon the sequentially alternative addition of Fe3þ and EDTA, the absorbance and color changes of the solution were almost reversible even after several cycles. These results showed that sensor 1 could be used as a recyclable sensor for Fe3þ. 3.2. Fluorescence and absorption studies of 1 toward Al3þ The fluorescence selectivity of sensor 1 toward various metal ions was also conducted in bis-tris buffer solution (10 mM, pH 7.0). Sensor 1 exhibited no fluorescence intensity upon excitation at 455 nm (Fig. 5). When 180 equiv of various metal ions (Al3þ, Ga3þ,

Scheme 1. . Synthetic procedure of 1.

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Fig. 1. (a) UV-vis absorption and (b) color changes of 1 (20 mM) upon addition of 1.4 equiv of different metal ions in buffer (10 mM bis-tris, pH 7.0).

Fig. 2. UV-vis absorption change of 1 (20 mM) with Fe3þ ions (0e1.2 equiv) in bis-tris buffer (10 mM, pH 7.0).





















þ

þ

In , Zn , Cd , Cu , Fe , Fe , Mg , Cr , Co , Ni , Na , K , Ca2þ, Mn2 and Pb2þ) were added to the solution of 1, only Al3þ

Fig. 3. Positive-ion ESI-mass spectrum of 1 (100 mM) upon addition of 1 equiv of Fe3þ.

induced a significant fluorescence enhancement at 489 nm. This result indicated that sensor 1 could act as a “turn-on” type fluorescence chemosensor for Al3þ in a near-perfect aqueous solution. Importantly, 1 could selectively detect Al3þ from Ga3þ and In3þ,

T.G. Jo et al. / Dyes and Pigments 139 (2017) 136e147

141

Scheme 2. . Proposed structure of Fe3þ-2∙1 complex.

Fig. 4. (a) UV-vis absorption (at 520 nm) and (b) color changes of 1 (20 mM) upon addition of Fe3þ (1.0 equiv) in the absence and presence of other metal ions (1.0 equiv).

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Table 1 Determination of Fe3þ in water samples.a Sample

Fe(III) added (mmol/L)

Fe(III) found (mmol/L)

Recovery (%)

R.S.D (n ¼ 3) (%)

Tap water

0.00 3.00 0.00 3.00 0.00 6.00

0.00 2.94 0.00 3.26 0.00 6.01

e 98.0 e 108.7 e 100.1

e 2.2 e 6.4 e 1.5

Drink water Artificial polluted waterb a b

Condition: [1] ¼ 20 mmol/L in bis-tris buffer (10 mM, pH 7.0). Prepared by deionized water, 6.0 mmol/L Zn2þ, Cd2þ, Pb2þ, Hg2þ and 9.0 mmol/L Naþ, Kþ, Ca2þ, Mg2þ.

Fig. 5. Fluorescence spectra of 1 (20 mM) upon addition of 180 equiv of different metal ions in buffer (10 mM bis-tris, pH 7.0).

while many chemosensors reported for Al3þ have difficulty in discriminating Al3þ from Ga3þ and In3þ due to their similar chemical behavior [62e64]. To understand the spectroscopic properties of 1 towards Al3þ, we conducted a fluorescence titration. Upon the addition of Al3þ into 1, the fluorescence intensity at 489 nm steadily increased until the amount of Al3þ reached at 180 equiv (Fig. S12). Then, UV-vis titration experiment of 1 with Al3þ was also conducted. The addition of Al3þ to a solution of 1 showed that the absorption band at 433 nm decreased and a new band at 464 nm increased gradually (Fig. S13). Two isosbestic points were observed at 336 nm and 443 nm, indicating that only one product was generated from 1 upon binding to Al3þ. The Job plot [58] was carried out to determine the binding mode between 1 and Al3þ (Fig. S14). The fluorescence intensity at 489 nm reached the maximum at the molar ratio of 0.5, indicating a 1:1 binding mode. Moreover, the binding mode between 1 and Al3þ was further confirmed by ESI-mass spectrometry analysis (Fig. S15). The positive mass spectrum showed that two peaks at m/z ¼ 470.7 and at m/z ¼ 490.7 were assignable to 1þ 2HþþAl3þþNO 3 þ Na þ2H2O [calcd, m/z: 471.12] and 1HþþAl3þþNO þ DMSO [calcd, m/z: 491.13], respectively. Based on 3 the fluorescence titration measurement, the association constant (K) of the 1-Al3þ complex was determined as 2.3  102 M1 through Benesi-Hildebrand equation [65] (Fig. S16), which was within the range of those (102~108) previously reported for Al3þ binding chemosensors [66e69]. The detection limit (3s/k) [60] of sensor 1 as a fluorescence sensor for the analysis of Al3þ ions was found to be 9.24 mM (Fig. S17). In order to check the possible interference from other metal ions in the detection of Al3þ, we examined the fluorescence intensity change of 1 to Al3þ in the presence of other competitive metal ions

(Fig. S18). In the presence of Cu2þ, Fe2þ and Fe3þ, the fluorescence emission intensity was inhibited, and some interference was observed in presence of Ga3þ. However, most other competitive metal ions did not interfere with the detection of Al3þ. To investigate the practicality of 1 as sensor, the detecting ability was tested in various pH (2e12). (Fig. S19). In the absence of Al3þ, sensor 1 exhibited no fluorescence intensity over the pH rage of 2e12. Upon the addition of Al3þ, an intense and stable fluorescence of 1-Al3þ was observed in the pH range of 4.0e11.0 which warrants its application under physiological conditions. To evaluate the feasibility of 1 for monitoring Al3þ in biological systems, we conducted the cell imaging experiments. Cells were preincubated with 1 for 10 min prior to addition of various concentrations of Al3þ (0e200 mM) (Fig. 6). The background fluorescence in the cells was not observed in absence of Al3þ. By contrast, the fluorescence intensity in the cells gradually increased as the Al3þ concentration increased from 0 to 200 mM. These observations showed that sensor 1 could be an appropriate and biocompatible detector to successfully sense Al3þ in biological systems. To get the insight into the binding mechanism of 1 with Al3þ, 1H NMR titration was conducted (Fig. 7). Upon the addition of Al3þ, the proton H9 disappeared and the integral of the proton H1 decreased by half. The rest protons H2, H3, H4, and H8 showed slightly downfield shifts. These results suggested that the N atom of the amide moiety and the O atom of the hydroxyl group of 1 might coordinate to Al3þ ion. When more than 1 equiv of Al3þ were added, there was

Fig. 6. Fluorescent responses of 1 to Al3þ in HeLa cells. Cells were preincubated with 1 for 10 min prior to addition of various concentrations of Al3þ. Conditions: [1] ¼ 20 mM; [Al3þ] ¼ 0, 100 and 200 mM; 37  C; 5% CO2. The scale bar is 50 mm.

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Fig. 7. 1H NMR titrations of 1 with the addition of Al3þ (0, 0.5 and 1.0 equiv).

no longer a spectral change, which indicates the 1:1 binding mode of 1 to Al3þ. To further elucidate the fluorescent sensing mechanism of 1 to Al3þ, geometric optimizations and theoretical calculations were performed for 1 and 1-Al3þ complex by utilizing the B3LYP/6-31g** methods with CPCM/water. The energy-minimized structure (1C, 2 N, 3C, 4 N ¼ 25.049 ) of 1 showed a twisted shape (Fig. 8). After combined with Al3þ, the structure o f 1-Al3þ complex was flattened (1C, 2 N, 3C, 4 N ¼ 0.343 ), which showed that 1 coordinated to Al3þ via two N atoms in the Schiff-base and the amide moiety and the O atom in the hydroxyl group. The singlet excited states of 1 and 1Al3þ complex were investigated using the TD-DFT (time dependent-density functional theory) methods. The first lowest excited state of both 1 and 1-Al3þ was determined for HOMO / LUMO transition (404.91 nm for 1 (Fig. S20) and 429.78 nm for 1Al3þ (Fig. S21)), which indicated an intramolecular charge transfer (ICT) transition. The calculations were consistent with the experimental absorption wavelengths of 1 and 1-Al3þ. On the other hand, there was no obvious change in the electronic transitions between 1 and 1-Al3þ complex except the hypochromic shift (404.91e429.78 nm) upon chelating of 1 with Al3þ (Fig. S22). Based on these calculations, the fluorescent sensing mechanism could be explained by chelation-enhanced fluorescence (CHEF) effect [44,70,71]. The chelation of 1 with Al3þ caused the rigid structure and the restriction of the free rotation around the C]N bond, which might inhibit the non-radiative process. Based on a Job plot, ESI-mass spectrometry analysis, 1H NMR titration, and theoretical calculations, we propose the binding structure of 1-Al3þ complex in Scheme 3.

3.3. Fluorescence and absorption studies of 1-Al3þ complex toward PPi The fluorescence variation of 1-Al3þ complex was examined upon the addition of various anions, such as PPi, AMP, ADP, ATP,   2   CN, AcO, F, Cl, Br, I, BzO, N 3 , SCN , H2PO4 , HS , NO3 , SO4 and PO3 in a near-perfect aqueous solution. Upon the addition of 4 36 equiv of various anions to 1-Al3þ solution, only PPi induced a fluorescence quenching (Fig. 9). These results suggested that 1-Al3þ complex could be a selective chemosensor for PPi over other various anions, especially including phosphate-based anions. The fluorescence titration of 1-Al3þ with PPi was investigated to understand fluorescence spectral variation (Fig. S24). Upon the addition of PPi into 1-Al3þ complex, the fluorescence intensity at 489 nm steadily decreased until the amount of PPi reached 36 equiv. The UV-vis titration experiments were also conducted (Fig. S25). Upon the addition of PPi to the solution of 1-Al3þ, the absorbance at 466 nm considerably decreased, and a new band at 433 nm appeared and reached a maximum at 36 equiv of PPi. Two isosbestic points were observed at 440 nm and 496 nm, indicating that only one species was formed by the reaction of 1-Al3þ with PPi. The final UV-vis spectrum of 1-Al3þ with PPi was almost identical to that of 1 itself. This result led us to propose that 1 was released from the 1-Al3þ complex by the chelation of PPi with Al3þ (Scheme 3). The binding mode of PPi and 1-Al3þ was determined by Job plot method, which revealed a 1:1 stoichiometric ratio (Fig. S26) [58]. Further, the demetallation of 1-Al3þ complex by PPi was confirmed by an ESI-mass spectrometry analysis (Fig. S27). When PPi was added into 1-Al3þ solution, the positive ion mass spectrum showed

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Fig. 8. The energy-minimized structures of (a) 1 and (b) 1-Al3þ.

Scheme 3. . Proposed sequential sensing process of Al3þ and PPi by 1.

that a peak at m/z 256.87 was assigned to P2O3 7 (PPi)þ Al3þþNaþþMeOH [calcd. 256.92], indicating the demetallation of 1-Al3þ complex by PPi. Based on UV-vis titrations, Job plot and ESImass spectrometry analysis, we proposed the sensing mechanism of the 1-Al3þ complex toward PPi (Scheme 3). The association constant was calculated to be 3.70  103 M1 from a Benesi-

Hildebrand plot (Fig. S27) [65]. The detection limit (3s/k) for PPi was found to be 20.5 mM (Fig. S28) [60]. To examine the practical applicability of 1-Al3þ complex as a PPi selective sensor, competitive experiments were carried out in the presence of PPi (36 equiv) with competing anions (36 equiv) (Fig. S29). There was no interference for the detection of PPi by 1-

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PPi in living cells. To the best of our knowledge, this is the first example to sequentially detect both Al3þ and PPi in living cells [33e37]. 4. Conclusion

Fig. 9. Fluorescence spectra of 1-Al3þ upon addition of 36 equiv of different anions in buffer (10 mM bis-tris, pH 7.0).

Al3þ, while ADP and ATP showed a slight inhibition. These results indicated that the detection of PPi by 1-Al3þ was not disturbed from various anions even including phosphate-based anions. In order to investigate the pH dependence of 1-Al3þ toward PPi, the pH effect was conducted in a wide range of pH. The optimal range for the fluorescent sensing of PPi by 1-Al3þ was turned out to be between pH 4 and pH 11 (Fig. S30). Based on the response of 1-Al3þ complex toward PPi at the physiological pH range in aqueous solution, we further investigated whether the complex could detect the PPi in live cells (Fig. 10). When the cells were preincubated with 1 and Al3þ, green fluorescence could be observed. After PPi (0e200 mM) was gradually added to the preincubated cells, the fluorescence in the cells reduced and finally disappeared. These results indicated that 1-Al3þ complex has a huge potential as a suitable detector for monitoring

We have developed a simple, selective and efficient sensor 1, which can detect Fe2þ/3þ via direct visualization and Al3þ and PPi by fluorescence change in a near-perfect aqueous media. The senor 1 could selectively and preferentially bind with Fe2þ/3þ among other competitive metal ions, which can detect Fe2þ/3þ ions at low concentration ca. 0.28 mM, which is lower than the EPA guidelines for drinking water of 5.37 mM. Based on the various experiments, we have demonstrated that Fe2þ of the 1-Fe2þ complex was rapidly oxidized to Fe3þ in air. In addition, 1 could successfully detect Fe3þ in real water samples and be also reusable simply by treatment with EDTA. Moreover, 1 could act as an ‘off-on-off’ type fluorescence sensor for sequential detection of Al3þ and PPi in a nearperfect aqueous solution. The sensor 1 showed high selectivity toward Al3þ over other competitive metal ions including Ga3þ and In3þ. The fluorescent sensing mechanism of 1 with Al3þ could be explained by the effect of CHEF with theoretical calculations. Furthermore, 1-Al3þ complex could selectively detect PPi in the presence of other various anions, especially including phosphatebased anions. The spectroscopic experiments demonstrated that 1-Al3þ complex detected PPi through a metal-complex displacement method. Importantly, this is the first example that sensor 1 could sequentially detect both Al3þ and PPi in living cells, to the best of our knowledge. Therefore, we believe that the sensor 1 might contribute to the development of the multiple target chemosensors in aqueous and biological environments. Acknowledgements Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2014R1A2A1A11051794, NRF2015R1A2A2A09001301 and NRF-2014S1A2A2028270) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.11.052. References

Fig. 10. Fluorescent imaging of HeLa cells incubated with 1 and Al3þ followed by addition of PPi. Fluorescence of the cells treated by 1 and Al3þ was quenched by PPi introduction. Cells incubated with 1 (for 5 min) followed by addition of Al3þ for 10 min were treated with various concentrations of PPi. Conditions: [1] ¼ 20 mM; [Al3þ] ¼ 200 mM; [PPi] ¼ 0, 100 and 200 mM; 37  C; 5% CO2. The scale bar is 50 mm.

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