Introduction

Zinc is the second most abundant transition metal ion in human body [1–7]. Zinc has attracted a great deal of attention [8–13], because it plays very important role in variety of physiological and pathological processes such as apoptosis, catalytic function of protein, enzyme regulation and so on [14–20]. Especially, labile Zn2+ has been implicated in signaling processes in the brain, immunological function and gene transcription [21–24]. Its deficiency generates unbalanced metabolism, which in turn can induce retarded growth in children, brain disorders and high blood cholesterol, and also be implicated in various neurodegenerative disorders such as Alzheimer’s disease, epilepsy, ischemic stroke, and infantile diarrhea. Excess zinc may also cause serious neurological disorders such as Alzheimer’s and Parkinson’s diseases [25–28]. Thus, a technique to detect and visualize free zinc ions would be highly demanded [29–34].

To date, many chemosensors have been reported to detect trace amount of Zn2+ . Many of them, however, have disadvantages such as insufficient sensitivity or selectivity, and inhibition problems from other transition metal ions, especially Cd2+, which is in the same group of the periodic table and shows similar properties to Zn2+ [35–38]. Thus, low cost and easily prepared Zn2+ selective fluorescence chemosensors are needed for convenience [39–45].

In view of this necessity and as part of our effort devoted to zinc ion recognition, we have considered the combination of a quinoline moiety known as having desirable photo-physical properties as a fluorophore group and a N, N′-dimethyl ethylene amine as a binding site (Scheme 1) [46–48]. Especially, we expected that the N, N′-dimethyl ethylene amine group, being hydrophilic in nature, would increase water-solubility of the chemosensor.

Scheme 1
scheme 1

Synthetic procedure of 1

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Herein, we report a new chemosensor 1 for Zn2+, composed of the quinoline and N, N′-dimethyl ethylene amine moieties. We have observed its prominent fluorescence enhancement in the presence of zinc ion, while there was no enhancement in the presence of other metal ions. In particular, it was able to distinguish Zn2+ from Cd2+.

Experiments

Reagents and Instrument

All the solvents and reagents (analytical grade and spectroscopic grade) were obtained commercially and used as received. 1H and 13C NMR spectra were recorded on a Varian 400 MHz and 100 MHz spectrometer, respectively and chemical shifts were reported in ppm, relative to tetramethylsilane Si(CH3)4. Absorption spectra were recorded at 25 °C using a Perkin Elmer model Lambda 25 UV/Vis spectrometer. The emission spectra were recorded on a Perkin-Elmer LS45 fluorescence spectrometer. Electrospray ionization mass spectra (ESI-MS) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument. 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.

Synthesis of Receptor 1

3-Chloro-N-(quinolin-8-yl)propanamide (1.17 g, 5 mmol) and potassium iodide (8 mmol, 1.33 g) were dissolved in MeCN (20 mL) and stirred for 1 h. Then, N,N′-dimethylethane-1,2-diamine (0.44 mL, 5 mmol) and sodium hydroxide (0.24 g, 6 mmol) were added in the resulting solution. It was stirred for 12 h at room temperature. The solvent was removed under reduced pressure to obtain bright yellow oil, which was dissolved in methylene chloride and washed twice with water. Then, the solution was purified by silica gel column chromatography (10:1 v/v CH2Cl2-CH3OH) (Scheme 1). The solvent was evaporated under vacuo. Yield: 0.97 g (68 %). 1H NMR (400 MHz, DMSO-d 6 , ppm): δ = 10.59 (s, 1 H), 8.85 (d, J = 4 Hz, 1 H), 8.51 (d, J = 8 Hz, 1 H), 8.27 (d, J = 8 Hz, 1 H), 7.52 (d, J = 4 Hz, 1 H), 7.50 (t, J = 4 Hz, 1 H), 7.38 (t, J = 8 Hz, 1 H), 2.97 (t, J = 6.4 Hz, 4 H), 2.79 (t, J = 6.4 Hz, 4 H), 2.41 (s, 6 H); 13C NMR (100 MHz, CD3CN, ppm): 163.09, 162.82, 162.52, 149.15, 137.13, 136.03, 127.05, 122.31, 122.12, 116.65, 55.77, 49.11, 48.73, 43.54, 35.09, 27.59. LRMS (ESI): m/z calcd for C16H22N4O-H++Zn2+: 547.23; found 547.20. Elemental analysis calcd (%) for C16H23N4: C, 67.11; H, 7.74; N, 19.56; found: C, 66.87; H, 7.92; N, 19.83.

Fluorescence Titration of 1 Toward Zn2+

The receptor 1 (1.72 mg, 0.006 mmol) was dissolved in MeCN (2 mL) and 20 μL of the receptor 1 (3 mM) was diluted to 2.98 mL MeCN/bis-tris buffer solution (3:7, v/v) to make the final concentration of 20 μM. Zn(NO3)2∙6H2O (11.9 mg, 0.04 mmol) was dissolved in MeCN (2 mL) and 3–36 μL of the Zn2+ solution (20 mM) was transferred to each receptor solutions prepared above. After mixing them for a few seconds, fluorescence spectra were taken at room temperature.

UV-vis Titration of 1 Toward Zn2+

The receptor 1 (1.72 mg, 0.006 mmol) was dissolved in MeCN (2 mL) and 30 μL of the receptor 1 (3 mM) were diluted to 2.97 mL MeCN/bis-tris buffer solution (3:7, v/v) to make the final concentration of 30 μM. Zn(NO3)2∙6H2O (11.9 mg, 0.04 mmol) was dissolved in MeCN (2 mL) and 0.9–9 μL of the Zn2+ solution (20 mM) were added to the receptor 1 solution prepared above. After mixing them for a few seconds, UV-vis spectra were obtained at room temperature.

Job Plot Measurements

The receptor 1 (1.72 mg, 0.006 mmol) was dissolved in MeCN (2 mL). 500 μL of the receptor solution was taken and diluted with MeCN/bis-tris buffer solution (3:7, v/v) to make the final concentration of 50 μM. The total volume of the receptor solution was 30 mL. Zn(NO3)2∙6H2O (11.9 mg, 0.04 mmol) was dissolved in MeCN (2 mL). 75 μL of the zinc solution was taken and diluted with MeCN/bis-tris buffer solution (3:7, v/v). The total volume of zinc solution was 30 mL. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the 1 solution were taken and transferred to vials. 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the zinc solution were added to each diluted 1 solution. Each vial had a total volume of 3 mL. After reacting them for a few seconds, fluorescence spectra were taken at room temperature.

Competition with Other Metal Ions

The receptor 1 (1.72 mg, 0.006 mmol) was dissolved in MeCN (2 mL) and 20 μL of this solution (3 mM) was diluted with 2.98 mL of MeCN/bis-tris buffer solution (3:7, v/v) to make the final concentration of 20 μM. MNO3 (M = Na, K, 0.04 mmol) or M(NO3)2 (M = Zn, Cd, Cu, Mg, Co, Ni, Ca, Mn and Pb, 0.04 mmol) or M(NO3)3 (M = Al, Fe and Cr, 0.04 mmol) or M(ClO4)2 (M = Fe, 0.04 mmol) was separately dissolved in MeCN (2 mL). 36 μL of Zn2+ solution and each metal solution were taken, respectively, and added to receptor 1 prepared above to give 12 equiv. After mixing them for a few seconds, fluorescence spectra were obtained at room temperature.

pH Effect Test of 1 Toward Zn2+

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, receptor 1 (1.72 mg, 0.006 mmol) was dissolved in MeCN (2 mL), and then 20 μL of the receptor 1 (3 mM) were diluted with 2.98 mL MeCN/bis-tris buffer solution (3:7, v/v) to make the final concentration of 20 μM. Zn(NO3)2∙6H2O (11.9 mg, 0.04 mmol) was dissolved in MeCN (2 mL). 36 μL of the Zn2+ solution (20 mM) were transferred to each receptor solution (20 μM) prepared above. After mixing them for a few seconds, fluorescence spectra were obtained at room temperature.

NMR Titration of 1 Toward Zn2+

Four NMR tubes of 1 (0.28 mg, 0.01 mmol) dissolved in CD3CN (0.7 mL) were prepared, and four different equiv. (0, 0.5, 0.8 and 1 equiv) of zinc nitrate dissolved in CD3CN (0.3 mL) were added separately to the solutions of 1. After shaking them for a few seconds, the 1H NMR spectra were taken.

Determination of Zn2+ in Water Samples

Fluorescence spectral measurements of water samples containing Zn2+ were performed by adding 20 μL of 3 mmol/L stock solution of 1 and 0.60 mL of 50 mmol/L bis-tris buffer solution to 2.38 mL sample solutions. After well mixed, the solutions were allowed to stand at 25 °C for 2 min before the test.

Theoretical Calculation Methods

All DFT/TDDFT calculations based on the hybrid exchange-correlation functional B3LYP [49, 50] were carried out using Gaussian 03 program [51]. The 6-31G** basis set [52, 53] was used for the main group elements, whereas the Lanl2DZ effective core potential (ECP) [54, 55] was employed for Zn. In vibrational frequency calculations, there was no imaginary frequency for the optimized geometries of 1 and 1-Zn2+, 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) [56, 57]. To investigate the electronic properties of singlet excited states, time-dependent DFT (TDDFT) was performed in the ground state geometries of 1 and 1-Zn2+. Thirty lowest singlet states were calculated and analyzed. The GaussSum 2.1 [58] was used to calculate the contributions of molecular orbital in electronic transitions.

Results and Discussion

Synthesis of 1

The compound 1 3-((2-(dimethylamino)ethyl)amino)-N-(quinolin-8-yl)propanamide was synthesized by substitution reaction of 3-chloro-N-(quinolin-8-yl)propanamide and N,N′-dimethyl ethylene amine in acetonitrile (Scheme 1), and characterized by 1H NMR, 13C NMR, elemental analysis and ESI-mass spectrometry.

Fluorescence and Absorption Spectroscopic Studies of 1 Toward Zn2+

The fluorometric behavior of the receptor 1 toward various metal ions was studied in a mixture of MeCN/bis-tris buffer solution (3:7, v/v). When excited at 370 nm, receptor 1 exhibited a weak fluorescence emission (λmax = 523 nm) compared to that (424 folds) in the presence of Zn2+ (Fig. 1). By contrast, upon addition of other metal ions such as Al3+, Cd2+, Cu2+, Fe2+, Fe3+, Mg2+, Cr3+, Hg2+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+, either no or slight increase in intensity was observed. These results indicated that the receptor 1 could be used as a fluorescence chemosensor for Zn2+ and discriminate Zn2+ from Cd2+ [59–63]. Moreover, we examined the fluorometric properties of 1 with Zn2+ in polar and non-polar solvents such as chloroform, methanol (MeOH), acetonitrile (MeCN) and N,N-dimethylformamide (DMF) (Fig. S1). 1 displayed strong fluorescence with Zn2+, which featured a red-shift with increase of the solvent polarity.

Fig. 1
figure 1

Fluorescence spectral changes of 1 (20 μM) in the presence of different metal ions (12 equiv) such as Al3+, Zn2+, Cd2+, Cu2+, Fe2+, Fe3+, Mg2+, Cr3+, Hg2+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+ with an excitation of 523 nm in a mixture of MeCN/bis-tris buffer solution (3:7, v/v)

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To further investigate the chemosensing properties of 1, fluorescence titration of the receptor 1 with Zn2+ ion was carried out. As shown in Fig. 2, the emission intensity of 1 at 523 nm gradually increased until the amount of Zn2+ reached 12 equiv. The binding properties of 1 with Zn2+ were further studied by UV-vis titration experiments (Fig. 3). UV-vis absorption spectrum of 1 showed two absorption bands at 240 nm and 310 nm. Upon the addition of Zn2+ ion to the solution of 1, the two bands have red-shifted to 257 and 367 nm, respectively. Meanwhile, three clear isosbestic points were observed at 246 nm, 283 and 336 nm, implying the undoubted conversion of free 1 to a zinc complex.

Fig. 2
figure 2

Fluorescence spectral changes of 1 (20 μM) in the presence of different concentrations of Zn2+ ions in a mixture of MeCN/bis-tris buffer solution (3:7, v/v). Inset: Fluorescence intensity at 523 nm versus the number of equiv. of Zn2+ added

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Fig. 3
figure 3

UV-vis titration of 1 (30 μM) with Zn2+ (0–2 equiv). Inset: Absorption titration profile of 1 with Zn2+ at 322 nm

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The Job plot showed a 1:1 complexation stoichiometry between 1 and Zn2+ (Fig. S2) [64], which was further confirmed by ESI-mass spectrometry analysis (Fig. 4). The positive-ion mass spectrum of 1 upon addition of 1 equiv. of Zn2+ showed the formation of the [1-H++Zn2++2IPA+DMSO]+ [m/z: 547.20; calcd, 547.23]. From the fluorescence titration data, the association constant for 1 with Zn2+ was determined as 1.4 x 104 M−1 using Benesi-Hildebrand method (Fig. S3) [65]. This value is within the range of those (1.0 ~ 1.0 x 1012) reported for Zn2+ sensing chemosensors [66–68]. The limit of detection was estimated to check the efficiency of the probe, which was based on the 3σ/slope (Fig. S4) [69, 70]. The detection limit for Zn2+ was determined as 7.1 μM, which was much lower than the WHO guideline (76 μM) for Zn2+ ions in drinking water [71, 72].

Fig. 4
figure 4

Positive-ion electrospray ionization mass spectrum of 1 (100 μM) upon addition of Zn(NO3)2 (1 equiv)

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To explore the ability of 1 as a fluorescence receptor for Zn2+, interference experiments were performed in the presence of Zn2+ (12 equiv) mixed with various metal ions (12 equiv) (Fig. 5). There was no interruption for the detection of Zn2+ in the presence of Mg2+, Hg2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+, while relatively low detectable responses were observed in the presence of Al3+, Cu2+, Fe2+, Fe3+, Cr3+ and Co2+. On the other hand, Cd2+ ion hardly inhibited the fluorescence intensity of 1-Zn2+. These results suggest that 1 could be a good sensor for Zn2+ and, indeed, distinguish Zn2+ from Cd2+ commonly having similar properties in the same group of the periodic table.

Fig. 5
figure 5

Competitive selectivity of 1 (20 μM) toward Zn2+ (12 equiv) in the presence of other metal ions (12 equiv) with an excitation of 370 nm in a mixture of MeCN/bis-tris buffer solution (3:7, v/v)

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The pH dependence of the 1-Zn2+ complex was examined. Over the pH range tested, the fluorescence intensity of 1-Zn2+ displayed strong pH dependence (Fig. 6). An intense and stable fluorescence of 1-Zn2+ found in the pH range of 7.0–12.0 warrants its application under physiological conditions, without any change in detection results.

Fig. 6
figure 6

Fluorescence intensity (at 523 nm) of 1 (20 μM) in the presence of Zn2+ at different pH values (2–12) in a mixture of MeCN/bis-tris buffer solution (3:7, v/v)

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1H NMR Spectroscopic Studies of 1 Toward Zn2+

The 1H NMR titration experiments were studied to further examine the binding mode between 1 and Zn2+ ion (Fig. 7). Upon addition of Zn2+ to receptor 1, H10 disappeared at 0.5 equiv. H8, H9, H11, H12 and H13 showed significantly downfield shift, while H7 shifted upfield and the protons in quinoline slightly shifted downfield or upfield. There was no shift in the position of proton signals on further addition of Zn2+ (>1.0 equiv). These results suggest that two nitrogen atoms in dimethyl ethylene amine might coordinate to Zn2+ ion (Scheme 2). Based on these results, we proposed that the low fluorescence of 1 could be due to photoinduced electron transfer (PET) from lone-pair electrons of receptor (dimethyl ethylene amine) to fluorophore (quinolone). Thus, ‘off-on’ fluorescence of 1 caused by Zn2+ might be attributed to the inhibition of PET (Scheme 2).

Fig. 7
figure 7

1H NMR titration of 1 with Zn(NO3)2∙6H2O

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Scheme 2
scheme 2

Fluorescence enhancement mechanism and proposed structure of 1-Zn2+ complex

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Determination of Zinc ion in Water Samples

We constructed a calibration curve for the determination of Zn2+ by 1 (Fig. 8). Receptor 1 showed a good linear relationship between the fluorescence intensity of 1 and Zn2+ concentration (0–120 μM) with a correlation coefficient of R2 = 0.9821 (n = 3). This result indicates that 1 is suitable for quantitative detection of Zn2+. In order to examine the applicability of the receptor 1 in environmental samples, the chemosensor was applied for the determination of Zn2+ in water samples. First, tap water samples were chosen. As shown in Table 1, one can see a satisfactory recovery and R.S.D. values of water samples. Also, we prepared artificial polluted water samples by adding various metal ions known as being involved in industrial processes into deionized water. The results were also summarized in Table 1, which exhibited a satisfactory recovery and R.S.D. values for the artificial water samples.

Fig. 8
figure 8

Fluorescence intensity (at 523 nm) of 1 as a function of Zn(II) concentration. [1] = 20 μmol/L, [Zn(II)] = 0–120 μmol/L. Conditions: all samples were conducted in a mixture of MeCN/bis-tris buffer solution (3:7, v/v). λex and λem were 370 and 523 nm, respectively

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Table 1 Determination of Zn(II) in water samples
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Theoretical Calculations

To gain an insight into fluorescent sensing mechanism for 1-Zn2+, time-dependent density functional theory (TD-DFT) calculations were performed at the optimized geometries (S0) of 1 and 1-Zn2+ complex (Fig. 9). In case of 1, the main molecular orbital (MO) contributions of the first lowest excited states were determined for HOMO → LUMO and HOMO - 1 → LUMO transition (332.82 nm, Fig. S5). As shown in Fig. S6, HOMO - 1 → LUMO of 1 indicates π → π* transition in quinoline moiety, which means radiative transition. HOMO → LUMO of 1 indicates PET from dimethyl ethylene amine to quionline, which could explain the non-radiative process of 1. For 1-Zn2+ complex (Fig. S7), the third lowest excited state was considered as main transition of 1-Zn2+ complex (oscillator strength = 0.1317), while its first lowest excited state showed minor transition (oscillator strength = 0.0284). The main molecular orbital (MO) contribution of the third lowest excited state was determined for HOMO - 1 → LUMO (330.48 nm). As shown in Fig. S5, it shows π → π* transition in quinoline moiety, which indicates radiative transition. Thus, these results suggested that the sensing mechanism of 1 toward Zn2+ was originated by inhibition of PET process [73].

Fig. 9
figure 9

Energy minimized structures of (a) 1 and (b) 1-Zn2+

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Conclusion

We have synthesized a new fluorescent chemosensor 1, which displays high sensitivity and selectivity toward zinc in aqueous media. The complexation of 1 with Zn2+ exhibited a pronounced enhancement in the fluorescence emission. Moreover, the detection limit (7.1 μM) is much lower than the WHO detection level (76 μM) for Zn2+ ions in drinking water. Most importantly, recovery studies of the water samples added with Zn2+ demonstrated its value in the practical application. Therefore, we believe that receptor 1 will be a prototype for the practicable system for detecting Zn2+ concentrations in environmental systems.