A highly selective colorimetric and fluorescent turn-on chemosensor for Zn2+and its logic gate behavior

Synthetic Metals 232 (2017) 17–24

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A highly selective colorimetric and fluorescent turn-on chemosensor for Zn2+and its logic gate behavior Na-Na Li, Yu-Qing Ma, Shuang Zeng, Ya-Tong Liu, Xue-Jiao Sun, Zhi-Yong Xing

MARK

⁎

Department of Applied Chemistry, College of Science, Northeast Agricultural University, Harbin 150030, PR China, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Quinoline Chemosensor X-ray crystallography Zn2+ Logic gate

A novel quinoline-based fluorescent chemosensor L was synthesized and characterized by 1H NMR, 13C NMR, HRMS and X-ray Crystallography. L showed a significant fluorescent enhancement (120-fold) and color change upon the addition of Zn2+ attributed to the joint contribution of the PET and CHEF processes. The binding ratio of the L–Zn2+ complex was 1:1 confirmed by the 1 HNMR, mass-spectral analysis and Job plot, and the detection limit for Zn2+ was as low as 5.2 × 10−7 M. L was successfully applied to the determination of Zn2+ in real water samples. Moreover, the fluorescent signals of L were utilized to built an INHIBIT logic gate.

1. Introduction

building block for construction of fluorescent chemosensors for transition metals due to favorable biocompatibility and interesting photophysical properties [65–67]. Some chemosensors for detection Zn2+ using that building block involved different mechanisms, such as photoinduced electron transfer (PET) [68–72], internal charge transfer (ICT) [73–77], chelation enhanced fluorescence (CHEF) [78], twisted intramolecular charge transfer (TICT) [79], excited-state intermolecular proton transfer (ESIPT) [80], and inhabition of C]N isomerization [81]. Moreover, several Zn2+ chemosensors [82–85] have been developed on the basis of multi-mechanism which will amplify recognition process by producing multiple signals to a greater extent to improve selectivity and sensitivity. However, to the best of our knowledge, the Zn2+ chemosensor based on the combination of intramolecular charge transfer (PET) and chelation enhanced fluorescence (CHEF) mechanism with the 8-aminoquinoline fluorophore as the signaling unit has been scarcely reported. Taking the above statements into consideration, chemosensor L was designed and synthesized by a facile condensation between compound 1 and 2 (Scheme 1). Optical properties were investigated using UV–vis and fluorescence response of the L to Zn2+. As we expected, L showed excellent selectivity and sensitivity with a fluorescence enhancement to Zn2+ over other tested cations, especially distinguishing Zn2+ from Cd2+ in CH3OHeH2O (4:1, v/v, containing 0.01 M HEPES buffer, pH = 7.0). Furthermore, chemosensor L was successfully applied in detection of Zn2+ in real water samples. We also found that the fluorescence signal of chemosensor L could be used as an INHIBIT logic gate controlled by two chemical inputs Zn2+ (Input 1) and Cu2+ (Input 2).

Zinc, the second most abundant transition metal after iron in the human body, plays an important role in numerous cellular functions including neural signal transmission, apoptosis, DNA synthesis, modulation of diverse ion channels and gene transcription [1–6]. Nevertheless, some serious diseases, such as Alzheimer, infantile diarrhea, cerebral ischemia and epilepsy are associated with the disorder of Zn2+ metabolism [7–12]. Moreover, pollution due to transition metals such as Zn2+ has been a threat to the human health and environment [13–16]. Therefore, it is crucial to develop a rapid and sensitive method for detection of Zn2+ in environmental and biological systems. Up to now, some analytical methods including atomic absorption spectrometry (AAS) [17], inductively coupled plasma mass spectroscopy (ICPMS) [18], inductively coupled plasma-atomic emission spectrometry (ICP-AES) [19], electrochemistry [20–26], ion selective membrane [27–52] have been developed for detection of metal ions. However, complicated pretreatment, consuming time, expensive instrumentation and large sample amount have limited their application. During the last decades, fluorescent chemosensors have attracted significant attention due to high sensitivity, simplicity, low-cost, convenience and real time bio-imaging [53–61]. Although a large number of fluorescent probes based on various fluorophores have been reported for selective detection of Zn2+, problems such as insolubility in water, complicated synthesis process, and disturbance from other transition metal ions (especially Cd2+, which is in the same group of the periodic table and shows similar properties to Zn2+) remains [62–64]. Therefore, the development of a highly selective sensing system for Zn2+ has become an attractive challenge. 8-aminoquinoline is a widely used ⁎

Corresponding author. E-mail address: [email protected] (Z.-Y. Xing).

http://dx.doi.org/10.1016/j.synthmet.2017.07.017 Received 19 April 2017; Received in revised form 17 July 2017; Accepted 23 July 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.

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Scheme 1. Synthesis of L.

2. Experimental 2.1. Materials and instrumentation All chemical solvents and reagents were used as received from commercial sources without further purification. 1H NMR and 13C NMR spectra were recorded on a Bruck AV-600 spectrometer. Chemical shifts (δ) are reported in ppm, relative to tetramethylsilane. Absorption spectra were recorded at 25 °C using a Pgeneral TU-1901 UV–vis spectrophotometer. Fluorescence measurements were performed on a Perkin Elmer LS55 fluorescence spectrometer. Mass spectra were measured on a Waters Xevo UPLC/G2-SQ TofMS spectrometer. The melting point was determined on a Beijing XT4-100X apparatus. Crystal data of L were collected on an Xcalibur, Eos, Gemini diffractometer with Mo Kα radiation (λ = 0.71073 Å) [86].

2.2. Synthesis

Fig. 2. Fluorescence spectra of L (50 μM) in the presence of various metal ions (200 μM) in CH3OHeH2O (4:1, v/v, containing 0.01 M HEPES, pH = 7.0), (λex = 360 nm). Inset: Fluorescence color changes of L solution before and after addition of Zn2+ under UV lamp at 365 nm.

2.2.1. Synthesis of L As shown in Scheme 1, according to the literature method [75], L was synthesized by coupling of 2-chloro-N-(quinol-8-yl)-Acetamide (1) and tert-butyl 1-piperazinecarboxylate (2) via one step reaction. N-bocpiperazine (0.447 g, 2.4 mmol), K2CO3 (0.276 g, 2 mmol) and KI

Fig. 1. X- ray crystal structure of L.

18

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Fig. 3. Fluorescence spectra of L (50 μM) on addition of different amount of Zn2+ in CH3OHeH2O (4:1, v/v, containing 0.01 M HEPES, pH = 7.0), (λex = 360 nm). Inset: fluorescence intensity at 484 nm versus the number of equiv. of Zn2+ added.

2.3. General information Stock solution of L (50 μM): 25 mL of the chemosensor L (0.5 mM) were diluted to 225 mL CH3OHeH2O (7:2, v/v, containing 2.5 mmol HEPES, pH = 7.0) solution to make the final concentration of L of 50 μM. Stock solutions (10 mM) of the cationic salts of NaClO4, KClO4, Pb (NO3)2, Mg(ClO4)2, Zn(ClO4)2·6H2O, Cu(ClO4)2·6H2O, Co(NO3)2·6H2O, Ba(ClO4)2, Ni(NO3)2·6H2O, Cd(NO3)2, Ca(NO3)2·4H2O, AgNO3, Al (NO3)3·9H2O, MnSO4·H2O, FeCl2·4H2O or Fe(ClO4)3·H2O were prepared with ultrapure water, respectively. In selectivity experiments, the test solutions were prepared by adding appropriate amounts of the cations stock solution (50 μM) into 10 mL of L stock solution for the UV–visible and Fluorescence Spectroscopic Studies. For fluorescence measurements, excitation was set at 360 nm, emission was acquired from 400 nm to 650 nm and the excitation and emission slit widths were 10 nm and 10 nm, respectively.

Fig. 4. UV–vis spectral changes of L (50 μM) in the presence of different concentrations of Zn2+ ion in CH3OHeH2O (4:1, v/v, containing 0.01 M HEPES, pH = 7.0). Inset: color changes of L solution before and after addition of Zn2+.

3. Results and discussion (10 mg, 0.06 mmol) were added to a stirred solution of compound 1 (0.44 g, 2 mmol) in acetonitrile (20 mL) and refluxed for 6 h. After the reactants were fully consumed (monitored by TLC), the reaction mixture was allowed to room temperature (25 °C) and the solvent was removed under reduced pressure. The residue was further purified by silica gel column chromatography using methanol/dichloromethane (1:20, v/v) as the eluent to afford white solid. Yield: 0.52 g (70.2%). Mp: 165–166 °C. 1H NMR (Fig. S1) (600 MHz, DMSO-d6) δ 11.30 (s, 1H), 8.97–8.90 (m, 1H), 8.63 (d, J = 7.6 Hz, 1H), 8.39 (d, J = 8.3 Hz, 1H), 7.66 (d, J = 8.2 Hz, 1H), 7.62 (dd, J1 = 8.2 Hz, J2 = 4.2 Hz, 1H), 7.57 (t, J = 7.9 Hz, 1H), 3.48 (s, 4H), 3.27 (s, 2H), 2.59–2.51 (m, 4H), 1.40 (s, 9H). 13C NMR (Fig. S2) (151 MHz, DMSO-d6) δ (ppm) 168.85, 154.27, 149.82, 138.52, 137.03, 134.38, 128.31, 127.49, 122.74, 122.28, 115.89, 79.43, 62.08, 53.07, 40.54, 28.55. HRMS m/z (TOF MS ES+) (Fig. S3): calcd for C20H26N4O3 + H+, 371.2083 [M + H]+, found: 371.2175.

3.1. X-ray diffraction studies In order to obtain accurate structural information, single crystal of L was grown and the molecular structure was determined by single crystal X-ray diffraction. All data were collected at room temperature. The structures were solved by direct methods and refined on F2 by fullmatrix least-squares using the SHELXTL-2014 program. All non-hydrogen atoms were refined anisotropically. The molecular structure of L was shown in Fig. 1, and the most significant crystal data was listed in Table S1 and Table S2, respectively. Structural information was deposited with the Cambridge Crystallographic Data Center (CCDC 1494263). 3.2. Spectral responses of probe L 3.2.1. Selective optical response of L toward various metal ions To gain an insight into the metal affinity of chemosensor L toward different metal ions, the fluorescence spectroscopy was measured with various metal ions such as Na+, K+, Ag+, Ba2+, Fe2+ Co2+, Ni2+, Ca2+, Zn2+, Cd2+, Cu2+, Mn2+, Mg2+, Pb2, Fe3+ and Al3+ in the solution of CH3OHeH2O (4:1, v/v, containing 0.01 M HEPES buffer, pH = 7.0). The chemosensor L alone exhibited a very weak

2.2.2. Preparation of crystal L L (37 mg, 0.1 mmol) was dissolved in 15 mL of hot ethanol. After cooling and filtration, the solution was stored at room temperature for 7 days to yield single crystals of L suitable for X-ray analysis. 19

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Fig. 5. a Faluorescence response of L (50 μM) upon addition of various metal ions (200 μM) in and without the presence of Zn2+ (200 μM) in CH3OHe H2O (4:1, v/v, containing 0.01 M HEPES, pH = 7.0), (λex = 360 nm). bThe absorption spectra of L (50 μM) upon addition of various metal ions (200 μM) in the presence of Zn2 ± (200 μM) in CH3OHe H2O (4:1, v/v, containing 0.01 M HEPES, pH = 7.0).

fluorescence emission when it was excited at 360 nm. Upon addition of Zn2+ to the solution of L, a 120-fold fluorescence enhancement was observed and the solution of L showed a significant color change from colorless to cyan, which could easily be detected by the naked-eye under UV light of 365 nm (Fig. 2). The significant fluorescence enhancement was due to the inhibition of the intramolecular photoinduced electron transfer (PET) from piperazine “N” to quinoline moiety and an additional contribution from chelation enhanced fluorescence (CHEF) upon coordination with zinc [82,87,88]. In contrast, all other metal ions under investigation show little change of fluorescence of L except for Cd2+, which responded with a negligible increase in the fluorescence intensity because the intensity was significantly lower than that obtained with Zn2+ alone under the same conditions. These results suggest that L could act as “turn-on” chemosensor for Zn2+ particularly from Cd2+ which is a major obstacle [62–64] for the Zn2+ chemosensor.

increased until the amount of Zn2+ reached 10 equiv. and there was a good linear relationship between the maximal fluorescence intensity of L and the concentration of Zn2+ varied from 5 μM to 60 μM. Moreover, UV–vis titration experiments were also conducted and the results were shown in Fig. 4. Upon the gradual addition of Zn2+ ion (0–16 equiv.), the absorption bands centered at 240 nm and 308 nm gradually decreased while two new absorption bands at 256 nm and 354 nm gradually appeared along with the solution color changed from colorless to pale yellow. Three well-defined isosbestic points at 246, 277, and 334 nm indicated the clean conversion of L into L–Zn2+ complex.

3.2.3. Competition experiment In order to evaluate the practical possibility used as a selective fluorescent sensor for Zn2+, competition experiments of Zn2+ ion with other competition metal ions were carried out according the fluorescence intensity data at 484 nm (Fig. 5a). Tested metals, Na+, K+, Ag+, Ba2+, Fe2+ Co2+, Ni2+, Ca2+, Zn2+, Cd2+, Cu2+, Mn2+, Mg2+, Pb2, Fe3+ and Al3+ (200 μM), were added separately to the solution of L (50 μM) and Zn2+ (200 μM). The results revealed that most coexistent metal ions had negligible or a small effect on the emission intensity of the L- Zn2 ± complexation except for Ni2 ± , Fe3 ± and Cu2 ± . The fluorescence of the L-Zn2 ± complexation was almost completely

3.2.2. Fluorescence and UV–vis titration experiments In order to investigate the binding properties of the chemosensor L towards Zn2+ ion, fluorescence titration of L with Zn2+ in the solution of CH3OHeH2O (4:1, v/v, containing 0.01 M HEPES buffer, pH = 7.0) was carried out (Fig. 3). The emission intensity of L at 484 nm gradually 20

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Fig. 6. ESI–MS spectrum of L (50 μM) upon addition of 4 equiv. of Zn2+ in CH3OH.

quenched due to the paramagnetic properties of Cu2 ± . As for Ni2 ± and Fe3 ± , in order to confirm whether the transmetallation processes would occur during the addition of Ni2 ± or Fe3 ± into the solution of L-Zn2 ± complexation, the absorption spectra of the systems L ± Zn2 ± ± Mn ± (Mn ± = Ni2 ± , Fe3 ± , Na ± , Ag ± , Ba2 ± , Co2 ± , Ca2 ± and Al3 ± ) were measured, respectively (Fig. 5b). The results showed that the absorption spectra of the systems L ± Zn2 ± ± Ni2 ±

and L ± Zn2 ± ± Fe3 ± was obviously changed comparing with those systems L ± Zn2 ± ± Mn ± (Mn ± Na ± , Ag ± , Ba2 ± , Co2 ± , Ca2 ± and Al3 ± ), indicated that the transmetallation processes occurred in the L ± Zn2 ± ± Ni2 ± and L ± Zn2 ± ± Fe3 ± systems [89–91]. According to these results, L could achieve high selectivity for Zn2+ in the presence of most competing metal ions.

Fig. 7. 1H NMR spectra of L with Zn2+ in DMSO-d6.

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Scheme 2. Proposed sensing mechanism of L for Zn2+.

Table 1 Determination of Zn2+ in water samples from different water sources by standard-addition method (n = 4). Water samples studied

Amount of standard Zn2+ added (μg mL−1)

Total Zn2+ found (n = 4) (μg mL−1)

Recovery of Zn2+ (n = 4) added (%)

RSD (%)

Relative error (%)

ultrapure water

1.56 2.60 1.56 2.60

1.53 2.60 1.52 2.54

98.33 100.20 97.82 97.72

2.01 6.07 1.19 2.70

−1.67 0.20 −2.18 −2.28

Tap water (Department of Chemistry)

Fig. 8. Fluorescence spectra of L (50 μM) at various pH values in CH3OHeH2O (4:1 v/v, containing 0.01 M HEPES, pH 7.0) in the absence and presence of Zn2+ (4 equiv).

Fig. 9. Fluorescence spectral changes of L (50 μM) after the sequential addition of Zn2+ and EDTA in the solution of CH3OHeH2O (4:1 v/v, containing 0.01 M HEPES, pH 7.0), (λex = 360 nm).

Fig. 10. Operation of the INHIBIT logic gate. (a) The fluorescence intensities of L at λem = 484 nm in the presence of different inputs. (b) INHIBIT logic scheme.

3.3. Binding stoichiometry and sensing mechanism

Table 2 Truth table for the INHIBIT logic gate. 2+

comIn order to investigate binding stoichiometry of the L- Zn plex, the Job’s method for fluorescence measurement was applied. The total concentration of Zn2+ and L was 5 × 10−5 mol/L with the molar ratio of Zn2+ varied from 0.1 to 0.9 (Fig. S4). As shown in Fig. 7, the fluorescence emission intensity reached a maximum when the molar fraction of Zn2+ was 0.5, showing a 1:1 complex binding stoichiometry between L and Zn2+. According to the Benesi–Hildebrand equation [92], the association constant was determined from fluorescence titration experiments and calculated as 3466 M−1 (Fig. S5). The detection limit was 0.52 μM (Fig. S6) based on the equation DL = K × SD/S [58], 22

Input 1 Zn2+

Input 2 Cu2+

Output If484 nm

0 1 0 1

0 0 1 1

0 1 0 0

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which is far below the WHO guideline (76 μM) for Zn2+ ions in drinking water [63]. To further confirm the binding stoichiometry of L with Zn2+, HRMS of L in the presence of Zn2+ was carried out (Fig. 6). The peaks at 371.2098 and 533.0794 were attributed to the [L + H+]+ (calcd m/z 371.2083) and [L + Zn2+ + ClO4−]+ (calcd m/z 533.0781), respectively. These results support the 1:1 binding stoichiometry of L with Zn2+ which concluded from the Job’s plot analysis. To explore the specific binding site, the 1H NMR titration experiments were conducted by adding different equiv. of Zn2+ to the DMSOd6 solution of L (Fig. 7). During the titration, the concentration of L was kept constant and a gradual increase was brought in the concentration of Zn2+ to vary the mole ratio from 0 to 2 equiv. Upon the addition of Zn2+, the singlet signal of methylene (Ha) protons were broadened along with a slight upfield shift from 3.30 ppm to 3.29 ppm, the methylene (Hb) protons of piperzine ring became a wide singlet peak from a triplet peak and the methylene (Hc) protons of piperzine ring were broadened with no obvious shift, all of these indicated that the two nitrogen atoms of piperazine ring might coordinate to Zn2+ [93]. Furthermore, the signals of eNH- and quinoline ring protons showed no obvious changes, which indicated that nitrogen atoms of amide and quinoline ring may not coordinate with Zn2+. As shown in Scheme 2, we proposed the following signaling mechanism for detection Zn2+. L alone showed no emission at 484 nm, when Zn2+ ion coordinates with the nitrogen atom of the piperazine unit, an enhancement of the emission can be achieved due to the inhibition of PET process.

campus. The solution of Zn2+ at different concentrations levels were spiked in all real samples and the experimental results were summarized in Table 1. The concentration of Zn2+ detected was close to that of the added Zn2+ and further verified L has potential application for Zn2+ detection in environmental analysis. 3.8. Logic gate behavior of L It was known that the molecular logic gates had been achieved in the fabrication chemical circuits [94–96]. According to the fluorescence response of chemosensor L with Zn2+ and Cu2+ in its maximum emission intensity (λem = 484 nm), chemosensor L can be used to construct the INHIBIT logic gate, and the results were shown in Fig. 10 and Table 2, respectively. The fluorescent intensity output values (If 484 nm) below the threshold level are assigned as logic “0”, while the output values above the threshold are assigned as logic “1”. As shown in Fig. 10. (a), the output is logic “0” with three possible input combinations such as (Zn2+ = 0, Cu2+ = 0, bar a), (Zn2+ = 0, Cu2+ = 1, bar c), (Zn2+ = 1, Cu2+ = 1, bar d) and the output is only logic “1” when the input signal is (Zn2+ = 1, Cu2+ = 0, bar b). Considering the above concepts had meet the requirements of the INHIBIT logic gates, the maximum emission of L at 484 nm with Zn2+ and Cu2+ as inputs could be regarded as a molecular circuit exhibiting an INHIBIT logic function (Fig. 10 b). 4. Conclusion In conclution, a quinoline-based fluorescent chemosensor L was synthesized and applied for selective detection of Zn2+ ions. L showed high selectivity and sensitivity for Zn2+ by a large enhancement (120fold) of fluorescent intensity along with color change which can be detected by naked eye. A 1:1 binding stoichiometry for the L- Zn2+ complex was confirmed, and the detection limit of L for Zn2+ (0.52 μM) was considerably lower to detect micromolar concentration of Zn2+. L was successfully applied in real sample detection and could be utilized to build an INHIBIT logic gate based on two input Zn2+ and Cu2+.

3.4. pH effect test of L toward Zn2+ For practical applications, the sensing ability of L and L-Zn2+ complex at different pH values was investigated. As shown in Fig. 8, the fluorescent intensities of L alone are almost unaffected with pH variation. However, the fluorescence intensity of L–Zn2+ displayed a strong pH dependence. The fluorescence intensity of the L-Zn2+ complex increased gradually from pH 6 to 7.5, reaching a maximum value around pH 7.5, and decreased gradually from pH 7.5 to 10. This result indicated that the binding ability of L with Zn2+ occurred effectively between pH from 6 to 8 and the optimum detection range of chemosensor L to Zn2+ was between pH from 7 to 7.5.

Acknowledgements This work was supported by Heilongjiang Postdoctoral Funds for scientific research initiation (No. LBH-Q14023) and State Scholarship Fund of China Scholar Council (No. 201408230115). We would like to thank Professor Jinlong Li (Qiqihar University) for ESI-mass running and Professor Zhendong Jin’s (The University of Iowa) instruction in writing.

3.5. Response time studies For evaluation a probe in practical sensing, response time is an important factor besides the high selectivity. The effect of the reaction time on the binding process of Zn2+ to L was studied and the results were shown in Fig. S7. Upon the addition of Zn2+, the fluorescence intensity of L was enhanced instantly and reached the maximum within 30 s and maintained constant in the following 10 min. This result shows that L could be used as a Zn2+ chemosensor with the real-time response ability.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet.2017.07.017.

3.6. Reversibility of L for Zn2+studies

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In order to study the reversibility of the chemosensor L, the titration experiments were carried out by adding the Na2EDTA agent. As shown in Fig. 9, upon the addition of EDTA to the solutions of L- Zn2+, the fluorescent spectra of L- Zn2+ almost backed to the original state without the Zn2+, indicating the regeneration of the free L. The results showed that the Zn2+ recognition process was reversible by the treatment with EDTA.

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3.7. Determination of zinc ion in water samples In order to examine the applicability of the chemosensor L in environmental samples, chemosensor L was examined by the determination of spiked Zn2+ in real water samples collected from local region of 23

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