A dual-channel sensor containing multiple nitrogen heterocycles for the selective detection of Cu2+, Hg2+ and Zn2+ in same solvent system by different mechanism.

Dyes and Pigments 170 (2019) 107651

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A dual-channel sensor containing multiple nitrogen heterocycles for the selective detection of Cu2+, Hg2+ and Zn2+ in same solvent system by different mechanism.

T

Chu-Ming Panga,1, Shi-He Luoa,b,1, Kai Jianga,∗∗, Bo-Wen Wanga, Si-Hong Chena, Neng Wanga, Zhao-Yang Wanga,b,∗ a

School of Chemistry and Environment, South China Normal University, Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, Guangzhou Key Laboratory of Analytical Chemistry for Biomedicine, Guangzhou, 510006, China Key Laboratory of Functional Molecular Engineering of Guangdong Province, South China University of Technology, 381 Wushan Road, Guangzhou, 510640, China

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Molecule containing multiple nitrogen heterocycles Bivalent metal ions Dual-channel Naked eyes Different mechanism

A molecule 3 containing multiple nitrogen heterocycles has been designed and synthesized. The systematical studies on sensing behaviors of compound 3 in DMSO/H2O by fluorescent and UV–vis absorption spectra indicate that this molecule shows the selectivity to Cu2+, Hg2+ and Zn2+ by dual-channels. Compound 3 shows significant fluorescence quenching to both Cu2+ and Hg2+ but with a red-shift for Hg2+, and only for Zn2+ there are fluorescence colorimetric and ratiometric changes. Furthermore, the absorption spectrum of 3 to Cu2+ also exhibits ratiometric change with the color change of solution from colorless to yellow, while no similar response to Hg2+ and Zn2+. These changes can be recognized via naked eyes. Three kinds of different mechanism have been convincingly proposed through ESI-Mass, 1H NMR titration, FT-IR spectra and DFT calculation. The suitable pH range of sensor 3 to Cu2+, Hg2+, Zn2+ has been determined to be 5.1–13.3, 6.9–11.2, 5.1–12.2, respectively. And time response experiments show that, as a fast-responsive fluorescent sensor for three kinds of cations, it can be used for the real-time analysis of Hg2+. Particularly, the reversible detection of Cu2+ in 5 cycles indicates that sensor 3 is expected to be a molecular logic gate for Cu2+.

1. Introduction In recent years, the application of organic conjugates in chemosensors has been obtained much attention [1–5], especially large numbers of the sensors with high selectivity and sensitivity for detecting Cu2+ [6–8], Hg2+ [9,10], Zn2+ [11,12] have been reported. The amount of these metal ions in environment increases drastically with the development of industry, which might damage our health and ecosystem [13–15]. Copper ion is the third most abundant transition metal ion in the human body after iron and zinc, and it plays a vital role in human physiology for its' unique redox active nature [8,14]. However, the exposure to or ingestion of excessive copper can cause gastrointestinal disturbance, Wilson or Alzheimer's disease [6,7]. Due to the non-biodegradable property, mercury ion is one of the most poisonous heavy metal ions. Especially, it can be accumulated by food

chains and might be transformed to high toxic methylmercury species that are easy to go through the cell membrane. Excessive accumulation of Hg2+ might cause serious diseases, like schizophrenia, kidney damage and even death when severe mercury poisoning to the immune system [9,10]. Zn2+ is the second most abundant essential trace element in human body and important for lots of physiological processes [12,15]. Therefore, the design and synthesis of organic conjugate chemosensors used to rapidly and efficiently detect metal ions have been becoming a growing concern in recent years [16–23]. However, plenty of the reported small molecular sensors for ionic detection often only serve to single analyte, specifically one metal ion, which may limit the application of sensors in complicated environment and increase the extra spending. The multifunctional sensors conform to the atomic economy and meet the requirement of highly spatial integration of the devices, so

∗ Corresponding author. School of Chemistry and Environment, South China Normal University, Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, Guangzhou Key Laboratory of Analytical Chemistry for Biomedicine, Guangzhou, 510006, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (K. Jiang), [email protected] (Z.-Y. Wang). 1 Chu-Ming Pang and Shi-He Luo contributed equally to this work.

https://doi.org/10.1016/j.dyepig.2019.107651 Received 26 March 2019; Received in revised form 15 June 2019; Accepted 19 June 2019 Available online 24 June 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.

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Scheme 1. Synthetic route for compound 3.

J = 8.0 Hz), 8.15 (1H, s), 8.18 (1H, s), 8.49 (2H, d, J = 8.0 Hz), 8.56 (2H, d, J = 8.0 Hz), 13.27 (2H, b); IR (film), ν, cm−1: 3493, 3048, 1662, 1595, 1422, 1407, 1298, 1100, 1052, 836, 798, 758; ESI-MS, m/z (%): Calcd for C32H21N6+ ([M+H]+): 489.17 (100), Found: 489.21 (100); Anal. Calcd for C32H20N6: C 78.66, H 4.14, N 17.20, Found: C 78.59, H 4.13, N 17.28.

that the research on the sensors for the detection of three analytes in multiple signals has been a significant trend recently [24–28]. Furthermore, a sensor for multiple analytes that conducts its’ sensing performance in the same solvent system is able to reduce sample handling and cost as well as improve the efficiency compared with conventional sensors. Now, more and more attention has been attracted to the sensors capable to detect three kinds of metal ions [29–34]. Among them, especially based on small molecules, there are some deficiencies. For example, the output signals are similar, which can hardly be distinguished by naked eyes [29,31]. Hence, it is still important to develop new multifunctional sensors for the detection via naked eyes for various metal ions, such as Cu2+, Hg2+, and Zn2+. Herein, a molecule 3 containing multiple nitrogen heterocycles has been designed and synthesized via the reaction of 3,3′-diaminodiphenylamine and quinaldic acid catalyzed by polyphosphoric acid (PPA, Scheme 1). The N atoms owning lone pair electrons and the half-enclosed structure formed by the benzimidazole and quinoline rings in compound 3 endow its abilities to combine with diverse metal ions by coordination, which might lead to unique changes of the spectral signal for different cations. Thus, the compound 3 is expected to be a selective chemosensor to detect multiple metal ions in the solvent system by dual channels via different mechanism. It is proved by the experimental results indeed, and the sensor based on benzimidazole and quinoline rings is less reported to the best of our knowledge.

2.3. Methods 2.3.1. General procedure for spectra measurements The compound 3 was dissolved in DMSO to acquire 1 mM stock solution. The measurements of both UV–vis absorption and fluorescence spectra were conducted in the solution of DMSO/H2O (v/v, 9/1, 10 μM) at room temperature. And referring to the literature [5,35–38], the limit of detection was calculated. 2.3.2. Computational methods As the reported method [16,39], all of the calculations were obtained using density functional theory (DFT) with the B3LYP/6-31G (d, p) level for sensor of the Gaussian 09 program. For metal complexes, the C, N and H is calculated in B3LYP/6-31G level and the metal ions is calculated in LANL2DZ basis set. 3. Results and discussion

2. Experimental

3.1. Synthesis and optical properties of 3

2.1. Chemicals and apparatus

Lab-on-a-molecule type sensor containing single or multiple combining sites is expected to be a well-structured chemosensor for detecting various analytes through different interaction or reaction with them and to achieve its sensing performances with distinguishable output signals [40]. With its development [28,29,34,41], some reported sensors are mainly for the detection of two kinds of anions [42–45], or one metal ion and another anion [11,46], or two kinds of metal ions [47]. Thus, it is urgent to design and synthesize the lab-on-a-molecule type sensor based on small organic molecular conjugate to recognize multiple metal ions. Hence, a molecule 3 containing multiple conjugate benzimidazole and quinoline units has been synthesized via a one-step reaction (Scheme 1). The electron density redistribution and changes of optical signals of 3 induced by the coordination between nitrogen atoms with different metal ions may be observed, so it is able to be a sensor for highly selective detection of multiple metal ions by various output signals. The structure was well characterized (its 1H NMR, MS and FT-IR spectra figures, Figs. S1–S3, can be seen in Supplementary Material), and the data are similar to the reported [48]. Then, the optical properties of the compound 3 were studied in 8 kinds of solvent with different polarity (toluene, DCM, CHCl3, EtOAc, EtOH, MeCN, DMSO and DMF). As shown in Fig. S4, the fluorescence emission spectra of 3 in different solvents change slightly with the variation of their polarity. However, it's worth noting that the fluorescence of 3 changes from blue in toluene to greenish-blue in DMSO, and for the fluorescence emission peak, there is a bathochromic shift from 452 nm to 465 nm. Furthermore, the fluorescence intensity of compound 3 in the DMSO is the strongest, because of the well solubility of 3 in DMSO. The studies on UV–vis absorption spectra of compound 3 in different solvent show that its absorption peak is mainly located at 368 nm due to the π-π* electronic transitions in aromatic rings and the peak

Melting point was tested on an X-5 digital melting point apparatus without correcting. 1H NMR spectra were measured on BRUKER DRX400 spectrometer using TMS as an internal standard. Positive ions mass spectrum (ESI-MS) was recorded on AB sciex (qtrap) liquid chromatography tandem mass spectrometer. Elemental analysis was obtained with PerkinElmer Series II 2400. UV–vis spectra were measured by using a Shimazu UV-2700 ultraviolet absorption detector at room temperature. The fluorescence spectra were obtained with a Hitachi F4600 spectrophotometer at room temperature, with the slit width was 5 nm for excitation and 10 nm for emission. The pH values were measured by a PHS-25C meter. Fluorescence lifetime was measured by FLS 920 Fluorescence Spectrometer. The FT-IR spectra were given by German platinum Elmer Spectrum Two Fourier transform infrared spectrometer. All chemicals used within this work were of analytical grade purity. 2.2. Synthesis According to the literature [4,5], 3.3 mmol quinaldic acid (1), 1.5 mmol 3,3′-diamino- diphenylamine (2) and 25 mL polyphosphoric acid (PPA) were added into a round bottom flask, stirring 48 h at 170 °C under the atmosphere of N2. After the reaction was stopped and cooled to room temperature, the dark green mixture was obtained by adding water and regulating pH to the range of 9–10 with sodium hydroxide. Then, the product was purified by column chromatography on silica gel with gradient eluents of petroleum ether and ethyl acetate. 2,2′-Di(quinolin-2-yl)-3H,3′H-5,5′-bibenzo[d]imidazole (3), 0.63 g, yield 86%; yellow solid, m.p. 183.3–185.1 °C; 1H NMR (DMSO‑d6-TMS, 400 MHz), δ, ppm: 7.55–7.75 (6H, m), 7.80–7.89 (4H, m), 8.07 (2H, d, 2

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528 nm as well. While the color of the solution changes from greenishblue to yellow after 10 equiv. Zn2+ being added, which mainly due to the increase of the fluorescence intensity at 540–700 nm. Therefore, the compound 3 can be used to recognize Cu2+, Hg2+ and Zn2+ through the fluorescence channel and monitor these metal ions via naked eyes. To further distinguish Cu2+ from Hg2+, the selectivity experiments were carried out by UV–vis absorption spectra in the solution of 3 (10 μM DMSO/H2O, v/v, 9/1) also. As shown in Fig. 3a, upon the addition of Cu2+, there is a manifest red-shift of the UV–vis absorption peak from 368 nm to 417 nm, and the color of solution changes from colorless to pale yellow (Fig. 3c). To further analyze Fig. 3b, for Cu2+, the ratio value (A417/A368) of the absorbance at 417 nm and 368 nm is up to 1.78, which is obviously higher than other metal ions added into the sensing system. With the addition of Hg2+ or Zn2+, only a slight red-shift in absorption spectra of sensor 3 can be found and the change is out of the visible region. Hence, the color changes of their solutions under room light are hardly visualized. These results demonstrate that Cu2+ and Hg2+ are able to be distinguished by UV–vis absorption channel. As reported methods recently [33,35,49,50], the anti-interference ability of chemosensor 3 to Cu2+, Hg2+ and Zn2+ was further measured in the competition experiments under the condition of two analytes, containg one target analyte and one interferent specie. As depicted in Fig. S6, for Cu2+ most of metal ions have no obvious interference in the value of A417/A368, expect for some bivalent or trivalent metal ions (Fig. S6a). For example, Al3+ and Fe3+ have certain interferences to the sensing of Cu2+ [49]. For Hg2+, the fluorescence quenching efficiency of the system is down to about 60% when Hg2+ is coexisting with Al3+ and Fe3+ [50], and there is significant fluorescence quenching with the quenching efficency above 91% in most cases (Fig. S6b). For Zn2+, Al3+ and Fe3+ have a little effect on the I/I0 ratio after the mixture of compound 3 and Zn2+ in the presence of them, moreover there is an obvious decrease of I/I0 upon the addition of Cu2+ (Fig. S6c). In a word, some metal ions disturb the performance of compound 3 due to their strong coordination abilities. This might be owing to the fact that some bivalent or high valent metal ions can combine with 3 in a certain. Now, the system faces this kind of deficiencies, eliminating the interference of high valent metal ions is still a problem in the field of multifunctional sensor. Of course, these competitive experiments were tested by pairs of analytes. There are certain deficiencies in this method also, because it is difficult to judge the performance when multiple analytes are coexisting. So, it is still a challenge to achieve simultaneous detection and avoid the interference of high valent metal ions currently. Thus, though some metal ions have certain interference to the performance of compound 3 due to their different coordination abilities in the competition experiments, the compound 3 still shows the selectivity for Cu2+, Hg2+ and Zn2+ in most cases.

Fig. 1. Fluorescence spectra of 3 (10 μM in DMSO/H2O, v/v, 9/1) after the addition of 10 equiv. of various metal ions (λex = 377 nm).

changes slightly in different solvent (Fig. S5). Photophysical properties of compound 3 in different solutions were also analyzed and summarized in Table S1. It can be obtained that, the relative fluorescence quantum yield (ΦR) of compound 3 in DMSO using quinine sulphate as the internal reference is up to 0.869. This datum is higher than that in the other solvent, and the lifetime of 3 in DMSO is 3.22 ns. Both endow its potential to be an optical chemosensor in DMSO. 3.2. Selectivity of chemosensor 3 to Cu2+, Hg2+ and Zn2+ In order to investigate the sensing performance of chemosensor 3 in the same solvent system, the selectivity experiments of compound 3 in response to various metal ions were measured by fluorescence and UV–vis absorption experiments in DMSO/H2O (v/v, 9/1) solution at room temperature. The fluorescence titrations were conducted in the presence of 14 kinds of different metal ions (Na+, K+, Ag+, Mg2+, Ca2+, Ba2+, Pb2+, Fe2+, Cu2+, Hg2+, Zn2+, Cr3+, Al3+ and Fe3+). It can be found that the fluorescence emission peak of 3 in DMSO/H2O (v/ v, 9/1) solution is located at 475 nm (Fig. 1) due to the intramolecular charge transfer (ICT) effect produces between benzimidazole and quinoline moiety. The fluorescence at 475 nm (I475) decreases distinctly after the addition of 10 equiv. Cu2+ or Hg2+ into the solution of 3 (10 μM), with the fluorescence quenching efficiency up to 97% and 92% for Cu2+ and Hg2+ respectively. Therefore, this is a fluorescent “turnoff” system for Cu2+ and Hg2+. Meanwhile, a red-shift of fluorescence emission peak of 3 can be observed upon the addition of Hg2+ (10 equiv.), but the same phenomenon is not found in the emission spectra of 3 when adding Cu2+. To our delight, though the decrease of I475 is also observed upon the addition of 10 equiv. Zn2+, the fluorescence quenching efficiency for Zn2+ (50%) is obviously lower than that toward Cu2+ and Hg2+. More importantly, the fluorescence intensity at 540–700 nm increases gradually and there is a change from greenish-blue to luminous yellow in the presence of Zn2+, indicating that chemosensor 3 has potential to be a fluorescent ratio- and colorimetric sensor for Zn2+. Furthermore, the investigations of fluorescence changes under the 365 nm UV-lamp after the addition of 10 equiv. different metal ions show that the fluorescence of 3 in DMSO/H2O (v/v, 9/1) is greenish-blue, and it changes significantly after Cu2+, Hg2+, or Zn2+ being added (Fig. 2). It can be found that the addition of Cu2+ (10 equiv.) exhibits a remarkable quenching effect on the fluorescence of the solution of 3. Of course, the quenching phenomenon is also observed in the presence of Hg2+, but the fluorescence is pale yellow. It may ascribe to the decreased fluorescence intensity and the red-shift of emission peak from 475 nm to

3.3. The fluorescence and UV–vis absorption titration of sensor 3 The dose-dependent fluorescence responses of sensor 3 to Cu2+, Hg , Zn2+ were determined by fluorescence titration experiments in DMSO/H2O (v/v, 9/1) at room temperature. It can be seen that, the fluorescence intensity of 3 (10 μM) at 475 nm decreases grdually with the increase of Cu2+ in the sensing system (Fig. S7). The fluorescence quenching efficiency of 3 is up to 97% when the amount of Cu2+ is added to 2 equiv. The relationship of I475 and Cu2+ concentration is exponential, as shown in Fig. S8. Within the low concentration range (c < 7.0 ✕ 10−6 M), the fluorescence intensity shows a good linear relationship with the Cu2+ concentration, and the correlation coefficient is 0.9809. Similarly, we also explored the response of sensor 3 toward Hg2+ and Zn2+ by fluorescence titration experiments (Fig. 4). The fluorescence intensity of 3 decreases remarkably upon the addition of Hg2+. It tends to be a stable value and the fluorescence quenching efficiency at 2+

3

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Fig. 2. Fluorescence changes of 3 (10 μM in DMSO/H2O, v/v, 9/1) after the addition of 10 equiv. of various metal ions under a 365 nm UV lamp.

Fig. 3. UV–vis absorption spectra (a), A417/A368 (b) and color changes in visible light (c) of 3 (10 μM in DMSO/H2O, v/v, 9/1) after the addition of 10 equiv. of Cu2+ or 10 equiv. of other metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

intensity at 550–700 nm continues to increase. The fluorescence of 3 is still bright yellow. Of course, with the increase of Zn2+, the ratio value (I580/I475) changes exponentially (Fig. S10). Within the range of low concentration (1–6 μM), it presents a good linear relationship. To investigate the interactions between sensor 3 and Cu2+, Hg2+, 2+ Zn , the UV–vis absorption spectra of 3 in the presence of them respectively were further studied. The solution of 3 in DMSO/H2O (v/v, 9/1) is colorless and its absorption peak is located at 368 nm due to ππ* transition (Fig. 5a). With the addition of Cu2+, the absorbance at 368 nm significantly decreases and the absorbance near 417 nm gradually increases. It tends to be stable after 1.8 equiv. of Cu2+ being added. And a clear isosbestic point can be observed at 385 nm, indicating that there is a new product generated upon the addition of Cu2+ to 3 in solution [21,22,47]. Importantly, there is a larger red-shift of absorption peak, making the solution color of 3 changed from

475 nm is 92% when the amount of Hg2+ is added to 2 equiv. At the same time, there is a gradual red-shift of the emission peak, so that the fluorescence changes from greenish-blue to dark yellow, as shown in Fig. 4a. Compared with Fig. S7, this is an important difference for sensor 3 to distinguish Hg2+ from Cu2+. Of course, the relationship between I475 and the concentration of Hg2+ shown in Fig. S9, is exponential too. On the other hand, the response of chemosensor 3 to Zn2+ is obviously different (Fig. 4b). The fluorescence intensity of the sensing system at 400–550 nm declines gradually and the fluorescence intensity at 550–700 nm increases simultaneously in the presence of Zn2+. It's noteworthy that the fluorescence color of solution starts to change to bright yellow when Zn2+ is added to 10 equiv., as the illustration shown in Fig. 4b. When the amount of Zn2+ is up to 80 equiv., the emssion peak red shifts from 475 nm to 545 nm and the fluorescence

Fig. 4. Fluorescence spectra changes of compound 3 (10 μM in DMSO/H2O, v/v, 9/1) upon the addition of different concentration of Hg2+ (a), Zn2+ (b), (λex = 377 nm, Insert: Fluorescence color changes of 3 before and after addition of Hg2+ or Zn2+). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4

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Fig. 5. UV–vis absorption spectra changes (a) of sensor 3 (10 μM in DMSO/H2O, v/v, 9/1) and the plot (b) of A417/A368 of the sensor 3 upon the addition of different concentration of Cu2+ dissolved in water (Insert: Color changes of 3 before and after addition of Cu2+). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

colorless to yellow when the amount of Cu2+ is up to 0.6 equiv., as the illustration shown in Fig. 5b. What's more, the ratio of the absorbance at 417 nm and 368 nm (A417/A368) shows an exponential relationship to the concentration of Cu2+. The UV–vis absorption titration experiments were also carried out to investigate the interaction between sensor 3 and Hg2+ or Zn2+ (Figs. S11–S14). Though the isosbestic point at different wavelength can be found, the changes of red-shift induced by Hg2+ and Zn2+ are inconspicuous for the non-visible range absorption peaks. Thus, sensor 3 is able to recognize Cu2+ via naked-eyes under visible light.

According to the reported method [37,38], the LOD for Hg2+ is determined as 0.2 μM (Fig. S16). Compared with some reported sensors for Hg2+, this value is better than some sensors for Hg2+ (Table S5) [17,18]. To our delight, the fluorescence intensity at 475 nm declines drastically in the presence of 2 equiv. Hg2+, but shows only slight change after 3.3 min, indicating that sensor 3 can be a fast-responsive sensor to detect Hg2+ [15,18]. What's more, the LOD for Zn2+ was also investigated as the method mentioned in the literature [4]. As shown in Fig. S17, the LOD is 0.94 μM for Zn2+. This implies that sensor 3 has the potential to sensitively detect Zn2+ (Table S6) [12,31,53]. After adding Zn2+ into the solution of 3 about 4.3 min, the ratio value of I580/I475 becomes stable. This response time is shorter than that of the general fluorescent sensor for metal ions [51]. Thus, sensor 3 has desirable sensitivity and response time for the detection of Cu2+, Hg2+ and Zn2+.

3.4. The sensitivity of 3 to Cu2+, Hg2+ and Zn2+ The limit of detection (LOD) is an important indicator to measure the sensitivity of the sensor. According to the corresponding references [35,36], the LOD of sensor 3 toward Cu2+ can be calculated to be 1.12 nM (Fig. 6) by the formula, LOD = 3δ/K, which is much lower than the LOD of some reported sensors for detecting Cu2+ (Table S4) [1,6,8,13]. What's more, the response time for the detection of Cu2+ also has been investigated. The changes in the absorption spectra are tending to be stable in 8 min after the addition of Cu2+ (Fig. S15a). This response time is comparable to some conventional metal ion sensors [51,52]. Therefore, these results manifest that sensor 3 has better sensitivity than most of sensors for Cu2+ detection.

3.5. The probable mechanism of sensor 3 toward Cu2+, Hg2+ and Zn2+ To further get insight into the interaction, the sensing mechanism of sensor 3 to Cu2+, Hg2+ and Zn2+ were determined. Firstly, the stoichiometry and binding constants of 3 with these metal ions were measured. The stoichiometry of sensor 3 with Cu2+ can be obtained by the Job's plot (Fig. 7), the appearance of maximum fluorescence emission at the 0.5 molar fractions clearly indicates the interaction between 3 and Cu2+ as ratio 1:1. According to the corresponding references

Fig. 7. Job's plot of binding stoichiometry between 3 and Cu2+ in DMSO/H2O (v/v, 9/1) solution (λex = 377 nm), the total concentration of [Cu2+] and [3] are 10 μM.

Fig. 6. The plot of the absorbance ratio of the sensor 3 between 417 nm and 368 nm (A417/A368) upon the addition of different concentration of Cu2+ (c < 4.5 ✕ 10−6 M) dissolved in water. 5

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[16,54], the combining constant of chemosensor 3 and Cu2+ is determined by fluorescence titration study, which can be calculated through the Benesi-Hildebrand equation: 1/(I0-I) = 1/{Ka (I0-Imin) [Cu2+]} + 1/(I0-Imin). The binding constant Ka of sensor 3 with Cu2+ is 2.56 ✕ 108 M−1 (Fig. S18). Similarly, the stoichiometry of sensor 3 with Hg2+ or Zn2+ was also determined by the Job's plot through experiments (Figs. S19–S20). As shown in Fig. S19, the presence of maximum fluorescence emission at the 0.33 molar fractions indicates the interaction between 3 and Hg2+ as ratio 1:2. While for Zn2+, the combination ratio is 1:1 (Fig. S20). Thus, the binding constant is determined respectively. For Hg2+, it can be calculated by the Benesi-Hildebrand equation: 1/(I0 - I) = 1/{Ka (I0 Imin) [Hg2+]2} + 1/(I0 - Imin) as the combination ratio of 1 : 2, which is 1.58 ✕ 107 M−2. For Zn2+, it is calculated to be 6.36 ✕ 104 M−1 by the Benesi-Hildebrand equation: 1/(I0 - I) = 1/{Ka (I0 - Imin) [Zn2+]} + 1/ (I0 - Imin). Compared with the binding constants of Cu2+, Hg2+ and Zn2+ from the fluorescence titrations (Table S2), it can be found that the binding affinity of Cu2+ with 3 is the biggest among the three metal ions. And for binding 3 with Zn2+, it is the smallest. These mean that, due to the good binding affinity of Cu2+, the signals of sensor 3 in responding to Cu2+ might be less disturbed by Zn2+. Contrarily, the signal of 3 in response to Zn2+ might be disturbed by Cu2+ and Hg2+ in some extent. These are in accordance with competitive experiments. In the previous selectivity experiments, the fluorescence quenching efficiency for Cu2+ or Hg2+ can be over 92%, this quenching phenomenon is needed to be further investigated. As we know that the quenching processes might be static, dynamic quenching mechanism or both of them, which can be studied by the Stern-Volmer equation: “I0/I = Ksv[Mn+] + 1” [21]. However, the nonlinear Stern-Volmer plots at higher Cu2+ or Hg2+ concentration (Figs. S21–S22) means that more than one type of quenching mechanism might work simultaneously [55]. To further get insight the nature of the quenching process, the fluorescence decay curves of sensor 3 were obtained with the addition of different equivalents of Cu2+ or Hg2+ by the time-correlated single photon counting (TCSPC) experiments (Fig. 8), with the measured lifetime data in details (Table S3). The average lifetime (τ) of sensor 3 in DMSO/H2O (v/v, 9/1) is 1.12 ns. When the Cu2+ is added, the τ is still remained to be 1.12 ns. This indicates that the lifetime of the sensor 3 is not affected by the static quenching, owing to the ground state nonfluorescent complexation between luminophore and quencher formed. The lifetime of sensor 3 is not affected by Hg2+ in low concentration (less than 1 equiv.), which means a static quenching mechanism also. When the amount of Hg 2+ is equal to or more than 1 equiv., the lifetime of the solution of sensor 3 gradually decreases (Table S3), indicating that there is a dynamic quenching process. It might be caused by the collisional energy transfer between excited luminophore in sensor 3 to the quencher (Hg2+), or the external heavy atom (Hg2+) induced emission quenching [21,56].

To explore the sensing mechanism, the complexes of sensor 3 and metal ions have been confirmed by the LC-MS (Figs. S23–S25). In the tests, the metal ion (Cu2+, Hg2+, Zn2+, respectively) is added into the solution of sensor 3 to obtain complex, using MeOH/HCOOH (v/v, 1/1) solution as the mobile phase. When Cu2+ is added into the solution of sensor 3, there is a peak at m/z 1099.84 (Fig. S23). This indicates that a 2:2 copper complex may be formed in the sensing system [57,58], combining with the above other analysis results. Similarly, for Hg2+, the peak at m/z 1000.32 reveals that a 1:2 complex has been formed (Fig. S24), being in accordance with Job's plot. And the peak at m/z 689.17 is found for the complex of 3 with Zn2+ (Fig. S25), confirming that a complex has been formed also [59–61]. As the reported method [62–65], adding different equivalents of metal ions in D2O into the solution of sensor 3, the 1H NMR titration experiments were conducted in DMSO‑d6 to further determine the interaction of sensor 3 with the Cu2+, Hg2+ and Zn2+. As depicted in Fig. 9, the chemical shift of the benzimidazole moiety can be found at 13.27 (H1, H1′), 8.15 ppm (H4), 8.18 ppm (H4′), 7.80–7.89 (H6, H6′, H7, H7′) ppm, respectively. And the partial signals in quinoline moiety can be found at 8.56 (H10, H10′), 8.49 (H9, H9′), 8.07 (H11, H11′) ppm, respectively. These signals gradually shift to downfield in the presence of Cu2+, especially the chemical shift of N-H (H1, H1′) nearly disappears with the increase of Cu2+. These changes might be attributed to the combination of Cu2+ with N atoms in the benzimidazole and quinoline rings and the deprotonation of N-H in imidazole ring in compound 3. Thus, the photoinduced electron transfer (PET) process appears in the complex (3 + Cu2+), inducing the fluorescence quenching in the sensing system [58,66]. On the other hand, the proton signals of 3 are also affected by the interference of the paramagnetism of Cu2+ [16]. Similarly, it can be found in Fig. S26, the proton signals in benzimidazole (H1, H4, H6, H7) and quinoline (H9, H10, H11) moiety gradually shift to downfield when Hg2+ is added, indicating that Hg2+ ion may be bonded to N atoms in benzimidazole and quinoline moiety of sensor 3. Afterwards, a complex is formed between 3 and Hg2+ by coordination, resulting in the fluorescent quenching. These changes might be assigned to ligand-to-metal charge transfer (LMCT) process occurring from sensor 3 to Hg2+ ion [23,67,68]. What's more, the chemical shift of H also shifts to downfield upon addition of Zn2+ (Fig. S27), which is mainly due to the combination of Zn2+ with N atoms in the benzimidazole and quinoline rings, resulting in a significant enhancement of the weak intramolecular charge transfer (ICT) effect in the sensor 3. Thus, there is a fluorescence change [11,22,69]. The interaction between sensor 3 and metal ions has been further confirmed by the FT-IR spectra. As shown in Fig. S28, the compound 3 shows an imine stretching band of imidazole and quinoline rings at 1662 cm−1 and C-N stretching band at 1100 and 1052 cm−1. After the addition of Cu2+, Hg2+ or Zn2+ respectively, there is a red-shift for C=N stretching band in a degree. Meanwhile, the C-N stretching band at 1052 cm−1 disappears and there is a blue-shift for the signal at

Fig. 8. Time-correlated single photon counting (TCSPC) plot for sensor 3 (10 μM) and 3 with different amounts of Cu2+ (a) or Hg2+ (b). 6

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Fig. 9. Partial 1H NMR spectra changes of sensor 3 (0.025 M in DMSO‑d6) in the presence of Cu2+.

1100 cm−1. These may be due to the coordination between metal ions and C=N and C-N in sensor 3, resulting in the decrease of density of conjugated molecular electron cloud [7,58]. According to the above mentioned experimental evidences, we can clearly know that there should be strong interaction between sensor 3 and Cu2+, Hg2+or Zn2+ ion. To explore the combining model and sensing mechanism of sensor 3 toward Cu2+, Hg2+, Zn2+, the structures of sensor 3 and metal complexes were simulated and optimized by the corresponding methods [16,39,70]. It can be found in Fig. 10 that, the highest occupied molecular orbital (HOMO) of sensor 3 is mainly located at benzimidazole rings and few is located at the quinoline rings. On the contrast, the

lowest unoccupied molecular orbital (LUMO) of sensor 3 is mainly distributed on quinoline rings and little on benzimidazole rings, with the energy gap 3.56 eV. When a 2:2 binding complex is formed in the sensing system upon the addition of Cu2+, the HOMO of Cu2+-complex (3 + Cu2+) is located at one of the sensor molecule in 3 + Cu2+, while the LUMO is located at the other sensor molecule in the complex. The energy gap is 0.33 eV, which is much lower than the energy gap of sensor 3 and benefits to the generation of PET process. However, because of hydrogen bonding network formed in sensor 3, the PET process is restricted, endowing it well fluorescent properties [5]. When the Cu2+-complex is formed by the coordination with Cu2+, the deprotonation appears in compound 3 and makes the hydrogen bonding

Fig. 10. Frontier molecular orbitals of sensor 3 and complexes of 3 combined with Cu2+, Hg2+, Zn2+ (calculated using DFT and optimized with the B3LYP/6-31G(d, p) basis set for 3 and B3LYP/6-31G basis set for C, H and N, LanL2DZ for metal ions in complexes). 7

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network broken. Therefore, the PET process occurs in Cu2+-complex, leading to the fluorescence quenching [58,71]. It also can be seen in Fig. 10 that, for the solution of sensor 3 in the presence of Hg2+, the complex (3 + Hg2+) may be easily generated due to its lower energy. The distribution of HOMO in 3 + Hg2+ changes a little compared with sensor 3, but the LUMO of 3 + Hg2+ totally transfers to the Hg2+, which are different from those observed in 3 + Cu2+. These results demonstrate that the electron cloud redistribution occurs after the combination of sensor 3 with Hg2+, inducing the LMCT process between sensor 3 and Hg2+. Thus, the fluorescence quenches remarkably in the presence of Hg2+ [23,67,68]. In addition, the electron of Zn2+-complex (3 + Zn2+) is also redistributed (Fig. 10). It's well to be reminded that the HOMO-LUMO of the 3 + Zn2+ is located at the two sides of the sensor (benzimidazole and quinoline moiety) respectively. And the energy gap is 0.87 eV, which is lower than that of the sensor 3. Both are beneficial to improve the ICT effect in the sensing system, leading to the fluorescence change from greenish-blue to luminous yellow [11,22]. Based on the above systematical investigations, we determine that sensor 3 is able to combine with different metal ions by various models via different mechanism (Fig. 11). When Cu2+ is added into the solution of sensor 3, the fluorescence quenches because the PET process produces in the Cu2+-complex. For Hg2+ ion, the LMCT effect occurs in the complex (3 + Hg2+), inducing the fluorescence quenching. For Zn2+ ion, the fluorescence color change is due to the enhancement of ICT effect in the complex (3 + Zn2+).

Fig. 12. The value of A417 in different pH before and after the addition of Cu2+ into the solution of sensor 3 in DMSO/H2O (10 μM, v/v, 9/1).

3.6. Effects of pH and reversibility of sensor 3 toward Cu2+, Hg2+, Zn2+ The pH value of environment may affect the performance of sensor in the practical application [72]. To investigate the pH tolerance of sensor 3 toward Cu2+, the absorption spectra of the sensor was studied in the absence or presence of Cu2+ in different pH range. As shown in

Fig. 11. The possible mechanism of sensor 3 toward Cu2+, Hg2+ or Zn2+ in solution system. 8

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Fig. 13. The relative absorbance ratio (A417/A368) obtained during the titratition of sensor 3 with Cu2+ (10 μM) and EDTA (10 μM) in DMSO/H2O (v/v, 9/1) solution (a), the fluorescence intensity at 475 nm showing the reversity and reusability of sensor 3 for sensing Cu2+ (b) and the corresponding truth table (c); the general representation of an IMPLICATION gate (d).

leading to the dissociation of Cu2+ from the complex (3 + Cu2+). They result in the recovery of the sensing system [45], which occurs in the 5 times repeated experiment. Thus, it is confirmed that compound 3 shows a reversible ratiometric and colorimetric changes to Cu2+ ion and then sensor 3 can be efficiently recycled for the detection of Cu2+ by the treatment of EDTA. Similarly, the reversibility of sensor 3 towards Hg2+ can be achieved with the assist of DETA, and the reversible detection for Hg2+ is able to perform in 5 cycles (Fig. S30). According to the method in the literature [8,21], the value (A417/ A368) of sensor 3 can be defined as the logic gate output while two chemical inputs are Cu2+ and EDTA respectively. For input, the addition and the absence of Cu2+ and EDTA can be defined as “High” and “Low”, respectively. For output, the color change (colorless to yellow) of sensor 3 is defined as “High”, while change (yellow to colorless) is defined as “Low” (Fig. 13c). Thus, the changes of sensing system are observed upon the addition of Cu2+, so that the output is read as “High”. Under other circumstances, the color of the solution is not affected, making the output read as “Low”. Therefore, it can be designed as molecular logic gate for the detection of Cu2+ (Fig. 13d).

Fig. 12, the initial absorption of sensor 3 at 417 nm is relatively stable. Under different pH conditions, when 2 equiv. Cu2+ is added into the solution of sensor 3, the absorption at 417 nm has no evident change in the pH range of 1.1–3.1, which may be due to the protonation of the N atom on benzimidazole or quinoline and the decrease of electron cloud density in 3 under a strongly acidic environment. In the pH range of 5.1–13.3, there is a clear red-shift from 368 nm to 417 nm for the absorption peak of sensor 3 and the absorbance is bascially stable (about 0.115). These indicate that sensor 3 can recognize the Cu2+ in the pH range of 5.1–13.3, which is relatively wide compared with some reported sensors [3,16]. The pH toluence studies of sensor 3 toward Hg2+ and Zn2+ were conducted by fluorescence experiments. As depicted in Fig. S29, the fluorescence of sensor 3 obviously decreases in the pH range of 6.9–11.2 after the addition of 2 equiv. Hg2+, and this appropriate pH range is wider than that reported before [20]. For Zn2+, it's suitable pH range is 5.1–11.2, which is wider than that of some previously reported zinc ion fluorescent sensors [12,54]. With the guidance of green chemistry, the reversibility of sensor has attracted much attention [63,64]. To identify the reusability of sensor 3 for detection of Cu2+, we tested the reversible absorption and fluorescence response of sensor 3 by the titration of 3 with Cu2+ followed by addition of ethylenediamine tetra-acetic acid (EDTA) as a powerful chelator for metal ions. As shown in Fig. 13, when Cu2+ is added into the solution of sensor 3, it absorption signal is ratiometric (A417/ A368 = 1.68) with the color changed from colorless (off) to yellow (on). The ratio value decreases upon addition of EDTA. And after mixture is becoming homogeneous, the recovery rate of A368 is up to 92% with the solution color change from yellow to colorless. The color changes of “colorless-yellow-colorless” can be recognized by naked eyes under the visible light (Fig. 13a). Furthermore, we can obtain that the fluorescence intensity of sensor 3 decreases obviously in the presence of Cu2+, and the fluorescence changes from dim yellow to greenish-blue after EDTA solution is added into the mixture. The fluorescence recovery rate is up to 91% during 5 times reversible experiments (Fig. 13b). Importantly, the fluorescence changes are easily monitored under 365 nm UV lamp. These are attributed to the fact that EDTA as a powerful chelator can coordinate with metal ions easily. So, once adding EDTA solution into the mixture of 3 + Cu2+, a stable complex Cu2+-DETA is formed subsequently,

4. Conclusions In summary, sensor 3 containing multiple benzimidazole and quinoline moieties has been synthesized. Its sensing performance toward Cu2+, Hg2+ and Zn2+ was conducted by dual channels in DMSO/H2O (v/v, 9/1). It shows high selectivity and good pH tolerability to them. The LOD of sensor 3 for Cu2+, Hg2+ and Zn2+ are 1.12 nM, 0.2 μM, and 0.94 μM respectively. The experimental and theoretical investigations of sensing mechanism demonstrate that 3 can recognize Cu2+, Hg2+ and Zn2+ by various combining models via different mechanism. The fluorescence quenching phenomenon occurs by PET or LMCT effect when Cu2+ or Hg2+ is added into the solution of 3 respectively. While in the presence of Zn2+, the ratiometric and colorimetric changes can be found due to ICT effect. More importantly, the detection of three metal ions can be fully identified in naked eyes according to their different output signals. Thus, the multifunctional sensor based on multiple nitrogen heterocycles containing both N combining sites and aryl fluorophores for the detection of three metal ions (Cu2+, Hg2+ and Zn2+) in multiple signals can be realized by brief design and simple synthesis. Even so, in the later, we hope to make efforts to achieve 9

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simultaneous detection of multiple analytes and thoroughly avoid the interference of high valent metal ions.

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