A turn-on fluorescent sensor for relay recognition of two ions: from a F⁻-selective sensor to highly Zn²⁺-selective sensor by tuning electronic effects

Abstract

A simple ion sensor bearing quinoline and an amide group was designed and synthesized, which showed both colorimetric detection for F⁻ and a fluorescence turn-on response for Zn²⁺. Moreover, sensor L2 can distinguish F⁻ and Zn²⁺ via different sensing mechanisms (deprotonation for F⁻; inhibition of photo-induced electron transfer (PET) and excited-state intramolecular proton transfer (ESIPT) for Zn²⁺). Meanwhile the distinct color change and the rapid enhancement of fluorescence emission provide naked eye detection. This sensor achieved the detection of two ions which does not need to rely on two different probes: utilization of the innate reactivity of only one probe could achieve a dual recognition purpose in a tandem fashion.

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Ion recognition by artificial receptors has attracted increasing attention because of the important roles played by ions in both environmental and biological systems.^1–9 Fluorescent sensors for zinc ions have received wide attention because of the indispensable roles of zinc in living matter. Zinc is the second most abundant transition metal in mammals. Zinc ions play vital roles in various biological processes, e.g., as a cofactor in metalloproteins, in neurotransmission, in signal transduction, and as a regulator of gene expression and cellular apoptosis.^10–12 However, the disorder of zinc ions metabolism has been linked to several diseases such as Alzheimer's disease (AD), prostrate cancer, cerebral ischemia, and epilepsy.^13,14 At the same time, design and synthesis of fluoride anion receptors that are able to sense fluoride anion by the naked eye has currently been evolving as a forefront research topic because of the significant role of fluoride anion in the broad range of biological, environmental, and chemical processes and is important in public health and medicine.^15–20 For example, the fluoride anion has been utilized in many countries as an additive to water supplies and toothpaste because of its beneficial but still controversial effects on dental health.^21–25 In addition, the fluoride anion has been reported as a potential treatment of osteoporosis.^26–28 On the other hand, high levels of the fluoride anion, as found in certain well waters and other environmental sources, have been implicated in several types of human pathologies, including dental and skeletal fluorosis, osteoporosis, metabolic and neurological dysfunctions, and kidney failure.^29,30

The development of highly selective and ratiometric fluorescent sensors for zinc ions is still an important task, although a great of efforts have been devoted in recent years to explore fluorescent sensors for zinc ions by the ratio of the emission intensity changes.^31–33 More importantly, the greatest challenge for detecting zinc ions comes from the interference of other transition-metal ions, in particular cadmium ions. According to our previous work,³⁴ herein, we report a new amide derivative as a selective fluorescent sensor for zinc, which achieve the identification purpose by inhibition of photoinduced electron transfer (PET) and excited-state intramolecular proton transfer (ESIPT) effect. Notably, most of the previously reported relay recognition function sensing anions by zinc ion complex, however, the sensor we report presently can be recognized from anion-selective sensor to highly Zn²⁺-selective sensor.^35,36 Photoinduced electron transfer (PET) and excited-state intramolecular proton transfer (ESIPT) effect are very widely used mechanisms of fluorescence signal modification used in the design of zinc sensors.^37–41 PET sensors isolation of the signaling and sensing parts through aliphatic spacers cannot give a shift of the fluorescence maxima. On the contrary, it can provides significant enhancement of a fluorescence signal with increased concentration of the metal ions and gives “off–on” sensors. In ESIPT sensors the proton transfer process occurs between neighboring proton donor and receptor in one stimulated molecule, and the process is completed by intramolecular hydrogen bond. ESIPT process can effectively reduce the possibility of stimulated molecules light reaction and significantly enhance the light fastness of molecules. At the same time, we can observe an obvious Stokes shift. A combination of these two techniques promises better results in “turn-on” ratiometric sensor design since the effect of intensity increase due to the inhibition of PET and ESIPT. We are very gratifying to see that the results go as we expect, sensor L2 could allow selectively recognition of zinc in DMSO. The detection limit for zinc ion was as low as 0.21 mM.

The recognition profiles of the sensor L2 toward various anions, F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻ and CN⁻, were primarily investigated by UV-vis spectroscopy and fluorescence spectroscopy in DMSO. When 50 equivalents of fluoride were added to a DMSO solution of sensor L2 (2.0 × 10⁻⁵ M), there was a significant color change, from colorless to yellow, which was visible to the naked eyes (ESI, Fig. S1†). The color changes of solution mainly because the damaged of intramolecular hydrogen bonding by the addition of fluoride. In the UV-vis spectrum of a solution of L2 in DMSO (2.0 × 10⁻⁵ M), a strong absorption at 348 nm was decreased, meanwhile, a broad absorption was appearing at 400 nm when 50 equivalents of fluoride were added (Fig. 1).

Fig. 1 Absorption spectra of target compound L2 (2.0 × 10⁻⁵ M) in DMSO in the presence of fluoride anion (50 equiv.). Inset: photograph showing the change in color of the solution of L2 in DMSO after addition of various anions (50 equiv.).

To further investigate the interaction between sensor L2 and fluoride, the UV-vis spectroscopy variation of sensor L2 (2.0 × 10⁻⁵ M) in DMSO was monitored during titrations with different concentrations of fluoride from 0 to 30 equivalents (Fig. 2). With an increasing amount of fluoride, the absorbance peak at 348 nm gradually decreased. At the same time, a new peak appeared at 400 nm and two isosbestic points at 300 nm and 373 nm was clearly observed which indicated that a reaction taking place between L2 and fluoride.

Fig. 2 Absorption spectra of L2 (2.0 × 10⁻⁵ M) in the presence of different concentrations of fluoride anions (0–30 equiv.) in DMSO.

And then, the recognition profiles of the L2–F were primarily tested by addition of various metal cations (Fe³⁺, Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Cr³⁺, and Mg²⁺) by UV-vis spectroscopy in DMSO. When 20 equivalents of zinc were added to a DMSO solution of sensor L2–F, there was a significant color change, from yellow to colorless, which can be clearly observed by naked eyes (Fig. 3). The variation in the UV-vis absorption spectral of L2–F in DMSO was recorded during the titrations with different concentrations of zinc from 0 to 3 equivalents. As shown in Fig. 4, a strong absorption at 348 nm was increased and an absorbance peak at 400 nm was decreased gradually, and two isosbestic points at 300 nm and 374 nm were clearly observed with the increasing concentrations of zinc ions.

Fig. 3 Absorption spectra of target L2–F in DMSO in the presence of zinc ions (20 equiv.). Inset: photograph showing the change in color of the solution of L2–F in DMSO after addition of zinc ions.

Fig. 4 Absorption spectra of L2–F in the presence of different concentrations of zinc ions (0–3 equiv.) in DMSO.

Furthermore, L2–F expressed a very weak emission at 475 nm (λ_ex = 400 nm) in fluorescence spectroscopy. This is mainly due to that the nitrogen-atom of 8-aminoquinoline contain lone pair electrons which occur excited-state intramolecular proton transfer (ESIPT) with excited-state of naphthofuran fluorophore, zinc ion cannot corporate with L2 directly because of the strong intramolecular hydrogen bonding in L2, which lead to the sensor L2 cannot identified zinc by itself (Fig. 5).

Fig. 5 Fluorescence spectra upon excitation at 400 nm in DMSO of L2 (2.0 × 10⁻⁵ M), L2–F, L2–Zn and L2–F–Zn. Inset: photographs showing the change in the fluorescence of L2 after addition of fluoride anions and zinc ions in DMSO.

Nevertheless, with the addition of 20 equivalents of zinc ions in the L2–F solution, zinc ion cooperated with heteroatom (such as N and O) which blocked the photoinduced electron transfer (PET) and the fluorescence intensity at 500 nm increased rapidly. The color change from colorless to yellow green could be distinguished by the naked eyes under the UV-lamp (365 nm) as shown in Fig. 6.

Fig. 6 Fluorescence spectra upon excitation at 400 nm in DMSO of L2–F before and after addition of various ions (20 equiv.). Inset: photographs showing the change in the fluorescence of L2–F after addition of zinc ions (20 equiv.) in DMSO using UV lamp at room temperature.

To validate the selectivity of L2–F, the same tests were carried out using Fe³⁺, Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Cr³⁺ and Mg²⁺ metal ions. None of these ions induced any significant changes in the fluorescence spectrum of the sensor. The selectivity of L2–F for zinc ion over other metal ions was examined. The results revealed that all potentially competitive metal ions exerted no or little influence on the fluorescence detection of zinc ion in DMSO (Fig. 7).

Fig. 7 Fluorescence emission data for a 1 : 20 mixture of L2–F and different metal ions as their perchlorate salts in DMSO (excitation wavelength = 400 nm).

To further investigate the interaction between L2–F and zinc ion, the fluorescence emission spectral variation of L2–F in DMSO was monitored during titrations with different concentrations of zinc ions from 0 to 4 equivalents (Fig. 8). With an increasing amount of zinc ion, the emission peak at 500 nm gradually increased. The scatter plot indicated that the reaction basic achieved balance when the concentration of zinc ions increased to 1.0 equivalent and the reaction carried out in the combination of 1 : 1 (ESI, Fig. S1†). Furthermore, the detection limit of the fluorescent spectrum changes calculated on the basis of 3σ/m is 2.1 × 10⁻⁷ M for zinc, indicating that the sensor can detect very low concentration of zinc in the environment (ESI, Fig. S3†). Meanwhile, the stability and reproducibility of the sensor L2 were investigated by the change of UV-vis spectroscopy and fluorescence spectroscopy in DMSO during 2 h by the addition of fluoride and zinc ions (ESI, Fig. S4†). The experiment demonstrated that L2 could rapidly (within a short time of 3 s) detect fluoride and zinc and had an extraordinary stability in DMSO.

Fig. 8 Fluorescence spectra of L2–F in the presence of different concentration of zinc ions (0–4 equiv.) in DMSO.

The recognition mechanism of the sensor L2 with fluoride and zinc were investigated by IR spectra and ¹H-NMR titration methods. In the IR spectra of L2, the stretching vibration absorption peaks –NH– appeared at 3338 cm⁻¹. However, when L2 reaction with fluoride anion, the stretching vibration absorption peaks of amide –NH– disappeared, while, the stretching vibration absorption peaks of –NH– appeared at 3441 cm⁻¹ after the addition of zinc in L2–F solutions which indicated that fluoride took away the H proton of amide, and a coordination reaction occurred between L2, zinc and fluoride, which made the amide H proton return (ESI, Fig. S5 and S6†).

The results of ¹H-NMR titration experiments also support this presumption. As shown in Fig. 9, after adding 0.1 equivalents of fluoride anions, the single amide –NH– peak at 10.94 ppm decreased and gradually disappeared. At the same time, the other peaks didn't have any shift upon addition of fluoride, indicating that fluoride could taking the H proton away via deprotonating in the amide moiety of L2. However, with the addition of zinc later, the amide –NH– peak at 10.94 ppm enhanced again because of the coordination between L2 and zinc. In addition, the formation of 1 : 1 combine between L2–F and zinc ion was further confirmed by a Job plot (ESI, Fig. S7†) and the appearance of a peak at m/z 339 and m/z 423, assignable to [L2 + H⁺] and [L2 + F⁻ + Zn²⁺ + H⁺] in the ESI/MS as shown in ESI, Fig. S8 and S9.†

Fig. 9 Partial ¹H-NMR spectra of L2 (DMSO-d₆) and in the presence of varying amounts of fluoride anions and zinc ions.

Thus, the results of the IR spectra, ¹H-NMR titration and Job plot suggested that there are strong intramolecular hydrogen bonding between imide, furan O and quinoline N in the L2 molecules, which cause the ESIPT process and result in the fluorescent quenching. Afterwards, with the addition of fluoride anions in the L2 solution, fluoride took the H proton away from amide, and destroyed the intramolecular hydrogen bond at the same time, which enable the chemical bond of N-quinoline was able to rotate freely. Meanwhile, after adding of zinc ions, the cooperation reaction occurs between heteroatom (such as N and O) of L2–F and zinc ion which blocked the PET effect, and made the fluorescent intensity increased little by little (Fig. 10).

Fig. 10 Cartoon representation of the recognition process of sensor L2.

To investigate the practical application of sensor L2, test strips were prepared by immersing filter papers into a DMSO solution of L2 (0.1 M) and then drying in air. The test strips containing L2 was utilized to sense fluoride anions and the test strips containing L2–F was utilized to sense zinc ions. As shown in Fig. 11, when fluoride anions and zinc ions were added on the test kits, respectively, the obvious color change was observed only with fluoride anions in visible light. And the test strip containing L2–F has a significant color change with the addition of zinc ions. The same tests were also done by fluorescent method at the same time. Only L2–F–Zn displayed a dramatic fluorescent enhancement under the 365 nm UV lamp. Therefore, the test strips could conveniently detect fluoride anions and zinc ions in solutions.

Fig. 11 Photographs of L2 and the response of fluoride anions and zinc ions on test papers in visible light and under irradiation at 365 nm.

A single sensor L2, in which can distinguish F⁻ and Zn²⁺ via different sensing mechanisms (deprotonation for F⁻; inhibition of photo-induced electron transfer (PET) and excited-state intramolecular proton transfer (ESIPT) for Zn²⁺), has been well designed. This sensor exhibited an obvious color change in fluoride anion solution, and after the addition of zinc ions, the dramatically changes of UV-vis and fluorescent could be observed by naked eyes. Thus, it is believed that sensor L2 will have a role to play in the sensing, detection, and recognition of fluoride anions and zinc ions. In addition, simple but efficient test strips could conveniently and rapidly (within a short time of 3 s) detect fluoride and zinc in solutions. We believe that the turn on, ratiometric fluorescence and the involvement of the ESIPT and PET process in a single system is certainly a promising feature in the sensing of anions.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21161018; 21262032; 21574104), the Program for Chang jiang Scholars and Innovative Research Team in University of Ministry of Education of China (No. IRT1177), the Natural Science Foundation of Gansu Province (No. 1010RJZA018), the Youth Foundation of Gansu Province (No. 2011GS04735) and NWNU-LKQN-11-32.

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Footnote

† Electronic supplementary information (ESI) available: Complete experimental procedures and some of the spectroscopic. See DOI: 10.1039/c6ra05381e

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