Sequential recognition of zinc ion and hydrogen sulfide by a new quinoline derivative with logic gate behavior

Abstract

A Schiff base-type fluorescent chemosensor L has been synthesized and characterized to relay recognition of Zn²⁺ and H₂S. Among various metal ions, only Zn²⁺ induces the fluorescence enhancement of the sensor L and results in an “Off–On” type sensing with excellent selectivity and high sensitivity in aqueous solution. The lowest detection limit for Zn²⁺ is 1 × 10⁻⁷ M. Density functional theory calculation for L and the resultant zinc complex is also performed in this study. The chemosensor also exhibits fluorescence quenching with Cu²⁺ in aqueous solution. Fluorescent changes of L upon the addition of Zn²⁺ and Cu²⁺ is utilized as an INHIBIT logic gate at the molecular level, using Zn²⁺ and Cu²⁺ as chemical inputs and the fluorescence intensity signal as the output. On the other hand, the consequent product of L and Zn²⁺, L–2Zn, is an excellent indicator for H₂S for displacement of Zn²⁺ from the complex L–2Zn. Its H₂S sensing behavior is not interfered with by reduced glutathione (GSH), L-cysteine (L-Cys), and even bovine serum albumin (BSA) indicates that L–2Zn is able to detect H₂S without any distinct interference from these biological thiols. The addition of H₂S leads to the fluorescence quenching of L–2Zn, forming an “On–Off” type sensing system. Therefore, the sensing process for Zn²⁺ and sequential detection of H₂S is a reversible one, and also constitutes an “Off–On–Off” type fluorescence monitoring system. This Zn²⁺ and H₂S sequential recognition via fluorescence relay enhancement and quenching give probe L the potential utility for Zn²⁺ and H₂S detection in aqueous media and biological samples.

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Introduction

Fluorescent sensors for the selective detection of various biologically and environmentally relevant metal ions, such as the zinc ion (Zn²⁺) have recently attracted great attention. Zn²⁺ is the second most abundant metal ion in the human body, which plays an important role in physiological and metabolic processes.^1–3 However, a lack of Zn²⁺ could result in an increasing risk of different diseases.^4,5 Thus the development of new and efficient systems for sensing Zn²⁺ is of great current interest. Among several chemical tools, fluorescent probes provide the optimal choice for the detection of Zn²⁺ in biological samples due to their simplicity and high sensitivity. In this regard, a variety of fluorescent Zn²⁺ probes have been developed based on various fluorophores.^6–9 From another viewpoint, a number of fluorescent sensors have been reported as molecular logic gates.^10,11 The use of molecular logic gates has become very popular for miniaturization of the information process using ions, photons, pH, temperature or certain molecules as inputs, whereby the observable changes in optical or electrochemical signals are the output data.¹²

On the other hand, organic compounds as sensors for metal ions and resultant metal complexes as sensors for anions or biomolecules have gained popularity recently.^13–19 Some of the sequential detection systems work through a cation displacement assay. For instance, the fluorescent sensor binding with the metal ion displays a significant change in its fluorescence intensity. Then the resultant metal ion complex acts as an ideal candidate for the recognition of anions through the liberation of the sensor from the metal complex due to the strong affinity of the anion towards the metal ion; consequently this brings a considerable change in the fluorescence profile of the receptor–metal complex.²⁰

Among most of the essential biomolecules of the human body, hydrogen sulfide (H₂S) acts as the third endogenous gaseous signaling compound (gasotransmitter) after NO and CO.^21–24 Altered levels of H₂S have been linked to many diseases;^25,26 for example, at a low concentration, H₂S can produce personal distress, whereas at a higher concentration, it can result in loss of consciousness, permanent brain damage, or even death through asphyxiation.²⁷ Therefore, the detection of H₂S in living systems has attracted great attention recently.^28–30 A few fluorescent sensors for H₂S have been reported over the last two years.^31–34 However, most of the reported sensors are based on specific H₂S-induced reactions and have some drawbacks such as having too long a response time and not being easy to recover. To solve these problems, fluorescent sensors based on cation displacement sensing mechanisms would be the best choice, because the cation displacement process usually occurs within a matter of seconds. The recovered organic fluorophores could coordinate with cations and be reused to recognize H₂S. Therefore, it would be highly desirable to employ an organic metal complex as a fluorescence probe for H₂S. In this regard, pioneering work has been reported from Nagano's laboratory,³⁵ where they developed a novel fluorescent probe with high selectivity and sensitivity based on azamacrocyclic Cu²⁺ complex chemistry for H₂S.

However, fluorescent sensors that can relay recognition of metal ions and H₂S are very rare. Inspired by the importance of sensing of Zn²⁺ and H₂S in physiological samples, the design of fluorescent sensors that can sequentially detect Zn²⁺ and H₂S is very meaningful.

Bearing the above statement in mind, in this work, a fluorescent sensor for Zn²⁺ and the resultant Zn²⁺ complex as a sensor for H₂S has been designed. Firstly, a quinoline-based Schiff base compound L was synthesized for the detection of Zn²⁺ since many quinoline-based fluorescent sensors have excellent selectivity for zinc sensing.^36,37 Density functional theory calculations for L and the resultant zinc complex are also carried out in this study. The fluorescent changes of L upon the addition of Zn²⁺ and Cu²⁺ are utilized as an INHIBIT logic gate at the molecular level. Secondly, the resultant complex L–2Zn was employed as an H₂S sensor via the removal of Zn²⁺ from the complex L–2Zn. As is well known that in the aqueous state under the physiological pH, the major form of H₂S exists as HS⁻, where the ratio of HS⁻ : H₂S is approximately 3 : 1.³⁸ Thus, in this work, HS⁻ was used as the equivalent of H₂S. The L–2Zn ensemble can detect H₂S within 2 min, which is much faster than other reported sensors based on H₂S-induced reactions, such as azide reduction^21,23,39 and nucleophilic reaction to achieve strong fluorescence.^22,30 Furthermore, this monitoring process for both Zn²⁺ and H₂S is an “Off–On–Off” monitoring system and could be extended to the design of new receptors for the detection of cations and anions in aqueous solutions.

Experimental

Reagents and instrumentation

8-Aminoquinoline, glycine, 4-methylphenol, all the cationic compounds and anionic compounds were purchased from Aldrich and used as received. All other chemicals were of the reagent-grade purchased from Tianjing Guangfu Chemical Company and used as supplied. All solvents used for synthesis and measurements were redistilled before use.

¹H NMR and ¹³C NMR spectra were obtained using a 400 MHz Varian Unity Inova spectrophotometer. Solid-state ¹³C CP MAS NMR was carried out using a Bruker Avance IIWB 400 M spectrometer at room temperature. All absorption and emission spectra were recorded at room temperature. UV-Vis absorption spectra were obtained using a Varian UV-Cary100 spectrophotometer. Steady-state luminescence spectra were measured using an Hitachi F-4500 fluorescence spectrophotometer with an excitation wavelength of 360 nm. Quantum yields were determined by an absolute method using an integrating sphere on an Edinburgh Instruments FLS920 spectrometer. All absorption and fluorescence spectra were recorded 2 min after the addition of cations or anions.

UV-Vis and fluorescence titrations

Solutions of compound L were prepared in dry DMF (10⁻² M). Metal perchlorates and anions were prepared in H₂O. In the titration experiments, each time 2 mL of solution of ligand L (10 μM) were added to a quartz cuvette (path length, 1 cm) and metal ions were added to the quartz cuvette using a micro-pipette. For fluorescence measurements, excitation was provided at 360 nm, and emission was collected from 410 to 630 nm.

Synthesis of compound L

The synthesis of chemosensor L was achieved from the condensation of compound 1 and compound 2 in a yield of 70% (Scheme 1, for the detailed experimental sections see ESI†).

Scheme 1 Synthetic process of L.

Results and discussion

The condensation of glycine-N-8-quinolylamide with 2,6-diformyl-4-methylphenol in ethanol provided the quinoline-based compound L in 70% yield (Scheme 1). It is well known that many quinoline-based fluorescent sensors have excellent selectivity for zinc sensing.^40–47 Therefore, the synthesized compound L was firstly used as a zinc probe. Subsequently the sequential complex L–2Zn was used as a hydrogen sulfide sensor based on a cation removal sensing mechanism.³⁵

Absorption spectroscopy of L and Zn²⁺

The binding behavior of compound L toward Zn²⁺ as its chloride salt was examined by UV-Vis spectroscopy. The titration experiment was carried out in CH₃CH₂OH–Tris–HCl buffer solution (50 mM Tris, 50 : 50, v/v, pH 7.2) by adding aliquots of Zn²⁺. The absorption spectrum of compound L (10 μM) was characterized by the presence of a typical absorption band at 320 nm due to the quinoline moiety (Fig. 1). Upon addition of Zn²⁺ ions (0–3 equiv.), the band at 320 nm was decreased slightly, while a new band at 260 nm increased gradually. This intensity changes of the absorption peaks were likely due to the coordination of L with Zn²⁺.⁴⁸ No obvious absorbance enhancement was observed at 260 nm when the concentration of Zn²⁺ was increased from 2 to 3 equiv., indicating the exclusive formation of a 1 : 2 complex between L and Zn²⁺ (Fig. 1a inset). A Job plot also confirmed the 1 : 2 stoichiometric ratio of L with Zn²⁺ (Fig. 1b).

Fig. 1 (a) UV-Vis absorption of L (10 μM) upon the addition of different concentrations of Zn²⁺ (0–3 equiv.) in CH₃CH₂OH–Tris–HCl buffer solution (50 mM Tris, 50 : 50, v/v, pH 7.2), (b) Job's plot of Zn²⁺ and L with a total concentration of 30 μM.

Fluorescence response of L and Zn²⁺

The fluorescence spectroscopic study is also an important method to investigate the zinc-binding behavior of the fluorescent probe L. In the fluorescence spectrum, compound L (10 μM) showed a weak emission at 482 nm when it was excited at 360 nm (Fig. 2). The weak fluorescence emission of compound L can be explained by two reasons as described in other reported quinoline-based sensors^42,49,50 and Schiff base-type sensors.⁵¹ Firstly, the six nitrogen atoms of L can form intramolecular hydrogen bonds with hydrogen atoms in the absence of metal ions, which results in a photo-induced electron-transfer, and the de-excitation of the resulting tautomer occurs mainly through a non-radiative pathway.^42,49,50,52 Secondly, C=N isomerization is also a decay process of excited states in compounds with an unbridged C=N structure so those compounds are often non-fluorescent.⁵¹ As a result, compound L emits only weak fluorescence. Upon addition of 2.0 equiv. of Zn²⁺ ions to the solution of sensor L, the intramolecular hydrogen bond is inhibited, the C=N isomerization is also inhibited, thus leading to a significant increase in the fluorescence emission. The fluorescence intensity of L increased gradually as a function of Zn²⁺ ion concentration (Fig. 2 inset). When more than 2.0 equiv. of Zn²⁺ was added, the fluorescence intensity became constant afterwards. The detection limit of L as a fluorescent sensor for the analysis of Zn²⁺ was found to be 0.1 μM from the titration experiment. The fluorescence quantum yield of compound L in the free state was found to be 7.9% and for the Zn²⁺-bound state was found to be 11.33%.

Fig. 2 Fluorescence spectra of L (10 μM) in CH₃CH₂OH–Tris–HCl buffer solution (50 mM Tris, 50 : 50, v/v, pH 7.2) in the presence of different concentrations of Zn²⁺ (0–5 equiv.). λ_ex = 360 nm. Inset: fluorescence intensity at 482 nm of L as functions of Zn²⁺ concentration.

Cation selectivity assays were performed for sensor L with an excitation energy of 360 nm while keeping other experimental conditions unchanged (Fig. 3, red bars). Fig. 3 illustrates the fluorescence intensities of sensor L in the presence of various metal ions under physiological conditions (CH₃CH₂OH–Tris–HCl buffer solution, pH 7.2). The results showed that, among the metal ions studied, only Zn²⁺ has a significant effect on the fluorescence intensity of the sensor L. Other cations which exist at high concentrations in living cells, e.g., Ca²⁺, Mg²⁺, Na⁺, and K⁺, did not enhance the fluorescence intensity, as shown in Fig. 3. These results were presumably due to the poor complexation of alkali metals or alkaline earth metals with sensor L. However, a fluorescence increase of Cd²⁺ was also observed; this is because Zn²⁺ and Cd²⁺ are in the same group of the periodic table and cause similar spectral changes when coordinated with fluorescent sensors.⁴⁵ However, Cd²⁺ is rarely present in biological systems and would not cause any problem when using sensor L in biological applications.

Fig. 3 Fluorescence response of L (10 μM) and different metal ions (5 equiv.) in CH₃CH₂OH–Tris–HCl buffer solution (50 mM Tris, 50 : 50, v/v, pH 7.2). The red bars represent the addition 5 equiv. of various metal ions to a 10 μM solution of L. The green bars represent the change of the emission that occurs upon the subsequent addition of 5 equiv. Zn²⁺ to the above solution. λ_ex = 360 nm, λ_em = 482 nm.

The fluorescence intensity of sensor L in the presence of 5 equiv. of Zn²⁺ mixed with various metal ions (5 equiv.) was measured (Fig. 3, green bars) in cation competition studies. In the presence of most of the investigated cations, the emission intensity of Zn²⁺-bound L was unperturbed. However, Cu²⁺, Ni²⁺, Co²⁺ and Fe³⁺ caused fluorescence quenching to some extent, due to the displacement of Zn²⁺ by these cations.^49,53 However, Cu²⁺, Ni²⁺ and Co²⁺ ions would have little influence in vivo, since they exist at very low concentrations and are negligible in normal biological samples. Thus, Zn²⁺ can be distinguished from these investigated ions under physiological conditions.

In addition to metal ion selectivity, the fluorescence intensity of sensor L at various pH values in the presence and absence of Zn²⁺ was also measured (Fig. 4). In the absence of Zn²⁺, protons did not induce any obvious fluorescence enhancement in the range of pH < 6.5, but slight increase was observed at pH > 6.5. The reason was that the formation of the intramolecular hydrogen bond was forbidden in alkaline solution. When Zn²⁺ was added, sensor L showed no appreciable sensing ability for Zn²⁺ at pH < 6.5, which could be due to the competition of H⁺ below pH 6.5. However, sensor L exhibited satisfactory Zn²⁺ sensing abilities when the pH was increased to the range 6.5–9. At pH = 7.2, the fluorescence intensity reached its maximum value, indicating that sensor L possessed the highest sensing ability in an environment similar to the physiological environment (pH = 7.2).

Fig. 4 Fluorescence intensity of L (10 μM) at various pH values in CH₃CH₂OH–Tris–HCl buffer solution (50 mM Tris, 50 : 50, v/v) in the absence and presence of Zn²⁺ (5 equiv.), λ_ex = 360 nm.

Logic gates based on L with Zn²⁺ and Cu²⁺ as the inputs

It has been reported that fluorescence sensors can act as logic gates.¹⁰ Based on the interaction of L with Zn²⁺ and Cu²⁺ ions with subsequent changes of its emission intensity, a binary INHIBIT type logic gate by employing Zn²⁺ and Cu²⁺ as inputs was constructed. In order to elucidate the design of the logic gate, binary “0” and binary “1” are assigned to the inputs and outputs. For the input signals, binary “0” stands for no addition of Zn²⁺/Cu²⁺. Binary “1” denotes addition of Zn²⁺/Cu²⁺. The output values (fluorescence intensity) below a pre-defined threshold level are translated into binary “0”, while the fluorescence output values above the threshold correspond to binary “1”, in accordance with positive logic convention. The four possible input combinations are (0,0), (1,0), (0,1) and (1,1) shown in Fig. 5. Free L shows a weak fluorescence emission at 482 nm (“0”). In the presence of Zn²⁺ ion the fluorescence is strong (“1”). In the presence of Cu²⁺ ion the fluorescence is low (“0”). In the simultaneous presence of both Zn²⁺ and Cu²⁺ ions, the system still exhibits a low fluorescence (“0”). Therefore, a two-input INHIBIT logic gate was successfully mimicked for L.

Fig. 5 The combinatorial logic scheme (top) of INHIBIT logic operations and the truth table (bottom).

Quantum mechanical calculations

Moreover, quantum mechanical calculations were carried out to confirm the configuration of L–2Zn, in which the B3LYP^54,55 function was used. For atoms C, H, N, Cl and O in this compound, the 6-31G(d,p) basis set was used, whereas the LANL2DZ basis set was used for the Zn atom. The calculations were performed using the Gaussian 09 suite of programs.⁵⁶ The optimized configuration of L and L–2Zn complex are depicted in Fig. 6a and 6b. The results show that there are two Zn²⁺ ions binding to L via five coordination sites, and the two Zn²⁺ ions coordinate with the O atom of para-methyl-substituted phenol and one Cl atom simultaneously. The Zn–O bond length is 2.090 Å (Zn–O_{phenol–oxygen}), the Zn–Cl bond length is 2.457 Å, and the Zn–N bond lengths are 2.058 Å (Zn–N_{amino–nitrogen}), 2.182 Å (Zn–N_{quinoline–nitrogen}) and 2.206 Å (Zn–N_{Schiff base–nitrogen}). It can be concluded that L can provide suitable space to accommodate the Zn²⁺ and Cl⁻ ions, forming a stable compound.

Fig. 6 The optimized configuration of (a) L, (b) L with Zn²⁺. HOMO–LUMO energy gaps for respective compounds and interfacial plots of the orbitals: (c) free L and (d) L–2Zn.

Additionally, the electronic properties of L and the L–2Zn complex were also analyzed. It has been shown that the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as the electronic distributions may be used to explain the changes in the fluorescent properties upon metal cation coordination and therefore detect the ratiometric response to metal cations.^57,58 With respect to L, the HOMO is mainly located on one of its aminoquinoline units, whereas the LUMO is mainly located on the phenol unit (Fig. 6c). But for the binding of two Zn²⁺ and one Cl⁻ ions to L, the HOMO is mainly located on the two aminoquinoline units, whereas the LUMO is mainly located on the phenol unit (Fig. 6d). Evidently, the binding of two Zn²⁺ and one Cl⁻ ions to L forces L to redistribute its electron density to stabilize the resulting L–2Zn complex formed. Compared to L, the energy level of the HOMO ofthe L–2Zn complex increases, but the LUMO decreases, so the HOMO–LUMO energy gaps for the L–2Zn complex are smaller than L. These changes can be ascribed to the electron redistribution after the binding. Thereby, it is indicated that the enhanced fluorescence spectra upon the binding of two Zn²⁺ and one Cl⁻ ions to L arises from the energy level changes caused by the electron redistribution.

Fluorescence spectroscopy of L–2Zn and H₂S

Recovery and reuse are important properties for a chemosensor toward the analyte.^59–61 In this study, considering the fact that Zn²⁺ can coordinate with sulfide anions to form a highly stable species ZnS, which has an extremely small solubility product constant (1.2 × 10⁻²³), the obtained fluorescence on the L–2Zn system is regarded as a promising ensemble for the fluorescent “On–Off” detection of H₂S via the Zn²⁺ displacement approach. Also in the H₂S selectivity experiment, it is very exciting and noteworthy that compound L can be regenerated only by adding HS⁻ to the solution containing L–2Zn. Thus, a sequential detection system is constructed, not only for Zn²⁺ detection, but also for the sensing of H₂S, as HS⁻ is equivalent to H₂S in physiological solution.³⁸

In addition, competition experiments were also carried out to explore the anti-interference ability of L–2Zn. Complex L–2Zn (10 μM) was treated with 10 equiv. of various anions, including AcO⁻, Br⁻, Cl⁻, I⁻, CN⁻, NO₃⁻, NO₂⁻, SO₃²⁻, PO₄³⁻ and SO₄²⁻. As shown in Fig. 7 (black bars), only HS⁻ caused considerable fluorescence quenching, whereas PO₄³⁻ brought about a slight decrease in the fluorescence intensity. In the anion competition experiments, the L–2Zn ensemble (10 μM) was treated with HS⁻ (10 equiv.) in the presence of various tested anions (10 equiv.) in pH 7.2 CH₃CH₂OH–Tris–HCl (50 mM Tris, 50 : 50, v/v) buffer. Also, the results showed that all of the relevant anions tested have virtually no influence on the fluorescence detection of HS⁻ (Fig. 7, red bars). In addition, some thiol-containing amino acids such as reduced glutathione (GSH), L-cysteine (L-Cys) and even bovine serum albumin (BSA) were also examined to further evaluate the H₂S selectivity. As with other reported organic metal complex fluorescent probes for H₂S,³⁵ the addition of these thiol-containing amino acids to L–2Zn did not induce any noticeable fluorescence intensity changes. Thus, the complex L–2Zn has a very high selectivity for H₂S.

Fig. 7 Fluorescence response of L–2Zn (10 μM) and different anions (10 equiv.) in CH₃CH₂OH–Tris–HCl buffer solution (50 mM Tris, 50 : 50, v/v, pH 7.2). The black bars represent the addition 10 equiv. of the various anions to a 10 μM solution of L–2Zn. The red bars represent the change of the emission that occurs upon the subsequent addition of 5 equiv. HS⁻ to the above solution. λ_ex = 360 nm, λ_em = 482 nm.

The mechanism of the sensing of H₂S can be described in Scheme 2. When Zn²⁺ is added, the intramolecular hydrogen bond of compound L is broken, and the lone pair of electrons on the nitrogen atoms is involved in coordination with the Zn²⁺, leading to a significant increase in the fluorescence emission. And the fluorescence is recovered by the addition of H₂S to L–2Zn ensemble, which is attributed to the low solubility product constant of ZnS. With alternately adding Zn²⁺ and H₂S, compound L shows an “Off–On–Off” type fluorescence change, meaning that the detection process is a reversible one for sequential sensing.⁶²

Scheme 2 The proposed mechanism of the sensing of Zn²⁺ and H₂S.

To further understand the sensing behavior of L–2Zn for H₂S, an H₂S titration experiment was conducted. The fluorescence titration experiments of L–2Zn with H₂S were recorded in pH 7.2 CH₃CH₂OH–Tris–HCl (50 mM Tris, 50 : 50, v/v) buffer solution. As can be seen in Fig. 8, in the absence of H₂S, the free L–2Zn displayed quite a high fluorescence. Importantly, with the addition of NaHS (0–10 equiv.), the fluorescence intensity of L–2Zn decreased gradually at 482 nm due to the displacement of Zn²⁺ from the complex L–2Zn. The detection limit of H₂S could reach the 10⁻⁶ M level from the titration experiment.

Fig. 8 Fluorescence spectra of L–2Zn (10 μM) in CH₃CH₂OH–Tris–HCl buffer solution (50 mM Tris, 50 : 50, v/v, pH 7.2) in the presence of different concentrations of H₂S (0–5 equiv.). λ_ex = 360 nm. Inset, fluorescence intensity of L–2Zn at 482 nm as a function of H₂S concentration.

Conclusion

In conclusion, we have synthesized a multi-responsive and selective sensor L as a probe to monitor Zn²⁺ and for the sequential detection of H₂S. The receptor showed high sensitivity and selectivity for Zn²⁺ detection with a detection limit of 10⁻⁷ M. And the density functional theory calculation of sensor L and L–Zn was further studied: the results obtained here were very consistent with the experimental results. The fluorescent intensity of compound L can be recovered by adding HS⁻ to the Zn²⁺ sensing system via the Zn²⁺ displacement approach. Therefore, the Zn²⁺ complex can be used for H₂S recognition. With alternately adding Zn²⁺ and H₂S, compound L shows an “Off–On–Off” type fluorescence change, meaning that the detection process is a reversible one. And the cation displacement approach employing a metal-complex for anion recognition allows for the design new receptors for the sensitive detection of anions in aqueous solutions.

Acknowledgements

The authors acknowledge financial support from the NSFC (Grants 21301082), the Natural Science Foundation of Gansu (no. 1308RJYA028) and the Fundamental Research Funds for the Central Universities (lzujbky-2013-61). The technical support from the Super Computing Center of Gansu Province is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47755j

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