Multi-analyte, ratiometric and relay recognition of a 2,5-diphenyl-1,3,4-oxadiazole-based fluorescent sensor through modulating ESIPT

Abstract

A new two dipicolyl amine (DPA) dangled 2,5-diphenyl-1,3,4-oxadiazole derivatized sensor L has been designed and synthesized. Sensor L displays excited state intramolecular proton transfer (ESIPT) properties in buffered water solution (HEPES 10 mM, pH = 7.4). L shows highly selective and ratiometric fluorescence responses to Zn²⁺ in a 1 : 2 binding ratio. Fluorescence spectra and ¹H NMR studies demonstrate that L binds Zn²⁺ ions through its amide form, which promoted the ESIPT emission enhancement. The in situ generated L–2Zn²⁺ complex can serve as a relay recognition sensor toward PPi and S²⁻ via further complexation and Zn²⁺ sequestering processes respectively. Thus, multi-analyte and relay recognition of L through modulating ESIPT has been achieved.

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1. Introduction

Zn²⁺, as the second most abundant transition metal ion in the human body, plays fundamental roles in a variety of biological processes including gene transcription, signal transmission, and mammalian reproduction.¹ Early studies proved that disruption of Zn²⁺ concentration in cells can result in various pathological processes such as Alzheimer's disease, epilepsy and infantile diarrhea.² Therefore, considerable efforts have been focused on the development of a Zn²⁺ specific fluorescent sensor due to the high sensitivity and easy operability of fluorescence techniques.³

Pyrophosphate (PPi) detection has attracted immense attention of chemists due to its critical roles in several bioenergetic and metabolic processes.⁴ PPi is released during the DNA polymerase chain reaction, and higher PPi levels in biological fluids such as blood serum are pertinent to cardiovascular disease and acute renal failure.⁵ Recent investigations reveal that high synovial fluid PPi is associated with some diseases such as formation of calcium pyrophosphate dehydrate crystals and chondrocalcinosis.⁶ As a consequence, development of chemosensors for highly selective detection of PPi aroused considerable interest and a great number of PPi selective sensors have been documented.⁷

On the other hand, sulfide detection has also received considerable attention because of its notorious toxicity. Sulfide has variable utilities in the fields of manufacturing of sulfuric acid, dyes and cosmetics,⁸ however, exposure to high levels of sulfide is dangerous because over inhalation of sulfide can lead to various physiological and biochemical problems including irritation in mucous membranes, unconsciousness, and respiratory paralysis.⁹ Consequently, a number of sulfide selective chemosensors have been developed.¹⁰ Although sulfide sensing utilizing a metal complex via S²⁻-induced metal ion displacement approach is a simple and effective method, fluorescent sulfide detection with Zn²⁺-complexes are still rare.¹¹

Recently, relay recognition, especially the sequential metal ion and anion recognition has received increasing more attention due to the operation simplicity and labor-saving, because the metal ion and anion recognition can accomplished sequentially under the identical conditions.¹² Fluorescent sensors with ESIPT property are very attractive due to the two wavelength emissions and large Stokes shift.¹³ Recent studies demonstrate that it is an effective approach to realize ratiometric recognition through modulating the ESIPT process.^11,14

With these ideas in mind, we herein report the design and synthesis of a new oxadiazole derivatized fluorescent sensor L, which is expected to serve as an effective relay recognition sensor for Zn²⁺ and PPi/S²⁻ through modulating the ESIPT behavior. The designed principles for sensor L are based on the following verities and hypothesis: (1) due to well-known DPA binding units, sensor L may exhibit highly selective and ratiometric response to Zn²⁺ along with Zn²⁺ binding induced enhancement of ESIPT;¹⁵ (2) the in situ generated L–Zn²⁺ complex has potential applicability to serve as an effective sensor for some anions such as PPi or S²⁻ through anion direct binding or anion elicit Zn²⁺ sequestering;^10c,14a,16 (3) it is rational to realize ratiometric recognition of Zn²⁺ and a certain anion through perturbing the ESIPT behavior of the sensor. As we expected, sensor L indeed displays highly selective and ratiometric fluorescence recognition to Zn²⁺ in water solution, the in situ formed L–Zn²⁺ complex can serve as a dual fluorescence responses sensor to PPi and S²⁻ through different fluorescence emission properties.

2. Results and discussion

2.1 Fluorescence recognition of L to Zn²⁺

Sensor L was readily prepared as depicted in Scheme 1, and the structure of L was fully characterized by NMR and HRMS spectrometry. In virtue of the good water solubility of L, we could examine its fluorescence properties in HEPES buffered (10 mM, pH = 7.4) solution. Upon excitation at 315 nm, L (10 μM) displays two emission bands appeared at 378 nm and 445 nm, they are attributed to the normal state shorter-wavelength and excited state longer-wavelength emission, respectively. This result indicates that our previous hypothesis that incorporation one more amide group may promote the ESIPT process is true. Subsequently, the metal ion selectivity of L was evaluated (Fig. 1). Upon addition 2.0 equiv. of Zn²⁺ to L solution, a significant fluorescence intensity enhancement was observed at 445 nm along with a decrease in intensity at 378 nm, indicating that the Zn²⁺ binding leading to a great enhancement of ESIPT emission of L. However, upon individual addition of other metal ions including Cu²⁺, Co²⁺, Hg²⁺, Ni²⁺, Ag⁺, Pb²⁺, Sr²⁺, Ba²⁺, Fe²⁺, Mn²⁺, Fe³⁺, Al³⁺, Cr³⁺, Mg²⁺, K⁺, Cd²⁺, and Na⁺ caused significant fluorescence quenching at 445 nm, particularly Cu²⁺, Co²⁺, Hg²⁺ and Ni²⁺ induced almost complete fluorescence quenching of L at 445 and 378 nm due to their paramagnetic properties.¹⁷ These results demonstrate that the ESIPT process of L was suppressed on interaction with the examined metal ions except Zn²⁺. Therefore, an excellent selectivity of L to Zn²⁺ was achieved. In addition, L solution also behaves strong blue fluorescence on addition of Zn²⁺ under illumination with a portable UV lamp at 254 nm (inset in Fig. 1). The time-dependent fluorescence changes of L solution in the presence of 2.0 equiv. of Zn²⁺ were then examined. The results reveal that the fluorescence intensity increases with prolonging the stocking time and reaches a plateau after 1.5 h of Zn²⁺ addition (Fig. S1†). Thus, in the following investigations, the fluorescence spectra were checked after 1.5 h of Zn²⁺ addition.

Scheme 1 Synthesis of sensor L. Reagents and conditions: (a) chloroacetic chloride, AcONa/AcOH; (b) DPA, KI, DIPEA, dry CH₃CN, reflux 10 h.

Fig. 1 Fluorescence spectra changes of L (10 μM) in HEPES buffered (10 mM, pH = 7.4) water solution upon respective addition 2.0 equiv. of different metal ions (λ_ex = 315 nm). Inset: fluorescence color changes of L solution before and after addition of Zn²⁺ under illumination with a portable UV lamp at 254 nm.

Subsequently, the fluorescence titration experiment was conducted to evaluate the sensing behavior of L to Zn²⁺. As shown in Fig. 2, upon incremental addition of Zn²⁺ (0 to 2.0 equiv.) to L solution, the emission intensity at 378 nm decreased gradually accompanied with significant enhancement of emission band at 445 nm. This ratiometric fluorescence emission variation terminated when 2.0 equiv. of Zn²⁺ was employed, indicative of a 1 : 2 binding stoichiometry of L and Zn²⁺. Benesi–Hildebrand plot analysis based on 1 : 2 binding ratio was carried out and resulted in a nice linear straight line (linear coefficient greater than 0.99) (Fig. S2†), further advocates the proposed 1 : 2 binding ratio of L with Zn²⁺. The apparent association constant of L and Zn²⁺ was estimated to be 1.67 × 10¹⁰ M⁻². Based on the titration data, the detection limit of L to Zn²⁺ was then calculated. Plotting the normalized fluorescence intensity against log[Zn²⁺] provided a nice linear relationship (Fig. 3), and the point of this line crossed the ordinate axis is regarded as the detection limit,¹⁸ which was found to be 2.77 × 10⁻⁶ M.

Fig. 2 Fluorescence spectra changes of L (10 μM) in HEPES buffered (10 mM, pH = 7.4) water solution upon incremental addition of Zn²⁺ (0 to 2.0 equiv.). λ_ex = 315 nm.

Fig. 3 Changes of normalized fluorescence intensity (F − F_min)/(F_max − F_min) of L (10 μM) against log[Zn²⁺] in water solution (HEPES 10 mM, pH = 7.4). λ_ex = 315 nm, λ_em = 445 nm.

To further verify the binding ratio of L with Zn²⁺, Job's plot analysis was performed. The results show that the emission intensity of the tested solution reaches a maximum when the mole ratio of Zn²⁺ appeared at 0.67, indicating the 1 : 2 binding stoichiometry of L and Zn²⁺ (Fig. S3†). A solid evidence for this binding ratio comes from the high-resolution mass spectroscopy (HRMS) analysis of L–Zn²⁺ solution, the significant peak appeared at m/z 893.1190 is assignable to [L–2H⁺ + 2Zn²⁺ + Cl⁻]⁺ (Fig. S4†).

To obtain better insights into the binding behavior of L and Zn²⁺, ¹H NMR spectra of L with and without Zn²⁺ were compared in DMSO-d₆, because the Zn²⁺ induced enhanced emission band of L in DMSO (λ_em = 462 nm) is very close to that in buffered water solution (λ_em = 445 nm) (Fig. S5†), indicating the similar coordination modes of L with Zn²⁺ in both H₂O and DMSO solutions. As shown in Fig. 4, the amide NH proton in free L signaling at 11.50 ppm up-field shifted to 10.82 ppm (Fig. 4b) on addition of Zn²⁺, suggesting that L binds Zn²⁺ through an amide form.^3d,19 The methylene protons (H_a) adjacent to C=O appeared at 3.44 ppm (Fig. 4a) downfield shifted to 3.86 ppm after binding with Zn²⁺ (Fig. 4b). Concomitantly, the singlet peak of methylene protons (H_b) in DPA moiety at 3.87 ppm (Fig. 4a) down-field shifted to ca. 4.40 and divided into two nonequivalent groups (Fig. 4b), indicating that the free rotation of sigma bonds in DPA unit of L are restricted after coordination with Zn²⁺. The proton signal at 8.51 ppm (Fig. 4a) could be tentatively assigned to H_d due to the possible hydrogen bonding between H_d and amide O atom.¹⁵ After binding with Zn²⁺, this signal up-field shifted to ca. 8.0 ppm (Fig. 4b), indicative of the chelation of amide O with Zn²⁺, which suppressed the hydrogen bonding. Protons neighboring pyridine N atom (H_c) signaling at 8.38 ppm (Fig. 4a) downfield shifted to 8.72 ppm (Fig. 4b) on addition of Zn²⁺. These results are very similar to those we previously reported sensor OXD for Zn²⁺ recognition.¹⁵ Thus, the ¹H NMR investigations demonstrate that L binds with Zn²⁺ through an amide form. Based on these results, the proposed binding mode of L and Zn²⁺ is illustrated in Fig. 4.

Fig. 4 ¹H NMR spectrum of L (DMSO-d₆) in the absence (a) and presence (b) of Zn²⁺.

As a good sensor, the anti-interference ability is also important. Thus, the influences of other potential competitive metal ions were assayed. As we previously described, individual addition of other metal ion except Zn²⁺ produced different extent emission quenching of L at 445 nm. On further addition of Zn²⁺ to L solution containing other metal ion (except Cu²⁺), noticeable fluorescence enhancement at 445 nm could be observed (Fig. 5). Even though Co²⁺ and Hg²⁺ induced moderate interferences on Zn²⁺ recognition event, the substantial emission signal enhancements are clear enough to signify the existence of Zn²⁺. Coexistence of Cu²⁺ could totally suppress the fluorescence emission of L, indicating that L binds Cu²⁺ much more tightly than that of Zn²⁺, which excludes the metal ion exchange. These results demonstrate that the Zn²⁺ recognition process has a good anti-interference ability to other metal ions except Cu²⁺.

Fig. 5 Fluorescence intensity (at 445 nm) changes of L (10 μM) in water solution (HEPES 10 mM, pH = 7.4) in the presence of various metal ions. The gray bars represent the fluorescence intensity of L solution in the presence of 2.0 equiv. of miscellaneous metal ions; the red bars represent the fluorescence intensity of the above solution upon further addition of 2.0 equiv. of Zn²⁺. λ_ex = 315 nm.

To further confirm the practicability of L for Zn²⁺ detection, pH effect on fluorescence changes of L (at 445 nm) with and without Zn²⁺ was then examined (Fig. 6). L solution exhibits weak fluorescence emission within pH range from 2 to 13. In the presence of 2.0 equiv. of Zn²⁺, the solution displays strong fluorescence emission from pH 7 to 10, suggesting that sensor L is suitable for Zn²⁺ detection at near neutral pH conditions.

Fig. 6 The fluorescence intensity changes (at 445 nm) of L and L–2Zn²⁺ solution at various pH conditions. λ_ex = 315 nm.

2.2 Relay recognition of L–2Zn²⁺ to S²⁻ and PPi

With the relay recognition idea in mind, we then examined the fluorescence response of L–2Zn²⁺ complex to different anions. L–2Zn²⁺ solution for anion sensing was in situ prepared by simply mixing 2.0 equiv. of Zn²⁺ with L. As depicted in Fig. 7, addition of S²⁻ to L–2Zn²⁺ solution elicited fluorescence band recovery of L, suggesting the S²⁻-induced Zn²⁺ ion sequestering event. On addition of PPi, a new emission band at 396 nm with 49 nm blue shift was observed. Other interested anions including F⁻, Cl⁻, Br⁻, I⁻, SCN⁻, PO₄³⁻, S₂O₃²⁻, H₂PO₄⁻, HPO₄²⁻, NO₂⁻, NO₃⁻, AcO⁻, ClO₄⁻, CN⁻, C₂O₄²⁻, SO₄²⁻, P₂O₇⁴⁻, HSO₄⁻, CO₃²⁻, and HCO₃⁻, induced almost negligible fluorescence emission changes of L–2Zn²⁺ solution. These results demonstrate that L–2Zn²⁺ complex can selectively detect S²⁻ and PPi through different fluorescence responses.

Fig. 7 Fluorescence spectra changes of the in situ generated L–2Zn²⁺ solution (10 μM, HEPES 10 mM, pH = 7.4) upon respective addition 5.0 equiv. of different anions. λ_ex = 315 nm.

2.2.1 S²⁻ recognition by L–2Zn²⁺ complex. To evaluate the S²⁻ sensing behavior of L–2Zn²⁺, fluorescence titration experiments of L–2Zn²⁺ solution with various amounts of S²⁻ was then conducted. Upon gradual addition of S²⁻ to L–2Zn²⁺ solution, the fluorescence intensity at 445 nm gradually decreased alone with increase in emission intensity at 378 nm (Fig. 8). When 3.0 equiv. of S²⁻ was added, the emission bands restored to the emission state of L, indicating that the bound Zn²⁺ in L–2Zn²⁺ complex was sequestered by S²⁻ and releasing the free L. This S²⁻-induced Zn²⁺ sequestering process was further proved by ¹H NMR spectra comparison. The ¹H NMR spectrum (in DMSO-d₆) of L–2Zn²⁺ in the presence of S²⁻ almost exactly restored to that of free L (Fig. S6†), this result strongly supports the Zn²⁺ displacement event. According to the titration profile, the detection limit of L–2Zn²⁺ to S²⁻ was evaluated to be 2.28 × 10⁻⁶ M following the aforementioned method (Fig. S7†). Competition experiments were then conducted and the results indicate that the S²⁻ recognition process is hardly influenced by other coexisting anions except PPi (Fig. 9).

Fig. 8 Fluorescence spectra changes of the in situ generated L–2Zn²⁺ solution (10 μM, HEPES 10 mM, pH = 7.4) upon addition of different amount of S²⁻ (0 to 3.0 equiv.). λ_ex = 315 nm.

Fig. 9 Fluorescence intensity (at 445 nm) changes of L–2Zn²⁺ (10 μM) in water solution (HEPES 10 mM, pH = 7.4) against various anions. The red bars represent the fluorescence intensity of L–2Zn²⁺ solution in the presence of 3.0 equiv. of miscellaneous anions; the gray bars represent the fluorescence intensity of the above solution upon further addition of 3.0 equiv. of S²⁻. λ_ex = 315 nm.

2.2.2 Recognition of PPi by L–2Zn²⁺ complex. As depicted in Fig. 7, addition of PPi to L–2Zn²⁺ solution results in the appearance of a blue-shifted emission band instead of recovery of the emission band of free L, which suggests that PPi may further bind with Zn²⁺ in L–2Zn²⁺ complex and hampered the ESIPT. To get further insights into the sensing behavior of the complex to PPi, fluorescence titration experiments were conducted (Fig. 10). On stepwise increase the added PPi concentration to L–2Zn²⁺ solution, the emission band centered at 445 nm gradually decreased and the newly formed emission band at 396 nm gradually increased. The fluorescence spectra changes terminated when 5 equiv. of PPi was employed.

Fig. 10 Fluorescence spectra changes of the in situ generated L–2Zn²⁺ solution (10 μM, HEPES 10 mM, pH = 7.4) upon addition of increasing amounts of PPi (0 to 5.0 equiv.). λ_ex = 315 nm. Inset: fluorescence color changes of L–2Zn²⁺ solution in the absence and presence of PPi under illumination with a portable UV lamp at 254 nm.

Non-linear least squares fitting of the titration profile based on a 1 : 1 binding equation provided a nice non-linear curve (Fig. S8†), which supports the 1 : 1 binding stoichiometry of PPi and L–2Zn²⁺. The association constant of PPi with L–2Zn²⁺ was estimated to be 1.11 × 10⁵ M⁻¹. Another powerful evidence for this binding ratio was obtained from the HRMS analysis of L–2Zn²⁺ solution in the presence of PPi. The prominent peak appeared at m/z 1057.0739 is assignable to [L + 2Zn²⁺ + P₂O₇⁴⁻ + Na⁺]⁺ (Fig. S9†), which supports not only the 1 : 2 binding ratio of L with Zn²⁺, but also the 1 : 1 binding stoichiometry of L–2Zn²⁺ and PPi. The plausible binding mode of L–2Zn²⁺ and PPi is illustrated in Scheme 2. Based on the titration data, the detection limit of L–2Zn²⁺ to PPi was evaluated to be 7.27 × 10⁻⁶ M utilizing the aforementioned method (Fig. S10†). Similarly, fluorescence competitive experiments were also examined to assay the anti-interference ability of PPi recognition (Fig. 11). The results demonstrate that the PPi recognition process is hardly hampered by other coexisting anion except S²⁻.

Scheme 2 Proposed binding mode of L–2Zn²⁺ with PPi.

Fig. 11 Fluorescence intensity (at 396 nm) changes of L–2Zn²⁺ (10 μM) in water solution (HEPES 10 mM, pH = 7.4) to various anions (λ_ex = 315 nm). The gray bars represent the fluorescence intensity of L–2Zn²⁺ solution in the presence of 5.0 equiv. of miscellaneous anions; the red bars represent the fluorescence intensity of the above solution upon further addition of 5.0 equiv. of PPi.

Recently, the development of molecular logic gates based on conversion of chemically encoded information into fluorescent signals has received considerable interest of chemists.^3c,20 On the basis of the competition experiments described in Fig. 9 and 11, we noticed that the emission intensity of L–2Zn²⁺ solution restored to the emission state of free L in the presence of both PPi and S²⁻ no matter the adding sequence of these two anions. Thus, an INHIBIT logic gate can be constructed when the fluorescence intensity of L–2Zn²⁺ solution at 396 nm was examined. Depending on the two chemical inputs, namely, input 1 (PPi) and input 2 (S²⁻), L–2Zn²⁺ can switch between high and low emission states. When the threshold value at 396 nm is set as 100, the output is recorded as 1 and 0 corresponding to the strong and weak emission intensity respectively. Input 1 elicits strong emission at 396 nm, represent as output ‘1’. On the contrary, input 2 results in a weak emission signal, represent as output ‘0’. Therefore, monitoring the emission intensity at 396 nm, upon addition of PPi and S²⁻, and their mixture results in an INHIBIT logic gate (Fig. 12).

Fig. 12 Corresponding truth table of the logic gate (a) and illustration of the INHIBIT logic gate (b).

3. Conclusion

In summary, we have developed a new DPA dangled oxadiazole derivative L possessing ESIPT property for highly selective and ratiometric recognition of Zn²⁺. L binds two Zn²⁺ ions through its amide form, which can promote the ESIPT emission enhancement. The in situ generated L–2Zn²⁺ complex behaves relay recognition behavior to PPi and S²⁻ via different response mechanisms. Addition of S²⁻ to L–2Zn²⁺ solution results in Zn²⁺ sequestering and releasing of L, however, introducing PPi can lead to further complexation with L–2Zn²⁺. Thus, multi-analyte and relay recognition of L through modulating ESIPT has been achieved.

4. Experimental

4.1 Materials and instruments

Unless otherwise specified, solvents and reagents were of analytical grade from commercial suppliers and were used without further purification. Compounds 1²¹ and 2²² were prepared according to reported methods. ¹H NMR and ¹³C NMR spectra were performed on an Agilent 400 MR spectrometer, and the chemical shifts (δ) were expressed in ppm and coupling constants (J) in Hertz. HRMS were measured on a Bruker micrOTOF-Q mass spectrometer (Bruker Daltonik, Bremen, Germany) or a matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometer (MALDI micro MX, Waters, USA). Fluorescence measurements were carried out on a Sanco 970-CRT spectrofluorophotometer (Shanghai, China). The pH measurements were made with a model PHS-25B meter (Shanghai, China).

4.2 Synthesis of sensor L

Dipicolyl amine (0.391 g, 1.96 mmol), potassium iodide (104.4 mg, 0.63 mmol) and 1.72 ml of diisopropylethylamine were added to 2 (0.4 g, 0.988 mmol) in 140 mL dry acetonitrile. The mixture was then heated to reflux and stirred for 10 h under nitrogen atmosphere. After removing the solvent, the residue was purified by column chromatograph with ethyl acetate and methanol (10 : 1, v/v) to afford 440 mg of sensor L. Yield: 60%. M.p. 101–102 °C. ¹H NMR (400 MHz, DMSO-d₆) δ 11.50 (s, 2H), 8.51 (d, J = 8.4 Hz, 2H), 8.38 (d, J = 4.4 Hz, 4H), 8.18 (d, J = 7.6 Hz, 2H), 7.63–7.54 (m, 10H), 7.33 (t, J = 7.6 Hz, 2H), 7.13 (t, J = 6.0 Hz, 4H), 3.87 (s, 8H), 3.44 (s, 4H); ¹³C NMR (100 MHz, DMSO-d₆) δ 170.49, 162.86, 157.87, 149.13, 137.63, 137.04, 133.25, 129.41, 124.31, 123.83, 122.80, 121.44, 112.01, 60.43, 58.37; HRMS (MALDI-TOF-MS, positive mode) calcd for C₄₂H₃₈N₁₀NaO₃ 753.3026 [L + Na⁺]⁺, found 753.2984.

4.3 General procedure for spectroscopic analysis

Doubly distilled water was used for all experiments. Sensor L was dissolved in HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, 10 mM, pH = 7.4) buffered water to afford the test solution (10 μM). Titration experiments were carried out in 10 mm quartz cuvettes at 25 °C. Metal ions (as chloride or nitrate salts, 10 mM) or anions (as sodium or potassium salts, 10 mM) in water (HEPES 10 mM, pH = 7.4) were added to the host solution and used for the titration experiment.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (no. 21476029, 21176029), the Natural Science Foundation of Liaoning Province (no. 20102004) and the Program for Liaoning Excellent Talents in University (no. LJQ2012096) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR and HRMS of sensor L, and other supplementary data. See DOI: 10.1039/c4ra16347h

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