A fluorescence chemosensor based on imidazo[1,2-a]quinoline for Al³⁺ and Zn²⁺ in respective solutions

Abstract

A new chemosensor, N′-[(2-hydroxyphenyl)methylidene]imidazo[1,2-a]quinoline-2-carbohydrazide (L2), was developed, which could detect Al³⁺ in DMSO/H₂O HEPES buffer and Zn²⁺ in EtOH/H₂O HEPES buffer. The chemosensor exhibits high selectivity and sensitivity for sensing Al³⁺ and Zn²⁺ with a fluorescence “turn-on” mode.

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Introduction

Serving as the most abundant metal on earth and the second most abundant transition metal in human body, aluminium and zinc, respectively, carry irreplaceable weight in living systems. As a potentially toxic ion, aluminium is a non-essential element for biological processes that has been implicated in various neurodegenerative and neurological disorders such as Alzheimer's disease, dialysis encephalopathy, and problems in bone and muscles.^1–3 According to a WHO (World Health Organization) report, the average daily intake of aluminium is approximately 3–10 mg per day for human beings, which has been widely used in food additives, aluminium-based pharmaceuticals, and aluminium containers and cooking utensils.⁴ Zinc is an essential element for life and plays critical roles in many biochemical processes such as gene expression, apoptosis, immune system responses and neurotransmission.^5–8 However, the disordered cellular zinc level can induce various diseases, for instance, Alzheimer's disease, epilepsy and infantile diarrhea.^9,10 Consequently, the recognition and quantification of aluminium and zinc ions are significant goals in both biological and environmental research fields.

With regards to the detection of environmentally and biologically relevant ions, fluorescence measurement is considered to be a versatile technique with high sensitivity, a rapid response time, and easy to perform.^11,12 In particular, molecular chemosensors that show fluorescence responses upon selective binding with metal ions have received great interest because it is cost-effective, rapid, facile, and amenable to real time-monitoring.¹³ In recent years, fluorescent chemosensors for the detection of zinc^14–20 or aluminium ions^21–24 have been emerging continuously. However, most of the fluorescence chemosensors are studied in single organic solvents such as THF, DMSO and EtOH. Till date, the effect of different solvents on chemosensors for the selective and sensitive metal-ion detection has been rarely reported.²⁵ Therefore, there is still an urgent need to develop new sensors that are able to recognize different metal ions selectively independent of the solvent environment. Based on the above mentioned needs and the studies on quinoline-based ligands,^26–33 a new and simple sensor (L2) for Al³⁺ and Zn²⁺ that shows distinctly different optical properties in different solvent systems was designed and synthesized (Scheme 1). Sensor L2 exhibits enhanced fluorescence with high selectivity upon binding to Al³⁺ in DMSO/H₂O HEPES buffer, but selectivity changes to Zn²⁺ in EtOH/H₂O HEPES buffer.

Scheme 1 Synthetic route of L1 and L2. Conditions: (1) CH₃CN, catalyst MoO₃, H₃PO₄, at 50 °C for 12 h; (2) benzotrifluoride, at 70 °C for 5 h; (3) THF, reflux for 16 h; (4) MeOH, at room temperature for 1 h; (5) EtOH, at room temperature for 12 h.

Experimental section

Materials and instruments

All organic reagents were obtained from commercial suppliers and used without purification. UV-Vis spectra were obtained on a Shimadzu 3100 spectrometer. Fluorescence measurements were carried out using an Edinburgh Instruments Ltd-FLS920 fluorescence spectrophotometer. ¹H NMR spectra were obtained on a Bruker AV III 400 MHz NMR spectrometer and ¹³C NMR spectra were obtained on a Bruker AV III 100 MHz NMR spectrometer with tetramethylsilane (TMS) as an internal standard. Infrared spectra were obtained using a Bruker Vertex 70 FT-IR spectrometer with KBr pellets.

Methods for the preparation of the receptor

Synthesis of compound imidazo[1,2-a]quinoline-2-carbohydrazide (L1). Compound L (ethyl imidazo[1,2-a]quinoline-2-carboxylate) was synthesized following a series of previously reported methods.^34–36 Hydrazine hydrate (80%, 4 mL) was added dropwise to a methanol solution (20 mL) of ethyl imidazo[1,2-a]quinoline-2-carboxylate (0.91 mmol, 0.22 g). The reaction mixture was stirred for 12 h at room temperature. The solvent was then removed under reduced pressure, and the residue was dissolved in chloroform (10 mL). The organic layer was washed with water, dried over anhydrous Na₂SO₄, filtered, and concentrated. The residue was purified by silica gel chromatography with chloroform/methanol (v/v, 15 : 1) as the developing solvent to give compound L1 as a gray solid (0.16 g, 77%). ¹H NMR [400 MHz, DMSO-d₆, J = Hz, δ (ppm)]: 9.58 (1H, s), 9.16 (1H, s), 8.50 (1H, d, J = 8.4), 8.01 (1H, dd, J = 7.9, 1.2), 7.76 (2H, ddd, J = 12.3, 9.7, 5.5), 7.61–7.51 (2H, m), 4.56 (2H, s). ¹³C NMR [101 MHz, DMSO, δ (ppm)]: 161.81, 142.99, 138.78, 132.86, 129.93, 129.63, 128.13, 126.03, 123.42, 117.14, 116.74, 114.76. [M + H]⁺: 227.1.

Synthesis of compound N′-[(1E)-(2-hydroxyphenyl)methylidene] imidazo[1,2-a]quinolie-2-carbohydrazide (L2). Salicylic aldehyde (0.98 mmol, 0.12 g) was added to an ethanol solution (30 mL) of imidazo[1,2-a]quinoline-2-carbohydrazide (0.43 mmol, 0.10 g). Then, the solution was stirred for 12 h at room temperature and a white precipitate appeared. The precipitate was filtered and then washed with ethanol to isolate L2 in pure form (0.09 g, 62%); ¹H NMR [400 MHz, DMSO-d₆, J = Hz, δ (ppm)]: 12.39 (1H, s), 11.51 (1H, s), 9.38 (1H, s), 8.80 (1H, s), 8.59 (1H, d, J = 8.3), 8.05 (1H, d, J = 7.9), 7.87 (1H, d, J = 9.6), 7.78 (1H, t, J = 7.8), 7.62 (2H, dd, J = 12.6, 5.9), 7.47 (1H, d, J = 8.0), 7.32 (1H, t, J = 7.8), 6.94 (2H, t, J = 7.6). ¹³C NMR [101 MHz, DMSO, δ (ppm)]: 158.85, 158.03, 149.41, 143.17, 137.98, 132.84, 131.75, 130.37, 130.11, 129.72, 128.74, 126.36, 123.52, 119.81, 119.13, 117.02, 116.93, 116.67. ESI-MS: [M + H]⁺: 331.1.

Preparation of test solutions

Stock solutions of the probe L2 (2.0 × 10⁻⁵ M) were prepared in two solvent systems: DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v) and EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). Stock solutions of various ions were prepared in deionized water.

Results and discussion

The properties of L2 in DMSO/H₂O HEPES buffer

As shown in Fig. S9,† the complexation time in the system of DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v) was investigated by monitoring the fluorescent emission intensity of L2 (20 μM) binding with Al³⁺ (10 equiv.) at an excitation wavelength of 305 nm. After the addition of Al³⁺, the fluorescence intensity of L2 at λ = 450 nm was enhanced to a relatively stable value after 6 h. Therefore, the complexation time of 6 h was used for this system.

The selectivity of L2 for different metal ions (Cu²⁺, Ag⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Co²⁺, Pb²⁺, Mn²⁺, Li⁺, Ni²⁺, Fe³⁺, Ca²⁺, Cr³⁺, Zn²⁺, Mg²⁺, and Al³⁺) was investigated by fluorescence emission spectroscopy. L2 (20 μM) exhibited a weak fluorescence intensity at 375 nm when it was excited at 305 nm in DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). The addition of Al³⁺ to the solution induced a remarkable increase in the fluorescence intensity along with a significant red shift of 75 nm (Fig. 1A). Moreover, there was a sharp change from colorless to blue in the presence of Al³⁺ ions (Fig. 1A inset). In contrast, addition of other metal ions caused almost negligible fluorescence increases. To further understand the properties of L2 as a receptor for Al³⁺, a titration experiment was performed with increasing concentration of Al³⁺ (Fig. 1B). Upon increasing addition of Al³⁺, the fluorescence emission maximum at 450 nm gradually increased and reached a plateau when the concentration of Al³⁺ was at 25 equivalents. The fluorescence intensity of L2 (20 μM) at λ = 450 nm increased linearly with the concentration of Al³⁺ from 1 to 10 μM. A good linear relationship was observed between the fluorescence intensity and [Al³⁺] (Fig. S10†). The detection limit (3σ/slope) for Al³⁺ was calculated to be 1.73 × 10⁻⁷ M. The fluorescence quantum yield of L2 in DMSO/H₂O HEPES buffer was 0.36 and was increased to 0.77 by Al³⁺ addition.

Fig. 1 (A) Fluorescence spectra of L2 (20 μM) upon the addition of metal salts (10 equiv.) of Cu²⁺, Ag⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Co²⁺, Pb²⁺, Mn²⁺, Li⁺, Ni²⁺, Fe³⁺, Ca²⁺, Cr³⁺, Zn²⁺, Mg²⁺, and Al³⁺ in DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). Λ_ex = 305 nm. Inset: color of L2 and L2 + Al³⁺ system under a UV lamp. (B) Fluorescence titration spectra of L2 (20 μM) upon increasing addition of Al³⁺ (up to 25 equiv.) in DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). Λ_ex = 305 nm.

To further check the selectivity of receptor L2 towards Al³⁺, competitive experiments were carried out in DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). When L2 was treated with 1 equivalent of Al³⁺ in the presence of the same concentration of other metal ions (Fig. S12†), several metal ions (Ni²⁺, Cu²⁺ and Cd²⁺) decreased the emission intensity and some other metal ions (Pb²⁺, Fe³⁺, Zn²⁺ and Mg²⁺) increased the emission intensity. However, the result of Fig. 1A confirms that addition of other metal ions alone caused no significant florescence increases. Thus, L2 can be used potentially to qualitatively detect Al³⁺ in this specified condition.

The UV-Vis spectrum of L2 in DMSO/H₂O HEPES buffer (20 μM) in the presence of 10 equiv. of a variety of metal ions (Cu²⁺, Ag⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Co²⁺, Pb²⁺, Mn²⁺, Li⁺, Ni²⁺, Fe³⁺, Ca²⁺, Cr³⁺, Zn²⁺, Mg²⁺, Al³⁺) was detected, as shown in Fig. S14.† The result shows that these metal ions (Fe³⁺, Ni²⁺, Cu²⁺) could be distinguished easily over other ones using UV-Vis spectroscopy. In addition, the activity of L2 toward Al³⁺ was examined with absorption spectroscopy. Sensor L2 displayed four characteristic peaks at 291, 301, 326 and 339 nm in DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v).

Titration experiments with increasing addition of Al³⁺ ions resulted in the decrease of the four intrinsic peaks along with the emergence of two new peaks at 382 nm and 407 nm. A distinct isosbestic point at 354 nm implied the complete conversion of L2 to its Al³⁺ complex (Fig. 2). To confirm the chelation structure, DFT calculations were carried out with B3LYP/6-31G(d) basis sets using the Gaussian 09 suite of programs. Structure optimization and energy calculations provide the best binding mode between L2 and Al³⁺ (Fig. 3). The DFT calculations also revealed that there is a reasonable decrease in the HOMO to LOMO energy gap from L2 to its aluminium complex (Fig. 4), which is consistent with the emerging of the new red shifted absorbance peaks upon the addition of Al³⁺ to L2.

Fig. 2 Changes in the absorption spectra of L2 (20 μM) with increasing addition of Al³⁺ in DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v).

Fig. 3 Optimized structure of L2 + Al³⁺ at B3LYP/6-31 + G(d,p).

Fig. 4 Energy diagrams of the HOMO and LOMO orbitals of L2 and L2 + Al³⁺ complex calculated with DFT using a B3LYP/6-31G(d) basis set within the Gaussian 09 program.

The effect of pH was studied dependently. Over the pH range tested, L2 has nearly no fluorescence. A sharp decrease in fluorescence was observed at acidic (pH < 7) and basic (pH > 8) conditions in the presence of Al³⁺. At lower pH (2–7), the decrease probably attributes to the protonation of the imidazole nitrogen, which interferes with the coordination between metal ions and nitrogen. At pH greater than 8, the decrease of the fluorescence intensity is possibly due to the formation of salts.

The properties of L2 in EtOH/H₂O HEPES buffer

The complexation time in the system of EtOH/H₂O HEPES buffer was very short and can be ignored. The selectivity of L2 for different metal ions (Cu²⁺, Ag⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Co²⁺, Pb²⁺, Mn²⁺, Li⁺, Ni²⁺, Fe³⁺, Ca²⁺, Cr³⁺, Zn²⁺, Mg²⁺, and Al³⁺) in EtOH/H₂O HEPES buffer was detected by fluorescence spectroscopy. As is evident from Fig. 5A, a solution of L2 (20 μM) showed a low intensity at 345 nm when it was excited at 315 nm in EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). The addition of Zn²⁺ to the solution induced a large increase in the fluorescence intensity, along with a significant red shift of 144 nm. There was also a dramatic color change from colorless to yellow-green in the presence of Zn²⁺ ions (Fig. 5A inset). Other metal ions induced inconspicuouss fluorescence increases. Similar to the above mentioned studies with Al³⁺, fluorescence titration was also conducted with Zn²⁺ (Fig. 5B). Upon increasing addition of Zn²⁺, the fluorescence emission maximum at 489 nm gradually increased and reached a plateau when the concentration of Zn²⁺ was 7 equivalents. The fluorescence intensity of L2 (20 μM) at λ = 489 nm increased linearly with the concentration of Zn²⁺ from 0.1 to 1 μM. A good linear relationship was observed between the fluorescence intensity and [Zn²⁺] (Fig. S11†). The detection limit (3σ/slope) for Zn²⁺ was calculated to be 6.36 × 10⁻⁸ M. The fluorescence quantum yield of L2 in EtOH/H₂O HEPES buffer was 0.25 and was increased to 0.41 by Zn²⁺ addition.

Fig. 5 (A) Fluorescence spectra of L2 (20 μM) upon the addition of metal salts (7 equiv.) of Cu²⁺, Ag⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Co²⁺, Pb²⁺, Mn²⁺, Li⁺, Ni²⁺, Fe³⁺, Ca²⁺, Cr³⁺, Zn²⁺, Mg²⁺, and Al³⁺ in EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). Λ_ex = 315 nm. Inset: color of L2 and L2 + Zn²⁺ system under UV lamp. (B) Fluorescence titration spectra of L2 (20 μM) upon increasing addition of Zn²⁺ (up to 7 equiv.) in EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). Λ_ex = 315 nm.

Competition experiments were carried out in EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). When L2 was treated with 1 equivalent of Zn²⁺ in the presence of the same concentration of other metal ions (Fig. S13†), several metal ions (Ni²⁺, Fe³⁺ and Cu²⁺) decreased the emission intensity and most of the metal ions (Pb²⁺, Co²⁺, Ag⁺, Hg²⁺, Al³⁺, Cr³⁺ and Mg²⁺) increased the emission intensity. Nonetheless, the result of Fig. 5A confirmed that the addition of other metal ions alone caused no significant fluorescence enhancement. Thus, L2 can potentially be used to qualitatively detect Zn²⁺ in this specified condition.

The UV-Vis spectrum of L2 (20 μM) in EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v) in the presence of 7 equivalents of a variety of metal ions (Cu²⁺, Ag⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Co²⁺, Pb²⁺, Mn²⁺, Li⁺, Ni²⁺, Fe³⁺, Ca²⁺, Cr³⁺, Zn²⁺, Mg²⁺, and Al³⁺) was detected, as shown in Fig. S15.† It is not easy to differentiate various metal ions using UV-Vis spectroscopy. The UV-spectral properties of L2 and L2 + Zn²⁺ were researched in EtOH/H₂O HEPES buffer. The receptor L2 also has four absorption peaks at 290, 298, 322, and 333 nm in EtOH/H₂O HEPES buffer, which is similar to L2 in DMSO/H₂O HEPES buffer.

Titration experiments with increasing amounts of Zn²⁺ ions resulted in the decreasing of the four intrinsic peaks along with the emergence of a new peak at 390 nm. A distinct isosbestic point at 354 nm established the transformation of a free receptor to its zinc complex (Fig. 6). The DFT calculations of L2 + Zn²⁺ have similar results with L2 + Al³⁺ (Fig. 7 and 8).

Fig. 6 Changes in absorption spectra of L2 (20 μM) with the increasing addition of Zn²⁺ in EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v).

Fig. 7 Optimized structure of L2 + Zn²⁺ at B3LYP/6-31 + G(d,p).

Fig. 8 Energy diagrams of the HOMO and LOMO orbitals of L2 and the L2 + Zn²⁺ complex calculated with DFT using a B3LYP/6-31G(d) basis set with the Gaussian 09 suite of programs.

The fluorescence changes of L2 (20 μM) in the absence and presence of Zn²⁺ in different pH values were examined. The influence of pH for L2 and L2 + Zn²⁺ in EtOH/H₂O solution was analogous to L2 and L2 + Al³⁺ in DMSO/H₂O solution. The fluorescent effect of L2 + Zn²⁺ is best when pH values approach neutrality, which could have a similar explanation to the L2 + Al³⁺ study.

The proposed mechanism

The low fluorescence of the free receptor may be attributed to a large extent of intramolecular charge transfer (ICT). Upon interaction with the target analytes, the intramolecular charge transfer (ICT) changes following the principle of forming a stable structure. Therefore, the chelation enhanced fluorescence (CHEF) process occurs in the presence of the analytes with an accompanying large Stokes shift. Moreover, a difference in the charge density of the cations and solvent effect are likely to affect the ICT mechanism. This may account for the different emission spectra of the probe upon interaction with Al³⁺ and Zn²⁺ in different solvents (Scheme 2).

Scheme 2 The proposed mechanism of L2 with Al³⁺ in DMSO and Zn²⁺ in EtOH.

Conclusions

In summary, we designed and synthesized a new fluorescent probe, L2, which selectively senses Al³⁺ and Zn²⁺ ions with a switch on response in its fluorescence spectra. L2 shows prominent fluorescent selectivity toward Al³⁺ over other common metal ions in DMSO/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v) and shows excellent fluorescent selectivity toward Zn²⁺ over other common metal ions in EtOH/H₂O HEPES buffer (10 mM, pH = 7.4, 9 : 1, v/v). The detection limits for Al³⁺ and Zn²⁺ were found to be as low as 1.73 × 10⁻⁷ M and 6.36 × 10⁻⁸ M, respectively.

Acknowledgements

The authors thank Henan Sanmenxia Aoke Chemical Industry Co., Ltd for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21055k

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