A highly selective and sensitive fluorescent turn-on Al³⁺ chemosensor in aqueous media and living cells: experimental and theoretical studies

Abstract

A highly selective and sensitive fluorescent chemosensor (1) based on a 2-aminobenzoic acid Schiff base has been synthesized in one step. The binding properties of 1 with metal ions were investigated by fluorescence, UV-vis, ¹H NMR and electrospray ionization mass spectra. This sensor exhibited enhanced fluorescence in the presence of Al³⁺ in aqueous solution and also in living cells. The sensing mechanism of 1 toward Al³⁺ was explained by DFT/TDDFT calculations. The binding constant was determined to be 3.1 × 10⁸ M⁻¹ which is one of the highest binding affinities among such simple Schiff bases for Al³⁺ in buffer solution. The calibration curve for the analytical performance of Al³⁺ was found to be linear over a concentration range with a very low detection limit of 290 nM (3σ).

---

1. Introduction

The development of selective and sensitive chemosensors for the detection of metal ions has received considerable attention because of their important roles in medicine, living systems and the environment.^1–7 Among metals, aluminum is the third most prevalent metallic element in the Earth. People are widely exposed to aluminum because of its widespread use in food additives, aluminum-based pharmaceuticals and storage/cooking utensils.^8–13 The World Health Organization (WHO) prescribed the average human intake of aluminum to be around 3–10 mg day⁻¹ and limited its drinking water concentration to 7.41 μM. Tolerable weekly aluminum intake in the human body is estimated to be 7 mg kg⁻¹ body weight.^14–16 The superfluous ingestion of aluminum influences the absorption of calcium in the bowel, causing softening of the bone, atrophy and even aberrance, and also affects the absorption of iron in blood, causing anemia. In addition, the toxicity of aluminum causes damage to the central nervous system, is suspected to be involved in neurodegenerative diseases such as Alzheimer's and Parkinson's, and is responsible for intoxication in hemodialysis patients.^17–19

In recent years, much effort has been devoted to design various chemosensors specific for Al³⁺ detection.^20–61 However, many of them have suffered from the cost of synthesis, the number of synthesis steps, selectivity, binding constants, detection limits, use of harmful organic solvents, and poor water solubility. Therefore, it is necessary to develop new sensors for selectively detecting Al³⁺ in aqueous solution. To increase the water solubility, we used 2-aminobenzoic acid with a carboxylic group acting as a hydrophilic character. To the best of our knowledge, very few studies have been reported on the use of aminobenzoic acid as a fluorescent chemosensor for the detection of metal ions.^32,62–65

Herein, we report an aminobenzoic acid-based Schiff base 1 as a highly selective and sensitive fluorescent chemosensor for Al³⁺. 1 showed one of the highest binding affinities (K_a = 3.0 × 10⁸ M⁻¹) among such simple Schiff bases for Al³⁺ in aqueous solution and quite a low detection limit (2.9 × 10⁻⁷ M) of a nanomolar concentration level.

2. Experimental section

2.1. Materials and instrumentation

All the solvents and reagents (analytical grade and spectroscopic grade) were obtained from Sigma-Aldrich and used as received. The Live/Dead assay (LIVE/DEAD® Viability/Cytotoxicity Kit) kit was purchased from Life Technologies Company. ¹H NMR measurements were performed on a Varian 400 MHz spectrometer and chemical shifts were recorded in ppm. Electrospray ionization mass spectra (ESI-MS) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQ™ Advantage MAX quadruple ion trap instrument. Elemental analysis for carbon, nitrogen, and hydrogen was carried out by using a Flash EA 1112 elemental analyzer (thermo) in the Organic Chemistry Research Center of Sogang University, Korea. Absorption spectra were recorded at 25 °C using a Perkin Elmer model Lambda 2S UV/vis spectrometer. Fluorescence measurements were performed on a Perkin Elmer model LS45 fluorescence spectrometer.

2.2. Synthesis of the fluorescence sensor 1

A solution of 2-aminobenzoic acid (0.14 g, 1 mmol) in ethanol was added to a solution containing 4-(diethylamino)-2-hydroxybenzaldehyde (0.19 g, 1 mmol) in ethanol and the mixture was stirred for 5 min. Then, two drops of HCl were added into the reaction solution and it was stirred for 3 h at room temperature. The solvent was evaporated and a bright yellow product was recrystallized from acetonitrile. The yield: 0.23 g (74.3%). ¹H NMR (DMSO-d₆, 400 MHz) δ: 8.72 (s, 1H), 7.93 (d, 1H, J = 7.6 Hz), 7.58 (d, 2H, J = 6.8 Hz), 7.44 (d, 1H, J = 8.8 Hz), 7.29 (m, 1H, J = 7.6 Hz), 6.41 (d, 1H, J = 8.4 Hz), 6.39 (s, 1H), 3.45 (q, 4H, J = 6.8 Hz), 1.14 (t, 6H, J = 7.0 Hz). ESI-mass m/z [M − H⁺]⁻: calcd, 311.14; found, 311.33. Anal. calcd for C₁₈H₂₀N₂O₃ (312.15): C, 69.21; H, 6.45; N, 8.97. Found: C, 69.60; H, 6.30; N, 8.70%.

2.3. Fluorescence and UV-vis measurements

For fluorescence measurements, sensor 1 (1.6 mg, 0.005 mmol) was dissolved in methanol (5 mL) and 30 μL of sensor 1 (1 mM) were diluted with 2.97 mL bis-tris buffer (10 mM, pH 7.0, water : CH₃OH = 1 : 1 (v/v)) to make the final concentration of 10 μM. Al(NO₃)₃·9H₂O (37.5 mg, 0.1 mmol) was dissolved in methanol (5 mL). 1.5–28.5 μL of the Al³⁺ solution (20 mM) were transferred to each sensor solution (10 μM) prepared above. After mixing them for a few seconds, fluorescence spectra were obtained at room temperature.

For UV-vis measurements, all experimental conditions were identical as above. 1.5–21 μL of the Al³⁺ solution (10 mM) were transferred to each sensor solution (10 μM) prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

2.4. Job plot measurements

Sensor 1 (1.6 mg, 0.005 mmol) was dissolved in methanol (5 mL). 30, 27, 24, 21, 18, 15, 12, 9, 6, 3 and 0 μL of the sensor 1 solution were taken and transferred to vials. Each vial was diluted with bis-tris buffer (10 mM, pH 7.0, water : CH₃OH = 1 : 1 (v/v)) to make a total volume of 2.97 mL. Al(NO₃)₃·9H₂O (1.9 mg, 0.005 mmol) was dissolved in methanol (5 mL). 0, 3, 6, 9, 12, 15, 18, 21, 24, 27 and 30 μL of the Al(NO₃)₃ solution were added to each diluted sensor 1 solution. Each vial had a total volume of 3 mL. After shaking the vials for a few seconds, fluorescence spectra were taken at room temperature.

2.5. NMR titration

For ¹H NMR titration of sensor 1 with Al³⁺, four NMR tubes of sensor 1 (31.2 mg, 0.1 mmol) dissolved in DMSO-d₆ (700 μL) were prepared and then four different concentrations (0, 0.02, 0.05 and 0.1 mmol) of Al(NO₃)₃·9H₂O dissolved in DMSO were added to each solution of sensor 1. After shaking them for a minute, ¹H NMR spectra were recorded at room temperature.

2.6. Competition with other metal ions

Sensor 1 (1.6 mg, 0.005 mmol) was dissolved in methanol (5 mL) and 30 μL of sensor 1 (1 mM) were diluted with 2.97 mL bis-tris buffer (10 mM, water : CH₃OH = 1 : 1 (v/v)) to make the final concentration of 10 μM. MNO₃ (M = Na, K, 0.1 mmol) or M(NO₃)₂ (M = Mn, Co, Ni, Cu, Zn, Cd, Hg, Mg, Ca, Pb and Cr³⁺, 0.1 mmol) or M(NO₃)₃ (M = Fe, Al, Ga, In, 0.1 mmol) were separately dissolved in methanol (5 mL). 27 μL of each metal solution (20 mM) were taken and added into 3 mL of each sensor 1 solution (10 μM) prepared above to make 18 equiv. Then, 27 μL of Al(NO₃)₃ solution (20 mM) were added into the mixed solution of each metal ion and sensor 1 to make 18 equiv. After mixing them for a few seconds, fluorescence spectra were recorded at room temperature.

2.7. Methods for cell imaging and cytotoxicity

Human dermal fibroblast cells in low passage were cultured in FGM-2 medium (Lonza, Switzerland) supplemented with 10% fetal bovine serum, 1% penicillin/streptomycin in an in vitro incubator with 5% CO₂ at 37 °C. Cells were seeded onto a 12 well plate (SPL Lifesciences, Korea) at a density of 2 × 10⁵ cells per well and then incubated at 37 °C for 4 h after the addition of various concentrations (0–200 μM) of Al(NO₃)₃ dissolved in DMSO. After washing with phosphate buffered saline (PBS) two times to remove the remaining Al(NO₃)₃, the cells were incubated with 1 dissolved in a medium/DMSO solution (FGM-2-medium : DMSO = 9 : 1 v/v) at room temperature for 30 min. The cells were observed using a microscope (Olympus, Japan). The fluorescent images of the cells were obtained using a fluorescence microscope (Leica DMLB, Germany) at the excitation wavelength of 410 nm.

To observe cell viability, a live & dead assay was performed for 1 and the 1-Al³⁺ complex. Fibroblasts (P = 5) were in vitro cultured to reach 70% confluence. The cells were incubated with 1 (10 μM) dissolved in a medium/DMSO solution (FGM-2-medium : DMSO = 9 : 1 v/v) for 1 h, 12 h and 24 h, respectively. The reagent (400 μL) of the live & dead assay was added into each cell culture plate. In case of the 1-Al³⁺ complex, the cells were incubated with Al(NO₃)₃ (150 μM) dissolved in MeCN. After washing with phosphate buffered saline (PBS) two times to remove the remaining Al(NO₃)₃, the cells were incubated with 1 dissolved in a medium/DMSO solution (FGM-2-medium : DMSO = 9 : 1 v/v) at room temperature for 1 h, 12 h and 24 h, respectively. The reagent (400 μL) of the live & dead assay was added into each cell culture plate. Both viability and morphological changes of the cells were observed using a fluorescence microscope (Leica DMLB, Leica; Wetzlar, Germany).

2.8. Theoretical calculations

All theoretical calculations were performed by using the Gaussian 03 suite.⁶⁶ The singlet ground states (S₀) of 1 and the Al³⁺–2·1 complex were optimized by DFT methods with Becke's three parameterized Lee–Yang–Parr (B3LYP) exchange functional^67,68 with the 6-31G** basis set.^69,70 In vibrational frequency calculations, there was no imaginary frequency for 1 and the Al³⁺–2·1 complex, suggesting that the optimized 1 and the Al³⁺–2·1 complex represented local minima. For all calculations, the solvent effect was considered by using the Cossi and Barone's CPCM (conductor-like polarizable continuum model).^71,72 To investigate the electronic properties of singlet excited states, time-dependent DFT (TDDFT) was performed in the ground state geometries of 1 and the Al³⁺–2·1 complex. The 20 singlet–singlet excitations were calculated and analyzed. The GaussSum 2.1⁷³ was used to calculate the contributions of molecular orbitals in electronic transitions.

3. Results and discussion

3.1. Spectroscopic measurements of 1 toward Al³⁺

Sensor 1 was obtained by coupling 2-aminobenzoic acid with 4-(diethylamino)-2-hydroxybenzaldehyde with 74% yield in ethanol (Scheme 1), and characterized by ¹H NMR, ESI-mass spectrometry, and elemental analysis.

Scheme 1 Synthesis of sensor 1.

We studied the fluorescence response behaviors of 1 towards various metal ions in bis-tris buffer (10 mM, pH 7.0, water : CH₃OH = 1 : 1 (v/v)). As shown in Fig. 1, sensor 1 alone displayed a very weak fluorescence emission upon excitation at 410 nm. The addition of Al³⁺ resulted in a prominent fluorescence enhancement (11-fold) at 460 nm. In contrast, other metal ions such as Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Pb²⁺ and Cr³⁺ caused almost no fluorescence increase. Importantly, the addition of Ga³⁺ or In³⁺, which belong to the same group with Al³⁺ in the periodic table, had no significant effect on the fluorescence, demonstrating that sensor 1 was even highly selective for Al³⁺ over Ga³⁺ and In³⁺. So far, only a few selective sensors which can discriminate Al³⁺ from both Ga³⁺ and In³⁺ have been reported.^20–22,26 The selectivity of 1 toward Al³⁺ in bis-tris buffer alone was also observed, but fluorescence enhancement was not high enough (Fig. S1, ESI†).

Fig. 1 Fluorescence spectra of sensor 1 (10 μM) upon addition of metal nitrate salts (18 equiv.) of Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Ga³⁺, In³⁺, Pb²⁺ and Cr³⁺ with an excitation of 410 nm in bis-tris buffer (10 mM, pH 7.0, water : CH₃OH = 1 : 1 (v/v)). Inset: Picture of the fluorescence corresponding to 1 and 1-Al³⁺ (λ_ex = 365 nm).

To further study the fluorescence sensing behavior of 1, a quantitative investigation of the binding affinity of 1 with Al³⁺ was studied by fluorescence titration (Fig. 2). When sensor 1 was titrated with Al³⁺, the fluorescence intensity increased up to 18 equiv. and then no change was observed, which might be due to a combinational effect of the inhibition of C=N isomerization and the activation of chelation enhanced fluorescence (CHEF).^45,74–76 Imines are poorly fluorescent in part due to isomerization of the C=N double bond in the excited state. The C=N isomerization might be inhibited upon complexation with certain metal ions. Therefore, the stable chelation of 1 with Al³⁺ ions made the C=N isomerization of 1 inhibited, resulting in fluorescence enhancement. Moreover, the complexation of 1 with Al³⁺ ions induced rigidity in the resulting molecule and tended to produce a chelation-enhanced fluorescence detection. The fluorescence quantum yields of sensor 1 without and with Al³⁺ were 0.0062 and 0.4240, respectively.⁷⁷

Fig. 2 Fluorescent titration spectra of sensor 1 (10 μM) in the presence of different concentrations of Al³⁺ in bis-tris buffer (10 mM, pH 7.0, water : CH₃OH = 1 : 1 (v/v)). Inset: The fluorescence at 460 nm (λ_ex = 410 nm) of 1 as a function of Al³⁺ concentration.

The binding properties of 1 with Al³⁺ were further studied by UV-vis titration experiments (Fig. 3). On treating Al³⁺ with the solution of 1, the absorption peak at 425 nm gradually decreased while the peak at 352 nm increased. Two clear isosbestic points were observed at 395 and 468 nm, indicative of a clean conversion of 1 into the 1-Al³⁺ complex.

Fig. 3 Absorption spectral changes of 1 (10 μM) in the presence of different concentrations of Al³⁺ ions (5, 10, 15, 20, 25, 30, 35, 40, 50, 60 and 70 μM) in bis-tris buffer (10 mM, pH 7.0, water : CH₃OH = 1 : 1 (v/v)). Inset: Absorbance of 1 at 425 nm as a function of Al³⁺ concentration.

The Job plot for the binding between 1 and Al³⁺ exhibited a 2 : 1 stoichiometry (Fig. 4).⁷⁸ Also, the negative-ion mass spectrum for the reaction of 1 with Al³⁺ showed the formation of the [2·1(-4H⁺) + Al³⁺]⁻ complex [m/z: 647.27; calcd, 647.24] (Fig. S2, ESI†). From the fluorescence titration, the association constant was calculated to be 3.1 × 10⁸ M⁻¹ from Li's equations (see the ESI,† and Fig. S3).⁷⁹ This value is one of the highest binding affinities among such simple Schiff bases for Al³⁺ in buffer solution, indicating that the carboxylic group showed the most effective binding property among other functional groups such as imine, amine, and quinolone (Table S1, ESI†).^80–82

Fig. 4 Job plot of sensor 1 and Al³⁺, where the intensity at 460 nm was plotted against the mole fraction of aluminum ions. The total concentration of aluminum ions with sensor 1 was 10 μM in bis-tris buffer (10 mM, pH 7.0, water : CH₃OH = 1 : 1 (v/v)).

By using the above-mentioned fluorescence titration results, the detection limit of 1 for the analysis of Al³⁺ ions on the basis of 3σ/K was calculated to be 2.9 × 10⁻⁷ M (Fig. S4, ESI†).⁸³ The detection limit of 1 was quite lower than the WHO limit of Al³⁺ (7.41 × 10⁻⁶ M) acceptable in drinking water, which means that sensor 1 is a powerful tool for the detection of Al³⁺ in aqueous solution. The analytical performance characteristics of the proposed Al³⁺-sensitive chemosensor were investigated. Fig. 5 shows a calibration curve obtained from the plot of fluorescence intensity with the added Al³⁺ concentration. The curve equation was I = 0.08063C_Al + 13.924 where the C_Al unit was nM (R² = 0.996). The linear range of quantitative detection for aluminum ions was determined to be 40–1000 nM with a detection limit of 290 nM (3σ). The detection limit appeared to be within the range of quantitative detection of aluminum ions. Therefore, sensor 1 was not only selective towards aluminum(III) ions but also possessed a linear response range with a low limit of detection.

Fig. 5 Calibration curve based on fluorescence intensity with added Al³⁺ concentration.

The ¹H NMR titration experiments of sensor 1 with Al³⁺ ions were studied to further examine the binding mode (Fig. 6). The protons of carboxyl and hydroxyl groups did not appear maybe due to the intra- or inter-molecular hydrogen bonds. Upon addition of 0.5 equiv. of Al³⁺, the H_a proton of the imine moiety and the H_c and H_d protons of the ethyl groups were shifted to downfield by 0.32, 0.10, and 0.06 ppm, respectively. Also, the protons of the two benzene rings were more complicated and shifted to downfield except for H_b, which was shifted to upfield by 0.15 ppm. Further addition of Al³⁺ showed no spectral change. These results suggested that the two oxygen atoms of the carboxyl and hydroxyl groups and the nitrogen atom of the imine moiety might be involved in Al³⁺ coordination. Based on ¹H NMR titration, the Job plot, and ESI-mass spectrometry analysis, we proposed the structure of a 2 : 1 complex of 1 and Al³⁺, as shown in Scheme 2.

Fig. 6 ¹H NMR titration of 1 with Al(NO₃)₃: (I) 1; (II) 1 with 0.2 equiv. of Al³⁺; (III) 1 with 0.5 equiv. of Al³⁺; and (IV) 1 with 1 equiv. of Al³⁺.

Scheme 2 Proposed structure of a 2 : 1 complex of 1 and Al³⁺.

To further check the practical applicability of sensor 1 as an Al³⁺ selective fluorescent sensor, we carried out competition experiments. When 1 was treated with 18 equiv. of Al³⁺ in the presence of the same concentration of other metal ions, the fluorescence enhancement caused by Al³⁺ was retained with Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ga³⁺, In³⁺, and Pb²⁺ (Fig. 7). Instead, Fe³⁺, Cu²⁺ and Cr³⁺ inhibited about 80%, 80% and 100% of the interaction between 1 and Al³⁺, which are the well known strong quenchers of fluorescence.^84,85 Thus, sensor 1 can be used as a selective fluorescent sensor for Al³⁺ in the presence of most competing metal ions.

Fig. 7 (a) Competitive selectivity of 1 toward Al³⁺ in the presence of other metal ions (18 equiv.) with an excitation of 410 nm in bis-tris buffer (10 mM, water : CH₃OH = 1 : 1 (v/v)). (b) The fluorescence images of 1 upon addition of Al³⁺ (18 equiv.) in the absence and presence of various metal ions (18 equiv.) with an excitation of 365 nm in bis-tris buffer (10 mM, water : CH₃OH = 1 : 1 (v/v)).

For biological applications, the pH dependence of the Al³⁺–2·1 complex was examined in bis-tris buffer (10 mM, water : CH₃OH = 1 : 1 (v/v)) solution (Fig. 8). The fluorescence intensity of the Al³⁺–2·1 complex displayed a significant response between pH 5 and 10, which includes the environmentally relevant range of pH 7.0–8.4.⁸⁶ These results indicated that Al³⁺ could be clearly detected by the fluorescence spectra measurement using 1 over the environmentally relevant pH range (pH 7.0–8.4).

Fig. 8 Fluorescence intensity (at 460 nm) of the Al³⁺–2·1 complex at different pH values (3–12) in bis-tris buffer (10 mM, water : CH₃OH = 1 : 1 (v/v)).

3.2. Application for cell imaging

Further experiments were conducted to test whether intracellular Al³⁺ could be monitored by fluorimetry. Adult human dermal fibroblasts were first incubated with various concentrations of Al³⁺ (0, 30, 50, 100, 150 and 200 μM) for 4 h and then exposed to 1 for 30 min before imaging, so that 1 could permeate easily through the living cells without any harm. The fibroblasts that were cultured with both Al³⁺ and 1 exhibited fluorescence (Fig. 9), whereas the cells cultured without Al³⁺ or 1 did not exhibit fluorescence. The intensity and region of the fluorescence within the cell containing 1 gradually increased with an increase in the Al³⁺ concentration from 0 to 200 μM and the fluorescence intensity of the cells persisted even after 5 h from exposure to 1. Moreover, the biocompatibility of 1 was also examined with the living cells (Fig. S5, ESI†). All the fibroblasts were still alive for 24 h. These results showed that the new sensor 1 was efficient to image Al³⁺ in cells, and could be useful in the determination of the exposure level of cells to Al³⁺.

Fig. 9 Fluorescence images of fibroblasts cultured with Al³⁺ and 1. Cells were exposed to 0 (A and G), 30 (B and H), 50 (C and I), 100 (D and J), 150 (E and K) and 200 μM (F and L) Al(NO₃)₃ for 4 h and then later with 1 for 30 min. The images (A–F) were observed using a light microscope and the images (G–L) were taken using a fluorescence microscope. The scale bar is 50 μm.

3.3. Theoretical calculations for 1 and the Al³⁺–2·1 complex

To gain an insight into the fluorescent sensing mechanism for the Al³⁺–2·1 complex, time-dependent density functional theory (TD-DFT) calculations were performed at the optimized geometries (S₀) of 1 and the Al³⁺–2·1 complex (Fig. S6, ESI†). The optimized geometries were calculated based on the Job plot, ESI-mass and ¹H NMR titration. In case of 1, the main molecular orbital (MO) contribution of the first lowest excited state was determined for the HOMO → LUMO transition (394.61 nm, Fig. S7, ESI†), which indicated an intramolecular charge transfer (ICT) transition from the julolidine moiety to the benzoic acid moiety. For the Al³⁺–2·1 complex, the first lowest excited state was determined for the HOMO → LUMO transition (499.43 nm, Fig. S8, ESI†), which indicated the forbidden transition (oscillator strength: 0.0045). The main molecular orbital (MO) contributions of the fifth lowest excited states were determined for HOMO → LUMO+1 and HOMO−1 → LUMO+1 transitions (367.42 nm, Fig. S8, ESI†), which showed the allowed transition and was similar to the intramolecular charge transfer (ICT) transition of 1. There were no obvious changes in the electronic transitions between 1 and the Al³⁺–2·1 complex. Only, hypochromic shift (394.61 to 367.42 nm) was observed upon chelation of 1 with Al³⁺. These results suggested that the sensing mechanism of 1 toward Al³⁺ originated from the rigid structure of the Al³⁺–2·1 complex, which might inhibit the non-radiative process such as C=N isomerization. Thus, the CHEF effect and the inhibition of C=N isomerization plays an important role in fluorescence enhancement.^87,88

4. Conclusion

We have developed a simple, cost-effective, fluorescent aminobenzoic acid-based chemosensor 1 for selective determination of Al³⁺ in aqueous solution. Sensor 1 displayed a remarkable fluorescence enhancement in the presence of Al³⁺ ions and can be successfully applied to living cells for detecting Al³⁺. The large enhancement of fluorescence was explained by the inhibition of C=N isomerization and the activation of chelation enhanced fluorescence (CHEF), which were explained by theoretical calculations. On the basis of the Job plot, ¹H NMR titration and ESI-mass spectrometry analysis, the formation of the 2 : 1 (1 : M) complex was proposed. Moreover, sensor 1 showed one of the highest binding constants (3.1 × 10⁸) among such simple Schiff bases in aqueous solution and displayed a linear response range with a low limit of detection (290 nM) for the analytical performance of Al³⁺ ions.

Acknowledgements

Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2014R1A2A1A11051794 and NRF-2015R1A2A2A09001301) are gratefully acknowledged.

References

1. C. Liang, W. Bu, C. Li, G. Men, M. Deng, Y. Jiangyao, H. Sun and S. Jiang, Dalton Trans., 2015, 44, 11352–11359.
2. H. Wang, B. Wang, Z. Shi, X. Tang, W. Dou, Q. Han, Y. Zhang and W. Liu, Biosens. Bioelectron., 2015, 65, 91–96.
3. C. Li, J. Qin, G. Wang, B. Wang, A. Fu and Z. Yang, Inorg. Chim. Acta, 2015, 430, 91–95.
4. R. R. Koner, S. Sinha, S. Kumar, C. K. Nandi and S. Ghosh, Tetrahedron Lett., 2012, 53, 2302–2307.
5. S. Sinha, T. Mukherjee, J. Mathew, S. K. Mukhopadhyay and S. Ghosh, Anal. Chim. Acta, 2014, 822, 60–68.
6. Z. Liu, C. Zhang, Y. Li, Z. Wu, F. Qian, X. Yang, W. He, X. Gao and Z. Guo, Org. Lett., 2009, 11, 795–798.
7. X. Qu, C. Li, H. Chen, J. Mack, Z. Guo and Z. Shen, Chem. Commun., 2013, 49, 7510–7512.
8. D. Maity and T. Govindaraju, Inorg. Chem., 2010, 49, 7229–7231.
9. D. Maity and T. Govindaraju, Chem. Commun., 2010, 46, 4499–4501.
10. A. Sahana, A. Banerjee, S. Lohar, A. Banik, S. K. Mukhopadhyay, D. A. Safin, M. G. Babashkina, M. Bolte, Y. Garcia and D. Das, Dalton Trans., 2013, 42, 13311–13314.
11. A. Banerjee, A. Sahana, S. Das, S. Lohar, B. Sarkar, S. K. Mukhopadhyay, A. K. Mukherjee and D. Das, Analyst, 2012, 137, 2166–2175.
12. J. Y. Jung, S. J. Han, J. Chun, C. Lee and J. Yoon, Dyes Pigm., 2012, 94, 423–426.
13. S. Yun, Y.-O. Kim, D. Kim, H. G. Kim, H. Ihm, J. K. Kim, C.-W. Lee, W. J. Lee, J. J. Yoon, K. S. Oh, J. Yoon, S.-M. Park and K. S. Kim, Org. Lett., 2003, 5, 471–474.
14. G. Berthon, Coord. Chem. Rev., 2002, 228, 319–341.
15. Z. Krejpcio and R. W. Wojciak, Pol. J. Environ. Stud., 2002, 11, 251–254.
16. J. Barcelo and C. Poschenrieder, Environ. Exp. Bot., 2002, 48, 75–92.
17. P. Nayak, Environ. Res., 2002, 89, 101–115.
18. J. Kumar, M. J. Sarma and P. Phukan, Dalton Trans., 2015, 44, 4576–4581.
19. C. Exley, Coord. Chem. Rev., 2012, 256, 2142–2146.
20. J. Lee, H. Kim, S. Kim, J. Y. Noh, E. J. Song, C. Kim and J. Kim, Dyes Pigm., 2013, 96, 590–594.
21. M. Mukherjee, S. Pal, S. Lohar, B. Sen, S. Sen, S. Banerjee and P. Chattopadhyay, Analyst, 2014, 139, 4828–4835.
22. S. Hu, J. Song, G. Wu, C. Cheng and Q. Gao, Spectrochim. Acta, Part A, 2014, 136, 1188–1194.
23. Y. J. Jang, Y. H. Yeon, H. Y. Yang, J. Y. Noh, I. H. Hwang and C. Kim, Inorg. Chem. Commun., 2013, 33, 48–51.
24. L. Wang, L. Yang and D. Cao, Sens. Actuators, B, 2014, 202, 949–958.
25. W. Cao, X.-J. Zheng, J.-P. Sun, W.-T. Wong, D.-C. Fang, J.-X. Zhang and L.-P. Jin, Inorg. Chem., 2014, 53, 3012–3021.
26. J. Y. Noh, S. Kim, I. H. Hwang, G. Y. Lee, J. Kang, S. H. Kim, J. Min, S. Park, C. Kim and J. Kim, Dyes Pigm., 2013, 99, 1016–1021.
27. K. Tiwari, M. Mishra and V. P. Singh, RSC Adv., 2013, 3, 12124–12132.
28. C.-H. Chen, D.-J. Liao, C.-F. Wan and A.-T. Wu, Analyst, 2013, 138, 2527–2530.
29. S. Kim, J. Y. Noh, K. Y. Kim, J. H. Kim, H. K. Kang, S. W. Nam, S. H. Kim, S. Park, C. Kim and J. Kim, Inorg. Chem., 2012, 51, 3597–3602.
30. L. Fan, J. Qin, T. Li, B. Wang and Z. Yang, Sens. Actuators, B, 2014, 203, 550–556.
31. Y. K. Jang, U. C. Nam, H. L. Kwon, I. H. Hwang and C. Kim, Dyes Pigm., 2013, 99, 6–13.
32. O. Alici and S. Erdemir, Sens. Actuators, B, 2015, 208, 159–163.
33. Y. Jiang, L.-L. Sun, G.-Z. Ren, X. Niu, W.-Z. Hu and Z.-Q. Hu, ChemistryOpen, 2015, 4, 378–382.
34. S. Erdemir, O. Kocyigit and S. Malkondu, J. Fluoresc., 2015, 25, 719–727.
35. N. R. Chereddy, P. Nagaraju, M. V. N. Raju, V. R. Krishnaswamy, P. S. Korrapati, P. R. Bangal and V. J. Rao, Biosens. Bioelectron., 2015, 68, 749–756.
36. S. Gui, Y. Huang, F. Hu, Y. Jin, G. Zhang, L. Yan, D. Zhang and R. Zhao, Anal. Chem., 2015, 87, 1470–1474.
37. L. Peng, Z. Zhou, X. Wang, R. Wei, K. Li, Y. Xiang and A. Tong, Anal. Chim. Acta, 2014, 829, 54–59.
38. S. K. Shooraa, A. K. Jaina and V. K. Gupta, Sens. Actuators, B, 2015, 216, 86–104.
39. H. Kim, B. A. Rao, J. W. Jeong, S. Mallick, S.-M. Kang, J. S. Choi, C.-S. Lee and Y.-A. Son, Sens. Actuators, B, 2015, 210, 173–182.
40. J. W. Jeong, B. A. Rao and Y.-A. Son, Sens. Actuators, B, 2015, 208, 75–84.
41. W.-H. Ding, D. Wang, X.-J. Zheng, W.-J. Ding, J.-Q. Zheng, W.-H. Mu, W. Cao and L.-P. Jin, Sens. Actuators, B, 2015, 209, 359–367.
42. S. Paul, A. Manna and S. Goswami, Dalton Trans., 2015, 44, 11805–11810.
43. J. Wang and Y. Pang, RSC Adv., 2014, 4, 5845–5848.
44. S. Goswami, A. Manna, S. Paul, A. K. Maity, P. Saha, C. K. Quah and H.-K. Fun, RSC Adv., 2014, 4, 34572–34576.
45. J. J. Lee, G. J. Park, Y. S. Kim, S. Y. Lee, H. J. Lee, I. S. Noh and C. Kim, Biosens. Bioelectron., 2015, 69, 226–229.
46. Y. Lu, S. Huang, Y. Liu, S. He, L. Zhao and X. Zeng, Org. Lett., 2011, 13, 5274–5277.
47. S. H. Kim, H. S. Choi, J. Kim, S. J. Lee, D. T. Quang and J. S. Kim, Org. Lett., 2012, 12, 560–563.
48. S. Sen, T. Mukherjee, B. Chattopadhyay, A. Moirangthem, A. Basu, J. Marek and P. Chattopadhyay, Analyst, 2012, 137, 3975–3981.
49. V. P. Singh, K. Tiwari, M. Mishra, N. Srivastava and S. Saha, Sens. Actuators, B, 2013, 182, 546–554.
50. S. Guha, S. Lohar, A. Sahana, A. Banerjee, D. A. Safin, M. G. Babashkina, M. P. Mitoraj, M. Bolte, Y. Garcia, S. K. Mukhopadhyay and D. Das, Dalton Trans., 2013, 42, 10198–10207.
51. S. Das, A. Sahana, A. Banerjee, S. Lohar, D. A. Safin, M. G. Babashkina, M. Bolte, Y. Garcia, I. Hauli, S. K. Mukhopadhyay and D. Das, Dalton Trans., 2013, 42, 4757–4763.
52. A. Sahana, A. Banerjee, S. Das, S. Lohar, D. Karak, B. Sarkar, S. K. Mukhopadhyay, A. K. Mukherjee and D. Das, Org. Biomol. Chem., 2011, 9, 5523–5529.
53. L. Wang, W. Qin, X. Tang, W. Dou, W. Liu, Q. Teng and X. Yao, Org. Biomol. Chem., 2010, 8, 3751–3757.
54. S. Sinha, R. R. Koner, S. Kumar, J. Mathew, P. V. Monisha, I. Kazi and S. Ghosh, RSC Adv., 2013, 3, 345–351.
55. Y.-W. Wang, M.-X. Yu, Y.-H. Yu, Z.-P. Bai, Z. Shen, F.-Y. Li and X.-Z. You, Tetrahedron Lett., 2009, 50, 6169–6172.
56. A. Dhara, A. Jana, S. Konar, S. K. Ghatak, S. Ray, K. Das, A. Bandyopadhyay, N. Guchhait and S. K. Kar, Tetrahedron Lett., 2013, 54, 3630–3634.
57. T.-J. Jia, W. Cao, X.-J. Zheng and L.-P. Jin, Tetrahedron Lett., 2013, 54, 3471–3474.
58. B. K. Datta, C. Kar, A. Basu and G. Das, Tetrahedron Lett., 2013, 54, 771–774.
59. D. Karak, S. Lohar, A. Sahana, S. Guha, A. Banerjee and D. Das, Anal. Methods, 2012, 4, 1906–1908.
60. K. Kaur, V. K. Bhardwaj, N. Kaur and N. Singh, Inorg. Chem. Commun., 2012, 26, 31–36.
61. Z. Liu, Z. Yang, Y. Li, T. Li, B. Wang, Y. Li and X. Jin, Inorg. Chim. Acta, 2013, 395, 77–80.
62. J. Isaad and A. E. Achari, Tetrahedron, 2011, 67, 4939–4947.
63. S. Mukherjee, P. Mal and H. Stoeckli-Evans, J. Lumin., 2014, 155, 185–190.
64. Z. Yu, L. A. Bracero, L. Chen, W. Song, X. Wang and B. Zhao, Spectrochim. Acta, Part A, 2013, 105, 52–56.
65. Z. Li, J. Pan and J. Tang, Anal. Lett., 2002, 35, 167–183.
66. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03, Revision D.01, Gaussian, Inc., Wallingford CT, 2004.
67. A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
68. C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785–789.
69. P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213–222.
70. M. M. Francl, W. J. Petro, W. J. Hehre, J. S. Binkley, M. S. Gordon, D. F. DeFrees and J. A. Pople, J. Chem. Phys., 1982, 77, 3654–3665.
71. V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–2001.
72. M. Cossi and V. Barone, J. Chem. Phys., 2001, 115, 4708–4717.
73. N. M. O'Boyle and A. L. Tenderholt, J. Comput. Chem., 2008, 29, 839–845.
74. L. Wang, W. Qin, X. Tang, W. Dou and W. J. Liu, J. Phys. Chem. A, 2011, 115, 1609–1616.
75. S. A. Lee, G. R. You, Y. W. Choi, H. Y. Jo, A. R. Kim, I. Noh, S.-J. Kim, Y. Kim and C. Kim, Dalton Trans., 2014, 43, 6650–6659.
76. Y. W. Choi, G. J. Park, Y. J. Na, H. Y. Jo, S. A. Lee, G. R. You and C. Kim, Sens. Actuators, B, 2014, 194, 343–352.
77. S. C. Burdette, G. K. Walkup, B. Spingler, R. Y. Tsien and S. J. Lippard, J. Am. Chem. Soc., 2011, 123, 7831–7841.
78. P. Job, Ann. Chim., 1928, 9, 113–203.
79. R. Yang, K. Li, K. Wang, F. Zhao, N. Li and F. Liu, Anal. Chem., 2003, 75, 612–621.
80. F. Yu, L. J. Hou, L. Y. Qin, J. B. Chao, Y. Wang and W. J. Jin, J. Photochem. Photobiol., A, 2016, 315, 8–13.
81. J. Qin, Z. Yang and P. Yang, Inorg. Chim. Acta, 2015, 432, 136–141.
82. W.-H. Ding, D. Wang, X.-J. Zheng, W.-J. Ding, J.-Q. Zheng, W.-H. Mu, W. Cao and L.-P. Jin, Sens. Actuators, B, 2015, 209, 359–367.
83. Y.-K. Tsui, S. Devaraj and Y.-P. Yen, Sens. Actuators, B, 2012, 161, 510–519.
84. B. Bodenant, F. Fages and M.-H. Delville, J. Am. Chem. Soc., 1998, 120, 7511–7519.
85. A. Torrado, G. K. Walkup and B. Imperiali, J. Am. Chem. Soc., 1998, 120, 609–610.
86. R. M. Harrison, D. P. H. Laxen and S. J. Wilson, Environ. Sci. Technol., 1981, 15, 1378–1383.
87. D. Maity and T. Govindaraju, Eur. J. Inorg. Chem., 2011, 5479–5485.
88. D. Maity and T. Govindaraju, Chem. Commun., 2012, 48, 1039–1041.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj02334c

---