An Acylhydrazone-Based Fluorescent Sensor for Sequential Recognition of Al3+ and H2PO4−

Abstract

A novel acylhydrazone-based fluorescent sensor NATB was designed and synthesized for consecutive sensing of Al³⁺ and H₂PO₄⁻. NATB displayed fluorometric sensing to Al³⁺ and could sequentially detect H₂PO₄⁻ by fluorescence quenching. The limits of detection for Al³⁺ and H₂PO₄⁻ were determined to be 0.83 and 1.7 μM, respectively. The binding ratios of NATB to Al³⁺ and NATB-Al³⁺ to H₂PO₄⁻ were found to be 1:1. The sequential recognition of Al³⁺ and H₂PO₄⁻ by NATB could be repeated consecutively. In addition, the practicality of NATB was confirmed with the application of test strips. The sensing mechanisms of Al³⁺ and H₂PO₄⁻ by NATB were investigated through fluorescence and UV–Visible spectroscopy, Job plot, ESI-MS, ¹H NMR titration, and DFT calculations.

1. Introduction

Al³⁺, the third most abundant metallic element in nature [1,2], is broadly employed in daily life in packaging materials, pharmaceuticals, food additives, machinery, clinical medicines, and water purification [3,4]. Owing to its widespread usage, Al³⁺ could be readily accumulated in the body, which leads to the development of diverse diseases such as Parkinson’s and Alzheimer’s disease [5,6]. Dihydrogen phosphate (H₂PO₄⁻) is an important component related to many intercellular activities, such as signaling mediation, protein phosphorylation, enzymatic reactions, ion-channel regulation, and so on [7,8,9]. However, excessive agricultural use of phosphate causes eutrophication or massive algal growth, leading to a deficiency in oxygen levels [10,11,12]. For these reasons, there has been a strong demand for the development of sensing and detection methods for Al³⁺ and H₂PO₄⁻.

The traditional analytical methods reported for the analysis of cations and anions, such as ICP-AES, AAS, and electrochemical methods, have been largely restricted due to their expensive instruments, complicated procedures, and the need for highly trained operators [13,14,15]. In contrast, fluorescence methods have shown the advantages of cost-effectiveness, simplicity, easy operation, and high sensitivity [16,17,18]. While numerous fluorescent chemosensors for a single analyte have been reported, fluorescent chemosensors that allow the sequential sensing of multiple analytes with great selectivity and sensitivity are still needed [19,20,21] because they are more cost-effective, recyclable and practical [22,23,24]. Several fluorescent sensors have been addressed for consecutive sensing of Al³⁺ and several anions [25,26,27,28] or several cations and H₂PO₄⁻ [29,30,31]. In addition, Kumar et al. reported a fluorescent sensor for sequential sensing of Al³⁺ and H₂PO₄⁻/HSO₄⁻ [32]. The practical importance of sequential sensing may have potential applications such as logic gates and molecular switches. Nevertheless, a sequential fluorescent sensor that can exclusively detect Al³⁺ and H₂PO₄⁻ has not been reported to date.

As Al³⁺ is a hard cation, chemosensors containing hard base units, such as nitrogen or oxygen atoms, prefer to coordinate with Al³⁺ [33,34,35]. In this regard, acylhydrazone derivatives, having oxygen and nitrogen atoms, are expected to be a suitable functional group to design an Al³⁺ chemosensor [36,37,38]. Naphthalene moieties have been widely applied for the design of fluorescent sensors because of their excellent photophysical properties as a fluorophore [39,40,41]. Hence, we expected that a compound including both acylhydrazone and naphthalene may operate as a fluorescence chemosensor for Al³⁺.

In the current study, we designed an acylhydrazone-based fluorescent sensor, NATB, which showed green fluorescence emissions with Al³⁺ and could sequentially detect H₂PO₄⁻ through fluorescence quenching with high sensitivity and selectivity. A sensing mechanism of NATB to Al³⁺ and H₂PO₄⁻ was illustrated by fluorescence and UV–Vis spectroscopy, Job plot, ESI-MS, ¹H NMR titration, and calculations.

2. Experimental Section

2.1. Materials and Equipment

All solvents and reagents were commercially obtained from TCI (TCI, Nihonbashi-Honcho, Tokyo, Japan) and Sigma-Aldrich (MilliporeSigma, Burlington, MA, USA). NMR experiments were conducted using DMSO-d₆ as the solvent, and the data were recorded on a Varian spectrometer (Varian, Palo Alto, CA, USA). Fluorescence and UV–Visible spectra were measured with Perkin Elmer machines (Perkin Elmer, Waltham, MA, USA). The quantum yields of NATB and NATB-Al³⁺ were relatively determined with quinine (Φ = 0.54 in 1 × 10⁻¹ M H₂SO₄) as a reference. ESI-MS data were recorded on a Thermo Finnigan machine (Thermo Finnigan LLC, San Jose, CA, USA).

2.2. Synthesis of N′-[(E)-(3-tert-butyl-2-hydroxyphenyl)methylidene]-3-hydroxynaphthalene-2-carbohydrazide (NATB)

The intermediate compound, 3-hydroxy-2-naphthohydrazide (2), was synthesized following a previously reported method [42]. The excess amounts of 3-(tert-butyl)-2-hydroxybenzaldehyde (1, 1.8 mmol) and 3-hydroxy-2-naphthohydrazide (2, 0.3 mmol) were mixed in absolute EtOH (10 mL) with a catalytic amount of HCl and stirred at room temperature for 1 day. A yellow precipitate was filtered, rinsed with cold absolute EtOH, and dried (77.2 mg, 70.1%); ¹H NMR in DMSO-d₆: δ 12.42 (s, 1H), 12.24 (s, 1H), 11.19 (s, 1H), 8.63 (s, 1H), 8.45 (s, 1H), 7.93 (d, 1H), 7.77 (d, 1H), 7.53 (t, 1H), 7.38 (t, 1H), 7.35 (s, 1H), 7.32 (d, 1H), 7.30 (d, 1H), 6.91 (t, 1H), 1.43 (s, 9H). ¹³C NMR in DMSO-d6: δ 163.23 (1C), 156.90 (1C), 153.75 (1C), 151.45 (1C), 136.36 (1C), 135.84 (1C), 130.31 (1C), 129.53 (1C), 128.59 (1C), 128.54 (1C), 128.20 (1C), 126.69 (1C), 125.75 (1C), 123.75 (1C), 119.86 (1C), 118.70 (1C), 117.54 (1C), 110.54 (1C), 34.39 (1C), 29.16 (3C). ESI-MS (m/z): [NATB + H⁺]⁺ calcd 363.17, found 363.04.

2.3. Preparation of Spectroscopic Experiments

An NATB stock (10 mM) was prepared in DMSO. The stock solutions (20 mM) of varied cations were prepared using their nitrate salts (Al³⁺, Na⁺, Cr³⁺, Fe²⁺, Ca²⁺, Cd²⁺, Zn²⁺, Pb²⁺, Co²⁺, Cu²⁺, In³⁺, Mn²⁺, Ga³⁺, Ni²⁺, Mg²⁺, Ag⁺, Hg²⁺ and K⁺) or perchlorate salt (Fe³⁺). The concentrated solutions (20 mM) of varied anions were prepared using their tetrabutylammonium salts (H₂PO₄⁻, SCN⁻, BzO⁻, N₃⁻, OAc⁻ and NO₂⁻), tetraethylammonium salts (F⁻, Cl⁻, Br⁻, I⁻ and CN⁻), sodium salts (S²⁻ and ClO⁻), or potassium salts (HPO₄²⁻, PO₄³⁻, HSO₄⁻ and P₂O₇⁴⁻(PPi)). All spectroscopic experiments were conducted in MeOH.

2.4. Competitive Experiments

For Al³⁺, 6 μL (10 mM) of an NATB stock in DMSO was mixed into MeOH (2 mL) to make 30 μM. A total of 4.5 μL of various cations (20 mM) in DMF was diluted in NATB to make 45 μM. Finally, 4.5 μL (20 mM) of an Al³⁺ stock in DMF was mixed into each solution to produce 45 μM, and their fluorescent spectra were measured.

For H₂PO₄⁻, 6 μL (10 mM) of an NATB stock in DMSO and 4.5 μL (20 mM) of an Al³⁺ stock in DMF were diluted into MeOH (2 mL) to produce 30 μM of NATB-Al³⁺. We added 4.5 μL of various anions (20 mM) in H₂O to NATB-Al³⁺ to produce 45 μM. A total of 4.5 μL (20 mM) of an H₂PO₄⁻ stock was diluted into each solution to produce 45 μM. Their fluorescent spectra were measured.

2.5. Determination of Association Constant (K)

The association constant (K) was calculated using Li’s method [43]. If the ligand (L) and the analyte (M) form an m-n complex, M_mL_n, the equilibrium constant of the corresponding complex, K, can be expressed by the following equation:

${\left[M\right]}^m=\frac{1}{nK}\frac{1}{{\left[L\right]}_T^{n-1}}\frac{1-\alpha }{{\alpha }^n}$

where,

$$\begin{gathered} \left[M\right]=the concentration of analyte \\ {\left[L\right\}}_T=the total concentration of ligand \end{gathered}$$

and α could be described as:

$\alpha =\frac{I-I_{max}}{I_{min}-I_{max}}$

where,

$I=the fluorescence intensity of complex$

2.6. Calculations

Calculations were achieved with the Gaussian 16 program [44]. Optimal geometries of NATB and NATB-Al³⁺ were provided with the DFT method [45,46]. B3LYP was selected as the hybrid functional basis set. The 6–31G(d,p) basis set was implemented to all atoms except Al³⁺ [47,48], and the LANL2DZ basis set was employed for applying ECP to Al³⁺ [49,50,51]. No imaginary frequency was found in the optimized states of NATB or NATB-Al³⁺, indicating their local minima. The solvent effect of MeOH was considered with IEFPCM [52]. Based on the energy-optimized structures of NATB and NATB-Al³⁺, the plausible UV–Vis transition states were calculated by the TD-DFT method with 20 lowest singlet states.

3. Results and Discussion

The synthesis of NATB was conducted as depicted in Scheme 1. The condensation reaction of 3-(tert-butyl)-2-hydroxybenzaldehyde (1) and 3-hydroxy-2-naphthohydrazide (2) produced the desired product, N’-[(E)-(3-tert-butyl-2-hydroxyphenyl)methylidene]-3-hydroxynaphthalene-2-carbohydrazide (NATB), which was verified with ¹H NMR, ¹³C NMR (Figures S1 and S2), and ESI-MS.

3.1. Spectroscopic Examination of NATB to Al³⁺

To confirm the fluorescence selectivity of NATB, the fluorescence emission was studied with a variety of cations in MeOH (Figure 1). As a result, NATB exhibited notable fluorescence emission at 526 nm with Al³⁺, while NATB and NATB with other cations showed negligible or no fluorescence emission (λ_ex = 358 nm). These outcomes demonstrated that NATB could be utilized as a fluorescent probe for the selective sensing of Al³⁺. On the other hand, NATB was soluble in aqueous media, but it did not show any selectivity to Al³⁺. In addition, the fluorescence emission of NATB was examined with various anions including dihydrogen phosphate. NATB had no selectivity for the anions.

To check the concentration-dependent properties of NATB to Al³⁺, fluorescence titration was carried out (Figure 2). NATB exhibited little fluorescence with a tiny quantum yield (Φ = 0.008). However, the continuous increase in Al³⁺ up to 1.5 equiv significantly enhanced the green fluorescence emission at 526 nm (Φ = 0.162). UV–Vis spectrometry was also conducted with Al³⁺ to examine its photophysical characteristics (Figure 3). Upon the addition of Al³⁺, the absorption of 310 nm clearly decreased, while a new absorption of 325 nm constantly increased up to 1.5 equiv. An explicit isosbestic point was observed at 315 nm, verifying that the coordination of NATB with Al³⁺ produced a stable complex.

A 1:1 stoichiometric coordination between NATB and Al³⁺ was suggested by the Job plot experiment (Figure S3), which was explicitly supported by ESI-MS analysis (Figure S4). The positive ion mass displayed a large peak of 596.16 (m/z), which was correlated to [NATB(-H⁺) + Al³⁺ + 2 DMF + NO₃⁻]⁺ (calcd. 596.23). The association constant (K) of NATB-Al³⁺ was confirmed to be 3.6 × 10⁴ M⁻¹ (Figure S5) based on Li’s method [43]. The detection limit of NATB toward Al³⁺ was 0.83 μM, based on 3σ/slope (Figure S6).

The ¹H NMR titrations were achieved to investigate the binding mechanism of NATB toward Al³⁺ (Figure 4). Upon the addition of Al³⁺ to NATB, the proton H₁₄ continually disappeared and the protons H₅ and H₆ were deshielded. These results indicate that the deprotonated oxygen on the tert-butylphenol group and the oxygen and nitrogen on the acylhydrazone group may be coordinated to Al³⁺ (Scheme 2).

To verify the practicability of NATB as a probe for Al³⁺, an interference experiment was conducted (Figure S7). NATB could detect Al³⁺ with other cations without significant interferences, except for In³⁺, Fe³⁺ and Cu²⁺. These three cations bound more tightly to NATB than Al³⁺. For the practical application of NATB, test kits were prepared by immersing filter paper strips in the NATB solution. When NATB-coated test kits were immersed in a range of concentrations of Al³⁺ solutions, the obvious green fluorescence emission showed up above 2 mM of Al³⁺ under UV light (Figure 5a). However, the fluorescence was not displayed when those strips were applied to the same concentration of other cations (Figure 5b). These results indicate the potential applications of NATB in easily recognizing Al³⁺ without any complicated tools.

3.2. Calculations

To comprehend the Al³⁺-sensing property of NATB, DFT calculations were performed with the Gaussian 16 program (Figure 6). As the Job plot, ESI-MS, and ¹H NMR titration implied a 1:1 stoichiometric coordination of NATB with Al³⁺, all calculations were conducted with 1:1 stoichiometry. NATB showed a dihedral angle of 0.013° (1O, 2C, 3N, and 4C) with a planar structure (Figure 6a). The coordination of NATB with Al³⁺ distorted its structure, showing a dihedral angle of 98.875° (Figure 6b).

Based on the energy-minimized structures of NATB and NATB-Al³⁺, TD-DFT calculations were conducted to inspect the transition energies and molecular orbitals. NATB featured the main absorption induced from the HOMO → LUMO (347.28 nm), showing intra-charge transfer (ICT) transition from the tert-butylphenol to the naphthol (Figure S8). The major absorption of NATB-Al³⁺ derived from the HOMO-1 → LUMO transition (412.27 nm) also showed a similar ICT transition (Figures S9 and S10). The reduction in the energy gap was consistent with the red-shift of the experimental absorption. These outcomes led us to conclude that the fluorescence turn-on mechanism of NATB to Al³⁺ may be a chelation-enhanced fluorescence (CHEF) effect [53]. Based on experimental data and theoretical calculations, an appropriate binding structure of NATB-Al³⁺ is proposed in Scheme 2.

3.3. Spectroscopic Examination of NATB-Al³⁺ to H₂PO₄⁻

We studied the fluorescence selectivity of NATB-Al³⁺ to a range of anions such as H₂PO₄⁻, Cl⁻, CN⁻, OAc⁻, F⁻, ClO⁻, I⁻, N₃⁻, BzO⁻, SCN⁻, Br⁻, NO₂⁻, S²⁻, HPO₄²⁻, PO₄³⁻, HSO₄⁻, and PPi in MeOH (Figure 7). Most of the anions did not affect the fluorescence emission of NATB-Al³⁺, while the addition of H₂PO₄⁻ toward NATB-Al³⁺ resulted in significant fluorescence quenching (λ_ex = 358 nm). The result demonstrated that NATB-Al³⁺ could be used as a chemosensor for H₂PO₄⁻ with fluorescence quenching.

The fluorescence titration experiments were conducted to verify the fluorescence quenching ability of H₂PO₄⁻ toward NATB-Al³⁺ (Figure 8). The fluorescence of NATB-Al³⁺ consistently diminished with the addition of H₂PO₄⁻ up to 1.5 equiv (Φ = 0.005). UV–Vis spectroscopy showed that the continuous addition of H₂PO₄⁻ increased the absorbance at 310 nm, while those at 270 and 325 nm decreased with the explicit isosbestic points at 253 and 315 nm (Figure 9). The UV–Vis spectrum of H₂PO₄⁻ with NATB-Al³⁺ is analogous to that of free NATB, implying that the addition of H₂PO₄⁻ released Al³⁺ from the NATB-Al³⁺ complex (Figure S11).

The stoichiometry of H₂PO₄⁻ toward NATB-Al³⁺ was determined by the Job plot experiment (Figure S12), which exhibited a 1:1 stoichiometry. The mass spectral analysis displayed a peak of 395.06 (m/z), which demonstrated the regeneration of NATB ([NATB + H⁺ + MeOH]⁺; calcd. 395.20) (Figure S13). These outcomes supported the mechanism that the addition of H₂PO₄⁻ released Al³⁺ from NATB-Al³⁺, which resulted in the loss of fluorescence. Based on Li’s method [43], the association constant (K) for H₂PO₄⁻ with NATB-Al³⁺ was calculated as 1.2 × 10⁴ M⁻¹ (Figure S14). The detection limit of NATB-Al³⁺ toward H₂PO₄⁻ was determined as 1.7 μM, based on 3σ/slope (Figure S15). Importantly, NATB is the first fluorescent sensor for the consecutive sensing of Al³⁺ and H₂PO₄⁻ (Table S1). On the other hand, NATB showed higher detection limits for Al³⁺ and H₂PO₄⁻ compared to Kumar’s work [32], but it could solely detect H₂PO₄⁻ without the interference of HSO₄⁻.

The reversibility in the response of NATB was verified through the alternative additions of Al³⁺ and H₂PO₄⁻ (Figure 10). The fluorescence emission of NATB repeated its enhancing and quenching processes several times without fluorescence efficiency loss. To verify that NATB-Al³⁺ is an effective fluorescence probe for H₂PO₄⁻, the interference of other anions was tested (Figure S16). The results indicated that the presence of other anions (1.5 equiv) did not interfere with the fluorescence quenching of NATB-Al³⁺ toward H₂PO₄⁻.

4. Conclusions

An acylhydrazone-based chemosensor NATB was developed and its sequential recognition of Al³⁺ and H₂PO₄⁻ was studied. NATB showed a strong fluorescence increase with Al³⁺, and its complex NATB-Al³⁺ sequentially detected H₂PO₄⁻ by releasing Al³⁺ with turn-off fluorescence. Importantly, NATB is the first sequential fluorescent probe for selective sensing of Al³⁺ and H₂PO₄⁻. Detection limits of NATB for Al³⁺ and H₂PO₄⁻ were calculated as 0.83 and 1.7 μM, respectively, based on 3σ/slope. NATB could repeat sequential recognition of Al³⁺ and H₂PO₄⁻ several times and could be applied to detect Al³⁺ in test strips. The sensing mechanism of NATB toward Al³⁺ and H₂PO₄⁻ was demonstrated with a Job plot, ESI-MS, ¹H NMR spectroscopy, and theoretical calculations. The detection mechanism of NATB toward Al³⁺ is suggested to be a CHEF effect through DFT calculations.

Author Contributions

D.C. and C.K. provided the initial idea for this work; D.C. contributed to the collection and analysis of field test data; D.C. and C.K. wrote the paper. All authors have read and agreed to the published version of the manuscript.

Funding

National Research Foundation of Korea (2018R1A2B6001686) is acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Schemes

Scheme 1. Synthesis of NATB.

Figure 1. Fluorescence changes of NATB (30 μM) with a variety of cations (1.5 equiv) in MeOH. Photograph: the fluorescent images of NATB and NATB-Al3+ (λex: 358 nm).

Figure 2. Fluorescence titration of NATB (30 μM) with varied amounts of Al3+ (0–1.5 equiv) in MeOH.

Figure 3. UV–Vis changes of NATB (30 μM) with varied amounts of Al3+ (0–1.5 equiv) in MeOH.

Figure 4. 1H NMR titration of NATB with Al3+ in DMSO-d6.

Scheme 2. Sequential recognition mechanism of Al3+ and H2PO4− by NATB.

Figure 5. Detection of Al3+ by NATB-coated test kits (10 mM). (a) NATB-coated test kits immersed in the solution of different Al3+ concentrations; (b) NATB-coated test kits immersed in 2 mM of various cation solutions.

Figure 6. Energy-optimized forms of (a) NATB and (b) NATB-Al3+.

Figure 7. Fluorescence changes of NATB-Al3+ (30 μM) with various anions (45 μM) in MeOH (λex: 358 nm). Photograph: the fluorescent images of NATB-Al3+ and NATB-Al3+-H2PO4− (λex: 358 nm).

Figure 8. Fluorescence titration of NATB-Al3+ (30 μM) with various amounts of H2PO4− (0–1.5 equiv) in MeOH.

Figure 9. UV–Vis changes of NATB-Al3+ (30 μM) with various amounts of H2PO4− (0–1.5 equiv) in MeOH.

Figure 10. Change in fluorescence of NATB (30 μM) upon alternate addition of Al3+ and H2PO4− in MeOH (λex: 358 nm).

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/ma14216392/s1, Table S1: Examples of chemosensors for successive detection related to Al³⁺ or H₂PO₄⁻ or both; Figure S1: ¹H NMR spectrum of NATB in DMSO-d₆; Figure S2: ¹³C NMR spectrum of NATB in DMSO-d₆; Figure S3: Job plot for the binding of NATB with Al³⁺ (50 μM) in MeOH. Fluorescence intensity at 526 nm is plotted as a function of the molar ratio of [Al³⁺]/([Al³⁺] + [NATB]); Figure S4: Positive-ion ESI mass spectrum of NATB (100 μM) in MeOH upon the addition of 1 equiv of Al³⁺ in DMF; Figure S5: Li’s equation plot (at 526 nm) of NATB (30 µM) in MeOH, based on fluorescence titration, assuming 1:1 stoichiometry for association between NATB and Al³⁺; Figure S6: Calibration curve of NATB as a function of Al³⁺ concentration in MeOH. [NATB] = 30 μM and [Al³⁺] = 0–18 μM (λ_ex = 358 nm); Figure S7: Competitive experiments of NATB (30 µM) toward Al³⁺ (45 µM) in the presence of other metal ions (45 µM, λ_ex = 358 nm) in MeOH; Figure S8: (a) The theoretical excitation energies and the experimental UV–Vis spectrum of NATB. (b) The major electronic transition energies and molecular orbital contributions of NATB; Figure S9: (a) The theoretical excitation energies and the experimental UV–Vis spectrum of NATB-Al³⁺. (b) The major electronic transition energies and molecular orbital contributions of NATB-Al³⁺; Figure S10: The major molecular orbital transitions and excitation energies of NATB and NATB-Al³⁺; Figure S11: UV–Vis spectra of NATB and NATB-Al³⁺ with H₂PO₄⁻ in MeOH, respectively; Figure S12: Job plot for the stoichiometry of NATB-Al³⁺ with H₂PO₄⁻ (30 μM) in MeOH. Fluorescence intensity at 526 nm is plotted as a function of the molar ratio of [NATB-Al³⁺]/([NATB-Al³⁺] + [H₂PO₄⁻]); Figure S13: Positive-ion ESI mass spectrum of NATB-Al³⁺ (100 μM) in MeOH upon the addition of 1 equiv of H₂PO₄⁻ in H₂O; Figure S14: Li’s equation plot (at 526 nm) of NATB-Al³⁺ (30 µM) based on fluorescence titration in MeOH, assuming 1:1 stoichiometry for association between NATB-Al³⁺ and H₂PO₄⁻; Figure S15: Calibration curve of NATB-Al³⁺ in MeOH as a function of H₂PO₄⁻ concentration. [NATB-Al³⁺] = 30 μM and [H₂PO₄⁻] = 0.0–18.0 μM (λ_ex = 358 nm); Figure S16: Interference studies of NATB-Al³⁺ (30 µM) toward H₂PO₄⁻ (45 µM) in the presence of other anions (45 µM, λ_ex = 358 nm) in MeOH.

References

1. Pungut, N.A.S.; Saad, H.M.; Sim, K.S.; Tan, K.W. A turn on fluorescent sensor for detecting Al³⁺ and colorimetric detection for Cu²⁺: Synthesis, cytotoxicity and on-site assay kit. J. Photochem. Photobiol. A Chem.; 2021; 414, 113290. [DOI: https://dx.doi.org/10.1016/j.jphotochem.2021.113290]

2. Maity, D.; Govindaraju, T. Pyrrolidine constrained bipyridyl-dansyl click fluoroionophore as selective Al³⁺sensor. Chem. Commun.; 2010; 46, pp. 4499-4501. [DOI: https://dx.doi.org/10.1039/c0cc00119h]

3. Kim, S.Y.; Lee, S.Y.; Kang, J.H.; Kim, M.S.; Kim, A.; Kim, C. Colorimetric detection of Fe^3+/2+ and fluorescent detection of Al³⁺ in aqueous media: Applications and DFT calculations. J. Coord. Chem.; 2018; 71, pp. 2401-2414. [DOI: https://dx.doi.org/10.1080/00958972.2018.1478086]

4. Kaur, R.; Saini, S.; Kaur, N.; Singh, N.; Jang, D.O. Rhodamine-based fluorescent probe for sequential detection of Al³⁺ ions and adenosine monophosphate in water. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2020; 225, 117523. [DOI: https://dx.doi.org/10.1016/j.saa.2019.117523]

5. Cheah, P.W.; Heng, M.P.; Izati, A.; Ng, C.H.; Tan, K.W. Rhodamine B conjugate for rapid colorimetric and fluorimetric detection of aluminium and tin ions and its application in aqueous media. Inorg. Chim. Acta; 2020; 512, 119901. [DOI: https://dx.doi.org/10.1016/j.ica.2020.119901]

6. Maity, D.; Govindaraju, T. A differentially selective sensor with fluorescence turn-on response to Zn²⁺ and dual-mode ratiometric response to Al³⁺ in aqueous media. Chem. Commun.; 2012; 48, pp. 1039-1041. [DOI: https://dx.doi.org/10.1039/C1CC16064H]

7. Zhou, J.; Yuan, Y.-F.; Zhuo, J.-B.; Lin, C.-X. Synthesis and characterization of cyclophane: The highly selective recognition of Fe³⁺ in aqueous solution and H₂PO₄⁻ in acetonitrile solution. Tetrahedron Lett.; 2018; 59, pp. 1059-1064. [DOI: https://dx.doi.org/10.1016/j.tetlet.2018.02.005]

8. Singh, A.; Nishith, U.; Trivedi, D.R. Spectroscopic studies of colorimetric receptors for detection of biologically important inorganic F⁻, AcO⁻ and H₂PO₄⁻ anions in organo-aqueous medium: Real-life application. Inorg. Chem. Commun.; 2020; 115, 107874. [DOI: https://dx.doi.org/10.1016/j.inoche.2020.107874]

9. Goswami, S.; Maity, S.; Maity, A.C.; Das, A.K.; Khanra, K.; Mandal, T.K.; Bhattacharyya, N. A macrocyclic piperazine linked extremely Zn²⁺ selective fluorescent chemosensor with bio-imaging and for H₂PO₄⁻ sensing. Tetrahedron Lett.; 2014; 55, pp. 5993-5997. [DOI: https://dx.doi.org/10.1016/j.tetlet.2014.09.014]

10. Cao, C.; You, X.; Feng, L.; Luo, G.; Yue, G.; Ji, X. Synthesis of new chromogenic sensors containing thiourea and selective detection for F^–, H₂PO₄^–, and Ac^– anions. Can. J. Chem.; 2020; 98, pp. 659-666. [DOI: https://dx.doi.org/10.1139/cjc-2020-0120]

11. Choi, J.H.; Pandith, A.; Chakradhar, D.; Kim, H.R.; Kim, H.S. Al³⁺-Morpholine-appended Anthracene Ensemble as a Dual Photonic Switch for H₂PO₄⁻ and CN⁻ Ions and its Biological Applications. Bull. Korean Chem. Soc.; 2019; 40, pp. 138-145. [DOI: https://dx.doi.org/10.1002/bkcs.11657]

12. Pandith, A.; Uddin, N.; Choi, C.H.; Kim, H.S. Highly selective imidazole-appended 9,10-N,N’-diaminomethylanthracene fluorescent probe for switch-on Zn²⁺ detection and switch-off H₂PO₄⁻ and CN⁻ detection in 80% aqueous DMSO, and applications to sequential logic gate operations. Sens. Actuators B Chem.; 2017; 247, pp. 840-849. [DOI: https://dx.doi.org/10.1016/j.snb.2017.03.112]

13. Theetharappan, M.; Neelakantan, M.A. A Water-Soluble Schiff Base Turn-on Fluorescent Chemosensor for the Detection of Al³⁺ and Zn²⁺ Ions at the Nanomolar Level: Application in Live-Cell Imaging. J. Fluoresc.; 2021; 31, pp. 1277-1290. [DOI: https://dx.doi.org/10.1007/s10895-021-02756-7]

14. Chandra, R.; Manna, A.K.; Sahu, M.; Rout, K.; Patra, G.K. Simple salicylaldimine-functionalized dipodal bis Schiff base chromogenic and fluorogenic chemosensors for selective and sensitive detection of Al³⁺ and Cr³⁺. Inorg. Chim. Acta; 2020; 499, 119192. [DOI: https://dx.doi.org/10.1016/j.ica.2019.119192]

15. Jeong, H.Y.; Lee, S.Y.; Han, J.; Lim, M.H.; Kim, C. Thiophene and diethylaminophenol-based “turn-on” fluorescence chemosensor for detection of Al³⁺ and F⁻ in a near-perfect aqueous solution. Tetrahedron; 2017; 73, pp. 2690-2697. [DOI: https://dx.doi.org/10.1016/j.tet.2017.03.069]

16. Wang, M.; Lu, L.; Song, W.; Wang, X.; Sun, T.; Zhu, J.; Wang, J. AIE-active Schiff base compounds as fluorescent probe for the highly sensitive and selective detection of Al³⁺ ions. J. Lumin.; 2021; 233, 117911. [DOI: https://dx.doi.org/10.1016/j.jlumin.2021.117911]

17. Park, S.M.; Saini, S.; Park, J.E.; Singh, N.; Jang, D.O. A benzothiazole-based receptor for colorimetric detection of Cu²⁺ and S²⁻ ions in aqueous media. Tetrahedron Lett.; 2021; 73, 153115. [DOI: https://dx.doi.org/10.1016/j.tetlet.2021.153115]

18. So, H.; Cho, H.; Lee, H.; Tran, M.C.; Kim, K.T.; Kim, C. Detection of zinc (II) and hypochlorite by a thiourea-based chemosensor via two emission channels and its application in vivo. Microchem. J.; 2020; 155, 104788. [DOI: https://dx.doi.org/10.1016/j.microc.2020.104788]

19. Feng, X.; Fu, Y.; Jin, J.; Wu, J. A highly selective and sensitive fluorescent sensor for relay recognition of Zn²⁺ and HSO₄⁻/H₂PO₄⁻ with “on-off” fluorescent responses. Anal. Biochem.; 2018; 563, pp. 20-24. [DOI: https://dx.doi.org/10.1016/j.ab.2018.09.018]

20. Du, K.; Niu, S.; Qiao, L.; Dou, Y.; Zhu, Q.; Chen, X.; Zhang, P. A highly selective ratiometric fluorescent probe for the cascade detection of Zn²⁺ and H₂PO₄⁻ and its application in living cell imaging. RSC Adv.; 2017; 7, pp. 40615-40620. [DOI: https://dx.doi.org/10.1039/C7RA08020D]

21. Hwang, S.M.; Kim, M.S.; Lee, M.; Lim, M.H.; Kim, C. Single fluorescent chemosensor for multiple targets: Sequential detection of Al³⁺ and pyrophosphate and selective detection of F⁻ in near-perfect aqueous solution. N. J. Chem.; 2017; 41, pp. 15590-15600. [DOI: https://dx.doi.org/10.1039/C7NJ03575F]

22. Liu, Y.; Wang, X.; Feng, E.; Fan, C.; Pu, S. A highly selective sequential recognition probe for Zn²⁺ and HSO₄⁻/H₂PO₄⁻ based on a diarylethene chemosensor. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2021; 246, 119052. [DOI: https://dx.doi.org/10.1016/j.saa.2020.119052] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33075705]

23. Jain, H.; Deswal, N.; Joshi, A.; Ramachandran, C.N.; Kumar, R. Triazole-appended pyrano[2,3-c]pyrazolone based colorimetric chemosensors for recognition of Fe³⁺ ions and their molecular logic gate behavior. Anal. Methods; 2019; 11, pp. 3230-3243. [DOI: https://dx.doi.org/10.1039/C9AY00515C]

24. Qu, W.J.; Yan, G.T.; Ma, X.L.; Wei, T.B.; Lin, Q.; Yao, H.; Zhang, Y.M. “Cascade recognition” of Cu²⁺ and H₂PO₄⁻ with high sensitivity and selectivity in aqueous media based on the effect of ESIPT. Sens. Actuators B Chem.; 2017; 242, pp. 849-856. [DOI: https://dx.doi.org/10.1016/j.snb.2016.09.173]

25. Li, S.; Cao, D.; Meng, X.; Hu, Z.; Li, Z.; Yuan, C.; Zhou, T.; Han, X.; Ma, W. A novel schiff base fluorescent probe based on coumarin and benzothiazole for sequential detection of Al³⁺ and PPi and its applicability in live cell imaging. J. Photochem. Photobiol. A Chem.; 2020; 392, 112427. [DOI: https://dx.doi.org/10.1016/j.jphotochem.2020.112427]

26. Fu, J.; Li, B.; Mei, H.; Chang, Y.; Xu, K. Fluorescent schiff base probes for sequential detection of Al³⁺ and F⁻ and cell imaging applications. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2020; 227, 117678. [DOI: https://dx.doi.org/10.1016/j.saa.2019.117678] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31676151]

27. Sun, X.J.; Liu, T.T.; Fu, H.; Li, N.N.; Xing, Z.Y.; Yang, F. A Naphthalimide-Based Fluorescence “Off-on-Off” Chemosensor for Relay Detection of Al³⁺ and ClO⁻. Front. Chem.; 2019; 7, 549. [DOI: https://dx.doi.org/10.3389/fchem.2019.00549]

28. Huang, M.X.; Lai, J.P.; Sun, H.; Wu, W.Z. A simple, highly selective and ultra-sensitive “off-on-off” fluorescent chemosensor for successive detection of aluminum ion and phosphate in water samples. Microchem. J.; 2019; 151, 104195. [DOI: https://dx.doi.org/10.1016/j.microc.2019.104195]

29. Zhang, Y.M.; Chen, X.P.; Liang, G.Y.; Zhong, K.P.; Yao, H.; Wei, T.B.; Lin, Q. A water-soluble fluorescent chemosensor based on Asp functionalized naphthalimide for successive detection Fe³⁺ and H₂PO₄⁻. Can. J. Chem.; 2018; 96, pp. 363-370. [DOI: https://dx.doi.org/10.1139/cjc-2017-0451]

30. Meng, X.; Li, S.; Ma, W.; Wang, J.; Hu, Z.; Cao, D. Highly sensitive and selective chemosensor for Cu²⁺ and H₂PO₄⁻ based on coumarin fluorophore. Dyes Pigments; 2018; 154, pp. 194-198. [DOI: https://dx.doi.org/10.1016/j.dyepig.2018.03.002]

31. Purkait, R.; Das Mahapatra, A.; Chattopadhyay, D.; Sinha, C. An azine-based carbothioamide chemosensor for selective and sensitive turn-on-off sequential detection of Zn(II) and H₂PO₄⁻, live cell imaging and INHIBIT logic gate. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2019; 207, pp. 164-172. [DOI: https://dx.doi.org/10.1016/j.saa.2018.09.019] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30227347]

32. Kumar, A.; Kumar, V.; Upadhyay, K.K. An Al³⁺ and H₂PO₄⁻/HSO₄⁻ selective conformational arrest and bail to a pyrimidine-naphthalene anchored molecular switch. Analyst; 2013; 138, pp. 1891-1897. [DOI: https://dx.doi.org/10.1039/c3an36697a] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23389672]

33. Anu, D.; Naveenm, P.; Rajamanikandan, R.; Kaveri, M.V. Development of hydrazide based fluorescence probe for detection of Al³⁺ ions and application in live cell image. J. Photochem. Photobiol. A Chem.; 2021; 405, 112921. [DOI: https://dx.doi.org/10.1016/j.jphotochem.2020.112921]

34. Sun, J.; Li, Y.; Shen, S.; Yan, Q.; Xia, G.; Wang, H. A squaraine-based fluorescence turn on chemosensor with ICT character for highly selective and sensitive detection of Al³⁺ in aqueous media and its application in living cell imaging. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2020; 228, 117590. [DOI: https://dx.doi.org/10.1016/j.saa.2019.117590]

35. Xu, Y.; Kong, L.; Bai, L.; Chen, A.; Li, N.; Cheng, L.; Liu, W.; Sun, X.; Tao, F.; Wang, L. et al. A new water-soluble polymer fluorescent chemosensor with thiophene Schiff base site for selectively sensing Al³⁺ ions. Tetrahedron; 2021; 79, 131888. [DOI: https://dx.doi.org/10.1016/j.tet.2020.131888]

36. Li, Z.; Wang, Q.; Wang, J.; Jing, X.; Wang, S.; Xiao, L.; Li, L. A fluorescent light-up probe for selective detection of Al³⁺ and its application in living cell imaging. Inorg. Chim. Acta; 2020; 500, 119231. [DOI: https://dx.doi.org/10.1016/j.ica.2019.119231]

37. Liao, Z.; Liu, Y.; Han, S.F.; Wang, D.; Zheng, J.Q.; Zheng, X.J.; Jin, L.P. A novel acylhydrazone-based derivative as dual-mode chemosensor for Al³⁺, Zn²⁺ and Fe³⁺ and its applications in cell imaging. Sens. Actuators B Chem.; 2017; 244, pp. 914-921. [DOI: https://dx.doi.org/10.1016/j.snb.2017.01.074]

38. Zhu, G.; Huang, Y.; Wang, C.; Lu, L.; Sun, T.; Wang, M.; Tang, Y.; Shan, D.; Wen, S.; Zhu, J. A novel coumarin-based fluorescence chemosensor for Al³⁺ and its application in cell imaging. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.; 2019; 210, pp. 105-110. [DOI: https://dx.doi.org/10.1016/j.saa.2018.11.006]

39. Das, A.K.; Goswami, S. 2-Hydroxy-1-naphthaldehyde: A versatile building block for the development of sensors in supramolecular chemistry and molecular recognition. Sens. Actuators B Chem.; 2017; 245, pp. 1062-1125. [DOI: https://dx.doi.org/10.1016/j.snb.2017.01.068]

40. Immanuel David, C.; Bhuvanesh, N.; Jayaraj, H.; Thamilselvan, A.; Parimala Devi, D.; Abiram, A.; Prabhu, J.; Nandhakumar, R. Experimental and Theoretical Studies on a Simple S-S-Bridged Dimeric Schiff Base: Selective Chromo-Fluorogenic Chemosensor for Nanomolar Detection of Fe²⁺ & Al³⁺ Ions and Its Varied Applications. ACS Omega; 2020; 5, pp. 3055-3072. [DOI: https://dx.doi.org/10.1021/acsomega.9b04294]

41. Das, B.; Dolai, M.; Dhara, A.; Ghosh, A.; Mabhai, S.; Misra, A.; Dey, S.; Jana, A. Solvent-Regulated Fluorimetric Differentiation of Al³⁺ and Zn²⁺ Using an AIE-Active Single Sensor. J. Phys. Chem. A; 2021; 125, pp. 1490-1504. [DOI: https://dx.doi.org/10.1021/acs.jpca.0c10518]

42. Singh, R.; Das, G. “Turn-on” Pb²⁺ sensing and rapid detection of biothiols in aqueous medium and real samples. Analyst; 2019; 144, pp. 567-572. [DOI: https://dx.doi.org/10.1039/C8AN01624K]

43. Yang, R.; Li, K.; Wang, K.; Zhao, F.; Li, N.; Liu, F. Porphyrin assembly on β-cyclodextrin for selective sensing and detection of a zinc ion based on the dual emission fluorescence ratio. Anal. Chem.; 2003; 75, pp. 612-621. [DOI: https://dx.doi.org/10.1021/ac020467n] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/12585492]

44. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Petersson, G.A.; Nakatsuji, H. et al. Gaussian 16, Revision C.01; Gaussian, Inc.: Wallingford, CT, USA, 2016.

45. Becke, A.D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys.; 1993; 98, pp. 5648-5652. [DOI: https://dx.doi.org/10.1063/1.464913]

46. Lee, C.; Yang, W.; Parr, R.G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B; 1988; 37, pp. 785-789. [DOI: https://dx.doi.org/10.1103/PhysRevB.37.785] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/9944570]

47. Hariharan, P.C.; Pople, J.A. The influence of polarization functions on molecular orbital hydrogenation energies. Theor. Chim. Acta; 1973; 28, pp. 213-222. [DOI: https://dx.doi.org/10.1007/BF00533485]

48. Francl, M.M.; Pietro, W.J.; Hehre, W.J.; Binkley, J.S.; Gordon, M.S.; DeFrees, D.J.; Pople, J.A. Self-consistent molecular orbital methods. XXIII. A polarization-type basis set for second-row elements. J. Chem. Phys.; 1982; 77, pp. 3654-3665. [DOI: https://dx.doi.org/10.1063/1.444267]

49. Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys.; 1985; 82, pp. 270-283. [DOI: https://dx.doi.org/10.1063/1.448799]

50. Wadt, W.R.; Hay, P.J. Ab initio effective core potentials for molecular calculations. Potentials for main group elements Na to Bi. J. Chem. Phys.; 1985; 82, pp. 284-298. [DOI: https://dx.doi.org/10.1063/1.448800]

51. Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitale. J. Chem. Phys.; 1985; 82, pp. 299-310. [DOI: https://dx.doi.org/10.1063/1.448975]

52. Klamt, A.; Moya, C.; Palomar, J. A Comprehensive Comparison of the IEFPCM and SS(V)PE Continuum Solvation Methods with the COSMO Approach. J. Chem. Theory Comput.; 2015; 11, pp. 4220-4225. [DOI: https://dx.doi.org/10.1021/acs.jctc.5b00601] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26575917]

53. Jo, T.G.; Bok, K.H.; Han, J.; Lim, M.H.; Kim, C. Colorimetric detection of Fe³⁺ and Fe²⁺ and sequential fluorescent detection of Al³⁺ and pyrophosphate by an imidazole-based chemosensor in a near-perfect aqueous solution. Dyes Pigments; 2017; 139, pp. 136-147. [DOI: https://dx.doi.org/10.1016/j.dyepig.2016.11.052]