A simple sensitive ESIPT on-off fluorescent sensor for selective detection of Al³⁺ in water

Abstract

A highly selective and sensitive fluorescent sensor for Al³⁺ has been developed. The sensor shows great fluorescence turn-on upon binding Al³⁺ in complete water, giving strong blue emission. In addition, the sensor's turn-on exhibits excellent selectivity to the Al³⁺ cation, with only a slight interference from Zn²⁺. These findings suggest that the developed Al³⁺ sensor could be a useful molecular probe for practical applications.

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Aluminum is the most abundant metal in the Earth's crust and extensively used in modern life.¹ The soluble form of aluminum (Al³⁺), however, is highly toxic to plant growth.² Excessive aluminum, especially when deposited in the brain even in small amounts, has also been shown to be toxic to humans, and is believed to cause neurodementia such as Parkinson's disease, Alzheimer's disease and dialysis encephalopathy, osteoporosis, etc.³ For these reasons, the development of Al³⁺ sensors for its facile detection is of great importance in both environmental monitoring and biological assays.

Despite the strong interest, the fluorescent detection for Al³⁺ cations remains to be a challenging problem. Owing to its weak coordination with ligands and strong hydration ability in water,⁴ the detection of Al³⁺ cation is often affected by the existence of interfering metal ions. So far, very few fluorescent chemosensors have been reported for detection of Al³⁺ with moderate success to date compared to the transition metal ions.^5–25 The majority of the reported Al³⁺ sensors, however, have limitations such as tedious synthetic efforts and/or lack of practical applicability in aqueous solutions.¹¹ Today, almost all the reported dyes for Al³⁺ have been tested in organic solvents or mixed solvents. In order to enable evaluation of Al³⁺ ions in aqueous environments, it is highly desirable to develop new sensors, which not only recognize Al³⁺ ions selectively but also compete effectively with the strong hydration of Al³⁺ ion during the application in aqueous.

One option is to integrate the Al³⁺ binding event with the excited state intramolecular proton transfer (ESIPT) in the sensor design. Recently, ESIPT has attracted attention from both theoretical and experimental viewpoints, because it shows a uniquely large Stokes' shifted fluorescence emission (6000–12 000 cm⁻¹).²⁶ In addition, the ESIPT turn-on or turn-off events will usually lead to a large change in fluorescence wavelength,²⁷ which is of great importance in their practical applications. In general, the ESIPT process requires a proton donor (–OH, –NH₂) and a proton acceptor (–C=O, –N=) group in close proximity in order to form the intramolecular hydrogen bond (a necessary condition for ESIPT).²⁸

In order to demonstrate the concept of using ESIPT in Al³⁺ sensing, we decide to explore the synthesis of Schiff base 1. In the sensor design, the hydroxyl group in 1 forms an intramolecular hydrogen bond with the adjacent imine bond (–CH=N–), which gives ESIPT. The hydroxyl and adjacent “acetohydrazide” groups also provide a strong binding cavity to host the Al³⁺ cation. As a consequence, the new sensor integrates the following functions into a single molecule: (a) containing sufficient polar groups to improve water solubility; (b) including an amine group for photoinduced electron transfer (PET) effect to suppress the background signal; and (c) utilizing the Al³⁺ binding to switch the excited-state intramolecular proton transfer (ESIPT), thereby inducing a large spectral shift. Herein, we report the fluorescence response of sensor 1, which exhibits remarkable fluorescence turned on (by ∼73 fold) upon binding Al³⁺ ion. In addition, the Al³⁺ binding also induced a large spectral shift (by 40 nm) (Fig. 1), as the cation binding turned off the ESIPT.

Fig. 1 (a) Fluorescent spectra of 1 (20.0 μM) with 5.0 equiv. of various metal ions in pure water: Na⁺, K⁺, Ag⁺, Mg²⁺, Ca²⁺, Hg²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Ni²⁺, Co²⁺, Cu²⁺, Fe²⁺, Zn²⁺, Cr³⁺, Fe³⁺. (b) Fluorescent images of 1 in the presence of different cations. (c) Change ratio ((F_A − F_A0)/F_A0) of fluorescence of 1 (20.0 μM) in pure water containing various metal ions (5.0 equiv.): Na⁺, K⁺, Ag⁺, Mg²⁺, Ca²⁺, Hg²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Ni²⁺, Co²⁺, Cu²⁺, Fe²⁺, Zn²⁺, Cr³⁺, Fe³⁺, Al³⁺.

Chemosensor 1 was synthesized in over 90% yield by simple coupling of 2-hydroxybenzaldehyde with acetohydrazide (A) (Scheme 1). Compound 1 could exist in the isomers 1a and 1b, whose ratio was dependent on the equilibrium in different solvents (See ESI Fig. S1–S3†). The structure of the major isomer 1a was determined by X-ray diffraction (ESI Fig. S14†). In aqueous, the free ligand 1 gave very weak green fluorescence (the emission λ_em = 485 nm, ϕ_fl = 0.01), partly attributing to the PET effect from the amine. As expected, the emission of 1 exhibited a large Stocks' shift in water (Δλ = 495 (λ_em) − 317 (λ_max) ≈ 168 nm), as a consequence of ESIPT process. Upon addition of Al³⁺ cation, however, the solution gave bright blue fluorescence, with its quantum efficiency reaching as high as ϕ_fl = 0.73 (Fig. 1a). In addition, the Al³⁺ binding also shifted the emission signal (around 40 nm shift from the weak green fluorescence to strong blue fluorescence), which could be used for naked eye detection (See Fig. 1a and b). Clearly, two “oxygen” and one “nitrogen” atoms in sensor 1 provided a strong Al³⁺ binding (via hard acid–base interaction) to compete with the Al³⁺ hydration, thereby allowing the reliable fluorescence turn-on in the aqueous solution.

Scheme 1 Synthesis of 1 and its Al³⁺ complex.

The fluorometric behaviour of 1 was further investigated by addition of the other metal ions Na⁺, K⁺, Ag⁺, Mg²⁺, Ca²⁺, Hg²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Ni²⁺, Co²⁺, Cu²⁺, Fe²⁺, Zn²⁺, Cr³⁺, and Fe³⁺ in pure water. As shown in Fig. 1a and c, the addition of 5.0 equiv. of Na⁺, K⁺, Ag⁺, Mg²⁺, Ca²⁺, Hg²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Mn²⁺ and Cr³⁺ has no obvious effect on the fluorescence emission. The metal ions Ni²⁺, Co²⁺, Cu²⁺, Fe²⁺ and Fe³⁺ quenched the fluorescence. Although sensor 1 responded to Zn²⁺ cation with a slight increase in the fluorescent intensity, the emission wavelength of 1-Zn was only shifted to 473 nm (around 10 nm blue shift). Therefore, sensor 1 is highly selective and sensitive for Al³⁺ detection, which makes it feasible for biological and environmental applications.

The absorption peak of 1 (λ_max = 317 nm) was progressively decreased upon addition of Al³⁺ (Fig. 2a), which was accompanied with a new band at about 352 nm. The large spectral bathochromic shift indicated the deprotonation, as a consequence of Al³⁺-binding to phenol. Observation of the distinct isosbestic point at 333 nm suggests the complex formation with one new chemical species. On the basis of Job plot, the complex was assumed to have a ligand-to-metal ratio of 2 : 1. The assumption was supported by high resolution mass spectroscopy (HRMS), which detected m/z 381.1135, corresponding to [2(1-H⁺)+Al³⁺]⁺ (See Fig. 4: the calcd mass for C₁₈H₁₈AlN₄O₄: 381.1143). Furthermore, mass spectra detected no Al³⁺ complex with 1 : 1 ligand-to-metal ratio from the aqueous solution of 1 and Al³⁺ (ESI Fig. S5†).

Fig. 2 Titration of 1 (10 μM) in water by addition of different amount of Al³⁺.

The cation binding was further examined by ¹H NMR titration. The free ligand exhibited two imine (–CH=N–) signals at 8.25 and 8.15 ppm, corresponding to the major and minor isomers 1a and 1b respectively (Fig. 3). Upon addition of Al³⁺ cation, a broad imine signal was developed at 8.45 ppm, attributing to the formation of 1-Al³⁺ complex. The triplet signal at 7.27 ppm (aromatic H_c) disappeared when ∼0.5 equiv. molar Al³⁺ was added, indicating that all ligand was consumed to bind Al³⁺ cation, forming 1-Al³⁺ complex. Further addition of Al³⁺ (0.5–1.0 equiv. molar) did not reveal significant change. Since all the ligand was used to bind Al³⁺, the broad peak at 8.28 ppm was also attributed to 1-Al³⁺, in addition to the peak at 8.45 ppm. The two peaks at 8.28 and 8.45 ppm suggested possible existence of two different isomers in the metal complex, as the mass spectra detected only 1-Al³⁺ complex in 2 : 1 ligand-to-metal ratio.

Fig. 3 ¹H NMR of 1 in CD₃OD upon addition of various equiv. of Al³⁺ cation.

In order to further evaluate the selectivity of 1 for Al³⁺ sensing, competition experiments were carried out with other metal cations. The fluorescence response of 1 to Al³⁺ in pure water in the presence of 5 equiv. of various cations including Na⁺, K⁺, Ag⁺, Mg²⁺, Ca²⁺, Hg²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Ni²⁺, Co²⁺, Cu²⁺, Fe²⁺, Zn²⁺, Cr³⁺ and Fe³⁺ are given in Fig. 4, respectively. There were insignificant changes in the fluorescence of “1 + Al³⁺” in the presence of other competing metal cations. Except for Fe³⁺ and Cu²⁺, most competing metal ions did not interfere with detection of Al³⁺ by 1 in H₂O (Fig. 4), indicating that 1 can be used as a selective chemosensor for the Al³⁺ cation.

Fig. 4 Fluorescence intensity of 1 (20.0 μM) in pure water (column 1), and sensor 1 in the presence of different metal ion(s) (5.0 equiv.): (2) Al³⁺; (3) Al³⁺ + Na⁺; (4) Al³⁺ + K⁺; (5) Al³⁺ + Ag⁺; (6) Al³⁺ + Mg²⁺; (7) Al³⁺ + Ca²⁺; (8) Al³⁺ + Hg²⁺; (9) Al³⁺ + Ba²⁺; (10) Al³⁺ + Pb²⁺; (11) Al³⁺ + Cd²⁺; (12) Al³⁺ + Mn²⁺; (13) Al³⁺ + Ni²⁺; (14) Al³⁺ + Co²⁺; (15) Al³⁺ + Zn²⁺; (16) Al³⁺ + Cr³⁺; (17) Al³⁺ + Fe²⁺; (18) Al³⁺ + Cu²⁺; (19) Al³⁺ + Fe³⁺.

When using 1 in low concentration (120 nM), the water scattering signal and the background noise could be brought to a observable level (Fig. 5) to evaluate the detection limit. Sensor 1 exhibited a good linear response towards Al³⁺ in the concentration range of 1.0 to 60 nM in water, showing its analytical value. The detection limit for Al³⁺ was determined to be as low as 5.0 × 10⁻¹¹ M (0.5 nM) in pure water (ESI Fig. S15†), which was defined as the three-fold standard deviation of the fluorescence obtained from a blank sample (dye 1 in the absence of Al³⁺).^29,30a The detection limit of 1 for Al³⁺ cation was about 3 times lower than the highest detection limit reported in DMSO,^30b representing a significant advance in sensing Al³⁺ cation in water.

Fig. 5 Fluorescent intensity of Dye 1 (120 nM) upon addition of different concentration of Al³⁺. The sharp signal at ∼410 nm was from the scattering of water. The inset at the top right shows that the linear response of 1 to Al³⁺ concentration.

In an effort to seek the potential biological application, sensor 1 was applied to zebrafish. When zebrafish was exposed to dye 1 and Al³⁺ in fish tank, bright fluorescence was observed in the fish head and tail (Fig. 6 and ESI Fig. S6†), suggesting that this molecule could be used in organisms at certain conditions.

Fig. 6 Fluorescent images of Al³⁺ in Zebrafish (a) fish exposed to dye 1 only in the fish tank at the concentration of 10 μM for 2 hours and (b) fish exposed to 10 μM dye 1 and Al³⁺ for 2 hours.

In conclusion, a highly selective and sensitive fluorescent sensor for Al³⁺ has been developed. The sensor shows great fluorescence turn-on upon binding Al³⁺ in aqueous solution, giving strong blue emission. In addition, the sensor's turn-on exhibits excellent selectivity to Al³⁺ cation, with only a slight turn-on effect observed from Zn²⁺. These findings suggest that the developed Al³⁺ sensor could be a useful probe for the application in the biological systems.

Acknowledgements

This work was supported by National Institute of Health (Grant no: 1R15EB014546-01A1). We also thank the Coleman endowment from the University of Akron for partial support, and thank Dr Qin Liu at the University of Akron for assistance in the zebrafish experiment.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: CCDC 970383. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra47104g

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