Efficient sensing of fluoride ions in water using a novel water soluble self-assembled supramolecular sensor based on pillar[5]arene

Abstract

The development of novel strategies to achieve efficient detection of ions in water is an intriguing challenge. Moreover, due to its high hydration enthalpy, the detection of F⁻ in water is a very difficult task. Herein, we report a novel and efficient method for highly selective detection of ions in water. This novel approach can be illustrated as follows. First, water soluble cationic pillar[5]arene (P5) and aromatic acid derivatives (N1) that were self-assembled in water formed supramolecular sensors (P5N1). Then, competitive coordination was rationally introduced into these supramolecular sensors by adding metal ions (such as Fe³⁺) to form new supramolecular sensors (P5N1Fe). Interestingly, the supramolecular sensor P5N1Fe showed a high selectivity for F⁻ in water solution. Therefore, this suggests that this method is a novel and facile way to design sensors for the efficient sensing of ions in water.

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Introduction

As we all know, ions play a fundamental role in many chemical, biological, medical and technological processes.^1–4 Generally, most biological and environmental procedures are carried out in water systems. Therefore, it is very important to detect ions in water solutions. To date, there have been various useful strategies or methods, such as the design of water soluble receptors or using host/guest complexes as water soluble sensors, employed to develop chemosensors that could efficiently work in water.^5–8 However, these methods typically suffer from a number of problems. First, owing to the poor water solubility of most organic groups, it is very difficult to simultaneously introduce a signal group and binding sites into a single water soluble chemosensor. Second, host/guest complex chemosensors easily suffer from interference from other guest ions. Finally and more importantly, numerous ions have a high hydration enthalpy, which blocks the binding between ions and sensors. For instance, the hydration enthalpy (ΔH°) of fluoride (F⁻) is −504 kJ mol⁻¹.^9–12 Consequently, the development of novel strategies or approaches to achieve efficient detection of ions in water is still an intriguing challenge.

Recently, pillararene chemistry has experienced rapid development^13–18 and pillararene derivatives are now widely used in the field of molecular recognition,^19–22 drug delivery,^23–27 molecular devices^28–34 and so on. It's worth mentioning that there are two types of important water soluble pillararenes developed to date: cationic pillararene³⁵ and anionic pillararene.^36,37 Due to their good water solubility, inclusion and self-assembly properties, these types of pillararene derivatives provide a great opportunity for the design of water soluble chemosensors.

In view of these and based on our research in ion recognition,^38–44 by rationally introducing competitive coordination into a self-assembled supramolecular chemosensor, we developed a novel and efficient way to design chemosensors that could work efficiently in water. Our strategy was as follows. First, as shown in Scheme 1, we used water soluble cationic pillar[5]arene (P5) as the host and the aromatic acid derivative N1 as the guest. In N1, the naphthyl groups act as fluorescent signal groups and π–π interaction sites, whereas, the acid groups act as hydrophilic groups and binding sites. Second, the pillararene P5 and aromatic acid N1 could self-assemble into a supramolecular system, which could act as a supramolecular chemosensor P5N1. Finally, in order to achieve high selectivity, we rationally introduced competitive coordination by adding metal ions (such as Fe³⁺) into the supramolecular system to form a new supramolecular sensor P5N1Fe (Scheme 2). Interestingly, this strategy achieved great success. By the competitive coordination between Fe³⁺, F⁻ and the aromatic acid N1 in the supramolecular sensors, P5N1 could separately sense Fe³⁺ in water, whereas P5N1Fe could sense F⁻ in water with high selectivity and sensitivity.

Scheme 1 Self-assembly process of the supramolecular sensor P5N1.

Scheme 2 Self-assembly process of the supramolecular sensor P5N1Fe and the proposed ions sensing mechanism based on competitive coordination.

Results and discussion

The cationic pillar[5]arene (P5) was synthesized according to Scheme S1.† The self-assembly properties of P5 with N1 were investigated via ¹H MNR spectroscopy in D₂O. As shown in Fig. 1 and S1–S3,† when P5 was mixed with N1, the protons' peaks on N1 and P5, respectively, showed evident shifts. More specifically, the triplet peaks of H^e and H^f on N1 showed dramatic upfield shifts (Δδ = −1.01 and −0.92 ppm) and became broad. A possible reason for this was that these protons were located in the cavity of P5 and were thus shielded by the electron-rich cylinder. Moreover, the double peaks of H^d and H^c on N1 showed moderate upfield shifts (Δδ = −0.20 and −0.39 ppm), which could be attributed to the slipped-parallel type π–π interactions⁴⁵ between the naphthyl group of N1 and the P5 electron-rich cylinder. These slipped-parallel type π–π interactions also induced single peaks of H¹, H² and H³ on P5 to carry out moderate upfield shifts (Δδ = −0.17 and −0.23 ppm). Moreover, the single peak of H^a on N1 showed a moderate downfield shift (Δδ = 0.29 ppm), which could be attributed to the electrostatic interaction between the carboxylate group on N1 and the quaternary ammonium group on P5. These electrostatic interactions induced charge transfer from the naphthyl carboxylate group to the quaternary ammonium group on P5. For the same reason, the peaks of H^3–8 on P5 showed upfield shifts. According to these results, the proposed self-assembly mechanism is shown in Scheme 1, where the partial naphthyl group of N1 was located in the cavity of P5, and slipped-parallel type π–π interactions exist between the naphthyl group of N1 and the P5 cylinder. Moreover, the carboxylate groups of N1 were bonded to the quaternary ammonium groups on P5 via electrostatic interactions. By this means, P5 and N1 self-assembled into a supramolecular sensor P5N1.

Fig. 1 Partial ¹H NMR of the D₂O solution of (a) P5 (1 × 10⁻³ mol L⁻¹), (b) mixture of P5 (1 × 10⁻³ mol L⁻¹) and N1 (1 : 2) (c) N1 (2 × 10⁻³ mol L⁻¹).

Mass spectrometry experiments also supported the above proposed self-assembly mechanism. In the ESI-MS of the supramolecular sensor P5N1 (Fig. S5†), a peak at m/z 611.55 for [P5 + 2N1 + 4Br⁻ + 2H⁺]⁴⁺ was monitored, which proved the 1 : 2 complexation stoichiometry between P5 and N1. Moreover, the Job plot (Fig. S5†) also supported this result.

Further evidence for the formation of the self-assembled supramolecular sensor P5N1 was received from fluorescent spectroscopy. As shown in Fig. 2, the water solution of N1 showed green fluorescence at λ_em = 510 nm, whereas the water solution of P5 had no fluorescence. Upon the addition of the P5 solution into the N1 solution, the fluorescence of N1 showed a significant increase. This phenomenon indicated that P5 carried out a self-assembly with N1 and formed the corresponding supramolecular chemosensor P5N1. The association constant for the complexation between P5 and N1 was estimated by means of fluorescence titrations (Fig. S6†) at room temperature in water. The association constant (K_a) for P5N1 was calculated to be 9.94 × 10⁹ M⁻².

Fig. 2 Fluorescence spectra of P5, N1 and P5N1 (λ_ex = 376 nm).

The stability of the supramolecular sensor P5N1 was also examined over a wide range of pH values in HEPES buffered water solution (Fig. 3). From the results, the supramolecular sensor P5N1 showed good stability in the pH range 5.0–11.0.

Fig. 3 Influence of pH on the fluorescence of P5N1 in HEPES buffered water solution (1 × 10⁻⁵ mol L⁻¹).

In order to investigate the ions' response property of the supramolecular sensor P5N1, a series of metal ions, including Al³⁺, Mg²⁺, Ca²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Cr³⁺, Tb³⁺, Eu³⁺ and La³⁺, were separately added into the water solution of P5N1. Interestingly, the supramolecular sensor P5N1 showed a selectivity response for Fe³⁺. Indeed, when adding various metal ions into the P5N1 water solution, only Fe³⁺ could induce the quenching of the fluorescence of P5N1 at λ_em = 510 nm (Fig. 4 and S7†).

Fig. 4 (a) Photograph of P5N1 water solutions (2.0 × 10⁻⁵ mol L⁻¹) upon adding 10 equiv. of various cations in water; from left to right: free, Al³⁺, Mg²⁺, Ca²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Cr³⁺, Tb³⁺, Eu³⁺ and La³⁺; (b) fluorescence spectra of the supramolecular sensor P5N1 in water solution (1 × 10⁻⁵ mol L⁻¹) upon the addition of 10 equiv. of Fe³⁺, Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Co²⁺, Cr³⁺, Mg²⁺, Tb³⁺, Eu³⁺, La³⁺ and Al³⁺ ions (λ_ex = 376 nm).

Moreover, the competitive experiments (Fig. 5) indicated that other cations could not induce any significant interference into the sensing process of P5N1 for Fe³⁺. Therefore, P5N1 could act as a selective Fe³⁺ sensor. In addition, the supramolecular sensor P5N1 showed high sensitivity for Fe³⁺. Based on fluorescent titrations (Fig. S8 and S9†), the detection limit of P5N1 for Fe³⁺ was 8.24 × 10⁻⁷ mol L⁻¹.

Fig. 5 Fluorescence intensities of the supramolecular sensor P5N1 water solution (1 × 10⁻⁵ mol L⁻¹) in the presence of 10.0 equiv. of various ions containing 10.0 equiv. of Fe³⁺ in H₂O (λ_ex = 376 nm).

According to the above results, we rationally introduced competitive coordination into the supramolecular sensor P5N1 via separately adding Fe³⁺ into the P5N1. The Fe³⁺ could be coordinated with the carboxylate groups in P5N1 to form the corresponding new supramolecular sensor P5N1Fe.

Interestingly, by competitive coordination, the supramolecular sensor P5N1Fe showed a selective fluorescence response for F⁻ in water solution. As shown in Fig. 6, with the addition of water solutions of various anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, HSO₄⁻, SCN⁻, CN⁻ and ClO₄⁻) into the supramolecular sensor P5N1Fe, only F⁻ could induce the P5N1Fe water solution to show a dramatic fluorescence increase at λ_em = 510 nm and emit green fluorescence (Fig. S10†).

Fig. 6 (a) Photograph of P5N1Fe water solutions (1.0 × 10⁻⁵ mol L⁻¹) upon adding 2 equiv. of various anions in water; from left to right: free, Cl⁻, Br⁻, I⁻, F⁻, Ac⁻, H₂PO₄⁻, HSO₄⁻, SCN⁻, CN⁻, and ClO₄⁻; (b) fluorescence spectra changes of the supramolecular sensor P5N1Fe solution (1 × 10⁻⁵ mol L⁻¹) in the presence of various anions upon the addition of F⁻, Cl⁻, Br⁻, I⁻, Ac⁻, H₂PO₄⁻, HSO₄⁻, SCN⁻, CN⁻, and ClO₄⁻ (2 equiv., respectively) in water solutions (λ_ex = 376 nm).

Moreover, the competitive experiments (Fig. 7) indicated that other anions could not induce any significant interference into the sensing process of P5N1 for Fe³⁺ or P5N1Fe for F⁻. Therefore, P5N1Fe could act as a selective F⁻ sensor. In addition, the supramolecular sensor showed high sensitivity for F⁻. Based on fluorescence titrations (Fig. S11 and S12†), the detection limit of P5N1Fe for F⁻ was 4.07 × 10⁻⁸ mol L⁻¹.

Fig. 7 Fluorescence intensity changes of the supramolecular sensor P5N1Fe water solution (1 × 10⁻⁵ mol L⁻¹) ensemble in the presence of other anions (2 equiv.) followed by the addition of F⁻ (2 equiv.) in a mixed aqueous medium (λ_ex = 376 nm).

In order to verify the fine F⁻ recognition properties coming from the supramolecular sensor P5N1Fe, a series of controlled experiments were carried out. As shown in Fig S13,† no separate component of the supramolecular sensor P5N1Fe, such as P5, N1, or P5N1, could sense F⁻. These results confirmed that the self-assembled supramolecular sensor P5N1Fe was the recognition host.

The reversible experiments also supported these results. By alternating the addition of Fe³⁺ and F⁻ to the supramolecular sensor P5N1Fe water solution, the fluorescence emission of the tested solution performed alternate reviving and quenching processes (Fig. 8). This off–on–off switching process could be repeated several times with little fluorescent efficiency loss. These results confirmed that the F⁻ recognition process is a competitive coordination process (Scheme 2).

Fig. 8 Fluorescence intensity reversible switching cycles of the P5N1 water solution (1 × 10⁻⁵ mol L⁻¹) on alternate addition of Fe³⁺ and F⁻ (λ_ex = 376 nm).

For ease of use, test kits for ions, such as ion responsive films (Fig. 9) or ion test strips (Fig. 10), based on these supramolecular sensors could be easily prepared. For example, by pouring the solution of these supramolecular sensors onto a clean glass surface and drying in air, we could obtain the corresponding ion responsive supramolecular films. Moreover, ion test strips could be fabricated by immersing filter papers into a water solution of the supramolecular sensors and then drying them in air.

Fig. 9 Writing and erasing of a natural light invisible image on P5N1 and P5N1Fe films. The photographs were taken at room temperature under nature light (N) and exposure to 365 nm ultraviolet light (UV).

Fig. 10 Photographs of the supramolecular sensor P5N1-based test strips after immersion in (A) 1 × 10⁻⁸ M Fe³⁺ water solutions; (B) 1 × 10⁻⁷ M Fe³⁺ water solutions; (C) 1 × 10⁻⁶ M Fe³⁺ water solutions; (D) 1 × 10⁻⁵ M Fe³⁺ water solutions; (E) 1 × 10⁻⁴ M Fe³⁺ water solutions under irradiation at 365 nm. Photographs of the supramolecular sensor P5N1Fe-based test strips after immersion in (F) 1 × 10⁻⁵ M F⁻ water solutions; (G) 1 × 10⁻⁴ M F⁻ water solutions; (H) 1 × 10⁻³ M F⁻ water solutions under irradiation at 365 nm.

As show in Fig. 9 and 10, these ion responsive films or test strips could act as convenient test kits for the detection of Fe³⁺ and F⁻ in water. Moreover, as shown in Fig. 10, the P5N1 film has green fluorescence emission, such that when writing on the film with a writing brush dipped in Fe³⁺ water solution, a dark writing image appeared under UV conditions. Furthermore, the P5N1Fe film has no fluorescence, such that when writing on the film with a writing brush dipped in F⁻ water solution, a brilliant green fluorescent writing image appeared. These fluorescent images could be erased by brushing F⁻ or Fe³⁺ on the corresponding films again. Therefore, these films could act as security display materials.

In summary, by rationally introducing competitive binding interactions into a self-assembled cationic pillar[5]arene-based supramolecular sensor, water soluble supramolecular sensors P5N1 and P5N1Fe were successfully developed. These supramolecular sensors could sense Fe³⁺ and F⁻ with high selectivity and sensitivity in water solution, respectively. It is worth mentioning that, due to the high hydration enthalpy of F⁻, the detection of F⁻ in water is still a very difficult task, while P5N1Fe showed high selectivity for F⁻ in water. Moreover, convenient test kits for ions and erasable security display materials based on these supramolecular sensors were prepared. Therefore, this is an efficient and simple way to develop chemosensors that could selectively detect ions in water.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (No. 21574104; 21161018; 21262032) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT 15R56).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23878e

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