Novel functionalized pillar[5]arene: synthesis, assembly and application in sequential fluorescent sensing for Fe³⁺ and F⁻ in aqueous media

Abstract

We designed and synthesised a novel copillar[5]arene PF5 that can through self-inclusion produce strong blue fluorescence. The pillar[5]arene-based chemosensor PF5 could be a sequential fluorescence sensor for ferric ions (III) followed by fluoride ions with high sensitivity and selectivity in aqueous solutions. When Fe³⁺ was added to the solution of the sensor PF5, the blue fluorescence emission was quenched. After the addition of F⁻, the blue fluorescence emission of the PF5–Fe³⁺ system returned to the original level. PF5 has specific selectivity to Fe³⁺ and common cations (Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Co²⁺, Cr³⁺, Mg²⁺, Fe²⁺, and Al³⁺) could not interfere with the detection process. In addition, PF5–Fe³⁺ has specific selectivity to F⁻ and common anions (Cl⁻, Br⁻, I⁻, AcO⁻, NO₃⁻, HSO₄⁻, ClO₄⁻, SCN⁻, and CN⁻) and does not interfere with the detection process. The detection limit of the sensor PF5 for Fe³⁺ was 9.0 × 10⁻⁷ mol L⁻¹, and the detection limit of F⁻ was 2.59 × 10⁻⁸ mol L⁻¹. Moreover, test strips based on the sensor were fabricated, which could be very good sequential test kits for ferric ions (III) and fluoride ions. Moreover, the sensor PF5 could also sequentially detect Fe³⁺ in tap water and F⁻ in toothpaste.

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Introduction

Among the different types biologically important metals and nonmetals, both iron and fluorine are the most plentiful necessary elements found in the human body and are critical for maintaining significant physiological processes. Given the physiological implications of ferric and fluoride, their detection are very important. The detection of Fe(III) at minute quantities is very meaningful because iron, with its chemical diversity, is critical for the appropriate functioning of most organisms in the entire spectrum of the biological system.¹ In the human body, iron is one of the most necessary trace elements; a deficiency of ferric ion (Fe(III)) in the body causes anaemia, liver damage, hemochromatosis, Parkinson's disease, and cancer.² Ferric ions also play critical roles in the metabolism and growth of living cells and catalyse many physiological processes.³ A safe limit for the Fe³⁺ ions was restricted to 2 mg L⁻¹ by the World Health Organization. Therefore, it is important to find simple, rapid, and efficient methods for the detection of Fe³⁺ at trace levels in biological and food specimens.⁴ To date, several fluorescent receptors for iron and fluoride ions have been reported, but the realization of both Fe³⁺ and F⁻ for fluorescence measurements is still a challenge. Fluoride ions are one of the most attractive targets because of their significant meaning for environmental and health concerns. The fluoride ion has unique chemical properties and widespread use in pharmacy and toothpaste to prevent tooth decay, enamel demineralization while wearing orthotic devices, and as a treatment for osteoporosis. However, a high intake of fluoride can result in serious side effects, namely, fluorosis, which may damage the kidneys in both humans and animals and result in urinary stones. The United States Environmental Protection Agency (EPA) obtains an executable drinking water standard for fluoride of 4 mg L⁻¹ to prevent skeletal fluorosis.⁵ Accordingly, the design of new chemosensors for the simple and easy detection of Fe³⁺ and F⁻ has attracted increasing attention.

Pillararenes, a new macrocyclic subject, have a stiff architecture with an overall cylindrical or pillar-like shape.⁶ They have two openings of the tubular structures, and exhibit distinguished recognition capabilities towards a variety of guests.⁷ Functional groups introduced on pillararenes often bring about unique properties that have greatly stimulated the interest of chemists of various fields.⁸ Owing to the hydrophobic nature of these macrocycles, host–guest complexation and self-assembly of pillararenes derivatives have been investigated widely in organic media. However, as many recognition events in nature occur in aqueous media, efforts have also been directed towards the development of pillararene based receptors capable of recognizing guests in the aqueous phase.⁹ Therefore, we imagine that by proper functionalization, pillararenes can serve as platforms for self-assembly and pre-organizing chelating groups for new ion sensors.¹⁰ Pillar[5]arene-based recognition receptors as a continuous recognition system for Fe³⁺ and F⁻ ions has not been reported.

In view of this, and as a part of our research interest in pillararenes chemistry and molecular recognition,¹¹ we designed and synthesised a novel pillar[5]arene-based ion receptor PF5 linked 2-aminobenzothiazole at one site, which further self-organize into annularity supramolecular polymers at the solution state utilizing pillar[5]arene-based self-assembly interactions. Furthermore, the chemosensor PF5 could be a sequential fluorescence sensor for ferric ions followed by fluoride ions with high sensitivity and selectivity in aqueous solutions.

Results and discussion

The synthesis of sensor PF5 is shown in Scheme 1 and the synthesis details are presented in Scheme S1.† Sensor PF5 and its intermediate have been characterized by ¹H NMR, ¹³C NMR, and ESI mass spectrometry (Fig. S1–S6†). The monomeric pattern DMP5 and G were also prepared as a control for comparison (Scheme S2†). The target molecules and intermediates were characterized by ¹H NMR spectroscopy (Fig. S7 and S8†).

Scheme 1 Synthesis of the functionalized pillar[5]arene PF5.

To determine if the inclusion behavior was produced, the host–guest interaction between DMP5 and G was first studied. As shown in Fig. S9,† the ¹H NMR spectra of DMP5 with the addition of different equivalents of G showed that the chemical shifts of H_G2–G8 on G gradually shifted upfield, and H_G1 slightly moving downfield, suggesting that the G deeply threaded into the cavity of the DMP5. Subsequently, variable concentration ¹H NMR spectroscopy of PF5 in CDCl₃ was carried out (Fig. 1). It was found that the aggregates formed by self-inclusion were very stable in a CDCl₃ solution (5 mL). As the concentration increased, the proton resonances did not exhibit obvious changes even at a high concentration of 50 mM, suggesting that PF5 did not form intermolecular complexes in CDCl₃, but as the concentration increased, the subtle signals of protons H_a shifted downfield. Furthermore, all the signals such as the aromatic protons H_b–f became broad and the alkyl chain protons H_j show peak splitting, which indicated that the self-inclusion function of PF5 was very weak. This is similar to the previous report by Wang's group.¹²

Fig. 1 ¹H NMR spectra (600 MHz, CDCl₃, 298 K) of PF5 at various concentrations in CDCl₃: (a) 0.5 mM, (b) 1.0 mM, (c) 5 mM, (d) 20 mM, (e) 50 mM.

Furthermore, the correlation of the protons were further validated by a NOESY NMR spectrum of PF5, the aromatic protons H_b–c have strong correlations with methyl protons (H_h) and methylene protons (H_g) on the pillar as well as alkyl chain protons (H_i), indicating that the monomers self-organization into strong fluorescence aggregates driven by the self-assembled between the pillar[5]arene units and benzothiazole units (Fig. S10†). After adding iron, the cavity of pillar[5]arene units was occupied by Fe³⁺, which with oxygen atoms of pillar[5]arene units undergoing complexation, making benzothiazole units free from the cavity of pillar[5]arene units. The self-inclusion of PF5 appeared collapsed which can bring about the fluorescence quenching. The further addition of F⁻, which combined the Fe³⁺, the PF5 was self reinclusion, bring about fluorescence recovery (Scheme 2). Therefore, the achieved assembly and application in sequential fluorescent sensing for Fe³⁺ and F⁻.

Scheme 2 Schematic of the self-assembly process of monomer PF5 and the production of supramolecular polymer fluorescence, metal–ligand interactions cause fluorescence quenching and the fluorine ions complexed fluorescence recovery.

To evaluate the binding ability of compound PF5 toward Fe³⁺ ions, UV-vis and fluorescence experiments was carried out in DMSO/H₂O (8 : 2, v/v) by adding divisible of Fe³⁺ as its perchlorate salt. The absorption spectrum of compound PF5 (1.0 mM) exhibited a maximum absorption band at 360 nm. However, an obvious intensity increase occurred upon treatment with 10 equivalents of Fe³⁺ (Fig. S11†). Conversely, the fluorescence emission band of compound PF5 (1.0 mM) in the 380–540 nm range showed an obvious decrease when increase occurred upon a treatment with 10 equivalents of Fe³⁺ (Fig. S12†). These phenomena confirmed the binding behaviour of Fe³⁺ by PF5. To investigate the Fe³⁺ recognition abilities of the sensor PF5 in DMSO/H₂O (8 : 2, v/v) solution, a series of host–guest recognition experiments were carried out. The recognition profiles of the sensor PF5 toward various cations (including Fe³⁺, Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Co²⁺, Cr³⁺, Mg²⁺, Fe²⁺ and Al³⁺) were investigated primarily using fluorescence spectroscopy. In the fluorescence spectrum, the maximum emission of PF5 appeared at 432 nm in DMSO/H₂O (8 : 2, v/v) while excited at λ_ex = 360 nm. When 10.0 equivalents of the various anions were added to the solution of sensor PF5, only Fe³⁺ showed one of the most obvious decreases in the fluorescence emission band in the 380–540 nm range. And when sensor PF5 suffered with selected cations, such as Hg²⁺, Fe²⁺, and Ag⁺ (10 equiv.), in DMSO/H₂O (8 : 2, v/v) showed little change, which singularity not be affected to identifying Fe³⁺ (Fig. 2). Therefore, in DMSO/H₂O (8 : 2, v/v), PF5 showed specific fluorescence selectivity to Fe³⁺. Furthermore, sensor PF5 could be considered a good on–off Fe³⁺ fluorescent switch.

Fig. 2 Fluorescence spectra responses of PF5 (1.0 mM) in DMSO/H₂O (8 : 2, v/v) upon addition of 10.0 equivalents of Fe³⁺, Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Co²⁺, Cr³⁺, Mg²⁺, Fe²⁺ and Al³⁺ ions (λ_ex = 360 nm). Inset: image of PF5 (1.0 mM) upon adding 10.0 equivalents of various ions, which was observed under a UV-lamp (365 nm).

To further examine the interaction between the sensor PF5 and Fe³⁺, fluorescence spectrum variation of sensor PF5 was monitored during the titration with different concentrations of Fe³⁺ (Fig. 3). In a DMSO/H₂O (8 : 2, v/v) solution of PF5, with an increasing amount of Fe³⁺, the fluorescence emission bands at 432 nm decreased by ∼88.4%. The fluorescent titration curve of the PF5 toward Fe³⁺ showed a good linear correlation over the concentration range of 0–40.0 equivalents, from which the detection limit for Fe³⁺ was estimated to be 9.0 × 10⁻⁷ M (Fig. S13†). The stability constant K_a between PF5 and Fe³⁺ was 1.02 × 10⁶ M⁻¹.

Fig. 3 Fluorescence spectra of PF5 (0.4 μM) in the presence of different concentrations of Fe³⁺ in DMSO/H₂O (8 : 2, v/v) (excitation wavelength = 360 nm). Inset: image of PF5 (1.0 mM) upon the addition of 10.0 equivalents of Fe³⁺, which was observed under a UV-lamp (365 nm).

The selectivity of PF5 to Fe³⁺ was also examined over a wide range of pH values (Fig. S14†). The detection of Fe³⁺ can work well in the pH range of 1.0–14.0 in DMSO/H₂O (8 : 2, v/v). In addition, the changes in the fluorescence intensity depending on the reaction time were recorded from 0 to 40 seconds, for a 1 : 10 mixture of PF5 and Fe(ClO₄)₃ in DMSO/H₂O (8 : 2, v/v) at room temperature (Fig. 4). This clearly shows that the reaction was complete within 30 s after the addition of Fe³⁺.

Fig. 4 Time-dependent of PF5 (1.0 × 10⁻³ M) upon addition of Fe³⁺ (10.0 × 10⁻³ M) in DMSO/H₂O (8 : 2, v/v) with a plot of the fluorescence intensity that is estimated as the peak height at 432 nm.

To explore the utility of sensor PF5 as an ion-selective chemosensor for Fe³⁺, competitive experiments were carried out in the presence of 10 equivalents of Fe³⁺ and 10 equivalents of the other ions (Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Co²⁺, Cr³⁺ Mg²⁺, Fe²⁺ and Al³⁺) in DMSO/H₂O (8 : 2, v/v). The results of these studies showed that these competing ions have no or little influence on the fluorescence emission spectra of sensor PF5 with Fe³⁺, which further indicated that PF5 has specific selectivity to Fe³⁺ (Fig. 5). To further verify the resistance to interference of PF5 with Fe³⁺, metal ion competitive experiments were performed in the presence of 20 equivalents of Fe³⁺ and 20 equivalents of various other ions (Hg²⁺, Ag⁺, Ca²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Co²⁺, Cr³⁺, Mg²⁺, Fe²⁺ and Al³⁺) in DMSO/H₂O (8 : 2, v/v). The results showed that these competing ions had no or little influence on the fluorescence emission spectra of sensor PF5 with Fe³⁺ (Fig. S15†). In addition, the fluorescence of the Job's plot indicated that PF5 and Fe³⁺ formed a 1 : 1 complex (Fig. S16†).

Fig. 5 Fluorescence intensities of PF5 (1.0 mM) in the presence of 10.0 equivalents of various ions containing 10.0 equivalents of Fe³⁺ in DMSO/H₂O (8 : 2, v/v) (λ_ex = 360 nm).

Because the in situ generated PF5–Fe³⁺ complex exhibited almost complete fluorescence quenching and F⁻ binds strongly to Fe³⁺ ions, this study examined whether this ensemble system can be exploited as a turn-on fluorescent sensor for F⁻ anions, which is known to play important roles in a wide range of chemical and biological processes. Thus, the PF5–Fe³⁺ complex prepared by mixing an equal amount of PF5 and Fe(ClO₄)₃ (10.0 mM) in the same mixed aqueous solution, was treated separately with 1.5 equivalents of different anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, NO₃⁻, HSO₄⁻, ClO₄⁻, CN⁻ and SCN⁻). When a fluoride ion was added to the PF5–Fe³⁺ system, the fluorescence intensity is completely regenerated (Fig. 6), as also indicated visually by the fluorescent color change (Fig. 6, inset). However, a series of host–guest recognition experiments were carried out. The other anions (Cl⁻, Br⁻, I⁻, AcO⁻, NO₃⁻, HSO₄⁻, ClO₄⁻, CN⁻ and SCN⁻) show very little response (Fig. S17†). The observed fluorescence and color regeneration can be ascribed to the capture of Fe³⁺ from its chelated complexes by F⁻, resulting in the formation of more stable species [FeF₆]³⁻ and the release of free ligand PF5, PF5 was re-assembled into the annulus fluorescent polymer.

Fig. 6 Fluorescence emission spectra of PF5–Fe³⁺ (10 mM) in the presence of 1.5 equiv. F⁻ in a mixed aqueous medium (DMSO/H₂O, 8 : 2, v/v); (λ_ex = 360 nm). Inset: image of PF5–Fe³⁺ (10 mM) upon the addition of 1.5 equivalents F⁻, which was observed under a UV-lamp (365 nm).

The selectivity of the PF5–Fe³⁺ ensemble as a fluorescent sensor for F⁻ was also examined over a wide range of pH values, as shown in Fig. 7. The detection of F⁻ can work well over the pH range of 1.0–12.0 in DMSO/H₂O (8 : 2, v/v).

Fig. 7 Influence of pH on the fluorescence of PF5–Fe³⁺ complex (10 mM) with fluoride ion (1.5 equiv.) in DMSO/H₂O (v/v = 8 : 2). Inset: pH fluorescence of a full scan.

To further investigate the interaction between sensor PF5–Fe³⁺ and F⁻, the fluorescence spectrum variation of sensor PF5–Fe³⁺ was monitored during a titration with different concentrations of F⁻ (Fig. 8). The fluorescent titration curve of the PF5–Fe³⁺ complex toward the fluoride ion offers a good linear correlation at the concentration range of 0–1.5 equivalents, from which the detection limit for fluoride was estimated to be 2.59 × 10⁻⁸ M (Fig. S18†). This value is much lower than the limit concentration level (4.00 mg L⁻¹) in drinking water set by USEPA.¹³

Fig. 8 Fluorescence titration of the PF5–Fe³⁺ complex (10 mM) with fluoride ion in a mixed aqueous medium (DMSO/H₂O (8 : 2, v/v); λ_ex = 360 nm). Inset: image of PF5–Fe³⁺ (10 mM) upon the addition of 1.5 equivalents of F⁻, which was observed under a UV-lamp (365 nm).

Control experiments of PF5–Fe³⁺ even in the presence of 1.5 equivalents of each of the anions indicated the absence of an interaction between PF5–Fe³⁺ and anions in the same solvent system. Competition experiments were conducted by adding fluoride (1.5 equiv.) to the solution of PF5–Fe³⁺ in the presence of 2.0 equivalents of other anions (Fig. 9). The fluorescence emission spectra displayed a similar pattern to that with F⁻ alone, suggesting that all the tested anions do not interfere in the sensing of F⁻. In addition, the fluorescent intensity could be turned off and on repeatedly with the alternate addition of Fe³⁺ and F⁻ ions at least in six cycles (Fig. S19†). The above results suggest that the PF5–Fe³⁺ ensemble could serve as an outstanding sensitive and selective fluorescent off–on sensor for F⁻.

Fig. 9 Fluorescence intensity changes of PF5–Fe³⁺ ensemble (10 mM) in the presence of other anions (1.5 equiv.) followed by the addition of F⁻ (1.5 equiv.) in a mixed aqueous medium (DMSO/H₂O (8 : 2, v/v), λ_ex = 360 nm).

To further investigate the practical applications of chemosensor PF5, test strips were fabricated by immersing filter paper into a DMSO/H₂O (v/v = 8 : 2) solution of PF5 (2 × 10⁻³ M) followed by drying them in air. The test strips containing PF5 were utilized to sequential sense Fe³⁺ and F⁻. As shown in Fig. 10, when Fe³⁺ was first added to the test strips, an obvious color change was observed. After the addition of F⁻, the color of the test strips once again changed, which served as a convenient and efficient sequential Fe³⁺ and F⁻ test kits.

Fig. 10 Images of PF5 on the test strips only PF5 and alternately added Fe³⁺ and F⁻.

The practical utility of the probe in a daily life sample was investigated using Fe³⁺ in tap water and F⁻ in toothpastes. The fluorescence emission spectra of PF5 (1.0 mM in 5 mL DMSO) is just as line 1 shown in Fig. 11: (a) upon the titration of concentrated tap water (1.0 mL) to obtain line 2; (b) soluble components of Colgate toothpaste sample (10 equiv.) added to solution of (2) to obtain line 3; (c) soluble components of Colgate toothpaste sample (10 equiv.) added to solution of (3) to obtain line 4; (d) soluble components of Colgate toothpaste sample (10 equiv.) added to a solution of (4) to obtain line 5. Significant color change can be observed directly on the sensor PF5; the sensor could detect sequentially Fe³⁺ in tap water and F⁻ in real samples such as toothpastes (Fig. 11), confirming that PF5 is a promising Fe³⁺ and F⁻ probe for practical applications. Therefore, this novel functionalized pillar[5]arene PF5 can be used as an fluorescent sensor for the sequential detection of Fe³⁺ and F⁻ ions, which showed excellent stability, reversibility, and repeatability (Fig. S20†).

Fig. 11 Fluorescence emission spectra of PF5 (1.0 mM in DMSO) sequential detect Fe³⁺ in tap water and F⁻ in real samples such as toothpastes. Inset: images illustrating the optical changes upon the addition of the analyte. Excitation wavelength is 360 nm.

Conclusions

In summary, a novel functionalized pillar[5]arene was synthesized, which can undergo self-inclusion to produce a strong blue fluorescence. PF5 could act as a chemosensor for ferric (III) and fluoride ions through a competitive complexation reaction. For the first time, the recognition ability of specific binding to Fe³⁺ among 14 metal ions is due to the formation of a weak fluorescent PF5–Fe³⁺. The resulting PF5–Fe³⁺ ensemble can also act as a turn-on fluorescent sensor for fluoride ions over other anions in the same media without interference. Moreover, test strips based on the sensor were fabricated, which served as convenient and efficient sequential Fe³⁺ and F⁻ test kits; it could be further used for practical applications to the sequential detection Fe³⁺ in tap water and F⁻ in real samples such as toothpastes. The study shown herein not only highlights the use of pillararenes for selective fluorescent recognition towards both ferric ions (III) and fluoride in a sequential fashion, but more importantly, it implicates the potential of pillararene macrocycles for the design of fluorescent sensors towards other metal cations with tailored properties.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (NSFC) (no. 21574104; 21161018; 21262032), the Natural Science Foundation of Gansu Province (1308RJZA221) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1177).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26240b

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