A novel functionalized pillar[5]arene for forming a fluorescent switch and a molecular keypad

Abstract

We have synthesized a novel functionalized pillar[5]arene (PC5) and used it for fluorescent detection of iron ions (Fe³⁺). It displays a specificity response for iron ions over other common cations (Hg²⁺, Co²⁺, Ca²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Cr³⁺, Cu²⁺, Mg²⁺ and Ag⁺) in a solution of DMSO/THF (1 : 4, v/v). Competitive cations did not show any significant changes in emission intensity and the fluorescence spectra detection limit was 1.25 × 10⁻⁸ M, indicating the high selectivity and sensitivity of the sensor to Fe³⁺. It is well known that H₂PO₄⁻ has a binding ability with Fe³⁺ to form the complex (Fe(H₂PO₄)₃), so we designed a fluorescent switch and a molecular keypad of PC5 between Fe³⁺ and H₂PO₄⁻. Furthermore, a thin film based on the PC5 was prepared, which was confirmed to be a convenient test kit for detecting iron ions.

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Introduction

Pillar[n]arenes are a new generation of macrocyclic hosts after crown ethers,¹ cyclodextrins,² calixarenes,³ and cucurbiturils⁴ since they were found in 2008.⁵ They are highly symmetrical and rigid, therefore they have shown interesting host–guest binding properties with plenty of guest molecules.⁶ Pillar[n]arenes have novel properties due to being easier to functionalize by different substituents on the benzene rings,⁷ thus functionalized pillar[n]arenes have attracted a lot of attention from scientists. Therefore, designed and synthesized unique functionalized pillar[n]arenes are particularly important.

Detection of transition metals has become important due to their vital role in biological and environmental applications.⁸ Among all the transition metals, iron, the physiologically most abundant and versatile transition metal in biological systems, is no doubt one of the most important because of the crucial roles it plays in oxygen uptake, oxygen metabolism, electron transfer, and transcriptional regulation.⁹ An Fe³⁺ deficiency can lead to the permanent loss of motor skills, and its excess can lead to diseases such as Parkinson's and Alzheimer's.¹⁰ Therefore, monitoring of cations is a prerequisite for studying the physiological functions and the diagnosis of diseases and their prevention.¹¹ Furthermore, Fe³⁺ is a well known fluorescence quencher due to its paramagnetic nature, which makes it difficult to develop a turn-off-on fluorescent sensor and more noteworthy to develop a sensitive one.¹² Therefore, intense research efforts have been devoted to the development of fluorescent sensors for Fe³⁺ ion detection.¹³

In recent years, the fluorescent switch and the molecular keypad attracted a lot of attention of scientists, however, the fluorescent switch and the molecular keypad based on pillar[n]arenes have rarely been reported. This may greatly impede the application of pillar[n]arenes. Therefore, it is quite necessary to design and synthesize novel functionalized pillararene for forming the fluorescent switch and the molecular keypad.

Herein, we present a novel, reversible fluorescence turn-off-on sensor PC5 for Fe³⁺ and H₂PO₄⁻. The sensor PC5 was synthesized via a 3-step pathway (Scheme 1). According to a previous report,¹⁴ a mixture of 8.0 equiv. of 1,4-dibromobutane, 1.0 equiv. of hydroquinone, 6.0 equiv. K₂CO₃, 2.0 equiv. KI and 0.5 equiv. PEG-400 was stirred in acetone for 36 h to give 1,4-bis(4-bromobutoxy)benzene 1 in 80% isolated yield. Next, a mixture of 4.0 equiv. of 1,4-dimethoxybenzene, 1.0 equiv. of 1,4-bis(4-bromobutoxy)benzene, 5.0 equiv. of paraformaldehyde, and 5.0 equiv. of [BF₃·O(C₂H₅)₂] was stirred in 1,2-dichloroethane for 4 h to give copillar[5]arene 2 in 34% yield. Finally, the target compound PC5 was successfully obtained in 80% yield by etherification reaction of compound 2 and 2-mercaptobenzothiazole. The target compound and intermediates were characterized by ¹H NMR spectrum, ¹³C NMR spectrum and ESI-MS (Fig. S1–S9†).

Scheme 1 Synthesis of functionalized pillar[5]arene PC5.

Results and discussion

In order to investigate the Fe³⁺ recognition abilities of the sensor PC5 in DMSO/THF (1 : 4, v/v) solution, a series of host–guest recognition experiments were carried out. The sensor PC5 toward various cations (including Fe³⁺, Hg²⁺, Co²⁺, Ca²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Cr³⁺, Cu²⁺, Mg²⁺ and Ag⁺) were primarily investigated using fluorescence spectroscopy. The free sensor PC5 showed an absorption peak at 330 nm (Fig. S10†). In the fluorescence spectrum, the maximum emission of PC5 appeared at 380 nm in DMSO/THF (1 : 4, v/v) while excited at λ_ex = 330 nm. The addition of Fe³⁺ resulted in a decrease in the fluorescence intensity of PC5 and an approximate 80% fluorescence vanish was observed (Fig. S11†). The same tests were applied using Fe³⁺, Hg²⁺, Co²⁺, Ca²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Cr³⁺, Cu²⁺, Mg²⁺ and Ag⁺cations, and only Fe³⁺ have significant changes in the fluorescence spectrum of the sensor, and none of those other cations induced any significant changes in the fluorescent spectrum of the sensor. Upon addition of Fe³⁺, the band peak significantly weaken at 350–600 nm. The apparent color change from blue to colorless could be distinguished under the UV lamp (Fig. 1).

Fig. 1 Fluorescence spectra responses for PC5 (2 × 10⁻⁴ M) and each of the various cations (4 × 10⁻⁴ M) as the perchlorate, in the DMSO/THF (1 : 4, v/v) solution. Inset: color changes observed for PC5 upon the addition of Fe³⁺, Hg²⁺, Co²⁺, Ca²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Cr³⁺, Cu²⁺, Mg²⁺ and Ag⁺ cations in DMSO/THF (1 : 4, v/v) solution.

To further investigate the efficiency of the sensor PC5 toward Fe³⁺ detection, we carried out fluorescence emission titration experiments. In the fluorescence spectrum, upon addition of increasing amounts of Fe³⁺ ions (0–0.57 equiv.) to the solution of PC5 in DMSO/THF (1 : 4, v/v), directly leading to a blue emission quenching, the fluorescence emission at 380 nm is gradually red-shifted to 450 nm (Fig. 2). The detection limit of the fluorescent spectrum changes calculated on the basis of 3δ/S is 1.25 × 10⁻⁸ mol L⁻¹ (Fig. S12†), indicating the high sensitivity of the sensor to Fe³⁺.

Fig. 2 Fluorescence spectra of PC5 (2 × 10⁻⁴ M) in the presence of different concentration of Fe³⁺ in DMSO/THF (1 : 4, v/v) solution.

In order to quantify the complexation ratio between PC5 and Fe³⁺, the fluorescence of Job's plot measurement was conducted by varying the concentration of both PC5 and Fe³⁺ (Fig. 3). The inflection point appears at the mole fraction of 0.5 which indicates that PC5 and Fe³⁺ were formed a 1 : 1 complex.

Fig. 3 The Job's plot examined between Fe³⁺ and PC5, indicating the 1 : 1 stoichiometry, which was carried out by fluorescence spectra (λ_ex = 330 nm).

To further exploit the utility of the sensor PC5 as cation selective sensor for Fe³⁺, competitive experiments were carried out in the presence of 2.0 equiv. of Fe³⁺ and 2.0 equiv. of various cations in (DMSO/THF, 1 : 4, v/v) solution. The fluorescence selectivity was examined at an emission wavelength of 380 nm, all the competing cations did not interfere in the detection of Fe³⁺ (Fig. 4). This result displayed high selectivity of the sensor PC5 toward Fe³⁺ over other analytes mentioned above.

Fig. 4 Fluorescence of the sensor PC5 at 380 nm with addition of 2.0 equiv. of Fe³⁺ in the presence of 2.0 equiv. of other cations in (DMSO/THF, 1 : 4, v/v).

It is well known that H₂PO₄⁻ has binding ability with Fe³⁺ to form a complex (Fe(H₂PO₄)₃), so we investigated the fluorescence behavior of the sensor PC5 between Fe³⁺ and H₂PO₄⁻. Upon addition H₂PO₄⁻ to the solution of PC5 and Fe³⁺, the strong brilliant blue fluorescence of PC5 at 380 nm emerged again (Fig. S13†). Fig. 5 shows the repeated switching behavior when altering adding amounts of H₂PO₄⁻ and Fe³⁺ to PC5, which evidently proved the excellent reusability and stability of PC5 toward Fe³⁺ at least for four successive cycles.

Fig. 5 Emission spectra showing the reversible complexation between PC5 and Fe³⁺ (2.0 equiv.) by introduction of H₂PO₄⁻ (7.0 equiv.).

The reversibility and selectivity of PC5/(PC5 + Fe³⁺) toward Fe³⁺/H₂PO₄⁻ ion prompted us to consider the present system as a sequence dependent molecule keypad lock using the PC5, H₂PO₄⁻ and Fe³⁺ as three different chemical inputs. These chemical inputs designated as “H”, “P” and “G”, respectively. Among the possible six input combinations, HGP, HPG, GHP, GPH, PHG, and PGH, the combination HPG give minimum fluorogenic output (Fig. 6). The keypad contains various keys, like that in the electronic keypad (containing A–Z) and follows correct password order (HPG).

Fig. 6 Output for H (PC5), corresponding to six possible input combinations at 380 nm. Inset shows a molecular keypad lock generating emission at 380 nm when a correct password, namely, HPG, is entered keys H, P, and G hold the relevant inputs PC5, H₂PO₄⁻ and Fe³⁺, respectively.

The possible mechanism was also put forward by emission spectra. As can be seen from the emission spectra, when adding iron ion, acyclic monomeric analog 3 fluorescence is quenched (Fig. S14†). This shows that iron ion is coordinated with N atom on the thiazole ring and S atom for etherification. However, fluorescence intensity of PC5 was significantly higher than 3 (Fig. S15†), because PC5 has π electronic effect.

Based on the above experiments and analysis, we put forward the complex mechanism of PC5 and Fe³⁺. The sensor PC5 is most likely to band with Fe³⁺ via N atom on the thiazole ring and S atom for etherification formed two four-membered rings, thereby forming a 1 : 1 complex (Scheme 2).

Scheme 2 The sensing mechanism of the sensor PC5 to Fe³⁺.

Finally, to investigate the practical application of PC5, a thin film was prepared by immersing a glass sheet into a high concentration solution of PC5 (100 mM) and then drying it in air. The thin film was utilized to sense Fe³⁺. As shown in Fig. 7, when Fe³⁺ were added onto the thin film, the obvious color change was observed under irradiation at 365 nm using a UV lamp. Therefore, the thin film could be a convenient test kit for detecting Fe³⁺. Same manner, a glass sheet was immersed into a high concentration solution of PC5 + Fe³⁺ (100 mM) and then drying it in air, when H₂PO₄⁻ were added onto this thin film, the obvious fluorogenic change was observed under irradiation at 365 nm using a UV lamp.

Fig. 7 Photos of (A) the thin film utilized to sense Fe³⁺ cation and H₂PO₄⁻ anion were added onto the (B) thin film under irradiation at 365 nm using a UV lamp.

Conclusions

In conclusion, we have designed and synthesised a functionalized pillar[5]arene (PC5). It can sense iron ion through fluorescent quenching with specific selectivity and high sensitivity. It's worth noting that the competitive cations did not afford any obvious interference response. The fluorescence spectra detection limit was 1.25 × 10⁻⁸ M, which is far lower than the WHO guideline of Fe³⁺ (less than 5.3 × 10⁻⁶ M). Furthmore, we designed the fluorescent switch and the molecular keypad of PC5 between Fe³⁺ and H₂PO₄⁻. Moreover, the thin film based on PC5 were fabricated, which could serve as practical fluorescence test kits to detect Fe³⁺ for simple and fast measurement.

Acknowledgements

This work 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).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14899a

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