Pillar[5]arene-based fluorescent polymer for selective detection and removal of mercury ions

A novel pillar[5]arene-based thioacetohydrazone functionalized fluorescent polymer was designed and synthesized. This polymer not only contains pillar[5]arene units as the fluorophore (signal transducer) but also embedded the thioacetohydrazone group as the ionophore (cation receptor). Therefore, it displays specificity response for mercury ion over other common cations (Mg, Ca, Zn, Co, Fe, Pb, Cd, Ni, Tb, Cu, Eu, Fe, Cr, Ag and La) in DMSO/H2O (1 : 1, v/v). Competitive cations did not show any significant changes in emission intensity and the fluorescence spectra detection limit was 8.12 10 7 M, indicating the high selectivity and sensitivity of the polymer towards Hg. Meanwhile, this polymer can efficiently remove Hg from water.


Introduction
Because of their high toxicity and bioaccumulation, heavy metal ions released into the environment can lead to a wide range of severe diseases. 1 Considerable efforts are accordingly being devoted to developing new methodologies for detection and removal of specic toxic metal ions. 2 For example, many types of small molecules have been designed as uorescent sensors for selective and sensitive detection of metal ions. 3 On the other hand, various absorbents, such as porous silicas, 4 hydrogels, 5 nanoparticles, 6 and metal-organic frameworks (MOFs), 7 are being tested for the possible removal of toxic ions. However, most of the developed methods can only perform either the detection or the removal tasks separately, which limits their practical applications.
Pillararenes, a new kind of macrocyclic compounds, are composed of hydroquinone units linked by methylene bridges at the para positions. 8 They have novel host-guest binding properties due to being easier to functionalize by different substituents on the benzene rings, 9 thus functionalized pillararenes attracted a lot of attention of scientists. 10 Since it has multiple benzene ring units, it has a certain luminescent properties. 11 Therefore, pillararenes are used for uorescence detection of ions, 12 but most of them are used to identify iron ions. 13 In addition, these reports rarely mentioned the corresponding ion can be removed. In order to extend the application of pillararenes, it is quite necessary to design and synthesize novel pillararenes for detection and removal of ions.
Polymers have been widely used as organic light-emitting diodes (OLEDs), 14 thin-lm transistors, chemical sensors, 15 and in various photonic and electronic devices. 16 However, the pillar[5]arene-based polymer have rarely been reported. 17 On the other hand, the use of pillar[5]arene-based polymers to detect and remove ions hasn't been reported.
Herein, we designed and synthesized a pillar[5]arene-based uorescent polymer for detecting and removing the metal ions. As a proof-of-concept, a thioacetohydrazone functionalized uorescent polymer, PP5, was designed and applied for the selective sensing and effective removal of the toxic Hg 2+ (Scheme 1). Although we have previously studied the selective detection and removal of mercury ions, these sensors are small molecules. 18 However, this report is the use of pillar[5]arenebased polymer for the selective detection and removal of mercury ions. Although it has previously been reported that the use of pillar[5]arene-based pseudorotaxane to detect and remove mercury ions, it is the host-guest complex (no polymer). Importantly, the real-time uorescence response did show efficient quenching of PP5 upon the addition of Hg 2+ . Excellent sensitivity and selectivity toward the Hg 2+ detection were veri-ed in the presence of other competitive cations. The removal ability of PP5 was further elucidated by the effective separation of Hg 2+ from water. Meanwhile, PP5 can recycle detection and removal of Hg 2+ .

Materials and instruments
1,4-Dimethoxybenzene, boron triuoride ethyl ether complex, 1,4-dibromobutane and ethyl mercaptoacetate were reagent grade and used as received. Solvents were either employed as purchased or dried by CaCl 2 . 1 H NMR spectra were recorded on a Mercury-600BB spectrometer at 600 MHz and 13 C NMR spectra were recorded on a Mercury-600BB spectrometer at 151 MHz. Chemical shis are reported in ppm downeld from tetramethylsilane (TMS, d scale with solvent resonances as internal standards). Melting points were measured on an X-4 digital melting-point apparatus (uncorrected). Mass spectra were performed on a Bruker Esquire 3000 plus mass spectrometer (Bruker-FranzenAnalytik GmbH Bremen, Germany) equipped with ESI interface and ion trap analyzer. The morphologies and sizes of the polymer were characterized using eld emission scanning electron microscopy (FE-SEM, JSM-6701F) at an accelerating voltage of 8 kV. The X-ray diffraction analysis (XRD) was performed in a transmission mode with a Rigaku RINT2000 diffractometer equipped with graphite monochromated CuKa radiation (l ¼ 1.54073Å). The infrared spectra were performed on a Digilab FTS-3000 Fourier transforminfrared spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-5301PC spectrouorophotometer.

Adsorption experiments
The adsorption experiments were performed at different mercury(II) concentration, corresponding adsorbent and room temperature (Table S1 †). The experiments were carried out in 25 mL round-bottom asks, with continuously stirring (usually for 5 h). The residual concentration of mercury(II) was determined by the inductively coupled plasma (ICP) analysis.

Results and discussion
The polymer PP5 shown in Scheme 1 and the synthesis details are presented in Scheme S1. † The model compound M1 (as a soluble-molecule counterpart of PP5, structure shown in Scheme S2 †) also been synthesized. Their intermediate and the model compound M1 have been characterized by 1 H NMR, 13 C NMR, and ESI-MS ( Fig. S1-S11 †). This polymer not only contains pillar[5]arene units as the uorophore (signal transducer) but also embedded the thioacetohydrazone group as the ionophore (cation receptor). These unique characteristics are expected to be benecial for the performance in selective detection and facile removal of Hg 2+ . PP5 also been characterized by IR (Fig. S10 †).
With the robust hydrazone linkage in its structure, PP5 is insoluble and stable in common organic solvents, such as DMF, THF, DMSO, acetone, acetonitrile, ethanol and CHCl 3 . Importantly, PP5 is also insoluble and very stable in water. Thermogravimetric analysis (TGA) indicates that PP5 is thermally stable up to 285 C (Fig. S12 †). The scanning electron microscopy (SEM) images showed that PP5 possessed the irregular granular morphology (Fig. S13 †). A characteristic vibrational band appeared at 1620 cm À1 in the FT-IR spectrum of PP5 (Fig. S10 †), indicating the successful condensation of 1,4phthalaldehyde and 4 via the formation of -C]Nbonds. Meanwhile, we also investigated the crystallinity of this polymer in the solid state using powder XRD measurements (Fig. S14 †). However, the peak of the polymer is the bread peak and signicantly lower than compound 4, which indicates that PP5 is amorphous.
To further exploit the utility of the polymer PP5 as cation selective sensor for Hg 2+ , competitive experiments were carried out in the presence of 10.0 equiv. of Hg 2+ and 10.0 equiv. of various cations in DMSO/H 2 O (1 : 1, v/v). The uorescence selectivity was examined at an emission wavelength of 470 nm, neither of the competitive metal ions showed an appreciable inuence on the Hg 2+ detection (Fig. 2). These results further identied that PP5 exhibits a satisfactory selectivity toward Hg 2+ detection.
The sensitivity of PP5 toward the Hg 2+ detection was evaluated via the real-time uorescence response. For this purpose, the stock solution of Hg(ClO 4 ) 2 was gradually added to the suspension of PP5 in DMSO/H 2 O (1 : 1, v/v), and the corresponding uorescence spectra were measured immediately (Fig. 3). The detection limit of the uorescent spectrum changes calculated on the basis of 3d/S is 8.12 Â 10 À7 mol L À1 (Fig. S16 †), indicating the high sensitivity of the sensor to Hg 2+ .
To further illustrate the effective removal of Hg 2+ from water, PP5 (8 mg) was suspended in a dilute aqueous solution of Hg(ClO 4 ) 2 (100 ppm in 10.0 mL). Aer the mixture was stirred at room temperature for 5 h. The inductively coupled plasma (ICP) analysis veried that the concentration of the residual Hg 2+ in  water was 3.75 ppm, that is, 96.25% of the mercury was removed by polymer PP5. Therefore, this polymer could effectively remove Hg 2+ from water. In addition, the adsorption capacity of adsorbent have been calculated by the adsorption experiment data (Table S1 †). The average value of adsorption capacity is 108 mg g À1 at room temperature.
On the basis of the excellent Hg 2+ uptake capacity, we further explored the recycle use of PP5. Upon the simple treatment with 5 equiv. of aqueous Na 2 S solution to exchange the adsorbed Hg 2+ out, 19 the uorescence of PP5 could be easily recovered. As shown in Fig. 4, this Hg 2+ adsorption-desorption cycle could be repeated at least four times without signicant loss of the sensitivity and responsiveness of PP5.
We used model compound M1 to study the possible mechanism by 1 H NMR titration experiments because PP5 is difficult to be dissolved. Partial proton NMR spectra of M1 is shown in Fig. 5(a), and the signal assignments are depicted on the top. Upon the adsorption of Hg 2+ , H 1 and H 3 signals of M1 showed a obvious downeld shi ( Fig. 5(b)). This result identied Hg 2+ have the strong interaction with S and N atoms in Hg/M1. Meanwhile, aer the treatment of Hg/M1 with Na 2 S, the 1 H NMR spectrum recorded thereaer (Fig. 5(c)) closely resembles that of fresh M1. The results also illustrate the formation of complexes [HgS 2 ] 2À . Therefore, the selective detection and effective removal of Hg 2+ for PP5 stems indeed from Hg 2+ have the strong interaction with S and N atoms.
The recognition mechanism of the polymer PP5 with Hg 2+ was also investigated by IR spectroscopy. In the IR spectrum of PP5 (Fig. 6), the N-H bond show the stretching vibrations absorption peak at 3217 cm À1 . However, aer the addition of Hg 2+ , this peak shis to 3448 cm À1 . Meanwhile, the n C-S-C band at 955 cm À1 was also changed, which indicates that PP5 bonds to Hg 2+ via S and N atoms. In addition, the n C]O band at 1679 cm À1 and the n C]N band at 1620 cm À1 were almost unchanged, implying that Hg 2+ does not bind to the -C]O bonds and -C]N bonds (instead, to the S atoms and the N atom of N-H groups) in PP5. According to powder XRD measurements ( Fig. S14 †), no clear new peaks were detected when mercury ions are added to the polymer PP5, indicating Fig. 4 Recycle use of PP5 for selective detection and facile removal of the toxic Hg 2+ . Upon treatment in aqueous Na 2 S solution, PP5 was easily recovered and could be repeatedly used.   that the amorphous of the polymer is not destroyed. Therefore, the mercury ions should be coordinated with S and N atoms.

Conclusions
In summary, a novel pillar[5]arene-based thioacetohydrazone functionalized uorescent polymer has been synthesized, and it is used for uorescence detection and removal of the toxic mercury ions. Meanwhile, this polymer exhibits high selectivity and sensitivity (8.12 Â 10 À7 M), and it can the efficient removal of Hg 2+ from water. This research not only explored a new method for the synthesis of pillararene-based polymers but also expanded the pillararene applications about cation sensing/ adsorption/removal. Thus, this good example might stimulate wide interest of scientists for further development of new pillararene-based polymers. We also expect that our research will help the environment and industry.

Conflicts of interest
There are no conicts to declare.