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

Article abstract of DOI:10.1039/c7ra10326c

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 t

Full text of DOI:10.1039/c7ra10326c

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PAPER
Pillar[5]arene-based fluorescent polymer for
selective detection and removal of mercury ions†
Cite this: RSC Adv., 2017, 7, 47709
*
Jin-Fa Chen, Bing-Bing Han, Jin-Feng Ma, Xi Liu, Qing-Yu Yang, Qi Lin,
Hong Yao,
*
You-Ming Zhang
and Tai-Bao Wei
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 (Mg2+, Ca2+, Zn2+, Co2+, Fe3+, Pb2+
,
Received 17th September 2017
Accepted 5th October 2017
Cd2+, Ni2+, Tb3+, Cu2+, Eu3+, Fe2+, Cr3+, Ag+ and La3+) 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 ꢀ 10ꢁ7 M, indicating the high selectivity and sensitivity of the polymer towards Hg2+. Meanwhile,
this polymer can efficiently remove Hg2+ from water.
DOI: 10.1039/c7ra10326c
rsc.li/rsc-advances
of pillararenes, it is quite necessary to design and synthesize
novel pillararenes for detection and removal of ions.
Introduction
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 functional-
ized uorescent polymer, PP5, was designed and applied for the
selective sensing and effective removal of the toxic Hg2+
(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]arene-
based polymer for the selective detection and removal of
mercury ions. Although it has previously been reported that the
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 pillar-
arenes 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 corre-
sponding ion can be removed. In order to extend the application
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education
of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry
and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu, 730070, P.
R. China. E-mail: weitaibao@126.com; linqi2004@126.com; Fax: +86 9317973191;
Tel: +86 9317973191
† Electronic supplementary information (ESI) available: 1H NMR date and other
Scheme 1 The pillar[5]arene-based polymer PP5 was applied for both
materials. See DOI: 10.1039/c7ra10326c
fluorescent detection and removal of Hg2+
.
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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 Hg2+. Excellent
sensitivity and selectivity toward the Hg2+ detection were veri-
ed in the presence of other competitive cations. The removal
ability of PP5 was further elucidated by the effective separation
of Hg2+ from water. Meanwhile, PP5 can recycle detection and
removal of Hg2+.
Synthesis of a copillar[5]arene 2. To a solution of 1,4-bis(4-
bromobutoxy)benzene (1.90 g, 5.0 mmol) and 1,4-dimethox-
ybenzene (2.76 g, 20.0 mmol) in 1,2-dichloroethane (200 mL),
paraformaldehyde (0.75 g, 25.0 mmol) was added under
nitrogen atmosphere. Then boron triuoride diethyl etherate
(6.75 mL, 25 mmol) was added to the solution and the mixture
was stirred at room temperature for 4 h and concentrated by
rotary evaporation. The resultant oil was dissolved in CH2Cl2
and washed twice with H2O. The organic layer was dried over
anhydrous Na2SO4 and evaporated to afford the crude product,
which was isolated by ash column chromatography using
petroleum ether/ethyl acetate (20 : 1, v/v) to give 2 (1.69 g, 34%)
as a white solid. Mp 187–189 ꢂC. 1H NMR (600 MHz, CDCl3)
d 6.84–6.74 (m, 10H), 3.87 (t, J ¼ 5.9 Hz, 4H), 3.83–3.78 (m, 10H),
3.72 (t, J ¼ 19.9 Hz, 24H), 3.33 (s, 4H), 1.94 (s, 4H), 1.84 (s, 4H).
13C NMR (151 MHz, CDCl3) d 150.80 (s), 150.75 (s), 150.70 (s),
Experimental
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 CaCl2. 1H NMR spectra were recorded on 150.58 (s), 149.84 (s), 128.44 (s), 128.30 (s), 128.08 (s), 114.89 (s),
a Mercury-600BB spectrometer at 600 MHz and 13C NMR 114.15 (s), 113.92 (s), 113.71 (s), 67.32 (s), 55.95 (d, J ¼ 3.6 Hz),
spectra were recorded on a Mercury-600BB spectrometer at 55.76 (s), 55.70 (s), 33.34 (s), 30.55 (s), 29.75 (s), 29.48 (d, J ¼ 5.2
151 MHz. Chemical shis are reported in ppm downeld from Hz), 29.19 (s), 28.32 (s). ESI-MS m/z: (M + NH4)+ calcd for
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 spec-
C
51H64O10Br2N 1010.2871; found 1010.2878.
Synthesis of functionalized pillar[5]arene 3. Copillar[5]arene
2 (0.5 g, 0.5 mmol) K2CO3 (0.28 g, 2 mmol) and ethyl mercap-
toacetate (0.3 mL, 2.75 mmol) were added to acetone (80 mL).
trometer (Bruker-FranzenAnalytik GmbH Bremen, Germany) The solution was reuxed overnight. Aer the solid was ltered
equipped with ESI interface and ion trap analyzer. The off, the solvent was evaporated and the residue was dissolved in
morphologies and sizes of the polymer were characterized using CH2Cl2. Column chromatography (silica gel; petroleum
1
eld emission scanning electron microscopy (FE-SEM, JSM- ether : CH2Cl2 ¼ 20 : 1) afforded a white solid (6.0 g, 80%). H
6701F) at an accelerating voltage of 8 kV. The X-ray diffraction NMR (600 MHz, CDCl3) d 6.78 (dd, J ¼ 14.3, 7.1 Hz, 10H), 3.85 (t,
J ¼ 5.8 Hz, 4H), 3.79–3.74 (m, 14H), 3.71 (s, 6H), 3.69 (s, 12H),
analysis (XRD) was performed in a transmission mode with
3.66 (s, 12H), 3.24 (s, 4H), 2.72 (t, J ¼ 7.0 Hz, 4H), 1.87 (ddd, J ¼
a Rigaku RINT2000 diffractometer equipped with graphite
26.5, 14.7, 7.9 Hz, 8H). 13C NMR (151 MHz, CDCl3) d 170.88 (s),
˚
monochromated CuKa radiation (l ¼ 1.54073 A). The infrared
spectra were performed on a Digilab FTS-3000 Fourier transform- 151.71–148.89 (m), 129.38–126.85 (m), 116.55–112.46 (m), 67.71
infrared spectrophotometer. Fluorescence spectra were recorded (s), 55.73 (dd, J ¼ 10.4, 5.9 Hz), 52.34 (s), 41.19 (s), 33.42 (s),
on a Shimadzu RF-5301PC spectrouorophotometer.
32.52 (s), 29.46 (s), 28.86 (s), 25.72 (s).
Synthesis of pillar[5]arene 4. Functionalized pillar[5]arene 3
(0.5 g, 0.5 mmol) and hydrazine hydrate (0.3 mL, 6 mmol) were
added to ethanol (20 mL). The solution was reuxed overnight.
Then the solvent was removed by evaporation, you can afford
a white solid. Aer white solid was washed by ethanol to obtain
Adsorption experiments
The adsorption experiments were performed at different mer-
cury(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 deter-
mined by the inductively coupled plasma (ICP) analysis.
1
4 as a white solid (0.48 g, 93%). H NMR (600 MHz, DMSO-d6)
d 9.11 (s, 2H), 6.78 (d, J ¼ 3.4 Hz, 10H), 4.34 (s, 4H), 3.84 (s, 4H),
3.75–3.61 (m, 34H), 3.06 (s, 4H), 2.67 (s, 4H), 1.80 (dd, J ¼ 29.0,
7.3 Hz, 8H). 13C NMR (151 MHz, DMSO-d6) d 168.92 (s), 150.32
(s), 127.90 (s), 113.69 (s), 67.84 (s), 55.89 (s), 32.99 (s), 32.09 (s),
29.42 (s), 28.80 (s), 25.96 (s). ESI-MS m/z: (M + H)+ calcd for
Synthetic procedures
Synthesis of 1,4-bis(4-bromobutoxy)benzene 1. Hydroqui-
none (2.3 g, 20.0 mmol), K2CO3 (16.6 g, 120 mmol), KI (6.6 g,
40 mmol), 1,4-dibromobutane (34.6 g, 160 mmol) and acetone
(400.0 mL) were added in a 500 mL round-bottom ask stirred
at room temperature. The reaction mixture was stirred at reux
for 1.5 days. Aer the solid was ltered off, the solvent was
evaporated and the residue was dissolved in CH2Cl2. Column
chromatography (silica gel; petroleum ether : CH2Cl2 ¼ 10 : 1)
afforded a white solid (6.0 g, 80%). Mp 83–85 ꢂC. 1H NMR
(600 MHz, CDCl3) d 6.83 (d, J ¼ 0.8 Hz, 4H), 3.96 (t, J ¼ 6.0 Hz,
4H), 3.52–3.25 (m, 4H), 2.10–1.88 (m, 8H).
C
55H71O12N4S2 1043.44; found 1043.2296.
Synthesis of polymer PP5. To a solution of pillar[5]arene 4
(0.21 g, 0.2 mmol) in ethanol (20 mL), terephthalaldehyde
(0.027 g, 0.2 mmol) and acetic acid (0.05 mmol, as a catalyst)
was added at room temperature. The mixture was reuxed for
48 h, and yellow solids were precipitated. The solid was isolated
by sucking ltration, washed with MeOH, dried at 80 ꢂC under
vacuum for 12 h to yield PP5 as a yellow powder (0.18 g, 75%
yield). IR (powder, KBr, cmꢁ1) 3217 (N–H), 1679 (C]O), 1620
(C]N), 834 (Ar–H).
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Synthesis of the model compound M1. To a solution of pillar
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those other cations induced any signicant changes in the
uorescent spectrum (Fig. 1).
[5]arene 4 (0.21 g, 0.2 mmol) in ethanol (20 mL), benzaldehyde
(45 mL, 0.44 mmol) and acetic acid (0.05 mmol, as a catalyst) was
added at room temperature. The mixture was reuxed for 48 h,
and white powder were precipitated. The solid was isolated by
sucking ltration, washed with MeOH, dried at 80 ꢂC under
vacuum for 12 h to yield M1 as a white powder (0.21 g, 85%
yield). 1H NMR (600 MHz, DMSO-d6) d 11.47 (s, 1H), 11.37
(s, 1H), 8.18 (s, 1H), 7.97 (s, 1H), 7.65 (dd, J ¼ 17.4, 6.0 Hz, 4H),
7.49–7.26 (m, 6H), 6.75 (s, 10H), 3.81 (s, 4H), 3.63 (s, 34H), 3.29
(s, 4H), 2.71 (d, J ¼ 5.7 Hz, 4H), 1.80 (s, 8H).
To further exploit the utility of the polymer PP5 as cation
selective sensor for Hg2+, competitive experiments were carried
out in the presence of 10.0 equiv. of Hg2+ and 10.0 equiv. of
various cations in DMSO/H2O (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 Hg2+ detection (Fig. 2). These results further
identied that PP5 exhibits a satisfactory selectivity toward Hg2+
detection.
The sensitivity of PP5 toward the Hg2+ detection was evalu-
ated via the real-time uorescence response. For this purpose,
the stock solution of Hg(ClO4)2 was gradually added to the
suspension of PP5 in DMSO/H2O (1 : 1, v/v), and the corre-
Results and discussion
The polymer PP5 shown in Scheme 1 and the synthesis details sponding uorescence spectra were measured immediately
are presented in Scheme S1.† The model compound M1 (as (Fig. 3). The detection limit of the uorescent spectrum changes
a soluble-molecule counterpart of PP5, structure shown in calculated on the basis of 3d/S is 8.12 ꢀ 10ꢁ7 mol Lꢁ1
Scheme S2†) also been synthesized. Their intermediate and the (Fig. S16†), indicating the high sensitivity of the sensor to Hg2+
.
model compound M1 have been characterized by 1H NMR, 13
C
To further illustrate the effective removal of Hg2+ from water,
NMR, and ESI-MS (Fig. S1–S11†). This polymer not only PP5 (8 mg) was suspended in a dilute aqueous solution of
contains pillar[5]arene units as the uorophore (signal trans- Hg(ClO4)2 (100 ppm in 10.0 mL). Aer the mixture was stirred at
ducer) but also embedded the thioacetohydrazone group as the room temperature for 5 h. The inductively coupled plasma (ICP)
ionophore (cation receptor). These unique characteristics are analysis veried that the concentration of the residual Hg2+ in
expected to be benecial for the performance in selective
detection and facile removal of Hg2+. PP5 also been character-
ized 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 CHCl3. Impor-
tantly, PP5 is also insoluble and very stable in water. Ther-
mogravimetric analysis (TGA) indicates that PP5 is thermally
ꢂ
stable up to 285 C (Fig. S12†). The scanning electron micros-
copy (SEM) images showed that PP5 possessed the irregular
granular morphology (Fig. S13†). A characteristic vibrational
band appeared at 1620 cmꢁ1 in the FT-IR spectrum of PP5
(Fig. S10†), indicating the successful condensation of 1,4-
phthalaldehyde and 4 via the formation of –C]N– bonds.
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.
Fig. 1 Fluorescence spectra responses for PP5 ([RU] ¼ 4 ꢀ 10ꢁ5 M)
and each of the various cations (4 ꢀ 10ꢁ4 M) in DMSO/H2O (1 : 1, v/v).
In order to investigate the luminescence properties of PP5,
the insoluble polymer PP5 be dispersed in DMSO/H2O (1 : 1,
v/v), and a series of host–guest recognition experiments were
carried out. PP5 toward various cations (including Mg2+, Ca2+,
Zn2+, Co2+, Fe3+, Pb2+, Hg2+, Cd2+, Ni2+, Tb3+, Cu2+, Eu3+, Fe2+,
Cr3+, Ag+ and La3+) were primarily investigated using uores-
cence spectroscopy. In the uorescence spectrum, the
maximum emission of PP5 appeared at 470 nm while excited at
lex ¼ 340 nm in DMSO/H2O (1 : 1, v/v). When 10.0 equiv. of Hg2+
was added to the dispersed solution of PP5, the uorescence
emission band was quenched (Fig. S15†). The same tests were
applied using Mg2+, Ca2+, Zn2+, Co2+, Fe3+, Pb2+, Cd2+, Ni2+, Tb3+,
Fig. 2 Fluorescence of PP5 ([RU] ¼ 4 ꢀ 10ꢁ5 M) at 470 nm with
Cu2+, Eu3+, Fe2+, Cr3+, Ag+ and La3+ cations, and only Hg2+ have
addition of 10.0 equiv. of Hg2+ in the presence of 10.0 equiv. of other
signicant changes in the uorescence spectrum, and none of cations in DMSO/H2O (1 : 1, v/v).
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Fig. 3 Fluorescence spectra of PP5 ([RU] ¼ 4 ꢀ 10ꢁ5 M) in the pres-
ence of different concentration of Hg2+ in DMSO/H2O (1 : 1, v/v). Inset:
Fig. 5 1H NMR spectra of M1 (a), Hg/M1 (b), and Hg/M1 after the
treatment with Na2S (c). H1 and H3 signals of M1 was downfield shifted,
which verified that mercury ions have a stronger coordination ability
with S and N atoms.
A plot of emission at 470 nm versus number of equivalents of Hg2+
.
water was 3.75 ppm, that is, 96.25% of the mercury was removed
by polymer PP5. Therefore, this polymer could effectively
remove Hg2+ 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 gꢁ1 at room temperature.
Meanwhile, aer the treatment of Hg/M1 with Na2S, the 1H
NMR spectrum recorded thereaer (Fig. 5(c)) closely resembles
that of fresh M1. The results also illustrate the formation of
complexes [HgS2]2ꢁ. Therefore, the selective detection and
effective removal of Hg2+ for PP5 stems indeed from Hg2+ have
the strong interaction with S and N atoms.
On the basis of the excellent Hg2+ uptake capacity, we further
explored the recycle use of PP5. Upon the simple treatment with
5 equiv. of aqueous Na2S solution to exchange the adsorbed
Hg2+ out,19 the uorescence of PP5 could be easily recovered. As
shown in Fig. 4, this Hg2+ 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 mech-
anism by 1H 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 Hg2+, H1 and H3 signals of M1 showed
a obvious downeld shi (Fig. 5(b)). This result identied Hg2+
have the strong interaction with S and N atoms in Hg/M1.
The recognition mechanism of the polymer PP5 with Hg2+
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 cmꢁ1. However, aer the addition of
Hg2+, this peak shis to 3448 cmꢁ1. Meanwhile, the nC–S–C band
at 955 cmꢁ1 was also changed, which indicates that PP5 bonds
to Hg2+ via S and N atoms. In addition, the nC]O band at
1679 cmꢁ1 and the nC]N band at 1620 cmꢁ1 were almost
unchanged, implying that Hg2+ 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 Hg2+. Upon treatment in aqueous Na2S solution, PP5 was
easily recovered and could be repeatedly used.
Fig. 6 IR spectra of PP5 (red) and PP5–Hg2+ (black) in KBr disks.
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that the amorphous of the polymer is not destroyed. Therefore,
the mercury ions should be coordinated with S and N atoms.
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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 ꢀ 10ꢁ7 M), and it can the efficient removal
of Hg2+ 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.
Acknowledgements
This work was supported by the National Natural Science 10 (a) W. Cheng, H. Tang, R. Wang, L. Wang, H. Meier and
Foundation of China (NSFC) (No. 21662031; 21661028;
21574104; 21262032) and the Program for Changjiang Scholars
and Innovative Research Team in University of Ministry of
Education of China (IRT 15R56).
D. Cao, Chem. Commun., 2016, 52, 8075; (b) Y. Cao, X. Hu,
Y. Li, X. Zou, S. Xiong, C. Lin, Y. Shen and L. Wang, J. Am.
Chem. Soc., 2014, 136, 10762; (c) X. Chi, X. Ji, D. Xia and
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