Reaction-based AIEE-active conjugated polymer as fluorescent turn on probe for mercury ions with good sensing performance

Article abstract of DOI:10.1016/j.dyepig.2018.03.060

Based on Hg²⁺-promoted deprotection reaction of thioketal, a type of thioketal decorated conjugated polymer (PTS) with the feature of aggregation induced emission enhancement (AIEE) was successfully synthesized as fluorescent turn on probe for

Full text of DOI:10.1016/j.dyepig.2018.03.060

DyesandPigments156(2018)1–7
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Reaction-based AIEE-active conjugated polymer as fluorescent turn on probe
for mercury ions with good sensing performance
Yuanrong Shan1, Wenjie Yao1, Ziqin Liang, Lihong Zhu, Shuibin Yang, Zhijun Ruan∗
Hubei Key Laboratory of Processing and Application of Catalytic Materials, College of Chemistry and Chemical Engineering, Huanggang Normal University, Huanggang
438000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Based on Hg2+-promoted deprotection reaction of thioketal, a type of thioketal decorated conjugated polymer
(PTS) with the feature of aggregation induced emission enhancement (AIEE) was successfully synthesized as
fluorescent turn on probe for Hg2+. With the introduction of trace aqueous Hg2+, fluorescence of PTS in THF-
H2O mixture (water fraction of 98%) exhibited significant enhancement. The response was very fast with good
Aggregation induced emission enhancement
Conjugated polymer
Chemodosimeter
Mercury ions
Fluorescence probe
selectivity towards Hg2+ due to the specific chemical reaction. Other common metal ions (Ag+, Cr3+, Al3+
,
Fe3+, Ca2+, Ni2+, Co2+, Pb2+, Cu2+, Zn2+, Mg2+, Fe2+, Mn2+, Cd2+, Ba2+, Li+, Na+ and K+) gave nearly no
disturbance to the sensing process. Furthermore, the detection limit of Hg2+ reached 2.3 × 10−7 mol L−1 for
this sensing system.
1. Introduction
(CPs) have displayed huge superiority in detection sensitivity owing to
their large delocalized molecular structures with unique signal ampli-
Mercury ions are one of the most toxic heavy metal ions, have many
adverse effects on the endocrine system, immune system and nervous
system of organisms [1–3]. Even more seriously, some lower organisms
can transform inorganic mercury into methylmercury, a potent neuro-
toxin for central nervous system, the harmful influence will dramati-
cally increase [4–6]. Moreover, the mercury pollutant will be stored in
animal tissues rather than excreted, the concentration may be very high
in animals that at the top of the food chain. Thus, global mercury
pollution from human activities and nature has become a serious threat
to living beings and environment [7,8]. It is urgent to develop con-
venient methods for selective and sensitive detection of mercury ions.
Traditional analytical methods include atomic absorption spectrometry,
inductively coupled plasma mass spectroscopy, and chromatography.
Whereas, most of them are incapable to meet the requirement of easy-
to-use detection due to complicated sample pretreatment and expensive
instruments. Great efforts have been invested in developing straight-
forward methods, such as colorimetric, fluorescent and electrochemical
probes. Among these methods, fluorescence-based probe has appeared
as a powerful tool to detect mercury ions, due to its simple operation,
high sensitivity and low-cost [9–13]. In the past several years, various
fluorescent mercury ions probes based on polymers [14–18] or small
organic molecules [19–28] have been developed. Compared to small
molecular probes, probes based on fluorescent conjugated polymers
fication effect [29–31].
Recently, Tang and coworkers have found that some organic mo-
lecule was weak emissive in solution but strongly emissive in ag-
gregated state [32,33]. These aggregation induced emission enhance-
ment (AIEE) luminogens exhibited many advantages, such as intrinsic
strong emission in aggregated state, good photobleaching-resistance
and high sensitivity when used as fluorescent probes [34–40]. Since
then, lots of turn on fluorescent Hg2+ chemosensors based on AIEE
small molecules have been successfully constructed [41–46].
To achieve high selectivity, fluorescent mercury ions probes based
on specific chemical reactions have been developed [47–54]. In pre-
vious work, we have designed a novel AIEE-active fluorescent small
molecular probe TPE-S towards mercury ions based on the Hg2+-pro-
moted deprotection reaction of mercaptal (Chart S1 and S2) [55]. The
nitrobenzene moieties in TPE-S was highly twisted, so the nitro group
was close to the fluorophore (tetraphenylethylene, TPE), quenching its
fluorescence emission. While the nitro group in TPE-O was far from the
TPE moiety, weakening the quenching effect (the crystal structures seen
Chart S1). Thus, once triggered by Hg2+, the conversion of TPE-S to
TPE-O could give a turn on signal output. However, the fluorescence
intensity and fluorescence enhancement value of this sensor were very
limited, regardless of its high selectivity (Inset photos in Chart S1).
In this work, taking advantage of the excellent fluorescence
∗
Corresponding author.
E-mail address: ruanzhijun87@126.com (Z. Ruan).
Contributed equally.
1
https://doi.org/10.1016/j.dyepig.2018.03.060
Received 17 February 2018; Received in revised form 25 March 2018; Accepted 25 March 2018
Availableonline30March2018
0143-7208/©2018ElsevierLtd.Allrightsreserved.

Y. Shan et al.
DyesandPigments156(2018)1–7
Fig. 1. Chemical structure and the Hg2+ sensing
process of PTS.
amplification effect of CPs, TPE-S was copolymerized with fluorene
moieties to construct a new conjugated polymer of PTS with the aim of
obtaining better sensing performance. So far, the AIEE-active con-
jugated polymers as fluorescence turn on probes for Hg2+ are very rare
[56–59]. The chemical structure and the sensing process of PTS are
shown in Fig. 1. In fact, PTS emits very weak fluorescence in THF-H2O
mixtures with the water fraction of 98%. After the addition of trace
Hg2+ ions to PTS, the deprotection reaction happened immediately,
accompanying with the emission becomes dramatically strong. Other
solvent was removed under vacuum, the residue was dissolved in a bit
of chloroform, added dropwise into 200 mL of methanol through a
cotton filter, the precipitates were collected by filtration. Then, the
polymer was washed with methanol and dried to a constant weight.
Yellow solid was obtained in 75% yield. Mw = 11240; Mw/Mn = 1.74.
1H NMR (300 MHz, CDCl3) δ (ppm): 8.20 (d, J = 7.2, 2H, ArH), 7.82
(m, 4H, ArH), 7.55 (m, 11H, ArH), 7.19 (br, 10H, ArH), 4.35 (s, 2H),
2.00 (br, 4H), 1.03 (br, 12H), 0.74 (br, 10H). 13C NMR (100 MHz,
CDCl3) δ (ppm): 195.4, 151.6, 149.9, 146.9, 143.3, 142.1, 142.0, 139.8,
139.1, 133.8, 131.8, 131.7, 131.3, 130.6, 129.0, 128.0, 126.5, 126.3,
125.8, 123.6, 121.0, 120.0, 55.2, 44.7, 40.4, 32.0, 29.6, 23.7, 22.5,
14.0.
metal ions (Ag+, Cr3+, Al3+, Fe3+, Ca2+, Ni2+, Co2+, Pb2+, Cu2+
,
Zn2+, Mg2+, Fe2+, Mn2+, Cd2+, Ba2+, Li+, Na+ and K+) gave nearly
no disturbance to the sensing process. Herein, we present the synthesis,
characterization, sensing behavior of PTS in detail.
2.4. Synthesis of poly[2-(4-(2-(4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl)-
1,2-diphenylvinyl)phenyl)-2-(4-nitrobenzyl)-1,3-dithiolane] (PTS)
2. Experimental
2.1. Material
PTO (1.00 equiv) and 1, 2-ethanedithiol (3.00 equiv) were dissolved
in dry dichloromethane (10 mL), then BF3·Et2O (6.00 equiv) as the
Lewis acid was added, stirring at room temperature for 2 days. Then,
the reaction mixture was filtered through a cotton filter, the filtrate was
collected. After the solvent was removed, the residue was dissolved in a
bit of chloroform, added dropwise into 300 mL of methanol through a
cotton filter, the precipitates were collected by filtration. Then, the
polymer was washed with methanol and dried to a constant weight.
Yellow solid was obtained in 62% yield. Mw = 12835; Mw/Mn = 1.69.
1H NMR (300 MHz, CDCl3) δ (ppm): 8.00 (d, J = 7.2, 2H, ArH), 7.70
(br, 2H, ArH), 7.55 (m, 8H, ArH), 7.24 (m, 15H, ArH), 3.58 (s, 2H), 3.30
(m, 4H), 1.97 (br, 4H), 1.03 (br, 12H), 0.74 (br, 10H). 13C NMR
(100 MHz, CDCl3) δ (ppm): 151.6, 151.5, 146.8, 144.6, 143.5, 143.2,
142.7, 142.4, 141.7, 140.5, 140.3, 140.1, 140.0, 139.5, 139.4, 139.3,
139.1, 132.0, 131.89, 131.8, 131.4, 130.9, 128.7, 128.4, 127.8, 127.1,
126.7, 126.6, 126.2, 125.9, 125.8, 122.4, 120.9, 120.0, 119.9, 55.1,
52.4, 40.4, 39.2, 31.4, 29.6, 23.7, 22.5, 13.9.
Tetrahydrofuran (THF) was dried over and distilled from K-Na alloy
under an atmosphere of dry nitrogen. Dichloromethane (DCM) was
dried over and distilled from CaH2. The solutions of various metal ions
were prepared by double distilled water. Compounds 1 and 3 were
synthesized according to previous literatures [60,61]. All other reagents
were used as received without further purification.
2.2. Synthesis of 1-(4-(2,2-bis(4-bromophenyl)-1-phenylvinyl)phenyl)-2-
(4-nitrophenyl)ethanone (2)
SOCl2 (1 mL) was added to a solution of 2-(4-nitrophenyl)acetic acid
(0.181 g, 1 mmol) in nitrobenzene (10 mL) and stirred at 60 °C over-
night. Excess SOCl2 was stripped off under vacuum at room tempera-
ture. Compound 1 (0.490 g, 1 mmol) was added to the resultant solu-
tion with an ice bath under nitrogen atmosphere, then AlCl3 (0.133 g,
1 mmol) was added. After 4 h, the reaction mixture was extracted with
DCM for several times, the organic layer was combined and dried over
anhydrous Na2SO4. After evaporation of the DCM solvent, the crude
product was purified by column chromatography using PE (petroleum
ether)-EA (ethyl acetate) (10:1, V/V) as eluent to afford a pale-yellow
solid (0.451 g, 69%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.22 (d,
J = 8.7, 2H, ArH), 7.79 (d, J = 8.7, 2H, ArH), 7.42 (d, J = 8.7, 3H,
ArH), 7.24 (m, 2H, ArH), 7.16 (m, 6H, ArH), 6.98 (br, 2H, ArH), 6.88
(d, J = 7.2, 4H, ArH), 4.35 (s, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm):
195.3, 148.8, 147.0, 142.1, 142.0, 141.4, 140.8, 140.2, 134.2, 133.0,
132.8, 132.8, 131.6, 131.4, 131.2, 131.0, 130.6, 128.1, 128.0, 127.3,
123.6, 121.3, 121.1, 44.8. MS (EI), m/z [M+]: 653.4, calcd: 653.1.
Anal. calcd for C34H23Br2NO3: C 62.50, H 3.55, N 2.14; found: C 62.81,
H 3.66, N 2.08.
2.5. Preparation of the solutions of various metal ions
One millimole of inorganic salt: Hg(ClO4)2·3H2O, AgNO3, Cr
(NO3)3·9H2O, Al(NO3)3 9H2O, Fe(NO3)3·9H2O, CoCl2·6H2O, Ca
(NO3)2·4H2O, Ba(NO3)2, Pb(NO3)2, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, Cu
(NO3)2·3H2O, MnSO4·H2O, Cd(NO3)2·4H2O, Mg(ClO4)2, Fe(SO4)2·7H2O,
KNO3, NaNO3 and LiNO3 was dissolved in double distilled water
(10 mL) to afford 1 × 10−1 mol/L aqueous solution, respectively. The
stock solutions were diluted to desired concentrations with double
distilled water when needed.
2.6. Fluorescence intensity changes of PTS with different metal ions
2.3. Synthesis of poly[1-(4-(2-(4-(9,9-dihexyl-7-methyl-9H-fluoren-2-yl)
phenyl)-1-phenyl-2-p-tolylvinyl)phenyl)-2-(4-nitrophenyl)ethanone] (PTO)
A solution of PTS (1 × 10−3 mol/L) in THF was prepared. Different
metal ions (1 × 10−1 mol/L, 4.5 μL) were added to the solution of PTS
(60 μL) in a quartz tube respectively, then distilled water was added to
help the formation of aggregation state (with water fraction of 98%).
The resultant solutions (3 mL) were placed in a quartz cell (10.0 mm
width), and the changes of the fluorescence intensity were recorded at
room temperature each time (excitation wavelength 368 nm).
A mixture of compound 2 and compound 3 (1.00 equiv), K2CO3
(20.00 equiv), tetrakis (triphenylphosphine) palladium (Pd(PPh3)4)
(5 mol %) and THF/H2O (2:1 in volume), was charged with argon. The
reaction was stirred under reflux for 2 days. Then, the reaction mixture
was filtered through a cotton filter, the filtrate was collected. After the
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Y. Shan et al.
DyesandPigments156(2018)1–7
Scheme 1. Synthetic pathway of probe PTS.
2.7. Instrumentation
3.2. UV–vis absorption spectra
1H and 13C NMR spectroscopy was conducted with a Varian
Mercury 300 or Bruker ARX400 spectrometer using tetramethylsilane
(TMS; δ = 0 ppm) as internal standard. EI-MS spectra were recorded
with a Finnigan PRACE mass spectrometer. Elemental analyses (EA)
were performed by a CARLOERBA-1106 by a micro-elemental analyzer.
Gel permeation chromatography (GPC) was performed on an Agilent
1100 series HPLC system and a G1362A refractive index detector.
Polystyrene standards were used as calibration standards for GPC. The
The UV/Vis absorption spectra of PTS and PTO are shown in Fig.
S1. The dilute solution of PTO and PTS in THF were colorless with
maximum absorption wavelength (λmax) at about 355 and 353 nm, re-
spectively. As presented in the textbook of basic organic chemistry, the
ketones could be transformed to enols in the presence of chemical base
[62–64]. Thus, after the addition of t-BuOK to the solution of PTO in
THF, the ketone-enol isomerization reaction occurred with a new ab-
sorption peak (λmax = 569 nm) appeared. At the same condition, the
addition of t-BuOK to PTS caused nearly no changes due to the absence
of the methyl ketone group. However, when t-BuOK was added to the
solution of PTS + Hg2+, the ketone-enol isomerization reaction oc-
curred immediately, with an apparent color change from colorless to
red purple, and a new absorption peak (λmax = 569 nm) appeared (Fig.
S1b). Therefore, Hg2+ could promote the deprotection reaction of
mercaptal very well, and caused the followed ketone-enol isomerization
with the aid of t-BuOK.
Fourier transform infrared (FTIR) spectra were recorded on
a
PerkinElmer-2 spectrometer in the region of 4000–400 cm−1. UV/Vis
spectra were obtained by using a Shimadzu UV-2550 spectrometer.
Photoluminescence spectra were performed on
a Hitachi F-4500
fluorescence spectrophotometer.
3. Results and discussion
3.1. Synthesis and characterization
3.3. AIEE properties
The synthetic procedure of the polymers is shown in Scheme 1.
Compounds 1 and 3 were synthesized according to previous literatures
[60,61]. Compound 2 was synthesized by the acylation reaction be-
tween 1 and 2-(4-nitrophenyl) acetyl chloride. The precursor polymer
PTO was prepared by the palladium-catalyzed Suzuki coupling reaction
between 2 and 3. Then, the target polymer PTS was synthesized by the
protection reaction between PTO and 1, 2-ethanedithiol. All these final
products and intermediates were fully verified by 1H and 13C NMR
(Figs. S3–S8), mass spectrometry, elemental analysis, GPC and Fourier
transform infrared spectra.
PTO and PTS have good solubility in common organic solvents,
such as dichloromethane, chloroform and THF, but insoluble in water.
THF (good solvent) and H2O (poor solvent) were chosen as the solvent
pair for their miscibility. As shown in Fig. 3, Photoluminescence (PL)
spectra of PTO and PTS were measured in THF-H2O mixtures with
different water fractions (fw) to investigate their AIEE properties. Their
dilute solution in pure THF showed weak emission. Upon the addition
of water, the aggregates of these two polymers were formed step by
step. Their PL intensity gradually increased with the increase of the
water fraction in the mixture solvents, exhibiting the obvious AIEE
phenomenon. As to PTO, the highest PL intensity was achieved when
fw = 98%. As shown in the inset photos, its THF solution emitted weak
light, but strong emissions were observed with fw = 98% (Fig. 3a).
However, as to PTO, the PL intensity declined swiftly, even lower than
that of its pure THF solution, when the water fraction was higher than
90% (Fig. 3b). Precipitates were formed and could be observed by
naked eyes when fw = 98%, which would induce a decrease of the ef-
ficient concentration of fluorophores during the PL measurement. Thus,
the fluorescence decline should be caused by the occurrence of amor-
phous aggregates and the quick formation of precipitate due to the
introduction of large amount of water [65–68]. On the other hand, Yin
et al. have demonstrated that fluorescence of dyes could be significantly
quenched through efficient energy transfer from TPE to an additional
fluorophore (via a non-conjugated linkage) [69]. Similarly, as men-
tioned above, the nitrobenzene moieties in TPE-S were highly twisted
and close to TPE, quenching its fluorescence. Thus, molecular structure,
The 1H NMR spectra of PTS and PTO are shown in Fig. 2. After the
protection reaction between PTO and 1, 2-ethanedithiol, the singlet at
4.35 ppm assigned to the methylene (Ha in PTO) attached to carbonyl
group shifted from 4.35 to 3.58 ppm (Ha’ in PTS) as the result of the
transformation of the carbonyl group to thioketal group. Meanwhile,
the multiplet at the range of 3.33–3.21 ppm assigned to the methylene
(-SCH2CH2S-, Hb in PTS) attached to the sulfur atoms appeared.
Moreover, great changes had also taken place in aromatic areas after
the protection reaction. The hydrogen atoms ratio between Ha’ and Hb
in PTS was 1:2 confirming that carbonyl groups in PTO have been
completely reacted with 1, 2-ethanedithiol (Fig. S7). The 13C NMR
spectra of PTO and PTS are shown in Fig. S6 and Fig. S8. After in-
corporation with thiol, the obvious signal shift at 195.39 ppm for
carbon of the carbonyl group in PTO was disappeared in that of PTS.
3

Y. Shan et al.
DyesandPigments156(2018)1–7
Fig. 2. 1H NMR spectra of PTO and PTS (in CDCl3), the solvent peaks are marked with asterisks.
such as nitrobenzene moieties in the polymer could be another reason
that quenched the fluorescence of PTS. Thus, once triggered by Hg2+
with the concentration of Hg2+ ranging from 5 to 30 μM (R2 = 0.992,
,
inset in Fig. 4b). The detection limit of Hg2+ could be evaluated to be
the conversion of PTS (weak PL intensity) to PTO (high PL intensity)
inTHF-H2O mixture (VTHF/Vwater = 2/98), could give out the fluor-
2.3 × 10−7 M (S/N = 3). In addition, as shown in Fig. S2, the Hg2+
-
promoted deprotection reaction of PTS occurred quickly within 5 min.
To evaluate the specificity of PTS towards Hg2+, the influence of
various common metal ions were investigated in parallel under the
escent turn on signal, to report the presence of Hg2+
.
same conditions. Thus, various metal ions (Hg2+, Ag+, Cr3+, Al3+
Fe3+, Ca2+, Ni2+, Co2+, Pb2+, Cu2+, Zn2+, Mg2+, Fe2+, Mn2+, Cd2+
,
,
3.4. Hg2+ sensing properties
Ba2+, Li+, Na+ and K+) were added to the solution of PTS. As shown in
Fig. 5, only Hg2+ ions could led the remarkable PL enhancement, other
metal ions brought slight influence on the PL intensity, indicating the
specific chemical reactions between PTS and Hg2+. This result is rea-
sonable due to the strong affinity between Hg2+ and thioketal groups in
PTS. The high selectivity of PTS towards Hg2+ could be further con-
firmed by the competition experiments. To the solution of PTS with one
of the competitive metal ions was added Hg2+ subsequently, the
fluorescence was still turn-on rapidly (Fig. 6), illustrating the nice anti-
The PL spectra of PTS in the presence of Hg2+ ions at different
concentrations are shown in Fig. 4. Upon the addition of Hg2+ ions,
then water was added for the formation of aggregation state. The
emission peak (λem) of PTS gradually enhanced and slight red-shift with
the increasing concentration of Hg2+. And an emission enhancement of
about 11.3-fold at 520 nm was observed until the concentration of
Hg2+ reached to 1.5 × 10−4 M (Fig. 4a). In sharp contrast, fluorescence
of PTS exhibited significant enhancement after the addition of
Hg2+ions, accompanying with the quantum yield of PTS increased from
0.64% to 7.67%. Under a normal UV lamp, the remarkable PL en-
hancement could be visually observed by the naked eyes (Inset photos
in Fig. 4a). Also, there was a good linear relationship between the in-
tensity change and the concentration of Hg2+ ions at both low and high
concentration (Fig. 4b). Values of I/I0 possess a good linear relationship
interference and excellent selectivity of PTS towards Hg2+
.
3.5. Mechanism of the sensing process
This probe was designed according to the Hg2+-promoted
Fig. 3. PL spectra of PTO (A) and PTS (B) in THF-H2O mixtures (20 μM) with different water fractions. Inset: Photos of luminogens in THF/H2O mixtures taken under
the illumination of a 365 nm UV lamp.
4

Y. Shan et al.
DyesandPigments156(2018)1–7
Fig. 4. (A) PL spectra of PTS (20 μM) in the presence of different amounts of Hg2+, excited at 368 nm in THF-H2O mixtures with the water fraction of 98%. Inset:
corresponding fluorescence photos of PTS before and after reacted with Hg2+ (1.5 × 10−4 M). (B) PL enhancement values (I/I0) versus Hg2+ concentration, the plots
for Hg2+ are shown as an inset.
Fig. 5. (A) PL spectra of PTS (20 μM) in the presence of various metal ions (1.5 × 10−4 M) excited at 368 nm in THF-H2O mixtures with the water fraction of 98%.
(B) Corresponding fluorescence responses of PTS to various metal ions.
deprotection reaction of thioketal, transforming PTS to PTO in the
presence of Hg2+. FTIR spectrum was used to confirm the sensing
mechanism. As shown in Fig. 7, there was a very strong absorption peak
at about 1686 cm−1 in PTO, corresponding to the stretching vibration
of the carbonyl group (C=O), which has completely disappeared in
PTS, for the absence of carbonyl group (carbonyl group was trans-
formed to thioketal). After the addition of Hg2+ ions, the typical ab-
sorption peak of carbonyl group at 1686 cm−1 appeared
(PTS + Hg2+), indicating the successful deprotection reaction of PTS
and the formation of PTO.
4. Conclusion
In conclusion, considering TPE-S has good performance towards
Hg2+, it was utilized to design a conjugated polymer probe PTS with
the aim to enhance the fluorescence signal. Due to the signal amplifi-
Fig. 6. Fluorescence spectra profiles of PTS (20 μM) in the presence of various
cation effect of conjugated polymer, a fluorescent turn on probe for
metal ions (1.5 × 10−4 M, black line), followed by added Hg2+ (1.5 × 10−4 M,
aqueous Hg2+ with good sensing performance was realized. Once
red line), in THF/H2O (2/98, v/v) solution. (For interpretation of the references
to color in this figure legend, the reader is referred to the Web version of this
article.)
triggered by mercury ions, remarkable PL enhancement could be vi-
sually observed by naked eyes. The response was very fast with good
selectivity towards Hg2+ due to the specific chemical reaction. Other
metal ions (Ag+, Cr3+, Al3+, Fe3+, Ca2+, Ni2+, Co2+, Pb2+, Cu2+
,
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DyesandPigments156(2018)1–7
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.
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