Hg2+ detection by aniline-based conjugated copolymers with high selectivity

Article abstract of DOI:10.1016/j.polymer.2011.04.017

Conjugated polymers (P1, P1L and P2-P5) constructed by alkynyl-substituted aniline and substituted arene analogs could be synthesized through Pd-catalyzed Sonogashira coupling. The responsive optical properties of poly(2-ethynyl aniline) (P1 and P1L for d

Full text of DOI:10.1016/j.polymer.2011.04.017

Polymer 52 (2011) 2537e2541
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Hg2 detection by aniline-based conjugated copolymers with high selectivity
þ
Bin Yang, Guangzhao Li, Xu Zhang, Xin Shu, Anning Wang, Xiaoqing Zhu, Jin Zhu*
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory
of Microstructures, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 November 2010
Received in revised form
Conjugated polymers (P1, P1L and P2eP5) constructed by alkynyl-substituted aniline and substituted
arene analogs could be synthesized through Pd-catalyzed Sonogashira coupling. The responsive optical
properties of poly(2-ethynyl aniline) (P1 and P1L for different molecular weights, P1L with longer chains
22 March 2011
2þ
þ
2þ
2þ
3þ
2þ
2þ
þ
2þ
2þ
on average) toward various metal ions (including Ni , Ag , Cu , Zn , Fe , Fe , Mn , Na , Ca , Pb ,
Accepted 11 April 2011
Available online 15 April 2011
þ
3þ
3þ
2þ
2þ
3þ
3þ
2þ
K , Cr , Al , Cd , Pt , Au
and Tl ) were investigated. Hg exhibited the most pronounced
fluorescence response of both P1 and P1L without interference from those coexistent ions due to aniline
in the polymer backbone as the metal binding ligand, while other metal ions does not cause obvious
Keywords:
change of fluorescence. Compared with P1, P1L exhibits better sensitivity toward Hg2 . Introducing
þ
Fluorescence sensor
Mercury detection
Conjugated polymer
pyridyl, thienyl or phenyl groups into the polymer backbone would weaken the quenching responses to
2þ
Hg compared with P1. The results indicated P1 and P1L could be used as a selective fluorescence
sensor toward Hg2
þ
.
Ó 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
synthesis of aniline-based conjugated polymers (P1, P1L and
P2eP5) (Scheme 1) used as fluorescence sensors for the detection
2
þ
Conjugated polymers as fluorescence sensors have gained
of transition metal ions. Compared with other cations, such as Ni
,
,
þ
2þ
2þ
3þ
2þ
2þ
þ
2þ
2þ
þ
3þ
3þ
increasing attention due to their high sensitivity and ease of
measurement. The high sensitivity of the conjugated polymers
originates from the sensory signal amplification which is attributed
to energy migration along the polymer backbone upon excitation.
An additional advantage of polymer-based fluorescence sensors is
that the structure and sequence of the conjugated polymers can be
systematically modified to suit diverse targets by the introduction
of different functional groups (based on their steric and electronic
properties) at well-defined molecular levels [1,2]. The research on
polymer-based fluorescence sensors for the detection of heavy and
transition metal ions is emerging as an area of current interest in
the recent years [3e5]. Among them, mercury and its derivatives
are dangerous and widespread global pollutants which have caused
serious environmental and health problems [6e9]. Many fluores-
cence sensors for mercury detection have been reported. Most of
them are based on small molecules [3,10], while polymer-based
chemosensors are very few [11e25]. Therefore, the development
of highly selective and sensitive polymer-based chemosensors for
mercury detection is still attractive. In this paper, we report the
Ag , Cu , Zn , Fe , Fe , Mn , Na , Ca , Pb , K , Cr , Al
2
þ
2þ
3þ
3þ
2þ
Cd , Pt , Au and Tl , Hg exhibits the most pronounced effect
upon the fluorescence property of P1. Additionally, P1L with larger
2
þ
molecular weight exhibit better sensitivity toward Hg compared
with P1. The results indicate that poly(2-ethynyl aniline) (P1 and
2
þ
P1L) could be used as a selective fluorescence sensor for Hg
detection.
2. Experimental part
2.1. Materials and measurements
NMR spectra were obtained using a Bruker DRX-500 NMR
spectrometer with chemical shifts reported as ppm (TMS as the
internal standard). Fluorescence spectra were obtained from
a RF-5301PC spectrometer. Mass spectra were determined on
a Micromass GCT machine. Molecular weights of the polymers
were determined by gel permeation chromatography (GPC) with
Waters-244 HPLC pump with THF as the solvent and polystyrene as
the relative standard. All reagents were of analytical grade. All
perchlorates were purchased from Alfa Aesar. PtCl
2
(98%) was
purchased from Alfa Aesar. TlCl $4H O (98%) was purchased from
3
2
SigmaeAldrich. All compounds were prepared according to the
literature [16,26].
*
Corresponding author. Fax: þ86 25 8331 7761.
E-mail address: jinz@nju.edu.cn (J. Zhu).
0
032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.04.017

2538
B. Yang et al. / Polymer 52 (2011) 2537e2541
Scheme 1. Synthesis procedures of the polymer sensors.
2
.2. General procedure for the synthesis of aniline-based conjugated
P1L: Yield: 87%. M
P2: Yield: 76%. M
500 MHz,
2.13e2.20 (m, 3H).
P3: Yield: 80%. M
(500 MHz, CDCl
P4: Yield: 73%. M
(500 MHz, d -DMSO): 7.15e7.49 (m, 4H), 5.66 (s, 2H), 3.82e3.92
(m, 6H), 2.19 (s, 3H).
P5: Yield: 71.2%. M
(500 MHz, CDCl
w
¼ 7149, M
n
¼ 2359, PDI ¼ 3.0.
¼ 5438, M
n
¼ 2753, PDI ¼ 1.9. 1H NMR
polymers (P1, P1L and P2eP5, P1 as an example) (Scheme 1)
w
(
6
d -DMSO): 7.23e7.93 (m, 5H), 5.50 (m, 2H),
To a flame-dried 50 mL Schlenk flask, 2 (0.167 g, 0.559 mmol), 1
¼ 3948, M
n
¼ 1079, PDI ¼ 3.6. 1H NMR
(
0.149 g, 0.562 mmol), (PPh
3
)
2
PdCl
2
(50.0 mg, 0.071 mmol), CuI
w
(
50.0 mg, 0.260 mmol), and anhydrous tetrahydrofuran (5 mL) were
3
): 7.01e7.23 (m, 4H), 4.87 (s, 2H), 2.22 (s, 3H).
1
added under a slow stream of nitrogen. A saturated ethanol solution
of KOH (1 mL) was then added. Piperidine (7 mL) was introduced to
the reactionvia syringe. The reactionwas stirredand held at 30 C for
w
¼ 2630, M ¼ 1213, PDI ¼ 2.1. H NMR
n
6
ꢀ
¼ 1934, M
n
¼ 758, PDI ¼ 2.5. 1H NMR
4
8 h. The solution was poured into a mixture of chloroform (75 mL)
and water (100 mL). The organic layer was washed three times with
water (100 mL) followed by NH OH (30%, mass content,150 mL). The
w
3
): 7.19 (s, 2H) 7.03 (s, 2H), 3.91e4.09 (m, 4H),
2.23 (s, 3H), 1.89e1.54 (m, 8H), 0.98e1.05 (m, 12H).
4
aqueous layer was washed twice with chloroform (20 mL). The
organic layers were combined. To the resultant organic layer,
methanol (400 mL) was added to precipitate the polymer. Suction
2.3. Metal ion titration
filtration afforded a brown polymer (P1). P1: Yield: 82%. M
w
¼ 6394,
1
M
n
¼ 1353, PDI ¼ 4.6. H NMR (500 MHz, d
6
-DMSO): 7.28 (s, 2H), 5.52
Each metal ion titration experiment was started with a DMSO
(
s, 2H), 2.19 (s, 3H). P1L was synthesized by increasing the reaction
solution (3.0 mL) of P1, P1L, P2, P3, P4 and P5 with a known
ꢀ
ꢂ5
time to 72 h at 60 C. P2eP5 were synthesized in a similar fashion
concentration (3.0 ꢁ 10 M). Mercury perchlorate salt and other
ꢂ2
with the corresponding reagents.
perchlorate salts (1.0 ꢁ 10 M, aqueous solution) were used for the

B. Yang et al. / Polymer 52 (2011) 2537e2541
2539
ꢂ5
Fig. 3. Fluorescence quenching efficiencies (1 ꢂ F/F
0
) of P1 (3.0 ꢁ 10 M) in the
ꢂ5
Fig. 1. UVevis absorption spectra of a DMSO solution of P1 (1 ꢁ10 M, with respect to
the receptor unit) after the addition of Hg2 : [Hg ] ¼ (1, 2, 3, 4, 5) ꢁ 10 M.
ꢂ5
presence of various main group and transition metal ions (5.3 ꢁ 10 M for each metal
þ
2þ
ꢂ4
0
ion). F and F are taken as the fluorescence intensity of P1 at the maximum emission
wavelength (458.5 nm). Excitation wavelength: 411 nm.
ꢂ2
titration. PdCl
2
was dissolved by DMSO (1.0 ꢁ 10 M). TlCl
3 2
$4H O
ꢂ2
was dissolved in an aqueous solution (1 ꢁ10 M). Titration exper-
iments were performed by adding aliquots of a solution of the
selectedmetalsalttotheDMSOsolutionof P1, P1L, P2, P3, P4andP5.
The fluorescence response behaviors of the polymer on various
main group and transition metal ions have been investigated. Fig. 2
and Fig. S2 show the change of the fluorescence spectra of P1
ꢂ5
2þ
(aqueous
(3.0 ꢁ 10
M in DMSO) upon the addition of Hg
3
. Results and discussion
solution). As is evident from Fig. 2, the maximum fluorescence band
of the polymer appears at 458.5 nm with the excitation at 411 nm.
Obvious fluorescence quenching could be observed upon the
3
.1. Synthesis and features of the conjugated polymers (P1, P1L and
P2eP5)
2þ
2þ
addition of Hg . Hg leads to an intense decrease (97.3%) of
2
þ
fluorescence intensity for P1. The lowest concentration of Hg to
cause an effective fluorescence quenching of P1 was measured to
The synthesis procedures of the monomers 2, 3 and the conju-
gated polymer are outlined in Scheme 1. In this paper, a typical
Sonogashira reaction condition was applied to the synthesis of the
polymers [16,26]. The polymerization could be carried out under
mild reaction conditions in THF in the presence of a catalytic
ꢂ7
be 4.5 ꢁ 10 M (3
s method).
2þ
Beyond Hg , the fluorescence response properties of P1 toward
other metal ions were also examined (Fig. 3 and Fig. S3). The fluo-
rescence intensities of P1 are almost not affected by the addition of
amount of Pd(PPh
a moderate yield of 71.2%e87%.
3 2 2
) Cl (10% mol) and CuI (5% mol) with
2þ
þ
2þ
2þ
2þ
3þ
2þ
2þ
þ
2þ
2þ
þ
3þ
Ni , Ag , Cu , Zn , Fe , Fe , Mn , Na , Ca , Pb , K , Cr
,
3þ
2þ
3þ
3þ
Al , Cd , Pt , Au and Tl , respectively. Normally, an excellent
fluorescence sensor with highly sensitive and selective features
needs to meet one of the essential requirements that minor or no
interference from other metal ions should occur. In this paper, we
3
.2. The selective and sensitive recognition of the polymer sensor
2
þ
on Hg
further investigated the utility of P1 as an ion-selective fluorescence
Ion-responsive properties of P1 are investigated by UVevis
absorption spectroscopy. P1 exhibits a strong absorption band at
2þ
sensor for Hg
. Herein, the fluorescence responses of P1
ꢂ5
2þ
ꢂ5
(
3.0 ꢁ 10 M) to a mixture of Hg (5.3 ꢁ 10 M) and other metal
3
95 nm, which is associated with
segment (Fig. 1). The maximum absorption wavelengths (
-conjugated polymers are dictated by both the degree of conju-
gation and conjugation length.
p
e
p
* transition of the conjugated
ꢂ5
ions (5.3 ꢁ 10 M for each metal ion) were recorded as Fig. 4 and
lmax) of
Fig. S4. In the coexisting metal experiment, while an intense fluo-
p
2þ
rescence quenching was caused by the addition of Hg no matter
2þ
3þ
þ
2þ
2þ
3þ
2þ
2þ
þ
2þ
other metal ions (Ni , Ag , Cu , Zn , Fe , Fe , Mn , Na , Ca
,
The change of the absorption spectrum in the case of addition of
2þ
þ
3þ
Pb , K , Cr and Al included) existed or not, no obvious inter-
ference was observed in the presence of mixed metal ions without
Hg2 is presented in Fig. 1. Most notably, the maximum absorption
band experienced a shift toward the shorter wavelength. The iso-
sbestic point with an “out of focus” impression (around 361 and
þ
2þ
2þ
2þ
3þ
3þ
Hg . The existence of Cd , Pt , Au and Tl affected the fluo-
rescence intensity a little. These results indicated P1 is specifically
2
þ
4
32 nm, respectively) implied the interaction of Hg with the
2þ
sensitive toward Hg but not other metal ions. Thus the polymer
chelating site of P1.
ꢂ5
2þ
2þ
ꢂ5
Fig. 2. A: Emission spectra of a DMSO solution of P1 (3.0 ꢁ 10 M) after the addition of Hg . [Hg ] ¼ (0, 0.01, 0.02, 0.03, 0.13, 0.23, 0.33, 1.33, 2.33, 3.33, 4.33, 5.33) ꢁ 10 M.
2
þ
Excitation wavelength: 411 nm. B: The change of fluorescence intensity of P1 when the concentration of Hg added changed (data correspond to Fig. 2A).

2540
B. Yang et al. / Polymer 52 (2011) 2537e2541
ꢂ5
Fig. 6. Fluorescence quenching efficiencies (1 ꢂ F/F
0
) of P1L (3.0 ꢁ 10 M) in the
ꢂ5
presence of various main group and transition metal ions (5.3 ꢁ 10 M for each metal
ꢂ5
0
ion). F and F are taken as the fluorescence intensity of P1L at the maximum emission
Fig. 4. Metal ion specificity: emission spectra of a DMSO solution of P1 (3.0 ꢁ 10 M,
2þ
2þ
þ
2þ
wavelength. Excitation wavelength: 411 nm.
solution A) without metal ion (red); Hg added to solution A (blue); Ni , Ag , Cu
,
2
þ
3þ
2þ
2þ
þ
2þ
þ
2þ
2þ
2þ
þ
2þ
3þ
3þ
3þ
2þ
2þ
2þ
2þ
þ
3þ
2þ
3þ
þ
Zn , Fe , Fe , Mn , Na , Ca , Pb , K , Cr , Al , Cd , Pt , Au and Tl added
2
þ
2þ
3þ
to solution A (green); Ni , Ag , Cu , Zn , Fe , Fe , Mn , Na , Ca , Pb , K , Cr ,
3þ
2þ
2þ
3þ
3þ
ꢂ7
Al , Cd , Pt , Au , Tl and Hg added to solution A (cyan). The concentration of
1.8 ꢁ 10 M (3
s method), about 40% of that of P1. These results
ꢂ5
each metal ion added is 5.3 ꢁ 10 M. Excitation wavelength: 411 nm. (For interpre-
indicated that polymers with the same backbone exhibit analogous
fluorescence quenching properties and selectivity toward various
metal ions. Additionally, better sensitivity toward metal ion might
be obtained for polymer of longer backbones. This may derive from
the more efficient energy migration along the longer polymer
tation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
2þ
could be used as a sole Hg probe with high selectivity. The sensi-
2
þ
2þ
backbone upon the addition of metal ions [1,2]. The Hg center
coordinated with the polymer backbone provided an efficient
quenching site for the whole polymer.
tivity toward Hg could be attributed to several factors, the large
radius of the Hg2 ion (compared with many other metal ions) and
þ
2þ
coordination diversification of Hg , its soft acid property, the
binding ability of aniline unit and strong HgeN binding [17]. Because
of its 5d106s electronic configuration and lack of ligand field stabi-
2
3.4. Effect of different polymer backbones
2þ
lization energy, Hg can accommodate a range of coordination
numbers and geometries. Two-coordinate linear and four coordinate
To gain further insight into the sensing mechanism, P2eP5 with
distinct chemical structures (e.g., side chain bulkiness, heteroatom
binding site) are examined. Polymers P2eP5 exhibit similar
2þ
tetrahedral species are common. Hg is a soft acid andthe useof soft
donor atoms (e.g. nitrogen, sulfur and phosphorus) in a chelating
2þ
2þ
unit will generally increase its affinity and selectivity for Hg [3].
responses to Hg in the absorption spectra (Figs. S6eS13). Fluo-
2
þ
rescence quenching responses of P2eP5 to Hg
were also
observed (Fig. 7) which might derive from the analogous chain unit.
But, compared with P1, the quenching responses were much
weaker. From P2 to P5, the quenching intensity decreased gradu-
3
.3. Effect of molecular weight
In order to investigate the effect of molecular weight on the
2þ
ally as a result of the decreased coordination ability with Hg
aniline > pyridine > thiophene > alkoxy substituted benzene).
quenching properties of poly(2-ethynyl aniline), P1L was synthe-
sized through increasing reaction time (72 h) at raised temperature
(
ꢀ
(
60 C). The yielded polymer exhibit a larger molecular weight and
lower polydispersity compared with P1.
4. Conclusion
As expected, P1L also displayed pronounced fluorescence
quenching toward Hg2 (Fig. 5), analogous to P1. Hg leads to
a decrease of fluorescence intensity (97.2%) for P1L, while the
addition of other metal ions (Fig. 6) has weak influence on the
fluorescence intensity of P1L. The coexisting metal experiment
þ
2þ
In conclusion, aniline-based conjugated polymers (P1, P1L and
P2eP5) were synthesized through Pd-catalyzed Sonogashira
coupling. P1 and P1L exhibited intense fluorescence quenching
2þ
response to Hg . The polymer was also proved to possess excellent
2
þ
(
Fig. S5) indicated that the fluorescence quenching response of
P1L could not be easily affected by other disturbing metal ions.
A detection limit of P1L toward Hg
sensitivity and selectivity toward Hg . The detection limit of P1 and
2
þ
ꢂ7
ꢂ7
P1L to Hg was measured to be 4.5 ꢁ 10 M and 1.8 ꢁ 10 M,
2
þ
was measured to be
respectively (3
s
method). Weaker quenching responses of P2eP5 to
ꢂ5
2þ
2þ
ꢂ5
Fig. 5. A: Emission spectra of a DMSO solution of P1L (3.0 ꢁ 10 M) after the addition of Hg . [Hg ] ¼ (0, 0.01, 0.02, 0.03, 0.13, 0.23, 0.33, 1.33, 2.33, 3.33, 4.33, 5.33) ꢁ 10 M.
2
þ
Excitation wavelength: 411 nm. B: The change of fluorescence intensity of P1L when the concentration of Hg added changed (data correspond to Fig. 5A).

B. Yang et al. / Polymer 52 (2011) 2537e2541
2541
Appendix. Supplementary information
Supplementary data related to this article can be found online at
doi:10.1016/j.polymer.2011.04.017.
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