Highly sensitive conjugated polymer fluorescent sensors based on benzochalcogendiazole for nickel ions in real-time detection

Article abstract of DOI:10.1039/c4tc01112k

Reports about fluorescent sensors for the highly sensitive and selective detection of Ni²⁺ are rare compared with Hg²⁺, Ag ⁺, Cd²⁺, Pb²⁺, Cr³⁺, etc. Herein, we report the synthesis and appl

Full text of DOI:10.1039/c4tc01112k

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Highly sensitive conjugated polymer fluorescent
sensors based on benzochalcogendiazole for nickel
ions in real-time detection†
Cite this: J. Mater. Chem. C, 2014, 2,
7402
Yunxiang Lei, Hui Li, Wenxia Gao, Miaochang Liu, Jiuxi Chen, Jinchang Ding,
*
*
Xiaobo Huang and Huayue Wu
Reports about fluorescent sensors for the highly sensitive and selective detection of Ni2+ are rare compared
with Hg2+, Ag+, Cd2+, Pb2+, Cr3+, etc. Herein, we report the synthesis and application of two conjugated
polymer fluorescent sensors, P-1 and P-2, using benzochalcogendiazole and triazole as cooperative
receptors of Ni2+. The synthesis was carried out by the polymerization of 4,7-diethynylbenzoselenadiazole
(M-2) and 4,7-diethynylbenzothiadiazole (M-3) with 2,7-diazido-9,9-dioctyl-9H-fluorene (M-1) via copper-
catalyzed azide–alkyne cycloaddition. P-1 and P-2 exhibit emission peaks at 535 nm and 514 nm, and
show yellow and green fluorescence, respectively. These two polymers showed outstanding fluorescence
response behavior toward Ni2+ during real-time detection. Compared with benzothiadiazole-based P-2,
benzoselenadiazole-based P-1 showed higher sensitivity and selectivity, and its detection limit could reach
2.4 nM, which might be due to the specific electronegativity of selenium. This work increases the
application scope of the click reaction in regard to the design and synthesis of novel conjugated polymer
fluorescent sensors based on the benzochalcogendiazole unit.
Received 28th May 2014
Accepted 8th July 2014
DOI: 10.1039/c4tc01112k
www.rsc.org/MaterialsC
uorescent sensors for the detection of Ni2+ remains a challenge
for researchers.
Introduction
Recently, the copper-catalyzed azide–alkyne cycloaddition
reaction (CuAAC) has had an enormous impact on the eld of
polymer materials science because of its high efficiency, mild
reaction conditions and technical simplicity.7 In the click
reaction, a thiazole ring is directly generated. The click-formed
triazole can be used as a metal-binding ligand, and it is more
than a simple linker group. Many uorescent sensors based on
a triazole unit exist for metal ion detection.8 However, these
uorescent sensors are mostly based on small organic mole-
cules,8,9 and only a few are based on conjugated polymers.6e,10
Benzochalcogendiazoles, such as 2,1,3-benzothiadiazole
(BT) and 2,1,3-benzoselenadiazole (BSe), play very important
roles in the synthesis and application of uorescent sensors
with p-conjugated structure, because they are excellent uo-
rophores owing to their strong and stable uorescence, good
thermal stability and good electron-withdrawing ability.11
Comparisons between the effects of the heteroatoms S and Se
are common since Se is much larger and less electronegative
than S. It has been reported that BSe-based compounds have
obvious absorption and emission redshis compared with BT-
based compounds, because BSe can more effectively lower band
gaps, and some of these compounds exhibit interesting
photoelectric properties.12 Additionally, benzochalcogendiazole
units containing C]N bonds can also be used as recognition
moieties which display high affinity for transition metal ions.13
Over the last two decades, conjugated polymer uorescent
sensors for heavy and transition metal (HTM) ions, anions, and
organic molecules have attracted much interest owing to their
high sensitivity, selectivity, rapidity and ease of measurement.1
Their excellent sensitivity is attributed to sensory signal
amplication that originates from energy migration along the
conjugated backbone upon light excitation, and this was
pointed out by Swager and co-workers.2 Ni2+ is a dangerous and
widespread global pollutant,3 but reports concerning uores-
cent sensors for Ni2+ detection are still relatively scarce
compared with Hg2+, Ag+, Cd2+, Pb2+, Cr3+, etc.4 Ni2+-sensitive
uorescent sensors are oen based on small molecules, and
some of these sensors encounter serious interference problems
from other HTM ions like Cu2+, Pb2+, Co2+, Hg2+, Zn2+, Fe2+, etc.5
Very few uorescent sensors based on conjugated polymers for
Ni2+ detection have been reported,6 and they oen show
unsatisfactory selectivity because their structures mostly
incorporate bipyridyl or oligopyridyl groups, which exhibit
strong ability to coordinate with several metal ions. The devel-
opment of highly selective and sensitive conjugated polymer
College of Chemistry & Materials Engineering, Wenzhou University, Wenzhou 325035,
P. R. China. E-mail: xiaobhuang@wzu.edu.cn; huayuewu@wzu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc01112k
7402 | J. Mater. Chem. C, 2014, 2, 7402–7410
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
In this work, we report the synthesis and application of two nitric acid was added dropwise. The reaction mixture was
new conjugated polymer uorescent sensors, P-1 and P-2, which allowed to warm to room temperature and then it was stirred for
incorporate a benzochalcogendiazole unit and a triazole unit by 2 h. The reaction mixture was poured on ice and then extracted
the polymerization of 4,7-diethynylbenzoselenadiazole (M-2) with CHCl3. The organic layer was washed with water, saturated
and 4,7-diethynylbenzothiadiazole (M-3) with 2,7-diazido-9,9- NaHCO3 solution, and then brine, dried over anhydrous
dioctyl-9H-uorene (M-1) via the CuAAC reaction, respectively. Na2SO4, and then evaporated under vacuum to dryness. The
Both these two polymers are soluble in common organic residue was puried by silica gel column chromatography
solvents and are highly emissive. The sensing properties of (petroleum ether–CH2Cl2) (1 : 5, v/v) to obtain compound 3 as a
these two polymers were investigated using benzochalco- yellow solid in 50% yield. 1H NMR (500 MHz, CDCl3): d 8.33 (dd,
gendiazole and triazole as cooperative receptors for metal ions J1 ¼ 8.5 Hz, J2 ¼ 2.0 Hz, 2H), 8.26 (d, J ¼ 2.0 Hz, 2H), 7.93 (d, J ¼
by obtaining uorescence spectra for real-time detection. 8.5 Hz, 2H), 2.12–2.08 (m, 4H), 1.17–1.01 (m, 20H), 0.78 (t, J ¼
Although both these polymers were highly sensitive to Ni2+
,
7.0 Hz, 6H), 0.58–0.52 (m, 4H); 13C NMR (125 MHz, CDCl3): d
compared with P-2 based on BT, P-1 based on BSe showed 153.5, 148.4, 144.7, 123.5, 121.5, 118.5, 56.4, 39.7, 31.6, 29.7,
higher sensitivity and selectivity because of the specic elec- 29.1, 29.0, 23.8, 22.5, 13.9.
tronegativity of selenium.
9,9-Dioctyl-9H-uorene-2,7-diamine (4) was synthesized
according to a reported method.16 A mixture of compound 3
(0.50 g, 1.2 mmol), zinc powder (2.00 g), anhydrous CaCl2 (0.30
g), ethanol (30 mL) and water (10 mL) was reuxed for 5 h, and
then the hot mixture was ltered to remove the zinc powder.
The mixture was extracted with CHCl3, and the organic layer
was washed with water and brine, dried over anhydrous Na2SO4,
and then evaporated under vacuum to dryness. The residue was
puried by silica gel column chromatography (petroleum ether–
ethyl acetate) (1 : 5, v/v) to give compound 4 as a yellow oil in
90% yield. 1H NMR (500 MHz, CDCl3): d 7.35 (d, J ¼ 8.0 Hz, 2H),
6.63 (s, 2H), 6.61 (d, J ¼ 2.0 Hz, 2H), 3.64 (br, 4H), 1.86–1.83 (m,
4H), 1.25–1.06 (m, 20H), 0.85 (t, J ¼ 7.0 Hz, 6H), 0.67 (br, 4H);
13C NMR (125 MHz, CDCl3): d 151.6, 144.4, 133.1, 119.0, 113.8,
110.0, 54.6, 40.9, 31.8, 30.2, 29.3, 29.2, 23.7, 22.6, 14.0.
Experimental
Measurements and materials
1H NMR and 13C NMR spectra were recorded in CDCl3 solutions
on a Bruker DRX 500 NMR spectrometer with tetramethylsilane
(TMS) as the internal standard. The chemical shi was recorded
in ppm, and the following abbreviations were used to indicate
multiplicities: s ¼ singlet, d ¼ doublet, m ¼ multiplet, and br ¼
broad. EI mass spectra were recorded on an Agilent 5975C DIP/
MS mass spectrometer. C, H and N elemental analyses were
performed on an Elementar Vario MICRO analyzer. UV-vis
absorption was recorded on a Shimadzu UV-1700 spectrometer,
and uorescence spectra were recorded on a RF-5301PC
uorometer. Thermogravimetric analysis (TGA) was performed
on a Perkin-Elmer Pyris-1 instrument under a N2 atmosphere.
Molecular weight was determined by gel permeation chroma-
tography (GPC) with a Waters-244 HPLC pump. THF was used
as the solvent and the results are relative to polystyrene
standards. THF and Et3N were puried by distillation from
sodium in the presence of benzophenone. The other solvents
were commercially available and were of analytical reagent
grade. The other materials were of common commercial grade
and were used as received.
2,7-Diazido-9,9-dioctyl-9H-uorene (M-1) was synthesized
according to a reported method.16 Compound 4 (0.4 g, 1.1
mmol) was dissolved in a mixture of 30 mL water and 10 mL
concentrated hydrochloric acid in a round-bottom ask at 0 ꢀC.
Sodium nitrite (0.18 g, 2.3 mmol) was added dropwise to the
above solution. Aer 1 h of stirring, sodium azide (0.16 g, 2.5
mmol) was added dropwise. The reaction mixture was stirred at
ꢀ
0 C for 4 h. The mixture was extracted with CH2Cl2, and the
organic layer was washed with water and brine, dried over
anhydrous Na2SO4, and then evaporated under vacuum to
dryness. The residue was puried by silica gel column chro-
matography by eluting with petroleum ether to give M-1 as a
yellow solid in 93% yield. 1H NMR (500 MHz, CDCl3): d 7.60 (d,
Preparation of monomer M-1
9,9-Dioctyl-9H-uorene (2) was synthesized according to a J ¼ 8.0 Hz, 2H), 7.00 (dd, J1 ¼ 8.0 Hz, J2 ¼ 2.0 Hz, 2H), 6.95 (d, J ¼
reported method.14 1-Bromooctane (4.26 g, 22 mmol) was added 2.0 Hz, 2H), 1.94–1.90 (m, 4H), 1.23–1.04 (m, 20H), 0.83 (t, J ¼
using a syringe to a mixture of uorene (1) (1.66 g, 10.0 mmol) 7.5 Hz, 6H), 0.59–0.57 (m, 4H); 13C NMR (125 MHz, CDCl3): d
and KOH (5.6 g, 100 mmol) in THF (25 mL) at room tempera- 152.6, 138.7, 137.6, 120.5, 117.9, 113.6, 55.4, 40.4, 31.7, 29.9,
ture. The reaction mixture was stirred at room temperature for 29.2, 29.1, 23.6, 22.6, 14.0.
12 h. Upon evaporation to dryness, the residue was extracted
with CHCl3 and washed with water and then brine, dried over
anhydrous Na2SO4, and then evaporated under vacuum to
Synthesis of monomer M-2
dryness. The residue was puried by silica gel column chro- 3,6-Dibromobenzene-1,2-diamine (7) (532 mg, 2.0 mmol),
matography (petroleum ether–ethyl acetate) (200 : 1, v/v) to Pd(PPh3)2Cl2 (140.2 mg, 0.20 mmol), CuI (38.2 mg, 0.20 mmol),
obtain compound 2 as a yellow oil in 82% yield.
PPh3 (52.4 mg, 0.20 mmol) and trimethyl silyl acetylene
2,7-Dinitro-9,9-dioctyl-9H-uorene (3) was synthesized (1.72 mL, 12.00 mmol) were dissolved in a solvent mixture of
according to a reported method.15 Compound 2 (5 g, 12.8 mmol) Et3N (25 mL) and THF (25 mL). The reaction mixture was stirred
ꢀ
was dissolved in 10 mL glacial acetic acid in a round-bottom at 80 C for 12 h under N2. Aer cooling to room temperature,
ꢀ
ask. The solution was chilled to 0 C and then 10 mL fuming the solvent was removed using a rotary evaporator, and the
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 7402–7410 | 7403

View Article Online
Journal of Materials Chemistry C
Paper
residue was extracted with CHCl3. The organic extract was
washed with water and then brine, dried over anhydrous
Na2SO4, and then evaporated under vacuum to dryness. The
residue was puried by silica gel column chromatography
(petroleum ether–ethyl acetate) (30 : 1, v/v) to give compound 8
as a yellow solid in 42% yield. MS (EI, m/z): 300 (M+).
Preparation of the conjugated polymers P-1 and P-2
A mixture of M-1 (118.2 mg, 0.25 mmol), M-2 (57.8 mg,
0.25 mmol), 10 mol% sodium ascorbate (4.95 mg, 0.025 mmol)
and 5 mol% CuSO4$5H2O (3.12 mg, 0.0125 mmol) was dissolved
in a 10 mL THF, 2 mL t-BuOH and 2 mL H2O solvent mixture.
ꢀ
The reaction mixture was stirred at 30–36 C for 2 days under
To a solution of compound 8 (112.2 mg, 0.38 mmol) in
reuxing ethanol (40 mL) was added a solution of SeO2 (43.2
mg, 0.4 mmol) in hot water (20 mL). The reaction mixture was
stirred under reux for 2 h. Aer cooling to room temperature,
the solvent was removed using a rotary evaporator and the
residue was extracted with CHCl3. The organic layer was washed
with water and then brine, dried over anhydrous Na2SO4, and
then evaporated under vacuum to dryness. The residue was
puried by silica gel column chromatography using petroleum
ether as an eluent to give compound 9 as a yellow solid in 75%
N2. The solvents were removed under reduced pressure and the
residue was extracted with CHCl3 (3 ꢂ 50 mL). The combined
organic layers were washed with an aqueous NH4OH solution,
water and then dried over anhydrous Na2SO4. Aer the solution
was removed, the resulting polymer was precipitated in meth-
anol, and then ltered and washed with methanol several times.
Further purication was conducted by dissolving the polymer in
CHCl3 to precipitate in methanol. The polymer was dried under
vacuum to give 120.0 mg P-1 as an orange solid in 57.8% yield.
1
GPC results: Mw ¼ 7560, Mn ¼ 5880, PDI ¼ 1.29. H NMR (500
1
yield. H NMR (500 MHz, CDCl3): d 7.62 (s, 2H), 0.33 (s, 18H);
Hz, CDCl3): d 9.26 (br, 2H), 8.76–7.58 (m, 6H), 7.05–6.94 (m, 2H),
2.02–1.91 (m, 4H), 1.25–1.08 (m, 20H), 0.79–0.64 (m, 10H). FT-IR
(KBr, cmꢁ1): 2926, 2851, 1610, 1582, 1465. Anal. calcd for
13C NMR (125 MHz, CDCl3): d 158.0, 132.8, 118.2, 102.3, 99.5,
ꢁ1.2.
Compound 9 (100 mg, 0.26 mmol) was dissolved in a mixed
solvent of THF (20 mL) and MeOH (20 mL), and then K2CO3
(26 mg, 0.266 mmol) was added to the solution. The reaction
mixture was stirred at room temperature for 1 h. The solvent
was removed under reduced pressure and the residue was
extracted with CHCl3. The organic layer was washed with water
and then brine, dried over anhydrous Na2SO4 and then evapo-
rated under vacuum to dryness to give the yellow solid M-2 in
C
39H44N8Se: C, 66.56; H, 6.30; N, 15.92. Found: C, 63.45; H, 6.01;
N, 13.49.
Using the same procedure as for the preparation of P-1, P-2
was obtained as a brown solid in 75.5% yield from M-3 and M-1.
GPC results: Mw ¼ 9400, Mn ¼ 7200, PDI ¼ 1.31. P-2 spectro-
scopic data: 1H NMR (500 Hz, CDCl3): d 9.29–9.25 (m, 2H), 8.84–
7.71 (m, 6H), 7.06–7.02 (m, 2H), 2.18–2.01 (m, 4H), 1.25–1.11
(m, 20H), 0.79–0.66 (m, 10H). FT-IR (KBr, cmꢁ1): 2924, 2852,
1610, 1588, 1475. Anal. calcd for C39H44N8S: C, 71.31; H, 6.75; N,
17.06. Found: C, 68.56; H, 6.21; N, 15.32.
1
82% yield. H NMR (500 MHz, CDCl3): d 7.66 (s, 2H), 3.67 (s,
2H); 13C NMR (125 MHz, CDCl3): d 159.1, 133.4, 118.5, 85.2,
79.4.
Metal ion titrations with P-1 and P-2
Synthesis of monomer M-3
Each metal ion titration experiment was started using a 3.0 mL
polymer uorescent sensor in a CHCl3 solution of known
concentration (10 mmol Lꢁ1). Mercury perchlorate salt and
various other metal salts (nitrates) in CH3CN were used for the
titration. Because the minimum response time that can be
measured is limited by the manipulation time, all measure-
ments were obtained 10 s aer the addition of the metal salt to
the polymer solutions for real-time detection.
4,7-Diethynylbenzothiadiazole (M-3) was synthesized as previ-
ously reported.11c,17 4,7-Dibromobenzothiadiazole (6) (1.10 g,
3.75 mmol), Pd(PPh3)4 (87.5 mg, 0.075 mmol), CuI (27.5 mg,
0.15 mmol) and trimethylsilyl acetylene (1.5 mL, 11.25 mmol)
were dissolved in 50 mL Et3N. The reaction mixture was stirred
at 75 ꢀC for 5 h under N2. Aer cooling to room temperature, the
solvent was removed by rotary evaporation and the residue was
extracted with CH2Cl2 (2 ꢂ 30 mL). The combined organic
layers were washed with water and 1.2 mol Lꢁ1 HCl (30 mL), and
then dried over anhydrous Na2SO4. Upon evaporation to
dryness, the residue was puried by silica gel column chroma-
Results and discussion
Synthesis and features of the conjugated polymers
tography using petroleum ether–ethyl acetate (15 : 1, v/v) as an The synthesis procedures of the monomers M-1, M-2 and M-3
eluent to give compound 10 as a pale yellow solid in 95% yield. and the conjugated polymers P-1 and P-2 are outlined in
Compound 10 was directly used for the synthesis of M-3 without Scheme 1. Monomer M-1 was synthesized from uorene (1) in a
further purication.
ve-step reaction as reported previously.11–16 Monomers M-218
Compound 10 (450 mg, 1.35 mmol) in a KOH methanol and M-311c,17 were synthesized from benzothiadiazole (5) in a
solution (1 mol Lꢁ1, 35 mL) was stirred at room temperature for three- and ve-step reaction, respectively, as previously repor-
1 h. The solvent was removed under reduced pressure and the ted. The terminal alkynes M-2 and M-3 are unstable and need to
residue was extracted with CHCl3. The organic layer was washed be kept in the dark under a N2 atmosphere at ꢁ4 ꢀC before use.
with water and then brine, dried over anhydrous Na2SO4, and
M-1, M-2 and M-3 were used as monomers for the synthesis
then evaporated under vacuum to dryness to give M-3 as a of the target polymers. We used a typical click reaction for the
yellow solid in 90% yield. 1H NMR (500 MHz, CDCl3): d 7.75 (s, synthesis of these polymers.10a,19 The polymerization was
2H), 3.67 (s, 2H); 13C NMR (125 MHz, CDCl3): d 154.3, 133.2, carried out under mild reaction conditions in a THF/t-BuOH/
116.7, 85.3, 78.9.
H2O solution in the presence of a catalytic amount of sodium
7404 | J. Mater. Chem. C, 2014, 2, 7402–7410
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
Scheme 1 Synthetic routes to M-1, M-2, M-3 and the conjugated polymers P-1 and P-2.
ascorbate (10 mol%) and CuSO4$5H2O (5 mol%). The number-
average molecular weight (Mn) was determined to be 5880 for P-
1 and 7200 for P-2 by GPC using a polystyrene standard in THF.
The GPC results of these two polymers show a moderate
molecular weight. The resulting polymers show good solubility
in common solvents including CHCl3, CH2Cl2, toluene and
THF, and this can be attributed to the exible n-octyl group on
the uorene units, which are side chains on the polymers. The
TGA of the polymers was carried out under a N2 atmosphere at a
heating rate of 10 ꢀC minꢁ1 (Fig. 1). As shown in Fig. 1, although
the P-1 and P-2 repeating units have a similar polymer compo-
sition and backbone chain structure, their TGA plots are
different. The TGA curves reveal that the 5% weight loss
temperatures (Td) for P-1 and P-2 are 211 ꢀC and 334 ꢀC, Fig. 1 TGA curves of P-1 and P-2.
respectively. There is a total loss of about 60% and 58% for P-1
and P-2 when heated to 700 ꢀC, respectively. By comparison with
wavelength region, 300–350 nm, is assigned to a localized p–p*
transition, and the other band at around 360–500 nm is
assigned to the effective p electronic transition of the conju-
gated units in the main-chain backbone.12e P-1 and P-2 exhibit
emission peaks at 535 nm and 514 nm, and show yellow and
green uorescence, respectively. A red shi (21 nm) was
observed for the uorescence spectra of P-1 by comparison with
P-1, P-2 has higher stability, which can be ascribed to the high
thermal stability of the BT unit. These results indicate that these
two polymers have desirable thermal properties for use in
uorescent materials.
Optical properties
The UV-vis absorption and uorescence spectra of P-1 and P-2 that of P-2, which is related to the narrower p–p* gap of the
were recorded in CHCl3 as shown in Fig. 2. As is evident from aromatic Se-containing heterocycle.12a The uorescence
Fig. 2, these two polymers exhibit two distinct absorption bands quantum yields (FF) of these two polymers were determined in
in the range of 300 nm to 500 nm. The band in the shorter CHCl3 at room temperature using the quinine sulfate solution
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 7402–7410 | 7405

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 2 Absorption spectra of P-1 (-,-) and P-2 (-B-), and fluores-
cence spectra of P-1 (---, lex ¼ 425 nm) and P-2 (-C-, lex ¼ 408 nm)
in CHCl3.
Fig. 3 Fluorescence spectra of P-1 (10.0 mmol Lꢁ1 in CHCl3) with
increasing concentrations of Ni2+ (0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6,
1.8, 2.0, 2.5, 3.0, 4.0, 5.0 mmol Lꢁ1 in CH3CN) (lex ¼ 425 nm). Inset:
visible fluorescence of P-1 before (left) and after (right) the addition of
Ni2+ (5.0 mmol Lꢁ1) under a 365 nm UV lamp.
as a uorescence reference (Fr ¼ 0.55 in 0.5 mol Lꢁ1 H2SO4).
The FF values could be calculated using the equation: FF ¼ Fr
(Fs/Fr)(Ar/As)(hs/hr)2, where s and r denote the sample and
reference, respectively, F is the integrated uorescence inten-
almost linearly with the concentration of Ni2+ in the range of 0–
1.8 mmol Lꢁ1. The following curve equation was obtained: F ¼
sity, A is the absorbance at the excited wavelength, and h is the 2.44 ꢂ 105 to 9.33 ꢂ 1010[Ni2+] (Fig. 4, inset), which indicates
refractive index of the solvent.20 P-1 based on BSe has a low that Ni2+ can be quantitatively detected. According to this
uorescence quantum yield (FF ¼ 0.13) compared with P-2 equation and the standard deviation of the blank, the uores-
cence detection limit of P-1 for Ni2+ was determined to be 2.4
based on the BT polymer (FF ¼ 0.27), and the results are similar
to those previously reported,12m,18 which could be due to the
nM. The detection limit was sufficiently low to detect the
nanomolar concentrations of Ni2+ found in many chemical
fact that the narrow energy bandgap may increase the proba-
bility of singlet excitons decaying to the ground state in a
nonradiative way.12m
systems.
The uorescence-quenching efficiency of the P-1 sensor can
be described by the Stern–Volmer equation: F0/F ¼ 1 + KSV[Q]
and, herein, F0 and F are the uorescence intensities in the
absence and presence of a quencher, respectively. KSV is the
quenching constant and [Q] is the quencher concentration. At
Ni2+ concentrations from 0 to 5.0 mmol Lꢁ1, a good linear Stern–
Volmer plot of the uorescence intensity of P-1 versus the
concentrations of Ni2+ was obtained with a KSV value of 1.17 ꢂ
106 molꢁ1 L, as shown in Fig. 5.
Fluorescence-sensing properties of P-1 and P-2 toward Ni2+
We rst investigated the UV-vis absorption response behaviors
of P-1 and P-2 (see Fig. S1 and S2 in the ESI†) in CHCl3 toward
various metal ions in CH3CN, including K+, Pb2+, Cd2+, Co2+,
Ag+, Fe3+, Hg2+, Zn2+, Ni2+, Cu2+ and Mn2+. Although the addi-
tion of some metal ions to the polymer solution could cause
changes in the UV-vis absorption spectra of P-1 and P-2, these
two polymers could not show an outstanding response toward a
specic metal ion. And so, in this work, we mainly studied
the uorescence-sensing properties of these two polymers for
metal ions.
To determine the selectivity of the sensor toward Ni2+, the
uorescence response behavior of P-1 toward other important
To provide a potential “zero-wait” detection method for
metal ions, we rst investigated the uorescence-sensing
properties of P-1 upon the addition of various metal ions in real
time. All the measurements were obtained 10 s aer the addi-
tion of the metal salt to the host molecule solutions. Fig. 3
shows the uorescence spectra of P-1 (10.0 mmol Lꢁ1 in CHCl3)
upon the addition of Ni2+ in CH3CN and excitation at 425 nm.
Emission quenching upon the addition of Ni2+ was observed.
The emission quenching may be attributed to energy or electron
transfer between the metal complexes and the polymer back-
bone.6b The addition of Ni2+ led to 82.9% uorescence
quenching of P-1 at a concentration of 4.0 mmol Lꢁ1. Addi-
tionally, the yellow P-1 uorescence disappeared aer the
addition of Ni2+, and this was easily detected by the naked eye
Fig. 4 Calibration curve obtained from a plot of the fluorescence
intensity of P-1 with added Ni2+ from 0 to 5.0 mmol Lꢁ1. Inset: linear
(Fig. 3, inset). Notably, the uorescence intensity changed plot obtained at low concentrations of Ni2+ (0 to 1.8 mmol Lꢁ1).
7406 | J. Mater. Chem. C, 2014, 2, 7402–7410
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
Fig. 7 Metal specificity: the concentration of P-1 was 10.0 mmol Lꢁ1
The concentration of Ni2+ and the other metal ions was 4.0 mmol Lꢁ1
.
.
Fig. 5 Stern–Volmer plot of P-1 emission upon quenching by Ni2+
(0 to 5.0 mmol Lꢁ1).
Mix: mixture of Co2+, Cd2+, Hg2+, Fe3+, Ag+, Cu2+, Zn2+, Mn2+, K+ and
Pb2+
.
metal ions at a concentration of 4.0 mmol Lꢁ1 was determined
(Fig. 6). The degree of uorescence quenching (1 ꢁ F/F0) of P-1
We also investigated the possible use of P-2 for the deter-
mination of Ni2+ (Fig. 8). We found that Ni2+ resulted in 83.8%
uorescence quenching at a concentration of 14.0 mmol Lꢁ1. We
obtained a linear equation for the uorescence intensity of P-2
was no more than 10% by other metal ions including K+, Pb2+
,
Cd2+, Co2+, Ag+, Fe3+, Hg2+, Zn2+ and Mn2+. Fluorescence
quenching of the P-1 solution was observed upon the addition
of Cu2+, but the degree of uorescence quenching (30%) was
much smaller than that by Ni2+ ions. These results indicate
that P-1 shows good selectivity toward Ni2+ sensing compared
with other ions. For effective Ni2+ detection, another essential
requirement is that other metal ions exhibit no or little
interference. Competition experiments were performed on the
P-1 solution in the presence of 4 mmol Lꢁ1 of other metal ions
(Fig. 7). As shown in Fig. 7, the deviation in uorescence
quenching in the presence of other metals ions was less than
6%. These results conrm the ability of P-1 to sense Ni2+ with
high selectivity even in the presence of other metal ions. The
sensitivity and selectivity of P-1 may be ascribed to the
building block receptors from the BSe and triazole units in the
main-chain backbone. These t well, leading to the formation
of a more stable Ni2+–polymer complex compared with other
metal ions.
Fig. 8 (a) Fluorescence spectra of P-2 (10.0 mmol Lꢁ1 in CHCl3) with
increasing concentrations of Ni2+ (0, 1.2, 1.8, 2.4, 3.0, 4.0, 6.0, 8.0,
10.0, 14.0, 17.0, 20.0 mmol Lꢁ1 in CH3CN) (lex ¼ 408 nm). Inset: visible
fluorescence of P-2 before (left) and after (right) the addition of Ni2+
(20.0 mmol Lꢁ1) under a 365 nm UV lamp. (b) Fluorescence intensity of
Fig. 6 Relative fluorescence quenching of P-1 (10.0 mmol Lꢁ1) in the P-2 versus Ni2+ concentration from 0 to 20.0 mmol Lꢁ1. Inset: linear
presence of various metal ions (each 4.0 mmol Lꢁ1).
plot obtained at low concentrations of Ni2+ (0 to 6.0 mmol Lꢁ1).
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 7402–7410 | 7407

View Article Online
Journal of Materials Chemistry C
Paper
vs. Ni2+ concentration in the range of 0 to 6.0 mmol Lꢁ1 Ni2+: F ¼
1.10 ꢂ 105 to 1.29 ꢂ 1010[Ni2+] (Fig. 8b, inset). The limit of
detection was determined to be 11 nM. Furthermore, the KSV
value of P-2 was calculated to be 3.43 ꢂ 105 molꢁ1 L from the
Stern–Volmer plot, as shown in Fig. 9a. Additionally, the results
of the metal-binding properties of P-2 toward the other metal
ions showed that Co2+, Hg2+ and Cu2+ could signicantly
quench P-2 uorescence, and the quenching ratios were 57.9,
50.0 and 70.0% at the same concentrations as those used for
Ni2+, respectively (Fig. 9b). According to these results, it can be
concluded that P-1 has higher sensitivity and selectivity toward
Ni2+ detection than P-2.
Fig. 10 Possible P-1 nickel ion detection mechanism.
benzochalcogendiazole or triazole as the recognition unit,
which indicate that the individual BT11a,b,e, BSe11f or triazole6e,10
units tend to bind the mercury ion. According to Bunz and co-
workers,13 a binding pocket for Ni2+ is formed between the
nitrogen of the triazole and the nitrogen of the benzochalco-
gendiazole in the polymer backbone. The higher sensitivity and
selectivity of P-1 may be due to the lower electronegativity of
selenium compared with sulfur, which is favorable for an
adjustment of the aromaticity of the benzoselenadiazole cycle,
the size of the binding pocket and the binding capacity of
nitrogen.13
The stoichiometry for Ni2+ with P-1/P-2 can be determined on
the basis of the Benesi–Hildebrand expression:21 1/(F ꢁ F0) ¼
ꢁ1/{K(F0 ꢁ Fmin)[Ni2+]n} ꢁ 1/(F0 ꢁ Fmin), where Fmin is the
minimum uorescence intensity in the presence of a quencher,
and K is the associated constant. According to the uorescence
titration experiment data, the linear correlation coefficients of
Benesi–Hildebrand plots for Ni2+-bound P-1 and Ni2+-bound P-2
(see Fig. S3 and S4 in the ESI†) were 0.9958 and 0.9939 using the
Benesi–Hildebrand equation of linear tting based on 1 : 1 (n ¼
1), respectively. The results indicate that there is a 1 : 1 binding
mode between P-1/P-2 and Ni2+. In addition, the associated
constants obtained from the slope and intercept of the line were
2.03 ꢂ 106 molꢁ1 L for P-1 and 4.13 ꢂ 105 molꢁ1 L for P-2. A
possible mechanism for P-1 nickel ion detection is shown in
Fig. 10. We and others had reported several conjugated polymer
uorescent sensors for the selective detection of Hg2+ using
Conclusions
Two conjugated polymer uorescent sensors P-1 and P-2 using a
benzochalcogendiazole unit and a triazole unit as cooperative
receptors were synthesized by the CuAAC reaction. The result-
ing polymers show good thermal stability and solubility in
common organic solvents. P-1 and P-2 exhibit emission peaks at
535 nm and 514 nm and are characterized by yellow and green
uorescence, respectively. These two polymers show
outstanding uorescence response behavior toward Ni2+ in real-
time detection, and compared with BT-based P-2, the BSe-based
P-1 showed higher sensitivity and selectivity because of the
specic electronegativity of selenium. The quenching of P-1
uorescence gave a linear response to Ni2+ in the range of 0–1.8
mmol Lꢁ1, with a detection limit of 2.4 nM. The results indicate
that P-1 can be used as a highly sensitive and selective sensor for
the detection of Ni2+. This study opens up new opportunities for
the design of conjugated polymer uorescent sensors based on
the benzoselenadiazole unit.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21204066), the Commonwealth
Project of Science and Technology Department of Zhejiang
Province (no. 2012C23030) and the Bureau of Science and
Technology of Wenzhou (no. S20100007).
Notes and references
1 (a) D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev.,
2000, 100, 2537–2574; (b) S. W. Thomas, G. D. Joly and
T. M. Swager, Chem. Rev., 2007, 107, 1339–1386; (c)
H. N. Kim, Z. Guo, W. Zhu, J. Yoon and H. Tian, Chem. Soc.
Rev., 2011, 40, 79–93; (d) A. Alvarez, A. Salinas-Castillo,
Fig. 9 (a) Stern–Volmer plot of P-2 emission quenched by Ni2+ (0 to
20.0 mmol Lꢁ1). (b) Relative fluorescence quenching of P-2 (10.0 mmol
L
ꢁ1) in the presence of various metal ions (each 14.0 mmol Lꢁ1).
J. M. Costa-Fernandez, R. Pereiro and A. Sanz-Medel, TrAC,
´
7408 | J. Mater. Chem. C, 2014, 2, 7402–7410
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Trends Anal. Chem., 2011, 30, 1513–1525; (e) L. J. Fan,
Journal of Materials Chemistry C
8 J. J. Bryant and U. H. F. Bunz, Chem.–Asian J., 2013, 8, 1354–
1367.
9 Y. H. Lau, P. J. Rutledge, M. Watkinson and M. H. Todd,
Chem. Soc. Rev., 2011, 40, 2848–2866.
Y. Zhang, C. B. Murphy, S. E. Angell, M. F. L. Parker,
B. R. Flynn and W. E. Jones, Coord. Chem. Rev., 2009, 253,
410–422; (f) X. Feng, L. Liu, S. Wang and D. Zhu, Chem. Soc.
Rev., 2010, 39, 2411–2419; (g) H. Shi, H. Sun, H. Yang, 10 (a) Y. Wu, Y. Dong, J. Li, X. Huang, Y. Cheng and C. Zhu,
S. Liu, G. Jenkins, W. Feng, F. Li, Q. Zhao, B. Liu and
W. Huang, Adv. Funct. Mater., 2013, 23, 3268–3276; (h)
Chem.–Asian J., 2011, 6, 2725–2729; (b) L. Zheng, X. Huang,
Y. Shen and Y. Cheng, Synlett, 2010, 453–456.
H. Shi, X. Chen, S. Liu, H. Xu, Z. An, L. Ouyang, Z. Tu, 11 (a) S. J. Liu, C. Fang, Q. Zhao, Q. L. Fan and W. Huang,
Q. Zhao, Q. Fan, L. Wang and W. Huang, ACS Appl. Mater.
Interfaces, 2013, 5, 4562–4568; (i) H. Shi, S. Liu, Z. An,
H. Yang, J. Geng, Q. Zhao, B. Liu and W. Huang, Macromol.
Biosci., 2013, 13, 1339–1346; (j) H. F. Shi, S. J. Liu, H. B. Sun,
W. J. Xu, Z. F. An, J. Chen, S. Sun, X. M. Lu, Q. Zhao and
W. Huang, Chem.–Eur. J., 2010, 16, 12158–12167.
2 (a) Q. Zhou and T. M. Swager, J. Am. Chem. Soc., 1995, 117,
7017–7018; (b) Q. Zhou and T. M. Swager, J. Am. Chem.
Soc., 1995, 117, 12593–12602.
3 (a) W. M. Ashton, Nature, 1972, 237, 46–47; (b) C. Prophete,
E. A. Carlson, Y. Li, J. Duffy, B. Steinetz, S. Lasano and
J. T. Zelikoff, Fish Shellsh Immunol., 2006, 21, 325–334.
4 M. Dutta and D. Das, TrAC, Trends Anal. Chem., 2012, 32,
113–132.
5 (a) G. B. Li, H. C. Fang, Y. P. Cai, Z. Y. Zhou, P. K. Thallapally
and J. Tian, Inorg. Chem., 2010, 49, 7241–7243; (b) S. Ghosh,
R. Chakrabarty and P. S. Mukherjee, Inorg. Chem., 2009, 48,
549–556; (c) H. Wang, D. Wang, Q. Wang, X. Li and
C. A. Schalley, Org. Biomol. Chem., 2010, 8, 1017–1026; (d)
N. Aksuner, E. Henden, I. Yilmaz and A. Cukurovali, Sens.
Actuators, B, 2012, 166–167, 269–274; (e) W. Lin, L. Yuan,
Z. Cao, J. Feng and Y. Feng, Dyes Pigm., 2009, 83, 14–20; (f)
F. A. Abebe, C. S. Eribal, G. Ramakrishna and E. Sinn,
Tetrahedron Lett., 2011, 52, 5554–5558; (g) S. Maisonneuve,
Q. Fan and J. Xie, Tetrahedron, 2008, 64, 8716–8720; (h)
C. Ma, A. Lo, A. Abdolmaleki and M. J. MacLachlan, Org.
Lett., 2004, 6, 3841–3844; (i) Y. D. Fernandez,
A. P. Gramatges, V. Amendola, F. Foti, C. Mangano,
P. Pallavicini and S. Patroni, Chem. Commun., 2004, 1650–
1651; (j) Y. Xiao and X. Qian, Tetrahedron Lett., 2003, 44,
2087–2091; (k) I. Grabchev, J. M. Chovelon and X. Qian,
New J. Chem., 2003, 27, 337–340; (l) F. Bolletta, I. Costa,
L. Fabbrizzi, M. Licchelli, M. Montalti, P. Pallavicini,
L. Prodi and N. Zaccheroni, J. Chem. Soc., Dalton Trans.,
Macromol. Rapid Commun., 2008, 29, 1212–1215; (b)
X. B. Huang, J. Meng, Y. Dong, Y. X. Cheng and C. J. Zhu,
J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 997–1006; (c)
X. B. Huang, Y. Xu, L. F. Zheng, J. Meng and Y. X. Cheng,
Polymer, 2009, 50, 5996–6000; (d) J. Li, J. Meng,
X. B. Huang, Y. X. Cheng and C. J. Zhu, Polymer, 2010, 51,
3425–3430; (e) X. Ma, F. Y. Song, L. Wang, Y. X. Cheng and
C. J. Zhu, J. Polym. Sci., Part A: Polym. Chem., 2012, 50,
517–522; (f) D. Q. Li, H. Li, M. C. Liu, J. X. Chen,
J. C. Ding, X. B. Huang and H. Y. Wu, Macromol. Chem.
Phys., 2014, 215, 82–89.
12 (a) R. Yang, R. Tian, W. Yang, Q. Hou and Y. Cao,
Macromolecules, 2003, 36, 7453–7460; (b) R. Yang, R. Tian,
J. Yan, Y. Zhang, J. Yang, Q. Hou, W. Yang, C. Zhang and
Y. Cao, Macromolecules, 2005, 38, 244–253; (c) J. Hou,
T. L. Chen, S. Zhang, H. Y. Chen and Y. Yang, J. Phys.
Chem. C, 2009, 113, 1601–1605; (d) J. Hou, M. H. Park,
S. Zhang, Y. Yao, L. M. Chen, J. H. Li and Y. Yang,
Macromolecules, 2008, 41, 6012–6018; (e) T. Yasuda,
T. Imase and T. Yamamoto, Macromolecules, 2005, 38,
7378–7385; (f) J. Yang, C. Jiang, Y. Zhang, R. Yang,
W. Yang, Q. Hou and Y. Cao, Macromolecules, 2004, 37,
1211–1218; (g) S. Uchiyama, K. Kimura, C. Gota, K. Okabe,
K. Kawamoto, N. Inada, T. Yoshihara and S. Tobita, Eur. J.
Chem., 2012, 18, 9552–9563; (h) S. Das, P. B. Pati and
˙
S. S. Zade, Macromolecules, 2012, 45, 5410–5417; (i) M. Içli-
¨
¨
˙
Ozkut, H. Ipek, B. Karabay, A. Cihaner and A. M. Onal,
Polym. Chem., 2013, 4, 2457–2463; (j) G. Qian, H. Abu and
Z. Y. Wang, J. Mater. Chem., 2011, 21, 7678–7685; (k)
F. B. Koyuncu, A. R. Davis and K. R. Carter, Chem. Mater.,
2012, 24, 4410–4416; (l) G. L. Gibson, T. M. McCormick
and D. S. Seferos, J. Am. Chem. Soc., 2012, 134, 539–547;
(m) L. Chen, L. Wang, X. Jing and F. Wang, J. Mater.
Chem., 2011, 21, 10265–10267.
1999, 1381–1385; (m) N. Chattopadhyay, A. Mallick and 13 J. J. Bryant, B. D. Lindner and U. H. F. Bunz, J. Org. Chem.,
S. Sengupta, J. Photochem. Photobiol., A, 2006, 177, 55–60; 2013, 78, 1038–1044.
(n) S. C. Dodani, Q. He and C. J. Chang, J. Am. Chem. Soc., 14 S. J. Liu, Q. Zhao, R. F. Chen, Y. Deng, Q. L. Fan, F. Y. Li,
2009, 131, 18020–18021.
6 (a) Y. Zhang, C. B. Murphy and W. E. Jones, Macromolecules,
L. H. Wang, C. H. Huang and W. Huang, Chem.–Eur. J.,
2006, 12, 4351–4361.
2002, 35, 630–636; (b) J. Feng, Y. Li and M. Yang, J. Polym. 15 S. P. Dudek, M. Pouderoijen, R. Abbel, A. P. H. J. Schenning
Sci., Part A: Polym. Chem., 2009, 47, 222–230; (c) Y. Liu,
and E. W. Meijer, J. Am. Chem. Soc., 2005, 127, 11763–
S. Zhang, Q. Miao, L. Zheng, L. Zong and Y. Cheng,
11768.
Macromolecules, 2007, 40, 4839–4847; (d) X. Huang, Y. Xu, 16 D. Li, X. Wang, Y. Jia, A. Wang and Y. Wu, Chin. J. Chem.,
Q. Miao, L. Zong, H. Hu and Y. Cheng, Polymer, 2009, 50, 2012, 30, 861–868.
2793–2805; (e) X. Huang, Y. Dong, J. Meng, Y. Cheng and 17 B. A. D. Neto, A. S. Lopes, G. Ebeling, R. S. Goncalves,
C. Zhu, Synlett, 2010, 12, 1841–1844.
7 (a) A. Qin, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2010,
V. E. U. Costa, F. H. Quina and J. Dupont, Tetrahedron,
2005, 61, 10975–10982.
39, 2522–2544; (b) A. Qin, J. W. Y. Lam and B. Z. Tang, 18 D. Q. Li, H. Li, M. C. Liu, J. X. Chen, J. C. Ding, X. B. Huang
Macromolecules, 2010, 43, 8693–8702.
and H. Y. Wu, Polymer, 2013, 54, 6158–6164.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 7402–7410 | 7409

View Article Online
Journal of Materials Chemistry C
Paper
19 V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B. Sharpless,
Angew. Chem., Int. Ed., 2002, 41, 2596–2599.
20 W. Lin, L. Yuan, Z. Cao, J. Feng and Y. Feng, Dyes Pigm., 2009,
83, 14–20.
21 (a) H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949,
71, 2703–2707; (b) J. Wen, L. Dong, J. Tian, T. Jiang,
Y. Q. Yang, Z. Huang, X. Q. Yu, C. W. Hu, S. Hu, T. Z. Yang
and X. L. Wang, J. Hazard. Mater., 2013, 263, 638–642; (c)
Y. Zhang, G. Wang and J. Zhang, Sens. Actuators, B, 2014,
200, 259–268; (d) Z. H. Xu, L. Zhang, R. Guo, T. Xiang,
C. Wu, Z. Zheng and F. Yang, Sens. Actuators, B, 2011, 156,
546–552; (e) S. R. Liu and S. P. Wu, J. Fluoresc., 2011, 21,
1599–1605.
7410 | J. Mater. Chem. C, 2014, 2, 7402–7410
This journal is © The Royal Society of Chemistry 2014