Synthesis of water-soluble, fluorescent, conjugated polybenzodiazaborole for detection of cyanide anion in water

Article abstract of DOI:10.1016/j.polymer.2013.05.002

A polymer sensor (P1) containing benzodiazaborole moieties for recognition of cyanide anion and sulfonate groups for water-solubility was synthesized by Suzuki cross-coupling polymerization, which enables cyanide detection in aqueous solution. The polymer showed visible fluorescence quenching upon exposure to cyanide anion concentrations as low as ～2 μM. Selective detection of cyanide anions can be accomplished via fluorescence quenching, while other anions exhibited fluorescence enhancement upon exposure, mainly due to the difference in the nucleophilicity of anions, affecting benzodiazaborole group.

Full text of DOI:10.1016/j.polymer.2013.05.002

Polymer 54 (2013) 3542e3547
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of water-soluble, fluorescent, conjugated
polybenzodiazaborole for detection of cyanide anion in water
1
1
*
Ji Hye Son , Geunseok Jang , Taek Seung Lee
Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and Textile System Engineering, Chungnam National
University, Daejeon 305-764, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A polymer sensor (P1) containing benzodiazaborole moieties for recognition of cyanide anion and sul-
fonate groups for water-solubility was synthesized by Suzuki cross-coupling polymerization, which
enables cyanide detection in aqueous solution. The polymer showed visible fluorescence quenching upon
Received 6 February 2013
Received in revised form
27 April 2013
Accepted 1 May 2013
Available online 9 May 2013
exposure to cyanide anion concentrations as low as w2 mM. Selective detection of cyanide anions can be
accomplished via fluorescence quenching, while other anions exhibited fluorescence enhancement upon
exposure, mainly due to the difference in the nucleophilicity of anions, affecting benzodiazaborole group.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Conjugated polymers
Sensors
Fluorescence
1. Introduction
detection. Along with the aforementioned methods of cyanide
detection, electron-deficient boron-containing compounds have
Recognition and sensing systems for anions have received a
great deal of interest because of their widespread use in biological,
chemical, and environmental processes [1]. Among various anions,
the cyanide anion is one of the most detrimental due to its toxicity
toward biological and environmental systems [2]. The cyanide
anion is extremely dangerous and can be absorbed through the
lungs, skin, and gastrointestinal tract, leading to vomiting, loss of
consciousness, and eventual death [3]. Because of the extreme
toxicity of cyanide, the maximum contaminant level for free cya-
nide in drinking water should not exceed 2.7 ꢀ 10ꢁ6 M according to
the World Health Organization (WHO) [4].
As a result, a variety of methods for cyanide detection have
been reported using colorimetric and fluorometric detection
methods. Various approaches related to the cyanide-complexes
with transition metals [5aed], CdSe quantum dots [5e,f], boronic
acid derivatives [5g,h], and gold nanoclusters [5i,j]. Nucleophilic
attack of cyanide to electron-deficient benzothiadiazole [6a],
pyrylium salt [6b], oxazine [6c], acyl triazene [6d], acridinium [6e],
and salicylaldehyde [6f,g] have been developed for cyanide
proven to be highly efficient in colorimetric and fluorescent cya-
nide detection [7aed]. Anion recognition in the colorimetric and
fluorescence response can be also dictated by the basicity of the
anions, working on amino (NeH) groups such as urea moieties
[7eeg].
It is known that conjugated polymers have remarkable
efficiencies of signal amplification compared with the weak sig-
nals of their corresponding monomers, due to
pep* transitions of
p
-conjugated properties [8]. In general, conjugated polymers are
hydrophobic in nature due to their rigid main chain, and thus,
water-soluble conjugated polymers can be obtained by the
incorporation of polar functional groups such as carboxyl groups,
sulfonates, phosphonates, and quaternary ammonium salts onto
the polymer backbone [9]. Water-soluble conjugated polymers,
i.e. conjugated polyelectrolytes are being intensively studied for
applications in biosensors or chemical sensors in an aqueous
environment, because of their detection capabilities for DNA,
proteins, and other biologically and chemically relevant molecules
[10].
As mentioned above, most of the developments of cyanide
sensors are focused on small molecules, while polymeric sensory
materials are rarely reported. Furthermore, the majority of polymer
sensory materials for cyanide anion are not soluble in water due to
their organic and hydrophobic nature, preventing their application
to aqueous samples [11].
* Corresponding author. Tel.: þ82 42 821 6615.
E-mail address: tslee@cnu.ac.kr (T.S. Lee).
URL: http://www.onom.re.kr
1
These authors contribute equally.
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.05.002

J.H. Son et al. / Polymer 54 (2013) 3542e3547
3543
Herein, we report a rational design of a water-soluble conju-
gated polymer for the detection of cyanide anions via turn-off
mode based on the interaction with benzodiazaborole group,
which is composed of electron-deficient boron and acidic NeH
units, which will be deprotonated by cyanide attack. Sulfonate
groups were introduced in the side chains of the conjugated
polymer to enhance water-solubility, thus making this design
applicable to aqueous samples. The detection mechanism in-
volves the fluorescence quenching of P1 in the presence of cya-
nide anions, presumably due to the combined interactions both
between electron deficient boron and nucleophilic cyanide anions
[5h,7aed,11a,b,12] and between acidic amines and basic cyanide
anions [7f,g]. To our knowledge, this is the first report to exploit
both effects in cyanide anion sensing. As a result, the newly
designed polymer could interact with cyanide anions with a high
degree of selectivity (even in the presence of other anions) and
vacuo. Yield 3.95 g (90.8%). 1H NMR (300 MHz, DMSO-d6):
d
7.43 (s,
2H), 4.00 (t, 4H), 3.08e3.03 (t, 4H), 1.78e1.69 (m, 8H) ppm. FT-IR
(KBr, cmꢁ1): 3063 (CeH), 1479 (C]C), 1399 (SO3), 586 (CeBr).
2.3.3. 4,7-Dibromo-2,1,3-benzothiadiazole (3)
2,1,3-Benzothiadiazole (1 g, 73.7 mmol) was dissolved in 47%
hydrobromic acid (10 mL) in a 50 mL three-necked round-
bottomed flask. A solution of bromine (1.14 mL, 22.02 mmol) in 47%
hydrobromic acid (10 mL) was added dropwise into the solution.
The mixture was refluxed and stirred for 4 h. Precipitates were
isolated by filtration washed with water. The product was dried in
vacuo. Yield 1.57 g (72.56%). 1H NMR (300 MHz, CDCl3):
d 7.73 (s,
2H) ppm. FT-IR (KBr, cmꢁ1): 3077e3046 (CeH), 1870 (C]N), 1479
(C]C), 586 (CeBr). Anal. Calcd for (C6H2Br2N2S): C, 24.5; H, 0.69; N,
9.53; S, 10.9. Found: C, 24.3; H, 0.70; N, 9.21; S, 11.0.
sensitivity (w2
m
M) based on the change in their fluorescence
2.3.4. 3,6-Dibromobenzene-1,2-diamine (4)
intensity.
3 (1 g, 3.40 mmol) was dispersed in ethanol (33 mL) in a 100 mL
round-bottomed flask. Sodium borohydride (2.4 g, 63.4 mmol) was
added portionwise at 0 ꢂC. The mixture was stirred at room tem-
perature for 20 h. After evaporating the solvent under vacuum, the
solid product was dissolved in ether. The ether solution was washed
with water several times, dried on sodium sulfate, and was finally
removed by evaporation. The product was dried in vacuo. Yield 0.3 g
2. Experimental
2.1. Materials
All the chemicals were purchased from Aldrich and Acros
and were used without further purification. 2,5-Dibromobenzene-
1,4-diol (1) [13], 1,4-dibromo-2,5bis(4-sulfonatobutoxy) benzene
sodium salt (2) [14], 4,7-dibromo-2,1,3-benzothiadiazole (3) [15],
3,6-dibromobenzene-1,2-diamine (4) [16], and 4,7-dibromo-2,3-
dihydro-2-phenyl-1H-benzo[d] [1,3,2]diazaborole (5) [12] were
synthesized according to previously published methods.
(33%). 1H NMR (300 MHz, CDCl3):
d 7.27 (2H), 4.0 (4H) ppm. FT-IR
(KBr, cmꢁ1): 3385 (NH2), 3079 (CeH), 1648 (NeH), 1452 (C]C),
1240 (CeN). Anal. Calcd for (C6H6Br2N2): C, 27.1; H, 2.27; N, 10.53.
Found: C, 27.5; H, 2.32; N, 10.4.
2.3.5. 4,7-Dibromo-2,3-dihydro-2-phenyl-1H-benzo[d] [1,3,2]
diazaborole (5)
2.2. Characterization
4 (0.3 g, 1.13 mmol) and phenylboronic acid (0.17 g, 1.356 mmol)
were dissolved in toluene (8 mL) in a 50 mL three-necked round-
bottomed flask. The mixture was refluxed and stirred for 3 days.
After removing the solvent by evaporation under vacuum, a solid
product was crystallization from hexane. The product was dried in
The 1H, 13C, and 5B NMR spectra were obtained using a Bruker
DRX-300 spectrometer with tetramethylsilane as an internal
standard (Korea Basic Science Institute). The FT-IR spectra were
obtained from a Bruker Tensor 27 spectrometer. The UV-vis ab-
sorption spectra were recorded on a PerkinElmer Lambda 35
spectrometer. Elemental analysis was performed on an Elemental
Analyzer EA 1108 (Fisons Instruments). The photoluminescence
spectra were obtained from a Cary Eclipse (Varian) fluorescence
spectrophotometer utilizing a xenon lamp as the excitation
source.
vacuo. Yield 0.2 g (50%). 1H NMR (300 MHz, DMSO-d6):
d 8.02
(2H, s), 7.95 (2H, s), 7.77 (2H, d), 7.38 (1H, m), 7.32 (2H, t) ppm. 5B
NMR (DMSO-d6):
1870 (C]N), 1479 (C]C), 1420 (BeN), 586 (CeBr).
d
ꢁ3.50 ppm. FT-IR (KBr, cmꢁ1): 3405 (NeH),
2.4. Polymerization of P1
2 (0.2109 g, 0.361 mmol), 5 (0.0317 g, 0.09 mmol), and 1,4-
benzenediboronic acid bis(pinacol) ester (0.1489 g, 0.451 mmol)
were dissolved in a mixture of DMF (6 mL) and 2 M K2CO3 aqueous
solution (9 mL) in a three-necked round-bottom flask. After
addition of tetrakis(triphenylphosphine)palladium(0) (0.0266 g,
0.023 mmol) as a catalyst, the reaction mixture was stirred at 90 ꢂC
for 48 h under argon. After the reaction, the reaction mixture was
cooled, poured into acetone (500 mL), and the precipitates were
then isolated by filtration. Finally, the polymer was purified
through dialysis against water (Millipore NanopureÔ) using a 12.4
kD MWCO cellulose membrane for 3 days. After the dialysis, the
polymer solution was freeze-dried and a dark yellow solid was
2.3. Synthesis
2.3.1. 2,5-Dibromohydroquinone (1)
Hydroquinone (10 g, 90.8 mmol) was dissolved in acetic acid
(80 mL) in a 500 mL three-necked round-bottomed flask. A solution
of Br2 (9.34 g, 181.6 mmol) in acetic acid (30 mL) was added
dropwise to the hydroquinone solution. The mixture was stirred at
room temperature for 4 h after which water was added. The pre-
cipitate was isolated by filtration, and was crystallized from the
aqueous solution. Yield 7.14 g (29%). 1H NMR (300 MHz, DMSO-d6):
d
9.82 (s, 2H), 7.04 (s, 2H) ppm. FT-IR (KBr, cmꢁ1): 1482 (C]C), 581
(CeBr).
obtained. 1H NMR (300 MHz, DMSO-d6):
d 8.0 (s, amine group),
7.8e6.9 (m, phenyl group), 4.1e3.6 (m, alkyl group), 3.1e2.5 (m,
2.3.2. 2,5-Dibromo-1,4-bis(4-sulfonatobutoxy)phenylene sodium
salt (2)
alkyl group), 2.0e1.6 (m, alkyl group) ppm. 13C NMR (75 MHz,
DMSO-d6):
d 159.67, 151.69, 141.25, 136.10, 135.69, 130.87, 127.46,
Sodium hydroxide (0.895 g, 22.4 mmol) and 1 (2 g, 7.46 mmol)
were dissolved in ethanol (17.5 mL) in a 250 mL three-necked
round-bottomed flask. After the addition of 1,4-butane sultone
(2.3 mL, 22.4 mmol), the reaction mixture was stirred and refluxed
for 20 h. After the reaction, the crude product was isolated by
filtration and was washed with ethanol. The product was dried in
123.63, 120.14, 107.78, 55.35, 55.26, 25.38, 24.33, 24.10, 22.94,
22.88 ppm. 5B NMR (DMSO-d6):
d
ꢁ3.54 ppm. FT-IR (KBr, cmꢁ1):
3405 (NeH), 3063 (CeH), 1870 (C]N), 1479 (C]C), 1420 (BeN),
1399 (SO3),. Anal. Calcd. (%) for C19.6H21.5N0.33S1.67O5.0K1.67: C,
51.01; H, 4.69; N, 0.99; S, 11.56; Found: C, 50.00; H, 4.58; N, 1.0;
S,10.98.

3544
J.H. Son et al. / Polymer 54 (2013) 3542e3547
3. Results and discussion
250
200
150
100
50
0.4
3.1. Synthesis and photophysical properties
The synthetic procedures for the preparation for monomers
and the polymer P1 are illustrated in Scheme 1. 1,4-Dibromo-2,5-
bis(4-sulfonatobutoxy)benzene sodium salt (2) was synthesized
according to the previously published methods [13]. 4,7-Dibromo-
2,1,3-benzothiadiazole (3) was easily obtained via bromination of
commercially available 2,1,3-benzothiadiazole with a good yield
[15]. 4,7-Dibromo-2,3-dihydro-2-phenyl-1H-benzo[d] [1,3,2]dia-
zaborole (5) was obtained via the addition of phenylboronic acid
into 4, in toluene [12]. P1 was synthesized with corresponding
monomers via the palladium-catalyzed Suzuki cross-coupling re-
action. Monomer 5 was used in this experiment to obtain 100%
diazaborole units in the polymer, instead of the more common
and simple usage of post-functionalization. According to the
previous published method of post-functionalization with phe-
nylboronic acid to a polymer, it was reported that only 73e85% of
the transformation of diamino units to diazaborole units could be
attained [12].
0.3
0.2
0.1
0.0
0
300
400
500
600
700
Wavelength (nm)
Fig. 1. Absorption (dotted) and fluorescence (solid) spectra of P1 in aqueous solution
([P1] ¼ 1 ꢀ 10ꢁ6 M for absorption spectrum). Excitation wavelength 323 nm.
The molar composition in P1 was determined by elemental
analysis, and was found to be 0.18:0.82 as a mole fraction (m and n).
The chemical structure of the polymer was confirmed by 1H, 13C,
and 5B NMR spectra.
attempted, but it was not possible to obtain a feasible data
[10h,i,17]. However, it should be noted that the molecular weight of
P1 should be more than 12,400 because P1 was dialyzed using a
membrane of 12,400 cutoff.
Due to the large amounts of sulfonate groups, P1 shows good
solubility in water. However, it was not possible to obtain a reliable
molecular weight of P1. Determination of molecular weight of P1
by gel permeation chromatography (GPC) using an aqueous
mobile phase was not possible, because of the adsorption of the
polymer to the polystyrene gel in the GPC column. Matrix-assisted
laser desorption/ionization time of flight (MALDI-TOF) was also
The optical absorption and fluorescence emission spectra of
the polymers in aqueous solution are shown in Fig. 1. P1 has an
absorption maximum at 323 nm in aqueous solution. The polymer
exhibits blue emission in aqueous solution under UV irradiation
and shows an emission maximum around 428 nm. The quantum
yield of P1 was found to be 3.4% in aqueous solution, which was
determined by comparing with rhodamine B as a standard.
SO3 Na+
-
O
O
O
S
OH
Br
O
OH
1,4-butane sultone
EtOH
Br2
AcOH
Br
Br
Br
HO
N
O
HO
2
1
+Na-O3S
B
S
S
HN
NH
H2N
NH2
Br
N
N
N
phenylboronic acid
Toluene
NaBH4
EtOH
Br
Br
Br
Br
Br
5
4
3
SO3 K+
-
B
HN
NH
O
O
O
Pd(0)
B
B
2
+
5
+
O
K2CO3, DMF
O
n
m
O
+K-O3S
P1
Scheme 1. Synthetic routes for monomers and polymer.

J.H. Son et al. / Polymer 54 (2013) 3542e3547
3545
0.6
0.4
0.2
0.0
200
150
100
50
0
300
400
500
600
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
(a)
(b)
200
150
100
50
0
0
1
2
3
-
log [CN /μM]
(c)
Fig. 2. Changes in absorbance (a) and emission (b) spectra of P1 (1.5 ꢀ 10ꢁ5 M) with cyanide concentration of 0 (C); 1.0 ꢀ 10ꢁ5
(
); 1.0 ꢀ 10ꢁ4 (A); 1.0 ꢀ 10ꢁ3 M (9) in aqueous
7
solution; (c) changes in the fluorescence intensity of P1 at 428 nm according to the concentration of cyanide anion.
3.2. Cyanide sensing properties
The effect of the cyanide anion on the UV-vis absorption spectra
is shown in Fig. 2a. As can be seen from the spectra, there were
negligible changes in the absorbance and in the absorption
maximum upon the addition of cyanide anions to the P1 solution.
In contrast, progressive emission quenching of P1 at 428 nm was
observed in the fluorescence spectra after the addition of cyanide
anions as shown in Fig. 2b and c. A linear relationship between the
intensity changes and logarithmic concentrations of cyanide anion
The interaction of P1 and cyanide anions was investigated in
aqueous media under ambient condition. The concentrations of P1
was 1.5 ꢀ 10ꢁ6 M. Standard solutions of tetrabutylammonium
cyanide with various concentrations of 1.0 ꢀ 10ꢁ7 to 1.0 ꢀ 10ꢁ3
M
were added to the P1 solution. Changes in the optical properties of
P1 were examined in 10 mm quartz cuvette.
Fig. 3. Partial 1H (left) and 5B NMR (right) spectra of 5 (a) and (b); 5 þ CNꢁ (2 equiv) (c) and (d) in DMSO-d6.

3546
J.H. Son et al. / Polymer 54 (2013) 3542e3547
can be observed with the naked-eye (inset photographs in Fig. 2c)
under UV irradiation. The limit of detection (LOD) was determined
The crucial feature of P1 lies in its high selectivity toward cya-
nide anion over the other competitive anions. Changes in the
fluorescence spectra of P1 (1.5 ꢀ 10ꢁ5 M) are completely different
between cyanide and other anions such as Fꢁ, Brꢁ, Clꢁ, Iꢁ, AcOꢁ, and
POꢁ4 in aqueous solution. As can be seen in Fig. 4, the competitive
anions show noticeable enhancement in fluorescence intensity at
428 nm, while only cyanide anion lead to quenching the fluores-
cence of P1. This phenomenon can be clearly seen by measuring the
relative intensity changes in Fig. 4b in the presence of cyanide and
of other anions. It is likely that the alteration in hydrogen bonding
between the electron-rich amine and anions is responsible for the
increase in the emission intensities when P1 was exposed to Fꢁ,
Brꢁ, Clꢁ, Iꢁ, AcOꢁ, and POꢁ4 [19].
The high selectivity of P1 is much highlighted by competition
experiments in mixed anions (cyanide þ anion). In the presence of
other competitive anions, cyanide can still quench the fluorescence
of P1, meaning there is no interference by other anions (Fig. 5a).
This negligible interference from other anions suggests that the P1
has significant selectivity toward cyanide anion. Judging from less
to be about 2
method [18].
mM, as estimated according to a previously reported
1H and 5B NMR titrations of monomer 5 were carried out upon
addition of 2 equiv of cyanide anion to elucidate the deprotonation
of NeH and electronic perturbation in boron atoms as shown in
Fig. 3. It was possible to observe both disappearance of NeH peak at
8.02 ppm and chemical shift of boron peak (ꢁ3.50 to ꢁ3.95 ppm) in
the presence of cyanide anion in DMSO-d6 solution of 5. Based on
these observations, it is presumed that the fluorescence quenching
was caused by the combined effects of electronic perturbation of
boron atoms as well as deprotonation of NeH in the presence of
cyanide anion.
300
P1
-
P1+ CN
200
-
-
-
-
P1+ CN + F
P1+ CN + Cl
-
-
P1+ CN + PO
4
-
-
100
P1+ CN + I
-
-
P1+ CN + Br
350
450
500
400
550
-
-
P1+ CN + AcO
0
400
450
500
550
600
650
700
Wavelength (nm)
(a)
-
CN + anion
anion alone
6
4
2
0
-2
-
-
-
-
-
-
-
CN
F
Cl
Br
I
AcO
PO4
Anions
(b)
Fig. 4. (a) Changes in the emission spectra, (b) relative emission changes, and (c)
Fig. 5. (a) Changes in the fluorescence and (b) relative intensity of P1 in aqueous
photographs of P1 (1.5
ꢀ
10ꢁ5 M) in aqueous solution by addition of anions
solution (1.5
ꢀ
10ꢁ5 M) with mixed anions ([anion]
¼
1.0
ꢀ
10ꢁ5 M;
(1.0 ꢀ 10ꢁ5 M). Excitation wavelength 323 nm. (
DI ¼ I0 ꢁ I, I0 ¼ initial intensity of P1
[CNꢁ] ¼ 1.0 ꢀ 10ꢁ5 M). Excitation wavelength 323 nm (
D
I ¼ I0 ꢁ I, I0 ¼ initial intensity
solution at 428 nm, I ¼ intensity of P1 þ anion at 428 nm).
of P1 at 428 nm and I ¼ intensity of P1 þ anions at 428 nm).

J.H. Son et al. / Polymer 54 (2013) 3542e3547
[6] (a) Qian G, Li X, Wang ZY. J Mater Chem 2009;19:522e30;
3547
than 10% interference by other anions on the emission intensity,
this enables P1 to use as a highly selective material toward cyanide
anion (Fig. 5b).
(b) García-Acosta B, Martínez-Máñez R, Sancenón F, Soto J. Chem Commun
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A new water-soluble conjugated polybenzodiazaborole based
on boron moieties with sulfonate side chains was synthesized by
Suzuki coupling polymerization for the creation of turn-off fluo-
rescence sensor for cyanide anions. The conjugated polymer can
interact with cyanide anions leading to fluorescence quenching,
which was attributed to the interaction with electron-deficient
boron atoms and cyanide anions. In contrast, anions other than
cyanide had obvious response signals of fluorescence enhance-
ment. This reveals that P1 has good selectivity towards the cyanide
anions based on the efficient fluorescence quenching. This specific
approach enables us to attain a versatile tool for detecting cyanide
anions. The newly synthesized water-soluble polymer is suitable
for the sensitive and selective determination of cyanide anions.
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Acknowledgments
This research was supported by Basic Science Research Program
(2012R1A2A2A01004979) and Nuclear R&D Project through the
National Research Foundation of Korea (NRF) funded by the Korean
government.
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