A glucose-selective fluorescent water-soluble hyperbranched polymer sensor with boronic acid end groups

Article abstract of DOI:10.1080/15421401003598090

We have synthesized water-soluble hyperbranched poly(p-phenylene)s having boronic acid groups capable of reacting with diol-containing saccharides. The polymerization was carried out by palladium-catalyzed Suzuki cross-coupling reaction using tribromo monomer for the preparation of hyperbranched type structure. The water-soluble hyperbranched polymer HP exhibited much higher sensitivity toward glucose compared with the linear polymer LP presumably due to the difference in the number of the boronic acids which are present in the end groups of the polymers.

Full text of DOI:10.1080/15421401003598090

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A Glucose-Selective Fluorescent Water-
Soluble Hyperbranched Polymer Sensor
With Boronic Acid End Groups
Dai Geun Kim a , Minjung Lee b & Taek Seung Lee c
a Organic and Optoelectronic Materials Laboratory, Department of
Nanotechnology , Chungnam National University , Daejeon, Korea
b Department of Oriental Medicine and Food Biotechnology ,
Joongbu University , Chubu-myon, Kumsan-gun, Chungnam, Korea
c Organic and Optoelectronic Materials Laboratory, Department of
Advanced Organic Materials and Textile System Engineering and
Graduate School of Analytical Science and Technology (GRAST),
Chungnam National University , Daejeon, Korea
Published online: 19 Apr 2010.
To cite this article: Dai Geun Kim , Minjung Lee & Taek Seung Lee (2010) A Glucose-Selective
Fluorescent Water-Soluble Hyperbranched Polymer Sensor With Boronic Acid End Groups, Molecular
Crystals and Liquid Crystals, 519:1, 54-61, DOI: 10.1080/15421401003598090
To link to this article: http://dx.doi.org/10.1080/15421401003598090
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Mol. Cryst. Liq. Cryst., Vol. 519: pp. 54–61, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003598090
A Glucose-Selective Fluorescent Water-Soluble
Hyperbranched Polymer Sensor With Boronic
Acid End Groups
DAI GEUN KIM,1 MINJUNG LEE,2
AND TAEK SEUNG LEE3
1Organic and Optoelectronic Materials Laboratory, Department of
Nanotechnology, Chungnam National University, Daejeon, Korea
2Department of Oriental Medicine and Food Biotechnology, Joongbu
University, Chubu-myon, Kumsan-gun, Chungnam, Korea
3Organic and Optoelectronic Materials Laboratory, Department of
Advanced Organic Materials and Textile System Engineering and
Graduate School of Analytical Science and Technology (GRAST),
Chungnam National University, Daejeon, Korea
We have synthesized water-soluble hyperbranched poly(p-phenylene)s having
boronic acid groups capable of reacting with diol-containing saccharides. The
polymerization was carried out by palladium-catalyzed Suzuki cross-coupling
reaction using tribromo monomer for the preparation of hyperbranched type struc-
ture. The water-soluble hyperbranched polymer HP exhibited much higher sensi-
tivity toward glucose compared with the linear polymer LP presumably due to the
difference in the number of the boronic acids which are present in the end groups
of the polymers.
Keywords Fluorescence; glucose sensor; water-soluble hyperbranched polymer
Introduction
In the past decade, one of the important challenges in the management of diabetes is
the monitoring the glucose concentrations [1]. The most commonly used technology
for blood glucose concentration determination is an enzyme-based method, which
requires frequent collection of blood samples [2]. However, use of enzyme shows
some practical limitations in the development of implantable sensors for continuous
glucose monitoring in blood or in interstitial tissue.
The boronic acids have been known for a few decades for their ability to interact
with diol-containing compounds such as carbohydrates [3,4]. They have been used
for the development of receptor and fluorescent probes for sugars [5,6]. The main
advantage of using boronic acid as ligand group for sugars are the fast and reversible
Address correspondence to Taek Seung Lee, Organic and Optoelectronic Materials
Laboratory, Department of Advanced Organic Materials and Textile System Engineering,
Chungnam National University, Daejeon 305-764, Korea. E-mail: tslee@cnu.kr
54

Hyperbranched Polymer for Glucose Sensing
55
interaction with sugars [7,8]. Besides, many substituted phenylboronic acids are
commercially available allowing the development of a variety of synthetic fluorescent
probes for sugars with slight modification. Different mechanisms have been
employed to induce spectral change following the interaction of the boronic acid and
the sugars.
Hyperbranched polymers have also been employed to reduce intermolecular
interaction because of their highly branched and globular molecular structures [9].
Hyperbranched structure is of greater advantage over its linear counterpart in high
solubility, easiness in processing, and minimization of unfavorable intermolecular
interactions and crystallization [10]. Furthermore, by controlling the nature of the
peripheral groups in hyperbranched polymers, it is possible to endow them with
functions such as enhanced adhesion and energy harvesting as well as biomolecular
sensing characteristics.
Recently water-soluble conjugated polymers have received a great deal of
attention due to their unique optoelectronic properties, which may serve as a basis
for a new generation of optoelectronic devices and biochemical detection [11–13].
Their special characteristics stems from the combination of optoelectronic proper-
ties of conventional polymers and water solubility induced by ionic nature of
polyelectrolytes [14,15].
In this work, we are reporting the first water-soluble hyperbranched poly(p-
phenylene) containing boronic acid that showed high selectivity and the significant
fluorescence intensity changes within the range of physiological glucose concen-
tration. The number of glucose-binding ligand, boronic acid was maximized through
the synthesis of the hyperbranched polymer structure. The structure-sensing
property of the hyperbranched polymer was investigated in detail by comparison
with linear counterpart. The results demonstrated that the hyperbranched structure
is effective in enhancing sensitivity and selectivity toward glucose.
Experimental
Reagents and Instrumentation
1,4-Butane sultone, benzene-1,4-diboronic acid, 4-bromobenzaldehyde, tris(4-
bromophenyl)amine, and tetrakis(triphenylphosphine)palladium(0) were purchased
1
from Aldrich. All the chemicals were used without further purification. H NMR
spectra were obtained on a Bruker DRX-300 spectrometer (Korea Basic Science
Institute). The elemental analysis was determined with a CE Instruments EA-1110
elemental analyzer. FT-IR spectra were obtained from Mattson Genesis II
spectrometer. UV-vis absorption spectra were recorded on a PerkinElmer Lambda
35 spectrometer. Photoluminescence spectra were taken from a Varian Cary Eclipse
equipped with a xenon lamp excitation source.
Synthesis of 2,5-Dibromohydroquinone (1)
To a 500 ml round-bottomed flask were added 10 g (90.8 mmol) of hydroquinone
and 100 ml of acetic acid. 9.32 g (181.6 mmol) of Br2 in 10 ml of acetic acid was added
dropwise to the hydroquinone mixture. The mixture was stirred at room temperature
for 2 h and then water was added. Precipitate was isolated by filtration and crystal-
1
lized from water. Yield 8.71 g (35.8%). H NMR (300 MHz, DMSO-d6): d 9.84 (s,
2H), 7.04 (s, 2H) ppm.

56
D. G. Kim et al.
Synthesis of 1,4-Dibromobenzene-2,5-bis-4-butoxysulfonic acid (2)
To a 250 ml round-bottomed flask were added 1.96 g (49.09 mmol) of NaOH, 3.3 g
(12.27 mmol) of 1 and 150 ml of ethanol. 5 ml (49.09 mmol) of 1,4-butane sultone
was added dropwise. The mixture was refluxed for 20 h. After the reaction, the crude
product was filtered and washed with ethanol. The white solid was dried in vacuum.
To a 500 ml beaker were added 3 g of the white product and 90 ml of water. 35% HCl
was added dropwise to the mixture. The precipitate was formed and isolated by
filtration. The product was dried in vacuum. Yield 6.9 g (96%). 1H NMR
(300 MHz, D2O): d 7.43 (s, 2H), 4.18-4.14 (t, 4H), 3.08-3.03 (t, 4H), 2.00-1.96 (m,
8H) ppm. FT-IR (cmꢀ1): 719 (S–O), 1018 (aryl C–Br), 1204, 1358 (S O), 1489
=
=
(C C), 2936 (C–H), 3351 (OH). Anal. Calcd. For C14H20Br2O8S2: C, 31.12%; H,
3.73%; S, 11.87%. Found: C, 30.38%; H, 3.86%; S, 11.35%.
Synthesis of HP
To a 100 ml round-bottomed flask were added 1 g (1.85 mmol) of 2, 0.38 g
(2.31 mmol) of benzene-1,4-diboronic acid, 0.148 g (0.308 mmol) of tris(4-bromophe-
nyl)amine, 18 ml of 2 M aqueous Na2CO3 solution, and 30 ml of dry DMF under
nitrogen. After addition of tetrakis(triphenylphosphine)palladium(0) (5 mol %) as
a catalyst the mixture was heated up to 85ꢁC. The mixture was stirred for 40 h.
The reaction mixture was poured into acetone. Precipitate was redissolved in
deionized water and the solution was dialyzed using a membrane (3500 cutoff) for
3 days. The polymer was obtained after drying in freeze drier. Yield 0.41 g
1
(40.2%). H NMR (300 MHz, D2O): d 7.7-6.8 (m, 8H), 4.2-3.7 (m, 6H), 3.2-2.8 (t,
6H), 2.1-1.6 (d, 12H) ppm. FT-IR (cmꢀ1): 727 (S–O), 1155 (aryl C–N), 1379
=
=
(S O), 1611 (C C), 2944 (C–H), 3428 (OH). Anal. Found. For C14H20Br2O8S2:
C, 39.72%; H, 4.17%; S, 10.66%.
Synthesis of LP
To a 100 ml round-bottomed flask were added 0.5 g (0.825 mmol) of 2, 0.183 g
(1.11 mmol) of benzene-1,4-diboronic acid, 18 ml of 2 M aqueous Na2CO3 solution,
and 30 mL of dry THF under nitrogen. After addition of 5 mol% of Pd(0) catalyst
the mixture was heated up to 85ꢁC. Then the mixture was stirred for 48 h. The
reaction mixture was poured into acetone. Precipitate was redissolved in deionized
water and the solution was dialyzed using a membrane (3500 cutoff) for 3 days.
1
The polymer was obtained after drying in freeze drier. Yield 0.203 g (39.84%). H
NMR (300 MHz, D2O): d 7.6-6.7 (m, 6H), 4.1-3.5 (m, 4H), 3.2-2.8 (t, 4H), 2.1-1.6
(d, 8H) ppm. FT-IR (cmꢀ1): 725 (S–O), 1381 (S O), 1615 (C C), 2930 (CꢀH),
3430 (OH). Anal. Calcd.: C, 50.18%; H, 5.18%; S, 10.31%; Found: C, 49.15%; H,
5.48%; S, 10.48%.
=
=
Glucose Sensing Test
Monosaccharide sensing studies were carried out in a 10 mm quartz cuvette. The
fluorescence spectral changes in HP and LP (8 ꢂ 10ꢀ7 M) in the presence of each
monosaccharide were recorded in their 6 mM sodium phosphate buffer solutions
(pH 7.4).

Hyperbranched Polymer for Glucose Sensing
Results and Discussion
57
The synthetic routes for monomer and polymers are depicted in Figure 1. Monomers
1 and 2 for the preparation of the polymers were synthesized according to the litera-
ture procedures [16,17]. Poly(p-phenylene)s are typically prepared by palladium-
catalyzed Suzuki cross-coupling of diboronic acid and dibromo compound (plus
tribromo monomer for hyperbranched polymer, HP). For the preparation of control
LP, 2 and slight excess of 1,4-phenylenediboronic acid in the presence of catalytic
amount of Pd(0) were reacted in THF with aqueous sodium carbonate solution.
Figure 1. Synthetic routes for HP and LP.

58
D. G. Kim et al.
In the case of HP, the polymerization time was selected experimentally before
the appearance of precipitates under the same polymerization condition. HP
obtained before precipitation was still soluble in water. However, the product
precipitated was not soluble in water mainly due to its crosslinked structure. Thus
precise control of the reaction time and the content of tribromo monomer, tris(4-
bromophenyl)amine is essential to obtain water-soluble hyperbranched polymer.
LP showed good solubility in water even for the prolonged reaction time. HP and
1
1
LP were characterized by H NMR, IR, and elemental analysis. H NMR spectra
of HP and LP exhibited chemical shifts of phenylene group and alkylene group at
7.7-6.7 and 4.2-1.6 ppm, respectively, thus, which the polymers were successfully
synthesized.
The optical properties of the polymers were investigated with a spectrophot-
ometer and spectrofluorometer in 6 mM sodium phosphate buffer solution at
physiological pH of 7.4. Due to the presence of water-soluble sulfonic acid group
at the side chain of HP and LP, they showed good solubility in the buffer solution.
Their absorption and fluorescence spectra in the buffer solutions are shown in
Figure 2. The linear LP shows absorption band at 307 nm with a shoulder band
around 330 nm. However, the shoulder band was strengthened in the case of
HP, which exhibited kmax at 333 nm. This corresponds to the intensified push-pull
structure of HP, which had electron-donating triphenylamine group.
On the while, the fluorescence bands of the HP and LP are concentrated at
418 nm, indicating that the conjugation lengths of the emitting species are almost
identical. This should be attributed to effective excitation energy transfer from the
short conjugated species to the larger ones which are the main emitting species [18].
Fluorescence experiments were performed to evaluate the affinity of the polymer
for glucose. Specifically, 2 ml of the polymer aqueous solution (8 ꢂ 10ꢀ7 M, 6 mM
sodium phosphate buffer, pH 7.4) was mixed with aqueous saccharide solution in
the same buffer at various concentrations, and then the fluorescence intensity was
recorded (Fig. 3). Fluorescence intensity decreased upon exposure to the saccharides.
Figure 2. Absorption and emission spectra of LP (dotted, excited at 307 nm) and HP
(solid, excited at 333 nm) in aqueous solution (polymer concentration: 10ꢀ5 M for absorption
spectra).

Hyperbranched Polymer for Glucose Sensing
59
Figure 3. Fluorescence spectra of HP (8 ꢂ 10ꢀ7 M) upon addition of (a) glucose; (b) galactose;
(c) fructose (0, 6, 12, 25, 50, 100, and 200 mM) in 6 mM aqueous sodium phosphate buffer at
pH 7.4; kex ¼ 333 nm.
Among the saccharides investigated, HP showed highest selectivity for glucose over
fructose and galactose in terms of the fluorescence intensity changes. It needs to be
noted that the boronic acid has more affinity to fructose over glucose [3,19]. Sensory
HP represents an improvement of sensitivity for glucose over fructose compared
with fluorescence quenching.
Based on the complex formation between boronic acid and diol groups in
glucose, the hyperbranched HP offers
a great advantage over a linear
counterpart LP, because a number of end groups (boronic acid) can be incorporated
in the backbone. Figure 4 illustrates the fluorescence change was saturated at the
concentration of 6 mM of glucose (even in fructose) due to the limited number of
diol-binding sites, which are existing only at two end groups per polymer chain.
It should be noted that HP showed the most sensitive fluorescence intensity
changes to glucose in mM region (Fig. 5(a)). Compared with the case of LP shown
in Figure 5(b), consecutive fluorescence quenching can be observed in HP according
to the concentration of glucose, while all the three saccharides can quench the
fluorescence intensity of LP at the similar level. The results suggests that a number
of boronic acid functional group on HP provide a potential binding with various
concentration of glucose, whereas LP has low tendency for the complex formation
between boronic acid and glucose exhibiting saturated detection limit of ꢃ6 mM.

60
D. G. Kim et al.
Figure 4. Fluorescence spectra of LP (8 ꢂ 10ꢀ7 M) upon addition of (a) glucose; (b) galactose;
(c) fructose (0, 6, 12, 25, 50, 100, and 200 mM) in 6 mM aqueous sodium phosphate buffer at
pH 7.4; kex ¼ 307 nm.
Figure 5. Emission intensity changes (DI=I0) of (a) HP and (b) LP as a function of the sacchar-
ide concentration; 8 ꢂ 10ꢀ7 M in 6 mM aqueous sodium phosphate buffer at pH 7.4;
&
kex ¼ 333 nm and kem ¼ 418 nm for HP; kex ¼ 307 nm and kem ¼ 418 nm for LP;
glucose;
~
. galactose; fructose.

Hyperbranched Polymer for Glucose Sensing
61
Conclusion
We synthesized
a
blue emitting hyperbranched water-soluble conjugated
poly(p-phenylene) with a number of boronic acid functional groups in its ends via
Suzuki coupling polymerization. As a control, linear water-soluble conjugated
poly(p-phenylene) with boronic acid only at both chain ends. The polymers showed
similar absorption and emission bands in their aqueous solutions. According to the
investigations on their sensing properties toward saccharides, hyperbranched type
polymer exhibited more selective and sensitive toward glucose than its linear
type counterpart.
Acknowledgment
This work was supported by the National Research Foundation (NRF) grant funded
by the Korea government (MEST) (No. R01-2007-000-10740-0 and 2009-608146).
References
[1] The Diabetes Control and Complications Trial Research Group. (1997). Diabetes,
46, 271.
[2] Heller, A. (1999). Annu. Rev. Biomed. Eng., 1, 153.
[3] Lorand, J. P. & Edwards, J. O. (1959). J. Org. Chem., 24, 769.
[4] Sugihara, J. M. & Bowman, C. M. (1958). J. Am. Chem. Soc., 80, 2443.
[5] Hartley, J. H., James, T. D., & Ward, C. J. (2000). J. Chem. Soc., Perkin Trans.1,
2000, 3155.
[6] James, T. D., Sandanayake, K., & Shinkai, S. (1996). Angew. Chem., Int. Ed. Engl.,
35, 1911.
[7] DiCesare, N. & Lakowicz, J. R. (2001). Chem. Commun., 2022.
[8] Karnati, V. V., Gao, X., Gao, S., Yang, W., Ni, W., Sankar, S., & Wang, B. (2002).
Bioorg. Med. Chem. Lett., 12, 3373.
[9] Shirota, Y., Kuwabara, Y., Inada, H., Wakimoto, T., Nakada, H., Yonamoto, Y.,
Sawai, S., & Imai, K. (1994). Appl. Phys. Lett., 65, 807.
[10] Robinson, M. R., Wang, S., Bazan, G. C., & Cao, Y. (2000). Adv. Mater., 12, 1701.
[11] Bauer, J. W., Kim, S., Balanda, P. B., Reynolds, J. R., & Rubner, M. F. (1998).
Adv. Mater., 10, 1452.
[12] Gaylord, B. S., Heeger, A. J., & Bazan, G. C. (2002). Proc. Acad. Sci. USA, 99, 10594.
[13] Lee, J. H., Choi, M. S., & Lee, T. S. (2008). Mol. Cryst. Liq. Cryst., 492, 192.
[14] Kim, H. J., Lee, J. H., Kim, T. H., Lyoo, W. S., Kim, D. W., Lee, C., & Lee, T. S. (2007).
J. Polym. Sci. Part A, Polym. Chem., 45, 1546.
[15] Kim, H. J., Lee, J. H., Lee, M., & Lee, T. S. (2008). React. Funct. Polym., 68, 1696.
[16] Fkyerat, A., Dubin, G.-M., & Tabacci, R. (1999). Helv. Chim. Acta, 82, 1418.
[17] Kim, S., Jackiw, J., Robinson, E., Schanze, K. S., & Reynolds, J. R. (1998). Macromo-
lecules, 31, 964.
[18] Tsai, L.-R. & Chen, Y. (2008). Macromolecules, 41, 5098.
[19] Springsteen, G. & Wang, B. (2002). Tetrahedron, 58, 5291.