A facile strategy for polymers to achieve glucose-responsive behavior at neutral pH

Article abstract of DOI:10.1039/c1sm05978e

A novel type of amphiphilic block polymer with phenylboronate ester as a leaving group in the hydrophobic block was designed and synthesized as a nanocarrier to successfully realize glucose-responsiveness at neutral pH.

Full text of DOI:10.1039/c1sm05978e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Soft Matter
Cite this: Soft Matter, 2011, 7, 7948
www.rsc.org/softmatter
COMMUNICATION
A facile strategy for polymers to achieve glucose-responsive behavior at
neutral pH†
*
Yuan Yao, Xuemin Wang, Tianwei Tan and Jing Yang
Received 26th May 2011, Accepted 30th June 2011
DOI: 10.1039/c1sm05978e
acid–acyclic diol complexation may act as effective sugar-responsive
materials. Due to saccharide molecules having predominant associ-
ation force with organoboron, the presented pinacol phenylboronate
moieties on the polymer are prone to combine with sugar molecules
and detach from the polymer architecture as a phenylboronate–
saccharide complex. The disengagement of phenylboronate groups
will trigger the polarity transfer of the polymer from amphiphile to
double hydrophile, thus leading to disruption of the initial nano-
aggregates formed via self-assembly of the amphiphilic polymer, and
A novel type of amphiphilic block polymer with phenylboronate
ester as a leaving group in the hydrophobic block was designed and
synthesized as a nanocarrier to successfully realize glucose-respon-
siveness at neutral pH.
Recently, considerable impetus has been given to the research on
saccharide-responsive materials due to their practical significance in
the development of sensors and drug releasing carriers for glucose-
related human disorders such as diabetes, a life-threatening disease
with an increasing tendency around the world.1,2 Boron-containing
polymers, in particular, polymeric phenylboronic acids, have attrac-
ted growing scientific attention as potential insulin-delivering candi-
dates because of being prone to form water-soluble phenylborate
derivatives between phenylboronic acid and sugar molecules.3–8 To
realize their application as drug delivery vehicles under physiological
conditions, one of the most challenging problems in the development
of organoboron polymers is how to achieve the saccharide-respon-
siveness at neutral pH. Although the incorporation of the electron-
rich nitrogen atom into the polymers has been evidenced to be an
effective method to lower the pKa value of phenylboronic acid resi-
dues via intra- or intermolecular interaction between B and N atoms,
thus realizing sugar-responsiveness at neutral pH,9,10 it is still an
extremely important and arduous job to explore more organoboron
macromolecules to satisfy various requirements of the sensing and
delivery applications.
It is well-known that the rigid cis-diols found in many saccharides
generally exhibit higher affinities with organoboronic acids through
reversible boronate formation than acyclic diols like ethylene glycol,11
which suggests that the boronic acid–acyclic diol complexes are easily
dissociated in the presence of appropriate saccharide molecules.4e,12–14
This competition mechanism between sugar molecule and acyclic diol
provides an excellent opportunity for developing a novel type of
sugar-responsive materials. Inspired by the competition concept, we
envision that amphiphilic block polymers with hydrophobic block
bearing pinacolboronate ester units generated via the phenylboronic
State Key Laboratory of Chemical Resource, College of Life Science and
Technology, Beijing University of Chemical and Technology, Beijing,
P. R. China. E-mail: yangj@mail.buct.edu.cn; Fax: +86 10 64427578;
Tel: +86 10 64427578
† Electronic supplementary information (ESI) available: Experimental
details, NMR spectra, FTIR spectra and critical micelle concentration
determinations. See DOI: 10.1039/c1sm05978e
Fig. 1 (A) Schematic diagram for sugar-responsive behavior. (B)
Synthesis of monomer and block copolymer.
7948 | Soft Matter, 2011, 7, 7948–7951
This journal is ª The Royal Society of Chemistry 2011

View Article Online
releasing the loaded molecules (Fig. 1A). Alternatively, boronate ester
architecture exhibits a relatively lower pKa than boronic acid itself,11
thus also providing one potential to explore smart sugar-responsive
polymers available to physiological need. Herein, we employed this
polarity switch mechanism induced by the competition between sugar
and acyclic diol to develop one type of block copolymer with pina-
colboronate moieties and evaluate their stimuli-responsive ability in
the presence of D-glucose at neutral pH. Fig. 1B schematically shows
our synthesis approach of monomer and corresponding amphiphilic
block copolymer.
Fig. 3 TEM graph of particles formed via self-assembly of the block
polymer 3 (left, scale bar stands for 900 nm); hydrodynamic diameter
distribution of particles in water measured by DLS (right).
The acrylate monomer 2 containing pinacol phenylboronate was
prepared from phenylboronic acid and 1,1,1-tris(hydroxymethyl)
propane as starting materials in a two-step simple transformation.
Subsequently, the amphiphilic block polymer 3 was fabricated via
ATRP of monomer 2 using monomethoxy poly(ethylene glycol)–Br
(MPEG–Br, Mn ¼ 2100 g molꢀ1) as macroinitiator. Polymerization
was conducted at 90 ꢁC in anisole at the monomer concentration of
1.10 g mLꢀ1, and the experimental molecular weight of the polymer
was linearly increased with polydispersity index controlled in the
range of 1.20–1.30 over the course of the polymerization (Fig. 2A).
The resulting block polymer MPEG-block-PpBDEMA via 18 h
polymerization had hydrophilic and hydrophobic repeating unit ratio
of 44 to 64 (calculated by 1H NMR in Fig. S9†, Mw/Mn ¼ 1.22). This
successful preparation is further confirmed by a GPC trace which
showed a sharp signal free from any residual MPEG–Br (Fig. 2B).
Compared to the previously reported synthetic method of organo-
boron macromolecules,6,9 the current synthesis was of ease and well
operation.
techniques using nile red as probe molecule and cargo over time. The
releasing percentage was calculated by a comparison of the fluores-
cence intensity at determined time and initial time. In Fig. 4A, upon
the addition of glucose, the nanoparticles from MPEG-block-
PpBDEMA promptly generated response, and nearly 40 percent of
the encapsulated nile red escaped from the nanoparticles in 3 min at
pH 7.8. Although the environmental pH decrease would weaken the
affinities of boronate and sugar molecules, rapid release of nile red
also occurred at pH 7.4 as well, and 20 percent of the encapsulated
molecules was liberated during the experimental time. The blank
experiment without glucose was conducted, and no obvious nile red
release trend was detected in 6 min incubation regardless of pH
values.
In the human body, the blood glucose concentration is tightly
controlled in a normal range of 4.4–6.6 mM. The blood glucose
concentration undulation would induce insulin response to different
degrees. To confirm the controllable release ability of these nano-
particles with the change of environmental stimuli, we investigated
the effect of glucose concentration on the stimuli-responsiveness of
nanoparticles in aqueous solution at pH 7.4 (Fig. 4B). At the given
The critical micelle concentration (CMC) measured by the fluo-
rescence technique15 was 49 mg Lꢀ1 for the polymer 3 (Fig. S11 in the
ESI†). Particles can be fabricated from the polymer using the
reported technique.16 A transmission electron microscopy (TEM)
graph revealed that these particles were close to spherical micelles
having core–shell structure with a hydrophobic core composed of
PpBDEMA and the overall size about 30 nm (Fig. 3). The size
distribution was corroborated by dynamic light scattering (DLS)
measurements in water, which indicated that the particles had an
average diameter of 90 nm.
In this study, the dissociation of pinacolboronate ester from the
polymer would unmask the hydroxyl groups of the polymers and
change the polymer polarity, which would trigger the release of the
molecules encapsulated in the hydrophobic core of nanocarriers. To
test the release mechanism induced by the competition effect of the
sugar molecules on pinacolboronate ester, particles formed from
MPEG-block-PpBDEMA were suspended in aqueous solutions at
pH 7.4 and 7.8 with or without 8 mg mLꢀ1 glucose. Their glucose-
responsive behavior at 37 ꢁC was investigated by fluorescence
Fig. 4 (A) Nile red release from the particles with and without 8 mg
mLꢀ1 glucose at various pH environments as measured by fluorescence
intensity decrease at 620 nm; (B) time dependence of nile red release at
various glucose concentrations at pH 7.4 and 37 ꢁC; (C) nile red release
from the particles under the stimuli of glucose in batched addition, each
batch of glucose at 4 mg mLꢀ1; (D) particles size changing measured by
DLS during incubating at various concentrations of glucose for 30 min at
pH 7.4 and 37 ꢁC.
Fig. 2 (A) Mn and polydispersity index versus polymerization time for
ATRP of monomer 2; (B) GPC traces of the block polymer 3 compared
to MPEG–Br (Mn ¼ 2100 g molꢀ1).
This journal is ª The Royal Society of Chemistry 2011
Soft Matter, 2011, 7, 7948–7951 | 7949

View Article Online
glucose concentration of 4 mg mLꢀ1 (22.0 mM), 10 percent of the
encapsulated nile red was released in 2 min. As more glucose was
added to the particle aqueous solution, the competition driving force
increase for glucose–boronate complexation is of benefit to more
phenylboronate detachment from the hydrophobic core of nano-
particles. Therefore, higher release rates were observed at glucose
concentrations of 12 mg mLꢀ1 (66.0 mM) and 16 mg mLꢀ1
(88.0 mM) in the similar period of incubation. Moreover, nile red
release percentage was almost linearly increased depending on the
initially given glucose concentration (Fig. S11†). Meanwhile, it is
interestingly found that the release of nile red triggered by glucose at
any given concentration reached a plateau within 5 min.
Fig. 5 FITC-insulin release from the nanoparticles at the glucose
concentration of 4 mg mLꢀ1 at pH 7.4 and 37 ꢁC (left). The images of
nanoparticle solution before and after insulin release (right, yellow one
was prior to release and the colorless one was after releasing).
Besides, glucose-responsive behavior of the nanoparticles was
further investigated by consecutive addition of glucose at pH 7.4 and
ꢁ
37 C. When the added glucose of each batch was maintained at
4.0 mg mLꢀ1 in the measured solution, interestingly, it is detected that
10 percent of the encapsulated nile red was released during 4 min
observation under the stimuli of the first 4 mg mLꢀ1 glucose, then
following one release plateau (Fig. 4C). Upon the addition of more
4.0 mg mLꢀ1 glucose, additional 10% nile red escaped from the
nanoparticles in the second 4 min incubation. The similar release
behavior occurred when the third and fourth batch of glucose at the
given concentration were added into the above particles solution. The
nanoparticle solution turned cloudy after the fourth glucose-
responsive release. The overall release percentage of nile red reached
40% at the cumulative glucose concentration of 16.0 mg mLꢀ1, which
is similar to the results upon the addition of the identical glucose
concentration in one portion (Fig. 4B). This effect of the consecutive
release under the given glucose concentration indicates that the
studied nanocarriers have potential to be one type of on–off glucose-
responsive switch which can controllably release the encapsulated
molecules depending on the changing glucose concentration.
The size change of the nanoparticles in response to glucose was
followed by DLS measurement. It is noteworthy that the addition of
glucose molecule led to the rapid and remarkable expanding of the
nanoparticles at pH 7.4 and 37 ꢁC (Fig. 4D). The nanoparticle size
increased from the initial 90 nm to 140 nm after 30 min incubation at
the given glucose concentration of 4 mg mLꢀ1. More obviously, the
micellar bulk was increased to 450 nm and 760 nm when the glucose
concentration was determined at 12 mg mLꢀ1 and 20 mg mLꢀ1,
respectively. In contrast, the micellar size remained stable in the
absence of glucose during 30 min observation. Under the stimuli of
glucose, the micellar bulk expansion instead of disruption was likely
to result from the removal of pinacolboronate ester from the nano-
particles’ hydrophobic core uncovered the hydroxyl groups which are
highly hydrophilic but not water-soluble,17 thus resulting in the
maintenance of micellar structures with bigger size, as shown in
Fig. 1A.
loading FITC-insulin exhibited quite low leaching in the absence of
glucose stimuli, suggesting a good capping efficiency (Fig. 5, left). In
contrast, obvious insulin release triggered by 4 mg mLꢀ1 glucose was
observed within 2 h, and showed a similar diffusion-controlled kinetic
profile. The color of the micellar solution turned from light yellow to
colorless with FITC-insulin escaping from the polymeric compart-
ment, as shown in the right image of Fig. 5. Further exploration of
insulin release under glucose-stimuli is underway in our laboratory.
In conclusion, utilizing the affinity disparity of saccharide mole-
cules and acyclic diol binding with organoboron and the character-
istics of boronate ester having relatively low pKa, the polymeric
nanocarriers containing pinacolboronate ester architecture can
indeed release its payload in the presence of glucose at neutral pH.
This study provides one well-operation method to realize glucose-
responsive release under physiological conditions. Further work will
investigate the detailed smart regulation of sugar-responsive behavior
and the biocompatibility of the polymers and dissociated products
during the responsiveness.
Acknowledgements
This work was supported by National Natural Science Foundation
of China (NSFC, Grant No. 20804003), Scientific Research Foun-
dation for the Returned Overseas Chinese Scholars, State Education
Ministry and Chinese Universities Scientific Fund.
Notes and references
1 T. D. James and S. Shinkai, Top. Curr. Chem., 2002, 218, 159.
2 F. Jakle, J. Inorg. Organomet. Polym. Mater., 2005, 15, 293.
3 (a) J. N. Cambre, D. Roy, S. R. Gondi and S. B. Sumerlin, J. Am.
Chem. Soc., 2007, 129, 10348; (b) D. Roy, J. N. Cambre and
B. S. Sumerlin, Chem. Commun., 2008, 2477; (c) D. Roy,
J. N. Cambre and S. B. Sumerlin, Chem. Commun., 2009, 2106; (d)
P. De, S. R. Gondi, D. Roy and B. S. Sumerlin, Macromolecules,
2009, 42, 5614.
4 (a) Y. J. Zhang, Y. Guan and S. Q. Zhou, Biomacromolecules, 2006, 7,
3196; (b) Y. J. Zhang, Y. Guan and S. Q. Zhou, Biomacromolecules,
2007, 8, 3842; (c) T. Hoare and R. Pelton, Biomacromolecules, 2008,
9, 733; (d) X. Jin, X. Zhang, Z. Wu, D. Teng, X. Zhang, Y. Wang,
Z. Wang and C. Li, Biomacromolecules, 2009, 10, 1337; (e)
Z. B. Ding, Y. Guan, Y. J. Zhang and X. X. Zhu, Soft Matter,
2009, 5, 2302.
In addition, the glucose-triggered insulin release from the poly-
meric micelles was also preliminarily conducted in this study. After
fluorescein-labeled insulin (FITC-insulin, 3.0 mg) and the polymer
MPEG-block-PpBDEMA (5.0 mg) dissolved in 0.3 mL THF was
added to 5.0 mL deionized water with stirring at room temperature
overnight, the insulin-encapsulated micellar solution transferred into
dialysis membranes (molecular weight cut off 12 000) was dialyzed
against distilled water until the fluorescence signal of the dialysis bag
inside at the emission wavelength of 519 nm was stable. The insulin
release behavior was tested with or without glucose at the concen-
tration of 4 mg mLꢀ1 at pH 7.4 and 37 ꢁC, respectively. The micelles
5 (a) V. Lapeyre, C. Ancla, B. Catargi and V. Ravaine, J. Colloid
Interface Sci., 2008, 327, 316; (b) T. Chen, D. P. Chang, T. Liu,
R. Desikan, R. Datar, T. Thundat, R. Berger and S. Zauscher,
J. Mater. Chem., 2010, 20, 3391.
7950 | Soft Matter, 2011, 7, 7948–7951
This journal is ª The Royal Society of Chemistry 2011

View Article Online
€
6 (a) Y. Qin, G. Cheng, A. Sundararaman and F. Jakle, J. Am. Chem.
Soc., 2002, 124, 12672; (b) Y. Qin, G. Cheng, O. Achara, K. Parab and
10 (a) K. T. Kim, J. J. L. M. Cornelissen, R. J. M. Nolte and J. C. M. van
Hest, J. Am. Chem. Soc., 2009, 131, 13908; (b) K. T. Kim,
J. J. L. M. Cornelissen, R. J. M. Nolte and J. C. M. van Hest, Adv.
Mater., 2009, 21, 2787.
€
F. Jakle, Macromolecules, 2004, 37, 7123; (c) Y. Qin, V. Sukul,
D. Pagakos, C. Cui and F. Jakle, Macromolecules, 2005, 38, 8987.
€
7 (a) V. Ravaine, C. Ancla and B. Catargi, J. Controlled Release, 2008,
132, 2; (b) D. Wang, T. Liu, J. Yin and S. Y. Liu, Macromolecules,
2011, 44, 2282.
8 (a) B. Wang, R. Ma, G. Liu, Y. Li, X. Liu, Y. An and L. Shi,
Langmuir, 2009, 25, 12522; (b) B. Wang, R. Ma, G. Liu, X. Liu,
Y. Gao, J. Shen, Y. An and L. Shi, Macromol. Rapid Commun.,
2010, 31, 1628.
9 (a) I. Hisamitsu, K. Kataoka, T. Okano and Y. Sakurai, Pharm. Res.,
1997, 14, 289; (b) A. Matsumoto, S. Ikeda, A. Harada and
K. Kataoka, Biomacromolecules, 2003, 4, 1410; (c) A. Matsumoto,
R. Yoshida and K. Kataoka, Biomacromolecules, 2004, 5, 1038; (d)
A. Matsumoto, K. Yamamoto, R. Yoshida, K. Kataoka, T. Aoyagi
and Y. Miyahara, Chem. Commun., 2010, 46, 2203.
11 J. P. Lorand and J. O. Edwards, J. Org. Chem., 1959, 24, 769.
12 G. Springsteen and B. H. Wang, Chem. Commun., 2001, 1608.
13 C. Cannizzo, S. Amigoni-Gerbier and C. Larpent, Polymer, 2005, 46,
1269.
14 Y. Zhao, B. G. Trewyn, I. I. Slowing and V. S. Y. Lin, J. Am. Chem.
Soc., 2009, 131, 8398.
15 I. Astafieva, X. F. Zhong and A. Eisenberg, Macromolecules, 1993,
26, 7339.
16 (a) L. Zhang and A. Eisenberg, Macromol. Symp., 1997, 113, 221; (b)
A. Choucair and A. Eisenberg, Eur. Phys. J. E, 2003, 10, 37; (c)
D. E. Discher and A. Eisenberg, Science, 2002, 279, 967.
17 E. J. Vandenberg and D. Tian, Macromolecules, 1999, 32,
3613.
This journal is ª The Royal Society of Chemistry 2011
Soft Matter, 2011, 7, 7948–7951 | 7951