A supramolecular hydrogel containing boronic acid-appended receptor for fluorocolorimetric sensing of polyols with a paper platform

Article abstract of DOI:10.1039/c2cc17503g

A boronic acid-appended fluorescent receptor was incorporated into self-assembled nanofibers containing a hydrophobic FRET-paired dye to develop a gel-based fluorocolorimetric sensor for polyols. We demonstrated that the gel-based sensor is capable of det

Full text of DOI:10.1039/c2cc17503g

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A supramolecular hydrogel containing boronic acid-appended receptor for
fluorocolorimetric sensing of polyols with a paper platformw
Masato Ikeda,a Keisuke Fukuda,a Tatsuya Tanida,a Tatsuyuki Yoshiia and Itaru Hamachi*ab
Received 1st December 2011, Accepted 12th January 2012
DOI: 10.1039/c2cc17503g
A boronic acid-appended fluorescent receptor was incorporated
into self-assembled nanofibers containing a hydrophobic FRET-
paired dye to develop a gel-based fluorocolorimetric sensor for
polyols. We demonstrated that the gel-based sensor is capable of
detecting polyols such as catechol and dopamine not only under
semi-wet conditions, but also under dry conditions using a paper
platform.
FRET (fluorescence resonance energy transfer)-based fluorescence
sensing enables us to monitor a specific analyte accurately by
two-wavelength emission ratiometry as well as visually by
fluorescence color change.1 Recently, we have developed a
unique approach for constructing a fluorescence sensing system
endowed with FRET readout by taking advantage of cooperative
action of fluorescent molecular receptors and self-assembled
nanofibers in supramolecular hydrogels.2–4 The sensing principle
is simple, that is based on the translocation of the receptors in the
gel matrix, when an analyte induces localization change of the
receptors between hydrophobic nanofibers and aqueous phase, a
ratiometric fluorescence signal change will be observed by the
substantial change in the average distance between the fluorescent
receptor and a corresponding FRET-paired dye embedded in the
nanofibers.
In this study, we introduced a boronic-acid appended
fluorescent receptor (2) into a supramolecular hydrogel 15
Fig. 1 (a) Chemical structures of boronic acid receptor 2 and FRET
with an aim of sensing polyols such as dopamine and sugars.
The boronic acid moiety of 2 can form ester complexes with
these polyols in aqueous media concurrently with an anionic
charge generation on its boron atom,6 which makes the
receptor more hydrophilic. Therefore, the receptor should
migrate from a hydrophobic site of nanofiber 1 to the more
hydrophilic sites or aqueous phase upon complexation with
polyols, which induces fluorescence color change (Fig. 1c).
Here we describe that the gel-based sensing system is capable of
fluorocolorimetrically detecting polyols such as catechol derivatives.
Moreover, we succeeded in incorporating the gel-based sensing
donor 3. (b) Self-assembly of hydrogelator 1 to form supramolecular
fibers (supramolecular hydrogel) induced by Ca2+ ion complexation.
Confocal laser scanning microscopic (CLSM) image of supramolecular
nanofiber 1ÁCa2+ containing 3 ([1] = 0.2 wt%, [Ca2+/1] = 1.0,
[3] = 30 mM, 50 mM HEPES (pH 7.2)). (c) Schematic representation
of translocation of receptor 2 upon the binding of a polyol from
hydrophobic interior of the nanofiber 1ÁCa2+ containing FRET donor
3 to aqueous phase, which leads to the change in FRET efficiency.
system into a filter paper without notable loss of its sensing
ability even after drying, which can be envisioned as a lightweight
and portable sensor chip for monitoring the polyols with the
naked eye.
a Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, Katsura, Kyoto,
615-8510, Japan. E-mail: ihamachi@sbchem.kyoto-u.ac.jp;
Fax: +81-75-383-2759; Tel: +81-75-383-2754
As a gel matrix, we employed a phosphoric acid appended
supramolecular hydrogelator 1 which forms a stable hydrogel in
the presence of a Ca2+ ion (Fig. 1b).5 A boronic acid receptor
bearing 7-nitrobenzoxa[1,2,5]diazole (NBD) 2 (NBD-B) for
polyol binding and a hydrophobic coumarin dye 3 (Coum-C12)
for a suitable FRET donor of NBD were introduced into the
b Japan Science and Technology Agency (JST), CREST,
5 Sanbancho, Chiyoda-ku, Tokyo, 102-0075, Japan
w Electronic supplementary information (ESI) available: Supplementary
method and figures (Fig. S1–S4). See DOI: 10.1039/c2cc17503g
c
2716 Chem. Commun., 2012, 48, 2716–2718
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supramolecular hydrogel 1ÁCa2+ (Fig. 1a and c). Fig. 2a shows
typical fluorescence spectral change of the supramolecular
hydrogel 1ÁCa2+ containing NBD-B 2 and Coum-C12 3 upon
polyol addition. Without polyols, two emission peaks at
454 nm and 534 nm assignable to coumarin and NBD dye,
respectively, were observed with almost the same intensity,
typical for the FRET system (F454/F534 = 0.95, solid line).7
With the increase in polyol concentration, the peak at 534 nm
gradually decreased and the peak at 454 nm was concurrently
intensified, so that the ratio of F454/F534 increased to 2.5 for
fructose (dotted line) and 2.3 for catechol (dashed line).8 This
seesaw type of spectral change is ascribable to cancellation of
FRET upon polyol binding to 2, suggesting that the average
distance between 2 and 3 increased, most probably due to
migration of 2 to the more hydrophilic space upon binding of
the polyols. In contrast, NBD-B 2 alone in an aqueous
solution showed only a slight change in its fluorescence by
the addition of catechol (Fig. S2, ESIw). These results indicate
that the self-assembled nanofiber 1ÁCa2+ is essential as a nano-
platform for the present polyol sensing system.
Next, we sought to integrate this sensing system into a filter
paper to prepare a thin, lightweight, and portable gel-based
sensor paper. Integration of a sensing system into a robust
solid matrix such as paper and polymer without loss of sensing
function is practically important to fabricate sensor devices
and is still challenging.9 Here, the gel-based sensor paper was
prepared by simply spotting a gel precursor aqueous solution
containing 2 and 3 on a cellulose-based filter paper, on which
aqueous CaCl2 solution was spotted beforehand (Fig. 3a).10
When an aqueous analyte solution (10 mM, 5 mL) was
dropped on the gel-based sensor paper, followed by excitation
with UV light (a handy LED lamp, lex = 395 nm), blue
emission at the spots corresponding to catechol, dopamine,
and catechin could be distinguished by the naked eye within
15 min, whereas green emission was almost unchanged at spots
corresponding to tyrosine, glucose, and fructose (Fig. 3b). This
selective color change agrees well with the above-mentioned
fluorescence spectral changes, demonstrating that the gel-based
sensor paper can sense catechol derivatives visually and rapidly.
Even after 1 week, comparable color change was observed,
suggesting that the gel-based sensor paper can be stored. In
addition, as shown in Fig. 3c, change in the fluorescence colors
was clearly distinguished from green to blue in a concentration
range of catechol comparable to that of the titration curve in
Fig. 2b, suggesting no notable loss of sensitivity under dry
conditions. In the absence of the supramolecular nanofiber
1ÁCa2+, the initial color of the spot on the filter paper was not
Fig. 2b summarizes titration curves of the gel-based sensing
material for various substances, which were obtained using
a 384-well plastic micro-plate. The titration curves provided
the apparent binding constants for each polyol as follows:
155 Æ 28 MÀ1 for catechol, 157 Æ 32 MÀ1 for dopamine,
22 Æ 2.7 MÀ1 for fructose and 3.3 Æ 1.6 MÀ1 for mannitol. The
sensing selectivity, in order of catechol, dopamine > fructose,
mannitol c glucose, is in good agreement with the selectivity
of boronic acid derivatives in aqueous solution reported
previously.6
Fig. 3 (a) Schematic illustration showing a fabrication process of a
gel-based sensor paper. The sensing spot comprises a porous matrix of
hydrophilic cellulose fibers and supramolecular nanofibers 1ÁCa2+
containing 2 and 3. (b) Photograph (lex = 395 nm) of a gel-based
sensor paper upon the addition of various substances (10 mM, 5.0 mL).
The spotted position of substances is shown in the right panel.
(c) Photograph of a gel-based sensor paper for monitoring catechol
Fig. 2 (a) Fluorescence spectral change (lex = 400 nm) of hydrogel
1ÁCa2+ containing 2 and 3 upon the addition of polyols ([Fructose] =
85 mM, [Catechol] = 8.5 mM). Inset shows a fluorescence spectral
change upon the addition of catechol (0–8.5 mM). (b) Plots of the
fluorescence intensity ratio (F454/F534) of hydrogel 1ÁCa2+ containing
2 and 3 upon the addition of various substances. Reaction conditions:
[2] = 38 mM, [3] = 50 mM, [1] = 0.2 wt%, [Ca2+/1] = 1.0, 50 mM
HEPES (pH 7.2) containing 2 vol% DMSO, room temperature.
and the corresponding changes in the pixel intensity ratio (Iblue/Igreen
)
and fluorescence intensity ratio (F460/F520). Error bars represent standard
deviation (n = 3). Reaction conditions: spotted aqueous CaCl2 solution
(1.0 mL): [CaCl2] = 324 mM, spotted sol (1.0 mL): [2] = 20 mM,
[3] = 30 mM, [1] = 0.2 wt%, 50 mM HEPES (pH 7.2) containing
6 vol% DMSO.
c
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greenish as that in the presence of nanofiber 1ÁCa2+ (Fig. S4,
ESIw), supporting that efficient FRET between 2 and 3 didn’t
occur without the nanofiber 1ÁCa2+. These results demonstrate
that the supramolecular nanofiber 1ÁCa2+ can function as a
robust and versatile nano-platform not only in a semi-wet gel
system but also under dry conditions.
3 (a) S. Kiyonaka, K. Sada, I. Yoshimura, S. Shinkai, N. Kato and
I. Hamachi, Nat. Mater., 2004, 3, 58–64; (b) Y. Koshi, E. Nakata,
H. Yamane and I. Hamachi, J. Am. Chem. Soc., 2006, 128,
10413–10422; (c) A. Wada, S.-i. Tamaru, M. Ikeda and
I. Hamachi, J. Am. Chem. Soc., 2009, 131, 5321–5330; (d) S.-i.
Tamaru, M. Ikeda, Y. Shimidzu, S. Matsumoto, S. Takeuchi and
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ncomms1018; (e) M. Ikeda, R. Ochi and I. Hamachi, Lab Chip,
2010, 10, 3325–3334; (f) M. Ikeda, T. Yoshii, T. Matsui, T. Tanida,
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4 Recent reviews on supramolecular hydrogel: (a) L. A. Estroff and
A. D. Hamilton, Chem. Rev., 2004, 104, 1201–1217; (b) M. de Loos,
B. L. Feringa and J. H. van Esch, Eur. J. Org. Chem., 2005,
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5 (a) H. Komatsu, S. Matsumoto, S.-i. Tamaru, K. Kaneko,
M. Ikeda and I. Hamachi, J. Am. Chem. Soc., 2009, 131,
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H. Komatsu, Y. Shimoyama, I. Hamachi and S. Takeuchi,
Angew. Chem., Int. Ed., DOI: 10.1002/ange.201104043, in press.
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Recognition, The Royal Society of Chemistry, Cambridge, 2006;
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2010, 14, 758–766.
7 FRET phenomenon was clearly evident from the comparison
of fluorescence spectra among supramolecular hydrogel 1ÁCa2+
containing either 2 or 3 and the both (see Fig. S1, ESIw).
8 The total fluorescence intensity decreased upon the addition of
catechol derivatives, suggesting that they act as quenchers.
9 (a) S. Arimori, M. L. Bell, C. S. Oh, K. Frimat and T. D. James,
Chem. Commun., 2001, 1836–1837; (b) J. N. Cambre and
B. S. Sumerlin, Polymer, 2011, 52, 4631–4643; (c) H. Shibata,
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and S. Takeuchi, Proc. Natl. Acad. Sci. U. S. A., 2011, 108,
13399–13403.
We demonstrated that the gel-based sensor is capable of
detecting polyols such as catechol, dopamine, and catechin not
only under semi-wet conditions, but also under dry conditions
using a paper platform. Paper matrices such as paper chromato-
graphy, litmus paper, and so on have been used extensively in
analytical and clinical chemistry. Very recently, patterned paper
devices bearing microfluidic channels inside them have been
actively developed to control the flow of solutions.11 Although
the present sensor still needs to be improved further, especially
in terms of sensitivity and color discrimination for practical
applications,12 we expect that supramolecular nanofiber platforms
enable us to fabricate hydrophobic nano-domains inside paper
devices and thus offer a unique opportunity to develop rapid
and inexpensive diagnostic tools.
This work was supported in part by the JST (Japan Science
and Technology Agency), CREST program and the global
COE program, ‘‘Integrated Materials Science’’ of the Ministry of
Education, Culture, Science, Sports, and Technology (Japan).
MI thanks Terumo Life Science Foundation for financial
supports. We acknowledge Dr K. Kuwata (Kyoto University)
for HRMS measurements.
Notes and references
1 For recent reviews: (a) K. M. Sapsford, L. Berti and I. L. Medintz,
Angew. Chem., Int. Ed., 2006, 45, 4562–4588; (b) S. L. Wiskur,
H. Ait-Haddou, J. J. Lavigne and E. V. Anslyn, Acc. Chem. Res.,
2001, 34, 963–972; (c) L. Basabe-Desmonts, D. N. Reinhoudt and
M. Crego-Calama, Chem. Soc. Rev., 2004, 36, 993–1017;
(d) J. F. Callan, A. P. de Silva and D. C. Magri, Tetrahedron,
2005, 61, 8551–8588; (e) P. Anzenbacher, Jr., P. Lubal, P. Bucek,
M. A. Palacios and M. E. Kozelkova, Chem. Soc. Rev., 2010, 39,
3954–3979.
2 (a) I. Yoshimura, Y. Miyahara, N. Kasagi, H. Yamane, A. Ojida
and I. Hamachi, J. Am. Chem. Soc., 2004, 126, 12204–12205;
(b) S. Yamaguchi, T. Yoshimura, T. Kohira, S.-i. Tamaru and
I. Hamachi, J. Am. Chem. Soc., 2005, 127, 11835–11841.
10 CLSM images of the gel-based sensor paper showed fluorescence
of 3 along the surface of cellulose fibers in the filter paper matrix
(Fig. S3, ESIw), suggesting that nanofiber 1ÁCa2+ is entangled with
the cellulose fibers.
11 (a) A. W. Martinez, S. T. Phillips, M. J. Butte and
G. M. Whitesides, Angew. Chem., Int. Ed., 2007, 46, 1318–1320;
(b) A. W. Martinez, S. T. Phillips and G. M. Whitesides,
Anal. Chem., 2010, 82, 3–10 and references cited therein.
12 For polyphenol sensing in green tea using synthetic pores and
boronic acid derivatives, see: S. Hagihara, H. Tanaka and
S. Matile, J. Am. Chem. Soc., 2008, 130, 5656–5657.
c
2718 Chem. Commun., 2012, 48, 2716–2718
This journal is The Royal Society of Chemistry 2012