Continuous Glucose Sensing with a Fluorescent Thin-Film Hydrogel

Article abstract of DOI:10.1002/anie.200352405

Sweetness and light: A fluorescent thin-film hydrogel that contains a covalently bound boronic-acid-functional quencher (Q) and a fluorescent dye (F; see Figure) continuously senses glucose under physiological conditions through binding of the sugar (S) to a labeled receptor (R).

Full text of DOI:10.1002/anie.200352405

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Chemie
itoring would be highly beneficial for the diabetic commun-
ity.[2]
Many different approaches to developing a continuous
glucose monitor (CGM) are being pursued.[3] Among them,
the enzymatic method, which uses fluorescence-based bio-
sensors, has attracted much attention.[3b,c] In contrast, our
approach involves the use of a chemosensor that utilizes the
interaction between a negatively charged fluorescent dye and
a positively charged boronic-acid-functional quencher.[4]
Boronic acids are known to bind to glucose reversibly
under aqueous conditions.[5] When attached to a fluorophore,
the acid molecules can modulate the fluorescence as a
function of saccharide concentration.[6] In our approach, the
boronic acid group is attached to a quencher instead of a dye,
and the fluorescence is modulated upon binding of glucose to
the boronic-acid-functional quencher. The signal transduction
in this two-component system is based on the electrostatic
attraction between the fluorophore and the quencher and is a
function of the charge on the boron moiety.[4]
For a glucose sensor to be useful in a device, the sensing
components must be immobilized to allow real-time monitor-
ing. For in vivo use, the device must operate at physiological
temperature, ionic strength, and pH values. In addition, it is
preferable to use visible light for optical sensors to avoid
complications associated with UV light and autofluorescence.
To date, there has been no report of a boronic-acid-based
glucose sensor that fulfils all these criteria.[7] We report the
development of a two-component sensing system immobi-
lized in a thin film hydrogel. The combination of a cationic
boronic-acid-functional quencher 5 and an anionic dye 10 is
shown to function as a CGM under physiological conditions.
Viologen 5 was prepared as indicated in Scheme 1.
Commercially available 3,5-dimethylphenylboronic acid was
first protected as a 1,3-propanediol ester in dichloroethane
and then treated with NBS to give, after recrystallization from
CH3OH, the dibrominated species 3 in 52% yield. Reaction
of 3 with vinylpyridine in a CH2Cl2/CH3OH solution[8] gave
Carbohydrate Sensors
Continuous Glucose Sensing with a Fluorescent
Thin-Film Hydrogel**
Jeff T. Suri, David B. Cordes, Frank E. Cappuccio,
Ritchie A. Wessling, and Bakthan Singaram*
Diabetes mellitus is a chronic disease that impairs the ability
of the body to manufacture or use insulin, a hormone
necessary to metabolize glucose. Although there is no cure
for the disease, tight glucose control substantially reduces
morbidity and mortality among diabetes patients.[1] Conse-
quently, there is a consensus that continuous glucose mon-
[*] Prof. Dr. B. Singaram, J. T. Suri, D. B. Cordes, F. E. Cappuccio,
Dr. R. A. Wessling
Department of Chemistry and Biochemistry
University of California, Santa Cruz
Santa Cruz, CA 95064 (USA)
Fax: ( +1)831-459-2935
E-mail: singaram@chemistry.ucsc.edu
Scheme 1. Synthesis of positively charged quencher monomer 5.
a) 1,3-propanediol, C2H4Cl2, CaH2, reflux3 h; b) N-bromosuccinimide
(NBS), azobisisobutyronitrile, C2H4Cl2, reflux, 3 h, 52%, 2 steps;
c) vinylpyridine, CH2Cl2/CH3OH (3:1), 408C, 22 h, 52%; d) 4,4’-bipyri-
dine, CH3OH/dimethyl formamide (3:1), 408C, 42 h, 39%.
[**] We thank the BioSTAR Project and the Industry–University Coop-
erative Research Program with Glumetrics for their financial
support. Julie Congdon is thanked for her assistance in the
synthesis of starting materials.
Angew. Chem. Int. Ed. 2003, 42, 5857 –5859
DOI: 10.1002/anie.200352405
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
the monobromomethylphenylboronate 4. Further quaterni-
zation with 4,4’-dipyridyl and chromatographic purification
on C18-silica gel afforded tetracationic viologen 5 in 39%.
Monomeric dye 10 was synthesized in five steps from the
commercially available sodium salt 6 (Scheme 2). Chlorina-
Figure 1. Fluorescence emission spectra of 10 (4”10À6 m) with increas-
ing concentrations of 5. Inset: Stern–Volmer plot, where F is the fluo-
rescence intensity of 10 in the presence of 5 and F0 is the fluorescence
intensity of 10 in the absence of 5. lex =470 nm.
glucose decreased the quenching efficiency of 5 (Ksv = 7.2”
103 mÀ1).
The relative affinity of 5 towards different saccharides was
then determined. Titration of solutions containing 10 (4 ”
10À6 m) and 5 (1.2” 10 À4 m) were carried out with fructose,
glucose, and galactose. Binding constants for each sugar were
calculated from the binding isotherms.[10] The measured
affinities are ranked in the order: fructose (2063mÀ1) > glu-
cose (275mÀ1) > galactose (210mÀ1). Phenyl boronic acids
normally show a 3:1 greater affinity for galactose over that for
glucose[6] but 5 binds glucose more strongly than galactose.
With the quenching and sugar-sensing ability of the dye/
quencher combination established, the sensing elements were
immobilized in a thin film (Scheme 3). A 50% aqueous
solution containing 5, 11, 2-hydroxyethyl methacrylate, poly-
ethyleneglycol dimethacrylate, and an initiator was injected
between two glass plates separated by a 25.4-mm teflon spacer.
Free-radical polymerization was carried out at 408C for 15 h.
The resultant film was leached in pH 9 buffer for one day and
equilibrated in pH 7.4 buffer for one day. The swollen
Scheme 2. Synthesis of negatively charged dye monomer 11. a) SOCl2, reflux,
3 h, then H2O, 81%; b) Boc-lysine, NaOH, terahydrofuran/CH3CN/H2O
(2:7:1), RT, 24 h, then HCl, 94%; c) trifluroacetic acid, RT, 2 h, 65%; d) meth-
acroyl chloride, H2O, NaOH, RT, 1 h, 17%; e) acetic anhydride, NaOAc, 24 h,
70%. Boc=tert-butoxycarbonyl; Ac=acyl.
tion followed by sulfonamide formation gave the Boc-
protected derivative 8 in 88%. Deprotection and reaction
with methacryol chloride under standard conditions afforded
the trifunctionalized derivative 10, which was converted into
the acetoxy-protected monomer 11 for use in polymerization.
Pyranine derivatives functionalized
with three sulfonamides have shifted
excitation and emission peaks in com-
parison to unfunctionalized pyranine
(lex = 454 nm and lem = 510 nm)[9] and
this shift is observed for 10, for which
lex = 491 nm and lem = 540 nm.
To evaluate the ability of 5 to
quench the fluorescence of 10 in the
absence and presence of different
saccharides, fluorescence spectros-
copic measurements were carried
out in aqueous buffer solutions at
pH 7.4. Upon titrating 10 with 5,
significant fluorescence quenching
was observed (Figure 1), with an appa-
rent Stern–Volmer quenching con-
Scheme 3. Synthesis of a glucose-sensing polymer 12 with its sensing elements in proximity to
one another. a) 2-Hydroxyethyl methacrylate, polyethylene glycol dimethacrylate (MW =1000),
VA-044 (initiator), 408C, 15 h; b) NaOH solution, pH 9, 24 h.
stant of Ksv = 1.3 ” 104 mÀ1. Titration
of 10 with 5 in the presence of 5 mm
5858
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Chemie
hydrogel was found to contain 40% water (by weight). The
deprotected film was subsequently tested for its sugar-sensing
ability.
glucose in the physiologically important range of 2.5–20 mm
and 2) operate under physiological conditions (378C, 0.1 mm
ionic strength, pH 7.4). Moreover, the fluorescence signal
obtained is completely reversible, which allows real-time
monitoring of glucose levels. Further studies are underway to
determine the scope of this approach in continuous glucose
monitoring.
The hydrogel was mounted into a flow cell and phosphate
buffer of ionic strength 0.1m was circulated through the cell.
The film was excited at 470 nm by front-face illumination and
the emission at 540 nm was monitored over time. The
temperature was kept constant at 378C. After a stable
baseline had been obtained, the buffer solution was replaced
with saccharide solution and the change in fluorescence
intensity was measured. As indicated in Figure 2, the infusion
Received: July 17, 2003 [Z52405]
Keywords: boronic acid · carbohydrates · fluorescence · gels ·
.
sensors
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[7] Boronic acid based sensors have been prepared that use UV
light as the excitation source, see A. E. Colvin, U.S. Pat. Appl.
Publ. 2002039793, 2002.
[8] Vinylpyridinium salts are very unstable and are known to
spontaneously polymerize, see a) T. Kanbara, M. G. Mikhael,
A. B. Padias, H. K. Hall, J. Polym. Sci. Part A 1997, 35, 2787;
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[9] a) E. Koller, R. Moser, Appl. Fluoresc. Technol. 1989, 1, 15;
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[10] K. A. Connors, Binding Constants—The measurement of Molec-
ular Complex Stability, Wiley, New York, 1987.
[11] UV/Vis measurements in solution studies confirmed the for-
mation of a ground-state complex between 5 and 10.
Figure 2. Relative fluorescence intensity change of emmission from 12
over time in the presence of a range of concentrations of glucose. F is
the fluorescence intensity of 12 in the presence of glucose and F0 is
the fluorescence intensity of 12 in the absence of glucose. lex =470 nm
and lem =540 nm.
of glucose into the hydrogel resulted in a stepwise change in
fluorescence intensity that was dependent on sugar concen-
tration. Importantly, the sensor detected glucose in the
physiological range of 2.5–20 mm. Moreover, the changes in
fluorescence were completely reversible. Similar profiles
were obtained for fructose and galactose and the apparent
binding constants for each sugar were determined. The
selectivity for each saccharide changed once the components
were immobilized in a polymer. The immobilized system is
more selective for glucose and less selective for fructose and
galactose than the free components; the relative affinities are:
fructose (666mÀ1) > glucose (333mÀ1) > galactose (111mÀ1).
Hydrogels are known to mimic the behavior of polymer
chains in solution in that a high degree of segmental mobility
is possible even though long-range diffusion cannot occur.
Functional groups attached to the polymer chains are free to
move about and interact at least locally. This movement
appears to occur in 12. Since the quenching mechanism
appears to be predominately static,[11] the dye and quencher
units in the polymer chain must have the freedom to associate
and dissociate within the polymer matrix depending on the
local saccharide concentration.
The saccharide-sensitive hydrogel reported herein shows
promise as the basis for a continuous glucose monitoring
system. Through a reversible electrostatic interaction within
its polymer matrix, the chemosensor 12 is able to 1) detect
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