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Table 1: Composition of selected responsive copolymers and IOHs
based on 1 and 2 (fixed molar ratio 85/15).
Sample functional
comonomer
cross-linker
cloud
comonomer content [mol%] content [mol%] point [8C][a]
5a
4a
4a
4b
4b
15
7
0
19
0
0
19
0
27.0
IOH-a
5b*
5b
IOH-b 4b
5c
10
10
14
10
13
–
39.2[b]
42.4[c]
4c
4c
none
34.5
30.9
IOH-c
5d
19
0
Figure 1. Monomers used for producing the hydrogel matrix (1 and 2),
its cross-linking (3), and functionalization (4a–c). The mannose
moiety of 4b was protected (R=acetyl) during polymerization, and
deprotected (R=H) in the final copolymer and IOH systems.
IOH-d none
–
19
[a] Cloud points measured by turbidimetry in 0.3 wt% PBS solution
(pH 7.4). [b] Before deprotection. [c] After deprotection.
binding modifies the overall hydrophilicity, the LCST of such
systems is shifted.[7]
content (Table 1). Moreover, in order to obtain IOH films of
sufficient mechanical stability for handling, the thin mono-
mer-filled opal template films were covered by a compara-
tively thick comonomer film of about 100 mm thickness,
resulting in about 5 mm thin IOH layer supported by the
corresponding bulk hydrogel film after curing and template
removal.
In addition to the functional inverse opal series IOH-a to
IOH-c, we prepared analogously IOHs devoid of recognition
units as references (series IOH-d, Table 1). Also for com-
parative model studies in aqueous solution, we synthesized
the analogous soluble copolymers without cross-linker (sam-
ples 5a–d, Table 1).
Cloud points of the soluble copolymer analogues of the
IOHs in aqueous buffer solution (phosphate buffered saline,
PBS) are listed in Table 1. They indicate an LCST-type phase
transition of the basic comonomer composition at 30.98C.
Functionalization by monomers 4a–c inevitably modulated
the overall hydrophilicity of the copolymers and thus changed
the cloud points to a small extent. Importantly, the addition of
the corresponding ligands (see Figure 2 for a selection)
induced a notable shift of the cloud points. For instance, the
addition of fructose or of the xylose-derived copolymer PXM
(see below) increased the cloud point of 0.3 wt%
This dual responsiveness allows an induced phase tran-
sition under isothermal conditions, if the binding of the
analyte shifts the phase transition temperature from below to
above the sensing temperature, or vice versa.[7b] Within an
appropriately chosen temperature window, this should pro-
duce much larger volume changes than simple swelling/
deswelling effects because of the primary modulation of the
hydrophilicity of the system by the binding of an analyte.
For a proof of concept, we focused on established model
recognition systems, such as the binding of vicinal diols as in
certain glycopolymers by benzoboroxol,[8] of lectins by
specific sugars,[9] and of avidin by biotin.[7b,10] Hence, we
synthesized the recognition group bearing comonomers 4a–c,
functionalized by a boronic acid half-ester,[8a] an azidoman-
nose derivative, and
a
biotin moiety,[11] respectively
(Figure 1). The functional IOHs were produced in three
steps. First, monodisperse silica nanoparticles were prepared
through the Stçber process[12] and assembled into a colloidal
crystal. Then, the voids were infiltrated by appropriate
monomer mixtures and polymerized. Finally, the template
nanoparticles were removed (Scheme 2).
solutions in PBS of copolymer 5a by 8.48C or by
5.98C, respectively, while the weakly or non-
binding related molecules ribose, maltopentaose
(PMA), or poly(vinylalcohol) (PVA) produced
only marginal shifts (< 0.58C). Similarly, the
addition of the protein avidin increased the
Scheme 2. Synthesis of inverse opal hydrogels (IOHs): a) Monodisperse silica par-
ticles (diameter 420 nm) are assembled into a synthetic opal of about 5 mm
thickness. b) The template opal is infiltrated by the precursor monomer mixture,
which is subsequently photopolymerized to give a filled hydrogel. c) Removal of the
template particles by dilute HF produces the dual responsive IOH.
cloud points of 5c by 6.78C, while the addition
of other, non-binding proteins, such as bovine
serum albumin (BSA), does not affect the cloud
point. These results demonstrate the basic double
responsiveness of the functionalized copolymers
toward the temperature as well as molecular
The nature and content of the cross-linker in the sensor
IOH must be carefully adapted, to allow effective contact/
penetration of the macromolecular analytes onto/into the
hydrogel walls, while ensuring a still sufficient refractive index
difference between the hydrogel matrix and the water-filled
voids. Good results were obtained with the oligomeric cross-
linker 3 in quantities of 19 mol% of the total comonomer
recognition processes.
When appropriately cross-linked, the thermoresponsive-
ness of these copolymer systems is preserved in inverse opal
hydrogels. This results in their pronounced shrinking upon
heating, with the concomitant marked shift of the Bragg peak
position and of the corresponding color. This is exemplified
for IOH-a in Figure 3. Characteristic for copolymers of 1 and
2
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Angew. Chem. Int. Ed. 2015, 54, 1 – 5
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