By Yun-Ju Lee and Paul V. Braun*
Synthesis of three-dimensional (3D) functional structures
through colloidal templating is a topic of growing interest in
materials chemistry, especially in the fields of optics, chemical
sensing, and microfluidics. Colloidal crystals with 3D translational order can be formed from suspension by self-assembly
methods such as patterned sedimentation,[1] controlled evaporation,[2,3] sonication,[4] and other methods.[5] These colloidal
crystals have been utilized to template the growth of mesostructured semiconductors,[3,6,7] metals,[8±10] and polymers.[11±21]
Because the colloidal template has a characteristic spacing of
the order of hundreds of nanometers, the resulting structure interacts strongly with visible and infrared light, leading to optical diffraction that follows Bragg's law:
!1=2
1=2
P 2
8
2
k ˆ 2dneff ˆ
D
ni Vi sin u
(1)
3
i
where d is the characteristic spacing, D is the center-to-center
distance of the colloidal crystal composite, n and V are, respectively, the refractive index and the volume fraction of
each component phase, and u is the angle between the incident beam and the sample normal. Periodic hydrogel structures are especially interesting because hydrogels may be
functionalized to respond to a variety of physical and chemical stimuli by changing their dimensions.[22±25] When tailored
correctly, such response in a 3D structure can lead to large
shifts in the Bragg diffraction wavelength, thus creating functional materials with tunable optical properties.[26±28] Applications for such materials may include chemical and biological
sensing, optical switching, and microfluidic flow-control elements. Here, using commercially available colloidal suspensions and chemicals, we demonstrate the synthesis and optical
diffraction of robust inverse opal hydrogel sensors with a tunable pH sensitivity of potentially 0.01 pH units. In addition,
the swelling kinetics of this mesoporous polymer film was
studied.
There have been several recent publications on the formation of self-assembled 3D hydrogel mesostructures. One method involves the synthesis of colloidal particles of the hydrogel
poly(N-isopropyl acrylamide) (PNIPAM), which were subse-
±
[*] Prof. P. V. Braun, Y.-J. Lee
Department of Materials Science and Engineering
Beckman Institute for Advanced Science and Technology
and Frederick Seitz Materials Research Laboratory
University of Illinois at Urbana-Champaign
1304 West Green St., Urbana, IL 61801 (USA)
E-mail: [email protected]
[**] This material is based upon work supported in part by the U.S. Department of Energy, Division of Materials Sciences under Award No.
DEFG02-91ER45439, through the Frederick Seitz Materials Research
Laboratory at the University of Illinois at Urbana-Champaign, and carried out in part in the Center for Microanalysis of Materials, University of
Illinois, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439. We thank an anonymous reviewer for
pointing out several important points on hydrogel behavior.
Adv. Mater. 2003, 15, No. 7±8, April 17
DOI: 10.1002/adma.200304588
quently organized into ordered arrays.[17,29±31] These periodic
structures displayed thermally induced shifts in optical diffraction due to transformation of the hydrogel from a swollen
hydrophilic network below the lower critical solution temperature (LCST) to a collapsed hydrophobic network above
LCST. Another method utilizes a dried colloidal crystal to
template the polymerization of PNIPAM.[32] After etching
away the template, the remaining PNIPAM inverse opal
exhibited color changes which were dependent on the applied
temperature. A third method, pioneered by Asher and coworkers, generates hydrogel±colloidal array composites using
photopolymerization of dilute acrylamide precursors inside an
ordered matrix of charge stabilized colloids. When copolymerized with crown ethers, the colloidal composite demonstrated
excellent diffraction sensitivity to heavy metal ions.[18±20] A
similar hydrogel exhibited diffraction shifts with applied mechanical stress.[11,33] A pH- and ionic-strength-sensitive hydrogel±colloidal array composite was synthesized using the same
acrylamide±colloidal array system followed by partial hydrolysis,[34] and its optical response was successfully modeled
using free energy equations of charged polyelectrolyte networks first described by Flory.[35] When the colloidal template
was etched away, an ordered array of isolated water-filled
pores was generated, and entropic entrapment of macromolecules inside these pores were observed by diffraction and
spectral absorption.[36,37] However, each of the above methods
contains limitations which we believe can be circumvented
with our system. Hydrogel colloidal crystals, like most wet colloidal crystals, are not robust to shock or drying, which can
cause irreversible collapse of the ordered structures. Hydrogel±colloidal array composites require the use of non-ionic hydrogel precursors to avoid disrupting the charge stabilized colloidal array. Thus, ionic moieties such as pH-sensitive groups
must be incorporated after polymerization through hydrolysis.
By using highly charged colloidal arrays as templates, bicontinuous structures are not formed, and thus the diffusion of
ions and water may be slow unless the hydrogel phase is fairly
dilute. Finally, due to the softness of hydrogel colloids and
some hydrogel±colloidal array composites, integration with a
substrate or into a microfluidic system may be problematic.
We synthesized bicontinuous pH-sensitive inverse opal hydrogels with tunable optical response using colloidal crystal
templates. Among the advantages for this system are facile
diffusion and thus rapid response owing to interconnected
pore structure and good mechanical stability of the concentrated hydrogel. Furthermore, a large variety of functional
groups may be incorporated to confer sensitivity to different
stimuli within the hydrogel structure. A mixture of 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AA) was selected as the building block for the pH-sensitive hydrogel because such a system has been successfully employed to
generate mechanically stable pH-sensitive flow-control valves
on the micrometer scale.[38] Several different concentrations
of AA in HEMA were chosen to determine possible effects
on pH sensitivity. The precursor mixtures were photopolymerized inside dried colloidal crystals made from polystyrene la-
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Tunable Inverse Opal Hydrogel pH Sensors**
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tex suspension. Following colloidal template removal, visibly
opalescent inverse opal hydrogel films were formed (see Experimental). Scanning electron microscopy (SEM) revealed a
highly ordered array of pores (Fig. 1); the average center-to-
(a)
(b)
Fig. 1. Structure of templated inverse opal hydrogel films. a) SEM image of the
2.5 % AA inverse opal hydrogel. Defect structures of the colloidal templates,
such as vacancies and point defects, were duplicated with high fidelity, yielding
single domains ~ 10 lm across. b) High-magnification SEM image of the same
hydrogel, showing the expected interconnected triplet pore structure.
center spacing between nearest neighbors was 243 nm, which
represents ~ 10 % shrinkage compared with the nominal particle diameter (270 nm). The persistence of the periodic structure under vacuum and the moderate shrinkage suggest that
the hydrogel is mechanically robust.
The pH-dependent optical diffraction of the mesoporous
hydrogel films was characterized using a vis-NIR microspectrometer. Diffracted light from the hydrogel films in phosphate buffers at pH ³ 2 was collected by a reflected light microscope coupled to a photodiode array spectrometer via an
optical fiber, allowing fast collection of spectra from one or a
few domains (Fig. 2a). Figure 2b presents the pH dependence
of diffraction for hydrogels containing various AA concentrations. At pH £ 4, the 5 % AA hydrogel film diffracted 544 nm
light, which is quite close to the value calculated using the
Bragg equation (547 nm), assuming an inverse opal of hydrogel (n = 1.50) in water. As the pH increased, the diffraction
steadily red-shifted until it reached 850 nm at pH 6.94. The
unitless diffraction shift (Dk/k0) for the film is equal to 0.56. In
comparison, the 2.5 % AA hydrogel diffraction red-shifted
from 517 nm to 652 nm at pH 7.01, a Dk/k0 of 0.26. The ratio
of the Dk/k0 values (2.15) is in good agreement with the acid
concentration ratio of the two samples (~ 2). Because diffraction probes the spacing between (111) planes, this result suggests that the swelling is largely confined to one dimension.
Such behavior is reasonable because the hydrogel film is an564
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Fig. 2. Optical spectroscopy of pH-sensitive inverse opal hydrogel films.
a) Schematic diagram of the microspectrometer. b) Reflection spectra of inverse opal hydrogel films containing different concentrations of AA. Inset: Reflection spectra of inverse opal hydrogel with 5 % AA at different pH. c) Ionic
strength dependence of 5 % AA hydrogel in unbuffered 1 mM and 0.01 mM
HCl (aq.) solutions. Inset: Reflection spectra of inverse opal hydrogel with 5 %
AA in solution of 0.01 mM HCl and different concentrations of KCl (in mM).
chored on one side to the substrate, and thus can only swell
vertically. A control sample containing only HEMA exhibited
a Dk/k0 of 0.03, which can be explained by small quantities of
acidic impurities in as-received HEMA.[39] When the direction
of pH change was reversed, the diffraction of the AA containing films blue-shifted until the original diffraction wavelengths were recovered. Interestingly, an apparent hysteresis
was present in the diffraction response for increasing and decreasing pH sweeps (Fig. 2b). As we will explain shortly, this
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Adv. Mater. 2003, 15, No. 7±8, April 17
2
s
k
k0
@1‡
!3
l02
0
k
Cp
Ka
A
(2)
Dion
where k and k0 are the diffraction wavelengths for the swollen
and unswollen gel, l0 is the thickness of the unswollen hydrogel, Cp is the concentration of acidic moiety in the unswollen
hydrogel, Ka is the acid dissociation constant for the acidic
moiety, and Dion is the diffusivity of H+ or OH± ions in water.
Figure 3a presents the diffraction shift kinetics of the 5 % AA
inverse opal hydrogel between pH 4 and 5.4. Increasing pH
from 4.03 to 5.40 (diamonds) caused diffraction to slowly redshift from 557 nm at t = 0 s to 599 nm at t = 480 s, followed by
a fast red-shift to equilibration at 693 nm by t = 1200 s. The kinetics of the fast diffraction shift points to a diffusion limited
process (Dk/kat1/2). Decreasing pH from 5.39 to 4.08 resulted
in a similar behavior, where diffraction slowly blue-shifted
from 696 nm at t = 0 s to 682 nm at t = 320 s followed by a fast
blue-shift to equilibration at 562 nm by t = 1120 s. Figure 3b
shows the diffraction shift kinetics of the same hydrogel as pH
red-shifted from 5.42 to 6.81. Again, a delayed diffusion-lim-
Adv. Mater. 2003, 15, No. 7±8, April 17
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hysteresis disappears after about 20 min and can most likely
be attributed to the kinetics of counter-iron diffusion, however, the slow relaxation of entangled crosslinks in the hydrogel structure may also contribute to the hysteresis.
The ionic strength dependence of the pH-sensitive inverse
opal hydrogel was probed by varying the KCl concentration
in an unbuffered HCl (aq.) solution. When the 5 % AA hydrogel in unbuffered 1 mM HCl/0.3 M KCl (aq.) solution was
diluted with 1 mM HCl (aq.), its diffraction red-shifted from
549 nm to 565 nm (Fig. 2c). Because the hydrogel was in its
neutral state at this acid concentration, the observed swelling
(dk/k0 = 0.03) is most likely a result of increasing nonspecific
hydrophilic interactions between the hydrogel network and
water molecules as KCl concentration decreases, also known
as the ªsalting outº effect.[40] When KCl concentration was
varied at 0.01 mM HCl, the 5 % AA hydrogel, which was then
negatively charged, exhibited reversible diffraction shift between 615 nm at 300 mM KCl and 825 nm at 0.1 mM KCl.
The diffraction shift agrees qualitatively with the increasing
Donnan potential inside the polyelectrolyte networks as the
ionic strength decreases.
The pH-sensitive hydrogel films were thin (~ 23 lm) and
contained an interconnected network of pores. As a result,
pH-dependent diffraction-shift kinetics of the order of minutes was observed. By making several simplifying assumptions, the kinetics of swelling for a charged polyelectrolyte
network during pH change may be modeled as a hindered diffusion-limited process, where H+ and OH± ions, diffusing from
the bulk solution into the gel phase as a result of a concentration gradient, are annihilated when they come into contact
with immobilized deprotonated/protonated acid moieties on
the network.[41] Thus, the effective diffusion coefficient is decreased by the local pH condition, and the time for hydrogel
equilibration (s) is of the order of:
1
0
k
Fig. 3. pH-Dependent diffraction-shift kinetics of the 5 % AA inverse opal hydrogel film. a) Diffraction shift kinetics during pH increase from 4.03 to 5.40
(diamonds) and during pH decrease from 5.39 to 4.08 (squares). Both exhibit
diffusion-limited response with an initial time delay. b) Kinetics of pH-induced
diffraction shift as pH increased from 5.42 to 6.81. The initial time delay is
smaller, while the overall equilibration time remains unchanged. Inset: final diffraction wavelengths of 5 % AA hydrogel at various pH. When diffraction shifts
are allowed to equilibrate, the diffraction wavelengths of the hydrogel as pH increased (diamonds) overlap those as pH decreased (triangles), demonstrating
that the hysteresis in diffraction observed in Figure 2 is due to diffusion-limited
kinetics.
ited diffraction shift was observed: the diffraction red-shifted
from 703 nm at t = 0 s to 705 nm at t = 60 s, followed by a large
red-shift to 782 nm by t = 1120 s. Several conclusions can be
drawn from the diffraction shift kinetics. First, the equilibration times are quite close to one another, suggesting that the
overall diffusion kinetics in the three cases is similar. In addition, the actual equilibration times are of the same order as,
but larger than, the calculated equilibration time (s ~ 400 s).
Because the ions are diffusing in a dense hydrogel (20 % v/v
in water) rather than a dilute aqueous solution, their diffusivity is probably lower than the bulk value (~ 8 ” 10±5 cm2 s±1),
leading to an increase in equilibration time.[41] Second, the diffraction shift kinetics in all three cases consisted of a slow initial shift followed by a fast shift which was approximately diffusion-limited. This phenomenon may be explained by a
combination of the following increasing counter-ion diffusivity
as the dense hydrogel swells, and diffraction optics, where the
hydrogel layer that contributes most to the reflection spectra
is also the layer that is farthest away from the bulk solution
(Fig. 2a). Finally, as the inset in Figure 3b shows, when the hydrogel swelling is allowed to equilibrate, the diffraction wavelengths of the hydrogel during the increasing pH sweep (dia-
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monds) overlap those during the decreasing pH sweep (triangles), suggesting that the hysteresis in diffraction observed in
Figure 2b results from the short time allowed between pH
steps (~ 400 s). Work is underway to determine the relative
contributions of hydrogel density and optical factors to the kinetics of diffraction shift.
In this communication, we present the synthesis and characterization of mechanically robust, optically tunable inverse
opal hydrogels through colloidal crystal templating. Mesoporous hydrogels based on HEMA±AA copolymers exhibit pHdependent shifts in optical diffraction, the magnitude of which
can be tailored by varying the AA concentration. Such hydrogels also demonstrate ionic-strength-sensitive diffraction corresponding to both Donnan potential-induced swelling and
nonspecific polymer±water interactions. The kinetics of hydrogel expansion/contraction followed a delayed diffusionlimited process with equilibration time of ~ 1200 s. Efforts are
underway to accelerate and fine tune the diffraction response
by changing film dimensions and chemistry, with a goal of
creating diffraction-based chemical and biological sensors.
Experimental
The colloidal crystals were formed using a method modified from Lu et al.[4]
25 mm ” 25 mm glass slides (ªbottom slidesº) were cut from standard microscope slides (Fisher Scientific). 2.5 mm diameter holes were drilled through
some of the glass slides using diamond-tipped drill bit (ªtop slidesº). All slides
were then cleaned using H2SO4/Nochromix mixture for 12 h, rinsed with deionized (DI) H2O, submerged in 0.05M KOH (aq.) for 5 min, rinsed again with
DI H2O, then dried under flowing N2. Top slides were submerged in 2 mM (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylchlorosilane (Gelest) in toluene for
2 min to render them hydrophobic. A glass tube (5 mm ID, 25 mm length) was
epoxied to each top slide to form a reservoir. Spacers were made by cutting
15 mm ” 15 mm square holes inside 25 mm ” 25 mm ” 23 lm Mylar films (Dupont), which were then cleaned by sonication in ethanol. Each spacer was submerged in a 0.05 % v/v suspension of polystyrene colloids (diameter = 270 nm,
4 % solids, Interfacial Dynamics) in ethanol for 30 s. A cell was then assembled
by clamping together a bottom slide, a colloid decorated spacer, and a top slide/
reservoir with butterfly clips. 400 lL of undiluted polystyrene colloidal suspension (diameter = 270 nm, 4 % solids) was injected into the reservoir and capped
tightly with a standard rubber pipette bulb. Two strips of duct tape were affixed
on top of a sonicator (Fisher Scientific F530) in a crossed diagonal pattern. The
filled cell was placed at 30 to the horizontal on the edge of a glass Petri dish on
top of the tape. After 24 h of sonication, the cell was removed from the sonicator, excess colloidal suspension was decanted, and the cell was allowed to dry
overnight under a covered crystallization dish. 2-Hydroxyethyl methacrylate
(Acros), acrylic acid (Acros), ethylene glycol dimethacrylate (Aldrich), and Irgacure-651 (Ciba Specialty Chemicals) were used as received. 2.5 g HEMA,
AA (5 mol-% or 2.5 mol-% vs. HEMA), 25 mg EGDM, 75 mg Irgacure-651,
and 0.625 g H2O were mixed and then dispensed into the reservoir until it was
full. The reservoir was then capped with rubber bulb to allow the monomer
mixture to infiltrate the colloidal crystal. Once the colloidal crystal became
translucent, indicating successful infiltration, excess precursors were removed
from the reservoir, and the remaining mixture was photopolymerized at 365 nm
(Spectroline 11SC-1OP with long-wave UV filter) for 2 h. The two slides were
separated and then placed in chloroform (Acros) for at least 24 h to fully dissolve the polystyrene colloids. Phosphate buffer solutions of different pH values
were made by mixing 0.1 M KH2PO4 (aq.) (Acros) with different quantities of
0.1 M HCl (aq.) or 0.1 M NaOH (aq.), and their pH were measured with a pH
meter (Fisher Accumet AR10). The slide containing the mesoporous polymer,
usually the bottom slide, was dipped in ethanol for 10 s to remove the CHCl3,
after which it was placed in phosphate buffer solution at pH ~ 1.5.
SEM characterization of the samples was performed in a Hitachi S-4700
field-emission scanning electron microscope. Vis±near-IR spectroscopy was carried out using a microspectrometer consisting of a reflected-light microscope
(Zeiss Axiovert 135) with light output coupled to a linear photodiode array
spectrometer (Control Development, Inc., South Bend, IN) via an optical fiber.
566
Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
A 10”/0.25NA objective was used in combination with a 100 lm pinhole placed
directly in front of the optical fiber, resulting in a nominal spot size of 10 lm.
All spectra were referenced to a silvered mirror. Each sample was placed, hydrogel side up, in a polystyrene Petri dish, followed by immersion with solution.
To measure pH dependence of diffraction, pH was adjusted by removing ~ 50 %
of solution from the Petri dish, then adding phosphate buffer solutions to give
appropriate pH values. Reflection spectra were acquired for 10 s at ~ 400 s after
each pH change, and the corresponding pH was measured. To determine ionic
strength dependence of diffraction, unbuffered HCl (aq.) solution containing
0.3 M KCl was diluted by adding a solution containing the same concentration
of HCl with no KCl, and reflection spectra were collected for 10 s at ~ 400 s
after each dilution. For kinetic studies, individual spectra were integrated for
2 s every 10 s.
±
Received: October 5, 2002
Final version: December 30, 2002
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