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Humidity-Sensing Inverse Opal Hydrogels Robert A. Barry and Pierre Wiltzius* Department of Materials Science and Engineering, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ReceiVed July 14, 2005. In Final Form: NoVember 9, 2005 Soft material hydrogel sensors have seen increased interest recently. Most of these sensors are used in an aqueous environment. In this study, we depart from this trend and analyze the ability of a periodic hydrogel structure to respond to variations in ambient humidity through an optical change. First, a polyacrylamide inverse opal hydrogel structure was created from a colloidal crystal template. Next, this material was tested under various humidity conditions and responded to these changes by shifting its optical reflection peak noticeably within the visible wavelength range. This effect opens the doors for these materials as humidity sensors. The kinetics of the peak shifts was also observed, showing a rapid response to ambient humidity changes. Finally, the structural dimension change is compared through peak shifts, Fabry-Perot fringes of the optical cavity, and scanning electron microscopy observations.
Introduction Considerable attention has recently been given to photonic crystals. The periodic refractive index of these materials gives rise to interesting optical properties.1-3 These effects can be utilized in several ways, one of which is using them as sensors to measure various environmental changes: pH, temperature, and others.4-11 A method that was devised to do this is forming a completed colloidal crystal12-14 and then back-filling it with hydrogel precursor material, cross-linking it, and finally removing the original colloids.15 The remaining inverse structure retains the opal characteristics of the original lattice of the photonic crystal. Due to the elastic properties of the low-density hydrogel material, the lattice constant of the inverse crystal can be changed fairly easily. Here we report a sensing scheme that also has the potential to be used in situations where others would not be feasible. While other, nonopaline optical hydrogel based humidity sensors that rely on optical power dissipation have been developed,16 the goal of the work presented in this paper is to produce thin film humidity sensors with response times on the order of seconds that can be placed in areas with restrictive geometric constraints or in systems where a reflection peak shift optical response is preferable to optical power dissipation in transmission or (1) Krieger, I. M.; O’Niell F. M. J. Am. Chem. Soc. 1968, 90, 3114-3120. (2) Rundquist, P. A.; Photinos P.; Jagannathan, S.; Asher, S. A. J. Chem. Phys. 1989, 91, 4932-4941. (3) Biswas, R.; Sigalas, M. M.; Subramania, G.; Ho, K. K. Phys. ReV. B 1998, 57, 3701-3705. (4) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829-832. (5) Arsenault A. C.; Kitaev, V.; et al. J. Mater. Chem. 2005, 15, 133-138. (6) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959-960. (7) Asher, S. A.; Alexeev, V. L.; Goponenko, A. V.; Sharma, A. C.; Lednev, I. K.; Wilcox, C. S.; Finegold D. N. J. Am. Chem. Soc. 2003, 125, 3322-3329. (8) Qian, W.; Gu, Z. Z.; Fujishima, A.; Sato, O. Langmuir 2002, 18, 45264529. (9) Pan, G.; Kesavamoorthy, R.; Asher, S. A. Phys. ReV. Lett. 1997, 78, 38603863. (10) Pan, G.; Kesavamoorthy, R.; Asher, S. A. J. Am. Chem. Soc. 1998, 120, 6525-6530. (11) Debord, J. D.; Lyon, L. A. J. Phys. Chem. B 2000, 104, 6327-6331. (12) Braun, P. V.; Zehner, R. W.; White, C. A.; Weldon, M. K.; Kloc, C.; Patel, S.; Wiltzius, P. AdV. Mater. 2001, 13, 721-724. (13) Xia, Y. N.; Gates, B.; Li, Z. Y. AdV. Mater. 2001, 13, 409-413. (14) Xia, Y. N.; Gates, B.; Yin, Y. D.; et al. AdV. Mater. 2000, 12, 693-713. (15) Park, S. H.; Xia, Y. AdV. Mater. 1998, 10, 1045-1048. (16) Arregui, F. J.; Ciaurriz, Z.; Oneca, M.; et al. Sensors Actuators BsChem. 2003, 96, 165-172.
techniques using electrical signals. The flexibility and dimensions of these inverse opal hydrogel (IOH) sensors allow their application in such systems where other sensors may not be suited. In tightly enclosed spaces, the 10 µm clearance needed in the vertical direction coupled with the small potential footprint of the sensor allow it to be included into enclosed spaces that would otherwise be inaccessible. The sensor can also be attached to a curved surface or one that may be bent; it does not always have to rest on a flat substrate. Work thus far on IOH sensors has focused on the detection of pH, ionic strength, or specific chemical groups.17-24 These systems operate on charge screening, solvent quality, or chemical attractions via specific chemical groups covalently attached to the hydrogel network. Some examples are the introduction of acrylic acid into a polymer to give it sensitivity to other charged groups and the addition of sugar-binding proteins and bound sugars or other chemicals in an effort to detect free sugar molecules.25,26 These methods require the hydrogel system to be immersed in water for structural support and for proper screening effects of the detected solute. Unlike these, however, the sensors discussed in this paper can operate outside of a supporting solvent. The methods of producing these previous hydrogel-based sensors are similar to these reported in this work. The specific additive for each system is included with the hydrogel precursor and integrated into the cross-linked network. However, like in the previous works that take advantage of solvent quality, the hydrogel itself may show intrinsic sensitivities without any additives. This study makes use of polyacrylamide’s natural hydrophilicity to induce dimensional change and create a novel humidity sensor that, unlike previous samples, does not need to be supported in (17) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534-9537. (18) Lee, Y. J.; Braun, P. V. AdV. Mater. 2003, 15, 563-566. (19) Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 9554-9557. (20) Fudozi, H.; Xia, Y. AdV. Mater. 2003, 15, 892-896. (21) Jiang, P.; Smith, D. W.; Ballato, J. M.; Foulger, S. H. AdV. Mater. 2005, 17, 179-184. (22) Walker, J. P.; Asher, S. A. Anal. Chem. 2005, 77, 1596-1600. (23) Asher, S. A.; Sharma, A. C.; Goponenko, A. V.; Ward, M. M. Anal. Chem. 2003, 75, 1676-1683. (24) Saito, H.; Takeoka, Y.; Watanabe, M. Chem. Commun. 2003, 21262127. (25) Lee, Y. J.; Pruzinsky, S. A.; Braun, P. V. Langmuir 2004, 20, 30963106. (26) Kataoka, K.; Miyazaki, H.; Bunya, M.; Okano, T.; Sakurai, Y. J. Am. Chem. Soc. 1998, 120, 12694-12695.
10.1021/la0519094 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/23/2005
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an aqueous environment, removing this constraint from the work done thus far with IOH. Experimental Section Preparation of the Flow Cell. The flow cell is constructed similarly to previous methods27 by sandwiching a 12-µm-thick Mylar film (Dupont) that has been cut into a 1 in. square outline 2 mm wide between two glass squares. The glass squares have both been cleaned overnight in a sulfuric acid-Nochromix (Godax Laboratories) solution and then thoroughly rinsed in deionized water. The bottom glass square is used shortly after soaking in a 0.05 M NaOH solution to make it uniformly hydrophilic. The top glass slide has its underside treated with a 2.5% octadecyltrichlorosilane (Acros) solution in hexane (Fisher) so that its exposed surface is hydrophobic enough to prevent sticking to either the hydrogel or colloids when removed from the cell. This step prevents the slide from sticking to the hydrogel layer and generates better crystals. The top slide also has a 2-mm hole drilled in it and a tube attached there for introduction of the colloidal mixture and the hydrogel solution after crystallization. The top and bottom slides are held together by a set of four clips, one on each corner. Before the structure is assembled, the Mylar spacer is dipped in a very dilute (0.08% PS colloids (IDC) in 3:1 water/ethanol) solution of colloids, causing colloids to stick to its surface. When compressed between the glass slides, these squished colloids create flow channels for the water to exit through the sides while being too small for other colloids to leave. Preparation of the Inverse Opal. PS colloidal solution (400 µL of 2.05% 450 nm) (IDC) was introduced into the tube over the cavity of a prepared flow cell. This reservoir tube is sealed on top with a rubber bulb, which is gently moved down over the opening to apply a slight pressure to the cell. This cell is placed on a “masking tape bridge” spanning a sonicator. This allows it to receive some sonication, but not so much to disrupt the crystallization. Water leaves through the sides and the colloids crystallize inward. After the entire space inside the spacer is filled with a colloidal crystal, the excess colloidal solution is removed from the reservoir and the flow cell is placed in a desiccator for drying. After drying is complete, the cell is then backfilled by an acrylamide solution of 47.9% acrylamide (Acros), 47.9% water, 3.8% bisacrylamide crosslinker (ICN Biomedicals), and 0.4% diethoxyacetophenone photoinitiator (Acros), which is allowed to permeate for a controlled time on the basis of the desired properties of the final gel. After the solution permeation is complete, the monomer is then cross-linked by 40 min of exposure to ultraviolet light from a 100-W shortwave UV mercury lamp (UVP, B-100A). Conventional Humidity Shift Measurement Procedure. Infrared (905-1680 nm) and visible (292-1056 nm) range spectrum analyzers (Control Development) are connected by a fiberoptic cable to a Zeiss inverted light microscope. These spectrum analyzers have low fluctuations in measurement and are self-consistent over time with a standard deviation of 0.02%-0.04% in the region of interest. The sample is held in a small flow cell sealed by an O-ring around the edges and vacuum grease under the glass slide on which the sample rests. A dry nitrogen tank supplies the flow to the system, which is split into two lines. One of these lines flows through a series of three bubblers designed to increase its humidity. The other line goes straight to the flow controller (Omega). Both of these lines are mixed in the flow controller, allowing the determination of the humidity of the air flowing through the sample cell. A traceable hygrometer (FisherSci) was used to measure the output and received air directly from the outlet of the humidity-controlling flow cell. During the humidity shift procedure, the spectrometers and lamp were allowed to warm for 3 h. For the kinetics experiments, measurements were taken every minute. Other experiments had time increments of 20 min for “stepping cycles” that involved relative humidity shifts of 50% or less. Large shifts of more than 50% relative (27) Lu, Y.; Yin, Y. D.; Gates, B.; et al. Langmuir 2001, 17, 6344-635.
Figure 1. Reflectance spectra of typical IOH under high (80% RH) and low (20% RH) humidity conditions. humidity were allowed 30 min to equilibrate. During these long equilibration times, the sample was kept shielded from direct illumination by a shutter to prevent long-term overheating of the hydrogel. To actually change the inflowing humidity the ratio of flow rates of the dry and wet streams were adjusted. While the total combined flow from the controller was maintained at 40 mL/min. The humidity was verified with the hygrometer, although a slight lag was present due to the exchange of air of the entire chamber. Measurement time of the spectra was kept at 2 s to minimize extra noise. The reflection percent of the results is calibrated to a silver mirror for the visible wavelength spectrometer and a gold mirror for the infrared. Direct Flow Humidity Shift Procedure. The same infrared (9051680 nm) and visible (292-1056 nm) range spectrum analyzers and their warm and measurement procedures were used in this method. The light source and its warm procedures were kept the same as well. The flow in this technique is applied from an unregulated source, which has been flown through a series of bubblers filled with water to raise its humidity. After this step the air is at 80% relative humidity. The flow is then directed through a stretch of hose and an empty bubbler to catch any droplets of water that might be in the stream. Finally, this flow is directed over the sample for the desired amount of time and then removed. This method was adopted due to the fact that, while the flowcontrolled techniques gave good results for equilibrium shifting, their kinetics is severely limited by the low flow rate of the system.
Results The optical properties of the hydrogel films were measured with spectrometers as described in the Experimental Section. Specifically, the reflectance of the films was measured as a function of wavelength under a variety of environmental conditions. Figure 1 illustrates the change in reflectance as the relative humidity (RH) varies from 20% to 80%. The peak reflectance of the inverse opal hydrogel at 20% RH lies at 538 nm. When the relative humidity is increased to 80% RH, the peak in reflection increases to 580 nm. The kinetics of this shifting will be discussed later. Figure 2 shows the peak position and shift difference between hydrogels in which the acrylamide solution was allowed to infiltrate for 0.5 and 24 h. “Infiltration time’”denotes how long the acrylamide hydrogel solution is left in the colloidal crystal prior to cross-linking (see Experimental Section for more detail). The hydrogel with the longer infiltration time shows a 20% RH reflectance peak at much larger wavelength (670.5 nm) than in the sample with the shorter infiltration time (519.7 nm). The wet peak position of these spectra has moved as well. The sample
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Figure 4. Spectra of IOH with 24-h infiltration time extended into longer wavelengths for Fabry-Perot analysis under 20% RH and 80% RH conditions.
Figure 2. Spectra shift of IOH for sample with (a) 24-h infiltration time and (b) 1.5-h infiltration time. The shift occurred in both cases by changing the relative humidity from 20% to 80%.
Figure 5. SEM image of an IOH lattice of the type used in this work.
Discussion
Figure 3. IOH spectra shift with time from ambient conditions (50%) to 80% RH. Humid air flow was applied at 0 s and removed at 50 s. The sample cycles back to the original peak position after the flow is removed.
with the longer infiltration time has a wet reflectance peak at 693.3 nm, while the shorter infiltration time produces a wet reflection peak of 550.5 nm. Figure 3 shows the peak shift variation with respect to time when taking an IOH from ambient conditions (50.2% RH) to 80% RH. This kinetics test was carried out using the direct flow mechanism as explained in the Experimental Section. The hydrogel’s reflectance peak begins at 535.7 nm and ends after 50 s at 568.5 nm. Spectra were taken every 2 s, and can be seen broken down into a time analysis in Figure 9. Figure 3 also shows the reflection spectra and reversibility for the full cycling of the sample. The flow is removed at 50 s and the peak rapidly decreases back to its ambient level. As can be seen in Figure 4, the reflection spectrum has significant features beyond the primary peak. These smaller peaks, which will be addressed later, are due to Fabry-Perot interference between the top and bottom surfaces and can be used to determine the thickness of the hydrogel film.
IOH sensors were synthesized that are hydrophilic and maintain structural integrity in a nonaqueous environment, allowing them to react to the relative humidity of the air. These humidity sensitive hydrogels work via the change in optical properties. Other IOH sensors require an aqueous environment to be functional and rely on charge screening from water solvated ions or relatively weak interactions.10-12 These approaches require the hydrogel to be supported in an aqueous solution. The humidity sensing IOH structures developed here are robust and can retain their periodic structure under vacuum conditions (Figure 5). The periodicity from the original colloidal template pictured here extends through the structure, though the Z direction has a shorter periodicity due to contraction. These inverse opal hydrogel structures are able to have this useful response to humidity due to the chemical structure of their constituents. The acrylamide that makes up the polymer has excellent hydrophilicity and water absorption thanks to its chemical structure.28 When the ambient humidity is increased, the hydrophilic acrylamide hydrogel absorbs water from the air to reach a new equilibrium. As the structure absorbs this water, it deforms from its previous equilibrium contracted structure, attempting to swell in all directions. However this acrylamide hydrogel is well bound to the hydrophilic glass substrate. The other inner surface of the flow cell is hydrophobically treated, preventing adhesion. The exact method of adhesion to the hydrophilic surface is not known, but due to the surface attachment, considerable swelling occurs primarily in the direction perpendicular to the surface. The shift of the reflectance spectrum peak is a result of this swelling. The periodic refractive index contrast of the vertically periodic layers results in constructive interference of light (28) Day, J. C.; Robb, I. D. Polymer 1981, 22, 1530-1533.
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Figure 6. Cross sectional SEM image of contracted dry IOH structure.
according to the Bragg equation.29 The swelling causes a spacing increase between these layers of pores and hydrogel. When this hydrogel is absorbing water from the air, the water brought into the polymer structure will change that structure’s refractive index, bringing the average of the water-hydrogel composite down with more water added. The dry, collapsed hydrogel has a refractive index of about 1.49, while water has a value of 1.33. As the hydrogel absorbs more water, the average will steadily shift down toward the value of water. This effect will be addressed in more detail later. The Bragg reflection that occurs in these inverse opals is caused by interference of reflected light from the regular spaced alternating layers of refractive index. Because all of the measurements in this study were taken at a 90° normal angle to the surface, the angular aspect of the Bragg equation reduces to 1 and the equation can be simplified. The increased distance between the layers causes a red-shift in the reflection peak, as predicted by Bragg’s equation, shown here simplified for normal incidence.
λ ) 2dneff
(1)
Here λ is the wavelength of reflected light, d is the interlayer spacing, and neff is the effective refractive index of the structure. As the structure swells the Bragg wavelengths get longer as well, causing the reflected peak to red-shift. When the structure shrinks, this process reverses and the peak is blue-shifted back toward its dry equilibrium position. The contracted structure, which is the equilibrium state for the hydrogel in the absence of water, can be seen in Figure 6. This structure varies from the initial state in that the spacing of layers in the Z direction has reduced while the dimensions along the other axes remain the same. Figure 5 shows what the periodicity in all directions looks like in a “noncontracted” system. The relative humidity in the environment controls the amount of water absorbed into this structure and the degree of swelling. This sets up a balance between structure strain due to deformation and absorbance of water. Increasing the accessibility of water to the system changes this balance and relocates the corresponding equilibrium for the IOH to a reproducible point. This balance also drives the structure to contract back down when water is removed and allows cycling and reversibility of the sensor system. While the position of the peak shifts with varying humidity, the range of that shift can be adjusted by tuning the density of the polymer making up the hydrogel. One way to control the amount of the final hydrogel is by varying the solution infiltration time. This is the time that the precursor mixture is allowed to (29) Hiltner, P. A.; Krieger, I. M. J. Phys. Chem. 1969, 73, 2386-2389.
Figure 7. Peak position shift with time under direct humid air flow from 0 to 50 s. At t ) 0, a stream of humid air is applied, the flow is removed at 50 s, and the sample returns to room conditions.
flow into the dry colloidal crystal. As the solution is left in the colloid matrix, water will continue to evaporate out at the edges while polyacrylamide constituents are retained. This slowly increases the concentration of both monomer and cross-linker in the crystal matrix, yielding a more densely packed hydrogel matrix when cross-linked. Preliminary results also suggest that the response of the hydrogel can be tuned by controlling the elastic modulus of the structure. This can be done by varying the initial ratio of monomer to cross-linker before polymerization. Modifications of this sort allow some customizability in the final matrix composition and control over the final swelling and reflection properties. The initial position of the peak at a given humidity varies according to original size of colloids used to make the photonic crystal, the hydrogel composition, and the solution infiltration time. After formation of the colloidal crystal, the hydrogel is infiltrated into this preform with identical microstructure dimensions. The etching of the original colloids removes this constraint and the removal of the water causes the hydrogel to contract to a new equilibrium size. This size is determined by the amount of hydrogel material that remains after cross-linking and drying. Because the hydrogel solution is roughly 50% water, a typical dried form has microstructure characteristics in the surfaceperpendicular dimension about half the size of the original colloids. The entire range of wavelength shifting due to humidity is dependent on the starting location of the peak. If an inverse opal hydrogel is more collapsed and has its dry peak at a lower position, then its swollen peak shift will be lower than the corresponding swollen peak from an IOH with a higher dry peak. The shifting range tends to decrease slowly as the dry peak position increases for structures made from equally sized colloids. Figure 7 shows a more detailed analysis of the peak shift kinetics for a sample under the direct flow procedure. For this graph, the reflection spectra were taken every 2 s and analyzed to determine the peak positions. When exposed to a change in relative humidity, the IOH undergoes its most drastic change shortly after exposure. When the sample is moving from lower to higher humidity, most of the shift occurs in the first 10-20 s, with change dropping off quickly after that. A similar trend is seen in the case of decreasing humidity. There is a difference between the cases, however. When drying out, the inverse opal structure responds more slowly than when it is taking up water. This is a result of the hydrophilicity of the acrylamide hydrogel. The structure absorbs water more readily than it releases it. This basic behavior with time holds for other optical properties as well. Figure 9 illustrates how this change is mirrored in the normalized full width at half-maximum (fwhm) and peak height of the spectra of the IOH. The similarity of the change in these
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Langmuir, Vol. 22, No. 3, 2006 1373 Table 1. Time Taken for the Respective Values to Make 90% of Their Total Shift (a) 0% f 90% and 100% f 10% shift times shift time (s) t90 t90(100f10)
peak pos (s)
normalized fwhm (s)
peak height (s)
13.8 18.7
20.4 11.5
13.4 18.1
(b) peak position shifts with extreme infiltration times infiltration time (h)
t0-90 (s)
t100-10 (s)
1.5 24
17.8 9.4
13.3 15.0
a Peak position, fwhm, and peak height for a typical IOH. b Peak position shifts for short- and long-time infiltration IOH samples.
Figure 9. (a) SEM cross section of dry IOH and (b) ESEM image of IOH in humid environment (6.3 Torr of H2O pressure at 4 °C).
shift from 10% to 90% of the overall change in Figures 7 and 8 can be seen in Table 1. This shows the rapid time scale over which the bulk of the shift occurs for these various properties. From section a of Table 1, it can be seen that the time scale of the peak position and peak height shifts are very similar while the fwhm shift is markedly different. This close correspondence of the peak position and peak height shifts implies that they are derived from the same structure transition. This is most likely the result of the structure swelling in the Z direction with its absorption of water. The fwhm is more an indication of structure regularity and may lag behind as the structure’s smaller features change with contraction and expansion. Section b of Table 1 shows the shift time of the peak for long and short infiltration time IOH sensors. The initial reaction to high humidity seems much faster in the long infiltration time sample. This is possibly due to it being less contracted and having an easier time mechanically swelling. The return of the peak to the dry state is slightly quicker for the shorter infiltration time sample, which may be related to its equilibrium “dry” structure being more collapsed. It should be remembered that, as the structure absorbs more water, the average refractive index of the IOH will decrease, approaching that of water. This will lower the contrast between the phases in the IOH, bringing the refractive index of composite hydrogel/water phase closer to that of air. Reducing the refractive index contrast in this IOH will reduce the reflection efficiency of the effective Bragg grating and lower the height of the IOH’s reflection peak, in accordance with eq 1. The lowering of the composite structure’s average refractive index will also affect the wavelength position of the peak. There are three key refractive indices to keep in mind in this system: the refractive index of air (1), water (1.33), and cross-linked polyacrylamide (1.49). This value of n for polyacrylamide was obtained from a refractometer and verified with SEM thickness measurements and reflection spectra of our system and is very close to other published values.30 Because the pores are always filled with air, their refractive index does not change, but as water is absorbed into the inverse opal structure, its refractive index decreases. This reduction in refractive index is small, however. The refractive index of a two-phase structure can be approximated by eq 2, where neff is the effective refractive index of the structure, and Vo,p and no,p are the volume fractions and refractive indecies of the opal structure and pores, respectively.
properties to the peak position shift implies that they are all affected similarly by the swelling state of the IOH. The time to
neff ) (Vono2 + Vpnp2)0.5
Figure 8. (a) Normalized fwhm and (b) peak height shift with time. Both under direct humid air flow started at t ) 0 and removed at t ) 50 s.
(2)
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Table 2. Comparison of Peak Position and Fabry-Perot Shifts dry f wet interlayer swelling infiltration time (h)
% peak pos. shift
% Fabry-Perot shift
Fabry-Perot/ peak size ratio
24 0.5
3.6 6.2
2.5 5.1
0.989 0.990
In an inverse opal face-centered cubic structure such as this one, the open pores constitute 74% of the volume, while the cross-linked structure is 26%. If an initially dry IOH swells 5% (a typical figure for the change in wavelength peak seen when moving to higher humidity), then it can be assumed to have taken up another 5% of its volume in water, causing a change in the refractive index of the polymer phase from 1.49 of cross-linked acrylamide to 1.483. This in turn causes a change in the overall composite refractive index, moving it from 1.1477 to 1.1452, a relatively minor change. This change of 0.2% alone would only account for a peak shift of 1-2 nm. This is not a large contributing factor to the peak position movement upon exposure to humidity. Also, this minor drop in refractive index is counter to the swelling effect, reducing the degree of the reflectance peak shift. The shift to longer wavelengths is due purely to structure swelling, since it is working against this refractive index change phenomenon. To verify that the peak shift is due to structure swelling, FabryPerot fringes were employed to measure the change in structure thickness. These fringes are a result of interference between the top and bottom layer of the hydrogel and should reflect the same uniform swelling ratio as that between the layers which we assume causes the direct reflection shift. This is because the structure should be swelling uniformly in the Z direction, increasing the spacing between the layers and the thickness of the entire structure by the same factor. Table 2 shows the swelling results as derived from the FabryPerot fringes for samples with long and short infiltration times. The data gathered from these interference fringes shows that the structure does in fact swell in the bulk. While both methods show similar swelling, there is some discrepancy between the swelling amounts from peak measurements and those generated by the Fabry-Perot fringes. This could be due to differences in the structure; the less dense structure has more freedom to swell, while the denser 24-h infiltrated gel will have more cross-links and restrictions to increasing its size. Overall though, both methods of determining swelling agree that the structure does increase its interlayer spacing. Both methods also give a similar magnitude of change, in agreement to within 1-2% of total swelling. Another test conducted on the IOH was to swell it by increasing the relative humidity in an environmental SEM (ESEM) while monitoring its cross section to examine the size change under different humidity conditions. The ESEM can operate under nonhigh-vacuum conditions, so water vapor can be used. This allows us to swell our gel while monitoring it. Due to operating conditions (30) Franklin, J.; Wang, Z. Y. Chem. Mater. 2002, 14, 4487-4489.
for the ESEM, the humidity was only on target directly adjacent to the cooled surface on which the sample was fixed. We cannot assume that the relative humidity was the same for all exposed areas of the IOH. As a consequence of this, water was condensed onto the IOH. Inside the chamber the partial pressure of water vapor was maintained at 6.3 Torr, while the sample stage was held at 4 °C, this combination of factors serves to raise the relative humidity of the air directly at the cooled surface to 100% and begins condensation. When the humidity passes the necessary condensation point, the IOH begins to swell, more than doubling in size in less than 1 min. This dimensional change is much more drastic than any seen in the humidity variation tests, where changes rarely exceed 10%. This effect can be seen in Figure 9. We speculate that the dramatic thickness increase is a result of physical forces caused by oversaturation of water in the structure. Instead of simply absorbing water into the polymer area, the water is filling the structure in its entirety, including the pores. This exerts additional forces to expand and accommodate all of the additional water that normally would not be taken into the IOH simply as a result of ambient humidity.
Conclusions We have created humidity sensitive inverse opal hydrogels of acrylamide polymer using colloidal opal structures. These IOH structures exhibit reflectance spectra with highly differentiable peaks due to their layered structure and Bragg reflection properties. The IOH structures respond to varying levels of relative humidity by causing a rapid shift in their reflectance spectra due to interlayer swelling and shrinking of the structure. The thin film and flexible nature of these hydrogel sensors allows a large degree of versatility in environment and application. Because they can be tailored in shape and thickness, these hydrogel sensors could be put in constrained environments where an internally contained humidity sensor would be of value. In fact, these hydrogels could be put into nearly any position or geometry as long as there was a line of sight to monitor their reflection spectra. These structures present an elegant method for humidity detection. The displacement of the peak position from some base would be useful to form these robust IOH films into an optical humidity sensor system. Also, the range over which the peak shifts can be tuned by adjusting the conditions of the original hydrogel synthesis. Due to their structural flexibility and tunability, these sensors may be used in humidity-sensing situations in which conventional sensors cannot. Acknowledgment. This material is based upon work supported by, or in part by, the U.S Army Research Laboratory and the U.S. Army Research Office under contract/grant number DAAD19-03-1-0227. We would like to acknowledge the Beckman ITG for assistance with microscopy, spectroscopy, and facilities. We would also like to extend thanks to Stephanie Pruzinsky and Paul Braun for helpful discussions. LA0519094
