Thermally Responsive Hydrogel Films Studied by Surface Plasmon

Feb 24, 2009 - Institute of Materials Research and Engineering, A-Star (Agency for Science, Technology and Research), 3 Research Link, Singapore 11760...
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Anal. Chem. 2009, 81, 2611–2617

Thermally Responsive Hydrogel Films Studied by Surface Plasmon Diffraction Nan Zhang† and Wolfgang Knoll*,†,‡ Institute of Materials Research and Engineering, A-Star (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, and Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Thermally responsive hydrogel gratings were micropatterned and immobilized on a gold coated glass substrate using a photochemical cross-linking and attachment method. The thermoresponsive behavior of hydrogels with different thicknesses was studied in MilliQ water by the surface plasmon diffraction (SPD) technique. The diffraction intensity can be substantially increased by surface plasmon field enhancement. Because of its self-reference property, SPD offers considerable advantages in the characterization of the thin hydrogel film. The real-time kinetic observation of the volume phase transition in thin hydrogel films can be achieved by monitoring the diffraction intensity variation as a function of temperature. Little diffraction intensity of the hydrogel grating was observed in the swollen state while strong intensity peaks were observed in the collapsed state owing to the large change in the optical contrast upon the volume phase transition. Stimuli-responsive polymers are very promising materials in biorelated applications such as drug delivery,1-3 biotechnology,4-6 and chromatography.7-9 For some devices, switchable surface properties (e.g., wettability or exposure of surface moieties) may be of interest, while in other platforms switching of some properties of the entire polymer film (e.g., its thickness, volume, or absorbance) may be desired. Poly(N-isopropylacrylamide) (PNIPAAm) is one of the most studied thermosensitive polymers that exhibits a volume phase transition in water upon increasing the temperature. This behavior is attributed to the hydrophobic character of the N-isopropyl side groups. Individual PNIPAAm chains swell with a random coil conformation at lower temperatures due to the “iceberg structure” formation of water surround* To whom correspondence should be addressed. Phone: +49 6131 379 161. Fax: +49 6131 379 360. E-mail: [email protected]. † A-Star (Agency for Science, Technology and Research). ‡ Max-Planck-Institute for Polymer Research. (1) Gupta, P.; Vermani, K.; Garg, S. Drug Discovery Today 2002, 7, 569–579. (2) Qiu, Y.; Park, K. Adv. Drug Delivery Rev. 2001, 5, 321–339. (3) Yokoyama, M. Drug Discovery Today 2002, 7, 426–432. (4) Jeong, B.; Gutowska, A. Trends Biotechnol. 2002, 20, 305–311. (5) Galaev, L. Y.; Mattiasson, B. Trends Biotechnol. 2000, 17, 335–340. (6) Sharma, S.; Kaur, P.; Jain, A.; Rajeswari, M. R.; Gupta, N. M. Biomacromolecules 2003, 4, 330–336. (7) Kikuchi, A.; Okano, T. Prog. Polym. Sci. 2002, 27, 1165–1193. (8) Kobayashi, J.; Kikuchi, A.; Sakai, K.; Okano, T. J. Chromatogr., A 2002, 958, 109–119. (9) Anastase-Ravion, S.; Ding, Z.; Pelle, A.; Hoffam, A. S.; Letourneu, D. J. Chromatogr., B 2001, 761, 247–254. 10.1021/ac802527j CCC: $40.75  2009 American Chemical Society Published on Web 02/24/2009

ing the hydrophobic N-isopropyl groups,10 but they collapse into a globule resulting from the aggregation of the PNIPAAm chain if the solution temperature is higher than the lower critical solution temperature (LSCT). This then leads to the melting of the iceberg structure. In bulk water solutions, this volume phase transition occurs around 32 °C over a narrow temperature range (1-2 °C). The collapse of PNIPAAm chains grafted onto a surface has been studied before,11-16 some reported a real-time observation of the kinetics of the phase transition.11,16 Optical diffraction at periodic spatial structures has been introduced as a detection principle to the field of sensor development for the past decade. However, the sensitivity of a conventional diffraction-based sensor in the label-free mode is usually poor.17 In 1987, it was proposed that the diffraction efficiency could be greatly enhanced by using plasmon surface polaritons (PSP)18,19 as the diffracted electromagnetic wave. In this case, light of a laser source is coupled to a surface plasmon mode through a prism (Kretschmann configuration) and a dielectric grating on the planar metal surface is used to diffract the nonradiative PSP field, which then couples out again via the prism. This is different from the way in which a surface plasmon is excited by a grating structure. The intensity of the diffraction orders were shown to be about 20 times stronger than that in the normal total internal diffraction (TID) mode.20 Moreover, the diffraction intensity is dependent only on the optical contrast at the interface which is a function of the dielectric constant of the layer and the grating amplitude. On the basis of this finding, surface plasmon enhanced diffraction (SPD) for biosensing was proposed recently21 and it has been used in oligonucleotide hybridization studies.22 In a conventional surface plasmon resonance (SPR) sensor, the reflectivity floats (10) Tanford, C. Physical Chemistry of Macromolecules; Wiley: New York, 1961. (11) Balamurugan, S.; Mendez, S.; Balamurugan, S. S.; O’Brien, M. J.; Lopez, G. Z. Langmuir 2003, 19, 2545–2549. (12) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163–6166. (13) Zhang, J.; Pelton, R.; Deng, Y. Langmuir 1995, 11, 2301–2302. (14) Kidoaki, S.; Ohya, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17, 2402– 2407. (15) Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. Adv. Mater. 2002, 14, 1130–1134. (16) Nayak, S.; Debord, S. B.; Lyon, L. A. Langmuir 2003, 19, 7374–7379. (17) Goh, J. B.; Tam, P. L.; Loo, R. W.; Goh, M. C. Anal. Biochem. 2003, 313, 262–266. (18) Rothenha¨usler, B.; Knoll, W. Opt. Commun. 1987, 63, 301–304. (19) Rothenha¨usler, B.; Knoll, W. Appl. Phys. Lett. 1987, 51, 783–785. (20) Yu, F.; Knoll, W. J. Nonlinear Opt. Phys. Mater. 2005, 14, 149–160. (21) Yu, F.; Tian, S. J.; Yao, D.; Knoll, W. Anal. Chem. 2004, 76, 3530–3535. (22) Yu, F.; Yao, D.; Knoll, W. Nucleic Acids Res. 2004, 32, e75–e81.

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Figure 1. (a) Adhesion promoter and poly(N-isopropylacrylamide) terpolymer used in this study. (b) Schematic of the experimental setup with a hydrogel grating on the LaSF9 glass substrate: (1) high refractive index glass substrate LaSF9 with 2 nm Cr and 50 nm Au coating, (2) hydrogel grating, (3) temperature sensor connected with temperature controller, (4) sealing glass, (5) hot plate with temperature sensor connected to temperature controller. (c) Schematic representation of the samples preparation for OWS and SPD measurement.

with temperature variation, leaving the film unchanged. Compared to this, it was found that SPD is insensitive toward this kind of temperature change.23 Tian et al.24 fabricated an electrochemically tunable grating and used it for the detection of the electrooxidation of β-nicotinamide adenine dinucleotide (NADH) which further demonstrated the potential of this technique. Since SPD is sensitive to a change of the optical contrast at the interface, it is suitable to characterize the thin hydrogel layers which have thickness changes associated with the volume phase transitions. In this paper, PNIPAAm terpolymer hydrogel film layers with different thicknesses were micropatterned, and the SPD technique was applied to study the thermal response behavior of the patterned hydrogel film in a kinetic mode. The real-time observation of the kinetics of the volume phase transition of a PNIPAAm thin film on a planar surface will provide more valuable information for possible applications. EXPERIMENTAL SECTION Materials. The (3-thiopropyl)oxybenzophenone (BP-thiol) used as an adhesion promoter and poly(N-isopropylacrylamide) terpolymer were synthesized by Matthias Junk at the Max Planck Institute for Polymer Research, Mainz, Germany. Their molecular structures are given in Figure 1a. For the assembly of the promotor monolayer, a concentration of 5 mmol/L BP-thiol in ethanol was used. The terpolymer of NIPAAm, methacrylic acid (MAA), and 4-methacryloyloxybenzophenone (MABP) was synthesized by free radical polymerization. The MAA was used to overcome the “skin barrier” effect25 when collapsing, and the MABP was chosen as the cross-linking unit. The composition of (23) Yu, F.; Knoll, W. Anal. Chem. 2004, 76, 1971–1975. (24) Tian, S. J.; Armstrong, N. R.; Knoll, W. Langmuir 2005, 21, 4656–4660. (25) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249.

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the polymer was PNIPAAm/MAA/MABP ) 100:18:10. The molecular weight (Mw) and the molecular weight distribution (polydispersity index, Mw/Mn) of the poly(N-isopropylacrylamide) terpolymer were 150 000 g/mol and 2.4, respectively, as determined by gel permeation chromatography (GPC). When used, the polymer was dissolved in ethanol. Sample Preparation. The samples preparation process is illustrated in Figure 1c. The SPR substrates were high-refractive index glass slides (LaSFN9) coated with 2 nm chromium and a 50 nm gold film, which were deposited by a thermal evaporator (Biemtron, Japan). The adhesion promoter BP-thiol was immobilized on the gold surface at room temperature from a 5 mM ethanol solution in a self-assembly process. The substrate was incubated in the solution overnight and then rinsed by ethanol to remove any unbound molecules and blown dry with nitrogen gas. Polymer films were deposited on the substrate by spin-coating the solution of the terpolymer onto the adhesion promoter layer. Typically, a spin speed of 1000 rpm was used for 5 s, followed by 3000 rpm for 1 min. Then the samples with the polymer films were dried in an oven at 60 °C and irradiated with UV light at an intensity of 4 mW/cm2 at 365 nm for 30 mins. A 330 nm thick hydrogel film was obtained from a 3 wt % polymer solution and a 25 nm hydrogel film from a 0.5 wt % polymer solution. Photochemical attachment is an effective tool to create microstructured substrates. The change in the solubility of a polymercoated substrate after exposure to the cross-linking radiation allows for the selective removal of some parts of the polymer coating. The photochemical attachment and cross-linking in this study is based on the reaction between the benzophenone group and the C-H bonds in the polymer backbone.26 The attachment (26) Prucker, O.; Naumann, A. C.; Ru ¨ he, J.; Knoll, W.; Frank, W. C. J. Am. Chem. Soc. 1999, 121, 8766–8770.

Figure 2. SPS/OWS measurements of a homogeneous hydrogel layer of 330 nm dry thickness at different temperatures.

of the benzophenones to a polymer chain leads to the covalent attachment of the chains to the substrate surface. A 400 mesh transmission electron microscopy (TEM) grid with parallel bars was used to cover parts of the spin-coated polymer film during the UV irradiation. The polymer film covered by the grid bars remained uncross-linked and, hence, could be removed by ethanol, while the areas between the grid bars exposed to the UV light resulted in a cross-linked polymer which was insoluble in ethanol. After the UV irradiation and rinse with ethanol, a hydrogel grating pattern was formed in the area of the TEM grid, while all other areas were covered with a homogeneous hydrogel film. Instrumental and Measurement. Surface Plasmon Spectroscopy (SPS)/Optical Waveguide Spectroscopy (OWS) Measurements. The setup used for the experiments is based on the prism-coupling mode SPR which is also called Kretschmann configuration. It has been described in great detail elsewhere.27 Briefly, a p-polarized HeNe laser with a wavelength of λ ) 633 nm modulated by a frequency chopper is used to excite the PSP. The light is reflected off the Au-coated base of the prism, and the reflected intensity is measured by a photodiode detector connected to a lock-in amplifier. The sample holder and the detector are controlled by two coaxial goniometers, respectively, which can turn their angular position independently. Both of them rotate in a θ/2θ mode in a SPS angular scan. If the film is thick enough, OWS measurements can be conducted with such a setup as well. SPD Measurements. The setup used for SPD measurements was based on the Kretschmann configuration. Figure 1b shows the schematic geometry for the SPD measurement in this work. Three SPD measurements modes were used in this work. First, in the diffraction angular mode, the incident angle of the laser was fixed at the SPR minimum angle which corresponds to surface plasmon mode excitation. The diffraction intensity was recorded with the detector motor rotated at angle φ, sweeping through the different diffraction orders. A 1 mm slit was used, resulting in an angle resolution of ∆φ ) 0.08°. Second, if the detector was fixed (27) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569–638.

at the angle corresponding to the first diffraction order and a θ/2θ scan was recorded as in a normal SPS angular scan, the diffraction intensity variation as a function of the incident angle was obtained. Third, if the incident angle of the laser was fixed at the SPR minimum angle and the detector was fixed at the first diffraction order, the diffraction intensity could be recorded as a function of time. If simultaneously, the ramped temperature was also recorded, this time dependence could be converted to a corresponding temperature change. We call this measurement a diffraction kinetic mode. Swelling Experimental. In order to control the temperature of the water in contact with the hydrogel, a home-built SPR flow cell holder with a hot stage was used. The substrate (LaSFN9 glass slide) and the flow cell were fixed on the hot stage, as shown in Figure 1b. The temperature of the water in the flow cell was measured with a temperature sensor connected to a temperature controller. Another temperature sensor connected to the temperature controller monitored the temperature of the hot stage. Both temperatures were read by a computer along with the recorded SPR data. The temperature in the flow cell could be adjusted by activating the hot stage and/or by cooling with water surrounding the hot stage. In order to operate with a stable system, the hydrogel film layer was left to swell for 1 h at room temperature (25 °C), followed by a temperature-dependent swelling and collapse cycle carried out before the collection of SPS/OWS and SPD data. After this initial treatment, the film composition and its structure do not change over time and the swelling/ collapse process is fully reversible. The hydrogel was left to swell for 15 mins at each measured temperature to reach thermal equilibrium before SPS/OWS measurement. The temperature inside the flow cell was accurate within 0.1 °C. The response time of the gels is on the order of seconds, much faster than the time scale of the temperature change in the flow cell. RESULTS AND DISCUSSION Homogenous Hydrogel Layers. It has been demonstrated that SPS/OWS is a suitable tool for the characterization of polymer Analytical Chemistry, Vol. 81, No. 7, April 1, 2009

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films with the thickness ranging from several hundred nanometers to a few micrometers.28-30 In SPS measurements, the reflected intensity of the light is recorded as a function of incident angle and decreases dramatically as the light couples to the plasmon resonance mode of the metal or to the waveguide modes of the dielectric layer. The evanescent tail of the plasmon decays exponentially into the dielectric and therefore SPR is very surface sensitive. For a film of known thickness, this can be used to determine the refractive index of the dielectric film. Conversely, assuming a constant refractive index, this method can also be used to determine the film thickness. If the dielectric film is sufficiently thick so as to act as a planar waveguide, the SPS setup can also be used for OWS. For these thicker films, the position of the reflectivity minimum is sensitive only to the refractive index of the polymeric material near to the interface to the solid substrate, while the position of the waveguide modes depends on both film thickness and refractive index. If more than one waveguide mode can be found in the OWS spectrum, generally, the higher order waveguide modes being found at lower incident angles are more sensitive to thickness changes while the lower order waveguide modes located at higher incident angles are sensitive to refractive index changes. Thus, the thickness and the refractive index can be determined independently. More details can be found elsewhere.27 As a reference system to the patterned hydrogel grating, the volume phase transition of a homogeneous hydrogel film layer without (lateral) pattern was studied by SPS/OWS. Figure 2 shows the SPS/OWS spectra of the hydrogel layers at different temperatures. The scans show two important features, the SPR minimum (between 58° and 80°) and the first waveguide mode (between 48° and 49°). A simple box model, assuming a constant thickness and a homogeneous refractive index throughout the whole hydrogel layer, was used to fit the results according to Fresnel calculations in order to determine the refractive index n and the layer thickness d of the hydrogel. As can be seen in Figure 2, the calculated spectra fit the corresponding experimental measurements results quite well. The parameters from the spectra fitting were as following: the thickness of the hydrogel layers swollen at 16 °C was d ) 1220 nm with a refractive index of n ) 1.36 and the thickness of the hydrogel layer collapsed at 40 °C was d ) 330 nm with a refractive index of n ) 1.46. The results are in good agreement with our previous studies31,32 and comparable with another group.33 Patterned Thick Hydrogel Film. On the surfaces covered by a TEM grid, the polymer cross-linked only in the areas exposed to the UV light while the unexposed areas with the unchanged PNIPAAm terpolymer could be removed by ethanol. Thus, after UV illumination and rinsing with ethanol, a hydrogel grating pattern was obtained on the TEM covered areas. An optical microscopy image of a hydrogel grating after an ethanol rinse is (28) Prucker, O.; Christian, S.; Bock, H.; Ruhe, J.; Frank, C. W.; Knoll, W. Macromol. Chem. Phys. 1998, 199, 1435–1444. (29) Pareek, P.; Adler, H. P.; Kuckling, D. Prog. Colloid Polym. Sci. 2006, 132, 145–151. (30) Biesalski, M.; Ruhe, J. Langmuir 2000, 16, 1943–1950. (31) Harmon, M. E.; Jakob, T. A. M.; Knoll, W.; Frank, C. W. Macromolecules 2002, 35, 5999–6004. (32) Beines, P. W.; Klosterkamp, I.; Menges, B.; Jonas, U.; Knoll, W. Langmuir 2007, 23, 2231–2238. (33) Schmaljohann, D.; Nitschke, M.; Schulze, R.; Eing, A.; Werner, C.; Eichhorn, K. Langmuir 2005, 21, 2317–2322.

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shown in Figure 3a. A highly periodic grating is clearly observed across the whole area originally covered by the TEM grid, representing the reverse replica of the used grid. The light parts correspond to the hydrogel areas while the gray parts correspond to the gold surface with the adhesion promoter self-assemblying monolayer (SAM). The inset shows the same hydrogel grating pattern at a higher magnification. SPS angular scan measurements were carried out in the areas of patterned hydrogel films (with a thickness of d ) 330 nm in the collapsed state). Figure 3b shows the SPS spectra in the swollen state (19 °C) and in the collapsed state (50 °C). It clearly shows two SPR minima in the angular scan in the collapsed state. The one at lower angles is due to the SPR excited in the hydrogel free areas, while the other one at higher angles is due to the SPR excited in the hydrogel areas. This can be simply explained by the fact that PSPs excited at the Au-water areas laterally decay before they reach the boundary line to the neighboring regions consisting of hydrogel-water interfaces. By calculation of the reflectivity according to Rg ) xfRf + (1 - xf)Rc with Rf being the reflectivity recorded in the SPR measurement with bare Au, Rc being the reflectivity recorded in the SPR measurement with a homogeneous hydrogel layer, xf being the fraction of the hydrogel-uncovered area, and Rg being the calculated reflectivity. Assuming xf ) 0.48, Rg as a function of incident angle is shown in Figure 3c. The comparison with the experimentally measured reflectivity variations (Figure 3b) shows good agreement. Since the higher minimum angle of the SPR spectrum in the collapsed state reflects SPR excitation in the hydrogel-covered area, we fixed the incident angle at 73° and a diffraction kinetic measurement was recorded with simultaneous temperature variation. Figure 3d shows the diffraction intensity variation as a function of temperature for the hydrogel film layer during the heating and cooling processes which correspond to the collapsing and swelling processes. It can be seen that the transition is fully reversible. It also shows a hysteresis with the transition temperature being T ) 37 °C in the heating process while T ) 33 °C in the cooling process. The refractive index values and the thickness variation of the hydrogel layer obtained by fitting the SPR spectra (cf. Figure 2) of the homogeneous hydrogel layer are also shown in Figure 3d. It can be seen that the corresponding transition roughly follows the thickness change. However, we have to note that the intensity of the SPD signal also depends on the incident angle, as it has been demonstrated in a previous study.19 Figure 4 gives a direct impression of the incident angle-dependence of the diffraction intensity variation. The data presented in Figure 4 show how the first order diffraction intensity varies with the incident angle variation when keeping the detector at the angle of the first order diffraction in a normal SPS angular scan (θ/2θ mode). In the swollen state, only one peak is observed which is located at lower angles, while in the collapsed state, two peaks are observed. It can be seen that the angle positions of the peaks correspond to the minimum angles of the SPS angular scans (cf. Figure 3b), indicating that the diffraction intensity is greatly enhanced by the surface plasmon field excitation. On the basis of this, the results shown in Figure

Figure 3. (a) Optical microscopy image of a hydrogel grating film layer of 330 nm dry thickness. (b) Experimental and (c) simulation SPR results of a patterned thick hydrogel grating film layer. (d) Diffraction intensity variations of the 1st order SPD intensity, the refractive index n, and thickness d of a hydrogel grating variation with the temperature variation, fitted from the experimental data in Figure 2.

Figure 4. First order diffraction intensity variation as a function of the incident angle θ in the case of a patterned hydrogel grating of 330 nm dry thickness.

3d are not bare variations of the diffraction intensities; they represent combinations of SPR coupling efficiency and SPD intensity variations. In the phase transition of the thick hydrogel film layer, the SPR shift plays a dominated role because the SPR minimum angle shift is huge during the swelling-collapsing cycle.

The kinetics shown in Figure 3d should be consistent with that measured by recording the SPR minimum angle variations during the swelling and collapse process. Thus, in the case of a thick grating hydrogel film, SPD does not give additional advantage over the classical SPS measurements. Self-Referencing Property of SPD. Yu et al.23 demonstrated the self-referencing property of SPD by carrying out an experiment with a stepwise temperature increase. It was shown that applying higher temperatures induced a detuning of the SPR signal due to the refractive index change of the bulk solution, while virtually no changes were seen in the diffraction signal. Here, we describe an experiment that relies on this self-referencing property. Two refractive index oils were used to induce a refractive index change of the bulk solution, while keeping the hydrogel grating unaffected. Two different refractive index oils were sequentially applied to the same sensor surface (cf. Figure 3a) and both the SPR and the diffraction signals were recorded, as shown in Figure 5. It can be seen that changing the refractive index of the bulk solution induced a significant variation of the SPR signal; however, no change was found for the diffraction signal, which confirmed the self-referencing mechanism in SPD. Patterned Thin Hydrogel Film. In thick hydrogel films, SPD does not offer additional advantage compared to classical SPR Analytical Chemistry, Vol. 81, No. 7, April 1, 2009

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Figure 5. SPS and SPD measurements of a patterned hydrogel grating of 330 nm dry thickness with the refractive index variation of the bulk solution.

measurements because of the huge shift of the resonance angle during the phase transition. However, owing to its self-referencing feature, it allows for the recording of small thickness changes of very thin hydrogel film layers. Compared to SPR which combines the effect of variation of the bulk solution refractive index and of film properties, SPD will only reflect changes of the film itself. Hence, the thermal response of a very thin hydrogel film, e.g., dry hydrogel layer thickness of 25 nm in our experiment, was studied next. Figure 6a shows SPS measurements on homogeneous and on patterned hydrogel film in the swollen and in the collapsed state. Here only one SPR minimum in the collapsed state is seen because the film is so thin that the two PSPs excited along the hydrogel-covered and the uncovered areas merged into virtually one minimum. It can be seen that the minimum angle in the patterned film is smaller than that in the homogeneous film because of the lower averaged coverage. We fixed the incident angle at 59°, which corresponds to the SPR resonance angle of the thin grating hydrogel film. In Figure 6b, it can be seen that the SPD signal is stronger in the collapsed state than in the swollen state. The refractive index of the hydrogel in the swollen state is closed to that of water; hence, the diffraction signal originating from the small amplitude variation between the bare Au (plus the adhesion promoter layer) and the thin swollen hydrogel stripes is rather weak. With an increase in the temperature, inducing the collapse of the gel, stronger diffraction peaks are observed because of the larger difference of the refractive index between the polymeric hydrogel to that of water. For such a thin hydrogel film layer, the SPR minimum angle shift during the phase transition is attributed to three factors: (i) the thickness, (ii) the refractive index of hydrogel film, and (iii) the refractive index of the bulk solution. All of them change simultaneously with temperature variation. However, SPR can not differentiate between any change of the hydrogel film and that in the bulk solution, whereas SPD can kick the variation of the bulk solution. A diffraction kinetic measurement was conducted while varying the temperature by keeping the detector at the angular position of the first order diffraction intensity. The result is shown in Figure 6c. The transition temperature of the fully reversible transition from the swollen to the collapsed state is higher than that of the reversed process. The general trend of the variation of the diffraction intensity is the same as that of an SPR 2616

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Figure 6. (a) SPS measurements of a homogeneous and a patterned hydrogel grating in the swollen and in the collapsed states. (b) SPD measurements of the hydrogel grating in the swollen and in the collapsed state. (c) First order diffraction intensity variation of the same grating with temperature variation. Here the hydrogel’s dry thickness is 25 nm.

measurement reported by another group.11 However, it must be pointed out that the SPD result shown here exclusively reflects the transition of the hydrogel film without any contribution from a refractive index change of the bulk solution. CONCLUSIONS The thermal-responsive behavior of PNIPAAm hydrogel film layers with different thickness was studied by the SPD technique. The volume phase transition can be observed in real-time diffraction kinetic measurement by monitoring the diffraction intensity with temperature variation. For a thick patterned hydrogel film, OWS measurement was used. There are two minima at

the collapsed state in the SPR angular scan, which responded to the SPR excited on the areas with or without a hydrogel film, which is consistent with the simulation result. However, since SPD intensity is enhanced by the surface plasmon field excitation, it is angle dependent. Thus, it would not offer an advantage to the classical SPR in characterizing the thick hydrogel film layer due to the huge SPR shift in the phase transition. In the case of the thin patterned hydrogel film, SPR measurement was used. Only one SPR minimum in the collapsed state can be seen, and the two PSPs excited along the hydrogel-covered and the uncovered areas merged into virtually one minimum. Thanks to the selfreference property of the SPD technique, the effect of the bulk solution variation during the swelling/collapse process is excluded, Thus, the SPD only reflects the changes of the hydrogel film itself, which makes it useful in the thin hydrogel film layer.

ACKNOWLEDGMENT We thank Matthias Junk and Dr. Ulrich Jonas for providing the adhesion promoter and the poly(N-isopropylacrylamide) terpolymer used in this study. We are grateful to Dr. Yun Zong and Dr. Bernhard Menges for helpful discussions on OWS measurements. This work was partially supported by the Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research) under the Polymer Program with Grant Number IMRE/04-8R0303.

Received for review November 29, 2008. Accepted January 29, 2009. AC802527J

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