Photonic Crystal Chemical Sensors: pH and Ionic Strength

Diffraction from a photonic crystal material composed of a hydrolyzed polymerized crystalline colloidal array (PCCA) can be used to sense pH and ionic strength. The PCCA is a polyacrylamide hydrogel which embeds a polystyrene crystalline colloidal array (CCA). The diffracted wavelength of the PCCA changes as the PCCA volume changes due to the alterations in the CCA lattice constant. We examine the pH and ionic strength dependence of the hydrolyzed PCCA volume by monitoring the Bragg diffracted wavelength. We also develop a zero free parameter quantitative model to describe the pH and ionic strength dependence of the hydrogel volume. A major challenge to the fields of chemistry, physics, and materials science over the next decade is to utilize the increased potential richness of nanoand mesoscale materials, compared to small molecules, to create smart materials that can be used to fabricate complex and sophisticated devices. One especially useful class of materials results from the self-assembly of colloidal particles (Figure 1) to form crystalline colloidal arrays (CCA),1,2 which can then be polymerized to form polymerized CCA (PCCA).3 The CCA self-assembly results from electrostatic repulsion between monodisperse, highly charged colloidal particles, which forms a soft fluid material where the particles occur in a face-centered cubic or body-centered cubic array.1,2 These CCA can form large single crystals which intensely Bragg diffract visible light.1,2 Polymerizing an acrylamide hydrogel network around the CCA forms a highly unusual, soft material4 that possesses all the rich volume-phase transition phenomenology of polymer gels.5 It also contains a periodic lattice whose Bragg diffraction sensitively reports on the hydrogel volume.4 These CCA and PCCA are examples of photonic crystal materials, which hold the promise to serve numerous applications in optics and physics.6 Our group demonstrated the first CCA photonic crystal application in 1984, where we fabricated large CCA single crystals for use as optical filters for spectroscopic instrumentation.2 More recently, we demonstrated the use of PCCA for nanosecond optical switching and optical limiting7 and demonstrated that these materials could be used for chemical sensing applications.4 We attached, for example, crown ethers to the PCCA hydrogel to selectively bind Pb2+ ions. The binding of Pb2+ cations to the crown ether-hydrogel formed a new class of soft materials known as ionic gels. This immobilization of cations localizes their counterions, which results in a Donnan potential, which creates an osmotic pressure, which swells the hydrogel in proportion to the Pb2+ bound. This results in Bragg diffraction shifts that sense the concentration of Pb2+ in solution, for example. These photonic crystal sensing materials have the potential utility to sense any analyte, provided recognition elements can be synthesized and attached to the hydrogel to couple analyte binding to hydrogel volume-phase transitions. Even though hydrogels play a central role in chemistry, biology, and medicine, a fundamental understanding of their volume-phase transitions is incomplete, despite years of extensive theoretical and experimental investigations.5,8-16 This is especially unfortunate since workers such as Tanaka and others have demonstrated an increasingly rich volume-phase transition phenomenology of hydrogels which potentially can be used for artificial muscles, actuators, etc.5,8,9 One major hurdle faced by previous investigations was the difficulty of measuring volume changes and the requirement for macroscopic hydrogels to have sufficient measurement precision. Thus, perturbations of the hydrogel environment required long incubation times for diffusion to reach equilibrium. * To whom correspondence is to be addressed: (phone) 412-624-8570; (fax) 412-624-0588; (e-mail) [email protected]. † Present address: Department of Chemical Engineering, Inje University, 607 Obangdong, Kimhae, Kyongnam, 621-749, Korea. (1) Krieger, I. M.; O’Neill, F. M.; J. Am. Chem. Soc. 1968, 90, 31143120. Hiltner, P. A.; Krieger, I. M.; J. Phys. Chem. 1969, 73, 2386-2389. (2) Carlson, R. J.; Asher, S. A. Appl. Spectrosc. 1984, 38, 297-304. Flaugh, P. L.; O’Donnell, S. E.; Asher, S. A. Appl. Spectrosc. 1984, 38, 847-849. Asher, S. A. U.S. Patent 4,627,689, 1986. (3) Asher, S. A.; Holtz, J. H.; Liu, L.; Wu, Z.; J. Am. Chem. Soc. 1994, 116, 4997-4998. Asher, S. A. U.S. Patent 5,281,370, 1994. (4) Holtz, J. H.; Asher, S. A.; Nature 1997, 389, 829-832. Holtz, J. H.; Holtz, J. S. W.; Munro, C. H.; Asher, S. A. Anal. Chem. 1998, 70, 780791. Asher, S. A.; Holtz, J. H. U.S. Patent 5,854,078, 1998. (5) Tanaka, T. Sci. Am. 1981, 244, 124-138. in Structure & Dynamics of Biopolymers; Nicolini, C., Ed.; NATO ASI Ser. E; NATO: Dordrecht, 1986; pp 237-257 and references therein. (6) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (7) Pan, G.; Kesavamoorthy, R.; Asher, S. A. Phys. ReV. Lett. 1997, 78, 3860-3863. (8) Annaka, M.; Tanaka, T. Nature 1992, 355, 430-432. Mafe, S.; Manzanares, J. A.; English, A. E.; Tanaka, T. Phys. ReV. Lett. 1997, 79, 3086-3089. (9) English, A. E.; Tanaka, T.; Edelman, E. R. J. Chem. Phys. 1997, 107, 1645-1654. English, A. E.; Tanaka, T.; Edelman, E. R. Polymer 1998, 39, 5893-5897. (10) Dusek, K.; Patterson, D. J. Polym. Sci. 1968, A-26, 1209-1216. Dusek, K. J. Polym. Sci.: Symp. 1973, 42, 701-712. Hasa, J.; Ilavsky, M.; Dusek, K. J. Polym. Sci.: Polym. Phys. Ed. 1975, 13, 253-262. (11) Konak, C.; Bansil, R. Polymer 1989, 30, 677-680. (12) Marchetti, M.; Prager, S.; Cussler, E. L. Macromolecules 1990, 23, 1760-1765. Marchetti, M.; Prager, S.; Cussler, E. L. 1990, 23, 34453450. (13) Baker, J. P.; Hong, L. H.; Blanch, H. W.; Prausnitz, J. M. Macromolecules 1994, 27, 1446-1454. (14) Kudaibergenov, S. E.; Sigitov, V. B. Langmuir 1999, 15, 42304235. (15) Ilavsky, M. Macromolecules 1982, 15, 782-788. (16) Janas, V. F.; Rodriguez, F.; Cohen, C. Macromolecules 1980, 13, 977-983. 9534 J. Am. Chem. Soc. 2000, 122, 9534-9537 10.1021/ja002017n CCC: $19.00 © 2000 American Chemical Society Published on Web 09/13/2000 In the work here, we use Bragg diffraction from carboxylated PCCA hydrogels to monitor their volume-phase transitions in response to pH and ionic strength changes. This work follows the previous studies by Tanaka and others, which clearly demonstrated the pH and ionic strength dependence of the ionic gel volumes.5,8,9 We also measured the Young’s modulus of the hydrogel from the PCCA Bragg diffraction to determine the elastic restoring forces.12,17 We describe here a detailed hydrogel volume-phase model which accurately models swelling with no adjustable parameters. Finally, the results demonstrate that our carboxylated PCCA photonic crystals are excellent pH and ionic strength sensors. Experimental Section The PCCA was prepared by dissolving 5 wt % acrylamide, 0.5 wt % N,N′-methylenebisacrylamide, and 20 μL of diethoxyacetophenone in a liquid CCA solution (∼8 wt % colloids) prepared from 100-nmdiameter highly charged (sulfonated) monodisperse polystyrene colloids. This mixture was injected between two quartz plates separated by a 125-μm-thick Parafilm spacer and was photopolymerized with 365nm illumination. The PCCA was removed from between the quartz plates and washed with water, and the amide groups in the PCCA were partially hydrolyzed by a 3-min treatment with 1 N sodium hydroxide containing 10 wt % N,N,N′,N′-tetramethylethylenediamine (TEMED; Sigma). The PCCA was then extensively washed with deionized water. Diffraction spectra were measured by using a reflectance fiber-optic probe coupled to a SI 400 (Spectral Instruments, Inc.) UV-visible spectrophotometer. The experimental stochastic error in the diffraction measurements derives from ∼10-min fluctuations in the sample diffraction, which appears to mainly result from IPCCA flexing associated with the solution movement. We expect a < 2-nm standard deviation in the wavelength maximum measurement. Results and Discussion We used Braggs law to relate the diffracted wavelength to the lattice spacing and to the hydrogel volume. The use of the simple Braggs law relationship should lead to insignificant error.25 Figure 1 shows the pH dependence of the hydrolyzed PCCA diffraction. At normal incidence, the hydrolyzed PCCA in deionized water at pH 6.7 diffracts 681-nm light. As the pH increases from neutrality, the diffraction monotonically red shifts until pH 9.6, whereupon it blue shifts with further pH increases; by pH 11, the diffraction blue shifts to 604 nm. The diffraction monotonically blue shifts as the pH decreases from pH 6.7 until 506-nm light is diffracted at pH 2.0. The diffraction peaks remain symmetric and relatively narrow for all pH values, which indicates preservation of ordering of the CCA as the gel volume changes. The pH dependence of diffraction shown in Figure 1 results from the ionic gel response to changes in protonation and ionic strength. When the PCCA is hydrolyzed, some amide groups hydrolyze to carboxyl groups, which ionize as determined by their pKa and solution pH. Ionization of these covalently attached carboxyl groups immobilizes counterions inside the gel. This results in an osmotic pressure, which swells the gel against its restoring elastic constant.18 Thus, an increased pH increases the ionization; the gel swells and the diffraction red shifts. Since ionization is complete by pH 9, further pH increases only increase ionic strength. This decreases the osmotic pressure and shrinks the gel. This maximum in hydrogel volume, which occurs near pH 8.5, has also been observed for other ionic gels.8,14 The ionic strength dependence of diffraction was studied by using pH 6.7 and 8.5 solutions containing 1 mM NaCl. These solutions have ionic strengths essentially identical to that of the pH 11 solution titrated using a minimum amount of NaOH. Figure 2 shows that the increased ionic strength shrinks the hydrolyzed PCCA and blue shifts the diffraction to a value close to that from the pH 11 solution. For nonionic gels, the hydrogel volume is determined by only the free energy of mixing of the polymer and the solvent, ∆GM, and the counterbalancing free energy associated with network elasticity, ∆GE. (17) Mark, J. E.; Erman, B. Rubberlike Elasticity, A Molecular Primer; John Wiley & Sons: New York, 1988; pp 29-52. (18) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; pp 432-594. Figure 1. Inset, upper right: a PCCA where the colloidal particles occur in a cubic array embedded within an acrylamide hydrogel. The pH dependence of the hydrolyzed PCCA diffraction wavelength (see the inset on the upper left) is shown by the triangles. The solid line was calculated from the affine model and the broken line from the phantom model. Photonic Crystal Chemical Sensors J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9535