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Conformational Dynamics of Poly(acrylic acid). A Study Using Surface Plasmon Resonance Spectroscopy Diptabhas Sarkar and P. Somasundaran* NSF/IUCR Center for Studies in Novel Surfactants, Langmuir Center for Colloids and Interfaces, Columbia University, 911 Mudd Building, 500 West 120th Street, New York, New York 10027 Received September 15, 2003. In Final Form: February 25, 2004 The conformational dynamics of poly(acrylic acid) induced by pH change is reported here. Poly(acrylic acid) immobilized on gold surface was exposed to pH changes, and the conformational changes thus induced were followed in real time using surface plasmon resonance spectroscopy. The temporal profile of the stretching-coiling phenomenon showed a minimum point, which was proposed to be arising due to the contradictory behavior of two different property changes in the polymeric system. Normally surface plasmon resonance (SPR) response would be a convoluted effect of the thickness and refractive index changes, but the behavior observed here, where the SPR response is predominantly governed by either one of the two, is unique and to the author’s knowledge is a feature that is observed for the first time. Analysis of the kinetics of the angle change revealed that it takes longer for the polymer to stretch than it takes for it to collapse, with the kinetic rate constants varying by at least an order of magnitude. The SPR angle change as well as the kinetic constants increased linearly with molecular weight. Effect of Ca2+ was studied, and it was found that the polymer was locked in its conformation due to the binding of the multivalent cations.
1. Introduction With the progress of science, there is a constant demand for newer and technologically advanced materials, which will have the capability of sensing an external stimulus and respond with an appropriate action. These materials can potentially be used in the development of drug delivery systems, biomimetic energy transducing devices,1,2 data storage devices,3 and miniature circuits and microelectronics4,5 and in the cosmetics industry. In view of nature’s amazing capabilities for intelligent designs based on the dynamics of various structures, it is clear that a good understanding of how these nanomaterials behave and how fast they do so under external perturbations, controlled or otherwise, is the key to major breakthroughs in this area. Polymers and surfactants, because of their inherent chemical reactivity and interesting physical properties, offer the maximum scope for modifications to respond to different external stimuli, and thus knowledge of the fundamental interactions governing their dynamic behavior under controlled external perturbations is essential in the pursuit of these smart materials. Much work has been done to understand the physicochemical interactions that govern the formation of adsorbed colloidal layers on surfaces or surface colloids (solloids).6-9 The traditional methods, namely, surface * Corresponding author. E-mail:
[email protected]. (1) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. (2) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (3) Hayashi, S.; Kumamoto, Y.; Suzuki, T.; Hirai, T. J. Colloid Interface Sci. 1991, 144, 538. (4) Roescher, A.; Moller, M. Adv. Mater. 1995, 7, 151. (5) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000. (6) Surface contamination: genesis, detection, and control; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vols. 1 and 2. (7) Aplan, F.; Fuerstenau, D. W. Froth Flotation; AIME: New York, 1962. (8) Hanna, H. S.; Somasundaran, P. Improved Oil Recovery by Surfactant and Polymer Flooding; Shah, D. O., Schecter, R. S., Eds.; Academic Press: New York, 1977; p 253.
tension measurement, study of adsorption isotherms, viscosity measurements, zeta potential measurements, wettability, and calorimetric techniques,10,11 have led to a better understanding and characterization of polymers and surfactants at interfaces. However studies regarding the dynamics of polymeric materials have been hard to come by as the experimental determination of the conformation and orientation of adsorbed macromolecules in real time is a difficult task. 2. Theory 2.1. Present Scheme. Poly(acrylic acid) can be protonated (below the pKa) or deprotonated (above the pKa) according to the following equation pKa
RCOOHsurface {\} RCOO-surface + H+solution where “R” represents the poly(acrylic acid) backbone. Deprotonation leads to the development of electrostatic repulsion in the poly(acrylic acid) backbone (Figure 1). Experimental evidence does suggest that PAA can exist in different conformations depending on the solvent, pH, and ionic strength of the system.12 Such flexibility also influences its adsorption characteristics on solids and in turn affects subsequent suspension behavior. Using a fluorescent labeled polymer and by monitoring the extent of excimer formation, it was shown previously that a polymer could have a stretched or coiled conformation at the interface depending on the pH. These studies have further demonstrated that the PAA adsorbed in the stretched form on alumina at high pH could not become coiled13 under the test conditions and duration. In contrast, (9) Somasundaran, P.; Chandar, P. Solid-Liquid Interactions in Porous Media Paris; 1985; p 411. (10) Hough, D. B.; Rendall, H. M. Adsorption from Solution at the Solid-Liquid Interface; Parfitt, G., Rochester, C. H., Eds.; Academic Press: New York, 1983. (11) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68, 3562. (12) Arora, K. S.; Turro, N. J. Polymer 1986, 27, 783.
10.1021/la035727q CCC: $27.50 © 2004 American Chemical Society Published on Web 04/24/2004
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in solution. The present design is closer to real life polymer brush systems, such as the ones encountered in flocculation studies, where polymers simply adsorb onto the colloidal particles according to the thermodynamics of the systems. It is well-known that two contrary tendencies govern the spatial distribution of a polymer at an interface. The tendency to maximize configurational entropy causes the polymer to coil up so as to adopt a random walk configuration and the tendency to maximize enthalpy of solvation tends to stretch out the polymer. Using a selfconsistent field method, Milner has shown that the concentration profile of a polymer brush approaches a parabolic form19,20 rather than the step-function as suggested by Alexander21 and de Gennes.22 The profile can be described mathematically as
φ(z) ) (B/ω)((h*)2 - z2)
(1)
h* ) hmax ) (12/π2)1/3(σω)1/3N
(2)
B ) π2/8N2
(3)
where Figure 1. Conformations of poly(acrylic acid).
the polymer adsorbed in the coiled form at low pH did stretch out when the pH was increased. With this concept of hydrogen bonds, a large body of literature has been built up to induce supramolecular ordering in polymeric systems.14-17 Even though a lot of papers have been published on the use of surface plasmon resonance (SPR) to study interactions in biopolymeric systems, almost all of them focus on measuring the binding affinities and kinetic constants of reversible interactions. The studies on polymers essentially looked into the kinetics of binding of the polymeric species on to the metal surface. However here we report the conformational dynamics of poly(acrylic acid) (PAA) as induced by pH changes. The PAA molecule was bound to the SPR sensor surface (gold), and the conformational change (Figure 1) was followed in real time. The effect of pH change on these polymers was studied in this work as a function of their molecular weight. A unique feature was observed in the temporal profiles of pH change experiments, where the effects due to refractive index changes and thickness changes could be distinguished, even though they were not totally deconvoluted. From a study of the kinetic rate constants, it was found that it takes longer for the molecule to stretch out than it takes for it to collapse. A direct relationship was found in the magnitude of the change observed to the molecular weight of the polymer used. 2.2. Polymer Brush Analogy. To explain the observed behavior, an analogy has been drawn to the theory of polymer brushes. Polymer brushes refer to an assembly of polymer chains, which are tethered by one end to a surface.18 The tethering should be sufficiently dense such that the polymer chains are crowded and forced to stretch away from the interface to avoid overlapping, sometimes much further than the typical unstretched size of a chain, which would normally adopt a random walk configuration (13) Chandar, P.; Somasundaran, P.; Turro, N. J.; Waterman, K. C. Langmuir 1987, 3, 298. (14) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (15) Krische, M. J.; Lehn, J.-M.; Kyritsakas, N.; Fischer, J. Helv. Chim. Acta 1998, 81, 1909. (16) Kato, T.; Frechet, J. M. J. Macromolecules 1989, 22, 3818. (17) Aksenova, N. I.; Kemenova, V. A.; Kharenko, A. V.; Zezin, A. B.; Bravova, G. B.; Kabanov, V. A. Polym. Sci. 1998, 40, 226. (18) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677.
and
N is the molecular weight, F is the surface coverage, and T is the excluded volume parameter. 2.3. SPR as a Probe of Dynamics of Polymer Behavior. Surface plasmons are electromagnetic waves trapped at an interface between a metal and a dielectric, with exponentially decaying fields in both media. The presence of exponentially decaying fields make it sensitive to changes in refractive indices next to the interface, a property that is exploited for optical monitoring of perturbations in the local environment. Over the past decade SPR has grown into a versatile technique with applications in studies of self-assembled monolayers,23 metallic nanoparticles,24 conformational changes of surface immobilized proteins,25 and biosensing measurements.26 Excellent reviews have been written on the physics27 of surface plasmons and on its applicability as a sensor28,29 for optical monitoring of subtle chemical changes. Launching of surface plasmons requires the angular frequency and momentum of the incident photons to match that of the normal modes of the metal plasma, and this can be done by using a prism coupler. It is well-known that under total internal reflection conditions, there exists an evanescently decaying wave in the optically rarer medium. If the metal layer is attached to the base of the prism, these evanescent waves can couple with the surface plasmon modes of the metal, which causes the incident photon energy to be transferred to the metal with a consequent decrease in the reflectance profile. (19) Milner, S. T.; Witten, T. A.; Cates, M. E. Macromolecules 1988, 21, 2610. (20) Milner, S. T. Science 1991, 251, 905. (21) Alexander, S. J. Phys. (Les Ulis Fr.) 1977, 38, 983. (22) de Gennes, P.-G. J. Phys. (Les Ulis Fr.) 1976, 37, 1443. (23) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (24) Hutter, E.; Fendler, J. H.; Roy, D. J. Phys. Chem. B 2001. (25) Salamon, Z.; Cowell, S.; Varga, E.; Yamamura, H. I.; Hruby, V. J.; Tollin, G. Biophys. J. 2000, 79, 2463. (26) Zacher, T.; Wischerhoff, E. Langmuir 2002, 18, 1748. (27) Raether, H. Excitation of Plasmons and Interband Transitions by Electrons; Springer: Berlin, Heidelberg, New York, 1980; Vol. 88. (28) Homola, J.; Yee, S. S.; Gauglitz, G. Sens. Actuators, B 1999, 54, 3. (29) McDonnell, J. M. Curr. Opin. Chem. Biol. 2001, 5, 572.
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Figure 2. Four-layer structure: θSPO, surface plasmon resonance angle; g, a, and s, dielectric constant of glass (prism material), polymer, and solution, respectively; mr and mi, real and imaginary components of the dielectric constant of metal, respectively; dm and da, thickness of the metal and the polymer layer, respectively. Note: The thickness of the metal layer has been exaggerated with respect to the polymer layer, to show the evanescent wave. In reality the metal layer is at least an order of magnitude thinner than the polymer layer.
A schematic diagram of the four-layer system applicable to the present study is shown in Figure 2. The different phases involved are labeled as follows: 1, glass slide (SF10, n ) 1.78) optically coupled to a hemicylindrical prism of the same material; 2, a layer of gold (50 nm ( 2 nm) which acts as the sensor surface (a chromium layer ca. 2-3 Å is used as an adherent for the gold on glass, but owing to its extremely thin thickness, it is neglected from the simulation studies); 3, the polymer layer; 4, the overlayer solution of the pH-controlled water. For a given wavelength of the driving beam, the reflectance (reflectivity “R” as a function of the angle of incidence “θ”) goes through a minimum at a certain angle θSPO. At this angle the wavevector of the incident radiation matches the wavevector of the surface plasmon oscillations at the metal/ dielectric interface and the incident radiation couples into the surface plasmon modes, the external manifestation of which is the attenuated reflection. Due to the evanescent nature of the surface plasmons (the probing depth is of the order of 2000 Å), the angle of minimum reflectance, θSPO, is extremely sensitive to the thickness and dielectric constant of the material close to the metal surface. A change in the polymer conformation is expected to cause minute changes in these two properties, which would cause a change in θSPO. Following the magnitude of the change and the temporal profile of the same, it is possible to get some information on the dynamics of the polymeric system. 2.4. Duality in SPR Response. Any response in experimental techniques involving an optical probe is governed by the thickness and the dielectric constant of the layers, if the system under study is approximated as a stacked Fresnel’s system. For a four-layer system as defined in Figure 2, the angle change in the SPR response can be approximated as30,31
∆θSPO ≈
( )( ) ( ) ( )
a - s mrs 180 1 k 0 a k0(g1/2) cos(θSPO) π mr + s r
a - m
s - mr
(-mrs)-1/2
2
2πda (4) λ
Figure 3. Changes in surface plasmon resonance (SPR) angle due to changes in refractive index (RI) and thickness (Th). Theoretical calculations were performed using the complete four-layered Fresnel’s matrix. For the RI variation calculations, the thickness of the polymer layer was held constant at 400 nm while the RI was varied from 1. 38 to 1.44. For the thickness change experiments, the RI was held constant at 1.40 and the thickness was varied from 100 to 600 nm, gold ) -11 + 1.09i; thickness of gold ) 50 nm; l ) 632.8 nm; RIprism ) 1.78; RIsolvent ) 1.33.
As can be seen from the above equation, the SPR resonance angle increases both with increasing thickness and with increasing refractive index (Figure 3). For a four-layer stacked structure such as the one presented here, decrease in the thickness of the polymer layer will lead to a downshift in the angle of minimum reflection, if the refractive index is assumed to be held constant during the process. On the other hand the increase in the refractive index of a layer held at constant thickness leads to an increase in the SPR resonance angle. 3. Materials and Methods Commercially available poly(acrylic acid) samples, with molecular weights 50K, 90K, 450K, 750K, 1250K, and 3000K were obtained from Aldrich and used without any further purification. All inorganic chemicals, for pH control experiments (hydrochloric acid and sodium hydroxide) and for experiments using sodium chloride and calcium chloride, were purchased from Fisher Scientific and were either analytical grade or reagent grade. All solutions were made using freshly prepared triple distilled water. All experiments were done at 25 °C. Experiments reported here were done on an in-house-built setup using the angle-scan method32,33 in a converging beam configuration.34 A schematic representation of the instrument is shown in Figure 4 and has been described in detail elsewhere.35 The sensor surface was created by electron beam evaporating 50 ( 5 nm of gold onto the glass slide with an intervening layer of chromium (
