Langmuir 2006, 22, 4589-4593
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Surface Plasmon Resonance Spectroscopy Study of Electrostatically Adsorbed Layers Vale´rie J. Gandubert and R. Bruce Lennox* Department of Chemistry and Centre for Self-Assembled Chemical Structures (CSACS), McGill UniVersity, 801 Sherbrooke Street West, Montre´ al, Que´ bec, H3A 2K6, Canada ReceiVed October 12, 2005. In Final Form: March 5, 2006 The use of 4-(dimethylamino)pyridine to form an adhesion layer for the adsorption of anionic polyelectrolytes on gold surfaces is investigated. In situ surface plasmon resonance spectroscopy is used to monitor the changes in thickness of the adsorbed layers as a function of pH changes. Weak (poly(acrylate)) and strong (poly(styrenesulfonate)) polyelectrolytes have been studied. Although 4-(dimethylamino)pyridine is weakly bound to gold, it is not displaced by these polyelectrolytes and acts as an adhesion layer. The relationship of the interaction of anionic polyelectrolytes with 4-(dimethylamino)pyridine-modified planar gold and gold nanoparticles is discussed.
1. Introduction Thin films of organic molecules, biological molecules, or polymers can adsorb onto surfaces via covalent bonding, electrostatic interaction, hydrogen bonding, and/or hydrophobic interactions. When the adsorption relies on electrostatic interactions, the surfaces are usually chemically modified with a monolayer of molecules bearing charged functional groups. Selfassembled monolayers (SAMs) of alkyl thiols have been used as adhesion layers for polymers on gold surfaces. This system benefits from the robustness of the gold-sulfur bond and the variety of functionalized thiols that are available.1-4 The more labile ligand 4-(dimethylamino)pyridine (DMAP) has recently been used as an adhesion layer for polyelectrolytes on gold nanoparticles (NP).5 Poly(acrylate) (PAA) and poly(styrenesulfonate) (PSS) were found to adsorb onto DMAP-capped gold (DMAP-Au) NP (d ≈ 6 nm) and to thus stabilize the NP even at low pH values.5 The interactions between the polyelectrolyte chains and DMAP-Au NP were probed by monitoring changes in the optical properties of the NP. In this work, the analogous planar system was investigated by surface plasmon resonance (SPR) spectroscopy. SPR spectroscopy is very specific to the interface region and can probe the adsorption of molecules at the interfacial. It is often used to detect biomolecules, study the kinetics of reactions, or follow layer-by-layer growth of multilayer assemblies.2,6-9 This technique provides thickness values of the adsorbed layer and, by inference, conformational details. For example, the changes in conformation of immobilized poly(acrylic acid) on gold has been investigated using SPR spectroscopy.10 * To whom correspondence should be addressed. E-mail: bruce.lennox@ mcgill.ca. (1) Nabok, A. V.; Hassan, A. K.; Ray, A. K. Mater. Sci. Eng., C 1999, C8-C9, 505. (2) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422. (3) Li, L.; Chen, S.; Jiang, S. Langmuir 2003, 19, 2974. (4) Zhu, M.; Schneider, M.; Papastavrou, G.; Akari, S.; Mo¨hwald, H. Langmuir 2001, 17, 6471. (5) Gandubert, V. J.; Lennox, R. B. To be submitted to Soft Matter. (6) Green, R. J.; Frazier, R. A.; Shakesheff, K. M.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B. Biomaterials 2000, 21, 1823. (7) McDonnell, J. M. Curr. Opin. Chem. Biol. 2001, 5, 572. (8) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (9) Georgiadis, R.; Peterlinz, K. P.; Peterson, A. W. J. Am. Chem. Soc. 2000, 122, 3166. (10) Sarkar, D.; Somasundaran, P. Langmuir 2004, 20, 4657.
The SPR experiments described here were performed to understand the nature of the interactions of anionic polyelectrolyte chains with a cationic DMAP self-assembled monolayer (SAM). The specific goals were to (i) monitor the adsorption of the polyelectrolyte chains onto the surface as a function of pH, (ii) study the conformational changes of the adsorbed chains as a function of pH, (iii) monitor the desorption of the chains as they become neutralized, and (iv) determine whether the polyelectrolyte chains adsorb onto the DMAP SAM, or displace it. The results are compared to those obtained in the parallel polyelectrolyte/nanoparticle study.5 2. Experimental Section Materials. 4-(Dimethylamino)pyridine (99%), poly(styrenesulfonate) (sodium salt, Mw ) 7 × 104), and 2-(dimethylamino)ethanethiol hydrochloride (98%) were received from Aldrich. Poly(acrylate) (sodium salt, Mw ) 1.8 × 105) was obtained from Prof. A. Eisenberg. Phosphoric acid and sodium hydroxide were purchased from Fisher Scientific. All compounds were used as received without further purification. The water used in all experiments was Millipore-purified water with a resistivity of 18 MΩ. DMAP, 2-(Dimethylamino)ethanethiol, and Polyelectrolyte Solutions. 10 mM DMAP or 2-(dimethylamino)ethanethiol aqueous solutions were used to form a SAM on the gold surface. The pH of 10 mM aqueous polyelectrolyte (residue concentration) solutions was adjusted (1 < pH < 10) by addition of sodium hydroxide or phosphoric acid. Solutions containing both 10 mM DMAP and 10 mM polyelectrolyte were also adjusted to desired pH values. These solutions were used for the adsorption of polyelectrolyte on the first adsorbed layer or directly on the gold, and in the study of the effect of pH on the assembly. Aqueous phosphoric acid solutions of various pH values were used to study the effect of pH on the 2-(dimethylamino)ethanethiol monolayer and on unmodified gold. 10 mM aqueous DMAP solutions with pH adjusted to the desired values were used to study the effect of pH on the DMAP monolayer. To keep the experimental conditions as similar as possible to those used with the gold NP,5 no buffer solutions were used. Phosphoric acid was used to decrease the pH of the solutions, instead of acetic acid, because small amounts are required to achieve the same pH change. A lesser change in the refractive index of a given solution thus results. SPR Instrument. A computer-controlled scanning angle apparatus was used to perform the SPR measurements (Resonant Probe Microscopy GmbH, Goslar, Germany). A Kretschmann configuration was used (Supporting Information). The incident light was a linearly polarized HeNe laser (λ ) 632.8 nm, JDS Uniphase), attenuated,
10.1021/la052751q CCC: $33.50 © 2006 American Chemical Society Published on Web 04/14/2006
4590 Langmuir, Vol. 22, No. 10, 2006 and p-polarized with respect to the plane of incidence on the gold surface (Glan-Thompson polarizing prism, Halle). The optical signal was modulated at a frequency of 1 kHz (optical chopper, PerkinElmer 197) and correlated to the detector through a lock-in amplifier (EG&G PAR 5210). The goniometer was a stepper-motor driven θ/2θ goniometer with angular resolution of 0.01° (Huber 414a). The light reflected at the metal/prism interface was focused on a silicon photodiode detector, and the photodiode signal was measured with the lock-in amplifier in phase with the incident light source. The goniometer rotation and data acquisition were controlled through an IEEE interface board (Keithey KPC-488.2), and the software used was provided by Resonant Probe. SPR Flow Cell and Gold Substrate Preparation. A thin gold layer (thickness ∼50 nm) was produced on a cleaned LaSFN9 glass slide (n632.8 nm ) 1.845, Hellma Optik) by thermal evaporation, at a rate of ca. 1.2 Å/s at 3 × 10-7 Torr. The film thickness was monitored via a quartz crystal microbalance. Gold adheres well to LaSFN9, and no precoating with titanium or chromium was required. The slide was index-matched to a right-angle LaSFN9 glass prism (Hellma Optik), using a Cargille Series B matching liquid (1-iodonaphthalene, n ) 1.700, SPI). A Teflon flow cell (volume ∼1 mL) was pressed against a clean microscope slide. The prism/ gold slide assembly was then pressed on top of the cell, allowing the gold film to be exposed to the filling solution (Supporting Information). A tight seal was ensured by Kalrez O-rings. The goniometer position at which the back-reflected beam overlapped with the incident beam was used to define the angle of incidence of 45°. The full assembly was mounted on the stage of the goniometer, with the center of the gold slide on the axis of rotation. In Situ SPR Measurements. The solutions were manually added to the flow cell. The formation of the first monolayer and the adsorption and conformational changes of the polyelectrolyte chains were monitored using the kinetic and angular reflectivity modes. The kinetic adsorption data were obtained by tracking the SPR angle (incident angle of minimum reflectivity) over time, after addition of each solution. Once a plateau was reached (constant SPR angle after 40-60 min for the polyelectrolytes), an angular reflectivity curve (SPR curve) was acquired by measuring the reflectivity as a function of the incidence angle of the laser. All experiments were performed at room temperature. An example of the SPR curves thus obtained is provided in the Supporting Information. Thickness Calculations. The quantitative treatment of the SPR curves, based on the Fresnel theory for a multilayer system, was performed using the Fresnel Modeling software provided by Resonant Probes (Mainz, Germany). The theoretical curves were computer generated and manually fitted to the SPR curves. The SPR cell is initially filled with MilliQ water. The system consists of three layers: the LaSFN9 prism, the gold layer, and the water above the metal. The prism has a refractive index () of 3.404, which is constant through the experiment. The refractive index of the water was fitted and is 1.7735 ( 0.0015. The thickness of the gold layer (ca. 50 nm) and its complex refractive index (typically ′ ) -12.9, ′′ ) 1.2) were determined via fitting. These values were kept constant throughout the fitting of all the subsequent SPR curves acquired on the same gold slide, without disassembling the cell. The system consists of four layers after adsorbing DMAP, 2-(dimethylamino)ethanethiol, or polyelectrolyte onto the unmodified gold substrate: the LaSFN9 prism ( ) 3.404); the gold layer (parameters from initial fitting with water); the DMAP, thiol, or polyelectrolyte layer ( ) 2.25);1,10 and the solution above the adsorbed layer ( between 1.7720 and 1.7790). Although the refractive index of the polyelectrolyte was assumed constant, swelling-induced conformational changes of the polyelectrolyte tend to decrease it.11 An of 2.40 is a more accurate value for DMAP; using of 2.25 leads to a very small difference in calculated thickness (50%) are protonated at pH e 4. Once the interresidue repulsions are decreased, the chains can coil and a thickness increase results. The extent of adsorption of a weak polyelectrolyte onto an oppositely charged surface is in fact (17) Weber, M.; Nart, F. C. Electrochim. Acta 1996, 12, 4723. (18) Cumberland, S. L.; Strouse, G. F. Langmuir 2002, 18, 269. (19) Leong, Y. K. Colloid Polym. Sci. 1999, 277, 299. (20) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309.
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Figure 3. Thickness vs pH for the adsorption of PSS on (2) DMAPmodified, (9) 2-(dimethylamino)ethanethiol-modified, and (b) unmodified gold surfaces. The surfaces were exposed to a series of 10 mM PSS solutions of decreasing pH. A DMAP-modified surface was also exposed to PSS solutions of decreasing pH containing 10 mM DMAP (4). The asterisk (/) indicates the thickness of the adsorbed layer before addition of PSS solutions. The lines are added to aid the eye.
predicted to be maximal at a pH value one unit lower than the pKa of the polyelectrolyte.21 Therefore, both conformational changes and additional PAA adsorption might contribute to the thickness increase at low pH. The change of thickness over the 1.6 e pH e 4 range is 1.4 nm for the 2-(dimethylamino)ethanethiol layer and 0.9 nm for the DMAP layer. In the latter case, the thickness increase caused by the coiling of the PAA chains could be lessened by the desorption of PAA chains and/or DMAP molecules. As mentioned earlier, DMAP molecules without a polyelectrolyte coating desorb at low pH (Figure 1). The addition of a DMAP-free solution might trigger desorption of some DMAP, as a small thickness decrease is observed at 7 e pH e 10. However, a small thickness decrease is also noted when solutions containing both PAA and DMAP are added, even though the quantity of nonprotonated DMAP present is larger than the amount required to form a full monolayer at the exposed gold surface.22 This suggests that the small thickness decrease at 7 e pH e 10 is not in fact due to the loss of adsorbed DMAP. PSS Adsorption. Again, when comparing the results obtained when exposing a DMAP-modified, a 2-(dimethylamino)ethanethiol-modified, or an unmodified gold surface to a solution of pure PSS, a clear trend is evident (Figure 3). The adsorption of PSS onto the three surfaces is limited at high pH (pH 6-10). The thickness increases at pH e 6, with a more pronounced effect at pH e 3. PSS is a strong polyelectrolyte and is expected to be fully ionized and highly stretched over the 3 e pH e 10 range (pKa in solution ) 119). When stretched PSS chains adsorb onto the surface, small thickness changes result. The larger thickness increase observed over the 1 e pH e 4 range could be due to both the adsorption of more polyelectrolyte chains at the surface and conformational changes. Because the ionic strength of the polyelectrolyte solutions increases as the acid content increases, the adsorption of more PSS can occur via screening of the inter-residue repulsive forces. The inter-residue repulsions also decrease upon neutralization of the sulfonates. This facilitates the adsorption of additional (21) Blaakmeer, J.; Bo¨hmer, M. R.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1990, 23, 2301. (22) Given that the gold surface exposed to the solution has an area of 1 cm2, the estimated footprint of a DMAP molecule is 15 Å2, and the volume of the SPR cell is 1 mL, the DMAP concentration required to form a full monolayer at the exposed gold surface is 1 µM, assuming Kads is large.
Gandubert and Lennox
polyelectrolyte on the surface and induces conformational changes (coiling). The constant thickness increase down to pH 1 indicates that the PSS chains remain at the surface even at these low pH values. In the case of the adsorption of PSS onto a DMAP layer, the thickness remains constant at 6 e pH e 10. To test whether this constant thickness is due to the limited adsorption of PSS over this pH range or due to DMAP desorption, PSS solutions containing DMAP were added. The adsorbed film thickness remains constant at 6 e pH e 10 although the quantity of neutral DMAP in solution is sufficient to form a full monolayer at the gold surface.22 This suggests that the constant thickness is due to the limited adsorption of fully ionized PSS onto the cationic surface rather than to the loss of DMAP over this pH range. The near-constant thickness at 3 e pH e 6 is likely due to the loss of DMAP upon protonation of the pyridine nitrogen, which is compensated by the adsorption of some PSS chains. The DMAP molecules present in solution in the control experiment (PSS + DMAP) are all in the protonated form over this pH range and are thus unavailable for binding. The thickness increases by 1.25 and 1.45 nm for PSS on DMAP and on 2-(dimethylamino)ethanethiol respectively over the 1 e pH e 6 range. The similarity in these values, given the resolution of the experiment, suggests that PSS adsorbs onto the DMAP layer, as per that on the 2-(dimethylamino)ethanethiol layer. Displacement of DMAP by PSS is therefore not indicated. Planar Systems vs Nanoparticles. Before they are compared in detail, it is important to note that there are obvious differences between planar systems and NP relevant to these polyelectrolyte binding studies. First, the curvature of the NP surface may be a limiting factor in the adsorption of a polyelectrolyte, especially at high pH where the charged polyelectrolyte chains are less flexible. The extent of polyelectrolyte adsorption will determine the pH dependence of conformational changes of the polyelectrolyte chains. Second, DMAP binding may be sensitive to whether the gold is a flat surface or is a small sphere, given that alkyl amines are kinetically less stable on a planar surface than on NP.23 The enhanced stability of polyelectrolyte-associated DMAPAu NP to low pH conditions parallels that observed here for planar systems. Whereas DMAP-Au NP aggregate at pH e 4, DMAP-Au NP in the presence of PAA or PSS are stable at pH 2.7 and 1.8, respectively.5 In these SPR experiments on planar gold, the PAA and PSS layer thicknesses progressively increase at pH < 7. A parallel process on the NP would impart increased sterics, consistent with the observed stability toward coalescence. At high pH values (7 < pH < 10) DMAP-Au NP exhibit a broadened and red-shifted plasmon band upon addition of PAA (10 nm red shift) or PSS (20 nm red shift). These spectral changes may be due to polymer chain mediation of interparticle plasmon coupling, most likely from proximity effects. The question arises as to whether the polyelectrolytes are indeed surface-associated at these pH values. These SPR experiments on planar gold establish that PAA and PSS are indeed adsorbed onto the DMAPmodified surface at high pH values.
4. Conclusions SPR spectroscopy studies confirm that anionic polyelectrolyte chains adsorb onto a DMAP-Au monolayer rather than displace it. The polyelectrolyte layer stabilizes the DMAP SAM formed on a planar gold surface at low pH values, by preventing desorption (23) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723.
SPR of Electrostatically Adsorbed Layers
of DMAP molecules from the gold surface. The same stabilizing effect was also observed in the DMAP-Au NP system, where PAA and PSS were found to enhance the stability of DMAPAu NP at low pH values. The weak polyelectrolyte PAA adsorbs onto a DMAP planar SAM at higher pH values compared to the strong polyelectrolyte PSS. The PSS layer remains adsorbed onto the DMAP planar SAM even at very low pH values, which explains the stability of PSS-DMAP-Au NP at 1.8 e pH.
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Acknowledgment. We thank Dr. A. Badia for sharing her expertise in SPR spectroscopy and Mr. O. Tanchak for useful comments. Supporting Information Available: SPR spectroscopy principles, the SPR instrument configuration, the experimental configuration of the SPR flow cell, and examples of SPR curves. This material is available free of charge via the Internet at http://pubs.acs.org. LA052751Q
