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Specific Hydrogen-Bond-Mediated Recognition and Modification of Surfaces Using Complementary Functionalized Polymers Tyler B. Norsten, Eunhee Jeoung, Raymond J. Thibault, and Vincent M. Rotello* Department of Chemistry, The University of Massachusetts, Amherst, Massachusetts 01003 Received May 12, 2003 Specific hydrogen-bonding interactions between polymers and surface-tethered recognition elements were used to selectively modify self-assembled monolayers (SAMs) on gold. The interfacial recognition processes were followed by observing frequency changes of thymine-SAM modified quartz crystal microbalance (QCM) chips during adsorption of diamidopyridine-functionalized (DAP) polystyrene from a nonpolar solvent. QCM studies combined with X-ray photoelectron spectroscopy (XPS), water contact angle, and ellipsometry measurements of the polymer-modified surfaces demonstrate the selectivity of the polymer-surface hydrogen-bonding interactions. These studies also indicate that the degree of recognition element functionalization of both the polymer and the surface is crucial in determining the rate, selectivity, and coverage of polymer on the surface.
Introduction The use of noncovalent interactions at the solid-liquid interface provides an efficient and effective means of controlling the physical and chemical properties of surfaces.1 The ability to regulate noncovalent interactions between the surface and the modifier is an important tool for controlling the selectivity of the adsorption process. To this end, techniques such as chemisorption2 and Langmuir-Blodgett deposition3 employing various interactions, such as hydrophobic,4 electrostatics,5 hydrogen bonding,6 and ionic/dative coordination,7 have all been exploited in an effort to provide efficient routes to effectively modify surfaces. For this purpose, SAMs provide versatile surfaces to study interfacial phenomenon, allowing the incorporation of specific amounts of diverse functionality.8 Electrostatic self-assembly using polymers has emerged as a powerful technique for surface modification.9 This (1) Huisman, B. H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Pure Appl. Chem. 1998, 70, 1985. (2) (a) Ulman, A. Chem. Rev. 1996, 96, 1533. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (3) (a) Petty, M. C. Lagmuir-Blodgett films; Cambridge University Press: Cambridge, UK, 1996. (4) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 2229. (5) (a) Ginzburg, M.; Galloro, J.; Ja¨kle, F.; Power-Billard, N. K.; Yang, S.; Sokolov, I.; Lam, C. C. N.; Neumann, W. A.; Manners, I.; Ozin, G. A. Langmuir 2000, 16, 9609. (b) Godı´nez, L. A.; Lin, J.; Mun˜oz, M.; Coleman, A. W.; Rubin, S.; Parikh, A.; Zawodzinski, T. A., Jr.; Loveday, D.; Ferraris, J. P.; Kaifer, A. E. Langmuir 1998, 14, 137. (6) (a) Garcia-Lopez, J. J.; Zapotoczny, S.; Timmerman, P.; van Veggel, F. C. J. M.; Vancso, G. J.; Crego-Calama, M.; Reinhoudt, D. N. Chem Commun. 2003, 352. (b) Credo, G. M.; Boal, A. K.; Das, K.; Galow, T. H.; Rotello, V. M.; Feldheim, D. L.; Gorman, C. B. J. Am. Chem. Soc. 2002, 124, 9036. (c) Jeoung, E.; Carroll, J. B.; Rotello, V. M. Chem. Commun. 2002, 1510. (d) Park, J. S.; Lee, G. L.; Lee, Y.-J.; Park, Y. S.; Yoon, K. B. J. Am. Chem. Soc. 2002, 124, 13366. (e) Miura, Y.; Xu, G.-C.; Kimura, S.; Kobayashi, S.; Iwamoto, M.; Imanshi, Y.; Umenura, J. Thin Solid Films 2001, 393, 59. (f) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206. (g) Arias, F.; Godı´nez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086. (7) (a) van Manen, H.-J.; Auletta, T.; Dordi, B.; Scho¨nherr, H.; Vancso, G. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Funct. Mater. 2002, 12, 811. (b) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 8518. (c) Maskus, M.; Abrun˜a, H. D. Langmuir 1996, 12, 4455. (8) Isaacs, L.; Chin, D. N.; Bowden, N.; Xia, Y.; Whitesides, G. M. In Supramolecular Materials and Technology; Reinhoudt, D. N., Ed.; Wiley: Weinheim, Germany, 1999; pp 14-24.
technique is characterized by the adsorption of polyanions and polycations onto complementary charged substrates. The approach, while highly effective, relies on the strong nondirectional electrostatic attraction between cations and anions and is thus fundamentally a binary self-assembly process. The use of designed recognition elements such as specific hydrogen-bond dyads provides a means for expanding the “alphabet” of surface modification strategies while at the same time enhancing the interfacial specificity between the surface and adsorbate. Engineering specific neutral hydrogen-bonding elements into this approach would also provide a facile route to the creation of chargeneutral surfaces, an important issue when designing protein resistive and biocompatible materials.10 Use of these inherently weaker interactions, however, raises important issues that must be addressed with respect to specific versus nonspecific surface adsorption. Specific interfacial hydrogen bonding has been explored in the design of small11 and large biomolecule12 sensor systems;13 however, its application as a more general polymer-based surface modification strategy has yet to be explored. We have previously demonstrated that the thymine-diamidopyridine (Thy-DAP) hydrogen-bonding motif is a simple and effective interaction when incorporated into SAM-based systems.6b,c We report here the use of Thy-DAP specific hydrogen bonding to selectively modify SAM surfaces with complementary functionalized polymers. (9) For recent reviews on electrostatic self-assembly, see: (a) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (b) Decher, G. Science 1997, 277, 1232. (10) (a) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605. (b) Sandana, A. Chem. Rev. 1992, 92, 1799. (11) (a) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1998, 120, 7328. (b) Mirsky, V. M.; Hirsch, T.; Piletsky, S. A.; Wolfbeis, O. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 1108. (12) (a) Cao, Y. C.; Jin, R.; Mikin, C. A. Science 2002, 30, 1536. (b) Gerion, D.; Parak, W. J.; Williams, S. C.; Zanchet, D.; Micheel, C. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7070. (c) Rao, J.; Yan, L.; Lahiri, J.; Whitesides, G. M.; Weis, R. M.; Warren, H. S. Chem. Biol. 1999, 6, 353. (13) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315.
10.1021/la034809b CCC: $25.00 © 2003 American Chemical Society Published on Web 07/12/2003
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Figure 1. Schematic illustration of the binding of porphyrintagged polymer 3 to thymine-(NH)/octanethiol-(OT) functionalized SAM 1 and methylthymine-(NMe)/octanethiol-(OT) functionalized SAM 2a.
Experimental Section Preparation of Monolayers. Gold substrates (50 ÅTi/ 1000 Å Au, 15 mm × 15 mm × 1.5 mm) for XPS, ellipsometry, and contact angle measurements were purchased from Evaporated Metal Films (Ithaca, NY). QCM chips (fundamental resonance frequency, fo ) 10 MHz) were purchased from International Crystal Manufacturing Company. Prior to use, gold substrates and QCM chips were cleaned for 10 min in a piranha etch (H2SO4 concentrated + 30% H2O2, 4:1, 70 °C. CAUTION: Strong oxidant), rinsed with distilled/deionized H2O and EtOH, respectively, and placed directly into the thiol solution. Deposition solutions were prepared by mixing appropriate ratios of 0.01 M ethanol solutions of thymine thiol4b and octanethiol to yield SAMs 1a-d and 2a-b. Monolayers were formed by immersion of the gold substrates into the deposition solution. To minimize air oxidation during the 16-20 h assembly process, the solutions were thoroughly purged with argon and stored under an argon atmosphere at room temperature. Physisorbed materials were then removed by repeated rinses with fresh washes with ethanol then chloroform. QCM Apparatus. The QCM studies were conducted using a 10.000 MHz lever oscillator (#35366) purchased from International Crystal Manufacturing Company and a Metex MXC-1600 universal counter with power supply. The frequency counter was interfaced to a standard PC, and Benchview V1.0 software was used to receive and view the data. X-ray Photoelectron Spectroscopy. XPS spectra were obtained on a Physical Electronics Inc., model 5100, XPS spectrophotometer using the Mg KR source with an electron beam power of 400 W. Spectra were acquired at takeoff angles of 15° and 75° from the surface. Highresolution spectra were recorded with a 35.75 eV pass energy and acquisition times ranging from 2 to 4 min. Results and Discussions Preparation of Polymers and SAMs. The diamidopyridine-functionalized polymer 314 (Figure 1) was used for preliminary QCM and surface characterization studies. For XPS studies, porphyrin 415 was attached to provide (14) IIhan, F.; Gray, M.; Rotello, V. M. Macromolecules 2001, 34, 2597.
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a sensitive dual-element XPS tag (F and Zn).16 The thymine SAM 1a (Figure 1) was prepared by immersing thin films of evaporated gold on the appropriate substrates (glass or quartz) into a 1:3 mixed ethanolic thymine-thiol(NH)/octanethiol (OT) solution.6c SAMs with different ratios of thymine on the surface (1b-1d) (vide infra) were similarly prepared using the appropriate ratio of thyminethiol to octanethiol. SAMs featuring N-methylthymine (NMe) functionality (2a and 2b) were prepared in a similar fashion to provide a control surface incapable of threepoint hydrogen bonding with polymer 3. QCM Studies. In contrast to polyelectrolyte deposition where repulsion between like charges limits nonspecific adsorption, van der Waals contacts between the polymer backbone and the surface can compete with the engineered hydrogen-bonding interactions. To determine the selectivity that can be obtained through three-point specific hydrogen bonding, the modification of thymine- and N-methylthymine-functionalized SAMs 1a and 2a was examined at several different concentrations of polymer 3. In these experiments SAMs of 1a and 2a were prepared as described above. These chips were mounted onto the QCM oscillator and submerged into a solution containing fresh chloroform. When a steady baseline had been achieved, a chloroform solution containing the appropriate concentration of 3 was injected to give the corresponding final polymer concentrations. Polymer adsorption on the surface was then followed by monitoring the corresponding change in resonant frequency of the modified quartz chips.17 At all concentrations studied, SAM 1a adsorbs polymer 3 substantially faster than control SAM 2a (Figure 2). The kinetics of both specific and nonspecific adsorption were strongly concentration dependent. At the highest concentration studied (15 µM, Figure 2a), very rapid deposition of polymer 3 was observed with surface 1a. There was likewise significant deposition observed for control surface 2a. It is interesting to note that after the initial rapid uptake of polymer 3 observed with surface 1a, the rate of deposition between the two surfaces was essentially identical, indicating that nonspecific adsorption was occurring in both cases. While a high degree of selectivity could be obtained at the highest concentration by limiting deposition time, the best selectivity was obtained by a 10-fold lowering in concentration (1.5 µM, Figure 2b). As the polymer concentration was reduced further (0.15 µM, Figure 2c), the ratio of the frequency change for nonspecific versus specific binding is similar; however, both processes become quite slow. XPS, Ellipsometry, and Wettability of Adsorbed Polymer Films. QCM experiments indicated that polymer concentrations in the range of 1.5 µM yielded rapid polymer adsorption to the SAM with excellent selectivity observed between surfaces 1a and 2a. This selectivity was verified by XPS of the modified surfaces, monitoring for the presence of fluorine. In these studies modified SAMs 1a and 2a were dipped into 1.5 µM chloroform solutions of polymer 3 for a periods of 30 and 120 s. The SAMs were then thoroughly rinsed with fresh chloroform to remove any physisorbed polymer. As expected, the F1s peak (689 (15) See Supporting Information. (16) (a) Niemz, A.; Jeoung, E.; Boal, A. K.; Deans, R.; Rotello, V. M. Langmuir 2000, 16, 1460. (b) Su, Z.; Wu, D.; Ling Hsu, S.; McCarthy, T. J. Macromolecules 1997, 30, 840. (17) For papers describing QCM surface adsorption studies, see: (a) Ha, T. H.; Kim, K. Langmuir 2001, 17, 1999. (b) Kim, D. H.; Noh, J.; Hara, M.; Lee, H. Bull. Korean Chem. Soc. 2001, 22, 276. (c) Sastry, M. Bull. Mater. Sci. 2000, 23, 159. (d) Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 9645. (e) Roush, J. A.; Thacker, D. L.; Anderson, M. R. Langmuir 1994, 10, 1642.
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Figure 4. XPS survey scans of surface 2a recorded with takeoff angles of 15° for (a) a 30 s dip-time in 3 and (b) a 2 min dip-time in 3. [3] ) 1.5 µM in CHCl3. The insets show the F and Au regions of the spectra. The boxed insets highlight the Zn region for the 75° takeoff angle of the same sample.
Figure 2. QCM frequency responses to SAMs 1a and 2a in CHCl3 after addition of polymer 3 at varying concentrations: (a) [3] ) 15 µM, (b) [3] ) 1.5 µM, and (c) [3] ) 0.15 µM.
Figure 3. XPS survey scans of SAM 1a recorded with takeoff angles of 15° for (a) 30 s and (b) 2 min deposition time in CHCl3 solution of 3. [3] ) 1.5 µM in CHCl3. The insets show the F and Au regions of the spectra. The boxed insets highlight the Zn region for the 75° takeoff angle of the same sample.
eV) appears in the 30 s deposition time with surface 1a (Figure 3a) becoming stronger after 120 s (Figure 3b). Concomitantly, the Au4f doublet (centered at 86 eV)
decreases with increasing deposition time, as expected due to the polymer coating. Further evidence of polymer deposition was provided by the increase in the peak corresponding to the Zn3/2p of the metalloporphyrin (1022 eV) observed in the 75° takeoff angle XPS spectra of polymer-modified SAM 1a (Figure 3b, boxed inset). Collectively, these data indicate the formation of an overlayer of polymer 3 covering the immobilized SAM. In contrast, N-methylthymine SAM 2a showed very little uptake under identical conditions (Figure 4a and 4b), as demonstrated by the absence of the Zn3/2p signal, the very weak F1s peak, and little or no change in the Au4f peak. These results substantiate the data observed in the QCM experiments, further corroborating the ability of hydrogen bonding to provide selectivity for interfacial interactions. An important issue in the deposition process is the thickness of the polymer layer. To probe this issue, ellipsometry was used to determine the thickness of surfaces 1a and 2a after exposure to polymer 3. Virtually no film was detected by ellipsometry for the 30 s deposition period (1.5 µM 3) on SAM 1a (2.4 ( 2.2 Å), indicating sparse coverage after this period. However, a 120 s deposition period yielded a film thickness of ca. 12.3 ( 0.3 Å, comparable with other ultrathin polymer films adsorbed from solution18 and consistent with complete polymer monolayer coverage with 3. In all cases, no film was observed with N-methylthymine SAM 2a, Table 1. One of the major goals of surface modification is the alteration of the chemical and physical properties of the interfacial boundary.19 The dynamic water contact angle measurements of SAM 1a and 2a prior to polymer deposition are similar to previously reported values.4c In all cases polymer deposition with surface 1a results in modest increases in the contact angles of SAM 1a. The final contact angle values of the polymer 3 modified surfaces are in line with other reported polystyrene films.20 The contact angle hysteresis is informative as it gives an indication of chemical heterogeneity and overall surface (18) Motschmann, H.; Stamm, M.; Toprakcioglu, Ch. Macromolecules 1991, 24, 3681. (19) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (20) Extrand C. W. Langmuir 1993, 9, 475.
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Table 1. Dynamic Water Contact Angles, QCM Frequency Shifts, and Ellipsometric Analysis for Surfaces 1a and 2a for Various Deposition Periods and Concentrations of Polymer 3 dynamic water contact anglesa
QCM
ellipsometric analysisa,b
SAM
[polymer 3] (µM)
depositi on period (s)
∆υ (Hz)
advancing (θ°)
receding (θ°)
hysteresis (θ°)
film thickness (Å)
1a
0 15 1.5 1.5 0 15 1.5 1.5
0 30 30 120 0 30 30 120
0 210 101 159 0 25 0 4
75.0 84.0 84.5 85.0 80.0 80.3 81.0 80.3
51.5 50.8 37.6 49.6 63.8 65.0 65.0 63.5
23.5 33.2 46.9 35.4 16.2 15.3 16.0 16.8
0c 10.4 ( 0.4 2.4 ( 2.2 12.8 ( 0.8 0c 0c 0c 0c
2a
a Values reported represent an average of five separate measurements and are (1-2°. b Values for the film thicknesses were calculated using the refractive index of octanethiol and polystyrene and represent an average of five measurements at multiple positions on the surface. c No film was detected.
Figure 5. NH-, Nme-, and OT-functionalized SAMs and polystyrene polymers 5a-e with varying amounts of DAP substitution.
roughness. The water contact angle results indicate that the film becomes more uniform with increasing deposition times, as demonstrated by the decrease in the hysteresis with longer deposition periods. When taken together with the QCM and ellipsometry results for the same deposition periods and concentrations, these data are consistent with a picture of incomplete surface coverage resulting in scattered polymer on the surface for short deposition periods (t ) 30 s). This presents a rough and chemically nonhomogeneous surface to the receding edge of the water droplet, resulting in the larger measured hysteresis. As the length of deposition time and/or concentration of polymer is increased, the polymer fills the interstitial voids creating a smoother, thicker more uniform film. As expected, SAM 2a gave no indication of polymer uptake by water contact angle analysis under similar conditions, once again and reaffirming the results obtained by QCM and ellipsometry. Polymer Adsorption as a Function of Recognition Element Density. With specificity characterized, we next turned our attention to the role of functional group density on polymer adsorption. For these experiments, we parametrically varied the density of recognition elements along the polymer and in the SAM. QCM experiments were then employed to follow the differences in polymer adsorption to the surface. The effect of DAP functional group density on the polymer was determined using the 1:3 thymine surface
Figure 6. (a) QCM frequency responses for functionalized DAP polymers 5a-e adsorbing onto SAM 1a. Polymer concentration in all cases was 1.5 µM in CHCl3. (b) A plot of the initial rate of adsorption onto SAM 1a as a function of percent DAP substitution on the polymer. Curve is intended to lead the eye.
1a, with the amount of DAP substitution on the polymer varied from between ∼2% and 40%.21 Polymers 5a-e (Figure 5) without porphyrin tags were employed for these experiments.22 The deposition process was strongly dependent upon the degree of DAP substitution. Figure 6a shows the QCM profiles of adsorption of polymers 5a-e on SAM 1a. These data indicate that as the number of DAP groups on the polymer increases, the ability of the polymer to adsorb to the surface also increases. It is notable that a relatively low degree of functionalization (5%, corresponding to an average of ∼2 DAP units/polymer chain) resulted in appreciable polymer binding. Higher loadings of DAP on the polymer (15-40%) result in fairly similar adsorption profiles and ∆υmax values. A closer look at the frequency (21) Increasing the amount of DAPsubstitution beyond 40% lead to solubility problems of the polymer in chloroform. (22) It was established that the porphyrin group had little affect on polymer uptake: QCM profiles with and without the porphyrin were virtually identical. See Supporting Information for QCM profiles.
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but similar amounts of adsorbed polymer over longer deposition periods. Consistent results were obtained when the degree of thymine was varied on the surface while the amount of recognition element on the polymer remained constant at 25% DAP for polymer 5d (Figure 7). Surprisingly, very low thymine loadings in the SAM (1:500 NH:OT) results in more adsorption of polymer 5d onto the surface than compared to the 100% N-methylthymine control SAM 2b. As the amount of thymine on the surface is increased from 0.2% for SAM 1d to 100% for SAM 1b, the relative maximum change in frequency also increases. The kinetic dependence of deposition of functional group density is bimodal with large increases in rate observed at low loadings and a slower essentially linear behavior at higher thymine loadings (Figure 7b). Combined, these results clearly illustrate that the degree of hydrogen-bonding elements immobilized within the SAM as well as those located along the polymer backbone affect both the rate of adsorption and amount of polymer adsorbed on the surface. Conclusions
Figure 7. (a) QCM frequency responses for SAMs 1a-d and 2b from adsorption of polymer 5d. Polymer concentration in all cases was 1.5 µM CHCl3. (b) A plot of the initial rate of adsorption of polymer 5d as a function of percent thymine on the surface. Curve is intended to lead the eye.
change early in the adsorption process (between t ) 0-100 s), however, reveals that the number of DAP groups on the polymer plays an important role in the initial rate of polymer adsorption. As the amount of DAP on the polymer is systematically increased (Figure 6b), the initial rate of adsorption increases substantially. It appears that under these conditions, approximately 15% DAP functionalization (or about 5 DAP per polymer strand) is required to achieve fairly rapid and complete polymer adsorption. Less functionalization leads to slow and incomplete polymer adsorption while more DAP results in faster initial rates
These studies indicate that the highly specific Thy-DAP hydrogen-bonding motif provides rapid and selective adsorption of polymer on the surface. Controlling the number of recognition sites on both the polymer and the surface affects both the rate of polymer deposition and the relative amount of polymer adsorbed. This approach to noncovalent surface modification provides a facile route to the formation of neutral surfaces employing specificdirectional hydrogen-bonding interactions between modified surfaces and polymers. Further studies applying this approach to the formation of neutral multilayer films using layer-by-layer self-assembly are underway and will be reported in due course. Acknowledgment. This research was supported by the National Science Foundation [(DMR-9809365, MRSEC) and (CHE-0213554)]. T.B.N. acknowledges the NSERC (Canada) for their support in the form of a postdoctoral fellowship. LA034809B
