© Copyright 2004 by the American Chemical Society
VOLUME 108, NUMBER 17, APRIL 29, 2004
LETTERS Depletion Effect on Supermolecular Assembly: A Control of Geometry of Adsorbed Molecules Liang Li, Dongshan Zhou, Junfeng Zhang, and Gi Xue* State Key Laboratory of Coordination Chemistry, Department of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: February 9, 2004; In Final Form: March 8, 2004
Depletion interaction of added additional polymer latex was reported to drive self-aggregation of particles in suspension or on a hard wall (Yodh, A. G.; Lin, K. H.; Crocker, J. C.; Dinsmore, A. D.; Verma, R.; Kaplan, P. D. Philos. Trans. R. Soc. London A 2001, 359, 921). We describe an experimental investigation of controlling the optimal geometrical arrangement for the adsorbed molecules by depletion interaction between the adsorbent and the metal wall. Bipyridine and a polymer of amino acid were found to form ordered structures that covered silver after adding a depletant of low molecular weight poly(ethylene glycol) (PEG) into their solutions in ethanol. The adsorbed molecules were investigated by surface enhanced Raman scattering (SERS) spectroscopy, which provided information about morphology at the molecular level. The PEG-mediated depletion interaction with the wall contributed to the entropic force for supramolecular assembling of these adsorbents on substrate.
Introduction The optimal geometrical arrangement of adsorbed organic thin film remains one of the most interesting and puzzling problems which have been of relevance in diverse disciplines such as biology, chemistry, and physics. The unexpected properties of ultrathin films represent one of the major mysteries of current science and technology.2 The technological drive to place into surface ever-thinner films cast onto solid substrates has advanced beyond the current level of fundamental scientific understanding.3 Entropic forces between particles in suspension are sometimes produced by the addition of other constituents to the background solvent.1 These added constituents are often other particles, or polymers, which are larger than the background solvent. The physical origin of this induced attraction is both simple and general.4,5 In suspension, for each large particle that may be a * Corresponding author:
[email protected]. Fax: 86-25-83317761.
colloid,6 a protein,7 or a bacteria DNA in a cell,8 there surrounds a crust that is unreachable for the center of the smaller spheres. For entropic reasons they avoid the space between two close large particles, or between a particle and a planar wall, and create an effective attraction among the large particles, or a push of the particle toward the walls.9 Recently, this “attraction by repulsion” underlies a wide spectrum of entropically driven phase separation and assembly phenomena and has been extensively studied theoretically,8 experimentally,1,6-9 and by computer simulations.10 The supramolecular assembly of organics and polymer chain segments on metal is of great interest both in science and in technology, including applications such as biosensors, adhesives, coatings, paints, printed circles, and engineering materials. It is very important to understand the surface molecular structure and underline physics for the adsorption. The development of surface enhanced Raman scattering (SERS) spectroscopy provide a unique method to measure the nearest layer of adsorbents
10.1021/jp0494178 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/02/2004
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to the metal.11 Here we describe SERS measurements of bipyridine and poly(L-histidine) on silver. We try to contribute the solvent-mediated depletion interaction to the molecular orientation of the adsorbents on the substrate. Experimental Section Silver foil, 0.025 mm thick, was immersed into vigorously agitated 3.5 M HNO3 at ambient temperature, for about 2-3 min until the surface of the foil became milky. After etching, the foil was thoroughly rinsed with distilled water and dried in air. The roughened foils were ready for sample doping, and then for the SERS study. Silver foil after such a roughening procedure was reported to exhibit a strong enhanced factor for Raman scattering.12 2,2′-Bipyridine solution was prepared in ethanol at 0.01 M. Poly(ethylene glycol) with molecular weight 300 (PEG-300) was obtained from Aldirich. A controlled amount of PEG-300 was added to the solution to keep it clear and transparent. A piece of silver foil was immersed into the solution for 10 min. The specimen was then washed with ethanol and dried for Raman measurement. Poly(L-histidine) was purchased from Aldirich (Mn ) 15000), and was dissolved in ethanol to make a solution of 0.03 wt % concentration. A controlled amount of PEG was added to the solution with stirring to keep the solution transparent because an excessive amount of PEG would result in phase separation of the solution. A certain amount of poly(L-histidine) solution in ethanol/PEG was spread uniformly onto a 2 cm2 silver foil, and the foil was placed into a container to evaporate the ethanol slowly. The foil was then washed with water to clean the PEG residue. Thickness of the film after drying was controlled by the volume of the solution spread on the silver foil to a range of 50-100 nm. The prepared sample was ready for SERS measurement. SERS spectra were recorded with a SPEX-1403 Raman spectrometer. The incident excitation wavelength was 647.1 nm from a Kr+ laser source, with output power 20-100 mW. A backscattering geometry in air was used for all samples. Results and Discussion The observation of enormously enhanced cross-sections (up to 106) for Raman scattering from molecules adsorbed on metal surface was one of the most important discoveries in the field of surface chemistry in the last two decades of the past century.11 Surface enhanced Raman scattering (SERS) was first observed for pyridine monolayer adsorbed onto metal electrodes.11 Figure 1 shows the SERS spectra of a monolayer of adsorbed 2,2′bipyridine on silver along with a normal Raman spectrum (Figure 1a) of bipyridine in the solid state. The SERS spectrum of Figure 1b was recorded for a specimen of bipyridine on silver adsorbed from a solution in ethanol and PEG oligomer. Figure 1c illustrates the SERS spectrum of bipyridine on silver prepared from ethanol solution. Bipyridine on silver electrode or silver sols was studied extensively. It was reported that the Ramanactive ring breathing mode located at 993 cm-1 is sensitive to its environment. In the SERS spectra of Figure 1b and 1c, the ring breathing mode was shifted to 1005 and 1025 cm-1, respectively. The SERS spectrum of Figure 1b indicates weak signal intensities of the 1587, 1570, 1452, and 1301 cm-1 bands due to ring in-plane deformation, or in-plane CH bending modes, when compared with their counterparts at 1587, 1570, 1444, and 1299 cm-1 in Figure 1a, respectively. The strong 1480 cm-1 band in Figure 1a is completely lost in Figure 1b. It is remarkable that the 855 and 223 cm-1 bands, attributed to CH out-of-plane wagging and ring out-of-plane deformation, re-
Figure 1. (a) Normal Raman spectrum of bipyridine in solid state, (b) SERS spectrum of bipyridine adsorbed on silver from ethanol/PEG solution, (c) SERS spectrum of bipyridine adsorbed on silver from ethanol solution.
spectively, are intensified in Figure 1b. The appearance of a strong band at 1392 and 1274 cm-1 in the SERS spectrum implies that the presence of the surface has led to reduction in symmetry and made these Raman-inactive vibration modes (au and bu) SERS-active. We propose that the bypyridine rings are oriented parallel to the surface in Figure 1b if the SERS selection rule based on electromagnetic theory is applied. The notable feature in the SERS spectrum of bipyridine on silver in Figure 1c is the loss of the 855 cm-1 band in seen in Figure 1b. In addition, the in-plane ring modes between 1600 and 700 cm-1 in Figure 1c appear very strong and have shifted considerably. The band observed at 233 cm-1 is assigned to a Ag-N stretching mode. Therefore, the SERS spectrum in Figure 1c reveals a change of adsorption configuration showing that bipyridine interacts uniquely with the surface via σ donation from one of the nitrogen atoms in a standing-chelating fashion. It was to our great surprise that the SERS spectrum in Figure 1b could change to the same spectrum as Figure 1c after the sample was heated at 100 °C for 1 h. There are many interactions between the polymer and the surface, or the polymer and the adsorbate. In our previous work, we have calculated the energy curves for the different adsorption geometries of imidazole on the silver surface.13 It was found that the standing-chelating fashion was the most stable adsorbing geometry owing to a bonding through the nitrogen lone pair (N-bonded through σ-donation), while the flat geometry was in a metastable minimum owing to the aromatic ring π-bonded to the surface. The energy difference between the flat bonding geometry and standing-up geometry is about 0.18 eV (∼7 kBT). However, Kaplan et al.9 discovered that the instability arising in binary particle mixtures was phase separation into ordered surface phases near a hard wall and a disordered bulk liquid phase. Ordered and disordered surface phases can also be understood using simple excluded-volume entropy arguments,
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Figure 2. (a) Schematic of the depletion effect in solution and on the wall of various shapes. The dark shaded region represents the gain of small-sphere excluded volume (entropy).1 The centers of small spheres are excluded from a depletion zone (hashed regions) outside the large spheres and corrugated walls. Spheres are preferentially drawn to interior corners.7 (b) Cartoon of bipyridine on surface of the wall. The flat orientation provides the largest dark shaded region which represents the gain of excluded volume (entropy).
in this case between the large particle and the wall (illustrated in Figure 2a). This is so-called the “colloid limit”. If the added polymer is larger than the particle, the depletion interaction was often called the “protein limit”.14 The resulting entropic forces are of considerable importance in a wide variety of practical materials ranging from frozen desserts to paints to motor oils to living cells. The depletion potential in the isotropic hard spheres is more than 5 kBT by direct measurement.15a The entropic force on a particle near the wall surface was calculated to be roughly twice as large as that between two large particles in the bulk.1,9 However, its effect to change the orientation of anisotropic particles may be significant. The quantitative estimation was reported of a depletion-induced significant modulus of the “entropic troque” of the order of about 20 kBT rad-1 to drive the rod to lie parallel with the wall.15b So the depletion interaction is fairly close to, or larger than, the energy difference between two geometries and can play an important role on the change of the orientation. Figure 2b shows a cartoon for the adsorption of bipyridine from an ethanol/PEG solution onto silver, based on the spectroscopic evidence shown in Figure 1b. It can be seen that the flat orientation of the molecule on the surface provides the largest overlapping of the excluded volumes of the wall and the molecules, indicating that PEGmediated depletion interaction induced entropic force for the adsorption geometry of bipyridine on silver. However, annealing the dried sample at high-temperature drives the metastable flat geometry to the more stable standing-chelating fashion, as shown in Figure 1c, since there is no longer a depletion effect to counteract such a flip-flop. The depletion interaction effect for controlling surface geometry of the adsorption layer on metal can be extended to polymeric thin films. Figures 3B and 3C show SERS spectra recorded from poly(L-histidine) films prepared from ethanol and ethanol/PEG solutions, respectively. For comparison, a normal Raman spectrum for solid poly(L-histidine) was shown in Figure 3A. The relative intensities of bands in the two SERS spectra are different. We propose that these differences are due to a geometry change of the chain segments on the surface of silver. The most intense bands in the SERS spectrum (Figure 3C) of the polymer film prepared from ethanol/PEG solution are at
Figure 3. Dependence (A) normal Raman spectrum of poly(L-histidine) in solid state; (B) SERS spectrum of poly(L-histidine) film on silver prepared from ethanol solution; (C) SERS spectrum of poly(L-histidine) film on silver prepared from ethanol/PEG solution.
1015 and 770 cm-1, the relative intensities of the bands in the spectrum (Figure 3B) of the polymer film from ethanol solution are almost similar to the normal Raman spectrum of pure polymer. In the Raman spectrum of monosubstituted imidazole, the band near 760 cm-1 is due to out-of-plane C-H deformation, and the band at 1600 cm-1 is due to ring stretching modes.16 The SERS spectrum of Figure 3C shows an enhanced and shifted band at 770 cm-1 and a very weak band at 1600 cm-1, indicating that the imidazole rings are “lying down” flat on the surface, while the SERS spectrum in Figure 3B indicates a random orientation. In particular, we would like to mention that the signal-to-noise ratio for 1331 and 1007 cm-1 bands in Figure 3B are about one-tenth that of their counterparts in Figure 3C, indicating that enhancement for the spectrum adsorbed from ethanol is weaker than that from ethanol/PEG solution. It seems that ordered geometry of the adsorbent on the surface could provide much stronger enhancement in Raman intensity. The addition of polymers to suspensions of micro- and nanoparticles induces depletion interactions that profoundly affect their physical properties. Demixing or ordered organization of large particles will overlap their crusts and result in extra free volumes for smaller particles to translate and contribute extra translation entropy for them. The instable arising binary particle mixture was phase separated into an ordered surface phase near a hard wall and a disordered bulk liquid phase. Ordered surface phases can also be understood using simple excluded-volume entropy arguments,1,9 in this case between the large particle and the wall (illustrated in Figure 2). The entropic force on a particle near the wall surface was reported to be roughly twice as large as that between two large particles in the bulk.1 In our SERS experiments, the addition of PEG oligomer to bipyridine or polymer solution developed entropic
5156 J. Phys. Chem. B, Vol. 108, No. 17, 2004 forces and “entropic torque” to aggregate the polymers or to push them to the wall.17 The experimental evidences clearly show that PEG-mediated depletion interaction between these polymers and the metal substrates resulted in ordered aggregations which covered the surfaces. It is clear that depletion phenomena play a role in supramolecular assembling of thin films. Acknowledgment. We gratefully acknowledge financial support by National Natural Science Foundation of China (NNSFC, No. 20374027, 50133010 and 90103036) and the Doctorial Study Fund of Chinese University. References and Notes (1) Yodh, A. G.; Lin, K. H.; Crocker, J. C.; Dinsmore, A. D.; Verma, R.; Kaplan, P. D. Philos. Trans. R. Soc. London A 2001, 359, 921. (2) Reiter, G.; de Gennes, P. G. Eur. Phys. J. E. 2001, 6, 25. (3) Sanchez, I. C. Physics of polymer surfaces and interfaces; Butterworth-Heinemann: Boston, 1992. (4) Colloid Physics; Proceedings of the Workshop on Colloid Physics; University of Konstanz, Germany, 1995 [Physica A 235, 1997]. (5) (a) Asakura, S.; Oosawa, F. J. Chem. Phys. 1954, 22, 1255. (b) Asakura, S.; Oosawa, F. J. Polym. Sci. 1958, 33, 183. (6) Adams, M.; Dogic, Z. Keller, S. L.; Fraden, S. Nature 1998, 393, 349.
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