© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 32, AUGUST 17, 2000
LETTERS Temperature-Driven Phase Transitions in Oligo(ethylene glycol)-terminated Self-Assembled Monolayers Ramu j nas Valiokas,† Mattias O 2 stblom,† Sofia Svedhem,‡ Stefan C. T. Svensson,‡ and ,† Bo Liedberg* DiVisions of Applied Physics and Chemistry, Department of Physics and Measurement Technology, Linko¨ ping UniVersitet, S-581 83 Linko¨ ping, Sweden ReceiVed: April 21, 2000; In Final Form: June 15, 2000
This letter explores the phase behavior of oligo(ethylene glycol) self-assembled monolayers using temperatureprogrammed infrared reflection absorption spectroscopy. The monolayers are formed by self-assembly of hexa(ethylene glycol) (EG6) and tetra(ethylene glycol) (EG4)-terminated and amide group containing alkanethiols on polycrystalline gold. The ethylene glycol portions of the two monolayers are known to exist in two different conformations at room temperature: EG6 in helical and EG4 in all-trans (zigzag). The helical phase of the EG6 gradually diminishes upon increasing the temperature and a pronounced conformational transition occurs around 60 °C, leading to a rapidly increasing population of all-trans conformers along the EG6 chain. The EG4 SAM exhibits a much simpler phase behavior. The oligomer conformation is marginally affected upon increasing the temperature to 75 °C, displaying the dominating all-trans phase, which possibly coexists with a small fraction of gauche-rich (disordered) regions. The reported conformational changes are reversible upon returning to 20 °C after stepwise heating to 70 °C.
Introduction Self-assembled monolayers (SAMs) on solid surfaces1,2 are becoming more and more important for fundamental studies of phenomena at surfaces and interfaces, as well as for a variety of technological applications. For example, the use of SAMs, prepared from tailor-made bifunctional compounds bearing sulfur headgroups on gold and silver, to mimic the surface properties of polymeric bulk materials is a very attractive approach in the biomaterials science community.3,4 In organic electronics, several investigators have used oligomers as building blocks to generate well-defined and stable interfacial SAM structures with novel and unique properties.5-9 * Corresponding author. Tel.: +46 13 281877. Fax: +46 13 137568. E-mail:
[email protected]. † Department of Physics and Measurement Technology. ‡ Divisions of Applied Physics and Chemistry.
In the research on artificial biopolymers, one type of oligomers that has been synthetically incorporated into SAMs is the oligo(ethylene glycols) (OEGs). The polymeric analogue, poly(ethylene glycol),10 is well-known for its protein- and cellrepulsive properties.11,12 The approach of using OEG SAMs was introduced by G. Whitesides and co-workers as a model system in studies of protein adsorption on organic surfaces.13 In other applications, the OEG SAMs have been utilized as a convenient spacer arm to facilitate biomolecular recognition events at surfaces.14-16 Further on, several different types of supported lipid bilayers have been designed taking advantage of the flexible (hydrogel-like) nature of the OEG portion of the SAMs.17-21 Naturally, a successful application of the OEG SAMs in any of the above-mentioned cases requires a good understanding of their structural properties. In a detailed study by Harder et al.22 it was shown that the terminal OEGs can
10.1021/jp001536+ CCC: $19.00 © 2000 American Chemical Society Published on Web 07/15/2000
7566 J. Phys. Chem. B, Vol. 104, No. 32, 2000
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exist in different conformations, and that these conformations could be controlled by varying the lattice parameters of the substrate. More recently, a structural study of a series of SAMs formed by direct pinning of the OEG to gold was presented by Vanderah et al.23 Such an “inverted” structure with alkyl chains on top of the OEG layer displayed a remarkably well-defined helical crystalline phase of the hexaethylene glycol portion. In an earlier work we introduced a new class of OEG SAMs formed by self-assembly of HS(CH2)15CONH-EGn, where n ) 1, 2, 4, 6.24 The distinctive feature of the compounds is the amide moiety between the alkylthiol chain and the OEG chain, resulting in lateral hydrogen bonding in the SAMs. This type of SAMs showed an interesting chain-length-dependent phase behavior: the EG2 and EG4 chains adopted the all-trans and the EG6 chain the helical conformation. In the present letter, we report on an infrared reflection absorption spectroscopy (IRAS) study of the temperature-driven OEG phase transitions in these laterally hydrogen-bonded SAMs. This is to our knowledge the first temperature-programmed infrared investigation of the phase behavior of OEGs in SAMs. We believe that the study will contribute to deeper understanding of their structural properties, inter- and intramolecular interactions, and stability. In particular, we hope that this basic approach will improve the knowledge about the temperature dependence of protein adsorption on flexible organic substrates, which is necessary for the development of novel protein-resistant surfaces. It will also be very helpful for the design of supporting matrixes for biomimetic lipid membranes.
incubation beakers. After at least 48 h of adsorption, the samples were rinsed in ethanol, ultrasonicated for 3 min, and rinsed again. Finally, they were blown dry in nitrogen gas and immediately transferred into the measurement system. Temperature-Programmed Infrared Reflection Absorption Spectroscopy. The spectroscopic measurements were performed in an ultrahigh vacuum (UHV) system which has been described in detail elsewhere.25 A modified Bruker IFS PID 22 spectrometer aligned at a grazing angle of 82° was equipped with f/16 transfer optics and a liquid-nitrogen-cooled narrow band MCT detector. The spectral resolution was 2 cm-1 and 500 scans were collected. A three-term Blackmann-Harris apodization function was applied to the interferograms before Fourier transformation. The data were analyzed using Bruker OPUS software. The temperature of the sample was changed by resistive heating or liquid-nitrogen-cooling of the solid copper sample holder in the UHV measurement chamber (base pressure was 10-10 mbar). The temperature was measured by a Pt100 element in the sample holder and controlled by an Eurotherm controller. After setting the temperature to the chosen value, the system was allowed to stabilize for 5 min, then the sample spectrum was recorded. When the spectroscopic characterization of the sample at the different temperatures was completed, it was set back to room temperature to check the reversibility. Finally, the sample was transferred into a preparation chamber for ion sputtering and then the background spectrum was recorded. Results and Discussion
Experimental Section Sample Preparation. A synthetic pathway for preparing the OEG-terminated and amide group-containing alkanethiol compounds (Scheme 1) was briefly described in a previous paper.24 For adsorption of SAMs, fresh ethanolic solutions of 20 µM EG4 and EG6 were prepared in plastic beakers from 1 mM stock solutions stored in glass vials at room conditions. As a substrate for the SAMs, 2000 Å thick gold films were electron-beam deposited via a 25 Å titanium adhesion layer on standard (100)silicon wafers. The electron-beam evaporation of the metals was done in a Balzers UMS 500 P system operating at a base pressure of 10-9 mbar and an evaporation pressure of about 10-7 mbar. The evaporation rate was kept constant at 10 Å/s for gold. Prior to SAM adsorption, the gold surfaces were cleaned in a 5:1:1 mixture of deionized (MilliQ) water, 25% hydrogen peroxide, and 30% ammonia for 5 min at 80 °C, followed by rinsing in deionized water. The efficiency of this washing procedure was tested before the adsorption experiment by measuring the optical characteristics of the gold samples using an automatic Rudolph Research AutoEL ellipsometer with a He-Ne laser light source of λ ) 632.8 nm, at an angle of incidence of 70°. A gold surface treated in this way gave routinely an ellipsometric angle ∆ >110.00°. The cleaned samples were soaked in ethanol and then transferred into the
The reflection-absorption (RA) spectra at room temperature (20 °C) confirm the presence of highly ordered EG4 and EG6 SAMs (Figures 1 and 2). The spectra are almost identical to those published previously,24 except for some small differences in peak position and intensity. These differences are attributed to the SAM formation from 20 µM solutions. In our earlier work, 1 mM solutions gave SAMs with less pronounced crystalline OEG phases. A detailed discussion about these differences will be reported separately. The SAMs are investigated in the temperature range of 20-90 °C, with 5 °C intervals, using IRAS. However, annealing of the SAMs above 75 °C results in a significant increase in the disorder of the underlying alkyl part of the SAM, which in turn affects the conformation of the OEGs. Since the present study focuses entirely on the phase behavior of the OEG part, we would like to avoid this contribution. RA spectra are therefore only reported up to 75 °C. CH Stretching Region. The evolution of the CH stretching peaks with temperature is shown for the EG6 SAM in Figure 1. The peaks due to the alkyl CH2 asymmetric (d-) and symmetric (d+) stretching modes appear at 2918 and 2851 cm-1, respectively, indicating an excellent all-trans crystallinity of the alkyl part of the SAM. The symmetric CH2 stretching mode of the EG6 methylenes, which also appears in this part of the spectra
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J. Phys. Chem. B, Vol. 104, No. 32, 2000 7567
Figure 1. Temperature-induced changes in the CH stretching region of EG6-terminated alkanethiolates on gold in the temperature range 2075 °C, with intervals of 5 °C. For EG4, the spectra are shown only at 20 °C (upper curve) and 75 °C (lower curve), as only a slow drop in the intensity of the peaks could be observed upon the stepwise increase of the temperature.
as a strong peak at 2892 cm-1, is characteristic for the crystalline helical phase of the oligomer.10,22,24,26 Upon increasing the temperature, changes in the intensity of this peak dominate in the 3000-2800 cm-1 region. First, it gradually decreases in intensity with every 5 °C. Later, the peak vanishes quickly around 65 °C and disappears in the background of overlapping contributions from the tails of the alkyl peaks and othersymmetric CH2 stretching modes of the EG6 portion of the SAM. It also can be observed that a weak but well-resolved jump occurs in the broad shoulder around 2950 cm-1 on going from 60 to 65 °C. Contrary to the CH2 stretching modes of the EG6 part of the SAM, the two alkyl peaks undergo no significant change within the reported temperature range. The slight shift of the asymmetric stretch to 2919 cm-1 at the highest temperatures is approximately due to the appearance of one gauche defect per alkyl chain. The intensity of the d- peak is slightly reduced with increasing temperature. However, because of the strongly varying and overlapping intensity contribution from the 2892 cm-1 peak, it is hard to say whether the observed gradual lowering of the intensity of the d- peak can be attributed to a change in tilt(rotation) angles of the alkyl chains.27,28 The RA spectra of the EG4 SAM are also shown in Figure 1. The upper spectrum is taken at 20 °C and the lower one at 75 °C. The alkyl CH2 stretching peaks also demonstrate a high degree of order in the SAM at room temperature (2918 cm-1 (d-) and 2851 cm-1 (d+)). However, the completely different signature from the EG4 CH2 symmetric stretches, between the two sharp alkyl peaks, indicates a structural difference between the two compounds. Further on, the structure of the EG4 SAM is retained within the investigated range of temperatures, with only a weak shift of the (d-) peak to 2919 cm-1 and a small drop in intensity, which in this case can be interpreted as a decrease in the tilt(rotation) angles of the alkyl chains with respect to the surface normal.27,28
Figure 2. (a) Fingerprint region of EG6-terminated alkanethiolates on gold in the temperature range 20-75 °C, with intervals of 5 °C. (b) Fingerprint region of EG4-terminated alkanethiolates on gold in the temperature range 20-75 °C, with intervals of 5 °C.
7568 J. Phys. Chem. B, Vol. 104, No. 32, 2000 Fingerprint Region. Temperature-driven changes are also seen in the fingerprint region of the EG6 SAM spectra, Figure 2a. The features in the EG6 SAM spectrum previously assigned to CH2 scissoring 1464, wagging 1349, twisting 1244, and rocking modes at 963 cm-1, respectively, as well as the strong and sharp skeletal COC stretching peak at 1114 cm-1, are all due to the presence of the helical crystalline phase.10,22-24,26 The modes have their transition dipole moments parallel to the helical axis,26 and the strong intensity of these peaks and the CH symmetric stretch discussed above confirm a very good orientation of the EG6 chain perpendicularly to the surface. Furthermore, they all follow the response seen in the CH2 stretching region upon the increasing temperature. Namely, the peaks gradually decrease with every 5 °C until they disappear at around 70 °C, indicating a continuous loss of the helical conformation of the EG6. However, in addition to this, a distinctive change occurs in the region of the skeletal COC modes between 1200 and 1050 cm-1 around 60-65 °C. When the characteristic signatures due to the helical phase disappear, a new spectral feature suddenly appears at 1144 cm-1. The 1144 cm-1 feature continues to gain intensity and becomes the dominating peak in the region at 75 °C. In fact, it remains almost unchanged up to 90 °C (not shown). The change in the spectral signature reflects a dramatic conformational transition occurring in the EG6 part of the SAM. Note that the new strong COC peak at 1144 cm-1 is not identical to the broader and weaker one around 1126 cm-1, previously observed in disordered OEG phases of EG3 SAMs.22 It must therefore have a different origin. We believe that the 1144 cm-1 peak corresponds to the 1145 cm-1 peak seen in the RA spectrum of the EG3 SAMs on silver, which was attributed to an all-trans conformation of the oligomer.22,29 In our previous paper we also reported on EG4 and EG2 SAMs with their COC skeletal stretching modes at 1146 and 1143 cm-1, respectively, and based on IRAS, ellipsometric, and contact angle data, we concluded that these peaks belonged to the all-trans phase.24 The same strong all-trans peak at 1148 cm-1 is dominating the fingerprint region in the RA spectrum of the EG4 SAM at 20 °C, Figure 2b. The small shift in peak position as compared to our earlier work is, as mentioned before, due to the µM solutions and the better packing of the resulting SAMs. The nature of this peak can be better understood from this temperature-programmed study. Thus, one can easily observe that only marginal changes occur in the fingerprint of the EG4 as the temperature increases. In addition to the slight rearrangement of the alkyl chains seen in the CH2 stretching region, the strong COC stretch peak gradually shifts toward the final position at 1143 cm-1 at 75 °C. This behavior is understandable if the all-trans conformers are assumed to dominate the EG4 phase in the SAM. Similarly to the alkyl chains, the densely packed and extended EG4 oligomers appear to be restricted from any additional phase transitions within the chosen range of temperatures. Since a strong resemblance is found between the EG4 and EG6 COC modes above 65 °C, it is reasonable to believe that both compounds exist in a similar, possibly slightly disordered, but still predominantly all-trans state. We have to point out again that the OEGs always coexist as a mixture of different conformers in the SAMs under investigation, as can be seen from the broad shoulders on the COC stretching peak. However, either the helical crystalline (for EG6) or all-trans phase (for EG4) is dominating for the particular compound at room
Letters
Figure 3. Evolution of normalized integrated intensities as a function of temperature for the EG6 fingerprint peaks due to the helical (open symbols) and all-trans (filled symbol) conformations: wagging 1349 cm-1 (triangles), COC stretching 1114 cm-1 (open circles), rocking 963 cm-1 (diamonds), and COC stretching 1144 cm-1 (filled circles) modes.
Figure 4. The reversibility of the temperature-induced conformational changes of the EG6- and EG4-SAMs. The thick lines show the COC stretching modes at 20 °C, before the heating cycle. The spectra in thin lines were recorded after raising the temperature in 5 °C steps to 70 °C and then returning directly to 20 °C.
temperature. Most importantly, the helical conformers of EG6 can be switched to the all-trans state by increasing the substrate temperature. The integrated intensities (normalized) of the helical and alltrans peaks in the spectra of the EG6 SAM, Figure 2a, can also be plotted as a function of temperature to reveal the phase transition temperature, Figure 3. A subtraction procedure was used before the integration in order to separate the intensities of the skeletal COC peaks characteristic of the helical (1114 cm-1) and all-trans phases (1144 cm-1). For example, the spectrum obtained at 75 °C, Figure 2a, was subtracted from the spectra obtained at the lower temperatures (using different scale factors) to access the COC peak intensity at 1114 cm-1. Then, the intensities of the helical peaks were determined by
Letters integrating between 1360 and 1338 (1349) cm-1, 1200-1050 (1114) cm-1, and 980-950 (963) cm-1, respectively. Similarly, the intensity of the all-trans COC peak at 1144 cm-1 was obtained by subtracting the spectrum recorded at 20 °C from those at the higher temperatures, and then integrating between 1200 and 1050 cm-1. The intensities of the three helical peaks display exactly the same temperature dependence, confirming a gradual loss of the helical phase just above room temperature. At around 55 °C the peaks due to the helical phase start to rapidly decrease in intensity, and the temperatures corresponding to 50% loss of the original peak intensities are all observed in a narrow range 60 ( 2 °C. This process is paralleled by a sharp increase in intensity of the 1144 cm-1 peak, and the corresponding 50% value occurs close to 60 °C. Although the subtraction procedure is slightly uncertain, the present approach clearly suggests that the oligomer portion of the EG6 SAM undergoes a helical to all-trans phase transition around 60 °C. Moreover, the overall phase transition is completed at 70-75 °C. The reversibility of the observed conformational transition was checked in a separate series of measurements by first raising the temperature to 70 °C and then returning to 20 °C. The same stabilization time of 5 min was used before the measurements. The SAMs were found to relax into the initial conformations, giving spectral signatures nearly identical to those at 20 °C before heating, as it is shown for the COC stretch peak in Figure 4. The difference in the composition of the OEG phase before the SAMs are subjected to the temperature cycle (thick lines) and after it (thin lines) is marginal. Particularly, the relatively high degree of reversibility in the helical EG6 phase is surprising keeping in mind the high complexity of the compounds forming the SAMs. However, we found that the losses in the initial phase somewhat increase for both compounds when the temperature is raised to 90 °C (the spectra not shown), and this can be due to the irreversible rearrangements in the underlying alkyl part of the SAMs. Conclusions We have studied the temperature driven phase behavior of the EG6- and EG4-terminated and amide group containing alkanethiolates on gold. First, it can be concluded that in the EG4 SAM no significant conformational transitions take place between 20 and 75 °C, indicating a good packing and stability of both the alkyl and EG4 parts of the molecules. This finding is consistent with a dominating all-trans phase of the oligomer within the chosen temperature range. Second, the EG6 in the SAM is found to undergo a distinctive conformational transition from the dominating helical-crystalline phase, to a phase similar to that of EG4, instead of the expected amorphous disordered state. The conformational transition to the all-trans state occurs around 60 °C, and it is reversible in the range of temperatures 20-70 °C. A detailed study on the mechanism of this transition and the role of the intermolecular interactions including lateral hydrogen bonding between the amide groups will be reported separately. Finally, the present study demonstrates the power of temperature-programmed IRAS measurements for obtaining
J. Phys. Chem. B, Vol. 104, No. 32, 2000 7569 a better understanding of the phase behavior of oligomers in SAMs. The importance of the OEG conformation for design of protein-resistant surfaces and supported lipid membranes will be addressed in subsequent papers. Acknowledgment. R.V. and S.S. are Ph.D. students at the Graduate School Forum Scientum which is founded by the Swedish Foundation for Strategic Research (SSF). The authors also thank the Swedish Research Council for Engineering Sciences (TFR). References and Notes (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (2) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (3) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (4) Ostuni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B 1999, 15, 3-30. (5) Ba¨uerle, P.; Go¨tz, G.; Hiller, M.; Scheib, S.; Fischer, T.; Segelbacher, U.; Bennati, M.; Grupp, A.; Mehring, M.; Stoldt, M.; et al. Synth. Met. 1993, 61, 71-79. (6) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (7) Liedberg, B.; Yang, Z.; Engquist, I.; Wirde, M.; Gelius, U.; Go¨tz, G.; Ba¨uerle, P.; Rummel, R. M.; Ziegler, C.; Go¨pel, W. J. Phys. Chem. 1997, 101, 5951-5962. (8) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J. Y.; Gozin, M.; et al. J. Am. Chem. Soc. 1999, 121, 1059-1064. (9) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir 1999, 15, 1121-1127. (10) Bailey, F. E., Jr.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976. (11) Lee, J. H.; Kopecek, J.; Andrade, J. D. J. Biomed. Mater. Res. 1989, 23, 351-368. (12) Desai, N. P.; Hubbell, J. A. Biomaterials 1991, 12, 144-153. (13) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20. (14) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009-12010. (15) Lahiri, J.; Isaacs, L.; Tien, J.; Whitesides, G. M. Anal. Chem. 1999, 71, 777-790. (16) Lahiri, J.; Isaacs, L.; Grzybowski, B.; Carbeck, J. D.; Whitesides, G. M. Langmuir 1999, 15, 7186-7198. (17) Heyse, S.; Stora, T.; Schmid, E.; Lakey, J. H.; Vogel, H. B. B. A.-ReV. Biomembranes 1998, 1376, 319-338. (18) Heyse, S.; Ernst, O. P.; Dienes, Z.; Hofmann, K. P.; Vogel, H. Biochemistry 1998, 37, 507-522. (19) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197-210. (20) Raguse, B.; Braach-Maksvytis, V.; Cornell, B. A.; King, L. G.; Osman, P. D. J.; Pace, R. J.; Wieczorek, L. Langmuir 1998, 14, 648-659. (21) Williams, L. M.; Evans, S. D.; Flynn, T. M.; Marsh, A.; Knowles, P. F.; Bushby, R. J.; Boden, N. Langmuir 1997, 13, 751-757. (22) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. 1998, 102, 426-436. (23) Vanderah, D. J.; Meuse, C. W.; Silin, V.; Plant, A. L. Langmuir 1998, 14, 6916-6923. (24) Valiokas, R.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. Langmuir 1999, 15, 3390-3394. (25) Engquist, I.; Lundstro¨m, I.; Liedberg, B. J. Phys. Chem. 1995, 99, 12257-12267. (26) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37, 2764-2776. (27) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (28) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (29) Pertsin, A. J.; Grunze, M.; Garbuzova, I. A. J. Phys. Chem. 1998, 102, 4918-4926.
