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Langmuir 1997, 13, 2740-2748
Functionalization of Hydroxyl and Carboxylic Acid Terminated Self-Assembled Monolayers David A. Hutt and Graham J. Leggett* Department of Materials Engineering and Materials Design, The University of Nottingham, University Park, Nottingham NG7 2RD, U.K. Received January 22, 1997. In Final Form: March 12, 1997X The functionalization of monolayers of mercaptopropanol (MPL) and mercaptopropanoic acid (MPA) adsorbed onto surfaces of gold has been studied using X-ray photoelectron spectroscopy and contact angle goniometry. The terminal hydroxyl goups of the MPL monolayers were derivatized with the vapor from either trifluoroacetic anhydride, pentafluoropropanoic anhydride, or heptafluorobutyric anhydride to yield highly hydrophobic, CF3 functionalized surfaces. For the MPA monolayers the carboxylic acid groups at the surface were derivatized by reaction with the vapor from trifluoroethanol, pyridine, and di-tertbutylcarbodiimide. This resulted in only ∼60% of the surface carboxyl groups undergoing reaction in contrast to the 100% derivatization of poly(acrylic acid), indicating a difference in reactivity of these two materials.
Introduction Self-assembled monolayers (SAMs), formed typically by the adsorption of alkanethiols onto gold surfaces, or by the adsorption of alkylsilanes onto silica surfaces, have attracted widespread interest in recent years in a range of applications including fundamental studies of interfacial phenomena, such as wetting1-3 and biological interactions,4-6 and the development of novel functional molecular thin film architectures.7-9 We are interested in the construction of molecular films for electronic and sensor applications, utilizing microstructured SAMs as templates. We are currently exploring a variety of strategies by which SAM patterning by photooxidation may be combined with the functionalization of reactive tail groups to yield the desired film chemistry. In the present study, we have explored the efficacy of two procedures which may prove valuable as means by which functional molecules may be attached to hydroxyl and carboxylic acid tail groups, a particularly convenient approach because of the ease with which SAMs with these tail chemistries may be prepared. The approaches which we have explored have been utilized elsewhere to derivatize polymer surfaces prior to analysis by X-ray photoelectron spectroscopy (XPS). A further objective of our research has therefore been to compare the reactivities of SAM and polymer surfaces with similar chemistries. Such studies may yield valuable insights not only into the * Corresponding author. X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (2) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (3) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (4) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877. (5) DiMilla, P. A.; Folkers, J. P.; Biebuyck, H. A.; Harter, R.; Lopez, G. P.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 2225. (6) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. N.; Wang, D. I. C.; Whitesides, G. M.; Ingber, D. E. Science 1994, 264, 696. (7) Spinke, J.; Liley, M.; Guder, H.-J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821. (8) Knoll, W.; Angermaier, L.; Batz, G.; Fritz, T.; Fujisawa, S.; Furuno, T.; Guder, H.-J.; Hara, M.; Liley, M.; Niki, K.; Spinke, J. Synth. Met. 1993, 61, 5. (9) Willner, I.; Doron, A.; Katz, E.; Levi, S.; Frank, A. J. Langmuir 1996, 12, 946.
S0743-7463(97)00069-3 CCC: $14.00
functionalization of SAMs but also into the fundamental nature of surface reactivity both for SAMs and polymer surfaces. A range of labeling techniques have been developed for polymer surfaces,10-22 utilizing reactive molecules which yield prominent, characteristic peaks in the XPS spectra, and may have wider utility in the functionalization of organic surfaces. The majority of these procedures involve the use of reagents which are in the vapor phase, rather than the liquid phase, to prevent absorption of material into the polymer. In fact it has frequently been found that common solution phase chemistry does not work at surfaces for the attachment of labeling groups.10 Fluorinated molecules are particularly valuable, because of both the high photoelectron yield from fluorine and the magnitude of the chemical shifts induced by F in the C 1s spectrum. Trifluoroacetic anhydride (TFAA) reacts with hydroxyl groups to form an ester and is therefore a useful probe for these species, although Chilkoti and Ratner17 have shown that it may also be used to derivatize epoxide functionalities. In both cases, the yield is close to 100% and the use of TFAA has been widely investigated by a number of groups for several model polymers.13,14,16-22 For carboxylic acid groups, trifluoroethanol (TFE) has been used in conjunction with a carbodiimide reagent to activate the carbonyl carbon of the acid group toward nucleophilic attack.13,15,18 The efficacy of this approach has been investigated using poly(acrylic acid) (PAA) as a model system, and for this polymer, 100% derivatization has been achieved. However, recent work by Alexander et (10) Batich, C. D. Appl. Surf. Sci. 1988, 32, 57. (11) Reilley, C. N.; Everhart, D. S. In Applied Electron Spectroscopy; Windawi, H., Ho, F., Eds.; Wiley-Interscience: New York, 1982. (12) Briggs, D. In Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; J. Wiley and Sons: New York, 1990. (13) Chilkoti, A.; Ratner, B. D.; Briggs, D. Chem. Mater. 1991, 3, 51. (14) Sutherland, I.; Sheng, E.; Brewis, D. M.; Heath, R. J. J. Mater. Chem. 1994, 4, 683. (15) Popat, R. H.; Sutherland, I.; Sheng, E.-S. J. Mater. Chem. 1995, 5, 713. (16) Ameen, A. P.; Ward, R. J.; Short, R. D.; Beamson, G.; Briggs, D. Polymer 1993, 34, 1795. (17) Chilkoti, A.; Ratner, B. D. Surf. Interface Anal. 1991, 17, 567. (18) Everhart, D. S.; Reilley, C. N. Anal. Chem. 1981, 53, 665. (19) Dickie, R. A.; Hammond, J. S.; deVries, J. E.; Holubka, J. W. Anal. Chem. 1982, 54, 2045. (20) Chilkoti, A.; Castner, D. G.; Ratner, B. D.; Briggs, D. J. Vac. Sci. Technol., A 1990, 8, 2274. (21) Ameen, A. P.; Short, R. D.; Ward, R. J. Polymer 1994, 35, 4382. (22) Rinsch, C. L.; Chen, X.; Panchalingam, V.; Eberhart, R. C.; Wang, J.-H.; Timmons, R. B. Langmuir 1996, 12, 2995.
© 1997 American Chemical Society
SAM Functionalization
al.23 has shown that the selection of PAA as the model system may have been fortuitous. In a comparative study of the derivatization of PAA and poly(methacrylic acid) (PMAA), they found that only 60% of the carboxylic acid groups in PMAA were derivatized under circumstances in which complete derivatization of PAA would be expected. They attributed this difference to structural effects and suggested that the reaction was much slower for PMAA than for PAA. Nevertheless, the ease with which carboxylic acid terminated SAMs may be produced and the range of molecules which may be attached to them (including not only alcohols but also amine-containing molecules such as proteins) mean that this procedure is worthy of further investigation. In the present study, we have functionalized monolayers of two short-chain thiols, 3-mercaptopropanol (MPL) and 3-mercaptopropanoic acid (MPA). Exposure to gas-phase fluorinated anhydrides was employed to functionalize monolayers of MPL, while gas-phase TFE combined with a carbodiimide in the presence of base was employed in the case of MPA. The compositions of the resulting functionalized monolayers were determined using X-ray photoelectron spectrocopy, and comparison was made with results obtained for polymeric model systems. Experimental Section Preparation of Monolayers. Monolayers of 3-mercaptopropanol (98%, Aldrich) and 3-mercaptopropanoic acid (>99%, Aldrich) were prepared on evaporated gold films supported on glass coverslips (Chance, No. 2 thickness). The coverslips and all other glassware used in the experiments were cleaned by immersion in hot (∼90 °C) “piranha” solution24 for 30 min, followed by rinsing with copious amounts of reverse osmosis water and drying in an oven at 60 °C. The gold films were deposited in a General Engineering bell jar vacuum system pumped by a liquid nitrogen trapped diffusion pump, providing a base pressure of 99%), pentafluoropropanoic anhydride (PFPA, 95%), and heptafluorobutyric anhydride (HFBA, >99%) were obtained from Fluka and used without further purification. Monolayers were derivatized using the procedure described by Chilkoti et al.,13 by exposing them to the vapor from 600 to 700 µL of the anhydride placed in the bottom of a sample tube. The samples were added to the tube and prevented from coming into contact with the liquid by a small piece of glass coverslip. A polythene stopper was fitted to prevent evaporation of the anhydride, and the samples were exposed for varying periods of time. Following exposure, samples were removed from the tube and analyzed directly by either water droplet contact angle measurements or XPS. Derivatization Using Trifluoroethanol. Again the method of Chilkoti et al.13 was followed. The samples to be derivatized were placed in a sample tube to which 0.9 mL of 2,2,2trifluoroethanol (>99%, Fluka) was added. As for the anhydride derivatization, the slides were prevented from contacting the (23) Alexander, M. R.; Wright, P. V.; Ratner, B. D. Surf. Interface Anal. 1996, 24, 217. (24) Piranha solution is a mixture of 30% hydrogen peroxide solution and concentrated sulfuric acid (95%) in the ratio 3:7. CAUTION: Great care must be taken when using Piranha solution as it has been found to detonate spontaneously on contact with organic material.
Langmuir, Vol. 13, No. 10, 1997 2741 liquid by a small piece of glass coverslip placed in the bottom of the tube. After 15 min 0.4 mL of pyridine (99%, Lancaster) was added, followed 15 min later by 0.3 mL of N,N′-di-tert butylcarbodiimide (di-tBuC) (>99%, Fluka). The tube was sealed between additions and on completion with a polythene stopper to prevent evaporation of the reagents. Samples were then left for periods up to several days for the reaction to take place. XPS Analysis. XPS analysis of samples was carried out using a Vacuum Generators ESCALAB instrument equipped with a twin anode (Mg and Al) unmonochromated X-ray source and 100 mm radius hemispherical electron energy analyzer. Samples were mounted on stainless steel stubs with a small silver paint contact applied between the gold layer and the stub to prevent charging of the film as a result of the insulating glass substrate. Spectra were recorded using the Mg KR source, with the analyzer operated at a fixed pass energy. In order to avoid electron channeling effects, a take-off angle of 70° (relative to the sample surface) was used throughout, which does not correspond to the tilt angle of the carbon chains in the monolayers (63°).25 For each sample, several spectra were recorded: a low-resolution survey scan using a pass energy of 50 eV and high-resolution scans of the C 1s, F 1s, O 1s/Au 4p3/2 and Au 4f(5/2+7/2) regions of the spectrum at 10 eV pass energy. Many scans were recorded for each region to improve the signal to noise ratio, but the number was limited to prevent significant damage to the monolayers during acquisition. XPS spectra were fitted using the VGX900 software that was used to collect the data. A Shirley background was first removed, before fitting each component with a Gaussian-Lorentzian (2030%) curve with no asymmetry. As many parameters as possible were left free during the iteration process; however for C and O, the full width at half maximum (fwhm) often had to be fixed to maintain realistic values due to the fairly poor signal to noise ratio for these data sets. Contact Angle Measurements. Water droplet contact angle measurements were performed using a Rame-Hart goniometer equipped with a micro liter syringe. The sessile drop method was used throughoutsa droplet of water (1-2 µL) was formed on the end of the syringe and lowered onto the surface, the syringe was then retracted until the drop detached itself and the contact angle was measured after the drop had come to rest. This method has been shown to give equivalent values to the advancing contact angle obtained when the syringe is left in contact with the droplet and used to expand the drop across the surface.25 All angles quoted are subject to an error of (2°. Preparation and Imaging of Patterned Monolayers. Monolayers with spatially defined regions of different chemical functionality were prepared using a photolithographic procedure described in detail elsewhere.26-28 A mercaptopropanol layer was exposed in air to light from a medium-pressure mercury arc lamp, using masks to produce the patterns. In this process, the thiolate (RS-) species bound to the Au surface is photooxidized26-29 to a sulfonate (R-SO3-) which is readily replaced at the surface by a second thiol adsorbed from solution. The photooxidized sample was placed in a solution of MPA for approximately 7 min to produce a monolayer consisting of areas of hydroxyl functionality that were masked during photooxidation and surrounding areas that were covered with MPA molecules. Imaging secondary ion mass spectrometry (SIMS) analysis of this sample was performed using an instrument equipped with a VG MM 12-12S quadrupole analyzer and a Ga liquid metal ion source. A primary particle current of ∼5 nA, with a beam energy of 10 keV, was employed throughout.
Results Anhydride Derivatization of Mercaptopropanol Monolayers. Monolayers of mercaptopropanol (MPL) on gold were derivatized for different lengths of time by exposure to TFAA vapor following the reaction shown in (25) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (26) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (27) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (28) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F. Anal. Chem. 1994, 66, 2170. (29) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342.
2742 Langmuir, Vol. 13, No. 10, 1997
Hutt and Leggett
Figure 1. XPS spectra of mercaptopropanol monolayers (a) before and (b) after exposure to TFAA vapor for 4 h. Scheme 1. Derivatization of Mercaptopropanol Monolayers by Trifluoroacetic Anhydride
Scheme 1, which is expected to yield a completely CF3 functionalized surface. Figure 1 shows the changes in the C 1s, F 1s, and O 1s/Au 4p3/2 regions of the XPS spectrum on derivatization for 4 h. It is apparent that the signal to noise ratio for the data, particularly C, is quite poor. This is a result of both the small quantities of material under analysis (some of the peaks correspond to only a single layer of atoms) and, especially in the case of C, the small photoionization cross sections. However, despite this difficulty, it was still possible to fit the data to produce meaningful information, although the peak positions are subject to errors of approximately (0.2 eV as a result. For an as-prepared MPL layer the C 1s spectrum consists of two components, with peak areas in the ratio 2:1. The largest peak at a binding energy (B.E.) of 284.7 eV is attributed to the two methylene C atoms nearest to the S atom (S produces little or no shift to a neighboring C atom30) and the peak at 286.5 eV is due to the C atom attached to the OH group at the surface. The B.E. shift (1.8 eV) observed here for the C atom attached to the hydroxyl group is a little bigger than the value of 1.47 eV quoted in the literature for polymeric materials,30 possibly a result of the high noise level within the spectra. The O 1s/Au 4p3/2 region of the spectrum shows three peaks: an O 1s peak at 532.8 eV, the Au 4p3/2 peak at 546.7 eV, and, between these, a Au 4p3/2 satellite peak generated by the unmonochromated Mg X-ray source. As expected, the F region of the spectrum does not show any peaks. Following derivatization with TFAA several changes take place in the XPS spectra. Firstly, a large, highly (30) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; J. Wiley and Sons: New York, 1992.
symmetrical F 1s peak is observed at 688.3 eV indicating a single environment for the F atoms. Secondly, two additional peaks appear in the C 1s spectrum at higher B.E.: a peak at 289.4 eV (4.9 eV chemical shift) due to the carboxyl C atom in the molecule and a peak at 292.6 eV (8.1 eV chemical shift) due to the C atom in the CF3 group. These peak shifts are similar to those observed for TFAAderivatized poly(vinyl alcohol) (PVA)16 when the β shift of the CH2 polymer backbone is included. The C peak area ratios of approximately 2:1:1:1 are in good agreement with those expected for a completely derivatized surface. Finally, the O 1s peak has increased in size and contains two components of approximately equal area, one due to the linking O atom and the other due to the carbonyl O atom. Table 1 summarizes the peak positions and peak assignments for these spectra, with reference to the labels given in Scheme 2. Figure 2 shows C 1s and F 1s XPS spectra for MPL derivatized with pentafluoropropanoic anhydride (PFPA) and heptafluorobutyric anhydride (HFBA), TFAA spectra have been included for comparison. As expected, for PFPA-derivatized surfaces, an additional peak (absent from the TFAA-labeled material) is observed in the C spectrum at 290.9 eV, due to the additional CF2 group in the chain. Confirmation of this assignment can be found by observing that this peak doubles in size for the HFBAderivatized samples and shifts to higher B.E. by 0.2-0.3 eV due to the presence of a third CF2 group. As the number of CF2 groups increases through the series, the highest B.E. component, attributed to the terminal CF3 group, shifts to even higher energies indicating the highly electronegative environment of this C atom. Examination of the F 1s peaks shows the expected increase in size with the progression from TFAA to HFBA, the peaks remaining symmetrical with a constant fwhm. It would appear however, that there is a small shift of ∼0.4 eV in the peak position to higher B.E. with increasing F content in the sample, the cause of which is unclear. The peak position data and assignments are summarized in Table 1. The XPS data showed that for TFAA, exposures as short as 15 min were enough to produce spectra indicative of complete derivatization of the surface, while for HFBA this time period was not long enough. Figure 2 shows that the F 1s peak for HFBA-derivatized monolayers is significantly smaller after 15 min than the peak recorded after extended exposure. Further information about the time dependence of the reaction was obtained by following the change in water droplet contact angle as a function of anhydride vapor exposure time, Figure 3. For freshly prepared MPL samples an advancing contact angle of
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Langmuir, Vol. 13, No. 10, 1997 2743
Table 1. Binding Energies of C 1s, F 1s, and O 1s XPS Peaks Obtained from As Prepared and Derivatized Mercaptopropanol Monolayers on Golda binding energies/eV (chemical shifts relative to peak 1 given in parentheses) peak assignment
1
2
mercaptopropanol
284.7
TFAA derivatized
284.5
PFPA derivatized
284.6
HFBA derivatized
284.9
286.5 (1.8) 286.2 (1.7) 286.5 (1.9) 286.6 (1.7)
a
C 1s 3
O1s 4
5
1
2
F1s 1
532.8 289.4 (4.9) 289.5 (4.9) 289.6 (4.7)
290.9 (6.3) 291.2 (6.3)
292.6 (8.1) 293.3 (8.7) 293.5 (8.6)
532.7 532.8 532.7
533.9 (1.2) 534.2 (1.4) 534.0 (1.3)
688.3 688.6 688.7
Peak assignment numbers refer to those shown in Scheme 2.
Scheme 2. Structures of Mercaptopropanol Monolayers Derivatized using TFAA, PFPA and HFBAa
a Numbers adjacent to atoms correlate with the XPS peak assignments given in Table 1.
Figure 3. Variation in water droplet contact angle with exposure time for mercaptopropanol monolayers exposed to vapor from either TFAA, PFPA, or HFBA. Lines have been added to guide the eye.
Figure 2. XPS spectra of mercaptopropanol monolayers derivatized with different anhydride vapors: (a) 4 h with TFAA; (b) 4 h with PFPA; (c) 15 min with HFBA; (d) 4 h with HFBA.
between 3° and 7° was obtained, in good agreement with values observed by other groups for hydroxyl-terminated surfaces. After exposure to the anhydride for as little as 30 s, the contact angle for the sample was >100°. For TFAA the contact angle rose rapidly to 107° and did not increase further, even after >4 h of exposure. For PFPA the contact angle rose immediately to ca. 112° and then increased a little to 115° for samples exposed for 60 min or more. Exposure to HFBA for 70 min. The greater exposure times required to saturate the surface with PFPA and HFBA, compared to TFAA, may be due to the greater steric hindrance introduced by the longer chains. However, a
simpler explanation may be the reduced vapor pressure of these higher boiling point liquids. Extended exposure (>5 h) of the monolayers to the anhydrides resulted in a reduction in contact angle to values much less than 100°. The XPS spectra of these samples also showed a larger F/Au ratio than expected, and it is thought that this is explained by the presence of physisorbed anhydride and its decomposition products (i.e., carboxylic acids) on the surface. The contact angle for the saturated HFBA surface of 119° is in excellent agreement with the value of 118° obtained for SAMs composed of perfluorinated molecules, HS-(CH2)2-(CF2)5-CF3 (Bain et al.25) and HS-(CH2)4O-(C6H4)-S-(CH2)-(CF2)10-CF3 (Evans et al.31) indicating that the derivatized surfaces have comparable order to SAMs prepared directly from fluorinated molecules. The lower values for the shorter chain anhydrides would indicate either greater disorder in these layers or greater “sensing” of the polar ester group just below the surface.32 An important point to consider, especially when dealing with fluorinated molecules, is damage induced in the sample by X-rays and associated photoelectrons/secondary electrons. This phenomenon has been studied previously for polymers and SAMs,33-35 frequently by following the (31) Evans, S. D.; Flynn, T. M.; Ulman, A.; Beamson, G. Surf. Interface Anal. 1996, 24, 187. (32) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663. (33) Wheeler, D. R.; Pepper, S. V. J. Vac. Sci. Technol. 1982, 20, 226.
2744 Langmuir, Vol. 13, No. 10, 1997
Hutt and Leggett Scheme 3. Mechanism of Derivatization by TFE of Mercaptopropanoic Acid Monolayersa
Figure 4. Effect of exposure to X-ray radiation in the XPS spectrometer of the F 1s to Au 4f(5/2+7/2) XPS peak area ratio for mercaptopropanol monolayers derivatized with TFAA, PFPA, and HFBA.
variation in F XPS peak intensity as a function of irradiation time. In order to quantify the extent of any damage that may have been occurring in this study, the F/Au XPS peak area ratio was measured for samples derivatized with saturation quantities of TFAA, PFPA, and HFBA as a function of time. To do this, F 1s spectra and Au 4f spectra were recorded repeatedly for periods similar to those used in the conventional analysis. Fewer scans were taken for each region than usual so that many data points could be obtained in the time available. Figure 4 shows the change in F/Au ratio as a function of time for the three different derivatization agents. It is clear from these data that considerable loss of F occurs from the samples over the time period of a typical high-resolution analysis (∼1 h), with the F/Au ratio decaying to approximately 75% of the starting value. However, this decrease is accentuated slightly by using the ratio, as loss of fluorine will result in a stronger Au peak (due to less attenuation by the overlayer), at the same time as reducing the size of the F peak. Derivatization of Carboxylic Acid Terminated Monolayers. Mercaptopropanoic acid monolayers were derivatized for different periods of time, using the vapor phase procedure described above and shown in Scheme 3. Figure 5 shows XPS spectra for MPA monolayers as prepared and following exposure to TFE, pyridine, and di-tBu C for 42 and 90 h. The C 1s region for the as prepared monolayer shows three peaks. The first, at 284.7 eV, is attributed to the CH2 group bound to the S atom near the Au surface, while the highest binding energy (289.0 eV) component originates from the terminal carboxyl C atom. The chemical shift of 4.3 eV observed here is in good agreement with that seen for carboxyl groups in polymers and self-assembled monolayers.25,30 The peak area ratio of 3.8:1 for these two components is, however, much larger than the expected 1:1 ratio. The origin of the intermediate component in the spectrum is less clear. One possibility is that this arises from the methylene adjacent to the carboxyl group in MPA. However, on the basis of data recorded for PAA30 this species would only be expected (34) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1993, 97, 9456. (35) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981.
a Figures adjacent to atoms correlate with the XPS peak assignments in Table 3.
to be shifted by 0.4 eV relative to the other methylene group. XPS spectra obtained by Bain et al.25 of mercaptoundecanoic acid SAMs on gold required an additional component at 0.8-1.1 eV chemical shift to fit the data correctly, which they attributed to the CH2 group adjacent to COOH. Neither of these shifts would account for the 1.7 eV shift measured for the intermediate component in our spectra and a shift of 0.4 eV is unlikely to be resolved by our spectrometer. We therefore feel that the methylene group adjacent to COOH is contributing to the 284.7 eV peak, partly explaining the unexpectedly large area of this feature, rather than being represented by the intermediate component. Overall the total C 1s to Au 4f(5/2+7/2) peak area ratio for MPA is found to be 0.045 ( 0.003, which is larger than the value of the same ratio for MPL (0.022(0.003) and propanethiol36 (0.017 ( 0.003) layers. This leads us to assign the intermediate peak in the spectrum, together with the additional area of the 284.7 eV peak to organic contaminants adsorbed from the laboratory ambient onto the high-energy acid-terminated surface. Contamination of this nature produces contrast in scanning electron microscopy (SEM) images of surfaces of patterned monolayers of methyl- and acid-terminated molecules37,38 (low and high surface energies, respectively) and has been noted in XPS studies23 of PAA where the (36) Hutt, D. A.; Leggett, G. J J. Phys. Chem. 1996, 100, 6657. (37) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (38) Lopez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir, 1993, 9, 1513.
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Langmuir, Vol. 13, No. 10, 1997 2745
Figure 5. XPS spectra of mercaptopropanoic acid monolayers exposed to derivatizing agents (TFE/pyridine/di-tBuC) for (a) 0 h, (b) 42 h, and (c) 90 h.
hydrocarbon peak was also observed to be much larger than expected. Significantly, water droplet contact angles for freshly prepared MPA monolayers were less than 10°, in agreement with the findings of other workers for monolayers terminated with carboxylic acid groups1-3. Whatever the nature of the adsorbed contaminants, they are presumably displaced by the advancing water droplet during contact angle measurement. It is possible in our work that a significant component of this contamination is physisorbed ethanol from the preparation solution. This is supported, in part, by the chemical shift of the intermediate component which is very similar to that expected for a CH2OH group. Adsorbed ethanol may be the origin of the additional component observed by Bain et al.25 in their C 1s XPS spectrum of a mercaptoundecanoic acid SAM. Molecular dynamics simulations of hydroxylterminated SAMs by Sprik et al.39 indicated that ethanol may adsorb to these surfaces via hydrogen bonds. Such hydrogen bonding interactions would be likely on the carboxylic acid terminated surfaces and, indeed, may be more effective than those modeled by Sprik et al. for hydroxyl-terminated surfaces. Some evidence for an enlarged C peak in the spectra of mercaptopropanol can be seen in Figure 1 but is much less significant than was the case for MPA; if ethanol adsorption is ocurring, our data would therefore suggest that it is more extensive on the acid-terminated surfaces. Also shown in Figure 5 is the O 1s/Au 4p3/2 region of the XPS spectrum for the as prepared MPA monolayer. This shows two peaks in the O region separated by 1.5 eV indicative of the two different bonding environments of the O atoms in the layer. This is in good agreement with studies of PAA, which show a chemical shift of 1.3 eV between them. The chemical composition of the MPA layer derived from these spectra is shown in Table 2 and further demonstrates the larger than expected C content of the samples. The C 1s, O 1s, and F 1s XPS spectra obtained from monolayers of MPA that had been exposed to the vapor from the derivatizing reagents for 42 and 90 h are also displayed in Figure 5. As can be seen from the F region of the spectra, a considerable amount of TFE has been incorporated into the samples during this time period. As a result of this the C 1s spectra have also changed with a new peak observed at 292.9 eV, attributed to the CF3 group. In addition, after exposure to the reagents for 90 (39) Sprik, M.; Delamarche, E.; Michel, B.; Rothlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116.
Table 2. Composition (Atomic %) of As Prepared and TFE-Derivatized Mercaptopropanoic Acid Monolayers (Experimental Values (5%)a measured composition/atom % (expected values given in parentheses) C mercaptopropanoic acid experimental composition theoretical composition TFE derivatized experimental composition 42 h 90 h theoretical composition 100% derivatization 60% derivatization poly(acrylic acid) experimental composition theoretical composition TFE derivatized experimental composition theoretical composition (100% derivatization)
O
71 (60)
29 (40)
61 64
24 16
(50) (52.5)
(20) (25)
63 (60)
37 (40)
50 (50)
22 (20)
F
15 20 (30) (22.5)
28 (30)
a Data obtained from a sample of poly(acrylic acid) is shown for comparison. Values obtained from XPS spectra after applying experimentally determined sensitivity factors of 0.22, 0.6, and 1 for C 1s, O 1s, and F 1s, respectively. The contribution from S has not been included in either the experimental or theoretical values and effects due to electron attenuation within the monolayers have not been accounted for.
h, the intermediate peak in the as prepared spectrum appears to have increased in size and shifted slightly to higher B.E. This peak now has a chemical shift of 2.2 eV and is similar in size to the CF3 component indicating that this is generated by the -CH2O- group adjacent to CF3. This assignment is in agreement with the work of Popat et al.15 for TFE-derivatized PAA, for which a 2.2 eV shift was also observed for this species. The component due to the -COO- group has remained unchanged in size, but the CH2 peak at 284.8 eV has been greatly reduced as the surface is derivatized, producing a peak area ratio for these two features of 2.4:1, much closer to the expected value of 2:1. This would seem to indicate that the contaminating layer has been displaced by the reagents and would explain the increase in size and shift to higher B.E. of the intermediate peak in the spectrum as the initial contamination feature is removed and subsequently replaced by the -CH2O- feature. The O 1s/Au 4p3/2 region of the spectrum has remained essentially unchanged upon derivatization of the surface, except for a slight increase in the B.E. of the second O 1s component, which is the
2746 Langmuir, Vol. 13, No. 10, 1997
Hutt and Leggett
Table 3. Bindng Energies of C 1s, F 1s, and O 1s XPS Peaks Obtained from As Prepared and TFE-Derivatized Mercaptopropanoic Acid Monolayersa binding energies/eV (chemical shifts relative to peak 1 given in parentheses) C 1s
a
O 1s
peak assignment
1
2
3
mercaptopropanoic acid
284.7
TFE derivatized 42 h TFE derivatized 90 h
284.7
286.4 (1.7) 286.4 (1.7) 287.0 (2.2)
289.0 (4.3) 288.6 (3.9) 288.7 (3.9)
284.8
4
293.0 (8.3) 292.9 (8.1)
1
2
532.1
533.6 (1.5) 533.5 (1.5) 533.7 (1.7)
532.0 532.0
F 1s 1
688.6 688.6
Peak assignment numbers refer to those shown in Scheme 3.
Figure 7. C 1s and F 1s XPS spectra of (a) as prepared and (b) TFE-derivatized (14 h of exposure) poly(acrylic acid).
Figure 6. Fluorine uptake (expressed as F 1s to Au 4f(5/2+7/2) XPS peak area ratio) as a function of time, for mercaptopropanoic acid monolayers exposed to derivatizing agents. Ratios were determined from high-resolution spectra. Dotted line indicates the ratio obtained from similar spectra of a mercaptopropanol monolayer derivatized with TFAA.
linking atom, attributed to the electron-withdrawing effect of the distant CF3 group. The peak positions and assignments for all these spectra are summarized in Table 3 and a compositional analysis is given in Table 2. It is apparent from the F 1s spectra of the derivatized materials shown in Figure 5 that the time period for reaction is very long. In addition to this, in the C 1s spectra after 90 h of exposure, the components resulting from the TFE group are smaller than those from the underlying MPA molecule, indicating incomplete reaction with all the available COOH sites in the original monolayer. In order to determine if this was due to insufficient reaction time, the F 1s/Au 4f(5/2+7/2) ratio was measured from a number of samples exposed to the reagents for periods of several days. These are shown in Figure 6 and indicate a slow steady uptake of TFE for the first 80 h of exposure, after which no more is incorporated into the monolayer. The saturation ratio, when compared to that obtained for TFAA-derivatized MPL, indicates only ∼60% coverage of the COOH sites with TFE. The hydrophobicity of the derivatized layers was much less than that obtained from TFAA derivatized MPL; water droplet contact angle measurements performed on samples exposed to the vapor for more than 90 h gave values of 70 ( 7°, compared to 107° for TFAA. The slow and incomplete derivatization of the MPA monolayer is in disagreement with the published literature concerning the derivatization of PAA. In order to confirm
that the correct experimental procedure was being applied to the samples, the derivatization was repeated using a sample of PAA.40 Figure 7 shows XPS spectra of PAA before and after derivatization for 14 h using the procedure described above for SAMs. The C 1s spectrum of the as prepared sample shows good agreement with literature data, with two peaks due to the C polymer backbone and carboxyl groups. These are separated by 4.6 eV in agreement with the MPA monolayer spectra. However, in addition, there is a small peak at 286.4 eV which would suggest the presence of hydroxyl groups in the sample, perhaps deriving from adsorbed organic contamination from the laboratory ambient, as suggested above for the intermediate component in the MPA C 1s spectrum. On derivatization, two new peaks appear in the C spectrum at higher B.E. and the hydroxyl peak is no longer needed in the curve fit. A large F peak is also evident. The similarity in area of the C 1s components indicates complete derivatization by TFE, and this is supported by the compositional analysis displayed in Table 2 in excellent agreement with the work of Chilkoti et al.13 These results indicate that the correct experimental procedure was carried out and reasons for the reduced efficiency of the derivatization with mercaptopropanoic acid monolayers will be discussed later. Selectivity of Derivatization. For derivatization reactions to be useful in the labeling of chemical groups for XPS analysis, it is important that the reaction is selective for the group of interest. To this end the selectivity of the derivatization reactions for hydroxyl and carboxyl groups was determined by using each procedure for the opposite functional group. Exposure of a mercaptopropanol layer to the vapor from TFE, pyridine, and di-tBuC for 16 h (data not shown) resulted in no incorporation of F into the sample and no additional components in the C 1s region of the XPS spectrum. From this it can be concluded that these reagents do not derivatize hydroxyl (40) PAA was drop cast onto a glass slide from a 25 wt % solution in water (Aldrich).
SAM Functionalization
Langmuir, Vol. 13, No. 10, 1997 2747 Table 4. Experimentally Determined and Theoretical Composition (atom %) of Mercaptopropanol Monolayers, As Prepared and after Derivatization with TFAA, PFPA, and HFBA (Experimental Values (5%)a measured composition/atom % (expected values given in parentheses)
Figure 8. F- SIMS image of a patterned SAM composed of mercaptopropanol and mercaptopropanoic acid derivatized with TFAA. The image shows clearly the MPL areas, functionalized by F, surrounded by MPA which has not reacted with the TFAA. The widths of the bars are 110 and 55 µm.
groups, in agreement with earlier studies employing poly(vinyl alcohol).13 The XPS spectra obtained after reaction of MPA with TFAA for 16 h indicated that some F was present in the sample, although only approximately 9% of a monolayer. Some reaction of TFAA with PAA has been noted by Chilkoti et al.13 and Sutherland and co-workers14 in their studies of derivatization reactions. The reaction of TFAA with a carboxylic acid is expected to occur, but the product is itself an anhydride which is likely to decompose on exposure to water vapor in the air to the acid and TFE. The F present in the sample may be a result of physisorbed fluorinated material left on the surface. Over the period of a typical TFAA derivatization of hydroxyl groups (∼30 min) the reaction of carboxylic acid functionalities is likely to be very low. Confirmation of the selectivity of the TFAA reaction was demonstrated by the preparation of a patterned monolayer containing spatially well defined areas of hydroxyl and carboxylic acid functional groups. A transmission electron microscopy (TEM) grid (Sjostrand pattern of thick and thin bars, obtained from Agar) was used as a mask, to create areas of hydroxyl functionality that were not illuminated during photo-oxidation, surrounded by regions of carboxylic acid terminated molecules. This sample was exposed to TFAA vapor for 15 min to derivatize the hydroxyl groups in the material. Figure 8 shows a FSIMS image of the sample and shows the spatial distribution of fluorine in the monolayer reproducing the TEM grid pattern used as a mask. Clearly, there has been no significant reaction of TFAA with the MPA areas of the surface, highlighting the selectivity of this procedure. Discussion The reaction of TFAA with mercaptopropanol surfaces results in extensive derivatization of the monolayer, with a large proportion (possibly all) of the hydroxyl groups being functionalized, in agreement with previous studies using model polymer systems. After 4 h of exposure to PFPA and HFBA, the MPL samples are also extensively derivatized with a high proportion of surface -OH groups reacted leading to hydrophobic surfaces with water droplet contact angles indicative of close-packed arrays of fluoroalkyl chains. Experimentally determined compositional data for these samples are compared with the theoretical
mercaptopropanol experimental composition theoretical composition TFAA derivatized experimental composition theoretical composition 100% derivatization 80% derivatization PFPA derivatized experimental composition theoretical composition 100% derivatization 80% derivatization HFBA derivatized experimental composition theoretical composition 100% derivatization 80% derivatization
C
O
F
77 (75)
23 (25)
47
18
35
(50) (52)
(20) (21)
(30) (27)
46
15
39
(46) (48)
(15) (16)
(39) (36)
44
10
46
(44) (46)
(12) (13)
(44) (41)
a Sensitivity factors determined from polymeric materials of 0.22, 0.6, and 1 for C 1s, O 1s, and F 1s, respectively, have been applied to the experimental data to obtain these values. The contribution from S has not been included in either the experimental or theoretical compositions and effects due to electron attenuation within the monolayers have not been accounted for.
values in Table 4. These data do not include the contribution from the S atom as this was too small and noisy to quantify reliably and they do not allow for effects due to the attenuation of the photoelectrons produced by the atoms “buried” beneath the surface layers. Furthermore, it must be remembered that these figures were obtained from peak areas measured from the highresolution spectra and therefore contain effects due to damage induced by X-ray irradition during analysis. This would have the effect of reducing the size of peaks recorded later in the sequence (F 1s and O 1s) compared to C 1s near the start. However, despite these limitations it can be seen that the compositional data clearly indicate that a substantial fraction (possibly all) of the monolayers have been derivatized. The C and O compositions are slightly smaller than expected, while the F composition is slightly larger, as expected for atoms nearest to the surface. Consideration of the effects that the greater size of the CF2 group compared to a CH2 group may have on the packing of the derivatized chains indicates that some of the -OH groups may not be derivatized. Structural studies of self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM)41,42 have resolved a closepacked arrangement of molecules in a (x3×x3)R30° structure on the Au(111) surface. This arrangement produces a spacing between the S head groups of 4.99 Å, which is larger than the diameter of the methylene chains (∼4.5 Å).43 As a result of this, simple calculations show that the molecules are expected to tilt to an angle of 27° from the surface normal to maximize the van der Waals interactions between chains and produce a close-packed structure. This tilting has been confirmed by studies employing IR spectroscopy and ellipsometry.2 For a perfluorinated molecule the diameter of the chain is approximately 5.6 Å, larger than the S head group spacing (41) Anselmetti, D.; Baratoff, A.; Guntherodt, H.-J.; Delamarche, E.; Michel, B.; Gerber, Ch.; Kang, H.; Wolf, H.; Ringsdorf, H. Europhys. Lett. 1994, 27, 365. (42) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (43) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.
2748 Langmuir, Vol. 13, No. 10, 1997
in a conventional SAM.44 This increased size has been shown to force the molecules of a SAM composed of CF3(CF2)7(CH2)2SH on Au(111) to a form a p(2 × 2) structure instead, with a tilt of ∼16° from the surface normal.45 This structure results from the close similarity of the chain diameter to twice the Au(111) close packed distance of 2.88 Å. However, more recent work46 has shown that although the lattice spacings are very similar, the monolayer surprisingly does not appear to be commensurate with the substrate. As a result of this, the density of perfluorinated SAMs on gold is ∼33% less than that of the methylene chain SAMs on gold (28.7 Å2/chain compared to 21.6 Å2/chain, respectively). It is therefore expected that derivatization of MPL with fluorinated molecules such as TFAA, PFPA, and HFBA will result in some of the -OH groups remaining unreacted due to the inability of the fluorinated chains to pack into the space available. In one of the few studies to employ TFAA labeling of SAMs, Bertilsson and Liedberg47 used a solution-based procedure to tag hydroxyl groups for IR analysis. They concluded that the alkyl chain was unaffected by the labeling procedure and that the yield for the reaction was 80-90%, although they were unable to detect any underivatized -OH groups. The density of close packed perfluorinated chains unattached to a surface is 27.2 Å2/chain (i.e., 25% less dense than that of the (x3×x3)R30° structure), and it is therefore expected that approximately one -OH group in every four or five sites will be unreacted, assuming that the -OH groups are in a close packed (x3×x3)R30° structure. Table 4 compares the compositions obtained for the surfaces in our study with the expected compositions for 100% and 80% derivatization of -OH groups. It is clear that in terms of the atomic percent composition there is very little difference between 80 and 100% derivatization and that it is not possible to distinguish between them on the basis of these data alone (given the error limitations). Instead, we must consider other data in order to obtain a more precise indication of the extent of derivatization. The strongest evidence for incomplete derivatization comes from the F/Au XPS peak ratios for samples that have received minimal X-ray exposures (for example, see the t ) 0 data in Figure 4), where effects due to sample degradation can be excluded. At zero exposure, the expected F/Au XPS peak area ratios (excluding small effects due to photoelectron attenuation) for completely derivatized layers are 3:5:7 for TFAA, PFPA, and HFBA, respectively, compared to the measured ratios of 3:4:5.5. However, the PFPA:HFBA F ratio is 1:1.4, as expected. The discrepancy with the TFAA F signal suggests higher levels of fluorination of the TFAA-functionalized monolayer, indicating that derivatization is more extensive than is the case for PFPA and HFBA. Comparison of the F ratios suggests that levels of derivatization by PFBA and HFBA are 80% of the level achieved using TFAA. Examination of the C 1s spectra revealed that the ratios of the various components matched the values expected for complete derivatization closely for TFAA-treated MPL, while for PFBA- and HFBA-treated monolayers, the CF2 and CF3 components were smaller than would be expected (44) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (45) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (46) Liu, G.-y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (47) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141.
Hutt and Leggett
for complete derivatization. This leads us to speculate that the MPL layer is almost completely derivatized by TFAA as there are sufficient degrees of freedom for the CF3 groups to pack at the same density as the underlying monolayer. However, for the materials treated with the longer PFPA and HFBA molecules, derivatization is ca. 80% due to the larger diameters of the CF2 groups. The slow rate and incompleteness (only ∼60% of surface sites reacted) of the TFE/pyridine/carbodiimide reaction on the MPA surface was quite surprising as it is highly efficient for PAA, proceeding to complete stoichiometric reaction with all COOH groups in only 14 h. However, our finding would appear to agree with the results of Alexander et al.23 for the derivatization of poly(methacrylic acid) which was only 60% complete after 80 h of exposure. In this work steric hindrance between neighboring COOH groups caused by the isotactic nature of the polymer was thought to prevent the approach of the bulky di-tBuC species. A similar effect is thought to be occurring with the MPA monolayers: the close packed environment of the COOH groups makes approach of the carbodiimide difficult, especially after neighboring molecules have already been derivatized. Similarly, after the formation of the intermediate on the acid surface, the next step in the mechanism involves the “back-side” attack on the carboxyl C atom by the O in TFE, an even more sterically hindered process due to the surrounding, highly oriented MPA molecules. If this explanation is correct, it might be expected that the reactivity of long chain acid-terminated SAMs to the TFE derivatization procedure will be even less as these monolayers are more highly ordered than SAMs of MPA. An alternative explanation for the poor MPA derivatization may be the presence of adsorbed contaminants or ethanol from the sample preparation on the surface. It is clear from the C 1s XPS spectra that an appreciable quantity of contamination is removed from the surface upon derivatization and this may be the rate-determining step in the process. Theoretical and STM studies by Sprik et al.39 have suggested that ethanol may be hydrogen bonded to the carboxylic acid surface and the presence of this may firstly prevent the attack of the carbodiimide reagent, but may also mean that the unfluorinated ethanol is bonded to the surface in preference to TFE. Conclusions The reactivity of monolayers of mercaptopropanol and mercaptopropanoic acid to vapor-phase reagents has been examined with a view to controlled surface modification. Mercaptopropanol monolayers are derivatized extensively by TFAA, with possibly all of the surface hydroxyl groups being functionalized, but there are indications that only ∼80% derivatization occurs with PFPA and HFBA, probably due to steric effects. For MPA functionalized with TFE, only approximately 60% of the carboxyl surface sites are reacted, in stark contrast to the same process applied to poly(acrylic acid) for which 100% derivatization occurs. Again, steric hindrance is thought to be the cause of the reduced reactivity, preventing the approach of the reagents to the carboxyl carbon atom. In agreement with model studies using polymers, the reactions are found to be selective only to the desired surface functional group. Acknowledgment. The authors are grateful to the EPSRC (Grant GR/K28671) for funding this research. LA970069T
