pubs.acs.org/Langmuir © 2010 American Chemical Society
Halogenation of Carbon Substrates for Increased Reactivity with Alkenes Matthew R. Lockett† and Lloyd M. Smith* Department of Chemistry, University of Wisconsin;Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States. † Current address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States. Received July 31, 2010. Revised Manuscript Received September 17, 2010 Carbon substrates are readily functionalized with alkene-containing molecules via an ultraviolet-light-catalyzed reaction, resulting in the formation of a carbon-carbon bond with the surface. This reaction is typically performed on hydrogen-terminated carbon substrates, limiting its utility as alkene molecules with low electron affinities do not readily attach to this surface. Recently, a wet-chemical method for preparing bromine- and chlorine-terminated carbon substrates has been developed. Replacing the terminal hydrogen atoms with a halogen analog increases the surface’s reactivity with alkene-containing molecules, affording a means of modifying the carbon substrate with the alkene molecules that do not readily attach to the hydrogen-terminated surface and with a greatly reduced reaction time.
The ability to functionalize carbon-based materials provides a means of tailoring their chemical properties and reactivities. Alkene-containing molecules have been attached to a variety of carbon-based materials via an ultraviolet (UV)-light-catalyzed reaction in which neat alkene molecules are placed in direct contact with the carbon surface and irradiated until the surfaces are fully functionalized. This reaction scheme has been applied to carbon nanofibers1,2 as well as to planar glassy carbon,3,4 amorphous carbon thin film,5 and nanocrystalline diamond substrates.6,7 A number of alkene molecules have been employed in this reaction, affording a means of preparing aldehyde-,8 amine-,5,6 carboxylic acid-,2,9 and hydroxyl-terminated3,10 surfaces. There are pronounced differences in the reactivity of individual alkenes with their respective carbon substrates, with a majority of the functionalization reactions requiring prolonged reaction times. These differences can be attributed to the nature of the carbon substrate as well as to the alkene molecule. It has been shown that the oxidation state of the carbon substrate (H- vs O-terminated) modulates its reactivity toward a given alkene molecule:11-13 the rate of attachment of trifluoracetic *Corresponding author. E-mail:
[email protected]. Tel: 608 263-2594. Fax: 608 265-6780 (1) Baker, S. E.; Colavita, P.; Metz, K.; Nichols, B. M.; Sun, B.; Uhlrich, J.; Wang, X.; Kuech, T. F.; Hamers, R. J. Chem. Mater. 2006, 18, 4415–4422. (2) Landis, E. C.; Hamers, R. J. J. Phys. Chem. C 2008, 112, 16910–16918. (3) Phillips, M. F.; Lockett, M. R.; Rodesch, M. J.; Shortreed, M. R.; Cerrina, F.; Smith, L. M. Nucleic Acids Res. 2008, 36, e7. (4) Yang, W. S.; Auciello, O.; Butler, J. E.; Cai, W.; Carlisle, J. A.; Gerbi, J.; Gruen, D. M.; Knickerbocker, T.; Lasseter, T. L.; Russell, J. N.; Smith, L. M.; Hamers, R. J. Nat. Mater. 2002, 1, 253–257. (5) Sun, B.; Colavita, P. E.; Kim, H.; Lockett, M.; Marcus, M. S.; Smith, L. M.; Hamers, R. J. Langmuir 2006, 22, 9598–9605. (6) Strother, T.; Knickerbocker, T.; Russell, J. N.; Butler, J. E.; Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 968–971. (7) Yang, L. J.; Li, Y. B.; Erf, G. F. Anal. Chem. 2004, 76, 1107–1113. (8) Lockett, M. R.; Shortreed, M. R.; Smith, L. M. Langmuir 2008, 24, 9198– 203. (9) Lockett, M. R.; Carlisle, J. C.; Le, D. V.; Smith, L. M. Langmuir 2009, 25, 5120–6. (10) Lockett, M. R.; Weibel, S. C.; Phillips, M. F.; Shortreed, M. R.; Sun, B.; Corn, R. M.; Hamers, R. J.; Cerrina, F.; Smith, L. M. J. Am. Chem. Soc. 2008, 130, 8611–3. (11) Colavita, P. E.; Sun, B.; Wang, X. Y.; Hamers, R. J. J. Phys. Chem. B 2009, 113, 1526–1535. (12) Nichols, B. M.; Russell, J. N.; Butler, J. E.; Hamers, R. J. J. Phys. Chem. B 2005, 109, 20938–20947. (13) Rezek, B.; Shin, D.; Nebel, C. E. Langmuir 2007, 23, 7626–7633.
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acid-protected amino-10-undec-1-ene (TFAAD) decreases upon oxidation of nanocrystalline diamond12,13 but is increased for oxidized amorphous carbon substrates.11 Colavita et al. have also shown that different alkene molecules attach to the surface at different rates.11,14 This reactivity correlates with the electron affinity of the alkene molecule, dictated largely by the structure of the terminal functional group distal to the alkene, where the rate of attachment increases with increasing electron affinity. The need for molecules with high electron affinities limits the utility of this functionalization strategy, restricting the number of molecules that will readily attach to the carbon surface. Increasing the number of compatible molecules that one can attach to the carbon surface requires the chemical and physical properties of the substrate to be modifed. Recently, a wet chemical means of chlorinating and brominating amorphous carbon substrates has been described.15 Halogenation has afforded a means of easily modifying amorphous carbon substrates with Grignard reagents15 as well as with alkylthiol molecules.16 The successful attachment of these molecules to the surface suggests that the presence of the halogen increases the surface’s reactivity toward substitution-like chemical reactions. A similar trend is likely for the photochemical attachment of alkenes to carbon surfaces because the average bond dissociation energy of a carbon-halogen bond is much lower than that of its carbon-hydrogen and carbon-oxygen analogs.17 Here, a series of experiments were performed to determine if terminating an amorphous carbon substrate with chlorine or bromine modulates the surface’s reactivity with the following alkenecontaining molecules: (Figure 1a) dodec-1-ene, (1b) methyl undec10-enoate (UA-Me), (1c) undec-10-enoic acid (UA), and (1d) trifluoracetic acid-protected amino-10-undec-1-ene (TFAAD). These four alkene molecules were attached to bromine-, chlorine-, and hydrogen-terminated amorphous carbon substrates and characterized with X-ray photoelectron (XP) and (14) Colavita, P. E.; Sun, B.; Tse, K.-Y.; Hamers, R. J. J. Am. Chem. Soc. 2007, 129, 13554–13565. (15) Lockett, M. R.; Smith, L. M. Langmuir 2009, 25, 3340–3343. (16) Lockett, M. R.; Smith, L. M. J. Phys. Chem. C 2010, 114, 12635–12641. (17) CRC Handbook of Chemistry and Physics, 89th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2008; p 2736.
Published on Web 10/06/2010
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Figure 1. Alkene-containing molecules used in this work: (a) dodec-1-ene, (b) methyl undec-10-enoate (UA-Me), (c) undec-1enoic acid (UA), and (d) trifluoroacetic acid-protected amino-10undec-1-ene (TFAAD).
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Figure 2. Methylene stretching region of (a) TFAAD and (b) dodec-1-ene after a 16 h attachment reaction on bromine-, chlorine-, and hydrogen-terminated carbon-on-metal substrates. The following assignments are provided for reference: (1) v(CH2)a, (2) v(CH2)s, and (3) v(CH3)a. Scheme 1. Attaching Alkene Molecules to Halogenated Carbon Substrates
polarization-modulated Fourier transform infrared reflectionabsorption spectroscopy (PM-FTIRRAS) measurements. Each spectroscopic value presented is the average of three surface sample values. The carbon substrates were prepared by applying a thin film of amorphous carbon onto gold-coated glass substrates. This layered carbon-on-metal substrate has been described previously,10,18 and a detailed procedure of the preparation can be found in the Supporting Information section. Amorphous carbon thin films are an attractive material because they are readily deposited at room temperature, providing a means of easily incorporating the physical and chemical robustness of carbon materials with a wide variety of materials and devices.19 Recently, amorphous carbon thin films have been applied to electrodes20,21 and micromechanical devices5 as well as to surface plasmon resonance supporting substrates.10,18 Prior to attaching the alkene molecules to the surface, each carbon-on-metal substrate was first hydrogen terminated in an RF-generated hydrogen plasma. Raman studies of the amorphous carbon substrates have shown that the material is in fact disordered, containing both G and D bands, and that the film is composed of a large number (>75%) of sp2-hybridized carbon atoms with no detectable amount of hydrogen present on the surface.5 This is supported by the absence of methylene stretches (3100-2800 cm-1) in the infrared spectra. Hydrogen plasma treatment not only hydrogen terminates the substrate but also reduces the heterogeneity of amorphous carbon thin films that are prepared at different times.18 The halogenated substrates were incubated in benzene containing 100 mM PX5 (X = Br or Cl) and a catalytic amount of benzoyl peroxide. This method of halogenation attaches approximately 1.44 1015 bromine and 2.54 1015 chlorine atoms/cm2, corresponding to 0.39 and 0.61 of the theoretical monolayer density.16 Neat alkene liquids were then placed on the bromine-, chlorine-, or hydrogen-terminated carbon-on-metal substrates, covered with a quartz coverslip, and placed in a nitrogen-purged reaction chamber sealed with a quartz window. The reaction chamber was then irradiated with UV light from a mercury grid lamp with a primary emission line at 254 nm. The functionalization steps utilized in this work are summarized in Scheme 1.
The attachment of each alkene molecule (Figure 1) to the bromine-, chlorine-, and hydrogen-terminated carbon-on-metal substrates was monitored over a 16 h period with infrared measurements. The average intensity of the symmetric and asymmetric stretching modes (v(CH2)s at 2925 and v(CH2)a at 2854 cm-1, respectively) of the methylene groups was used to compare the relative number of molecules attached to each substrate. Each of the alkene molecules contains the same number of methylene groups (except for dodec-1-ene, which contains an additional methylene unit), affording a means of comparing (a) the relative number of molecules attached to the bromine-, chlorine-, and hydrogen-terminated substrates for a given alkene molecule and (b) the relative number of molecules of each alkene molecule for a given substrate. The orientation of the molecules on the surface can affect the intensity of their infrared peaks;22 however, it has been shown previously that the net absorbance of the methylene stretching peaks provides a reasonable estimate of the molecular surface coverage on carbon substrates.11,14,23 A set of control experiments were also performed in which the neat alkene molecules were placed on the bromine-, chlorine-, and hydrogen-terminated substrates and incubated for 16 h in the absence of ultraviolet light. The infrared spectra of these substrates were the same as unmodified amorphous carbon substrates, with no new distinguishable infrared signatures evident.24 Figure 2a shows the methylene stretching region (3000-2800 cm-1) of the TFAAD molecule on the bromine-, chlorine-, and hydrogen-terminated carbon-on-metal substrates after a 16 h reaction. Figure 2b contains the methylene stretching regions for dodec-1-ene after a 16 h attachment reaction. The relative intensities of the TFAAD methylene stretches on each of the substrates are very similar and suggest that the molecule readily attaches to the surface, regardless of termination. In contrast to
(18) Lockett, M. R.; Smith, L. M. Anal. Chem. 2009, 81, 6429–37. (19) Robertson, J. Adv. Phys. 1986, 35, 317–374. (20) Luo, J. K.; Fu, Y. Q.; Le, H. R.; Williams, J. A.; Spearing, S. M.; Milne, W. I. J. Micromech. Microeng. 2007, 17, S147–S163. (21) Hauert, R. Diamond Relat. Mater. 2003, 12, 583–589.
(22) Tolstoy, V. P. Handbook of Infrared Spectroscopy of Ultrathin Films; Wiley-VCH: Hoboken, NJ, 2003. (23) Colavita, P. E.; Streifer, J. A.; Sun, B.; Wang, X. Y.; Warf, P.; Hamers, R. J. J. Phys. Chem. C 2008, 112, 5102–5112. (24) Data not shown.
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TFAAD, the nature of the surface termination results in significant differences in the methylene stretch intensities (i.e., the number of molecules that attach to the surface) for the dodec1-ene molecule. The dodec-1-ene molecule readily attaches to the halogenated surfaces, but there are no distinguishable infrared signatures on the hydrogen-terminated surface. Figure 3 contains the methylene stretching regions for each alkene molecule after 16 h of attachment on the (a) brominated and (b) hydrogenated surfaces. It is worth noting that a similar number of molecules attach to the carbon-on-metal substrates after 16 h, regardless of surface termination, when the terminal functional groups distal to the alkene moiety, such as TFAAD and UA, have a high electron affinity. This is not the case for UA-Me and dodec-1-ene, for which there is a pronounced enhancement of the attachment reaction upon halogenation of the surface,11,14 as shown in Figure 3. The increased reactivity of UA-Me and dodec-1-ene with the halogenmodified surface expands the variety of molecules that can be attached to the carbon-on-metal substrates. The net absorbance values of the methylene asymmetric stretch (2925 cm-1) for each alkene molecule, after a 16 h reaction, are presented in Table 1. XP measurements afford an alternative means of monitoring alkene molecule attachment to the surface. High-resolution
Figure 3. Methylene stretching region of TFAAD, UA, UA-Me, and dodec-1-ene after a 16 h attachment reaction on (a) bromineand (b) hydrogen-terminated carbon-on-metal substrates. Table 1. Net Absorbance of the v(CH2)a Peak (2925 cm-1) of Each Alkene Molecule on Bromine-, Chlorine-, and Hydrogen-Terminated Carbon-on-Metal Substrates after a 16 h Attachment Reaction net absorbance v(CH2)a
TFAAD UA UA-Me dodec-1-ene
Br-terminated
Cl-terminated
H-terminated
0.00194 0.00172 0.00030 0.00021
0.00200 0.00159 0.00018 0.00014
0.00186 0.00163
spectra of the C 1s region were used to monitor UA-Me, UA, and TFAAD attachment over a 16 h period because each molecule contains a unique carbon signature that is readily distinguished from the sp2- and sp3-hydridized carbon components associated with the bulk material. Each C 1s high-resolution spectrum was aligned (285.0 eV), and the raw data were fit to Voigt functions after Shirley baseline corrections25 prior to analysis. Figure 4 shows the C 1s high-resolution spectra for (a) UA, (b) UA-Me, and (c) TFAAD molecules on bromine-terminated carbon-on-metal substrates after 16 h of attachment. Each spectrum contains four components, three of which are also present in the bulk material: the C-C bonds (sp2- and sp3hybridized species, 284.9 and 285.6 eV) and oxidized carbon species (C-O bonds, 286.4 eV). The peak at 289.5 eV is attributed to the CdO bond present in each of the alkene molecules (Figure 1). The TFAAD spectrum contains an additional component at 294.2 eV that is attributed to the molecule’s C-F bonds. It is very difficult to determine if dodec-1-ene has attached to the carbon-on-metal substrates with XP measurements as it possess no unique functional groups that are readily distinguishable from the bulk material. XP measurements are more quantitative than surface IR measurements. Here, the relative numbers of UA-Me, UA, and TFAAD molecules attached to the surface were determined from the C 1s region by means of the CdO/C-C component ratio. The C-C components (284.9 and 285.6 eV) were assumed to arise solely from the bulk material, and the number of carbon atoms on the surface was determined using a density of 2.2 g/cm2 (graphite) and an X-ray penetration depth of 2.0 nm.26 The C-F component (294.2 eV) of the TFAAD C 1s high-resolution spectrum was also used to calculate the number of TFAAD molecules attached to the surface; this number was compared to the results obtained for the CdO component.27 Table 2 shows the number of molecules attached to the bromine-, chlorine-, and hydrogen-terminated carbon-on-metal substrates at two time points, after 8 and 16 h of attachment. Each of the carbon-on-metal substrates, independent of surface termination, contained a similar number of TFAAD molecules after 16 h of illumination. The average TFAAD coverage, 1.57((0.001) 1015 molecules/cm2, is similar to the results previously obtained on amorphous carbon substrates.5 Likewise, a similar number of UA molecules (1.43((0.001) 1015 molecules/cm2) attached to the carbon substrate, independent of surface termination. This is not surprising because UA has been shown to attach readily to hydrogen-terminated amorphous carbon substrates.2,9 There are significant differences, however, in the number of UA-Me molecules that attached to each surface, with the bromineterminated substrates containing the largest number of molecules.
Figure 4. C 1s high-resolution XP spectra of (a) UA-Me, (b) UA, and (c) TFAAD molecules on a bromine-terminated carbon-on-metal substrate after a 16 h attachment reaction. 16644 DOI: 10.1021/la103050z
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Table 2. Number of Molecules Attached to Bromine-, Chlorine-, and Hydrogen-Terminated Carbon-on-Metal Substrates after Alkene Attachment number of molecules attached to the carbon-on-metal substrates (# of molecules/cm2) Br-terminated
TFAAD UA UA-Me
Cl-terminated
H-terminated
8h
16 h
8h
16 h
8h
16 h
1.56 1015 1.47 1015 1.14 1015
1.58 1015 1.46 1015 1.14 1015
1.39 1015 1.18 1015 1.11 1015
1.58 1015 1.46 1015 1.13 1015
1.21 1015 9.63 1014 4.86 1013
1.57 1015 1.44 1015 2.12 1014
Figure 6. Attachment of dodec-1-ene molecules to bromine- (2), chlorine- (b), and hydrogen-terminated (9) carbon-on-metal substrates monitored via the net absorbance of the v(CH2)a peak at 2925 cm-1 with PM-FTIRRAS measurements.
Figure 5. Attachment of (a) TFAAD, (b) UA, and (c) UA-Me molecules to bromine- (2), chlorine- (b), and hydrogen (9)terminated surfaces as a function of time (hours) as determined with XP high-resolution measurements of the C 1s region. The reaction progress was monitored with the ratio of CdO (UA-Me and UA) and C-F (TFAAD) peaks to the 285.5 eV peak.
After 16 h of illumination, the bromine-terminated surfaces contain 5.4 times the number of molecules as their hydrogenterminated analogs. This difference is even more pronounced at 8 h, in which there is a 23.5-fold difference between the two substrates. (25) Shirley, D. A. Phys. Rev. B 1972, 5, 4709–4714. (26) Powell, C. J.; Jablonski, A. NIST electron effective attenuation-length database. In National Institute of Standard and Technology, 2001. (27) The number of molecules obtained from the C-F component is similar to that obtained from the CdO component and is within the error associated with the measured data. Data are not shown.
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The number of molecules that attach to the bromine-terminated surface after 16 h (i.e., the point of saturation (as shown in Figure 5)) varies between alkene species with 1.15 1015, 1.46 1015, and 1.58 1015 molecules/cm2 for the UA-Me, UA, and TFAAD molecules, respectively. These differences are also observed in the PM-FTIRRAS spectra of the methylene stretch region (Figure 3). Similar results have been reported previously,11,14,23 and it has been proposed that the differences in the number of molecules attached to the surface are a result of the alkenes forming multilayers.28 The C 1s XP spectra were also used to monitor the rate of attachment of the UA-Me, UA, and TFAAD molecules to the carbon-on-metal substrates. Here, the CdO/C-C component ratio was determined and plotted as a function of illumination time. Figure 5 shows the reaction plots for the (Figure 5a) UA-Me, (Figure 5b) UA, and (Figure 5c) TFAAD molecules on bromine- (2), chlorine- (b), and hydrogen-terminated substrates (9). The presence of the halogen species on the surface increases the rate of alkene molecule attachment. This pattern of reactivity, Br- > Cl- > H-terminated substrates, corresponds to the relative ease of C-R bond (R = Br, Cl, or H) cleavage. In each case, the time required to saturate the carbon surface with alkene molecules is significantly reduced for bromine-terminated substrates, reducing the reaction time needed, compared to that for their hydrogen analogs, by at least a factor of 2 (∼8 h). The rate of dodec-1-ene attachment was monitored over a 16 h period with infrared measurements. The dodec-1-ene molecules attach only to the carbon-on-metal substrates that were first bromine- or chlorine-terminated. A similar reactivity pattern is observed for this attachment reaction, with the dodec-1-ene molecules attaching most quickly to the bromine-terminated surface and most slowly to the hydrogen-terminated substrate. (28) Wang, X.; Colavita, P. E.; Metz, K. M.; Butler, J. E.; Hamers, R. J. Langmuir 2007, 23, 11623–11630.
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There is no detectable amount of dodec-1-ene attached to the hydrogen-terminated substrate after 16 h of illumination, as previously reported by Colavita et al.14 Figure 6 shows the rate of dodec-1-ene attachment via the net absorbance of the v(CH2)a peak at 2925 cm-1. The net absorbance of an infrared peak cannot accurately be used to calculate the number of molecules attached to a surface; however, it does provide a means of comparing the relative differences in dodec-1ene attachment to the bromine-, chlorine-, and hydrogen-terminated surfaces. Figure 6 shows that none of the carbon surfaces are saturated with dodec-1-ene molecules after 16 h, independent of substrate termination, which is attributed to the relatively slow attachment of this low electron affinity molecule.
Conclusions The introduction of a halogen (bromine or chlorine) onto the carbon-on-metal substrate surface via the formation of a carbon-halogen bond15 enhances its reactivity toward alkenecontaining molecules. This increased reactivity further expands the utility of alkene functionalization of carbon surfaces by (1)
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increasing the variety of alkene molecules that will readily attach to the surface and (2) increasing the reactivity of the surface toward the alkene molecules and thus decreasing the time required to functionalize the surface. An additional advantage of the reaction scheme outlined here is that the halogenation of the carbon surfaces is accomplished in a solution-phase reaction, which does not require specialized equipment and/or harsh reaction conditions. Acknowledgment. We thank Professor Robert J. Hamers for access to and use of his PM-FTIRRAS instrument. This research utilized an NSF-supported shared facility at the University of Wisconsin and was funded by NSF grant CHE-0809095, which was cofunded by the MPS/CHE and BIO/MCB divisions. L.M.S. has a financial interest in GWC Technologies. Supporting Information Available: Detailed information on the methods and materials used in this work. This material is available free of charge via the Internet at http://pubs.acs.org.
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