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Helical versus All-Trans Conformations of Oligo(ethylene glycol)Terminated Alkanethiol Self-Assembled Monolayers Phillip S. Johnson,† Mohit Goel,‡ Nicholas L. Abbott,‡ and F. J. Himpsel*,† †

Department of Physics, University of Wisconsin-Madison, 1150 University Avenue, Madison, Wisconsin 53706, United States Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, United States

‡

S Supporting Information *

ABSTRACT: The complex mixture of conformational states exhibited by oligo(ethylene glycol)-terminated alkanethiols on Ag and Au surfaces is explored by polarization-dependent Xray absorption spectroscopy. Three self-assembled monolayers (SAMs) with known helical or all-trans conformations are used as references to characterize a SAM with unknown conformations. This case study is used as a prototype for developing a systematic framework to extract the conformations of SAMs from the polarization dependence of several orbitals. In the case at hand, these are associated with the C− H/Rydberg bonds of the alkane, the C−H/Rydberg bonds of ethylene glycol, and the C−C bonds of the backbone. The C−H/ Rydberg orbitals of the alkane and ethylene glycol are distinguished via the chemical shift of the corresponding C 1s core levels.

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INTRODUCTION

complemented by IR data, which are shown in the Supporting Information. Various methods have been used to characterize the organization and orientation of organic oligomers such as alkanethiols, arachidates, organosilanes, and OEGs, combined with Au or Ag substrates, including IR spectroscopy,16−20 density functional theory,14,21,22 sum frequency generation,23 Xray photoelectron spectroscopy,18,24 and XAS.25−33 It is generally understood that (EG)n SAMs of sufficient length (n ≥ 6) on Au will prefer a helical conformation independent of alkane chain length. This includes direct attachment of OEG to the surface by a thiol group. Those on silver surfaces will prefer an all-trans orientation due to the closer packing of the Ag atoms in the substrate.20,25 The S−S spacing on Ag is 4.77 Å, whereas it is 4.97 Å on Au, thus providing increased flexibility to EG moieties.6 At low coverage, the SAMs are disordered.12,25,34 Surface defects and nonuniform domain structure produce regions of amorphous conformation, deviating from the ideal structure. Quantification of the helical, all-trans, or amorphous conformations in a given SAM remains difficult, particularly because the SAMs are in general a mix of the various conformations that is affected by humidity and coadsorbed species. In order to devise methods to characterize an OEGterminated alkanethiol SAM of unknown conformation, we have investigated an alkane SAM and three OEG-terminated

Self-assembled monolayers (SAMs) of oligo(ethylene glycol) (OEG)-terminated alkanethiols are widely used in the process of functionalizing surfaces for immobilization of proteins and other biomolecules. They are particularly useful in minimizing nonspecific adsorption and cell attachment.1−5 The order, orientation, and conformation of these molecules all influence the reactivity of the surface. Control over the interfacial structure of the SAM is necessary for reproducible functionalization of surfaces.6,7 The structure of the OEG monolayers can also affect their interaction with the immobilized biomolecules, thereby defining their functional properties.8,9 A given OEG SAM in ambient conditions coexists as a mixture of mainly two conformational states: the helical conformation in which the OEG tails assume a conformation close to a 7/2 helix and a planar arrangement of OEG tails referred to as all-trans conformation (see Figure 1 in ref 10). The ratio between the two conformations in a given OEG SAM is a function of many factors such as hydration levels,11 substrate used,3,6 surface coverage,11,12 EG repeats,6,12 and solvent.13 The ability of these monolayers to resist nonspecific protein adsorption depends upon their ability to associate with water molecules, which in turn depends upon the order, orientation, and conformation of OEG moieties in a given OEG SAM.14,15 As such, it is important to characterize these quantities for a well-defined set of experimental conditions. In the present study, we characterize the conformations of OEG-terminated alkanethiols on Ag and Au surfaces by polarization-dependent X-ray absorption spectroscopy (XAS), an element- and bond-specific technique. These results are © 2014 American Chemical Society

Received: March 12, 2014 Revised: July 21, 2014 Published: August 5, 2014 10263

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X-ray Absorption Measurements. All XAS spectra were taken using the U2 VLS-PGM at the Synchrotron Radiation Center (SRC) in Madison, Wisconsin. The C 1s edge was calibrated to the CC π* transition in graphite at 285.35 eV. The absolute accuracy at this edge is ±0.1 eV, and the relative accuracy between different peaks in the same spectrum is ±0.05 eV. All spectra presented are taken in the surface-sensitive total electron yield (TEY) mode, with a probing depth of a few nanometers. Radiation damage was minimized by using the narrowest possible exit slits and tested by a series of scans on the same spot. All spectra exhibit a small pre-edge dip at 285 eV with little polarization dependence, which is due to hydrocarbon contamination on the Au mesh used for monitoring the photon flux. This was corrected by dividing the mesh-normalized spectrum for each SAM by a spectrum of a clean Au sample normalized to the same mesh. This “double divison” eliminates the C signal from the mesh. Some residual fine structure near 285 eV could be due to a small fraction of a monolayer of π-bonded carbon in the samples. The Au reference was prepared in a separate vacuum system and transferred under inert atmosphere. For the Ag substrate, we performed double division with both Au and Ag reference samples. However, the Ag reference was contaminated by a small amount of carbon (possibly because Ag is more reactive than Au). This caused an artifact at the CC π* transition (285 eV), which led us to adopt the Au reference. The difference between the normalization to Au and Ag references is insignificant for our analysis, since the C−H/Rydberg σ* transitions occur above 285 eV. Also, the X-ray absorption spectra of clean Ag and Au are both featureless at the C 1s edge. Our major uncertainty in the determination of the polarization dependence is the realignment of the sample position when changing the angle of incidence, particularly when going toward grazing incidence. The light spot changes its size and moves slightly. (The visible part of the synchrotron radiation could not be used for alignment, since the work was performed at an undulator beamline.) As a result, we are sampling slightly different parts of the sample at different angles. That affects both signal and background. For these reasons, we have decided to fit and normalize the spectra directly at each angle rather than difference spectra taken at different angles. Data Processing. In order to isolate the polarization-dependent components of the spectra, it is necessary to remove the polarizationindependent components such as Rydberg excitations and the continuum step. These features are approximated by fitting a common background function to all spectra from a given sample. This function has been adapted from the literature (see eq 7.8 in ref 37) by adding an exponentially decaying background to the usual step function:

alkanethiol SAMs: (a) undecanethiol on Au, (b) EG4 on Ag (known to be all-trans), (c) EG6 on Au (known to be helical), and (d) EG4 on Au (a mix of helical and all-trans conformations). The first three SAMs serve as references, while the fourth becomes a test case for characterizing a more complex structure. The conformation of the molecules is extracted from the polarization dependence of the transitions from the C 1s core level to three unoccupied orbitals. These are associated with the C−H/Rydberg bonds of the alkane, the C−H/Rydberg bonds of ethylene glycol, and the C−C bonds of the backbone. Since the reference molecules have distinctly different polarization dependence, we can quantify the proportion of helical to alltrans character in the EG4-terminated SAM. The use of three independent orbitals together with three reference SAMs allows a stringent discrimination between various structures. Finally, we wish to emphasize that the development of a methodology to characterize the conformations of OEG SAMs is important, because past studies have suggested that the conformational state of the SAM influences the extent to which a given OEG SAM will resist nonspecific adsorption of proteins. Past studies have suggested that resistance to nonspecific protein adsorption may be mediated by water binding to different conformations of the OEG groups.35 In particular, it has been proposed that the helical conformation of OEG interacts more strongly with water.14 Thus, characterization of the helical content of an OEG SAM is important for understanding the interactions of water with these SAMs.

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EXPERIMENTAL SECTION

Materials. Materials used in the current study were used as they were received. Methoxy-terminated tetraethylene glycol alkanethiol (HS−C11−EG4−OCHH3) and amine-terminated hexaethylene glycol alkanethiol (HS−C11−EG6−NH2HCl) were obtained from Prochimia (Poland). Ti (10 nm) and Au (100 nm) were deposited on Si wafers using physical vapor deposition to prepare gold films used in the current study.36 These Au films were used as soon as they were prepared (within 24 h of preparation). SAMs were prepared by immersing the Au films into the desired thiol solution (1 mM in ethanol). After 18 h of incubation, the SAMs were rinsed using ethanol and water and dried using N2 gas immediately before being placed in vacuum for XAS measurements. Idealized structures of the EG4- and EG6-terminated alkanethiol SAMs are shown in Figure 1.

⎛ ⎛ ⎞⎞ x−p 1 1 Istep(x) = ⎜⎜ + erf⎜ σ ⎟⎟⎟(A + B e−k(x − p)) ⎟ 2 2 ⎜⎝ ⎝ 2 ln2 ⎠⎠ The amplitude A is the height of the continuum far above the edge, A + B represents the maximum of the background at threshold, p the position of the step, σ the width of the leading edge, and k the decay constant for the exponentially decaying part of the background. Such a background fit is subtracted from a series of polarization-dependent spectra after they have been normalized to the height of the continuum at 320 eV. The background-subtracted spectra are then fit with Gaussians to determine changes in the peak areas with the polarization. Typical fits are shown in the Supporting Information. Figure 2 shows the normalized C 1s spectra for the EG4 SAM on Au and the three reference SAMs at both normal and grazing incidence. Also shown is the background given by the equation above for each SAM. Subtraction of this background extracts the polarizationdependent near-edge features that are shown in Figure 3 and in the Supporting Information.

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Figure 1. Conformations of the SAMs studied in this work. The EG6 SAM on Au has a helical conformation in the ether groups, while the EG4 SAM on Ag has an all-trans conformation. The EG4 SAM on Au has a mix of helical and all-trans conformations in unknown proportions.

RESULTS The near-edge spectra in Figure 3 demonstrate the polarization dependence of the three orbitals that characterize the four 10264

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Figure 3. C 1s absorption spectra (TEY) for (a) an ordered alkanethiol SAM, (b) an EG4-terminated alkanethiol SAM on Ag, (c) an EG6-terminated alkanethiol SAM on Au, and (d) an EG4terminated alkanethiol SAM on Au. A background has been subtracted from the spectra to isolate the polarization dependent C−C and C− H/Rydberg features (see Figure 2). The EG6 SAM exhibits reduced polarization dependence compared to the EG4 SAMs on Ag and Au, consistent with its helical conformation. The angle of incidence is measured from the surface.

Figure 2. C 1s absorption spectra (TEY) for (a) an ordered alkanethiol SAM on Au, (b) an EG4-terminated alkanethiol SAM on Ag, (c) an EG6-terminated alkanethiol SAM on Au, and (d) an EG4terminated alkanethiol SAM on Au. Spectra at normal and grazing incidence are shown, as well as the background (dashed) that is subtracted from each spectrum to isolate polarization-dependent features.

0.1 eV). The more electronegative O removes negative charge density from the ether C, thereby increasing the C 1s binding energy and the transition energy to the C−H/Rydberg orbital. With high-resolution and well-ordered SAMs, we observe a splitting of the C−H/Rydberg transition into a double peak for both the alkane and ether portions of the SAMs, as shown in Figure 3. Such a splitting has been observed before in pure alkane films and even in the gas phase.38 Several possible assignments have been proposed, for example, two C−H orbitals with even and odd symmetry about the plane of the molecule,32,39,40 two closely spaced Rydberg states,41,42 and a combination of Rydberg states and a C−H orbital.38,43 The van der Waals interaction between alkane molecules in a molecular solid can also modify the line shape.44 The main features of the ordered alkanethiol on Au in Figure 3a are the split C−H/Rydberg peak and a broad C−C peak at higher energy. They show opposite polarization dependence, as expected from the orthogonal orientation of their dynamic dipole moments. The strong polarization dependence with a maximum of the C−H/Rydberg double peak at normal incidence is consistent with an orientation of the alkane chains

SAMs investigated here, that is, (a) undecanethiol on Au, (b) EG4-terminated alkanethiol on Ag, (c) EG6-terminated alkanethiol on Au, and (d) EG4-terminated alkanethiol on Au. The incident soft X-rays are p-polarized, with the angle of incidence defined from the surface (i.e., 90° is normal incidence). The double peak at 287.4 and 288.0 eV is characteristic of the C−H/Rydberg orbitals in the alkane section, the double peak at 288.7 and 289.4 eV corresponds to similar C−H/Rydberg orbitals in the ethylene glycol section, and the broad peak at about 293.6 eV represents C−C backbone orbitals. The higher energy C−H/Rydberg double peak is absent in the pure alkanethiol (Figure 3a), which provides a clear assignment of the two C−H/Rydberg transitions. The energy difference is due to the chemical shift between the C 1s core levels of the alkane and the ether (1.3 ± 10265

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nearly perpendicular to the surface. A weaker feature appears near normal incidence at 291.5 eV, just below the main C−C peak, but its polarization dependence is too much affected by the larger C−C peak to be useful for a quantitative analysis. Figure 3b shows similar spectra for an EG4-terminated alkanethiol SAM on Ag. The Ag substrate forces both the alkane chain and the EG 4 into a nominally all-trans conformation due to the denser packing of the molecules on Ag compared to Au. Immediately evident in these spectra compared to those in Figure 3a is the appearance of a split ether C−H/Rydberg peak. Strong polarization dependence can be seen for the alkane C−H/Rydberg and C−C peaks, which is consistent with that in Figure 3a. In both cases, the molecules have an all-trans conformation. Figure 3c shows the C 1s spectra for an EG6-terminated alkanethiol SAM on Au. In contrast to the EG4 SAM on Ag, the EG6 SAM on Au has a nominally helical conformation. Indeed, the modulation of the spectra with incident angle is reduced compared to that of the EG4 SAM on Ag in Figure 3b. This reduced polarization dependence is to be expected, since the helical conformation results in the C−C and C−H/Rydberg orbitals having a less uniform orientation than in an all-trans conformation. Furthermore, we find the intensity of the ether C−H/Rydberg peaks in the EG6 SAM to be enhanced due to the additional two EG moieties. The limited probing depth of TEY spectra means that the deeper alkanethiol portion of the SAM contributes less to the spectrum. Figure 3d shows the spectra for an EG4-terminated alkanethiol SAM on Au. The conformation of this SAM is a mix of helical and all-trans in unknown ratio. Indeed, the spectra exhibit a polarization dependence between that of helical EG6 on Au in Figure 3c and that of all-trans EG4 on Ag in Figure 3b. A comparison with the polarization dependence for the two extreme cases of all-trans and helical provides a semiquantitative measure of the ratio between helical and alltrans in the mixed SAM. As mentioned in the literature,25,34 some degree of amorphous or disordered conformations are to be expected in mixed samples as well, though these should have a lower contribution than the helical or all-trans conformations.

Figure 4. Peak areas of the two C−H/Rydberg transitions and the C− C transition, plotted versus cos2 θ (the angle of incidence measured from the surface) and normalized to a maximum of 1 for the straight line fits. The slopes of the fit lines are listed in Table 1. They characterize the conformation of the SAM quantitatively and can be used to optimize structural models. The slope for the C−C orbital in (d) is about halfway between those in (b) and (c), indicating a 1:1 mix of all-trans and helical conformation in EG4 on Au.

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DISCUSSION A more quantitative assessment of whether a SAM is closer to being helical or all-trans is obtained by plotting the intensities of each peak versus cos2 θ, where θ is the angle of incidence measured from the surface. Dipole transition intensities for 1s core level excitations are proportional to cos2 δ, where δ is the angle between the electric field vector of the X-rays and the dynamic dipole moment of the transition, that is, the axis of the p orbital for the s-to-p transition.37 Various analysis methods have been used in the literature to determine average tilt angles of molecules in SAMs and extract information on their conformation.25,33,37 In Figure 4, the normalized peak areas of the three orbitals are plotted versus cos2 θ for each of the samples in Figure 3. Corresponding to the polarization dependence of a dipole transition, a straight line fit is applied to the data and normalized to a maximum of 1. The slope of this line is used as a quantitative measure of the polarization dependence. The peak areas are determined by fitting the background-subtracted spectra with Gaussians, as shown in the Supporting Information. For the alkane and ether C−H/Rydberg data, the intensities of the double peaks are added, since their polarization dependence is similar. The C−C curves in Figure 4

include contributions from the C−C backbone of both the alkane and ether regions. The normalization chosen here makes the polarization dependence of different orbitals and different SAMs comparable by eliminating differences in the numbers of alkane or ether moieties. The slope of each line provides a quantitative measure for characterizing the conformation (see the compilation of the slopes in Table 1). The slope of a particular orbital is not directly related to a conformational parameter. For example, a small slope could be indicative of either disorder in the orbital orientation or perfect order in the polar orientation with a tilt at the magic angle.37 When the magnitude of the slope approaches unity, on the other hand, it is safe to assume that there is a high degree of order in the polar orientation. By having such line plots available for three different orbitals, one is able to characterize the conformation of a SAM much better. This can be accomplished by comparing 10266

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We have found a COC peak position at ∼1131 cm−1 (Figure S1, Supporting Information), which is consistent with ethylene glycol moieties exhibiting a mixed helical and all-trans conformation. This observation is again in agreement with our XAS measurements. Moreover, previous PM-IRRAS studies of oligoethylene SAMs on gold found that alkyl CH2 stretching vibrations exhibited peak positions higher than 2918 cm−1, which indicates the presence of gauche defects in the alkyl region of the OEG SAMs.11,46

Table 1. Slopes of the Fit Lines in Figure 4 sample

transition

slope

undecanethiol on Au

alkane C−H/Rydberg C−C alkane C−H/Rydberg ether C−H/Rydberg C−C alkane C−H/Rydberg ether C−H/Rydberg C−C alkane C−H/Rydberg ether C−H/Rydberg C−C

−0.66 0.94 −0.79 −0.25 0.90 −0.38 −0.12 0.42 −0.61 −0.14 0.62

EG4 on Ag

EG6 on Au

EG4 on Au

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CONCLUSIONS This work addresses the conformations of SAMs, which are widely used for immobilization of proteins at surfaces to prevent nonspecific attachment. Polarization-dependent XAS is used to determine the order, orientation, and conformation of these SAMs, factors that play an important role during the immobilization process. Of particular interest to this study is the conformation of an EG4-terminated alkanethiol SAM on Au, which has been hypothesized to be a mix of the helical and all-trans conformations. Using three SAMs with known helical or all-trans conformations as reference systems, we are able to quantify the unknown mix of a fourth SAM. On the basis of this finding, we propose a general method for how to optimize structural models for the conformations of SAMs. The polarization dependence of several characteristic orbitals is calculated for various models and compared to the observed polarization dependence. The bond-selective nature of XAS along with its polarization dependence and surface sensitivity are the keys to making this a useful tool. In particular, the interaction of water molecules with OEG SAMs has been the focus of a number of recent studies. The measurements presented here establish a methodology for using X-ray absorption measurements in the future to understand the effect of water vapor on the conformations of ethylene glycol moieties. The principles developed in this study for EG4 SAMs can also be extended to other classes of SAMs, including those comprising oligopeptides and oligonucleotides, in which the conformational states of the molecules underlie their functional properties.

the calculated polarization dependence of the three orbitals for various structural models, applying an R-factor optimization similar to that used in low energy electron diffraction. In previous work, we have developed an algorithm to calculate the polarization dependence of a complex molecule from the orientation of its individual bond orbitals.45 In addition to this general framework, we extract a few specific observations from Figure 4. There are clear differences in the orientational order, as evidenced by the different slopes of the fit lines for both the alkane C−H/Rydberg and the C−C transitions (shown in black and blue, respectively). Both exhibit smaller slopes in Figure 4c compared to those in a and b. That suggests less orientational order for the EG6 SAM on Au, which is consistent with a helical conformation. For the ether group, the C−H/Rydberg transition (shown in red) has a smaller slope. Another effect reducing the slope is disorder in longer chains,25,34 but that should not make a significant difference between EG4 and EG6. Comparing Figure 4d to the three others provides information about the unknown conformation of the EG4 SAM on Au. In this case, we can again use the slopes of two independent transitions for characterization (alkane C−H/ Rydberg and C−C). Both are about halfway between the corresponding slopes of all-trans EG4 on Ag and helical EG6 on Au. This observation suggests a roughly 1:1 mixture of helical and all-trans conformations for the EG regions of the EG4 SAM on Au. Here, we wish to comment that the conformations of OEG monolayers prepared in different substrates (Ag versus Au) with varying EG repeats have been previously studied using IR spectroscopy. In these past studies, the peak position of the asymmetric COC stretching vibration has been used as an indicator of the conformational state of the ethylene glycol moieties in a given SAM. In particular, a COC stretching vibration peak at ∼1142 cm−1 has been assigned to an all-trans form, whereas a peak at ∼1116 cm−1 has been attributed to a helical form of the ethylene glycol moieties.6,12 Of specific relevance to our paper, in a previous study, Harder et al. used Fourier-transform IR reflection absorption spectroscopy to show that SAMs with three ethylene glycol moieties (EG3) prepared on Ag exhibit an all-trans conformation.6 Furthermore, in a more recent polarizationmodulation IR reflection absorption spectroscopy (PMIRRAS) study by Zorn et al., EG6 SAMs on Au were shown to exhibit a predominantly helical conformation.12 Both these prior studies are consistent with the results reported in the current study. In addition, we have performed PM-IRRAS measurements of an EG4 SAM on Au in 0% relative humidity.

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ASSOCIATED CONTENT

S Supporting Information *

PM-IRRAS measurement of asymmetric COC stretching vibration of EG4 SAM in 0% relative humidity, details of the PM-IRRAS measurement, and example of Gaussian fits to XAS spectra for an EG4 SAM on Au. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (608) 263-5590; fax: (608) 265-2334; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The X-ray absorption measurements were supported by the National Science Foundation under award no. DMR-1121288 (MRSEC). The Synchrotron Radiation Center is primarily funded by the University of Wisconsin-Madison with supplemental support from facility users and the University of Wisconsin-Milwaukee. N.L.A. acknowledges support from 10267

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the Army Research Office (W911NF-11-1-0251 and W911NF14-1-0140).

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