A Thiol-Substituted Carotenoid Self-Assembles on Gold Surfaces

J. Phys. Chem. B 2002, 106, 2933-2936

2933

A Thiol-Substituted Carotenoid Self-Assembles on Gold Surfaces Dezhong Liu, Gregory J. Szulczewski,* and Lowell D. Kispert* Department of Chemistry, UniVersity of Alabama, Tuscaloosa, Alabama 35487-0336

Alex Primak, Thomas A. Moore, Ana L. Moore, and Devens Gust Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604 ReceiVed: May 16, 2001; In Final Form: December 17, 2001

A thiol-substituted carotenoid, 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene, (AMC) was allowed to selfassemble on polycrystalline gold surfaces. The stability, packing density, optical absorption, and hydrophobicity of the resulting monolayers were investigated with X-ray photoelectron spectroscopy (XPS), Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS), UV-Vis spectroscopy, spectroscopic ellipsometry, and contact angle measurements. XPS and spectroscopic ellipsometry measurements reveal the saturation coverage is ∼4 × 1014 AMC molecules/cm2 and the film thickness is ∼20 Å, respectively. FTIR-RAS and contact angle measurements suggest that the polyene chains are packed well enough to create a hydrophobic interface.

Introduction The formation of self-assembled monolayers (SAMs) from organosulfur compounds on metal surfaces has been a topic of great interest over the past two decades.1 The majority of these investigations concern the self-assembly of various alkanethiols and dialkyl disulfides on gold surfaces. Currently there is increasing interest in studying the properties of electroactive SAMs for potential applications in molecular devices.2 We have previously shown that a synthetic carotenoid containing a thiol group, 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (AMC), could be embedded into a docosanethiol SAM on Au.3 The structure of AMC is shown below.

Using a dilute mole fraction of AMC in the SAM we were able to measure the electrical properties of single molecules in an insulating matrix. We found that AMC molecules were over 1 million times more conductive than the surrounding alkane chains, suggesting the carotenoids act as a molecular wire. Carotenoids are an important class of molecules since they play an active role in photodriven electron transport processes.4 A large amount of published literature5-9 on the electrochemical behavior of carotenoids in solution indicates that the first oxidation potential occurs between 0.53 and 0.8 V vs a saturated calomel electrode resulting in the formation of a radical cation. A second oxidation potential occurs at slightly higher potential to produce a dication. An exception to this occurs for β-carotene where the formation of the dication is favored at a lower * Authors to whom correspondence should be addressed at University of Alabama. (L. D. Kispert) Phone: (205) 348-8436. Fax: (205) 348-9104. E-mail: [email protected]. (G. J. Szulczewski) Phone: (205) 3480610. Fax: (205) 348-9104. E-mail: [email protected].

oxidation potential than the formation of the radical cation due to the lowering of the energy upon solvation of the dication by a polar solvent.6 A side reaction occurs where the dication (unstable in aqueous media or in the presence of a trace amount of moisture) losses a proton to form a neutral radical upon reduction. Some evidence exists that a trace amount of such a product is also due to proton loss from the carotenoid cation radical.7 The presence or absence of this side reaction determines whether a reversible or irreversible cyclic voltammogram can be obtained. Although there is a large body of electrochemical data for carotenoids measured on electrode surfaces, very little is known about the molecular orientation of adsorbed carotenoids.10 Since AMC was able to pack moderately well into a docosanthiol SAM in our earlier work, we wanted to explore the formation of SAMs composed of only AMC molecules. We present the results of studies on the preparation and characterization of molecular films of AMC on Au surfaces by FTIR-RAS, XPS, UV-Vis spectroscopy, spectroscopic ellipsometry, and contact angle measurements. Experimental Section Chemicals. The synthesis of 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene (AMC) has been previously described.3 Anhydrous dichloromethane (99.8%, CH2Cl2) and anhydrous ethanol (EtOH) were obtained from Aldrich. Other chemicals were of analytical grade and were used as received. Ultrapure water with a resistivity of 18.2 MΩ cm was used for preparing aqueous solutions. Organic solutions were prepared in a drybox under a nitrogen atmosphere. Self-Assembly of AMC on Gold Surfaces. Gold substrates (thickness of ∼200 nm) were prepared by thermal evaporation onto Si(100) wafers (Virginia Semiconductor, Fredricksburg, VA) that were first coated with a layer (∼5 nm) of chromium under high vacuum.11 The gold-coated wafers were cut into 20 × 20 mm pieces (for FTIR-RAS measurements) or 10 × 12 mm pieces (for other measurements), and stored in pure ethanol

10.1021/jp011876a CCC: $22.00 © 2002 American Chemical Society Published on Web 02/16/2002

2934 J. Phys. Chem. B, Vol. 106, No. 11, 2002 for future use. Prior to monolayer formation, the Au films were rinsed with deionized water, dried in a stream of N2, and held in a freshly prepared Piranha solution (concentrated H2SO4:30% H2O2 ) 3:1 V/V) for 10 min. CAUTION: The Piranha solution is highly oxidizing and should be handled with extreme care. The solution should be disposed of upon completion of substrate cleaning to aVoid explosions. The surfaces were then washed thoroughly with deionized water and anhydrous ethanol. After being dried in a stream of argon, the gold electrodes were transferred immediately to a 0.1 mM AMC solution prepared in a mixture of anhydrous CH2Cl2 and EtOH (1:4 V/V), and incubated for 24 h in the dark at ambient temperature under pure N2. Just before measurement, an AMC-modified Au electrode was rinsed three times with the solvent mixture, and then dried in a gentle stream of high purity argon (99.998%, Post Airgas, Inc.). Infrared Spectroscopy. We used a Nicolet 560 spectrometer with a liquid nitrogen cooled mercury cadmium telluride detector and purge system to remove water and carbon dioxide from the sample chamber. A single reflection accessory at an 85° incidence angle was used to perform Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS). The spectrometer resolution was set to 4 cm-1 and 512 scans were averaged to improve the signal-to-noise ratio. A spectrum of a bare gold surface was subtracted from the sample spectrum during data processing. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectra were obtained with a Kratos Analytical Axis 165 Scanning Auger/X-ray Photoelectron Spectrometer using monochromatic Al KR radiation (1486.7 eV) as the excitation source. All spectra were acquired at a pass energy of 80 eV with the anode operated at ∼270 W. The base pressure in the analysis chamber was less than 1 × 10-9 Torr. Quantitative analysis was performed using a proprietary program from Kratos Analytical, which uses instrument-specific sensitivity factors. The Shirley method was used to subtract the inelastic scattering contribution from the raw data.12 Contact Angle Measurements. Contact angles were determined with a Rame´-Hart model 110-00 goniometer at room temperature and ambient relative humidity using ultrapure water as the probing liquid. Advancing contact angles (θ) were measured by applying a ∼5 µL water drop onto the surface from a blunt-ended needle attached to a 2 mL syringe. The contact angle was recorded immediately after the drop detached from the needle tip. At least four measurements were taken for each sample and the average value was calculated. Results and Discussion Optical Studies. Optical absorption spectra of AMC SAMs on an optically transparent Au surface (∼18 nm Au film on glass) and AMC in bulk CH2Cl2 solution are displayed in Figure 1. For AMC monolayers deposited on the Au surface, the peak shape is broadened, the maximal absorption is 20 nm red-shifted relative to AMC in the bulk solution, and the spectral fine structure disappears. The molar extinction coefficient of AMC in CH2Cl2 was estimated to be 1.5 ((0.4) × 105 (λmax ) 471 nm), which is comparable to the reported data of the following common carotenoids: β-carotene: 1.3 × 105 (λmax ) 465 nm, in CHCl3); canthaxanthin: 1.2 × 105 (λmax ) 480 nm, in benzene).13 We can roughly estimate the surface coverage from the absorbance and assumption that the extinction coefficient for AMC does not change significantly from the solution phase. Using this approach we calculate ∼8 × 1014 AMC molecules/ cm2.

Liu et al.

Figure 1. UV-Visible absorption spectra for AMC deposited on the semi-transparent Au surface (dotted line) and 41 µM AMC in CH2Cl2 solution.

Ellipsometry measurements were made using a variable-angle spectroscopic ellipsometer (J. A. Woollam Co., Inc.) at an incident angle 70° over a wavelength range of 300-1000 nm. The thickness of the AMC monolayer was determined to be 20 ( 2 Å using the software accompanying the Woollam ellipsometer. X-ray Photoelectron Studies. Figure 2 shows X-ray photoelectron spectra of an AMC SAM on a gold electrode. In the C(1s) region (Figure 2B), it can be seen that the binding energy for C(1s) is 284.6 eV, which is consistent with the fact the majority of the carbon atoms are sp2 hybridized.14 To investigate the interaction of the AMC molecules with the Au substrate, it is essential to scrutinize the S(2p) region. Figure 2C shows an asymmetric peak that we could not resolve at a pass energy of 80 eV. Since the photoemission cross section for sulfur is small and the photoelectrons must escape the ∼2 nm AMC film (a distance comparable to their mean free path15), we had to sacrifice resolution for sensitivity. The asymmetric peak contains the S(2p3/2) and S(2p1/2) peaks, which are separated by 1.2 eV. The measured S(2p3/2) binding energy of 162.0 eV is in excellent agreement with other studies.16 We did not observe a sulfonate peak at ∼168 eV, which suggest that AMC adsorbs on Au as a thiolate. We measured a series of XPS spectra for long-chain alkanethiols to determine the C(1s)-to-S(2p) and S(2p)-to-Au(4f) peak area ratio. The C(1s):S(2p) ratio will increase with increasing alkyl chain length, but also due to attenuation of the sulfur signal. The theoretical surface coverage1 for a (x3×x3)R30° alkanethiol SAM on a perfect Au(111) surface is 0.33 monolayer (ML) sulfur atoms with respect to gold (1.5 × 1015 Au atoms cm-2). On the basis of our XPS measurements of AMC films we calculate a surface coverage of ∼0.23 ML S or 3.5 × 1014 AMC molecules/cm2. The measured coverage is consistent with the fact that each AMC molecule has a larger cross-sectional area than an alkanethiol. The large area per molecule likely arises from (a) steric interactions between the β-ionone ring group that interfere with strong packing interactions, and (b) the side methyl groups that promote disorder in the monolayers. The thickness of the monolayer (d) was determined by measuring the Au(4f) intensity before and after sputtering away the AMC molecules. The equation Id ) I0 exp (-d/λ) relates

Thiol-Substituted Carotenoid on Gold Surfaces

J. Phys. Chem. B, Vol. 106, No. 11, 2002 2935

Figure 3. Comparison of (A) reflection FT-IR spectrum of the AMC modified Au surface and (B) transmission FT-IR spectrum of AMC in KBr pellet in the C-H stretching region.

TABLE 1: Contact Angle Measurements

a

Figure 2. XPS spectra of a self-assembled monolayer of AMC on gold. (A) survey scan, (B) C(1s) region, and (C) S(2p) region.

the attenuation of a signal from the Au substrate to the thickness of a monolayer at a takeoff angle of 90° (defined as the angle between the surface plane and entrance to the analyzer) where Id and I0 are the Au(4f) intensity of the AMC-covered and clean Au surfaces, respectively. The value of inelastic mean free path (λ) for the Au(4f) photoelectrons through the organic layer was taken from the literature (42 Å).16 Using this approach we calculate the AMC monolayer to be 20 ( 1 Å. The thickness determined by ellipsometry and XPS in this work is in excellent agreement with previous scanning tunneling microscopy measurements.3 Contact Angle Measurements. The wetting behavior is one of the most important properties of a monolayer. The surface of a homogeneous monolayer that is densely packed and possesses a high degree of orientation demonstrates characteristic wetting properties. The wetting contact angle yields information about the ordering of the monolayer chains. Table 1 shows the water contact angle data for bare Au, AMC-Au, and octadecanethiol-Au. The water contact angle for AMCAu is fairly high, but not as large as that of the alkanethiol. This indicates that (a) the SAMs of AMC present a hydrophobic surface to the probing liquid, and (b) the carotenoid is not densely packed on the Au surface. We expect that AMC SAMs would most likely be less ordered than the simple n-alkanethiols since AMC contains a large hydrophobic terminal group that provides steric inhibition to ordering.

substrate

contact angle (( 2°)

bare Au AMC-Au ODT-Aua

36 73 98

Octadecanethiol SAM on Au.

FTIR-RAS Studies.The molecular structure of AMC SAMs was investigated with infrared spectroscopy. The FTIR-RAS spectrum of AMC on an Au electrode surface is shown in Figure 3A and the FTIR transmission spectrum of AMC in KBr is shown in Figure 3B. Only the C-H stretching region is shown. The peak assignments for AMC in KBr are summarized as follows: 3025 cm-1 (alkene C-H stretch); 2950 cm-1 (asymmetric CH3 stretch); 2924 cm-1 (asymmetric CH2 stretch); 2862 cm-1 (symmetric CH2 stretch); 2826 cm-1 (symmetric CH3 stretch). It is well-known that application of the surface dipole selection rule17 can provide information about molecular orientation and the degree of order in the alkyl chains in SAMs.18 In this study we do not expect the AMC molecules to adopt an ordered monolayer due to steric hindrance along the polyene backbone and presence of the bulky terminal β-ionone ring group. Here we applied the surface dipole selection rule in a qualitative fashion to deduce the orientation of an AMC molecule within the monolayer. A comparison of the IR spectra of AMC in the KBr matrix indicates the peak frequencies are slight higher (∼5-10 cm-1) in the monolayer film. Examination of the relative intensity of asymmetric and symmetric methyl stretching modes suggests that the AMC polyene backbone mostly lies along the surface normal because the intensity ratio of the asymmetric to symmetric methyl stretch is largest in the film. In such a bonding geometry the local C3V symmetry axis of the methyl groups lies roughly parallel to the surface. The result is an increase in the intensity of the out-of-plane asymmetric methyl stretching mode and decrease in the in-plane

2936 J. Phys. Chem. B, Vol. 106, No. 11, 2002

Liu et al. Dr. Tatyana Konovalov for calculating the structure of AMC and preparing Figure 4. We also thank Todd Morris for making the ellipsometry measurements. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the U.S. Department of Energy under Grant No. DE-FG02-86-ER13465 to L.K. and DE-FG02-93ER14404 to D.G., T.M., and A.M. References and Notes

Figure 4. Proposed structure of AMC in monolayer films on Au surfaces.

stretching mode. To account for the C:S ratio determine by XPS we expect the benzene rings must be aligned parallel with respect to each other (i.e., the plane of the ring lies roughly along the surface normal). A schematic diagram of the proposed molecular orientation19 of AMC in the SAM on Au is shown in Figure 4. We do not intend to imply that all AMC molecules are in the same orientation. There is certainly some twist and tilt of the polyene backbone. Conclusions The infrared spectroscopy, contact angle, UV-Vis spectroscopy, ellipsometry, and XPS data indicate that AMC molecules self-assemble on gold surfaces. The monolayers are less ordered than alkanethiols due to steric hindrance along the polyene backbone and the bulky terminal β-ionone ring group, but still present a hydrophobic interface. Analysis of the positions of the C-H stretching modes indicates that the polyene chains are disordered. XPS spectra suggest that AMC molecules adsorb on Au surfaces as thiolates. Acknowledgment. We thank Dr. Elli Hand for helpful discussions. We are grateful to Professor Shane C. Street’s group for their help with FTIR-RAS measurements. Contact angles were measured in Professor M. Robert Metzger’s lab. We thank

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