ARTICLE pubs.acs.org/Langmuir
Formation Process and Solvent-Dependent Structure of a Polyproline Self-Assembled Monolayer on a Gold Surface Ying Han,† Hidenori Noguchi,†,‡ Kazuyasu Sakaguchi,† and Kohei Uosaki*,†,‡ † ‡
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan International Center for Materials Nanoarchitectonics (MANA), National Institute of Materials Science (NIMS), Tsukuba 060-0810, Japan ABSTRACT:
The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.
’ INTRODUCTION Ordered organic molecular layers on solid surfaces have attracted considerable attention because of their wide range of potential applications, such as sensors and molecular and biomolecular electronic devices. Very organized and compact molecular layers, i.e., self-assembled monolayers (SAMs), are spontaneously formed on a solid surface in liquid and gas phases through the adsorption of the surface-active group to the solid surface and lateral interaction among adsorbed molecules. Thiol SAMs on gold are the most studied system among the numerous reports on SAMs on various solid surfaces.15 To obtain SAMs with highly controlled functions, it is essential to form the ordered SAMs without defect and, therefore, to understand the formation process of SAMs. Formation processes of alkanethiol SAMs have been studied using various techniques, such as infrared (IR) spectroscopy,68 quartz crystal microbalance (QCM),9,10 scanning tunneling microscopy (STM),1114 second harmonic generation (SHG) spectroscopy,1517 sum frequency generation (SFG) spectroscopy,18,19 ellipsometry,20,21 contact-angle measurement,20,21 surface plasmon resonance (SPR),2224 and reductive desorption.25,26 It has been confirmed that the SAM formation is proceeded first by rapid adsorption followed by slow reorganization.9,25 r 2011 American Chemical Society
Peptides are one of the most important candidates for surface modification of solid surfaces because they are expected to form various well-defined structures, such as α helix and β sheet. There are many reports about modification of solid surfaces by peptides with various functions,2754 although only a few reports are available on the formation process of peptide SAM.5559 Kimura et al. introduced functionalized α-helix peptide SAMs on a gold surface to study the electron-transfer process and photocurrent generation.2736 Kraatz et al. studied various helices on a gold surface.3743 Interestingly, they found that the electron-transfer property is strongly affected by secondary structures. Polyproline is a very interesting peptide. A study on the electron-transfer property of ferrocenoyl-oligoproline-cystamine [Fc-Pron-CSA]2 (n = 06) SAMs37 showed a very low decay constant, β, for the polyproline chain. Furthermore, its conformation varies dramatically with the solvent. For example, it takes PPI and PPII helices in methanol and water, respectively. The PPI helix is a right-handed helix with an axial translation of 1.9 Å, composed of 3.3 residues per turn, and the PPII helix is a Received: June 5, 2011 Revised: August 20, 2011 Published: September 08, 2011 11951
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Langmuir left-handed helix with an axial translation of 3.1 Å, composed of 3.0 residues per turn.6064 Consequently, it is expected that the electron-transfer property of the polyproline-modified electrode can be controlled by changing the solvent. Thus, it is very important to investigate the formation process and solventdependent structure of the proline SAM. In this work, we have investigated the formation process of the lipoic-acid-terminated polyproline SAM on a gold surface using various techniques. The amount of polyproline molecules on a gold surface was determined by the area of the reductive desorption peak, and orientation and thickness of polyproline SAM with adsorption time were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry, respectively. The kinetics of the polyproline SAM formation process was discussed on the basis of these results. Finally, the conformation change of the polyproline SAM by changing the solvent from water to methanol and methanol to water was confirmed by in situ IR measurement.
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Figure 1. Currentvoltage curves of the Au(111) single-crystal electrode, which was modified by immersing in aqueous solution containing 10 μM Lip-G-P15-W-NH2 for various time periods, measured in 0.5 M KOH. The sweep rate is 10 mV/s.
’ EXPERIMENTAL SECTION Gold wire (99.999%, ϕ = 1 mm) was obtained from Tanaka Precious Metal. KOH (semiconductor grade) was purchased from Aldrich. Fmocamide resin was purchased from Novabiochem. Fmoc-L-Trp(Boc)-OH, Fmoc-L-Pro-OH, Fmoc-Gly-OH, L-proline-N-Fmoc-15N, lipoic acid, N-ethyldiisopropylamine (DIEA), piperidine, trifluoroacetic acid (TFA), dichloromethane, and 1-methyl-2-pyrrolidine (NMP) were purchased from Wako Pure Chemicals. O-(Benzotriazol-1-yl)-N,N,N0 , N0 -tetramethyluronium hexafluorophosphate (HBTU) and 1-hydroxybenzotriazole, anhydrous (HOBT) were purchased from Watanabe Chemical. The synthesis of lipoic acid-glycine-proline15-tryptophan-NH2 (Lip-G-P15-W-NH2) was performed by Fmoc solid-phase synthesis.65 The protecting group of the side chain was deprotected when the peptide was cleaved from the resin by the reaction with 95% TFA and 5% H2O solution. The crude peptide was purified by reversed-phase high-performance liquid chromatography (RP-HPLC). The peptide was identified by matrixassisted laser desorption ionizationtime of flight (MALDITOF) mass spectrometry. The 15N isotopic polyproline, lipoic acid-proline(15N)9-NH2 (Lip-P(15N)9-NH2), was also synthesized using the same method to assign the IR band. A Au(111) single-crystal electrode for electrochemical measurement was prepared as described before.44 A Au(111) single-crystal disk for ellipsometry measurement was purchased from Surface Preparation Laboratory. The Au(111) surface was flame-annealed in a hydrogen flame and gradually cooled to room temperature in air prior to each measurement. An electroless deposited gold film on silicon prism66 was used in the ATR-IR measurement. Lip-G-P15-W-NH2 SAM was prepared by immersing the gold substrate in H2O or D2O solution containing 110 μM Lip-G-P15-WNH2. Water was purified using a Milli-Q water purification system (Yamamoto, WQ-500). Electrochemical measurements were carried out in a three-compartment electrochemical cell with a hanging meniscus configuration in a 0.5 M KOH aqueous solution using a potentiostat (Hokudo Denko, HA-151) and a function generator (Toho Technical Research). Linear sweep voltammograms (LSVs) were recorded using an X-Y recorder (Rikadenki, RW-21). The electrode potential was referred to as an Ag/ AgCl (saturated KCl) reference electrode, and a Pt wire was used as a counter electrode. The electrolyte solution was deaerated by bubbling ultrapure Ar gas (99.9995%, Air Water) for at least 30 min before each experiment. An in situ real-time ellipsometric measurement was carried out at an angle of incidence of 65° on the Au(111) surface with a light of 450 nm wavelength from a xenon lamp using Sopra GESP-5. A Teflon cell with
Figure 2. Immersion time dependencies of reductive charge density (b) and desorption peak potential (2) of SAM prepared in 10 μM Lip-G-P15-W-NH2 aqueous solution. The inset shows the initial stage of the relation. two quartz crystal windows, which are perpendicular to the incident and reflect light, was used. After we made sure that the values of tan Ψ, cos Δ, and R were keep constant for bare gold in water, 1.14 mL of 150 μM Lip-G-P15-W-NH2 H2O solution was added in 16 mL of water to make 10 μM diluted solution. An in situ ATR-IR measurement was carried out at incidence angle of 60°, using a Bio-Rad, FTS-30 spectrometer with a MCT (HgCdTe) detector cooled by liquid nitrogen. The integration time for one spectra was 80 (ca. 1 min). After a stable signal of bare gold was obtained in D2O, 72 μL of 15 μM Lip-G-P15-W-NH2 D2O solution was added to 1 mL of D2O to prepare a D2O solution containing 1 μM Lip-G-P15-W-NH2. A relatively low concentration was chosen to follow the growth process in detail because it takes 1 min to obtain one spectrum.
’ RESULTS AND DISCUSSION Adsorption Kinetics of Lip-G-P15-W-NH2 by Reductive Desorption Measurement. Figure 1 shows typical LSVs of a
Au(111) single-crystal electrode, which was modified by being immersed in a 10 μM Lip-G-P15-W-NH2 aqueous solution for various periods of time at room temperature, obtained by sweeping the potential from 0 to 1.2 V in 0.5 M KOH aqueous solution at the sweep rate of 10 mV s1. Sharp reductive peaks were observed between 0.8 and 0.9 V. These peaks should 11952
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correspond to the desorption of Lip-G-P15-W-NH2 SAM from the gold surface as
It is clear that the area and the position of peak increased and shifted negatively, respectively, with the immersion time, and the results are summarized in Figure 2. The reductive charge increased quickly initially up to ca. 10 min and then slowly and reached 35 μC/cm2, which corresponds to the coverage of 1.1 1014 molecules/cm2.67 On the basis of this value, the crosssection of each molecule is calculated to be 92 Å2. Thus, if we consider that the polyproline molecule is well-packed on the gold surface as a cylindrical structure, the diameter of one polyproline molecule adsorbed on the gold surface is ca. 10 Å, which is a little larger than that of the polyproline helix with a PPII secondary structure that is 89 Å.63 This suggests that the polyproline molecule did not stand vertical to surface normal. The reductive desorption peak shifted negatively with the immersion time and reached 0.9 V at the maximum coverage. The negative shift of the reductive desorption peak with immersion shows that there exists an interaction among adsorbed polyproline molecules. The change of the peak shift was clearly observed for 60 min, which shows that this process is slower than the adsorption kinetics determined by reductive desorption charge. Porter et al. reported that the reductive peak position of alkanethiol SAM on a Au(111) surface is linearly related with the chain length with a slope of 20 mV/carbon number when the data were recorded at 100 mV/s in 0.5 M KOH.69 As reported in the previous work, the reductive desorption peak position of decanethiol SAM was 1.0 V.26 Using these data and the relation between the reductive peak position and n-alkanethiol chain length, the peak position of the polyproline SAM at the maximum coverage is close to that of pentanethiol (C5SH) SAM. It means that the interaction between the peptide chains are much less than that between alkyl chains. We assume that the formation process of SAM can be described by simple Langmuir adsorption kinetics in the initial stage as described by dθ ¼ kobs ð1 θÞ dt
Figure 3. Immersion time dependencies of ln(1 θ) of the SAM.
ð2Þ
where θ is the monolayer coverage and kobs is the apparent rate constant. By integrating eq 2, one obtains lnð1 θÞ ¼ kobs t
ð3Þ
Figure 3 shows the relation between ln(1 θ) and the immersion time. A linear relation between ln(1 θ) and the immersion time is obtained in the initial stage, where we can assume that there is no interaction between the adsorbed molecule. Deviation from the linear relation was observed when θ was higher than 0.8. The value of kobs is 0.44 min1, which is very close to the value obtained for decanethiol SAM formation on a Au(111) surface in 10 μM ethanol solution (0.52 min1). Structural Developments of Polyproline SAM Studied by Ellipsometry. In situ ellipsometry at the Au(111) surface in 10 μM Lip-G-P15-W-NH2 aqueous solution was carried out. The Au(111) disk was mounted in a cell, and ellipsometry was carried out in Milli-Q water. Using Ψ = 27.96 (tan Ψ = 0.531) and Δ = 87.68 (cos Δ = 0.041) obtained in water, the complex refractive index of the bare gold electrode at 450 nm is calculated
Figure 4. Immersion time dependencies of (a) tan Ψ, (b) cos Δ, and (c) ΔR/R of the Au(111) surface in 10 μM Lip-G-P15-W-NH2 aqueous solution.
to be n = 1.585 1.839j, which is in good agreement with the previously reported value, i.e., n = 1.562 1.904j at 442.8 nm.70 After stable signals were obtained, the concentrated solution of Lip-G-P15-W-NH2 was added to the cell, so that the concentration of Lip-G-P15-W-NH2 became 10 μM. Figure 4 shows the time dependencies of ellipsometric parameters of tan Ψ, cos Δ, and ΔR/R after the addition of concentrated Lip-G-P15-W-NH2 aqueous solution. δΨ, δΔ, and ΔR/R increased immediately after the injection of concentrated Lip-G-P15-W-NH2 aqueous solution. From the values of δΨ, δΔ, and ΔR/R, the thickness, df, and the complex refractive index, n = nf jkf, of the layer can 11953
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Table 1. Peak Position of Amide I and CN Stretching Bands of Lip-G-P15-W-NH2 under Various Conditions amide I
CN stretching
band/cm1
band/cm1
powder in D2O
1645 1622
1430 1458 1392
in methanol-d4
1640
SAM formed in D2O
1630
1453
SAM formed in methanol-d4
1645
1449
SAM formed in D2O and then
1641
1448
1640
1454
immersed in methanol-d4 SAM formed in methanol-d4 and then immersed in D2O
Figure 5. Immersion time dependencies of (a) nf, (b) kf, and (c) df of the SAM formed in a 10 μM Lip-G-P15-W-NH2 aqueous solution.
Figure 7. Transmission IR spectra of the powder of Lip-G-P15-W-NH2 (top) and Lip-P(15N)9-NH2 (bottom).
Figure 6. IR spectra of the gold surface during the adsorption process in 1 μM Lip-G-P15-W-NH2 D2O solution. Spectra obtained 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 17, 21, 26, 31, 36, 44, 52, and 60 min after Lip-G-P15W-NH2 was introduced are shown from the bottom to the top.
be calculated using a three-phase model with solution, a flat homogeneous layer, and Au.71 Figure 5 shows the immersion time dependencies of calculated values of df, nf, and kf of the SAM. All of the values increased with immersion time as a result of the formation of the polyproline SAM on the Au(111) surface. While the thickness of the SAM became constant at 5 nm after 12 min, nf and kf increased more slowly and took 2 h to reach the constant values of 1.52 and 0.02, respectively. The different behavior between the thickness and refractive index curve shows that the reorganization of the monolayer requires a very long time. Structural Developments of Polyproline SAM in D2O Studied by ATR-IR. To investigate the detail of the formation process, an ATR-IR measurement was performed. Figure 6
shows Fourier transform infrared (FTIR) spectra of the gold surface in the region of 18001200 cm1 during the formation process of the Lip-G-P15-W-NH2 SAM in 1 μM Lip-G-P15-WNH2 D2O solution. Various absorption bands, including two dominant bands around 1628 and 1453 cm1, were observed, and they grew with immersion time. The band around 1628 cm1 can be assigned to the amide I in PPII conformation.6163 In comparison to the position of the peak corresponding to amide I in PPII conformation in water, that is 1622 cm1, the peak appeared at higher frequency. It is known that the peak position of amide I of peptide shifts to a higher frequency if the peptide becomes solid. Actually, the peak position of Lip-G-P15-W-NH2 powder was observed at 1645 cm1, as shown in Table 1. Thus, the peak shift to a higher frequency upon the formation of the SAM shows that the Lip-GP15-W-NH2 SAM is well-packed. Furthermore, the peak of the amide I band slightly shifted from 1628 cm1 at the beginning of the SAM formation to 1630 cm1 after 60 min, showing that the order of the SAM became slightly higher as the coverage increased. There are, however, two possibilities in the assignment of the peak at 1453 cm1. This band could be due to either CN stretching of the amide bond or CH2 bending. To assign this peak, transmission IR spectrum of Lip-G-P15-W-NH2 powder was compared to that of the 15N-labeled polyproline molecule, Lip-P(15N)9-NH2, powder as shown in Figure 7. The band observed at 1430 cm1 for Lip-G-P15-W-NH2 shifted to 1415 cm1 for Lip-P(15N)9-NH2, while the position of other 11954
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Figure 9. Immersion time dependence of the tilt angle of the polyproline helix axis of Lip-G-P15-W-NH2 SAM to surface normal in 1 μM Lip-G-P15-W-NH2 D2O solution.
Figure 8. Immersion time dependencies of the absorbance of (a) amide I band and (b) CN stretching and (c) the ratio between the absorbance of amide I and that of CN stretching bands of SAM in 1 μM Lip-G-P15-W-NH2 D2O solution.
peaks remained the same, suggesting that the band at 1430 cm1 has the contribution of a N atom. To confirm the band to be due to CN stretching, simple consideration based on Hooke’s law is carried out as follows. The peak frequency of the CN stretching band, ν, is given by sffiffiffi 1 k v¼ ð4Þ 2π μ where k is the force constant and μ is the reduced mass of C and N, which is given by μ¼
mC mN mC þ mN
ð5Þ
where mC and mN are the masses of C and N atoms, respectively. Thus, the ratio between the frequency of C14N stretching and that of C15N stretching is given by sffiffiffiffiffiffiffiffiffi v14 N m15 N ð6Þ ¼ v15 N m14 N According to eq 6, the C15N stretching band should be redshifted by 22 cm1 compared to the C14N stretching band. Rothschild et al. showed that incorporation of 15N-proline in bacteriorhodopsin resulted in an isotopic red shift of the CN stretching band in the 14201440 cm1 region by 15 cm1.72
Figure 10. IR spectra of Lip-G-P15-W-NH2 SAMs (a) formed in 1 μM Lip-G-P15-W-NH2 D2O solution (solid line) and after replacing solvent by pure methanol-d4 (dashed line) and (b) formed in 1 μM Lip-G-P15W-NH2 methanol-d4 solution (solid line) and after replacing solvent by pure D2O (dashed line).
Thus, it is reasonable to consider that the peak at 1453 cm1 is mainly due to CN stretching of the amide bond. Figure 8 shows immersion time dependencies of absorbance of the (a) amide I and (b) CN stretching band, respectively. Absorbance of amide I and CN stretching peaks increased with immersion time and became constant after 6080 min of immersion. The adsorption rate followed by the IR measurement 11955
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Figure 11. Schematic model of the formation process of Lip-G-P15-W-NH2 SAM on a gold surface in Lip-G-P15-W-NH2 D2O solution.
is slower than those determined by the reductive desorption (Figure 2) and ellipsometry (Figure 5) because the concentration of the polyproline molecule in the IR measurement (1 μM) was much lower than those used for the reductive desorption and ellipsometry (10 μM). It is interesting to note that the amide I peak increased and reached the saturated value faster than the CN stretching peak, resulting in the monotonous decrease of the ratio of the integrated absorbance of amide I to that of the CN stretching peak with immersion time, as shown in Figure 8c. This result indicates that the orientation of the polyproline SAM changed during the formation process. The tilt angle of the polyproline helix axis of Lip-G-P15-W-NH2 SAM to surface normal, which can be determined by the ratio of the integrated absorbance of amide I to the CN stretching peak,27,50,59 became smaller with the immersion time and reached a constant value of 32° after ca. 80 min, as shown in Figure 9. Conformational Change of Polyproline SAM Induced by Solvent Exchange. It is interesting to see whether the conformation of polyproline SAM changes by changing solvent because it is known that the conformation of polyproline depends upon the solvent, as mentioned before. The positions of both the amide I6063 and CN stretching73 bands are known to depend upon PPII and PPI conformations. Peak positions of these peaks of Lip-G-P15-W-NH2 obtained in D2O (PPII conformation) and methanol-d4 (PPI conformation) solution are summarized in Table 1. Figure 10 shows the spectra change of polyproline SAM by replacing solvent from (a) D2O to methanol-d4 and (b) methanol-d4 to D2O. The peak positions of both amide I and CN stretching bands changed when the solvent was exchanged. The amide I band shifted toward a higher frequency by 13 cm1, and the CN stretching band shifted toward a lower frequency by 5 cm1, when the solvent was replaced from D2O to methanol-d4 for SAM formed in 1 μM Lip-G-P15-W-NH2 D2O solution, as shown in Figure 10a. On the other hand, the amide I band shifted toward a lower frequency only by 5 cm1, and the CN stretching band shifted toward a higher frequency by 5 cm1, by replacing solvent from D2O to methanol-d4 for SAM formed in 1 μM Lip-G-P15-W-NH2 methanol-d4 solution, as shown in Figure 10b. These shifts show that the SAM changes its conformation from the one formed in one solvent to other in the replaced solvent. However, although the peak positions after the solvent was changed from D2O to methanol-d4 are very close to those of the SAM formed in methanol-d4, those after the solvent was changed from methanol-d4 to D2O are not so close to those of the SAM formed in D2O, showing that the conformation change for PPII to PPI takes place more readily. This may be due to the stronger interaction among peptide chains in the SAM formed in methanol-d4.
’ CONCLUSION The formation process of Lip-G-P15-W-NH2 SAM on a gold surface was investigated by several techniques, including
electrochemical reductive desorption, ellipsometry, and ATR-IR. The reductive desorption charge increased with the immersion time and reached 90% of full coverage in 10 min, followed by a very slow increase, reaching a saturated coverage. The reductive peak position shifted negatively with the immersion time, showing that there are attractive interactions between polyproline molecules. The change of the peak shift was clearly observed for 60 min, which shows that this process is slower than the adsorption kinetics determined by reductive desorption charge. The position of the reductive peak at the maximum coverage was 0.9 V, which is almost the same as that of C5SH SAM on the Au(111) surface, suggesting that the interaction between polyproline molecules is smaller than that between alkylthiol molecules. The thickness of polyproline SAM was increased with the immersion time and became constant at 5 nm in 12 min, but the refractive index of polyproline SAM increased very fast in a few minutes, followed by a slow increase to 1.53. The IR results show that the polyproline helix axis with respect to surface normal became smaller as the amount of the adsorbed molecules increased and the packed monolayer with the orientation of 32° to surface normal at the maximum coverage was formed after prolonged immersion. The SAM formation process in D2O was schematically shown in Figure 11. Both polyproline SAMs formed in D2O and methanol-d4 solutions show conformation change by exchanging solvent but to different extents, with the larger change for the SAM forming in the D2O solution.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +81-29-860-4301. Fax: +81-29-851-3362. E-mail:
[email protected].
’ ACKNOWLEDGMENT The present work was partially supported by a Grant-in-Aid for Scientific Research on the Innovative Area of “Molecular SoftInterface Science” (2005), the Global COE Program (Project B01: Catalysis as the Basis for Innovation in Material Science), and the World Premier International (WPI) Research Center Initiative on Materials Nanoarchitectonics from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. Ying Han is supported by a MEXT scholarship for foreign students. ’ REFERENCES (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105 4481–4483. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (3) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152–7167. (4) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510–1514. 11956
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