ARTICLE pubs.acs.org/JPCC
Phenylboronic Ester- and Phenylboronic Acid-Terminated Alkanethiols on Gold Surfaces Cecilia Vahlberg, Mathieu Linares, Patrick Norman, and Kajsa Uvdal* The Division of Molecular Surface Physics and Nano Science and Division of Computational Physics, Department of Physics, Chemistry and Biology (IFM), Link€oping University, SE-581 83 Link€oping, Sweden
bS Supporting Information ABSTRACT: In this work, it is shown that a well-organized monolayer of phenylboronic ester-terminated thiol (BOR-capped) on gold surfaces can be prepared. Our results also show that the BOR-capped molecular system can be cleaved directly on the surface, resulting in an unprotected BOR-uncapped monolayer with the boronic acid functional groups available for coordination to diol molecules in the ambient media. The monolayers of BOR-capped and BORuncapped were characterized using infrared spectroscopy, near edge X-ray absorption fine structure spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and contact angle goniometry. The X-ray photoelectron spectroscopy results showed that both BOR-capped and BOR-uncapped are chemically linked to the gold substrate. According to the infrared spectroscopy results, the main component of the CdO vibrational mode present in the amide moiety is perpendicular oriented relative to the gold surface normal for the BOR-capped molecular system. The near edge X-ray absorption fine structure spectroscopy resonance peak located at approximately 285 eV, assigned to π1* transitions, was used to estimate the average tilt angle of the vector parallel to the π* orbitals of the aromatic ring relative to the gold surface normal. The average tilt angle is estimated to be approximately 63° for the BOR-capped monolayer on gold surfaces. The aromatic ring of the BOR-uncapped molecule has a more tilted orientation compared to the BOR-capped one. The experimental infrared spectroscopy and near edge X-ray absorption fine structure spectroscopy results were supported with theoretical modeling including calculations of vibrational modes and of excitation processes. monolayers terminated with methyl groups.15 Consequently, a stable and inert monolayer, which could be easily changed to a more reactive surface, would be valuable for biosensing and biorecognition studies. In this study, we show that well-ordered and stable monolayers terminated with a phenylboronic ester could be formed on gold substrates. Second, we show that the protection group (the pinacolyl group) of BOR-capped, chemisorbed on the surfaces could be removed, resulting in an unprotected boronic acid terminated thiol (BOR-uncapped) monolayer. The BOR-capped and BOR-uncapped monolayers on gold surfaces were investigated using infrared spectroscopy (IR), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle goniometry. This study is part of a series of spectroscopic investigations where we use experimental and theoretical studies to increase our knowledge of the characterized biomolecular systems.16
1. INTRODUCTION Many naturally occurring biological reactions and processes take place at the interfaces between surfaces and the surrounding media. Well-defined and highly ordered molecularly based surfaces are useful, e.g., biomolecular interaction studies at surfaces and interfaces, that is, biomolecular controlled selective processes as well as biorecognition studies.15 In order to fully understand the biointeractions investigated, characterization studies of the functionalized surfaces are valuable.15 The technique to use thiol chemistry to form self-assembled monolayers (SAMs) is well-established and has frequently been used for modifications of surfaces as alkane thiols easily form wellorganized molecular structures on surfaces.1,68 The terminating tail can be chosen and/or modified to control the properties of the SAM-modified surface. Boronic acids can form covalent bonds with organic molecules containing 1,2-diol and 1,3-diol groups such as neurotransmitters and different saccharides.9 Consequently, molecules based on boronic acid have a high potential for biosensor applications.1014 Surface immobilized boronic acid derivatives have recently been used to detect different compounds containing diol molecules such as glycated hemoglobin, galactose, and dopamine.1014 However, surfaces functionalized with boronic acid-terminated alkanethiols have a higher surface energy compared to surfaces functionalized with methyl-terminated alkanethiols, which results in that monolayers terminated with boronic acid groups are more sensitive for contamination compared to the more robust r 2011 American Chemical Society
2. MATERIALS AND METHODS 2.1. Preparation of Gold Substrate, Multilayers, and Monolayers. A phenylboronic ester-terminated thiol with the full Received: September 6, 2011 Revised: November 23, 2011 Published: December 02, 2011 796
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to the surface normal. The curve fit was done using the XPSPEAK95 program (version 2.0) with a GaussianLorentzian function (ratio 80:20). The Au (4f7/2) peak, positioned at 84.0 eV, was used to align all the XPS spectra. The near edge X-ray absorption fine structure spectroscopy measurements were performed at beamline D1011 (MaxLab synchrotron light facility, Lund, Sweden) using partial retardation yield. The retardation voltage was set to 150 V for the C Kedge spectra. The energy resolution was ∼0.1 eV. The degree of linear polarization was >95%. The incidence angle of the plane polarized light used for the 4-APBE multilayer was 55° relative to the surface normal. Three different incidence angles of the X-ray light were used for the monolayer samples: 90° (E-vector parallel to the gold surface), 55° and 20° (E-vector near the surface normal) relative to the surface normal. Each NEXAFS spectrum was normalized to corresponding spectrum from sputtered clean gold surfaces, recorded at the very same angle. 2.3. Computational Details. All theoretical calculations were performed on molecules in vacuum adopting molecular structures that were optimized by means of density functional theory (DFT) in conjunction with the B3LYP19 exchange-correlation functional and Dunning’s correlation consistent triple-ζ basis set (cc-pVTZ).20 All stationary points were characterized as minima by evaluation of the analytic molecular Hessians to be followed by the determination of the infrared (IR) absorption spectra at the same level of theory. The presented theoretical IR frequencies represent values that are scaled by a factor of 0.97 to account for anharmonic corrections in an ad hoc manner. These calculations were performed with use of the Gaussian03 program.21 Calculations of pre-edge X-ray absorption spectra (XAS) were performed in the framework of the complex polarization propagator approach.22,23 In these calculations, an accurate valence core hole interaction is achieved by employment of the Coulomb attenuated method B3LYP (CAMB3LYP) functional with a parameter setting (α = 0.19; β = 0.81;, and μ = 0.33) that guarantees a correct asymptotic limit, see refs 2426 for a more detailed discussion and motivation. The augmented double-ζ basis set (aug-cc-pVDZ)20 was adopted for the XAS calculations, which is a choice that enables a good description of corevalence excitations. Due to the limited basis set chosen in these calculations, the description of Rydberg transitions is less accurate. However, it is less relevant for this molecular system, since Rydberg transitions are known to be quenched for large molecular system on surfaces.27 The theoretical XAS spectra have been overall shifted as to align the first strong pre-edge peak of each spectrum with that found in the corresponding experimental spectrum. These calculations were performed with use of the Dalton program.28
name 8-mercapto-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)phenyl)octanamide (BOR-capped) was kindly synthesized by L. Johansson (IFM, Link€oping University). Information regarding the synthesis can be found in the Supporting Information. Gold samples were prepared and monolayers were formed through the following procedures: Si(100) surfaces were washed in TL1 solution containing Milli-Q water/NH3/H2O2 (5:1:1) at approximately 80 °C for 15 min, to remove organic contaminations, and then carefully rinsed with Milli-Q water and dried in N2 gas before being inserted in an electron beam evaporation system. The Si(100) surfaces were first precoated with a 2.5 nm thick Ti layer at a rate of ∼0.2 nm/s and then coated with a 200 nm thick Au layer at a rate of ∼1 nm/s. The base pressure was ∼109 Torr, and the evaporation pressure was ∼107 Torr. The gold surfaces were washed in TL1 solution, to remove possible hydrocarbon contamination, and then carefully rinsed with Milli-Q water. Ellipsometry is used to confirm the cleanness of the TL1 washed surfaces. The gold surfaces were rinsed with ethanol and then immediately placed in the ethanol based BORcapped incubation solution (0.1 mM). The incubation was done in room temperature, and the solution was kept in darkness to avoid possible light initiated oxidation processes. The incubation time was approximately 24 h. The pinacolyl functional group of BOR-capped was removed after chemisorption by incubation in a deprotection solution (9:1 CH3CN/1 M HCl, excess of 3-carboxyl phenyl boronic acid)17 for 24 h. After the surfaces have been incubated in the BOR-capped solution and the deprotection solution, they were rinsed in ethanol, sonicated for 15 min in ethanol, rinsed again, tried in nitrogen gas, and then, immediately placed in the instruments. 4-APBA is diluted in ethanol. Multilayers for transmission IR measurements were prepared by making a solution cast film on a CaF2 window. 2.2. Monolayer and Multilayer Characterization. Infraredreflection absorption spectroscopy measurements were performed on a Fourier transform spectrometer (Bruker, model IFS66). This spectrometer is equipped with a mercury cadmium telluride (MCT) detector and has a grazing angle of incidence reflection accessory aligned at 85°. Liquid nitrogen was used to cool the detector before and during the measurements. Transmission IR measurements were performed on a Vertex 70 instrument. The analysis chambers were purged with nitrogen gas before and during the measurements to reduce the contribution of carbon dioxide and water. The resolution was 2 cm1 for both the IRAS and TR measurements. A three-term BlackmannHarris apodization function was applied to the interferograms before Fourier transformation. The thicknesses of the monolayers of BOR-uncapped and BOR-capped were measured with single wavelength (λ = 632.8 nm, HeNe laser) null ellipsometry. The ellipsometer is an automatic Rudolph Research AutoEl ellipsometer with an angle of incidence of 70°. All measurements were performed in air. The refractive index of clean gold surfaces was measured prior to the incubation. The refractive index of the monolayers of BOR-capped and BOR-uncapped is assumed to be 1.5.18 The static water contact angles were measured using a CAM200 goniometer in air. High-resolution core level X-ray photoelectron spectroscopy measurements were performed using a Scienta ESCA 200 spectrometer at Link€oping University. The pressure in the analysis chamber was ∼1010 Torr. The measurements were based on photoelectrons with takeoff angles of 20° with respect
3. RESULTS AND DISCUSSION In this work, it is studied how a protected phenylboronic acid molecular monolayer such as BOR-capped can be prepared and how the protecting group in a second step can be cleaved directly from the monolayer on the surfaces, to obtain a fresh unprotected phenylboronic acid-terminated molecular monolayer, that is, BOR-uncapped. This results in a molecular monolayer, with functional groups, in this case boronic acid, available for further linking procedures. Multilayer samples of 4-aminophenyl boronic acid pinacol ester (4-APBA) were measured as a reference. The chemical structures of BOR-capped, BOR-uncapped, and 797
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Figure 1. Chemical structures of (A) 4-APBA, (B) BOR-capped, and (C) BOR-uncapped.
4-APBA are presented in Figure 1. A monolayer of BOR-capped was anchored to the gold surfaces using thiolate formation. In a second step, a monolayer of BOR-uncapped was formed by removing the pinacolyl functional group of the BOR-capped monolayer on the surface by transesterification.17 The molecular orientation, the moleculesurface interaction, the thickness, and wettability were investigated, before and after the deprotection procedure, using infrared spectroscopy (IR), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle goniometry. The IR and NEXAFS results are compared with theoretical calculations to further strengthen and support our experimental measurements and our assignments. 3.1. X-ray Photoelectron Spectroscopy. High-resolution XPS was used to investigate the molecularsurface interaction of both BOR-capped and BOR-uncapped adsorbed on gold surfaces. The XPS S(2p) spectra of BOR-capped and BORuncapped recorded in bulk sensitive mode are shown in Figure 2A,B. The S(2p) spectrum for BOR-capped shows two peaks with the binding energy positions at 162.0 and 163.2 eV (Figure 2A). We assign these peaks to S(2p3/2) and S(2p1/2), respectively. This split of the S(2p) peak is due to spinorbit interactions.29 The binding energy positions of the two S(2p) peaks are characteristic for thiolate sulfur species strongly bound to the gold substrate, and this is in good agreement with previous high-resolution XPS results on self-assembled monolayers on gold.3033 In the case of BOR-uncapped, the following results are obtained. The S(2p) core level XPS spectrum of BOR-uncapped adsorbed on a gold surface shows a line shape close to what was
Figure 2. S(2p) core level XPS spectra of (A) BOR-capped and (B) BOR-uncapped on gold surfaces recorded using bulk sensitive mode.
observed for BOR-capped (Figure 2B). For BOR-uncapped adsorbed on gold, the same type of pronounced S(2p) spinorbit doublet is observed with binding energy peak positions for S(2p3/2) and S(2p1/2) found at 162.0 and 163.2 eV, respectively. This is consistent with the results for BOR-capped and indicates that the monolayer is still anchored to the surface after the deprotection procedure. Consequently, the strong molecular surface interaction is retained during the removal of the pinacolyl functional group. It should be noted that only thiolate sulfur species are detected for the monolayers of BOR-capped and BOR-uncapped. Neither oxidized species nor unbounded sulfur species are present. Pure BOR-capped and BOR-uncapped monolayers are thus present on the surfaces. This indicates a nice and gentle preparation technique to produce robust BORcapped monolayers, followed by an effective deprotection and cleaning procedure to obtain pure BOR-uncapped monolayers. The O(1s) spectra of BOR-capped and BOR-uncapped, recorded in bulk sensitive mode, are shown in Figure 3A,B. The O(1s) XPS spectrum of BOR-capped (Figure 3A) shows one main peak and a pronounced shoulder at the low binding energy side of the main peak. Two peaks are fitted to the experimental data, and corresponding binding energy peak positions, estimated to be about 532.6 and 531.1 eV, are obtained based on the curve fit. We assign the main O(1s) peak with binding energy position at 532.6 eV to the oxygen present in the boronic ester functional group. The peak with the binding energy 798
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Figure 3. O(1s) core level XPS spectra of (A) BOR-capped and (B) BOR-uncapped on gold surfaces recorded using bulk sensitive mode. Figure 4. IR spectra of 4-APBA: (A) experimental transmission spectrum of multilayer on a CaF2 window and (B) theoretical spectrum obtained at the DFT/B3LYP/cc-pVTz level of theory.
position at 531.1 eV is assigned to the oxygen present in the amide moiety, in good agreement with earlier XPS studies of CH3(CH2)11NHCOCH2SH34 and of a tyrosine terminated thiol32 on gold surfaces. The O(1s) XPS spectrum of BORuncapped (Figure 3B) also shows one main peak and a pronounced shoulder at the low binding energy side of the main peak. Two peaks are fitted to the experimental data. The binding energy positions of the two peaks are estimated to be about 532.4 and 531.1 eV. We assign the peak at 532.4 eV to the oxygen present in the boronic acid functional group. This is in good agreement with a previous XPS study on HS(CH2)11B(OH)2 SAMs on Au surfaces by Carey et al.15 The peak located at 531.1 eV is assigned to the oxygen present in the amide moiety, in agreement with our assignment for BOR-capped. 3.2. Infrared Spectroscopy, Ellipsometry, and Contact Angle Goniometry. Infrared spectroscopy was used to investigate BOR-capped and BOR-uncapped monolayers on gold substrates and to identify functional groups present in the two molecules. Molecular orientation for both molecular systems are obtained using the surface selection rule35 and complementary results from XPS and NEXAFS. 4-Aminophenylboronic acid pinacol ester (4-APBA) (see chemical structure in Figure 1A) was measured as a reference. The experimental IR results and assignments were supported with theoretical IR calculations. In the experiment, we should be aware that not only intramolecular vibrations but also intermolecular
interactions are present and introduce modifications on the spectral results as shifts, intensity variations, or new additional vibrational modes. As a consequence, there are shifts and variations in the intensities when comparing the results obtain from the experiment and the calculated IR data for some of the resonances. This is expected, since the theoretical calculations are based upon single isolated molecules. The transmission (TR) spectrum for 4-APBA multilayers and the corresponding theoretical IR spectrum are shown in Figure 4A and B, respectively. A detailed summary of the proposed assignments is presented in Table 1. There are two vibrational bands near 1629 and 1604 cm1. We assigned these vibrational bands to the NH2 scissoring vibrational mode36 and the aromatic CC symmetric stretching vibrational mode.16 The stretching CN vibrational mode also contributes to the band centered at 1629 cm1 The vibrational bands located at 1399 and 1389 cm1 are assigned to the bending vibrational mode of the methyl groups.37,38 There is a large difference in the intensities of the vibrational bands centered at approximately 1399 and 1389 cm1 when comparing the experimental and calculated spectra. The vibrational bands are clearly observed in the experimental spectrum but are only noticeable in the calculated spectrum. There is a strong intensity vibrational band detected at 799
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Table 1. Proposed IR Assignments of Multilayers of 4-APBA on a CaF2 Windowa experimental ν (cm1) CaF2 tablett (TR)
theoretical ν (cm1) intensity
assignment
1629
1611
0.36
NH2 sci + CN str +
1604 1399
1597
0.11
NH2 sci + Ar CC sym str CH3 umbrella
1342
1.00
Ar CB str + BO sym str
Ar CC sym str
CH3 umbrella
1389 1359 1311 1301
1304
0.14
BO asym str
1271
1274
0.17
CN str
1144
1136
0.34
CO str
1088
1079
0.17
BO sym str + Ar CC str
a
The experimental measurements and assignments are supported with theoretical calculations.
approximately 1359 cm1. We assign this vibrational band to the BO stretching vibrational mode39 and the aromatic CB stretching mode. The bands located at approximately 1311 and 1301 cm1 are assigned to the BO stretching vibrational mode based on our theoretical calculations. The vibrational band at 1271 cm1 is assigned to the stretching vibrational mode in the CN bond36 and the vibrational band found at 1144 cm1 is assigned to the stretching vibrational mode in the CO bond.40 The vibrational band at approximately 1088 cm1 is assigned to the BO stretching vibrational mode and the aromatic CC stretching vibrational mode based on our theoretical calculations. The experimental IR spectrum of the multilayer of BORcapped measured in transmission mode is presented in Figure 5A, and an IRAS spectrum of the BOR-capped monolayer on gold surfaces is presented in Figure 5B. The theoretical calculated IR spectrum of BOR-capped is presented in Figure 5C. Information can be obtained regarding the orientation of the amide moiety and the aromatic ring relative to gold surface normal by investigating intensity variations of these vibrational bands in the TR spectrum and the IRAS spectrum. This is based on the surface selection rule, which states that only vibrational modes with a transition dipole moment vector component that has a parallel orientation relative to the surface normal will be observed.35 Vibrational bands detected in the spectra that contribute with information regarding the molecular orientation are discussed in the following section. A detailed summary of the proposed assignments is presented in Table 2. The vibrational band located 1664 cm1 in the TR spectrum of BOR-capped, see Figure 5A, is assigned to the CdO stretching vibrational mode in the amide moiety.41 The corresponding vibrational band is detected at approximately 1668 cm1 in the IRAS spectrum. The intensity of this vibrational band is low in the IRAS spectrum but high in the TR spectrum of multilayer of BOR-capped. This result indicates that the main component of the CdO vibrational mode is oriented perpendicular to the normal of the surface, according to the surface selection rule. There is a vibrational band observed at approximately 1609 cm1 in the TR spectrum, and there is a vibrational band at about 1608 cm1 in the IRAS spectrum. We assign these bands to the CC stretching vibrational mode42,43 the and NH bending vibrational mode in the amide moiety in agreement with our
Figure 5. IR spectra of BOR-capped: (A) experimental transmission spectrum of multilayer on a CaF2 window, (B) experimental IRAS spectrum of monolayer on gold substrates, and (C) theoretical spectrum obtained at the DFT/B3LYP/cc-pVTZ level of theory.
theoretical calculations. The main contribution to these vibrational bands originates from the aromatic CC stretching vibrational mode. In this case, the direction of the dipole transition moment of the aromatic CC stretching vibrational mode is parallel to the plane of the ring. The relative intensity of the vibrational band in the IRAS spectrum is higher compared to the 800
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Table 2. Proposed IR Assignments for BOR-capped Multilayers on a CaF2 Window (TR) and Chemisorbed on Gold Substrates (RA)a experimental ν (cml) CaF2 tablett (TR)
theoretical
Au (RA)
ν (cm1)
intensity
assignment
1664
1668
1708
0.18
1609
1608
1600
0.18
CdO str Ar CC sym str + NH bend
1593
1594
1570
0.15
NH bend + Ar CC str
1529
1536
1508
0.09
Ar CH bend + Ar CN str + Ar CB str + NH bend
1513
1514
1488
0.38
NH bend + NC str + Ar CN str + Ar CC str + Ar CH bend
1390
0.10
1400
Ar CC asym str + NH bend + Ar CH bend CH3 umbrella
1360
1378 1356
1340
BO str Ar CB str + BO str
1.00
1309
1310
0.10
BO asym str + Ar CC asym str + Ar CB str + Ar CH bend
1275
1266
0.07
Ar CC str + Ar CN str + CH2 wag + BO str
1227
0.09
1247
1250
Ar CN str + Ar CH bend + NH bend
1189 1144
Ar CH bend 1135
0.35
CO sym str
1120 1089 a
Ar CH bend 1082 1075
0.11 0.12
BO sym str + Ar CC str + CC str BO sym str + Ar CC str + CC str
The experimental measurements and assignments are supported with theoretical calculations.
vibrational band in the TR spectrum. This result indicates that the ring has a preferential orientation of the plane of the aromatic ring toward the gold surface normal according to the surface dipole selection rule. There is a vibrational band at approximately 1529 cm1 in the TR spectrum and a vibrational band at about 1536 cm1 in the IRAS spectrum. We assign these bands to the aromatic CH bending vibrational mode, the aromatic CN stretching vibrational mode, the aromatic CB stretching vibrational mode, and the NH bending vibrational mode in the amide moiety. The directions of the transition dipole moments for the aromatic ring vibrations are parallel to the ring plane. The relative intensity of the vibrational band in the IRAS spectrum is higher than the vibrational band in the TR spectrum. Consequently, this result also indicates that the ring has a preferred orientation of the plane of the aromatic ring toward the normal of the gold surface. The vibrational band observed at approximately 1144 cm1 in the TR spectrum is assigned to the CO stretching vibrational mode.40 This vibrational band is not detected in the IRAS spectrum (Figure 5B). This result indicates that the main component of the CO stretching vibrational mode has a parallel orientation relative to the gold surface when BOR-capped is adsorbed on the surface. There is a vibrational band centered at approximately 1120 cm1 in the IRAS spectrum, which we assign to aromatic CH bending.43 This vibrational band is not detected in the TR spectrum (Figure 5A). The orientation of the transition dipole moment for the aromatic CH bending is parallel to the plane of the aromatic ring. Thus, this result also indicates that the ring has an upright orientation relative to the gold surface. An experimental IRAS spectrum of a BOR-uncapped monolayer on gold surface and a theoretical IR spectrum of BORuncapped are presented in Figure 6A,B. Vibrational bands present in the experimental spectrum that contribute with new information regarding the result of the reorientation of the amide moiety and the ring structure are discussed in this following
section. A detailed summary of the proposed assignments of the vibrational bands is presented in Table 3. There is a vibrational band centered at approximately 1672 cm1. We assign this band to the CdO stretching vibrational mode in the amide moiety in agreement with our previous assignment for the TR and IRAS measurements on BOR-capped. This vibrational band is clearly observed in the IRAS spectrum of BOR-uncapped but had a very low intensity in the IRAS spectrum of BOR-capped, which indicates that the orientation of the amide moiety has changed during the deprotection procedure. Based upon the results, we estimate the angle between the main component of the CdO vibrational mode and the surface normal to be less than 90° for the BOR-uncapped molecular system. There are four vibrational bands observed in the region 15001610 cm1 in the IRAS spectrum of BOR-capped (Figure 5B). There are also four vibrational bands observed in the same region in the IRAS spectrum of BOR-uncapped (Figure 6A). Aromatic ring vibrations contribute to vibrational bands in this region; see the assignments in Tables 2 and 3. The main component of the aromatic CC stretching vibrational mode is parallel oriented relative to μ for the vibrational bands centered at approximately 1608 cm1 (Figures 5B and 6A). The direction of the dipole moment of the main component of the aromatic CC stretching vibrational mode is perpendicular to μ for the aromatic CC vibrational modes contributing to the vibrational bands at approximately 1594 cm1 (Figures 5B and 6A). There is a larger difference the intensity for the vibrational bands at approximately 1608 and 1594 cm1 in the IRAS spectrum of BOR-capped (Figure 5B) compared to the IRAS spectrum of BOR-uncapped (Figure 6A). This result indicates that the plane of the ring has a preferred orientation toward the surface normal for the BOR-capped molecular system compared to the BOR-uncapped molecular system, which has aromatic ring with a more tilted orientation relative to the gold surface normal. The same trend is observed for the vibrational bands centered at approximately 1530 and 1510 cm1 in the IRAS spectra of BOR801
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capped and BOR-uncapped (Figures 5B and 6A). The aromatic CN stretching vibrational mode and the aromatic CB
stretching vibrational mode contribute to the vibrational band at 1530 cm1. The main components of these vibrational modes have a parallel orientation relative to μ. Aromatic CC stretching vibrational modes contribute to the vibrational bands centered at approximately 1510 cm1 in the IRAS spectra of BORcapped and BOR-uncapped (Figures 5B and 6A). The main components of these aromatic CC stretching vibrational modes are perpendicular oriented relative to μ. There is a larger difference observed in the intensities between the two vibrational bands in the region 15301510 cm1 in the IRAS spectrum of BOR-capped (Figure 5B) compared to the IRAS spectrum of BOR-uncapped (Figure 6A). Consequently, this result also indicates a reorientation of the aromatic ring during the deprotection procedure and a preferential orientation of the plane of the aromatic ring toward the surface normal for the BOR-capped molecular system compared to the aromatic ring of the BORuncapped molecular system, which prefer a more tilted orientation relative to the surface normal. The static contact angles of water for BOR-capped and BORuncapped molecular monolayers on the gold substrates were measured by contact angle goniometry (see Figure 7 for schematic of BOR-capped adsorbed onto a gold surface). The static contact angles obtained were 69.5° ( 1.7° and 31.5° ( 1.9° for BOR-capped and BOR-uncapped, respectively. The static contact angle of water is reduced for the BOR-uncapped monolayer compared to the BOR-capped monolayer consistent with that the boronic acid functional group is now terminating the molecular monolayer. The thicknesses for BOR-capped and BORuncapped molecular monolayers on gold surfaces were measured using null ellipsometry. The thicknesses were measured to be 2.19 ( 0.07 nm and 1.93 ( 0.08 nm for BOR-capped and BORuncapped, respectively. This shows that the thickness of the BOR-uncapped monolayer is slightly reduced compared to the BOR-capped. We estimate the size of the pinacolyl group to be 0.20.3 nm based upon the length of a CC bond, which is in good agreement with our difference in thickness between the BOR-capped and BOR-uncapped monolayers. 3.3. Near Edge X-ray Absorption Fine Structure Spectroscopy. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements on molecular monolayers on surfaces deliver both information of the electronic structure of the
Figure 6. IR spectra of BOR-uncapped: (A) experimental IRAS spectrum of monolayer on gold surfaces and (B) theoretical spectrum obtained at the DFT/B3LYP/cc-pVTZ level of theory.
Table 3. Proposed IR Assignment for BOR-uncapped Monolayer on Gold Substrates (RA)a experimental ν (cm1)
theoretical 1
Au (TR)
ν (cm )
intensity
1672
1709
0.37
1608
1599
0.30
Ar CC sym str + NH bend
1592 1531
1571 1505
0.29 0.28
NH bend + Ar CC str Ar CH bend + Ar CN str + Ar CB str + NH bend
assignment CdO str
1514
1487
0.64
NH bend + NC str + Ar CN str + Ar CC str + Ar CH bend
1403
1391
0.32
Ar CC asym str + NH bend + Ar CH bend
1378
1339
0.57
BO asym str + BO sym str
1336
1.00
Ar CB str + BO sym str
1286
1276
0.14
Ar CC str + Ar CN str + CH2 twist + BO str
1250
1228
0.18
Ar CN str + Ar CH bend + NH bend
0.34
Ar CH bend NC str
1189 1147 114 a
Ar CH bend
The experimental measurements and assignments are supported with theoretical calculations. 802
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Figure 7. Schematic figure of BOR-capped adsorbed onto a gold surface.
molecular monolayer as well as the orientation of different functional groups such as aromatic rings and alkane chains within the monolayer relative to the surface.44,4546 The transitions of electrons from core levels into unoccupied valence states or continuum states are probed.47 The intensity of the resonances depends on the orientation of the electric field vector (the E-vector) relative to the orientation of the transition moment vector of the final state molecular orbital that is probed and the X-ray absorption cross section. The intensity will be maximized when the vector defining the final state molecular orbital and the electric field vector are parallel. The intensity (I) of the resonances, for vector-type orbitals, is related to the average tilt angle (α) of the corresponding orbital and the X-ray incidence angles (θ) with respect to the surface normal by 1 Iðθ, αÞ µ 1 þ ð3cos2 θ 1Þð3cos2 α 1Þ 2
Figure 8. Experimental carbon K-edge NEXAFS spectra of multilayer of 4-APBA recorded using an incidence angle (θ) of 55° (the angle between normal of the surface and the beam polarization). A comparison between the experimental results and the corresponding theoretical spectrum is included in the enlargement (284290 eV).
The experimental and calculated NEXAFS spectra of multilayer of 4-APBA are presented in Figure 8 (chemical structure, see Figure 1A). The gray solid line in the theoretical spectrum represents absorption due to X-ray radiation that is polarized parallel with the main axis of the molecule. The black solid line represents the isotropic average of the absorption. The z-direction is chosen to be parallel with the normal of the plane of the aromatic ring, thus parallel with the vector along the π* orbitals. Consequently, the resonances shown for which the gray and black solid lines overlap can be assigned as pure π* transitions whereas a separation of the lines indicates both π* and σ* character of the transitions. The proposed assignments are presented in Table 4. There are four resonances observed in the experimental spectrum positioned at 284.4, 285.0, 285.4, and 286.3 eV. We assign these resonances to π1* transitions associated with carbons in the aromatic ring structure (for details see Table 4).
ð1Þ
if a substrate with a three-fold or higher symmetry is used.47 Information of molecular orientation relative to the surface, that is, the orientation of specific molecular orbitals can be extracted from measuring a set of NEXAFS spectra as a function of (θ) and applying the experimental data to eq 1. 803
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Table 4. Experimental Positions and Proposed Assignments of Resonances Present in the C K-edge NEXAFS Spectra of 4-Aminophenyl Boronic Acid (4-APBA), BOR-capped, and BOR-uncapped energy (eV) 4-
BOR-
BOR-
APBA
capped
uncapped
assignment
284.4
284.3
284.3
C1s f π1*(CdC)
285.0 285.4
285.0 285.4
285.0 285.4
C1s f π1*(CdC) C1s f π1*(CdC)
286.3
286.2
286.2
C1s f π1*(CdC)
287.4
287.4
C1s f σ*(C—H)
288.0
288.0
C1s f π*(CdO)
C 1s f σ*(C—H), C1s f π2*(CdC)
287.9 288.3
288.3
288.3
C1s f σ*(C—H), C1s f π2*(CdC)
290.0
290.0
290.2
C1s f π2*(CdC), C1s f σ*(C—H)
293 302
292 302
292 302
C1s f σ*(C—C, C—O, C—N, C—B) C1s f σ*(CdC, CdO)
4-APBA has a para-disubstituted aromatic ring with a carbon to nitrogen bond (carbon 1) and a carbon to boron bond (carbon 4). There is a difference in electronegativity between nitrogen, hydrogen, and boron (N > H > B). Consequently, the closest chemical environment of the carbons present in the aromatic ring is dependent on the electronegativity of the nearest neighbors. As a consequence, there is a shift in the observed photon energy positions of the resonances assigned to the π1* transitions. The π1* resonance associated with the carbon that has a carbon to nitrogen bond is observed at a higher photon energy position (286.3 eV) compared to the π1* resonance associated with carbon bonded to the boron atom, which is observed at 284.4 eV. Carbons 2,6 are closer positioned in space to the nitrogen atom compared to carbons 3,5 (see chemical structure, Figure 1A). Since the closest chemical environment is different also for the aromatic carbons 2,6 compared to carbons 3,5, there is a shift observed for the photon energy positions of the resonances associated also with these aromatic carbons, in good agreement with our theoretical calculations. The resonance associated with carbons 2,6 was observed at 285.4 eV, and the resonance associated with carbons 3,5 was observed at 285.0 eV. There are two resonances observed at 287.9 and 288.3 eV. We assign these two resonances to σ*(C—H) transitions associated with the protection group48 and to π2*(CdC) transitions associated with the aromatic ring.45 The low intensity resonance observed at approximately 290 eV is assigned to π2*(CdC) transitions.45 The resonance found at approximately 293 eV and the resonance centered at approximately 302 eV are assigned to σ*(C—C, C—O, C—B, C—N) and σ*(CdC) transitions, respectively.49 Experimental C K-edge NEXAFS spectra of BOR-capped monolayer recorded at three incidence angles of X-ray (90°, 55°, and 20°) are presented in Figure 9. The proposed assignments are summarized in Table 4. The resonances at 284.3, 285.0, 285.4, and 286.2 eV are assigned to π1* transitions associated with carbons in the aromatic ring structure (for details see Table 4), similarly with our assignments for the multilayer of 4-APBA.45,48 The resonance at 285.0 eV was used to calculate the average tilt angle of the aromatic ring plane relative to the normal of the gold surface. This was done by comparing the spectra recorded at normal and grazing X-ray incidence angle (I90/I20).
Figure 9. Experimental carbon K-edge NEXAFS spectra of a monolayer of BOR-capped adsorbed on a gold surface. The spectra are recorded at three different incidence angles (θ, the angle between the surface normal and the beam polarization); θ = 90°, 55°, and 20°.
The intensity of the resonance at 285.0 eV is enhanced in the spectrum recorded using the normal X-ray incidence angle (I90) compared to the spectrum recorded using the grazing incidence angle (I20). This result indicates that the plane of the aromatic ring of the BOR-capped molecule has a preferential orientation toward the surface normal. The average tilt angle of the vector parallel to the π* orbitals of the aromatic ring relative to the surface normal is estimated to be approximately 63° for the BORcapped molecules adsorbed onto gold substrates. The resonance at approximately 287.4 eV is assigned to C1s(CH) to σ* transitions.48,49 This resonance may also have contributions from σ*(CS) and Rydberg (R*) transitions.50,51 The resonance at about 288.0 is assigned to C1s(CdO) to π* transitions (amide moiety).16 There is a resonance at 288.3 eV. We assign this resonance to σ* transitions49 and π2* transitions associated with 804
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average tilt angle of the π* orbitals of the aromatic ring relative to the gold surface normal. The intensity of the resonance at 285.0 eV is enhanced in the spectrum recorded using a grazing incidence angle of the X-ray light compared to when a normal incidence angle is used, contrary to our NEXAFS results for the BORcapped molecular system. This result indicates that the orientation of the aromatic ring has changed to a more tilted orientation relative to the gold surface normal after the pinacolyl functional group has been removed. The average tilt angle of the π* orbitals of the aromatic ring relative to the gold surface normal is estimated to be approximately 47°, based on calculations (eq 1) using the intensity ratio (I90/I20) of the states in the spectra shown in Figure 10. We have observed that the orientation of the aromatic ring for the BOR-uncapped molecule is found to be in the range of approximately 4755°. The resonance at about 287.4 eV is assigned to C1s(CH) to σ* transitions48,49 and may also have contributions from σ*(CS) and Rydberg (R*) transitions.50,51 The resonance at about 288.0 eV is assigned to C1s(CdO) to π* transitions (amide moiety), in agreement with our assignment for BOR-capped. The resonances at approximately 288.3 and 290.2 eV are assigned to C1s(CH) to σ* transitions and π2* transitions associated with the aromatic ring. The excitation sites for the σ* transitions are the carbons in the alkane chain according to our theoretical calculations. The resonances at about 292 and 303 eV are assigned to σ* transitions (see the proposed assignments in Table 4).49 Calculated NEXAFS spectra of BOR-capped and BORuncapped derivatives can be found in the Supporting Information. These spectra are shown to be valuable for the assignments of the resonances detected in the experimental NEXAFS spectra. From the theoretical calculations, it is also possible to distinguish resonances that originates from pure π* transitions from transitions with both π* and σ* character, which is important when NEXAFS is used to investigate the average tilt angle of a specific functional group.
4. CONCLUSIONS Organized molecular monolayer of a boronic pinacol ester analogue (BOR-capped) is formed on the gold surface. Molecular monolayer of the boronic acid analogue (BOR-uncapped) is obtained by removing the pinacolyl group of BOR-capped on the gold substrates. These molecular systems are chosen as simple mimicry of a receptor. The unprotected BOR-uncapped has a boronic acid group available for further linking coordination with diol molecules in the ambient media. It is shown in this work that BOR-capped molecules are linked to the gold substrates through thiolate formation and this strong chemical linkage is preserved after the deprotection procedure. There is a reorientation of the molecular monolayer during the deprotection procedure according to the IR and NEXAFS results. We will continue this line of work to investigate the coordination of diol molecules to the BOR-uncapped molecular system.
Figure 10. Experimental carbon K-edge NEXAFS spectra of a monolayer of BOR-uncapped adsorbed on a gold surface. The spectra are recorded using three different incidence angles (θ, the angle between the surface normal and the beam polarization); θ = 90°, 55°, and 20°.
the aromatic ring structure.45 We assign the resonance at approximately 290.0 eV to C1s(CH) to σ* transitions (carbons in the alkane chain) and π2* transitions. The resonances at about 292 and 302 eV are assigned to σ*(C—C, C—O, C—N, C—B) and σ*(CdC, CdO) transitions, respectively.49 Experimental C K-edge spectra of the BOR-uncapped monolayer recorded at three incidence angles of X-ray (90°, 55°, and 20°) are presented in Figure 10. There are three resonances observed at 284.3, 285.0, 285.4, and 286.2 eV, in good agreement with our measurements for 4-APBA and BOR-capped. We assign these resonances to π1* transitions associated with the carbons in the aromatic ring (see assignments in Table 4). We use the resonance observed at approximately 285.0 eV to calculate the
’ ASSOCIATED CONTENT
bS
Supporting Information. Theoretical carbon K-edge NEXAFS spectra of a BOR-capped derivative and a BOR-uncapped derivative as well as a description about the synthesis of the BORcapped molecule. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B05; Gaussian: Pittsburgh, PA, 2003. (22) Norman, P.; Bishop, D. M.; Jensen, H. J. A.; Oddershede, J. J. Chem. Phys. 2001, 115, 10323–10334. (23) Norman, P.; Bishop, D. M.; Jensen, H. J. A.; Oddershede, J. J. Chem. Phys. 2005, 123, 194103-1–194103-18. (24) Ekstr€om, U.; Norman, P.; Carravetta, V.; Ågren, H. Phys. Rev. Lett. 2006, 97, 143001-1–143001-4. (25) Tu, G.; Rinkevicius, Z.; Vahtras, O.; Ågren, H.; Ekstr€om, U.; Norman, P.; Carravetta, V. Phys. Rev. A 2007, 76, 022506-1–022506-7. (26) Ekstr€om, U.; Norman, P. Phys. Rev. A 2006, 74, 042722-1– 042722-7. (27) H€ahner, G. Chem. Soc. Rev. 2006, 35, 1244–1255. (28) Dalton, a molecular electronic structure program, release 2.0 (2005). http://www.kjemi.uio.no/software/dalton/dalton.html (accessed 2005). (29) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.;Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, MN, 1995. (30) Tai, Y.; Shaporenko, A.; Rong, H. T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 16806–16810. (31) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058–4061. (32) Petoral, R. M.; Uvdal, K. J. Electron Spectrosc. Relat. Phenom. 2003, 128, 159–164. (33) Petoral, R.; Uvdal, K. J. Phys. Chem. B 2003, 107, 13396–13402. (34) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371–4382. (35) Tengvall, P.; Lundstr€om, I.; Liedberg, B. Biomaterials 1998, 19, 407–422. (36) Coleman, W. M.; Gordon, B. M. Appl. Spectrosc. 1988, 42, 1049–1056. (37) Petoral, R.; Uvdal, K. J. Phys. Chem. B 2005, 109, 16040–16046. (38) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678–688. (39) Barriet, D.; Yam, C. M.; Shmakova, O. E.; Jamison, A. C.; Lee, T. R. Langmuir 2007, 23, 8866–8875. (40) Valiokas, R.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. Langmuir 1999, 15, 3390–3394. (41) Jung, C. J. Mol. Recognit. 2000, 13, 325–351. (42) Engquist, I.; Lestelius, M.; Liedberg, B. Langmuir 1997, 13, 4003–4012. (43) Wesch, A.; Dannenberg, O.; W€oll, C.; Wolff, J. J.; Buck, M. Langmuir 1996, 12, 5330–5337. (44) Rong, H.-T.; Frey, S.; Yang, Y.J.; Zharnikov, M.; Buck, M.; W€uhn, M.; W€oll, C.; Helmchen, G. Langmuir 2001, 17, 1582–1593. (45) Solomon, J. L.; Madix, R. J.; St€ohr, J. Surf. Sci. 1991, 255, 12–30. (46) Fu, J.; Urquhart, S. G. J. Phys. Chem. A 2005, 109, 11724–11732. (47) Stohr, J. NEXAFS Spectroscopy; Springer: Berlin, Germany, 1992. (48) Kaznacheyev, K.; Osanna, A.; Jacobsen, C.; Plashkevych, O.; Vahtras, O.; Ågren, H.; Carravetta, V.; Hitchcock, A. P. J. Phys. Chem. A 2002, 106, 3153–3168. (49) Graf, N.; Yegen, E.; Gross, T.; Lippitz, A.; Weigel, W.; Krakert, S.; Terfort, A.; Unger, W. E. S. Surf. Sci. 2009, 603, 2849–2860. (50) Zubavichus, Y.; Shaporenko, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. A Lett. 2005, 109, 6998–7000. (51) Yan, C.; G€olzh€auser, A.; Grunze, M.; W€oll, C. Langmuir 1999, 15, 2414–2419.
Corresponding Author
*Fax: 46-13-137-568; E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by grants from the Swedish research council (Grant no. 621-2010-5014) Carl Tryggers Foundation, CeNano at LiU, and grants for computing time at National Supercomputer Centre (NSC), Sweden. We thank L. Johansson, Division of Biochemistry, IFM, Link€oping University for synthesizing the BOR-capped molecule as well as A. Preobrajenski, manager for Beamline D1011 at MaxLab in Lund, for the assistance during our NEXAFS measurements. ’ REFERENCES (1) Castner, D. G.; Ratner, B. D. Surf. Sci. 2002, 500, 28–60. (2) Chaki, N. K.; Vijayamohanan, K. Biosens. Bioelectron. 2002, 17, 1–12. (3) Ariga, K.; Hill, J. P.; Endo, H. Int. J. Mol. Sci. 2007, 8, 864–883. (4) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; G€opel, W. Science 1994, 265, 1413–1415. (5) Rickert, J.; Weiss, T.; G€opel, W. Sens. Actuators, B 1996, 31, 45–50. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (7) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (8) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481–4483. (9) Lorand, J. P.; Edwards, J. O. J. Org. Chem. 1959, 24, 769–774. (10) Wannapob, R.; Kanatharana, P.; Limbut, W.; Numnuam, A.; Asawatreratanakul, P.; Thammakhet, C.; Thavarungkul, P. Biosens. Bioelectron. 2010, 26, 357–364. (11) Pribyl, J.; Skladal, P. Biosens. Bioelectron. 2006, 21, 1952–1959. (12) Ho, J.; Hsu, W.-L.; Liao, W.-C.; Chiu, J.-K.; Chen, M.-L.; Chang, H.-C.; Li, C.-C. Biosens. Bioelectron. 2010, 26, 1021–1027. (13) Rick, J.; Chou, T.-C. Biosens. Bioelectron. 2006, 22, 329–335. (14) Freeman, R.; Bahshi, L.; Finder, T.; Gill, R.; Willner, I. Chem. Commun. 2009, 764–766. (15) Carey, R. I.; Folkers, J. P.; Whitesides, G. M. Langmuir 1994, 10, 2228–2234. (16) Vahlberg, C.; Linares, M.; Villaume, S.; Norman, P.; Uvdal, K. J. Phys. Chem. C 2011, 115, 165–175. (17) Pennington, T. E.; Kardiman, C.; Hutton, C. A. Tetrahedron Lett. 2004, 45, 6657–6660. (18) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45–52. (19) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (20) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007–1023. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; M. C. Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; 806
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