PM-IRRAS Investigation of Self-Assembled Monolayers Grafted onto

ARTICLE pubs.acs.org/Langmuir

PM-IRRAS Investigation of Self-Assembled Monolayers Grafted onto SiO2/Au Substrates Micha€el A. Ramin, Gwena€elle Le Bourdon, Nicolas Daugey, Bernard Bennetau, Luc Vellutini, and Thierry Buffeteau* Institut des Sciences Moleculaires (UMR 5255
CNRS), Universite Bordeaux 1, 351 Cours de la Liberation, 33405 Talence, France

bS Supporting Information ABSTRACT:

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to characterize self-assembled monolayers (SAMs). Novel ester-terminated organosilicon coupling agents possessing a trialkoxysilyl headgroup and a urea group in the linear alkyl chains (4) were synthesized and grafted onto SiO2/Au substrates (SiO2 film of 200 Å thickness deposited on gold mirror). This composite substrate allowed the anchoring of SAMs and preserved the high reflectivity for infrared radiation. PMIRRAS spectra with very high signal-to-noise ratios have been obtained in the mid-infrared spectral range allowing monitoring of the grafted SAMs. Quantitative analysis of the measured signal is described to compare PM-IRRAS and conventional IRRAS spectra. This quantitative analysis has been validated since the band intensities in the corrected PM-IRRAS and conventional IRRAS spectra are identical. Orientation information on the different functional groups has been obtained comparing the corrected PM-IRRAS spectrum with the one calculated using isotropic optical constants of ester-terminated organosilicon coupling agents 4. The carbonyls of the urea groups are preferentially parallel to the substrate surface favoring intermolecular hydrogen bonding and consequently a close packing of the molecules attached to the surface. By contrast, the alkyl chains present gauche defects and are poorly oriented.

1. INTRODUCTION Progress in the fields of chemical sensors, biosensors, and biochips depends, in particular, on the capacity to functionalize surfaces with suitable functional groups to immobilize the probe biomolecules.1
3 Self-assembled monolayers (SAMs) offer one of the highest quality routes for the preparation of chemically and structurally well-defined organic surfaces. Indeed, the surface properties can be tailored by introducing adequate functional groups at the terminus of the monolayers. However, the robustness of the monolayers is very dependent on its anchoring onto the substrate (adsorption, covalent bonds). For example, thiolbased SAMs are relatively fragile both thermally and chemically, restricting their use in biosensor applications. On the other hand, the covalent anchoring of SAMs on oxide surfaces, based on the chemistry of organosilanes, leads to a strong siloxane bonding with the substrate producing robust systems with uniform surface coverage. The stability of organosilane-based SAMs comes not only from the covalent anchoring with the hydroxyl groups of the substrate, but also from the partial cross-linking of the molecules through Si
O
Si bonds. The molecular self-assembly of these systems is mainly controlled by the intermolecular van der Waals interactions of the alkyl chains. However, the introduction of functional groups such as amide,4 urea,5 sulfone,6 aromatic,7 or r 2011 American Chemical Society

diacetylene8 into the linear alkyl chains may constitute an alternative way to control the molecular self-assembly of SAMs. Indeed, these functional groups can interact laterally by hydrogen bonding (amide, urea), dipole interaction (sulfone), π-stacking (aromatic), or covalent bonding (diacetylene), improving the stability and the mechanical properties of SAMs. A large number of techniques have been used to investigate and characterize self-assembled films,1a such as FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Brewster angle microscopy (BAM), ellipsometry, and contact angle measurements. Among these techniques, FTIR spectroscopy is a particularly simple and nondestructive way of acquiring molecular information on these bidimensional systems, such as the formation of chemical bonds with the substrate, the hydrogen-bonded structures, the conformation of the alkyl chains, and the orientation of the functional groups when polarization of the IR radiation is used. Depending on the nature of the substrate (metal, semiconductor, or dielectric), the characterization of SAMs can be performed in different ways. Received: February 17, 2011 Revised: March 24, 2011 Published: April 12, 2011 6076

dx.doi.org/10.1021/la2006293 | Langmuir 2011, 27, 6076–6084

Langmuir

ARTICLE

Scheme 1. Synthesis of Silylated Coupling Agenta

(a) SOCl2 (4 equiv), MeOH (0.2 M), 20 °C, 16 h. (b) 1,8-Bis(dimethylamino)naphthalene (proton sponge), diphenylphosphoryl azide (DPPA), THF, reflux, 6 h. (c) NEt3 (1.1 equiv), CH2Cl2, 10-isocyanatohexene (1.2 equiv). (d) HSi(OMe)3 (5 equiv), cat. Karstedt (0.025 equiv Pt), toluene, 60 °C, 20 h. a

Transmission is generally used to investigate SAMs deposited onto glass substrates, but the molecular information is limited to the stretching vibrations of the alkyl chains.9 Silicon substrates allow the analysis over the entire mid-IR spectral range10 and can be used as an internal reflection element (IRE) for attenuated total reflection (ATR) experiments. Since the multiple reflections inside the silicon IRE significantly increase the signal-tonoise (S/N) ratio of the IR spectra, this technique has been widely used to investigate the molecular organization and the chemical modification of SAMs.4m,o,11 On the other hand, infrared reflection
absorption spectroscopy (IRRAS) is a wellestablished technique to study monolayers and ultrathin films deposited onto metallic surfaces.12 A very interesting feature of this method is the surface selection rule, which states that only the vibrations with a component of the transition dipole moment aligned perpendicularly to the surface plane can interact with the stationary electric field and contribute to the IRRAS spectrum.13 As a consequence, IRRAS provides information about the orientation of the molecules with respect to the surface. However, when the sample is an ultrathin film (thickness less than 50 Å) or a grafted monolayer, the surface detectivity of the IRRAS method is generally not sufficient, and the measurement time necessary to get an acceptable S/N ratio becomes very long (several hours). To overcome the limitation of the IRRAS method (detectivity, changes in the sample environment), PM-IRRA spectroscopy was developed in the 1980s.14 The PM-IRRAS method conserves the IRRAS advantages of the electric field enhancement and of the surface selection rule, but additionally presents the tremendous advantages of high sensitivity in surface absorption detection and the ability to do in situ experiments even in infrared absorbing isotropic media. PM-IRRAS has been used successfully to obtain vibrational spectra of Langmuir
Blodgett (LB) monolayers, lipid bilayers, and SAMs deposited onto metallic substrates.15 In all these studies, the quality of the PM-IRRAS spectra has allowed a qualitative analysis of the monolayers and, in few cases, the acquisition of valuable information about the orientation of the molecules with respect to the metal surface. However, the normalized differential form of the PM-IRRAS signal as well as the demodulation of its high frequency part by a lock-in amplifier, which generates Bessel functions in the expression of the signal, prevents a straightforward interpretation of the PM-IRRAS spectra. A calibration procedure has been published to quantify the PM-IRRAS signal obtained with a lock-in amplifier and to express it in terms of the IRRAS signal,16 but generally authors publish PM-IRRAS spectra in arbitrary units which depend on the experimental conditions (time constant and sensitivity of the lock-in amplifier, gain factor due to the different amplifications).

On the other hand, very few PM-IRRAS experiments have been performed for SAMs grafted onto oxide surfaces and, in particular, on glass. Indeed, for such substrates, the values of Rp and Rs (reflectances polarized parallel and perpendicular to the incidence plane, respectively) are very different at grazing incidence, and the characteristic absorption bands of the monolayers are hidden by the substrate contribution. A method has been proposed to increase the sensitivity of PM-IRRAS for the study of ultrathin films deposited onto dielectric substrates,17 but the S/N ratio is not sufficient to detect a single monolayer or submonolayer in a reasonable acquisition time. However, the possibility of depositing ultrathin film oxide films (SiO2 or TiO2) on metallic substrates allows the grafting of SAMs onto these composite substrates while benefiting from the electric field enhancement as well as the surface selection rule associated with reflecting surface. PM-IRRAS experiments of lipid bilayers deposited onto SiO2/Au and TiO2/Au substrates have been reported in the literature,18 demonstrating that these composite substrates can be successfully used for PM-IRRAS investigations of organized organic monolayers deposited onto oxide surfaces. Moreover, Szunerits et al. have shown that stable thin silicate films deposited on gold surfaces can easily be coupled to organosilane molecules.19 In this article, the PM-IRRAS technique has been used to characterize a SAM containing a urea group in the linear alkyl chains and terminated by an ester group, grafted onto SiO2/Au substrate. Quantitative analysis of the measured signal is described to propose a simple calibration procedure comparing PM-IRRAS and conventional IRRAS spectra. Optical constants of the SAM have been determined to obtain information about the orientation of the alkyl chains and the urea groups.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Silylated Coupling Agent. The synthesis of a novel trialkoxysilane 4 with a urea group in the alkyl chain is reported in Scheme 1. This synthesis is performed in 4 steps from commercially available starting compounds. To avoid interactions between the terminal group and the silanols on the surface, it is important to protect the carboxylic acid group under the ester form (compound 1, step a). The unsaturated isocyanate 2 is synthesized by a modified Curtius reaction which is simpler than the classical one using the intermediate acid chlorides to obtain the acyl azides.20 Direct conversion of carboxylic acids to isocyanate (step b) is achieved using diphenylphosphoryl azide (DPPA) in the presence of 1,8-bis(dimethylamino)naphthalene (Proton sponge) as a base.21 The unsaturated precursor 3 was obtained upon reaction of 1 with 2 in CH2Cl2 (step c) in high yield (80%). Addition of the Si(OMe)3 group was achieved (step d) by hydrosilylation reaction of the unsaturated precursor 3 with a large excess of HSi(OMe)3 (5 equiv) in the presence of platinum catalyst 6077

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084

Langmuir (Karstedt catalyst). Experimental details and spectroscopic data are given in the Supporting Information (S1
S4). 2.2. Surface Modification. The SiO2/Au substrates were supplied by Optics Balzers AG. They correspond to Goldflex mirror with SiO2 protection layer (Goldflex PRO, reference 200785). Their absolute reflectance was higher than 98% in the 1.2
12 μm spectral range. The thickness of SiO2 layer, measured by ellipsometry, was 215 ( 7 Å, using a refractive index of 1.46 (I-elli2000 NFT ellipsometer, λ = 532 nm). A homogeneous surface was observed by atomic force microscopy (AFM) with a root-mean-square (rms) roughness of 9 Å (Thermomicroscope Autoprobe CP Research, Park Scientific). The substrates were cleaned and activated just before the grafting. They were treated successively with milli-Q water (18 MΩ.cm) and hot chloroform (10 min at least). Then, the substrate were exposed to UV-ozone (homemade apparatus, λ = 185
254 nm) for 30 min and introduced into the silanization flask immediately. The SAM was prepared as follows: to a solution of freshly synthesized compound 4 (11.90 mg; 2.5  10
4 mol.L
1) in anhydrous toluene (100 mL), trichloracetic acid (TCA; 0.4 mg, 10 mol %) as catalyst was added. This solution was introduced into the silanization flask at 18 °C under inert atmosphere, and substrates were immersed for 12 h. The samples were sonicated in toluene (5 min), chloroform (5 min), and milli-Q water (10 min), then dried under vacuum for 10 min. Contact angle measurements for the obtained SAM give a value of 70 ( 1° with water. This value was similar to those previously reported in the literature for ester-terminated SAMs.22 2.3. ATR Experiments. The ATR spectra of trialkoxysilane 4 were recorded with a ThermoNicolet Nexus 670 FTIR spectrometer equipped with a liquid nitrogen cooled narrow-band mercury cadmium telluride (MCT) detector using a Silver-Gate (germanium crystal) ATR accessory (Specac). The electric field of the infrared beam was polarized either perpendicular or parallel to the plane of incidence with a BaF2 wire grid polarizer (Specac). Each spectrum was obtained from the acquisition of 200 scans at a resolution of 4 cm
1. 2.4. IRRAS and PM-IRRAS Experiments. IRRAS and PMIRRAS spectra were recorded on a ThermoNicolet Nexus 670 FTIR spectrometer at a resolution of 4 cm
1, by coadding several blocks of 1500 scans (30 min acquisition time) with an optical mirror velocity, V, of 0.63 cm/s. IRRAS and PM-IRRAS spectra presented in this article correspond to 4 and 1 h acquisition times, respectively. IRRAS experiments were performed at an incidence angle of 75° using an external homemade goniometer reflection attachment. The parallel beam was directed out of the spectrometer with an optional flipper mirror and made lightly convergent with a ZnSe lens (191 mm focal length). A BaF2 wire grid polarizer (Specac) was used to select the p-polarized radiation. The efficiency of the polarizer was specified to be better than 99% for wavenumbers lower than 3300 cm
1. A second ZnSe lens (38.1 mm focal length) was used to focus the reflected beam onto a photovoltaic MCT detector (Kolmar Technologies, model KV104) cooled at 77 K. The MCT detector was set onto X, Y, and Z microcontroller stages to optimize the intensity of the focused IR beam. PM-IRRAS experiments were carried out on the same reflection attachment, adding a ZnSe photoelastic modulator (PEM, Hinds Instruments, type III) after the polarizer. The PEM oscillates at ωm = 37 kHz and changes the polarization from parallel to perpendicular at 74 kHz. The polarization modulated signal IAC was separated from the low frequency signal IDC (ωi between 500 and 5000 Hz) with a 40 kHz high pass filter and then demodulated with a lock-in amplifier (Stanford model SR 830). The output time constant was set to 100 μs to pass all the frequencies ωi. The two interferograms are high-pass and low-pas filtered (Stanford model SR 650) and simultaneously sampled in the dual channel electronics of the spectrometer. In all experiments, the PEM was adjusted for a maximum efficiency at 2500 cm
1 to cover the mid-IR range in only one spectrum. For calibration measurements, a second linear polarizer (oriented parallel or perpendicular to the first

ARTICLE

preceding the PEM) was inserted between the sample and the second ZnSe lens.

2.5. Spectral Simulations and Determination of the Optical Constants. The computer program used to calculate the IRRAS spectra of 4 deposited onto SiO2/Au substrates is based on the Abeles' matrix formalism,23 which has been generalized for anisotropic layers.24 Several parameters must be fixed in the program such as the number of layers, the angle of incidence (set to 75°), and the polarization of the infrared beam. The anisotropic optical constants (refractive index and extinction coefficient in the three space directions) of 4 have to be determined beforehand (vide infra). Because the grafting of SAMs does not induce orientation in the xy plane (substrate surface), uniaxial symmetry of molecular orientation can be assumed for the calculations (nx = ny = nxy and kx = ky = kxy). The optical constants of 4 have been determined from polarized ATR spectra, using the interdependence of n(ν h) and k(ν h) by the Kramers
Kronig relations. Dignam et al. have shown how the Kramers
Kronig relations can be applied to polarized ATR spectra.25 The in-plane optical constants (nxy and kxy) were calculated from the s-polarized ATR spectrum, whereas the out-of-plane optical constants (nz and kz) were obtained from the p-polarized ATR spectrum and the previously determined nxy and kxy. We have recently used this methodology to determine the optical constants of ionic liquids and proteins.26 Then, the isotropic optical constants of 4 have been calculated from the in-plane and out-of-plane optical constants (niso = (2nxy þ nz)/3 and kiso = (2kxy þ kz)/3).

3. CALIBRATION PROCEDURE 3.1. Expression of the Experimental PM-IRRAS Signal. As previously reported, the expression of the experimental PM-IRRAS signal of a thin film deposited onto a substrate is given by15b,16

SPM
IRRAS ¼

2 3 Gj½R P ðdÞ
R S ðdÞJ 2 ðj0 Þj ½R P ðdÞ þ R S ðdÞ ( ½R P ðdÞ
R S ðdÞJ 0 ðj0 Þ ð1Þ

where RP(d) and R S(d) are the reflectances of the film/ substrate system, polarized parallel and perpendicular to the incidence plane, respectively, d is the film thickness, J0(j0) and J2(j0) are the zero- and second-order Bessel functions of the maximum dephasing j0 introduced by the PEM, and G = 10 3 (GAC/(S 3 GDC))(exp[
2 Vνhτ]) is a gain factor due to the different amplifications (GAC and GDC) on the two channels and to the demodulation adjustments (sensibility, S, and output time constant, τ). The absolute value in eq 1 comes simply from the usual computer algorithm of phase correction (Mertz) used on FTIR spectrometers that always stores and plots Fourier transforms of interferograms as positive single-beam spectra. Finally, the ( sign in eq 1 depends on the state of the polarization selected by the polarizer before the PEM (þ for p-polarization and
for s-polarization). For metallic substrates, since [RP(d) þ RS(d)] is much larger than [RP(d)
RS(d)] and |J0(j0)| is less than 1, it is reasonable to neglect the [RP(d)
RS(d)]J0(j0) term in eq 1. Moreover, since RS(d) > RP(d), eq 1 becomes ½R S ðdÞ
R P ðdÞ SPM
IRRAS ¼ 2 3 G 3 J 2 ðj0 Þ ½R ðdÞ þ R ðdÞ P

ð2Þ

S

The polarized reflectances, RP(d) and RS(d), of the film/ substrate system have been calculated by McIntyre and Aspnes.27 When the thickness of the film is much less than the wavelength 6078

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084

Langmuir

ARTICLE

Table 1. Calculation of the PM-IRRAS Signal, SPM-IRRAS, by the Simplified eq 6 and by the Abeles Matrix Formalism SSub

Sfilm

(Sfilm/SSub) (%)

SPM-IRRAS (eq 6)

SPM-IRRAS (simulation)

error (%)

Au a

0.0539

0.0088

16.4

0.0627

0.0619

1.3

SiO2/Au b,c

0.0535

0.0088

16.4

0.0623

0.0614

1.4

SiO2c

1.1345

0.0088

0.8

1.1433

1.1366

0.6

substrate

a

nAu = 5.423 and kAu = 37.5.28 b Thickness of SiO2 = 200 Å. c nSiO2 = 1.48.

of the incident radiation (d , λ), it is possible to make some approximations, leading to the expressions " !# ^εS
^ε R S ðdÞ ¼ R S ð0Þ 1 þ 8πνd cos θ 3 Im ð3aÞ 1
^εS R P ðdÞ 2

0 13  
1 2 2 2 2 ^ ^ ^
ε
sin θÞ
ε sin θ ε ð^ ε S S C7 3 ^ε 6 B 8πνd 6 B 3 S C7 C7 ImB ¼ R P ð0Þ61 þ 3 2 4 @ A5 ð^εS
1Þ 3 ð^εS
tg θÞ cos θ

ð3bÞ where RP(0) and RS(0) are the polarized reflectances of the bare substrate, θ is the incidence angle, νh is the wavenumber, and ^ε and ^εS are the dielectric constants of the film and the substrate, respectively. For high reflecting metals, ^εS . ^ε and ^εS . 1, leading to the expressions R S ðdÞ ¼ R S ð0Þ  # 8πνd sin2 θ
1 Im R P ðdÞ ¼ R P ð0Þ 1
3 ^ε cos θ

ð4aÞ

"

ð4bÞ

Substitution of eqs 4a and 4b into eq 2 and assuming that 2 RP(0) 3 [(8πν hd sin θ)/cos θ] 3 Im(
1/ε^) , RP(0) þ RS(0) ≈ 2 leads to the simplified expression of the PM-IRRAS signal ½R S ð0Þ
R P ð0Þ SPM
IRRAS ¼ 2 3 G 3 J 2 ðj0 Þ ½R P ð0Þ þ R S ð0Þ   8πνd sin2 θ
1 Im þ G J ðj Þ 3 ^ε 3 3 2 0 cos θ

ð5Þ

Considering that ^ε = n^2 = (n þ ik)2and assuming that the refractive index, n, is much larger than the extinction coefficient, k, for organic compounds, eq 5 can be written ½R S ð0Þ
R P ð0Þ 16πνkd sin2 θ þ G J ðj Þ SPM
IRRAS ¼ 2 3 G 3 J 2 ðj0 Þ ½R ð0Þ þ R ð0Þ n3 cos θ 3 3 2 0 P

S

strength (n = 1.44 and k = 0.3 at νh = 1620 cm
1) deposited onto various substrates (Au, SiO2/Au, and glass). These calculations have been performed for G 3 J2(j0) = 1. The contribution of SSub is similar for SiO2/Au and Au substrates. As shown by Zawisza et al.,18a the metallic properties of the substrate are preserved even though an ultrathin film (i.e., lower than 400 Å) of SiO2 is deposited onto a gold mirror. Moreover, the enhancement of the normal component of the electric field of the p-polarization light is comparable to that on the Au substrate. The contribution of the monolayer signal, Sfilm, with respect to the substrate signal, SSub, is around 16.4% for SiO2/Au and Au substrates, whereas it is divided by 20 for a glass substrate. This last feature explains the difficulty performing PM-IRRAS experiments on a glass substrate. Finally, the values of the PM-IRRAS signal, SPM-IRRAS, calculated by the simplified eq 6 and by the Abeles matrix formalism differ by less than 1.4%, indicating that eq 6 can be used to model the experimental PM-IRRAS signal of an ultrathin film deposited onto metallic substrate. 3.2. Calibration Procedure. The calibration procedure consists of expressing the PM-IRRAS spectrum as the more conventional IRRAS spectrum (RP(d)/RP(0)), usually represented in the literature in terms of the pseudo absorption spectrum: 1
(RP(d)/RP(0)). This last quantity, which can be calculated from eq 4b, is given by 1


ð7Þ

The treatment of PM-IRRAS spectra can be performed in two steps. First, we extract the contribution of the film by subtracting the PM-IRRAS spectrum of the bare substrate from the PM-IRRAS spectrum of the sample. The two PM-IRRAS spectra should be recorded under the same experimental conditions (amplifications, sensitivity, and time constant of the lock-in amplifier). Second, we determine the G 3 J2(j0) function by using calibration measurements. These calibration measurements are performed by adding, between the sample and the ZnSe lens, a second linear polarizer oriented parallel and perpendicular to the first polarizer located in front of the photoelastic modulator. The two calibration spectra (Cpp and Cps) obtained from these measurements can be expressed by setting RP(d) and RS(d) equal to zero in the general eq 1. This gives

ð6Þ The expression of the PM-IRRAS signal contains two terms: The first term, SSub = 2 3 G 3 J2(j0)([RS(0)
RP(0)]/[RP(0) þ RS(0)]), represents the contribution of the substrate and depends only on the optical properties of the substrate (metal, semiconductor, or dielectric); the second term, Sfilm = [(16πνhkd sin2 θ)/(n3 cos θ)] 3 G 3 J2(j0), represents the contribution of the film and is linear with respect to the thickness d and the extinction coefficient k of the film. The contributions of SSub and Sfilm as well as the value of SPM-IRRAS calculated by our computer program have been reported in Table 1 for a monolayer (d = 30 Å) with medium oscillator

R P ðdÞ 16πνkd sin2 θ Sfilm ¼ ¼ R P ð0Þ n3 cos θ G 3 J 2 ðj0 Þ

Cpp ¼

2 3 G0 3 jJ 2 j 1 þ J0

ð8aÞ

Cps ¼

2 3 G0 3 jJ 2 j 1
J0

ð8bÞ

and

where G0 is the gain factor due to the different amplifications on the two channels and to the demodulation adjustments associated with the calibration measurements. Generally, G0 differs from G because different sensibilities of the lock-in amplifier are used for PM-IRRAS and calibration measurements. The G 3 J2(j0) function can be easily 6079

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084

Langmuir

ARTICLE

Figure 1. PM-IRRAS spectra of 4 grafted onto SiO2/Au substrate (black) and a SiO2/Au substrate (red), in the 4000
800 cm
1 spectral range.

obtained by combining eqs 8a and 8b G 3 J 2 ðj0 Þ ¼

G Cpp Cps G0 3 Cpp þ Cps

ð9Þ

4. RESULTS AND DISCUSSION

experiments. The decrease of the S/N ratio in the conventional IRRAS spectra is more marked in the 1800
1400 cm
1 spectral range, due to the change in the sample environment (bad compensation of the water vapor absorptions) between the recording of the sample and background single-beams. This feature clearly shows that PM-IRRAS is a particularly powerful technique for the characterization of a single or submonolayer deposited or grafted onto metallic substrates. 4.2. Determination of the Molecular Orientation. The orientations of the adsorbed molecular groups can be easily calculated from the IRRAS (or corrected PM-IRRAS) spectra because of the anisotropy of the surface electric field (only the component of the electric field normal to the surface Ez is different from zero in the vicinity of the metallic surface). Since the intensity of a given vibrational mode i is proportional to the square of the scalar product of the electric field E and the transition dipole moment Mi, the IRRAS intensity is given by µ ðMi 3 Ez Þ2 µ M 2iz ¼ Mi 2 cos2 θ I IRRAS i

where θ is the angle between Mi and the surface normal. For randomly oriented molecules, the projection of Mi onto the axes X, Y, and Z is the same (M2ix = M2iy = M2iz = M2i /3), leading to cos2 θ = 1/3. The orientations of adsorbed molecular groups can therefore be determined by calculating the IRRAS spectrum for randomly oriented molecules and by using the following relation:12a,e,15k
15m,18b

4.1. Comparison of IRRAS and PM-IRRAS Spectra. Figure 1

shows the experimental PM-IRRAS spectrum of a SiO2/Au substrate, in the 4000
800 cm
1 spectral range (red spectrum). This PM-IRRAS spectrum is essentially dominated by the J2 Bessel function introduced by the PEM (only one arch is observed since the PEM was adjusted for a maximum efficiency at 2500 cm
1 to cover the mid-IR range in only one spectrum) and by a strong band close to 1240 cm
1. This band is attributed to the longitudinal component of the asymmetric stretching vibration, νas(Si
O
Si), of the silica film.29 We have also reported in the same figure the experimental PM-IRRAS spectrum of silylated coupling agent 4 grafted onto SiO2/Au substrate. The bands of 4 appear with a very weak intensity upward from the J2 Bessel function. The PM-IRRAS spectrum of 4, expressed in IRRAS units, is reported in Figure 2a,b (red spectra) in the 3100
2800 cm
1 and 1850
1400 cm
1 spectral ranges, respectively. This spectrum has been corrected using the calibration procedure described in the previous section. The bands associated with different functional groups of the monolayer are clearly visible in Figure 2a and b. The bands located at 2927 and 2855 cm
1 are assigned to the asymmetric (νasCH2) and symmetric (νsCH2) stretching vibrations of the CH2 groups of the alkyl chains, respectively. The band at 1741 cm
1 is assigned to the carbonyl stretching vibration of the terminal ester group, while the bands at 1633 and 1577 cm
1 are attributed to the amide I and amide II modes of the urea group, respectively. The IRRAS spectrum of the same sample, recorded in the conventional manner, is also reported in Figure 2a and b (black spectra). Identical intensities of the bands are observed in the corrected PM-IRRAS and conventional IRRAS spectra, which validates our methodological approach in the treatment of the PM-IRRAS spectra. However, as shown in Figure 2b, the signal-to-noise ratio of the conventional IRRAS spectrum is drastically affected, even though the acquisition time was four times longer than for PM-IRRAS

ð10Þ

cos2 θ ¼

I IRRAS i 3 3 I random i

ð11Þ

where Irandom is the IRRAS intensity of the mode i calculated i from the isotropic optical constants of the studied molecule and using the same monolayer thickness. This method can be applied for LB monolayers or SAMs deposited onto metallic substrates because their thicknesses are generally perfectly known and because experimental IRRAS and calculated spectra do not differ in band shapes and vibrational frequencies. In the particular case of SAMs grafted onto oxide surfaces, this method cannot be used since the self-assembly may differ between grafted molecules and molecules deposited onto the germanium IRE used to determine the isotropic optical constants from ATR experiments. Moreover, the anchoring of SAMs modifies the siloxane group of the molecule. The situation may be improved significantly if two vibrational modes with different orientations of their transition dipole moments are considered.15e The orientation of a transition dipole moment M can be described with respect to the space coordinate system by using the Euler angles as shown in Scheme 2. The space coordinate system is formed by the X, Y, and Z axes, where Z is the normal to the substrate surface. The u, v, and w axes form the molecular coordinate system in which w is the direction of the first transition dipole moment Mi associated with the vibrational mode corresponding to the functional group of interest (reference axis). The orientation of Mi with respect to the space coordinate system is described by the two first Euler angles, the tilt (θ) and azimuthal (φ) angles, and the orientation of the second transition dipole moment Mj with respect to Mi is defined by the R angle and by the third Euler angle, the twist angle (ψ). Assuming a uniaxial orientation of the molecules with respect to the normal to the substrate surface (integration over the φ angle) and a cylindrical symmetry of Mj with respect to Mi (integration over the ψ angle), the IRRAS intensities of the two modes are 6080

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084

Langmuir

ARTICLE

Figure 2. Conventional IRRAS (black) and corrected PM-IRRAS (red) spectra of 4 grafted onto SiO2/Au substrate, in the (a) 3100
2700 and (b) 1850
1400 cm
1 spectral ranges.

Scheme 2. Space Coordinate System (X, Y, Z), Molecular Coordinate System (u, v, w), and Euler Angles Used to Describe the Orientation of Transition Dipole Moments Mi and Mj

given by30 I IRRAS µ M i 2 cos2 θ i  I IRRAS µ M 2j j

1 2 sin θ sin2 R þ cos2 θ cos2 R 2

ð12aÞ  ð12bÞ

Combining eqs 12a and 12b, the orientation of the reference axis is given by rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 3 ðR
cos2 RÞ ð13Þ tgθ ¼ sin2 R 2 IRRAS 2 where R = [(IIRRAS j 3 Mi )/(Ii 3 Mj )] can be determined from the intensities of the two bands measured in the experimental IRRAS spectrum and in the IRRAS spectrum calculated from the isotropic optical constants. The calculated IRRAS spectra of a compact monolayer of 4 deposited onto SiO2/Au substrate is reported in Figure 3a and b in the 3100
2700 cm
1 and 1850
1400 cm
1 spectral ranges, respectively. The widths of

all the bands are significantly narrower than those observed in the experimental IRRAS spectra presented in Figure 2a and b. Moreover, a little red shift is observed for the frequencies of most bands. These spectral modifications clearly show that the self-assembly differs between grafted molecules and molecules deposited onto the germanium IRE used to determine the isotropic optical constants. Nevertheless, this change in the molecular packing does not matter in the calculation of the relative intensities of two modes (M2i /M2j ). 4.2.1. Urea Groups. The orientation of the carbonyl of the urea groups can be determined from the amide I and amide II bands, using eq 13. The relative intensities of the two bands have been measured in the experimental and calculated IRRAS spectra, leading to a value of R equal to 2.03. The R angle between amide I and amide II bands has been already determined in the literature, and its value is around 75°.31 The calculated tilt angle between the carbonyl of the urea groups and the normal of the substrate surface makes an angle of θ = 64°. Therefore, the CdO and the N
H bonds are preferentially oriented parallel to the surface, favoring intermolecular hydrogen bonding. The presence of hydrogen bonding between the molecules is also revealed by the frequency difference (Δνh) between the amide I and II modes. Indeed, a Δνh value of 132 cm
1 has been measured for a diluted solution (5  10
3 M) of 3 in CDCl3, for which any intermolecular hydrogen bonding is possible. This Δνh value decreases up to 43 cm
1 in the solid state (see Figure 3b). The Δν h value of 56 cm
1 observed in the IRRAS spectrum (see Figure 2b) confirms the presence of strong intermolecular hydrogen bonding for 4 grafted onto SiO2/Au substrates. 4.2.2. Alkyl Chains. The determination of the orientation of the alkyl chains implies that they adopt an all-trans conformation. Moreover, a vibrational mode along the chain axis (like CH2 wagging modes) in addition to the νasCH2 (or νsCH2) mode is necessary to use eq 13. The frequencies observed in the IRRAS spectrum of 4 for the νasCH2 (2927 cm
1) and νsCH2 (2855 cm
1) bands clearly indicate the presence of a large number of gauche conformations in the grafted molecules.32 It is therefore impossible to make a quantitative determination of the orientation of the alkyl chains. Nevertheless, the frequencies as well as the relative intensities of the νasCH2 and νsCH2 bands observed on the experimental IRRAS, which are similar to that calculated for an isotropic orientation of the molecules, indicate that the alkyl chains are essentially disordered. 6081

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084

Langmuir

ARTICLE

Figure 3. Calculated IRRAS spectrum of a compact monolayer of 4 deposited onto SiO2/Au substrate, in the (a) 3100
2700 and (b) 1850
1400 cm
1 spectral ranges.

Scheme 3. Scheme of the Orientation of 4 onto SiO2/Au Substrate

4.2.3. Organization of 4 onto SiO2/Au Substrate. Considering the orientation information about the urea groups and alkyl chains, a schematic representation of the organization of 4 onto SiO2/Au substrate is proposed in Scheme 3. The carbonyls of the urea groups, preferentially oriented parallel to the surface, allow hydrogen bonding between the molecules. This feature as well as covalent bonding with the substrate through the siloxane groups improve the stability of the self-assembled monolayer. However, the distance between urea groups (4.5
5 Å),33 larger than the distance between two Si atoms of siloxane bridge (∼3.2 Å),34 may explain the disorder observed for the alkyl chains. Indeed, the distance between the alkyl chains is not sufficient enough to establish van der Waals interactions and consequently does not favor all-trans conformations. Moreover, the terminal ester groups may generate steric hindrance, favoring also the disorder of the alkyl chains.

The SAMs cross-linked internally via hydrogen bonding have already demonstrated their potential to improve the stability and control the tribological properties of the films.4d,g,m However, the introduction of buried groups into straight hydrocarbon chains such as single amide4d or sulfone6 is known to generate disorder of the alkyl chains. In this article, we have shown that the presence of a urea group in the molecule also produces disorder of the alkyl chains. The spacing of the molecules caused by the urea group certainly increases the mobility and the flexibility of the chains. On the other hand, highly ordered and dense SAMs limit the motion of the molecules, and the reactivity of the terminal head groups is significantly reduced due to the steric constraints. Commonly, the steric hindrance can be avoided by the formation of mixed monolayers. However, this method can generate surfaces with nanoislands of chemical functionality.35 Consequently, functionalized hydrogen-bonding SAMs with more accessible head groups could improve the surface reactivity. Thus, biological applications are strongly dependent on surface properties of SAMs which result directly from the design of the monolayers and their packing. In the field of biosensors, the SAMs described in this article could constitute a new platform for the covalent grafting of biomolecules.

5. CONCLUSION In this article, we have employed PM-IRRAS to characterize a SAM, obtained with a novel silylated coupling agent 4 containing a urea group in the alkyl chain and terminated by an ester group, grafted onto SiO2/Au substrates. A quantitative analysis of the measured signal has been performed to relate PM-IRRAS and conventional IRRAS spectra. This quantitative analysis shows that the PM-IRRAS signal is the sum of two terms: the first term is only dependent on the optical properties of the substrate, and the second term reveals the contribution of the film which is linear with respect to the thickness and the extinction coefficient of the film. The second term can be directly related to the IRRAS signal by using a simple calibration procedure. PM-IRRAS spectra with very high signal-to-noise ratios have been obtained in the mid-infrared spectral range allowing the detection of the different functional groups (alkyl chains, urea and ester groups) of 4. Thanks to the surface selection rule of PM-IRRA spectroscopy, it is possible to obtain orientational information about the different functional groups, by comparing experimental spectra 6082

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084

Langmuir with those calculated considering an isotropic orientation of the molecules. Thus, we have found that the carbonyls of the urea groups are preferentially parallel to the substrate surface favoring intermolecular hydrogen bonding and consequently a close packing of the molecules attached to the surface. By contrast, the alkyl chains present a large number of gauche conformations and are poorly oriented. This experimental approach will be used in the near future to investigate various SAMs containing a urea or amide group in the linear alkyl chains and terminated by different functional groups (ester, nitrobenzyl ester, vinyl, glycidyl). Chemical modifications of the terminal functions will be easily followed by PM-IRRAS, providing useful information for the development of chemical sensors or biosensors.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details, elemental analysis, and NMR data for the synthesis of compounds 1, 2, 3, and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: t.buff[email protected].

’ ACKNOWLEDGMENT The authors are indebted to the CNRS (Chemistry Department) and to Region Aquitaine for financial support in FTIR and optical equipments. ’ REFERENCES (1) (a) Ulman, A. Chem. Rev. 1996, 96, 1533. (b) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282. (c) Senaratne, W.; Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6, 2427. (2) Yan, L.; Huck, W. T. S.; Whitesides, G. M. Supramolecular Polymers, Ciferri, A., Ed.; Marcel Dekker: New York, 2000. (3) Mrksich, M. Chem. Soc. Rev. 2000, 29, 267. (4) (a) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4, 1214. (b) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaki, T.; Hosoki, A.; Ichimura, K. Langmuir 1992, 8, 1007. (c) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (d) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371. (e) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 17, 5239. (f) Siewierski, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Langmuir 1996, 12, 5838. (g) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486. (h) Fang, J.; Chen, M.-S.; Shashidhar, R. Langmuir 2001, 17, 1549. (i) Ren, S.; Yang, S.; Zhao, Y. Langmuir 2003, 19, 2763. (j) Monsathaporn, S.; Effenberger, F. Langmuir 2004, 20, 10375. (k) Song, S.; Ren, S.; Wang, J; Yang, S.; Zhang, J. Langmuir 2006, 22, 6010. (l) Song, S.; Zhou, J.; Qu, M.; Yang, S.; Zhang, J. Langmuir 2008, 24, 105. (m) Song, S.; Chu, R.; Zhou, J.; Qu, M.; Yang, S.; Zhang, J. J. Phys. Chem. C 2008, 112, 3805. (n) Anderson, A. S.; Dattelbaum, A. M.; Montano, G. A.; Price, D. N.; Schmidt, J. G.; Martinez, J. S.; Grace, W. K.; Grace, K. M.; Swanson, B. I. Langmuir 2008, 24, 2240. (o) Kim, J.; Cho, J.; Seidler, P. M.; Kurland, N. E.; Yadavalli, V. K. Langmuir 2010, 26, 2599. (5) (a) Ching, J.; Voivodov, K. I.; Hutchens, T. W. J. Org. Chem. 1996, 61, 3582. (b) Kim, J. H.; Shin, H. S.; Kim, S. B.; Hasegawa, T.

ARTICLE

Langmuir 2004, 20, 1674. (c) Nam, H.; Granier, M.; Boury, B.; Park, S. Y. Langmuir 2006, 22, 7132. (6) (a) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (b) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A. Langmuir 1991, 7, 2700. (7) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662. (8) Mowery, M. D.; Kopta, S.; Ogletree, D. F.; Salmeron, M.; Evans, C. E. Langmuir 1999, 15, 5118. (9) Dinh, D. H.; Vellutini, L.; Bennetau, B.; Dejous, C.; Rebiere, R.; Pascal, E.; Moynet, D.; Belin, C.; Desbat, B.; Labrugere, C.; Pillot, J.-P. Langmuir 2009, 25, 5526. (10) Monsathaporn, S.; Effenberger, F. Langmuir 2004, 20, 10375. (11) (a) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. Langmuir 1995, 11, 1190. (b) Gershevitz, O; Sukenik, C. N. J. Am. Chem. Soc. 2004, 126, 482. (12) (a) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (b) Golden, W. G.; Snyder, C. D.; Smith, B. J. Phys. Chem. 1982, 86, 4675. (c) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (d) Rabolt, J. F.; Jurich, M.; Swalen, J. D. Appl. Spectrosc. 1985, 39, 269. (e) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (f) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (g) Arndt, T.; Bubeck, C. Thin Solid Films 1988, 159, 443. (h) Dote, J. L.; Mowery, R. L. J. Phys. Chem. 1988, 92, 1571. (i) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (j) Hasegawa, T.; Takeda, S.; Kawaguchi, A.; Umemura, J. Langmuir 1995, 11, 1236. (k) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (l) Marguerettaz, X.; Fitzmaurice, D. Langmuir 1997, 13, 6769. (m) Myrzakozha, D. A.; Hasegawa, T.; Nishijo, J.; Imae, T.; Ozaki, Y. Langmuir 1999, 15, 6890. (n) Arnold, R.; Terfort, A.; W€oll, C. Langmuir 2001, 17, 4980. (o) Stoycheva, S.; Himmelhaus, M.; Fick, J.; Korniakov, A.; Grunze, M.; Ulman, A. Langmuir 2006, 22, 4170. (p) Nogues, C.; Lang, P. Langmuir 2007, 23, 8385. (q) Shin, H. S.; Kim, S. B.; Jung, Y. M. Langmuir 2007, 23, 10567. (13) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (14) (a) Golden, W. G.; Dunn, D. S.; Overend, J. J. Catal. 1981, 71, 395. (b) Dowrey, A. E.; Marcott, C. Appl. Spectrosc. 1982, 36, 414. (c) Golden, W. G.; Kunimatsu, K; Seki, H. J. Phys. Chem. 1984, 88, 1275. (d) Buffeteau, T.; Desbat, B.; Turlet, J. M. Mikrochim. Acta [Wien] 1988, II, 23. (15) (a) Golden, W. G.; Saperstein, D. D.; Severson, M. W.; Overend, J. J. Phys. Chem. 1984, 88, 574. (b) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc. 1991, 45, 380. (c) Barner, B. J.; Green, M. J.; Saez, E. I.; Corn, R. M. Anal. Chem. 1991, 63, 55. (d) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (e) Blaudez, D.; Buffeteau, T.; Desbat, B.; Orrit, M.; Turlet, J. M. Thin Solid Films 1992, 210/211, 648. (f) Frey, B.; Hanken, D. G.; Corn, R. M. Langmuir 1993, 9, 1815. (g) Blaudez, D.; Buffeteau, T.; Castaings, N.; Desbat, B.; Turlet, J. M. J. Chem. Phys. 1996, 104, 9983. (h) Fukushima, H.; Seki, S.; Nishikawa, T.; Takigushi, H.; Tamada, K.; Abe, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417. (i) Shon, Y. S.; Lee, S.; Colorado, R., Jr.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (j) Nogues, C.; Lang, P.; Vilar, M. R.; Desbat, B.; Buffeteau, T.; El-Kassmi, A.; Garnier, F. Colloids Surf., A 2002, 198, 577. (k) Zamlynny, V.; Zawisza, I.; Lipkowski, J. Langmuir 2003, 19, 132. (l) Zawisza, I.; Lipkowski, J. Langmuir 2004, 20, 4579. (m) Bin, X.; Zawisza, I.; Goddard, J. D.; Lipkowski, J. Langmuir 2005, 21, 330. (n) Swanson, S. A.; McClain, R.; Lovejoy, K. S.; Alamdari, N. B.; Hamilton, J. S.; Scott, J. C. Langmuir 2005, 21, 5034. (o) Spori, D. M.; Venkataraman, N. V.; Tosatti, S. G. P.; Durmaz, F.; Spencer, N. D.; Z€urcher, S. Langmuir 2007, 23, 8053. (p) Nogues, C.; Lang, P.; Desbat, B.; Buffeteau, T.; Leiserowitz, L. Langmuir 2008, 24, 8458. (q) Zhang, S.; Jamison, A. C.; Schwartz, D. K.; Lee, T. R. Langmuir 2008, 24, 10204. (r) Fonder, G.; Cecchet, F.; Peremans, A.; Thiry, P. A.; Delhalle, J.; Mekhalif, Z. Surf. Sci. 2009, 603, 2276. (s) Rajalingam, K.; Hallmann, L.; Strunskus, T.; Bashir, A.; W€oll, C.; Tuczek, F. Phys. Chem. Chem. Phys. 2010, 12, 4390. (16) Buffeteau, T.; Desbat, B.; Blaudez, D.; Turlet, J. M. Appl. Spectrosc. 2000, 54, 1646. 6083

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084

Langmuir

ARTICLE

(17) Saccani, J.; Buffeteau, T.; Desbat, B.; Blaudez, D. Appl. Spectrosc. 2003, 57, 1260. (18) (a) Zawisza, I.; Wittstock, G.; Boukherroub, R.; Szunerits, S. Langmuir 2007, 23, 9303. (b) Zawisza, I.; Wittstock, G.; Boukherroub, R.; Szunerits, S. Langmuir 2008, 24, 3922. (c) Zawisza, I.; Nullmeier, M.; Pust, S. E.; Boukherroub, R.; Szunerits, S.; Wittstock, G. Langmuir 2008, 24, 7378. (19) Szunerits, S.; Coffinier, Y.; Janel, S.; Boukherroub, R. Langmuir 2006, 22, 10716. (20) Shioiri, T.; Ninomiya, K.; Yamada, S. J. Am. Chem. Soc. 1972, 94, 6203. (21) Gilman, J. G.; Otonari, Y. A. Synth. Commun. 1993, 23, 335. (22) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205–1209. (23) (a) Abeles, F. Ann. Phys. (Paris) 1948, 3, 504. (b) Buffeteau, T.; Desbat, B. Appl. Spectrosc. 1989, 43, 1027. (24) Yamamoto, K.; Ishida, H. Appl. Spectrosc. 1994, 48, 775. (25) Dignam, M. J.; Mamicheafara, S. Spectrochim. Acta, Part A 1988, 44, 1435. (26) (a) Buffeteau, T.; Grondin, J.; Lassegues, J. C. Appl. Spectrosc. 2010, 64, 112. (b) Boulet-Audet, M.; Buffeteau, T.; Boudreault, S.; Daugey, N.; Pezolet, M. J. Phys. Chem. B 2010, 114, 8255. (27) (a) McIntyre, J. D. E.; Aspnes, D. E. Surf. Sci. 1971, 24, 417. (b) McIntyre, J. D. E. Electrochemistry and Electrochemical Engineering; Wiley: New York, 1973; Vol. 9, p 61. (28) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1985. (29) Galeener, F. L.; Lucovsky, G. Phys. Rev. Lett. 1976, 37, 1474. (30) (a) Pelletier, I.; Laurin, I.; Buffeteau, T.; Pezolet, M. J. Phys. Chem. B 2004, 108, 7162. (b) Anglaret, E.; Brunet, M.; Desbat, B.; Keller, P.; Buffeteau, T. Macromolecules 2005, 38, 4799. (31) (a) Tsuboi, M. J. Polym. Sci. 1962, 59, 139. (b) Rothschild, K. J.; Clark, N. A. Biophys. J. 1979, 25, 473. (32) (a) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (b) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (33) Dieudonne, P.; Wong Chi Man, M.; Pichon, B.; Vellutini, L.; Bantignies, J. L.; Blanc, C.; Creff, G.; Finet, S.; Sauvajol, J. L.; Bied, C.; Moreau, J. J E Small 2009, 5, 503. (34) Stevens, M. J. Langmuir 1999, 15, 2773. (35) Fan, F.; Maldarelli, C.; Couzis, A. Langmuir 2003, 19, 3254.

6084

dx.doi.org/10.1021/la2006293 |Langmuir 2011, 27, 6076–6084