J. Phys. Chem. B 2009, 113, 1559–1568
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Structural Changes in a Polyelectrolyte Multilayer Assembly Investigated by Reflection Absorption Infrared Spectroscopy and Sum Frequency Generation Spectroscopy Peter J. N. Kett,† Michael T. L. Casford,† Amanda Y. Yang,‡ Thomas J. Lane,‡ Malkiat S. Johal,‡ and Paul B. Davies*,† Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom and Department of Chemistry, Pomona College, 645 North College AVenue, Claremont, California 91711 ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: October 21, 2008
The structure of polyelectrolyte multilayer films adsorbed onto either a per-protonated or per-deuterated 11mercaptoundecanoic acid (h-MUA/d-MUA) self assembled monolayer (SAM) on gold was investigated in air using two surface vibrational spectroscopy techniques, namely, reflection absorption infrared spectroscopy (RAIRS) and sum frequency generation (SFG) spectroscopy. Determination of film masses and dissipation values were made using a quartz crystal microbalance with dissipation monitoring (QCM-D). The films, containing alternating layers of the polyanion poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]1,2-ethanediyl, sodium salt] (PAZO) and the polycation poly(ethylenimine) (PEI) built on the MUA SAM, were formed using the layer-by-layer electrostatic self-assembly method. The SFG spectrum of the SAM itself comprised strong methylene resonances, indicating the presence of gauche defects in the alkyl chains of the acid. The RAIRS spectrum of the SAM also contained strong methylene bands, indicating a degree of orientation of the methylene groups parallel to the surface normal. Changes in the SFG and RAIRS spectra when a PEI layer was adsorbed on the MUA monolayer showed that the expected electrostatic interaction between the polymer and the SAM, probably involving interpenetration of the PEI into the MUA monolayer, caused a straightening of the alkyl chains of the MUA and, consequently, a decrease in the number of gauche defects. When a layer of PAZO was subsequently deposited on the MUA/PEI film, further spectral changes occurred that can be explained by the formation of a complex PEI/PAZO interpenetrated layer. A per-deuterated MUA SAM was used to determine the relative contributions from the adsorbed polyelectrolytes and the MUA monolayer to the RAIRS and SFG spectra. Spectroscopic and adsorbed mass measurements combined showed that as further bilayers were constructed the interpenetration of PAZO into preadsorbed PEI layers was repeated, up to the formation of at least five PEI/PAZO bilayers. Introduction Understanding the intrinsic structure-property relationships in ordered molecular assemblies has been motivated by developments in organic electro-optic materials.1-5 Despite some shortcomings, self-assembly has become the method of choice in achieving preferred architectures in organic thin film assemblies. The desire to guide molecular self-assembly into useful macro-structures has been driven by the realization that specific molecular geometries favor specialized functions, such as the nonlinear optical (NLO) effect observed in asymmetrically ordered multilayers. 2-6 In particular, azobenzene moieties containing electron donor and acceptor groups have been studied in self-assembled bilayers,7,8 Langmuir-Blodgett films,9 and in multilayer assemblies10,11 with the goal of fabricating materials with a large NLO response. Here we assembled polyelectrolyte films containing the polyanion poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) and the polycation poly(ethylenimine) (PEI) from aqueous solution using the layer-by-layer electrostatic self-assembly (ESA) method pioneered by Decher and co-workers.12,13 The polyanion PAZO contains an NLO-active moiety, and ESA films containing this * Author to whom correspondence should be addressed. E-mail: pbd2@ cam.ac.uk. † University of Cambridge. ‡ Pomona College.
polyelectrolyte show promise as potential NLO-active materials.14 The films were constructed on a gold substrate coated with a self-assembled monolayer (SAM) of 11-mercaptoundecanoic acid. The schematic representation of a single PAZO and PEI bilayer, as deposited on the SAM, is shown in Figure 1. ESA is a process that involves the sequential deposition of a polycation and a polyanion to yield charge-alternating multilayers. The success of this alternating deposition technique is based upon the electrostatic adsorption of a polyelectrolyte from aqueous solution onto a substrate primed with a polyelectrolyte of opposite charge. Charge overcompensation and entropic gain due to counterion expulsion are generally cited as the driving mechanisms behind layer formation.15 Recently, Lane et al. have observed the multistep kinetics of the ESA process in situ using real-time dual-beam polarization interferometry (DPI).16 Their results show that equilibrium between the two oppositely charged polymer layers has been reached well within the time scale of the spectral measurements reported here. In this work we use two forms of surface vibrational spectroscopy, namely, reflection absorption infrared spectroscopy (RAIRS) and sum frequency generation (SFG) spectroscopy, to examine the molecular-level ordering in electrostatically bound multilayer assemblies composed of the weak polyelectrolytes PEI and PAZO. Multilayer assemblies were constructed layer-by-layer and characterized by quartz crystal microbalance
10.1021/jp807453w CCC: $40.75 2009 American Chemical Society Published on Web 01/16/2009
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Figure 1. Schematic representation of the adsorption sequence of PEI and PAZO onto an MUA SAM.
with dissipation monitoring (QCM-D) experiments from which masses were obtained using the Sauerbrey approximation.17 In RAIRS, infrared light is shone onto a metal surface at a grazing angle of incidence. The vibrational spectrum of molecules adsorbed onto the surface is obtained by comparing the intensity of the reflected light from a clean surface to one covered by a thin film. The vibrational modes that are observable in RAIRS are governed by the metal surface selection rule. This states that only modes that have a component of their transition dipole moment perpendicular to the metal surface appear in a RAIRS spectrum. Hence, the appearance of a RAIRS spectrum indicates that the transition dipole moments of specific functional groups in the film molecules have a component parallel to the surface normal. SFG spectroscopy makes use of the nonlinear optical phenomenon of sum-frequency generation to provide a vibrational spectrum of molecules at a surface or interface.18 A surface containing the adsorbed film is irradiated simultaneously with a fixed-frequency pulsed visible laser (ωvis) and a tuneable pulsed infrared laser(ωir). Light emitted at the sum frequency ωSF (ωSF ) ωvis + ωir) is detected as the infrared laser frequency is scanned. Resonant features occur from functional groups with a net polar orientation at the interface, provided they also contain molecular vibrations that are both infrared and Raman active. Because SFG emission only occurs from noncentrosymmetric environments, the technique is highly interface specific. Hence, the SFG spectrum potentially carries information on the orientational and conformational ordering of the PEI/PAZO multilayer assembly. This information is complementary to that available from RAIRS. Experimental Section The polyanion PAZO (average MW ≈ 65 000-100 000 g mol-1, CAS No. 219957-04-7) and polycation PEI (MW ) 25000 g mol-1, CAS No. 9002-98-6, mixture of linear and branched chains) were purchased from Aldrich and were used
Kett et al. as received. Aqueous solutions (10 mM, based on monomeric molecular weight) of both polyelectrolytes were prepared using ultrapure water (resistivity >18.2 MΩ cm). For both solutions, the pH was 7 and the ionic strength was unadjusted. 11Mercaptoundecanoic acid (h-MUA, 99% purity), also obtained from Aldrich, was dissolved in ethanol to yield a 1 mM solution. Per-deuterated 11-mercaptoundecanoic acid, d-MUA, in which the hydrocarbon chain was >99% deuterated, was supplied by C. K. Gas Products (96.8% purity) and was also made up in 1 mM solutions. Details on solution and substrate preparations for polyelectrolyte multilayer assembly can be found elsewhere.19-22 Substrates used for the RAIRS and SFG experiments were prepared by thermally evaporating 200 nm gold coatings onto chromium-primed silicon wafers. The gold substrates were subsequently immersed in ethanolic h-MUA or d-MUA (1 mM) solutions for 24 h. PEI and PAZO were deposited layer-bylayer on the MUA monolayer using the ESA method. Briefly, clean Au substrates containing the MUA monolayer were immersed in a PEI solution for 5 min at room temperature, rinsed with ultrapure water, and dried under a stream of nitrogen gas. The substrates were then immersed in the PAZO solution for 5 min at room temperature, rinsed, and dried as before. This procedure was repeated until a predetermined number of PEI/ PAZO bilayers was constructed. Both SFG and RAIRS spectra were recorded layer-by-layer with PEI or PAZO as the topmost layer in films containing up to 5 bilayers. Single reflection RAIRS spectra were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer, using a liquid-nitrogen cooled MCT detector and a Specac grazing angle accessory set to an incidence angle of 85° and fitted with a ZnSe supported gold grid polarizer. The scans were background corrected against a clean gold sample, and the final spectra were obtained by averaging a minimum of 128 scans. Transmission spectra of solvent cast films on a Germanium ATR crystal were recorded on the same spectrometer as the RAIRS spectra, and a minimum of 16 scans were averaged to obtain the final spectra. SFG data were recorded on the Cambridge nanosecond laser spectrometer, which has been described in detail elsewhere.23 In brief, a Nd:YAG laser generates an infrared beam at 1064 nm that is frequency doubled to 532 nm using a KDP crystal. 90% of this beam is used to pump a dye laser that generates tunable visible light that is shifted into the infrared region by stimulated Raman scattering in a 34 atm H2 pressurized cell. The remainder of the visible light beam is delayed through a Herriot cell to allow both beams to overlap spatially and temporally at the surface, generating a sum frequency signal that is detected by a photomultiplier tube (Electron Tubes 9813). Spectra in the PPP beam polarization combination (sum frequency, visible, infrared) were recorded in the C-H alkyl stretching region, from 2800 to 3100 cm-1. Due to the inherently low nonresonant SFG signal from a metal surface using the SSP polarization combination, no reliable spectra were obtained with the SSP polarization combination. (Even in PPP polarization the spectra were relatively weak.) Data points were averages of 60 laser pulses per point scanned continuously at intervals of 2 cm-1. Scans were accumulated for approximately 6 h to compose a final spectrum. The modeling of the SFG spectra used a least-squares Levenberg-Marquardt algorithm to fit the resonance profiles to a Lorentzian description of the second-order susceptibilities. The modeling allows the frequency, strength, and widths of the vibrational resonances to be determined as well as the strength and phase of the nonresonant susceptibilities. The intensity of
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the recorded signal, ISF, depends on both resonant χ(2) R , and (2) , susceptibilities thus, nonresonant, χNR
|
(2) ISF ∝ χR(2) + χNR
|
2
This equation can then be expanded into polar coordinates
|
(2) iε ISF ∝ |χR(2) |eiδ + |χNR |e
|
2
(2) 2 (2) ∝ |χR(2) | 2 + |χNR | + 2|χR(2) ||χNR |cos[ε-δ] (1)
where δ and ε are the phases of the resonant and nonresonant terms, respectively. χ(2) R is given by
χR(2) )
B (ων - ωIR - iΓ)
where B is the strength of the resonance, Γ-1 is the relaxation time of the vibrationaly excited state, ωIR is the wavenumber of the IR laser beam, and ων is the wavenumber of a SF active vibration. This equation rearranges to,
|χR(2) | )
B B × (ων - ωIR - iΓ) (ων - ωIR + iΓ) )
B2 (ων - ωIR)2 + Γ2
that has the height form of a Lorentzian line shape,
y)
HW2 (ωv - ωIR)2 + W2
and hence,
|χR(2) | )
HW2 (ωv - ωIR)2 + W2
with B ) √H · W and Γ ) W. Substituting in the Lorentzian parameters for the magnitude and phase of the second order susceptibility into eq 1 yields,
ISF ∝ 2
2
HW (2) 2 + |χNR | + (ωv - ωIR)2 + W2
[
(
-W HW2 |χ(2) |cos ε-arctan 2 2 NR (ω (ωv - ωIR) + W v - ωIR)
)]
The least-squares fitting routine uses this equation to simulate 2 the observed spectrum, changing the parameters H, W, ωv, |χ(2) NR| , and ε to minimize the error between the simulation and the experiment. QCM-D (E4, Q-Sense) was used to obtain the adsorbed mass of the PEI and PAZO in the multilayer assemblies. All aqueous polyelectrolyte solutions were allowed to flow over a 14 mm diameter AT-cut quartz resonator with either SiO2-coated gold electrodes or MUA/gold-coated electrodes. The total volume space above the crystal was 40 µL. Prior to use, the quartz crystals were thoroughly washed in 2% Hellmanex II solution, and then ultrapure water. The crystals were first dried under a stream of N2 and then cleaned with UV ozone for 30 min at 25 °C. Finally, ultrapure water was allowed to flow over the crystal for 30 min prior to the first polyelectrolyte deposition.
Figure 2. (a) Changes in frequency (filled squares) and dissipation values (open squares) during the assembly of PEI on a SiO2 surface (upper) and a functionalized MUA surface (lower). The time at which PEI is exposed to the surface and the subsequent water rinses are indicated. (b) Mass (filled squares) and dissipation values (open squares) as a function of polyelectrolyte layer number. The 10 bilayers of PEI and PAZO were constructed on a MUA-coated Au substrate.
Results Polyelectrolyte layers of PEI and PAZO were assembled on a MUA monolayer commencing with PEI. QCM-D was used to characterize the formation of a multilayer film in situ and in real time. Figure 2a shows how the QCM resonant frequency and the dissipation values change when a substrate is exposed to the 10 mM PEI solution. Frequency change (∆F) measurements are approximately inversely proportional to the mass deposited;17 however, the frequency will typically under-report the mass of a loosely coupled film.24 Dissipation (∆D) is a measure of the energetic loss of a film, and this value will increase if the film becomes “floppy”, or less rigid. For comparison purposes, the PEI was exposed to both a negatively charged silicon oxide surface and a gold-coated QCM crystal functionalized with the MUA SAM. In both cases the frequency drops suggesting a mass increase due to the adsorption of the polyelectrolyte. Interestingly, larger net frequency shift, and therefore mass deposition, is observed when PEI adsorbs to the solid SiO2 surface compared to the MUA layer. Figure 2b shows the linear increase in the mass of the PEI/PAZO film assembled on the MUA SAM as a function of layer number. The dissipation values increase up to 5 bilayers, beyond which no
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Figure 3. RAIRS spectra of MUA and ODT on gold, (a) h-MUA in the C-H stretching region, (b) d-MUA in the C-D stretching region, and (c) ODT in the C-H stretching region.
discernible trend is observed. The relative standard deviations of the masses, frequencies, and dissipation values are only a few percent. In fact, using substrates of varying roughness yields an upper limit to the relative standard deviation of about 15%. Detailed RAIRS and SFG spectra were recorded for h-MUA and d-MUA on Au, and after sequential additions of PEI or PAZO, layer-by-layer, up to 5 bilayers. RAIRS and SFG spectra were also recorded for an octadecane thiol (ODT) SAM on Au. The RAIRS spectrum of h-MUA in the C-H stretching region (Figure 3a) showed two bands at 2849 and 2924 cm-1 corresponding to the methylene d+ and d- resonances. The corresponding C-D vibrational bands in d-MUA at 2091 and 2195 cm-1 are shown in Figure 3b. The RAIRS spectrum of ODT exhibited five bands in the C-H region (Figure 3c). These can be assigned to the symmetric (d+, 2848 cm-1) and asymmetric (d-, 2916 cm-1) methylene modes, the asymmetric methyl mode (r-, 2963 cm-1), and the symmetric methyl mode split by a Fermi resonance into two bands at 2878 (r+) and 2938 cm-1 (r+FR). The presence of d modes in a RAIRS spectrum indicates that the methylene groups of the adsorbed molecule are orientated, at least in part, parallel to the surface normal. This could be due to the hydrocarbon chains being tilted away from the surface normal and/or because they are disordered and contain gauche defects. Corroboratory data concerning the degree of disorder in the alkyl chains of a monolayer can be determined by examining the position of the asymmetric methylene mode (d-). Qualitatively, it has been shown that as the number of gauche defects in a polymethylene chain decreases and the chains in a monolayer becomes more ordered, the frequency of the d- mode also decreases, with a frequency of about 2918 cm-1 denoting a qualitative change from ordered to disordered chains.25,26 As the asymmetric methylene mode in the RAIRS spectrum of ODT
Kett et al.
Figure 4. Sum Frequency spectra recorded in the C-H stretching region with the ppp beam polarization combination of (a) h-MUA, (b) d-MUA, and (c) ODT monolayers adsorbed onto gold.
is at 2916 cm-1, it implies that the alkyl chains in the monolayer are ordered. This is in agreement with previous studies that have shown that ODT forms regular monolayers with alkyl chains in all-trans conformations.27 The presence of methylene bands in the RAIRS spectrum is thus due to the alkyl chains of ODT being tilted relative to the surface normal. MUA has a large terminal carboxylic acid group connected to a C10 alkyl chain and is therefore not expected to form a close-packed monolayer on Au. In the RAIRS spectrum of the h-MUA SAM the dmode lies at 2924 cm-1, which implies that the alkyl chains in the monolayer contain gauche defects and hence are less ordered than in an ODT SAM. Figure 4a shows a SFG spectrum of h-MUA/Au in air. Three relatively strong resonances due to the alkyl chain methylene groups are observed at 2861, 2908, and 2932 cm-1. Asanuma et al. have considered the assignment of the SFG spectra of alkoxy-terminated dodecyl monolayers on silicon.28 Following their assignment, the three bands in Figure 4a correspond closely to the symmetric stretch, d+ (2855 cm-1), and the two asymmetric stretches, dω- (2905 cm-1) and d- (2934 cm-1). The strong methylene resonances that appear in the SFG spectra are indicative of the presence of a significant population of gauche defects in the alkyl chains, corroborating the results observed in the RAIRS spectra.29 The SFG spectrum of d-MUA in the C-H region is shown in Figure 4b. As expected, there are no resonances intense enough to be modeled. For comparison, the SFG spectrum of octadecane thiol (ODT) on Au is shown in Figure 4c. In contrast to h-MUA, the ODT/Au spectrum exhibits no CH2 features. This is because the d resonances are all SFG inactive, on account of ODT forming a close-packed monolayer with the alkyl chains in an almost all-
Structural Changes in a Polyelectrolyte SAM
Figure 5. (a) RAIRS spectrum of h-MUA on gold below 2000 cm-1, (b) h-MUA transmission spectrum below 2000 cm-1, (c) RAIRS spectrum of d-MUA below 2000 cm-1, and (d) d-MUA transmission spectrum below 2000 cm-1.
trans conformation, so that the methylene groups are locally centro-symmetric.30 Only resonances due to the terminal CH3 groups are therefore observed for ODT/Au. These resonances are the symmetric stretch split by Fermi resonance at 2873 cm-1 (r+) and 2932 cm-1 (r+FR), and the asymmetric stretch (r-) at 2962 cm-1. Figure 5 shows the RAIRS and transmission spectra of hand d-MUA between 800 and 2000 cm-1. In both the RAIRS spectra (Figures 5a and 5c), the carbonyl stretch of MUA at ∼1700 cm-1 is very weak in spite of the mode being strong in the transmission spectra (Figures 5b and 5d). This suggests that on adsorption to Au, either MUA becomes deprotonated, so there are carboxylate rather than carbonyl groups present, or the carbonyl group lies parallel to the metal surface, and hence, by the metal surface selection rule, is RAIRS inactive. The former interpretation is supported by the presence of two strong modes at 1458 and 1552 cm-1 in the RAIRS spectrum of both d- and h-MUA. By comparison to literature values, and as a result of them being absent from the transmission spectra, they can be assigned to the symmetric and asymmetric carboxylate modes, respectively.31 However, as it has previously been shown that on adsorption to gold MUA becomes less acidic, ionization of all the acid groups is unlikely.32 With this in mind, the absence of the carbonyl band in the RAIRS spectra therefore suggests that an MUA SAM contains a mixture of protonated and deprotonated acid groups with the protonated groups lying parallel to the gold surface. The changes in the RAIRS spectra in the C-H and C-D stretching regions on depositing a layer of PEI on the MUA film are shown in Figure 6, panels a, b, and c. The RAIRS spectrum of the C-H stretching region, when PEI is adsorbed
J. Phys. Chem. B, Vol. 113, No. 6, 2009 1563
Figure 6. RAIRS spectra of MUA and MUA/PEI adsorbed on gold (a) d-MUA (dashed red line) and PEI on d-MUA (solid black line) in the C-H stretching region, (b) h-MUA (dashed red line) and PEI on h-MUA (solid black line) in the C-H stretching region, and (c) d-MUA (dashed red line) and PEI on d-MUA (solid black line) in the C-D stretching region.
on d-MUA (Figure 6a), can be assigned to PEI itself and indicates that a significant proportion of the methylene groups of the polymer are orientated with a component of their dipole moment parallel to the surface normal. Comparison of the relative intensities and spectral positions of the two bands in the h-MUA/PEI layer (Figure 6b) with the corresponding d-MUA/PEI spectrum (Figure 6a) shows that the spectrum of the h-MUA/PEI bilayer is dominated by PEI, with little contribution from the h-MUA. In the C-D region the vibrational features due to the d-MUA almost completely disappear on adsorption of PEI (Figure 6c) and remain so for all subsequent layers of PEI or PAZO deposited. This decrease in intensity of the methylene stretching bands of MUA on adsorption of PEI can be explained by a structural change in the MUA layer causing a reduction in the number of MUA methylene groups oriented parallel to the surface normal. The SFG spectra in the C-H stretching region when a PEI layer is deposited on h-MUA or d-MUA are shown in Figure 7. (The decreased infrared laser beam intensity of the spectrometer at longer wavelengths meant that no SFG spectral features were detectable in the C-D region from the d-MUA or from d-MUA on which PEI had been deposited.) Figures 7a and 7b show the SFG spectrum of h-MUA and h-MUA/PEI, respectively, and correspond to the two RAIRS spectra shown in Figure 6b. When PEI is adsorbed onto the h-MUA SAM, the resonances weaken to approximately 25% of their intensity with MUA on its own, red shift, and broaden (Figure 7b). Although the resonances weaken, two of them can be readily modeled. Assuming they are the d+ and dω bands, the red shifts are 7 and 5 cm-1, respectively. The d- resonance is less easily
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Figure 7. Sum Frequency spectra of (a) h-MUA, (b) PEI on h-MUA, and (c) PEI on d-MUA.
modeled due to the broadening of the spectral features. Assuming it is the shoulder on the blue side of the 2904 cm-1 resonance, it yields a red shift of 7 cm-1. In the SFG spectrum of d-MUA in the C-H region, no resonances could be modeled as there was no significant resonant contribution to the sum frequency signal (Figure 4b). However, an SFG spectrum was measured when PEI was deposited on d-MUA (Figure 7c). The SFG spectrum in Figure 7c differs in profile somewhat from the corresponding h-MUA/PEI spectrum shown in Figure 7b, confirming that the latter spectrum has a small but detectable contribution from the h-MUA SAM. The spectrum in Figure 7c can be modeled with two spectral resonances at 2869 and 2923 cm-1, which can be assigned to the methylene groups of PEI; hence, it corresponds to the SFG spectrum of the PEI layer alone. The weakening of the d resonances of h-MUA on adsorption of PEI is indicative of a decrease in the number of gauche defects in the alkyl chains of MUA. This could only result from a straightening of the alkyl chains in the MUA layer, possibly induced by electrostatic interactions between the monolayer and adsorbed polymer. A decrease in intensity of the sum frequency resonances along with the observed red shifts and broadening could alternatively be explained in terms of a complete disordering of the MUA monolayer. However, this interpretation is inconsistent with the RAIRS results. If the SAM became more disordered on adsorption of PEI, then the methylene groups of the d-MUA would become randomly oriented and the intensity of the C-D stretching modes in Figure 6c would have remained constant or increased on adsorption of PEI. Instead, these modes become markedly less intense, implying that the monolayer has become more ordered. The RAIRS spectra between 600 and 2000 cm-1 of a PEI layer and a PEI/PAZO bilayer adsorbed onto a d-MUA SAM, along with the transmission spectra of PEI and PAZO, are shown
Kett et al.
Figure 8. (a) RAIRS spectrum of PEI on d-MUA below 2000 cm-1, (b) PEI transmission spectrum below 2000 cm-1, (c) RAIRS spectrum of PEI/PAZO bilayer on d-MUA below 2000 cm-1, and (d) PAZO transmission spectrum below 2000 cm-1.
in Figure 8. The transmission spectrum of PEI is shown in Figure 8b, and the major bands at 1599 and 1655 cm-1 can be assigned to the scissoring modes of primary and secondary amines. The bands at 1460 and 1342 cm-1 are due to the scissoring modes of methylene and methyne groups respectively, whereas the bands below 1300 cm-1 are due to C-C and C-N bond stretches.31 The RAIRS spectrum of PEI adsorbed onto a d-MUA SAM is shown in Figure 8a and can be seen to closely resemble the PEI transmission spectrum, albeit with a shift in the amine N-H scissoring modes to 1569 and 1635 cm-1 and the appearance of a band at 1406 cm-1. These band positions in Figure 8a are in very good agreement with those seen by Cai et al., who used infrared spectroscopy to look at supramolecular complexes of PEI and octadecanoic acid (OA).33 They attributed the shift in N-H scissoring modes to the protonation of the amine groups of PEI and assigned the band at 1406 cm-1 to the symmetric carboxylate stretch of the deprotonated acid. They also assigned the band at ∼1560 cm-1 to an overlap of the asymmetric carboxylate stretch and an N-H scissoring mode of a protonated amine. The shift in the symmetric carboxylate stretch from 1458 to 1406 cm-1 on adsorption of PEI can be attributed to a change in the cation coordinated to the carboxylate groups of MUA.34 Cai et al. also managed to detect the formation of hydrogen bonds between PEI and OA, by observing the position of the carbonyl mode of OA. Unfortunately, as the carbonyl mode is absent from the RAIRS spectra in Figure 8, it is not possible to determine whether there is any hydrogen bonding between the carboxylic acid groups of MUA and the amines of PEI. Figure 8c shows the RAIRS spectrum of a PEI/ PAZO bilayer adsorbed onto a d-MUA SAM. The bands observed are primarily from PAZO (Figure 8d) with little evidence of the bands seen in Figure 8c from the PEI and MUA. The RAIRS spectra arising from the addition of further alternating layers of PEI and PAZO on d-MUA in the C-H
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Figure 9. RAIRS spectra in the C-H region of PEI/PAZO multilayers with either PEI (solid black line) or PAZO (dashed red line) as the uppermost layer. The numbers on the spectra refer to how many polyelectrolyte layers have been adsorbed.
region are shown in Figure 9. The spectra, which showed good reproducibility (see the Supporting Information), consist of bands at approximately 2850 and 2930 cm-1. As PAZO only absorbs weakly in the C-H region, changes in the intensities of these bands must arise from changes to the structural orientation of the PEI in the film. Specifically, an increase in intensity indicates an increased orientation of the PEI methylene groups parallel to the surface normal. The spectra arising from the sequential deposition of the PEI and PAZO layers are indicated in the numerical order in which they were deposited, in Figure 9. The spectral intensity of the first two PEI layers, that is, spectrum 1 (PEI) and spectrum 3 (PEI-PAZO-PEI), are greater than the intensity of the film with PAZO as the topmost layer, namely, spectrum 2 (PEI-PAZO) and spectrum 4 (PEIPAZO-PEI-PAZO). This trend is repeated to a greater or lesser extent up to the tenth layer of polyelectrolyte added, namely, the final PAZO layer. The decrease in intensity on adsorption of PAZO could be due to a loss of PEI from the surface. However, as the QCM results in Figure 2 show that the adsorbed mass increases with each layer, the decrease in peak intensity on adsorption of PEI is almost certainly an orientation effect rather than due to concentration changes.15 The structural conclusion to be drawn from the observation is that the degree of orientation of the methylene groups of the PEI along the surface normal is reduced when the PAZO layer is added, by comparison to the orientation when a fresh PEI layer is the topmost layer of the film. The SFG spectra recorded after the addition of PAZO and further PEI/PAZO layers to the MUA/PEI film are shown in Figure 10. Figure 10a is reproduced from Figure 7b for ease of comparison. Figure 10b shows the absence of any significant resonant contribution to the SFG signal when a layer of PAZO is adsorbed onto an h-MUA/PEI film, hence no resonances could be modeled. PAZO is Raman inactive and only weakly IR active in the C-H region and would not be expected to exhibit a strong SFG spectrum if the air/film interface comprised solely a PAZO layer. The absence of any resonant contribution to the SFG signal from the h-MUA layer could indicate that further stretching of the h-MUA alkyl chains on adsorption of PAZO has occurred, and thus there has been a decrease in the number of gauche defects or, more likely, that the resonant SFG signals being observed primarily arise from the air/film interface rather than the film/metal interface. Figure 10c shows that the addition of the next layer, a PEI layer, to the h-MUA/PEI/PAZO film
Figure 10. Sum frequency spectra of (a) PEI on h-MUA, (b) PEI/ PAZO on h-MUA, (c) PEI/PAZO/PEI on h-MUA, and (d) PEI/PAZO/ PEI on d-MUA.
produces a weak but identifiable SFG spectrum in the C-H region. The spectrum in Figure 10c is, at least qualitatively, more similar to the spectrum of PEI on d-MUA (Figure 7c) than to the spectrum in Figure 10a, which is also a film having a topmost PEI layer. It also closely resembles the spectrum of a PEI/PAZO/PEI layer absorbed onto a d-MUA SAM (Figure 10d). This strongly suggests that there is no longer a contribution from the h-MUA SAM to the SFG spectra of films which contain more than one PEI/PAZO bilayer and that the spectrum of the h-MUA/PEI/PAZO/PEI film is solely that of the uppermost PEI layer. The sequential pattern of spectra, that is, the absence or presence of an SFG spectrum for films with topmost layers of PAZO or PEI, respectively, continues up to at least five bilayers. This observation can be compared to recent second harmonic generation (SHG) studies, where Casson et al. found that the SHG signal increased up to three PEI/PAZO bilayers before leveling off.14 The intensity of the SHG signal remained large only when PAZO was the outer layer, otherwise it was attenuated. These complementary results show that the two nonlinear optical techniques are essentially both probing the air/film interface. RAIRS spectra were also recorded in the region below 2000 cm-1 for up to 5 PEI/PAZO bilayers adsorbed onto a d-MUA SAM. For the films with more than one PEI/PAZO bilayer, the spectra bore close resemblance to the transmission spectrum of PAZO (Figure 8d) with numerous, overlapping bands. Two regions of these spectra (1050-1225 cm-1 and 1525-1800 cm-1) are reproduced in Figure 11. They were selected as the changes in intensity seen on adsorption of PEI or PAZO are representative of the spectrum as a whole. There is a repeating pattern of increased intensity on adsorption of PAZO and either no change or a small decrease in intensity on adsorption of PEI. Unlike in the C-H region, where only one of the polymers had strongly infrared active modes, both PEI and PAZO absorb
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Figure 11. RAIRS spectra from 1050-1225 cm-1 and 1500-1800 cm-1 of PEI/PAZO multilayers with either PEI (solid black line) or PAZO (dashed red line) as the uppermost layer.
in the regions shown in Figure 11, making it difficult to distinguish between the contributions to the spectra from the individual polymers. However, it is notable that when PEI is the top layer, the intensity of the bands does not increase in spite of the polymer having infrared active modes in these regions. This suggests that the contribution to the band intensity from the underlying PAZO layer must decrease in order to compensate for any absorbance by the PEI top layer. This could come about if the PAZO layer reorients on adsorption of PEI, so that more of the NLO-active moieties (which are primarily responsible for the bands seen in the infrared spectrum) are lying parallel to the metal surface. Discussion The QCM-D data shows that the formation of a PEI layer on the MUA SAM results in a decrease in frequency (increase in mass) and a significant drop in dissipation. This is in sharp contrast to what is observed when PEI is adsorbed to a hydrophilic silicon oxide surface, where a drop in frequency is accompanied by a significant increase in dissipation. The ratio ∆D/∆F is an excellent qualitative measure of film rigidity (or “floppiness”). Figure 12 shows a plot of ∆D/∆F during the formation of a PEI layer on both a SiO2 coated QCM substrate and a MUA-functionalized Au-coated QCM crystal. When PEI adsorbs on the MUA SAM, an increase in ∆D/∆F is observed. We interpret this increase as a strong coupling between the MUA layer and the PEI, resulting in a rigid and dense film that is strongly coupled to the QCM substrate. In contrast, when PEI adsorbs on the SiO2 surface, there is a significant decrease in ∆D/∆F. These observations are consistent with a strong interaction between PEI and the MUA SAM, resulting in the formation of a film that is highly interpenetrated and dense compared to the SAM alone. In both experiments, the value of ∆D/∆F increases when the assembly is rinsed with ultrapure water. This is likely to be due to the swelling of the film under a pure aqueous environment. The RAIRS and SFG spectra presented in Figures 3 and 4 indicate there is greater disorder in an MUA SAM compared to a corresponding ODT monolayer. The d resonances in the SFG spectrum and the high frequency methylene symmetric stretch seen in the RAIRS spectrum both point toward a monolayer in which the alkyl chains contain gauche defects. The absence of a carbonyl band and the strong carboxylate bands in the RAIRS spectra of h- and d-MUA showed that at least
Figure 12. Changes in frequency/dissipation ratio during the assembly of PEI on a SiO2 surface (upper) and a functionalized MUA surface (lower). The time at which PEI is exposed to the surface and the subsequent water rinses are indicated.
some of the carboxylic acid groups are deprotonated in the monolayer. As the carbonyl mode was absent from the RAIRS spectra of both h- and d-MUA (Figures 5a and 5c), it was not possible to determine whether complete ionization of the acid groups in the SAM had occurred. If, as anticipated, not all the acid groups are deprotonated, then any protonated acid groups would have to be parallel to the metal surface, so that the component of the transition dipole moment of the carbonyl stretch along the surface normal is zero. Having all the carbonyl groups lying in the same plane (or at least parallel to each other) would seem a surprising result, as it would imply that there is a reasonably high degree of ordering within the monolayer. However, evidence has been previously found of strong interactions, in the form of hydrogen bonds, between the head groups in acid-terminated monolayers.35 Therefore, it may be that in an MUA SAM the alkyl chains are bent over, forming the gauche defects observed in the SFG and RAIRS spectra, to allow the carboxylic acid groups to interact with each other. Whether or not a carboxylic acid is involved in hydrogen bonding, the type of hydrogen bonding that occurs (e.g., linear chains, cyclic dimers) could be determined from the position of the carbonyl stretching mode. It would be possible to confirm whether or not an MUA SAM is entirely composed of ionized acid groups, and whether there is any hydrogen bonding between the head groups in the monolayer, from the S-polarized spectrum of MUA (i.e., the spectrum when vibrations with a transition dipole moment parallel to the metal surface are active). The metal surface selection rule, however, means the S-polarized spectrum on gold cannot be recorded using an external reflection technique such as RAIRS. The distinctive changes in both the RAIRS and SFG spectra on adsorption of PEI (Figures 6b, 6c, and 7) demonstrate that there is a strong interaction between the polymer and the SAM that leads to structural changes in the MUA/PEI film. The decrease in the intensities of the methylene SFG resonances when the PEI layer is added implies a decrease in the number of gauche defects in the MUA chains. However, the similarity of the SFG spectra of the films of d-MUA/PEI and of h-MUA/ PEI (Figures 7b and 7c) shows that most of the methylene
Structural Changes in a Polyelectrolyte SAM resonance intensity is contributed by the methylene groups of PEI itself. The corresponding RAIRS spectra of the same films are the dashed lines in Figure 6b and 6a, which qualitatively confirm that PEI contributes to the spectrum of the h-MUA/ PEI film. The almost complete disappearance of the CD2 bands in the d-MUA RAIRS spectrum when a PEI layer is added (Figure 6c) and the sharp fall in intensity of the CH2 band in the MUA spectrum (Figure 6b) favor the conclusion that the SFG spectral changes are due to a reduction of gauche defects in the MUA chains when PEI is added. The reduction in gauche defects will lead to fewer methylene groups with a net orientation parallel to the surface normal and, hence, a reduction in the RAIRS intensity. In Figure 8, the N-H scissoring modes of PEI were seen to redshift on adsorption to an MUA SAM. This change was attributed to a protonation of the amine groups of PEI. An electrostatic interaction between the positively charged polymer and the carboxylate groups of the MUA SAM could result in the straightening of the alkyl chains that the RAIRS and SFG spectra have suggested. The redshift of the symmetric carboxylate stretch of MUA on adsorption of PEI provided direct evidence of such an electrostatic interaction. The disappearance of the methylene bands in the SFG spectrum when PAZO is added (Figure 10b) suggests one of two possibilities. Either that there is little PEI at the interface, the PAZO forming a topmost nonpenetrating film, or that the PAZO has penetrated the PEI layer due to the strong electrostatic interaction to give rise to an isotropic distribution of the PEI methylene groups residing at the air interface which, lacking a net polarization along the surface normal, do not give a SFG spectrum. It is well-known that layers within ESA multilayer assemblies are highly interpenetrated. In particular, previous studies have shown that PAZO adds about five times more mass to the assembly compared to PEI and leads to extensive interpenetration.20 This has been confirmed by the very recent DPI results that the final step of the adsorption of PAZO on a nominally PEI layer is the interpenetration of the PAZO strands. (Interestingly the former possibility to explain the absence of a SFG spectrum, namely that PAZO forms a flat nonpenetrating layer on top of the PEI, is proposed by Lane et al. as an intermediate step leading to the final interpenetration achieved at equilibrium.16) The RAIRS results support the latter of the two possibilities mentioned above. It should first be recalled that PAZO has negligible infrared band intensity in the C-H region, so changes in the multilayer RAIRS spectra must arise from changes in the structure of the PEI layer. Before the addition of the first complementary PAZO layer, RAIRS of the MUA/PEI layer indicates a finite number of methylene groups oriented parallel to the surface normal. According to the intensity of the RAIRS spectrum this number reduces when the PAZO layer is added. As previously stated, this can be interpreted as due to interpenetration of the PAZO molecules into the PEI layer, reducing the net orientation of the PEI methylene groups in the vertical direction. It is reasonable to assume that a nonpenetrating PAZO layer would result in a constant net orientation of the PEI methylene groups and hence no reduction in the PEI RAIRS intensity when the PAZO layer was added. The formation of a PEI/PAZO complex driven by the opposite charges of the two polymers leaves little residual positive charge for interacting with the next layer of PEI deposited on the surface. The intensity of the RAIRS spectrum when this PEI layer is deposited again increases, as its structure is expected to resemble that of the first PEI layer on the MUA SAM (before the addition of the
J. Phys. Chem. B, Vol. 113, No. 6, 2009 1567 PAZO layer). When the next PAZO layer is added it again reduces the intensity of the underlying PEI layer due to interpenetration. This mechanism accounts for the repetitive increase and decrease in spectral intensity shown in Figure 9, continuing up to five PEI/PAZO bilayers. The picture that emerges then is notionally of a series of bilayers of PEI and PAZO, the lower layer of which is PEI. When first deposited, the PEI forms a relatively ordered layer with a proportion of its methylene groups having a net orientation parallel to the surface normal as characterized by the RAIRS spectrum. However, the strong electrostatic interaction and consequent interpenetration when the oppositely charged PAZO layer is added causes a noticeable change in the structure of the underlying PEI layer, namely a reduction in the orientation of the PEI methylene groups and a decrease in the RAIRS intensity. Therefore, at equilibrium the film comprises a more homogeneous mixture of the two polymers than the initial layer-bylayer deposition would suggest. Finally, it is worth noting that UV/visible absorption measurements (not shown) suggest that the PAZO chromophores aggregate in a “head-to-tail” arrangement.19,36 These measurements also indicated that the degree of aggregation only increased slightly with the number of bilayers, suggesting that this type of ordering may primarily be a surface effect. Summary and Conclusion SFG and RAIRS spectra were obtained for multilayer assemblies of PEI and PAZO deposited on an 11-mercaptoundecanoic acid self-assembled monolayer (SAM). The formation of the multilayers was confirmed by QCM-D measurements. The SFG spectra of the SAM exhibited strong CH2 resonances, indicating the presence of gauche defects in the alkyl chains of the acid. On adsorption of a layer of PEI, these resonances remained, but with decreased intensity. By using a SAM of perdeuterated MUA, they were shown to arise mainly from the PEI component with a smaller contribution from the SAM. There was also a decrease in the intensity in the RAIRS spectrum of the methylene stretching modes of MUA on adsorption of PEI. These two results taken together indicated that electrostatic interactions between the PEI and MUA, probably involving penetration of the polyelectrolyte into the SAM, resulted in a straightening of the MUA alkyl chains and a corresponding loss of gauche defects. On addition of PAZO to the MUA/PEI film, the intensity of the vibrational modes in the RAIRS spectrum decreased, and the resonances in the SFG spectrum were lost. Because PAZO is only weakly infrared active in the C-H stretching region, changes in the RAIRS spectra must arise from changes in the structure of the PEI layer. Within these layers there is a substantial decrease in the net orientation of the PEI methylene groups parallel to the surface normal. This can be explained in terms of the formation of highly interpenetrated layers and a reorientation of the underlying PEI layer on adsorption of PAZO. This structure would also account for the absence of a SFG spectrum when the topmost added layer is PAZO. Hence, although the polyelectrolytes were deposited layer-by-layer, the resultant film resembles more of a homogeneous mixing of the two polymers rather than defined stratified layers. Acknowledgment. M.S.J. and A.Y.Y. would like to thank the Pomona-Downing College (Cambridge) Faculty Exchange Program and the Pomona College Department of Chemistry for financial support. A.Y.Y. also acknowledges support from the
1568 J. Phys. Chem. B, Vol. 113, No. 6, 2009 Rose Hills Foundation. P.J.N.K. thanks the EPSRC for a studentship, and T.J.L. thanks the Arnold and Mabel Beckman Foundation. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Molecular Optics: Materials, Phenomena, and DeVices. Zyss, J., Ed; Chem. Phys. 1999, 245 (Special Issue). (2) Marder, S. R.; Kippelen, B.; Jen, A. K. Y.; Peyghambarian, N. Nature 1997, 388, 845. (3) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. ReV. 1994, 94, 195. (4) Liu, M.; Ushida, K.; Kira, A.; Nakahara, H. J Phys. Chem. B. 1997, 101, 1101. (5) Owaku, K.; Goto, M.; Ikariyama, Y.; Aizawa, M. Sens. Actuators, B 1993, 13-14, 723. (6) Johal, M. S.; Cao, Y. W.; Chai, X. D.; Smilowitz, L. B.; Robinson, J. M.; Li, T. J.; McBranch, D.; Li, DeQuan Chem. Mater. 1999, 11 (8), 1962–1965. (7) Johal, M. S.; Parikh, A. N.; Lee, Y.; Casson, J. L.; Foster, L.; Swanson, B. I.; McBranch, D. W.; Li, D. Q.; Robinson, J. M. Langmuir 1999, 15, 1275. (8) Pedrosa, J. M.; Romero, M. T. M.; Camcho, L.; Mobius, D. J.Phys. Chem. B 2002, 106, 2583. (9) Calculitan, N. G.; Scudder, P. H.; Rodriguez, A.; Casson, J. L.; Wang, H. L.; Robinson, J. M.; Johal, M. S. Langmuir 2004, 20, 8735. (10) Whitten, D. G.; Chem, L.; Geiger, H. C.; Perlstein, J.; Song, X. J.Phys. Chem. B 1998, 102, 10098. (11) Kinoshita, T. J.Photochem. Photobiol., B 1998, 42, 12. (12) Decher, G. Science 1997, 277, 1232–1237. (13) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210211, 831–835. (14) Casson, J. L.; McBranch, D. W.; Robinson, J. M.; Wang, H.-L.; Roberts, J. B.; Chiarelli, P. A.; Johal, M. S. J. Phys. Chem. B. 2000, 104, 11996–12001. (15) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592.
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