Article pubs.acs.org/Langmuir
Probing Cavitand−Organosilane Hybrid Bilayers via Sum-Frequency Vibrational Spectroscopy A. Aprile,† P. Pagliusi,*,†,‡ F. Ciuchi,‡ M. P. De Santo,†,‡ R. Pinalli,§ and E. Dalcanale§ †
Dipartimento di Fisica and ‡Licryl Laboratory, CNR-IPCF UOS di Cosenza, Universitá della Calabria, Ponte Pietro Bucci 33B, 87036 Arcavacata di Rende Cosenza, Italy § Dipartimento di Chimica, Universitá di Parma and INSTM, Udr Parma, Viale G. Usberti 17/A, 43100 Parma, Italy ABSTRACT: Quinoxaline cavitands (QxCav) are transferred by Langmuir-Schaefer method on selfassembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) and N,N-dimethyl-N-octadecyl-3aminopropyltrimethoxysilyl chloride (DMOAP) on fused silica substrates. The molecular architectures of both the hydrophobic SAMs templates and the hybrid cavitand-organosilanes bilayers at the solid−air interface are investigated and correlated by sum-frequency vibrational spectroscopy. The results show that QxCav are always in the closed vase configuration and orient with their principal axis normal to the substrates. The role of the alkyl chains density in the SAM templates on the QxCav transfer ratio is pointed out.
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layers provide a large stabilizing force to the bilayer,20 while still preserving its two-dimensional fluidity. Here we investigate the molecular architecture of quinoxaline cavitand (QxCav)− organosilane SSHBs, obtained by deposition of a QxCav monolayer on top of fused silica substrates coated with different long-chain silane surfactants: octadecyltrichlorosilane (OTS) and N,N-dimethyl-N-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP). We take advantage of a versatile analytical nonlinear optical technique, namely sum-frequency vibrational spectroscopy (SFVS), which has the unique capability to study molecular monolayer at the interface with high sensitivity to polar order and surface-specificity.21 SFVS is able to provide direct structural information, by deducing the orientational distribution of selected moieties at the interfacial layers through their vibrational spectra.22 It has been successfully used to yield the molecular-level description of the conformational switching of a cavitand monolayer at water/vapor interface.23,24 In this work we report a quantitative analysis of the SFVS spectra obtained for both the self-assembled monolayer (SAM) of OTS and DMOAP, and the QxCav SSHBs at the solid/air interface. The aim is to correlate orientation and surface density of the alkyl chains belonging to the different hydrophobic templates with the corresponding quantities of the overlying QxCav monolayer.
INTRODUCTION In the last decades, inspired by biology, supramolecular chemists have designed and synthesized different types of macrocyclic receptors that mimic the exquisite specificity of biotic molecules.1−3 Among these, cavitands,4 organic hostmolecules with open-ended cavity of molecular dimension, are a promising class of supramolecular receptors with significant potential for chemical and biochemical recognition.5 The capability to devise them with the proper cavity shape, depth, and chemical functionalities allows a high level of selectivity toward a given class of analytes and offer advantages in many fields as chemical sensing,6 biology and medicinal chemistry,7,8 for drug detection,9 preventive diagnosis of diseases,10 study of synthetic membranes,11 detection of organic vapors in air,12 and so on. A fundamental step for potential analytical applications is their stable integration on a proper transducing substrate, typically at solid−liquid or solid−gas interfaces, where efficient and selective binding depends on the organization of receptors with proper surface density, preserving the functional conformation and orienting the binding fragments toward the analyte-containing medium. Indeed, proper interfacial architectures have demonstrated enhanced sensitivity12 and significant suppression of nonspecific dispersion interactions,13 which often misrepresent the recognition events.14 With proper substitutions at the lower rim, cavitands can be directly grafted on substrates of foremost technological relevance for robust electronic or photonic transduction (i.e., silicon, gold).10,15 In this work we concentrate on the solid supported hybrid bilayers (SSHB), which are becoming increasingly important for fundamental studies on cell membrane activities, biocompatible surfaces and biosensor devices.16−19 Such simplified model membrane typically allows to stabilize a lipid layer on top of a hydrophobic covalently tethered layer of alkane-derivatives on metal (i.e. gold) or dielectric (i.e. silica, silicon, PDMS, etc.) substrates. The hydrophobic interactions between the two © 2014 American Chemical Society
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BASIC THEORY
The principles of surface-specific SFVS are described in details elsewhere,22 and we briefly summarize it for convenience of later discussion. When two intense laser beams at frequency ωvis and ωir overlap at an interface, the SF signal (ωSF = ωvis + ωir) in reflection geometry is given by Received: August 9, 2014 Revised: October 9, 2014 Published: October 9, 2014 12843
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S(ωSF) ∝ |(eSF ̂ ·L⃡ SF)χ ⃡ (2) : (eVIS ̂ ·L⃡ VIS)(eIR ̂ ·L⃡ IR )|2 IVISI ′IR
DMOAP (CH3(CH2)17(CH3)2N+(CH2)3Si(CH3O)3Cl−, 72% in methanol, Sigma-Aldrich) on the hydroxylated substrates were prepared by chemisorption of the organosilanes from dilute solutions and their successive polymerization.27−29 The OTS and DMOAP SAMs were formed by immersing the clean susbtrates into 2 mM anhydrous solutions of OTS in 90% n-hexane +10% chloroform (by volume) and of DMOAP in isopropyl alcohol, respectively. After 20 min at room temperature, the substrates were rinsed with isopropyl alcohol and sonicated for 10 min in chloroform, aiming at removing possibly formed physisorbed clusters. Then the silane SAMs were cured at 110 °C for 45 min to promote two-dimensional crosspolymerization via condensation of the silanol groups. The SSHBs were arranged by transferring the C11H23-footed QxCav molecules to the hydrophobic templates exploiting Langmuir− Schaefer (LS) deposition from their Langmuir film on water.30 Being amphiphilic, the QxCav molecules adsorb on water with the polar quinoxaline-based headgroup at the air/water interface and the four alkyl chains protruding upward. A previous SFVS investigation of Langmuir monolayer of the same C11H23-footed QxCav molecules on water has demonstrated that their alkyl chains exhibits appreciable gauche defects.24 The LS deposition is promoted by dispersion interactions between the alkyl chains of the cavitands and of the organosilane SAMs, which stabilizes the QxCav SSHBs with the receptor binding sites pointing toward the air. The QxCav molecules were spread drop-by-drop from a 1μM solution in chloroform onto the ultrapure water subphase in a Langmuir trough and left for 30 min to allow for complete evaporation of the solvent. The molecules were then compressed to the liquid-condensed phase at surface pressure 20 mN/m, corresponding to 100 Å2 area per molecule. The surfactantcovered substrates, horizontally oriented, were lowered from the gas (air) phase toward the Langmuir monolayer of QxCav until a continuous meniscus was formed around the substrates. The contact duration was set to 5 min, which allows the adsorption of cavitands on both SAMs to reach the equilibrium. The susbtrates were raised at constant speed (3 mm/min) until the substrate−water interface was broken and then dried under a gentle nitrogen stream. During the whole LS deposition process the monolayer was held at constant surface pressure by a feedback control on the compressing barriers. Comparing the drop in the Langmuir monolayer area following the LS deposition and the substrate area, we estimated a QxCav transfer ratio ∼25% and ∼10% of a monolayer for the DMOAP- and OTS-coated substrate, respectively.
(1)
where êi, L⃡ i and Ii are the beam polarization, the transmission Fresnel coefficients at the interface and the beam intensity at ωi, respectively, and χ⃡(2) is the surface nonlinear susceptibility S tensor given by χS⃡ (2) = χNR ⃡ (2) +
∑ q
Aq⃡ ωIR − ωq + i Γq
(2)
⃡ Here, χ⃡(2) NR is the nonresonant contribution, and Aq, ωq, and Γq are the amplitude, frequency, and damping constant of the qth surface vibrational mode, respectively. The amplitudes Aq,ijk in the lab coordinates (x,y,z) are related to their counterparts aq,lmn of the molecular hyperpolarizability in the molecular coordinates (ξ,η,ζ) through a coordinate transformation and an average over the molecular orientational distribution function (ODF) f(Ω) Aq , ijk = NS
∫ ∑ aq,lmn(i ·̂ l )(̂ j ̂ ·m̂ )(k·̂ n)̂ f (Ω) dΩ l ,m,n
(3)
with NS being the surface density of the molecular group and Ω = (ϕ,θ,ψ) the Euler angles.25 Fitting the SF vibrational spectra of different input/output polarization combinations with eqs1 and 2 allows deduction of Aq,ijk, which then provide the orientational information on the moiety contributing to the qth vibrational mode through eq3. Because both the SAMs (OTS and DMOAP) and the QxCav SSHBs are azimuthally isotropic (C∞,V symmetry), we can reduce to only three independent (2) nonvanishing terms of the susceptibility tensor: χ(2) zzz, χxxz and χ(2) with z referring to the surface normal. We are able to xzx deduce these terms by three different polarization configurations, i.e., PPP, SSP and SPS, of the SF, VIS, and IR beams, respectively. S- and P-polarization are perpendicular and parallel to the plane of incidence, hence SPS and SSP polarization configurations can excite only the vibrational modes with a component perpendicular or lying in the incidence plane, respectively. Therefore, SFVS yields vibrational spectra for selected molecular moieties present at an interface, and from the dependence on input/output beam polarization, determines their average orientations. In this experiment we use a broad band (BB) scheme, in which the IR laser pulses have a large bandwidth (≃ 150 cm−1) that allows one to obtain a portion of the vibrational spectrum without tuning the IR central frequency. Instead the VIS laser pulses have a narrow band (≃ 10 cm−1) for a good resolution of the SF spectra. SF signal is generated in reflection geometry from two collinear beams at 12 500 cm−1 (VIS) and 2800−3100 cm−1 (IR), spatial and temporal overlapped on the sample at gas/solid interface and all spectra are normalized against the SF spectra from a reference z-cut quartz crystal, whose nonlinear susceptibility is well-known.26
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RESULTS AND DISCUSSION Contact Angle Measurements. Static contact angles with ultrapure water have been measured for both the DMOAP and OTS SAMs, as well as for the fused silica surface, using an optical contact angle meter (CAM 200, KSV Instruments). After cleaning with the oxidizing acid solution, the bare susbtrates are very hydrophilic, exhibiting a contact angle of 21° ± 2°, which is typical for highly hydroxylated surfaces. On the other hand, surfactant-coated susbtrates are expected to be strongly hydrophobic. Indeed, we have measured contact angles of 91° ± 2° and 102° ± 2° for the DMOAP and OTS SAMs, respectively. These are typical values for alkyl-chain-terminated surfaces31 and are consistent with an higher OTS surface density (about 3.5 chains/nm2) with respect to the DMOAP one (about 2.5 chains/nm2).32−34 Atomic Force Microscopy. Surface topography of the organosilane-coated substrates before and after LS deposition of the QxCav overlayer was acquired in air on a Nanoscope IIIa (Bruker, CA) operated in tapping mode. Silicon probes (Bruker, CA) with resonance frequency 150-300 kHz, cantilever length 125μm and tip radius of curvature 10 nm were used. Both DMOAP- and OTS-coated substrates exhibit a uniform topography (Figure 1a,b, respectively), with the DMOAP surface presenting a small amount of agglomerates
EXPERIMENTAL SECTION
The optically polished fused silica substrates (NHI-1200, Helios Italquartz s.r.l.) were first cleaned with acetone and then soaked for 24 h at room temperature in an oxidizing acid solution, that is, a mixture of a solid oxidizer (NoChromix, Godax Laboratories) and concentrated sulfuric acid (95-98%, Sigma-Aldrich), to remove organic and inorganic contaminants from the surface. The substrates were then rinsed thoroughly with ultrapure water (18.2 MΩ·cm@25°C, Synergy UV Millipore) and blown dry with a jet of nitrogen. Compact and stable SAMs of OTS (CH3(CH2)17SiCl3, 90%, Sigma-Aldrich) and 12844
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Figure 1. AFM images, of 2 × 2 μm2 region, showing the surface morphology of the DMOAP (a), OTS (b), DMOAP-QxCav (c), and OTS-QxCav (d) samples. Height profiles in the panels c and d were measured along the horizontal white lines.
few tens of nm in diameter and height of ≃6 nm. Similar roughness has been measured on the DMOAP (0.45 nm) and on the OTS layer (0.25 nm), which, in addition to the contact angle measurements, supports the hypothesis of homogeneity of the organosilane SAMs on the fused silica substrates. After the LS deposition of QxCav molecules, the surface morphology changes. The surface roughness for both the DMOAP-QxCav and the OTS-QxCav samples increases to 2.0 and 1.2 nm, respectively. Although AFM images show islandlike distribution of the QxCav film on both SAMs, with a typical size of few tens of nm, the QxCav layer on DMOAP appears more uniform and dense (Figure 1c) than the one on the OTScoated susbtrate (Figure 1d). The height profiles, reported as insets in Figure 1c,d, are in agreement with the nominal length of the QxCav molecule (2.4 nm), thus supporting the assumption of a single, albeit low-density, layer of QxCav on top of the organosilane SAMs. Few regions characterized by steps larger than 4 nm maybe due to bilayers of QxCav molecules. Sum-Frequency Vibrational Spectroscopy. Reported in Figure 2 are the SFVS spectra, and their best-fit curves, of both OTS- and DMOAP-coated fused silica substrates for the SSP, SPS, and PPP polarization configurations. We limit the spectral range to the aliphatic CH stretch region 2800−3000 cm−1 because it enables to deduce the average conformation of the long alkyl chains. Five vibrational peaks can be recognized in the DMOAP SSP spectrum, which are attributed to the symmetric (2851 cm−1, d+) and antisymmetric (2918 cm−1, d−) stretch modes of the methylene (CH2) groups, and the symmetric (2877 cm−1, r+), antisymmetric (2964 cm−1, r−) and Fermi resonance (2942 cm−1, r+FR) of the methyl (CH3) groups. The presence of relatively strong CH2 stretch modes proves appreciable trans− gauche defects which break the centrosymmetry of the alkyl chains. On the other hand, the SSP spectrum of the OTS exhibits predominantly the r+ and r+FR modes of CH3, while CH2 stretches are virtually absent. This experimental evidence suggests that the OTS alkyl chains are in all-trans configuration with few gauche defects, so that the CH2 vibrations cancel each
Figure 2. Fitted SF spectra for SSP, SPS, and PPP polarization configurations from the two templates. Open black squares (OTS) and red circles (DMOAP) are the experimental data, lines are the fitting curves.
other along the chain. Moreover, the fact that the r+ peak is much more prominent in the SSP spectrum than in the SPS configuration implies that the CH3 groups, and accordingly the OTS alkyl chains, are oriented with their principal axis close to the surface normal.33 In Figure 3, the SFVS spectra of the QxCav-OTS and QxCav-DMOAP hybrid bilayers are shown in the range 2800− 3100 cm−1, where the CH stretch modes of both the alkyl chains and the aromatic QxCav headgroup are present. In particular, the peaks at 3024 and 3062 cm−1 are associated with 12845
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and DMOAP ones. Then, if a full monolayer of QxCav was present on top of OTS and DMOAP SAMs, the surface densities for the alkyl chains belonging to the two facing layers were close and one would expect a significant reduction in the r+ peak, which has not been observed in neither QxCav−OTS and QxCav−DMOAP cases. On the contrary, the overall intensity of the SSP spectrum of the QxCav−OTS in the aliphatic range is roughly the same of the bare OTS one, with only a more pronounced d+ shoulder, while the SSP spectrum of the QxCav−DMOAP is appreciably higher than the DMOAP one. Moreover, the aromatic ν2 stretch peak is more intense for the QxCav−DMOAP than for the QxCav− OTS. Without resorting to calculation, these evidence readily confirm a greater surface density of QxCav molecules on top of DMAOP, with respect to OTS, which can be qualitatively understood from the difference in alkyl chain density between the two SAMs. Indeed, QxCav molecules uptake is expected to occur with the polar head pointing out from the substrates, because of favorable dispersion interaction between their alkyl chains and the organosilanes ones.35 If the QxCav chains could intercalate the surfactant layer, the chain−chain dispersion interaction would increase and, in turn, it would enhance the transfer of the QxCav layer. The DMOAP headgroup is larger and occupy a surface area about twice (40 Å2)32 of the OTS one (20 Å2).33 As a consequence a full monolayer of OTS would exhibit more straight and tightly arranged alkyl chains, confirmed by the larger water contact angle, which could hinder the intercalation with the QxCav alkyl chains more than the less packed, albeit less ordered, DMOAP template. On the other hand, the C11H23-footed QxCav molecules have much shorter alkyl chains than the organosilane ones (C18H37), which yields to a fair amount of trans−gauche defects in the Langmuir monolayer24 and, possibly, in the SSHB. This is in agreement with the appreciable increase in the d+ peaks reported in the SSP spectra for both the OTS- and DMOAP-QxCav SSHBs. Moreoever, significant gauche defects in the QxCav chains will also randomize the orientation of their terminal CH3 groups, thus diminishing their contribution in SF spectra. Together with the low transfer ratios, this can explain the absence of the destructive interference in the r+ peaks of the SSHBs. The qualitative arguments above are supported and further detailed by the following quantitative analysis of the spectra in Figures 2−3. The three independent nonvanishing amplitudes Aq,ijk for the r+ and ν2 modes are determined from fitting of the SFG spectra using eqs1 and 2. Then, the approximate ODFs f(Ω) for the CH3 and the Qx moieties are deduced via eq 3. By symmetry, there are only two nonvanishing independent elements of the molecular hyperpolarizabity tensor for the r+ mode (ar+,ζζζ and ar+,ξξζ = ar+,ηηζ) and the ν2 mode (aν2,ζ′ζ′ζ′ and aν2,ξ′ξ′ζ′) in the molecular frames (ξ,η,ζ) and (ξ′,η′,ζ′) of the CH3 and Qx groups, respectively. From eq 3 we obtain
Figure 3. Fitted SF spectra for SSP, SPS, and PPP polarization configurations from the two hybrid bilayers. Open black squares (QxCav−OTS) and red circles (QxCav−DMOAP) are the experimental data, lines are the fitting curves.
the ν7 and ν2 CH modes of the quinoxaline (Qx) wings, respectively (see inset of Figure 3).24 With respect to the SFVS spectra of the sole OTS and DMOAP SAMs, we do not observe major changes in aliphatic CH stretch range of the SSHB spectra, signifying that QxCav molecules do not form a full monolayer on the SAMs and that the LS deposition does not significantly alter the structural conformation and the orientation of the underlying alkyl chains. Indeed, the CH3 symmetric stretch mode (r+) of the QxCav alkyl chains should have destructive interference with the oppositely oriented OTS
1 NSar + , ζζζ [(1 + r r +)⟨cos θCH3⟩ − (1 − r r +)⟨cos3 θCH3⟩] 2 1 = NSar + , ζζζ (1 − r r +)[⟨cos θCH3⟩ − ⟨cos3 θCH3⟩] 2
A r + , xxz = A r + , xzx
A r + , zzz = NSar + , ζζζ [r r +⟨cos θCH3⟩ + (1 − r r +)⟨cos3 θCH3⟩]
(4)
and 12846
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Langmuir A ν2 , xxz =
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1 NSa ν2 , ξ ′ ξ ′ ζ ′⟨cos θQx[3 + cos 2θQx + 2 sin 2 θQx 8 (2rν2 − cos 2ψQx )]⟩
1 A ν2 , xzx = − NSa ν2 , ξ ′ ξ ′ ζ ′⟨cos θQx sin 2 θQx(1 − 2rν2 + cos 2ψQx )⟩ 4 A ν2 , zzz = NSa ν2 , ξ ′ ξ ′ ζ ′⟨cos θQx(cos2 ψQx sin 2 θQx + rν2 cos2 θQx )⟩
(5)
for the r and the ν2 mode, where θ and ψ are the polar and torsion Euler angles,25 rr+ ≡ ar+,ξξζ/ar+,ζζζ ≃ 2.2,36 rc2 ≡ av2,ζ′ζ′ζ′/ av2,ξ′ξ′ζ′ ≃ 2.4,24 and the angular brackets represent the ensemble average over the ODFs f(Ω). For the CH3 and the Qx groups, we assume f CH3(Ω) = f(θ) and f Qx(Ω) = f(θ)δ(ψ − π/2), respectively, where f(θ) = const. for θmin ≤ θ ≤ θmax and f(θ) = 0 elsewhere. In particular, f Qx(Ω) reflects the C4,V symmetry of the QxCav molecules and the polar angle describes the opening angle of the vase configuration.24 From fitting the SF spectra of the OTS and DMOAP SAMs and eqs4 we find 0° ≤ θCH3,OTS ≤ 54° and 30° ≤ θCH3,DMOAP ≤ 87°, respectively, which establish the wider polar distribution of the alkyl chains for the DMOAP template (Figure 4b). Moreover, we can estimate the ratio between the molecular densities NS of the OTS and DMOAP monolayer by comparing the corresponding values of Ar+,xxz estimated by the SSP spectra in Figure 2a. Taking in account the obtained ODFs in eqs4, we find N(DMOAP) /N(OTS) ≃ 0.7 that assesses the lower alkyl chains S S density of the DMOAP template with respect to the OTS one. Considering that ⟨θCH3,OTS⟩ ≃ 36° and that the CH3 group forms an angle of about 35° with respect to the principal axis of the alky chains,37 we can conclude that the latter are close to the surface normal for the OTS template (Figure 4a). Looking at the Aν2,ijk amplitudes, from fitting the SF spectra of the QxCav−OTS and QxCav−DMOAP hybrid bilayers in Figure 3, they have the same sign of the nearby SAM alkyl chain modes, in particular of the Ar+,ijk, supporting the hypothesis of QxCav heads pointing toward the air. Moreover, from eqs5, we obtain superimposable polar angular ranges for the aromatic Qx wings of the QxCav 0° ≤ θ Qx,QxCav−OTS ≤ 34° and 0° ≤ θQx,QxCav−DMOAP ≤ 33°. The results, in excellent agreement with the SFVS investigation of QxCav Langmuir film on water,24 proves that the QxCav molecules are in the vase configuration, with their principal axis perpendicular to the substrate, for both the OTS and DMOAP templates (Figure 4). The relative density NS of QxCav deposited onto the two /N(QxCav−OTS) ≃ 2.5 is also estimated templates N(QxCav−DMOAP) S S from the ratio of the Aν2,xxz. This result agrees with the QxCav transfer ratios measured for the OTS (≃ 10%) and the DMOAP (≃ 25%) templates during the LS deposition and confirm that QxCav are more efficiently transferred on top of the less packed aliphatic template. It is worth noting that, even if 2.5 times higher than on OTS, the fraction of QxCav monolayer transferred from the Langmuir trough on the DMOAP template is still quite small. The advantage given by the lower molecular density in the DMOAP SAM could be partially compensated by its lower ordering. Indeed, the presence of trans−gauche defects and the wider average polar angle of the outer fragments of the DMOAP alkyl chains could somewhat hamper, by steric hindrance, the intercalation of the cavitand molecules, thus reducing the potential upload of a conceivable template with +
Figure 4. Schematic view of (a) QxCav/OTS and (b) QxCav/ DMOAP hybrid bilayer.
comparable molecular density but with straight alkyl chains. A mixed SAM of long and short aliphatic organosilanes, with proper molar fraction and chains lengths, could improve the LS transfer efficiency, combining the benefits of loosely packed outer alkyl chain fragments, while still being straight and aligned with the surface normal. Moreover, we believe that an increase in the chain length of the cavitand molcules would also be beneficial in that it would reduce the amount of gauche defects and enhance the chain−chain dispersion interaction in the hybrid bilayer.
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CONCLUSIONS In summary, we investigated the hybrid bilayers of cavitand QxCav on top of two different hydrophobic templates, namely OTS and DMOAP, at air/fused silica interface. We used SFVS to describe their molecular architectures and to infer how the 12847
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density and ordering of the templates affect the transfer of the cavitand layer by Langmuir−Schaefer method. On both templates QxCav is in the closed vase configuration and orients the mouth of the cavity outward, with its axis perpendicular to the substrate. The loosely packed alkyl chains of the DMOAP SAM allow more efficient cavitands upload, achieving a 25% transfer ratio from a compact Langmuir monolayer, 2.5 times larger than on OTS SAM. Based on the reported results, a viable strategy has been proposed to improve the cavitands surface density in solid-supported hybrid bilayers, which is of paramount importance for the potential application of such architectures in chemical and biochemical sensing.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: pasquale.pagliusi@fis.unical.it. Phone: +39 0984 496121. Fax: +39 0984 494401. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Y.R. Shen for insightful discussions and valuable comments on the manuscript and N. Scaramuzza for contact angle measurements. A.A. acknowledges the John Mott Scholarship Foundation, E.D. and R.P. acknowledge FIRB “RINAME Rete Integrata per la NAnoMEdicina” (RBAP114AMK) for financial support.
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REFERENCES
(1) Cram, D. J.; Cram, J. M. Container Molecules and Their Guests; Royal Society of Chemistry: London, 1997. (2) Lehn, J. Supramolecular Chemistry: Concepts and Perspectives; Wiley-VCH: Weinheim, Germany, 1995. (3) Meyer, E. A.; Castellano, R. K.; Diederich, F. Interactions with Aromatic Rings in Chemical and Biological Recognition. Angew. Chem., Int. Ed. 2003, 42, 1210−1250. (4) Cram, D. J. Cavitands: Organic Hosts with Enforced Cavities. Science 1983, 219, 1177−1183. (5) Pirondini, L.; Dalcanale, E. Molecular Recognition at the Gas− Solid Interface: A Powerful Tool for Chemical Sensing. Chem. Soc. Rev. 2007, 36, 695−706. (6) Pinalli, R.; Dalcanale, E. Supramolecular Sensing with Phosphonate Cavitands. Acc. Chem. Res. 2013, 46, 399−411. (7) Ryan, D. A.; Rebek, J. J. A Carbohydrate-Conjugated Deep Cavitand Permits Observation of Caviplexes in Human Serum. J. Am. Chem. Soc. 2011, 133, 19653−19655. (8) Ghang, Y.-J.; Schramm, M. P.; Zhang, F.; Acey, R. A.; David, C. N.; Wilson, E. H.; Wang, Y.; Cheng, Q.; Hooley, R. J. Selective Cavitand-Mediated Endocytosis of Targeted Imaging Agents into Live Cells. J. Am. Chem. Soc. 2013, 135, 7090−7093. (9) Biavardi, E.; Federici, S.; Tudisco, C.; Menozzi, D.; Massera, C.; Sottini, A.; Condorelli, G. G.; Bergese, P.; Dalcanale, E. CavitandGrafted Silicon Microcantilevers as a Universal Probe for Illicit and Designer Drugs in Water. Angew. Chem., Int. Ed. 2014, 1−7. (10) Biavardi, E.; Tudisco, C.; Maffei, F.; Motta, A.; Massera, C.; Condorelli, G. G.; Dalcanale, E. Exclusive Recognition of Sarcosine in Water and Urine by a Cavitand-Functionalized Silicon Surface. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 2263−2268. (11) Liu, Y.; Liao, P.; Cheng, Q.; Hooley, R. J. Protein and Small Molecule Recognition Properties of Deep Cavitands in a Supported Lipid Membrane Determined by Calcination-Enhanced SPR Spectroscopy. J. Am. Chem. Soc. 2010, 132, 10383−10390. (12) Daly, S. M.; Grassi, M.; Shenoy, D. K.; Ugozzoli, F.; Dalcanale, E. Supramolecular Surface Plasmon Resonance (SPR) Sensors for Organophosphorus Vapor Detection. J. Mater. Chem. 2007, 17, 1809− 1818. 12848
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(35) Lagugné-Labarthet, F.; Yu, T.; Barger, W.; Shenoy, D.; Dalcanale, E.; Shen, Y. Orientation of Cavitands at Air/Water and Air/Solid Interfaces Studied by Second Harmonic Generation. Chem. Phys. Lett. 2003, 381, 322−328. (36) Sung, W.; Seok, S.; Kim, D.; Tian, C. S.; Shen, Y. R. SumFrequency Spectroscopic Study of Langmuir Monolayers of Lipids Having Oppositely Charged Headgroups. Langmuir 2010, 26, 18266− 18272. (37) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. Dependence of Alkyl Chain Conformation of Simple Ionic Surfactants on Head Group Functionality As Studied by Vibrational Sum-Frequency Spectroscopy. J. Phys. Chem. B 1997, 101, 6724−6733.
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Probing Cavitand−Organosilane Hybrid Bilayers via Sum-Frequency Vibrational Spectroscopy A. Aprile,† P. Pagliusi,*,†,‡ F. Ciuchi,‡ M. P. De Santo,†,‡ R. Pinalli,§ and E. Dalcanale§ †
Dipartimento di Fisica and ‡Licryl Laboratory, CNR-IPCF UOS di Cosenza, Universitá della Calabria, Ponte Pietro Bucci 33B, 87036 Arcavacata di Rende Cosenza, Italy § Dipartimento di Chimica, Universitá di Parma and INSTM, Udr Parma, Viale G. Usberti 17/A, 43100 Parma, Italy ABSTRACT: Quinoxaline cavitands (QxCav) are transferred by Langmuir-Schaefer method on selfassembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) and N,N-dimethyl-N-octadecyl-3aminopropyltrimethoxysilyl chloride (DMOAP) on fused silica substrates. The molecular architectures of both the hydrophobic SAMs templates and the hybrid cavitand-organosilanes bilayers at the solid−air interface are investigated and correlated by sum-frequency vibrational spectroscopy. The results show that QxCav are always in the closed vase configuration and orient with their principal axis normal to the substrates. The role of the alkyl chains density in the SAM templates on the QxCav transfer ratio is pointed out.
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layers provide a large stabilizing force to the bilayer,20 while still preserving its two-dimensional fluidity. Here we investigate the molecular architecture of quinoxaline cavitand (QxCav)− organosilane SSHBs, obtained by deposition of a QxCav monolayer on top of fused silica substrates coated with different long-chain silane surfactants: octadecyltrichlorosilane (OTS) and N,N-dimethyl-N-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP). We take advantage of a versatile analytical nonlinear optical technique, namely sum-frequency vibrational spectroscopy (SFVS), which has the unique capability to study molecular monolayer at the interface with high sensitivity to polar order and surface-specificity.21 SFVS is able to provide direct structural information, by deducing the orientational distribution of selected moieties at the interfacial layers through their vibrational spectra.22 It has been successfully used to yield the molecular-level description of the conformational switching of a cavitand monolayer at water/vapor interface.23,24 In this work we report a quantitative analysis of the SFVS spectra obtained for both the self-assembled monolayer (SAM) of OTS and DMOAP, and the QxCav SSHBs at the solid/air interface. The aim is to correlate orientation and surface density of the alkyl chains belonging to the different hydrophobic templates with the corresponding quantities of the overlying QxCav monolayer.
INTRODUCTION In the last decades, inspired by biology, supramolecular chemists have designed and synthesized different types of macrocyclic receptors that mimic the exquisite specificity of biotic molecules.1−3 Among these, cavitands,4 organic hostmolecules with open-ended cavity of molecular dimension, are a promising class of supramolecular receptors with significant potential for chemical and biochemical recognition.5 The capability to devise them with the proper cavity shape, depth, and chemical functionalities allows a high level of selectivity toward a given class of analytes and offer advantages in many fields as chemical sensing,6 biology and medicinal chemistry,7,8 for drug detection,9 preventive diagnosis of diseases,10 study of synthetic membranes,11 detection of organic vapors in air,12 and so on. A fundamental step for potential analytical applications is their stable integration on a proper transducing substrate, typically at solid−liquid or solid−gas interfaces, where efficient and selective binding depends on the organization of receptors with proper surface density, preserving the functional conformation and orienting the binding fragments toward the analyte-containing medium. Indeed, proper interfacial architectures have demonstrated enhanced sensitivity12 and significant suppression of nonspecific dispersion interactions,13 which often misrepresent the recognition events.14 With proper substitutions at the lower rim, cavitands can be directly grafted on substrates of foremost technological relevance for robust electronic or photonic transduction (i.e., silicon, gold).10,15 In this work we concentrate on the solid supported hybrid bilayers (SSHB), which are becoming increasingly important for fundamental studies on cell membrane activities, biocompatible surfaces and biosensor devices.16−19 Such simplified model membrane typically allows to stabilize a lipid layer on top of a hydrophobic covalently tethered layer of alkane-derivatives on metal (i.e. gold) or dielectric (i.e. silica, silicon, PDMS, etc.) substrates. The hydrophobic interactions between the two © 2014 American Chemical Society
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BASIC THEORY
The principles of surface-specific SFVS are described in details elsewhere,22 and we briefly summarize it for convenience of later discussion. When two intense laser beams at frequency ωvis and ωir overlap at an interface, the SF signal (ωSF = ωvis + ωir) in reflection geometry is given by Received: August 9, 2014 Revised: October 9, 2014 Published: October 9, 2014 12843
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S(ωSF) ∝ |(eSF ̂ ·L⃡ SF)χ ⃡ (2) : (eVIS ̂ ·L⃡ VIS)(eIR ̂ ·L⃡ IR )|2 IVISI ′IR
DMOAP (CH3(CH2)17(CH3)2N+(CH2)3Si(CH3O)3Cl−, 72% in methanol, Sigma-Aldrich) on the hydroxylated substrates were prepared by chemisorption of the organosilanes from dilute solutions and their successive polymerization.27−29 The OTS and DMOAP SAMs were formed by immersing the clean susbtrates into 2 mM anhydrous solutions of OTS in 90% n-hexane +10% chloroform (by volume) and of DMOAP in isopropyl alcohol, respectively. After 20 min at room temperature, the substrates were rinsed with isopropyl alcohol and sonicated for 10 min in chloroform, aiming at removing possibly formed physisorbed clusters. Then the silane SAMs were cured at 110 °C for 45 min to promote two-dimensional crosspolymerization via condensation of the silanol groups. The SSHBs were arranged by transferring the C11H23-footed QxCav molecules to the hydrophobic templates exploiting Langmuir− Schaefer (LS) deposition from their Langmuir film on water.30 Being amphiphilic, the QxCav molecules adsorb on water with the polar quinoxaline-based headgroup at the air/water interface and the four alkyl chains protruding upward. A previous SFVS investigation of Langmuir monolayer of the same C11H23-footed QxCav molecules on water has demonstrated that their alkyl chains exhibits appreciable gauche defects.24 The LS deposition is promoted by dispersion interactions between the alkyl chains of the cavitands and of the organosilane SAMs, which stabilizes the QxCav SSHBs with the receptor binding sites pointing toward the air. The QxCav molecules were spread drop-by-drop from a 1μM solution in chloroform onto the ultrapure water subphase in a Langmuir trough and left for 30 min to allow for complete evaporation of the solvent. The molecules were then compressed to the liquid-condensed phase at surface pressure 20 mN/m, corresponding to 100 Å2 area per molecule. The surfactantcovered substrates, horizontally oriented, were lowered from the gas (air) phase toward the Langmuir monolayer of QxCav until a continuous meniscus was formed around the substrates. The contact duration was set to 5 min, which allows the adsorption of cavitands on both SAMs to reach the equilibrium. The susbtrates were raised at constant speed (3 mm/min) until the substrate−water interface was broken and then dried under a gentle nitrogen stream. During the whole LS deposition process the monolayer was held at constant surface pressure by a feedback control on the compressing barriers. Comparing the drop in the Langmuir monolayer area following the LS deposition and the substrate area, we estimated a QxCav transfer ratio ∼25% and ∼10% of a monolayer for the DMOAP- and OTS-coated substrate, respectively.
(1)
where êi, L⃡ i and Ii are the beam polarization, the transmission Fresnel coefficients at the interface and the beam intensity at ωi, respectively, and χ⃡(2) is the surface nonlinear susceptibility S tensor given by χS⃡ (2) = χNR ⃡ (2) +
∑ q
Aq⃡ ωIR − ωq + i Γq
(2)
⃡ Here, χ⃡(2) NR is the nonresonant contribution, and Aq, ωq, and Γq are the amplitude, frequency, and damping constant of the qth surface vibrational mode, respectively. The amplitudes Aq,ijk in the lab coordinates (x,y,z) are related to their counterparts aq,lmn of the molecular hyperpolarizability in the molecular coordinates (ξ,η,ζ) through a coordinate transformation and an average over the molecular orientational distribution function (ODF) f(Ω) Aq , ijk = NS
∫ ∑ aq,lmn(i ·̂ l )(̂ j ̂ ·m̂ )(k·̂ n)̂ f (Ω) dΩ l ,m,n
(3)
with NS being the surface density of the molecular group and Ω = (ϕ,θ,ψ) the Euler angles.25 Fitting the SF vibrational spectra of different input/output polarization combinations with eqs1 and 2 allows deduction of Aq,ijk, which then provide the orientational information on the moiety contributing to the qth vibrational mode through eq3. Because both the SAMs (OTS and DMOAP) and the QxCav SSHBs are azimuthally isotropic (C∞,V symmetry), we can reduce to only three independent (2) nonvanishing terms of the susceptibility tensor: χ(2) zzz, χxxz and χ(2) with z referring to the surface normal. We are able to xzx deduce these terms by three different polarization configurations, i.e., PPP, SSP and SPS, of the SF, VIS, and IR beams, respectively. S- and P-polarization are perpendicular and parallel to the plane of incidence, hence SPS and SSP polarization configurations can excite only the vibrational modes with a component perpendicular or lying in the incidence plane, respectively. Therefore, SFVS yields vibrational spectra for selected molecular moieties present at an interface, and from the dependence on input/output beam polarization, determines their average orientations. In this experiment we use a broad band (BB) scheme, in which the IR laser pulses have a large bandwidth (≃ 150 cm−1) that allows one to obtain a portion of the vibrational spectrum without tuning the IR central frequency. Instead the VIS laser pulses have a narrow band (≃ 10 cm−1) for a good resolution of the SF spectra. SF signal is generated in reflection geometry from two collinear beams at 12 500 cm−1 (VIS) and 2800−3100 cm−1 (IR), spatial and temporal overlapped on the sample at gas/solid interface and all spectra are normalized against the SF spectra from a reference z-cut quartz crystal, whose nonlinear susceptibility is well-known.26
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RESULTS AND DISCUSSION Contact Angle Measurements. Static contact angles with ultrapure water have been measured for both the DMOAP and OTS SAMs, as well as for the fused silica surface, using an optical contact angle meter (CAM 200, KSV Instruments). After cleaning with the oxidizing acid solution, the bare susbtrates are very hydrophilic, exhibiting a contact angle of 21° ± 2°, which is typical for highly hydroxylated surfaces. On the other hand, surfactant-coated susbtrates are expected to be strongly hydrophobic. Indeed, we have measured contact angles of 91° ± 2° and 102° ± 2° for the DMOAP and OTS SAMs, respectively. These are typical values for alkyl-chain-terminated surfaces31 and are consistent with an higher OTS surface density (about 3.5 chains/nm2) with respect to the DMOAP one (about 2.5 chains/nm2).32−34 Atomic Force Microscopy. Surface topography of the organosilane-coated substrates before and after LS deposition of the QxCav overlayer was acquired in air on a Nanoscope IIIa (Bruker, CA) operated in tapping mode. Silicon probes (Bruker, CA) with resonance frequency 150-300 kHz, cantilever length 125μm and tip radius of curvature 10 nm were used. Both DMOAP- and OTS-coated substrates exhibit a uniform topography (Figure 1a,b, respectively), with the DMOAP surface presenting a small amount of agglomerates
EXPERIMENTAL SECTION
The optically polished fused silica substrates (NHI-1200, Helios Italquartz s.r.l.) were first cleaned with acetone and then soaked for 24 h at room temperature in an oxidizing acid solution, that is, a mixture of a solid oxidizer (NoChromix, Godax Laboratories) and concentrated sulfuric acid (95-98%, Sigma-Aldrich), to remove organic and inorganic contaminants from the surface. The substrates were then rinsed thoroughly with ultrapure water (18.2 MΩ·cm@25°C, Synergy UV Millipore) and blown dry with a jet of nitrogen. Compact and stable SAMs of OTS (CH3(CH2)17SiCl3, 90%, Sigma-Aldrich) and 12844
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Figure 1. AFM images, of 2 × 2 μm2 region, showing the surface morphology of the DMOAP (a), OTS (b), DMOAP-QxCav (c), and OTS-QxCav (d) samples. Height profiles in the panels c and d were measured along the horizontal white lines.
few tens of nm in diameter and height of ≃6 nm. Similar roughness has been measured on the DMOAP (0.45 nm) and on the OTS layer (0.25 nm), which, in addition to the contact angle measurements, supports the hypothesis of homogeneity of the organosilane SAMs on the fused silica substrates. After the LS deposition of QxCav molecules, the surface morphology changes. The surface roughness for both the DMOAP-QxCav and the OTS-QxCav samples increases to 2.0 and 1.2 nm, respectively. Although AFM images show islandlike distribution of the QxCav film on both SAMs, with a typical size of few tens of nm, the QxCav layer on DMOAP appears more uniform and dense (Figure 1c) than the one on the OTScoated susbtrate (Figure 1d). The height profiles, reported as insets in Figure 1c,d, are in agreement with the nominal length of the QxCav molecule (2.4 nm), thus supporting the assumption of a single, albeit low-density, layer of QxCav on top of the organosilane SAMs. Few regions characterized by steps larger than 4 nm maybe due to bilayers of QxCav molecules. Sum-Frequency Vibrational Spectroscopy. Reported in Figure 2 are the SFVS spectra, and their best-fit curves, of both OTS- and DMOAP-coated fused silica substrates for the SSP, SPS, and PPP polarization configurations. We limit the spectral range to the aliphatic CH stretch region 2800−3000 cm−1 because it enables to deduce the average conformation of the long alkyl chains. Five vibrational peaks can be recognized in the DMOAP SSP spectrum, which are attributed to the symmetric (2851 cm−1, d+) and antisymmetric (2918 cm−1, d−) stretch modes of the methylene (CH2) groups, and the symmetric (2877 cm−1, r+), antisymmetric (2964 cm−1, r−) and Fermi resonance (2942 cm−1, r+FR) of the methyl (CH3) groups. The presence of relatively strong CH2 stretch modes proves appreciable trans− gauche defects which break the centrosymmetry of the alkyl chains. On the other hand, the SSP spectrum of the OTS exhibits predominantly the r+ and r+FR modes of CH3, while CH2 stretches are virtually absent. This experimental evidence suggests that the OTS alkyl chains are in all-trans configuration with few gauche defects, so that the CH2 vibrations cancel each
Figure 2. Fitted SF spectra for SSP, SPS, and PPP polarization configurations from the two templates. Open black squares (OTS) and red circles (DMOAP) are the experimental data, lines are the fitting curves.
other along the chain. Moreover, the fact that the r+ peak is much more prominent in the SSP spectrum than in the SPS configuration implies that the CH3 groups, and accordingly the OTS alkyl chains, are oriented with their principal axis close to the surface normal.33 In Figure 3, the SFVS spectra of the QxCav-OTS and QxCav-DMOAP hybrid bilayers are shown in the range 2800− 3100 cm−1, where the CH stretch modes of both the alkyl chains and the aromatic QxCav headgroup are present. In particular, the peaks at 3024 and 3062 cm−1 are associated with 12845
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and DMOAP ones. Then, if a full monolayer of QxCav was present on top of OTS and DMOAP SAMs, the surface densities for the alkyl chains belonging to the two facing layers were close and one would expect a significant reduction in the r+ peak, which has not been observed in neither QxCav−OTS and QxCav−DMOAP cases. On the contrary, the overall intensity of the SSP spectrum of the QxCav−OTS in the aliphatic range is roughly the same of the bare OTS one, with only a more pronounced d+ shoulder, while the SSP spectrum of the QxCav−DMOAP is appreciably higher than the DMOAP one. Moreover, the aromatic ν2 stretch peak is more intense for the QxCav−DMOAP than for the QxCav− OTS. Without resorting to calculation, these evidence readily confirm a greater surface density of QxCav molecules on top of DMAOP, with respect to OTS, which can be qualitatively understood from the difference in alkyl chain density between the two SAMs. Indeed, QxCav molecules uptake is expected to occur with the polar head pointing out from the substrates, because of favorable dispersion interaction between their alkyl chains and the organosilanes ones.35 If the QxCav chains could intercalate the surfactant layer, the chain−chain dispersion interaction would increase and, in turn, it would enhance the transfer of the QxCav layer. The DMOAP headgroup is larger and occupy a surface area about twice (40 Å2)32 of the OTS one (20 Å2).33 As a consequence a full monolayer of OTS would exhibit more straight and tightly arranged alkyl chains, confirmed by the larger water contact angle, which could hinder the intercalation with the QxCav alkyl chains more than the less packed, albeit less ordered, DMOAP template. On the other hand, the C11H23-footed QxCav molecules have much shorter alkyl chains than the organosilane ones (C18H37), which yields to a fair amount of trans−gauche defects in the Langmuir monolayer24 and, possibly, in the SSHB. This is in agreement with the appreciable increase in the d+ peaks reported in the SSP spectra for both the OTS- and DMOAP-QxCav SSHBs. Moreoever, significant gauche defects in the QxCav chains will also randomize the orientation of their terminal CH3 groups, thus diminishing their contribution in SF spectra. Together with the low transfer ratios, this can explain the absence of the destructive interference in the r+ peaks of the SSHBs. The qualitative arguments above are supported and further detailed by the following quantitative analysis of the spectra in Figures 2−3. The three independent nonvanishing amplitudes Aq,ijk for the r+ and ν2 modes are determined from fitting of the SFG spectra using eqs1 and 2. Then, the approximate ODFs f(Ω) for the CH3 and the Qx moieties are deduced via eq 3. By symmetry, there are only two nonvanishing independent elements of the molecular hyperpolarizabity tensor for the r+ mode (ar+,ζζζ and ar+,ξξζ = ar+,ηηζ) and the ν2 mode (aν2,ζ′ζ′ζ′ and aν2,ξ′ξ′ζ′) in the molecular frames (ξ,η,ζ) and (ξ′,η′,ζ′) of the CH3 and Qx groups, respectively. From eq 3 we obtain
Figure 3. Fitted SF spectra for SSP, SPS, and PPP polarization configurations from the two hybrid bilayers. Open black squares (QxCav−OTS) and red circles (QxCav−DMOAP) are the experimental data, lines are the fitting curves.
the ν7 and ν2 CH modes of the quinoxaline (Qx) wings, respectively (see inset of Figure 3).24 With respect to the SFVS spectra of the sole OTS and DMOAP SAMs, we do not observe major changes in aliphatic CH stretch range of the SSHB spectra, signifying that QxCav molecules do not form a full monolayer on the SAMs and that the LS deposition does not significantly alter the structural conformation and the orientation of the underlying alkyl chains. Indeed, the CH3 symmetric stretch mode (r+) of the QxCav alkyl chains should have destructive interference with the oppositely oriented OTS
1 NSar + , ζζζ [(1 + r r +)⟨cos θCH3⟩ − (1 − r r +)⟨cos3 θCH3⟩] 2 1 = NSar + , ζζζ (1 − r r +)[⟨cos θCH3⟩ − ⟨cos3 θCH3⟩] 2
A r + , xxz = A r + , xzx
A r + , zzz = NSar + , ζζζ [r r +⟨cos θCH3⟩ + (1 − r r +)⟨cos3 θCH3⟩]
(4)
and 12846
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Langmuir A ν2 , xxz =
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1 NSa ν2 , ξ ′ ξ ′ ζ ′⟨cos θQx[3 + cos 2θQx + 2 sin 2 θQx 8 (2rν2 − cos 2ψQx )]⟩
1 A ν2 , xzx = − NSa ν2 , ξ ′ ξ ′ ζ ′⟨cos θQx sin 2 θQx(1 − 2rν2 + cos 2ψQx )⟩ 4 A ν2 , zzz = NSa ν2 , ξ ′ ξ ′ ζ ′⟨cos θQx(cos2 ψQx sin 2 θQx + rν2 cos2 θQx )⟩
(5)
for the r and the ν2 mode, where θ and ψ are the polar and torsion Euler angles,25 rr+ ≡ ar+,ξξζ/ar+,ζζζ ≃ 2.2,36 rc2 ≡ av2,ζ′ζ′ζ′/ av2,ξ′ξ′ζ′ ≃ 2.4,24 and the angular brackets represent the ensemble average over the ODFs f(Ω). For the CH3 and the Qx groups, we assume f CH3(Ω) = f(θ) and f Qx(Ω) = f(θ)δ(ψ − π/2), respectively, where f(θ) = const. for θmin ≤ θ ≤ θmax and f(θ) = 0 elsewhere. In particular, f Qx(Ω) reflects the C4,V symmetry of the QxCav molecules and the polar angle describes the opening angle of the vase configuration.24 From fitting the SF spectra of the OTS and DMOAP SAMs and eqs4 we find 0° ≤ θCH3,OTS ≤ 54° and 30° ≤ θCH3,DMOAP ≤ 87°, respectively, which establish the wider polar distribution of the alkyl chains for the DMOAP template (Figure 4b). Moreover, we can estimate the ratio between the molecular densities NS of the OTS and DMOAP monolayer by comparing the corresponding values of Ar+,xxz estimated by the SSP spectra in Figure 2a. Taking in account the obtained ODFs in eqs4, we find N(DMOAP) /N(OTS) ≃ 0.7 that assesses the lower alkyl chains S S density of the DMOAP template with respect to the OTS one. Considering that ⟨θCH3,OTS⟩ ≃ 36° and that the CH3 group forms an angle of about 35° with respect to the principal axis of the alky chains,37 we can conclude that the latter are close to the surface normal for the OTS template (Figure 4a). Looking at the Aν2,ijk amplitudes, from fitting the SF spectra of the QxCav−OTS and QxCav−DMOAP hybrid bilayers in Figure 3, they have the same sign of the nearby SAM alkyl chain modes, in particular of the Ar+,ijk, supporting the hypothesis of QxCav heads pointing toward the air. Moreover, from eqs5, we obtain superimposable polar angular ranges for the aromatic Qx wings of the QxCav 0° ≤ θ Qx,QxCav−OTS ≤ 34° and 0° ≤ θQx,QxCav−DMOAP ≤ 33°. The results, in excellent agreement with the SFVS investigation of QxCav Langmuir film on water,24 proves that the QxCav molecules are in the vase configuration, with their principal axis perpendicular to the substrate, for both the OTS and DMOAP templates (Figure 4). The relative density NS of QxCav deposited onto the two /N(QxCav−OTS) ≃ 2.5 is also estimated templates N(QxCav−DMOAP) S S from the ratio of the Aν2,xxz. This result agrees with the QxCav transfer ratios measured for the OTS (≃ 10%) and the DMOAP (≃ 25%) templates during the LS deposition and confirm that QxCav are more efficiently transferred on top of the less packed aliphatic template. It is worth noting that, even if 2.5 times higher than on OTS, the fraction of QxCav monolayer transferred from the Langmuir trough on the DMOAP template is still quite small. The advantage given by the lower molecular density in the DMOAP SAM could be partially compensated by its lower ordering. Indeed, the presence of trans−gauche defects and the wider average polar angle of the outer fragments of the DMOAP alkyl chains could somewhat hamper, by steric hindrance, the intercalation of the cavitand molecules, thus reducing the potential upload of a conceivable template with +
Figure 4. Schematic view of (a) QxCav/OTS and (b) QxCav/ DMOAP hybrid bilayer.
comparable molecular density but with straight alkyl chains. A mixed SAM of long and short aliphatic organosilanes, with proper molar fraction and chains lengths, could improve the LS transfer efficiency, combining the benefits of loosely packed outer alkyl chain fragments, while still being straight and aligned with the surface normal. Moreover, we believe that an increase in the chain length of the cavitand molcules would also be beneficial in that it would reduce the amount of gauche defects and enhance the chain−chain dispersion interaction in the hybrid bilayer.
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CONCLUSIONS In summary, we investigated the hybrid bilayers of cavitand QxCav on top of two different hydrophobic templates, namely OTS and DMOAP, at air/fused silica interface. We used SFVS to describe their molecular architectures and to infer how the 12847
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density and ordering of the templates affect the transfer of the cavitand layer by Langmuir−Schaefer method. On both templates QxCav is in the closed vase configuration and orients the mouth of the cavity outward, with its axis perpendicular to the substrate. The loosely packed alkyl chains of the DMOAP SAM allow more efficient cavitands upload, achieving a 25% transfer ratio from a compact Langmuir monolayer, 2.5 times larger than on OTS SAM. Based on the reported results, a viable strategy has been proposed to improve the cavitands surface density in solid-supported hybrid bilayers, which is of paramount importance for the potential application of such architectures in chemical and biochemical sensing.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: pasquale.pagliusi@fis.unical.it. Phone: +39 0984 496121. Fax: +39 0984 494401. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Y.R. Shen for insightful discussions and valuable comments on the manuscript and N. Scaramuzza for contact angle measurements. A.A. acknowledges the John Mott Scholarship Foundation, E.D. and R.P. acknowledge FIRB “RINAME Rete Integrata per la NAnoMEdicina” (RBAP114AMK) for financial support.
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REFERENCES
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dx.doi.org/10.1021/la503150z | Langmuir 2014, 30, 12843−12849
Langmuir
Article
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dx.doi.org/10.1021/la503150z | Langmuir 2014, 30, 12843−12849
