In Situ ATR FTIR Spectroscopic Study of the Formation and Hydration

Article pubs.acs.org/Langmuir

In Situ ATR FTIR Spectroscopic Study of the Formation and Hydration of a Fucoidan/Chitosan Polyelectrolyte Multilayer Tracey T. M. Ho,† Kristen E. Bremmell,‡ Marta Krasowska,† Stephanie V. MacWilliams,† Céline J. E. Richard,† Damien N. Stringer,§ and David A. Beattie*,† †

Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, South Australia 5095, Australia School of Pharmacy and Medical Sciences, University of South Australia, City East Campus, North Terrace, Adelaide, South Australia 5000, Australia § Marinova Pty. Ltd, 249 Kennedy Drive, Cambridge, Tasmania 7170, Australia ‡

S Supporting Information *

ABSTRACT: The formation of fucoidan/chitosan-based polyelectrolyte multilayers (PEMs) has been studied with in situ Fourier transform infrared (FTIR) spectroscopy. Attenuated total reflectance (ATR) FTIR spectroscopy has been used to follow the sequential build-up of the multilayer, with peaks characteristic of each polymer being seen to increase in intensity with each respective adsorption stage. In addition, spectral processing has allowed for the extraction of spectra from individual adsorbed layers, which have been used to provide unambiguous determination of the adsorbed mass of the PEM at each stage of formation. The PEM was seen to undergo a transition in growth regimes during build-up: from supra-linear to linear. In addition, the wettability of the PEM has been probed at each stage of the build-up, using the captive bubble contact angle technique. The contact angles were uniformly low, but showed variation in value depending on the nature of the outer polymer layer, and this variation correlated with the overall percentage hydration of the PEM (determined from FTIR and quartz crystal microbalance data). The nature of the hydration water within the polyelectrolyte multilayer has also been studied with FTIR spectroscopy, specifically in situ synchrotron ATR FTIR microscopy of the multilayer confined between two solid surfaces. The acquired spectra have enabled the hydrogen bonding environment of the PEM hydration water to be determined. The PEM hydration water is seen to have an environment in which it is subject to fewer hydrogen bonding interactions than in bulk electrolyte solution.

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INTRODUCTION Hydration of polyelectrolyte multilayers (PEMs) is a topic of significant interest in physical chemistry and interfacial science.1−3 One application area for which hydration is known to be critical is aqueous lubrication, the mechanism of friction reduction that is prevalent in biological systems.4,5 A related area for which hydration is expected to have an influence is controlled hydrophobicity and wettability of surfaces.6 Polyelectrolyte multilayers are being investigated as aqueous lubricants7−9 and as surface coatings to control hydrophobicity (for applications such as oil−water separation membranes).10,11 To optimize the use of PEMs as a platform technology in these two application areas, it is essential that multilayer hydration is determined and understood, and links are drawn between the hydration content/water structure and the properties of the multilayer. The amount of hydration water in PEMs is dependent on the nature of the interaction between the two polyelectrolytes (ionpairing, hydrogen bonding, hydrophobic interactions)12−16 and the identity of the outer polyelectrolyte layer (polyanion or polycation). 1,17−19 The amount of hydration water is © 2015 American Chemical Society

commonly determined using a combination of experiments with the quartz crystal microbalance (QCM, which is a gravimetric technique, sensing both polymer and hydration water) with other techniques that are sensitive only to the presence of the organic material (i.e., ellipsometry and surface plasmon resonance, SPR).20−22 While useful, these measurements do not provide any direct insight into the environment of the water in the films. More useful information can be obtained from neutron reflectivity measurements,18,23 carried out on PEMs in either the dry state or in various states of hydration. Such measurements have been used to distinguish between what is termed void water (i.e., water that is indistinguishable from bulk water) and swelling water (i.e., water that is intimately associated with the polymer components of the film). In addition, more recent studies with a novel neutron reflectivity/surface force apparatus device has allowed the hydration water in confined multilayers to be Received: May 17, 2015 Revised: September 30, 2015 Published: September 30, 2015 11249

DOI: 10.1021/acs.langmuir.5b01812 Langmuir 2015, 31, 11249−11259

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Figure 1. Schematic chemical structures of Undaria pinnatifida fucoidan (left−abbreviated to UP), chitosan (top right−abbreviated to CH), and polyethylene imine (bottom right−abbreviated as PEI).

more compact multilayer, due to the formation of additional inter- and intrapolymer hydrogen bonds. The additional binding interaction is made possible due to a degree of acetylation on the Undaria pinnatifida sugar monomers. It is the chemically/structurally more diverse Undaria pinnatifida fucoidan that has been chosen for this more detailed FTIR spectroscopic study of formation and hydration. The work we present here includes detailed ATR FTIR spectroscopy of the build-up of the PEMs using a conventional lab-based FTIR instrument. The spectra have been processed to enable the mass per layer and cumulative mass to be determined for every stage of build-up. The same system has been studied with in situ (while fully hydrated) captive bubble contact angle measurements, with contact angle reported as a function of layer number, and correlated with percentage hydration water within the PEM. Furthermore, these PEMs have been analyzed using ATR FTIR microscopy of a solid− solid contact, allowing for the acquisition of the hydration water spectra from within the PEM layers (without interference from bulk electrolyte solution).

interrogated. This allowed for the scattering profile of the hydration water to be determined in the absence of bulk water.24,25 Another technique that can be applied to the study of hydration water in polymer layers is Fourier transform infrared (FTIR) spectroscopy.26,27 PEMs have been studied routinely with in situ attenuated total reflection (ATR) FTIR spectroscopy, to observe the build-up and intralayer interactions in PEM systems.28−32 The technique allows for the quantitative analysis of adsorbed amount and information on the chemical environment within the PEM. ATR FTIR can therefore also be combined with techniques such as QCM to determine the percentage of hydration water.16 However, it is difficult to obtain unambiguous information on the hydration water environment due to the fact that the methodology probes both the water in the multilayer and bulk water present within the reach of the evanescent wave. It is possible to examine very thick multilayers without such interference33 (and can be used to study hydration as a function of multilayer composition and electrolyte content), but often the thickness of the multilayer will be determined by the application rather than the desire for a particular form of analysis. Recently, in work from this group, a methodology was developed to allow the hydration water within PEM films to be measured directly using in situ FTIR microscopy of PEMs confined within a solid−solid contact.8 FTIR spectroscopy can give access to important information regarding the environment of the hydration water, through interrogation of the water stretching band profile.26,34,35 We deploy our novel synchrotron FTIR methodology here to study the hydration water environment within a polysaccharide PEM formed from chitosan and fucoidan. Chitosan is a commonly studied polycation for biomolecule PEMs; fucoidan is a sulfonated polysaccharide that is extracted from seaweed. In an earlier publication from this group, we reported the formation and characterization of fucoidan/chitosan polyelectrolyte multilayers.36 We used the quartz crystal microbalance, ellipsometry, atomic force microscopy, and X-ray photoelectron spectroscopy to interrogate the differences in multilayers formed using two fucoidans extracted from two different species of seaweed (Fucus vesiculosus and Undaria pinnatifida). The work revealed that Undaria pinnatifida fucoidan produces a

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EXPERIMENTAL METHODOLOGY

Solutions and Substrates. Polyethylenimine (PEI; branched, 25 000 Da) and chitosan (CH; high molecular weight, ≥ 75% deacetylation) were purchased from Sigma-Aldrich. Undaria pinnatifida fucoidan (UP), from the Maritech range of extracts, was supplied by Marinova Pty. Ltd. (Tasmania, Australia). Details of the fucoidan chemistry/composition have been published previously,36 and are also supplied in the Supporting Information. Glacial acetic acid, potassium chloride (KCl, 99% AR), and ethanol (100% undenatured, 99.5% v/v, AR) were purchased from Chem-Supply (South Australia, Australia). KCl used for all solution preparation was calcined at 550 °C for over 8 h to remove any organic impurities, recrystallized, and then calcined at 550 °C for over 8 h again. OP-U polishing suspension and polishing pad were purchased from Struers. Solutions of UP fucoidan (500 ppm) were prepared in 0.1 M KCl and stirred overnight. PEI (500 ppm) was prepared in 0.1 M KCl. A stock solution of chitosan (10 000 ppm) was prepared in 0.1 M acetic acid and stirred overnight. The stock solution was then diluted to 500 ppm in 0.1 M KCl prior to experiments. UP fucoidan and chitosan solutions were pH adjusted with volumetric grade KOH and HCl to pH 6 prior to all experiments. All solutions were made with Milli-Q water with resistivity of 18.2 MΩ (Milli-Q), interfacial tension of 72.4 11250


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Langmuir mN m−1 at 22 °C, and a total organic carbon component of less than 4 mg L−1. All solutions were used within 24 h after preparation. Gold-coated silicon substrates were fabricated using physical vapor deposition (PVD). The detailed procedure for the PVD of the gold surfaces (and their characteristics) is reported elsewhere.37 The substrates and ZnSe hemisphere used in the synchrotron FTIR experiments were rinsed in ethanol and Milli-Q water, followed by drying in a stream of nitrogen gas prior to all experiments. The ZnSe crystal element used in the ATR-FTIR experiments was polished using OP-U suspension on a polishing pad in a “figure 8” motion for 2 min, followed by polishing in Milli-Q water on the pad for 2 min and then rinsed with ethanol and copious amounts of Milli-Q water, followed by drying in a stream of nitrogen gas prior to all experiments. ATR FTIR. FTIR spectroscopy measurements in attenuated total reflection mode (ATR FTIR) were performed with a FastIR single reflection ATR accessory (Harrick Scientific, USA) with an attached liquid flow cell in a Nicolet 6700 FTIR spectrometer (Thermo-Fisher Scientific). The PEM films were adsorbed onto a ZnSe crystal element in a liquid flow cell by the use of a peristaltic pump (Minipuls 3, Gilson, Inc.). Single channel spectra from 512 scans were recorded between 650 and 4000 cm−1 with 4 cm−1 resolution using the OMNIC v8.2.0.387 control software (Thermo Fisher Scientific). A background spectrum of the bare ZnSe crystal in air was recorded and used for all subsequent recorded spectra. The spectrum of the ZnSe crystal in contact with the background electrolyte was also recorded, and was used to manually subtract the contribution of the electrolyte from the adsorbed polymer layer spectra, and the concentrated solution spectra. In some cases, peaks due to water vapor were present in the recorded spectra, and these were subtracted using a spectrum of water vapor (a bare crystal spectrum was recorded 10−15 min after the background spectrum acquisition, which provides a spectrum of water vapor due to the difference in gas purge within the instrument). Spectral processing was performed using the OMNIC v8.2.0.387 software (Thermo Fisher Scientific). Spectral peak fitting/deconvolution of the water stretching band was performed using CasaXPS v2.3.16 (Casa Software Limited), using Gaussian peaks for the individual components, as commonly used for such deconvolution of the water vibrational spectrum.34,35 In all experiments, the PEM films were formed onto ZnSe by (i) the initial adsorption of PEI for 15 min; (ii) 5 min rinse in 0.1 M KCl pH 6 (rinsing step); (iii) adsorption either FV or UP fucoidan for 15 min; (iv) a second rinsing step (5 min); (v) adsorption of chitosan for 15 min; (vi) a third rinsing step (5 min). Steps (iii) to (vi) were repeated until 10 bilayers of fucoidan and chitosan were deposited. All adsorption steps were performed under flowing solution conditions. Captive Bubble Contact Angle Measurement. Contact angles of PEM coated silicon wafers were measured by the captive bubble technique using OCA 20 apparatus (DataPhysics). Although the base substrate is different from the ZnSe used in the FTIR experiments, the two surfaces are expected to yield near-identical multilayers, due to ZnSe having a significant negative charge at the deposition pH29 (as does the native oxide layer on silicon), the fact that both surface types are hydrophilic, and the fact that a PEI primer layer was deposited prior to any polysaccharide adsorption. A quartz cell with a Teflon sample holder, was filled with 0.1 M KCl, pH 6. The PEM-coated surface of the silicon wafer was placed on top of the Teflon holder in the quartz cell. The sample was in contact with the liquid. A thin steel needle (Hamilton) was placed below the surface of the substrate, and air was pushed through an airtight syringe (Hamilton Gastight) to create an air bubble at the orifice of the needle. The bubble was then pushed against the PEM modified silicon wafer surface. The silhouette of the bubble was captured and imaged by a progressive scan CCD camera (High Speed Camera HS3, DataPhysics). The static receding contact angles were determined by drawing the tangent close to the edge of the bubble in the SCA 20 software (DataPhysics). For each layer, the silicon wafers were dipped into polyelectrolyte solution in a glass Petri dish for 15 min, followed by rinsing in 0.1 M KCl for 5 min. Multiple measurements at different locations on the wafer were performed, resulting in at least 30 contact angle values measured for each adsorbed layer.

Infrared Microscopy and Contact Interface. Synchrotron FTIR experiments were performed at the Australian Synchrotron, Clayton, VIC, using procedures detailed in an earlier publication.8 However, brief details are included below. Spectra were collected in reflectance mode using a Bruker Vertex 80v spectrometer with a 32x Reflachromat objective (SpectraTech, Shelton CT, USA, 0.65 numerical aperture) with the microscope aperture set at 20 × 20 μm2, giving an analyzed sample area of 8.3 × 8.3 μm2, due to the beam focusing effect of the ZnSe element. The mapping of the region of interest (white light images) was performed on a Bruker Hyperion 2000 IR Microscope with a motorized sample stage. Spectral collection and the acquisition of white light images of the contact spot between the two solid surfaces were performed in the Opus 6.5 software. The PEM films were adsorbed onto a ZnSe hemisphere with two curved surfaces: one with a radius of curvature of 40 mm, used as the contact surface to producing a defined contact spot between the PEM coated hemisphere and the gold substrate; and one with the radius of curvature of 5 mm, used as the entry surface for the infrared light. The adsorption of the PEM film onto the ZnSe hemisphere was carried out by placing the hemisphere on an O-ring in different Petri dishes containing either polyelectrolyte or KCl solution as described above in the PEM film formation section. The O-ring was used to ensure that only the lower lesser-curved surface of the hemisphere was in contact with the solution in the Petri dish, and therefore that the top of the hemisphere was clear of any adsorbed film. Once the hemisphere was coated with the PEM film, it was carefully placed on top of a gold substrate with approximately 100 μL of 0.1 M KCl in a SpectraTech sample Micro Compression Cell (Thermo Fisher Scientific). The compression cell was used to hold and immobilize the two surfaces when in contact together. The position of the contact spot between the two surfaces was verified with an optical microscope to ensure the contact spot was centered. The contact spot was back-illuminated using white light from the infrared microscope. The radius of the contact region was determined from the microscope, which allowed for a determination of the pressure in the contact region.8,38

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RESULTS Polymer Solution Spectra. The ATR FTIR spectra of chitosan and fucoidan solutions used for the formation of the multilayers are given in Figure 3, and peak assignments are listed in Table 1. The solution spectra have been processed to remove the contribution from the background electrolyte, using the water bending mode region to ensure complete removal of water signal, without causing adverse line shape alteration to peaks in the amide region.39,40 The spectrum of chitosan in solution contains peaks due to the C−O−C and C−O−C/C− N linkages (1022 cm−1, 1078 cm−1, 1092 cm−1, and 1151 cm−1); also visible is the amide I band at 1641 cm−1.41,42 However, the spectrum also includes two large peaks that are due to the asymmetric and symmetric carboxyl stretch of deprotonated acetic acid (1549 and 1418 cm−1, respectively). These large peaks overlap with the peaks due to the −CH2 groups pendant from the chitosan repeat units (1415 cm−1), and the amide II band (1521 cm−1).41,42 The positions of these two chitosan peaks have been determined from collecting FTIR spectra of drop-cast chitosan films (from solutions made in acetic acid and from hydrochloric acid; see Supporting Information). The spectrum of the Undaria pinnatifida fucoidan also contains peaks due to polysaccharide C−O−C linkages (1003 cm−1, 1032 cm−1, 1055 cm−1, and 1169 cm−1),41,42 and a broad peak at approximately 1230 cm−1 due to stretching vibrations of the sulfonate group.41,43 Also present is a peak at 1631 cm−1 which is assigned to the CO stretching mode of the acetyl groups of the fucoidan.44 11251


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Figure 2. Schematic diagram of (top) ATR FTIR flow cell assembly for PEM build-up studies and (bottom) the optical arrangement for in contact FTIR microscopy of confined PEM films. Both setups use a ZnSe ATR element for light delivery and as the substrate for formation of the PEM film. The underlying substrate for contact FTIR microscopy is gold. Color scheme: red - chitosan; blue - fucoidan.

Figure 3. Solution ATR FTIR spectra of chitosan (top) and Undaria Pinnatifida fucoidan (bottom) for concentrated aqueous solutions of chitosan (5000 mg L−1, pH 6 and 0.1 M KCl) and Undaria pinnatifida fucoidan (10 000 mg L−1, pH 6 and 0.1 M KCl). Peak assignments are indicated alongside the major peaks (peaks marked with an asterisk are interference from acetic acid).

In Situ ATR FTIR Spectroscopy of PEM Build-up. The ATR FTIR spectra recorded for the build-up of the fucoidan/ chitosan multilayers are presented in Figure 4. Spectra were recorded after each adsorption/electrolyte flush stage for every polymer layer addition. Spectra recorded with the last layer being fucoidan are displayed in black; spectra recorded with the last layer being chitosan are displayed in light gray. The initial spectrum (dark gray) is the spectrum recorded with just the primer PEI layer attached to the ATR crystal. As with the concentrated solution spectra, the adsorbed layer spectra have been processed to remove the contribution from the background electrolyte. The spectral range of the top panel of Figure 4 contains the fingerprint region peaks (between 1700 and 750 cm−1). All peaks identified from the solution/drop-cast spectra are present, and one additional peak not clearly identified in the solution spectrum of fucoidan can be seen in the spectra of the multilayer (C−O−S stretch at approximately 830 cm−143). In addition, the sulfonate peak region is seen to be more clearly separated into two peaks, unlike the fucoidan solution spectrum in which this peak region has a single broad peak. The wavenumber positions and assignments for all peaks are given in Table 1. The sulfonate peak is solely attributable to the addition of one polymer: fucoidan. There is no increase in this region when chitosan is added (nor is there a decrease). The same observation can be made for the amide II peak when chitosan is added (with little/no increase when fucoidan is added), although the effect is not as pronounced. Another polymer specific trend is the shape of the C−O−C glycosidic linkage stretching region, which has a distinct profile alteration depending on which polysaccharide is added last. The clear

observation of polymer-specific intensity changes gives confidence that the spectra can be used to determine adsorbed mass for multilayer build-up. The lower panel of Figure 4 has the water OH stretching region and the CH stretching region. The CH stretching region contains peaks at 2940 and 2985 cm−1, and a pronounced peak shoulder at 2985 cm−1. The water stretching region contains a distorted band due to a difference between the water spectrum for bulk water and the water spectrum for multilayer hydration water. Whereas the water bending mode was able to be subtracted without any evidence of peak shape or baseline distortion from the fingerprint/amide region in the top panel in Figure 4, this was not the case for the stretching mode peak envelope. The subtraction factor for the removal of the signal from the background electrolyte solution was determined by removal of the signal from the bending mode. This subtraction factor progressively decreased from 1.0 to 0.91, as the film built up, and displaced water from the region probed by the evanescent wave. Therefore, alteration of the water stretching band envelope reflects the difference in the water environment within the multilayer, compared to the bulk electrolyte. The increase in intensity in the higher wavenumber region (centered around 3500 cm−1) and the decrease in intensity in the lower wavenumber region (centered around 3300 cm−1) would appear to indicate a less hydrogen-bonded environment in the multilayer.26,27 The multilayer build-up spectra provide the opportunity to quantify the adsorbed amounts of each individual polymer layer, and the overall adsorbed mass of the multilayer. Adsorption of macromolecules, and the quantification of their 11252


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Langmuir Table 1. Peak Maximum Positions and Assignments for ATR FTIR Spectrum of Chitosan Solution (5000 mg L−1, pH 6 and 0.1 M KCl), ATR FTIR Spectrum of Drop-Cast Chitosan (from 3700 mg L−1 in 0.015 M HCl), ATR FTIR Spectrum of Undaria pinnatifida Fucoidan (10 000 mg L−1, pH 6 and 0.1 M KCl), and ATR FTIR Spectrum of a 10Bilayer Chitosan/Fucoidan Polyelectrolyte Multilayer peak/ assignment

chitosan solution

chitoson drop-cast film (HCl)

Amide I41,42 CO (acetyl)44 amide II41,42 CH2 bending42 CH3 deformation42 sulfonate41,43 sulfonate41,43 C−O−C/C−O stretcha C−O−C/C−N stretch42 glycosidic linkage41 glycosidic linkage41 glycosidic linkage41 skeletal C−O stretchb skeletal C−O stretch42 C−O−S41,43

1641

1656

-c -c

fucoidan solution

UP/CH multilayer

1631

1624

1521 1411 1375

1529 1415 1379 1236

1245 1226

1169 1151

1151

1153

1092

1090

1078

1068 1060

1055

1030

1032

Figure 4. ATR FTIR spectra recorded for the fucoidan/chitosan multilayer at each stage of the multilayer build-up for a 10 bilayer multilayer (formation conditions as noted in the text). Top panel: fingerprint region. Bottom panel: CH and OH stretching region. Initial dark gray line: PEI adsorbed on the ATR crystal. Black lines: spectra recorded after fucoidan adsorption. Light gray lines: spectra recorded after chitosan adsorption.

1030

1022 830

a

Assigned in analogy to the C−O−C/C−N stretch of chitosan. b Assigned in analogy to the skeletal C−O stretch of chitosan. cPeak positions and intensities affected by acetic acid symmetric and asymmetric CO2 stretch.

attributable to one or other of the polymers, as determined by performing spectral processing to reveal the spectra of each individual layer. These spectra are presented in the Supporting Information. The individual layer spectra for chitosan and the solution spectrum for chitosan indicate that the peak at approximately 1150 cm−1 is a clear contender for use as a mass determining peak. It is sufficiently distinct and free from significant overlap with other chitosan peaks and fucoidan peaks. The sulfonate peak has been used for fucoidan in the calculations reported below (see Supporting Information for calibration plots for these two peaks). Peak areas of the two regions for the two polymers were determined by peak fitting/ deconvolution. The determined mass values are plotted in Figure 5. The top panel of Figure 5 contains the cumulative mass of the multilayer as a function of absolute layer number (1−20, with 20 being the outer chitosan layer of the completed multilayer). The data reveal that there is a two-stage growth profile for the build-up of the multilayer. The initial stage (bilayers 1−5) has a supralinear profile, following by a second stage that is overall linear in growth (bilayers 6−10), but with clear steps indicating that one of the two polymers contributes a greater share of the mass for each bilayer, with fucoidan being the polymer with the biggest mass contribution. The lower amount of chitosan for each layer is most likely due to extrinsic charge compensation of the fucoidan by potassium, which is seen to associate with the sulfonated polysaccharide in these multilayers (based on XPS data presented in our previous work36) . The change in the rate of growth, and the dominance

adsorbed amount using ATR FTIR can be performed in a number of ways. Crouzier et al.16 performed a calculation based on the equations of Harrick, but the equations they derived require the determination of a constant that is related to the layer refractive index and the layer density. A similar approach is advocated my Müller et al.31 However, one of the goals of the current work is to determine hydration water content of the multilayers, and thus a methodology that does not rely on an average refractive index or an average density, irrespective of potential changes in these values during build-up, would be preferred. For this reason we have chosen to follow the method of Pitt and Cooper45 (originally developed for protein adsorption studies; see full description in the Supporting Information). This methodology requires that the variation in adsorbate FTIR absorbance is linearly related to adsorbed mass. This assumption only holds for ATR FTIR if the adsorbing material/layer is thinner than approximately 300 nm.30 The UP/CH multilayer, as analyzed in our previous work from ellipsometry and atomic force microscopy,36 has an average layer thickness of less than 300 nm for 10 bilayers. It is therefore clear that we can use the method of Pitt and Cooper45 to determine adsorbed mass of the multilayer for the multilayer system studied here. Furthermore, to ensure that mass build-up can be determined unambiguously, we have chosen peaks for the chitosan and the fucoidan that are revealed as clearly 11253


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measured wet, the fully formed multilayer was seen to have a contact angle of 13.0 ± 0.3 degrees (average of >30 measurements, and standard deviation). Such low receding contact angles can be difficult to measure, as bubble attachment may not occur. For this system, in ∼10% of cases there was no bubble attachment to PEM modified surface. Captive bubble contact angle measurements were also made for each stage of the multilayer build-up. Figure 6 (upper panel)

Figure 5. Calculated mass for PEM build-up, quantified from ATR FTIR spectra (top) cumulative mass from fucoidan and chitosan layers, plotted as a function of absolute layer number; (bottom) calculated mass from each individual layer for fucoidan (black circles) and chitosan (gray triangles), plotted as a function of polyanion or polycation layer number (i.e., layer number 6 for chitosan refers to absolute layer number 12, and full formation of bilayer 6). Error bars are included on all data points, based on two independent measurements of film build-up. Inset in top panel is ellipsometric mass determination (mopt) for the same multilayer system (see Supporting Information for details). Figure 6. Top: the average value of receding contact angles for the PEM coated silicon wafer as a function of absolute layer number measured using the captive bubble technique; the inset is a captured image of an air bubble attached to the multilayer coated silicon wafer. Bottom: determined hydration content (as a percentage) of the total mass of the multilayer.

of the fucoidan in terms of the mass of the multilayer can be seen more clearly in the bottom panel of Figure 5, which contains a plot of individual layer mass (on two y-axes: left for fucoidan, and right for chitosan). For each polyelectrolyte, until the fifth layer is reached within the multilayer, the mass per layer increases steeply, indicating supralinear growth. The mass versus layer number behavior has been confirmed by ellipsometry measurements for the same system adsorbing onto a silicon wafer substrate (see inset in Figure 5, and Supporting Information). Captive Bubble Contact Angle Measurement. Measurement of the wettability of adsorbed polyelectrolyte multilayers when dry46−48 is of reduced relevance for applications that involve deployment in solution, as the contact angle of dry polymer adsorbed layers can differ significantly from wet polymer adsorbed layers.49 Nevertheless, static advancing contact angle measurements using the sessile drop technique were made on a dried 10 bilayer UP/CH multilayer (rinsed in Milli-Q water, dried using ultrapure filtered N2 gas, left overnight in a desiccator). The drop of water deposited on such a surface spread immediately forming an advancing contact angle too low to be measured (i.e., below 7°), indicating that the dried multilayer is completely hydrophilic. Captive bubble contact angle measurements were made to gain a better sense for the wettability of the multilayer when hydrated. When

contains a plot of multilayer contact angle, measured as a function of absolute layer number. On first inspection, there is no clear pattern in the variation of the contact angle with layer number. The contact angle measurements are between 13 and 18 degrees, with large unsystematic variation for the first 10−12 layers (5−6 bilayers). However, after the 11th layer, there is a consistent sawtooth profile in the contact angle, with marginally higher contact angles when fucoidan is the outer layer. The changes are small, but distinctive and statistically significant. Also plotted in Figure 6, in the lower panel, is the determined hydration percentage of the multilayer, as determined by combining the mass determined from the FTIR spectra (Figure 5) with mass determined from quartz crystal microbalance experiments (QCM-D; see Supporting Information). The determined hydration water is based on the average FTIR mass from two experiments and the average QCM mass from three measurements (producing the error bars seen on the plot). It can be seen that the percentage of hydration water drops sharply for the first few layers of the multilayer, from 70% 11254


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ZnSe8 and silicon;51 silicon is used as the lower substrate for this calculation, as the thin gold layer on top of the silicon lower substrate will not be the dominant factor in the mechanical properties of the solid−solid contact). Figure 7 (top panel) contains the FTIR spectrum of the glycosidic linkage and sugar monomer C−O−C stretching vibrations and the sulfonate stretching vibrations for a UP/CH 10 bilayer multilayer sample, deposited on the underside of the ZnSe hemisphere. Two spectra are plotted: one acquired within the contact spot, and one acquired from outside the contact spot. Although the signal-to-noise is poor, the peaks due to the multilayer are easily distinguished (compare with Figure 5); it should be noted that the spectrum of the in-contact multilayer is the first direct observation of a PEM in a tribological contact using FTIR spectroscopy. The signal intensity for the polymer multilayer peaks in both in- and out-of-contact spectra is similar (as expected, as the multilayer is on the ZnSe surface, and is present in both locations probed). However, there is a small decrease in the intensity of the spectrum of the in-contact multilayer relative to the out-of-contact. The altered signal intensity could be due to one of two possibilities: (i) different optical properties of the contact interface relative to the crystal−water interface; or (ii) the removal of some of the multilayer due to the forces present during the creation of the solid−solid contact. Nevertheless, it is clear that the multilayer is present in the contact region. The lower panel of Figure 7 contains the FTIR spectra of the water stretching and C−H stretching regions of the UP/CH 10 bilayer multilayer sample, from the same two sample spots as indicated in the picture inset in the upper panel. The spectra have been offset for clarity, to enable the accompanying peak deconvolution to be visible. The out of contact region of the spectrum is dominated by the water stretching envelope which extends from 2800 to 3700 cm−1, with a barely discernible C− H stretching shoulder between 2800 and 3000 cm−1. The dominance of the water signal is due to the sampling of not just the multilayer but also the bulk electrolyte (the evanescent wave will be approximately 500 nm in this region, and the multilayer is expected to only be less than 300 nm, based on ellipsometry and AFM data36). Deconvolution of the water stretching band for the out-ofcontact spectrum is displayed as five Gaussian peak components (dashed lines, lower gray trace). The fitting procedure is described in detail in the Supporting Information, but some details are also given here. Four of the five components are attributed to specific water hydrogen bonding environments (A1 at approximately 3280 cm−1; A2 at approximately 3440 cm−1; A3 at approximately 3550 cm−1; A4 at approximately 3630 cm−1), with A1 being for water that is most hydrogen bonded with its surroundings and A4 being for water that is least hydrogen bonded with its surroundings.26,27,35 The fifth peak component is a Fermi resonance of the A1 peak, caused by the overlap of this peak with an overtone of the water bending mode (at 1630 cm−1).34,52 The relative peak areas of the four components (with the area of the Fermi resonance included in that of the A1 mode), are given as percentages of the total region area in Table 2. Also given is the set of relative peak component areas for a deconvolution performed for the background electrolyte spectrum (pH 6, 0.1 M KCl; spectrum and deconvolution are given in the Supporting Information, acquired using the labbased instrument). It can be seen that the area of the most hydrogen bonded environment is slightly reduced relative to

(it is quite common for single polysaccharide layers adsorbed at surfaces to have such high degrees of hydration6,50) to below 50% once the absolute layer number reaches 10. Once the multilayer is halfway complete, the hydration content appears to stabilize with increasing layer number. However, a distinct variation with the nature of the outer polymer layer is observed, with marginally higher hydration for a fucoidan outer layer multilayer. This oscillation mirrors the “odd−even effect” seen with PEMs made from synthetic polymers where the polyanion is sulfonated (i.e., PSS).1,17,24 Synchrotron FTIR Microscopy of Confined PEMs. The hydration of the multilayer has also been investigated using synchrotron FTIR microscopy. The methodology was developed for studying confined boundary lubricant layers,8 and involves the use of a hemispherical ZnSe ATR crystal to form a confined circular contact region between a multilayer formed on the crystal and an underlying surface. The white light image of this solid−solid contact is shown in the inset in the top panel of Figure 7. The contact region is emphasized

Figure 7. Top: Synchrotron FTIR spectrum of the glycosidic linkage vibrations and sulfonate vibrations for the multilayer-coated ZnSe hemisphere in contact (black) and out of contact (gray) with the gold coated substrate; inset: white light image of the ZnSe hemisphere/gold surface contact spot (circled) showing the positions from which the spectra were acquired. Bottom: The five deconvoluted peak components (dotted lines) of OH stretching region (solid line) of the in contact (black) and out of contact (gray) positions.

with the black ring; the contact is not as distinct as in our previous studies8 (which used mica as the lower surface) due to the higher reflectivity from the gold surface used in the current work. The contact diameter for the data acquired was 500 μm, which corresponds to a mild applied pressure of approximately 130 MPa (average pressure in the contact region, as determined by the method outlined in our previous work,8 based on Hertzian contact mechanics,38 and the mechanical properties of 11255


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Langmuir Table 2. Position, Peak Width (FWHM), and Relative Percentage of the Deconvoluted OH Stretch Region 0.1 M KCl a

A1 A2 A3 A4

in contact

out of contact

position (cm−1)

fwhm (cm−1)

relative %

position (cm−1)

fwhm (cm−1)

relative %

position (cm−1)

fwhm (cm−1)

relative %

3243.5 3382.2 3497.0 3619.5

231.6 147.2 178.5 94.9

61.1 17.4 19.5 2.0

3281 3440 3550 3625

270.0 177.7 150.0 89.5

47.7 31.0 19.3 2.0

3289.0 3435.1 3553.1 3630.4

264.5 176.6 158.1 81.4

55.4 24.6 17.9 2.1

a

The relative percentage of A1 is the sum of the relative percentages of A1 and FR components. The FR fwhm is equal to the A1 fwhm. The peak positions for FR are 3048.1, 3070.0, 3074.7 cm−1 for 0.1 M KCl, in contact, and out of contact, respectively.

investigations of this multilayer system using AFM,36 and will thus not be affected by altered absorbance values as a function of distance from the crystal surface.30 It is also confirmed by in situ ellipsometry measurements for the same system (see Supporting Information and inset in Figure 5), which are not susceptible to sampling depth issues (although the mass values are subject to optical modeling assumptions). Dominant linear growth is most often associated with polyelectrolyte multilayers created from synthetic polymers, with a strong polyelectrolyte character. Many synthetic multilayers also have initial periods of supra-linear/exponential growth, when partial coverage of the substrate in the early stages of growth yields islands that grow nonlinearly in terms of adsorbed mass, until sufficient growth is achieved to result in coalescence of the individual regions of adsorption.54 Such a transition is expected to occur very early in the build-up process, when substrate effects are at their highest.54 Another mechanism for a switch between supralinear/exponential and linear growth for synthetic polyelectrolytes was put forward by the group of Pierre Schaaf,55−58 who determined that initially exponentially growing multilayers could be made to grow linearly, either by altering the time scale of layer formation (i.e., spray drying58) or by using mixtures of polyanions (one that encourages linear growth, plus one that results in exponential growth).55 Spray drying results in limited time for diffusion, eventually resulting in steady linear growth for each layer deposited in the surface. With polyanion mixtures, the transition was attributed to differential diffusion rates, and the dominance of the faster diffusing polyanion in the upper region of the multilayer. Later work from the group showed that for any system in which hindered diffusion occurs, a supralinear/ exponential to linear transition occurs during build-up,56,57 due to the formation of a restructuration zone in the middle of the multilayer, whose growth determines the increase in mass/ thickness during build-up. For natural polysaccharide polymers, the observed growth patterns have tended to favor supralinear/exponential growth throughout the entire build-up.59,60 Natural polymers are expected to yield greater interlayer diffusion and disorder, due to increased polydispersity and the weak nature of many natural polyelectrolytes (thus reducing the degree of polymer− polymer ion-pairing within a multilayer). The disorder and diffusion in such systems are expected to increase with increasing layer number.59 In spite of this general tendency, there are reported incidences of linear growth in natural polymer-based PEM systems. In studies of hyaluronic acid/ chitosan multilayers, Croll et al. saw linear growth using QCM measurements,61 but only when a cross-linker was used during build-up (thus hindering interlayer diffusion). In addition, Liu et al. saw linear growth during the formation of chondroitin sulfate/chitosan multilayers, using rhodamine-labeled chitosan and UV−vis transmittance measurements of film growth. In

that of the bulk water, indicating that the presence of the multilayer has altered the hydrogen bonding environment of the water sampled by the evanescent wave. This impact is seen even more clearly when we consider the spectrum of the multilayer sample acquired from within the contact region. This spectrum is much lower in intensity (although multiplied by 3 in the figure to allow easy comparison of the peak profiles), due to the exclusion of bulk electrolyte (also seen from the relative increase in the intensity from the C−H stretching region). The same peak deconvolution was performed for the spectrum acquired in contact, and the peak components are again denoted by dotted lines. Even without looking at the area contributions given in Table 2, it is clear that the water stretching region profile is different, with a smaller signal from the lower wavenumber (more hydrogen bonded) water stretching peak component, A1. This confirms the observation made above for the out-ofcontact spectrum. The water present within the fully hydrated multilayer is subject to less hydrogen bonding than that outside the multilayer. It should be noted that no significant signal is observed for the − NH stretching modes of the chitosan in either the out-ofcontact or the in-contact spectra in Figure 7. NH peaks can sometimes be seen as smaller sharp peaks atop −OH stretching bands in chitosan FTIR spectra.42,53 These peaks are smaller than the accompanying −CH stretching peaks of chitosan, and it is therefore not surprising that the −NH peaks are not visible in the spectra in Figure 7, given the small absorbance in the −CH stretching region. Furthermore, if −NH peaks are underneath the profile of the −OH stretching envelope, they would act to counter the observed trend of a decrease in the A1 component when comparing the spectra of the multilayer (in and out of contact) relative to that of bulk water (the A1 component is closest in position to the −NH stretching peaks42).

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DISCUSSION The data presented provide us with significant insight into the build-up of this multilayer system, and to the hydration of the multilayer. The ATR FTIR spectra not only allow us to detect the presence of the polymer layers, individually, as the build-up progresses, it also allows us to quantify the mass of the multilayer. Use of the peak absorbance of vibrations specifically attributable to each polymer (from regions with minimal spectral interference from the opposing polymer) determined from individual layer spectra, allows us to have a high degree of confidence in the mass values obtained. The mass values indicate an unambiguous transition in the build-up process, from a supralinear mode for bilayers 1−5, to a linear mode for bilayers 6−10. This is not an artifact from the measurement technique; the multilayer is within the first 300 nm of the evanescent wave, as proven by our earlier 11256


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FTIR microscopy measurements indicated an alteration to the water stretching mode of the layer hydration water. Whereas the standard ATR FTIR spectra obtained during build-up provided an indirect indication of this alteration (seen in the subtracted spectra in Figure 4), the synchrotron FTIR spectra acquired using the solid−solid ATR contact setup allow us to study the multilayer hydration without any interference (or need for spectral subtraction) from the surrounding electrolyte water. The water in the multilayer is subject to noticeably less hydrogen bonding than water in the bulk. Furthermore, the observation of water stretching band alteration in both ATR FTIR for a nonconfined multilayer and ATR FTIR microscopy of the confined layer rules out the influence of contact pressure on the balance of water environments in the multilayer (although it is likely that some water is squeezed out of the multilayer during the application of pressure, similar to that observed for neutron reflectivity measurements of confined multilayers24,25). It remains to be seen whether this variation in water environment also correlates with odd−even effects in water content due to the nature of the outer polymer layer. This will be investigated in future synchrotron studies. The change in hydrogen bonding environment for water in a polysaccharide multilayer was also observed for hyaluronic acid/chitosan multilayers studied in a similar manner,8 although in that case, the use of mica as one of the contacting surfaces prevented the detailed peak deconvolution reported in the current work. It is therefore expected that an alteration of the hydrogen bonding environment within polysaccharide PEMs is likely to be universal, and will be a factor in the performance of these multilayers in applications such as lubrication and antifouling.

this case, the linear growth may be attributed to significant removal of the chitosan during chondroitin sulfate addition, with chitosan removal occurring from the loosely bound outer region of the multilayer, which facilitates polyelectrolyte complex formation and desorption of the “neutral” complex from the multilayer. The data we have for our system (from this study and our previous work36) would indicate that our switch from supralinear to linear growth is most likely due to the mechanism of hindered diffusion, and the formation of an outer diffusive layer, and an inner restructured layer, as commonly seen in other multilayer systems.54,57 AFM images of UP/CH multilayers published in our earlier work36 show that the early stage of build-up (2 and 4 bilayers) yields multilayers that are rough (with some isolated island features) but that still cover the entire substrate with a significant layer thickness. This would argue against the island growth mechanism for a supralinear to linear transition as the substrate is covered, and adsorption will occur over the entire surface. Furthermore, our transition point is at the 5−6 bilayer (i.e., 10−12 absolute layer number) point, which is later than expected for the island coalescence mechanism.54 In addition, the switch to linear growth is observed even though the roughness of the multilayer (RMS and peak-to-valley distance) continues to increase for bilayers 6, 8, and 10.36 It is thus more likely that hindered diffusion is responsible for the growth transition observed here. As mentioned in the introduction, Undaria pinnatifida has additional acetyl groups that facilitate intra- and interpolymer hydrogen bonding,36 and this will contribute to a lower degree of interlayer diffusion of the components of the multilayer, resulting in linear growth once film build-up has reached a certain point. This would make our system comparable to the cross-linked hyaluronic acid/chitosan system studied by Croll et al.61 The switch in growth mode results in a stabilization of the mass per layer for each polycation/polyanion adsorption stage, and a steady (but alternating around a mean value) water content of around 50%. This degree of hydration is lower than many previously determined values for polysaccharide multilayers,59,62 which often have values as high as 90% for the entire build-up period. It is actually closer to the percentage hydration seen in synthetic polyelectrolyte multilayers, both in terms of the magnitude, and in terms of the tendency for the hydration content to decrease during build-up.18,62 When one considers the more ordered region of the multilayer growth at the start of the linear region (especially layers 11−15), it is clear from Figure 7 that increased hydration correlates with marginally higher contact angle, as measured using a captive bubble. This is at once counterintuitive (as one might initially suspect that a more hydrated polymer film would be more hydrophilic) but in agreement with the other contact angle determination method used in this work. The sessile drop method, for a dried/dehydrated film, yields complete wetting of the surface (i.e., contact angle below 7°), whereas for a fully hydrated multilayer, the captive bubble contact angle is approximately 13°. Furthermore, such behavior was observed for carboxymethyl cellulose adsorbed on a hydrophobic surface, where the contact angle measured using the captive bubble method was seen to decrease significantly when the polymer was dehydrated due to changing solution pH.6 The influence of hydration water content on hydrophobicity may be connected to the hydrogen bonding environment of the water in the multilayer. Both ATR FTIR spectroscopy and

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CONCLUSIONS The formation of a fucoidan/chitosan polyelectrolyte multilayer is seen to progress from an initially supralinear growth regime to one that is linear in terms of mass build-up. This switch is often seen for synthetic polyelectrolyte multilayers, and more rarely for polysaccharide multilayers. Also observed is an odd− even effect in the determined multilayer water content, similar to many PEM systems with sulfonated polyanions. The hydration water content of the multilayer affects the PEM hydrophobicity, and the hydration water is seen to be in an altered environment with respect to the balance of hydrogen bonding interactions. The ability to unambiguously determine water environment and correlate this with multilayer build-up characteristics is likely to yield further insights into the connection between multilayer composition and structure, and the applications of these soft matter surface coatings. Specifically, it is now possible to investigate water content and environment for relatively thin polysaccharide PEM lubricant layers, which will be critical for understanding aqueous lubrication in PEM systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01812. Chemical composition and molecular weight distribution for Undaria pinnatifida fucoidan; chitosan drop-cast film spectra; description of ATR FTIR mass quantification; ellipsometry/FTIR mass comparison for UP/CH 11257


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growth; determination of multilayer water content using FTIR and QCM-D data; water stretching band deconvolution and spectrum of 0.1 M KCl, bar chart comparing relative amounts of the individual peak components for electrolyte, in and out of contact multilayer spectra. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in part on the FTIR microscopy beamline at the Australian Synchrotron, Victoria, Australia. The authors warmly acknowledge Dr. Mark Tobin and Dr. Lauren Hyde, beamline scientists at the FTIR microscopy beamline, for their assistance in operating the FTIR microscope. This work was performed in part at the South Australian node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers. DAB acknowledges the financial support from the Australian Research Council (ARC: Future Fellowship FT100100393). TH acknowledges the financial support of Division of Health Sciences and the School of Pharmacy and Medical Sciences of UniSA for her scholarship support.

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In Situ ATR FTIR Spectroscopic Study of the Formation and Hydration

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