Molecular Interactions and Hydration States of Ultrathin Functional

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Letter

Molecular Interactions and Hydration States of UltraThin Functional Films at the Solid–Liquid Interface Andreas Furchner, Annika Kroning, Sebastian Rauch, Petra Uhlmann, Klaus-Jochen Eichhorn, and Karsten Hinrichs Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00208 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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Analytical Chemistry

Molecular Interactions and Hydration States of Ultra-Thin Functional Films at the Solid–Liquid Interface Andreas Furchner,∗,† Annika Kroning,† Sebastian Rauch,‡ Petra Uhlmann,‡ Klaus-Jochen Eichhorn,‡ and Karsten Hinrichs† †Leibniz-Institut für Analytische Wissenschaften – ISAS – e. V., Schwarzschildstraße 8, 12489 Berlin, Germany ‡Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, 01069 Dresden, Germany E-mail: [email protected]

Phone: +49 231 1392-3588. Fax: +49 231 1392-3544

ACS Paragon Plus Environment

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Abstract We significantly improve the infrared analysis of ultra-thin films in aqueous environments by employing in-situ infrared ellipsometry. Combining it with rigorous optical modeling avoids otherwise typical misinterpretations of spectral features and enables the simultaneous quantification of chemical composition, hydration states, structure, and molecular interactions. We apply this approach to study covalently end-grafted, nanometer-thin brushes of poly(N-isopropylacrylamide)—a thermoresponsive model polymer for proteins at solid–liquid interfaces. Quantitative analyses are based on a dielectric layer model that accounts for film swelling and deswelling, hydration of hydrophilic amide and hydrophobic isopropyl side groups, as well as molecular interactions of the polymer’s amide moieties. We thereby quantify the hydration and structure dependence of intra- and intermolecular C=O · · · H−N and C=O · · · H2 O hydrogen bonds, elucidating their role in the brush’s temperature-induced phase separation. The presented method is directly applicable to functional and biorelated films like polymer and polypeptide layers, which is of topical interest for interface studies, such as membrane processes and protein unfolding.

In-situ infrared-spectroscopic ellipsometry (IRSE) 1 is a powerful chemical analysis technique for surface and interface characterization that extracts detailed information about molecular structure and interactions without the need of labeling. Its chemical specificity enables the sensitive monitoring of conformational changes exhibited by ultra-thin polymer and protein films in response to stimuli like humidity, temperature variations, or changes in solvent properties. Thus far, IR-SE was successfully used to monitor the physicochemical properties of organic thin films qualitatively. 1–10 However, a major obstacle for quantification of biomolecular surfaces in their native aqueous environment is that film-related bands are often spectrally masked by solvent bands like the stretching and bending modes of water.

istry, composition, hydration, structure, function, and molecular interactions. The model accounts for optical effects like baseline drifts due to thinfilm interference, 13 band-shape distortions due to n/k mixing, 14 and dielectric effective-medium effects 15,16 from film hydration. It does not rely on manipulation procedures like ATR corrections 13 or subtraction of solvent background spectra.

In this study, we overcome this severe limitation by making use of comprehensive optical modeling based on a chemical and physical description of the films, thus unleashing the full analytical potential of in-situ IR-SE. We thereby demonstrate that molecular interactions and hydration states in stimuli-responsive, nanometer-thin films are quantitatively accessible at the solid–water interface.

We apply IR-SE to investigate the role of hydrogen-bond interactions on the temperatureinduced phase separation of poly(N-isopropylacrylamide) [PNIPAAm] brushes around their lower critical solution temperature (LCST) in water. 16,19 PNIPAAm—a versatile polymer for physicochemical and bioapplications like controlled drug release 20,21 and tunable protein adsorption 22–24 and resistance 9,19,25–27 —contains amide groups with C=O and N−H moieties that allow for both polymer–water and polymer–polymer hydrogen bonding. 28–30 The structural and chemical similarities with peptides and proteins render PNIPAAm an ideal model system with regard to investigations of intra- and intermolecular interactions in organic thin films at the solid–liquid interface.

Quantitative IR-SE automatically unravels the true film vibrational fingerprint even with overlapping water bands, thus avoiding the use of solvents like D2 O that potentially alter physical film properties 17,18 like swelling behavior, phase transitions, or aggregation and unfolding. In fact, IR-SE with rigorous optical modeling exploits both baseline drifts as well as film and solvent bands to probe film chemistry and physics in unprecedented depth.

Like standard ellipsometry, 11,12 in-situ IR-SE measures the ratio of the sample’s complex reflection coefficients parallel and perpendicular to the plane of incidence, r p /r s ≡ tan Ψ · ei∆ . Employing a chemical, physical, and optical model of the hydrated films enables us to reproduce the measured polarized IR spectra, and to gain quantitative access to a multitude of film properties like chem-


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Experimental Section

ing the wavenumber-dependent refractive and absorption index, respectively. The effective dielectric functions εeff of the hydrated polymer layers were modeled according to Bruggeman 15,36 after

Film Preparation. Homogenous thin PNIPAAm films (Mn = 56.0 kg/mol, PDI = 1.40) and ultrathin PNIPAAm brushes (Mn = 132.0 kg/mol, PDI = 1.28) were prepared in a grafting-to process onto poly(glycidylmethacrylate) [PGMA] functionalized 31–33 gold-coated glass substrates and IR-transparent silicon wedges, respectively. A (49.1 ± 0.5) nm thick film deposited on Au was used for studying interactions in dependence of humidity, whereas a (12.5 ± 0.8) nm ultra-thin endgrafted brush 16 was used for probing interactions temperature-dependently in aqueous environment. Infrared-Spectroscopic Ellipsometry. In-situ IR-SE was carried out in topside illumination under grazing incidence 13 for the films on Au, or in aqueous environment under non-ATR conditions 1 for the brushes on Si using a temperature-controlled flow cell 1,10 in backside illumination. All measurements were performed after equilibration of the films at the respective ambient conditions. To suppress systematic uncertainties and increase optical contrast and sensitivity, PNIPAAm spectra were referenced to spectra of clean substrates obtained under the same experimental conditions.

0 = fH2 O

εH2 O − εeff εPNI − εeff + (1 − fH2 O ) , (1) εH2 O + 2εeff εPNI + 2εeff

where fH2 O and εH2 O are volume fraction and dielectric function of water, respectively. Molecular vibrations and corresponding interactions of PNIPAAm’s amide groups were accounted for in the polymer dielectric function εPNI via a sum of filmchemistry sensitive Voigt oscillators. 37,38 Depending on the measurement geometry, 1 the high optical contrast of IR-SE spectra features characteristically downward- and/or upward-pointing polymer and solvent bands. Particularly the ν(H2 O) contrast and the tan Ψ baseline depend heavily on film swelling (d/ddry ) and hydration ( fH2 O ), enabling a sensitive fit on corresponding model parameters. This is demonstrated in Figure 1 by simulations in which all polymer oscillator amplitudes were set to zero (thin solid black lines), resulting in already good agreement with the water-stretching bands. Moreover, a fit on ν(H2 O) automatically accounts for δ(H2 O) and its overlap with the film’s amide bands. Artificial subtraction of a water background spectrum, which could cause serious errors in band interpretations, is therefore unnecessary.

Results and Discussion Figure 1 shows measured (and fitted) IR-SE tan Ψ spectra of a PNIPAAm brush in purified H2 O at different temperatures, as well as a zoom into the amide I region of the films in wet, dry, and humid state. Strong changes in amplitude and shape of the polymer and water bands are observed as the films are exposed to temperature and humidity stimuli that trigger dehydration or hydration. Especially the amide I band—associated mainly with carbonyl (C=O) stretching—undergoes marked alterations in band composition, which are tightly connected to variations in amide–amide and amide– water hydrogen-bond interactions. Fitting IR-SE spectra via an optical model enables one to quantify the measured band changes. The model deploys stratified-layer calculations 34,35 based on the thickness d of the hydrated polymer film, as well as on the dielectric functions of substrate, films, and ambient (air or water)—that is, on ε(˜ν) = [n(˜ν) + ik(˜ν)]2 , with n(˜ν) and k(˜ν) be-

As depicted in Figure 1 (bottom), the amide I band composition was identified and quantified as a set of five Voigt oscillators for fitting spectra from the solid–liquid interface. To reduce correlations between fit parameters (oscillator amplitudes, frequencies, line shapes), the first three of those oscillators, associated with free, single, and sequentially interacting carbonyl groups, were fitted from the dry-state measurement. Free C=O groups do not participate in any hydrogen bonding, whereas single interacting C=O groups couple to neighboring H−N groups via hydrogen bonds. Sequentially interacting refers to C=O groups that are coupled via consecutive hydrogen bonds over a longer range along the amide–amide sequence. Sequential, or cooperative, hydrogen bonding 18,39,40 is a known effect from the secondary structures of proteins, which considerably lowers the frequency of carbonyl stretching vibrations.


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Figure 1: Top: Measured and fitted in-situ IR ellipsometry spectra of swollen and collapsed PNIPAAm films in aqueous

and humid environment. Brush (de)swelling and changing molecular interactions are monitored via ν(H2 O) and PNIPAAm’s amide bands, respectively. Thin solid black lines are simulations without polymer dielectric oscillators. For dry and humid state, individual Voigt oscillator contributions to total amide I band envelopes are shown. All spectra are referenced to substrate measurements without polymer film. Presence of IR absorption bands allows for a fit on tan Ψ without need for additional (less sensitive) ∆ spectra. Small band around 1725 cm−1 is from carbonyl and ester contributions from PGMA linker sublayer and PNIPAAm–PGMA interface, respectively. 16,41 Hydration at linker interface was modeled with an additional PGMA oscillator around 1700 cm−1 . Complete film dehydration in dry state was verified by absence of water bands around 3400 cm−1 (not shown). Bottom: Carbonyl–amide and carbonyl–water interactions assigned from associated C=O-stretching amide I oscillators (with fitted center frequencies) in the optical model. With increasing number and strength of carbonyl hydrogen bonds, amide I components progressively shift to lower wavenumbers. 42 Polymer backbones indicated by thick black lines.

Two additional oscillators, related to carbonyl– water interactions, were fitted from humid-state spectra, keeping the dry-state oscillator frequencies and line shapes fixed. One oscillator is associated with weakly hydrated C=O groups that form a hydrogen bond with one H2 O molecule. The other oscillator corresponds to strongly hydrated C=O groups that form two hydrogen bonds, either in a carbonyl–water/water or in a mixed carbonyl– water/amide configuration. 18,43

ture, and molecular interactions. Figure 2 shows the hydration dependence of the five carbonyl species. As expected, the number of water-interacting C=O groups increases as the film becomes hydrated, whereas pure amide–amide interacting groups become less prevalent. The number of weakly hydrated carbonyl groups saturates at about 20 vol% water content, and decreases again at higher hydration levels. This substantiates the aforementioned band assignments. As more and more water penetrates into the film, most C=O groups hydrogenbond to at least one H2 O molecule. Further hydration leads to additional hydrogen bonds with water—that is, to a conversion from weakly to strongly hydrated C=O groups.

Importantly, reflection spectra like tan Ψ = |r p |/|r s | inherently depend on both absorption and refractive index. The total amide band envelopes in Figure 1 are therefore not just the mere sum of the individual oscillator components at the level of the thin-film IR-SE spectra.

Sequentially interacting amide groups— unperturbed by hydrogen bonding with water—are no longer present above 20 vol% water content,

Optical modeling of PNIPAAm spectra reveals a plethora of information about film hydration, struc-


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water molecules per NIPAAm monomer, which is not sufficient to achieve strong hydration for all carbonyl groups. This changes to 43 H2 O per monomer upon further hydration of the brush in aqueous environment, but can almost be reversed to about 7 H2 O per monomer by inducing a thermoresponsive collapse of the brush to above its LCST. In fact, this swelling–deswelling transition seems to be facilitated by only about 15% of carbonyl groups, whose hydrogen bonds with water are broken and replaced by entropically preferred 45,46 amide–amide hydrogen bonds. Most C=O groups remain strongly hydrated even in the collapsed state, despite the very pronounced changes in both swelling and water content. This suggests that additional water molecules above 25 vol% water content contribute mainly to the overall hydration of the brush, but not necessarily to specific interactions with PNIPAAm’s amide groups. Quantifying the hydration dependence of the hydrophobic isopropyl groups supports this claim: Fitted band positions of corresponding CH3 stretching modes (Figure 2, middle), which are known to be important hydration markers, 47–49 follow the same trend as the swelling and hydration behavior. Also, both νas (CH3 ) band shift, swelling, and hydration are more pronounced in the collapsed state at 52 vol% film water content than in the humid state at 25 vol%, whereas the composition of carbonyl interactions is almost the same in both states.

Figure 2: Humidity- and temperature-dependent interactions, hydration states, and structure of PNIPAAm films. Top: Amide–amide and amide–water interactions of hydrophilic carbonyl groups. Fits performed on individual oscillator amplitudes (fixed frequencies and line shapes). Uncertainties computed via error propagation of oscillator parameters, d, and fH2 O . 44 Middle: Hydration of hydrophobic isopropyl groups monitored via position of corresponding stretching band. 28 Bottom: Film hydration state and structure as monitored by number of water molecules per monomer and film swelling with respect to dry-state thickness (d/ddry ).

These results show that both the hydrophilic amide and the hydrophobic isopropyl groups are intrinsically involved in the brush’s temperatureinduced swelling–deswelling transition, and that the side-chain chemistry seems to be essential for the brush’s switching behavior.

neither in humid air nor in water below PNIPAAm’s LCST. However, as the brush switches from its highly swollen state at 25 ◦ C to the collapsed state at 45 ◦ C above the LCST, sequentially interacting carbonyl groups reform again, indicating that the brush becomes less hydrophilic. Concomitantly, a small fraction of strongly hydrated groups partially dehydrates, resulting in the presence of more weakly hydrated C=O groups. Alterations in hydrogen-bond interactions between dry, humid, and wet state are accompanied by drastic changes in swelling and water content (Figure 2, bottom). At 85% relative humidity, the slightly swollen PNIPAAm film contains about two

Our findings are consistent with IR studies on the switching behavior of PNIPAAm in solution. 45 Using deuterated water and an approximation of two amide I oscillators, the authors found similar trends in interactions and estimated that 35% of amide groups are involved in PNIPAAm’s coilto-globule transition. For PNIPAAm chains confined in the brush presented here, the end-grafted polymer chains are subject to steric effects preventing full hydration, resulting in a slightly less pronounced phase separation.


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Conclusions

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Acknowledgement Financial support by the Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, the Senatsverwaltung für Wirtschaft, Technologie und Forschung des Landes Berlin, and the Bundesministerium für Bildung und Forschung is gratefully acknowledged. This work was supported by the Deutsche Forschungsgemeinschaft (Grant DFG Hi 793/4-1). AF thanks Christoph Kratz for valuable comments on the manuscript.

The combination of polarized IR spectroscopy with a comprehensive physical and chemical film model is a powerful analytical method that delivers detailed insights into molecular interactions in biorelated organic thin films. Exemplarily, we quantified swelling, hydration, as well as carbonyl– amide and carbonyl–water hydrogen-bond interactions in functional PNIPAAm films and end-grafted brushes, both at the polymer–air and the polymer– water interface. Only a small percentage of amide groups was directly involved in the pronounced swelling–deswelling transition of the brush around its LCST in water. The probed PNIPAAm brushes can always be considered hydrophilic because strongly hydrated carbonyl species prevail in both the swollen and the collapsed state.

Notes The authors declare no competing financial interest.

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Using PNIPAAm as a model polymer for polypeptides and proteins, we demonstrated that IR-SE is applicable to studying functional and biorelated thin films in their native aqueous environment. Particularly relevant fields where quantitative IR-SE will be beneficial are structure analysis of polymer and protein films, 50–52 understanding and controlling protein adsorption on functional surfaces, 24,25 protein-resistant antifouling surfaces and membranes, 53–56 tunable cell growth, 24,57–59 cell and tissue characterization, 60 as well as thin films for sensing applications. 61 IR-SE’s high material-specific spectral contrast is crucial for such applications. In defined H2 O environments, the current detection limit for adsorbed ultra-thin films is about 0.05 mg/m2 , i. e., the submonolayer range (≈ 0.2 nm). Changes in film swelling and hydration of a few percent are detectable via the solvent bands. The minimum detectable film hydration is about 0.3 vol%. Future in-situ IR-SE research will focus on exploring the role of molecular interactions in different classes of stimuli-responsive polymers, as well as in composite functional polymer layers, peptide and protein films, and membranes. We also aim at improving the optical model to account for differences in the physical and optical properties of bulk-like and interacting water. This would reveal even further details on hydration and interactions of hydrophilic and hydrophobic side groups.


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Molecular Interactions and Hydration States of Ultrathin Functional

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