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. 2026 Jun 6;130(24):6097–6105. doi: 10.1021/acs.jpcb.6c02994

Thin but Strong Versus Thick but Soft: Contrasting Interfacial Water Barriers on Zwitterionic and Nonionic Antibiofouling Self-Assembled Monolayers

Zhentao Zhao †, Elisa Margarita Mendoza Zamarripa †, Shigeaki Morita ‡, Evan Angelo Quimada Mondarte †,*, Ryongsok Chang †, Chia-Ying Chiang §, Tomohiro Hayashi †,*
PMCID: PMC13288708  PMID: 42249852

Abstract

Both antibiofouling nonionic and zwitterionic self-assembled monolayers (SAMs) resist protein and cell adhesion similarly, despite having different chemical structures. This work elucidates their antibiofouling mechanisms by integrating surface force measurements with atomic force microscopy (AFM) and surface-enhanced infrared absorption (SEIRA) spectroscopy. The findings show that a very thin (<1 nm) water barrier near zwitterionic sulfobetaine (SB)-terminated SAMs causes strong short-range repulsion, whereas a thicker (>4 nm) water layer on nonionic hydroxy-terminated tri­(ethylene glycol) (EG3-OH) SAMs results in long-range repulsion. SEIRA spectroscopy identified different hydrogen-bonding states in interfacial water: intense interactions with SB groups and weaker bonds with EG3-OH groups. These SEIRA insights clarify the nature of surface forces influenced by interfacial water. The study discusses how variations in hydration, SAM–water interfacial interactions, and the role of interfacial water molecules contribute to the antibiofouling effectiveness of these two SAM types.

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Introduction

Surfaces that resist the attachment of biomolecules are in high demand in biomedical research, including biosensing and cell patterning. − A common strategy to suppress nonspecific adsorption of unwanted molecules onto artificial materials is by forming antibiofouling self-assembled monolayers (SAMs), which are often highly resistant to cells and proteins. − In general, antibiofouling SAMs are categorized into two types: nonionic and zwitterionic. The nonionic SAMs most widely used consist of poly­(ethylene glycol) (PEG), oligo­(ethylene glycol) (OEG)-, and saccharide-terminated alkanethiols on gold substrates. − As for zwitterionic antibiofouling SAMs, betaine- and phosphorylcholine (PC)-terminated alkanethiols are representative examples. −

Many works have been devoted to elucidating the mechanism underlying the antibiofouling property of protein- and cell-resistant SAMs, and several mechanisms have been proposed, including electrostatic repulsion, , entropic repulsion due to flexible molecular chains, and water-induced force. ,, Since most antibiofouling SAMs reject the adsorption of both negatively and positively charged proteins, electrostatic repulsion cannot fully explain their antibiofouling properties. Unlike polymer films, the flexibility of the molecular chains is limited for SAMs, owing to their high molecular packing density.

Recent surface-force analyses and computer simulations support the model of a water barrier formed near the SAMs. ,,− The formation of this water barrier originates from specific interactions between water molecules and the terminal groups. Surface force measurements with an atomic force microscope (AFM) have revealed the physical origin of the interfacial interaction responsible for protein and cell rejection and attachment. The water molecules in the interfacial region interacting with the terminal groups of the SAMs induce repulsive interactions from SAM-to-SAM separation distances of 4–8 nm in phosphate buffer solution (PBS). ,,− Molecular dynamics simulations have shown that a water barrier present on the surface of OEG- and sulfobetaine (SB)-terminated SAMs is responsible for the strong repulsion against approaching proteins, supporting the experimental results. ,

The hydration mechanisms of nonionic and zwitterionic groups are fundamentally different. In the case of PEG molecules, water molecules interact with the oxygen atoms of the EG units via hydrogen bonding. In zwitterionic molecules, hydration is intensified by the electrostatic attraction between water molecules and the charged groups within the zwitterionic molecule, resulting in a strongly bound hydration layer. Theoretical calculations of hydration free energy of single molecules of ethylene glycol (EG) and SB have reported values of −182 kJ/mol and −512 kJ/mol, respectively, indicating that dehydration of zwitterionic groups has a higher energetic cost compared to that of EG units.

To compare the strength of hydration of the two types of antibiofouling SAMs, Stein et al. investigated the layer of water in direct association with SB- and tetra­(ethylene glycol) (EG4)-terminated SAMs. By applying H/D exchange, they removed the water molecules loosely bound to the SAMs and subsequently examined the irremovable layer of water with sum-frequency generation (SFG) vibrational spectroscopy. Their approach demonstrated that water molecules are strongly trapped at the EG4 and SB terminal groups and enveloped by loosely bound water molecules. Moreover, the trapped water was strongly hydrogen-bonded to the surfaces of both SB and EG4 SAMs. Note that, throughout the present work, “EG3-OH” refers to the hydroxy-terminated tri­(ethylene glycol) alkanethiol used in our experiments, whereas “EG4” in the description above refers to the tetra­(ethylene glycol) chain length employed by Stein et al.

Previous research by Leng et al. applied SFG to investigate the structure of the interfacial water protecting the surfaces of SB methacrylate (SBMA) and OEG methacrylate (OEGMA) polymer films in contact with proteins. They reported that a strongly hydrogen-bonded water layer was present at both SBMA and OEGMA polymer surfaces, and only the pOEGMA surface had some water molecules associated via weak hydrogen bonding. The structure of the water layer adjacent to pOEGMA was sensitive to different proteins (bovine serum albumin, lysozyme, and fibrinogen), but was recovered after placing the film again in pure water. The difference in water structure between the films was attributed to the different nature of hydrogen-bonding interactions, suggesting that the hydration state of OEG and SB SAMs influences the mechanism underlying their protein and cell resistance. −

Our scientific question is why both OEG (or PEG)- and SB-based SAMs similarly exhibit antibiofouling properties despite their different hydration mechanisms and chemical structures (Figure ). The difference in the mechanisms underlying their antibiofouling properties and their hydration states has not been revealed experimentally. In this work, we performed surface force measurements in PBS between identical SAMs (same SAMs fabricated on both the probe and the substrate) of EG3-OH- and SB-terminated alkanethiols (denoted as EG3-OH and SB SAMs, respectively) and investigated the short- and long-range repulsive interactions induced by these SAMs. Furthermore, to understand the hydrogen bonding states of water in the vicinity of these SAMs, we performed vibrational spectroscopic analysis of water using SEIRA spectroscopy. − By combining these results, we attempt to clarify the mechanism underlying the antibiofouling property of the SAMs. For comparison, we also investigated a methyl-terminated hydrophobic SAM (denoted as C8 SAM), which is a typical protein-adsorbing SAM.

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Chemical structures of the alkanethiols used in this study: 1-octanethiol (C8), hydroxy-EG3-undecanethiol (EG3-OH), and sulfobetaine-3-undecanethiol (SB).

Experimental Section

Materials

We followed the protocol in our previous study to fabricate colloidal probes and substrates. ,,,,,, For the fabrication of SAMs, we prepared 1 mM ethanol solutions of 1-octanethiol (abbreviation: C8, Sigma-Aldrich, USA), zwitterionic sulfobetaine-3-undecanethiol (abbreviation: SB, HS­(CH2)11N+(CH3)2CH2SO3 –, Dojindo Molecular Technologies, Japan), and hydroxy-EG3-undecanethiol (abbreviation: EG3-OH, HS­(CH2)11(OCH2CH2)3OH, Dojindo Molecular Technologies, Japan). The fabricated probes and substrates were immersed into the solutions immediately after withdrawing from the evaporation chamber for metal deposition. Prior to use, the SAMs were rinsed with pure ethanol to remove physisorbed thiol molecules.

Surface Force Measurements

The details of the surface force measurements were described elsewhere. ,,,,,, The interaction forces between identical SAMs were measured using a commercial AFM system equipped with a liquid cell (MFP-3D Infinity, Oxford Instruments, U.K.). All force measurements were carried out in PBS at room temperature. Spring constants of the probes were calculated from their thermal fluctuations. Force mapping at three different positions of the substrate was performed to check the reproducibility of the recorded F–S curves. The probe velocity was set to 40 nm/s. The separation distance of zero was set as the position where the probe came in contact with the hard surface, displaying linearity in the constant compliance region of the force–displacement curve.

Surface-Enhanced Infrared Absorption Spectroscopy

The experimental details on surface-enhanced infrared absorption (SEIRA) spectroscopic measurements were described elsewhere. The infrared (IR) absorption spectra of the interfacial water molecules on SAMs were obtained by surface-enhanced infrared absorption spectroscopy (SEIRAS). We fabricated SAMs onto 20 nm-thick Au films (deposited at a rate of 0.01 nm/s) using the same procedure as for the substrates. The angle of incidence of the infrared beam to the silicon prism was 45 deg. To monitor the infrared absorption of the evanescent wave, a Fourier transform infrared spectrometer (Thermo Electron Nexus 470, Thermo Fisher Scientific, MA, U.S.) equipped with a liquid-nitrogen-cooled HgCdTe detector was used. SEIRA spectroscopic measurements were obtained under dry and wet conditions for each SAM. The wet condition consisted of the injection of water vapor with a relative humidity of 90% to the measurement cell. By subtracting the spectra measured in the wet condition from those measured in the dry condition, the IR absorption spectra of the interfacial water molecules on SAMs were obtained.

Results and Discussion

Surface Force Measurements

Figure shows the force–separation (F–S) curves of the interaction forces detected on approach between identical SAMs in PBS. An attraction was observed between C8 SAMs from the surface-to-surface distance of approximately 2 nm. Based on previous studies investigating interactions between hydrophobic surfaces, , we concluded that this attraction originated from the combination of hydrophobic and van der Waals interactions. This is consistent with our previous studies, which further evidenced the biofouling behavior of C8 SAMs.

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Force–separation (F–S) curves measured on approach between identical SAMs (EG3-OH, SB, and C8) in PBS. Solid and dashed black lines represent the best-fit curves obtained using the two-term exponential model (eq ) for EG3-OH and SB SAMs, respectively. The repulsive forces observed for EG3-OH and SB SAMs contrast with the attractive forces measured for C8 SAMs.

In contrast, the interactions between the bioinert EG3-OH and SB SAMs exhibited repulsive forces, contrasting with that of C8 SAM. As shown in Figure , repulsion was detected in the F–S curves from surface-to-surface distances of 8–12 nm. The decay length of the repulsion is much longer than the Debye length of PBS (0.7 nm). Considering that the net charges of these SAMs are zero, the contribution of the electrostatic double-layer force is very small. In previous work, we confirmed that the repulsion was induced by the interfacial water, because the repulsion was drastically decreased by adding alcohol, which disrupts the hydrogen bonding networks between water molecules in the interfacial region. ,,,,, We also reported that there is a clear correlation between the repulsion and the protein- and cell-resistance of the SAMs. The antibiofouling performance of the EG3-OH and SB SAMs prepared with the same protocol used here has been independently established in our previous work and that of others through BSA, fibrinogen, and lysozyme adsorption assays as well as platelet and cell adhesion assays; ,,,,, the present manuscript focuses on elucidating the molecular hydration mechanism that underlies these established performances.

Deconvolution of Short- and Long-Range Hydration Forces

Although the F–S curves for both EG3-OH and SB SAMs showed repulsive forces, the shapes of the curves are slightly different. To clarify the difference in SAM–water interactions between the antibiofouling SAMs, we fitted the F–S curves with a double exponential function. Previous vibrational analyses using SFG and computer simulations have revealed the presence of both strongly and weakly bound water molecules in the vicinity of the OEG and SB groups. , Therefore, we consider the contribution of two types of hydration: trapped water at the terminal groups (short-range) and loosely bound water extending further into the bulk (long-range). The F–S curves are fitted with the function expressed by eq

F(d)=Aexp(−d/λS)+Bexp(−d/λL) 1

where A and B are the fitting parameters describing the strength of the short- and long-range repulsion, d is the surface-to-surface separation distance, and λS and λL are the decay lengths of the short- and long-range repulsion, respectively. We confirmed that the two-term fitting is justified by comparing the RMS errors for one- and two-term fits (Figure S1 and Table S2). The RMS error for the two-term fit was considerably smaller than for the one-term fit, confirming that the two-term model provides a significantly better description of the measured curves. Beyond the empirical RMS comparison, the choice of a sum of two exponentials is rooted in the physical picture of the SAM–water interface. The Marčelja–Radić order-parameter theory of hydration forces predicts that the perturbation imposed on the local water structure by a hydrophilic surface decays exponentially with a correlation length set by the bulk water, , and surface-force apparatus and AFM measurements on hydrated phospholipid bilayers, mica surfaces, and bioinert SAMs have consistently reported short-range hydration decay lengths of 0.2–0.6 nm and a secondary, longer-range exponential of 1–4 nm originating from the cooperative reorganization of the H-bond network beyond the first hydration shell. , The two terms in eq thus correspond to two physically distinct contributions: the short-range component represents directly bound first-shell water at the SAM terminal groups, and the long-range component represents the propagation of this perturbation through several hydration shells. Alternative functional forms, a single exponential, a sum of two power laws F(d) = A/d n + B/d m with freely varied n and m, and an exponential plus a power-law combination, were also tested but failed to reproduce the data within the experimental noise. Power-law decays correspond to long-range van der Waals (∼d –2; estimated to be small and attractive in our system) or to osmotic compression of dense polymer brushes (Alexander–de Gennes, ∼d –9/4; not realized for short alkanethiolate SAMs); neither dominates here. The DLVO electrostatic double-layer contribution also obeys an exponential decay with a length close to the Debye length (κ–1 ≈ 0.7 nm in PBS), but EG3-OH and SB SAMs are charge-neutral, with reported zeta-potentials |ζ| of only a few millivolts at near-neutral pH, , and accordingly an electric-double-layer contribution cannot account for either of the long-range tails observed here.

The best-fit parameters are summarized in Table . Using these values, we reconstructed the individual short- and long-range repulsion components (Figure ). The differences in the characteristics of the short- and long-range repulsion between EG3-OH and SB SAMs are clearly evident from the fitting results. As shown in Figure A, the short-range repulsion is stronger for SB SAM than for EG3-OH SAM. The decay length λS is 0.34 nm for SB SAM and 0.54 nm for EG3-OH SAM, indicating that the short-range repulsion extends to approximately 1 nm for SB and less than 2 nm for EG3-OH from each surface. Previous theoretical calculations and molecular simulations indicated that the hydration free energy of zwitterionic molecules (−512 kJ/mol) is more than double that of OEG molecules (−182 kJ/mol). Therefore, the stronger SAM–water interaction of SB groups compared to EG3-OH groups explains the stronger short-range repulsion observed for SB SAMs.

1. Best-Fit Parameter Values Obtained with the Two-Term Exponential Model (eq )­ .

fit parameter EG3-OH SB
A (pN) 328 (7) 595 (12)
λS (nm) 0.54 (0.02) 0.34 (0.01)
B (pN) 550 (8) 540 (15)
λL (nm) 2.53 (0.03) 1.65 (0.03)
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a

Values in parentheses are the standard deviations.

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Reconstructed components of the surface forces from the two-term exponential fitting. (A) Short-range repulsion component (first term of eq ) and (B) long-range repulsion component (second term of eq ) for EG3-OH and SB SAMs. The short-range repulsion is stronger for SB SAM, while the long-range repulsion extends further for EG3-OH SAM.

In contrast, the long-range repulsion measured for EG3-OH SAMs extends to approximately 12 nm, which is longer than that of SB SAMs (approximately 8 nm) (Figure B). Smith et al. reported that PEG chains promote the ordering (clustering) of surrounding water molecules, indicating that hydrogen bonding networks are enhanced at the water–PEG interface. We expect that the promoted hydrogen bonding network causes the longer-range repulsion observed between EG3-OH SAMs. This is in agreement with our previous work in which the effect of the number of EG units on the thickness of the interfacial water was reported. As for SB, Kitano et al. reported that water molecules in the vicinity of SB moieties are strongly trapped; however, outside of these complexes, the hydrogen bonding state steeply returns to the bulk state as a function of the SB–water distance. We conclude, in agreement with these previous findings, that the different ranges of modulation of the hydrogen bonding network cause the difference in long-range repulsion.

It is informative to translate the fitted decay lengths into the number of water hydration shells. Taking the diameter of a water molecule (≈0.28 nm) as the thickness of one hydration shell, the short-range component corresponds to ∼1.2 shells for SB and ∼1.9 shells for EG3-OH, while the long-range component corresponds to ∼5.9 shells for SB and ∼9.0 shells for EG3-OH. These numbers are in good agreement with prior molecular-dynamics simulations and SFG/heterodyne-detected SFG observations on bioinert surfaces, which typically report a strongly oriented first-shell water layer, a perturbed secondary water layer extending 4–8 shells, and full relaxation to the bulk hydrogen-bond structure beyond ∼10 shells. ,,− This shell-level decomposition reinforces the physical assignment of eq : the short-range exponential corresponds to the directly bound first-shell water (one to two water diameters) and the long-range exponential corresponds to the cooperative perturbation of the H-bond network propagated through five to nine water layers.

Within the Marčelja–Radić framework, the pre-exponential of each hydration term scales with the square of the surface-imposed water order parameter (and hence with the binding strength between the surface and the first hydration shell), while the decay length is set by the bulk-water correlation length, modulated by the headgroup chemistry. , Within this framework, the more than 2-fold larger pre-exponential of the SB short-range component (A SB = 595 pN vs A EG3‑OH = 328 pN) is consistent with the much larger hydration free energy of the SB group, while the comparable pre-exponentials of the long-range component (B SB ≈ B EG3‑OH ≈ 540 pN) but very different decay lengths (1.65 vs 2.53 nm) reflect distinct propagation mechanisms: a localized but strong dipolar perturbation that decays rapidly for SB, and a cooperative reorganization of the bulk H-bond network that propagates further for EG3-OH.

Several alternative contributions to the F–S curves were considered and can be largely excluded. (i) Residual electrostatic/electric double-layer (EDL) contribution: the Debye length in PBS is κ–1 ≈ 0.7 nm; an EDL would therefore decay over ≈3κ–1 ∼ 2 nm. Because EG3-OH and SB SAMs are charge-neutral, an EDL cannot account for either of the long-range tails reported here, and the very fact that λL is shorter for SB than for EG3-OH is the opposite of what an EDL-only interpretation would predict. (ii) Local ion-pairing/Hofmeister effects near SB groups: at PBS ionic strength, partial association of buffer ions with the −SO3 – and −NMe3 + moieties may modulate the local water orientation, but published SFG and MD studies indicate that this primarily renormalizes the pre-exponential of the short-range component without introducing a new functional dependence. , Such effects therefore reside within the short-range exponential we already assign to SB–water binding. (iii) Steric/conformational contribution from the EG3-OH headgroup: The hydroxy-EG3 head is ∼1 nm in extended length, well below the brush regime, so a direct osmotic-brush contribution can be excluded; the indirect chain-mediated cooperative reorganization of the surrounding H-bond network induced by the ether oxygens does, however, contribute to the long-range component. (iv) Orientation and uniformity of the SB monolayer: for (CH2)11-SB-terminated alkanethiols on Au(111), grazing-incidence XRD, ellipsometry, RAIRS, and contact-angle measurements have established a well-ordered (√3 × √3)­R30° lattice with an upright alkyl backbone and SB termini predominantly oriented with the −SO3 – group pointing away from the gold and the −NMe3 + closer to the alkyl chain, but with a finite tilt and conformational distribution rather than a perfectly perpendicular dipole. ,, The time-averaged net charge of every surface molecule is zero, so the surface dipole rather than free charge is the source of any residual surface potential, and this dipolar field is screened within a few hydration shells in PBS. The reproducibility of our F–S curves at multiple positions on each substrate (Supporting Information), together with the consistency between our data and the previously characterized SAM packing, supports the interpretation of the two-term hydration force as an intrinsic property of the headgroup chemistry rather than a heterogeneity artifact. We therefore assign the short-range component primarily to first-shell, surface–water binding (electrostatic + H-bond for SB; H-bond for EG3-OH) and the long-range component to second- and further-shell water structuring modulated by the cooperative response of the H-bond network to the headgroup chemistry, with negligible EDL or steric brush contributions.

SEIRAS Analysis of Interfacial Water

To acquire a comprehensive picture of the hydrogen bonding states of the interfacial water, the OH stretching modes of the water molecules were analyzed by SEIRA spectroscopy. Since the decay length of the enhanced electromagnetic field in SEIRA measurements is approximately 5 nm, we analyzed the interfacial water within the distance range relevant to the repulsive forces observed in the AFM measurements.

The spectra obtained for C8, EG3-OH, and SB SAMs are different from the bulk water spectrum, indicating that the hydrogen bonding network in the interfacial region is modulated by SAM–water interactions (Figure ). In the case of C8 SAM, the spectrum showed a stronger signal in the region of 3200–3000 cm–1. In general, when the strength of hydrogen bonding is strong, the vibrational energy of the OH stretching mode decreases. Therefore, the signal in this lower-wavenumber region is attributed to water molecules with strong hydrogen bonding. Water molecules in the vicinity of hydrophobic surfaces cannot form hydrogen bonds with the surfaces; instead, they compensate by forming strong hydrogen bonds among themselves. This finding is in agreement with previous SFG results. −

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SEIRA spectra of the OH stretching region of interfacial water molecules on C8, EG3-OH, and SB SAMs, compared with the bulk water spectrum. The spectra reflect the different hydrogen bonding states of water at each SAM surface.

In the cases of EG3-OH and SB SAMs, the obtained spectra are different, indicating that their hydration states are different. In SEIRA measurements, the intensity of the signal decreases exponentially as a function of distance from the gold surface. Therefore, the signal from water molecules in the vicinity of the SAMs dominates the spectra. The results show that the OH-stretching vibrational energy of the interfacial water for SB SAM is lower than that for EG3-OH SAM. This is attributed to the strong interaction between SB groups and water molecules. Compared with SB, the interaction between EG3-OH and water molecules is weaker, and hence the vibrational energy is higher.

An apparent paradox arises if one assigns the OH-stretching peak position alone to the strength of the surface–water interaction: the C8 spectrum lies at lower wavenumber than the SB spectrum and is comparable to that of bulk water, which would naïvely imply that the hydrophobic C8 interface interacts with water more strongly than the zwitterionic SB. The peak position is in fact a measure of the average hydrogen-bond strength experienced by the water OH oscillators sampled by the SEIRA evanescent field; it does not by itself distinguish water–water from water–surface hydrogen bonding. For C8, the absence of any hydrogen-bond donor or acceptor on the surface forces the adjacent water molecules to compensate by forming an enhanced “ice-like” tetrahedral water–water network, with average OH-stretching energies similar to or lower than bulk water. ,− , This network is, however, only weakly anchored to the surface, which is consistent with the absence of a repulsive hydration force in our AFM data and with the well-known biofouling behavior of methyl-terminated SAMs. For SB, the dominant interaction is the direct binding of water to the −SO3 – and −NMe3 + moieties: this binding is energetically strong but geometrically constrained, so the average tetrahedral water–water hydrogen-bond network is weaker than at the C8 interface or in bulk, while the surface anchoring is much stronger. The intermediate OH-stretching energy of SB therefore reflects a competition between strong but localized water–surface binding and weakened water–water cooperativity. The lower wavenumber of SB compared with EG3-OH is consistent with stronger overall hydration of the zwitterionic group, in agreement with the much larger reported hydration free energy of SB, and with the larger pre-exponential of the short-range hydration force we observe by AFM. The complementary use of AFM and SEIRA is therefore essential: the SEIRA peak position reports the average hydrogen-bond strength of the interfacial water, while the AFM repulsion reports whether that water layer is, in addition, strongly anchored to the surface and therefore biologically protective. Independent SFG, isotopic-dilution SFG, and MD studies of SB and SB polymer surfaces have established that the water adjacent to SB groups is strongly bound to the headgroups, ,,,, and our combined AFM/SEIRA data are consistent with this picture.

Two technical caveats should be noted when comparing the SEIRA and AFM data sets. First, the SEIRA spectra were obtained as the difference between dry- and wet-condition (90% relative humidity, water vapor) measurements (Supporting Information), whereas the AFM force measurements were performed in PBS. Both EG3-OH and SB are strongly hydrophilic, and at 90% RH the SAMs are covered by an adsorbed water multilayer of several water layers, well within the ∼5 nm penetration depth of the SEIRA evanescent field; quartz-crystal-microbalance with energy dissipation (QCM-D) measurements of water condensation on the same SAM family in saturated water vapor further show that on the hydrophilic SB and EG3-OH SAMs the adsorbed water layer keeps growing in a film-wise manner and exceeds 10 nm, whereas on the hydrophobic C8 SAM it saturates at ≈2 nm; the relevant first- and second-shell water is therefore fully populated in the SEIRA experiment for all three SAMs.

Critically, although the absolute thickness of the adsorbed water differs between water vapor and PBS, the OH-stretching spectrum of the interfacial water itself is nearly insensitive to the surrounding medium: gap-controlled ATR-IR spectroscopy of the SAM–bulk-water interface in liquid water yields OH-stretching spectra for C8, OH-, and EG3-OH SAMs that are nearly identical in shape to those measured by SEIRA in 90% RH water vapor. The hydrogen-bond fingerprint that SEIRA captures is therefore representative of the SAM–water interfacial water that determines the AFM hydration force. Second, the absence of solvated buffer ions in the SEIRA experiment introduces some difference in the long-range solvation: in PBS, partial association of Na+/K+ and HPO4 2–/H2PO4 – ions with the −SO3 –/–NMe3 + groups can modify the directional polarization of the most strongly bound water layer. However, the Debye length in PBS is only 0.7 nm, much shorter than the SEIRA penetration depth and the AFM hydration decay length, so the screening of any local charge fluctuation is essentially complete; SFG studies of zwitterionic SB polymer brushes have explicitly shown that adding salt does not change the qualitative ordering pattern of the most strongly bound water layer but only shifts the OH-stretching frequency slightly. The SEIRA-derived hydrogen-bond strength and the AFM-derived hydration force can therefore be qualitatively correlated within these bounds, and the relative ordering of EG3-OH, SB, and C8 is robust against the choice of medium even though the absolute peak positions in SEIRA cannot be quantitatively converted to those in PBS without a separate calibration.

Correlation between Hydration State and Surface Forces

By combining the results from surface force measurements and SEIRA spectroscopy, we propose the following mechanism for the antibiofouling properties of EG3-OH and SB SAMs. The strengthened hydrogen bonding state of water on SB SAMs, as revealed by SEIRA, induces the strong short-range repulsion measured between SB SAMs, extending up to approximately 1 nm above each monolayer surface (half the short-range repulsion distance between identical SAMs). Inversely, in the case of EG3-OH SAMs, the weaker SAM–water interaction results in weaker short-range repulsion but enables the formation of a thicker water barrier layer extending to approximately 6 nm, facilitated by the promotion of water clustering observed for these SAMs. The distances quoted above and shown in the mechanism schematic (1, 4, and 6 nm) correspond to the half-width of each repulsion measured in the identical-SAM geometry: each distance is the surface-to-bulk separation at which the corresponding fitted exponential component decays to approximately 1% of its pre-exponential value (i.e., ≈5λ), which also coincides with the empirical AFM force-detection threshold of ∼5 pN observed in our F–S curves. With the fitted parameters in Table , this criterion gives 5λS/2 ≈ 0.85 nm for the SB short-range component (rounded to 1 nm), 5λL/2 ≈ 4.1 nm for the SB long-range component (4 nm), and 5λL/2 ≈ 6.3 nm for the EG3-OH long-range component (6 nm), in agreement with the SAM-to-SAM cutoffs of approximately 2, 8, and 12 nm visible in Figure . This criterion is now also stated in the schematic caption.

In other words, the local water structure at the SAM surface dictates the characteristics of the water-induced repulsion. The thick water barrier on EG3-OH SAMs serves to shield the SAM from interacting with proteins and cells through the extended hydrogen bonding network, whereas the strong hydration of SB SAMs in the short-range is crucially important for its antibiofouling property. We conclude that the different hydration states of the SAMs result in qualitatively similar water-induced repulsion because the short- and long-range hydration forces behave as complementary water layers that together form a barrier sufficiently thick to resist biofouling.

Conclusions

In this study, the hydration states of hydrophobic and antibiofouling (nonionic and zwitterionic) SAMs were investigated by surface force measurements using AFM and SEIRA spectroscopy. In the case of SB SAMs, the strong interaction between the SB groups and water molecules, as revealed by the low OH-stretching vibrational energy in SEIRA, results in strong short-range repulsion with a decay length of 0.34 nm. The EG3-OH SAMs interact with water molecules more weakly compared to SB SAMs, as indicated by the higher OH-stretching energy. However, the range of modulation of the hydrogen bonding state is longer for EG3-OH SAMs, resulting in longer-ranged water-induced repulsion with a decay length of 2.53 nm.

The main finding of this work is that the different states of hydration, as reported by several vibrational spectroscopic techniques and theoretical calculations, are reflected in the repulsive forces at the interface of the SB and OEG SAMs. Both types of antibiofouling SAMs resist biofouling through water-induced repulsion, but through complementary mechanisms: SB SAMs rely on strong, short-range hydration forces, while EG3-OH SAMs rely on weaker but longer-range forces arising from extended water structuring. These complementary hydration mechanisms explain why both nonionic and zwitterionic SAMs exhibit comparable antibiofouling performance despite their fundamentally different chemical structures. A natural extension of this work is to dissect the role of buffer ions and ionic strength on the SB hydration force by repeating the F–S measurements in pure water and in higher ionic strength solutions, and to perform SEIRA in PBS to enable a direct quantitative comparison with the AFM data. Such systematic measurements should help to disentangle the intrinsic headgroup hydration from ion-specific contributions and provide further insight into the design principles of bioinert surfaces.

Supplementary Material

jp6c02994_si_001.pdf (1MB, pdf)

Acknowledgments

We acknowledge Kazue Taki for her help in arranging this project. This work was supported by MEXT/JSPS KAKENHI Grant Numbers (JP26K01354, JP23H04059, and JP22H04530). It was performed under the Research Program for CORE lab of “Five-star Alliance” in “NJRC Mater. & Dev.” This work was also supported by the Japan–Taiwan Exchange Association. This work was supported by the Innovative Science and Technology Initiative for Security (ISTIS), Acquisition, Technology & Logistics Agency (ATLA) (JPJ013268), Japan. The World Research Hub (WRH) Program of Institute of Science Tokyo also supported this work.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcb.6c02994.

  • Comparison of one-term and two-term exponential fittings (Figure S1, Tables S1 and S2) (PDF)

⊥.

Physical Sciences Division, Pacific Northwest National Laboratory, Richland WA, USA 99354

∥.

Z.Z. and E.M.M.Z. equally contributed.

The authors declare no competing financial interest.

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