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May 15, 2017 - Key Laboratory of Plant Resources and Chemistry in Arid Regions, Xinjiang Technical Institute of Physics and Chemistry, Chinese. Academ...
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Adsorption and Orientation of 3,4-Dihydroxy‑L‑phenylalanine onto Tunable Monolayer Films Ting Chen,†,‡ Hui Yang,*,† Hongwei Gao,§ Mingkai Fu,‡,∥ Shizhe Huang,† Wei Zhang,† Guangxin Hu,† Fanghui Liu,† Aiqing Ma,⊥ Keji Sun,⊥ and Jinben Wang*,† †

Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Key Laboratory of Plant Resources and Chemistry in Arid Regions, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, P. R. China ∥ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ⊥ Oil Production Technology Research Institute, Shengli Oilfield Branch Company, Sinopec, Dongying, Shandong 257000, P. R. China S Supporting Information *

ABSTRACT: 3,4-Dihydroxy-L-phenylalanine (L-DOPA) is considered to be responsible for the mussel adhesion to a variety of surfaces. A molecular level understanding of the interactions between DOPA molecules and surfaces with different wettability and chemistry, however, posts significant challenges to control marine antifouling. Here, different self-assembled monolayers (SAMs) on gold surfaces were fabricated: (i) OH-, (ii) COOH-, and (iii) CH3-terminations. The effect of surface wettability and chemistry on the adsorption of DOPA upon the series of surfaces was investigated in situ, showing that the adsorbed mass was lower and the water content of DOPA layer was higher on hydrophilic surfaces (including OH- and COOH-terminated SAMs) than that on hydrophobic ones (including CH3-terminated SAMs and gold surface). Direct evidence regarding the DOPA orientation and the interaction between DOPA and film surfaces were obtained: on the OH-terminated surface a flexible and loose structure formed via coordinate hydrogen bonds of the hydroxyl end groups of the surface interacting with carboxyl groups of DOPA, while for the CH3-terminated surface, DOPA molecules mainly adopt a flat conformation due to the formation of hydrophobic “bonds” between the hydrophobic functional groups of alkyl chains on surface and aromatic rings of DOPA molecules. This study led a new insight into the adsorption mechanisms based on the adsorption processes and layer structures, and it proposed novel concepts for the design of antifouling and adhesive surfaces.

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INTRODUCTION The adhesion proteins secreted by marine mussels bind strongly to virtually all inorganic and organic surfaces in aqueous environments, accordingly serving as an intractable problem in the field of marine antifouling.1,2 A common feature of such proteins is that they contain a high content of 3,4dihydroxy-L-phenylalanine (L-DOPA), which is considered to be responsible for their capacity to compete successfully with water at the surface and cross-link under water.3,4 In the most recent decade, interaction mechanisms of DOPA side chains embedded in proteins and solid surfaces have been extensively studied, owing to a growing attraction in the respects of antifouling strategies and biomimetic adhesion.5−8 It can be concluded that DOPA plays an important part in the interactions through H-bonding,9,10 coordination with metal/ metal oxide,11,12 or covalent cross-linking,13,14 of which the © XXXX American Chemical Society

detailed binding mechanisms of DOPA to different surfaces is an extremely important research topic, attracting great interests of scientists. Although surface force apparatus (SFA) has been used to measure the macroscopic dissociation of two surfaces adhered by mussel proteins after 20−30 min absorption,15−18 yet the method cannot directly detect the dynamic adsorption behavior of such adsorbates in real time which provides a new insight into the adhesion mechanisms of mussel adhesive proteins. Till now, there has been a little knowledge about the adsorption processes of DOPA-containing proteins,19,20 however, the adsorption of isolated DOPA onto surfaces still remains Received: March 24, 2017 Revised: May 9, 2017 Published: May 15, 2017 A

DOI: 10.1021/acs.jpcc.7b02795 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C unclear. In particular, there is currently a lack of understanding about the relationship between surface chemistry and binding mechanisms of DOPA. Answering these questions is not only fundamentally important for the mechanisms behind mussel adhesion, but also practically valuable for the design of marine biofouling and biocompatibility of biomaterials. With this in mind, we chose 3,4-dihydroxy-L-phenylalanine (L-DOPA) as a model molecule that adhere to the surfaces which we constructed from a series of self-assembled monolayers (SAMs) on gold substrates, to study the adsorption behavior of DOPA on such monolayer films as well as the relationship between adhesion mechanism and surface properties, such as wettability and sorbed water content, by the use of quartz crystal microbalance with dissipation (QCM-D), spectroscopic ellipsometry (SE), and X-ray photoelectron spectroscopy (XPS) methods. Adhesion mechanism was revealed by the aid of density functional theory (DFT) method in terms of binding energy, density of states (DOS), and molecular configuration of DOPA on Au(111) surfaces with different thiol SAMs, which provided a new insight into the contribution of molecular conformation and orientation to the versatile adhesion of mussel onto surfaces owning different wettability and chemistry.

Figure 1. (a) Changes in frequency and dissipation of different alkyl thiol molecules immobilizing on gold surfaces at the third overtone. (b) QCM-D and ellipsometry mass of different thiol molecules immobilizing on gold surfaces.

for calculating the adsorbed mass on the sensor, as shown in eq 1:22 ρq tq Δf Δf Δm = − = −C f0 n n (1) where f 0 is the fundamental frequency (5 MHz); Δm is the adsorbed mass in ng/cm2; ρq is the density of quartz crystal (2648 kg·m−3); tq is the thickness of quartz crystal (3.3 × 10−4 m); n is the overtone number (1, 3, 5, ···13); C is a constant (17.7 ng·cm−2·Hz1−, in the case of n = 1 and f 0 = 5 MHz). For a nonrigid adsorbed film, e.g., in the case of ΔD/(Δf) > 1 × 10−6 /Hz, the layer essentially senses a viscoelastic “hydrogel” composed of adsorbate and coupled water.23,24 So, the Voigt model is chosen, in which the adsorbed mass can be modeled by the Q-tools software package (Biolin Scientific AB, Sweden). The parameters of the DOPA adsorbed layers studied were layer density (1200 kg·m−3),19,20 fluid density (1000 kg· m−3), layer viscosity (0.0005−0.1 kg·ms−1), layer shear modulus (103−105 Pa), and thickness (10−10−10−6 m). All the experimental data were fitted with Voigt model, as shown in Figures S1. Ex Situ Characterization of SAMs and DOPA Layers. After monitored by QCM-D method, samples were removed, washed with ethanol and blown dry with nitrogen, in the preparation for the following measurements at the temperature of 25 °C. SE (M-2000 V, J. A. Woollam) was carried out with an incidence angle of 70° and a wavelength scan from 370.1 to 999.1 nm. After the establishment of an optical constant of bare surface, the thickness of thiol monolayers was calculated by a Cauchy model with a resumed refractive index of 1.45.25 From the changes in ellipsometric angles (Δ, ψ) and refractive index (n), the optical thickness of the film was deduced as shown in Figure S2, based on which the adsorbed mass (Δme) was obtained, where the density of the bulk solution was chosen as 800 kg·m−3.26 The thickness of DOPA layers was determined in the same way and the refractive index was chosen to be 1.4, as shown in Figure S3.23 On the basis of the results of thickness, the adsorbed mass was evaluated, assuming that the DOPA solution density was chosen as 1000 kg·m−3.27 Surface topography was characterized through AFM (FASTSCAN, Bruker Instruments, USA) in a peakforce tapping mode using a silicon cantilever, with a nominal spring constant of 4 N/m and at a scan rate of 1.0 Hz. The cross section analysis and the roughness (Rq) of adsorbed layers in air were evaluated by the Nanoscope Analysis software. A multifunctional XPS system (ESCALab250Xi, VG, England) equipped with a 200 W

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EXPERIMENTAL SECTION Materials. 1-Octadecanethiol (96%) and 11-mercapto-1undecanol (97%) were purchased from Alfa Aesar and SigmaAldrich. 11-mercaptoundecanoic acid (95%) and amino acid 3,4-dihydroxy-L-phenylalanine (L-DOPA) were purchased from J & K Chemical Technology. All of the reagents except those especially mentioned were used without further purification and all of the solutions were prepared with Milli-Q water or absolute ethyl alcohol (99.9%) in this work. L-DOPA solution at 5 mmol was prepared by dissolving solid power in NaCl/ HCl solutions at a 1:1 volume ratio (pH = 5.5), and stirred on a vibrator for about 6 h in the dark conditions. They were stored in a dark and cool place to avoid oxidation before use. In Situ Monitoring of SAMs Formation and DOPA Adsorption. QCM-D (Q-sense E1, Biolin Scientific AB, Sweden) and a sensor of gold-coated quartz crystal with AT-cut (QSX 301, Biolin Scientific AB, Sweden) were employed. With a fundamental resonant frequency of ∼5 MHz, a sensor was mounted in a fluid cell with one side exposed to the solution. The sensor, with a root-mean-square (RMS) roughness of less than 2.0 nm, is approximately 14 mm in diameter. Prior to each experiment, the QCM chamber and connecting tubes were cleaned with 2% SDS solutions and Milli-Q water. Then, the crystal was mounted in a cell and a small amount of pure solvent was introduced to establish a stable baseline with a fluctuation being less than 0.5 Hz for several hours. The flow rate was set at a constant of 20 μL·min−1 within situ data acquisition. All of the experiments were repeated at least three times and the temperature was kept at 25 °C. The mass increase at the crystal surface is registered as a decrease in resonance frequency (Δf), whereas the change in dissipation (ΔD) depends on the viscoelastic property of the adsorbed film. For the sake of simplicity, only the profiles of Δf and ΔD versus time at the third overtone were exhibited in this work. Taking SAMs formation as an example (Figure 1a), a small dissipation trace (e.g., ΔD/(Δf) < 1 × 10−6 /Hz) indicates that the adsorbed film is relatively rigid and evenly distributed.21 Therefore, the Sauerbrey equation is appropriate B

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Figure 2. XPS spectrum, AFM topography, and contact angle of CH3- (a1, a2, a3, and a4), COOH- (b1, b2, b3, and b4), and OH-terminated (c1, c2, c3, and c4) SAMs, respectively.

Here EDOPA, Eslab, and E(DOPA+slab) are the energies of DOPA, alkyl thiol SAMs forming on Au(111), and DOPA adsorbed on SAMs over Au(111); ΔE is defined as a positive binding energy for each adsorption modes, signifying an attraction between DOPA and SAMs.

monochromatic Al Kα X-ray source was adopted. The X-ray spot of 500 μm was used and the takeoff angle (angle between the surface and the direction of the analyzer) was chosen at 90°. The background pressure in the analysis chamber was about 3 × 10−10 mbar. The hydrocarbon C 1s line at 284.8 eV from adventitious carbon was used for energy referencing. CA measurements were carried out with a 3 μL water drop using an optical contact angle meter (Kruss DSA CA goniometer, Germany). Calculation of Conformational Variation of Adsorbed DOPA. Electronic structure calculations were performed using DFT implemented in the CASTEP package.28,29 The exchangecorrelation function was calculated using the generalized gradient approximation (GGA) within the Perdew−Burke− Ernzerhof (PBE) formulation. The energy cutoff of plane-wave basis set was 750.0 eV within normal-conserving pseudopotential. The Monkhorst−Pack scheme (1 × 1 × 1) for the Brillouin Zone was chosen. Geometry optimization was taken to be converged when the maximal atomic force was smaller than 1.0 × 10−6 eV/Å. Au surface, consisting of three layers of Au atoms, was created by cutting a slab of bulk along the (111) plane, with the deepest layer of Au atoms frozen on its bulk position. In order to eliminate the mutual interactions, the slab was separated from its periodic images in z direction by a vacuum region of ∼40 Å. The calculations were performed on a supercell of size 17.58 × 11.72 × 44.79 Å3, containing one adsorbate molecule. Geometries of different SAMs over the gold substrates were optimized (see Table S1), suggesting that the electronic structure of DOPA cannot be disturbed by the underlying metal. An optimized DOPA molecule was placed on the top of the SAMs, with its phenylene ring either parallel (referring to “flat”) or perpendicular (referring to “upright”) to the surface and the bottom O atom or aromatic carbon placed 2.0 Å distant from the nearest surface O or H. The molecule− surface complex for these initial configurations was optimized and the geometries with the lowest energy were obtained, as shown in Table S2. After the optimization of all the geometries above, the binding energy (ΔE) of DOPA attaching to different SAMs was calculated via eq 2:30 ΔE = E DOPA + Eslab − E(DOPA + slab)

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RESULTS AND DISCUSSION SAMs Formation and Characterization. Figure 1a shows the shifts in frequency (Δf) and dissipation (ΔD) for 1octadecanethiol, 11-mercaptoundecanol, and 11-mercaptoundecanoic acid assembling on the gold surface. An immediate drop in Δf is observed after the thiol solution is introduced, suggesting a rapid transplant and adsorption of thiol from solution to surface. A slow period lasting several hours is proposed to be the rearrangement of the preadsorbed thiol molecules to maximize their contact and minimize their free energy.31,32 It approaches the equilibrium and the resonance frequency decreases about 11.5, 10.4, and 9.1 Hz for CH3-, COOH-, and OH-terminated SAMs, respectively. Correspondingly, the dissipation shifts are around 1 × 10−6 in the case of different thiol solutions, exhibiting a formation of rigid film. As shown in Figure 1b, the “wet” adsorbed mass of alkanethiol immobilizing on gold surface is about 203.2, 184.4, and 160.3 ng·cm−2 for CH3-, COOH-, and OH-terminated SAMs, separately, based on the Sauerbrey relation of eq 1. The “dry” adsorbed mass calculated from ellipsometry measurements does not show a big difference from the “wet” mass, further indicating the formation of rigid adsorbed layers and weak solvation effect. Figure 2a1 shows XPS spectra for the CH3-terminated SAMs, where the predominant peak is assigned to −CH2 and −CH3 of the alkyl chain. Similarly, there are double peaks in the C 1s spectra of COOH-terminated SAMs: one center at 284.8 eV due to C atoms in the alkane chain and the other center at 288.7 eV due to the carboxylic groups (Figure 2b1). For the OH-terminated layer, the C 1s peak can be decomposed into two contributions: the peak at 284.8 eV is attributed to the carbon atoms of the aliphatic chain, and the one at 286.5 eV is attributed to the C−OH tail (Figure 2c1). As shown in Figure 2, parts a2−c2, the S 2p spectra fits well with double peaks at about 162.2 and 163.2 eV, suggesting the formation of Au−S bonds for all the SAMs. The results confirm the presence of

(2) C

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Figure 3. (a) Shifts in frequency and dissipation for the adsorption of DOPA on CH3-, COOH-, OH-terminated SAMs and gold surface (n = 3); (b) D−f plot of DOPA adsorbed on OH- and CH3-terminated surfaces; (c) QCM-D mass, ellipsometry mass, and water content of DOPA adsorbed films on different surfaces; (d−g) N 1s spectrum for different thiol SAMs after DOPA adsorption.

loose and extended layer. It also indicates that the solvation effect on the adsorption process of DOPA may be stronger on hydrophilic surfaces than that on hydrophobic ones. The profiles of ΔD−Δf for OH- and CH3-terminated SAMs are plotted in Figure 3b, which indicates that the properties, such as surface wettability and chemistry etc., are considered to be the decisive factors dominating the adsorption configuration of DOPA on different surfaces. The adsorption process on both of the modified surfaces contains two regimes: in regime I, the slope of ΔD−Δf profile is bigger than that in regime II, due to the fast adsorption of DOPA on the underlying surfaces; regime II suggests a conformation adjustment, which is primarily dominated by intermolecular cross-linking and further adsorption of DOPA. In the case of the hydrophilic surface, a relatively high value of slope suggests that the DOPA molecules form an extended conformation, probably arising from the coordinate hydrogen bonds (between OH-terminated surface and catechol hydroxyl or carboxyl groups of DOPA) and leading to a low adsorption of DOPA. In the case of the hydrophobic surface, a flat conformation is revealed from a low value of ΔD/Δf, mainly because of the formation of hydrophobic “bonds” between CH3-terminated surface and aromatic rings of DOPA. Figure 3c shows the mass uptake of DOPA calculated from the results of QCM-D and ellipsometry measurements for each thiol surface. In the case of hydrophilic surfaces, such as OHand COOH-terminated surfaces, QCM-D mass of DOPA adlayers is about 1438.7 and 1558.3 ng·cm−2, respectively. In the case of hydrophobic surfaces, such as bare gold and CH3− SAMs, QCM-D mass goes to 1464.5 and 1321.8 ng·cm−2, separately. In comparison with the “wet” mass estimations, the

desired elements and chemical groups in the monolayers and indicate the successful formation of monolayers attaching to gold substrates. The morphologies of SAMs with similar average roughness (Rq) values around 1 nm are shown in Figure 2, parts a3−c3. A smooth and homogeneous surface can be observed after being coated with alkyl thiol molecules, indicating that SAMs are in the form of closely packed nanostructures. Contact angle of modified surfaces is presented in Figure 2, parts a4−c4, spanning the range from ∼19.6° for OH-terminated surface to ∼100.6° for CH3-terminated surface. XRD, AFM, and CA characterization results of bare gold surface are provided in Figure S4. Adsorption Behavior of DOPA. The shifts in frequency (Δf) and dissipation (ΔD) upon the adsorption of DOPA as a function of time are presented in Figure 3a, showing a continuous adsorption of DOPA at surface over 24 h. After being rinsed by buffer solution, a slight increase in frequency and a decrease in dissipation are observed, indicating that only some weakly bound components are removed from the surfaces. During the whole period, the frequency approaches ∼−14.0, ∼−15.6, ∼−18.3, and ∼−16.5 Hz for OH-, COOH-, and CH3-terminated SAMs and the gold surface, separately. Δf for the SAMs with alkyl end groups and gold surface is bigger than that for the SAMs with carboxylic and hydroxyl end groups. There is a slight shift in dissipation around 2.2 × 10−6 for the adsorption of DOPA on CH3-terminated SAMs, mainly ascribed to the formation of a relative rigid and compact layer. For the hydrophilic surface, such as OH- and COOHterminated SAMs, dissipation shifts are about 2.5 and 2.8 × 10−6, respectively, suggesting that DOPA adsorption leads to a D

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depicted in Table 1. ΔE values of DOPA upon OH-, COOH-, and CH3-terminated SAMs in the preferential bonding

“dry” mass from ellipsometry measurements is much lower, especially on OH- and COOH-terminated surfaces. The relative water content (RWC) can be calculated by the following equation: RWC = [(m dry − m wet )/m dry ] × 100%.33,34 RWC of the DOPA adsorbed on the modified hydrophilic surfaces exceeds 60 wt %, while decreases to about 40 wt % on the hydrophobic surfaces. It indicates that the DOPA adlayer couples a large amount of water and tends to be extended and loose on hydrophilic surfaces, and in contrast it consists of less water and has a rigid and compact conformation on hydrophobic surfaces. As can be seen from XPS results (Figure 3, parts d−g), the area of N 1s peak at 399.9 ± 0.5 eV (NH or NH2), denoted as the nitrogen content of adsorbed DOPA layers, enlarges with the increase of surface hydrophobility (from OH- to CH3-terminated surface), which provides another evidence that the adsorbates are easier to settle down on hydrophobic surfaces than on hydrophilic ones. Different from the topography of SAMs (Figure 2), the images of DOPA adsorbed layers on different surfaces show fluctuant and heterogeneous morphology in Figure 4, parts a−

Table 1. Binding Energies (ΔE) of Optimized DOPA Adsorbed on Different Underlying Surfacesa binding energyOpt (kcal/mol) adsorption configuration

SAM-OH

SAM-COOH

SAM-CH3

Au

flat upright-1 upright-2

0.9 0.9 3.7

2.6 1.0 3.6

4.0 −0.4 −0.7

38.5 35.7 36.9

“Flat” refers to the molecular binding configuration with a parallel orientation of the phenylene ring on the underlying surface. “Upright1” and “upright-2” refer to the molecular configuration in which DOPA interacts with the underlying surface through hydroxyl and carboxy group of DOPA, respectively, with the molecular axis perpendicular to the surface. Binding energies in italics are the preferential adsorption configurations of DOPA on each surface. a

configurations are 3.7, 3.6, and 4.0 kcal/mol, respectively, suggesting the configurations of “upright-2” for both of the hydrophilic surfaces and “flat” for the hydrophobic one as depicted in Figure 5. The determined values, in the presence of OH- and COOH-terminated SAMs, fall within the range of typical binding energies of hydrogen bond interaction (2.4−6.2 kcal/mol30) between the carboxy group of DOPA and the underlying surface; the value for CH3-terminated SAMs is close to the range of 2.7−3.8 kcal/mol17,30 referring to hydrophobic “bond” between the phenylene ring of DOPA and the alkyl group of surface. For gold surface, ΔE, ranging from 35.7 to 38.5 kcal/mol for different configurations, is higher than that in the presence of thiol terminated SAMs mainly due to the multiple “interaction sites” of Au surface as well as the formation of metal coordination bond, in agreement with the adsorption energy of DOPA on metal surfaces (∼30.0 kcal/ mol12,35). It indicates a complex configuration with the phenylene ring plane lying parallel to the surface or the hydroxyls and carboxyl orienting perpendicularly toward the surface (see Figure 5a), mainly due to the π-e interaction and coordinate bond between DOPA and Au(111) surface. Electronic structures can be reasonably speculated according to theoretical calculations from the projected density of states (PDOS) of DOPA before and after adsorption onto OH- and CH3-terminated surfaces using DFT method, as depicted in Figure 6. For a DOPA molecule (Figure 6a, the upper panel), the multiple peaks marked with black line are corresponding to 2s electron orbitals of O atoms, and those marked with red lines are assigned to O 2p states. After DOPA adsorbed on the OHterminated surface (Figure 6a, the lower panel), the states of O atoms, belonging to the adsorbate, shifts about 0.9 eV to the low energy compared with the bands in the upper panel, indicating the decrease of energy after DOPA adsorption and the formation of a more stable structure of adsorbate because of the interaction between DOPA and underlying surface. In addition, the 2p electron peak centering at ∼4.3 eV decreases to 0 after adsorption, suggesting that O atoms of DOPA participates in the interaction. In the case of CH3-terminated SAMs, a similar change in C 2s and 2p orbitals before and after DOPA adsorption can also be observed in Figure 6b, indicating that C atoms of DOPA participate in the interaction of DOPAsurface. The densities of states (DOS) of s and p orbitals of DOPA interacting with OH-terminated SAMs are shown in Figure 6c.

Figure 4. AFM images of DOPA adlayers on (a) gold surface, (b) CH3-, (c) COOH-, and (d) OH-terminated SAMs; (e) corresponding cross-sectional profiles; (f) water contact angle on DOPA adlayers.

d, with an average roughness of ∼2 nm, giving a further support regarding the intermolecular cross-linking of DOPA molecules. A comparison of the cross-sectional profiles along the whole images is shown in Figure 4e, exhibiting a smooth feature for the DOPA adlayer on hydrophobic surfaces whereas a rough feature on the hydrophilic surfaces. It is concluded that a more extended and loose adlayer forms on the OH- and COOHterminated surfaces than that on the gold and CH3-terminated surfaces. After DOPA adsorbed on different surfaces, all the contact angles go to ∼60° (Figure 4f), suggesting a similar wettability of DOPA adsorbed layers, mainly owing to the presence of intermolecular cross-linking of DOPA in all these cases. Adhesive Mechanism of DOPA. Binding energies (ΔE) of DOPA interacting with different underlying surfaces are E

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Figure 5. Optimized preferential adsorption configurations of DOPA on (a) gold surface, (b) OH-, and (c) CH3-terminated SAMs (light blue, carbon; white, hydrogen; blue, nitrogen; red, oxygen; yellow, gold atoms; dotted blue lines indicate the “bond” most likely forming in different adsorption modes).

Figure 6. PDOS of (a) O atom and (b) C atom of DOPA before and after adsorption onto OH- and CH3-terminated surfaces, respectively; (c) TDOS and PDOS plots of s and p orbitals of O and H atoms for DOPA adsorbed on OH-terminated surface; (d) TDOS and PDOS plots of s and p orbitals of C and H atoms for DOPA adsorbed on CH3-terminated surface.

Around the energy about −3 eV, O 2p states (inset ii overlap with H 1s (inset v assigned to DOPA and OH-terminated SAMs, respectively, indicating the hybridization between O 2p and H 1s orbitals, as well as the formation of hydrogen bond between them (O···H−O−). From −15 to 0 eV as shown in Figure 6d, C 2p orbitals of DOPA (inset ii) and CH3terminated surface (inset iv), as well as the H 1s orbital of the surface (inset v), contribute the valence band of TDOS and exhibit the hybridization between C and H atoms. It can be concluded that the C 2p states of DOPA interplay with the C 2p, and H 1s states of the CH3-terminated surface, resulting in

the hydrophobic interaction. In previous study, it was revealed that DOPA can form two surface species on inorganic surface (i.e., rutile): one species involving four attachment points via inner-sphere linkage and H-bond attachment (“lying down”) and another species involving only two attachment points via the phenolic oxygens (“standing up”).36,37 Here, we conclude the interaction mechanisms between DOPA and different organic surfaces: (i) in the presence of hydrophilic surface (such as OH-terminated SAMs), hydrogen bond forms between the carboxylate oxygen atoms of DOPA and the hydrogen atoms in the underlying surface, with a most likely F

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perpendicular orientation (Figure 5b); (ii) the hydrophobic surface, such as CH3-terminated surface, attracts the hydrophobic group of DOPA through hydrophobic interaction between the alkyl group of surface and the phenylene ring of DOPA, with a preferential parallel orientation (Figure 5c).

T.C. and H.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the projects of PetroChina Exploration and Development Research Institute (2015-40222000006 and RIPED-2017-JS-88), the National Natural Science Foundation of China (NSFC21603240), and the Important National Science and Technology Specific Project of China (2017ZX05013-003 and 2016ZX05025-003-009). We thank Dr. Min Wang from Biolin Scientific AB for the help in QCM measurements.

CONCLUSION The molecular configuration and binding mechanism of DOPA adsorbed on different wet surfaces, including OH-, COOH-, and CH3-terminated SAMs and the gold surface, were investigated through QCM-D, SE, XPS, and DFT calculations. Hydrophilic surfaces, such as OH- and COOH-terminated SAMs, showed a reduction of DOPA adhesion due to the ability in coupling a large amount of water in the adlayers. Hydrophobic surfaces, such as gold and CH3-terminated surfaces, exhibited a high adsorbed mass of DOPA, because of the formation of a relative rigid and compact conformation with a low water content. Preferential “upright” adsorption configuration was proposed for DOPA adsorbed on OHterminated SAMs via hydrogen bond interactions between the carboxylate oxygen atoms of DOPA and the hydrogen atoms of the underlying surface. For the CH3-terminated surface, DOPA mainly adopts a “flat” conformation with the phenylene ring paralleling to the surface, due to the formation of hydrophobic “bonds” between the aromatic rings of DOPA molecules and the alkyl groups of the surface. DOPA displayed adaptive adsorption behavior onto different surfaces, forming hydrogen bond interaction with hydrophilic surfaces (such as OHterminated surface) and generating hydrophobic interaction with hydrophobic surfaces (such as CH3-terminated surface). So, not only wettability but also surface chemistry had a direct effect on the binding mechanisms between DOPA and surfaces, which, to a large extent, determined the intermolecular interactions of the incoming DOPA molecules with the adsorbates. Our results demonstrated that building adapted molecular architectures of the surface can be used to exploit new strategies of marine antifouling.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02795. Figure S1 showing the shifts in frequency (Δf) and dissipation (ΔD) fitted with the Voigt model in the presence of different SAMs, Figure S2 showing ψ, Δ, and MSE fitted with the Cauchy model in the presence of different SAMs, Figure S3 showing ψ, Δ, and MSE fitted with the Cauchy model for DOPA adsorption on the corresponding SAMs, Figure S4 showing the X-ray powder diffraction spectrogram, AFM topography, and contact angle of bare gold surface, Table S1 exhibiting structural parameters of alkanethiol SAMs adsorbed on gold surface, and Table S2 exhibiting the binding energy of DOPA and different surfaces (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(H.Y.) E-mail: [email protected]. *(J.W.) E-mail: [email protected]. ORCID

Jinben Wang: 0000-0002-6360-9572 G

DOI: 10.1021/acs.jpcc.7b02795 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b02795 J. Phys. Chem. C XXXX, XXX, XXX−XXX