Surface-Independent Hierarchical Coatings with Superamphiphobic

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Surface-Independent Hierarchical Coatings with Superamphiphobic Properties Christoph Schlaich, Luis Cuellar Camacho, Leixiao Yu, Katharina Achazi, Qiang Wei, and Rainer Haag ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08487 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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ACS Applied Materials & Interfaces

Surface-Independent Hierarchical Coatings with Superamphiphobic Properties Christoph Schlaich,† Luis Cuellar Camacho, † Leixiao Yu, † Katharina Achazi, † Qiang Wei,* ,†,

‡

and Rainer Haag* ,† †

Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, 14195, Berlin,

Germany ‡

Department of Cellular Biophysics, Max-Planck Institute for Medical Research, Heisenbergstr.

3, 70569, Stuttgart, Germany

KEYWORDS: Superamphiphobic surfaces, hierarchical coatings, mussel-inspired adhesives, self-cleaning, protein resistant surfaces

ABSTRACT: Facile approaches for the fabrication of substrate independent superamphiphobic surfaces that can repel both water and organic liquids have been limited. The design of such super-repellent surfaces is still a major challenge of surface chemistry and physics. Herein, we describe a simple and efficient dip-coating approach for the fabrication of highly hierarchical surface coatings with superamphiphobic properties for a broad range of materials based on a mussel inspired dendritic polymer (MI-dPG). The MI-dPG coating process provides a precise roughness control and the construction of highly hierarchical structures was achieved either

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directly by pH-controlled aggregation or in combination with nanoparticles (NP). Moreover, the fabrication of coatings with a thickness and roughness gradient was possible via simple adjustment of the depth of the coating solution. Subsequent post-modification of these highly hierarchical structures with fluorinated molecules yielded a surface with superamphiphobic properties that successfully prevented the wetting of liquids with a low surface tension down to about 30 mN/m. The generated superamphiphobic coatings exhibit impressive repellency to water, surfactant containing solutions, and biological liquids, such as human serum, and are flexible on soft substrates.

INTRODUCTION In the last decades research on superhydrophobic coatings has received a great deal of interest because of their potential application in various areas such as self-cleaning surfaces.1-3 Superhydrophobic surfaces have been extensively analyzed and it is now well established that superhydrophobicity is a result of a combination of a micro-/nanostructured surface and a hydrophobic material with a low surface energy.4-7 In contrast to superhydrophobic surfaces, superamphiphobic surfaces extend these extreme anti-wetting properties to organic solvents and surfactant containing solutions, which have resulted in a much broader range of applications, such as solvent-free synthesis of polymeric microspheres on a surface or handling of biologically sensitive liquids.8-9 However, the fabrication of superamphiphobic surfaces is still a major challenge in surface chemistry and physics, since the established ways to fabricate superhydrophobic surfaces are no longer sufficient and an even more precise design of micro- to nano-scaled hybrid structures is required.10 In the last decade several attempts to engineer superamphiphobic/quasi-superamphiphobic surfaces by precisely design of surface topographies


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have been reported.11-18 The importance of hierarchical surface roughness and re-entrant structures have been described in several excellent reviews.19-22 However, most of these superamphiphobic/quasi-superamphiphobic surface coatings have been prepared on certain substrates and rely heavily on the underlying material properties. Recently Chen et al. reported a superamphiphobic surface coating that could be applied to several hard and soft materials with perfluorinated copper powder23 but this method requires an adhesive, since there is no direct chemical bonding between the coating and the substrate. Polydopamine (PDA), which is a well-known mussel-inspired polymer for substrate-independent surface coatings,24-26 was employed to prepare superhydrophobic surfaces.27-29 However, the aggregates of the PDA coating are limited to a narrow nano range and a very small height. In all these examples, the lack of a roughness control in the PDA-based coatings had to be overcome by combining PDA with various particles or meshes. Very recently, PDA was combined with silica nanoparticles to construct a microscale porous superamphiphobic surface coating via a combination of ice templation and chemical vapor deposition.30 Nevertheless, due to the narrow size range of the aggregate and the limited control of the surface roughness, PDA cannot be directly used to fabricate superhydrophobic or superamphiphobic coatings. It is still a great challenge to develop substrate-independent superamphiphobic/quasi-superamphiphobic coatings by facile approaches without any specialized equipment. Herein, we report a noninvasive, efficient dip-coating approach for the fabrication of highly hierarchical surfaces with superamphiphobic properties for a broad range of materials based on a mussel-inspired dendritic polymer (MI-dPG), which was introduced in our recent work as a 2nd generation mussel-inspired adhesive.31 The MI-dPG forms a rough surface coating in a two-step time-dependent deposition process to obtain superhydrophilic/superhydrophobic properties that


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despite their universal substrate adhesion failed to repel organic solvent or surfactant containing solutions. In the present work, the MI-dPG could be controlled to form different sized aggregates by varying the coating conditions. We precisely tuned the surface roughness either directly by aggregation or in combination with nanoparticles (NP) to get both highly hierarchical micro and nano surface structures at the same time. The surface structure for a superhydrophobic coating, which has a similar surface roughness to our previously reported coating,31 only requires a one-step deposition while the hierarchical coating with (quasi)-superamphiphobic properties can be achieved in a two-step deposition. Most of the reported superamphiphobic surfaces, including our presented one, exhibit superamphiphobic properties only in a certain degree. Because the presented coating successfully repels liquids with surface tension down to about 30 mN/m, we call it from now on a quasi-superamphiphobic coating. The fabrication process is based on a complete bottom-up approach and involves a simple dip-coating process without any specialized equipment. The obtained highly hierarchical structures are chemical active surfaces, and the resulting quasi-superamphiphobic surfaces can be easily fabricated by simple post-modification with fluorinated molecules. Moreover, the thickness and roughness of the coatings can be simply controlled by the depth of coating solution, which resulted in superamphiphobicsuperhydrophobic-hydrophobic gradient coatings. Finally, the generated coatings are long-term stable during the incubation in aqueous conditions and can bear bending and even strong folding.


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RESULTS AND DISCUSSION The MI-dPG was synthesized in a four step synthesis starting from dendritic polyglycerol (dPG) as reported previously.32 In brief, the hydroxyl groups of the dPG scaffold (Mn = 12,000 g·mol-1, Mw = 18,000 g·mol-1) were converted to amine groups, 40% of which were further functionalized with catecholic moieties. Under basic conditions, the MI-dPG can undergo covalent crosslinking and form a chemically and mechanically stable coating on virtually all types of materials (see Scheme 1). Amines can couple with catechols under oxidizing conditions via Michael addition or Schiff’s base reactions to enhance the crosslinking of the coatings.33 In the present study, we mainly focused on coatings on glass substrates and all the presented contact angles refer to droplets on coated glass slides. However, to exemplify the universality of the mussel-inspired coating, we prepared additional quasi-superamphiphobic surfaces on various materials such as gold, silica, polypropylene (PP), and Teflon representing different classes of materials such as metals, metalloids, polymeric and fluorinated materials (see Figure 1a).


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Scheme 1. General coating approach. (a) Left: structure of the mussel-inspired dendritic polyglycerol (MI-dPG). Right: Resulting MI-dPG coating and post-modification with heptadecafluoro-undecanoyl

chloride

(CF8).

Fabrication

approach

superamphiphobic hierarchical MI-dPG surface coatings (SAP

of

(b) quasi-

hMI-dPG), (c) quasi-

superamphiphobic MI-dPG with Ag/Au NPs surface coatings (SAP hMI-dPG), (d) microroughened, superhydrophobic MI-dPG (SHP MI-dPG) coating.

Depending on such coating parameters, as the pH of the coating solution, concentration of polymer, coating time, and/or the H2O/MeOH ratio of the coating solution, the crosslinking of the MI-dPG could be accelerated or decelerated, which achieved a control over the size of aggregates deposited on the surface and the final surface roughness and morphology. When the


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crosslinking of MI-dPG dissolved in MeOH (1 mg/ml) was triggered by adding 3-(Nmorpholino)propanesulfonic acid (MOPS) buffer solution with a pH-value of 8.5, a microroughened surface was obtained with an aggregate size of 0.5 - 2 µm in an overnight coating. Much smaller nanometer-sized aggregates were obtained by reducing the pH-value of the buffer to 7.5 and lowering the MI-dPG concentration during the coating (0.5 mg/ml). The micrometerand the nanometer-sized coatings were combined to achieve a “lotus-like” hierarchical structure (Scheme 1). In the first step, a micro-roughened surface was prepared that then served as the initial layer for the second layer with the nanometer-sized polymeric aggregates uniformly distributed over the whole surface. As a result, a highly hierarchical MI-dPG-coated surface (hMI-dPG) was obtained via a simple two-layer approach. It should be emphasized here that the MI-dPG aggregates could stably anchor onto almost any kind of material via mussel-inspired adhesion and that the second layer of the nano-aggregates, which was crosslinked with the first layer via catechol chemistry, resulted in highly stable coatings.31 Additionally, AgNPs and AuNPs were applied as a second layer to obtain a hierarchical micro and nanometer-sized coating. The NPs were generated by immersing the micro-roughened, MI-dPG-coated slides in either a AgNO3 for 5 h for the AgNPs or in a AuCl3 solution for 16 h in the case of the AuNPs. In both cases, no additional reducing agent was necessary and the particles were uniformly distributed over the entire surface, because the catechol moieties have a reducing effect and can coordinate the generated NPs.34


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Figure 1. (a) Contact angle (CA) for water and surfactant solution of sodium dodecyl sulfate (SDS, 4% w/w) for different hMI-dPG coated substrates. (b) Wetting property of SDS (4%) on a MI-dPG superhydrophobic (SHP) surface and (c) and (d) wetting properties of hMI-dPG quasisuperamphiphobic (SAP) surface for water and SDS (4%), respectively.

The formation of the hierarchical surface structures was confirmed by scanning electron microscopy (SEM). In Figure 2d, the highly porous aggregated polymeric cluster with two-tier roughness on both the micro and the nano levels is clearly shown. The diameter ratio of the smaller particles in the second layer and the micrometer-sized aggregates in the first layer is about 1/10. The same phenomenon was observed for the AuNP- or AgNP-coated surfaces. Plenty of smaller NPs with a size range from 45 to 55 nm were uniformly distributed over the complete polymeric cluster (see Figure 2b and 2c). However, in this case, the diameter ratio of


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the particles decreased to 1/50 for the AuNP and AgNP-coated surfaces. Thus, in all three cases, the fabrication of highly hierarchical structures similar to lotus leaves was confirmed, which were then responsible for the superamphiphobicity.

Figure 2. SEM images of (a) micro-roughened MI-dPG coated surface, (b) MI-dPG AgNP coated surface, (c) MI-dPG AuNP coated surface, and (d) hierarchical MI-dPG-coated surface (hMI-dPG). Insets: images of 3 µL water droplets on these surfaces and the results of the contact angle (CA) measurements.


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These highly hierarchical structures with optimized roughness highly affected the wetting of the surfaces. In all three cases, superhydrophilic surfaces with water contact angles (WCA) close to 0° were obtained regardless of the substrate’s nature. Even on extremely hydrophobic substrates like PP or Teflon, the coated surface was completely wetted by the droplet. Due to numerous active groups on top of the surface, these superhydrophilic coatings could be easily transferred into a highly repellent surface with superamphiphobic properties by further post-modification with a fluorinated acid chloride without affecting the morphology of the hybrid structure (see Scheme 1). A water droplet placed on the fluorinated hMI-dPG coating formed a WCA of 173° ± 1° and showed a rolling-off angle (RA) less than 2°. This extreme repellency to water was further represented by the extremely low contact angle hysteresis (the difference between the advancing and receding contact angles of a moving droplet) of less than 1°. Even under running tap water at a maximum pressure, the water repellency maintained and the surface stayed completely dry (Figure 3b and Movie M1). Similarly, the coatings jointly composed of MI-dPG and NPs also displayed extreme water repellency: The AgNP- and AuNP-coated surfaces showed a WCA of 164° ± 5° and 165° ± 2°, respectively. After post-modification of the coating with micrometer-sized roughness (Figure 3a), the WCA was still in the range of 155° but lower than for the hMI-dPG coating one. Changing the solvent from water to different organic solvents or surfactant containing solutions, the droplets deposited on the hMI-dPG coating showed contact angles ranging from 148° ± 4° for hexadecane up to 163° ± 3° for diodomethane (Figure 3a). Even the rolling off angle for surfactant containing solutions like sodium dodecyl sulfate (SDS 4% w/w) with a surface tension of γ = 34.2 mN/m was 10°. This extreme anti-wetting properties of the quasi-superamphiphobic hMI-dPG surface is highlighted in Figure 3b and Movie M1. The CA for droplets of diodomethane, glycerol, SDS (4%), and


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hexadecane on the AuNP coated surface were 157° ± 2°, 152° ± 3°, 151° ± 4°, and 146° ± 5°, respectively. Droplets of the same solvents on the AgNP coating yielded CAs ranging from 143° ± 5° for hexadecane to 154° ± 3° for diodomethane. Therefore the repellent properties for the hMI-dPG coating was significantly higher than for the metal-based NP coatings and the superamphiphobicity decreased upon reducing the diameter ratios of the nanometer- to micrometer-sized aggregates from 1/10 to 1/50. Since the chemical functionalization of all the coatings presented in this study was the same, the morphology is mainly responsible for the difference in wettability. The diameter ratio of 1/10 gave the best result regarding the superamphiphobicity of the surfaces, which is in accordance with previous studies in literature.35

Figure 3. (a) Contact angles of various solvents on different types of MI-dPG coatings and on flat PTFE. Pictures on the right: CA of serum (top) and hexadecane (bottom) on the hMI-dPG surface. (b) Time-sequence images representing the repellency of the hMI-dPG surface against


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water, water with high pressure, SDS (4%), and a torrent of SDS (4%) (from left to right). Images were collected from Movie M1. Most of the superamphiphobic shows certain limitation and repel liquids only down to a certain surface tensions. In general, a surface is considered superamphiphobic when the organic solvents such as hexadecane (27.5 mN/m) contacting the surface show a contact angle over 150° and rolling off angle less than 10°.19-20 In some examples in literature it was even possible to successfully repel liquids with surface tensions down to 20 mN/m (e.g. octane or heptane) by electrospinning microbeads of fluoropolymers on stainless steel meshes.13-14 Only a specific doubly reentrant structure that enables very low liquid-solid contact fraction repel liquids with the lowest known value of 10 mN/m (i.e., FC-72).36 In case of our substrate-independent approach, the fluorinated hMI-dPG surfaces successfully repelled liquids until a surface tension of 34 mN/m (e.g. SDS 4%) as shown in Table 1. For hexadecane the static CA still was close to 150°, but the droplet stayed pinned to the surface and did not roll-off the surface upon tilting of the surface. When the surface tension of the contacting liquid gets even lower, such as decane (23.4 mN/m) the CA is further decreased down to 95°. In case of hexane (18.4 mN/m) the three-phase system totally collapsed and the liquid fully wetted the surface. These limitations of liquid repellency are really typical for super-wetting systems. They represent the main future challenge of creating surfaces that repel liquids with even lower surface tension by facile approaches. However, in contrast to the superhydrophobic MI-dPG the fluorinated hMI-dPG surface showed enhanced liquid repellency regarding organic solvent and surfactant containing solutions and thus can be considered as an easy-to-fabricate quasi-superamphiphobic surface.


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Table 1: Static CA and roll off angle of the quasi-superamphiphobic hMI-dPG coating. Liquid

The

Surface Tension

Contact Angle

Roll-off angle

Water

72.1 mN/m

173° ± 3°

< 2°

Serum

56.9 mN/m

166° ± 3°

5°

Diodomethane

50.9 mN/m

163° ± 3°

7.5°

Ethylene glycol

34.5 mN/m

157° ± 3°

10°

SDS (4%)

34.2 mN/m

156° ± 3°

10°

Hexadecane

27.5 mN/m

148° ± 4°

-

Dodecane

23.4 mN/m

95° ± 4°

-

Hexane

18.4 mN/m

< 10°

-

superhydrophobic

MI-dPG

increase

of

the

WCA

from

the

simple

to

the

quasi-superamphiphobic hMI-dPG surface was rather small but significant (see Figure 3a). The difference became obvious, when organic solvents or surfactant containing solutions were just used. Based on the textured solid-air-liquid interface, the water droplets on a superhydrophobic surface remained in the Cassie-Baxter state, which prevented the transition in the energetically more favored and completely wetted Wenzel state.37-38 However, the balance of solid-air-liquid interface could only be maintained until a certain impalement pressure was reached, which is proportional to the surface tension of a liquid (Equation 1).10 Similar correlations of the impalement pressure and the surface tension for different shaped surface structures have also been reported by other groups.13, 39 ≈

× ×

(1)


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In Equation 1 γ is the surface tension of the liquid, R is the radius of the spherical aggregates, r is the minimal distance between protrusions, and ΘM is the advancing contact angle of the liquid on a flat fluorinated surface. Liquids with a low surface tension, e.g., SDS (4%, w/w, 34.2 mN/m) could easily penetrate the roughness groves, replace air, and wet the superhydrophobic MI-dPG surface (see Figure 1b). As a result, droplets hitting the surface stayed strongly pinned to the surface as represented in the Wenzel state or Cassie-Baxter to Wenzel transition state (Figure 4a). In contrast, the quasi-superamphiphobic hMI-dPG surface exhibiting overhanging structures decreased the area fraction of the projected liquid-solid interface. This fraction correlates very well with the apparent contact angle of the surface (Equation 2).40 cos ∗ = −1 + ∅ (cos + 1)

(2)

In Equation 2 the surface fraction ∅ s corresponds to the ratio of the surface on top of the roughness grooves (in contact with the liquid) and the apparent surface of the substrate (Figure 4c). In case of the quasi-superamphiphobic hMI-dPG and superhydrophobic MI-dPG coating ∅ can be approximately calculated by Equation 3.40

∅ =

(3)

In Equation 3 R refers to the average radius of the particles in the top layer of the coating and r to the average distance between protrusions (Figure 4d). In case of the superhydrophobic MI-dPG coating with r = 4 µm and R = 375 nm, the theoretical CA for water and SDS (4%) is 157° and 136°, respectively. In contrast, the theoretical CA for water and SDS (4%) increased up to 172° and 165°, respectively, for the quasi-superamphiphobic hMI-dPG coating (R = 40 nm). In the case of the quasi-superamphiphobic surface, these theoretical values were consistent with the measured CAs for both liquids and represent the decrease in the area fraction of the projected


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liquid-solid interface (Figure 4c and Table S4). For the superhydrophobic surface, the theoretical value still agreed with the measured CA for water. However, the theoretical value for SDS (4%) was far higher than actually measured (136° vs. 110°), because SDS (4%) penetrated into the surface structure, which makes the Cassie-Baxter model no longer suitable. The Wenzel CA (θW) can be calculated by Equation 4. cos θW = a cos θ

(4)

In Equation 3 cos θ refers to the Young equation and a to the surface roughness factor, which is defined as the ratio of real contact area of the liquid-solid interface to the apparent area. The CA of SDS (4%) on flat PTFE surfaces was only 50° ± 5°. For the superhydrophobic MI-dPG surface with a ≥ 1, the theoretical θW thus must be smaller than 50°. However, the experimental value is 104° and far higher than the theoretical value. As a result, the SDS (4%) droplet on the MI-dPG superhydrophobic surfaces was not at Wenzel state either. Moreover, the droplet can be described in a Cassie-Baxter to Wenzel transition state. It is reasonable to speculate that liquids with a surface tension between 27.5 mN/m and 50.9 mN/m can penetrate the roughness grooves of the superhydrophobic MI-dPG surfaces but still cannot completely wet the aggregates and show a certain oleophobicity (Figure 4b). In contrast, the quasi-superamphiphobic hMI-dPG coatings pinned the solid-liquid-air, three-phase contact line for all of the shown liquids on the topside of the surface structure and successfully prevented the wetting transition from the CassieBaxter state to the Wenzel state.


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Figure 4. Different states of anti-wetting. (a) The liquids with low surface tension cause the transition from the Cassie-Baxter state to the Wenzel state on MI-dPG superhydrophobic surfaces but not on hMI-dPG quasi-superamphiphobic surfaces. (b) Different wetting models: Wenzel mode (wetting state), Cassie-Baxter-to-Wenzel transition state, and Cassie-Baxter mode (lotus state). (c) The hierarchical structure of the hMI-dPG surfaces decreased the area fraction of the projected liquid-solid interface (∅s). (d) Schematic of structure features.

To understand the influence of the coating parameters on the film thickness, two possible coating formations have to be considered. First, the MI-dPG directly adsorbs and anchors onto the substrate and forms a film that constantly increases with time. Second, the MI-dPG starts to cross-link in the solutions and forms partially insoluble particles that will sediment to the bottom and cross-links with the pre-adsorbed MI-dPG film on the substrate. Due to the second mechanism, the number of the stacked particles on each other and their density can be controlled


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by adjustment of the depth during the coating (amount of particles over the substrate). Consequently a coating with a gradient layer thickness was obtained by simply tilting the substrate during the coating. Post-modification of this gradient coating yielded a quasisuperamphiphobic surface with anisotropic wettability, and the liquid repellency constantly decreased along the gradient from high to low thickness (Figure 5a and 5b). For instance, a droplet of SDS (4%) decreased from 154° for maximum height down to 73° for minimum height. The film thickness was confirmed by atomic force microscopy and typically ranged from 0.5 to 4 µm along the gradient (Figure S2, Supporting Information). In the AFM picture and SEM pictures (Figure 5c-h), an increase in layer thickness, aggregate density, surface roughness and the maximum peak height along the gradient is clearly visible. This observation is further reflected in the increase of the Rq and Rmax values for different spots on the gradient coating. For instance, the Rq values rose from 454 nm to 764 nm and the Rmax increased from 2.8 µm to 3.6 µm from the low to high layer thickness respectively. The minimum thickness necessary for the construction of a surface with superamphiphobic properties was 3 µm, which correlates to a coating depth of 0.5 cm. It has been proven in previous studies that gradients of wettability play a major role in water collection and water-oil separation.41-42


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Figure 5. (a) Photo of the gradient hMI-dPG coating with dye colored SDS (4%) on top. (b) Schematic illustration of the gradient hMI-dPG coating. SEM images of the gradient hMI-dPG coating for (c) low thickness, (d) medium thickness, and (e) high thickness. 100 x 100 µm AFM images of a scratched gradient hMI-dPG surfaces and corresponding cross section profile (f) and (i) for low thickness with Rq = 360 nm, Ra = 454 nm, and Rmax = 2.91 µm, (g) and (j) for medium thickness with Rq = 425 nm, Ra = 520 nm, and Rmax = 2.82 µm, (h) and (k) for high thickness with Rq = 639 nm, Ra = 764 nm, and Rmax = 3.64 µm. The listed R-values are the mean of three different cross sections.


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To investigate the long-term stability of the quasi-superamphiphobic coatings stability and abrasion tests were done. First, contact angle measurements were performed after immersing the coatings in water for up to 30 days. The water CAs for the MI-dPG NPs and hMI-dPG-coated surfaces remained constant for 1 month over 165° and 170°, respectively (Figure 6b). This high stability of the catechol-crosslinked clusters is further represented by the CAs for SDS (4%), which remained around 154°, 150°, and 146° for the hMI-dPG, MI-dPG AuNP, and MI-dPG AgNP-coated surfaces, respectively. To fully understand the robustness of the hMI-dPG coating a linear abrasion test was performed that was recently suggested as a standard method to evaluate durability of a superamphiphobic or superhydrophobic coating.43-45 The abrasion tests were carried out on a quasi-superamphiphobic hMI-dPG coated glass. During the test sandpaper was placed face-down on top of the coated glass slide and moved for 6 cm along the surface. This process was then considered as one abrasion cycle (see Figure 6a). In order to maintain the pressure constant of the sandpaper on the surface it was weighted down with a 100 g weight. The water contact angles after each abrasion cycle are shown in Figure 6c. The static CA of water, diiodomethane, and SDS (4%) decreases slowly with increasing number of abrasion cycles. Within the first three abrasion cycles the CA for SDS (4%) and diiodomethane decreased from 163° and 156° down to 137° and 148°, respectively. Reason for this dramatic breakdown of the CA and loss of superamphiphobic properties is the fragileness of the delicate nanoscale aggregates of the hybrid structure that are easily abraded under large external shear forces. In contrast, the micrometer-sized aggregates could bear longer abrasion, which is represented in the longer maintenance of water repellency. The same trend was observed by monitoring the roll-off after every abrasion cycle (see Figure 6d). The self-cleaning effect of the quasisuperamphiphobic hMI-dPG for diiodomethane and SDS (4%) was only maintained until 1 and 3


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abrasion cycles, respectively. Whereas water droplets could roll-down easily the surface until 10 abrasion cycles. As expected the coatings do partially degrade upon abrasion in comparison to state of the art durable coatings, because the polyglycerol based materials obviously softer than sandpaper.12, 46-47 However, the soft polymer materials exhibit advantages on flexible surface coatings.

Figure 6. (a) Photographs representing one cycle of the linear sandpaper abrasion test. (b) Longterm stability test: contact angles of water and SDS (4%) after different days under water. (c) and (d) Contact angle and rolling off angle changes of different solvents on the hMI-dPG quasisuperamphiphobic surface after different abrasion cycles.

To further exemplify the universality of the hMI-dPG, a flexible PP foil was coated and transferred into the quasi-superamphiphobic surface. The flexible surface showed the same


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superamphiphobic properties as the hMI-dPG coating on rigid PP. To investigate the robustness and wettability of the structure the surface was characterized by SEM and WCA measurement. After bending and even strongly folding the hMI-dPG coated PP foil several times, the anti-wetting properties still remained. The high repellency and the flexibility the fabricated surface are highlighted in Figure 7a and Movie M2. There was a slight decrease in the repellency just for solvents with low surface tension, such as hexadecane and SDS (4%) or hexadecane. The CA for SDS (4%) and hexadecane went down to 138° ± 5° and 141° ± 4°, respectively, after the mechanical stress. SEM images after bending and folding the surfaces showed that the structure of the hMI-dPG was only slightly damaged (Figure 7b). Although the structure had some minor cracks, the overall structure of the polymeric cluster remained stable and did not detach from the substrates. As a result, the hierarchical structure persisted and the superhydrophobicity of the surface remained, whereas the superamphiphobicity became slightly limited.

Figure 7. (a) Time-sequence images of the hMI-dPG coated PP foil representing the flexibility and anti-wetting property before and after bending and folding the surface. (b) SEM image of the


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flexible hMI-dPG structure after bending and folding the surface about 30 times. (c) Protein adsorption of the fluorinated hMI-dPG surface in comparison to glass and polystyrene (PS). The adsorbed amount of proteins on both hMI-dPG-coated PS and glass was close to zero (lower than the limit of detection). (d) The adhesion of Hela cells that express green fluorescent protein (GFP) on the bare and hMI-dPG modified glass after 3 days of cultivation.

Non-specific adsorption of proteins on material surfaces is still a major problem for the design of new biomedical devices that are in contact with biological fluids. As soon the fluid gets in contact with an uncoated material, severe biological responses can occur such as blood coagulation, inflammation, or irritation of the surrounding tissue.48 Therefore the improvement of a material’s surface properties and the prevention of bio-fouling plays a key role in improving the biocompatibility of materials. In recent decades, research on anti-fouling surfaces was mainly focused on modifying materials with hydrophilic coatings.49-50 By taking advantage of the nearly contact-free solid-liquid interface of a superamphiphobic surface, new approaches for an efficient bioinert coating were developed.8,

51-52

Therefore the anti-fouling properties of the

fluorinated hMI-dPG coating were tested using the BCA™ Protein Assay Reagent Kit (PIERCE), quantitated by QCM measurements and cell culture experiments. Figure 7b shows the protein adsorption of a hMI-dPG-coated glass in comparison to a polystyrene (PS) and a glass reference from undiluted blood serum. Preventing protein adsorption in a highly complex protein environment is one of the biggest challenges for the traditional hydrophilic coatings. Blood serum is a complex protein mixture and strongly fouls many of the current “protein resistant” hydrophilic coatings.53 Moreover, traditional superhydrophobic surfaces cannot efficiently prevent wetting by blood and blood serum, because the surface tension of blood and


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blood serum is lower than that of water (Figure 3a).7 The adsorbed amounts of protein for the PS and glass reference were 2.4 and 1.7 µg/cm2, respectively. In contrast, the hMI-dPG-coated glass showed a protein adsorption close to zero (lower than the limit of detection), because the liquid did not wet the surface at all and the contact area in the serum-coating interface was minimized. Cell adhesion studies were performed with GFP-modified Hela cells and NIH-3T3 mouse fibroblasts. In the case of the non-coated glass surfaces (Figure 7d and Figure S5), the cells spread regularly over the whole surfaces after 3 days of cultivation, whereas no cells were detected on the hMI-dPG modified surfaces. This high repellency for biological fluids with their rather complex composition clarifies the enormous potential of superamphiphobic surfaces for biological applications.

CONCLUSION In summary, the MI-dPG coating process provides a precise control over the aggregate size and the resulting surface roughness and morphology. Thus controlled hierarchical surface structures are accessible either directly or in combination with metal NPs on various substrates. Due to numerous functional groups on top of the coating, the highly hierarchical structures could be easily transformed into surfaces with certain superamphiphobility by perfluoralkylation. The generated hMI-dPG coatings show universal applicability with moderate mechanical robustness on various rigid and flexible solid substrates including plastic, ceramic, metal, and metal oxide materials.

Furthermore, the hMI-dPG coatings showed very good long-term stability and

therefore possess a high potential as non-fouling biointerfaces because of their nearly contactfree solid-bioliquid interface.


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EXPERIMENTAL SECTION Materials and Instrumentation. All chemicals were used as received without further purification and purchased from Sigma (Steinheim, Germany) unless stated otherwise. Solvents were either employed as purchased or dried prior to use by usual laboratory methods. NaCl, NaOH and HCl were bought from VWR International (Darmstadt, Germany). 1H and 13C-NMR spectra were recorded on a Bruker ECX 400 MHz spectrometer. Dialysis was performed in benzoylated cellulose tubes (D7884, width: 32 mm, molecular weight cut-off (MWCO) 2000 g·mol-1) from Sigma-Aldrich Munich, Germany. The deionized water used was purified using a Millipore water purification system with a minimum resistivity of 18.0 MΩ·cm. The surface morphology of the coatings was investigated with a field emission scanning electron microscope (FE-SEM, Hitachi SU8030, Japan) at an accelerating voltage (Vacc) of 20 kV, a current of 10 µA and a working distance (WD) of around 8.3. The samples were dried under high vacuum and coated with a 8-10 nm gold layer by using a sputter coater (Emscope SC 500, Quorum Technologies, UK) for 20 s at 30 mA, 10-1 Torr (1.3 mbar) in argon atmosphere.

Synthesis of MI-dPG. The hyperbranched glycerol (dPG) with Mn=12.000 g·mol-1 and Mw=16.000 g·mol-1 was polymerized by a one-step ring opening anionic polymerization (ROAP) as described in earlier publications.54-55 Amine functionalized dPG (dPG-NH2) was prepared according to previously published procedures of our group.32 800 mg PG-NH2 (4 mmol) were dissolved in a mixture of MeOH and pH 4.8 2-(N-morpholino)ethanesulfonic acid (MES, 0.1 M) buffer (25 ml, 1v/1v). After addition of 1.11 g 3,4-dihydroxyhydrocinnamic acid


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(DHHA, 6 mmol) and 0.93 g 1-Ethyl-3-(3-dimethyl-aminopropyl)carbodiimid (EDC, 6 mmol), the solution was stirred for 16 h at room temperature. After removal of the solvent in vacuum, the final residue was dialyzed in MeOH for 4 days. For higher stability and better storage 37% HCl was added before drying the MI-dPG. 1H NMR (500 MHz; CD3OD): δ = 6.68-6.53 (m, Ar); 4.21-3.02 (m, PG-backbone); 2.75 (m, COCH2CH2C); 2.52 (m, COCH2CH2C) ppm.

MI-dPG initial layer (micro-structured layer). The MI-dPG coatings were prepared by immersing the freshly cleaned slides in a MI-dPG (e.g. 1 mg /ml) dissolved in 4 ml of a mixture of MeOH and pH 8.5 3-(N-morpholino)-propanesulfonic acid (MOPS) buffer (2v/1v) for 16 h. The depth of the solution was 1 cm. Then the coated slides were thoroughly rinsed with water and methanol and dried by an Argon stream.

MI-dPG 2nd layer (nano-structured layer). The MI-dPG coatings were prepared by immersing the slides with the initial layer in a MI-dPG (e.g. 0.5 mg /ml) dissolved in 4 ml of a mixed solution of MeOH and pH 7.5 3-(N-morpholino)-propanesulfonic acid (MOPS) buffer (3v/1v) for 16 h. The depth of the solution was 1 cm. Then the coated slides were thoroughly rinsed with water and methanol and dried by an Argon stream.

AgNP 2nd layer (nano-structured layer). The MI-dPG-AgNP coatings were prepared by immersing the slides with the initial layer in a solution of AgNO3 (1 mg /ml) for 5 h. Then the coated slides were thoroughly rinsed with water and dried by an Argon stream.


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AuNP 2nd layer (nano-structured layer). The MI-dPG-AuNP coatings were prepared by immersing the slides with the initial layer in a solution of AuCl3 (0.1 mg /ml) for 16 h. Then coated slides were thoroughly rinsed with water and dried by an Argon stream.

Superamphiphobic surfaces. Quasi-superamphiphobic coatings were achieved by further perfluorination of the hierarchical surfaces. The hierarchical surfaces were prepared as described above and then dried in high vacuum for 2 h. The coatings were immersed in a solution of 1 mg/ml 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoyl chloride with 10 eq. of triethylamine in diethyl ether under argon atmosphere. After 5 hours, the perfluorinated coatings were thoroughly rinsed with diethyl ether and dried with an Argon stream.

Linear Abrasion Test. The quasi-superamphiphobic hMI-dPG surface was placed fixed with double-sided tape on a table. The sandpaper (standard grit No. 240 bought from Bauhaus Germany) was abraded over the surface under a weight of 100 g over the surface in one direction. Pulling the sandpaper once over the surface is considered as one abrasion cycle. 50 cycles of mechanical abrasion tests were carried out on and water contact angles and rolling off angles were measured after from cycle 1-10 every cycle, from cycle 11-20 every second cycle, and from cycle 21 -50 every fifth cycle.


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Atomic Force Microscopy. For the surface roughness studies an AFM Nanoscope MultiMode 8 from Bruker operated in Contact and tapping mode was used. All measurements were performed in dry state and tapping mode was used when small regions were scanned (1-5µm) while contact mode performed at best for larger areas (20-100µm). AFM tips SNL-10 (Bruker) with a nominal tip radius of 2-12 nm were used. Scan rates of 0.5-0.15 Hz were used during mapping with 512 points per scan.

Protein adsorption. Protein adsorption form undiluted blood serum was measured by BCA™ Protein Assay Reagent Kit (PIERCE). Normally the UV-Vis data of the BCA test results are quantitated by the standard concentration-absorbance curve for the adsorption of single proteins, but this method is not suitable for serum. Therefore, the amount of adsorbed proteins on bare PS and glass was quantitated by quartz crystal microbalance (QCM) with dissipation in our experiments. The online protein adsorption from serum was firstly tested on PS and glass-like SiO2 sensors by QCM (0.1 ml/min for 30 min adsorption followed by buffer rinsing). The adsorbed proteins were further collected by incubated these sensors in SDS solution (1% w/w) for BCA tests. The protein adsorption on the hMI-dPG coated sensors was only measured by BCA reagent, because they were not suitable for QCM measurement. The UV-Vis data from the BCA tests can be used to compare the amount of proteins adsorbed on bare and coated surfaces.

Cell experiments. For the cell culture experiment NIH-3T3 fibroblasts and Hela cells were collected from petri dishes by incubation in trypsin (dilution 1:250) for 5 minutes at 37 °C. The cell suspension was washed from trypsin by centrifugation, the top layer was removed, and the


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remaining cells were resuspended in fresh medium. The slides were incubated with 800000 cells in 4 mL of cell medium (cell number was determined via Neubauer chamber) for 3 days at 37 °C and 5% CO2 in a small petri dish. After 3 days the medium was removed and the slides rinsed with with 4 mL PBS to remove non-adherent cells. The remaining cells were observed by microscope directly (TELAVAL 31, Zeiss, Germany). The average number of the adhering cells was calculated from at least eight randomly chosen areas.

ASSOCIATED CONTENT Supporting Information. Detailed materials and methods and supplementary figures and videos. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the FU Berlin Graduate School “Fluorine as a Key Element” (GRK 1582) funded by the German Science Foundation (DFG). The authors thank the core facility BioSupraMol and Anke Schindler for her support with the SEM measurements, Dr. Manfred Gossen for providing us the Hela cells, and Dr. Pamela Winchester for proofreading of this manuscript.

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49. Banerjee, I.; Pangule, R. C.; Kane, R. S., Antifouling Coatings: Recent Developments in the Design of Surfaces That Prevent Fouling by Proteins, Bacteria, and Marine Organisms. Adv. Mater. 2011, 23, 690-718. 50. Wei, Q.; Becherer, T.; Angioletti-Uberti, S.; Dzubiella, J.; Wischke, C.; Neffe, A. T.; Lendlein, A.; Ballauff, M.; Haag, R., Protein Interactions with Polymer Coatings and Biomaterials. Angew. Chem., Int. Ed. 2014, 53, 8004-8031. 51. Ellinas, K.; Tsougeni, K.; Petrou, P. S.; Boulousis, G.; Tsoukleris, D.; Pavlatou, E.; Tserepi, A.; Kakabakos, S. E.; Gogolides, E., Three-Dimensional Plasma Micro–Nanotextured Cyclo-Olefin-Polymer Surfaces for Biomolecule Immobilization and Environmentally Stable Superhydrophobic and Superoleophobic Behavior. Chem. Eng. J. 2016, 300, 394-403. 52. Zhao, J.; Song, L.; Yin, J.; Ming, W., Anti-Bioadhesion on Hierarchically Structured, Superhydrophobic Surfaces. Chem. Commun. 2013, 49, 9191-9193. 53. Gunkel, G.; Huck, W. T. S., Cooperative Adsorption of Lipoprotein Phospholipids, Triglycerides, and Cholesteryl Esters Are a Key Factor in Nonspecific Adsorption from Blood Plasma to Antifouling Polymer Surfaces. J. Am. Chem. Soc. 2013, 135, 7047-7052. 54. Wilms, D.; Stiriba, S.-E.; Frey, H., Hyperbranched Polyglycerols: From the Controlled Synthesis of Biocompatible Polyether Polyols to Multipurpose Applications. Acc. Chem. Res. 2009, 43, 129-141. 55. Burakowska, E.; Haag, R., Dendritic Polyglycerol Core-Double-Shell Architectures: Synthesis and Transport Properties. Macromolecules 2009, 42, 5545-5550.

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Scheme 1. General coating approach. (a) Left: structure of the mussel-inspired dendritic polyglycerol (MIdPG). Right: Resulting MI-dPG coating and post-modification with heptadecafluoro-undecanoyl chloride (CF8). Fabrication approach of (b) superamphiphobic hierarchical MI-dPG surface coatings (SAP hMI-dPG), (c) superamphiphobic MI-dPG with Ag/Au NPs surface coatings (SAP hMI-dPG), (d) micro-roughened, superhydrophobic MI-dPG (SHP MI dPG) coating. 131x106mm (300 x 300 DPI)



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Figure 1. (a) Contact angle (CA) for water and surfactant solution of sodium dodecyl sulfate (SDS, 4% w/w) for different hMI-dPG coated substrates. (b) Wetting property of SDS (4%) on a MI-dPG superhydrophobic (SHP) surface and (c) and (d) wetting properties of hMI-dPG superamphiphobic (SAP) surface for water and SDS (4%), respectively. 90x85mm (300 x 300 DPI)


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Figure 2. SEM images of (a) micro-roughened MI-dPG coated surface, (b) MI-dPG AgNP coated surface, (c) MI-dPG AuNP coated surface, and (d) hierarchical MI-dPG-coated surface (hMI-dPG). Insets: images of 3 µL water droplets on these surfaces and the results of the contact angle (CA) measurements. 90x128mm (300 x 300 DPI)



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Figure 3. (a) Contact angles of various solvents on different types of MI-dPG coatings and on flat PTFE. Pictures on the right: CA of serum (top) and hexedecane (bottom) on the hMI-dPG surface. (b) Timesequence images representing the repellency of the hMI-dPG surface against water, water with high pressure, SDS (4%), and a torrent of SDS (4%) (from left to right). Images were collected from Movie M1. 201x123mm (300 x 300 DPI)


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Figure 4. Different states of anti-wetting. (a) The liquids with low surface tension cause the transition from the Cassie-Baxter state to the Wenzel state on MI-dPG superhydrophobic surfaces but not on hMI-dPG superamphiphobic surfaces. (b) Different wetting models: Wenzel mode (wetting state), Cassie Baxter-toWenzel transition state, and Cassie Baxter mode (lotus state). (c) The hierarchical structure of the hMI-dPG surfaces decreased the area fraction of the projected liquid-solid interface (∅s). (d) Schematic of structure features. 90x79mm (300 x 300 DPI)



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Figure 5. (a) Photo of the gradient hMI-dPG coating with dye colored SDS (4%) on top. (b) Schematic illustration of the gradient hMI-dPG coating. SEM images of the gradient hMI-dPG coating for (c) low thickness, (d) medium thickness, and (e) high thickness. 100 x 100 µm AFM images of a scratched gradient hMI-dPG surfaces and corresponding cross section profile (f) and (i) for low thickness with Rq = 360 nm, Ra = 454 nm, and Rmax = 2.91 µm, (g) and (j) for medium thickness with Rq = 425 nm, Ra = 520 nm, and Rmax = 2.82 µm, (h) and (k) for high thickness with Rq = 639 nm, Ra = 764 nm, and Rmax = 3.64 µm. The listed R-values are the mean of three different cross sections. 90x75mm (300 x 300 DPI)


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Figure 6. (a) Photographs representing one cycle of the linear sandpaper abrasion test. (b) Long-term stability test: contact angles of water and SDS (4%) after different days under water. (c) and (d) Contact angle and rolling off angle changes of different solvents on the hMI-dPG quasi-superamphiphobic surface after different abrasion cycles. 541x351mm (96 x 96 DPI)



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Figure 7. (a) Time-sequence images of the hMI-dPG coated PP foil representing the flexibility and antiwetting property before and after bending and folding the surface. (b) SEM image of the flexible hMI-dPG structure after bending and folding the surface about 30 times. (c) Protein adsorption of the fluorinated hMIdPG surface in comparison to glass and polystyrene (PS). The adsorbed amount of proteins on both hMIdPG-coated PS and glass was close to zero (lower than the limit of detection). (d) The adhesion of Hela cells that express green fluorescent protein (GFP) on the bare and hMI-dPG modified glass after 3 days of cultivation. 158x125mm (300 x 300 DPI)


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55x50mm (300 x 300 DPI)


Surface-Independent Hierarchical Coatings with Superamphiphobic

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