Bioinspired Universal Monolayer Coatings by Combining Concepts

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Bioinspired Universal Monolayer Coatings by Combining Concepts from Blood Protein Adsorption and Mussel Adhesion Leixiao Yu, Chong Cheng, Qidi Ran, Christoph Schlaich, PaulLudwig Michael Noeske, Wenzhong Li, Qiang Wei, and Rainer Haag ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15834 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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ACS Applied Materials & Interfaces

Bioinspired

Universal

Monolayer

Coatings

by

Combining Concepts from Blood Protein Adsorption and Mussel Adhesion Leixiao Yu,1 Chong Cheng,1 Qidi Ran,1,2,5 Christoph Schlaich,1 Paul-Ludwig Michael Noeske,4 Wenzhong Li,1 Qiang Wei,1,2,3, * Rainer Haag1,2, * 1

Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustr. 3, 14195,

Berlin, Germany 2

Multifunctional Biomaterials for Medicine, Helmholtz Virtual Institute, Kantstr. 55,

14513, Teltow-Seehof, Germany 3

Department of Biointerface Science & Technology, Max-Planck Institute for Medical

Research, Heisenbergstr. 3, 70569, Stuttgart, Germany 4

Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM),

Wiener Str. 12, 28359 Bremen, Germany 5

Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und

Energie GmbH, Hahn-Meitner-Platz 1, 14109, Berlin, Germany

KEYWORDS: Universal coatings, Monolayer, Block copolymer, Bioinspiration, Biofunctional surface

ACS Paragon Plus Environment


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ABSTRACT: Despite the increasing need for universal polymer coating strategies, only a few approaches have been successfully developed and most of them are suffering from color, high thickness, or high roughness. In this paper, we present for the first time a universal monolayer coating that is only a few nanometers-thick and independent of the composition, size, shape, and structure of the substrate. The coating is based on a bioinspired synthetic amphiphilic block copolymer that combines two concepts from blood protein adsorption and mussel adhesion. This polymer can be rapidly tethered on various substrates including both planar surfaces and nanosystems with high grafting density. The resulting monolayer coatings are, on the one hand, inert to the adsorption of multiple polymer layers and prevent bio-fouling. On the other hand, they are chemically active for secondary functionalization and provide a new platform for selective material surface modification.


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1. INTRODUCTION Surface modifications with polymers play an important role for solid materials in the physical, chemical, and biomedical sciences.1 Traditional surface modification technologies face many challenges due to the fast development and diversification in biomedical and chemical materials science. There is increasing need for universal coatings that are substrate-independent, regardless of the substrates’ chemical composition and physical/structural characteristics. To achieve such, appropriate interactions between the coating polymers and the substrate surfaces with intra-coating crosslinking to stabilize them are required. Moreover, the coatings should present reactive functionalities to be further functionalized.2 Due to these high demands, only a limited number of universal coating technologies have been developed, which include surface plasma irradiated polymerization,3 layer-by-layer (LbL) assembly,4,5 phase transited protein adhesion,6 spin coating,7 chemical vapor deposition (CVD),8 and mussel-inspired or plant phenol-inspired coatings.9-11 However, none of these methods can induce the smallest possible thickness and substrate-contouring morphology of a highly controllable monolayer coating. The most widely used and benchmark universal polydopamine mussel-inspired coatings can only result in aggregated and rough coatings with dark brown color and are unstable under sonication. A thin polydopamine coating is also suffering from a homogeneity problem.12 Although a dopamine derivative, norepinephrine, has been reported to fabricate a relative smooth coating,13 it just decreases the size of the aggregates in the coating and is still far from being comparable to monolayer coatings.



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Herein, we report on a simple anchoring and efficient crosslinking approach to fabricate universal monolayer coatings that are inspired from blood protein adsorption and mussel adhesion. Blood proteins spontaneously adsorb on almost all solid material surfaces by denaturing themselves to expose “anchor domains” to the surfaces.14 Hydrophobic interactions, hydrogen bond formation, ionic or electrostatic attractions, and coordinative interactions are recognized as the main forces causing and driving this adsorption. Among them, hydrophobic interactions occur during the adsorption on various solid surfaces.14 Proteins, e.g. albumin, adsorb to all surfaces due to their amphiphilic properties. Mussels can adhere to a broad range of solid surfaces and are especially stable on metal oxide and mineral surfaces because of mussel foot proteins (mfps). Although the adhesion of mfps is a very complicated process, catecholic anchoring and subsequent intra-molecular crosslinking are believed to play the most important role.15 Moreover, the hydrophobic amino acids in mfps, especially in mfp-3 “slow” (mfp-3s), enhance the hydrophobic interaction and shield the catechols from the water phase to provide a microenvironment that retards oxidation.16 Therefore, a biomimetic amphiphilic block copolymer with catecholic and amino groups in the short anchoring segment was developed to integrate the amphiphilic interaction and catecholic anchoring, with manifold attachment and chelation as well as subsequent intralayer crosslinking to stabilize the surface coating. A stepwise dip-coating process was further designed to achieve universal monolayer coatings with a few nanometer thicknesses on macro-scale and nano-scale surfaces or interfaces, and ensure that the polymers literally attach to practically all solid surfaces


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– except to surfaces covered by themselves. It must be emphasized that catechol alone is not a strict universal anchor, which only exhibits weak hydrophobic interactions on non-polar surfaces.17

Figure 1. Polyglycerol (PG)-based amphiphilic block copolymer that mimics blood protein adsorption and mussel adhesion. The polyglycerol block serves as the hydrophilic domain, while the anchor domain can be converted from poly (allyl glycidyl ether) block. The catechol groups mostly contribute to the coordinative and/or hydrogen bonding on polar surfaces, but can also serve as hydrophobic domains together with the phenyl groups for anchoring on non-polar surfaces. The amine groups, on the one hand, increase the crosslinking efficiency and, on the other, displace hydrated cations from the mineral surfaces to stabilize the coatings. The



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ω-N3 or ω-Br terminal groups serve as the functional site for secondary modification. The detailed information of the synthesized polymers and their abbreviations are listed in Table 1 of the synthesis section.

Table 1. The generated copolymers with different functional groups are listed in the table with the corresponding abbreviation. (b: block copolymer, r: random copolymer)

2. RESULTS AND DISCUSSION

2.1 Bioinspired amphiphilic block copolymer

As shown in Figure 1, the polyglycerol (PG)-based block copolymer (Mn = 9000 g·mol-1, polydispersity = 1.2) with a polyglycerol block (about 110 repeat units) and a poly(allyl glycidyl ether) block (12 repeat units) was synthesized by ring-opening anionic polymerization (ROAP).18 The polyglycerol block serves as a hydrophilic domain that prevents adsorption of excess polymers to maintain the monolayer coatings. Meanwhile, the poly (allyl glycidyl ether) block can be functionalized by amine (2 units), catechol (5 units), and phenyl (5 units) groups as the anchor domain (PG-CatPh) by thiol-ene chemistry. The catechol groups allow polymers to be stably anchored on the polar surfaces via coordinative and/or hydrogen bonding,19 while the


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hydrophobic domains, i.e., phenyl groups, initiate adsorption on non-polar surfaces to form monolayers as the first step. It must be emphasized that the oxidation of catechols, which induces crosslinking and aggregation,20 must be avoided in this step by using acidic conditions (Figure S1).

In fact, there is a long history of employing amphiphilic polymers to modify hydrophobic surfaces.21 However, there are several drawbacks to employ traditional amphiphilic polymers for hydrophobic surface modification: The molecules tethered by hydrophobic interaction can be invaded by water long-term or be replaced by some other hydrophobic molecule,14 and can be easily rinsed away by surfactant solutions or some organic solvents. Especially, body fluids contain plenty of amphiphiles, which limit the use of those polymer coatings in biomedical applications. Therefore, it is necessary to crosslink the coatings on the non-metal surfaces (only anchored by hydrogen bonding, hydrophobic interaction, and/or other weak interactions) by oxidizing catechols to quinone-catechol and quinone-amine bonds22 as the second step. This step extends the oligovalent anchoring to polyvalent anchoring,23 which significantly enhances the stability of the coatings. The amine groups in the polymers, on the one hand, increase the crosslinking efficiency24 and, on the other, displace hydrated cations from the mineral surfaces.25 Interestingly, like our stepwise coating process, mussel adhesion is dictated by a time-regulated secretion of mfps to optimize the surface anchoring.26 Overall, these three types of groups in the anchor domain integrate the inspiration from blood protein adsorption and mussel adhesion and synergistically contribute to surface anchoring. Besides the main function of the



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groups described above, catechol and amine groups also donate hydrogen bonding, electrostatic attraction, and coordinative interaction to mimic blood protein adsorption. Phenyl groups also mimic the hydrophobic environment as in mfp-3s, to prevent the facile autoxidation of catechols besides providing additional hydrophobic interactions. Moreover, due to the self-limiting mono-layer adhesion, this amphiphilic block copolymer is also suitable for the coating of nanosystems, e.g., sheets and particles. 2.2 Adsorption of bioinspired block copolymers on various substrates In order to identify the effects of the hydrophobic domain, the mussel adhesive group, and the crosslinker on the surface attachment, quartz crystal microbalance (QCM) with dissipation was employed to analyze the adsorption of the polymers on different substrates which were selected to cover a wide range of distinct polar or non-polar characteristics. Dynamic online coating results show that about 800-1000 ng/cm2 of the block copolymer PG-CatPh adsorbed on different types of sensor surfaces, including gold (Au), titania (TiO2), silica (SiO2), polystyrene (PS), as well as alkylated and fluorinated self-assembled monolayer (SAM) on Au sensors (Figure 2 and Figure S2) under acidic conditions. It must be emphasized that the concentration of the polymer solutions for coating is always 1 mg/mL, which is far smaller than the critical micelle concentrations of the polymers (>30 mg/mL). It has been proven that under this condition, catecholic polymers can only anchor on metal oxide surfaces including TiO2 surfaces as monolayer coatings.27,28 The amount of the adsorbed PG-CatPh on all the tested surfaces was similar to the amount found on TiO2 surfaces,


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which may indicate the formation of monolayer coatings on these surfaces. The stability of the coatings in surfactant solutions was tested by rinsing the adsorbed polymers with 1% w/w sodium dodecyl sulfate (SDS) or deconex aqueous solution (Figure 2d). The amount of the adsorbed PG-CatPh on Au and TiO2 surfaces only slightly decreased (30 mg/mL). After that, the slides were thoroughly rinsed with Milli-Q water and methanol and then dry by N2 stream. The protocol of online coating by using QCM has been described in the QCM part. For non-metal substrate, the interaction between catechol and substrate surface was not strong enough. The crosslinking treatment was conducted by using an oxidant in basic buffers. The coated slides were immersed into 10 mg/mL K2S2O8 in pH 8.6 MOPS buffer solution for 1h to trigger the crosslinking and self-polymerization of catechol/quinone groups. 4.2 Coating on Nanosheet-Interfaces and Nanoparticle-Interfaces. To make the coating on thermal reduced graphene oxide (TRGO), TRGO41 (1 mg) and PG-CatPh (5mg) were firstly transferred into a small flask and then 2 mL pH 6 MOPS buffer was added. The coating was rapidly completed under sonication for 10 min and the coated TRGO was further washed with Milli-Q water by centrifugation at least 4 times to remove the free PG-CatPh. The centrifuged PG-CatPh coated TRGO can be easily re-dispersed in water just by hand shaking; the oven-dried PG-CatPh-TRGO can be re-dispersed by sonication. The PG-CatPh coating on Fe3O4 nanoparticles (Poly (acrylic acid), Mn = 1800 Da, was used as the stabilizer, prepared according to the method reported by J.P. Ge, et. al.42) was conducted in a similar manner as described for TRGO. ASSOCIATED CONTENT


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Supporting Information. Methods and any associated references are available in the Supplementary Information.

AUTHOR INFORMATION Corresponding Authors * [email protected], *[email protected] Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This work was supported by the Chinese Scholar Council (CSC) and Helmholtz Virtual Institute on “Multifunctional Biomaterials for Medicine”. We thank Dr. Pamela Winchester for proofreading this manuscript. We thank Dr. Yan Lu and Prof. Dr. Matthias Ballauff for the helpful discussion on the nanosysterms. We thank Prof. Dr. Fredrik Höök for the helpful discussion on the QCM results and analysis. We thank Prof. Dr. Roland Netz for the helpful discussion on the coating physics.

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Figure 1. Polyglycerol (PG)-based amphiphilic block copolymer that mimics blood protein adsorption and mussel adhesion. The polyglycerol block serves as the hydrophilic domain, while the anchor domain can be converted from poly (allyl glycidyl ether) block. The catechol groups mostly contribute to the coordinative and/or hydrogen bonding on polar surfaces, but can also serve as hydrophobic domains together with the phenyl groups for anchoring on non-polar surfaces. The amine groups, on the one hand, increase the crosslinking efficiency and, on the other, displace hydrated cations from the mineral surfaces to stabilize the coatings. The ω-N3 or ω-Br terminal groups serve as the functional site for secondary modification. The detailed information of the synthesized polymers and their abbreviations are listed in Table 1 of the synthesis section. 180x129mm (300 x 300 DPI)



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Figure 2. QCM frequency (f) and dissipation (D) shift as a function of time during the adsorption of block copolymer PG-CatPh on TiO2 (a) and PS (b) sensor surfaces. (c) ∆D-∆f plots of the adsorption of PG-Cat, PG-Ph, PG-CatPh, and PG-Cat(r) on PS sensor surfaces (60 min online coating). The starting point of the dynamic coating process was defined as ∆D = ∆f = t =0. The arrow shows the break point of the two adsorption periods of PG-CatPh. (d) The amount of the adsorbed block copolymer PG-CatPh on different types of sensor surfaces with or without rinsing by surfactant solutions. (#: The F-Au and C-Au represent the perfluorinated and alkylated self-assembled surfaces on gold sensors, respectively.) 131x95mm (600 x 600 DPI)


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Figure 3. The [C]/[O] concentration ratio of the different surfaces before and after being coated with PGCatPh block copolymers. The data were collected from XPS survey spectra. More details are provided in Tables S1 and S2. 65x47mm (600 x 600 DPI)



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Figure 4. (a-d) AFM height images of TiO2 (a), coated TiO2 (b), PS (c), and coated PS (d) surfaces at the same scanning magnification. The inserted curves show the representative height differences for each sample. Images in smaller scanning magnifications are available in Figures S9-10. (e, f) The average surface roughness and the surface coating density of the PG-CatPh polymer chains on TiO2 (e) and PS (f) substrate. Error bars indicate SD. 180x86mm (300 x 300 DPI)


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Figure 5. (a) Photographic picture obtained 12 hours after preparing aqueous dispersions of the PG-CatPhTRGO (left) and TRGO (right) nanosystems. (b) Typical TEM images of PG-CatPh-TRGO nanosheets prepared from an aqueous dispersion. (c) AFM height images of PG-CatPh-TRG, the inserted curve shows the height of PG-CatPh-TRGO increases to 9.6 ± 1.6 nm. (d) The DLS data of the diameter and zeta potential of Fe3O4 nanoparticles before and after coated by PG-CatPh. (e) The TEM image of PG-CatPh coated Fe3O4 nanoparticles. The diameter of Fe3O4 nanoparticle cores was about 30 nm. (f) TGA curves of Fe3O4 nanoparticles and PG-CatPh coated Fe3O4 nanoparticles. The inserted figure was the corresponding DTG curves. 180x101mm (300 x 300 DPI)



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Figure 6. (a) Scheme of protein and cell resistance of PG-CatPh coated surfaces. (b) Adsorbed amount of fibrinogen (Fib) on the PG-CatPh coated TiO2 and PS surfaces, as well as the adsorption on these coatings that were immersed in PBS buffer or SDS solution (2%, w/w) for 1 week or 1 month. Three parallel samples were tested every time and Error bars indicate SD (c) NIH3T3 cell adhesion on the PG-CatPh coated TiO2 and PS surfaces after 3 days of cell culture. Images were collected without rinsing the surfaces. Scale bar: 50 µm. 180x119mm (300 x 300 DPI)


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Figure 7. (a) Scheme of post-functionalization with RGD peptide by azide-alkyne cycloaddition to PG-CatPh coatings. (b) Number of MC3T3 osteoblasts adhered on bare, PG-CatPh coated, and RGD immobilized PTFE surfaces after one-day culturing (*p< 0.0001). (c) Fluorescent micrographs of MC3T3 osteoblasts stained for paxillin (green), filamentous actin (red), and nucleus (blue) with lower (above) and higher (below) magnification. Cells were cultured on bare, PG-CatPh coated, and c(RGDfK) immobilized PTFE surfaces for one day. 180x162mm (300 x 300 DPI)



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90x80mm (300 x 300 DPI)


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Bioinspired Universal Monolayer Coatings by Combining Concepts

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