Fast and Simple Preparation of Patterned Surfaces with Hydrophilic

Nov 24, 2015 - Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Enges...
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Fast and Simple Preparation of Patterned Surfaces with Hydrophilic Polymer Brushes by Micromolding in Capillaries Benjamin Vonhören, Marcel Langer, Doris Abt, Christopher Barner-Kowollik, and Bart Jan Ravoo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03924 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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Fast and Simple Preparation of Patterned Surfaces with Hydrophilic Polymer Brushes by Micromolding in Capillaries

Benjamin Vonhören,a,⊥ Marcel Langer,b,c, ⊥ Doris Abt,b,c Christopher Barner-Kowollik,b,c* Bart Jan Ravooa* a

Organic Chemistry Institute and Center for Soft Nanoscience, Westfälische Wilhelms-

Universität Münster, Corrensstrasse 40, 48149 Münster, Germany b

Soft Matter Synthesis Laboratory, Institut für Biologische Grenzflächen, Karlsruhe Institute of

Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany c

Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie,

Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany ⊥

Both authors contributed equally to the current work.

KEYWORDS Surface Patterning; Polymer Brushes; Micromolding in Capillaries; Self-assembly

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Abstract: Micropatterns of hydrophilic polymer brushes were prepared by micromolding in capillaries (MIMIC). The polymers are covalently bound to the surfaces by a rapid hetero DielsAlder reaction, constituting the first example of polymers grafted to surfaces in a defined pattern by

MIMIC.

The

polymers

(poly(acrylic

acid),

poly(hydroxyethyl

acrylate)

and

poly(tetraethyleneglycole acrylate)) ranging in molecular weight from 1500 to 6000 g mol-1) were prepared with narrow dispersities via the reversible addition fragmentation chain transfer (RAFT) process using a highly electron deficient RAFT agent able to react with surface anchored dienes such as cyclopentadiene. We demonstrate that the anchoring method is facile to perform and highly suitable to prepare patterned surfaces which are passivated against biological impact in well-defined areas.

Introduction The functionalization of surfaces with polymer brushes is of key importance to materials scientists and finds widespread application in various fields.1–3 One of the most prominent examples is the generation of non-fouling surfaces by deposition of hydrophilic polymers.4–13 Such surfaces are desired in diverse fields ranging from marine coatings for sea vessels to implants in medical applications, where patterned non-fouling surfaces are widely used as templates for tissue engineering. The hydrophobicity of a surfaces is considered to have a significant impact on protein adsorption, which often – but not always – leads to fouling.7,14 Surfaces displaying contact angles above 65° tend to adsorb proteins, whereas surfaces with contact angles below 65° are protein repellant in many cases.7 Herein, we take advantage of this behavior to generate patterned surfaces with protein adhesive and protein repellant areas via the introduction of a rapid ligation technique based on hetero Diels-Alder chemistry combined with

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micromolding in capiliaries (MIMIC). Via this approach, hydrophilic polymers brushes were covalently bound to hydrophobic self-assembled monolayers (SAM) in a highly spatially resolved fashion. Polymer brushes can be attached to a surface by two different methods.15 They can be grafted from the surface by first modifying the surface with e.g. a chain-transfer agent for reversible addition-fragmentation chain transfer (RAFT) polymerization or an appropriate polymerization initiator capable of imparting living characteristics onto the polymerization process (e. g. an initiator for atom transfer radical polymerization (ATRP) or nitroxide mediated polymerization (NMP)) and subsequent polymerization. Alternatively, preformed polymers with a reactive end group can be grafted to a surface, which is decorated with suitable reaction partners.16 Both techniques have characteristic advantages and disadvantages. Polymers that are grafted to a surface can be fully characterized (e.g. by size exclusion chromatography (SEC), mass spectrometry and NMR spectroscopy) prior to grafting, whereas the analysis of polymers that are grafted from the surfaces is more challenging. However, the grafting densities and layer thickness are often higher for comparable polymers that are grafted from the surface than those that are grafted to. In the latter case, the initial polymers tethered to the surface hamper the binding of additional polymers, since the incoming polymers cannot easily diffuse through the already bound layer of polymers to react with the functionalized surface. Thus, the thickness of the generated polymer layer is – in many cases – lower than the length of the utilized polymer chains. Patterned polymer surfaces can be obtained by a range of methods.2 The most prominent procedures rely on photolithography17–20 or locally confined mechanical forces (e. g. induced by atomic force microscopy (AFM)).21,22 Microcontact printing, a cheap and simple soft lithography

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technique, was shown to be a well-suited alternative for the fast preparation of micropatterned polymer brushes.23–32 In the current contribution, we employ micromolding in capillaries (MIMIC), a soft lithography technique33–35 to generate patterned polymer brushes. To this end, an elastomeric stamp is placed on a surface to form a network of capillaries between the stamp and the surface. Subsequently, a solution of the molecular material to be patterned is passed into the open end of the capillaries, which are subsequently filled by capillary forces. The negative pattern of the stamp is obtained after lifting off the stamp. A large variety of materials has already been patterned on surfaces by the MIMIC approach,34–36 including organic polystyrene microparticles37

and

inorganic magnetic Fe3O4 nanoparticles38

as well

as

organic

semiconductors,39 biomolecules40,41 or inorganic salts.36 However, to the best of our knowledge, MIMIC has not been used to covalently graft polymers onto a surface.

It would be highly attractive if polymers could be covalently bound to the substrate without any post modification after polymerization, in a resource efficient and time saving fashion. Reversible addition-fragmentation chain transfer (RAFT) agents with strong electron withdrawing Z-groups (e. g. –PO(OEt)2) are appropriate, since they are known to undergo rapid hetero Diels-Alder (HDA) reactions with their electron deficient C=S double bond (Fig. 1).42,43 In addition, the HDA reaction is reversible at elevated temperatures.44 Barner-Kowollik and colleagues successfully employed HDA reactions in the field of polymer conjugation43 and demonstrated that they proceed rapidly in a range of solvents including in water at ambient conditions without the need of any catalyst.45 Moreover, they applied the HDA reaction to modify single walled carbon nanotubes46 and to graft polymers in a non-patterned fashion to silicon wafers and performed subsequent dynamic covalent bonding on the surface.47

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Figure 1. Reversible Hetero Diels-Alder reaction between cyclopentadiene and a RAFT agent as the dienophile.

In the current study, we demonstrate that covalent surface patterning with hydrophilic polymers (poly(acrylic acid), poly(hydroxyethyl acrylate) and poly(tetraethyleneglycole acrylate)) can be simply performed by MIMIC. To this end, the three different polymers were synthesized by RAFT polymerization using an electron deficient RAFT-HDA agent and subsequently grafted to cyclopentadiene (Cp) capped self-assembled monolayers (SAM) in defined line patterns. The brushes on the surface were analyzed by contact angle goniometry, water condensation experiments, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Moreover, the protein repellent character of the polymer brushes was assessed by exposing the structured surfaces to solutions of rhodamine labeled peanut agglutinin as a model protein (Fig. 2). Rhodamine labeled peanut agglutinin was chosen because of its availability and bright fluorescence. We could further prove the reversible character of our binding methodology by erasing the polymer pattern at elevated temperatures.

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Figure 2. Preparation of patterned polymer brushes via micromolding in capillaries (MIMIC) on surfaces displaying protein repellent properties in pre-determined regions.

Results and Discussion The general concept of the preparation of surfaces patterned with hydrophilic polymer brushes is depicted in Fig. 3. Silicon wafers were coated with a SAM of 11-bromoundeyltrichlorosilane (A). After substitution of the bromine with Cp (B), MIMIC was employed to pattern the surfaces with three different polymers (C). The polymers could be detached at elevated temperatures.

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Figure 3. Synthesis of patterned polymer brushes. Substitution of a Br SAM (A) with sodium cyclopentadienide (NaCp) yields a Cp SAM (B). Subsequent modification with RAFT polymers (C) and detachment of the polymers at elevated temperatures. (D) Monomer units of the applied polymers. Initially, three hydrophilic polymers were synthesized using a well-established, HDA capable phosphorous containing chain transfer agent for the RAFT polymerization: poly(hydroxyethyl acrylate) (PHEA, Mn = 1500 g mol-1), poly(acrylic acid) (PAA, Mn = 4400 g mol-1) and poly(triethyleneglycol methylether acrylate) (PTEGA, Mn = 6000 g mol-1) (Fig. 3D). The detailed synthetic experimental procedures as well as size exclusion chromatographic (SEC) traces and nuclear magnetic resonance (NMR) spectra can be found in the Supporting Information (Fig. SI 1 and Fig. SI 2). The generated polymers can directly be grafted to Cp-SAMs via the thiocarbonyl thio terminus of the RAFT polymer without any further post modification.

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Cp-SAMs on silicon wafers were prepared from bromine terminated surfaces by substitution with Cp in THF at ambient temperature overnight (Fig. 2A and 2B). The successful surface assembly of 11-bromoundecyltrichlorosilane was evidenced by the increase in contact angle from 285 eV (characteristic for oxidized carbons of the polymer) diminish. (B) The signal pattern of the characteristic negative secondary ion CNhas vanished. (C) Rhodamine labeled peanut agglutinin does not adsorb in a specific pattern.

Conclusions A facile method for the preparation of structured surfaces with spatial resolved protein repellant properties is presented. A hetero Diels-Alder (HAD) capable chain transfer agent was used to polymerize hydroxyethyl acrylate, triethylene glycol methyl ether acrylate and acrylic acid in a controlled fashion via the RAFT process. The synthesized polymers vary in size from 1500 to 6000 g mol-1. Micromolding in capillaries with oxidized PDMS stamps was established

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to attach the polymer brushes in a defined pattern on a dienophile functionalized surface (i.e. cyclopentadiene). The polymer brushes could be detached at elevated temperatures due to the reversibility of the HDA reaction. Without the need of any catalyst for the covalent attachment of the polymer chains on the surface, and water as the solvent of choice, the method described here is a suitable tool for biological applications. All generated surfaces were comprehensively analyzed via XPS, ToFSIMS, contact angle measurements and water condensation experiments. The patterned surfaces were exposed to solutions of peanut agglutinin to evidence the protein repellent nature of the patterned polymer brushes.

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ASSOCIATED CONTENT Supporting Information. Experimental details (polymer synthesis and surface functionalization); additional analytical data (polymer and surface analysis). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Benjamin Vonhören and Marcel Langer contributed equally to the current work. ACKNOWLEDGMENT B.V. and B. J. R. thank the IRTG 1143 Münster–Nagoya, funded by the Deutsche Forschungsgemeinschaft (DFG). C.B.-K. acknowledges continued support from the Karlsruhe Institute of Technology (KIT) in the context of the BIFTM and STN programs of the Helmholtz association as well as the BMBF for funding in the context of the BioCoBra project.

ABBREVIATIONS RAFT (reversible addition fragmentation chain transfer); MIMIC (micromolding in capillaries); SAM (self-assembled monolayers); ATRP (atom transfer radical polymerization); NMP (nitroxide mediated polymerization); SEC (size exclusion chromatography); NMR (nuclear

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magnetic resonance); AFM (atomic force microscopy); HDA (hetero Diels-Alder); Cp (cyclopentadiene); XPS (X-ray photoelectron spectroscopy); ToF-SIMS (time-of-flight secondary ion mass spectrometry); PAA (poly(acrylic acid)); PHEA (poly(hydroxyethyl acrylate)); PTEGA (poly(tetraethyleneglycole acrylate)); PNA (peanut agglutinin)

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