Reversibly Cross-Linking Polymer Brushes Using Interchain Disulfide Bonds

The introduction of interchain cross-links in surface-grafted polymer brushes increases the robustness and mechanical properties of these thin polymer films. In most cases, cross-linked polymer brushes contain permanent interchain cross-links. The use of reversible interchain cross-links, in contrast, provides opportunities to dynamically modulate the cross-link density and properties of surface-grafted polymer brushes. This study explores the use of disulfide bonds to reversibly cross-link poly(2-(dimethylamino)ethyl methacrylate) copolymer brushes. These brushes were prepared via surface-initiated atom transfer radical copolymerization of 2-(dimethylamino)ethyl methacrylate and an azide-containing comonomer, 3-azido-2-hydroxypropyl methacrylate, followed by copper(I)-catalyzed azide-alkyne cycloaddition of S-propargyl thioacetate and subsequent deprotection of the thioacetate moieties. Heating the polymer brushes to 60 degrees C under air for 2 h resulted in the formation of disulfide cross-links, which could be reduced to generate the corresponding free thiol groups upon brief exposure to tris(2-carboxyethyl)phosphine hydrochloride. The formation and cleavage of the interchain disulfide cross-links has a profound influence on the swelling and viscoelastic properties of the brushes. Cross-linking leads to a decrease in swelling ratio and a concomitant dehydration and loss in dissipative properties of the brush film. These changes were observed using ellipsometry and quartz crystal microbalance with dissipation monitoring experiments by exposing the polymer brushes to a sequence of successive cross-linking and uncross-linking steps. These experiments indicated that while cross-linking and uncross-linking were fully reversible during the first few cycles, the response of the brush films became less pronounced upon prolonged oxidation/reduction, which was attributed to the oxidation of thiol side-chain functional groups and a concomitant reduction in the cross-link density of the polymer brushes. The results presented in this study show that the incorporation of disulfide interchain cross-links allows access to polymer brush films that can be reversibly cross-linked and uncross-linked over many cycles.

Controlling topology, dynamics and function of polymer brushes via post-polymerization modification

Piotr Mocny

Polymer brushes are a class of thin coatings, where each of chains is tethered to the underlying substrate via one chain end. Densely grafted polymer brushes feature a stretched chain conformation, which in a unique way enhances their lubrication and non-biofouling properties. Polymer brushes also present a high density of functional groups, whose exposure in solvated brushes is useful for catalysis or in medical applications, including diagnostics. Advances in surface-initiated controlled radical polymerizations (SI-CRP) have facilitated the synthesis of high grafting density polymer brushes with a great control over film thickness / polymer molecular weight and composition. These techniques enable access to plethora of polymer architectures. Due to the fact that SI-CRP strategies allow the use of a wide range of monomers, multiple routes for the post-polymerization modification of polymer brushes are possible. Combination of SI-CRP with post-polymerization modification provides possibilities to systematically study architecture- and composition-property relationships of polymer brushes. This Thesis aims to manipulate the topology and crosslinking dynamics of polymer brushes, whose effect on properties of the coatings will help to understand these relationships. Additionally, functions of polymer brushes can be extended by their modification. As an example, incorporation of nanoparticles is a way to provide catalytic functions. Swelling of the nanoparticle â polymer brush assemblies can expose the catalytically active sites and potentially maximize its activities. Chapter 1 provides an overview of manipulation strategies over the composition and topologies of polymer brushes, routes toward their crosslinking and methods for assembly of nanoparticles within polymer brush matrices. Chapter 2 demonstrates a two-step post-polymerization synthetic approach to generate loop polymer brushes with potentially improved non-biofouling and lubrication properties. In the first step, olefin groups are installed at the polymer chain-ends and then metathesis is used to induce loop closure. Chapter 3 presents a synthetic strategy to install different crosslinks onto polymer brush platforms. It comprises a two step-process: surface-initiated atom transfer polymerization (SI-ATRP) to generate a copolymer platform bearing azide groups, and copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) using propargyl-modified thiol precursors. Multiple crosslinking and decrosslinking cycles are studied under oxidative and reductive conditions. Chapter 4 explores the catalytic properties of 3-dimensional (3D) assemblies of amorphous molybdenum sulfide in polymer brushes as a template. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes are grown from highly-oriented pyrolytic graphite (HOPG) and used to bind anionic tetrathiomolybdate through an anion exchange reaction. In a final oxidation step, the polymer-bound tetrathiomolybdate is converted into the amorphous MoSx catalyst. The performance of polymer brush-catalyst system during hydrogen evolution reaction (HER) is studied.

EPFL2019

Tuning the Properties of Hydrogel Microspheres by Adding Chemical Cross-linking Functionality to Sodium Alginate

Françoise Borcard, Léo Bühler, Virginia Crivelli, Sandrine Gerber, Redouan Mahou, François Rémi Pierre Noverraz, Solène Marcelle Françoise Marie Passemard, Christine Wandrey

Two novel types of hydrogel microspheres (MS) are presented. First, one-component microspheres (1-comMS) were produced from sodium alginate (Na-alg) equipped with thiolfunctionalized hydroxyl groups. The functionalization pathway included the conversion of Na-alg into tetrabutylammonium alginate, insertion of new carboxyl groups, grafting of α-amine-ω- thiol poly(ethylene glycol), and restoration of the sodium salt. This modification conserves all original carboxyl groups of Na-alg and allows for covalent disulfide bond formation in addition to ionic cross-linking. Second, two-component microspheres (2-comMS) were obtained from a mixture of Na-alg and Na-alg functionalized with cysteamine. This functionalization was achieved by grafting cystamine dihydrochloride on some carboxyl groups followed by the reduction to cysteamine. Using the one-step MS formation process developed for both MS types, very fast ionic gelation with calcium ions conserves the spherical shape of the polymer solution droplets upon extrusion into the gelation bath, while simultaneously occurring slow covalent cross-linking reinforces the hydrogels. The physical properties of both MS types are adjustable by varying the polymer concentration, the degree of grafting, and the mixing ratio. In vitro cell microencapsulation studies confirmed the cytocompatibility of 1-comMS and 2-comMS.

American Chemical Society2015

Polymer Brushes as Substrates for Microarray Applications

Raphaël Barbey

The development of innovative bioanalytical technologies that enable the discovery of new therapeutics or the creation of systems for rapid, accurate and parallel analyses has gained an extraordinary interest in the biomedical sector in the last 10-15 years. Since proteins are the key-targets for such essential discoveries and developments, a particular emphasis has been placed on proteomics (study of proteins), including the investigation of protein structures, changes of protein expression upon treatment of cellular systems, the role of post-translational protein modifications on cell signaling events and the validation of selected proteins as disease-specific markers or pharmaceutical targets. These studies usually aim at using crude biological samples as the starting materials, which typically contain a large variety of proteins at concentrations covering a wide dynamic range and where the low abundant proteins are often the biologically most relevant. Therefore, the challenge is to create analytical tools which are able to cope with this large protein concentration range and which provide sufficiently high sensitivity to measure low abundant analytes in the presence of a high abundant protein matrix. The information that is expected from such technologies may soon exert a drastic change on the pace of medical research and considerably impact on the care of patients. As protein microarrays offer a powerful tool for the rapid, parallel analysis of complex samples and only require a minimal amount of reagents, they are currently one of the hot topics of life sciences, which strive to provide such analytical solutions. One of the current limiting factors in the field of protein microarrays for achieving the required analytical sensitivity is the amount of proteins that can be immobilized on the proteomic chip surface. Polymer brushes, which consist of an arrangement of polymer chains that are attached by one end to a substrate in a stretched, well-defined chain conformation, can increase the surface area available for protein binding, and thus act as a new category of 3D protein microarray substrates. With the advent of surface-initiated controlled polymerization techniques, extremely precise control over the thickness, composition, architecture and grafting density can be achieved, which allow to finely tune the properties of the polymer brush. In addition, these polymer chains may contain, natively or after specific post-polymerization reaction of their side chains, a high density of functional groups that can react with proteins. The different approaches available for the preparation of such surface-tethered polymer chains, the strategies allowing control over their architecture and properties as well as examples of applications related to protein binding are presented in Chapter 1. This Thesis describes the use of polymer brushes to create a new type of 3D proteomic chips, with the ultimate goal to improve the protein binding capacity compared to existing chip surface, without altering the accessibility of the detection reagents or increasing the intrinsic fluorescence of the background. The objective of this research work is to prepare polymer brush-based substrates that can outperform the sensitivity (i.e. increase the signal-to-noise ratio) that is obtained with a commercial microarray system, which is used as a benchmark in this Thesis. In this regard, functional polymer brushes containing epoxide groups that can react with the nucleophilic moieties of proteins have been chosen as promising candidates. In a preliminary set of experiments, the potential of glycidyl methacrylate-containing brushes to bind (bio)molecules via a post-polymerization modification reaction under mild conditions, i.e. at room temperature in aqueous medium, has been studied (Chapter 2). To this end, poly(glycidyl methacrylate) brushes, as well as several poly(glycidyl methacrylate)-co-poly(2-(diethylamino)ethyl methacrylate) copolymer brushes of different composition have been prepared by surface-initiated atom transfer radical (co)polymerization. The kinetics of the post-polymerization modification reaction of these (co)polymer brushes were followed using various model primary amines, as well as proteins. The introduction of tertiary amine groups, incorporated in the polymer brushes such as 2-(diethylamino)ethyl methacrylate units, was demonstrated not only to enhance the rate of the epoxide ring-opening reaction but also to induce higher protein binding capacities as compared to poly(glycidyl methacrylate) homopolymer brushes. The improved loading capacities of the (co)polymer brushes were evaluated in Chapter 3 under conditions that more closely resemble real microarray applications, such as the use of automated nanospotters to deposit proteins and fluorescence readers to detect protein immobilization. In this regard, commercially-available proteomic chips were coated with the epoxide-containing (co)polymer brushes and the spotting conditions as well as the proteins were varied. These studies revealed that the loading capacity depends on the pH and on the investigated protein, but also that the (co)polymer brush-coated substrates show far higher loading capacities than the standard, unmodified proteomic chips. This investigation also ascertained the supremacy of brushes containing 2-(diethylamino)ethyl methacrylate units compared to homopolymer brushes, as well as the superiority of thick (co)polymer brushes in comparison to their thinner equivalents. These (co)polymer brushes were subsequently used as platforms for real, although simplified, protein assays. The modified proteomic chips were spotted with unlabeled proteins that were further recognized with detection antibodies via either a one-step or a two-step approach. The clear benefits observed for copolymer brushes as compared to the existing technology under these realistic assay conditions nicely support the fact that the interior volume of the brush remains accessible for the detection reagents. In conclusion, the combination of the high loading capacity and the remaining accessibility for the detection partners make these (co)polymer brushes, and particularly the poly(glycidyl methacrylate)-co-poly(2-(diethylamino)ethyl methacrylate) brushes, excellent candidates for the improvement of the sensitivity in protein microarray applications.