Romantic Surfaces: A Systematic Overview of Stable, Biospecific, and Antifouling Zwitterionic Surfaces

Abstract

This Feature Article focuses on recent advances in the bioconjugation of surface-bound zwitterionic polymers for biospecific antifouling surfaces. Various approaches for the functionalization of antifouling zwitterionic polymers are systematically investigated, such as chain-end and side-chain functionalization. Side-chain functionalization methods can be further classified as those that are achieved through homopolymerization of custom-synthesized zwitterionic monomers equipped with reactive groups, or those that are achieved via synthesis of random or block copolymers combining different monomers with antifouling functionality and others with reactive groups. Several of the pros and cons of these approaches are outlined and discussed. Finally, some perspective and future directions of research are presented toward long-term stable, generically repelling surfaces that strongly and specifically adhere to a single component in a complex mixture.

Figure 1

Romantic surfaces are stable coatings that generally repel all components in a complex mixture but yet display a strong interaction to a specific compound. [Picture attributions: Top picture: Old couple crossing the street, CC-BY/2.0 - Copyright Ivan Mlinaric, flickr.com/photos/eye1/4517295809; Bottom right picture: Odd one out beside the A30, CC-BY-SA/2.0 - Copyright Maigheach-gheal - geograph.org.uk/p/889321.]

Figure 2

(A) Commonly used zwitterionic polymer brushes featuring as zwitterionic moiety a carboxybetaine (top), sulfobetaine (middle), or phosphoryl choline (bottom) group. (B) Schematic depiction of different strategies for introducing reactive sites: chain-end functionalization (top, left), random side-chain modification (top right), hierarchically structured copolymer brushes (bottom, left), and the development of zwitterionic monomers with intrinsic reactivity (bottom, right).

Figure 3

SI-ATRP of sulfobetaine methacrylate, followed by nucleophilic substitution of the brush’s terminal halide to create an amine-terminated zwitterionic layer. After reaction with a bifunctional NHS-activated ester, anti-Salmonella antibodies could be covalently bound.

Figure 4

(A) Chain-end functionalization of bead-bound zwitterionic polymer brushes opens up novel modes of antifouling testing. (B) Biotin-functionalized zwitterionic polymer beads, obtained via the method outlined in (A), display a generic antifouling—i.e., no binding of fluorescently labeled BSA in a pure protein solution or upon contact with (fluorescently labeled) serum—while strongly and selectively binding to streptavidin in each of these conditions.

Figure 5

Two-step block copolymer brush synthesis, followed by covalent immobilization of protein concanavalin A (A) or covalent antibody binding mediated by SpA (B). (C) Different modes of antibody immobilization onto the block copolymer brush shown in (B).

Figure 6

(A) Structure of the random copolymer and subsequent antibody functionalization reported by Nishizawa et al. (B) Schematic representation of the antibody functionalization of polymer nanoparticles using the physisorbed polymer depicted in (A), yielding nanoparticles that can be used for affinity-based separation methods.

Figure 7

(A) Synthesis of random copolymer of SBMAA and azido-functional oligo(ethylene glycol) acrylamide with terminal block of 4-vinylimidazole. (B) Tagging this polymer to fluorescent quantum dots and functionalizing it with integrin-targeting arginylglycylaspartic acid (RGD) peptides enables selective targeting of integrin-presenting cell lines in vitro and in vivo as observed via fluorescence imaging.

Figure 8

Direct biofunctionalization of carboxybetaines by NHS/EDC activation of carboxyl groups.

Figure 9

Hierarchical two-layer structures based on carboxybetaines with a dense low-fouling base layer and a low-density polymer layer with high loading capacity for immobilization of antibodies using NHS/EDC chemistry, as shown in Figure 8.

Figure 10

(A) Random surface-initiated copolymerization of an azide-containing SBMA monomer and a nonfunctional SBMA monomer (q = 0, 1, or 5%), followed by a SPAAC reaction of the azides to immobilize biotin inside the brush. (B) Protein adsorption and binding onto bare and polymer-brush-coated (as described in A) surfaces as measured by reflectometry: Top, fibrinogen adsorption; middle, avidin adsorption; bottom, bar plot summarizing fibrinogen adsorption, the avidin binding of pure avidin, and a mixed protein solution of avidin and fibrinogen.

Figure 11

Phosphorylcholine-based zwitterionic monomer with an alkyne “click handle”.