"Clickable" Polymer Brush Interfaces: Tailoring Monovalent to Multivalent Ligand Display for Protein Immobilization and Sensing

Abstract

Facile and effective functionalization of the interface of polymer-coated surfaces allows one to dictate the interaction of the underlying material with the chemical and biological analytes in its environment. Herein, we outline a modular approach that would enable installing a variety of "clickable" handles onto the surface of polymer brushes, enabling facile conjugation of various ligands to obtain functional interfaces. To this end, hydrophilic anti-biofouling poly(ethylene glycol)-based polymer brushes are fabricated on glass-like silicon oxide surfaces using reversible addition-fragmentation chain transfer (RAFT) polymerization. The dithioester group at the chain-end of the polymer brushes enabled the installation of azide, maleimide, and terminal alkene functional groups, using a post-polymerization radical exchange reaction with appropriately functionalized azo-containing molecules. Thus, modified polymer brushes underwent facile conjugation of alkyne or thiol-containing dyes and ligands using alkyne-azide cycloaddition, Michael addition, and radical thiol-ene conjugation, respectively. Moreover, we demonstrate that the radical exchange approach also enables the installation of multivalent motifs using dendritic azo-containing molecules. Terminal alkene groups containing dendrons amenable to functionalization with thiol-containing molecules using the radical thiol-ene reaction were installed at the interface and subsequently functionalized with mannose ligands to enable sensing of the Concanavalin A lectin.

Scheme 1. Illustration of Diversification of Surface Functionality of Polymer Brushes for Fabrication of Functional Interfaces

Figure 1

(a) Fabrication of DEGMA polymer brushes, (b) FTIR spectra of CTA (black line) and polymer brush-coated surface (red line), (c) AFM cross section of a polymer brush, and (d) XPS analysis plots of a polymer brush-coated surface.

Figure 2

(a) Introduction of “clickable” azide groups on polymer brush surface, (b) FTIR spectra of DEGMA-containing brush (red line) and azide functionalized DEGMA-containing brush (green line), and (c) the high-resolution XPS N 1s spectrum of azide functionalized DEGMA-containing brush.

Figure 3

(a) Fabrication of maleimide-containing polymer brush, (b) FTIR spectra of maleimide-terminated polymer brushes (blue line) compared with parent polymer brush (red line), and (c) the high-resolution XPS scan of N 1s.

Figure 4

(a) Fabrication of alkene-containing polymer brush and (b) FTIR spectra of azobis-G0-ene (mint green line) and alkene (azobis-G0-ene) functionalized DEGMA-containing brush (purple line).

Figure 5

(a) Modification of azide-terminated polymer brushes with BODIPY-alkyne, DBCO-carboxyrhodamine, and DIBO-biotin followed by streptavidin-coated Qdot nanoparticles; fluorescence microscopy images of a micropatterned azide-terminated polymer brush after modification with (b) BODIPY-alkyne, (c) DBCO-carboxyrhodamine, and (d) DIBO-biotin/streptavidin-coated Qdot nanoparticles (scale bar is 100 μm). The insets show a lack of fluorescence in control experiments.

Figure 6

(a) Post-polymerization modification of maleimide-terminated DEGMA polymer brushes with BODIPY-SH as well as Biotin-SH/streptavidin-coated Qdot nanoparticles; fluorescence image after modification with (b) BODIPY-SH, and (c) Biotin-SH/streptavidin-coated Qdot nanoparticles (scale bar is 100 μm). The insets show a lack of fluorescence in control experiments.

Figure 7

(a) Post-polymerization modification of alkene-terminated DEGMA polymer brushes with BODIPY-SH as well as Biotin-SH/streptavidin-coated Qdot nanoparticles, (b) fluorescence image of a micropatterned alkene-terminated DEGMA polymer brush after modification with BODIPY-SH, and (c) fluorescence image of a micropatterned alkene-terminated DEGMA polymer brush after modification with Biotin-SH/streptavidin-coated Qdot nanoparticles (scale bar is 100 μm). The insets show a lack of fluorescence in control experiments.

Scheme 2. Structures of Dendritic Azo-Chain Terminators and Representative Surface Modification Using a G2-Alkene-Based Dendron-Grafted Brush

Figure 8

Treatment of mannose-terminated DEGMA polymer brushes with ConA. Fluorescence image of a micropatterned (A) DEGMA@G0ene-mannose polymer brush, (B) DEGMA@G1diene-mannose polymer brush, (C) DEGMA@G1diene/AIBN-mannose polymer brush, and (D) DEGMA@G2tetraene/AIBN-mannose polymer brush after immobilization of Texas Red conjugated ConA (scale bar is 100 μm). Statistical significance *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; ****P ≤ 0.0001; and ns: P > 0.05.

Figure 9

(a) Surface plasmon resonance (SPR) binding analysis indicating the interaction between mannose-bearing G2 polymer brushes with the two different proteins (FITC-ConA and BSA) and (b) fluorescence microscope image of SPR sensor chip after binding of FITC-ConA. (Scale bar is 100 μm).