Hydrophilic Aldehyde-Functional Polymer Brushes: Synthesis, Characterization, and Potential Bioapplications

Abstract

Surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) is used to polymerize a cis-diol-functional methacrylic monomer (herein denoted GEO5MA) from planar silicon wafers. Ellipsometry studies indicated dry brush thicknesses ranging from 40 to 120 nm. The hydrophilic PGEO5MA brush is then selectively oxidized using sodium periodate to produce an aldehyde-functional hydrophilic PAGEO5MA brush. This post-polymerization modification strategy provides access to significantly thicker brushes compared to those obtained by surface-initiated ARGET ATRP of the corresponding aldehyde-functional methacrylic monomer (AGEO5MA). The much slower brush growth achieved in the latter case is attributed to the relatively low aqueous solubility of the AGEO5MA monomer. X-ray photoelectron spectroscopy (XPS) analysis confirmed that precursor PGEO5MA brushes were essentially fully oxidized to the corresponding PAGEO5MA brushes within 30 min of exposure to a dilute aqueous solution of sodium periodate at 22 °C. PAGEO5MA brushes were then functionalized via Schiff base chemistry using an amino acid (histidine), followed by reductive amination with sodium cyanoborohydride. Subsequent XPS analysis indicated that the mean degree of histidine functionalization achieved under optimized conditions was approximately 81%. Moreover, an XPS depth profiling experiment confirmed that the histidine groups were uniformly distributed throughout the brush layer. Surface ζ potential measurements indicated a significant change in the electrophoretic behavior of the zwitterionic histidinefunctionalized brush relative to that of the non-ionic PGEO5MA precursor brush. The former brush exhibited cationic character at low pH and anionic character at high pH, with an isoelectric point being observed at around pH 7. Finally, quartz crystal microbalance studies indicated minimal adsorption of a model globular protein (BSA) on a PGEO5MA brush-coated substrate, whereas strong protein adsorption via Schiff base chemistry occurred on a PAGEO5MA brush-coated substrate.

Adsorption of Aldehyde-Functional Diblock Copolymer Spheres onto Surface-Grafted Polymer Brushes via Dynamic Covalent Chemistry Enables Friction Modification

“…50 Surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP) was then used to graft uniform PGEO5MA brushes of ∼120 nm dry thickness, as determined via ellipsometry (Table S4), from these modified substrates, following a procedure recently reported by our group. 36 While, ultimately, aldehyde functionality is required for diamine functionalization and subsequent nanoparticle adsorption, cis-diol-functional PGEO5MA brushes were first grown herein as they allow for preparation of more densely grafted surfaces compared to brushes synthesized directly from polymerization of the respective AGEO5MA monomer. 36 To produce the analogous aldehyde-functional PAGEO5MA brushes, such PGEO5MA brushes were exposed to an aqueous NaIO 4 solution for 30 min at 22 °C (Scheme 2).…”

“…36 While, ultimately, aldehyde functionality is required for diamine functionalization and subsequent nanoparticle adsorption, cis-diol-functional PGEO5MA brushes were first grown herein as they allow for preparation of more densely grafted surfaces compared to brushes synthesized directly from polymerization of the respective AGEO5MA monomer. 36 To produce the analogous aldehyde-functional PAGEO5MA brushes, such PGEO5MA brushes were exposed to an aqueous NaIO 4 solution for 30 min at 22 °C (Scheme 2). 36 Selective oxidation of the pendent cis-diol groups was confirmed by both ellipsometry and X-ray photoelectron spectroscopy (XPS) studies.…”

“…36 To produce the analogous aldehyde-functional PAGEO5MA brushes, such PGEO5MA brushes were exposed to an aqueous NaIO 4 solution for 30 min at 22 °C (Scheme 2). 36 Selective oxidation of the pendent cis-diol groups was confirmed by both ellipsometry and X-ray photoelectron spectroscopy (XPS) studies. Periodate oxidation should lead to a reduction in the molecular weight of each repeat unit owing to the loss of a formaldehyde molecule.…”

“…33,35 PAGEO5MAbased diblock copolymer worms form soft, free-standing gels that exhibit strong mucoadhesion, 34 while PAGEO5MA brushes grafted from a planar surface can be reacted with model globular proteins, such as bovine serum albumin (BSA). 36 This involves the formation of multiple imine bonds between the aldehyde groups on the brush chains and the primary amine groups located at the surface of the protein.…”

A guide to functionalisation and bioconjugation strategies to surface-initiated polymer brushes

“…For example, aldehyde-functionalised polymer brushes were generated using an aldehyde-terminated pentaethylene glycol methacrylate. 70,111 To simplify the synthesis and polymerisation, these brushes were also generated by polymerising a propylene glycol terminated monomer, prior to selective oxidation of the diol in mild condition (sodium periodate), to form the corresponding aldehyde. Subsequently, these aldehyde functions were applied to the functionalisation of brushes with histidine residues, directly via Schiff base chemistry, resulting in responsive polycationic brushes.…”