Engineering Nitroxide Functional Surfaces Using Bioinspired Adhesion

Abstract

We pioneer a versatile surface modification strategy based on mussel-inspired oxidative catecholamine polymerization for the design of nitroxide-containing thin polymer films. A 3,4-dihydroxy-l-phenylalanine (l-DOPA) monomer equipped with a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-derived oxidation-labile hydroxylamine functional group is employed as a universal coating agent to generate polymer scaffolds with persistent radical character. Various types of materials including silicon, titanium, ceramic alumina, and inert poly(tetrafluoroethylene) (PTFE) were successfully coated with poly(DOPA-TEMPO) thin films in a one-step dip-coating procedure under aerobic, slightly alkaline (pH 8.5) conditions. Steadily growing polymer films (∼1.1 nm h) were monitored by ellipsometry, and their thicknesses were critically compared with those obtained from atomic force microscopic cross-sectional profiles. The heterogeneous composition of surface-adherent nitroxide scaffolds examined by X-ray photoelectron spectroscopy was correlated to that examined by in-solution polymer analysis via high-resolution electrospray ionization mass spectrometry, revealing oligomeric structures with up to six repeating units, mainly composed of covalently linked dihydroxyindole along the polymer backbone. Critically, the reversible redox-active character of the nitroxide-containing polymer scaffolds was investigated by cyclic voltammetric measurements, revealing a convenient and facile access route to electrochemically active nitroxide polymer coatings with potential application in electronic devices such as organic radical batteries.

Dynamic Nitroxide Functional Materials

“…The dominant peak at 399.8 eV derives from various PDA and poly(DOPA) backbone structures with different environments of the nitrogen atoms, and the amide functional group of incorporated DOPA–TEMPO ( 2 ) building blocks. Detailed investigations of the polymer composition can be found elsewhere . The free radical nitrogen species was identified to be associated with higher binding energies (402.0 eV), yet with only a minor contribution to the overall N 1s signal (Figure c).…”

“…Nitroxide immobilization onto PGMA microparticles was performed in an oxidative copolymerization and in situ polymer surface deposition of dopamine ( 1 ) along with a nitroxide functional dopamine derivative, denoted as DOPA–TEMPO ( 2 ) (Scheme a). The nitroxide‐equipped coating agent 2 composed of 3,4‐dihydroxy‐ l ‐phenylalanine ( l ‐DOPA), amide‐linked to a 4‐amino‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) derivative has been previously introduced by our group for substrate‐independent nitroxide surface decorations …”

“…Detailed investigations of the polymer composition can be found elsewhere. [22][23] The free radical nitrogen speciesw as identified to be associated with higher binding energies( 402.0 eV), yet with only am inor contribution to the overall N1ss ignal (Figure 1c). SubsequentN RC reactions at the interface of MS1 were performed with alkyl halides as coupling agents, which generate in situ transientc arbon-centered alkyl radicals in the presence of copper.…”

“…The nitroxide-equipped coating agent 2 composed of 3,4-dihydroxy-l-phenylalanine (l-DOPA), amide-linked to a4 -amino-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivativeh as been previously introduced by our group for substrate-independentnitroxide surfacedecorations. [22] Employing dopamine (1)a sacomonomer resulted in the formation of more robust adherent polymer films, which is most likelya ttributed to multilateral cross-linking effects of dopamine (1)c ompared to DOPA-TEMPO (2), in which one reactive site is already occupied by the additional amide-linked TEMPO moiety. [22][23] Furthermore, ring-opening reactions between PGMA-incorporated epoxide functional groups andv arious amine species present in the encasing polymer coating also contributed to the overall coating stability.…”

“…[22] Employing dopamine (1)a sacomonomer resulted in the formation of more robust adherent polymer films, which is most likelya ttributed to multilateral cross-linking effects of dopamine (1)c ompared to DOPA-TEMPO (2), in which one reactive site is already occupied by the additional amide-linked TEMPO moiety. [22][23] Furthermore, ring-opening reactions between PGMA-incorporated epoxide functional groups andv arious amine species present in the encasing polymer coating also contributed to the overall coating stability. [24] Bare PGMA microspheres were suspended in an aqueous Tris-HCl buffer system (pH 8.5) containing coating agents 1 and 2 (molarr atio 9:1).…”