Abstract
Poly(2-dimethylamino)ethyl methacrylate) (PDMA) brushes are grown from planar substrates via surface atom transfer radical polymerization (ATRP). Quaternization of these brushes is conducted using 1-iodooctadecane in n-hexane, which is a non-solvent for PDMA. Ellipsometry, AFM, and water contact angle measurements show that surface-confined quaternization occurs under these conditions, producing pH-responsive brushes that have a hydrophobic upper surface. Systematic variation of the 1-iodooctadecane concentration and reaction time enables the mean degree of surface quaternization to be optimized. Relatively low degrees of surface quaternization (ca. 10 mol % as judged by XPS) produce brushes that enable the formation of supported lipid bilayers, with the hydrophobic pendent octadecyl groups promoting in situ rupture of lipid vesicles. Control experiments confirm that quaternized PDMA brushes prepared in a good brush solvent (THF) produce non-pH-responsive brushes, presumably because the pendent octadecyl groups form micelle-like physical cross-links throughout the brush layer. Supported lipid bilayers (SLBs) can also be formed on the non-quaternized PDMA precursor brushes, but such structures proved to be unstable to small changes in pH. Thus, surface quaternization of PDMA brushes using 1-iodooctadecane in n-hexane provides the best protocol for the formation of robust SLBs. Fluorescence recovery after photobleaching (FRAP) studies of such SLBs indicate diffusion coefficients (2.8 ± 0.3 μm s–1) and mobile fractions (98 ± 2%) that are comparable to the literature data reported for SLBs prepared directly on planar glass substrates.
Highlights
We explore the post-synthesis modification of atom transfer radical polymerization (ATRP)-synthesized poly[2-(dimethylamino)ethyl methacrylate] (PDMA) brushes grown from a planar surface.[77−79] This is achieved via reaction with
The experimental data and the curve fit yield a diffusion coefficient and mobile fraction of 2.80 ± 0.3 μm[2] s−1 and 98 ± 2%, respectively.[89]. This D value is comparable to those reported by other workers for supported lipid bilayers formed on alternative polymer brushes.[58−66] It is similar to that obtained for the Supported Lipid Bilayer (SLB) formed on the nonquaternized PDMA precursor brush at neutral pH, which suggests that 10 mol % quaternization has little or no deleterious effect on the fluidity of the lipid bilayer.[71,90]
A control experiment in which quaternization is conducted using a good solvent for the brush chains (THF) indicate that the pH-responsive character of the original brush layer is not retained under these conditions
Summary
There has been substantial interest in stimulus-responsive polymers for at least the past two decades.[1−5] Typical stimuli include changes in pH,[6−9] temperature,[10−14] light,[15] and electrolyte concentration,[16−18] and such responsive polymers offer potential applications in many areas, including biomedical sensing,[3,19−21] lubrication,[22] and electronic devices.[23−25] One of the most studied examples is pH-responsive polymers. The resulting surface-quaternized PDMA brushes (denoted “QPDMA”) were rinsed using nhexane, ethanol, and acetone, with further sonication in n-hexane for 10 min followed by drying under a stream of N2 gas. PDMA brush-coated silicon wafers (prepared as described above) were cut into small pieces (∼1 cm2) and immersed in this 1iodooctadecane solution for approximately 18 h at 20 °C. Ellipsometric data were fitted using a single slab model with a refractive index given by a linear effective medium approximation (EMA) between the PDMA or QPDMA brush and water.[48,85] Again, three measurements were recorded for each brush-coated wafer, and data are reported as the mean ± standard error. The Axelrod method was employed to calculate the diffusion coefficient and the mobile fraction of the supported lipid bilayer.[51]
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