Synthesis and characterization of nanoscale biomimetic polymer vesicles and polymer membranes for bioelectronic applications

An amphiphilic ABA triblock copolymer was synthesized using poly(2-ethyl-2-oxazoline) (PEtOz) as hydrophilic block [A] and poly(dimethylsiloxane) (PDMS) as hydrophobic block [B]. The cationic ring-opening polymerization of 2-ethyl-2-oxazoline was initiated by benzyl chloride in the presence of NaI. PEtOz–PDMS–PEtOz was characterized in aqueous solution using transmission electron microscopy (TEM). The block copolymers formed vesicles with a [B] block hydrophobic component thickness of 4 nm, which is thin enough for successful reconstitution of proteins. The mean diameters of the vesicles were measured to be in the range of 150–250 nm, with a narrow distribution. The electrochemical properties of planar PEtOz–PDMS–PEtOz membrane films spread across a Teflon aperture were investigated by electrochemical impedance spectroscopy (EIS). The impedance data showed an increase of planar membrane capacitance (CMEM) from 2.58 × 10−7 to 2.71 × 10−7 F cm−2 and a decrease of membrane resistance (RMEM) from 12 to 10.8 Ω cm2. The increase of CMEM over time in buffer solution can be explained by an increase of the dielectric constant as a result of membrane electrolyte incorporation and/or by the formation and growth of defect in the free-standing films. In contrast to the formation of thin-walled vesicles, the spread triblock copolymer formed a free-standing membrane with a 9 nm thickness. Based on the two possible conformations (bridge midblock conformation and loop midblock conformation) that the triblock copolymer can have, we can conclude that PEtOz–PDMS–PEtOz formed 4 nm thick polymer vesicles with intercalated loop midblock structure in aqueous solution, while 9 nm thick free-standing polymer films with bilayer loop midblock conformation or with bridge midblock conformation were formed by aperture spreading.