Electrochemical Characterization of Ferricyanide Retention by Polymerized Diacetylenic Phospholipid Vesicles

This report describes measurement of retention of electroactive ferricyanide (Fe(CN)<INF>6</INF>^3-) entrapped within structurally stable photopolymerized vesicles composed of diacetylenic lipid 1-palmitoyl-2-(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (PC<INF>8,9</INF>PC). Vesicle size, shape, and dispersity were assessed by dynamic laser light scattering and rates of permeation of ferricyanide measured by cyclic voltammetry. Gold disk electrodes modified with 6-mercaptohexanol or 2-mercaptoethanamine (cysteamine) respond with quantitative sensitivity to extravesicular ferricyanide over a concentration range of 10 μM to 0.1 M, are insensitive to entrapped ferricyanide even at high applied oxidative potentials, and resist fouling by vesicles, vesicle fragments, or vesicle-rupturing surfactant. Quantitative changes in ferricyanide peak current over time enabled straightforward determination of ferricyanide permeation rate constants as a function of pH and temperature. At 25 °C, ferricyanide permeability increased from 1.1 × 10^-12 to 2.5 × 10^-12 cm/s with increasing pH from 6 to 8. At pH 7, ferricyanide permeability temperature dependency was found to fit an Arrhenius rate expression, increasing exponentially from 1.6 × 10^-12 to 5.8 × 10^-11 cm/s with increasing temperature from 25 to 70 °C, yielding a calculated energy barrier for permeation of 65 kJ/mol and a half-life for intravesicular ferricyanide loss as high as 2.4 weeks. Greater permeability observed at 15 °C relative to 25 °C is attributed to membrane defects present in the gel-phase. These results are consistent with diffusion of Fe(CN)<INF>6</INF>^3- across intact vesicle walls rather than a pore type mechanism and demonstrate the ability to tune retention of entrapped species by robust polymerized vesicles.