Redox Reactions without Direct Contact of the Reactants. Electron and Ion Coupled Transport through Polyaniline Membrane

Abstract

It is demonstrated that Fe3+ in one solution can be reduced to Fe2+ by ascorbic acid in another solution when both aqueous solutions are separated by polyaniline membrane. This transmembrane redox process is possible due to electron/anion coupled counter transport through polyaniline membrane. It was demonstrated that at least one of the solutions must have acidic pH to initiate the transmembrane redox reaction. Both redox processes on the solution/membrane interfaces and the electron/ion coupled transport through the membrane play important role in determining the rate of transmembrane reaction. Possible kinetic mechanism is proposed. Apparent "diffusion coefficients" for redox equivalents inside polyaniline membrane and the rate constants of redox reactions on both solution/membrane interfaces are estimated. Maximal transmembrane reaction rate is 2 x 10(-9) mol/(s cm2) in terms of transport of redox equivalents through the membrane and formation of Fe2+. This value is much higher than the typical values of the rates of respiration in mitochondria expressed in the same units. For thin membranes, the rates of transmembrane redox reactions are determined by interface processes and characteristic times are comparable to those in biomembranes.

Polyaniline membrane based potentiometric sensor for ascorbic acid, other redox active species and chloride

“…5 shows typical kinetics of redox potentials changes in both solutions due to the transmembrane redox reaction across the PANI-CSA membrane, where FeCl 3 solution was used as the oxidizing agent, and FeCl 2 solution as the reducing agent. We have shown earlier that the rates of transmembrane redox reactions through HCl doped PANI are limited by Cl À diffusion though the polymer, which is coupled with electron transport and directed from an oxidant to reducing agent [26]. Probably the same mechanism is important for PANI membranes doped by CSA and active at neutral pH.…”

“…Previously we suggested an explanation for this thermodynamic paradox, based on the idea that interface redox reactions can be driven by the difference of electrochemical potentials for chloride anions between the aqueous solution and the polymer film [19]. Transport of transmembrane redox equivalents is possible due to the coupled counter transport of anions, which maintain the electroneutrality of both the solutions and the membrane [26].…”

“…Evidently at high Cl À concentration the membrane loses its sensitivity to oxidant. Experimental membrane potential is total of two Donnan potentials on both membrane surfaces and the diffusion potential inside the membrane, which is formed because of the difference between Cl À mobility and electron mobility [26,27]. Measured with electrochemical methods, polaron mobility in polyaniline is $0.1 cm 2 s À1 V À1 , which corresponds to the diffusion coefficient of 0.0025 cm 2 s À1 [29].…”

“…4 demonstrates good linear response of transmembrane potential vs. the logarithm of ascorbic acid concentration in all cases. The slopes at different pH were close to each other with a value around 59/2 mV, corresponding to the two-electron oxidation of ascorbic acid by PANI membrane [26]. The lower detection limit was as low as 0.05 mM, which is better than those for inorganic ions Fe 3+ , Fe 2+ , ferri-and ferrocianide.…”

Laser induced forward transfer of conducting polymers

“…Polyaniline is one of the most commonly used conducting polymers due to its high conductivity, excellent environmental stability, and ease of preparation. In the field of biotechnology, polyaniline is employed in biological sensors [43], biomembranes [44], and as a substrate for mammalian cell growth [45]. The practical use of polyaniline is hindered by its mechanical properties and its poor solubility in most common organic solvents.…”