Electrochemical studies on ion transport in gels with scanning electrochemical microscopy

Ion transport properties of two gels made from polyacrylate and polyacrylamide (PAAM) were studied with scanning electrochemical microscopy (SECM) by using three ferrocene derivatives, which carry different charges, as the probe molecules. Ion diffusion in these gels could be nearly as fast as in aqueous electrolytes and did not adhere to the Stokes-Einstein equation. Passivation of the electrode surface in polyacrylate gels reported previously was mainly caused by the electrophoretic deposition of gel particles on the surface of the electrode. The deposited gel film can act as a cation-exchange membrane and shows permselectivity, which affects both the electrochemical and SECM behavior. No detectable film formation was found in the PAAM gel perhaps because of its neutral charge and rigid three-dimensional network structure. The small change in diffusion coefficients as a function of PAAM gel concentration and charges of the probe molecules suggest much of the transport in the gel occurs via a water-filled domain. The transient (chronoamperometric) SECM response could be used to determine diffusion coefficients from the critical transient time as a function of tip displacement without knowing the concentration of the redox mediator, the tip radius, and the number of electrons transferred in the electrode reaction.