Electrochemical studies on ion transport in gels with scanning electrochemical microscopy

Ion transport properties of two gels made from polyacrylate and polyacrylam ide (PAAM) were studied with scanning electrochemical microscopy (SECM) by using three ferrocene derivatives, which carry different charges, as the pr obe molecules. Ion diffusion in these gels could be nearly as fast as in aq ueous electrolytes and did not adhere to the Stokes-Einstein equation. Pass ivation of the electrode surface in polyacrylate gels reported previously w as mainly caused by the electrophoretic deposition of gel particles on the surface of the electrode. The deposited gel film can act as a cation-exchan ge membrane and shows permselectivity, which affects both the electrochemic al and SECM behavior. No detectable film formation was found in the PAAM ge l perhaps because of its neutral charge and rigid three-dimensional network structure. The small change in diffusion coefficients as a function of PAA M gel concentration and charges of the probe molecules suggest much of the transport in the gel occurs via a water-filled domain. The transient (chron oamperometric) SECM response could be used to determine diffusion coefficie nts from the critical transient time as a function of tip displacement with out knowing the concentration of the redox mediator, the tip radius, and th e number of electrons transferred in the electrode reaction.