Redox properties of ferricyanide ion on layer-by-layer deposited poly(glutamic acid) film-coated electrodes and its use for electrocatalytic sensing of ascorbic acid

Layer-by-layer (LbL) thin films composed of poly(allylamine hydrochloride) (PAH) and poly(glutamic acid) (PGA) were prepared on the surface of a gold (Au) electrode to study the redox properties of [Fe(CN)6]3− ion on the modified electrodes. The LbL film-coated electrodes exhibited redox response to [Fe(CN)6]3− ion when the outermost surface of the LbL film was covered with PAH while the response was suppressed on the LbL film-coated electrodes whose outermost surface was covered with PGA due to an electrostatic repulsion between [Fe(CN)6]3− ions and the negatively charged PGA layer. The redox properties of [Fe(CN)6]3− ion on the LbL film-coated electrodes significantly depended on the thickness of the LbL film and on pH of the [Fe(CN)6]3− solution. The thicker film-coated electrodes gave higher response than the thinner film-coated electrodes. The higher response was observed in pH 9.0 solution than in pH 7.4 and 4.0 media. The PAH-terminated LbL film-coated electrodes which had been pretreated in the [Fe(CN)6]3− solution at pH 9.0 or 7.4 for 15 min exhibited redox response even in a [Fe(CN)6]3− ion-free buffer solution, suggesting that [Fe(CN)6]3− ions are confined in the films. The [Fe(CN)6]3− ion-confined electrodes can be used for the electrocatalytic determination of a mM range of ascorbic acid in solution.