Simultaneous cyclic voltammetry and electrochemical quartz-crystal microbalance study at polymer film-modified electrodes of molecular inclusion of ferrocene by β-cyclodextrin polymer and carboxymethylated β-cyclodextrin polymer as well as ferrocenecarboxylic acid by β-cyclodextrin polymer

Abstract

The molecular inclusion equilibria accompanying the ferrocene electrode charge transfer, FC/FC +, in host films of condensation polymers of β-cyclodextrin (β-CDP) and of carboxymethylated cyclodextrin (β-CDPA) were investigated. The inclusion equilibria which accompanied the ferrocenecarboxylic acid electrode processes, FCAH(O)/FCA(I) and FCA(O) −/FCA(I), in the β-CDP film host were also studied. Simultaneous cyclic voltammetry (CV) and electrochemical quartz-crystal microbalance (EQCM) experiments were performed for the relevant inclusion and redox systems by means of the polymer film-modified Au/quartz electrodes. The effects of the electrostatic charge of the guest species, controlled by means of the electrode potential and/or by adjusting the pH of the solution, i.e. the pH and/or redox switching, as well as the effect of counter-ions were investigated for the selective guest ingress and egress. It was found that the FC + electro-oxidation product of FC is partially released from the β-CDP film and is fairly well retained in the β-CDPA one, provided that the fixed cation-exchange carboxylic sites are accessible for ion exchange. Acid-base titration of the CDPA film with the use of EQCM as sensor yielded the acid dissociation constant of the pendant carboxymethyl groups, p K a (β-CDPA) = 4.15. The FCA(I) oxidation product of FCAH is partially released from the β-CDP film at pH > p K a(β-CDP/FCAH), and it precipitates inside the film at pH > p K a(β-CDP/FCAH). The acid dissociation constant of FCAH in the β-CDP film, determined by the EQCM sensor, was found to be p K a(β-CDP/FCAH) = 3.9.