Diffusion regimes at nanoelectrode ensembles in different ionic liquids

The electrochemical and diffusion behaviour of different redox probes in different ionic liquids is studied at gold nanoelectrode ensembles (NEEs) in comparison with millimetre sized gold (Au-macro) and glassy carbon (GC) disk electrodes. The redox probes are neutral ferrocene (Fc), the ferrocenylmethyltrimetylammonium cation (FA+) and the ferrocenylmonocarboxylate anion (FcCOO−). The ILs are the dicyanamide, [N(CN)2] or bis(trifluoromethylsulfonyl)amide), [N(Tf)2] salts of the following cations: 1-butyl-3-methylimidazolium, [BMIm], 1-butyl-3-methylpyrrolidonium, [BMPy], or tris(n-hexyl)tetradecylphosphonium [P14,666]. These ILs are characterized by different viscosities, ranging from 32 to 277 cP. The cyclic voltammetric behaviour of the redox probes is reversible and diffusion controlled at GC electrodes. Diffusion coefficients (D) calculated by the Randles–Sevcik equation scales inversely with the IL viscosity, ranging from 2 × 10−8 to 3 × 10−7 cm2 s−1. Ionic solutes, namely FA+ and FcCOO−, present slightly lower D values than neutral Fc. At the Au-macro the electrochemical behaviour of the redox probes is diffusion controlled in the ILs containing the [N(Tf)2] anion, while it involves relevant adsorption processes in the [N(CN)2] containing electrolyte. For this reason the diffusion at gold NEEs is studied only in the former ILs.The CVs of the redox probes at the NEEs are peak shaped at low scan rate (vv), while they are sigmoidally shaped at high vv, but with some shift between forward and backward patterns. This is indicative of the occurrence of a total overlap (TO) diffusion condition when vv is low which becomes a mixed diffusion layers (MDL) regime, with only a partial overlapping of individual diffusion layers, at high vv values. In the most viscous IL, namely [P14,666] [N(Tf)2], at vv higher than 0.8 V s−1, a plateau current independent on the scan rate is achieved, indicating the tendency to reach the pure radial regime in this IL. The vv values at which the transition between TO and MDL is observed scales directly with D and inversely with the IL viscosity. This behaviour is interpreted on the basis of the dependence of individual diffusion layers at each nanoelectrode on redox probe/IL interaction which fits with existing theoretical models very recently developed for nanoelectrode arrays.