Electron Transfer Processes in Ferrocene-Modified Poly(ethylene glycol) Monolayers on Electrodes

Electrochemistry
is a powerful tool to study self-assembled monolayers.
Here, we modified cystamine-functionalized electrodes with different
lengths of linear poly­(ethylene glycol) (PEG) polymers end-functionalized
with a redox-active ferrocene (Fc) group. The electron transport properties
of the Fc probes were studied using cyclic voltammetry. The Fc moiety
attached to the shortest PEG (Mn = 250
Da) behaved as a surface-confined species, and the homogeneous electron
transfer rate constants were determined. The electron transfer of
the ferrocene group on the longer PEGs (Mn = 3.4, 5, and 10 kDa) was shown to be driven by diffusion. For low
surface densities, where the polymer exists in the mushroom conformation,
the diffusion coefficients (D) and rate constants
were increasing with polymer length. In the loose brush conformation,
where the polymers are close enough to interact with each other, the
thickness of the layers (e) was unknown and a parameter D1/2/e was determined. This
parameter showed no dependence on surface density and an increase
with polymer length.