Electron Transfer in Methylene-Blue-Labeled G3 Dendrimers Tethered to Gold

Redox-modified branched 3D dendrimeric nanostructures are considered a proper tool for the wiring of redox enzymes because they provide both an enzyme-friendly environment and exquisite electron transfer (ET) mediation. ET rates in G3 poly- (amido)amine (PAMAM) dendrimers, covalently attached to gold electrodes and labeled with methylene blue (MB), approached 267s(-1) and decreased as the packing density of dendrimers on the electrode surface was increased. A mechanistic analysis of the ET kinetics and fitting to the Marcus relationship showed that as the PAMAM surface coverage was increased, the ET mechanism switched from surface-confined ET (electron tunneling) in dilute monolayers to diffusional ET (electron hopping) at higher surface populations of dendrimers. Structural changes in the positively charged dendrimers electrostatically compressed at negative charges of the electrode surface, and their dependence on the dendrimer surface packing, contribute to both mechanistic pathways. Electrical wiring of horseradish peroxidase and hexose oxidase by using MB-labeled dendrimers allowed the bioelectrocatalytic reduction of H2O2 and oxidation of glucose by these enzymes. The demonstrated electrical communication between MB groups, localized on the periphery of dendrimers and distanced 2 to 3nm from the electrode surface, and the electrodes opens new routes to prospective biosensor and bioelectronic applications.