Graphene-based amperometric and field-effect biosensing with supramolecular protein complexes

Graphene is one of the most promising materials for future nanoelectronic devices and a prime target for the electronics industry. Graphene can be employed in electronic devices such as atomically thin transistors, electronic waveguides, and is likely to lead to sensitive chemical and biological sensors. In fact, graphene has shown extreme (down to single-molecule) sensitivity to environmental changes. Progress in its synthesis now allows its growth on large areas, thus making it a potentially superior candidate for biosensing interfaces.
Here we are exploring graphene as a surface for enzyme immobilisation and activity detection. Protein immobilisation is central to the development of new bio-assays or sensing platforms as it is directly linked to such issues as protein conformation and subsequently to whether they remain active or not after immobilisation. The 2-oxo acid dehydrogenase protein complex (2-OADHC) is one of the largest enzyme complexes, which is central to energy metabolism. It consists of a dihydrolipoyl acetyl-transferase (E2) protein core, on which 2-oxo acid decarboxylase (E1) and dihydrolipoamide dehydrogenase (E3) proteins bind non-covalently, but specifically. In this work we succeeded in immobilising both the E2 monomer and E2 supramolecular complexes directly onto the graphene surface, upon which the whole 2-OADHC complex can be assembled. The immobilisation of the proteins was studied by atomic force microscopy, while the retention of their activity was tested through their redox electron transfer to the graphene. Binding of the whole complex was detected by amperometric and field-effect sensing.
The immobilisation onto graphene of a large, complex in structure, multi-function protein offers extensive flexibility for tailoring protein-protein interactions and resulting functions. The thermostable E2 protein constitutes a model system for scaffolding of complex, multidomain protein systems, which can be detected using graphene-based biosensors.