Electron transfer in protein-carbon nanotube hybrid structures

We have developed a method of site-specific attachment of proteins to pristine carbon nanotube (CNT) sidewalls using genetically encoded unnatural amino acids with functional moieties. Here, we incorporated an azido phenylalanine (AzPhe) group at different positions in the protein to assess the importance of different protein-CN configurations. This site-directed mutation of the protein structure has provided routes for direct covalent attachment to the sidewall of the CNT, as well as providing a functional group for modification with another molecule such as pyrene for non-covalent CNT attachment. We have employed a variety of techniques to study these nanoscale systems in order to gain an insight into the binding mechanism, the protein-CNT interaction dynamics and their electronic properties. Using atomic force microscopy (AFM), we found that the proteins bind regular patterns dictated by the position of the phenyl azide. By integrating CNTs into electrical transistor-like devices, we have performed electrical measurements across the CNTs to monitor the attachment of various proteins to the CNT sidewalls. Using these protein-CNT systems, we have also been able to study the interaction between proteins and CNTs using Raman spectroscopy and total internal reflection fluorescence microscopy (TIRFM). Here we present evidence of covalent attachment of proteins to CNTs using Raman spectroscopy to study the changes in vibrational modes present in the CNTs. TIRFM has provided evidence of post cross-linking activity of proteins on CNTs using super-folder green fluorescent protein (sfGFP) as a marker. By analysing the protein's fluorescent properties, we have produced evidence suggesting the importance of the orientation of the protein with respect to the CNT, which in turn determines the distance between the CNT interface and the active site of the protein. The approach developed here provides a versatile and convenient generic approach to interfacing proteins, in defined orientations, to CNTs that holds promise for exploitation in bioelectronics tools and biomolecular sensors