Electron Transfer and Biomolecular Electronics
In the last years our interest was particularly direct to get a deep insight in the electron transfer properties of copper and heme proteins. More recently we are extending this study at single molecule level. The Scanning Tunneling Microscope (STM) enables controlled two-terminal measurements, and its development has thus allowed to probe electron transport through individual redox molecules.
In this respect, the possibility to achieve a stable and oriented immobilisation was instrumental to investigate the electron transfer properties of the copper proteins. Such kind of tethering could be obtained by S-S and SH groups onto gold electrode. The following figure represent the disulphide bridge engineered poplar plastocyanin (PCSS) bound via S-S onto Au(111) surface.
In our investigation of the electron transfer properties of cupredoxins, we are also planning to include proteins which do not contain any metal in their natural structure (e.g., lysozyme), since, in general, this can aid the understanding of the mechanism of tunneling through a biomolecule which is not clarified yet.
The figure below shows STM images, of PCSS, azurin from Pseudomonas aeruginosa and lysozyme adsorbed onto Au surface via their S-S groups.
PCSS Azurin Lysozyme
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90 nm 95 nm 90 nm
Electrochemical STM
The imaging in electrochemical environment is a powerful tool to deepen the understanding of metalloproteins redox behaviour at a metal surface.
Performing STM imaging of PCSS as function of substrate potential (as shown in the figure below) image contrast variations can be revealed. Such response is consistent with an alignment of molecule redox levels with gold Fermi level, strongly supporting the involvement of the copper active site in the tunnelling process.
Substr potential +28 mV vs SCE Substr potential -222 mV vs SCE Substr potential +28 mV vs SCE
120 nm 120 nm 120 nm
Prospective applications
Bioelectronics
Biomolecular nanoelectronics can be suggested as a potential "step forward" in the direction of future nanoelectronics. This approach exploits the functional properties of biomolecules that are naturally engineered to perform a certain activity (e.g. electron transfer) in an effective way.
A biomolecular transistor, made of a single blue copper protein inserted in the gap between two microelectrodes, about 10 nm apart, can be envisaged: varying the value of the gate potential Vg with respect to that of the protein redox center in a dry environment, the azurin-based monomolecular transistor can be switched off (a), or switched on (b), giving rise to the characteristic curve (c) where the different device states are indicated by different colours.
The single protein transistor is expected to operate at room temperature, to show a power dissipation as low as some nW, to be characterized by a capacitance of the order of 10-19F and to have overall performances superior to those of single electron transistors made of semiconductor nanocrystals and carbon nanotubes.
Biosensors
Another fascinating application of bioelectronics is the development of sensing devices, which come up from the integration of biomaterials, such as enzymes and antigen-antibodies, and an electric support. Their working principle, schematically shown in picture below, is performing the electronic transduction of biorecognition events, or biocatalyzed transformation, on the transducers.
Indeed, metalloproteins are able to capture and transfer electrons by recognition of their natural partners: the immobilization on electrodes of such complexes is currently under investigation for the construction of new kinds of biological sensors.
More details about these activities can be found within the SAMBA project report