Electron transfer, conduction and biorecognition properties of the redox metalloprotein Azurin assembled onto inorganic substrates

The efficient implementation of functional biomolecules into hybrid devices is a central topic in current material science research, and redox metalloproteins often emerged as promising candidates. In particular, the blue copper protein Azurin (AZ) is highly suitable for future integrations in bio-optoelectronic nanodevices and biosensors, since it is characterized by a very fast and efficient intramolecular electron transfer, and it shows a marked structural robustness once adsorbed on surfaces; this allowing the preservation of its physiological functions. In addition, AZ bears an exposed disulfide group, suitable for covalent anchoring on gold, allowing thus a controlled orientation of the protein and a good electrical coupling with the electrode. Moreover, AZ shows an interesting interplay among its redox, optical and vibrational properties, somewhat connected with its biorecognition capabilities towards several substrates. We present here an extensive overview of our results on the functional properties of AZ once adsorbed on surfaces, together with a critical comparison with recently published studies. We mainly focus on its electron transfer, conduction and biorecognition capability, also in connection with external visible light and voltage excitation. We, moreover, outline the development of suitable surface assembling strategies that could ensure both the preservation of the biomolecule physiological characteristics and the establishing of a reliable connection with the electrical/optical read out. A particular emphasis is given to the extensive application of the Scanning Probe Microscopies that could allow a detailed characterization of the hybrid systems at the single-biomolecule level, disclosing aspects otherwise hidden when bulk techniques are being used.