Structural stability study of protein monolayers in air

The assessment of the folding and of the structural stability of a protein in air, upon immobilization in the solid state, represents a critical point from both a fundamental point of view and for the development of solid state nanobioelectronics. The recent demonstrations by Rinaldi et al. [R. Rinaldi et al., Adv. Mater. 14, 1453 (2002); Appl. Phys. Lett. 82, 472 (2003); Ann. (N.Y.) Acad. Sci. 1006, 187 (2003)] of protein-based solid state devices and transistors working in air have raised an intriguing question about the behavior of a biomolecule under nonphysiological conditions. The operation principle of the realized devices is based on the physiological electron transfer function of the metalloprotein azurin. This means that azurin should retain its shape and functionality also in the solid state when utilized in air and at room temperature. In this Brief Report, we prove this claim by analyzing the conformational state of the azurin monolayers developed for such devices by means of intrinsic fluorescence spectroscopy. We show that the immobilization of azurins in the solid state under nonliquid conditions, by means of a specific chemisorption process, does not necessarily lead to protein denaturation. This result is of great importance because it opens up interesting perspectives for the development of solid state hybrid nanodevices for electronic applications requiring nonliquid environments.