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AFM, STM and Ellipsometry Characterization of a Monolayer of

[align=center][b]AFM, STM and Ellipsometry Characterization of a Monolayer of Azurin Molecules Self-Assembled on a Gold Surface in Air[/b][/align][b][align=center]t,} fGE?cX%^t[
[/b]Anna Rita Bizzarri, Laura Andolfi, Michel Stchakovsky and Salvatore Cannistraro[/align][align=left]zy ?+Q2V4Zk%n
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Introduction1bI*?t;DYv_2N8qmA

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Results and Discussion
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Acknowledgments
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References
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[b]Abstract[/b]
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~mE5X We use several techniques to characterize the monolayer formed in air by the electron transfer protein, azurin, self-assembled on a gold substrate in a stable way.  Tapping Mode Atomic Force Microscopy and Scanning Tunnelling Microscopy scans reveal the presence of individual molecules highly packed and stably bound to gold. The height of the single molecules, as determined by Atomic Force Microscopy is consistent with the thickness of the protein monolayer as measured with spectroscopic ellipsometry. Scanning Tunnelling Microscopy, operating in constant current mode, is able to image single molecules with a lateral size in agreement with the crystallographic dimension. Sweeps of bias voltage in tunnelling conditions point out a good electric contact of the single protein molecules with some rectifying behaviour.
f,o#~_!R,U)Er`e Introduction
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:Z\ iW"E;DbI The assembling of proteins on metal surfaces, mainly noble metals (gold, silver, platinum), is central to new interdisciplinary areas that combine bionanotechnology with physics, chemistry and biology [1,2]. It represents the first step to achieving efficient construction of biosensors and biodevices for advanced medical diagnostics [3]. Electron Transfer (ET) proteins, which are part of chains where the conduction through the biomolecules occurs at the level of the single electron, are suitable candidates for incorporation in hybrid submicrometer-sized electronic components also for their fast and directional ET properties [4,5].9\;OlYD
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To assure an electrical communication with the conducting substrate with minimal perturbation of the macromolecular structure, the proteins may be immobilized on self-assembled monolayers of small organic molecules [5].  However, the presence of such spacers between the electrode and the biomolecules will severely reduce the ET rate.  To overcome such a limit, the protein may directly move to the metal surface by exploiting suitable protein linker groups [6-10]. This provides on one hand, a good electric contact between the molecules and the metal; on the other hand, it keeps the distance between the electrode and the redox centre within the range at which significant ET rates can occur [11].  Furthermore, a particularly relevant aspect in view of the development of protein-based solid state devices, is represented by the organization of protein on the metal surface in air, at which a low water content is to be expected.  The creation of protein monolayers onto a specific substrate is a crucial aspect in the construction of a nanodevice and its functionality strongly depends on the quality of the biomolecular film.8G%HgH?

9S2{8eEH4p In this connection, we are interested in fully characterizing in air a monolayer of the blue copper protein azurin (AZ), directly bound to gold with the main focus on the organization of the molecules at the surface as well as on their electric coupling to the electrode.;q2m8A&l r
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Due to its ET capability and its intrinsic stability, AZ, with peculiar and well characterized spectroscopic properties [12] has gained considerable interest for applicative studies [13-16].  Indeed, it is emerging as a good candidate for the realization of biodevices even working at the single molecule level [17-19]. Notably, by exploiting its native exposed disulfide group, AZ can be covalently anchored in a well-defined way on bare gold electrodes [6,7,17].  Even if AZ on gold has been widely studied by several techniques, a comprehensive investigation of self-assembled monolayers of AZ on gold in air, has not been performed. With such an aim, a number of complementary techniques (Tapping Mode Atomic Force Microscopy (TMAFM), spectroscopic ellipsometry, Scanning Tunnelling Microscopy (STM) and Spectroscopy (STS)) have been exploited. The single molecules are characterized by a homogeneous shape and appear to be stably and robustly anchored to gold forming a regular monolayer. The height of individual molecules with respect to the gold substrate, measured by TMAFM is consistent with the thickness of protein layers measured by ellipsometry. This data indicates that the protein molecules, bound through the SS to gold, assume a conformation partially tilted on the gold surface. Both the STM images and the STS measurements provide indication of good electric contact between the molecules and the electrode substrate.
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