Short communicationSTM study of morphology and electron transport features in cytochrome c and nanocluster molecule monolayers
Introduction
The control of electron current at the level of single electrons is a challenge in nanoelectronics with the potential for the creation of circuits with extremely low dimensions and energy dissipation. Physical limitations for three-terminal nanoelectronic devices are concerned with the wave behavior of an electron at small device dimensions and problems with the determination of the electrochemical potential of tunneling electrons due to Heisenberg's uncertainty principle. Such limitations can be circumvented in a system where a number (at least two) of tunnel junctions were incorporated in the device with the sequential localized discrete character of electron transfer between the electrodes. In biological systems, the electron transfer processes are principal components of the stepwise oxidation–reduction reactions in electron transport chains in the membranes of the mitochondria (metabolic reduction of dioxygen), chloroplasts and bacteria (photoinduced electron transport). The importance of tunneling mechanism for biological redox electron transfer is due to the rather fixed and localized site nature of electron transfer in metalloproteins [1], [2], [3]. Thus, nature gives an example of the realization of multi-junction tunnel electronic system where electron current through a single molecule can be potentially controlled at the level of a single electron at ambient temperature.
Earlier in our group, we have introduced a bioinspired approach to create reproducible stable planar supramolecular structures based on the formation of biomimetic mixed Langmuir–Blodgett (LB) films consisting of inert amphiphile molecular matrix and guest chemically synthesized nanocluster molecules. Such nanostructures can serve as a model for the investigations of basic mechanisms of membrane redox reactions and are perspective for the development of room-temperature controlled single-electron tunneling molecular elements and systems [4]. The double tunnel junction (DTJ) structure “graphite substrate–nanocluster molecule–scanning tunneling microscope (STM) tip” was studied [5], and the effects related to single electron tunneling and discrete electronic level spectrum were observed in such structures at room temperature by STM [6]. Using such an approach, a single-electron tunneling transistor (SET) based on a single nanocluster molecule was demonstrated at room temperature for the first time [6], [7].
STM allows to visualize the molecular nanostructures and study redox processes in the single molecules with high spatial and spectral resolution at ambient conditions [8], [9]. The application of such technique to investigate the electrochemical electron transfer processes in single biological electron carriers such as metalloproteins with nanometer resolution is a new approach in nanobioelectrochemistry. The molecular structure of the samples for the investigations of single redox centers by this technique has to be the monolayer on the conducting substrate. Recently, immobilized protein monolayers were investigated by STM [10], [11], [12]. In the present work, we have used such an approach to study the electron transport features in the organized monomolecular films of cytochrome c—a protein electron carrier in the natural electron transport chains of the mitochondria. Cytochrome c is a well-characterized heme protein with a heme–iron redox center that has two redox levels accessible under physiological conditions, the oxidized (3+) and reduced (2+) levels. Cytochrome c is rather small protein (MW=12 000) with a roughly spherical shape of 3.5 nm diameter. In this report, we present experimental results on the STM characterization of cytochrome c and the nanocluster Pt5(CO)7[P(C6H5)3]4 monolayer LB films on graphite substrates. The obtained experimental data on the STM study of the morphology and electron transport features in cytochrome c and the nanocluster molecular monolayers give evidence for close discrete electron-tunneling effects in such immobilized molecular structures.
Section snippets
Experimental
Cytochrome c (horse heart, MW=12 000) was purchased from Sigma. Nanocluster molecules Pt5(CO)7[P(C6H5)3]4 used in the work were synthesized by Prof. S.P. Gubin in accordance with known procedures [13]. Such clusters have a metal nucleus surrounded by an organic shell which provides stability and atomic reproducibility of the nanocluster structure and tunnel barrier parameters. The last is of principal importance for the development of quantum devices, thus making nanoclusters prospective
Results and discussion
Fig. 1 shows STM topographic images of Pt5(CO)7[P(C6H5)3]4 (picture (a)) and cytochrome c/AOT complex monolayers (pictures (b) and (c)) deposited by the horizontal lifting method onto the surface of the HOPG substrate. The STM images of the monolayers were reproducible and revealed globular structures with characteristic diameters of ∼12 Å for the nanocluster and about 3.5 nm for the protein corresponding to the geometries of these molecules known from the literature. The heights of cytochrome c
Conclusions
Monolayer LB films with cytochrome c and nanocluster molecules were deposited successfully on conducting HOPG substrate and were studied by the STM technique at ambient conditions. The high-resolution single molecule spectroscopic STM study was carried out by recording tunneling current–bias voltage (I–V) curves and non-symmetrical I–V curves with the steps of variable widths and heights that were observed to be dependent on the point of the STM tip position over the protein molecule. The
Acknowledgements
This work was supported by the Russian Foundation for Basic Researches (Grant 99-03-32218) and INTAS (Grant 99-864).
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