STM을 이용한 Cytochrome c/Viologen/GFP로 구성된 박막의 정류특성 분석

유 창준, 박 세정, 남 윤석, 최 정우, 이 원홍

서강대학교 화학공학과

Investigation of Rectifying Property of Cytochrome c/Viologen/GFP Hetero Thin Films with STM

Chang-Jun Yoo, Sei-Jeong Park, Yun Suk Nam, Jeong-Woo Choi

and Won Hong Lee

Department of Chemical Engineering, Sogang University

C.P.O. BOX 1142, Seoul 100-611, Korea

Introduction

Transfer of an electron from one site to another in a molecular or between molecules and/or electrodes is one of the most fundamental and ubiquitous process in chemistry, biology and physics. It has been focus of experimental and theoretical studies for decades and in many relatively simple systems the process is well understood. Control and exploiting of this process in organized molecular systems is a major proposition of molecular and bioelectronics. Progress in molecular electronics device engineering is still rather modest due to problems with elucidation and effective control of the structure and interactions at nanoscopic length scales and also delicacy and instability of complex supramolecular assemblies. In biological systems, the electron transfer processes are principal component of stepwise oxidation-reduction reactions in electron transfer chain. The importance of tunneling mechanism for the biological electron transfer process is due to rather fixed and localized site nature of electron transfer in metalloprotein[1-4].

In this study, the MIM structured device consisting of GFP/Viologen/Cytochrome c hetero-films, which used as an electron sensitizer, a mediator and an electron acceptor, were presented based on the biomimesis. Using GFP, viologen, and cytochrome c, the hetero-type molecular thin films were fabricated and the unidirectional electron flow from GFP to cytochrome c via viologen was confirmed by current-voltage characteristic with STM. The obtained experimental data on STM study of morphology and electron transport features in cytochrome c/viologen/GFP hetero thin films give the evidence for biomolecular photodiode with nano level.

Experimental details

Green fluorescent protein (GFP), N-allyl-N'-[3-propylamido-N",N"-di(n-octadecyl)]-4, 4'-bipyridium dibromide (viologen), and cytochrome c (extracted from horse heart, type VI) were used as an electron sensitizer, a mediator, and an electron acceptor, respectively. Organic mediator, viologen was synthesized, and GFP (rEGFP), cytochrome c was purchased from CLONTECH CO. (Palo Alto, California, USA) and Sigma Chemical Company (St. Louis, USA). Other miscellaneous reagents were purchased from Sigma Chemical Company and used without further purification.

After the deposition of cytochrome c LB film onto the gold substrate carried out using a circular langmuir trough (type 2022, Nima Tech, England), the viologen LB film was deposited onto the cytochrome c LB film by same technique. The Self-Assembly (SA) monolayer of GFP was constructed by dipping the hetero-LB film (cytochrome c/viologen) into the GFP solution. The surface morphology of cytochrome c LB film onto the gold substrate, cytochrome c/viologen hetero-type film and cytochrome c/viologen/GFP hetero-type film were analyzed using STM (AutoProbe CP, PSI, USA). The STM microtopographic images were obtained at a constant-current mode, tunnel current I = 0.4 nA, and a bias boltage V = 0.9 mV. The images were stable and reproducible. Rectifying property of the molecular bio-photodiode with nano scale was observed by current-voltage (I-V) measurement. The methods of hybrid STM/I-V measurement were shown in fig.1.

Figure 1. The Scheme of a current-voltage (I-V) measurement using STM tip

Results and Discussion

Fig. 2 shows STM images of cytochrome c LB films onto the gold substrate(a),  cytochrome c/viologen hetero film(b) and cytochrome c/viologen/GFP hetero film(c). The cytochrome c LB film deposited onto the gold substrate has uniformed surface morphology. The height of a film was about 50□ and the size of protein aggregation was about 0.1~0.2□m. Also, the viologen LB film was deposited  onto cytochrome c LB films. The height and the size cytochrome c/viologen hetero-type film were of  about 80□, 0.1□m, respectively. As shown in Fig. 2(c), the spherical type of aggregations of GFP molecules was observed, which indicated that the GFP molecules were partially adsorbed onto the viologen film surface. It is intimated that the GFP molecules were aggregated on the viologen surface. In Fig. 2(c), the aggregation size and height of GFP molecules on viologen film surface were about 0.2~0.3 □m, 160□.

(a)                      (b)                        (c)

Figure 2. STM Images of a homo-film and hetero-films deposited onto the gold               substrate; (a), Cytochrome-c LB film onto the gold substrate; (b), Viologen           LB film deposited onto cytochrome-c LB film; (c), GFP molecules                 aggregated on the viologen surface.

As shown in Fig. 3, the rectifying characteristic was observed from the measurement of current-voltage characteristics with the nano scale. In photo state, when a +0.1V (forward) bias was applied to the molecular photodiode, the photocurrent was generated with 2.3 pA. Otherwise a -0.1V (reverse) bias was applied to the molecular photodiode, the photocurrent was about 0.0 pA. This property was called rectification of photodiode. From these results, it can be concluded that the photodiode characteristics with nano scale using scheme of the substrate/hetero film/STM tip was verified and the proposed molecular array mimicking photosynthetic reaction center can be usefully applied as a model system for the development of the biomolecular photodiode.

The biomolecular device composed of functional proteins based on the electron transport in the biological photosynthesis process was investigated. The hetero-type films consisted of GFP, viologen and cytochrome c, which were used as a sensitizer, an electron mediator and an electron acceptor, respectively. Based on the topologies by STM measurement, the deposition of cytochrome c LB film and the adsorption of GFP film onto viologen/cytochrome c hetero-films were verified. The photodiode characteristic function of the proposed system with nano scale (substrate/hetro-type film/STM tip) were verified by current-voltage characteristic measurement. The directional flow of photocurrent through their redox potential difference occurred efficiently in the proposed molecular device. The diode characteristic of the proposed

system was verified with nano scale and thus the proposed molecular system  composed of biomolecules could be applied as biomolecular photodiode with nano scale.

Figure 3. The Scheme of a current-voltage (I-V) measurement using STM tip

Acknowledgement

This work was supported by grants from Korea Science and Engineering Foundation (KOSEF : 20006-307-03-2) and the National Program for Tera-level Nanodevices of the Ministry of Science and Technology as one of the 21 century Frontier Programs

References

[1] Chang A.M., Austin R.H., Electron tunneling in cytochrome c, Chem. Phys. 77,      5272 (1982)

[2] Sigel H., Sigel A., eds., Metal Ions in Biological Systems, Dekker, New York,      27 (1991)

[3] Bendall D.S., ed. Protein Electron Transfer, BIOS Publishers, Oxford (1996)

[4] Winkler J.R., Gray H.B., J. Biol. Inorg. Chem. 2, 399 (1997)