Hybrid STM/I-V시스템을 이용한 국부영역에서의 Cytochrome c /Viologen/GFP로 구성된 박막의 전류-전압 특성 분석

유창준, 박세정, 남윤석, 최정우, 이원홍

서강대학교 화학공학과

Investigation of Local Current-Voltage Characteristics of Cytochrome c /Viologen/GFP Hetero-Thin Film using Hybrid STM/I-V

Measurement System

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

Bioelectronics and molecular electronics are a breakthrough with great potential for generating new concepts and technologies for the development of electronic devices.[1] The concept in the development of new functional bioelectronic devices can be inspired from the biological systems such as an electron transfer chain or photosynthetic reaction center. By mimicking the organization of functional molecules in the biological electron transfer system, an artificial bioelectronic device can be realized. The specific energy and electron transfer takes place on a molecular scale due to the redox potential difference as well as the electron transfer property of the functional molecules, especially the electron acceptor, electron relay, sensitizer, and electron donor.[2,3] Various artificial photodiodes have been fabricated by mimicking the electron transport function of biological photosynthesis.[4,5]

In this study, the MIM(metal/insulator/metal) structured device consisting of GFP, viologen, cytochrome c hetero-thin film was presented based on the biomimesis. GFP, viologen, and cytochrome c was used as an electron sensitizer, a mediator, and an electron acceptor. Current-voltage (I-V) characteristics analysis of cytochrome c/viologen/GFP hetero-thin film was performed by hybrid STM/I-V measurement system.

Experimental details

1. Materials

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

2. Experiments

Cytochrome c was deposited onto gold substrate by LB technique using a circular langmuir trough (type 2022, Nima Tech, England), and the viologen was deposited onto cytochrome c LB film by same technique. The self-assembly (SA) monolayer of GFP was constructed by dipping the hetero LB film (viologen LB film deposited onto cytochrome c LB film) into the GFP solution. The surface morphology of cytochrome c LB film onto the gold substrate and cytchrome c/viologen/GFP hetero-thin film were analyzed using STM (AutoProbe CP, PSI, USA). The STM microtopographic images were obtained at a constant-current mode, set point I=0.4 nA, sample bias V=0.9 mV. Current-voltage measurements were performed by hybrid STM/I-V measurement system. The schematic illustration of the method of hybrid STM/I-V measurement was shown in figure 1.

Figure 1. The schematic illustration of the method of hybrid STM/I-V                   measurement

Results and Discussion

Figure 2 shows STM images of cytochrome c LB films onto the gold substrate. Cytochrome c LB film  deposited onto the gold substrate is uniform. Film height is about 60∼80Å, and protein aggregation size is about 0.2□∼0.1□. As shown in Figure 3(a) and (b), the spherical types of aggregation of GFP molecules was observed. It is intimated that GFP molecules were partially adsorbed onto the viologen/cytochrome c hetero LB film surface. The aggregation size and height of cytochrome c/viologen /GFP hetero-thin film were about 80∼90Å, 0.1□∼0.05□, respectively.

Figure 4(a) shows STM topographic image(scan size:0.3□×0.3□) of cytochrome c/viologen/GFP hetero-thin film onto the gold substrate. STM image of the film surface is reproducible. Surface shape is revealed globular structure, and a globular cluster size is about 0.1□∼0.05□. Figure 4(b) shows height profiles of point a, b, c, d, and e. The heights of point a, b and c on the line 1 are 80Å, 65Å, 60Å, respectively. And The heights of point d and e on the line 2 are 5Å, 1Å. This result might be due that amount of adsorbed GFP molecules at point a, b, and c were much more than point d and e.

(a)                               (b)

Figure 2. STM images of a cytochrome c LB film onto the gold substrate ;            (a), scan size : 1□×1□; (b), scan size : 0.5□×0.5□

(a)                               (b)

Figure 3. STM images of GFP molecules onto viologen/cytochrome c hetero-            LB film onto the gold substrate; (a), scan size : 1□×1□; (b),                scan size 0.5□×0.5□

As shown in Figure 5, at point a, b and c(the aggregation parts), the rectifying characteristics were observed from current-voltage measurements. In photo state, when a +0.1V (forward bias) was applied to point a, b, and c, the photocurrent was generated with 1.45∼1.6pA. Otherwise a -0.1V (reverse bias)  was applied to point a, b, and c, the photocurrent was about -0.2∼-0.3pA. This property was called rectification of photodiodes. Generated amount of photocurrent of point d and e(the hole parts) was totally lower than point a, b, and c.

From these results, it can be concluded that the local photodiode characteristics using scheme of the substrate/hetero thin 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(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 and thus the proposed molecular system composed of biomolecules could be applied as biomolecular photodiode with nano scale.

(a)                                (b)

Figure 4. STM image of GFP/viologen/cytochrome c hetero LB film onto the           gold substrate and height profiles; (a), STM images (scan size : 0.3           □×0.3□); (b), height profiles of the line 1 and line 2

Figure 5. Current-voltage (I-V) curves of point a, b, c, d and e

Acknowledgement

This work was supported by grants from Korea Science and Engineering Foundation (KOSEF : 20006-307-03-2)

References

[1] J. Deisenhofer, O. Epp, K. Miki, R. Huber, and H. Michel, Nature, 318,     618-624 (1985).

[2] G. J. Karvanos, Fundamentals of Photoinduced Electron Transfer (VCH,   New York, 1993).

[3] M. Fujihira, Thin Films Vol. 20, Organic Thin Films and Surfaces:        Direction for The Nineties, 239-277 (Academic Press, New York, 1995).

[4] S. Isoda et al., Thin Solid Films, 210/211, 290 (1992).

[5] M. Sakomura, and M. Fujihira, Thin Solid Films, 243, 616 (1994).