GFP/Cytochrome c 복합박막으로 구성된 생물분자 광 다이오드

최 정우, 박 세정, 유 창준, 이 원홍

서강대학교 화학공학과

Biomolecular Photodiode Consisting of GFP/Cytochrome c Hetero Films

Jeong-Woo Choi, Sei-Jeong Park, Chang-Joon Yoo, and Won Hong Lee

Department of Chemical Engineering, Sogang University

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

Introduction

Electron transfer between molecules is one of the most fundamental process in bioelectronics. Photoinduced electron transport processes in nature such as photoelectric conversion and long-range electron transfer in photosynthetic organisms are known to occur not only very efficiently but also unidirectionally through the functional groups of biomolecules[1]. The Concept or idea in the development of new functional devices can be inspired from the biological systems such as electron transfer chain or photosynthetic reaction center. By mimicking the organization of the functional molecules in the biological system, molecular electronic device can be realized artificially. The molecular thin film fabricated by appropriate techniques can be used as a model system corresponding to the photosynthetic reaction center in the biological system[2]. Progress in molecular electronic devices engineering is still rather modest due to problems of elucidation and effective control of the structure and interactions at nanometer scales, and also delicacy and instability of complex supramolecular assemblies[3].

Molecular photodiode consisting of green fluorescent protein (GFP) and cytochrome c hetero thin films fabricated by self-assembly technique was presented. Proteins (cytochrome c and GFP) were chosen and used in fabricating molecular array as an electron sensitizer and electron acceptor respectively. Protein formation onto the substrate was performed by the self-assembly technique. In order to make molecular assembly onto the gold substrate, cytochrome c was adsorbed onto the 11-mercaptoundecanoic acid (11-MUA) by electrostatic attractive force. GFP can also be adsorbed onto the cytochrome c surface by electrostatic attractive force. The unidirectional electron flow from GFP to cytochrome c was confirmed by current-voltage characteristic. To observe the photo-response of molecular electronic device, the photo-switching function of GFP/cytochrome c hetero-structured device was investigated.

Experimental

1. Materials

Cytochrome c (extracted from horse heart, type VI) and green fluorescent protein (GFP), and were used as an electron sensitizer and an electron acceptor, respectively. GFP (rEGFP) and cytochrome c were purchased from CLONTECH CO. (Palo Alto, California, USA) and Sigma Chemical Company (St. Louis, USA). 11-mercaptoundecanoic acid (11-MUA) was also purchased from Sigma Chemical Co. (St. Louis, USA) and used without further purification.

2. Methods

Electrochemically etched tungsten wire( 0.5mm diameter) was used as STM tip. STM tip etcher was TE-100 system(PSI, USA). The conditions for tip etching are that the current cutoff and voltage are 0.5AC amperes and 80V. STM microtopographic images were obtained by recording the tip height at a constant current mode (Park science Ins.) at an ambience temperature (21C). Tunnel current I=0.5nA, and a bias voltage Vbias =0.9mV. To measure the photoelectric response of the molecular electronic device, MIM structured device was constructed as shown in Fig. 1.

. FIGURE 1. The schematic illustration of I-V measurement using STM tip

Considering the redox potential of each molecule obtained by cyclic voltammetry measurement, the cytochrome c was deposited onto the gold substrate by self-assembly technique and subsequently adsorbing the GFP onto the cytochrome c film surface. The input light of 460nm wavelength was generated by the 500W xenon lamp (Oriel Co., USA) system, and then the photocurrent produced from the molecular electronic device was detected through a current-voltage (I-V) measuring unit (SMU Model 236, Keithley, USA) and personal computer.

Result and Discussion

The effect of photoswitching of the MIM device was shown in Fig. 2.

The photoswitching of the device was comparatively stable. With repeated step illumination, the reproducible photocurrent was generated accordingly. The photocurrents were stable and the level of response was consistent during the repeated cycle over 10sec period. The results indicate that the photoswitching function of the MIM device was achieved. In the proposed molecular device, the photoinduced unidirectional flow of electrons could be achieved due to the redox potential difference as well as electronic coupling between the functional molecules.

FIGURE 2. Photoswitching effect of MIM Device during the repeated cycle

over 10sec period; 0 voltage

To verify the rectifying property in the nano scale, STM tip was used as a top electrode and the rectifying characteristics were observed from the measurement of current-voltage characteristics. The complex features of electron tunneling in gold substrate/cytochrome c/GFP/STM tip system can be determined by the tunnel junction parameters, energy separation in molecular system. The resistance between STM tip and cytochrome c/GFP hetero film is 1M□. Typical I-V curves in different STM tip positions over the GFP molecules and STM images are shown on Fig. 3 and 4. As shown in Fig. 3, curves (green, red, and black) correspond to the position on Fig. 4.   In the STM image, the height of GFP molecule is about 40□ and diameter is 600□. These results indicate that the tunneling current is dependent on the tip position over the molecule and molecule space orientation with respect to the substrate surface and the electron transport features in individual protein molecule via STM.

FIGURE 3. Rectifying property with comparison of rectifying characteristic at three points

In photo state, when a +0.6V(forward) bias was applied to the molecular photodiode, the photocurrent was generated with 0.2nA. On the other hand, when a 0.6V(reverse) bias was applied to the molecular photodiode, the photocurrent was 0.0nA. Also, when a 460nm monochromatic light was irradiated, the photocurrent was generated with the appropriate bias voltage.

(a)               (b)                 (c)

FIGURE 4. GFP molecules on different STM tip position (a:Green, b:Red, c:

Black)

Otherwise, the current was less generated in the dark environment (without light irradiation) even though the bias voltage was increased as shown in Fig. 5.

FIGURE 5. Rectifying property with comparison of photo-current(●) and

dark-current(▼).

From these results, it can be concluded that the photodiode characteristics of the proposed device was verified with STM system. and the proposed method for investigation of rectifying property by the STM system can provide the applicability of molecular electronic device with nano level.

Acknowledgement

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

References

[1] Kuhn H., Hong F. T., 1993. Molecular Electronics-Biosensors and Biocomputers,     plenium Press, New York, 3.

[2] Fujihira M., Organic Thin Films and Surfaces: Direction for The Nineties, 20,        239-277,1995.

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