Langmuir-Blodgett 기법을 이용한 박테리오로돕신박막의 표면 특성

민준홍, 최현구, 최정우, 이원홍, 김의락*

서강대학교 화학공학과, 계명대학교 화학과*

Surface Characteristics of Bacteriorhodopsin Films Deposited by

Langmuir-Blodgett Technique

Junhong Min, Hyun-Goo Choi, Jeong-Woo Choi, Won Hong Lee, Ui Rak Kim*

Dept. of Chemical Engineering, Sogang University,

Dept. of Chemistry, Keimyung University*

INTRODUCTION

bR makes it possible that the bacteria can perform the anabolism by proton pumping from cytoplasmic surface to extracellular side when light illuminates, and the resulting gradient of proton in the purple membrane provides the electrical response differentially upon illuminated light intensity change [1,2]. The optical device and memories consisting of bR film for information processing have been developed according to its high responsivity and excellent stability in optical and photoelectric property. To construct the electronic devices with bR film, the formation of organized molecular film of bR has been required.

Langmuir-Blodgett (LB) film technique has been applied to the formation of organized molecular film of biological materials for the device [3,4]. The horizontal- and vertical-dipping technique to make bR LB film have been used widely [5,6]. But, based on its photocurrent characteristics and surface morphology measured by atomic force microscopy (AFM), the comparison of two techniques have not been reported.

In this study, the photocurrent of bR films fabricated by by horizontal- or vertical-dipping technique was investigated. The photoelectric response and surface topology measured by atomic force microscopy (AFM) were investigated to compare the dipping techniques of bR LB film.

EXPERIMENTAL DETAILS

1. Materials

bR was purchased from Sigma Co.(St. Louis, USA). ITO glass (surface resistance < 30 W) was obtained from Samsung Display Device Co. (Korea). The topographies of bR LB films were obtained by AFM (Autoprobe CP, Park Scientific Instruments, USA).

2. Preparation of bR LB films

Mixture of H2O/ Hexane/ DMF (volume ratio, 10: 10: 2) was used as the solvent for the spreading of bR. This mixture caused no dissolution of bR into the aqueous subphase [7], allowing direct measurement of pressure-area isotherms for the monolayer. bR was deposited by the vertical- and horizontal- dipping technique using LB trough (Nima Tech, Coventry, UK). bR films by the vertical dipping technique was deposited as z type by the control of dipping speed.

3. Measurement of  photocurrent of bR film

The photocurrent of BR films was measured using a signal analyzer (Keitheley 238, USA), an amplifiers (B&H, DC10000H2O, USA) and an oscilloscope (Lecroy 9400, USA). The 150W Xenon arc lamp (Oriel Co., USA) with an optical filter (500 - 600 nm) was used as a light source.

RESULTS AND DISCUSSIONS

The effect of the subphase pH on the surface-pressure isotherm was shown in Fig. 1. The molecular area was increased as pH of subphase was increased. It was considered that the repulsion between bR molecules caused by the their electrostatic forces due to its isoelectric point (pI: about 5 [8]) was increased along with the increase of pH of subphase. Also it might be due to the effects of subphase pH on the folding state of bR protein. This result can be conformed by the observation of surface topographies of bR LB film fabricated by vertical dipping technique using AFM in Fig. 2. As pH of subphase was increased, the size of bR aggregates did not much change but the coverage of bR onto the substrate were decreased. This result suggested that the surface density of molecule per area was decreased as pH of subphase was increased. Thus the optimal pH of subphase for high surface density of molecules might be around 5, near its isoelectric point, comparing with 6.5 - 7 as optimal pH for high photocurrent generation reported [1] .

The dipping surface pressure of bR monolayer is one of the key operating variables in deposition process by LB technique. Fig. 3 showed the surface topographies of bR LB films at various dipping surface pressures in vertical dipping. In the topographies, the higher parts corresponded to BR aggregate. The size of bR aggregate adsorbed onto the substrate was increased as the dipping surface pressure increased.

The coverage of bR monolayer at 40 mN/m of surface pressure (Fig. 3(d)) was more enlarged than that at 30 mN/m (Fig. 3(c)). Fig. 4 showed the surface topographies of bR LB films at various dipping surface pressures in horizontal dipping. Also the coverage of bR adsorbed onto the substrate was  increased as the dipping surface pressure was increased. The size of bR aggregate was more uniform than that in vertical dipping. It could be suggested that the horizontal dipping technique is more suitable than the vertical dipping technique in the fabrication of bR LB film.

Acknowledgment

This work was supported by grants from Korea Science and Engineering Foundation (KOSEF : 95-0502-02-01-3).

References

[1]  Miyasaka, T., Koyama, K. and Itoh, I.: Science, 225, 342(1992) .

[2] Takei, H., Lewis, A., Chen, Z. and Nebenzahl, I.: Applied Optics, 30, 500(1991) .

[3] Niemi, H. E. -M., Ikonen, M., Levlin, J. M. and Lemmetyinen, H.:  Langmuir, 9, 2436(1993).

[4] Choi, J. W., Bae, J. Y., Min, J., Cho, K. S. and Lee, W. H.: Sensors and Materials, 8, 493(1996).

[5] Furuno, T., Takimoto, K., Kouyama, T., Ikegami, A. and Sasabe, H.: Thin Solid Films, 160, 145(1988).

[6] Ikonen, M., Peltonen, J., Vuorimaa E. and Lemmetyinen, H.: Thin Solid Films, 213, 277(1992).

[7] Miyasaka, T. and Koyama, K.: Thin Solid films, 210/211 146(1992).

[8] Righetti, P. G., Tudor, G. and Ek, D.: Journal of Chromatography, 220, 115(1981).

Figure1. The effect of the pH on □-A isotherm : curve a, pH5.3; curve b, pH8; curve c, pH11.

Figure 2. AFM topographies of bR LB film fabricated by horizontal dipping technique. (a) pH 5.3, (b) pH 8, (c) pH 11.

Figure 3. AFM Topograpies of bR LB film fabricated by Vertical Dipping tecgnique on various Surface Pressure, (a) 10mN/m, (b) 20mN/m, (c) 30mN/m, (d) 40mN/m.

Figure 4. AFM Topograpies of bR LB film fabricated by Horizontal Dipping tecgnique on various Surface Pressure, (a) 10mN/m, (b) 20mN/m, (c) 30mN/m, (d) 40mN/m.