Gold표면에 형성시킨 자기조립박막 위에 흡착된

bacteriorhodopsin의 광전류특성

최 현구, 한 건국, 민 준홍, 최 정우, 이 원홍

서강대학교 화학공학과

Photocurrent Characteristics of Bacteriorhodopsin Adsorbed onto

the Modified Self-Assembled Monolayers on Gold Surface

Hyun-Goo Choi, Keon-Kook Han, Junhong Min, Jeong-Woo Choi, Won Hong Lee

Department of Chemical Engineering, Sogang University

Introduction

In the past few years, self-assembly (SA) technique has been emerged as one of the most challenging fields in the fabrication of highly-ordered molecular thin films [1]. The well ordered monolayer films, known as self-assembled monolayers (SAMs), have been widely studied focused on the monolayer formation and the basic properties of the adsorbed monolayers. A great many researchers have intended to reveal the interfacial phenomena at organic-inorganic interfaces and form the inorganic thin films at the surface of the self-assembled monolayers used as an active site for the surface nucleation of inorganic materials [2]. However, practical applications of SAMs to bioelectronics have been rarely reported.

Bacteriorhodopsin (bR), a light-driven proton pumping protein embedded in the purple membrane of Halobacterium salinarium (previously known as Halobacterium halobium), is one of the widely studied proteins due to its long-term stability and availability in many fields of applications. Photovoltage and concomitant photocurrent, which is normally very small, are detected as a result of charge displacement in the bR molecules initiated by the rapid photoisomerization of the chromophore retinal known as Schiff base [3]. Practical applications of bR have been demonstrated with the artificial photoreceptor, optical memory devices, and image processing systems, etc. [3-5].

In the present study, self-assembled monolayers were used as a template for the formation of bacteriorhodopsin thin films. The fabricated thin films of bR were used as the key component in the proposed artificial photoreceptor and the photocurrent characteristics of the device were investigated with the light illumination.

Materials and Methods

Materials

Bacteriorhodopsin (bR) extracted from Halobacterium salinarium was purchased from Sigma Chemical Co. (St. Louis, USA). 3-mercapto propionic acid, ethanol, poly-L-lysine, HEPES (N-[2-hydroxyethyl]piperazin-N'-[2-ethane sulfonic acid]), and other miscellaneous reagents were purchased from Sigma Chemical Co. (St. Louis, USA) and used without further purification. Gold plates composed of standard slide glass (1"×3"×0.04") deposited with 50 Å of Titanium and 1000 Å of gold were purchased from EMF Corp. (Ithaca, USA).

bR Adsorption onto Self-Assembled Monolayers

Gold plates were cut in the size of 1.5 cm × 2.5 cm to be used as a substrate for the SAMs formation, and cleaned in the ethanol by sonication for 5 min (3 times). After completely drying the gold plate, it was immersed in the 5 mM  ethanolic solution of 3-mercapto propionic acid for 18 hrs, and then rinsed with absolute ethanol and D.I. water before drying. 3-mercapto propionic acid-adsorbed gold plate was then immersed in the 1 mM poly-L-lysine solution, prepared by using sodium phosphate buffer (pH, 8), for 1 hr and then dried. The substrate composed of Au/SAMs/poly-L-lysine was treated with 18.5 □M of bR solution, prepared by using HEPES buffer (pH, 8), using microsyringe. After 30 min, the substrate was rinsed with the buffer solution and finally with D.I. water to remove any salt remaining in the bR film surface, and then dried in the petri dish. The bR-adsorbed SAMs were then used as a crucial component in fabricating an artificial photoreceptor.

Figure 1. A schematic diagram of the experimental setup for the photocurrent measurement. 1. Xenon lamp system; 2. Optical filter; 3. Shield box; 4. Electrochemistry cell; 5. Glass; 6. ITO layer; 7. Glass/Au/SAMs/poly-L-lysine/bR; 8. Low-noise current preamplifier; 9. Amplifier; 10. Oscilloscope

Photocurrent Measurement

bR-adsorbed SAMs deposited onto gold plates were set in an electrochemistry cell equipped with ITO glass as a counter electrode. 0.1 M KCl solution was used as a supporting electrolyte and the pH of electrolyte was adjusted to 7 with KOH solution. The artificial photoreceptor consisting of bR-adsorbed SAMs set in an electrochemistry cell was serially connected to a low-noise current preamplifier (Stanford Research System, SR570, USA), an amplifier (B&H Electronics, DC10000H2O, USA), and an oscilloscope (Gould, UK) as shown in figure 1. The light source was irradiated onto the bR-adsorbed SAMs through ITO glass, the counter electrode, by the Xenon lamp system (Oriel Co., USA) with an optical filter, and the resulting photoelectric responses were obtained as a photocurrent.

Results and Discussion

Adsorption of bR molecules onto Modified Self-Assembled Monolayers

Self-assembled monolayers (SAMs) of 3-mercapto propionic acid are spontaneously formed onto gold surface by the covalent bonding between the thiol group of 3-mercapto propionic acid and gold. Since the surface end group of SAMs of 3-mercapto propionic acid is carboxylic group, a negatively charged group, poly-L-lysine, which has amine group as the positively charged unique R group, can be adsorbed onto the surface of SAMs of 3-mercapto propionic acid on gold through electrostatic interaction and/or intermolecular hydrogen bonding. Furthermore, since the intermolecular hydrogen bonding can be relatively readily formed between the surface carboxylic groups of 3-mercapto propionic acid, it can make the monolayer stability much improved. The modified surface of SAMs can be, therefore, used as a template for the bR adsorption. A schematic illustration of the bR adsorbed-SAMs on gold is shown in figure 2.

Figure 2. A schematic illustration of bR-adsorbed self-assembled monolayers on gold

via poly-L-lysine as a bridging molecules.

Photocurrent Measurement of the Proposed Artificial Photoreceptor

A typical photoelectric response of bR obtained from the proposed photoreceptor is shown in figure 3. When the 550 nm light was illuminated on the photoreceptor, differential photoelectric response was generated as a photocurrent, and when the light was turned off, the reverse response was generated. As the concentration of bR solution increased, the magnitude of the photocurrent generation increased. It indicates that the much more bR molecules could be adsorbed onto the modified SAMs as the concentration of bR solution increased. Based on these results, it is concluded that the proposed bioelectronic device using bR-adsorbed SAMs on gold can be used as an artificial photoreceptor and the optimization for the fabrication of more efficient photoreceptor should be performed.

Figure 3. A photoelectric response of the proposed photoreceptor consisting of bR-

adsorbed self-assembled monolayers on gold.

References

[1] Ulman, A., An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly. Academic Press Inc., New York, pp 339-413, (1991).

[2] Mann, S., Archibald, D. D., Didymus, J. M., Douglas, T., Heywood, B. R., Meldrum, F. G., and Reeves, N. J., Crystallization at inorganic-organic interfaces : Biominerals and biomimetic synthesis. Science, 261, 1286-1292, (1993).

[3] Birge, R. R., Photophysics and molecular electronic applications of the rhodopsin. Annu. Rev. Phys. Chem., 41, 683-733, (1990).

[4] Choi, H. G., Min, J., Choi, J. W., and Lee, W. H., Molecular photoreceptor consisted of bacteriorhodopsin/flavin complex Langmuir-Blodgett films. Biosensors & Bioelectronics, in press, (1998).

[5] Miyasaka, T., Koyama, K., and Itoh, I., Quantum conversion and image detection by a bacteriorhodopsin-based artificial photoreceptor. Science, 255, 342-344, (1992)