Engineered Bacteriorhodopsin Film with Oriented Patterns for the Improvement of the Photoelectric Response

Abstract

The use of photosensitive proteins has become a competitive solar energy solution, owing to its pollution-free nature, high conversion efficiency, and good biocompatibility. Bacteriorhodopsin (bR) is an important light-sensitive protein that is widely used in the fabrication of photoelectronic devices. However, research on the optimization and comparison of the immobilization techniques is lacking. In this study, in order to obtain bR films with a high energy conversion efficiency, three immobilization techniques, namely dropcasting, electrophoretic sedimentation, and Langmuir-Blodgett deposition, were used to fabricate films, and their topographical and photoelectrical characteristics were compared. All three immobilization techniques can transfer bR molecules to substrates, forming functional photosensitive bR films. The absorption of the bR films at 568 nm reached the highest value of 0.3 under the EPS technique. The peak photocurrent for the EPS technique reached 5.03 nA. In addition, the EPS technique has the highest efficiency factor of 13.46, indicating that it can generate the highest value of photocurrent under the same light conditions, owing to the improved orientation, and no significant decrease in the peak photocurrent was observed after three weeks, which indicates the stability of the photoelectric response. These results indicate that the EPS technique has a great potential for the photoelectrical device fabrication and solar-energy conversion.

Keywords: bacteriorhodopsin; efficiency factor; energy conversion; immobilization technique; photoelectric response.

Figure 1

Topographical characterization of bacteriorhodopsin films immobilized with different techniques. (a) SEM image of the dropcasted film. A clear edge and uniformly distributed bR film indicates a good immobilization quality. (b) SEM image of the electrophoretic sedimented film. A clear edge and uniformly distributed bR film indicates a good immobilization quality. (c) AFM image of the Langmuir–Blodgett deposited film. The bR molecules are uniformly deposited on the substrate with a peak height of 97.3 nm, corresponding to three monolayers of bR.

Figure 2

(a) UV-VIS absorption spectrum of the bR solution. An absorption peak at approximately 568 nm and an A₂₈₀/A₅₆₈ of 2.87 indicate a high purity. (b) UV-VIS absorption spectrum of the bR films immobilized with different immobilization techniques. Dropcasted film and electrophoretic sedimented film show the absorption peak at around 568 nm, verifying the successful transferring of bR molecule; Langmuir–Blodgett deposited film showed a considerably low absorption through the entire range, indicating a small amount of protein transferred.

Figure 3

Photocurrent response of the bR films. (a) Photocurrent response of the bR film immobilized using the DC technique with a peak photocurrent of 2.07 nA. (b) Photocurrent response of the bR film immobilized using the EPS technique with a peak photocurrent of 5.38 nA. (c) Photocurrent response of the bR film immobilized using the LBD technique with a peak photocurrent of 1.61 nA. (d–f) Relationship between the power of the photoflash and peak photocurrent generated by the bacteriorhodopsin films immobilized with different techniques. One-way ANOVA analysis shows that the peak photocurrent increases as the power of the light source increases.

Figure 4

Schematics of the different immobilization techniques. (a) Schematics of dropcasting. Evaporation of the solvent leaves a dry bR film on the ITO glass. (b) Schematics of electrophoretic sedimentation. The bR molecules move toward the ITO glass under the effect of an electric field, simultaneously forming a unified orientation at the same time. (c) Schematics of the Langmuir–Blodgett deposition. The amphiphilicity of the bR molecules leads to an oriented monolayer on the aqueous subphase, which is then transferred onto the ITO glass substrate.

Figure 5

Schematics of the photocurrent measurement apparatus. The bR film initiates its proton pumping function under light stimulation, generating a photocurrent which is then detected by the electrochemical workstation and recorded by the computer.