Combining the advantages of high capacity and parallelism of photonics and the versatility of electronics,¹  optoelectronic devices exhibit great potential to impact the development of many applications, such as sensing, computing, and signal communication and processing. Biomacromolecules, especially proteins with specific functions in photosynthesis processes in vivo,²  offer researchers a rich natural source to explore optoelectronic integrated computing circuits.

Bacteriorhodopsin (BR), a chromoprotein, serves as a light-driven proton pump.³  It has been found in the purple membrane of Halobacterium salinarum, where BR is organized into trimers. Each protein is composed of seven transmembrane helices with a retinal chromophore that is covalently bound in the central region. By absorption of visible green light (∼570 nm), a BR molecule undergoes a multistep reaction cycle including seven intermediates with different isomerized states of retinal.⁴  It exhibits high efficiency of light conversion, long-term stability of structure, and desirable photoelectric and photochromic properties. More interestingly, it is capable of forming thin films or gels and maintaining its biological activity on solid supports. Therefore, BR has emerged as an outstanding material for biooptics and bioelectronics and has attracted much interest in recent years.