Bidirectional optoelectronic switching in protein by photoinduced phase transition

Protein polymers conduct electricity due to the delocalization of π electrons along the conjugated main chains, but their photoconductivity often shows unidirectional tunability. We report a protein polymer with bidirectional optoelectronic conductivity because of reversible light intensity-dependent secondary structural transformation. The protein device shows a positive photocurrent under high light intensity (2.22 to 3.18 μW·μm^-2 and the wavelength of 405-nanometer illumination) illumination and a negative photocurrent under low light intensity (0.32 to 1.91 μW·μm^-2). Under low light intensity light, the helix structures untwist into a low-conductivity β-turn because only partial hydrogen bonds are damaged by insufficient low-energy excitons and Joule heat, whereas high-intensity light drives a transition to a high-conductivity β sheet because most hydrogen bonds are destructed by high-energy excitons and sufficient Joule heat. The tunable low-conductivity state arises from an increase in β-turn content coupled with a decrease in β sheet content, while the inverse transition enables the high-conductivity state. This bidirectional conductivity enables the fibroin-based device to perform in-sensor computing for real-time perception of multiple object motions.