Fig. 2

a Photocurrent measurement equipment. (①: The electrodes in the chip were connected with the droplet. ②: The droplet contained E. coli BL21 (DE3). ③: Upon illumination, the rhodopsin pumped proton into periplasmic space, acidifying the buffer in the droplet). b The schematic of biosensor. Under the constant potential (o.1V), electrodes were able to detect the diffused proton pumped by Gloeobacter rhodopsin induced by illumination in the droplet, which consisted of E. coli BL21 (DE3) and unbuffered saline