Fig. 3.
Origin of ion pump phenomenon. a Schematic of surface charge distribution on the nanotube before illumination, in which condition low-density negative charge is homogeneously distributed on the nanotube. b Equivalent circuit of a, the concentration gradient potential (VCG) is the only ionic current source. c Negative charged molecular structure and photo-induced separation of electrons and holes of carbon nitride. d Schematic of surface charge distribution on the nanotube after illumination, in which condition the separation of electrons and holes results in the heterogeneous charge distribution. e Equivalent circuit of d, transmembrane potential (VCNNM) provides a contrary potential with VCG. f Zeta-potential of the CNNM shows the negative charged surface before illumination. g The voltage difference between illuminated and non-illuminated sides shows transmembrane potential. Error bars represent standard deviations of three independent experiments. h The calculated electrical potential profile along the axis of the nanotube when integrating 100-fold concentration gradient based on the two models illustrated in a and d, which are based on the 500-nm-long nanotube with 30-nm-diameter. The potential of low concentration side (CL) is preset as 0 mV