Fig. 2.

(A) DIC and (B) fluorescence (GFP-GCaMP6s, green) images of neurons, at day 8 in culture, on top of a graphene transistor. Zoom-in (C) DIC, (D) fluorescence, and (E) photocurrent images of the white square regions in Fig. 2A and 2B. Three synapses/spines are marked by blue, red, and green circles, respectively. (F) Schematic of band structures of graphene. The black dotted line denotes the Fermi level and the solid line shows the graphene band diagram. A local electrochemical potential change induced by a synapse/spine results in the local carrier concentration changes of graphene, leading to the graphene energy band bending and subsequent photocurrent generation. Fluorescence intensity changes when the neurons were exposed to 4 mM K⁺, 60 mM K⁺, 4 mM K⁺, and 60 mM K⁺, respectively. Red triangles and green spheres represent the fluorescence intensities of synapses/spines at spots two (G) and three (H) in Fig. 2D. Photocurrent responses of three graphene-synapse junctions upon three high-K⁺ stimulation cycles (4–60–4–60–4–60–4). (I) Blue squares, (J) red triangles, and (K) green spheres represent the photocurrent responses of graphene-synapse junctions at spots with the corresponding color in Fig. 2E. ΔI_pc is the difference between the maximum (red) and minimum (blue) photocurrent response.