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. 2016 Dec 7;8(45):19043-19049.
doi: 10.1039/c6nr07290a. Epub 2016 Nov 4.

Probing electrical signals in the retina via graphene-integrated microfluidic platforms

Yuchen Zhang  1 , Kirsten H Dodson, Rachel Fischer, Rui Wang, Deyu Li, Rebecca M Sappington, Ya-Qiong Xu
Affiliations

Probing electrical signals in the retina via graphene-integrated microfluidic platforms

Yuchen Zhang et al. Nanoscale. .

Abstract

Graphene has attracted extensive attention in biological and biomedical fields due to its unique physical properties and excellent biocompatibility. We combine graphene field-effect transistors and scanning photocurrent microscopy with microfluidic platforms to investigate electrical signals in mouse retina. Remarkable photocurrent signals were detected from the graphene underneath the optic nerve head (ONH) of the retina, where the electrical activity from this region can modulate the carrier concentration of the graphene and induce local potential gradients. These built-in electrical potential gradients can efficiently separate photo-excited electron-hole pairs, leading to strong photocurrent responses in the graphene underneath the ONH. We also show that no significant photocurrent signal was observed in the graphene underneath either dehydrated or fixed retinal tissues, verifying that the photocurrent responses generated in the graphene underneath the ONH were indeed induced by the electrical activity in living retina. This method not only provides a way to investigate electrical processes in living retinal tissues, but also offers opportunities to study many other cellular systems involving cell-cell interactions through electrical signaling.

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Figures

Figure 1
Figure 1
Overview of the experimental design. (A) Top: Exploded view of a graphene-integrated microfluidic platform. The orange plate is a slice of agar gel. The pink disk represents a retina. The green plane indicates a graphene film. The yellow bars represent Au electrodes. Bottom: Schematic diagram of scanning photocurrent measurements. A one micron diameter diffraction-limited laser spot goes through a transparent coverslip to scan over graphene transistors underneath a retinal tissue in a microfluidic platform. (B) Raman spectrum of graphene on a coverslip. (C) Electrolyte gate response of a typical graphene transistor. (D) Fluorescence image of a CTB-labeled retina on top of graphene transistors. There are 54 separated electrodes in an electrode array, where the edge-to-edge distances between two electrodes in the horizontal direction is 180 μm and between upper and lower electrodes are 20 μm, 40 μm, and 60 μm, respectively. A graphene film is placed on top of all electrodes. Any two electrodes can be used as source and drain electrodes, respectively. Here, 16 opaque electrodes near the ONH are imaged. Scale bar is 200 μm.
Figure 2
Figure 2
(A) Fluorescence, (B) optical, (C) reflection and (D) scanning photocurrent images of a fresh excised, CTB-labeled retina on top of a graphene transistor in the ONH region, respectively. (E) Fluorescence, (F) optical, (G) reflection and (H) scanning photocurrent images of the retina/graphene at the same location after it is dehydrated at room temperature for 24 hours, respectively. Source and drain electrodes are 45 μm by 15 μm. The edge-to-edge distance between two electrodes is 60 μm. Scale bar is 30 μm. The inverted ‘14’ near the upper electrode is a metal marker for location.
Figure 3
Figure 3
(A) Fluorescence image of a fixed retina. Scale bar is 200 μm. (B) Enlarged fluorescence, (C) optical, and (D) scanning photocurrent images of the retina in the ONH region, respectively. The edge-to-edge distance between two electrodes is 180 μm. Scale bar is 30 μm in B–D. ‘12’ and ‘0’ near electrodes are metal markers for location.
Figure 4
Figure 4
Schematic diagrams of band structures of a graphene transistor. (A) Photocurrent signals generate at graphene-metal junction areas due to the band structure bending. (B) A retina can modulate the carrier concentration of graphene underneath it. Green solid lines show potential profiles. Black dashed lines denote the Fermi levels. Note that in (A) and (B), the potential profiles are obtained through numerical integration of the scanning photocurrent profiles along the dashed lines in Figure 2H and 2D, respectively.
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