Microelectronic Devices Based on Nanomaterial-Carrier Cells - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 135

covered with the graphene sheets have been demonstrated where a decrease
in current across the graphene sheets is observed as they wrinkle in response
to a cell's surface stresses. 1

7.3.3 Observing Ion Flow
Cell-based devices will operate in an aqueous environment as required to
maintain a cell's biological activity. An omnipresent and routine cell process
is the exchange of ions between the cell and its surroundings. A device that
can detect this process can significantly expand our ability to monitor cell
dynamics in real time. This will also expand the cell types that can be studied
using such an electrical interface and also the range of stimuli that can be
deciphered from the recorded signals. The detection of such an ion flow is
challenging as the cell is already in an environment that has significant ionic
strength (e.g. the nutrient broths used to grow the cells). Below, we discuss a
proposed mechanism of sensing this ion flow on a graphene-covered cell.
The detection is based on a three-electrode system, a source and drain
across the graphene-covered FET cell and a reference (e.g. Ag/AgCl) dipped in
the electrolyte as the gate electrode. 25-27 A schematic of the system and the
corresponding circuit elements is presented in Figure 7.5. Considering the
graphene sheets as a planar surface, the capacitance across them, in relation
to the reference electrode, consists of the sheets quantum capacitance (C q )
and the electrical double layer capacitance (C d ) (Figure 7.5) in series. For
graphene sheets C q B

d n 8 y 4 n g | 6

500 nF/cm 2 (electrical double layer
(EDL) of 1 nm thickness for physiological conditions). 25,26,28 Being in series,
the smaller capacitance C d , dominates the total capacitance. Therefore the
gating characteristics of the sheets by the reference electrode will be
dependent on C d . The ions flowing across the cell boundary will alter the
local ion concentration in the EDL (being just 1 nm thick), resulting in
modulation of the electrical properties of the sheets on top due to two pri-
mary effects.

2 mF/cm 2 , and C d B

.

Figure 7.5

Schematic of a graphene-covered cell forming an ON device similar to
Figure 7.4b. The schematic on the right shows a simplified electrical
circuit for the device.