Microelectronic Devices Based on Nanomaterial-Carrier Cells - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 133

two species. With this basic formulation, if the external interface consists of
a series of tunneling barriers, the current through it will be modulated due
to any changes in the parameters governing the tunneling probability. 2,20,21

h i exp E c

E

1
2

Ppexp F

c a

d n 8 y 4 n g | 6

Consider a device set-up with the interface consisting of tunneling junc-
tion in contact with the cell (Figure 7.1). An example of such a set-up is the
use of Au nanoparticle chains to form a conductive pathway. The complete
assembly is immersed in a solution required to maintain cell activity in a
controlled environment. The cell is dynamic and this leads to modulation of
the tunneling parameters in the interface. For example, the energy barrier
can be altered due to cellular processes such as the flow of ions and small
molecules. The ions carry charge and this can alter the magnitude of the
energy barrier for tunneling. Further, the flow of ions will alter the local
electrical double layer (EDL) which will be present on the interface as it is
maintained at a potential (compared to the solution) for the flow of elec-
trons. The EDL affects the local electric field of tunneling. Hence, the
modulations in the current are reflective of the cell's behavior as we can
directly correlate the change in the tunneling parameters, as noted above
due to the cell's activity.
Further modulations in cells shape and size (e.g., linked to cell's growth
and cytoskeleton) can alter the physical dimensions of the tunneling gap.
This again has an exponential effect on the tunneling probability. Such
modulations in the physical size of the tunneling junction have been dem-
onstrated by altering the humidity of the cell's environment. 2 This can be
translated to devices where the cell is maintained in a controlled growth
environment and its size is monitored by the modulations in current.
A challenge in the tunneling-based interface for cells is the signal levels,
i.e. the magnitude of the current for voltages that can be applied under
aqueous conditions without having faradic reactions in the system. In the
case of using nanoparticles with micrometer-size cells this can lead to
challenges. However, using nanorods or wires can significantly reduce the
number of tunneling junctions in the device leading to a better signal-to-
noise ratio. The trade-off is that a larger number of junctions also increases
the sensitivity for detecting cellular modulations.

.

7.3.2 Gating Effect
A continuous interface made with nanomaterials such as nanowires or
graphene sheets can detect cellular modulations due to corresponding
changes in their charge-carrier dynamics. The interface offers a continuous
path way for flow of electrons (or holes in the case of p-type nanomaterials).
The nanomaterials in this case are usually a semiconductor such as doped Si
nanowires and carbon nanotubes. 2D materials such as graphene where the