Biomedical Engineering Reference
In-Depth Information
membrane, leaving the cells undamaged due to the extremely small diameter of
the tube. Multiple BIT-FETs can record multiplexed intracellular signals from
both single neurons and networks of neurons.
Figure 4.3 shows a nanotube piercing the cell membrane and the FET
structure measuring cellular potential changes.
In the case of metal nanoelectrodes, the Park group
15
at Harvard University
developed a vertical silicon nanowire array with individual nanowires 150 nm
thick and 3 mm high.
16
Several nanowires were grouped (2 mm spacing) to cover
a single neuron and an array of grouped nanowires was used to interrogate a
small neural circuit. A high signal-to-noise ratio on the order of 100 was
achieved with the measured signal amplitude on the order of a few mV.
d
n
4
t
3
n
g
|
0
4.3 Graphene
Ever since the first isolation of freestanding graphene sheets in 2004, this two-
dimensional (2D) carbon crystal has been highly anticipated to provide unique
and new opportunities for sensor applications. Graphene has already demon-
strated great potential in various novel sensors that utilize the exceptional
electrical properties of the material (extremely high carrier mobility and
capacity), electrochemical properties (high electron transfer rate), optical
properties (excellent ability to quench fluorescence), structural properties
(one-atom thickness and extremely high surface-to-volume ratio), or its
mechanical properties (outstanding robustness and flexibility). Graphene
nanostructures exhibiting such excellent properties are very suitable for use as a
channel material in FETs as discussed in Chapter 1.
The incorporation of graphene in FETs results in the insertion of a new
matrix with superior sensing properties in a structure of high sensitivity, simple
device configuration, low cost, high miniaturization and real-time detection. A
typical FET consists of a semiconducting channel between two metal elec-
trodes, the drain and source electrodes, through which the current is injected
and collected. Varying the gate potential through a thin dielectric layer,
typically 300 nm SiO
2
, can capacitively modulate the conductance of the
channel. In a typical p-type metal oxide semiconductor field-effect transistor
(MOSFET), the negative gate potential leads to the accumulation of holes
(majority charge carries) resulting in an increase of the channel conductance,
while the positive gate potential leads to the depletion of holes and hence a
decrease of the conductance. In the case of the electronic sensor, the adsorption
of molecules on the surface of the semiconducting channel either changes its
local surface potential or directly dopes the channel, resulting in change of the
FET conductance. This makes the FET a promising sensing device with easily
adaptable configuration, high sensitivity and real-time capability, provided that
the issue of non-specific adsorption is addressed by using the appropriate
chemistry to prevent the fouling of the surface when the device is exposed to
complex biological media such as human serum or blood.
In some cases, the gate electrode is removed to simplify the device structure,
hence forming a chemiresistor. Such configuration is suitable for fabrication of
n
3
.
