Biomedical Engineering Reference
In-Depth Information
graphene-based sensors on polymer substrates for flexible electronics appli-
cations. Despite the lack of modulation by the gate potential, the working
principle of the chemoresistor is same as a normal FET sensor.
Figure 4.4 shows a typical gas sensing system, in which the channel is directly
exposed to a target gas species. The adsorption of gas molecules results in
the doping of the semiconducting channel, leading to a conductance change of
the FET device. The charge transfer from the adsorbed gas molecules to the
semiconducting channel is the dominant mechanism for the current response,
which is similar to carbon nanotube based gas sensors.
In order to detect biospecies, the grapheme-FETs should operate in an
aqueous environment. As shown in Figure 4(b), the graphene channel is usually
immersed in a flow cell or sensing chamber, which is used to confine the
solution. The drain and source electrodes are electrically insulated to prevent
current leakage from ionic conduction. Different insulators including
poly(dimethylsiloxane) (PDMS)/silicone rubber, SiO
2
thin film, SU-8 pass-
ivation and silicone rubber are used in different device structures. The gate
electrode, usually Ag/AgCl or Pt, is immersed in the solution. The gate
potential is applied through the thin electric double layer capacitance formed
at the channel-solution interface. The double-layer thickness (or Debye length)
is determined by the ionic strength, typically within 1 nm. Normally, the
solution-gate FET is over two orders of magnitude more sensitive than the
typical backgate FET.
Two major sensing mechanisms have been proposed for graphene-based
biosensors in solution, i.e. the electrostatic gating effect and the doping effect.
The gating effect suggests that the charged molecules adsorbed on graphene act
as an additional gating capacitance, which alters the conductance of the
graphene channel. However, the doping effect suggests a direct charge transfer
between the adsorbed molecules and the grapheme channel, similar to gas
sensing. In a real case, the actual sensing mechanism might be a combination of
both mechanisms, or involve more complicated mechanisms.
d
n
4
t
3
n
g
|
0
n
3
.
Figure 4.4
(a) Typical back-gate graphene-FET on Si/SiO
2
substrate used as gas
sensor. (b) Solution-gate graphene-FET on flexible polyethylene tereph-
thalate (PET) substrate used as chemical and biological sensor in aqueous
solution.
17
(Reprinted by kind permission of the Royal Society of Chemistry.)
