Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 71

in Si NW FETs with channel lengths in the range of 2-20m, for example, which
are much longer than the mean-free path, of 10-20 nm. The presence of charged
immobilized biomolecules can be modeled as a uniform electrical surface density
due to which the NW charge increases or decreases with q D d. Thus,
the conductance change originating in molecular conjugation processes at the NW
surface is given by

G D d=L; (2.26)

and the sensitivity is

S D 4= edN D : (2.27)

From ( 2.27 ), it follows that good sensitivity is achieved for weakly doped and thin
NWs. The NW FET biosensor is displayed in Fig. 2.15 .
The NW FET biosensors have many applications ( Patolsky et al. 2007 ). For
example, the presence of a virus can be detected by monitoring the change in
conductance due to variations in the electrical charge when a single virus becomes
immobilized on the functionalized substrate. If the virus unbinds, we detect the G 0
value of the conductance, while in the presence of the virus, the conductance reaches
the G value; a temporal measurement of the conductance reveals the dynamic of the
virus interaction with the functionalized NW.
Finally, we present briefly the biosensing with the help of CNTs and graphene.
In principle, the CNT transistors (CNT FETs) are well described in ( Dragoman
and Dragoman 2009 ), and we do not repeat here the theory of these transistors.
The sensing mechanism is similar to NW FETs, i.e., the conductance of CNT
FET is modulated in time when a biomolecule is sensed by the functionalized
CNT channel. There are two basic configurations of CNT FETs for biosensing
applications ( Gruner 2006 ): a FET with a channel consisting of a single CNT, or
a FET with a channel formed by a network of CNTs, which are randomly oriented.
There are very simple methods to functionalize CNTs in order to immobilize
biomolecules ( Yang et al. 2007 ), and various biomolecules such as DNA, RNA,
or proteins are detected with CNT FETs. An example is presented in what follows.
A simple CNT-based biosensor to detect streptavidin (a protein) and the IgG
(immunoglobulin G) mouse antibody ( Atashbar et al. 2006 ) is represented in

functionalized channel

NW

biomolecules

D S

SiO 2

n -Si

Fig. 2.15 FET NW biosensor configuration