Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 72

Fig. 2.16 CNT biosensor
based on a CNT network

CNT
network

protein analyte

electrodes

glass

Fig. 2.16 . The CNT biosensor in this figure is a nanosized gap between two Ag
electrodes. The gap is covered with a CNT network consisting of a suspension
of functionalized single-walled CNTs. If we apply a voltage of 1 V across the
electrodes, the conductance is decreased with 40% (36%) when 1 M of protein
(antibody) solution was added over the CNT network. In this way, DNA, RNA,
proteins, or viruses can be electrically detected.
The CNT biosensor in Fig. 2.16 can be easily transformed into a CNT FET with a
liquid gate sensor by inserting a Pt electrode into a conducting liquid placed over the
CNT network and the source and drain electrodes. In this case ( Gruner 2006 ), the
detection is based on a shift of the drain-source current dependence as a function of
gate voltage ( Gruner 2006 ). The CNT FET with liquid gate is displayed in Fig. 2.17 .
The attachment of biomolecules such as a protein on the CNT network forming the
FET channel is detected in real time.
Moreover, the CNT FET in Fig. 2.17 is able to detect viruses or other bioentities,
and even a single biomolecule. CNT FETs working in solution, as above, are
able to detect the hybridization of a single molecule of DNA. A single ssDNA
probe molecule is covalently attached to a point defect engineered in CNT, and the
drain-source conductance is measured in the presence of the complementary DNA
target. By measuring the fluctuations in time of the conductance, the temperature-
dependent kinetics of DNA matching can be studied, and in this way, the rate
constants, activation energies, and melting curves can be determined ( Sorgenfrei
et al. 2011 ).
Graphene has impressive physical properties, as discussed in Chap. 1 . FETs
based on graphene are developing very fast ( Schwierz 2010 ) and their cutoff exceed
300 GHz ( Liao et al. 2010 ). A top gate graphene FET is depicted in Fig. 2.18 .The
SiO 2 layer is deposited on a doped silicon (not shown) which acts as back gate.
Recently, such graphene FETs were used for an electronic nose ( Lu et al. 2010 ).
The graphene FET was functionalized with two ssDNA sequences, and the drain-
source current variation in time was used to detect vapors of analytes such as
nitrogen, propionic acid, or methanol. Large variations of the drain current were