Biomedical Engineering Reference
In-Depth Information
accumulation of carriers within the transistor structures from the binding of the charged
biomolecule to the gate. The selectivity and sensitivity of the FET can be improved by
modifying the gate of the FET with (bio)receptors/(bio)recognition molecules such as anti-
bodies, antigens, and oligonucleotides [1]. While successful demonstration of the label-free
detection of biomolecules such as proteins and DNA using planar films has been reported,
the limited sensitivity of planar devices has precluded them from having a large impact.
Field effect transistors fabricated from one-dimensional (1-D) nanostructured materials
are based on a similar framework but are more sensitive because, unlike planar FETs, they
avoid the reduction in conductance changes caused by lateral current shunting. When
used as the gate of FET device, 1-D nanostructures offer significant advantages over 2-D
thin-film planar gates. First, binding to the surface of 1-D nanostructures can lead to deple-
tion or accumulation of carriers in the “bulk” of the nanometer diameter structure versus
only the surface region of a planar device
(
Figure 4.1
)
, giving rise to large resistance/con-
ductance changes to the point that single-molecule detection is possible. Also, as shown in
Figure 4.2, real-time monitoring can be achieved when the binding between the receptor
and the target is reversible. Second, the direct conversion of chemical information into an
electronic signal can take advantage of the existing microelectronic technology and lead to
miniaturized sensor devices. Finally, the small size of the nanostructures makes it possible
for the development of high-density arrays of individually addressable nanostructures for
simultaneous analysis of a range of different species and massive redundancy to reduce
false positives/negatives [2]. Also, 1-D nanostructures, such as nanowires, nanobelts,
nanosprings, and carbon nanotubes (CNTs), are extremely attractive for nanoelectronics
because they can function both as devices and as the wires that access them.
Until recently the advancement of 1-D nanostructures has been slow because of many
difficulties associated with the synthesis and fabrication of these materials with well-
controlled dimensions, morphology, phase purity, and chemical composition. Three
classes of 1-D nanostructured materials, namely CNTs, silicon nanowires (Si NWs), and
lately conducting polymer nanowires (CP NWs), have shown profound performance in
device fabrication in general and in label-free detection technology in particular.
4.1.1
Scope and Overview of the Chapter
Recently, extensive reviews covering the synthesis of Si NWs and CNTs and their applications
for label-free detection have been published [3-7]. We will therefore limit the scope of this
chapter to biosensors based on CP NWs. We begin by outlining the methods used to synthe-
size and fabricate CP NWs. We highlight the successes and limitations of various methods
(A) One-dimensional nanowire FET
(B) Two-dimensional thin-film FET
FIGURE 4.1
Advantages of one-dimensional (1-D) nanowire-based field effect transistor (FET) (A) over two-dimensional
(2-D) thin-film FET (B). Binding to 1-D nanowire leads to depletion or accumulation in the bulk of the nanowire
as opposed to only the surface in 2-D thin film case.
