Biomedical Engineering Reference
In-Depth Information
12.3.3
Properties of Nanowires
Compared with bulk materials, low-dimensional NWs, with their large surface area and
possible quantum confinement effects, exhibit distinct electric, optical, chemical, and ther-
mal properties. Boland and coworkers [59] recently demonstrated the ultrahigh strength
of gold NWs. They found that, compared with Au bulk nanocrystalline metals, the
Young's modulus of Au NWs is essentially independent of diameter, whereas the yield
strength is largest for the smallest diameter wires, with strengths of up to 100 times greater
than that of the bulk material. They attributed this to reduced defects and a fewer number
of grains across the diameter of NWs. Alivisatos and coworkers [60] showed how semi-
conducting NWs could be used to enhance the processibility and efficiency of solar cells.
They fabricated thin-film photovoltaic devices by blending CdSe NWs with polythio-
phenes to obtain hybrid materials. They also found that NWs were superior to quantum
dots in photovoltaic applications because they could provide a direct path for electrical
transport at much lower loading; a power conversion efficiency of as high as 1.7% has
been obtained. Based on the extremely high surface-to-volume ratios of NWs, Penner and
coworkers [61] fabricated hydrogen sensors with Pd NWs. Since each NW contains many
break junctions along their longitudinal axis resulting from hydrogen reduction, the resist-
ance of these NWs exhibited a strong dependence on the concentration of hydrogen gas.
Various other applications of semiconducting NWs have recently been exhibited, such as
building blocks for assembling a range of nanodevices including FETs,
p
-
n
diodes, bipolar
junction transistors, and various biosensors. The latter will be discussed in detail later on.
12.4
Functionalization of Carbon Nanotubes and Nanowires for Biosensor
Development
Integration of biology and materials at the nanoscale has the potential to revolutionize
nanobiotechnology. The small size, good charge transport properties, and large active sur-
face area of CNTs and NWs holds great promise for the design of biosensors. The electrical
properties of CNTs and NWs are very sensitive to surface charge transfer and changes in the
surrounding environment, and drastic changes can be caused by single-molecule interac-
tions [2,62,63]. With the development of nanofabrication technology, the potential of CNTs
and NWs as sensing elements and tools for biological applications as well as for chemical
analysis has been attracting considerable attention for sensor development [64,65]. To con-
trol the chemical and physical properties of CNTs for biosensor applications, it is obviously
important to develop reliable methods for CNT or NW functionalization [66-69].
12.4.1
Solubilization and Functionalization of Carbon Nanotubes
It is well known that chemical modification of nanomaterial surfaces is often necessary to
generate various functionality and biocompatibility of such nanomaterials [70]. In partic-
ular, functionalization of CNTs and NWs with different selected biomolecular compo-
nents may introduce functional units such as recognition sites, catalytic elements or
bioactive groups, which are essential for various applications [71-73]. The recent bloom
of chemical modification and functionalization methods [74] has made it possible to con-
jugate NWs with various biological and bioactive species such as proteins [75-80],
enzymes [81-83], peptides [84,85], porphyrin [68,86,87], carbohydrates [88-90], and
nucleic acids [32,91-93]. The construction of such functionalized CNTs and NWs has
