Biomedical Engineering Reference
In-Depth Information
ends and electrodes [62]. Hundred percent alignment of nanostructures to the imposed mag-
netic fields was achieved by applying low external magnetic field of 0.02 T. In addition,
directional controllability of magnetic assembling technique was demonstrated by assem-
bling nanostructures with angles from 45° to 135° with respect to the electrodes.
4.5
Label-Free Sensing using Nanowire-Based Field Effect Transistor
While there have been several reported demonstrations of the application of the CP NWs
as FET devices and chemical sensors, there has been only a couple of reports elucidating
the application of biologically functionalized CP NWs for label-free sensing. Using avidin-
functionalized PPY nanowires, we illustrated the label-free detection of biotin-DNA (20
mer oligo) conjugate and antiavidin IgG. Figure 4.12 shows the response of an avidin-func-
tionalized PPY nanowire when challenged with biotin-DNA. The resistance of the func-
tionalized nanowire increased as a function of concentration, while a control nanowire
without embedded avidin showed no response. The biosensor response was very rapid
and as low as 1 nM of biotin-DNA was detected in a few seconds. Similar response was
observed with antiavidin IgG. The nanomolar sensitivity of the present biosensor, while
lower than that for Si NW and CNT, was obtained with nanowires that were not optimized
with respect to the conductivity. A sensitivity of potentially single-molecule detection is
possible by adjusting the nanowire's conductivity to a value closer to the lower end of a
semiconductor. Other demonstration of label-free sensing using biologically functionalized
CP comes from Tao's group in which PANI functionalized with gly-gly-his and hexa-his
was used for the detection of Cu 2 and Ni 2 ( Figure 4.13 ) .
4.6
Conclusions and Future Perspectives
One-dimensional nanostructures and most of the applications derived from these materials
are still in their infancy. Hence, several issues including their chemical/thermal/mechanical
stability need to be addressed before these materials can be utilized to their full potential. This
is of crucial importance because 1-D nanostructures are known to be both less thermally and
150
Conc. (nM)
R / R (%)
Injection
0
3
FIGURE 4.12
Electrical responses of an unmodified
nanowire (A) to 100 nM biotin-DNA (single
stranded) and avidin-embedded PPY (200 nm)
nanowires to 1 nM (B) and 100 nM (C)
biotin-DNA. The responses were recorded
on two separate PPY-avidin nanowires. PPY
nanowire containing entrapped avidin were
grown using 25 nM pyrrole in 10 mM NaCl
and of avidin [59]. (From Ramanathan, K.,
Bangar, M., Yun, M., Chen, W., Myung, N.,
Mulchandani, A. (2005). Bioaffinity sensing
using biologically functionalized conduct-
ing-polymer nanowire. J . Am . Chem . Soc .,
127:496-497. With permission.)
1
13
100
50
100
1000
54
(C)
50
(B)
(A)
0
0
50
100
150
Time (s)
 
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