Nanomedicine - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 136

This change originates in the electrostatic force F el , which superimposes on the
gravitation force mg acting upon the cantilever, the net effect being an increase in
the effective mass. We have then

r

f
f 0 D

m
m C F el =g 1;

(4.5)

with g D 9:8 ms 2 . The mass per unit length of a CNT is equal to .r ext
r int /,where is the density of the CNT, and r int and r ext are, respectively, its
inner and out er radii, wh ile the electrostatic force per unit length is given by

2" 0 V act = p h.h C 2r ext /. In the last expression, " 0 is the vacuum permittivity,
V act D V V G is the actuation voltage, with V the voltage applied on the cantilever,
and h is the distance between the undeflected CNT and the bottom electrode. To get
an insight into the wideband tunability of the tunneling nanoradio, it can be easily
estimated that for h D 300 nm, and inner and outer CNT radii of 1 and 15 nm,
respectively, the frequency of the CNT nanoradio can be tuned reversibly in a broad
range of values, i.e., f=f 0 Š 0:6, by a very small actuation voltage, of less than
1 mV, as indicated in Fig. 4.13 . However, this performance can only be obtained if
the difference between the voltage on the cantilever and that applied on the gate is
controlled with high precision.
So, in principle, there is a possibility to obtain a CNT radio able to penetrate a
single cell. Moreover, as in the case of microchips, it is possible to remotely control
cellular processes by electrical means via wireless inductive coupling. This amazing
fact was demonstrated for the hybridization control of DNA through the inductive
coupling to an attached nanoparticle antenna, the crystalline nanoparticle having a
diameter of 1.4 nm ( Hamad-Schifferli et al. 2002 ). The nanoparticles are attached
to the DNA and increase locally the temperature via wireless coupling to values
high enough to denaturate the DNA, while other molecules in the neighborhood
are unaffected. The wireless inductive heating was performed with a microwave
field having a frequency of 1 GHz. The demonstration of the working principle of
this device consisted in the fabrication of 38-nucleotide hairpin loops, which are

D f / f

0.6

0.3

0.8 V (mV)

0.2 0.4 0.6

Fig. 4.13 Frequency tuning of the tunneling nanoradio