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Principles and Procedures to Assess Nanomaterial Toxicity 223

confirmation of induction of specific cellular response elements due to
exposure to various NM.

Nanoelectrodes
Electrodes made at the nanoscale can display altered properties simply
by the nature of their dimensionality [56, 59, 60]. Reducing something
to a smaller size can result in increased available reactive surface area,
as well as other effects such as focusing of the electric field due to the
rod-like geometric configuration [61–63]. These are desirable effects
that enhance the system through their physical aspects. Carbon nan-
otube (CNT) and metallic arrays have been used as electrodes to pro-
mote electron transfer in redox reactions [59–64].
Carbon nanotubes are unique and possess many interesting proper-
ties in the electronic, mechanical, chemical, and optical domains [65].
Particularly relevant to the work covered in this chapter is that they
have shown great promise as a biochemically compatible, nanoelectronic
interface to biomolecules. CNTs used as a bioelectronic interface with
a protein immobilized at one end offer a number of advantages. It max-
imally maintains the protein’s native conformation, hence its functional
properties, by minimizing the contact surface area in much the same
way as a straw holds a soap bubble at its tip. Its high electrical con-
ductivity provides an efficient conduit for electronic signals from or to
the protein, and its sharp curvature at the tip greatly enhances the
electrical field and thereby increases exponentially the electron trans-
fer between the protein and the nanotube via quantum mechanical tun-
neling whose rate is an exponential function of the local field. Functional
activated carboxyl groups are readily produced via acid-treatment at the
open end of the nanotube. This facilitates its conjugation with biomol-
ecules localized at the tip and, therefore, enables site and molecule spe-
cific immobilization. Recent results have demonstrated the viability of
linking an enzyme’s site specifically with enhancement of electron trans-
fer rates from the immobilized protein of two orders of magnitude over
the known rate in the solution phase [56, 59].
Arrays of vertically aligned MWNTs are grown from a hexagonally
patterned array of nanopores in an anodized aluminum oxide (AAO)
template that are virtually identical in length, diameter, and spacing
10 2
(Figure 6.3). Their dense packing, on the order of 10 cm , and unifor-
mity produce a tight, even plane of nanotube tips highly suitable for
interfacing with biomolecules. Within the array, each individual tube is
physically separated and electrically insulated by the insulating AAO
template, and a direct electrical contact of each tube can be made to a
macro-scale electrode by sputtering the backside of the array with a
layer of metal (e.g., gold). Details of the fabrication process have been