Biology Reference
In-Depth Information
CNTs present a larger surface area and outstanding charge-
transport characteristics and might therefore greatly promote
electron-transfer reactions which can dramatically improve elec-
trochemical performance compared to that of other carbonaceous
materials [68]. The open end of a CNT is expected to show a fast
electron-transfer rate similar to the graphite edge plane while the
sidewall is inert like the graphite basal-plane. Fast electron-transfer
rate isdemonstrated along the tubeaxis[69].
CNTs are expected to present a wide electrochemical window,
flexible surface chemistry, and biocompatibility, similar to other
widely used carbon materials.
Among the different classes of carbon allotropes, carbon-based
composites,suchascarbonpaste(CP),areusuallymadeofpolycrys-
talline graphite. A key property of polycrystalline graphite is poros-
ity. Most polycrystalline graphite—such as powdered carbon—is
made by heat treatment of high-molecular-weight petroleum frac-
tions at high temperatures to perform graphitization. The term
“graphite” is used to designate materials that have been subjected
to high temperatures, and thus have aligned the sp
2
planes parallel
to each other.
Commercially available microcrystalline graphite exists as
extremely hydrophobic 1 to 20
μ
m particles that aggregate into
thin films on contact with solvent. When treated under strongly
oxidizing acidic conditions, graphite oxide is formed. Structurally,
graphite oxide is an epoxidized form of the sp
2
-bonded carbon net-
work together with acidic functional groups at the edges with the
oxidants intercalated in the interlaminar space [63].
While the electrical conductivity is provided by the conducting
carbonaceousfiller,inordertoprepareacarboncomposite,abinder
is also needed. The binder will confer the conducting composite a
certain physical, chemical, or biological stability. One of the sim-
plest carbon composite approaches for electrochemical biosensor
is based on soft carbon pastes [70]. These pastes are built by mix-
inganinertconductor(e.g.,graphitepowder)withanonconducting
liquid (e.g., para
n oil, silicone, Nujol). This insulating liquid has a
specificviscosityandthepastehasacertainconsistency.Theresult-
ing devices are easy to prepare and inexpensive. However, these
pastes have limited mechanical and physical stability, especially in
