Biomedical Engineering Reference
In-Depth Information
to identify PG as a suitable material for constructing an indicator electrode. The crystal-
line structure of PG closely approximates that of the ideal graphite crystal. The prepara-
tion, properties, and structure of PG have been described in detail by Walker [12]. PG can
be obtained by the pyrolysis of gaseous hydrocarbons. PG materials usually consist of
poly(aromatic rings) with sp
2
-hybridized carbon atoms. These aromatic planar rings are
stacked together by π-π interaction of the electronic network [13]. A schematic representa-
tion of the layered structure of graphite is shown in Figure 3.2. The planar layer (i.e., the
ab
plane) formed by the aromatic ring system is usually termed as
basal plane
, whereas the
planes perpendicular to the graphitic layer are the
bc
and
ac
planes and are called
edge
planes
. Edge planes are usually active compared with the basal plane. The reactivity of
the edge plane is due to the presence of various reactive groups such as carboxylic acid,
ketonic, and hydroxyl groups [14]. Electron transport through
c
-axis accounts for the conduc-
tivity of PG. PG electrodes used in voltammetric studies can be prepared by cutting small
rods (diameter 4 mm, length 10 mm) from a block of graphite [11]. The graphitic block must
be positioned in such a way that it is cut exactly perpendicular to the
ab
plane [13]. The
resulting small piece of graphitic rod [15] is then cemented in to long glass tubing with
epoxy cement. The PG rod is mounted in such a way that only the
ab
planes are exposed.
Care must be taken to avoid the occurrence of air pockets between the air tube and the PG
rod. This is usually achieved by initially coating the inside of the glass tube with epoxy
cement followed by coating the rod with epoxy and finally sliding the rod carefully into
the tube. During this event, the rear end of the graphitic rod will be coated with epoxy
that prevents the electrical contact to the graphitic rod. In certain cases, depending on
the application, the end of the PG electrode can be polished using a silicon carbide paper.
Various other forms of PG such as vapor-deposited PG and wax-impregnated PG have also
emerged, and their preparation and properties have been described in detail [15-18].
Highly Oriented PG
Highly oriented PG (HOPG) is manufactured by the process of annealing PG under high
compression force [19]. It is otherwise known as stress-annealed PG. The material has
c
a
b
b
Edge plane
Basal plane
c
a
FIGURE 3.2
Schematic representation of the layered structure of pyrolytic graphite.
© 2011 by Taylor & Francis Group, LLC
