3.1. Device fabrication
Electrochemical biosensors based on graphene and CNTs can
broadly be divided into two device architectures: (i) conventional
three electrode set-up, (ii) chemiresistive/FET set-up
(Balasubramanian and Burghard, 2006).
The three electrode set-up, commonly used for amperometric,
impedimetric and potentiometric biosensors comprises of a working
(WE), reference (RE) and counter-electrode (CE). Fig. 4 shows
the schematic representation of a conventional three electrode
system for electrochemical detection of analytes. For working electrodes,
the commonly used glassy carbon electrode (GCE) surface is
modified by depositing a film of graphene or carbon nanotubes.
Graphene flakes obtained by exfoliation and dispersed in surfactant
solution are drop-casted onto the bare GCE surface, baked at
slightly elevated temperatures (40–60 C) and washed repeatedly
with deionized water. In the case of carbon nanotubes, due to their
low solubility in most solvents, CNT-modified electrodes are prepared
by casting of CNT/sulfuric acid solution on GCE surface.
Nafion, a perfluorosulfonated polymer, is also used extensively to
solubilize CNTs (Wang et al., 2003). The resulting biosensors
greatly benefit from the coupling of the unique ion-exchange and
antifouling properties of Nafion films with the efficient electrocatalytic
properties of CNTs.