Chemical and biological sensors based on electroactive inorganic polycrystals - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 423

monitoring of lower H 2 O 2 levels. The decrease in detection limits is possible by
electrode miniaturization. Miniaturization usually creates new scientifi c and techno-
logical fi elds, and electroanalysis is not an exception. Nano-technology is expected to
play a key role in accessing limiting performance characteristics of chemical and bio-
logical sensors.
Diffusion of electroactive species to the surface of conventional disk (macro-) elec-
trodes is mainly planar. When the electrode diameter is decreased the edge effects of
hemi-spherical diffusion become signifi cant. In 1964 Lingane derived the corrective
term bearing in mind the edge effects for the Cotrell equation [129, 130], confi rmed
later on analytically and by numerical calculation [131, 132]. In the case of ultramicro-
electrodes this term becomes dominant, which makes steady-state current proportional
to the electrode radius [133-135]. Since capacitive and other diffusion-unrelated cur-
rents are proportional to the square of electrode radius, the signal-to-noise ratio is
increased as the electrode radius is decreased.
In the case of a single electrode, however, the decrease of its dimensions requires
the measurement of very low currents. To overcome this problem it is convenient to
use microelectrode arrays [136, 137]. Despite the fact that in such arrays microelec-
trodes are electronically connected to each other, analytical properties of such assem-
blies are advantageous over those of a conventional macro-electrode [138, 139].
Micro- (and even nano-) electrode arrays are commonly produced with photolithog-
raphy and electronic beam techniques by insulating of macro-electrode surface with
subsequent drilling micro-holes in an insulating layer [136, 137]. Physical methods
are, however, expensive and, besides that, unsuitable for sensor development in certain
cases (for instance, for modifi cation of the lateral surface of needle electrodes). That's
why an increasing interest is being applied to chemical approaches of material nano-
structuring on solid supports [140, 141].
Nano-electrode arrays can be formed through nano-structuring of the electrocatalyst
on an inert electrode support. Indeed, if the current of the analyte reduction (oxidation)
on a blank electrode is negligible compared to the activity of the electrocatalyst, the
former can be considered as an insulator surface. Hence, for the synthesis of nano-
electrode arrays one has to carry out material nano-structuring. Recently, an elegant
approach [140] for the electrosynthesis of mesoporous nano-structured surfaces by
depositioning different metals (Pt, Pd, Co, Sn) through lyotropic liquid crystalline phases
has been proposed [141-143].
Prussian blue-based nano-electrode arrays were formed by deposition of the elec-
trocatalyst through lyotropic liquid crystalline [144] or sol templates onto inert elec-
trode supports. Alternatively, nucleation and growth of Prussian blue at early stages
results in nano-structured fi lm [145]. Whereas Prussian blue is known to be a superior
electrocatalyst in hydrogen peroxide reduction, carbon materials used as an electrode
support demonstrate only a minor activity. Since the electrochemical reaction on the
blank electrode is negligible, the nano-structured electrocatalyst can be considered as a
nano-electrode array.
The morphology of Prussian blue electrodeposited onto a mono-crystalline graph-
ite surface was investigated by atomic force microscopy (AFM) and is presented in