Chemical and biological sensors based on electroactive inorganic polycrystals - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 414

Prussian blue

0.2 0.0 0.2 0.4 0.6 0.8 1.0

E, V

FIGURE 13.2 Typical cyclic voltammogram of Prussian blue-modifi ed smooth (mirrored glassy carbon)
electrode; 0.1 M KCl, 40 mV s 1 .

the color, and the reduced form of the polycrystal is denoted as Prussian white. The
transfer of electrons is compensated by the entrapment of cations in the fi lm according
to the equation:

←→ Fe

4 III

[ Fe

II

( CN ) ] 44 e K ⎯⎯ K Fe

II

[ Fe

II

( C N ) 63

(1)

63 4 4

In the literature the term “soluble Prussian blue” introduced by Keggin and Miles
[5] to determine the KFeFe(CN) 6 compound is still widely used. However, it is impor-
tant to note, that the term “soluble” refers to the ease with which the potassium ion
can be “peptized” rather than to the real solubility of Prussian blue. Indeed, it can be
easily shown by means of cyclic voltammetry that the stability of Prussian blue fi lms
on electrode supports is nearly independent of their saturation by potassium cations.
Moreover, Itaya and coworkers [9] have not found any appreciable amount of potas-
sium ions in Prussian blue, which makes doubtful structures like KFeFe(CN) 6 . Thus,
the above equation fully describes the Prussian blue/Prussian white redox reaction.
The Prussian blue/Prussian white redox activity with potassium as the counter-
cation is observed in cyclic voltammograms as a set of sharp peaks with a separation of
15-30 mV. These peaks, in particular the cathodic one, are similar to the peaks of the
anodic demetallization. Such a set of sharp peaks in cyclic voltammograms correspond
to the regular structure of Prussian blue with homogeneous distribution of charge and
ion transfer rates throughout the fi lm. This obvious conclusion from electrochemical
investigations was confi rmed by means of spectroelectrochemistry [10].
The sharpness of Prussian blue/Prussian white redox peaks in cyclic voltammo-
grams can be used as an indicator of the quality of Prussian blue layers. To achieve a
regular structure of Prussian blue, two main factors have to be considered: the depo-
sition potentials and the pH of initial growing solution. As mentioned, the potential
of the working electrode should not be lower then 0.2 V, where ferricyanide ions are
intensively reduced. The solution pH is a critical point, because ferric ions are known
to be hydrolyzed easily, and the hydroxyl ions (OH ) cannot be substituted in their