Chemical and biological sensors based on electroactive inorganic polycrystals - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 415

coordination sphere in the course of Prussian blue crystallization. According to our
fi ndings initial solution with pH 1 is optimal for deposition [11, 12].
It is important to note that not all cations promote Prussian blue/Prussian white
electroactivity. Except for potassium, only ammonium (NH 4 ), cesium (Cs ), and
rubidium (Rb ) were found able to penetrate the Prussian blue lattice. Other mono-
and divalent cations are considered as blocking ones.
At high anodic potentials Prussian blue converts to its fully oxidized form as is
clearly seen in cyclic voltammograms due to the presence of the corresponding set of
peaks (Fig. 13.2). The fully oxidized redox state is denoted as Berlin green or in some
cases as “Prussian yellow”. Since the presence of alkali metal ions is doubtful in the
Prussian blue redox state, the most probable mechanism for charge compensation in
Berlin green/Prussian blue redox activity is the entrapment of anions in the course of
oxidative reaction. The complete equation is:

III II

←→

III III

(2)

Fe [ Fe ( CN ) ] 33 e A ⎯⎯ Fe [ Fe ( CN ) 63 ]

4 63 4

Except for deposition of Prussian blue from the mixture of ferric and ferricya-
nide ions, its electrosynthesis from the single ferricyanide solution is reported [13].
Ferricyanide ions are not extremely stable even in aqueous solution, which is noticed
in the change of color after a few days of storage. Thus, the coordination sphere can
be destroyed also in the course of electrochemical reactions. The mentioned processes
may lead to formation of ferric-ferricyanide complex or “free” ferric ions. The reduc-
tion of the resulting mixture leads to the formation of Prussian blue.
Just a few years after the discovery of the deposition and electroactivity of Prussian
blue, other metal hexacyanoferrates were deposited on various electrode surfaces.
However, except for ruthenium and osmium, the electroplating of the metal or its anodiz-
ing was required for the deposition of nickel [14], copper [15, 16], and silver [9] hexacy-
anoferrates. Later studies have shown the possibilities of the synthesis of nickel, cobalt,
indium hexacyanoferrates similar to the deposition of Prussian blue [17-19], as well as
palladium [20-22], zinc [23, 24], lanthanum [25-27], vanadium [28], silver [29], and
thallium [30] hexacyanoferrates.
As mentioned, potassium ion promotes the redox activity of Prussian blue, whereas
sodium ion blocks it. However, indium, cobalt, and nickel hexacyanoferrates were
successfully grown and then cycled in the presence of sodium as the counter-cation
[18]. There are two possible explanations of redox activity in the presence of sodium.
All these hexacyanoferrates give the single set of peaks in their cyclic voltammograms.
This set of peaks may be attributed to the redox reaction with the charge compensation
due to entrapment of anions rather than cations, similar to the Berlin green/Prussian
blue redox reaction. Alternatively, sodium ions may penetrate the lattice of these
hexacyanoferrates. Transition metal hexacyanoferrates can be deposited on a variety
of materials. The only requirement to support this is its stability at high anodic
potentials. Most commonly Prussian blue and its analogs are deposited on carbon
materials and gold, and in some cases on platinum. For optical measurements ITO
can be used as a substrate. More recently the reports on the deposition of nickel [31],